View Full Version : Intuitive content of Loop Gravity--Rovelli's program
In the "perche i nostri discorsi" thread, selfAdjoint gave a concise sketch of the direction that Carlo Rovelli sees in loop gravity
Originally posted by selfAdjoint
Rovelli's vision of how to move forward beyond present day physics boils down to this. Couple the standard model to the set of quantized QG loops so there will be interactions with eigenvalues of length and eigenvalues of momentum, etc. Combined states. Work on the theory non-perturbatively in that way. No infinity problems because no "classical points" - same advantage stringy physics gives, but with perhaps a more basic underpinning. The point for me is, if you could do this you could calculate numbers for the accelerators. The advantage over normal analysis would be non-perturbative calculations without infinities. The advantage over lattice would be, well it's proposed as real physics, no continuum limit required.
...
A few posts later in the thread I suggested trying to map out an intuitive description of what Loop Gravity is basically about and what makes its approach different. Perhaps it would be good to make a separate thread just to do that, without controversy as to comparative merits or any other distractions. Here is my post from the other thread proposing to do that:
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Probably what we need most now is an intuitive sketch of what makes Loop Gravity different from your typical field theory----how is the backgroundlessness implemented?
Things are coming to a head. Rovelli's new book "Quantum Gravity" is major and has IMO material for a bunch of PhD dissertations just expanding on details. It also contains the "Dialog" as a final chapter. You can connect the points made about loop gravity in the Dialog to chapters and sections in the main part of the book. Also Smolin's April 2003 paper lays out what has been accomplished and what remains to do and what the prospects are for getting loop gravity tested---it is a thorough review and comparison: "How far are we from a quantum theory of gravity?"
Plus we have good accounts of loop cosmology by Bojowald
like the recent paper "Quantum Gravity and the Big Bang", and in some of Ashtekar's papers. It appears progress in cosmology has been dramatic of late. New researchers have been getting into LQG at the level of cosmology.
Plus there is this month's Berlin symposium "Strings meets Loops" which will probably generate a series of overview talks
aimed at wider audience----e.g. another cosmology overview by Bojowald, another full theory overview by Ashtekar, a spin foam overview by Rovelli, and so on.
So there is more and more accessible information than there was a year ago, about loop gravity. It looks to me as if new research possibilities are coming into focus. For example, these days I keep seeing papers about the "low energy limit" or "semi-classical limit", another place where newcomers are getting in (like those Argentine people this month---Kozameh, Gleiser, Parisi)
It seems to me to be a good time to try to say what loop gravity is about, in the simplest possible way. I am apt to make several false starts on this. If someone else has been thinking about it and wants to try, go ahead....
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was practically laughing just now thinking of my trying to give an intuitive explanation of loop gravity. Meteor has a copy of Three Roads, which probably has a perfectly good one, and I am too lazy to go down to the library and get a copy---never seen it.
so meteor or selfAdjoint are probably better equiped to do a sketch of loop gravity
BTW I believe that to give a good conceptual description of something often requires deeper understanding, paradoxically, than to descrbe it technically, so it is a place where one's shortcomings become evident HOWever here goes
I told you before I would make several false starts, it is inevitable, so lets get started
in loop gravity the excitations of space (or geometry or gravity allee same bizness weiss da) are polymers----essentially ball-and-stick models like of very large protein molecules
so the first analogy is a drumhead with sand sprinkled on it. you know that every different way it can vibrate is shown by the lines that appear in the sand when you excite---I hope you did this at the science museum as a kid and know what I mean: nice diagrams of lines appear on the surface and these diagrams CATALOG all the modes of vibration or the excitation modes.
the next analogy is a sink full of soapsuds---you had to wash the dishes as a kid and you can imagine the whole sink or the whole universe (whats the difference) full of soapsuds. Now in the middle of every bubble put a dot, and if one bubble contacts another bubble then connect those two dots with a line.
Now you have a network (a ball-and-stick molecule, a polymer) that fills the whole sink or universe. And we are going to give a number Q to each point (or ball, or dot, or vertex) in the network.
And a number P to each connecting line.
Whatever for? Why label each point and line with a number? (Roger Penrose thought of doing it, he is the one to ask about it) Well, using that additional information the network can tell us the VOLUME of any region----just add up the numbers attached to each point in the region, somehow-----and the AREA of any surface---just add up the numbers, in some fashion, that are attached to every line that punctures the surface.
Furthermore, if this labeled network's sticks become flexy the whole thing can be squashed flat and stuffed into your dresser drawer where you ordinarily keep undershirts.
So here is a thing which you can squash any shape and store anywhere which nevertheless tells you everything about the geometry of the universe, or the kitchen sink I forget which.
Yes, I have a copy of the book, and in it a spin network is defined as a directed graph, jointly with a series of rules that guide the evolution of the graph. To each edge of the graph is assigned a number, and the area of a surface depends on the value of the numbers of the edges that punctures the surface.Btw, the area of a surface can be computed with a formula that includes the Immirzi parameter. The volume of an object is proportional to the number of nodes of the graph thatr are inside the object.
However, in various documents on Arxiv, i've found that the edges of the graph are labeled with group representations of Lie groups, and the vertices with intertwining operators(damned if i now what's an intertwining operator!)I don't know if these spin networks are the same that the cited in the book of Smolin
Spin networks are used in the canonical approach to quantum gravity, but another approach, the sum-over-histories approach, has adopted a particular version of them, called spin foams, that are cell complexes.
selfAdjoint
Oct18-03, 10:20 AM
I don't know if these spin networks are the same that the cited in the book of Smolin
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
Originally posted by selfAdjoint
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
I'm sure you are right---what a spin network is in the literature must be the same as what Smolin describes in his book (allowing for whatever minor naming conventions differ) but I should emphasize that the picture I gave here of a "spin network" (*NOT*) bears only a faint resemblance to what defines a basis for the quantum states of geometry in the theory. Im still trying to see how to introduce the ideas in as intuitive and non-technical way possible----I may have to break down and see how Smolin did it in "Three Roads".
here's another piece of the jigsaw puzzle
in classical mechanics things move along trajectories---curved paths parametrized by time---and when you quantize the trajectories go away.
the curved paths things travel along dont exist any more, you have to erase the trajectories (or in Feynmann sum over histories you "integrate" all possible ways of getting from here to there---in any case the clear picture of a path loses reality and dissipates)
in GR, the 4D manifold is not a real thing (individual points are not events and have no physical meaning) because of gauge invariance any point will do----to define an event you need matter, like the event that two particles cross paths (at some point, but which point doesnt make any difference it is an arbitrary choice.
arbitrary choices needed to express something mathematically are called "gauge"----extra physically meaningless information that gets unavoidably mixed in as part of keeping the books.
in GR the 4D manifold is there so that you can write down the trajectory of the gravitational field. GR does not suggest that 4D spacetime exists, it is a mathematical amenity used for defining evolution of the gravitational field and the matter that goes into shaping the field.
but when GR is quantized, the trajectory goes away and one simply has a 3Dmanifold, with a space of all possible geometries defined on it
again, as in classical case, the points of the 3D manifold have no physical meaning---they are just "gauge". One can define surfaces and volumes only using matter---Rovelli and Ashtekar both use examples like by a surface I mean for instance the top of this desk.
The quantum states are functions defined on the space of all possible geometries that the 3D manifold can have. Analogous to quantizing a particle moving on the line by "wave functions" defined on the line.
The curious thing is that no one started out thinking of the "wave functions" defined on the space of geometries as spinnetworks! Nobody was looking for spin networks or asking for them! It just turned out that they appeared as the best way to CATALOG the functions defined on the space of all possible gravitational fields or all possible geometries.
At first they tried defining these functions using loops and they got a hilbertspace of loop-functions, but they couldnt get an orthogonal basis: the loops were too redundant. So they eventually borrowed spinnetworks from Penrose and they turned out to give an orthogonal basis for the space.
Also I even believe that the basis is countable and the hilbertspace is separable----technical conveniences to be sure.
So when I mentioned this polymer network thing that describes the geometry of the whole universe, but that you can stuff into the dresser drawer, it is a quantum state of geometry (a functional like a wavefunction defined on the collection of all classical geometries) and all quantum states look like this or combinations of things like this.
and matter fields must be defined on things like this
and the quantum state can evolve! At noon by some clock it can be this one in the top drawer and then at one o'clock it can be like this other one in the bottom drawer.
But it is a disconnected hopping, and there is no absolute clock you just have to choose some physical PROCESS (essentially something involving matter) to serve as a clock. This clock is part of the world and there is a correlation between what you observe the clock says and where you observe the pendulum is, or how far away the galaxies are, or whatever else. There is no one absolute time that drives the rest only correlations between different processes
processes which include, among other things, the change in the state of the 3D geometry of space.
So, when you check things out using cosmology, the evolution equation is a finite DIFFERENCE equation! It is not a differential equation. when you do loop cosmology the Friedmann equation that all cosmologists depend on becomes a step by step difference equation----e.g. Bojowald uses the scalefactor as his clock, there must be some physical process to use as clock, and correlates other stuff like curvature and inflation and density with the scalefactor. And so does everybody else that has been doing loop cosmology that I have seen. For example "Quantum Gravity and the Big Bang" the talk Bojowald gave recently.
it is interesting how the concept of time changes.
In Rovelli's textbook "Quantum Gravity" there is a philosophical section on time which I found really interesting---he finds that different branches of physics use ideas of time that are actually different from each other and also from everyday vernacular time.
he is able to distinguish around 8 or 9 different ideas of time.
Quantizing General Relativity seems to exert a strong influence on the ideas of time because both QM and GR bring insights about time which, if you try to put them together, produce something that seems radically new.
(of course one can avoid having to think about it if one throws out GR and replaces it with a lobotomized form or if one is very careful to only use GR and QM in separate situations and never together on the same problem)
exciting business
ranyart
Oct19-03, 05:53 AM
Marcus, there are a vast number of papers that continue to reveal a discrete direction, I am really glad that you take the time to post the most interesting idea's from many fields.
You may have this link allready?..but if so others may find it interesting:http://uk.arxiv.org/abs/gr-qc?0306059
Rovelli for me seems to be an architect of new thinking.
Originally posted by ranyart
Marcus, there are a vast number of papers that continue to reveal a discrete direction, I am really glad that you take the time to post the most interesting idea's from many fields.
You may have this link allready?..but if so others may find it interesting:http://uk.arxiv.org/abs/gr-qc/0306059
Rovelli for me seems to be an architect of new thinking.
thanks for calling attention to that paper, Ranyart. For some reason I had just glanced at it earlier. He probably is.
http://arxiv.org/abs/gr-qc/0306059
I will quote the last 5 sentences in (the conclusions part of) this paper and try to say why I think it is interesting
"...We have studied the propagator of our model in detail. We have shown that in the semiclassical limit it has a simple relation with the Hamilton function of the classical theory, but this relation is not a simple exponential, as one might have expected.
Instead, the propagator is real. It is the sum of two exponential
terms complex conjugate to each other, that propagate backward and forward, respectively, along the motions. Accordingly, the physical Hilbert space splits between forward and backward propagating states.
We expect this structure to be the same in quantum general relativity."
this is the kind of simple example (a system like a springbob with only a couple of degrees of freedom) that physics teachers love to use when introducing a new method---try the new approach out on the simplest thing in sight: an harmonic oscillator, a single particle in a potential well, whatever. Then the maths do not obscure the ideas.
So this is Daniele Colosi (a grad student at Marseille) and Rovelli having fun with a toy that moves in a simple ellipse. Is this your reading too? I just looked at the paper. I like it. Maybe we should make a thread about this paper or just look at it in this thread
Originally posted by marcus
thanks for calling attention to that paper, Ranyart....
http://arxiv.org/abs/gr-qc/0306059
...should make a thread about this paper or just look at it in this thread
your bringing up this paper got me looking at this and several related ones over the past day or so.
A Simple Background Independent Hamiltonian Quantum Model (Colosi/Rovelli)
Minkowski Vacuum in Background Independent Quantum Gravity
(Conrady, Doplicher, Oeckl, Rovelli, Testa)
and several related 2003 spin foam papers
recent work involving the hamiltonian seems to connect (in ways I didnt anticipate and dont fully understand) to recent spin foam work
there is the fact that in August 2002 John Baez and some others posted computer results that some spin foam numbers were not what some people expected them to be----this seems to have lead to increased interest in spin foams: something new to understand about them----several new papers with new ideas
then there is the fact that at this months symposium it is Rovelli who is talking about spin foams (and he and his associates have recently, in late 2002 and in 2003) put out several papers on spin foams
then there is the fact that several of these recent papers link up the hamiltonian and spin foam approaches----they are or seem to be trying to cure problems in both the hamiltonian and discover how to use spin foam models properly in a way that suggests some underground connection between the two
i had till now not paid attention much to spin foam quantum gravity but now because of these little hints Ive been reading in the past day or so, and because Rovelli has chosen to do the spinfoam presentation, I am beginning to pay more attention and trying to understand a little better.
BTW at the symposium the loop lineup looks like this
Ashtekar: quantum geometry and applications (this means overview and application to big bang, inflation, black holes...)
Bojowald: loop quantum cosmology (a strong run of results by him and about 10 other researchers over past 3 years, giving guidance to development of the full theory by testdriving in the cosmology case)
Rovelli: spin foams (this is the one that I cannot antipate, it will have unexpected things)
Lewandowski: the hamiltonian (this presumably will be profoundly analytic/algebraic as is the way with people from Warsaw. maybe selfAdjoint will help us understand this one [:)],
it has now been 5 years since Lewandowski found fault with Thiemann's hamiltonian and there has been a great deal of work involving hamiltonians since then! Perhaps L will summarize some of this. As befits a growing theory, the main issue here remains unresoved and people are still discovering how it should look, as for example in the paper you gave the link to )
This is merely by way of saying thanks for the link to that paper. It has given me something to do during spare moments for the past day or so
Yes they are the same. Smolin said the edges were labelled by a number in order not to confuse his readers. The closer statement would be the edges carry a spinor. This spinor is not just a label, but a genuine bit of physics. The intertwiner functions are like black boxes - deterministically relating spin reps in to spin reps out. These again are physics, somewhat like Heisenbeg's S-matrix relating momenta in to momenta out.
I've reading the paper where spin networks where introduced in the first time, "Spin networks and quantum gravity"
http://arxiv.org/abs/gr-qc/9505006
In this paper spin networks are defined like trivalent graphs with edges labeled by positive integers.A trivalent graph is a regular graph with 3 edges arriving at each node. The spin network has to follow 2 rules:
-The sum of the 3 edges that converge at a given node has to be an even number
-Each of these 3 edges can't be superior to the sum of the other 2
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
Ok, Ok, very much questions but this is interesting stuff
selfAdjoint
Oct20-03, 02:31 PM
I'm not sure, but I think gr-qc/9707010 (http://xxx.lanl.gov/abs/gr-qc/9707010) is early. See also Baez's TWF #110.
Originally posted by meteor
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
Ok, Ok, very much questions but this is interesting stuff
Meteor I hope my replying does not preclude a PF mentor or other knowledgeable person responding.
to say irreducible representation of SU2 is sort of like saying "spin" because there is one for each dimension and so roughly speaking one for each integer (or half integer if you divide each integer by two according to the quaint ancient custom of physicists)
the papers where Penrose made up "spin network" idea are not online!
however I have read about these papers and my understanding is that ALREADY AT THE BEGINNING penrose thought of the graph as labeled by "spins" that is to say either halfintegers or, what is the same, irreducible reps of SU2
as an interesting insight into human, or at least Penrose, nature, he regularly FLIPFLOPPED at the beginning between having the labels be integers which he called "colours" and dividing them all by two and calling them "spins". As a civilized mathematician he wanted to call them colours but as a savage physicist driven by brute instinct and prejudice he needed to divide them by two---as is the custom---and call them spins.
so this ambiguity of labeling has been there from the start
remember also that as children, while others are taught to skip rope and play hopscotch, physicists are taught that SU2 is the "double cover" of the rotation group, which is why an electron can turn around 720 degrees before it looks normal again. the first time it turns around it appears to have pointed teeth and is wearing a Count Dracula costume but then it turns around another 360 degrees and is its old self. But doubtless you know all this already!
ranyart
Oct20-03, 03:18 PM
Originally posted by marcus
so this ambiguity of labeling has been there from the start
remember also that as children, while others are taught to skip rope and play hopscotch, physicists are taught that SU2 is the "double cover" of the rotation group, which is why an electron can turn around 720 degrees before it looks normal again. the first time it turns around it appears to have pointed teeth and is wearing a Count Dracula costume but then it turns around another 360 degrees and is its old self. But doubtless you know all this already!
I like it!
The inside of Fort knox has a safe where the rotation of the combination/wheel-number will dictate if one opens the safe or not?
The inside cogs and wheels are dictated by the outside combination wheel, one false turn and CPT kicks in and you are forever going to be turning..and turning..the dials. Yet if one were on the inside and the back of the door had a seethrough covering, one can guide a way through any amount of infinite combinations with ease!
I'm not sure, but I think gr-qc/9707010 is early. See also Baez's TWF #110
Thanks! In Baez 110 put that spin networks can have more than 3 edges meeting at a vertex, so they are not actually considered trivalent graphs (Baez 110 was written in 1997)
I'm trying know to fathom what's the meaning of the Poisson algebra. I will post something about it
selfAdjoint
Oct21-03, 11:23 AM
Good for you if you post on the Poisson algebra. This is a missing piece in our discussions here.
Originally posted by selfAdjoint
Good for you if you post on the Poisson algebra. This is a missing piece in our discussions here.
Amen to that!
I told you I would make several false starts. Eventually there should be a non-technical description of loop gravity in only one to ten pages. Let's keep this thread going until we have one, or find one in the literature.
the basic picture of any quantum theory is you have a space of possibilities (configurations, might be simply a set of possible positions and momentums) and then you define a kind of "(not)probability" function or wave-function on that space of possibilities.
If the space of all possibilities, of whatever it is (one particle, N particles, a field, a geometry of the universe) is called A, then the the wavefunctions or quantum states or "(not)probability" functions are just complex valued functions on
A
usually there is a measure defined on called A so you can integrate these functions and they are "square integrable" which means they dont run off to infinity too much and have finite integrals
and I have to say that in mathematics this is, in a certain way, as basic as things ever get----a space, some complex-number-valued functions defined on that space---and being able to integrate or sum each of them, so each one has a finite size.
a loaf of bread, a jug of wine, and hilbert space---this is all we ask and it does not seem like a lot----the rest is trimmings.
so in a certain sense if I could just tell you how to build the configuration space called A of loop gravity and then, if I could just explain how to define a function on that space----and get a hilbertspace consisting of all the complex-number-valued functions on called A then I would be done explaining. All the rest----the selfadjoint operators on the hilbertspace, their evolution, their spectra, and all----that all "hatches" from how the original hilberspace of wavefunctions, or quantum states or whatever you call them, is defined.
So here we are down in the basement and there are not even any "spin networks" or "loops" around. I have to tell you the space called A of loop gravity.
Psssst! It is the space of "connections". A connection is one way to clothe a bare manifold with geometry if it has no geometry. The whole destiny of loop gravity, win or lose, succeed or fail, is in this one choice----the geometry of the universe shall be represented by the possible "connections" on a 3D continuum, a 3D manifold.
Until 1986 the guys like John Wheeler who were trying to quantize GR used the space of "metrics" for their called A and it gave them headaches. After 1986 almost everybody switched over to representing the geometries by connections.
Hey, the whole thing could go into another iteration if some yet other set of variables for GR were found---something that captured the essence of the shape of the world that was not a metric and not a connection---then you could have a new configuration space called A and a new hilbertspace of complex-valued functions defined on it.
It took 50 years to get from a space of metrics to a space of connections---people have tried to quantize GR for a long time. I am not betting they come up with something to replace connections, but they might.
So we are looking at the most basic question---how do you describe the shape of the world, what is a connection, how do you arrange all the possibilites to make a configuration-space, a space of all possible connections, how do you define functions on that space, that have their values in the complex number plane?
what is a connection?
Hi here,
With respect to the Poisson algebra, the only thing that I've discovered is that in 1987, Smolin and Rovelli introduced an infinite set of gauge invariant loop variables on the phase space of the theory (called then the Ashtekar phase space). These variables form a closed Poisson algebra
I don't know if Poisson algebras are anymore important in LQG since loops were substituted by spin networks
Marcus reading your anterior post, there are some ideas that have popped up in my mind, could you clarify, please? [t)]
Is it possible that this Ashtekar phase space is really the Hilbert space of LQG?. I mean, is possible that the loop variables introduced by Rovelli and Smolin are functions in this Hilbert space?
Is it possible that before the introduction of the loop variables, the functions in the Hilbert space were the connections?
Best wishes and keep fighting the stringers!
nonunitary
Oct22-03, 05:40 PM
Originally posted by meteor
Hi here,
With respect to the Poisson algebra, the only thing that I've discovered is that in 1987, Smolin and Rovelli introduced an infinite set of gauge invariant loop variables on the phase space of the theory (called then the Ashtekar phase space). These variables form a closed Poisson algebra
I don't know if Poisson algebras are anymore important in LQG since loops were substituted by spin networks
Marcus reading your anterior post, there are some ideas that have popped up in my mind, could you clarify, please? [t)]
Is it possible that this Ashtekar phase space is really the Hilbert space of LQG?. I mean, is possible that the loop variables introduced by Rovelli and Smolin are functions in this Hilbert space?
Is it possible that before the introduction of the loop variables, the functions in the Hilbert space were the connections?
Best wishes and keep fighting the stringers!
Hi there,
I am kind of new in this forum and found this tread about LQG which interests me a lot. I don´t know if I am misleading but I think that poisson algebras are still important in LQG, but they are not so much
talked about. Rovelli and Smolin indeed had a Poisson algebra that was the starting point for quantization. I think that Ashtekar and co-workers wrote a paper showing that the Rovelli-Smolin algebra was not closed and changing the poisson algebra to some other algebra I don't remenber. Recently, Sahlmann, Lewandowski and Thiemann have
taken this proposal and expanded it.
Just a few more comments. The phase space of the theory is not the same as the Hilbert space. Normaly, this is constructed out of functions of the configuration space, in this case, connections.
The loop variables are functions of the connection, but labelled by loops (or graphs in the case of spin networks).
Originally posted by marcus
what is a connection?
Gravity
Well the question to me then raises what the foundation of this whole topic is built upon? Philospohically Smolin was able to unite three roads to form algebraic topology.
The basis of the all the maths including the geometries must also follow the logic? Venn logic, and geometrical Intuitional developement? Category theory and topos(Algebraic Topology) was a integration of Smolins Three Roads?
It had to begin from supersymmetry, and from this, the gravity is understood, as well as, weak field settling to boundaries and defintion in cooling and discrete forms?
But in all of this, there is a exchange between energy/matter and the mobius can see things turn over as well as the klien bottle turning inside out. Where is that? Twisting in differential rotations?
You had to be able to see it from Kaluza and Kliens perspective and how we got there. U(1)=5d......and this includes all the covers?
Kip thorne help us to visualize, and in this great distance Ligo reads and in the quantum world how much more is this energy that continuity says, listen, things seem very smooth. But we have discrete structures, and how is the gravity revealed from the perspective of tangible objects, but in the recogniton of the distances?
Intuitively it must come to the distances? We do not disregard the structure that arises from the movement of the energy into objects(crystalization)
Sol
selfAdjoint
Oct22-03, 06:28 PM
Sol, it was a rhetorical question. Marcus is going to tell us what a connection is. I'll bet he's working on his metaphors right now.
Originally posted by selfAdjoint
Sol, it was a rhetorical question. Marcus is going to tell us what a connection is. I'll bet he's working on his metaphors right now.
Oh good:) You know how intuition can be sometimes trying to find the right words to explain the essence of things.
Sol
Hi meteor, welcome nonunitary,
everything you two said in your posts seems right to me, and in particular this:
Originally posted by nonunitary
The phase space of the theory is not the same as the Hilbert space. Normaly, this is constructed out of functions of the configuration space, in this case, connections.
The loop variables are functions of the connection, but labelled by loops (or graphs in the case of spin networks).
This is a good thing to be clear about----the configuration space is the set of possible connections (reflecting all the possible geometries there could be on the manifold). In the process of quantizing a classical theory a hilbertspace is constructed consisting of (complex number valued) functions defined on the configuration space.
Spaces of functions are typically convenient to use because they are linear---you can add two functions just like you add two vectors and so on----spaces of functions are typically infinite dimensional vectorspaces and they are handy just the way vectorspaces are. (This is hardly news to you meteor and nonunitary but might as well be said) And a hilbertspace is a type of vectorspace that commonly comes up as a set of real or complex-valued functions defined on something---in this case the classical theory's configuration space: the set of all possible geometries, as represented by the connections associated with them.
A classical theory will have a poisson algebra of various readings off of phase space corresponding to classical deterministic measurements you can make and in the process of quantizing the theory one will want to find an algebra of OPERATORS on the hilbertspace that these things correspond to.
I agree with nonunitary about the role played the poisson algebras and the algebras of quantum observables that people seem able to discover corresponding to them. Also that Sahlmann, Thiemann, Lewandowski and others have been busy with these things recently.
In effect, you are ahead of me right now and there is no need to wait for me to catch up. Anything you know about the theory that you want to explain, you should go ahead! We have no special responsibilities to anyone and no need to follow any special order.
What iterests me right now is this: how would you explain to someone with a minimum of math a way to think about connections.
A manifold is just a set equiped with coordinates around any point so that you CAN use those coordinate patches to give it geometrical shape if you so desire. But a bare manifold is devoid of geometry----there is almost nothing interesting about it unless it has some kinky topological features.
One way to put some starch in your manifold is to define a METRIC on it (which you can do because you have the kind of minimal amenities, namely coordinate patches). Once you have said what the distance between each pair of points in the manifold is, the thing snaps to attention and takes on shape.
But you can also proceed a different way. Because you have this minimal structure of coordinates you can define the tangent "plane" at every point---actually for a 3D manifold it is a tangent 3-space not a tangent plane. And the thing still looks like a ruppled shirt or wet paper bag, it just has tangentspaces stuck to it. But NOW you can decide on the infinitesimal "roll" that happens as you go from one point to the nearest neighboring points. That is, you can pick a "connection". And by continuing along a path and integrating the tiny roll at each point you get
by the end of the path a reall substantive rotation. So a "connection" is basically a contraption that tells how tangent vectors are supposed to rotate as you move along a definite path thru the manifold.
Well that bespeaks geometry too, just like a metric does. Technically it doesn't completely determine it but intuitively it goes a long ways towards defining what shape the thing is.
Now physicists learn Lie groups and Lie algebras long before they put on long pants and start shaving and having dates, so for them it is a kneejerk response that an infinitesimal "roll" is an element of su2, the Lie algebra of SU2. It is like the things you have to learn to pass your drivers test. An element of su2 is a particularly cute kind of 2x2 matrix of complex numbers.
So what is a connection? It is a program I have on my palmpilot that if you show me a point in the manifold and show me a DIRECTION in which to set out from that point, I will tell you the infinitesimal "roll" that is I will tell you a cute 2x2 matrix of complex numbers which is a member of su2.
And in a very rough sense all the possible geometries the manifold can have are reflected usefully in the set of all possible connections that can be defined on it
Ambitwistor
Oct23-03, 01:59 AM
Originally posted by meteor
Do somebody know the paper where the labels passed from being numbers to group representations? Do the Lie groups have to be some specific Lie group? Are actually spin networks continued to be defined as trivalents graphs? Must the group representations be irreducible representations?
The labels of spin network have always been representations; it's just that in the case of SU(2), representations can be simply labeled by numbers.
The Lie group can be anything, but in the connection variables, an SU(2) spatial connection is typically used, which leads to a kinematical Hilbert space of SU(2) spin networks.
Spin networks don't have to be trivalent. In fact, in LQG, trivalent spin networks have zero volume.
The group representations are irreducible, since the point of spin networks is to form an orthonormal basis of the space of connections (modulo gauge transformations); for that, you want networks labelled with irreps, as follows from the Peter-Weyl theorem; see http://arXiv.org/abs/gr-qc/9504036.
Originally posted by Ambitwistor
Spin networks don't have to be trivalent. In fact, in LQG, trivalent spin networks have zero volume.
I checked this in Rovelli's textbook and you are right: he says a node must be at least quadrivalent to have nonzero volume. I like everything in your post and look forward to many more. welcome. there is a considerable need for loop-knowledgeable people here.
I guess I need to look again at the volume formula, for some reason until I checked just now, I thought that a trivalent vertex contributed a unit of volume. I know that any vertex can be broken down by a series of surgical steps into a collection of trivalent one, I must go back and try to understand what happens to the volume.
Ambitwistor
Oct23-03, 02:34 AM
Originally posted by marcus
In the process of quantizing a classical theory a hilbertspace is constructed consisting of (complex number valued) functions defined on the configuration space.
At this level, it might be worth emphasizing that these "complex-valued functions on configuration space" are what some people might know better as "wavefunctions": the configuration space describes the system (like the position of a particle), and the value of the wavefunction (a complex number) is the probability amplitude for finding the system in that particular configuration (like the probability of finding a particle in a particular location).
So a "connection" is basically a contraption that tells how tangent vectors are supposed to rotate as you move along a definite path thru the manifold.
Well that bespeaks geometry too, just like a metric does. Technically it doesn't completely determine it but intuitively it goes a long ways towards defining what shape the thing is.
I'm not sure that I understand this side remark correctly, but if you want to speak technically, a connection does completely determine the geometry: if you have a Levi-Civita connection, then that is equivalent to having a metric. If you have some other kind of connection, then it defines a more general kind of geometry (not Riemannian) that does not arise from a metric.
Originally posted by Ambitwistor
At this level, it might be worth emphasizing that these "complex-valued functions on configuration space" are what some people might know better as "wavefunctions": the configuration space describes the system (like the position of a particle), and the value of the wavefunction (a complex number) is the probability amplitude for finding the system in that particular configuration (like the probability of finding a particle in a particular location).
absolutely right! In another thread I stressed the term "wavefunction" for this. I should always mention that as a synonym. I like to try a few alternative ways of saying things to allow for people coming to the subject from different backgrounds.
....a connection does completely determine the geometry: if you have a Levi-Civita connection, then that is equivalent to having a metric. If you have some other kind of connection, then it defines a more general kind of geometry (not Riemannian) that does not arise from a metric.
I am so glad you are on hand, ambitwistor! I will try (if the board permits it) to edit some of my posts to remove the vagueness about that in accordance with what you say.
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
Ambitwistor
Oct23-03, 03:00 AM
Originally posted by marcus
I guess I need to look again at the volume formula, for some reason I thought that a trivalent vertex contributed a unit of volume. And that any vertex could be broken down by a series of surgical steps into a collection of trivalent one.
As mentioned in Baez's Week 55, Loll showed that trivalent vertices do not contribute volume (http://arXiv.org/abs/gr-qc/9511030).
As for breaking down vertices into trivalent vertices, you're probably thinking of those tangle diagrams, where the spin network edges are decomposed into "virtual" nodes and edges, wired up according to recoupling theory. You can decompose spin networks into trivalent diagrams of that sort, but they aren't spin networks -- though they are equivalent to (not-necessarily trivalent) spin networks.
See http://relativity.livingreviews.org/Articles/lrr-1998-1/node17.html#TheVirtualNode
I'm impressed. In 5 minutes Ambitwistor came up with two
precisely-on-target online references---one to original work by a remarkable woman named Renata Loll and one to the exact place in Rovelli's classic LivingReviews exposition. That means Ambitwistor is a pro. Probably his/her time is too valuable to hang around much. Came thru like a big train through a small station and that was it.
Well well
I'd stay up and see what else happens tonight, but its after midnight here and time to turn in
(today selfAdjoint plugged PF, and the loop gravity threads in particular, on SPR Usenet. this could explain unexpected visits.
nice if some of these people stayed around)
selfAdjoint
Oct23-03, 09:31 AM
Marcus,
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
I have two thoughts on this. One, Ashtekar doesn't use all of his connection, but only the "anti-self-dual" part. Two, the Ashtekar variables specify not only a geometry but a kinematics. Thiemann's intro does a lot of degree-of-freedom counting, maybe that would be a reference on this issue.
Originally posted by selfAdjoint
Marcus,
I have two thoughts on this. One, Ashtekar doesn't use all of his connection, but only the "anti-self-dual" part. Two, the Ashtekar variables specify not only a geometry but a kinematics. Thiemann's intro does a lot of degree-of-freedom counting, maybe that would be a reference on this issue.
I was counting on being able to edit some of this expository writing, but discovered yesterday that the PF rules have changed. there is a time limit of 30 minutes afterwhich I cannot edit a post
It makes it easier to write if you can put in placeholder stuff at some point, then go research it and fix it up if necessary.
If you have more extensive editing capability (a mentor perk?) than I do, you are welcome to fix vague points in my discussion, correct errors, improve style or whatever. editing is half of writing
I was a bit sleepy when Ambitwistor passed through---not sure but I got the impression of someone who may actually do research in quantum gravity----knows work of Baez and of Renate Loll with quick exactitude.
nonunitary
Oct23-03, 11:58 AM
quote:
--------------------------------------------------------------------------------
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
--------------------------------------------------------------------------------
Hi there,
I think there is a confusion here. It is true that a metric and its (Levi Civita) connection carry almost the same information (up to constant re-scaling of the metric). In the basic variables of LQG the metric information of the manifold is in the triad E. The connection "A" that is known as the Ashtekar-Barbero connection has more infromation than just the metric. It also knoes about the ADM conjugate variable, namely the extrinsic curvature $K_{ab}$. Then the connection $A$ is given by $A_a^i=\Gamma^i_a- (const) K^i_a$, where the constant in the formula is the infamous Immirzi parameter.
Thus, even when the connection knows about the metric, it also has information about the extrinsic curvature, and that is why it serves as a conjugate variable for the $E$'s (that is, if the Immirzi parameter were zero, the variables would be all "configuration variables", and its Poisson bracket would vanish).
Another comment. Te self dual connections were the original variables introduced in 1986 by Ashtekar, but they were replaced in the 90's by the Ashtekar-Barbero connection with a "real" Immirzi parameter (instead of $i$ for the original self-dual case). The nice geometrical interpretation is however, lost.
Ambitwistor
Oct23-03, 12:40 PM
Originally posted by marcus
My intuitive feel is that the connection describes the geometry, and I am puzzled that the Ashtekar variables are not simply A (the connection) but are various pairs, like [A, E] where E is the densitized triad or "electric field"(a term sometimes used depending on a possibly confusing analogy). A and E are presented as "conjugate" variables. Yet the configuration space is just the collection of all possible A's. what is the essential additional information given by E? You are probably familiar with the notation I'm using, for brevity omiting subscripts and so on.
(A,E) are a conjugate pair, i.e., a point in phase space (not configuration space). It's analogous to how in QM, position and momentum (x,p) are a conjugate pair (but you only pick one of them them to form your quantum Hilbert space). In the ADM geometrodynamic variables, the 3-metric and the extrinsic curvature form the conjugate phase space variables, and you generally form your Hilbert space over the 3-metric. In the Ashtekar variables, you pick the Ashtekar connection and the densitized 3-triad.
Classically, you use the configuration space variable to describe the kinematics, and the conjugate momentum to describe the dynamics. (e.g., position in QM or the 3-metric in geometrodynamics describes the system at a given instant of time, and momentum or the extrinsic curvature describes how that state will evolve).
You can reconstruct the (densitized) 3-geometry of a spatial slice from E, not A (since the triad is basically just the "square root" of the 3-metric, as nonunitary mentioned). A itself determines a "geometry" on space, just like any connection does, but it's not the kind of metric geometry that a Levi-Civita connection defines. A actually carries information that one can use to reconstruct the spacetime geometry -- like extrinsic curvature does, and in fact A involves the extrinsic curvature (as nonunitary also pointed out).
selfAdjoint
Oct23-03, 01:55 PM
In Thiemann's derivation of the Ashtekar variables he first enlarges the phase space of the Palatini action, spanning this larger space with canonical variables K and E, K will go away but E will remain in the Ashtekar variables. He shows that the new (K,E) coincide with the Palatini (p,q) variables when a constraint is satisfied; this constraint is satisfied identically in the Palatini geometry. Only then is the connection A introduced, and it replaces the nonce variable K, and the new variables (A,E) are canonical and span the big phase space.
In general is it really true that a connection by itself specifies a geometry? Recall that in traditional Riemann you have first a metric - specified by a symmetric tensor, which restricts your choice of geometries, and then define the connection as a function of your metric (through the Christoffel symbols). This then gives you the curvature tensor and all the rest. But the contribution of the symmetric metric was important.
Ambitwistor
Oct23-03, 02:17 PM
Originally posted by selfAdjoint
In general is it really true that a connection by itself specifies a geometry? Recall that in traditional Riemann you have first a metric - specified by a symmetric tensor, which restricts your choice of geometries, and then define the connection as a function of your metric (through the Christoffel symbols). This then gives you the curvature tensor and all the rest. But the contribution of the symmetric metric was important.
Only a Levi-Civita connection specifies a Riemannian geometry, because only L-V connections are compatible with metrics. But starting at least with Klein, and certainly since Cartan, the notion of "geometry" has been expanded to include geometries other than Riemann's. You can think of a connections as giving a generalized kind of geometry, a special case of which are the Riemannian (metric) geometries.
Ambitwistor
Oct23-03, 03:33 PM
(Oops, that should be "L-C connections", i.e. "Levi-Civita".)
Ambitwistor
Oct23-03, 07:21 PM
Example: Yang-Mills gauge theories are geometric theories, even though they're not Riemannian geometries.
The gauge field A (e.g., the the scalar and vector electromagnetic potentials, together forming the 4-potential) is given by a connection, and the field strength tensor F (e.g., the electric and magnetic fields, together forming the Faraday tensor) is the curvature of that connection.
So, in addition to gravity, the fields of the Standard Model (electromagnetic, weak, strong) are also given by geometric theories, but it's not the Riemannian spacetime geometry of general relativity. The Ashtekar variables exploit this similarity by recasting general relativity in a form more similar to the geometry of other gauge theories. You can also go the other way, and try to recast the gauge theories in a form more similar to the geometry of conventional general relativity, in which case you get Kaluza-Klein theory.
Originally posted by Ambitwistor
Only a Levi-Civita connection specifies a Riemannian geometry, because only L-V connections are compatible with metrics. But starting at least with Klein, and certainly since Cartan, the notion of "geometry" has been expanded to include geometries other than Riemann's. You can think of a connections as giving a generalized kind of geometry, a special case of which are the Riemannian (metric) geometries.
At the start of the thread here I was hoping to find a way of presenting an intuitive picture of loop gravity.
Now I'm recalling the explanatory job Baez did on a variety of formalisms for GR---Palatini, Ashtekar-Sen, Barbero variation---I believe it was in TWF with references to hardcopy (the book by Ashtekar, which I have not read having been spoiled by the internet). Now I am thinking that either it is impossible to do what I had in mind. Or Baez will do it and put it on his website one of these days. Or one of the others (of several talented writers in loop gravity.) Or else....the way to go is to start with what you just said "You can think of a connections as giving a generalized kind of geometry, " and (possibly by means of dervish-like handwaving) OMIT the construction of the new GR variables but just take as given that a manifold has a space of all possible connections which reflects all its possible geometries and just go from there. *Takes a deep breath*
Was delighted by one of the other poster's (Gale's) idea of a wickedly clever third grader---which you elucidated by classical anecdote--and am wondering if that approach to quantum gravity would fly with such a third grader.
eigenguy
Oct25-03, 11:31 AM
Originally posted by marcus
At the start of the thread here I was hoping to find a way of presenting an intuitive picture of loop gravity.
I am studying the following paper
http://arxiv.org/abs/gr-qc/0207106
Abstract: A program was recently initiated to bridge the gap between the Planck scale physics described by loop quantum gravity and the familiar low energy world. We illustrate the conceptual problems and their solutions through a toy model: quantum mechanics of a point particle. Maxwell fields will be discussed in the second paper of this series which further develops the program and provides details.
Here's an excerpt:
"We will begin with the usual Weyl algebra generated by the exponentiated position and momentum operators. The standard Schrodinger representation of this algebra will play the role of the Fock representation of low energy quantum field theories and we will construct a new, unitarily inequivalent representation called the polymer particle representation in which states are mathematically analogous to the polymer-like excitations of quantum geometry. The mathematical structure of this representation mimics various features of quantum geometry quite well; in particular there are clear analogs of holonomies of connections and fluxes of electric fields, non-existence of connection operators, fundamental discreteness, spin networks, and the spaces Cyl and Cyl*. At the basic mathematical level, the two descriptions are quite distinct and, indeed, appear to be disparate. Yet, we will show that states in the standard Schrodinger Hilbert space define elements of the analog of Cyl*. As in quantum geometry, the polymer particle Cyl* does not admit a natural inner product. Nonetheless we can extract the relevant physics from elements of Cyl* by examining their shadows, which belong to the polymer particle Hilbert space HPoly. This physics is indistinguishable from that contained in Schrodinger quantum mechanics in its domain of applicability.
These results will show that, in principle, one could adopt the viewpoint that the polymer particle representation is the `fundamental one'|it incorporates the underlying discreteness of spatial geometry|and the standard Schrodinger representation corresponds only to the 'coarse-grained' sector of the fundamental theory in the continuum approximation. Indeed, this viewpoint is viable from a purely mathematical physics perspective, i.e., if the only limitation of Schrodinger quantum mechanics were its failure to take into account the discrete nature of the Riemannian geometry. In the real world, however, the corrections to non-relativistic quantum mechanics due to special relativity and quantum eld theoretic effects largely overwhelm the quantum geometry e ects, whence the above viewpoint is not physically tenable. Nonetheless, the results for this toy model illustrate why an analogous viewpoint can be viable in the full theory: Although the standard, low energy quantum field theory seems disparate from quantum geometry, it can arise, in a systematic way, as a suitable semi-classical sector of loop quantum gravity."
I'm trying to learn what the different spaces of LQG are useful for,for example I more or less know the utility of the Hilbert space, the configuration space and the phase space. But, what's the utility of the state space?
My resumee:
In LQG the two basic variables are a connection and a densitized triad field(sometimes called electric field). The connections are functions defined in the configuration space of the theory, and each connection represents a quantum state of spacetime.This configuration space is a vector space of functions
The connection and the densitized triad field form a canonical pair in the phase space of LQG, that is a infinite dimensional space
The Hilbert space of the theory is constructed of the connections defined in the configuration space. Spin network states (previously were used loop states) form the basis of this Hilbert space.
Now, is this Hilbert space the unique Hilbert space of the theory? I've read that there's something called "kinematical Hilbert space", and othe thing called "diffeomorphism invariant Hilbert space". They both refer to the same thing?
Would be good if you could clarify this: It's true that actually the complex SU(2) connection of Ashtekar is not used in LQG, but is used the real SO(3) connection introduced by Barbero?
Ambitwistor
Oct26-03, 01:40 PM
Originally posted by meteor
In LQG the two basic variables are a connection and a densitized triad field(sometimes called electric field). The connections are functions defined in the configuration space of the theory, and each connection represents a quantum state of spacetime.
Each connection (modulo an SU(2) or SO(3) gauge transformation) represents a classical state of space, not a quantum state of spacetime. (Well, not even that: it only represents space once you impose the constraints.) We haven't quantized yet.
Spin network states (previously were used loop states) form the basis of this Hilbert space. Now, is this Hilbert space the unique Hilbert space of the theory? I've read that there's something called "kinematical Hilbert space", and othe thing called "diffeomorphism invariant Hilbert space". They both refer to the same thing?
No. The kinematical Hilbert space is L^2(A/G), i.e., the (complex) Lebesgue square-integrable functions over the space of connections modulo gauge transformations. It's like saying that the configuration space of a particle is R^3 (all of space), and then saying that the space of quantum states (wavefunctions) is L^2(R^3), the space of (square-integrable) complex functions over R^3.
However, then we have to start imposing constraints. e.g., for the free particle in QM we could construct the space of states L^2(R^3), but now suppose that we really only want to quantize a particle that's constrained to move on the surface of a sphere in R^3, or something. Then we have to start chopping down the kinematical Hilbert space to get the physical Hilbert space, the wavefunctions of particles that are constrained to move on the surface of a sphere.
In loop quantum gravity, we start with the kinematical Hilbert space, which has the spin networks as a basis. It is the quantum space of states of connections (modulo gauge transformations). However, not ANY connection corresponds to a solution of Einstein's equation! Only connections which obey the Gauss, diffeomorphism, and Hamiltonian constraints are "physical", connections that represent a gravitational field. So just like we discard connections in the classical configuration space A/G that don't obey the constraints of general relativity, we have to discard states in the kinematical Hilbert space L^2(A/G) that don't obey the quantized versions of those constraints.
So, the diffeomorphism-invariant Hilbert space is what you get when you apply the diffeomorphism constraint to the kinematical Hilbert space. If you also apply the Hamiltonian constraint, you get the physical Hilbert space.
(Note: we applied the Gauss constraint before quantizing by modding out by gauge transformations to consider the space A/G, because it's easy to do that. Then we applied the other constraints after quantizing.)
See also:
http://www.lns.cornell.edu/spr/1999-05/msg0016153.html
http://www.lns.cornell.edu/spr/1999-05/msg0016258.html
Would be good if you could clarify this: It's true that actually the complex SU(2) connection of Ashtekar is not used in LQG, but is used the real SU(3) connection introduced by Barbero?
Well, there are a lot of connections floating around, actually. Some people like Ashtekar's connection. Many use Barbero's nowadays, because you don't have to deal with the reality conditions. Barbero's connection is not SU(3), it is SO(3); you can use an SU(2) connection too, but it's not the same as Ashtekar's connection.
(SU(2) and SO(3) are pretty interchangeable as far as connections are concerned, because they have the same Lie algebra. It can make a difference when global effects are concerned, but loop quantum gravity physicists are usually sloppy about such things.)
I was hoping to arrive at some posts expressing intuitive content of loop gravity. Some of us have been reading Livine's thesis and/or work co-authored with Alexandrov or with Freidel.
I find the work admirable but difficult to assimilate. It seems to me that i am gradually having to confront a more completely lorentzian fourdimensional theory----they are extending the group to the whole lorentz group and raising the dimension. How to picture this. Maybe someone else---selfAdjoint, ambitwistor, ... has ideas about how to describe this. Or is it just plain a lot more difficult and tough to describe?
I am used to having 3D connections corresponding to a 3D spatial manifold. Quantum states of 3D geometry. Operators, which presumably can evolve a bit like the Heisenberg picture but without an absolute preferred time, only one operator you choose arbitrarily to serve as clock for the other processes. This is not too bad.
but now Livine etc make us consider 4D connections corresponding to all possible geometries on some 4D manifold. The wave functions are not just functions defined on the connections but on a pair consisting of a 4D connection and a vectorfield χ
any concerns or comments about this new material
Ambitwistor
Oct27-03, 07:01 PM
Originally posted by marcus
It seems to me that i am gradually having to confront a more completely lorentzian fourdimensional theory----they are extending the group to the whole lorentz group and raising the dimension.
Well, there are many approaches floating around. The 4D approaches are more related to spin foams the usual loop quantum gravity in the canonical approach. It's probably best to start by thoroughly understanding one model, such as canonical LQG with the Ashtekar-Barbero connection, or the Barrett-Crane spin foam model, rather than trying to simultaneously learn about all the different cutting-edge approaches.
whether or not it is wise, I would like to understand the role played by this vectorfield chi, let's see how to write it
c
χ
the quantum state or wave function is defined on a pair
consisting of a connection and a vectorfield
Ψ(A, c)
as you say, Ambitwistor, the connection to spinfoam is close, but also there is a connection to the SU(2) loop gravity of the people you mentioned.
I would like to understand how this vectorfield seems to serve as a bridge between the SU(2) and the covariant (i.e. SL(2,C) or lorentzian) approaces
eigenguy
Oct27-03, 07:41 PM
Originally posted by Ambitwistor
Well, there are many approaches floating around. The 4D approaches are more related to spin foams the usual loop quantum gravity in the canonical approach. It's probably best to start by thoroughly understanding one model, such as canonical LQG with the Ashtekar-Barbero connection, or the Barrett-Crane spin foam model, rather than trying to simultaneously learn about all the different cutting-edge approaches.
I agree, this is excellent advice ambitwistor.
Marcus, if you want your understanding of LQG to advance beyond the impressionistic level it's on now, you really need to commit to just one or two papers on a specific topic and really go over them with a fine tooth comb, proving every intermediate result you can (if you can).
I was advised, quite wisely as it turns out, to look just at the issue of relating polymer and fock states beginning with the pedagogically effective paper I referred you to. You should listen to ambitwistor and jeff. (I must say I'm having an increasingly hard time understanding how you managed that physics expert award thing. Maybe your true calling is politics?[6)])
the role of c[
see page 98 of the thesis
a bridge player is discussing the taking of a particular trick
right after equation (8.30) he says
"With the help of a gauge transformation, one notices that it's always possible to rotate a given c(x) to be the same fixed one eg. (1,0,0,0). So an invariant function is completely determined by its section at c=c0"
and he defines a restricted wavefunction that now depends only on the connection
instead of f(A,c) we are now looking at
fχ = χ0(A), which I will just call f(A) for the moment
"let us remark that f(A) has a residual SU(2) invariance.
Thus we are in the process of studying functions of a lorentz connection, effectively not invariant under SL(2,C) but simply
under the compact group SU(2)!"
Livine's italics and exclamation point. so this is one of the things this vectorfield chi does.
Originally posted by marcus
the role of c[
see page 98 of the thesis...
he calls it the "time normal" and makes it one of the configuration variables along with the connection.
he gives some more idea of how he thinks of it right there on page 98, before the part I quoted, before (8.30)
it's a vectorfield with values in the quotient SL(2,C)/SU(2)
that you can think of as a normal to the hypersurface
and he gives a reference to maybe the best article on this
chi "boost" gadget, "time normal" "internal time direction"
the reference is to
http://arxiv.org/gr-qc/0207084
Projected Spin Networks for Lorentz connection: Linking spin foams and loop gravity.
it is dated 12 April, 2003 tho the number suggests earlier.
this 15 page paper (along with the Alexandrov/Livine one we were reading earlier today gr-qc/0205109) might be the best
auxilliary reading to have handy when looking over the thesis. but the thesis is fairly self-contained as such go
selfAdjoint
Oct28-03, 11:15 AM
Well in the last paper you cite he says the chi field determines the imbedding of the 3-d space Σ in the 4-d spacetime. Which I can see, a field of little vectors normal to that hypersurface and by their direction determining just which shape it takes in 4-space. Then he goes to the network and only keeps the chis at the vertices. And by this he reduces the group action on them from Poincare SO(1,3) (he calls it Lorentz) to a product of rotation groups SO(3) over the vertices. So far so good, it seems to me. If you really want to see the origin of the chis spelled out I guess you would have to go back to Holst's paper (Red Queen, Red Queen!) or the earlier papers by Livine that he cites.
Originally posted by selfAdjoint
...you would have to go back to Holst's paper (Red Queen, Red Queen!)
even to stay in one place, in other words, I have to run faster than a bandersnatch, but hey no problem we are always doing that!
Getting back to the original purpose of the thread, how to introduce loop gravity and spin foams in plain terms---minimum of technicality.
In the "spin foam models" thread a new poster came in today and expressed some curiosity about what was being talked about and I said I would try to do this. Thing is quantizing geometry---in other words general relativity (a theory of changeable geometry that has tested out well)----is on the agenda and amounts to "putting QM and GR together".
Want to say how spin networks and spin foams work in this context.
Originally posted by marcus
...amounts to "putting QM and GR together".
Want to say how spin networks and spin foams work in this context.
Space in GR is dynamic--it can change as matter or energy flows around, it can undulate, effects can ripple outwards carrying the news of events that effected the shape of space in some locale. So in quantizing GR one needs a way of describing geometry and the change in geometry.
It will turn out that networks can be used to get a handle on geometries-----to assign probability-like "amplitudes" to the various possible geometries that space can have
a network is basically fairly simple---something like a net or a large ball and stick molecular model---Ashtekar likes to call networks "polymers". A network can have thousands of individual links and vertices---or balls and sticks---or trillions and jillions, so in that sense it is complicated. But in another sense it is simple because made up of simple elements.
have to go, but will get back to this later
...a network is...something like a net or a large...molecular model---Ashtekar likes to call networks "polymers". A network can have thousands of individual links and vertices...so in that sense it is complicated. But in another sense it is simple because made up of simple elements.
there are several different ways to tell the story, here's one. A network by itself is rather amorphous---it lacks geometry. all it is is a bunch of points with a list of which pairs of points are connected (in which direction, to make later work simpler there is a preferred direction, the links are one-way-street type)
it doesnt have fixed angles or lengths written onto its links so it doesnt define shape by itself, but you can get a whole lot of different possible geometries to live on it, by assigning data to the links, like angles but not exactly angles.
What's done is to choose a GROUP of ways to twist and turn as you run along any link from point A to point B. This group is called G and it is usually some group of 2x2 matrices---you can write down all kinds of rotations with a mere 2x2 matrix and even other simple actions like expansions and contractions etc. There are several different groups of 2x2 matrices some larger with more varied action and some smaller and there is some range of choice in doing the theory.
Think of the matrices in the group as rotations and imagine that you go all thru the network and label each link with a group element that describes a "rotation" or something more general that happens when you run along that link.
That assignment of one group element to each link in the web can be called a (discrete) "connection".
Intuitively it connects how things are oriented around one node in the web with how they are oriented at the neighbor node just down the street.
A discrete connection is a stripped-down version of a much fancier bigger machine that lives on a smooth manifold, a continuum. If you were to plunge our finite network into a continuum, where an official bigtime connection was living, then IT would induce a discrete connection on our network which would be a kind of meager skeleton or no-frills diagrammatic sketch of the original. But let's not bother to define exactly what a manifold is or what the usual idea of connection is in differential geometry. Our stripped-down finite skeletal idea is workable.
(Indeed I got the discrete connection idea from E.R.Livine's thesis and some of the articles around it, it may actually turn out to work better for some things!)
Imagine a web with E links and V nodes (the E stands the word "edge" which is sometimes used for the links in a network, and V for vertex). A connection, at least before it comes to live on the graph, is just a list (g1, g2,....gE) of group elements. So if you like cartesian product set-notation the space of all possible connections which could be chosen for this particular web is
GxGx....xG = GE
I am telling you this set of connections because the theory defines its core hilbertspace on it. That is the basic thing in any quantum theory---that space and the operators on it. But first, since that is a bit technical, can you see how that in some sense this collection of all possible connections (each one telling a specific way things twist and turn as you run thru the network) is tantamount to the range of possible geometries?
Things keep happening in quantum gravity and I dont have a sticky here where I can keep the useful source material handy or post recent
developments. So I'll try using this thread.
Rovelli just posted a new draft of his book "Quantum Gravity". It is the November 25 draft and is quite a bit changed from the August draft some of us were reading earlier. the contract for publication has been signed with Cambridge University Press.
We were discussing stuff from Livine's thesis in this and another thread.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
The last sentence of the "Discussion" section at the end of the paper reads: "Of course, one of the big problems in future is to clarify whether the Lorentzian Kodama state is normalizable under an appropriate inner product or not." It is clear that he is specifically interested in applications to general relativity---he mentions loop quantum gravity in the first paragraph and refers to gravity/GR at several points in the paper. This paper can be seen as Oda's careful response to an earlier paper by Witten gr-qc/0306083.
Witten said Kodama was not normalizable (in whatever inner product Witten thought was appropriate) and Oda does not buy this and says politely that the question is still open---is, in fact, the "big problem in future".
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev have posted a brief paper which writes the actions for Palatini GR and Ashtekar GR and BF topological QFT and also another (FΛF) type of TQFT all in the same formula. There is a dynamic variable which as it changes seems to make the system change smoothly from one theory to another
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
I found some family resemblance between this paper and Oda's--but both are quite recent and neither cites the other.
The cosmological constant occurs in a number of recent quantum gravity papers. The one by Girelli/Livine is one of the most recent. One of the most basic--perhaps a landmark---is Karim Noui and Philippe Roche "Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
A paper by a couple of New Zealanders
http://www.arxiv.org/abs/gr-qc/0311086
points out a discrepancy between two ways that people have used
to calculate BH quasi-normal modes ("ringing frequencies")
and attempts to resolve the discrepancy by "critically re-assessing"
the approach used by Motl and Neitzke
the issues are still shifting around the area operator in loop gravity
and the Immirzi parameter which occurs in some versions
but not all (e.g. the covariant version used by some people such as Livine/Alexandrov/Noui/others?) what happens to the spectrum of the area operator in covariant loop gravity and how does that compare with the results (themselves not yet conclusive) for BH vibration modes?
lumidek
Nov28-03, 08:23 PM
Dear Marcus,
I always appreciate your interest and your qualified comments about physics - and not only the technical ones. [;)] Let me just say a couple of words about your new explanation. The current paper
http://www.arxiv.org/abs/gr-qc/0311086
is pretty far from the original proposals by Shahar Hod and Olaf Dreyer because no results - except for the four-dimensional Schwarzschild black holes in the infinite space - provide us with evidence of Hod's and Dreyer's conjectures. Everything else seems to contradict the general predictions by Hod and Dreyer (about the asymptotic real part of the quasinormal modes).
When you wrote that the authors "critically re-assess the approach used by Motl and Neitzke", you are twisting the words to get a very different meaning. The New Zealanders do not claim that there is anything wrong with our monodromy calculation done with Andy Neitzke! They are saying that something is probably wrong with a paper by Castello-Branco and Abdalla who did not use our methods carefully.
To claim - today - that our results with Andy are wrong would not be the most reasonable thing to do because essentially all of our results have been confirmed numerically - for example, our prediction for the Reissner-Nordstrom black holes was confirmed beautifully by colleagues like Berti, Kokkotas, Cardoso, Lemos and others (after our paper).
I and Andy used the so-called monodromy method, and as far as I know everyone agrees that it works correctly in the contexts that we studied in our paper. Of course, not only our results, but also the method can be useful for other people and other problems. However we can't guarantee that the results obtained by other people, using our machinery, will be correct.
I found some of the results in the Schwarzschild-de-Sitter context a bit counter-intuitive, and there have been many papers about the Schwarzschild-de-Sitter black hole. (Our paper with Andy was never about de Sitter, all the black holes that we studied were in empty space.)
The New Zealanders mostly criticize the paper by Castello-Branco and Abdalla [19]
http://arxiv.org/abs/gr-qc/0309090
Yes, I also think that they used our method a little bit blindly. Their results looked too similar to the case of the flat space. The New Zealanders claim that the method can't be used - at least not in this way - if two horizons coincide, and I tend to agree with that. Moreover, there are many questions about the choice of the boundary conditions.
The quasinormal modes remain a lively topic. You can observe a rather complete list of the developments if you look which papers cite e.g. my first paper:
http://www.slac.stanford.edu/spires/find/hep/www?c=00203,6,1135
Once again, be sure that none argues that my results or our results with Andy (and the methods used to derive them in our context) are incorrect. The newer papers study more complicated cases where some errors have been done - but not by me and Andy. [6)]
Best wishes
Luboš
selfAdjoint
Nov28-03, 09:21 PM
Welcome to the Physics Forums, Lubos. We're honored to have you here.
Originally posted by selfAdjoint
Welcome to the Physics Forums, Lubos. We're honored to have you here.
Indeed so! and thanks for making the distinction between your work and the use of a similar method by others which, you say, the New Zealanders were examining critically. Any more explication you can give would, I suspect, be much appreciated.
best wishes,
marcus
Things keep happening in quantum gravity and there is no sticky here to keep handy links to source material about loop gravity developments. So I continue trying to use this thread.
The term "Loop Gravity" is used for want of a better one for a broad range of research approaches to quantizing general relativity.
Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an accident because current approaches are not so much concerned with loops. But no one has come up with a good alternative.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
A kind of merging among topological QFT ("TQFT") and non-commutative geometry (especially because of the Cosmological Constant) and spinfoams and (Lorentzian spin network-based) LQG seems to be in process. Some people seem to have found a way to do spin network analysis with non-compact groups---so they can use SL(2,C) for gauge in place of SU(2)---hep-th/0205268. Some of this may matter or may not, seems too early to judge. But it might help to keep some of the links handy for reference.
Today Lubos Motl posted a message to Non-unitary ("somewhere in the tropics") containing one link. This link was offered as a characterization of LQG. It was to a 5-year old 11-page paper by Rovelli and Upadhya which was intended as a quick into. They call it a "Primer" to the subject. It does not really characterize the field of loop gravity but Lubos might appreciate it if I include the link in this "sticky" list of links so here it is:
Rovelli/Upahya 5-year-old brief 11-page "primer" to the subject
http://arxiv.org/abs/gr-qc/9806079
Rovelli just posted a new draft of his book "Quantum Gravity". It is the November 25 draft and is quite a bit changed from the August draft some of us were reading earlier. the contract for publication has been signed with Cambridge University Press. The PDF file for Rovelli's book is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
It takes about 10 minutes to download and convert so that it can appear on the screen. It is 300 plus pages long.
The SPIRES database on citations is often handy. There is a topcited list for the smaller series GR-QG (general relativity and quantum gravity) as well as for the huge series HEP-TH.
http://www.slac.stanford.edu/library/topcites/topcites.review.2002.html
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
The last sentence of the "Discussion" section at the end of the paper reads: "Of course, one of the big problems in future is to clarify whether the Lorentzian Kodama state is normalizable under an appropriate inner product or not." This paper can be seen as Oda's careful response to an earlier paper by Witten gr-qc/0306083.
Witten said Kodama was not normalizable (in whatever inner product Witten thought was appropriate). Apparently Oda does not buy this and says politely that the question is still open---is, in fact, the "big problem in future".
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev have posted a brief paper which writes the actions for Palatini GR and Ashtekar GR and BF topological QFT and also another (FΛF) type of TQFT all in the same formula. There is a dynamic variable which as it changes seems to make the system change smoothly from one theory to another
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
I found some family resemblance between this paper and Oda's--but both are quite recent and neither cites the other.
The cosmological constant occurs in a number of recent quantum gravity papers. The one by Girelli/Livine is one of the most recent. One of the most basic--perhaps a landmark---is Karim Noui and Philippe Roche "Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
this is the substitute for a sticky to keep links to current
Quantum Gravity resources, so I will update it from time to time.
QG Phenomenology seems to be attracting research interest (as prospects emerge for testing planck-scale effects)
the best and most recent survey of it that I have found is a November article by Giovanni Amelino-Camelia called
"Quantum Gravity Phenomenology"
http://arxiv.org/physics/0311037
It is 8 pages and was prepared in tandem with an article he wrote for the November 2003 issue of "Physics World". This issue was devoted to the current state of affairs in Quantum Gravity and had 3 invited survey articles, one on phenomenology (prospects for testing the theories) by Giovanni A-C, one on LQG by Carlo Rovelli, and one on string by Leonard Susskind.
There is a major issue within LQG about how energy and momentum transform under the Lorentz group. The two different points of view are exemplified by
Rovelli and Speciale "Reconcile Planck-scale discreteness and the Lorentz-Fitzgerald contraction"
http://arxiv.org/gr-qc/0205108
this was published in 2003 in Physics Review D.
Magueijo and Smolin "Generalized Lorentz invariance with an invariant energy scale"
http://arxiv.org/gr-qc/0207085
this was also published in 2003 in Physics Review D.
The two versions yield different predictions about dispersion in arrival time of gammaray bursts. It is barely possible that GLAST may be able to distinguish between the two approaches to building loop quantum gravity theory when it starts up in 2006.
The nub of the issue is that the planck length (or area, or energy) plays a crucial role as marking where quantum gravity effects become dominant. But relatively moving observers see lengths and areas differently! How can one reconcile having a theory locally embodying the principle of relativity (no preferred frame, all observers equal) that nevertheless has a certain length (or energy) as an important landmark?
Intuitively, just as in 1905 SR where the speed of light is the same for all observers, this other landmark the planck energy should be the same for all observers! But how can one manage this? So there is this internal debate in LQG on how to accomodate this and maybe observational data will help settle it
[edit: I checked on the launch date for Gammaray Large Area Space Telescope, GLAST, and it is not until 2006]
According to Loren B's post on another thread, the January 2004
issue of Scientific American has an article that (at least in part)
deals with Loop Gravity. I haven't seen it. Don't know if there is a web reference.
It seems like it might be a useful article to add to the list here, as a recent survey for wide audience. Does anyone have more information? The article is by Lee Smolin
[edit: I found a link to a two-paragraph teaser for the article
http://www.sciam.com/article.cfm?chanID=sa006&colID=1&articleID=00012BDE-E7EA-1FD3-A7EA83414B7F012C
more than that, and the crafty editors of digital SciAm want us to pay for it]
nonunitary
Dec16-03, 06:02 PM
The Sciam issue seems to be interesting. Too bad one cannot get it online!
By the way, there is a link in the General Relativity Hyperspace
where forthcoming events are announced:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference
There is also a workshop on Loop Quantum Gravity announced there, apart from the preliminary programme for GR17.
Originally posted by nonunitary
By the way, there is a link in the General Relativity Hyperspace
where forthcoming events are announced:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference
There is also a workshop on Loop Quantum Gravity announced there, apart from the preliminary programme for GR17.
Thanks! I followed the link you gave to:
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
and saw the announcement of "Loops and Spinfoam" conference in May 2004 in Marseille, organized by Rovelli, Freidel, and Roche.
and also the program you mentioned, for the 17th International Conference on General Relativity at:
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Dec.8
to be held in Dublin on 18-24 July 2004.
nonunitary
Dec17-03, 12:45 PM
Hi,
One can also see the most recent announcements at:
http://www.maths.qmul.ac.uk/wbin/GRnewslist/conference?10
where another LQG event is advertised.
I followed the links at the site of the organizers and found the
page:
http://www.nuclecu.unam.mx/~corichi/lqg.htm
Bye
Originally posted by nonunitary
...
http://www.nuclecu.unam.mx/~corichi/lqg.htm
...
Thanks again! this Mexico City symposium on loop and spinfoam
is really interesting news. I like the list of people
who have already said they would attend. One could learn more about quantum gravity, maybe, in Mexico Jan30-Feb1 next year than one might in Dublin in July with the 17th International Conference. I will copy the program planned for this long-weekend loop/foam symposium, from your link:
1. Spin foam models
(to include 2+1 and 3+1 theories; limitations of the Barrett-Crane
model; relation to the canonical approach to dynamics; the role of
the Barbero-Immirzi parameter; canonical and spin foam geometries; issue
of the `continuum limit', renormalization group flows)
2. Status of the Hamiltonian constraint
(to include 2+1 and 3+1 theories; spin-foam and Thiemann-type
approaches; quantum cosmology; Semi-classical corrections to
Einstein equations; factor ordering; too many solutions?
issue of finding solutions and inner product, the "phoenix project").
3. Semi-classical issues
(to include relation between kinematical and dynamical semi-classical
states; quantum field theory on quantum geometry; quantum cosmology;
Minkowski coherent state and Minkowski spin foam)
4. Loop quantum phenomenology
(to include Lorentz invariance;`Double special relativity'; quantum
cosmology; Kodama state and de Sitter background)
5. Conceptual issues
(observables through matter coupling, string theory in polymer
representation; matter couplings on semi-classical states of
geometry and string theory; issue of time; meaning of histories used
in spin foam, role of quantum groups is LQG)
Originally posted by marcus
4. Loop quantum phenomenology
(to include Lorentz invariance;`Double special relativity'; quantum
cosmology; Kodama state and de Sitter background)
the programme for the Mexico City symposium is a good "weather-vane" to point out what the interesting questions are now.
Loop quantum phenomenology seems to have some important issues.
Especially around DSR and GLAST and possible variations in photon time of flight over cosmological distances.
the person who has the most to say about this for me now is
in Wroclaw Poland, name of Jurek (Jerzy) Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org./abs/hep-th/0304027
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org./abs/hep-th/0312140
http://www.ift.uni.wroc.pl/JK-G/
Jerzy K-G has published (in 2001) with Amelino-Camelia and he has published (in 2003) with Freidel and Smolin. I think he understands DSR more as a mathematician, more rigorously and clearly than Amelino-Camelia who initiated the research into it and originally conceived of it!! I am impressed by these two papers. It looks to me as if he basically just takes control of DSR and contradicts Smolin and Amelino-Camelia. the way Jerzy K-G develops DSR it does what it is supposed to--there are two observer-independent scales one of speed and one of energy--and also the speed of a gammaray photon does NOT change with energy: it stays c for all photons, indeed all massless particles. This "Polish" version of DSR will be destroyed if it turns out in 2006 that GLAST does after all see any speed-variation in high-energy photons, so the Polish version of the theory is firmly and directly falsifiable. Have to say I like it. Alejandro Corichi should make sure K-G comes to the Mexico City symposium for topic 4 "Loop quantum phenomenology" [:)]
selfAdjoint
Dec17-03, 10:48 PM
Say, Marcus, it really looks like we PFers are pretty up to speed on the stuff they are going to have on that program. Thanks mostly to you, I might add. Great going!
Originally posted by selfAdjoint
Say, Marcus, it really looks like we PFers are pretty up to speed on the stuff they are going to have on that program. Thanks mostly to you, I might add. Great going!
by "you" you have got to mean Meteor, Nonunitary, yourself, Ambitwistor as well, to mention only the first that come to mind. Thanks to all. I didnt realize until today what a key role Jerzy K-G is playing. Look at the program from the Wroczlaw Institute of Theoretical Physics for the "40th Winter School in Theoretical Physics" for Feb 4-14, 2004. Each year since 1964 they have had a winterschool gathering a dozen or so worldclass people at this
ski-resort. this year the topic chosen is "Quantum Gravity Phenom."
Steve Carlip is one of the organizers, and so is (you guessed it) Jerzy Kowalski-Glikman.
Speakers:
E. Alvarez Quantum Gravity
G. Amelino-Camelia Introduction to quantum gravity phenomenology
P. De Bernardis Cosmology with BOOMERANG, WMAP
A. Grillo Planck-scale kinematics and the Pierre Auger Observatory
T. Jacobson Astrophysical bounds on Planck-supressed Lorentz violation
J. Kowalski-Glikman Introduction to doubly special relativity
C. Laemmerzahl Tests of Lorentz symmetry in space and interferometry
P. Lipari Ultra-high-energy cosmic-rays
J. Martin Trans-Planckian cosmology
N. Mavromatos PCT symmetry and quantum gravity phenomenology
T. Piran Gamma-ray bursts
J. Pullin Canonical quantum gravity phenomenology
L. Smolin Cosmological constant in Quantum Gravity
The names Ted Jacobson and Nick Mavromatos especially ring a bell as they've been writing papers about QG phenomenology, various kinds of astronomical tests, that other people cite regularly. IIRC Mavromatos co-authors with Ellis. But I guess so does Pullin and Lipari and several others, ring a bell I mean
So much is going on! end-January Mexico City (loop/foam symposium)
first half of February Karpasz School (QG phenomenology)
then Freidel and Rovelli's Loop Gravity/Spinfoam conference at Marseille May 3-May 7
then the July thing at Dublin--17th International General Relativity conference
Good thing nonunitary provided the links to these things. Yours truly had not registered all the activity.
Well, given that my knowledge in differential geometry is rather poor, I've ordered the book "Differential geometry" of Schaum to Amazon. Hope that will be a good book, like all the other Schaum books that I've read
A question: Then the SO(2) connection used like a variable in Ashtekar's general relativity is a real connection or a complex connection? There are papers that say that is real and others that is complex. I'm dying in the doubt
I've just read that loop quantum gravity violates the "weak energy condition" at short distances, when the granularity of spacetime becomes significant. I've don't have the foggiest idea of what is the weak energy condition, so I'm going to read about it right now
Hi Meteor, I started a thread with your two questions
1. about Ashtekar's new variables
2. about "weak energy condition"
Both could lead to discussion and I am trying to save this thread as a kind of "sticky" for useful links, source material, conference news, and so on. Hope it is OK for me to make a separate thread for what you asked about. You will see it.
I called it "Loop gravity---two questions"
several people have expressed interest in the SciAm January 2004 article by Lee Smolin, "Atoms of Space and Time"
The complete article is probably worth a visit to your local public library. It is written for general audience but manages to give a fairly clear picture of the field and how it developed. Here is an exerpt, as a sample, from the section where Smolin is describing how he and some others got started:
-------quote page 68----
...In the mid-1980s a few of us...Ashtekar...Jacobson...Rovelli...decided to reexamine the question of whether quantum mechanics could be combined consistently with general relativity using the standard techniques. We knew that the negative results from the 1970s had an important loophole. Those calculations assumed that the geometry of space is continuous and smooth, no matter how minutely we examine it, just as people had expected matter to be before the discovery of atoms.
Some of our teachers and mentors had pointed out that if this assumption was wrong, the old calculation would not be reliable.
So we began searching for a way to do calculations without assuming that space is smooth and continuous. We insisted on not making any assumptions beyond the experimentally well tested principles of general relativity and quantum theory. In particular we kept two key principles of general relativity at the heart of our calculations.
The first is known as background independence. This principle says that the geometry of spacetime is not fixed. Instead the geometry is an evolving, dynamical quantity. To find the geometry, one has to solve certain equations that include all the effects of matter and energy. Incidentally, string theory, as currently formulated, is not background independent; the equations describing the strings are set up in a predetermined classical (that is, nonquantum) spacetime.
The second principle, known by the imposing name of diffeomorphism invariance, is closely related to background independence. This principle implies that, unlike theories prior to general relativity, one is free to choose any set of coordinates to map spacetime and express the equations. A point in spacetime is defined only by what physically happens at it, not by its location according to some special set of coordinates...
...By carefully combining these two principles with the standard techniques of quantum mechanics, we developed....[the means]...to do a calculation...
That calculation revealed, to our delight, that space is quantized. We had laid the foundations of...loop quantum gravity...
------end of exerpt-----
ranyart
Dec24-03, 04:35 AM
Amazing..
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0312/0312103.pdf
Originally posted by ranyart
Amazing..
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0312/0312103.pdf
thanks for the lead, ranyart! a Loop Quantum Cosmology
article. I will have a look. In case anyone wants the abstract:
http://uk.arxiv.org./abs/gr-qc/0312103
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
invited talk at the 10th Marcel Grossman meeting July 2003
18 pages, 5 figures
edit: this turned out to be more than the title suggests.
there are 45 references (it's a mini-survey article)
a thumbnail sketch of LQG and quick review of current work
in the general theory, not limited to cosmology
followed by another concise review of current Loop cosmology
developments
then, on pages 10-12, they present their results
relating to cosmological boundary conditions
(citing and comparing work of Hartle/Hawking and of Vilenkin)
finally, pages 12-15, they discuss open questions having to do with behavior around the cosmological singularity or bounce, and graph some results of calculation around the bounce.
it is an interesting paper from several standpoints---
for instance what they choose to emphasize in the overview of the general field: on page 4 at the top, they cite two papers by Sahlmann, another two by Sahlmann/Thiemann, and one by Lewandowski/Okolow.
the view of Loop gravity is on the abstract side, through the window of "representations of the classical algebra"
the paper connects to history by citing Hartle and Hawking "Wave Function of the Universe" (1983) and Vilenkin "Quantum Creation of Universes" (1984) and pointing out the central long-standing concern with cosmological boundary conditions in the Wheeler-DeWitt quantum cosmology model.
they use the algebraic representation-theory raised earlier, carried over and specialized to cosmology, to say how and why Loop quantum cosmology differs from vintage 1980s (Wheeler-DeWitt) quantum cosmology-----different Hilbertspace, different operators, discrete spectra instead of continuous---references to the Bohr compactification and the Stone-von Neumann theorem at bottom of page 6.
then starting on page 7 they focus on the dynamics of loop quantum cosmology---the Hamiltonian constraint and difference equation that determines evolution around the bounce---and
show how the loop model matches up with Wheeler-DeWitt: in effect has the right limiting behavior (see for example Figure 1).
They also discuss ways the modern theory differs from the vintage model
(eliminates the singularity, provides for varying degrees of inflation depending on assumptions, and gives rise to somewhat different boundary conditions, or to similar ones in a different way)
this paper ties a number of threads together.
the other research currents it draws on and connects to
are as significant as the research results
it relates current research in loop cosmology with the historical antecedents (connects it to Wheeler/DeWitt/Hawking/Hartle/Vilenkin) by addressing issues that were traditionally central to earlier work
and it points up linkage between the specialized field of loop cosmology and the algebraic approach to the broader field of LQG associated with Ashtekar/Lewandowski/Thiemann/Sahlmann
the bibliography is extensive and up to date, as you might want from a "mini-survey". For example, the Husain/Winkler "On Singularity Resolution" paper that ranyart just found posted a couple of days ago on arxiv (gr-qc/0312094) is their reference 25.
an earlier brief overview of loop cosmology this year
http://arxiv.org./abs/astro-ph/0309478
is only 6 pages and less abstract
that is Bojowald's
"Quantum Gravity and the Big Bang"
it is less hilbert spacey but gives a quick idea of what
the field is about and how the calculations are done
both papers are good, just different introductions to the same thing
ranyart
Dec24-03, 04:05 PM
Marcus I know someone asked for a good link for loop Quantum Gravity?
I presume you have this link somewhere?..if so I can always delete it:
http://arxiv.org./abs/gr-qc/0306008
I will post just to the Abstract as I automatically link directly to pre-print papers, but maybe the abstract is more usual practice.
Originally posted by ranyart
I presume you have this link somewhere?..if so I can always delete it:
http://arxiv.org./abs/gr-qc/0306008
"Cosmological applications of loop quantum gravity"
I know the paper but I didnt have the link handy, not handy to this thread anyway. Thanks for mentioning it. Bojowald co-authored that with Hugo Morales-Tecotl, in Mexico City. It's good because it is introductory, part of a loop gravity seminar taught for undergrad and grad students. I read somewhere that Rovelli was Morales-Tecotl's thesis advisor, which makes me think that Morales-Tecotl is also a young person too, like Bojowald----recent PhD or recent postdoc.
I will post just to the Abstract as I automatically link directly to pre-print papers, but maybe the abstract is more usual practice.
Whichever you prefer! We can provide links to either abstract and full text. I don't know that one is more useful or usual than the other. I always look at the abstract first because a long PDF download ties up my computer and the abstract tells me how many pages.
Right now I feel a bit to lazy to bother but probably all these links should be gathered in a list-----or two lists: one for the full theory and one for the specific application to cosmology. It gets tedious playing librarian but it is actually easier than having to go through piles of paper on my desk. Thanks again for contributing these good links!
Loop gravity is a planck-scale theory and planck units have a special place in it. This month the National Institute of Standards and Technology (NIST) posted new values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the 2002 CODATA recommended values of
planck mass
planck length
planck time
planck temperature
the uncertainties have been reduced by an order of magnitude since
the values of planck units were posted in the 1998 CODATA set.
Maybe this is no big deal but it is nice that the natural units for Loop Gravity are gradually beginning to look more like a recognized system of units
A good article on timekeeping, discussing GR effects on the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, and two others
"Phenomenology of Doubly Special Relativity"
dated 30 December 2003 (recent)
about 22 pages
http://arxiv.org/gr-qc/0312124
Giovanni A-C is the most eminent person in quantum gravity phenomenology and I believe the fastest riser is Jerzy K-G.
QG Phenomenology is a hot field with a lot of recent papers---both theoretical and observational. The theoretical part says what are the various quantum gravity models and what (in the case of those models that actually predict and can be tested by possible observation) do they predict and how---with planned space observatories etc---can they be tested.
Like pruning a tree, the observationalists can do the loop gravity/spin foam theorists a favor by chopping off the bad branches
(that actually make testable predictions but the predictions are wrong). So there is growing interest and visibility for this QG Phenomenology business.
And so when Giovanni A-C and Jerzy K-G get together on a paper and give the latest word on the subject it is apt to be worth paying some attention. So I posted it. I thought it was.
It is probably time to gather the links in this "surrogate sticky" thread into a single post----there are enough links now so they are too spread out
Loop Gravity looks like it going to have an active year in 2004,
getting progressively more visible and well-established, with even some recognizeable prospects for testing (phenomenology).
Here are some straws in the wind for 2004.
Smolin's January "Scientific American" article
Rovelli's book "Quantum Gravity" now at Cambridge University Press (but the 30 December 2003 draft is still online)
a spate of conferences and symposia:
Mexico City in January (loop/foam)
Polish Winterschool in February (quantum gravity phenomenology, DSR)
Marseille in May (loop/foam)
Dublin in July (the whole range of gen rel)
How broad a range should "Loop Gravity" cover?
It is actually a fleet of theories being developed which share
two key characteristics
These were underscored in Smolin's article and can be abbreviated DI-BI (diffeomorphism invariance-background independence)
All these formulations of quantum gravity attempt to quantize General Relativity and preserve these two key features of the original 1915 theory. I will quote Smolin's short description of DI and BI in a moment.
The distinction between loop and spin foam approaches has become somewhat artificial. They have always been two parts of a single enterprise, and Livine's 2003 thesis (Boucles et Mousses de Spin en Gravite Quantique) showed how to bridge the formal divide. As can be seen from the titles and programs at the various conferences, one no longer discusses loop as distinct from foam---instead there is apt to be a unified "loop/foam" conference, or loop/foam survey talk, or session of talks.
From a historical perspective, the main thing Loop Gravity does (and and stringy theories do not) is to actually quantize the theory of General Relativity itself and, in doing so, retain the essential features DI and BI, which stringy models lack. Accordingly the main criticism of the Loop Gravity approach(es) by string folk is to insist that attempts to retain the Background Independence and Diffeo-Invariance features of Relativity are doomed to fail (in essence because quantizing GR has so-far proven difficult.) The essential features of the classical 1915 theory are declared to be too radical---General Relativity must be somehow wrong and in need of replacement by a theory which on the one hand predicts the same numbers but on the other hand has room for absolute space and time---a fixed background, a uniform flow of time.
DI and BI actually imply that continuous time does not exist at planck scale. In Bojowald's Loop Quantum Cosmology papers, for instance, there is no time coordinate. The (quantized) scale-factor of the universe is used as a clock----its eigenvalues are the ticks of the clock---it is meaningless to ask if they are "uniformly spaced"----the progress of the big bang or bounce expansion proceeds in quantized steps--the size of the universe is its own clock.
Quantizing GR means doing physics without time. Or at least with a quantized time as Bojowald does cosmology. This indeed is radical and apparently has many people in a state of denial if not outright horror ("This can't be right!")
As Rovelli says, his book is as much about time as about gravity. The quantization of time (or disappearance of continuous time coordinates at the quantum level) was not something anyone "put in by hand". It arose on its own accord from taking GR seriously and quantizing it by standard approaches.
Well, I should quote Smolin about what DI and BI mean. I will post this now and edit in the quotes later.
---------Smolin SciAm quote----
In particular we kept two key principles of general relativity at the heart of our calculations.
The first is known as background independence. This principle says that the geometry of spacetime is not fixed. Instead the geometry is an evolving, dynamical quantity. To find the geometry, one has to solve certain equations that include all the effects of matter and energy. Incidentally, string theory, as currently formulated, is not background independent; the equations describing the strings are set up in a predetermined classical (that is, nonquantum) spacetime.
The second principle, known by the imposing name of diffeomorphism invariance, is closely related to background independence. This principle implies that, unlike theories prior to general relativity, one is free to choose any set of coordinates to map spacetime and express the equations. A point in spacetime is defined only by what physically happens at it, not by its location according to some special set of coordinates...
...By carefully combining these two principles with the standard techniques of quantum mechanics, we developed....[the means]...to do a calculation...
That calculation revealed, to our delight, that space is quantized. We had laid the foundations of...loop quantum gravity...
------------end quote----------------
In an earlier post on this thread I gave the program list for the 40th annual Polish Winterschool of Theoretical Physics, but I apparently didnt give a link
http://www.ws2004.ift.uni.wroc.pl/html.html
The tradition is every winter to choose a topic in Theoretical Physics and get together the world's top people at a Polish ski resort for a couple of weeks of tutorials, seminars, and talks on new research.
It is at Ladek Zdroi, a spa in SW Poland on the Czech border in the Sudeten mountain range.
The first winterschool was in 1964. I guess detente was part of the aim of getting scientists together from east and west, or maybe just good science.
This year the topic chosen is "Quantum Gravity Phenomenology".
That mostly means loop/spin foam/doubly special relativity stuff. Does not seem to be much in way of stringy phenomenology because there doesnt seem to be much testable stringy prediction.
But notice that E. Alvarez is on the program. He is a string theorist who presented a oft-cited wake-up paper "Loops versus Strings" at a conference of string (and other HEP) people a couple of years ago.
Also notice the central role of Jerzy Kowalski-Glikman, who is covering Doubly Special Relativity. Involving a bending of Lorentz symmetry, DSR seems to be taking a prominent place in quantum gravity phenomenology.
What is the key idea in DSR? I will try a separate post on that.
The key idea of DSR is to repeat the success of 1905
in an analogous situation
Lorentz transformations of 1905 SR look just like square old Galilean frame transformations but "bent" slightly by a factor which is ordinarily very close to one except at very high speeds.
So you take the block of numbers you would have used in a Galilean and tweak slightly by a factor sqrt(1 - beta2), and then it turns out that a certain physical quantity (c, the Planck unit of speed) is the same in all frames, that is, is unchanged by the new "deformed" Galilean transformations.
In DSR you tweak the transformation matrix even a bit more and you get that TWO physical quantities are unchanged, not only the Planck speed unit, c, is invariant but also the planck energy unit.
We have to look at the situations in 1905 and now about 100 years later. In 1905 they had square Galilean frame change matrices and they noticed that Maxwell equations predicted a definite speed for EM radiation. So they had two choices
1. there was a preferred frame ("aether") that the equations worked in and they didnt work in other frames ("moving observers")
2. there was no preferred frame ("Galilean relativity") and Maxwell worked in all the frames you could transform to with a straight Galilean framechange. But then there was something that should be traveling the same speed in all frames!!!! Paradox. So they tweaked, or bent, or "deformed" the Galilean symmetries matrices slightly in a way that wouldnt be noticeable at small boosts, in other words with small everyday speed changes.
Lorentz and Poincare saw how, but failed to take it seriously, so Einstein eventually did and gets the credit.
Now in 2004 we have Lorentz framechange matrices---they look like Galilean in the everyday cases and are "deformed" or maybe one should say "subtley adjusted" so that the planck unit speed is invariant.
And we have realized that there are other natural units BESIDES the natural unit of speed, namely there is a natural unit of energy Ep. This is a gateway to planck scale where physics is apt to be a good deal different and there is a growing realization that all observers should probably see the same Ep. Or wait, there is still the preferred frame or "aether" possibility.
1. there could be a preferred frame "breaking Lorentz symmetry" as they say, then we dont have to adjust the matrices, and whatever is true is only really true in the universe's preferred frame---what you see from other perspectives deviates more or less from what you would be seeing in the one true frame. This is boring.
(but when people do "observational tests of quantum gravity" it is this Lorentz symmetry breaking hypothesis that they are really testing nowadays)
2. DSR is the other alternative, where there is no preferred frame, no "Lorentz symmetry breaking". You subtley adjust the transformations so they leave TWO rather than only one of the planck units invariant.
Basically here I am just repeating the account given in this latest DSR paper
"Phenomenology of Doubly Special Relativity"
by Giovanni A-C, Jerzy K-G, and two other people
http://arxiv.org/gr-qc/0312124
There is no sticky for links to loop gravity source material. So this thread can serve as a surrogate. This post gathers links from several earlier posts, and shortens the comments. These links include some that I found and some that other PF posters have contributed, to whom thanks!
The term "Loop Gravity" is used for a broad range of background-independent approaches to quantizing general relativity. Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an accident because current approaches are not so much concerned with loops. But no one has come up with a designation that includes spin foams and the various models based on spin networks and is any more convenient.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
A kind of merging among topological quantum field theory ("TQFT") and non-commutative geometry (especially because of the Cosmological Constant) and spinfoams and (Lorentzian spin network-based) Loop Gravity seems to be in progress. In another direction Loop Gravity seems to be connecting up with Doubly Special Relativity (DSR). A way has been found to do spin network analysis with non-compact groups---using SL(2,C) for gauge instead of SU(2). It seems too early to judge which of these trends are significant in the long term, but it may help to keep some of the links handy for reference.
Rovelli just posted the 30 December 2003 draft of his book "Quantum Gravity". The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html.
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
The SPIRES database on citations is often handy. There is a topcited list for the smaller series GR-QG (general relativity and quantum gravity)here:
http://www.slac.stanford.edu/library/topcites/top40.2002.E.html
And the more extensive series HEP-TH here:
http://www.slac.stanford.edu/library/topcites/topcites.review.2002.html
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Starodubtsev
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
-------Quantum Gravity Phenomenology---------
two recent papers:
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
other fairly recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
--------upcoming conferences--------
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------projected observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Floyd Stecker is at the NASA Goddard Laboratory for High Energy Astrophysics and something of a world-class expert on gamma-ray bursts and cosmic ray research. It seems that man-made accelerators are not big or powerful enough to be very effective in providing empirical guidance to quantum gravity theory. So what is apt to take the place of accelerators is high energy astrophysics. Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2006, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
Loop Gravity is a theory under construction, so Rovelli's
Chapter 7 "Dynamics and Matter", pages 199-212, is describing
work in progress. Section 7.1 discusses the hamiltonian and
7.2 the inclusion of matter.
Table 7,1 on page 208 gives "Quantum numbers of the spin network states for gravity and matter."
The graph \Gamma with N nodes and L links, is like a big quantum number describing adjacency. Nodes correspond to regions or chunks of space and links to the surfaces between those volumes
\Gamma adjacency
i_n volume of node n
j_l area of surface l
F_n number of fermions at node n
S_n number of scalars at node n
w_n field strength at node n
k_l electric flux across surface l
In section 7.2.4 "The quantum states of space and matter", notation is given for |s> a quantum state of space and matter.
As one has come to expect, quantities like volume/area, fieldstrength/flux appear as irreducible representations/intertwiners.
More details about this on pages 208 and 209.
"thus we can write
|s> = |\Gamma,i_n,j_l,F_n,S_n,w_n,k_l>
This state describes a quantum excitation of the system that has a simple interpretation as follows. There are N regions n, that have volume and where fermions and Higgs scalars can be located. These are separated by L surfaces l, that have area and are crossed by flux of the (electric) gauge field. The quantum numbers are related to observable quantities as in Table 7.1. This completes the definition of the kinematics of the coupled gravity+matter system."
the next section, 7.3 "Matter: dynamics and finiteness" writes the hamiltonian compounded of four pieces.
H = H_{Einstein} + H_{YangMills} + H_{Dirac} + H_{Higgs}
One brief exerpt from Section 7.3, "...The fact that the total hamiltonian turns out to be finite is extremely remarkable. It is perhaps the major payoff of the background independent quantization strategy on which LQG is based..."
For the finiteness result Rovelli cites "Lectures on Quantum Gravity"
http://arxiv.org/gr-qc/0210094
these are notes at the grad student level prepared by Thomas Thiemann, which are to appear in a textbook series called
"Lecture Notes in Physics" (Springer, Berlin)
and also "Quantum Gravity as the Natural Regulator of the Hamiltonian Constraint of Matter Quantum Field Theories"
http://arxiv.org/gr-qc/9705019
It is a bit of luck that quantizing space makes the ordinary infinities of QFT go away:"...the ultraviolet divergences of ordinary quantum field theory can be directly interpreted as a consequence of the approximation that disregards the quantized, discrete, nature of quantum geometry..."
Labguy posted this in the Astronomy forum, under the heading "Albert is Still Looking Good". It bears on Loop Gravity so I'll copy it and add it to our links.
-----------
Recent release, passed through Ned Wright's Cosmology site:
http://www.astro.ucla.edu/~wright/cosmolog.htm#04Dec03
"A Double Radio Pulsar.
9 Jan 2004 - Lyne et al. (2004, Science in press) gives the details about PSR J0737-3039 A&B, the double radio pulsar binary with a relativistic orbit, previously reported as a single pulsar in a binary system on 4 Dec 2003. The mass of the 23 millisecond pulsar (A) is 1.337+/-0.005 M(sun) while the mass of the 2.8 second pulsar (B) is 1.250+/-0.005 M(sun). There are now 6 measured constraints on (MA,MB) and the values given above are consistent with all 6 constraints, providing a stringent test of General Relativity which GR passes with flying colors".
And:
"An amazing binary pulsar.
4 Dec 03 - Nature today published a paper (Burgay et al. 2003, Nature, 426, 531-533) about a newly announced millisecond pulsar, PSR J0737-3039, in a relativistic binary system. Radio pulsars are neutron stars (NS) which have a mass of about 1.4 solar masses and a radius of 10 km, magnetic fields billions to trillions of times larger than the Earth's magnetic field, and spin periods from 1.6 milliseconds to several seconds. PSR J0737-3039 is orbiting another neutron star every 2.4 hours and the two stars will merge in 85 Myr due to gravitational radiation. Hence LIGO will have many more detectable NS+NS merger events based on the statistics of two objects instead of the previous estimate based solely on the one merging binary pulsar PSR B1913+16 known earlier.
...the relative motion of the two stars is 14,000 km in 22 seconds,..."
-----------
The relevance of continued observational confirmation of GR to Loop Gravity is that the theory is distinguished by treating General Relativity (with its basic assumptions of background independence and diff-invariance) as a fundamental theory to be quantized.
By contrast certain alternatives to Loop Gravity do not treat GR as fundamental. Instead it is treated as the low-energy limit of some hoped-for but still unknown theory not requiring GR's basic assumptions (such as background independence).
The Nature article also points out that having found another binary system due to merge in the (astronomically) near future----85 million years in this case---is suggestive that the merger of a pair of neutron stars (the kind of thing LIGO would like to detect gravity waves from) may be a more frequent event than was estimated earlier. LIGO will itself be testing, and potentially offering further confirmation of, General Relativity.
The source article for the binary pulsar was posted a couple of
days ago, 7 January 2004.
http://arxiv.org/astro-ph/0401086
it is 21 pages
and discusses the implications for testing GR in detail
the article is information rich, with plenty of
tables (of orbit parameters etc.) and figures.
6 binary pulsars are known
this one was discovered by an Australian dish (Parkes)
in 2003
several different tests (testing several different predictions)
of GR are possible as more observations of the binary system
accumulate
It is a lucky find.
In another thread the question of demographics came up again.
Numbers of papers, or even numbers of "blockbuster" papers that get lots of follow-up citations, dont necessarily mean all that much but the issue gets raised now and then so we should have some kind of objective data. There is a small and increasing output of papers
in Loop Gravity:
I just went to arxiv.org "Search Physics Archives" page and
put in [ABS = loop quantum gravity]OR[ABS = spin foam]OR[ABS = loop quantum cosmology] since 2000 and it gave me
these numbers of papers:
2000 46
2001 48
2002 64
2003 70
These are the preprints at the archive that have somewhere in their ABSTRACTS either the words loop quantum gravity, or the words spin foam, or the words loop quantum cosmology.
--------------
Although I'm not especially interested in string/brane theories, some people seem interested in comparisons so here's the same numbers for
[ABS = string]OR[ABS = brane]OR[ABS = M-theory]
2000 1457
2001 1496
2002 1500
2003 1265
That is, those where the abstract summary of the paper has in it somewhere the word string, or the word brane, or the word M-theory.
The numbers speak for themselves. There's more to say about the current state of research in Loop Gravity---hope to get back to this later today.
A guy at Stanford-SLAC puts out a list of the top-cited papers each year in each archived category. He hasnt done it for 2003 yet, so his most recent list is as of end 2002.
I was interested in the most-cited papers in the category gr-qc
"General Relativity and Quantum Gravity"
http://www.slac.stanford.edu/library/topcites/2002.gr-qc.1.shtml
and, in particular, in RECENT papers (dated 2000, 2001 and 2002) in that category. So here's the "Top Ten" list for that category, with the older (pre-2000) papers winnowed out. Of course most string papers are over in the hep-th, high energy physics-theory, category. I am focussing just on gr-qc here.
---------------------
57 citations
RELATIVITY IN SPACE-TIMES WITH SHORT DISTANCE STRUCTURE GOVERNED BY AN OBSERVER INDEPENDENT (PLANCKIAN) LENGTH SCALE
By Giovanni Amelino-Camelia (Rome U.).
Published in Int.J.Mod.Phys.D11:35-60,2002 [PS file for arXiv: gr-qc/0012051]
54 citations
CLASSICAL BLACK HOLE PRODUCTION IN HIGH-ENERGY COLLISIONS
By Douglas M. Eardley, Steven B. Giddings (UC, Santa Barbara).
Published in Phys.Rev.D66:044011,2002 [PS file for arXiv: gr-qc/0201034]
36 citations
INTRODUCTION TO MODERN CANONICAL QUANTUM GENERAL RELATIVITY
By Thomas Thiemann (Potsdam, Max Planck Inst.). [PS file for arXiv: gr-qc/0110034]
31 citations
THE CONFRONTATION BETWEEN GENERAL RELATIVITY AND EXPERIMENT
By Clifford M. Will (Washington U., St. Louis).
Published in Living Rev.Rel.4:4,2001 [PS file for arXiv: gr-qc/0103036]
28 citations
EXTENDING THE LIFETIME OF 3-D BLACK HOLE COMPUTATIONS WITH A NEW HYPERBOLIC SYSTEM OF EVOLUTION EQUATIONS
By Lawrence E. Kidder, Mark A. Scheel, Saul A. Teukolsky (Cornell U., Radio. Space Res. Ctr.).
Published in Phys.Rev.D64:064017,2001 [PS file for arXiv: gr-qc/0105031]
28 citations
GEOMETRY AND DYNAMICS OF THE BRANE WORLD
By Roy Maartens (Portsmouth U.). [PS file for arXiv: gr-qc/0101059]
27 citations
AN ALTERNATIVE TO QUINTESSENCE
By Alexander Yu. Kamenshchik (Landau Inst. & Landau Network Centro Volta), Ugo Moschella (Insubria U., Como & INFN, Milan), Vincent Pasquier (Saclay).
Published in Phys.Lett.B511:265-268,2001 [PS file for arXiv: gr-qc/0103004]
23 citations
QUANTUM GRAVITY: A PROGRESS REPORT
By S. Carlip (UC, Davis).
Published in Rept.Prog.Phys.64:885,2001 [PS file for arXiv: gr-qc/0108040]
21 citations
QUANTUM GEOMETRY OF ISOLATED HORIZONS AND BLACK HOLE ENTROPY
By A. Ashtekar (Penn State U. & Santa Barbara, KITP), John C. Baez (UC, Riverside & Penn State U.), Kiriil Krasnov (UC, Santa Barbara & Santa Barbara, KITP).
Published in Adv.Theor.Math.Phys.4:1-94,2000 [PS file for arXiv: gr-qc/0005126]
19 citations
GENERALIZED LORENTZ INVARIANCE WITH AN INVARIANT ENERGY SCALE
By Joao Magueijo (Imperial Coll., London), Lee Smolin (Perimeter Inst. Theor. Phys. & Waterloo U.).
Published in Phys.Rev.D67:044017,2003 [PS file for arXiv: gr-qc/0207085]
---------------------
obviously there are different ways of gauging activity and what the
most-cited papers in a field are and what the cut-off should be for "recent" work, but here is one measure: the number of other papers that cited the given paper in their references. At least it gives an idea of who the people are that write the papers that get most often cited in this division of the archive (and some slight indication as to what topics are of lively current interest)
Loop Gravity-watchers will be familiar with Etera Livine, Laurent Freidel, and David Louapre. David has been a sometimes poster at PF and both Livine and Freidel are among those whose Lorentzian spinfoam papers we discussed. Freidel and Louapre (with help from Livine) are preparing a series of papers on 2+1 dimensional quantum gravity of which the first is now available.
The ancestor of this series is
Laurent Freidel, “ A Ponzano-Regge model of Lorentzian 3-Dimensional gravity ”, Nucl. Phys. Proc. Suppl. 88 (2000) 237-240 , gr-qc/0102098.
The announced series of four papers, of which the first was just posted:
L.Freidel and D. Louapre,"Ponzano-Regge model revisited I: Gauge fixing, observables and interacting spinning particles"
http://arxiv.org./hep-th/0401076
L.Freidel and D. Louapre, “Ponzano-Regge model revisited II: Mathematical aspects; relation with Chern-Simons theory, DSU(2) quantum group and link invariant". To appear.
L.Freidel, E. Livine and D. Louapre, “Ponzano-Regge model revisited III: The Field Theory limit”. To appear.
L.Freidel and D. Louapre, “Ponzano-Regge model revisited IV: Lorentzian 3D Quantum Geometry”. To appear.
------other papers of possible related interest-------
L. Freidel and D. Louapre, “Diffeomorphisms and spin foam models,” Nucl. Phys. B 662, 279
http://arxiv.org/gr-qc/0212001
K. Noui and A. Perez “Three dimensional loop gravity coupled to point particles”, to appear.
----------miscellaneous-----
Links to a couple of things at PF here:
interesting post from "notevenwrong" about testability
http://www.physicsforums.com/showthread.php?s=&postid=128657#post128657
I dont think there's any need for any of us to justify
or explain why we are interested in the kinds of physics that
we're interested in.
So I dont normally bother to explain why I like to
report current developments in Loop Gravity or discuss
new papers with anyone who might be interested. But
here I was being challenged to explain my excitement and
I talked about why it's interesting to me personally
(doesnt have to be to you, but you may be enthused by
some of the same things). It's a ramble though.
http://www.physicsforums.com/showthread.php?s=&postid=128783#post128783
The MIT Theoretical Physicist Frank Wilczek
gave the keynote address at a December 2003
Conference on space and particle physics
"SpacePart03"
the text and one slide from his address has just been posted
http://www.arxiv.org/abs/astro-ph/0401347
He sketches the current situation. Wilczek is always
being asked to deliver Particle Physics overviews and keynotes (at the opening of a new accelerator or the closing ceremony for an old one), probably because he is most clearsighted senior theoretician they've got.
His 3 article series "Scaling Mount Planck" in Physics Today
issues back in 2001 and 2002 was a masterful survey of
big questions in theoretical physics (well, I was impressed).
Here he is doing it again, so its worth a look.
Basically it is a simple message. There are now two "standard models"
The one of particle physics and the one of cosmology.
Each has a certain number of exogenous parameters
IIRC he identifies four independent inputs to standard model
comology. And he describes progress and prospects as regards
particle physics explaining certain of the inputs to cosmology.
You can guess a lot of what the elder statesman is going to say
but no other voice has the clarity and authoritiy AFAIK.
I note he puts General Relativity in a good light (something particle physicists may eventually have to believe in on its own terms, in spite of the different underlying spacetime concept). He specifically mentions Diffeomorphism Invariance (calling it by Einstein's name for it: "general covariance")
"General relativity manifestly provides a beautiful, conceptually driven theory of gravity. It has scored many triumphs, both qualitative (big bang cosmology, black hole physics) and quantitative (precession of Mercury, binary pulsar).
The low-energy effective theory of gravity and the other interactions is defined algorithmically by the minimal coupling prescription, or equivalently by restricting to low-dimension operators. In this context, “low” means compared to the Planck energy scale, so this effective theory is very effective indeed.
As in the gauge sector, symmetry---here, general covariance---greatly constrains the possible couplings, bringing us down to just two relevant parameters. Almost all the observed phenomena of gravity are described using only one of these parameters, namely Newton’s gravitational constant. We are just now coming to accept that the other parameter, the value of the cosmological term, plays an important role in describing late-time cosmology..."
This is just a brief exerpt and his forte is his sense of proportion so you have to read the whole thing to get a feel for the relative importance he gives to things.
Earlier articles (June, Novemember 2001, August 2002)
http://www.physicstoday.org/pt/vol-54/iss-6/p12.html
http://www.physicstoday.org/pt/vol-54/iss-11/p12.html
http://www.if.ufrgs.br/~jgallas/wilczek.html
--------marginal note-----
At the same December 2003 space and particle physics conference where Wilczek gave the keynote there were a couple of talks on "Lorentz Symmetry Violation".
a paper on the subject by one of the partcipants
http://arxiv.org/hep-th/0312310
have to follow up on this later, got to run now
(Lorentz violation active research topic, relates to DSR)
Four Koreans I never heard of before just posted a DSR paper
http://arxiv.org/gr-qc/0401078
"Propagation of Light in Doubly Special Relativity"
It draws a on work by Jerzy Kowalski-Glikman and by Joao Magueijo, in particular on a paper they call KMM (for Kimberly, Magueijo, Medeiros)
http://arxiv.org/gr-qc/0303067
The authors are Kim, Kim, Rim, and Yee.
They are from various universities in Korea.
The timing of this is remarkable.
There is a Loop Gravity conference in Mexico City scheduled for next week and Thiemann brings out a Loop-String paper
doing string theory related algebra in the background independent LQG fashion.
Thiemann is scheduled to give a talk at that conference.
Here is the program for the upcoming Mexico City conference
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
Here is the link for Thiemann's new paper, and exerpts from the abstract
http://arxiv.org/hep-th/0401172
The LQG -- String: Loop Quantum Gravity Quantization of String Theory I. Flat Target Space
We combine I. background independent Loop Quantum Gravity (LQG) quantization techniques, II. the mathematically rigorous framework of Algebraic Quantum Field Theory (AQFT) and III. the theory of integrable systems resulting in the invariant Pohlmeyer Charges in order to set up the general representation theory (superselection theory) for the closed bosonic quantum string on flat target space.
....solve some of the major puzzles of string theory such as the cosmological constant problem. The solution presented in this paper exploits the flatness of the target space in several important ways. In a companion paper we treat the more complicated case of curved target spaces.
I was not aware until recently that this year Cambridge University Press is expected to bring out two graduate-level LQG texts.
I know Rovelli's 350-page "Quantum Gravity" is at press and expected out this year.
But it also seems Cambridge is publishing a 300+ page book by Thiemann called "Modern Canonical Quantum General Relativity". This is listed as "at press" in the references of a recent paper by the author, with projected publication date 2004.
I believe that a draft of Thiemann's "Modern...Quantum...Relativity" may be online at
http://arxiv.org/gr-qc/0110034
In draft, the title also had the word "Introduction" but that may have now been dropped for the sake of brevity.
Rovelli has an online draft of "Quantum Gravity" at his website---the link was given in several earlier posts.
These two books would together form the basis of a hefty year course in grad school. They complement each other to some extent: Rovelli's is more conceptual and discursive. It discusses philosophical foundations and presents many of the ideas historically. I like the well-chosen graphic examples used to illustrate the ideas. By comparison, the exposition in Thiemann's book is predominantly abstract mathematics, going into mathematical detail with thoroughness and rigor.
I should mention that the Berlin publisher Springer Verlag has also brought out a Loop Gravity textbook as part of its "Lecture Notes" series. This is a shorter, and more introductory, set of notes by Thiemann called
"Lectures on Loop Quantum Gravity".
A draft of this is online at
http://arxiv.org/gr-qc/0210094
As far as I know these three books, all appearing at about the same time, are the first hardcopy textbooks for the new field of Loop Gravity. In that sense 2004 looks like a landmark year in the development of the subject.
Back in the middle of December I posted the organizer's list of topics (for talks and discussion) for the Loop symposium being held Jan 30 thru Feb 1 in Mexico City.
At that time I did not have a list of the talks or the people giving them. You might be interested. These are most of the people active in Loop Gravity research and many of the talks and the discussion sections center around delicate or key unresolved issues. It is clearly a symposium for people inside the field to share their latest ideas and results, "in house" so to speak. So here is the program:
[I made topics bold or caps for emphasis, so its easier to scan]
--------
friday
Welcome: Alejandro Corichi 1:00-1:10
Session: PHENOMENOLOGY
Chair: Hugo Morales
Sudarsky <End of New Ether>
Discussion
Freidel <Imprints of Planck scale structures and DSR>
Discussion
Session: SEMI-CLASSICAL Issues
Chair: Luca Bombelli
Sahlmann: <Brief survey of available frameworks>
Discussion
Ashtekar:<Physical Applications>
saturday
Session: SPIN FOAMS
Chair: Jose Antonio Zapata
Perez <Status Report>
Discussion
Crane <Beyond Barrett-Crane models>
Discussion
Session: HAMILTONIAN CONSTRAINT
Chair: Karim Noui
Freidel<Relation between spin-foams and canonical gravity>
Discussion
Ashtekar <viability of the Thiemann scheme>
Discussion
Thiemann <The Phoenix project>
Discussion
sunday
Conceptual Issues
Chair: Alejandro Corichi
Jorge Pullin <Future directions that will maximize impact>
Possible topics of discussion:
*Prospects for discrete formulations
*Status of non-compact gauge groups: Integration theory
*Role of supersymmetry
*Observables from particles in 2+1 gravity
*Quantum cosmology and observations
*Where is physics in spin-foams?
*Is Kodama state viable?
*Causality in quantum gravity
* ...
* ...
Abhay Ashtekar <Closing remarks>
LIST OF PARTICIPANTS:
A. Ashtekar (Penn State)
L. Bombelli (Mississippi)
L. Crane (Kansas State U.)
A. Corichi (UNAM. Mexico)
L. Freidel (Perimeter)
J.M. Garcia-Islas (CIMAT, Mexico)
F. Girelli (Perimeter)
E. Livine (Perimeter)
H. Morales-Técotl (UAM-I, México)
K. Nuoi (Penn State)
A. Perez (Penn State)
J. Pullin (Louisiana)
H. Sahlmann (Penn State)
D. Sudarsky (UNAM, Mexico)
T. Thiemann. (Perimeter)
J.A. Zapata (UNAM, Mexico)
---------
Given that marcus wants this thread like a sticky for new papers, I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole. We describe the black hole in general microcanonical and canonical area ensembles for these spectra. For one of these spectra - the equally-spaced spectrum - we show in light of a proposed connection of the black hole area spectrum to the quasinormal mode spectrum that this spectrum is completely consistent with this connection. This follows {\em without} requiring a change in the gauge group of the spin degrees of freedom in this formalism from SU(2) to SO(3).
Originally posted by meteor
Given that marcus wants this thread like a sticky for new papers, I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
One of the authors, G. Gour, has co-authored with Jacob Bekenstein and the approach to the area spectrum taken here goes back to
a paper of Bekenstein's:
http://arxiv.org/abs/hep-th/0107045
----------quote from abstract------------
The Case for Discrete Energy Levels of a Black Hole
Authors: Jacob D. Bekenstein
Comments: Invited talk at "2001: A Spacetime Odyssey", inaugural conference of the Michigan Center for Theoretical Physics, May 22-25, 2001, 11 pages, to appear in the proceedings published by World Scientific Publishing
Journal-ref: 2001: A Spacetime Odyssey, eds. M. J. Duff and J. T. Liu, (World Scientific Publishing, Singapore 2002), pp. 21-31
The adiabatic invariant nature of black hole horizon area in classical gravity suggests that in quantum theory the corresponding operator has a discrete spectrum. I here develop further an algebraic approach to black hole quantization which starts from very elementary assumptions, and proceeds by exploiting symmetry. It predicts a uniformly spaced area spectrum for all charges and angular momenta. Area eigenvalues are degenerate; correspondence with black hole entropy then dictates a precise value for the interval between eigenvalues.
--------end quote-----
it would be extremely interesting if the equal-spaced area spectrum gains credibility. this is a wonderful paper to have on our
Loop reference shelf "surrogate sticky".
at present measurement cannot distinguish between this version and the unequally-spaced spectrum, it would seem, so theorists can pursue either
if this equal-spaced (ES) notion of the spectrum is right then the Immirzi parameter is
ln 3/3pi
just for fun I will write it in tex.
\gamma_{ES} = \frac{ln 3}{3 \pi}
this paper by G. Gour and his buddy has potential for stirring up a controversy, I suspect
as you well know, a commonly assumed value is
\gamma = \frac{ln 2}{\sqrt{3} \pi}
lumidek
Jan29-04, 11:14 AM
Dear marcus,
I am not sure why you exactly think that the idea of the discrete area (or even energy) spectrum of the black hole should be gaining credibility. It contradicts all important things that we know about the black hole, for example the thermal character of the radiation and other semiclassical calculations. These well-established insights, initiated by Hawking in the 1970s, have been amazingly confirmed by the developments in string theory in the 1990s, so there is really little doubt that they're correct.
The energy spacing comparable to the Hawking temperature does not sound too reasonable because it would essentially imply that the energy that the black hole must emit must be an integer multiple of the Hawking temperature. The real spectrum copies the nice and smooth curve by Planck.
There is no serious support for the idea of this hugely discretized spectrum - perhaps some children's toy models and many overly speculative papers (which is the polite way to describe the crackpots). The precise numbers that should determine the scaling are totally unscientific, too. You only get a log of an integer in these formulae because some people who propose it are imagining that everything in the physical world is made from bits (or "trits") - simply because their imagination does not go beyond it.
All the best
Luboš
Originally posted by lumidek
Dear marcus,
I am not sure why you exactly think that the idea of the discrete area (or even energy) spectrum of the black hole should be gaining credibility.
Dear Lubos, of course I remember what you had to say in your very first QNM paper about evenly spaced area spectrum. Seems a long time ago now.
In a PF context we need to be careful not to say "discrete area spectrum" (which includes the unequal spacing case) when we mean the "evenly spaced spectrum".
There seems to be wide agreement with the notion that discrete area spectrum is compatible with the themal character of the Hawking radiation. (Indeed I had the impression that you were also in agreement with this.)
However you and many other people have argued against the evenly spaced area spectrum for precisely the reason you mention. I believe you are in agreement here with Loop authorities like Ashtekar, Rovelli, Smolin. Back in (when was it?) 1995 when Rovelli and Smolin derived the discrete area spectrum, it was not equally spaced----spacing between eigenv. got closer and closer as area got larger.
No doubt you find it reassuring to be in such good company (on the same side of the fence as Rovelli and Smolin) on this issue[;)]
-------
But my personal view is that Jacob Bekenstein is a brilliant and original mind-----he deserves credit at a fundamental level for black hole thermodynamics, Hawking radiation, BH temperature, entropy and all that good stuff that started in the 1970s. He kicked it off.
If he says to keep an open mind and think about equal-spaced (ES) spectrum then I respect him and I am going to keep an open mind about it. Surprises sometimes happen. What if Gilad Gour (also very smart) and Bekenstein are right and there is something nobody has thought of yet that makes it compatible. Have to run, but back later.
lumidek
Jan29-04, 12:09 PM
Dear Marcus,
it's great if we agree that the evenly spaced spectrum does not seem too good.
Concerning the discrete, non-evenly-spaced spectrum, it is not a physical question whether the exponentially dense spectrum of the black hole energy is discrete or not. Of course, if the number of states (exponential of entropy) is finite, it must be discrete "in some sense".
The density of the states is increasing exponentially with a power of the mass. It is naive to think that you can determine the black hole energy with the precision comparable to this expected exponentially small spacing - simply because the width of the black hole itself is exponentially bigger (the width behaves as a power of the mass, without any exponentiating). The width essentially tells you the minimal error with which you can measure the spectrum, and this minimal error is much bigger than the required resolution to determine the character of the exponentially fine spacing.
Once you admit that the gaps in the spectrum are not huge (such as the power law gaps of the evenly spaced spectrum), there is no other physical question to ask about the character of the spectrum.
All the models that give you the super-exact energies must neglect the interactions and Hawking radiation because Hawking radiation makes black holes unstable and energies undetermined (width). Quite generally, you may be worried that neglecting Hawking radiation means neglecting quantum mechanics, and therefore all the conclusions about the quantization are internally inconsistent.
The only exception are extremal, BPS black holes that don't decay (temperature vanishes), and their energy is determined exactly by the BPS bound.
All the best
Luboš
Originally posted by lumidek
Dear Marcus,
it's great if we agree that the evenly spaced spectrum does not seem too good...
We dont agree Lubos. You indicate that you have a firm opinion that evenly spaced (ES) area spectrum doesnt seem good.
I am trying to keep an open mind.
Originally posted by lumidek
The energy spacing comparable to the Hawking temperature does not sound too reasonable because it would essentially imply that the energy that the black hole must emit must be an integer multiple of the Hawking temperature. The real spectrum copies the nice and smooth curve by Planck.
There is no serious support for the idea of this hugely discretized spectrum -
We were talking about area eigenvalue spacing.
The Gilad Gour paper does not consider "huge" gaps in the area spectrum but, on the contrary, gaps comparable in size to
the Planck area---in other words very tiny.
You may be jumping to conclusions because I mentioned Bekenstein as one who has argued for considering ES area spectrum.
See equation (3) of the Gour Suneeta paper.
Evenly spaced does not imply huge gaps, or gap-size dependent (as you suggest) on the Hawking temp. of the hole.
By your own argument in the your preceeding post if the gaps are
small like in Rovelli-Smolin spectrum (Planck area size) then one would not be able to physically tell the difference from smooth black body. The same applies to Gour-Suneeta because similarly small gaps.
lumidek
Jan29-04, 01:38 PM
Dear Marcus,
I see, sorry. I made a too quick conclusion that you share the only approach that I consider reasonable in this question.
It's great to keep an open mind, but if Hawking is correct, a lot of ideas can evaporate from a mind that is too open. [6)]
All the best
Luboš
Originally posted by meteor
... I will post this one:
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole. We describe the black hole in general microcanonical and canonical area ensembles for these spectra. For one of these spectra - the equally-spaced spectrum - we show in light of a proposed connection of the black hole area spectrum to the quasinormal mode spectrum that this spectrum is completely consistent with this connection. This follows without requiring a change in the gauge group of the spin degrees of freedom in this formalism from SU(2) to SO(3).
This is the paper by Gilad Gour and V. Suneeta (both at U Alberta
Edmunton) that Meteor called to our attention.
An objection raised earlier seems not to apply since, while the area spectrum is discrete (as usual in LQG) and evenly spaced, the spacing is microscopic (planck-scale) and not fundamentally very different from the un-evenly spaced. This paper could be important and needs a more careful look. So let's look at Gour/Suneeta equations (2) and (3)
These are the two competing formulas for the area of a surface S.
the first is in the non-evenly spaced (NS) case
A_S = 8\pi l_P^2 \gamma \Sigma \sqrt{j_n(j_n + 1)}
the second is in the evenly spaced (ES) case
A_S = 8\pi l_P^2 \gamma \Sigma (j_n + 1/2)
We are talking about a spin network state and a physical surface, S, defined by some material object. The spin network state has N edges which intersect the surface and each edge contributes a bit of area to the sum. The intersecting edges are indexed n = 1,....,N.
Oh yeah, lP2 is the planck unit of area, the square of the planck length
Equation (2) is the standard 1994 result of Rovelli and Smolin. A priori one wonders how Gour/Suneeta could possibly be challenging this by proposing a different formula for the area, namely equation (3).
Furthermore their proposal may make some people nervous since it results in a value of ln 3/3pi for the Immirzi parameter. As it happens this doesnt bother me. Some versions of LQG dont even have an Immirzi parameter: Loop theories are under construction and there is still room for variation in the parameters and even some surprises. So I am not going to dismiss this out of hand just because of some number being an unfamiliar ln 3/3 pi instead of the more usual value that has been around longer.
Anyway Meteor posted the link to this a few days ago so lets see what it is about.
common sense suggests there may be pitfalls in Gour/Suneeta approach.
the other formula goes back to 1994. why would Rovelli/Smolin have chosen their more complicated formula unless there were some reason?
but so far so good.
anybody (Meteor?) who wants to help read thru this short (7 page)
paper is invited
a couple of more links on the area issue:
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
Alexkseev and Smedbaeck are at the Institute for Theoretical Physics, Upsala University, in Sweden
Polychronakos is at the University of Ioannina in Greece and also the
CUNY Physics Department in the USA.
These seem to be the dangerous people we were warned about earlier. They think that, just as an atom while it is in a heat bath in equilibrium at a given temperature can radiate with a black body spectrum but by itself radiates a line spectrum, so a black hole not surrounded by radiation and therefore not in equilibrium with its surroundings may in fact radiate a line spectrum. Over much of the spectrum it would strongly resemble black body but at low frequencies it would deviate and become more obviously liney.
Polychronakos says, in "Area spectrum and...", for instance)
-------quote from page 9----------
We do not feel that this is damning. The high-frequency exponential part of the spectrum is accurately reproduced, the discreteness there being inconsequential. This is the energy range in which the photons (or other emitted particles) behave essentially like classical particles, whose scattering properties are expected to be accurately reproduced by the classical black hole metric. For frequencies close to the thermal frequency, however, the wavelength of the photons becomes comparable to the size of the black hole and they sense global properties of its geometry. Backreaction due to geometry change at emission and absorption of such photons is expected to be important, the energy of these photons being of the same order as
the energy spacing of the black hole. A deviation from ideal black-body spectrum, which assumes a fixed metric and ignores back-reaction, would seem reasonable.
-----end quote----------
If my opinion counts, (I'm only an amateur, not a high-level physicist) I don't like the idea of the equally-spaced area spectrum, because it means that practically all the contributions to the black hole area comes from edges carrying a representation of 1. Would be desirable to know why the representations=1/2 are excluded from puncture the black hole area
Originally posted by meteor
If my opinion counts..
you found the paper and put it in for discussion
so your opinion doesnt count???
Gour/Suneeta cite Polychronakos (hep-th/0304135)
which I am finding the easiest to read and the
most helpful. (the subject is new to me, maybe to
you as well)
On page 8 Polychronakos says
"To summarize...if the standard counting formula for states (6) is
assumed, then the equidistant area spectrum as proposed in [14]
naturally explains the domination of spin-1 links
and reproduces the ringing mode properties of black holes, without
the need to eliminate half-integer spins..."
You say "...would be desirable to know why spin-1/2" are excluded. It is a good question but, according to him, they are not eliminated arbitrarily or "by hand".
He says there is no need to exclude them, because statistically the spin-1 punctures are more prevalent.
Why? this is what I am wondering. He says there is a natural explanation for why spin-1 is more prevalent and predominates over the other. The explanation is on page 5, I think. Around equations (12) and (13). It is very brief and does not seem hard.
It seems to depend on the semiclassical result of Hod!
Hod made an important contribution to Loop gravity with his number
4ln3, which Motl, fortunately enough, was able to confirm by a direct calculation. It has added interest to the field and given them something to work on, and here it is showing up again on page 5 of Polychronakos.
Another paper:
"Beyond space and time"
http://arxiv.org/abs/physics/0401128
Author: Ruediger Vaas
Abstract:
"The secret network of the universe: How quantum geometry might complete Einstein's dream. - An informal introduction to quantum geometry (loop quantum gravity), spin networks, quantum black holes, and the work of Abhay Ashtekar, Carlo Rovelli, Lee Smolin and others."
After a quick inspection, I'm amazed: You will not find a single equation in the text!
An interesting phrase of the text: "If one could observe nature with maximum possible enlargement, space and time would dissolve and the granular mesh of the spin network would come to light ( or more precisely: the quantum physical superposition of all possible configurations of these entities)"
Originally posted by meteor
Another paper:
"Beyond space and time"
http://arxiv.org/abs/physics/0401128
Author: Ruediger Vaas
My suspicion about that article is that
Rudy Vaas is a science-journalist, and it was
written for the German public to run in
the popular press. In English it sounds "over-the-top"
in places, but a good editor could fix that.
Amitabha Sen translated it into English.
Sen is a reputable theoretical physicist.
In 1982 he started the line of development that
resulted in Ashtekar's 1986 "new variables"
approach. Maybe the new variables should
be called "Ashtekar-Sen"
It could be a good popularization article if
it were edited to tone down the journalistic excess.
Originally posted by meteor
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Abstract:
We compare two area spectra proposed in loop quantum gravity in different approaches to compute the entropy of the Schwarzschild black hole....
I've come to the conclusion that we should have a thread about the "equidistant spectrum" version LQG area.
this is about the reasoned opposition of ideas
the majority Loop people say the Rovelli/Smolin spectrum
which is not equidistant or "evenly spaced".
But a few people say to look at the evenly spaced spectrum
and they seem to have a case to make
the papers by Polychronakos
and by him and Alekseev etc
seem pretty clear and interesting too
as well as the one Meteor brought.
here are those other two links:
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
http://xxx.soton.ac.uk/abs/gr-qc/0401122
The Pauli Exclusion Principle and SU(2) Versus SO(3) in Loop Quantum Gravity
Author: John Swain
Abstract:
"Recent attempts to resolve the ambiguity in the loop quantum gravity description of the quantization of area has led to the idea that j=1 edges of spin-networks dominate in their contribution to black hole areas as opposed to j=1/2 which would naively be expected. This suggests that the true gauge group involved might be SO(3) rather than SU(2) with attendant difficulties. We argue that the assumption that a version of the Pauli principle is present in loop quantum gravity allows one to maintain SU(2) as the gauge group while still naturally achieving the desired suppression of spin-1/2 punctures. Areas come from j=1 punctures rather than j=1/2 punctures for much the same reason that photons lead to macroscopic classically observable fields while electrons do not."
In this paper, Swain tries to incorporate some kind of Pauli exclusion principle to LQG: If only at most two edges of j=1/2 are allowed to puncture a given surface, then the predominance of edges j=1 puncturing the horizon of a BH is naturally explained
Meteor thanks for the John Swain article. It seems as if there are a number of interesting possible reasons that the spin 1 edges play such a prominent role in the black hole horizon area.
We are now at page 10 of this thread and many of the links were gathered back on page 7, so I will update things and bring the earlier links forward:
--------------------------------
So far there is no sticky for Loop Gravity reference links. And this thread is serving as a surrogate "reference library". Thanks to all who have contributed so far!
The term "Loop Gravity" is used for a broad range of background-independent approaches to quantizing general relativity. Rovelli briefly discusses "the name of the theory" on page (xvi) of his new book. The name "loop" is something of an historical accident---current approaches are not so much involved with loops as with spin network states. But no one has come up with a collective designation that includes spin foams and is more convenient.
The main things the new approaches seem to have in common is that they emerge from General Relativity (rather than Particle Physics) and that they aren't string/brane theories.
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html.
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
---------some current work------
We were discussing stuff from Livine's thesis in this and another thread. Here is Livine's thesis. He does a lot with explicitly covariant---SL(2,C)-style---spin networks and makes an explicit bridge from LQG to Lorentzian spinfoams.
http://arxiv.org/gr-qc/0309028
Girelli and Livine have come out with a paper about quantizing speed.
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Ichiro Oda has posted "A Relation Between Topological Quantum Field Theory and the Kodama State"
http://arxiv.org/hep-th/0311149
Daniele Oriti's thesis is out
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Smolin and Magueijo
"Gravity's Rainbow"
http://www.arxiv.org/abs/gr-qc/0305055
(this was revised and reposted 3 February 2004,
I haven't read the revised version yet)
Smolin and Starodubtsev
"General Relativity with a topological phase: an action principle"
http://arxiv.org/hep-th/0311163
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
-------Quantum Gravity Phenomenology---------
three recent papers:
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other fairly recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------projected observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
========
simply to have this link handy:
http://www.physicsforums.com/misc/howtolatex.pdf
In another thread the question of "majority consensus" (or one might say "research demographics") came up again.
Numbers of papers, or even numbers of blockbuster papers that get lots of follow-up citations, dont necessarily mean all that much but the issue gets raised now and then so we should have some kind of objective data. There is a small and increasing output of papers in Loop Gravity:
Curious about quantifying this, I went to arxiv.org "Search Physics Archives" page and put in [ABS = loop quantum gravity]OR[ABS = spin foam]OR[ABS = loop quantum cosmology] since 2000. The engine found
these numbers of papers:
2000 46
2001 48
2002 64
2003 70
Y(2/11) 73*
These are the preprints at the archive that have somewhere in their ABSTRACTS either the words loop quantum gravity, or the words spin foam, or the words loop quantum cosmology.
--------------
Although I'm not especially interested in string/brane theories, some people seem interested in comparisons so here's the same numbers for
[ABS = string]OR[ABS = brane]OR[ABS = M-theory]
2000 1457
2001 1496
2002 1500
2003 1265
Y(2/11) 911*
That is, those where the abstract summary of the paper has in it somewhere the word string, or the word brane, or the word M-theory.
*The search engine also has a "Past Year" option which gives the papers posted in the year-to-date: the preceding 12 months. On February 11 I ran the same check for year-to-date and got corresponding numbers for the "Year to 2/11" which I've listed here separately as Y(2/11).
Y(2/11) reflects activity in part of the calendar year 2003 and in part of the calendar year 2004.
Meteor started us collecting recent work bearing on
the vibration of black holes and the Loop Gravty area spectrum.
Gilad Gour and V. Suneeta
"Comparison of area spectra in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0401110
Alexios Polychronakos
Area spectrum and quasinormal modes of black holes
http://arxiv.org/hep-th/0304135
Alekseev, Polychronakos, Smedbaeck
On the area and entropy of a black hole
http://arxiv.org/hep-th/0004036
Gour and Suneeta are at the University of Alberta.
Alexkseev and Smedbaeck are at the Institute for Theoretical Physics, Upsala University, in Sweden
Polychronakos is at the University of Ioannina in Greece and also the
CUNY Physics Department in the USA.
Setare and Vagenas
"Area Spectrum of Kerr and extremal Kerr Black Holes from Quasinormal Modes"
http://arxiv.org/hep-th/0401187
Berti, Cardoso, Yoshida
"Highly Damped Quasinormal Modes of Kerr Black Holes: A Complete Numerical Investigation"
http://arxiv.org/gr-qc/0401052
Setare is in Iran, Vagenas at Barcelona.
Berti is in St. Louis, Cardosoo and Yoshida are in Portugal.
Here is the link for this post:
http://www.physicsforums.com/showthread.php?s=&postid=140731#post140731
selfAdjoint
Feb3-04, 08:51 PM
Didn't Lubos write or cowrite a paper on this subject? I seem to remember it being mentioned some months ago on s.p.r.
Originally posted by selfAdjoint
Didn't Lubos write or cowrite a paper on this subject? I seem to remember it being mentioned some months ago on s.p.r.
Sure did. One solo and one with Andy Neitzke that I know of.
would you like the links?
why dont I give them in any case:
Lubos' paper:
http://arxiv.org/gr-qc/0212096
Lubos and Andy's paper:
http://arxiv.org/hep-th/0301173
------------------------
what's exciting about the more recent paper by Gour and Suneeta
is that they challenge the Area operator spectrum derived by Rovelli and Smolin in 1994 and propose a quantum correction in the area.
this resolves a long-standing difficulty and produces some nice
results.
Meteor brought the paper in. Ive been reading it and like it quite a bit. (it is however "revisionist" in a sense)
afterthought edit: selfAdjoint, I put a longer discussion of the LQG area spectrum in the thread called "Loop Quantum Gravity". Didnt want to take extra space in this thread which is serving as a link-basket.
A couple of days ago Rovelli posted the program for the May 2004 Conference (at Luminy on the Mediterranean)
the organizers:
Laurent Freidel
Philippe Roche
Carlo Rovelli
----exerpted material in no particular order----
A tentative list of morning speakers, still to be confirmed, is as
follows
Loops:
Abhay Ashtekar (quantum geometry)
Thomas Thiemann (dynamics and low energy)
Lee Smolin (overall results)
Ted Jacobson (devil's advocate)
Applications:
Martin Bojowald (loop cosmology)
Alejandro Corichi (black holes)
Daniel Sudarsky (phenomenology)
Spin foams:
John Baez (spinfoams)
Laurent Freidel (GFT, sum over complexes)
John Barrett (BC model)
Alejandro Perez (spinfoams)
Related approaches:
Jorge Pullin (consistent discretization)
Peter Forcacs or Max Neidermair (fixed point)
Ian Ambjorn or Renate Loll (dynamical triangulations)
John Klauder (general covariant dynamics)
....
....
6. SCIENTIFIC PROGRAM
Aim of the conference is to make the point on where we are in the
loop/spinfoam approach to quantum gravity. In particular:
evaluate the results obtained so far, point out open problems,
and discuss the directions of development that appear to be most
promising. The conference is therefore mostly (but not
exclusively) addressed to our community. The four days will
focus on 1) Loops, 2) Applications, 3) Spinfoams, 4) Related
approaches.
The conference will be articulated in:
- morning talks of approximately 30 minutes, meant to summarize the
present state of the different aspects of the field, followed by
ample discussion time.
- afternoon presentations of novel results. The duration of
these will be decided dividing the available time by the
number of communications accepted.
- A panel session, followed by a general discussion, on the last
day.
----end of exerpts----
Philippe Roche at the University of Montpellier has this webpage about the conference:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
Other conferences: Here are some recent and upcoming ones mentioned in a previous post in this thread.
....
....
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
...
...
"Big crunch avoidance in k=1 loop quantum cosmology"
http://arxiv.org/abs/gr-qc/0312110
Authors: Parampreet Singh, Alexey Toporensky
Abstract:
"It is well known that a closed universe with a minimally coupled massive scalar field always collapses to a singularity unless the initial conditions are extremely fine tuned. We show that the corrections to the equations of motion for the massive scalar field, given by loop quantum gravity in high curvature regime, always lead to a bounce independently of the initial conditions. In contrast to the previous works in loop quantum cosmology, we note that the singularity can be avoided even at the semi-classical level of effective dynamical equations with quantum modifications, without using a discrete quantum evolution"
k refers to curvature. In the case k=1 it represents a flat universe. In the text there's discussion about the geometrical density operator (never heard of this operator before). The effective Friedmann equation for LQC is presented, also the Raychaudhuri equation
Originally posted by meteor
"Big crunch avoidance in k=1 loop quantum cosmology"
http://arxiv.org/abs/gr-qc/0312110
Authors: Parampreet Singh, Alexey Toporensky
k refers to curvature. In the case k=1 it represents a flat universe. In the text there's discussion about the geometrical density operator (never heard of this operator before). The effective Friedmann equation for LQC is presented, also the Raychaudhuri equation
Meteor, Bojowald recently gave a survey and cited this article. It was a plenary talk on loop quantum cosmology at a January 5-10
conference called "ICGC 2004"
So he must think it is a good paper. I have not read it. I think the flat case is k=0 and the k=1 case is positive curvature----which would normally lead to a big crunch (at least in the sort of ordinary cosmology we used to have before there was a cosmological constant)
Bojowald's survey talk at the conference is online at arxiv and it has links to lots of recent LQC papers which saves us trouble in that department. One link for many:
Bojowald
Loop Quantum Cosmology: Recent Progress
http://arxiv.org/gr-qc/0402053
There is an event going on now (Feb 4-14) in Poland
about Quantum Gravity Phenomenology
Here is the latest speaker list I could find:
--------quote from program-------
Speakers:
E. Alvarez---Quantum Gravity
G. Amelino-Camelia---Introduction to quantum gravity phenomenology
P. De Bernardis---Cosmology with BOOMERANG, WMAP
A. Grillo---Planck-scale kinematics and the Pierre Auger Observatory
T. Jacobson---Astrophysical bounds on Planck-supressed Lorentz violation
J. Kowalski-Glikman---Introduction to doubly special relativity
C. Laemmerzahl---Tests of Lorentz symmetry in space and interferometry
P. Lipari---Ultra-high-energy cosmic-rays
J. Martin---Trans-Planckian cosmology
N. Mavromatos---PCT symmetry and quantum gravity phenomenology
T. Piran---Gamma-ray bursts
J. Pullin---Canonical quantum gravity phenomenology
L. Smolin---Cosmological constant in Quantum Gravity
....to gather together world-leading scientists working on the field of quantum gravity, astrophysics, and cosmology along with a number of post-graduate students and young post-docs and to offer young scientists the opportunity to learn about recent developments in the theoretical investigation of Planck-scale physics that might be tested experimentally in the near future. The lectures presented at the School would provide a broad coverage of subjects relevant for this field, including models of the fate of Lorentz invariance in quantum space-time, loop quantum gravity and string theory, cosmology and astrophysics.
-----------end quote----------
the so-called "Bohr compactification" of the real line
is named after Harald Bohr (b.1877, Niels younger bro)
Hans Halvorson at Princeton is a quality mathematician-cum-philosopher and I like the way he writes. he seems to think philosophically about quantum mechanics but also do functional analysis and topology as well. dont know any more about him.
"Complementarity of representations in quantum mechanics"
http://arxiv.org/quant-ph/0110102
AFAIK no one here has already noted Halvorson or this paper.
The Bohr compactification is important to cosmology and not much is on line about it, but Halvorson's paper is online, and see bottom of page 9, around equation (11) for a brief discussion
**************************
To see how the Bohr compactification fits into quantum cosmology see
Viqar Husain and Oliver Winkler
"On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
Best kind of confirmation of Bojowald's work because derives similar results (removal of BB singularity) entirely outside of the Loop Gravity framework.
Bohr compactification enters at top of page 6, right after they introduce the almost periodic functions on the Reals:
"It is well-known that AP(R) is naturally isomorphic to C(RBohr), the algebra of continuous functions on the so-called Bohr-compactification of R. As the name suggests, RBohr is a compact group which can be obtained as the dual group of Rdiscr, the real line endowed with the discrete topology. This suggests that taking
L2(RBohr,dµ0),
where µ0 is Haar measure on RBohr, as the
Hilbert space for our theory is a viable option. This is the decisive point where we depart from the traditional approach in geometrodynamics, where the Hilbert space is the conventional Schroedinger space L2(R, dx). Once we adopt this new choice, basis states in our Hilbert space are..."
they dont use the Ashtekar variables! they dont use the connection! they use the same ADM variables that Wheeler and DeWitt tried to use!!!
but they still manage to remove the Big Bang singularity.
the key thing turns out not to be cooking down LQG to get LQC
but something about an idea Harald Bohr had about "almost periodic functions" and something about topology.
Like what they say on page 10:
"Our main result is that there is an alternative to the Schroedinger quantization of the FRW cosmology in the standard ADM geometrodynamical variables. This quantization leads to conclusions
qualitatively similar to those obtained in loop quantum cosmology starting from the connection triad variables: (i) the Hamiltonian constraint acts like a difference operator, and (ii) the inverse scale factor can be represented as a densely defined operator.
Thus it is the representation space and the realizations of the basic observables rather than the nature of the classical variables that are responsible for the similar conclusions for this model."
their italics, my bolding
thanks to Ranyart for calling Viqar Husain/Oliver Winkler's paper to my attention.
Two new papers were just posted on Doubly Special Relativity
http://arxiv.org/gr-qc/0402092
Giovanni Amelino-Camelia
"Some encouraging and some cautionary remarks on Double Special Relativity in Quantum Gravity"
dated 22 February
(based on a talk given at the 10th Marcel Grossmann meeting on GR)
http://arxiv.org/hep-th/0402117
Jerzy Lukierski
"Relation between quantum kappa-Poincare framework and Doubly Special Relativity"
dated 18 February
Jose Mourao has co-authored numerously with such folk as Ashtekar, Lewandowski, Marolf, Thiemann, Renate Loll, and was thesis advisor of Jose Manuel Velhinho
here is a picture of Mourao and some bio stuff
http://www.math.ist.utl.pt/~jmourao/textojm.html
Velhinho's work on disertation with Mourao covered 1995-2001
This looks to me like a good new paper by Velhinho
http://arxiv.org/math-ph/0402060
It presents a summary of how the development of LQG is going
and the style is efficient, not cumbersome. He writes as
a mathematician.
On page 19 is treated the issue of spatial diff invariance---which was the basis of the "nonstandardness" discussion with Urs and others in the TT Loop-String thread, or so I gather from Urs' recent posts.
it is an interesting issue and Velhinho provides a concise overview in a couple of pages.
Velhinho is at University of Beira in Portugal
Murao is in the Mathematics Department at Lisbon Tech (the Inst. Sup. Tech)
LQG connects with semiclassical study of black holes
(through the BH entropy formula, Hawking radiation and QN modes)
Here are two interesting papers about Hawking radiation
(predicting that it is not perfectly thermal when quantum effects are adjusted for, and describing the mechanism)
Frank Wilczek and Maulik Parikh
"Hawking Radiation as Tunneling"
http://arxiv.org/hep-th/9907001
Maulik Parikh
"Energy Conservation and Hawking Radiation"
http://arxiv.org/hep-th/0402166
6 pages, dated 23 February
"Dynamics of loop quantum gravity and spin foam models in three dimensions":
http://arxiv.org/abs/gr-qc/0402112
Authors: Karim Noui, Alejandro Perez
Abstract:
"We present a rigorous regularization of Rovellis's generalized projection operator in canonical 2+1 gravity. This work establishes a clear-cut connection between loop quantum gravity and the spin foam approach in this simplified setting. The point of view adopted here provides a new perspective to tackle the problem of dynamics in the physically relevant
3+1 case."
I just printed it out :)
ranyart
Feb26-04, 03:26 AM
Originally posted by meteor
"Dynamics of loop quantum gravity and spin foam models in three dimensions":
http://arxiv.org/abs/gr-qc/0402112
Authors: Karim Noui, Alejandro Perez
Abstract:
"We present a rigorous regularization of Rovellis's generalized projection operator in canonical 2+1 gravity. This work establishes a clear-cut connection between loop quantum gravity and the spin foam approach in this simplified setting. The point of view adopted here provides a new perspective to tackle the problem of dynamics in the physically relevant
3+1 case."
I just printed it out :)
Likewise here, but are you aware there are a number of other compatable papers? all preceeding and very relevant:
http://uk.arxiv.org/abs/gr-qc/0402110
http://uk.arxiv.org/abs/gr-qc/0402111
http://uk.arxiv.org/abs/gr-qc/0402112
http://uk.arxiv.org/PS_cache/gr-qc/pdf/0402/0402113.pdf
Just as an after thought I belive this can go here to?
http://uk.arxiv.org/abs/hep-th/0311030
Or maybe Marcus can place it into a relevant forum?
[a)]
selfAdjoint
Feb26-04, 10:25 AM
Working through their paper gr-qc/0402112, I see that Noui and Perez do a standard quantization, regulating to get a finite sum and then showing that the regulator can be eliminated in quantizing. They then show that their physical Hilbert space is the same as the one obtained in the LQG approach.
All of this depends crucially on the fact that they can represent their 2+1 geometry as the product of a Riemann surface and a line. It's not clear how, or if, this quantization could be extended to 3+1 spacetime. On the other hand, the fact that in this case the standard type of quantization agrees with the Ashtekar et al quantization is supportive for the LQG folks.
We are now at page 11 of the thread, so I will update things and bring the earlier links forward. So far there is no sticky thread for Loop Gravity reference links, and this thread is serving as a surrogate sticky "reference library". Thanks to all who have contributed so far!
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Feb.2
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
------links to an unselective assortment of current work------
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
By Carlo Rovelli and Winston Fairbairn
Abstract:"We study the separability of the state space of loop quantum gravity. In the standard construction, the kinematical Hilbert space of the diffeomorphism-invariant states is nonseparable. This is a consequence of the fact that the knot-space of the equivalence classes of graphs under diffeomorphisms is noncountable. However, the continuous moduli labeling these classes do not appear to affect the physics of the theory. We investigate the possibility that these moduli could be only the consequence of a poor choice in the fine-tuning of the mathematical setting. We show that by simply choosing a minor extension of the functional class of the classical fields and coordinates, the moduli disappear, the knot classes become countable, and the kinematical Hilbert space of loop quantum gravity becomes separable"
The Spring 2004 issue of Jorge Pullin's newsletter is out.
"Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
It has a number of QG conference reports
by Bojowald
Date
Corichi
and others
selfAdjoint
Mar15-04, 10:04 AM
Thank you Marcus, for pointing to this. John Baez used to keep us up to date on this important publication, but he seems to have moved out of QG. The reports were very interesting.
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
Janitor
Mar25-04, 12:08 AM
All I can ever remember seeing at J.B.'s website is QG and category theory. Is he now down to just category theory?
Many of the QG pioneers like Ashtekar, Lewandowski, Baez are not publishing much and there is a new crop (many of whom were unknown in the Nineties.)
Baez has not published in QG for several years, or only negligibly.
Ashtekar likewise. But both play an important role. Ashtekar is a big presence at conferences. Baez will be a principle figure at the Marseille QG conference in May and the Dublin conference in July.
I guess the people who started up LQG in the 1990s are already
over 40, maybe pushing 50. Ashtekar must be over 50. So with some exceptions it really seems to be the next wave of young LQGists that is making the field progress
maybe the term is "mathopause"----it gets mathematicians
you could have direct knowledge yourself so why elaborate
Janitor
Mar25-04, 12:35 AM
Yeah, I guess I'm at that age. Of course, I never had the right stuff in the first place!
Originally posted by Janitor
Yeah, I guess I'm at that age...
argh! bummer! confession is the pits.
I actually think Baez might blossom again, just in a different field. he is a remarkable guy.
and if he can maybe there is hope for the rest of us.
meanwhile there are up-and-coming people in LQG to watch
here are a few names off the top of my head in no
particular order
Etera Livine
Laurent Freidel
Phillipe Roche
Martin Bojowald
Karim Noui
Hanno Sahlmann
Velhinho
Kowalski-Glikman
selfAdjoint
Mar25-04, 04:34 PM
My thesis advisor, Ed Fadell, published important research in the later 1990's. He was pushing 40 when I knew him in the early 1960's, so probably he was 70 when he did that. Don't ever count anybody out.
My thesis advisor, Ed Fadell, published important research in the later 1990's. He was pushing 40 when I knew him in the early 1960's, so probably he was 70 when he did that. Don't ever count anybody out.
Algebraic topology, impressive, maybe abstract algebra too?
Had a student by the name of Bob Brown whom I may have met but am not sure (he did abstract algebra IIRC, was teaching galois theory)
just happened on this:
E. Fadell, Homotopy groups of configuration spaces and the string problem of Dirac, Duke Math.J. 29 (1962), 231-242.
Edward Fadell apparently had several topnotch students which is another
dimension---the vitality that goes into that, as well as research. It is possible you made a real good choice of advisor
genes, character
we should all have it whatever it is that never stops
Are grand old men more the norm in physics?
Hans Bethe
did you ever look at the "mathematical geneology" website
it is like the Begats in the bible
I looked up Marc Rieffel not long ago
George Mackey advised Rieffel
and so and so advised Mackey and...
back to Birkhoff at Harvard around 1905 if I remember right
selfAdjoint
Mar25-04, 08:47 PM
You met Bob Brown?! He and I studied together - in fact I was a fixture at his and his wife Brenda's apartment, since I had no significant other of my own then. Bob's had a standout career at UCLA.
You met Bob Brown?! He and I studied together - in fact I was a fixture at his and his wife Brenda's apartment, since I had no significant other of my own then. Bob's had a standout career at UCLA.
Im not sure it was the same Bob Brown
algebraist (I formed a high opinion of him as a teacher and person)
could be the same, I'll write you a PM later
this just out
http://arxiv.org./abs/gr-qc/0403106
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
"Loop quantum gravity (LQG) or quantum geometry is
at present the main background independent and non–
perturbative candidate for a quantum theory of gravity
(see for example [1, 2]). Key successes of this approach
have been the prediction of discrete spectra for geometrical
operators [3], the existence of well defined operators
for the matter Hamiltonians which provides a cure for
the ultraviolet divergences [4], and the derivation of the
Bekenstein–Hawking entropy formula [5].
Given that LQG effects are likely to have important
consequences in high energy and high curvature regimes,
early universe cosmology provides a natural environment
to test these new features....
From the loop quantum cosmology (LQC) perspective,
the evolution of the universe is comprised of the three distinct
phases. Initially, there is a truly discrete quantum
phase which is described by a difference equation [9, 10].
A key consequence of this discretization is the removal
of the initial singularity [9]. As its volume increases,
the universe enters an intermediate semi–classical phase
in which the evolution equations take a continuous form
but include modifications due to non–perturbative quantization
effects [12]. Finally, there is the classical phase
in which the usual continuous ODE/PDE cosmological
equations are recovered and quantum effects vanish..."
also just out, Clifford Will's paper, could be the same paper he gave last week at Ulm to the German Physical Society
http://arxiv.org./abs/gr-qc/0403100
"Testing Alternative Theories of Gravity Using LISA"
-----quote from abstract-------
We investigate the possible bounds which could be placed on alternative
theories of gravity using gravitational wave detection from inspiralling compact binaries with the proposed LISA space interferometer. Specifically, we estimate lower bounds on the coupling parameter ω of scalar-tensor theories of the Brans-Dicke type and on the Compton wavelength of the graviton λ_g in hypothetical massive graviton theories.
In these theories, modifications of the gravitational radiation damping formulae or of the propagation of the waves translate into a change in the phase evolution of the observed gravitational waveform. We obtain the bounds through the technique of matched filtering, employing the LISA Sensitivity Curve Generator (SCG), available online. For a neutron star inspiralling into a 103M⊙ black hole in the Virgo Cluster,
in a two-year integration, we find a lower bound ω > 3 × 10^5. For lower-mass black holes, the bound could be as large as 2 × 10^6. The bound is independent of LISA arm length, but is inversely proportional to the LISA position noise error. Lower bounds on the graviton Compton wavelength ranging from 10^15 km to 5 × 10^16 km can be obtained from one-year observations of massive binary black hole inspirals at cosmological distances (3 Gpc), for masses ranging from 10^4 to 10^7M⊙. For the
highest-mass systems (10^7M⊙), the bound is proportional to (LISA arm length)1/2 and to (LISA acceleration noise)^−1/2. For the others, the bound is independent of these parameters because of the dominance of white-dwarf confusion noise in the relevant part of the frequency spectrum. These bounds improve and extend earlier work which used analytic formulae for the noise curves.
---------end quote-------
Initially, there is a truly discrete quantum
phase which is described by a difference equation [9, 10].
A key consequence of this discretization is the removal
of the initial singularity [9]. As its volume increases,
the universe enters an intermediate semi–classical phase
in which the evolution equations take a continuous form
but include modifications due to non–perturbative quantization
effects [12].
Is there any paper that can says when exactly in time the evolution of the universe change to be described by a difference equation to be described by differential equations?
Is there any paper that can says when exactly in time the evolution of the universe change to be described by a difference equation to be described by differential equations?
IIRC Ashtekar's paper "Quantum Geometry in Action: Big Bang and Black Holes"
gives an estimate of several hundred steps (of the difference equation) to converge to the semi-classical model
it is the usual sort of limiting process
the quantum regime converges to the semiclassical (after a very short period on the order of 100 planck time units)
and the semiclassical converges thereafter more gradually to
the ordinary or partial differential equation model
but as with other kinds of convergence one cannot say with precision the exact moment when
the discrete model stops being appropriate and the semiclassical model
begins to apply
there is a transition period when both are giving approximately the same answer
So what one needs is a rough order of magnitude idea of when the transition between models happens. If it is not in that Ashtekar paper then I must be thinking of one by Ashtekar, Bojowald, Lewandowski called
"Mathematical Structure of Loop Quantum Cosmology"
I will try to get a link and page reference for the several hundred planck time units or DiffEq timesteps---it's in one or the other or both papers. May be other places as well so someone else could come up with yet another link.
------------LONG LAPSE OF TIME-----
I forgot to get the references, however the one I mentioned first has something.
See page 10 of gr-qc/0202008, last paragraph of section 3.1 "Big Bang".
Ashtekar says there that the semiclassical model (Wheeler-DeWitt) is recovered when the scale of the universe is a few hundred planck lengths. that is, very soon.
Also next to last paragraph on page 8.
I would like to find a more recent and more precise paper, in answer to your question. At the moment I dont have one. Perhaps someone else out there does.
"3.3 Chern-Simons perturbation theory.
Setting
\frac{3}{4} = \frac{2}{3}
our Lagrangian becomes the Chern-Simons-functional..."
there is a mathematician named Dror Bar-Natan
on page 19 of this paper
q-alg/9702009
"The Fundamental Theorem of Vassiliev Invariants"
he claims to prove something
by setting 3/4 equal to 2/3.
His paper is about the "Fundamental Theorem of Vassiliev Invariants"
and it is divided into four sections with four different ways of proving
the fundamental theory and at the end of each section he has
a concluding paragraph entitled
"Why are we not happy?"
This shows a philosophical concern with the problem of human happiness.
Also he proves the theorem by algebra, by physics (the oldest way, already almost 10 years old), by geometry, by topology. and he finds something always unsatisfying or wrong. in the middle of the proof by physics he says
"This is of course silly."
Dror Bar-Natan has an unusual expository style. Or at least I hope that it is unusual.
BTW he calls the topological method "combinatorial-topological" because doubtless he thinks of combinatorics and topology as very close neighbors or almost joined at the hip
He cites V.I.Arnold a russian mathematician. Fairbairn and Rovelli also cited a book by V.I. Arnold. It would be possible to suspect that something is going on with knot theory and Vassiliev invariants. the quirky Bar-Natan tone of voice even encourages this suspicion.
Perhaps it will be necessary to classify knots-with-nodes and I cannot at the moment visualize how this would be done.
I will get the LQG paper by Gambini and Pullin that cites this Bar-Nathan.
Nonunitary gave this link in another thread.
------quote from nonunitary post in "chunkymorphism" thread---
...As far as I know the first paper about the invariants was
gr-qc/9803018
but you are right about the chunkymorphisms. The are a new invention of Rovelli. I haven't read the paper so I can not comment.
-----endquote----
the Gambini/Pullin paper
http://arxiv.org/gr-qc/9803018
is called
"Vassiliev invariants: a new framework for quantum gravity"
selfAdjoint
Mar30-04, 10:40 PM
How do you get to those q_alg papers in the arxiv? I have been trying every trick in the book for half an hour now and nothing! Click on mathematics and get, but can't ge q-alg or QA. Click on 1997 and search, nothing.
How do you get to those q_alg papers in the arxiv? I have been trying every trick in the book for half an hour now and nothing! Click on mathematics and get, but can't ge q-alg or QA. Click on 1997 and search, nothing.
only have a minute to reply but try
http://arxiv.org/PS_cache/q-alg/pdf/9702/9702009.pdf
will get back in a few minutes and check that this works
Im back.
this should get the abstract:
http://arxiv.org/q-alg/9702009
now I understand. the problem is to use the search engine
to find a paper like this one, but hopefully more recent
------------------
go to arxiv
don't click on search immediately
because right beside the button that says "search" there is
a menu box where you can select "math"
select "math" and then click on "search"
you then get a form where you can type in Author and Keyword
I typed in Bar-Natan and knot
and got many QA papers including this sample
3. math.QA/0201043 [abs, ps, pdf, other] :
Title: On Khovanov's categorification of the Jones polynomial
Authors: Dror Bar-Natan
Comments: Published by Algebraic and Geometric Topology at this http URL, 34 pages with many figures, source contains associated program and data file
Subj-class: Quantum Algebra; Geometric Topology
MSC-class: 57M25
Journal-ref: Algebraic and Geometric Topology 2 (2002) 337-370
selfAdjoint
Mar31-04, 10:08 AM
Thanks. Using your hint, I fooled around and found it with"Vassiliev invariants" which is what I was interested in anyway. Bar-Natan's motive for "why are we not happy" is perfectly clear; he wants to impose on his students a careful understanding of what it means to have a "proved theorem" which you can use to prove other things, and what it does NOT mean - which is the status of what he calls the fundamental theorem of Vassiliev invariants.
Hi selfAdjoint, I concur with your description of Bar-Natan's
serious and commendable motive but I also delight immoderately in
his sense of humor
which he uses to the hilt in implementing his serious idea
thanks to nonunitary for this, I never would have seen the paper if
he had not referred to that one by Gambini and Pullin about LQG and
the Vassiliev invariants
you know diff manifolds are in a deep sense just gussied up Rn
and it just shows you what an enormously rich thing Euclidean space is
that you can have all these different variations on that theme
the theme of the continuum
the theme of the coordinate patch and the metric
all fundamentally Rn at the root
can knots and networks be comparably rich
why is there all this interest in them just now
well this is not purely a rhetorical question although it
sounds like it, I was actually wondering, but not expecting to
be able to get an answer
it was clever of you to study algebraic topology in grad school
maybe it will be useful after lo these many years
You might be interested to have a look at Frieder Lenz's
lecture notes on
"Topological concepts in gauge theories"
http://arxiv.org./hep-th/0403286
the whole thing is 83 pages
http://arxiv.org./PS_cache/hep-th/pdf/0403/0403286.pdf
They just came out.
he has a good historical sense and begins with a story about something that happened in 1833 involving Carl Gauss and a magnetic monopole :-)
these notes strike me as student-friendly
by someone who is considerate and puts in some nice pictures
Getting ready for a Brahms Req rehearsal tonite.
Up to you to decide if Frieder Lenz's notes are good or not and for what.
selfAdjoint
Apr1-04, 09:17 AM
Thanks for the links, Marcus.
You know, reading Bar Natan's account of the topological proof of his "fundamental theorem" and its defects, I couldn't help thinking here's a natural arena for spectral sequences. That's only because I'm reading A User's Guide to Spectral Sequences at the same time, but seriously there are his filtered graded algebra and all - by a theorem, there is guaranteed to be a spectral sequence with the 1-page E^{p,q}_1 isomorphic to the homology of the algebra. But that's no good unless you can compute the limiting page E^{p,q}_{\infty} . The differentials of the sequence encode non trivial information about the algebra. I can't believe somebody hasn't tried this.
Bolen, Bombelli, Corichi
http://arxiv.org./abs/gr-qc/0404004
"Semiclassical States in Quantum Cosmology: Bianchi I Coherent States"
"We study coherent states for Bianchi type I cosmological models, as examples of semiclassical states for time-reparametrization invariant systems. This simple model allows us to study explicitly the relationship between exact semiclassical states in the kinematical Hilbert space and corresponding ones in the physical Hilbert space, which we construct here using the group averaging technique. We find that it is possible to construct good semiclassical physical states by such a procedure in this model; we also discuss the sense in which the original kinematical states may be a good approximation to the physical ones, and the situations in which this is the case. In addition, these models can be deparametrized in a natural way, and we study the effect of time evolution on an "intrinsic" coherent state in the reduced phase space, in order to estimate the time for this state to spread significantly."
john baez
Apr3-04, 12:00 AM
Baez has not published in QG for several years, or only negligibly.
Well, I don't think my paper with Christensen and Egan on asymptotics of 10j symbols was negligible - it contained the results of literally billions of calculations, and it was the first detailed analysis of a spin foam model of quantum gravity. And that was back in August of 2002, which isn't several years yet, just a couple! "Several" means at least 3! :smile:
But, you're right in perceiving that I'm mainly interested in other things
these days.
I found out about this thread from Carlo Rovelli, who sent me an email teasing me about it. I couldn't resist replying to an article entitled "the great John Baez burnout"! I'll take it as a compliment, since it suggests there was a flame flickering there for a while.
Here's how I replied to Rovelli's email:
Dear Carlo -
Hi! I hadn't seen these... thanks. It's pretty funny.
You know you're getting old when you start getting emails
with subject headers like this.
I am in fact rather fed up with quantum gravity. One reason is that
nobody knows a spin foam model that approximates GR in the classical
limit, and I don't see how to get one, despite a lot of work. But
there's another, equally important *positive* reason: these days, work
on n-categories is really revolutionizing mathematics! The subject
is packed with incredible suprises; it goes all the way down to
the foundations of how we think, and there are huge wide-open fields
of fruit ripe for the picking. I can't help but wanting to spend
my time doing this: it's as cool almost as quantum gravity, but I *know*
it will work.
But I might switch back to quantum gravity if and when spin foam
models seem to start working... because I really love the *physical*
universe, and the most mysterious and exciting aspect of math
to me is how it lets us understand the physical universe.
It will be fun to see everyone in Marseilles and see what their
mood is. Probably rather different from mine!
Best,
jb
Just so nobody gets the wrong idea: while I'm tired of trying to find a spin foam model with something like GR as its classical limit, I don't see any reason this should be impossible. Christensen, Egan and I just looked at a few versions of the Barrett-Crane model, and we didn't even succeed in ruling those out, just showing that they were far stranger than anyone expected.
I'm even *more* pessimistic about string theory and M-theory - otherwise I might switch to that.
But really, what got me off quantum gravity was the knowledge that I won't live forever. I have a choice of working on quantum gravity, where nobody knows for sure what's right and what's not, and working on mathematics, where I'm *sure* what I'm doing is right. I spent about a decade working on the former; now I want to do more of the latter.
maybe the term is "mathopause"----it gets mathematicians.
Actually, the idea that mathematicians burn out early is a bit of a myth. Sure, some of them *die* early, like Abel and Galois and Riemann. But the ones who keep living often keep doing good stuff - although lots of them get tired of publishing and spend more time just thinking and talking to people, because it's easier and more fun. For example, take Dennis Sullivan, or Erdos (who got other people to do the writing).
In case anyone is interested, I have a new paper called "Quantum Quandaries: A Category-Theoretic Perspective", in which I argue that a lot of the puzzling things about quantum mechanics will become less puzzling when it becomes part of a theory of quantum gravity, because the category of Hilbert spaces is a lot like a category where the morphisms are spacetimes:
http://math.ucr.edu/home/baez/quantum.ps
This will appear in a volume edited by Steven French, Dean Rickles and Juha Saatsi, probably to be entitled "Structural Foundations of Quantum Gravity".
So, I'm not *completely* fed up with quantum gravity.
I'm also working a lot on the foundations of quantum theory:
http://math.ucr.edu/home/baez/qg-fall2003/
http://math.ucr.edu/home/baez/qg-winter2004/
http://math.ucr.edu/home/baez/qg-spring2004/
So, please don't count me out yet! :smile:
But, it's true that there's a nice new crop of people working on loop quantum gravity and spin foam models.
I won't live forever
Do you have the proof for that? :biggrin:
http://arxiv.org./abs/gr-qc/0404018
Background Independent Quantum Gravity: a Status Report
125 pages
Ashtekar and Lewandowski
cragwolf
Apr6-04, 04:39 AM
Just call me a Baez fanboy. I have spent countless hours at his web site undergoing significant neural rewiring. Because of him I've been inspired to learn mathematics (I mean really learn it, beyond the "mathematical methods for physics" course I took way back in my undergraduate years). Baez is on the cutting edge of physics and mathematics, but he kindly and humbly devotes some of his time to helping us lesser beings learn something about the wonders of these subjects. His web site is a pedagogical paradise.
"Flat spacetime vacuum in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0404021
Authors: A. Mikovic
Comments: 20 pages, 6 figures
"We construct a state in the loop quantum gravity theory with zero cosmological constant, which should correspond to the flat spacetime vacuum solution. This is done by defining the loop transform coefficients of a flat connection wavefunction in the holomorphic representation which satisfies all the constraints of quantum General Relativity and it is peaked around the flat space triads. The loop transform coefficients are defined as spin foam state sum invariants of the spin networks embedded in the spatial manifold for the SU(2) quantum group. We also obtain an expression for the vacuum wavefunction in the triad represntation, by defining the corresponding spin networks functional integrals as SU(2) quantum group state sums"
Looking at the text, he mentions something called "spin network invariants". Never heard of this before (though I'm familiar with things like knot invariants or manifold invariants)
john baez
Apr7-04, 11:49 PM
Looking at the text, he mentions something called "spin network invariants". Never heard of this before (though I'm familiar with things like knot invariants or manifold invariants)
A spin network invariant is a function that assigns a complex number to each spin network embedded in space, where the number doesn't change when you apply a diffeomorphism of space to your spin network. (Here "space" is some 3-dimensional manifold.)
In loop quantum gravity, quantum states are commonly taken to be spin network invariants. You can think of such a state as a big fat linear combination of spin networks, where the coefficients are the aforementioned complex numbers.
If you attach a spin 0, 1/2, 1,... to a knot, you get a spin network of a specially simple kind. So, any spin network invariant gives an infinite sequence of knot invariants. But it has more information.
selfAdjoint
Apr8-04, 10:50 AM
By the way Dr. Baez, I have printed of and am studying your new Quantum Quandries paper. (http://arxiv.org/abs/quant-ph/0404040). How neat! From a sufficiently high perspective, quantum physics and general relativity are more like each other than either of them is like set theory. I am always glad to see set theory marginalized, because of my prejudice for Tarski's theorem and the BSS-machine results.
I've pretty much decided that there isn't a Santa Claus, but is there really a John Baez? :biggrin:
The Bianchi IX model in Loop Quantum Cosmology
Authors: Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
Comments: 41 pages, 3 figures, revtex4
Report-no: IMSc/2004/04/16, AEI-2004-028
The Bianchi IX model has been used often to investigate the structure close to singularities of general relativity. Its classical chaos is expected to have, via the BKL scenario, implications even for the approach to general inhomogeneous singularities. Thus, it is a popular model to test consequences of modifications to general relativity suggested by quantum theories of gravity. This paper presents a detailed proof that modifications coming from loop quantum gravity lead to a non-chaotic effective behavior. The way this is realized, independently of quantization ambiguities, suggests a new look at initial and final singularities
http://arxiv.org/abs/gr-qc/0404039
The Bianchi IX model in Loop Quantum Cosmology
Authors: Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
Comments: 41 pages, 3 figures, revtex4
Report-no: IMSc/2004/04/16, AEI-2004-028
The Bianchi IX model has been used often to investigate the structure close to singularities of general relativity. Its classical chaos is expected to have, via the BKL scenario, implications even for the approach to general inhomogeneous singularities. Thus, it is a popular model to test consequences of modifications to general relativity suggested by quantum theories of gravity. This paper presents a detailed proof that modifications coming from loop quantum gravity lead to a non-chaotic effective behavior. The way this is realized, independently of quantization ambiguities, suggests a new look at initial and final singularities
http://arxiv.org/abs/gr-qc/0404039
How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?
Is the Belinskii-Khalatnikov-Lifschitz scenario mostly used in analyzing quantum gravity solutions for cosmological singularities?
Thanks for any help (am just a beginner :redface:) .
How does LQG get rid of cosmological singularities in homogeneous models for e.g. Bianchi V? Any good references to papers?
.
Maddy, almost all the papers I know of by Bojowald etc have dealt with the isotropic case.
I will find some URL for ones dealing with anisotropic, but it is a comparatively recent effort AFAIK.
I havent seen anything about Bianchi V. I only recall stuff about Bianchi IX.
People sometimes come here (stingray, nonunitary) who might be able to respond more usefully.
As for papers about the homogenous (but not isotropic) case here are some references:
the most recent review of progress in the field is
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
this contains only two refs that are explicitly to the homogeneous case and they are from last year
"Homogeneous loop quantum cosmology" gr-qc/0303073
"Homogeneous loop quantum cosmology: the role of the spin connection" gr-qc/0311004
maybe you can find more if you look in the references of these
there was also this, which I have not looked at so cannot say if it mentions Bianchi V,
"Quantum suppression of the generic chaotic behavior close to cosmological singularities" gr-qc/0311003
it looks to me as if the homog. case is just barely being scratched at
and that the papers mostly go back only to November of last year
but I am not knowledgeable about homog. case and you might find out things
are different if you take a closer look
most LQC papers involve homogenenous and isotropic simplification and
in effect use a quantized form of the Friedmann equation (which is what most cosmology depends on anyway)---again this is just my limited view and we could hopefully get some expert comment responding to your question
http://arxiv.org/abs/gr-qc/0404055
Ln(3) and Black Hole Entropy
Authors: Olaf Dreyer
Comments: Contribution to the Proceedings of the 3rd International Symposium on Quantum Theory and Symmetries, Cincinnati, September 2003
"We review an idea that uses details of the quasinormal mode spectrum of a black hole to obtain the Bekenstein-Hawking entropy of A/4 in Loop Quantum Gravity. We further comment on a recent proposal concerning the quasinormal mode spectrum of rotating black holes. We conclude by remarking on a recent proposal to include supersymmetry. "
This paper try to fix the Immirzi parameter
Give a look to page 2. It contains the formula for the entropy of a Black hole according to Loop Quantum Gravity
this thread is serving as a surrogate sticky "reference library". Thanks to all who have contributed so far!
-------Loop Gravity texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018
Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references
---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum κ-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
--------upcoming conferences--------
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?03Aug.1
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Feb.2
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?10
General Relativity Conference (Dublin) July 2004
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
------observational means for testing quantum gravity------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
------links to an unselective assortment of current work------
Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
Woit's blog:
http://www.math.columbia.edu/~woit/blog/
the responses are getting interesting too
------------------
Recent paper by Olaf Dreyer
http://arxiv.org./gr-qc/0404055
-------------------
this is spillover from the main page of links
which is full
-------------------
a new QG Phenomenology paper
http://arxiv.org./gr-qc/0404113
"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia
------------
there is also a new Quantum Gravity Phenomenology
paper by Ted Jacobson et al
dealing with QG predictions of Lorentz violation and
their testability
http://arxiv.org/gr-qc/0404067
"Quantum Gravity Phenomenology and Lorentz Violation"
Ted Jacobson, Stefano Liberati, David Mattingly
looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"
Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
"...d = 3 + 1, if it can indeed be constructed, will offer the same potential. This relationship is the main theme of the present article.
The special role of space-time dimension d = 3 + 1 in differential topology is summarized by the following result.
Theorem 1.1. Let M be a topological d-manifold, d = 1, 2, 3, 4,... If M admits an infinite number of pairwise inequivalent differentiable structures, then d = 4.
This is a corollary of several theorems by various authors. We explain in this article why this result is related to the search for a quantum theory of general relativity..."
here is something unique about the dimension 4
It is the only possible dimension for spacetime to be if you want to have
plenty of possible smooth-manifold structures.
In this sense, dim = 5 is not OK, and dim = 6 is not OK.
This is a surprising theorem. I didnt know that d = 3+1 was mathematically so special as that. It is so surprising that I think I must be failing to understand. but Pfeiffer is I think very good and there it is written in black and white as theorem 1.1, so will go back and try to understand.
-------had to do something else and just got back-----
Pfeiffer says, on page 16:
"In d ≥ 3 + 1, no analogous result is available. There exist countably infinite families of (compact) smooth 4-manifolds [13] which are pairwise non-diffeomorphic, but which have homeomorphic underlying topological manifolds. There is therefore a considerable discrepancy between C∞- and C0-QFTs in d = 3 + 1 space-time dimensions.
The most striking result even concerns the standard space R4 [14, 15].
Theorem 4.1. Consider the topological manifold Rd, d ε N.
• If d < 4, then there exists a differentiable structure for Rd which is unique up to diffeomorphism.
• If d = 4, then there exists an uncountable family of pairwise non diffeomorphic differentiable structure for Rd.
Non-uniqueness of differentiable structures persists in higher dimensions, for example, there are 28 inequivalent differentiable structures on the sphere S7, or 992 inequivalent differentiable structures on S11, [16], but in dimension d ≥ 4 + 1 (d ≥ 5 + 1 if the manifold has a non-empty boundary), there never exists more than a finite number of non-diffeomorphic differentiable structures on the same underlying topological manifold.
The space-time dimension d = 3 + 1 is distinguished by the feature that there can exist an infinite number of homeomorphic, but non-diffeomorphic smooth manifolds."
------then on page 19 Pfeiffer says------
"Scenario for quantum gravity.
We have reached a first goal: the diffeomorphism gauge symmetry of general relativity on a closed space-time manifold has been translated into a purely combinatorial problem involving triangulations that consist of only a finite number of simplices, and their manipulation by finite sequences of Pachner moves.
If not only the partition function, but also the full path integral of general relativity in d ≤ 5+1 is given by a PL-QFT, we know that all observables are invariant under Pachner moves.
The partition function of quantum general relativity is an invariant of PL-manifolds, too, and can be computed by purely combinatorial methods for any given combinatorial manifold.
A generic expression of such a partition function is the state sum,
Z =
\sum_{ colourings } \prod_{ simplices }
(amplitudes),
where the sum is over all labelings of the simplices with elements of some set of colours, and the integrand is a number that can be computed for each such labeling. In Section 5 below, we give examples and illustrate that the partition function of quantum general relativity in d = 2 + 1 is precisely of this form.
If quantum general relativity in d = 3+1 is indeed a PL-QFT, the following two statements which sound philosophically completely contrary,
• Nature is fundamentally smooth.
• Nature is fundamentally discrete.
are just two different points of view on the same underlying mathematical structure: equivalence classes of smooth manifolds up to diffeomorphism."
also on page 20, right after this, there is a picture which illustrates what are Pachner moves in 2 dimensions and 3 dimensions.
this business on pages 19 and 20 of Pfeiffer paper seems interesting. I never heard talk like this. I have bolded some exerpts for emphasis.
http://arxiv.org/abs/gr-qc/0404083
Spectrum of quantized black hole, correspondence principle, and holographic bound
Authors: I.B. Khriplovich
Comments: 9 pages
An equidistant spectrum of the horizon area of a quantized black hole does not follow from the correspondence principle or from general statistical arguments. On the other hand, such a spectrum obtained in loop quantum gravity (LQG) either does not comply with the holographic bound, or demands a special choice of the Barbero-Immirzi parameter for the horizon surface, distinct from its value for other quantized surfaces. The problem of distinguishability of edges in LQG is discussed, with the following conclusion. Only under the assumption of partial distinguishability of the edges, the microcanonical entropy of a black hole can be made both proportional to the horizon area and satisfying the holographic bound
selfAdjoint
Apr22-04, 09:38 AM
That's a new thought. All edges are equal but some edges are more equal.
I knew long time ago about these peculiarity of 4-dimensional manifolds. I thought it was a mainstream knowledge.
By some stupid reason i have no acces to arwiv for around a week so i couldn´t read the article. So while waitng i´ll ask about other, slighly related, things.
I have a few generic questions/refelxions aobut some of the themes LQG is addressing.
Let´s beging by the question of entropy. My deal is whether the concept of entropy makes sense in GR at all. A lest in the same sense that in ordinay statistichal mehcanics.
I know about two main results. The one, of wich i have a reasonable understanding , about the black hole area behaving like entropy. I also have notice about (but no understanding at all) results of Penrose relating the Weyl tensor to entropy, at least in cosmologicla scenarys
The question is that in the microcanonical device the entropy is reltaed to the number of micro-states compatible with an energy. But in GR there is no a good (and less local) definition of the energy of the gravitatory field.
I think these is commonly known. Anyway i would like to know how it has been addressed.
In order to get my own understanding i revised the whole idea (i never have had a deep basics in statistichal mecanics and it was a good exercice for me) of micro-states.
It works fine because it is used mainly for quantum mechanical systems with a discrete spectrum. But i wanted to understand how it could work incontinuous systems, other than the gravitatory field, an dalso in continous spectrum of quantum systems.
I begined revising the black-body radiation. There the thermal equiibrium is suposed to be achieved by influence of the walls of a cavity.
By general (non rigurous) arguments (kirchov laws and it´s subleties/consecuences) it is assumed that an arbitrary (as far as i know expermintally tested for simply connected) shaped body can be simulated by a box shaped resonant cavity.
Them the number of microstates of the EM field can be charazterized by thre numbers (k1.k2,k3). And by standard manipulations they can be related to the frecuency.
Ok, i will not reexplain all the remaining basic facts. My point is here. We can give a meaning to the number of microstates because we are dealing with a configuration in wich the EM field is in a compact region. ALso it is important to note that it seems that we are faced with a topological nature of the black body radiation. It is invariant under topological changes of the shap of the cavity.
So i try to think on the vibration numbers like something with a topologicla meaning. ¿woul the black body reaction could have another behaviour for non topologically trivial shapes of the caity?
Well, i know these hass addresed me far form the original question of the entropy of the gravitatory field. But my gues is that we are triying to aplly blindly a concept which works fine in non relativistic discret spectred hamiltonians. Maybe in some more general situations it needs some refinements.
For example, ¿whay about yang-mills fields?
Of course you will be saying ¿What about at finite temperature field theory?
I have not studied it in deep. But the whole idea seems to me so merely formal that doesn´t convince me as a good way to gain deep understanding of the problems. But of course probably it is my problem.
Well, i have more conceptual issues. But i´ll expose them in another moment.
this is interesting
QG has already challenged the Big Bang singularity
quantizing seems to remove it (Bojowald and others say)
maybe there is no black hole singularity, just a very deep well
maybe quantizing GR will remove this singularity also
also even without quantizing GR some alternative models avoid a singularity
http://arxiv.org/astro-ph/9908113
"Alternate Explosions: Collapse and Accretion Events with Red Holes instead of Black Holes"
html version:
http://arxiv.org./html/astro-ph/9908113
http://arxiv.org./astro-ph/9912322
"Red Hole Gamma-Ray Bursts: A New Gravitational Collapse Paradigm
Explains the Peak Energy Distribution And Solves the GRB Energy Crisis"
gammaray bursts are especially interesting and may even now not be
satisfactorily explained---awful lot of energy in them, more than supernovas
what has happened to these 1999 conjectures?
EDIT: by the way the author, Jim Graber, sometimes posts here at PF.
why doesnt someone ask him about this alternative picture of gravitational collapse
looks like some category theory may be needed to do
quantum gravity (e.g. Velhinho, also several Baez papers)
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"
not being a whiz with categories or else they're just not very familiar, I'm
a bit bothered by their infiltrating into QG
there was just this paper by John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
and this other recent paper by Hendryk Pfeiffer
is categorical in its approach
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
and back in February there was this paper by Velhinho
"On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
and now there is a woman mathematician who has published with Louis Crane and lives in New Zealand---her name is Marni Dee Sheppeard.
Her paper is so categorical that it seems the same to me whether I read it front to back or back to front. That is, I see a lot of diagrams with arrows and dont understand anything. but it purports to be about quantum gravity and I like the name Marni Dee so here's the link:
http://arxiv.org/gr-qc/0404121
On state sums, internalisation and unification
M. D. Sheppeard
35 pages
Abstract: "In this mostly expository article, elements of higher category theory essential to the construction of a class of four dimensional quantum geometric models are reviewed. These models improve current state sum models for Quantum Gravity, such as the Barrett-Crane model, in that they appear, for instance to remove degeneracies which swamp the partition function. Much work remains to be done before a complete construction is reached, but the crucial categorical notion of internalisation already illuminates the idea that a full unified model may result from few, albeit as yet poorly understood, additional principles. In particular, a spacetime and matter duality principle is employed through an understanding of the role of pseudomonoidal objects in categorified cohomology."
the good news: these models improve on the Barrett-Crane model which is pretty much the main spin-foam studied and which has some numerical crankiness (Baez published a paper in 2002 about this)
the good news: somehow putting spacetime and matter on the same footing?
the bad news: what the devil is pseudomonoidal objects in categorified cohomology?! it sounds like a new disease and I just hope it's not painful.
well fraid theres something else to learn about in QG
selfAdjoint
Apr30-04, 04:48 PM
The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.
I just love the sniffy prose style too, which I associate with Oxbridge and its colonies like The Economist :
"The first and simplest way of regarding a manifold M as a category is to give it, albeit rather trivially, a groupoid structure. That is, the points of the manifold are the objects of the category and each point has attached an identity arrow, which is of course invertible."
Of course!
The Sheppeard paper looks good! I am going to read it, and see if I can keep up with it.
I am much cheered by your positive reaction.
I think I might be able to get something out of pages 6 thru 10
as they are a basic expostion of category theory.
although I'm less than confident of getting much of the rest of the paper.
I didnt notice at first that she even had a prose style! when I first looked i was just very impressed: she seemed awesomely intelligent and mainly bewildering. Will try to attribute some of this to her English.
This is a nice efficient way to start talking about categories (maybe it is the standard definition):
"Whereas a set has elements, and a map between sets takes elements to elements, a category has both elements, called objects, and relationships between elements, called arrows. Every object A is equipped with at least an identity arrow 1A from A to A. Maps between categories, called functors, take objects to objects and arrows to arrows. Arrows may be composed f ◦g..."
she makes this point first off that I dont believe Ive heard put so clearly by anyone:
background independence is basic to QG, she says, and
if you want background independence
then you have to go categories.
rovelli never told me that, baez only hinted at it, she flatly asserts it.
there is a bold magesterial quality turn to her thinking. Doesnt mince words. Says right out front where she's going. E.g. here is the first of the introduction:
"1 Introduction
The philosophy behind the construction outlined here is that any reasonable
attempt to describe quantum gravity within a unified framework ought to
respect some quantum principle of general covariance. Recall that in coming
to accept general covariance in the first place [37], Einstein needed to rid
himself of the idea that spacetime points had physical meaning outside of
their use in the metric tensor field. Put another way: no gravitational field,
no spacetime.
It is argued here that categorical internalisation is an essential element
required of a successful mathematical description of such a principle. There is mounting support for this point of view from studies of, for instance,..."
selfAdjoint
Apr30-04, 09:55 PM
Yes. The only problem with doing the first pages is that she front loads the abstract stuff and expects her readers to have some problems with that. Of course with my weird background, that's the part I like!
http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04
In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.
Doesn't LQG propose disconnected quanta of space-time? Or are these quanta of space-time connected by infinitesimal strings between them? Does LQG really propose communication through something of no dimensional thickness?
Sorry for the off-topic.
As I told in the previous post I couldn´t download the last pdf´s because my acces to arxiv was temporarilly locked.
The nnouncement said "one week", but it was taking more time to be unlocked so I mailed arxiv and the lock was deletted.
But i have been warned about using prefetching software. ¿Wha´s that?, could it be the acrobat reader active-w wich executes from inside the explorer.
I ask because i wouldn´t want to be locked agian. I can´t follow properly these interesting threads without acces to the pdf´s, you know.
thanks in advance and sorry again for the off-topic.
Sauron I was also banned one week from arxiv, but it was because my acrobat reader was of an old version. They said that if i updated to the newer version of acrobat I wouldn't be banned again, so I did and so I haven't been banned anymore
Sauron and Meteor, I haven't yet experienced this difficulty with arxiv
but if it happens, knowing about the
possibility will make it less of a shock. thanks for the warning!
----------------
a September 2004 conference in quantum gravity has been announced
by Jonathan Halliwell of Imperial College London
"Current Themes in Quantum Gravity: A Two-day Conference
in Honour of the 60th Birthday of Chris Isham
Imperial College, September 6-7, 2004.
Inspired by the 60th birthday of Prof. Chris Isham, we are pleased to announce a conference whose aim is to take stock of the current status of quantum gravity, its mathematical foundations, conceptual problems and physical predictions for cosmology and black holes. Aspects of quantum theory related to gravity will also be considered. The main themes of the meeting will be as follows:
The canonical Hamiltonian approach to gravity and the loop variable approach to quantum gravity.
Alternatives to the canonical approach, such as causal sets and other discrete models.
The phenomenology of quantum gravity, in particular, quantum cosmology and quantum black holes.
Aspects of the foundations of quantum theory related to quantum gravity, in particular, the decoherent histories approach and topos-theoretic ideas.
Conceptual problems in quantum gravity, in particular, the problem of time.
The list of speakers will include,
Abhay Ashtekar (Penn State)
Stanley Deser (Brandeis)
Gary Gibbons (Cambridge)
Jim Hartle (Santa Barbara)
Stephen Hawking (Cambridge)
Karel Kuchar (Utah)
Renate Loll (Utrecht)
Roger Penrose (Oxford)
Carlo Rovelli (Marseille)
Rafael Sorkin (Syracuse)
"
the announcement says more detail will eventually be available on the
website
http://theory.ic.ac.uk.
http://www.maths.qmul.ac.uk/wbin/GRnews/conference?04Apr.11
a recent paper co-written by Renate Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)
they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isnt conclusive but it is a suggestive preliminary finding at the least
Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research
A few hours after I wrote this (about the AJL paper) John Baez posted the "Marseille Workshop" thread at PF calling attention to his "Week 206" which is mostly about the Ambjorn Jurkiewicz Loll paper. Here is a link to Baez Week 206:
http://math.ucr.edu/home/baez/week206.html
Judging from what Baez says, and from today's reaction by Thomas Larsson on SPR, it could be a landmark paper.
Larsson's comment on AJL in reply to Baez week 206 was:
"If the numerical evidence in this paper is true, and it seems quite strong, then we see a new field open up here...I would not be surprised if this is the next bandwagon and a lot of smart people will jump onto it."
Hi Marcus (ranyart here) you not be aware of this 'amazing' paper:http://uk.arxiv.org/abs/gr-qc/0405060
One question before the storm?..the Dynamical Volume Operator here proposed is 3-D, Volumes do not exist in 2-Dimensional Spacetimes.
Straight Lengths for 2-D spaces, Curved Lengths for 3-D!
we are on our way ;)
...not be aware of...:http://uk.arxiv.org/abs/gr-qc/0405060
another by Thomas Thiemann!
he is busy this year
hope all is well with you
thanks for the link
getting back what we were talking about earlier
here is a photograph of Renate Loll
http://www1.phys.uu.nl/wwwitf/fotopagina's/Medewerkers/Renate.htm
for some reason it does not form an active link and must be copy-pasted into the browser
the snapshot appears to have been taken at Utrecht institute for theoretical physics
with ears discretely concealed (in case of being a Vulcan)
a recent paper co-written by Renate Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
my unqualified&inexpert opinion: Renate Loll kicks butt, anybody studying quantum gravity would be lucky to get a chance to study with her at Utrecht (netherlands)
they (Ambjorn, Jurkiewicz, Loll) did a monte carlo simulation (massive randomized computer experiment) and 4 dimensions emerged probabilistically from their quantum gravity setup---they probably helped and it isnt conclusive but it is a suggestive preliminary finding at the least
Renate Loll is one of the featured speakers (with Stephen Hawking, Carlo Rovelli, Roger Penrose,...) at that Quantum Gravity conference happening in London this september. what odds she talks about this monte carlo QG research
the best introductory presentation of Loll's style of quantum gravity that I have found so far is
http://arxiv.org/hep-th/0212340
"A Discrete History of the Lorentzian Path Integral"
this is dated 13 January 2003 and describes results in d = 2 and d = 3.
These foreshadow the results just announced for d = 4.
On page 16 she says:
"... In d = 4, the first numerical simulations are currently being set up."
so this is a good pedagogical exposition of just before the recent major result
http://arxiv.org./hep-ph/0306/0306198
Measuring quantum states of neutrons in the Earth's gravitational field
this experiment was a first
(objects falling in the earth's grav field seem to descend in jumps from level to level because their grav potential energy is quantized)
it was a beautiful experiment
first announced in a 2-page note in Nature vol 415 (2002)
http://www.nature.com/nsu/020114/020114-8.html
but this 9-page (Phys Rev D) article is online and beautifully illustrated
and more complete than the earlier one in Nature
[this thread is serving as a "surrogate sticky" quantum gravity link-basket,
or a reference library of links to QG stuff, the quantization of energy levels of a falling neutron seems at least periferally related to quantum gravity]
Nesvizhevsky's team is at Grenoble France but now
according to ZapperZ in another thread
there may be related work at Mainz
selfAdjoint
May14-04, 10:00 AM
Umm, Marcus, that experiment has been interpreted as showing not that gravity is quantized but that the particle's ENERGY is quantized. So whatever the gravity potential is, they can only acquire energy from it in steps. But of course that's old news.
so does the link belong in General Physics?
and not in the Quantum Gravity link-basket?
I would like to keep tabs on it one place or the other
but dont care about the wording
if you suggest an editing change I will be glad to make it
[PS I think I see the objection so I went ahead and changed
the wording in the post to emphasize that the connection with QG may be
only periferal.]
pelastration
May14-04, 06:05 PM
Marcus,
This Loll et All approach reminds me somehow to Buckminster Fuller's work , but he didn't started from 'dust-type' particles.
Buckminster was a very gifted guy. He was very consequent in his idea's, also on politics. This is the offical website: http://www.bfi.org
In his giant standard work http://www.rwgrayprojects.com/synergetics/synergetics.html you will some find some amazing concepts. I believe you have to enter that link first (for copyright).
Then I find it helpful to go first to the index about images : http://www.rwgrayprojects.com/synergetics/findex/findex.html.
By these image numbers you can look for the real texts (paragraph numbers) in the Table of content http://www.rwgrayprojects.com/synergetics/toc/toc.html.
Buckminster: "Please now think of all the tensional forces of Universe as one single membrane containing all the radiational, explosive forces we have enumerated. Now think of the original compression sphere exploding into many ... individual, exploded-apart, spherical mass components, each of which is tightly embraced by the membrane - leaving only intervening perpendicular linear tubes."
I think you will appreciate his logic.
Dirk
A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084
A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085
any comments are welcome. I have to go out for part of the evening, so
may not get around to discussing these for a while. I would guess the
Livine/Oriti paper is interesting.
L/O:
----quote---
Does a quantum gravity theory with an invariant length and a discrete spectrum for geometric observables necessarily break Lorentz symmetry or necessarily require some sort of modification/deformation of it? The answer, as we will see, is simply “no”.
---end quote---
I believe they are Two Gems!
this thread is serving as a surrogate sticky "reference library" for useful LQG links. Thanks to all who have contributed so far!
the last time I updated the main list of references was post #163 about thirty posts back, so it's about time to update the list again
I will break it down into some categories, with textbooks and introductory survey lectures and such coming first
------- texts--------
Rovelli posted the 30 December 2003 draft of his book "Quantum Gravity", to be published this year by Cambridge University Press.
The PDF file is at his homepage
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
The book is around 350 pages long and takes a few (like ten?) minutes to download and convert.
To download the 30 December 2003 draft of the book directly:
http://www.cpt.univ-mrs.fr/~rovelli/book.pdf
Here are Thiemann's Lecture Notes (they have been published in Berlin by Springer Verlag)
"Lectures on Loop Quantum Gravity".
A draft is online at
http://arxiv.org/gr-qc/0210094
-----a recent review article----
http://arxiv.org./abs/gr-qc/0404018
Ashtekar and Lewandowski
"Background Independent Quantum Gravity: a Status Report"
125 pages
many references
another recent survey:
Enrique Alvarez
http://arxiv.org/gr-qc/0405107
"Quantum Gravity"
( Lectures given at Karpacz. 40 pages)
this next is older and interesting partly for historical and broader perspective.
it is a Rovelli survey at a 1997 GR conference (plenary at GR15)
and you get not just LQG and string but some other approaches that
were tried in the 1990s:
http://arxiv.org/gr-qc/9803024
Carlo Rovelli
"Strings, loops and others: a critical survey of the present approaches to quantum gravity"
" I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models...."
---------a newsletter: "Matters of Gravity"----
Jorge Pullin's newsletter "Matters of Gravity"
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
-------Quantum Gravity Phenomenology and DSR---------
some recent phenomenology and DSR papers:
A new paper by Amelino-Camelia
http://arxiv.org/gr-qc/0405084
http://arxiv.org./gr-qc/0404113
"On alternative approaches to Lorentz violation invariance in loop quantum gravity inspired models
Jorge Alfaro, Marat Reyes, Hugo A. Morales-Tecotl and L.F. Urrutia
Ted Jacobson, Stefano Liberati, David Mattingly
"Quantum Gravity Phenomenology and Lorentz Violation"
http://arxiv.org./abs/gr-qc/0404067
15 April 2004
Giovanni Amelino-Camelia
"A perspective on quantum gravity phenomenology"
http://www.arxiv.org/abs/gr-qc/0402009
dated 2 February 2004
Giovanni Amelino-Camelia, Jerzy Kowalski-Glikman, Gianlucca Mandanici, and Andrea Procaccini
"Phenomenology of Doubly Special Relativity"
http://arxiv.org/gr-qc/0312124
dated 30 December 2003
Jerzy Kowalski-Glikman
"Doubly Special Relativity and quantum gravity phenomenology"
http://arxiv.org/hep-th/0312140
dated 12 December 2003
Jerzy Lukierski
"Relation between quantum ?-Poincare framework and doubly special relativity"
http://arxiv.org./hep-th/0402117
dated 18 February 2004
other less recent ones:
Jerzy Kowalski-Glikman and Sebastian Nowak
"Doubly Special Relativity and de Sitter space"
http://arxiv.org/hep-th/0304101
dated 11 October 2003
M. Daszkiewicz, K. Imilkowska, J. Kowalski-Glikman
"Velocity of particles in Doubly Special Relativity"
http://arxiv.org/hep-th/0304027
dated 3 April 2003
---------Loop Quantum Cosmology-------
Martin Bojowald
"Loop Quantum Cosmology: Recent Progress"
http://arxiv.org/gr-qc/0402053
One of the invited plenary talks at the January 2004 ICGC
conference (see list of recent conferences)
The Bianchi IX model in Loop Quantum Cosmology
Martin Bojowald, Ghanashyam Date, Golam Mortuza Hossain
41 pages
http://arxiv.org/abs/gr-qc/0404039
"Inflationary Cosmology and Quantization Ambiguities in Semi-Classical Loop Quantum Gravity"
Martin Bojowald, James E. Lidsey, David J. Mulryne, Parampreet Singh, Reza Tavakol
15 pages, 8 figures
http://arxiv.org./abs/gr-qc/0403106
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology and Boundary Proposals"
http://arxiv.org/gr-qc/0312103
dated 23 December 2003
Martin Bojowald
"Quantum Gravity and the Big Bang"
http://arxiv.org./astro-ph/0309478
dated 17 September 2003, briefly summarizes how
LQG can serve to cure the big bang singularity and
motivate inflationary expansion. Short and less technical
than the other two papers.
Martin Bojowald and Kevin Vandersloot
"Loop Quantum Cosmology, Boundary Proposals, and Inflation"
http://arxiv.org/gr-qc/0303072
dated 19 March 2003
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
"Loop quantum gravity effects on inflation and the CMB"
http://arxiv.org/astro-ph/0311015
from the Tsujikawa/Singh/Maartens abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler "On singularity resolution in quantum gravity"
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using the older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity is "robust"---doesnt depend on using a particular formalism.
as a background reference for classical (non-quantum) cosmology:
Charles Lineweaver
"Inflation and the Cosmic Microwave Background"
http://arxiv.org/astro-ph/0305179
dated 12 May 2003
-----in case of category theory----
http://www.folli.uva.nl/CD/1999/library/pdf/barrwells.pdf
Barr is at McGill and Wells is at U Virginia
its >100 pages of lecture notes
http://www.dcs.ed.ac.uk/home/dt/CT/categories.pdf
these notes are by Daniele Turi at U. Edinburgh
they are based on Saunders Mac Lane book
"Categories for the working mathematician"[/QUOTE]
here are some recent conferences and other stuff (I still need to edit and bringh some of it up to date)
------recent conferences------
Strings meet Loops (Albert Einstein Institute, MPI-Potsdam) October 2003
http://www.aei-potsdam.mpg.de/events/stringloop.html
Loop Gravity Workshop (Mexico City) January 2004
http://www.nuclecu.unam.mx/~corichi/lqg.htm
International Conference on Gravity and Cosmology (India) January 2004
http://www.cusat.ac.in/icgc04/
Quantum Gravity Phenomenology, (40th annual Polish Winterschool in Theoretical Physics) February 2004
http://www.ws2004.ift.uni.wroc.pl/html.html
Loop/SpinFoam Conference (Marseille) May 2004
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
Baez report on it
http://math.ucr.edu/home/baez/week206.html
--------upcoming conferences------
General Relativity Conference (GR17) at Dublin 3 July 2004
http://www.dcu.ie/~nolanb/gr17.htm
more annoucements at
http://www.maths.qmul.ac.uk/wbin/GRnewsfind/conference?conference
Chris Isham's 60th Birthday conference
Imperial College London, around September 6-7th
for info go to the Imperial College site and click on
"Isham 60 Conference"
http://theory.ic.ac.uk/
------links to an unselective assortment of current work------
Ambjorn Jurkeiwicz Loll
http://arxiv.org./hep-th/0404156
"Emergence of a 4D World from Causal Quantum Gravity"
Carlo Rovelli and Winston Fairbairn
"Separable Hilbert space in loop quantum gravity"
http://arxiv.org/abs/gr-qc/0403047
John Baez
"Quantum Quandaries: A Category-Theoretic Perspective"
http://arxiv.org/quant-ph/0404040
Hendryk Pfeiffer has a new preprint on arxiv
called
"Quantum Gravity and the Classification of
Smooth Manifolds"
http://arxiv.org./gr-qc/0404088
Livine's thesis
http://arxiv.org/gr-qc/0309028
Girelli and Livine
"Quantizing speeds with the cosmological constant"
http://arxiv.org/gr-qc/0311032
Oriti's thesis
http://arxiv.org/gr-qc/0311066
"Spin Foam Models of Quantum Spacetime"
Karim Noui and Philippe Roche
"Cosmological Deformation of Lorentzian Spin Foam Models"
http://arxiv.org/gr-qc/0211109
The cosmological constant occurs in a number of recent quantum gravity papers, for instance the one by Girelli/Livine.
Velhinho "On the structure of the space of generalized connections"
http://arxiv.org/math-ph/0402060
Noui and Perez "Three dimensional loop quantum gravity: physical scalar product and spin foam models"
http://arxiv.org/gr-qc/0402110
Noui and Perez "Three dimensional loop quantum gravity: coupling to point particles"
http://arxiv.org/gr-qc/0402111
Noui and Perez "Dynamics of Loop Quantum Gravity and Spin Foam Models in Three Dimensions"
http://arxiv.org/gr-qc/0402112
Noui and Perez "Observability and Geometry in Three Dimensional Quantum Gravity"
http://arxiv.org/gr-qc/0402113
Freidel and Louapre "Ponzano-Regge model revisited, I."
http://arxiv.org/hep-th/0401076
Gambini and Pullin "Canonical Quantum Gravity..."
http://arxiv.org/gr-qc/0402062
Buffenoir, Henneaux, Noui, Roche
Hamiltonian Analysis of Plebanski Theory
http://arxiv.org./gr-qc/0404041
(spin foam, BF)
http://arxiv.org/abs/hep-th/0405036
Remarks on the black hole entropy and Hawking spectrum in Loop Quantum Gravity
Authors: A. Alekseev, A.P. Polychronakos, M. Smedback
Report-no: CCNY-HEP-04/3, UUITP-13/04
In this note we reply to the criticism by Corichi concerning our proposal for an equidistant area spectrum in loop quantum gravity. We further comment on the emission properties of black holes and on the statistics of links.
A new one by Livine and Oriti
http://arxiv.org/gr-qc/0405085
----------fundamental constants, planck units, time-keeping-------
Historical source for Planck units, the 1899 paper (thanks arivero!)
http://www.bbaw.de/bibliothek/digital/struktur/10-sitz/1899-1/jpg-0600/00000494.htm
In December 2003, the National Institute of Standards and Technology (NIST) posted new CODATA recommended values for the basic planck units
http://physics.nist.gov/cuu/Constants/
choose "universal" from the menu to find (among other things) the recommended values of
planck mass
planck length
planck time
planck temperature
A 1997 article on timekeeping, discussing GR effects allowed-for in the GPS
http://www.allanstime.com/Publications/DWA/Science_Timekeeping/TheScienceOfTimekeeping.pdf
------prospects for testing quantum gravity observationally------
Floyd Stecker
"Cosmic Physics: the High Energy Frontier
http://arxiv.org/astro-ph/0309027
dated September 2003
Stecker discusses the various earth-based and orbital instruments, currently operating, or under construction, or planned, or proposed, and the kind of data becoming available. Among many other things he discusses GLAST, planned to start operating 2007, which, if there are tiny energy-dependent differences in speed among gamma-ray-burst photons, may be able to detect same. Also discusses neutrino observation.
-------science journalism----
"The Duel: Strings versus loops"
http://arxiv.org/abs/physics/0403112
A translation of Rudy Vaas' article in the German
science magazine "Bild der Wissenschaft" roughly
comparable to the "Scientific American"
========
simply to have this link on LaTex handy:
http://www.physicsforums.com/misc/howtolatex.pdf
quotes about physics:
http://www.angelo.edu/faculty/kboudrea/cheap/cheap2_physics.htm
Michael Flohr's great set of notes on group theory in physics:
http://www.itp.uni-hannover.de/~flohr/lectures
(scroll down to "Physical Applications of Group Theory")
http://arxiv.org/hep-th/0405160
Maulik K. Parikh
A Secret Tunnel Through The Horizon
(First prize in the Gravity Research Foundation Essay Competition)
7 pages
A poster on SPR named Chris Weed has noted Parikh's paper and recommended it, together with another that has recently appeared
http://arxiv.org/gr-qc/0405111
some more new ones:
http://arxiv.org/hep-th/0405183
"No black hole information puzzle in a relational universe"
Rodolfo Gambini, Rafael Porto, Jorge Pullin
4 pages
http://arxiv.org/gr-qc/0405119
"Automorphism covariant representations of the holonomy-flux *-algebra"
Andrzej Okolow, Jerzy Lewandowski
32 pages
Lewandowski/Okolow abstract: "We continue an analysis of representations of cylindrical functions and fluxes which are commonly used as elementary variables of Loop Quantum Gravity. We consider an arbitrary principal bundle of a compact connected structure group and following Sahlmann's ideas define a holonomy-flux *-algebra whose elements correspond to the elementary variables. There exists a natural action of automorphisms of the bundle on the algebra; the action generalizes the action of analytic diffeomorphisms and gauge transformations on the algebra considered in earlier works. We define the automorphism covariance of a *-representation of the algebra on a Hilbert space and prove that the only Hilbert space admitting such a representation is a direct sum of spaces L^2 given by a unique measure on the space of generalized connections. This result is a generalization of our previous work (Class. Quantum. Grav. 20 (2003) 3543-3567, gr-qc/0302059) where we assumed that the principal bundle is trivial, and its base manifold is R^d."
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")
In the main search field if you type
find a ambjorn and topcite 50+
then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date
It turns out he has authored 34 papers which topped 50 citations.
Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.
The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
In the main search field if you type
find author ambjorn and topcite 50+
then it will list Jan Ambjorn's papers which have received 50 or more
citations to date
or you can say "topcite 100+" to restrict the search still further.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D world...")
It turns out Ambjorn has published several "100+" citations papers.
This is an lqg thread. Of course maybe your posting this because you now believe lqg is wrong and you've decided to change religions.
A new PF poster named setAI pointed out a good 6-page essay by Lee Smolin today, so I will add it to this collection of links:
Sample from page 5:
"The debate between proponents of background-dependent and background independent theories is in fact just the modern version of an ancient debate. Since the Greeks, the argument has raged between those who believed that space and time have an eternally fixed, absolute character and those who thought space and time are no more than relations between events that themselves evolve in time. Plato, Aristotle, and Newton were absolutists. Heraclites, Democritus, Leibniz, Mach, and Einstein were relationalists. When we demand that the quantum theory of gravity be background-independent, we are saying we believe that the triumph that general relativity represented for the relational point of view is final and will not be reversed.
Much of the argument between string and loop theorists is a continuation of this debate. Most string theorists were trained as elementary-particle physicists and worked their whole lives in a single fixed spacetime. Many of them have never even heard of the relational/absolute debate, which is the basic historical and philosophical context for Einstein's work. Most people who work in loop quantum gravity do so because at some point in their education they understood the relational, dynamical character of spacetime as described in general relativity, and they believe in it. They don't work on string theory because they cannot take seriously any candidate for a quantum theory of gravity that is background-dependent and hence loses (or at best hides) the relational, dynamical character of space and time."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
It is a good essay because it combines a clearsighted overview with a personal insider's take, and also tells the history of this approach to quantum gravity from a participant's perspective.
Sample from page 3:
"Loop quantum gravity started in the early 1980s with some discoveries about classical general relativity by Amitaba Sen, then a postdoc at the University of Maryland. These were made into a beautiful reformulation of Einstein's theory by Abhay Ashtekar, then at Syracuse University and now director of the Center for Gravitational Physics at Penn State—a reformulation that brought the mathematical and conceptual language we use to describe space and time closer to the language used in particle physics and quantum physics."
http://www.edge.org/3rd_culture/smolin03/smolin03_p3.html
Another quote from page 5:
"Another reason that string theory cannot be the final word is that in string theory one studies strings moving in a fixed classical spacetime. Thus, string theory is what we call a background-dependent approach. It means that one defines the strings as moving in a fixed space and time. This may be a useful approximation, but it cannot be the fundamental theory. One of the fundamental discoveries of Einstein is that there is no fixed background. The very geometry of space and time is a dynamical system that evolves in time. The experimental observations that energy leaks from binary pulsars in the form of gravitational waves—at the rate predicted by general relativity to the unprecedented accuracy of eleven decimal places—tells us that there is no more a fixed background of spacetime geometry than there are fixed crystal spheres holding the planets up. The fundamental theory must unify quantum theory with a completely dynamical description of space and time. It must be what we call a background-independent theory. Loop quantum gravity is such a one; string theory is not."
http://www.edge.org/3rd_culture/smolin03/smolin03_p5.html
From page 6:
"So while I disagree with the leading string theorists about methodology, this hasn't kept me from working on string theory. After all, they don't own it; its open problems are there for anyone to try to solve. So I decided a few years ago to ignore their advice and try to construct the background independent form of M theory. In the process of inventing loop quantum gravity, we gained a lot of knowledge about how to make quantum theories of space and time that are background-independent."
http://www.edge.org/3rd_culture/smolin03/smolin03_p6.html
the essay seems to have been written in latter half of 2003, so is fairly recent.
thanks to setAI for flagging this one
selfAdjoint
May29-04, 10:59 AM
Thinking about all this LQG vs. string business, I wonder if the "beauitiful reformulation" of Ashtekar isn't like the "beautiful reformulation of string theory" of Schwartz and Witten that has motivated so much string research. I am wondering whether in the final analysis, beauty is all it's cracked up to be as a search strategy.
The two great historical exemplars of beauty first were Einstein and Dirac. In each case their approach achieved a great success early but then led them into unproductive wastelands. And it is at least arguable that both string physics and LQG research in the Ashtekar tradition are right now spinning their wheels. Maybe it's time for a younger generation, playing Feynman and Dyson to the Witten - Ashtekar version of Einstein-Dirac to have their say. Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna).
This post is possibly not in line with your intent to have this as a colllection of documents, but I just couldn't resist, seeing the same old same old out of Smolin being posted once again.
Thinking about all this LQG vs. string business,...
...Which is why I am very interseted in the AJL paper, a possibly rough hewed (remember Feyman's early rep?) but undoubtedly novel approach to the problem of background independent quantum mechanics (and THAT, not just quantum gravity is the big kahuna)...
Amen to that.
Background independent quantum mechanics is the big kahuna.
this turns up the lights on something that was creeping around the edges of my mind also
I'm very interested in the AJL dynamical triangulations approach too. We could continue in the Marseille thread that Baez started (it is largely about AJL but Marseille was billed as a Loop+Foam conference---a lot of family resemblances: loop to foam and foam to simplicial QG---making too sharp distinctions could be a mistake.
Anyway, if we leave this thread as a catchbasket for LQG-and-related links we could followup on AJL etc at the Marseille thread if that suits you, or start a new one on the Big Kahuna!
http://arxiv.org/abs/hep-th/0405273
the title is "Introduction to DSR"
there was a Quantum Gravity symposium in poland
this February and Jerzy Kowalski-Glikman (the organizer) lectured on
DSR and its relation to QG
this paper was developed from his lectures at the Winterschool, and
submitted to Springer for publication in its "lecture notes in physics" series.
anything calling itself an Introduction could potentially
be useful.
the idea of DSR is that the usual minkowski space and lorentz group symmetries
of special relativity are what results from forcing c to be the same
for all observers
what if you force TWO physical quantities, not just the speed of light but also the planck length or the planck mass, to be the same for all observers.
----------------
more DSR news, this time from Alejandro's city of Zaragoza
just out:
http://arxiv.org/hep-th/0405285
Quantum Uncertainty in Doubly Special Relativity
Authors: Jose Luis Cortes, J. Gamboa
4 pages, no figures
The modification of the quantum mechanical commutators in a relativistic theory with an invariant length scale (DSR) is identified...
---------QG phenomenology-----
a new paper:
http://arxiv.org/quant-ph/0406007
"Could Energy Decoherence due to Quantum Gravity be observed?"
Christoph Simon, Dieter Jaksch
7 pages, no figures
Sample from abstract:
"It has recently been proposed that quantum gravity might lead to the decoherence of superpositions in energy, corresponding to a discretization of time at the Planck scale....
... We also show how local energy decoherence, which acts separately on system and phase reference, could be detected with remarkable sensitivity and over a wide range of length scales by long-distance Ramsey interferometry with metastable atomic states. The sensitivity of the experiments can be further enhanced using multi-atom entanglement."
the Spires database is an amazing resource for keeping track of activity in various research lines and seeing what topics are attracting interest
several links within that site have proven useful (for me) recently
Here is the 2003 edition of the topcites for all categories, gr-qc as well as hep-th and the rest
http://www.slac.stanford.edu/library/topcites/2003/eprints/index.shtml
Here is the general index for topcites for all the years 1992-2003:
http://www.slac.stanford.edu/library/topcites/
I posted earlier another special Spires feature which lets you find the most-cited papers by a particular author. To use it you need to know that in their code the letter A stands for author, so you say "a ambjorn" to find papers authored by ambjorn.
there is a handy utility at Spires that I just learned about today
namely a feature of their search engine that is especially
designed to find highly cited papers. It is at:
http://www.slac.stanford.edu/spires/hep/
to illustrate, suppose you want to find influential or highly cited papers by Jan Ambjorn.
Ambjorn is the "dynamical triangulations" researcher at Niels Bohr inst. and at Utrecht, who has published recently with Renate Loll. ("Emergence of a 4D World from Causal Quantum Gravity")
In the main search field if you type
find a ambjorn and topcite 50+
then it will list those of Jan Ambjorn's papers which have received 50 or more citations to date
It turns out he has authored 34 papers which topped 50 citations.
Or you can say "topcite 100+" to restrict the search still further,
and find several of Ambjorn's papers which have topped 100.
The search engine takes several different formats and for one of them instead of saying "find a thiemann" you have to say
"find author thiemann". but the default seems to be use the letter A to stand for author.
lot of good information to get out of Spires, bravo to Stanford and SLAC for hosting it, I feel I've just scratched the surface
PAM Dirac was (one of) the first to try
to construct a quantum version of
General Relativity------to quantize gravity.
So this thread being for LQG links we should have a Dirac link
and Pelastration just supplied one with two photos of Dirac and
a newspaper interview
http://faculty.rmwc.edu/tmichalik/dirac.htm
cant recommend but thought it interesting enough to keep tabs on
"Toward a Background Independent Quantum Theory of Gravity"
Authors: Vishnu Jejjala, Djordje Minic, Chia-Hsiung Tze
Comments: Awarded Honorable Mention, 2004 Gravity Research Foundation Essay Competition; 8 pages
http://arxiv.org/gr-qc/0406037
This just appeared on arXiv today.
It looks like a keeper:
http://arxiv.org/gr-qc/0406042
"Oscillatory Universes in Loop Quantum Cosmology and Initial Conditions for Inflation"
James E. Lidsey, David J. Mulryne, N. J. Nunes, Reza Tavakol
6 pages, 4 figures
Several of them wrote a paper with Martin Bojowald that posted a couple of months back. Otherwise I dont recall seeing their names before
Rovelli has redone his homepage.
The book is planned to hit the bookstores in October
there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.
He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version
If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?
Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Rovelli has redone his homepage.
The book is planned to hit the bookstores in October
there is a picture of the book and a link to the Cambridge U. P.
catalog entry for it
planned price is 45 pounds sterling
the draft is still available free at his site, by agreement with C.U.P.
He mentions another piece of writing---popular----
called "What is space? What is time?"
so far just in Italian. I would guess there will be an English version
If you want to look for it in Italian (which I understand some people can read) the title is
Che cos' e lo spazio? Che cos' e il tempo?
Rovelli homepage:
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Thanks Marcus, its a must buy!
I like the cover of the book, the use of Geometry is very evident.
The cover outline is framed with precise measurment at its edge, with the Backgound uniform colour.
As one moves inward the Author and Title are framed by a 'casimir-effect'..and a sea of points 'Quantum-Background' are pretty hazy?
Cool1 :smile: :approve:
http://assets.cambridge.org/0521837332/cover/0521837332.jpg
Quantum gravity is perhaps the most important open problem in fundamental physics. It is the problem of merging quantum mechanics and general relativity, the two great conceptual revolutions in the physics of the twentieth century. The loop and spinfoam approach, presented in this book, is one of the leading research programs in the field. The first part of the book discusses the reformulation of the basis of classical and quantum Hamiltonian physics required by general relativity. The second part covers the basic technical research directions. Appendices include a detailed history of the subject of quantum gravity, hard-to-find mathematical material, and a discussion of some philosophical issues raised by the subject. This fascinating text is ideal for graduate students entering the field, as well as researchers already working in quantum gravity. It will also appeal to philosophers and other scholars interested in the nature of space and time.
http://titles.cambridge.org/catalogue.asp?isbn=0521837332
It will be a must buy for myself as well.
But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
I am seeing similarities arising not only from this perspective but from the current link Marcus supplied on the cosmological association (LQC (http://www.physicsforums.com/showthread.php?t=30258) ).
Olias and Sol2, I too am glad to see "Quantum Gravity" well on its
way to being available, and I will certainly buy a copy although
45 pounds sterling is a fair-size chunk of cash.
It was thoughtful of Carlo to work out an agreement with Cambridge where
the publisher will allow him to keep a draft version available free online at his website. that way if someone can't afford the book they can at least get the draft and print it out at home, or just keep it on the computer---which is already pretty useful.
speculation is always risky but I am speculating that because it's a fast-moving field there will be several editions of this book
what is coming out this year (planned for October) will be the first edition
but----with ongoing developments in quantum cosmology and the simplicial or "dynamical triangulations" approach, and whatever else (so hard to predict)----there may be a second edition, and possibly others as years go along.
and then the draft on rovelli's website will be a kind of "zero-th edition".
my favorite page in the draft version of the book is page 7---the part about the whale. I also like some things around page 52
I also really like the philosophical essays at the end
and the historical accounts
although it has a lot for the general reader, the book is primarily for graduate students looking for PhD thesis work to do and for established researchers wanting to move into the field of QG.
That is to say, it has generally accessible portions (which are admirable and enlightening, in my opinion) but also (in case other people besides Olias and Sol2 are reading this thread I want to stress) plenty that is not so accessible.
...
But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
....
this quote is very interesting and raises an important issue. maybe we will eventually have a thread devoted to it. for starters
how about going to arXiv and putting "quantum geometry"
into the abstract box
and doing a search for articles that say "quantum geometry" in their
abstract summary
It would give an idea of what the experts mean by it, in a technical sense.
Also in the title box, for the arXiv search engine. To find whatever
books and articles have been written about quantum geometry have that in the title. (I know some, but most likely not all.)
I am not promising that a good thread would come of this, or a clear resolution of how the term is used, even. but it is something to think about
selfAdjoint
Jun11-04, 12:04 PM
Putting "quantum geometry" in the latest year search at hep-th brought up four papers, all of which used Quantum Geometry as a synonym for the Ashtekar program, aka LQG.
Putting "quantum geometry" in the latest year search at hep-th brought up four papers, all of which used Quantum Geometry as a synonym for the Ashtekar program, aka LQG.
go back to earlier papers
there is completely different stuff called quantum geometry
I seem to recall Majid using the term
and maybe Connes
most likely others
no clear connection with string tho
I think maybe Brian Greene was fantasizing a little
or looking ahead to a desirable future, but could be wrong
the development of a quantum geometry has to come but
may have no clear connection with string IMHO
this quote is very interesting and raises an important issue. maybe we will eventually have a thread devoted to it. for starters
how about going to arXiv and putting "quantum geometry"
into the abstract box
and doing a search for articles that say "quantum geometry" in their
abstract summary
It would give an idea of what the experts mean by it, in a technical sense.
Also in the title box, for the arXiv search engine. To find whatever
books and articles have been written about quantum geometry have that in the title. (I know some, but most likely not all.)
I am not promising that a good thread would come of this, or a clear resolution of how the term is used, even. but it is something to think about
The dimensional significance of this topic is really a difficult issue for myself as well, and the statistics really surpirsed me that you have offerred.
If such a geometry was to emerge what exactly are we describing? Jeff's comments in regard to supersymmetry are valid statements because of the complexity of the issue in regards to the metric. The complexity of points really seem to flow when you come to that level, yet it has encapsulated the ideas of dimension. So geometry has its work cut out for it no doubt.
What exactly is the hierarchy problem?The gist of it is that the universe seems to have two entirely different mass scales, and we don't understand why they are so different. There's what's called the Planck scale, which is associated with gravitational interactions. It's a huge mass scale, but because gravitational forces are proportional to one over the mass squared, that means gravity is a very weak interaction. In units of GeV [billions of electron volts], which is how we measure masses, the Planck scale is 10 to the 19th GeV. Then there's the electroweak scale, which sets the masses for the W and Z bosons. These are particles that are similar to the photons of electromagnetism and which we have observed and studied well. They have a mass of about 100 GeV. So the hierarchy problem, in its simplest manifestation, is how can you have these particles be so light when the other scale is so big. (http://www.physicsforums.com/showpost.php?p=231252&postcount=16)
I had mentioned in the topic of the new math thread (http://www.physicsforums.com/showpost.php?p=231852&postcount=17), that such attempts at a discription woud have to be formulated in much the same way Smolin did? Klein's Ordering of geometries is really quite interesting in terms of Quantum Evolution? :smile:
Because we understand this dynamical movement in plasmatic features as supersymmetical conisderation one would have to understand how gravity moves to supergravity. If we understand the gravity field can have its differences( dimensional relationship?) then how we look at the Q<--->Q measure becomes a interesting relation in terms of understanding the metric in a different way.
ds2 = (cdt)2 - dl2
On a cosmological level this directs my attention, yet I recognize the complexity of the movement in the quantum world. Why classically does this not fit at that quantum level and what do we have to reconsider here?
Do you understand how this subject might evolve in this context?
...But I wonder about the issue of quantum geometry. How will this be formulated into the LQG perspective, as it has in strings?
The Elegant Universe, by Brian Greene, pg 231 and Pg 232
"But now, almost a century after Einstein's tour-de-force, string theory gives us a quantum-mechanical discription of gravity that, by necessity, modifies general relativity when distances involved become as short as the Planck length. Since Reinmannian geometry is the mathetical core of genral relativity, this means that it too must be modified in order to reflect faithfully the new short distance physics of string theory. Whereas general relativity asserts that the curved properties of the universe are described by Reinmannian geometry, string theory asserts this is true only if we examine the fabric of the universe on large enough scales. On scales as small as planck length a new kind of geometry must emerge, one that aligns with the new physics of string theory. This new geometry is called, quantum geometry."
...
hi Sol2, you copied in this Brian Greene quote which could be the start of a new thread so I started one, and hope we can continue the discussion there (in a harmonious fashion! :smile: I might add.)
there is no collective name for the group of background independent QG approaches aimed at quantizing GR
Ashtekar says Quantum Geometry, but means Loop
Thiemann says Canonical Quantum General Relativity, but means Loop
Gambini says Canonical Quantum Gravity, meaning his type of Loop
the most widely used term is Loop---LQG for short
Ashtekar and Lewandowski recently used Background Independent Q.G.
in a review article, meaning Loop
then there are closely allied approaches called Spin Foam
and Simplicial Quantum Gravity (dynamical triangulations in particular)
and one of the Simplicial people has used the term Quantum Geometry
but it does not mean exactly the same as what Ashtekar means
these approaches got together at the May 2004 "loop/foam conference"
but there is no agreed on collective noun
So I have tried to construct a keyword search in arXiv that would turn up these things and this seems to work. Here are numbers of preprints
by year 1992-present.
Year 1992:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1992/0/1
Year 1993:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1993/0/1
Year 1994:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/2003/0/1
Last twelve months (e.g. 14 June 2003 to 14 June 2004):
http://lanl.arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/past/0/1
this is designed to catch:
loop quantum gravity
loop quantum cosmology
canonical quantum gravity
simplicial quantum gravity
nonperturbative quantum gravity
spin foam
dynamical triangulation
[EDIT afterthought]
BTW Rovelli has a new paper out on arXiv, together with Oriti and Speziale.
predictably, the last link in the above search list turned it up
http://arxiv.org/gr-qc/0406063
"...The model sheds light also on several other features of spinfoam quantum gravity: the reality of the partition function; the geometrical interpretation of the Newton constant; and the fact that the partition function of general relativity is finite in spite of the divergence of the BF one."
Here is a more inclusive version of the above search:
2001:
http://lanl.arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2001/0/1
2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2002/0/1
2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2003/0/1
Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/past/0/1
------------------
http://arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+simplicial+OR+c anonical+nonperturbative+abs:+OR+AND+spin+foam+AND +dynamical+triangulation/0/1/0/past/0/1
another Loop quantum cosmology paper at arxiv today. the page of links needs to be brought up to date:
Qualitative Approach to Semi-Classical Loop Quantum Cosmology
G.V. Vereshchagin
http://arxiv.org/abs/gr-qc/0406108
this thread is serving as a surrogate sticky for useful loop-and-related quantum gravity links
I update it periodically.
to get a picture of QG developments in the first half of 2004 it is helpful to quote Baez post after the May 2004 marseille conference:
I just got back from the Marseille conference on loop quantum gravity and spin foams:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
It was really great, so I devoted "week206" of my column This Week's Finds entirely to this conference:
http://math.ucr.edu/home/baez/week206.html
In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model - something that nobody had succeeded in doing before!
http://www.arXiv.org/abs/hep-th/0404156
I hope this lays to rest certain rumors here that I'd burnt out on quantum gravity. :devil:
a key paper mentioned here
Ambjorn Jurkiewicz Loll
"Emergence of a 4D World from Causal Quantum Gravity"
http://www.arXiv.org/abs/hep-th/0404156
other key papers, connecting quantum gravity with outgrowths of DSR namely "DDSR" or "TSR" (smolin's name: triply special relativity) and moffat's NGT an outgrowth of MOND
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
Moffat
Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter
http://arxiv.org/astro-ph/0403266
another interesting development, resolution of the "Black Hole Information Paradox" using relational time----a quantum mechanical clock rather than absolute ideal time
Gambini Porto Pullin
"Realistic clocks, universal decoherence and the black hole information paradox"
http://arxiv.org/abs/hep-th/0406260
also their earlier paper
“No black hole information puzzle in a relational universe,”
http://arxiv.org/hep-th/0405183.
it is interesting that this information-loss paradox has been worked on rather hard by some wellknown stringy people like Susskind and, it seems, Maldacena--but in stringy context it is still a challenging outstanding problem which people are working on. So it is a bit of a coup to resolve it as GPP do, to resolve it at all would be respectable and they do it, as well, with apparent ease and not a lot of mess.
Since this thread is a linkbasket for links to recent LQG stuff that might be useful I will put these recent things here:
Penrose book is about LQG to some degree. "The Road to Reality"
It came out in July.
Carlo Rovelli book "Quantum Gravity" is coming out in Fall 2004 from Cambridge Uni Press. this is the first comprehensive LQG graduate-level textbook.
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
Lee Smolin has posted "An Invitation to Loop Quantum Gravity" a 50-page survey and intro with FAQ for physicists in other fields who want to switch fields and do QG research. http://arxiv.org/hep-th/0408048
"Invitation" is intended for Reviews of Modern Physics.
John Baez gave an introduction and survey at the Dublin GR17 conference, which is available online at Baez site.
"Loop Quantum Gravity, Quantum Geometry and Spin Foams"
http://math.ucr.edu/home/baez/lectures.html#lqg
"Quantum Gravity Phenomenology" was the topic of the Winterschool-2004 (WS-2004) symposium this year, 4-14February, a 10-day conference on the initial and planned efforts to test QG by empirical observation.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures if you want slides from the various talks given at WS-2004
Observational tests of QG have already had a considerable impact as discussed by Smolin in the "Invitation" survey article starting page 27.
LQG is rapidly reaching a point where it can guide experiment---if one counts certain kinds of astronomical observation as experiment.
Cambridge Uni Press is also publishing "Universe or Multiverse" which will contain a chapter by Smolin called "Scientific Alternatives to the Anthropic Principle" where he offers an evolutionary Multiverse hypothesis that generates testable (numerical) predictions and therefore has meaning as a part of science.
http://arxiv.org/hep-th/0407213
Smolin's Multi is falsifiable, using today's tools.
------------------
In his recent paper Smolin cites "personal communication" from Martin Bojowald regarding work on eliminating the Black Hole singularity (by the appropriate quantization of gravity). So far Bojowald has only published his preliminary work on this, not the final result:
Martin Bojowald
Spherically Symmetric Quantum Geometry: States and Basic Operators
http://arxiv.org/abs/gr-qc/0407017
26 pages
"The kinematical setting of spherically symmetric quantum geometry, derived from the full theory of loop quantum gravity, is developed. This extends previous studies of homogeneous models to inhomogeneous ones where interesting field theory aspects arise. A comparison between a reduced quantization and a derivation of the model from the full theory is presented in detail, with an emphasis on the resulting quantum representation. Similar concepts for Einstein-Rosen waves are discussed briefly."
Martin Bojowald and Rafal Swiderski
The Volume Operator in Spherically Symmetric Quantum Geometry
http://arxiv.org/abs/gr-qc/0407018
25 pages
"The spherically symmetric volume operator is discussed and all its eigenstates and eigenvalues are computed. Even though the operator is more complicated than its homogeneous analog, the spectra are related in the sense that the larger spherically symmetric volume spectrum adds fine structure to the homogeneous spectrum. The formulas of this paper complete the derivation of an explicit calculus for spherically symmetric models which is needed for future physical investigations."
http://arxiv.org/hep-th/0407115
Loop Quantum Gravity and the Cyclic Universe
Martin Bojowald, Roy Maartens, Parampreet Singh
6 pages
"Loop quantum gravity introduces strong non-perturbative modifications to the dynamical equations in the semi-classical regime, which are responsible for various novel effects, including resolution of the classical singularity in a Friedman universe. Here we investigate the modifications for the case of a cyclic universe potential, assuming that we can apply the four-dimensional loop quantum formalism within the effective four-dimensional theory of the cyclic scenario. We find that loop quantum effects can dramatically alter the near-collision dynamics of the cyclic scenario. In the kinetic-dominated collapse era, the scalar field is effectively frozen by loop quantum friction, so that the branes approach collision and bounce back without actual collision."
--------------------
Representations of the Weyl Algebra in Quantum Geometry
Christian Fleischhack
63 pages
http://arxiv.org/abs/math-ph/0407006
substantial progress beyond where this was taken by Hanno Sahlmann, Thiemann, Lewandowski, Okolow. It may be that Fleishhack has reached to goals set by this earlier work
----------------------
In the next post, Meteor calls attention to the work of Date and Hossain
who showed that both Inflation and the Big Bounce were generic in isotropic LQC. I think Meteor mentioned two of their three recent papers and I will add the other one for completeness:
"Genericity of Big Bounce in isotropic loop quantum cosmology"
http://arxiv.org/gr-qc/0407074
http://arxiv.org/abs/gr-qc/0407073
Effective Hamiltonian for Isotropic Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0407069
Genericity of inflation in isotropic loop quantum cosmology
http://arxiv.org/abs/hep-th/0407072
Corrections to the Planck's radiation law from loop quantum gravity
I've been looking quickly the third article, and apart of a proposed modification of Planck's law, there's also a proposed modification for Wien's displacement law
this thread is serving as a surrogate sticky for useful loop-and-related quantum gravity links
I update it periodically.
for a concise and up-to-date survey of LQG and allied approaches see
John Baez talk at Dublin, given Tuesday 20July2004:
Loop Quantum Gravity, Quantum Geometry and Spin Foams
It is online at his website
http://math.ucr.edu/home/baez/lectures.html#lqg
For more QG developments in the first half of 2004 here is Baez post after the May 2004 marseille conference:
I just got back from the Marseille conference on loop quantum gravity and spin foams:
http://w3.lpm.univ-montp2.fr/~philippe/quantumgravitywebsite/
It was really great, so I devoted "week206" of my column This Week's Finds entirely to this conference:
http://math.ucr.edu/home/baez/week206.html
In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model ---something that nobody had succeeded in doing before!
http://www.arXiv.org/abs/hep-th/0404156
I hope this lays to rest certain rumors here that I'd burnt out on quantum gravity. :devil:
a key paper mentioned here
Ambjorn Jurkiewicz Loll
"Emergence of a 4D World from Causal Quantum Gravity"
http://www.arXiv.org/abs/hep-th/0404156
-----------------
for me, one of the most enlightening things that has come online recently is the slides from a talk Lee Smolin gave in Poland in February at the Winterschool WS-2004. This was a 10-day symposium and the topic this year was Quantum Gravity Phenomenology
there were talks by a dozen or so experts and they are online at the WS-2004 site. Smolin gave 3 lectures and it is the third I found especially interesting.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures and scroll down to Smolin's three.
-------------------
Roger Penrose's new book "The Road to Reality" just appeared at the bookstores. It is 1000 pages. Key ideas were presented at his public lecture "Fashion, Faith, Fantasy in Modern Physics" at Dublin last week. Also last October Penrose gave 3 evening lectures on these three themes at Princeton, they are online to listen, with sketchy video.
http://www.princeton.edu/WebMedia/lectures/
scroll down to October 2003 and find the three lectures by Penrose
-----------------------------
A number of papers have appeared recently connecting quantum gravity with extensions of Special Relativity. Examples are outgrowths of DSR such as "DDSR" or "TSR" (smolin's name: triply special relativity). Another aspect involves moffat's NGT an outgrowth of MOND
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
Moffat
Modified Gravitational Theory as an Alternative to Dark Energy and Dark Matter
http://arxiv.org/astro-ph/0403266
another interesting development, resolution of the "Black Hole Information Paradox" using relational time----a quantum mechanical clock rather than absolute ideal time
Gambini Porto Pullin
"Realistic clocks, universal decoherence and the black hole information paradox"
http://arxiv.org/abs/hep-th/0406260
also their earlier paper
“No black hole information puzzle in a relational universe,”
http://arxiv.org/hep-th/0405183.
this is a very incomplete listing of what has recently become available online, by way of Loop-and-related QG sources.
I really should mention Leonardo Modesto removing the Black Hole singularity before posting this.
http://arxiv.org/gr-qc/0407097
Disappearance of the Black Hole Singularity in Quantum Gravity
Also the popular series of 3 articles on LQG by Rudy Vaas (translated from German by Amitabha Sen and Martin Bojowald)
Beyond Space And Time
Ruediger Vaas
7 pages, English translation of "Jenseits von Raum und Zeit"
http://arxiv.org/physics/0401128
The Duel: Strings versus Loops
Ruediger Vaas
10 pages, English translation of "Das Duell: Strings gegen Schleifen"
http://arxiv.org/physics/0403112
The Inverted Big-Bang
Ruediger Vaas
8 pages, English translation of "Der umgestuelpte Urknall"
http://arxiv.org/physics/0407071
a more accurate translation of the title would be
"the turned-inside-out Big Bang" or "the everted Big Bang"
in Loop Quantum Cosmology the volume element gets
turned inside out at the moment of the quantum bounce
where there used to be a singularity
Should also include the recalculation of the Immirzi parameter by
Domagala, Lewandowski, and Meissner
"Black Hole Entropy from Quantum Geometry"
http://arxiv.org/gr-qc/0407051
"Black Hole Entropy in Loop Quantum Gravity"
http://arxiv.org/gr-qc/0407052
http://arxiv.org/abs/http://www.arxiv.org/abs/gr-qc/0408033
On the counting of black hole states in loop quantum gravity
Authors: Sergei Alexandrov
Comments: 4 pages
Report-no: SPIN-04/09, ITP-UU-04/15
We argue that counting black hole states in loop quantum gravity one should take into account only states with the minimal spin at the horizon
BTW, Alexandrov has posted a reply to Lubos Motl
http://www.physicsforums.com/showthread.php?t=39780&highlight=alexandrov
BTW, Alexandrov has posted a reply to Lubos Motl
http://www.physicsforums.com/showthread.php?t=39780&highlight=alexandrov
hee hee
Lubos, a Czech, has gotten himself embroiled with a Russian
and has already acheived the level of (East European equivalent to)
racial epithets. The Czechs have cause to remember the Soviet period
to which Lubos refers:
"It seems to me like a person who wants to get the result 1917 apples, but he gets, by an explicit counting, 1991. Well, the remaining 74 are anti-socialist macroapples and they should not be counted, should they? LM"
Meteor thanks so much for the link. Sergei is at Utrecht (where also that
Renate Loll is----simplicial model gravity) and I earlier got the impression of him that he is quite bright, if also somewhat of a maverick (or wild horse). the world is beautiful.
selfAdjoint
Aug21-04, 05:41 PM
It's one thing for a moderator or mentor to also be a discussant, as we all are here. It's another to use your technical moderator power to respond to somebody's post before they can respond to yours. It seems to me that LM misuses the moderator powers this way, at least in spirit.
It's one thing for a moderator or mentor to also be a discussant, as we all are here. It's another to use your technical moderator power to respond to somebody's post before they can respond to yours. It seems to me that LM misuses the moderator powers this way, at least in spirit.
and his spirit is mean. we know this. Lubos is a viper.
however he is extraordinarily charming and we must cherish him
as we do poisonous flowers
to me, that crack about the 74 anti-socialist macro-apples that should not be counted (well almost) makes up for all the times he has unfairly insulted John Baez (tho JB might not think so)
oh, I see better now. You mean [i]as a moderator[/b]
all right to be nettlesome in private, but must be the unbiased moderator
you are right
it does demean the office of moderator to be so biased and
to take such advantage.
but after all it is only SPS.
it would drive me crazy if I had to cope with something like that at PF
but the PF moderators (yrslf incl.) are pretty OK thank goodness
Forget lubos's personality. His posts are full of valuable insights that are difficult to come by for most people here. Anyway, he never commits the crime of pretending to be something he's not, which is more than I can say for some of the people here.
I guess we can make allowances for some personalites, hey Jeff? :smile: We make them for you? :rofl:
Self Adjoint made a interesting point about comments at the same time as the posting. It would appear to me that as a moderator the content is being look at, which I guess is the moderators job, but why not after its posted.
As to our opinons of Lubos's Knowkedge, I do not think anyone is questioning that.
Smolin and those involved in Loop were well aware of what was happening in strings?. It seems that they go hand in hand, and for Smolin, critical summations of where he had been might not of answered, where he would like to go.
I am not putting words in his mouth just pointing out the excellents papers that have come and summation of those other areas of research of theoretical positions.
Susskind and Smolin rebuttal towards each other, is very interesting to me. This kind of particpation under the Edge banner by John Brock request for letters, helps people decide what is going on.
New paper to consideration:
http://arxiv.org/abs/gr-qc/0408094
Time dependence in Quantum Gravity
Authors: Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
Comments: 33 pages, 17 figures
Report-no: AEI-2004-065, IGPG-04/8-2
The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales.
Plus the follow-up of Alexandrov after some days of reflection
http://www.physicsforums.com/showthread.php?p=300078#post300078
Sean Carroll was asked a good series of questions about the testing of GR and possible alternatives to the theory, and posted his answers:
http://preposterousuniverse.blogspot.com/2004/08/testing-general-relativity.html
The American Physical Society is having a meeting of
the division of Particles and Fields at riverside
(this gives a picture of what High Energy Physics, or Particle Physics,
is doing these days)
http://dpf2004.ucr.edu/program.html
a fair number of the talks were on astronomy, cosmology, astrophysics topics, you can see by running down the list of plenary talks
Sean Carroll's overview of cosmology (theory) was today Tuesday 31Aug,
it is online and takes about 4 minutes to download
lots of graphs and other visual data
Finally, do you think that GR will ultimately prove to be wrong (or incomplete) at some level?
Yes. Everybody (in their right mind) does. GR is a classical theory, fundamentally inconsistent with the quantum world in which we live. At the very least we will have to find a quantum version of GR; more likely, we will have to find some more profound theory that is intrinsically quantum-mechanical and reduces to GR in the appropriate circumstances. If experiments reveal deviations from GR at even the classical level, so much the better.
http://preposterousuniverse.blogspot.com/2004/08/testing-general-relativity.html
DA..... you think? :laughing:
http://arxiv.org/abs/gr-qc/0409006
"Semiclassical Quantum Gravity: Statistics of Combinatorial Riemannian Geometries"
Or "How to use statistical geometry to quantify uncertainties"
http://arxiv.org/abs/gr-qc/0409020
Causal Loop Quantum Gravity and Cosmological Solutions
Authors: Ali Shojai, Fatimah Shojai
Comments: 16 pages, 4 figures
"We shall present here the causal interpretation of canonical quantum gravity in terms of new variables. Then we shall apply it to the minisuperspace of cosmology. A vacuum solution of quantum cosmology is obtained, and the Bohmian trajectory is investigated. At the end a coherent state with matter is considered in the cosmological model. "
Seems like a new interpretation of LQG in terms of Bohmian mechanics.
Hidden variables in LQG?
Florian Conrady
Free vacuum for loop quantum gravity
http://arxiv.org/abs/gr-qc/0409036
Olaf Dreyer, Fotini Markopoulou, Lee Smolin
Symmetry and entropy of black hole horizons
http://arxiv.org/abs/hep-th/0409056
Fotini Markopoulou, Lee Smolin
Gauge fixing in Causal Dynamical Triangulations
http://arxiv.org/abs/hep-th/0409057
Olaf Dreyer
Background Independent Quantum Field Theory and the Cosmological Constant Problem
http://arxiv.org/hep-th/0409048
http://arxiv.org/abs/gr-qc/0409045
I've read a bit of the paper, and in a nutshell: in gr-qc/9401028, there were a pair of guys trying to unify LQG with Yang-Mills theory, but there were difficulties. In this new paper, the authors say that they achieve this unification in the Euclidean signature
Daniele Colosi, Carlo Rovelli
Global particles, local particles
http://arxiv.org/abs/gr-qc/0409054
brief sample from conclusions section:
---quote---
...the distinction between global and local states can therefore be safely neglected in concrete utilizations of QFT. However, the distinction is conceptually important because it bears on three related issues: (i) whether particles are local or global objects in conventional QFT; (ii) the extent to which the quantum field theoretical notion of particle can be extended to general contexts where gravity cannot be neglected; and furthermore, more in general, (iii) whether particles can be viewed as the fundamental reality (the “ontology”) described by QFT. Let us discuss these three issues separately. ...
...Can we base the ontology of QFT on local particles? Yes, but local particle states are very different from global particle states. Global particle states such as the Fock particle states are defined once and for all in the theory, while each finite size detector defines its own bunch of local particle states. Since in general the energy operators of different detectors do not commute ([HR, HR'] nonzero) there is no unique “local particle basis” in the state space of the theory, as there is a unique Fock basis. Therefore, we cannot interpret QFT by giving a single list of objects represented by a unique list of states. In other words, we are in a genuine quantum mechanical situation in which distinct particle numbers are complementary observables. Different bases that diagonalize different HR operators have equal footing. Whether a particle exists or not depends on what I decide to measure. In such a context, there is no reason to select an observable as “more real” than the others.
The world is far more subtle than a bunch of particles that interact.
---end quote---
selfAdjoint
Sep15-04, 11:14 AM
Apropos of this Colosi-Rovelli attempt to generalize particles, see today's post on Peter Woit's Not Even Wrong about Grothendiek and his toposes. He generalized the Nullstellenstatz view, which you have explicated so clearly, Marcus, in which the points of a continuum are represented as the prime ideals of the algebra of continuous functions on it. G. represents points of a space X as sheafs over X; a sheaf is a kind of category, and this leads to G.'s definition of topos, which we have had some discussion about in connection with Chris Isham's papers. G. was looking to define the "group of a point", and he actually reached a good definition.
Much of the perceived beauty of string theory is actually the beauty of G. and his generation's work in topology and algebraic geometry, which people like Witten have scarfed up and instantiated in physical models, orbifolds, for example. See the survey of this work by Jacques Cartier which Woit gives a link to.
I am glad to see the web of connections to this Rovelli-Colosi paper
extended in such a distinguished way.
On another matter, it has always been an impediment and a concern that there is no introductory textbook for LQG.
Alejandro Perez posted this, just today:
Introduction to Loop Quantum Gravity and Spin Foams
http://arxiv.org/abs/gr-qc/0409061
selfAdjoint
Sep16-04, 08:24 AM
Yes I saw Perez's paper. I have only just glanced at it but I am glad to see that he doesn't stint on discussing the quantization problems.
meteor noticed this short paper by Gambini, Olson, Pullin
http://arxiv.org/abs/gr-qc/0409045
It was posted 10September but didn't get noted at the time.
Unified model of loop quantum gravity and matter
4 pages, dedicated to Michael P. Ryan on the occasion of his sixtieth birthday
"We reconsider the unified model of gravitation and Yang--Mills interactions proposed by Chakraborty and Peldán, in the light of recent formal developments in loop quantum gravity. In particular, we show that one can promote the Hamiltonian constraint of the unified model to a well defined anomaly-free quantum operator using the techniques introduced by Thiemann, at least for the Euclidean theory. The Lorentzian version of the model can be consistently constructed, but at the moment appears to yield a correct weak field theory only under restrictive assumptions, and its quantization appears problematic."
wolram found this
http://jdc.math.uwo.ca/spin-foams/
it is a great QG resource
Franz Hinterleitner
Canonical DSR
http://arxiv.org/gr-qc/0409087
"For a certain example of a "doubly special relativity theory" the modified space-time Lorentz transformations are obtained from momentum space transformations by using canonical methods. In the sequel an energy-momentum dependent space-time metric is constructed, which is essentially invariant under the modified Lorentz transformations. By associating such a metric to every Planck volume in space and the energy-momentum contained in it, a solution of the problem of macroscopic bodies in doubly special relativity is suggested."
may have a solution to the "Soccer Ball" problem mentioned in several recent papers on multi-special relativity (links in this thread to papers on DSR by Smolin, Kowalski-Glikman, Livine, Girelli, Oriti etc. mentioning this problem of the momentum of macroscopic bodies sometimes called soccer ball problem) I have to go but will get back to this and start a separate thread about this paper if it looks to me like Hinterleitner has made some headway with this.
nonunitary
Oct8-04, 06:44 PM
Hi,
There are two upcoming activities related to loop quantum gravity:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/index.php
and
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Hi,
There are two upcoming activities related to loop quantum gravity:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/index.php
and
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Thanks nonunitary! I see there's an interesting lineup of talks at the "Quantum Gravity in the Americas" conference, reflecting people's current research:
---quote from the program---
Workshop on Quantum Gravity in the Americas: Status and future directions
October 29 - 31, 2004
Brunnemann: Volume Operator and Recoupling Theory
Chen: Quantum Liouville Theory and Black Hole
Conrady: Vacuum State for LQG
Dittrich: Status of the Master Constraint Programme
Henson: Consequences of space-time discreteness on wave propagation
Manrique: On the macroscopic limit of vacuum compact QED
Meusburger: Phase space quantization of 2+1 gravity in Chern-Simons formulation
Perrini: Asymptotic safety for quantum gravity
Reyes: Higgs propagation in loop quantum geometry
Singh: Phenomenological aspects of LQC
Terno: Entropy and Entanglement for LQG Black Holes
Willis: Some Obstructions to Spin Networks for Non-Compact Gauge
*
Alfaro: Loop Quantum Gravity and High Energy Cosmic Rays
Bombelli: Coherent and semiclassical states for systems
Corichi: About Semi-Classical Quantum Gravity
Dowker: The `Problem of Time’ in a Sum-Over-Histories framework?
Freidel: Particles in 3d quantum gravity
Husain: Black Hole geometrodynamics revisited
Lewandowski: Black hole entropy
Major: Quantum Geometry Phenomenology: A Discrete Machian Model
Markopoulou: Locality in Quantum Gravity
Morales-Tecotl: Possible phenomenological limits for semi-classical LQG
Oeckl: General boundaries and transition amplitudes in QG
Oriti: Feynman propagator in spin foam QG: causality without time
Perez: Dynamics and Spin Foams in non perturbative QG
Pullin: Semi-discrete solution to the dynamics of LQG
Reisenberger: Canonical GR on null hypersurfaces
Sahlmann: String Theory with LQG methods
Smolin: Physics from Loop Quantum Gravity
Sudarski: Space-time granularity and Lorentz Invariance Violation
Urrutia: Synchrotron radiation in Myers-Pospelov electrodynamics
Winkler: Particles, cosmology and spinfoams
Zapata: Coarse graining in loop quantization
Ashtekar: Quantum geometry and black holes
Baez: Spin foams, 2-vector spaces and categorification
Sorkin: Is a past finite order the inner basis of space-time?
LIST OF POSTERS
Cambiaso: Lorentz symmetry violations: Constraints from ultra-high energy astronomy
Carrion: Wilson loop dynamics without regularization
Cortez: On nonequivalence of representations in QFT
Garcia-Islas
Giesel
Montesinos: Covariant Hamiltonian dynamics
Noui: About the Plebanski action
Vandersloot: A model of loop quantum cosmology and its physical Hilbert space
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
Stanford SLAC Library collaborates with the German organization DESY
on the Spires HEP database which has a lot of physics papers available for keyword search. They have sharpened the focus on Quantum Gravity at Spires. You can see this by comparing how the search engine works for 2003 and 2004.
the main URL is
http://www.slac.stanford.edu/spires/hep/
Quantum Gravity is one of the Spires keyword options, and as of now if you try it for 2004 you get 132 papers. It is not that this is so many, quantity-wise, what impresses me is the quality of the pick (especially as compared with past years)
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+k+quantum+gravity+and+date+2004&SKIP=0
this gets all the papers on Quantum Gravity that appeared this year that they have cataloged as QG so far. It looks to me to be remarkably complete. the representation of LQG and allied approaches to QG is near total and a real improvement over the same search for 2002 or 2003
The above URL is what you get when you type this into the search engine box:
FIND K QUANTUM GRAVITY AND DATE 2004
K means "keyword", it is fairly self-explanatory and the "help" section is helpful. Spires has other modes of search as well, like by author, and you have the option of filtering the search for highly cited papers
For example this will get any Quantum Gravity paper which appeared in 2003 and which has been cited by 50 or more other papers:
FIND K QUANTUM GRAVITY AND TOPCITE 50+ AND DATE 2003
The result is:
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+QUANTUM+GRAVITY+AND+TOPCITE+50%2 B+AND+DATE+2003
Another useful feature Spires has is a "top 100" papers list for each year, that ranks papers according to number of citations received.
http://www.slac.stanford.edu/library/topcites/
BTW has anyone besides me listened to Michael Peskin's 8 October talk at Kavli. He was on the panel that came after talks by Steven Weinberg and Frank Wilczek
http://online.itp.ucsb.edu/online/kitp25/
His perspective was in sharp contrast to Weinberg's, as Peskin himself pointed out. Peskin does the annual Spires HEP review.
It is always hard to find an article about Quantum Gravity written for general audience, that is online.
Smolin's January 2004 Scientific American article was clear and accessible (really good i thought) but not online---required a trip to the library, or for you to have a subscription.
this one by Rovelli is not only good and written for general audience, but free for downloading. It prints out to 5 pages and even mention's Renate Loll's work (computer simulations of evolving geometry, with graphic output). I guess Abhay Ashtekar liked it so he scanned it and put it at his website :smile: for educational use.
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
This thread is serving as a surrogate sticky (which I update periodically) for links to useful sources about Loop-and-related quantum gravity research.
the last update was back on page 15 around post #217 and another pass is overdue.
Other people's collections of links:
wolram found this
http://jdc.math.uwo.ca/spin-foams/
Introductions, surveys, books:
Abhay Ashtekar
Gravity and the Quantum
http://arxiv.org/gr-qc/0410054
Carlo Rovelli's book
Quantum Gravity
http://www.cpt.univ-mrs.fr/~rovelli/rovelli.html
contains introduction, historical and philosophical perspective as well as technical stuff.
Best general audience article available online is probably
Rovelli's November 2003 "Physics World" article which Astekar has
at his website. The title was Loop Quantum Gravity
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
Another good general audience article (but not available online) is
Lee Smolin's Atoms of Space and Time in the January 2004 issue
of the "Scientific American".
Probably the best introduction for physics students and physicists who are not specialists in QG is by Lee Smolin
An Invitation to Loop Quantum Gravity
http://arxiv.org/hep-th/0408048
50-page survey and intro with FAQ for physicists in other fields who want to switch fields and do QG research. Intended for the annual Reviews of Modern Physics. Contains list of unsolved problems to work on. List of main results so far.
Alejandro Perez
Introduction to Loop Quantum Gravity and Spin Foams
http://arxiv.org/abs/gr-qc/0409061
John Baez gave an introduction and survey at the Dublin GR17 conference, which is available online at Baez site.
"Loop Quantum Gravity, Quantum Geometry and Spin Foams"
http://math.ucr.edu/home/baez/lectures.html#lqg
--------
Ways to find out what's currently happening:
Conferences:
the 29,30,31 October conferences at Perimeter
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
Search engines:
SPIRES search engine
http://www.slac.stanford.edu/spires/hep/
http://www.slac.stanford.edu/library/topcites/
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+k+quantum+gravity+and+date+2004&SKIP=0
The above URL is what you get when you type this into the search engine box:
FIND K QUANTUM GRAVITY AND DATE 2004
K means "keyword", the "help" section is helpful.
ARXIV search engine
2001:
http://lanl.arxiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2001/0/1
2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2002/0/1
2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/2003/0/1
Last Twelve Months:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+doubly+special/0/1/0/past/0/1
John Baez TWF, eg. his TWF #206 reports on the Marseille conference
http://math.ucr.edu/home/baez/week206.html
"In particular, I spend a lot of time giving a very simple nontechnical introduction to the recent work of Ambjorn, Jurkiewicz and Loll in which they seem to get a 4d spacetime to emerge from a discrete quantum model - something that nobody had succeeded in doing before!"
the paper mentioned here is
Ambjorn Jurkiewicz Loll
Emergence of a 4D World from Causal Quantum Gravity
http://www.arXiv.org/abs/hep-th/0404156
Wave's_Hand_Particle just called attention to the fact that Smolin has
co-authored a followup to this paper
Fotini Markopoulou, Lee Smolin
Gauge fixing in Causal Dynamical Triangulations
http://arxiv.org/hep-th/0409057
PAGE TWO (look back one post for the beginning)
NEWSLETTER: the APS Gravity newsletter
Jorge Pullin's Matters of Gravity
http://arxiv.org./abs/gr-qc/0403051
this is the Spring 2004 issue
the Fall 2004 issue is
http://arxiv.org/abs/gr-qc/0409046
==========
TEXTS:
Besides Rovelli's book there is a recent monograph by Ashtekar and Lewandowski
Background Independent Quantum Gravity: A Status Report
http://arxiv.org/gr-qc/0404018
and Thiemann's Lecture Notes
(they have been published in by Springer Verlag, Berlin)
Lectures on Loop Quantum Gravity
available online
http://arxiv.org/gr-qc/0210094
Rovelli's LivingReviews article
http://arxiv.org/abs/gr-qc/9710008
Rovelli and Upadhya "Primer"
http://arxiv.org/abs/gr-qc/9806079
Rovelli and Gaul lecture notes from WS-1999
http://arxiv.org/abs/gr-qc/9910079
========
SURVEY ARTICLES:
A survey of the whole field of approaches to QG
which is interesting partly for historical and broader perspective is
Carlo Rovelli's
Strings, loops and others: a critical survey of the present approaches to quantum gravity
http://arxiv.org/gr-qc/9803024
" I illustrate the main achievements and the main difficulties in: string theory, loop quantum gravity, discrete quantum gravity (Regge calculus, dynamical triangulations and simplicial models), Euclidean quantum gravity, perturbative quantum gravity, quantum field theory on curved spacetime, noncommutative geometry, null surfaces, topological quantum field theories and spin foam models...."
another broad survey
Enrique Alvarez
http://arxiv.org/gr-qc/0405107
Quantum Gravity
( Lectures given at Karpacz. 40 pages)
==============
QG PHENOMENOLOGY:
Efforts and proposals to test QG are of increasing importance.
QG testability (Phenomenology) was the topic of the Winterschool-2004 (WS-2004) symposium this year, 4-14February, at Karpacz.
http://ws2004.ift.uni.wroc.pl/html.html
click on lectures if you want slides from the various talks.
Many of the talks are now written up as journal articles and available
on arxiv----more complete and faster download.
Cambridge Uni Press is publishing "Universe or Multiverse" which will contain a chapter by Smolin called "Scientific Alternatives to the Anthropic Principle" where he offers an evolutionary Multiverse hypothesis that generates testable (numerical) predictions.
http://arxiv.org/hep-th/0407213
Giovanni Amelino-Camelia
Planck-scale Lorentz-symmetry test theories
http://arxiv.org/abs/astro-ph/0410076
====================
EXTENSIONS OF SPECIAL RELATIVITY
Much QG phenomenology focuses on modifications of Lorentz symmentry---connecting quantum gravity with DSR namely "DDSR" or "TSR" ( triply special relativity)
Kowalski-Glikman, Smolin
"Triply Special Relativity"
http://arxiv.org/abs/hep-th/0406276
Girelli Livine Oriti
"Deformed Special Relativity as an effective flat limit of quantum gravity"
http://arxiv.org/gr-qc/0406100
======================
LOOP QUANTUM COSMOLOGY (only a few of the many papers)
meteor recently flagged this one
Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
Time dependence in Quantum Gravity
http://arxiv.org/abs/gr-qc/0408094
This recent paper has an extensive bibliography with many arxiv links, so
I will refer to that instead of posting them.
Here is the abstract:
"The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales."
Martin Bojowald
Loop Quantum Cosmology: Recent Progress
http://arxiv.org/gr-qc/0402053
Martin Bojowald
Quantum Gravity and the Big Bang
http://arxiv.org./astro-ph/0309478
Shinji Tsujikawa, Parampreet Singh, Roy Maartens
Loop quantum gravity effects on inflation and the CMB
http://arxiv.org/astro-ph/0311015
from their abstract:
"In loop quantum cosmology, the universe avoids a big bang singularity and undergoes an early kinetic-dominated super-inflation phase, with a quantum-corrected Friedmann equation. As a result, an inflaton field is driven up its potential hill, thus setting the initial conditions for standard inflation. We show that this effect can raise the inflaton high enough to achieve sufficient e-foldings in the standard inflation era. We analyze the cosmological perturbations and show that loop quantum effects can leave a signature on the largest scales in the CMB, with some loss of power and running of the spectral index."
Viqar Husain and Oliver Winkler
On singularity resolution in quantum gravity
http://arxiv.org/gr-qc/0312094
this is especially interesting because they duplicate LQC results (for example by Bojowald) using an older version of quantum gravity, ADM variables, quantized metric. Shows that the removal of the big bang singularity doesnt depend on using a particular formalism.
PAGE THREE OF THE LINKS
Stingray told us about this talk by Ashtekar
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
The talk was given 20 September at Penn State and is called
Black Hole Evaporation and Information Loss: Recent Advances
As you listen to the audio you have to step from one slide to the next
in synch with the talk. In this talk you get a foretaste of two papers by Ashtekar and Bojowald which are not on arxiv yet.
Ashtekar refers to the papers in
http://arxiv.org/abs/gr-qc/0410054
They are:
Ashtekar A and Bojowald M 2004 Non-Singular Quantum Geometry of the Schwarzschild Black Hole Interior Preprint
Ashtekar A and Bojowald M 2004 Black hole evaporation: A paradigm Preprint
nonunitary provided a link to an article giving the definitions of dynamical horizon and isolated horizon. Ashtekar uses these concepts in his talk
Abhay Ashtekar, Badri Krishnan
Isolated and dynamical horizons and their applications
http://arxiv.org/gr-qc/0407042
other papers of interest
Gambini Porto Pullin
Realistic clocks, universal decoherence and the black hole information paradox
http://arxiv.org/abs/hep-th/0406260
also their earlier
No black hole information puzzle in a relational universe
http://arxiv.org/hep-th/0405183
==========================
Wave's_Hand_Particle
Oct21-04, 01:46 AM
Marcus, it may be of interest if you could collate papers that are pertaining to each other, for intstance these appeared a couple of days ago:http://uk.arxiv.org/abs/hep-th/0409057
http://uk.arxiv.org/abs/hep-th/0409056
This is a reply from Smolin-Dreyer-Markopoulou, to the Loll-Ambjorn paper?
Marcus, it may be of interest if you could collate papers that are pertaining to each other, for intstance these appeared a couple of days ago:http://uk.arxiv.org/abs/hep-th/0409057
http://uk.arxiv.org/abs/hep-th/0409056
This is a reply from Smolin-Dreyer-Markopoulou, to the Loll-Ambjorn paper?
Thanks for pointing this out WHP. After I read your post I went back and edited it into post #245, that is 3 or 4 back from here.
The Smolin-Markopoulou hep-th/0409057 is definitely a response to the Ambjorn-Jurkiewicz-Loll paper. it supports the idea that the AJL technique does not single out any particular time coordinate (which would be a weakness in their approach). So it is a friendly corroboration that AJL looks like it is on the right track. However more needs to be done in that direction, as they note. If you see more dynamical triangulations papers please flag them. I'm looking forward to seeing some appear in the next few months.
selfAdjoint
Oct21-04, 10:58 AM
From hep-th/0409057:
It appears that, while other features of sum-over-histories quantum gravity, such as locality or fundamental discreteness, appear desirable, they are not shared by Causal Dynamical Triangulations. As it is the latter that possesses the good low-energy behavior, this implies either that these desirable features are misguided, or that our previous expectations for the geometrical interpretation of the path-integral histories as the microscopic description of spacetime were naive.
I am coming to believe that all the present day theories, as viewed from say 2040, will be seen to be infected with galloping naivite. Mathematical sophistication without the deep insight that is going to be discovered one of these days.
From hep-th/0409057:
I am coming to believe that all the present day theories, as viewed from say 2040, will be seen to be infected with galloping naivite. Mathematical sophistication without the deep insight that is going to be discovered one of these days.
Could this more fundamental physical principle be a deeper insight into entropy? Could entropy and information have a more direct connection to forms of logic? Certainly, inductive logic considers the probabilities of events, and so does quantum mechanics. So could there be rules of entropy applied to inductive logic that might also give rise to QFT? I wonder. If so, then perhaps the laws of physics and be derived from logic.
selfAdjoint
Oct21-04, 05:53 PM
No I don't think it's entropy. I don't think it's anything that's obvious, because hundreds of physicists are searching, and everything obvious has been tried or soon will be. I think there's a missing piece of mathematics. When I was in grad school, there wasn't any such thing as K-theory; then it was developed, and within a few years Witten was using it to refine string theory. Unlike the situation when I was growing up, the advance of physics is closely tied to the advance of mathematics.
Dan Christensen, Louis Crane
Causal sites as quantum geometry
20 pages, 3 figures
http://arxiv.org/abs/gr-qc/0410104
A seminar talk at Penn State by Martin Bojowald
http://www.phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
Quantum Cosmology: An Overview
27 January 2003
audio with slides.
more informative in some ways than journal articles
e.g. more computer-graphic images.
The first slide is here:
http://www.phys.psu.edu/events/display.html?event_id=516&file=0
No I don't think it's entropy. I don't think it's anything that's obvious, because hundreds of physicists are searching, and everything obvious has been tried or soon will be. I think there's a missing piece of mathematics. When I was in grad school, there wasn't any such thing as K-theory; then it was developed, and within a few years Witten was using it to refine string theory. Unlike the situation when I was growing up, the advance of physics is closely tied to the advance of mathematics.
I appreciate your input as always, but I don't know, guy. We are talking about the emergence of structure amoung alternative possibilities. I thing entropy is inherent in those considerations. To date I don't think that entropy has been applied to the structure of particle/strings or the structure of spacetime itself. But if we imagine the very first possible structure in the universe, you might think that there is entropy involved with any structure.
As I understant it, we don't know the reason that QM is as it is. We are still looking for fundamental principles that give rise to the formulism of QM. I have to wonder if that might not be some sort of conservation of entropy or at least a restriction of it rate. I consider for example whether the amplitude/phase of QM might derive from the necessity of alternatives when each alternative has some structure. The structure would be neg-entropy, and the increase of alternative would be entropy, thus QM. Or something like that.
I mean, the mere fact that we are talking about various kinds of structures in the universe and how they interact makes probabilities and entropy relevant. And since we are considering all the possibilities in our investigations, I think it may be inevitable that the final formalism must relate how all possibilities interact/interfere with each other to product the final result. We are now looking for why QM gives us the way that these possibilities interact/interfere.
Hi Mike, do you think you could start a thread on this so we can keep focused here on collecting links to source material for Loop and allied approaches to QG?
Dan Christensen, Louis Crane
Causal sites as quantum geometry
20 pages, 3 figures
http://arxiv.org/abs/gr-qc/0410104
...
ordinary (loop) QG is done on a set of points called a differentiable manifold-----a continuum---analog of ordinary 3D space but without
a precommittment to some particular geomety---a floppy continuum
that was the basis for classic 1915 GR too.
Now Christensen and Crane want to replace the diff-manif.
they want to get rid of the point set continuum and replace it with a new mathematical arena called a Site.
Grothendieck made up Sites. A site is a category with a "Grothendieck topolopy"
you consider your old pointset topological space and you notice that the subsets A of X form a partially ordered-by-inclusion structure and you abstract this notion. Now you have a bunch of "subsets" but they dont have points they are just abstract entities with an ordering relation (taken from the old "order-by-inclusion")
that's not all, these things (A, B,....) are also ordered by causality. One of them can precede another, sometimes.
Grotend. made up a topology to put on this kind of thing, and various
superstructure---presheaf, gerbe, bundle, gadgetry---which he and his friends always enjoyed doing.
Along come christensen crane and notice it would be a neat thing to do QG on instead of doing it on a manifold.
Einstein always said that the points of the manifold had no physical existence. So maybe christensen crane are purifying. and sometimes
when you purify it is like throwing overboard the balast and the ship or balloon can get off the ground.
so i want to call attention to this paper. it has the beginnings of a new approach. mostlikely one that will fail!!! of course. that is the game. one must try anyway. good luck to them.
[edit: it might succeed too, might be a really good idea---can't tell at this point]
you consider your old pointset topological space and you notice that the subsets A of X form a partially ordered-by-inclusion structure and you abstract this notion. Now you have a bunch of "subsets" but they dont have points they are just abstract entities with an ordering relation (taken from the old "order-by-inclusion")
Is it possible that the sets of a causal site can be shrunk down to points? Are causal site a generalization of point sets?
Is it possible that the sets of a causal site can be shrunk down to points? Are causal site a generalization of point sets?
I started a thread about the christensen crane paper, in case there is some interest in discussing it. QG-wise it's really new mathematics, could have potential. Anyway I think it rates a thread.
Let's discuss it there, if you want, and not load this link-basket thread with discussion
Here's a picture of Martin Bojowald
from the May 2004 conference at Marseille
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Martin is the guy in the gray T-shirt and black frames, not looking at the camera.
Turns out he gave two seminar talks at Penn State last year, both on Quantum Cosmology.
To get recorded seminar talks at Penn State you go here:
http://phys.psu.edu/events/
and select whatever semester.
In "spring 2003" there is a long list that includes
Bojowald's talks Quantum Cosmology: An Overview
and Quantum Cosmology: Formalism
the links for the slides and audio for these two are
http://phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
http://phys.psu.edu/events/index.html?event_id=521;event_type_ids=0;span=2002-12-26.2003-05-31
Ashtekar's recent talk is part of the same collection
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
If I could, i would edit this into the DSR section of a page of links a few posts back, but those pages are closed to further editing, so i will just stick it on here:
Liberati, Sonego, Visser
Interpreting doubly special relativity as a modified theory of measurement
http://arxiv.org/gr-qc/0410113
Madhavan Varadarajan
The graviton vacuum as a distributional state in kinematic Loop Quantum Gravity
44 pages
http://arxiv.org/abs/gr-qc/0410120
---quote---
Abstract:
The quantum behaviour of weak gravitational fields admits an adequate, albeit approximate, description by those graviton states in which the expectation values and fluctuations of the linearised gravitational field are small. Such states must approximate corresponding states in full quantum gravity. We analyse the nature of this approximation for the graviton vacuum state in the context of kinematical Loop Quantum Gravity (LQG) wherein the constraints are ignored. We identify the graviton vacuum state with kinematically non-normalizable, distributional states in LQG by demanding that relations between linearised operator actions on the former are mirrored by those of their non-linear counterparts on the latter. We define a semi- norm on the space of kinematical distributions and show that the identification is approximate upto distributions which are small in this semi-norm. We argue that our candidate states are annihilated by the linearised constraints (expressed as operators in the full theory) to leading order in the parameter characterising the approximation. This suggests the possibility, in a scheme such as ours, of solving the full constraints order by order in this parameter. The main drawback of our considerations is that they depend on certain auxilliary constructions which, though mathematically well defined, do not arise from physical insight. Our work is an attempt to implement an earlier proposal of Iwasaki and Rovelli.
I. Introduction:
A Dirac constraint quantization of a Hamiltonian formulation of gravity is defined through the following steps. First, a “kinematical” representation of the Poisson bracket algebra of a large enough set of functions on the unconstrained phase space is constructed such that these functions act as linear operators on the representation space. Next, the constraints of the theory are represented as quantum operators in this representation and physical states are identified with their kernel. Finally, an inner product on the space of physical states is chosen which enforces hermiticity conditions on a complete set of operators corresponding to Dirac observables, thus converting the physical state space to a Hilbert space...
---end quote---
Varadarajan's 19 archived papers go back to 1993. He has co-authored with Abhay Ashtekar, and also at various times both with Rafael Sorkin, and with Fernando Barbero.
Here are the two Iwasaki-Rovelli papers he says were his point of departure
J. Iwasaki and C. Rovelli, Int.J.Mod.Phys.D1, 533 (1993);
J. Iwasaki and C. Rovelli, Class.Quant.Grav.11, 1653 (1994).
Since the graviton is supposed to be quantized gravity which is also supposed to be quantized geometry, would gravitons exist everywhere and constitute the construction of spacetime itself? This would be opposed to gravition moving through space. Instead, are graviton space itself, always motionless with respect to the observer?
... would gravitons exist everywhere and constitute the construction of spacetime itself?
I believe not, Mike. That is, this paper does not indicate that gravitons are real----rather that they are a mathematical fiction which plays a part in a possibly useful approximation. Analysis of the gravitational field using gravitons would be appropriate, as this paper suggests, in the weak field case.
We should start a thread about ontology. You seem to me to have an abiding interest in ontological questions---what is really there, what is reality made of. I would say, in answer to such questions, "the field is the field."
---here is a short essay that could serve to start an ontology thread---
To say it in a lot more words: the field is the field and it is not made of anything more basic---it is not made of gravitons or anything else, it is really there. It may or may not sometimes be useful (as an approximation) to describe it in some special case in terms of the mathematical device of postulated gravitons and then it may or may not be helpful to talk among ourselves about "gravitons". But that is an overlay of mathematics, it is not ontological. Ontologically speaking there is nothing more basic than the field.
The field is not something defined "in space and time" because space and time arise from the field. space and time are appearances or phenomena which emerge from the field. We must arrive at ways of describing the field which do not depend on imagining a prior space and time in which to define it.
The field is a quantum animal, living in a Hilbertspace of all possible fields. The central problem in physics today is to find a satisfactory formal way of to describe the gravitational field, and the Hilbertspace of its possibilities.
When and if that is found, everything else will be describable relative to, and on top of, the field.
----let's continue this in a separate thread, so as not to choke the linkbasket---
Viqar Husain and Oliver Winkler
Quantum resolution of black hole singularities
http://arxiv.org/abs/gr-qc/0410125
this can be seen as continuation of their work in 2003 where they resolved the big bang singularity (following Bojowald lead but by their own methods)
Viqar Husain and Oliver Winkler
On singularity resolution in quantum gravity
http://arxiv.org/gr-qc/0312094
(they duplicated Bojowald LQC results using an older version of quantum gravity, ADM variables, quantized metric, showing that the removal of the big bang singularity doesnt depend on using a particular formalism)
the history is like this: in 2001 Martin Bojowald used Loop QG to resolve the big bang singularity---his landmark paper is Absence of singularity in Loop Quantum Cosmology
in the intervening time people have repeatedly confirmed his result using variations of LQG, it has been shown not to depend on the details but to be a basic thing. when you quantize Einstein 1915 Gen Rel, then you get a bounce at the beginning.
and Hussain and Winkler used their QG methods to corroborate this.
Now everybody was wondering "What about black hole?" does the black hole singularity also go away when you quantize? Bojowald and Ashtekar have a paper about this, not yet posted on arxiv.
And already a Rovelli postdoc named Leonardo Modesto has published, getting rid of the BH singularity
And now Hussain and Winkler too!
Although Bojowald and Ashtekar have not yet posted their resolution of BH singularity, Ashtekar gave a talk relating to it last month at Penn State.
Ashtekar's 20 September talk:
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
You can see his slides and listen to audio. what they think is at the heart of BH, at the former singularity, is shown around slide #12 and #13
======
I mentioned this earlier, but will repeat incase anyone missed it: a picture of Martin Bojowald from the May 2004 conference at Marseille
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Martin is the one not looking at the camera.
Laurent Freidel, David Louapre
Ponzano-Regge model revisited II: Equivalence with Chern-Simons
http://arxiv.org/abs/gr-qc/0410141
Laurent Freidel is interested in putting matter into 3d gravity----the 2+1 toy model that has proven easier to develop than the full 3+1 theory and which people look to for hints of how to proceed in building 4d quantum gravity.
He is giving a talk Saturday 29 October, on Symmetry and particles in 3D quantum gravity.
This paper may have some bearing on the talk. Here is the abstract:
---quote---
We provide a mathematical definition of the gauge fixed Ponzano-Regge model showing that it gives a measure on the space of flat connections whose volume is well defined. We then show that the Ponzano-Regge model can be equivalently expressed as Reshetikhin-Turaev evaluation of a colored chain mail link based on D(SU(2)): a non compact quantum group being the Drinfeld double of SU(2) and a deformation of the Poincare algebra. This proves the equivalence between spin foam quantization and Chern-Simons quantization of three dimensional gravity without cosmological constant. We extend this correspondence to the computation of expectation value of physical observables and insertion of particles.
---end quote---
Also just out today:
Daniele Oriti
The Feynman propagator for spin foam quantum gravity
http://arxiv.org/abs/gr-qc/0410134
---quote---
We link the notion causality with the orientation of the 2-complex on which spin foam models are based. We show that all current spin foam models are orientation-independent, pointing out the mathematical structure behind this independence. Using the technology of evolution kernels for quantum fields/particles on Lie groups/homogeneous spaces, we construct a generalised version of spin foam models, introducing an extra proper time variable and prove that different ranges of integration for this variable lead to different classes of spin foam models: the usual ones, interpreted as the quantum gravity analogue of the Hadamard function of QFT or as a covariant definition of the inner product between quantum gravity states; and a new class of causal models, corresponding to the quantum gravity analogue of the Feynman propagator in QFT, non-trivial function of the orientation data, and implying a notion of ''timeless ordering''.
---end quote---
Freidel and Oriti are scheduled to give talks day after tomorrow at the PI conference
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/program2.php
Saturday 29 October, Oriti 2PM and Freidel 3PM
the titles of the talks shed a bit of extra light on the topics of these papers
Saturday at 14:00
Oriti: Feynman propagator in spin foam quantum gravity: causality without time
Saturday at 15:00
Freidel: Symmetry and Particles in 3d quantum gravity
(the phrase "causality without time" is suggestive, since time may be something that emerges from the theory rather than being one of the things on which it is built, it would be nice to have a way of representing the causal ordering of events without a notion of clock or time axis)
John Baez gives a talk tomorrow morning (Friday 10AM) at the PI conference
http://math.ucr.edu/home/baez/lectures.html#dynamics
click on PDF to get the slides----lecture notes, rather more complete than slides usually are.
The Problem of Dynamics in Quantum Gravity
delineates and highlights major problems in both loop and foam approaches. attempts to set attainable goals, to guide gradual progress,
without minimizing the difficulties.
thoughtful of Baez to post his lecture notes the day before he gives the talk, often you have to wait for the online stuff until days or weeks after
Wave's_Hand_Particle
Oct29-04, 04:34 AM
Has anyone had problems with the Perimiter Seminar/Lectures links?..I have tried every single link, and not a single lecture can be viewed?
Has anyone had problems with the Perimiter Seminar/Lectures links?..I have tried every single link, and not a single lecture can be viewed?
WHP, I have listened to seminar talks online at Penn State and several other places, but never so far from Perimeter. I dont know about video.
I can't recall having seen any Perimeter talks that have even the audio available! Maybe I just havent tried.
this seems a shame. If anyone comes across audio and slides for some PI talks, please tell me. Maybe they plan to make such files available but havent gotten around to it yet.
a new Bojowald paper today:
Spherically Symmetric Quantum Horizons
Martin Bojowald, Rafal Swiderski
http://arxiv.org/abs/gr-qc/0410147
4 pages
Marc-Thierry Jaekel, Serge Reynaud
Gravity tests in the solar system and the Pioneer anomaly
http://arxiv.org/abs/gr-qc/0410148
4 page
---exerpt from Bojowald, Swiderski, conclusions section---
The isolated horizon framework provides an unambiguous condition which is local at the horizon. This makes it possible to impose the condition without full knowledge of physical solutions, which to our knowledge results in the first implementation of horizon conditions fully at the quantum level. It is this isolated horizon condition which leads to strong simplifications in the quantum Hamiltonian constraint exploited here.
Our results verify some of the earlier expectations concerning fluctuating horizons and make them more detailed. Moreover, we can show that the horizon area is an approximate quantum observable in the sense that it commutes with the dominant contribution to the Hamiltonian constraint. These calculations test several aspects of the constraint operator, in particular those which did not play a role in homogeneous models [14, 16, 17]. As we have seen, going to the horizon simplifies the analysis of some aspects of quantum observables since a horizon is much easier to impose on quantum states than an asymptotic regime where one could test the ADM mass.
The framework introduced here allows, e.g., to answer questions related to black hole evaporation [1]. There are several new possibilities not yet studied when matter Hamiltonians are coupled: First, the horizon conditions need to be generalized to dynamical horizons ...
---end quote---
reference [1] for this paper is listed simply as
[1] A. Ashtekar and M. Bojowald, in preparation.
According to Hossain, LQG explains some features of the CMB which have been observed, and makes in-principle testable predictions which distinguish it from the standard inflationary scenario. Hossain was at Albert Einstein Institute for a while this year and I believe is now back at Chennai, in India.
Golam M. Hossain
Primordial Density Perturbation in Effective Loop Quantum Cosmology
http://arxiv.org/gr-qc/0411012
30 pages
Here are Hossain's papers
http://arxiv.org/find/gr-qc/1/au:+Hossain_G/0/1/0/all/0/1
It looks to me as if this is a snapshot of Hossain at the table with Bojowald at the 2004 Marseille conference. Resembles an older shot which I know is him.
http://perimeterinstitute.ca/images/marseille/marseille017.JPG
Hossain mentions something interesting on page 26. Inflation scenarios were devised largely as a way to cope with the "horizon problem" and the temp uniformity of the CMB has always been a big talkingpoint for inflation.
But Loop cosmology resolves the horizon problem by eliminating the singularity, as Hossain points out. It also provides for well-behaved inflation with less fine-tuning. Hossain refers back to a paper with Date about this, and gives some further discussion in this paper as well.
Here's a link to the Hossain/Date paper
“Genericity of inflation in isotropic loop quantum cosmology,”
http://arxiv.org/gr-qc/0407069
--quote from gr-qc/0411012 abstract--
It is widely believed that quantum field fluctuation in an inflating background creates the primeval seed perturbation which through subsequent evolution leads to the observed large scale structure of the universe. The standard inflationary scenario produces scale invariant power spectrum quite generically but it produces, unless fine tuned, too large amplitude for the primordial density perturbation than observed. Using similar techniques it is shown that loop quantum cosmology induced inflationary scenario can produce scale invariant power spectrum as well as small amplitude for the primordial density perturbation without fine tuning. Further its power spectrum has a qualitatively distinct feature which is in principle falsifiable by observation and can distinguish it from the standard inflationary scenario.
---end quote---
Some new papers:
Leonardo Modesto
The Kantowski-Sachs Space-Time in Loop Quantum Gravity
http://arxiv.org/abs/gr-qc/0411032
Thomas Thiemann
Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031
Bianca Dittrich
Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013
Modesto is comparatively new to LQG (was doing String research until around a year and a half ago). His first paper was about using QG to remove the black hole classical singularity.
Bianca Dittrich is a student of Thiemann's at the Albert Einstein Institute (Gölm). She has been giving seminar-talks on the master constraint program (this spring at penn state and last month at perimeter)
there are some more details but I have to go out for the evening, so will add to this tomorrow
This may be a stupid question but hopefully someone can clarify.
Can spin networks describe a unit volume that is "sphere-like"?
Lee Smolin's SciAm article "Atoms of Space and Time" seems to be describing unit volume and area in spin networks in terms of polygonal structures e.g. pyramids, cubes, etc.?
Can a sphere-like volume of space be considered a unit volume since it doesn't seem to allow discrete interfaces with neighbouring unit volumes?
Hi maddy,
I'm not sure I understand what you mean by a unit of volume having some assigned shape. I will make this bold to focus attention on it and try to understand what you mean.
...Can spin networks describe a unit volume that is "sphere-like"?
Lee Smolin's SciAm article "Atoms of Space and Time" seems to be describing unit volume and area in spin networks in terms of polygonal structures e.g. pyramids, cubes, etc.?
Can a sphere-like volume of space be considered a unit volume since it doesn't seem to allow discrete interfaces with neighbouring unit volumes?
Maddy, as far as I know a spin network is not a lattice.
A common type of lattice has a UNIT CELL which is repeated over and over to form the lattice. Like a cubical lattice has a repeating cubical cell.
AFAIK there is no analogous concept in spin networks.
In the context of spin networks the only unit of volume AFAIK is the Planck volume unit and it has no assigned shape.
Like a pint or a quart or a gallon----they are unit volumes and they have no special shape.
I dont remember Lee Smolin saying anything about unit volumes having some definite polyhedral shape. If you have the SciAm article you could type in the paragraph that suggests this to you.
It is a pity that the Smolin SciAm article is not available online.
Rovelli has a popular, introductory article that is available free online and is thus easier to discuss---in case you'd like to start an entry-level discussion that anyone could follow.
Here is Ashtekar's list of popular articles on the web
http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html
and among them I think the best is Rovelli's
from Physics World (November 2003)
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
ah! it is dawning on me what your question might be about.
it might be about a certain specialized idea of duality in simplicial complexes.
(replace each point by an n-simplex, each line segment by an n-1 simplex...)
If so, then I think that is too sophisticated for an elementary discussion.
If Smolin brought it up in the SciAm article then he probably made things more difficult than necessary.
The answer to your question is no. You dont get spheres in that context.
In spin foam and spin networks this idea of duality does come up, though, and it is a pretty construction----for example in 3D you replace every point by a tetrahedron, every line by a triangle, every triangle by a line...., and you get a second pattern that is dual to the first-----in this sense it makes sense to associate volume with points and area with lines.
That may seem far-fetched. I am stretching to try to grasp what you have in mind.
Anyway the direct answer is no, sorry, no spheres anywhere in this picture.
Just to save making a new post for this, here's a new link for this thread
(in its capacity as LQG-and-allied linkbasket)
http://arxiv.org/abs/hep-th/0411154
Quantum kappa-Poincare Algebra from de Sitter Space of Momenta
J. Kowalski-Glikman, S. Nowak
10 pages
Abstract:"There is a growing number of physical models, like point particle(s) in 2+1 gravity or Doubly Special Relativity, in which the space of momenta is curved, de Sitter space. We show that for such models the algebra of space-time symmetries possesses a natural Hopf algebra structure. It turns out that this algebra is just the quantum kappa-Poincare algebra."
A new Martin Bojowald paper appeared today
http://arxiv.org/gr-qc/0411101
On Loop Quantum Gravity Phenomenology and the Issue of Lorentz Invariance
Martin Bojowald, Hugo A. Morales-Tecotl, Hanno Sahlmann
16 pages,
"A simple model is constructed which allows to compute modified dispersion relations with effects from loop quantum gravity. Different quantization choices can be realized and their effects on the order of corrections studied explicitly. A comparison with more involved semiclassical techniques shows that there is agreement even at a quantitative level..."
A couple more samples of Bojowald's output this year
http://arxiv.org/abs/gr-qc/0408094
Time dependence in Quantum Gravity
Martin Bojowald, Parampreet Singh, Aureliano Skirzewski
33 pages, 17 figures
"The intuitive classical space-time picture breaks down in quantum gravity, which makes a comparison and the development of semiclassical techniques quite complicated. By a variation of the group averaging method to solve constraints one can nevertheless introduce a classical coordinate time into the quantum theory, and use it to investigate the way a semiclassical continuous description emerges from discrete quantum evolution. Applying this technique to test effective classical equations of loop cosmology and their implications for inflation and bounces, we show that the effective semiclassical theory is in good agreement with the quantum description even at short scales."
http://arxiv.org/abs/gr-qc/0402053
Loop Quantum Cosmology: Recent Progress
Martin Bojowald
17 pages, 2 figures, Plenary talk at ICGC 2004
"Aspects of the full theory of loop quantum gravity can be studied in a simpler context by reducing to symmetric models like cosmological ones. This leads to several applications where loop effects play a significant role when one is sensitive to the quantum regime. As a consequence, the structure of and the approach to classical singularities are very different from general relativity: The quantum theory is free of singularities, and there are new phenomenological scenarios for the evolution of the very early universe including inflation. We give an overview of the main effects, focussing on recent results obtained by several different groups."
======
this thread serves to collect useful links to LQG-and-allied articles, and it needs periodic updating.
There are a lot of good links and at the moment I dont have time for a complete update right now. So I will just assemble a few specially good ones here:
Ashtekar's recent seminar talk at Penn State:
Black Hole Evaporation and Information Loss: Recent Advances
http://www.phys.psu.edu/events/index.html?event_id=934;event_type_ids=7;span=2004-08-20.2004-12-25
Ashtekar's list of links to online popular Loop Gravity articles
http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html
Ashtekar's recent survey article is excellent, it presents the whole QG
picture in understandable concise terms:
http://arxiv.org/abs/gr-qc/0410054
Gravity and the Quantum
"A general review of quantum gravity addresed non-experts. To appear in the special issue "Space-time a Hundred Years Later" of the New Journal of Physics; J. Pullin and R. Price (editors)."
Thiemann and Dittrich may have found a handle on LQG dynamics
(successfully modified the Hamiltonian)
Thomas Thiemann
Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031
Bianca Dittrich
Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013
Gambini Pullin may also have a handle on the dynamics, by a
discretization that replaces the Hamiltonian constraint by a stepwise unitary evolution operator:
http://arxiv.org/abs/gr-qc/0409057
Consistent discretization and loop quantum geometry
Just keeping tabs on Ganashyam Date and Golam Hossain (one of several papers)
Genericity of inflation in isotropic loop quantum cosmology
http://arxiv.org/gr-qc/0407069
Parampreet Singh has 3 seminar talks on LQG Phenomenology
two of which are online (Fall 2004 semester at phys.psu.edu):
Phenomenological Issues in Loop Quantum Cosmology I, II
http://phys.psu.edu/events/index.html?event_id=935&event_type_ids=0&span=2004-08-20.2004-12-25
http://phys.psu.edu/events/index.html?event_id=936&event_type_ids=0&span=2004-08-20.2004-12-25
Jerzy Lewandowski has a recent seminar talk on BH entropy in LQG,
clearest thing on that I have seen so far:
Black Hole Entropy
http://phys.psu.edu/events/index.html?event_id=938&event_type_ids=0&span=2004-08-20.2004-12-25
Survey by Lee Smolin
http://arxiv.org/abs/hep-th/0408048
An invitation to loop quantum gravity
Lee Smolin
50 pages
"We describe the basic assumptions and key results of loop quantum gravity, which is a background independent approach to quantum gravity. The emphasis is on the basic physical principles and how one deduces predictions from them, at a level suitable for physicsts in other areas such as string theory, cosmology, particle physics, astrophysics and condensed matter physics. No details are given, but references are provided to guide the interested reader to the literature. The present state of knowledge is summarized in a list of 35 key results on topics including the hamiltonian and path integral quantizations, coupling to matter, extensions to supergravity and higher dimensional theories, as well as applications to black holes, cosmology and Plank scale phenomenology. We describe the near term prospects for observational tests of quantum theories of gravity and the expectations that loop quantum gravity may provide predictions for their outcomes. Finally, we provide answers to frequently asked questions and a list of key open problems."
the Debate between Lee Smolin and string-theorist Lenny Susskind
http://www.edge.org/3rd_culture/smolin_susskind04/smolin_susskind.html
that took place this summer (2004) under auspices of the online magazine Edge
Simulating the evolution of the geometry of the universe by Monte Carlo computer runs----AJL (Ambjorn, Jurkiewicz, Loll)
http://arxiv.org/abs/hep-th/0404156
Emergence of a 4D World from Causal Quantum Gravity]
and the follow-up paper
http://arxiv.org/abs/hep-th/0411152
Semiclassical Universe from First Principles
============
To an increasing extent the seminar talks at Penn State are turning out to be helpful. in some sense more up-to-date than preprint postings on ArXiv.
So here is how you go there:
http://phys.psu.edu/events/
and select whatever semester.
Mostly I have been referring to "this semester" (Fall 2004) but some earlier ones are good too, like:
in "spring 2003" there is a long list that includes
Bojowald's talks Quantum Cosmology: An Overview
and Quantum Cosmology: Formalism
The links for the slides and audio for these two are
http://phys.psu.edu/events/index.html?event_id=516;event_type_ids=0;span=2002-12-26.2003-05-31
http://phys.psu.edu/events/index.html?event_id=521;event_type_ids=0;span=2002-12-26.2003-05-31
We should also keep track of some parallel developments, as noted in this thread
http://physicsforums.com/showthread.php?t=47209
Running Newton Constant (no dark matter)
It is basically about papers of Martin Reuter to whom Ashtekar drew attention in his recent LQG survey Gravity and the Quantum:
M. Reuter, H. Weyer
Running Newton Constant, Improved Gravitational Actions, and Galaxy Rotation Curves
http://arxiv.org/abs/hep-th/0410117
"A renormalization group (RG) improvement of the Einstein-Hilbert action is performed which promotes Newton's constant and the cosmological constant to scalar functions on spacetime. ... It is found that a power law running of Newton's constant with a small exponent of the order 10^-6 would account for their non-Keplerian behavior without having to postulate the presence of any dark matter in the galactic halo."
M. Reuter, H. Weyer
Quantum Gravity at Astrophysical Distances?
http://arxiv.org/abs/hep-th/0410119
"Assuming that Quantum Einstein Gravity (QEG) is the correct theory of gravity on all length scales we use analytical results from nonperturbative renormalization group (RG) equations as well as experimental input in order to characterize the special RG trajectory of QEG which is realized in Nature and to determine its parameters. ...could provide a solution to the astrophysical missing mass problem which does not require any dark matter. We show that an extremely weak power law running of Newton's constant leads to flat galaxy rotation curves similar to those observed in Nature. Furthermore, a possible resolution of the cosmological constant problem is proposed by noting that all RG trajectories admitting a long classical regime automatically give rise to a small cosmological constant."
Also there's the Time in Quantum Gravity thread
http://physicsforums.com/showthread.php?t=48492
where Edgar1813 was discussing with us at some length about
Gambini and Pullin stuff.
I guess one can say there is developing a kind of non-string "Quantum Gravity Scene" consisting of several approaches to directly quantizing General Relativity----not what is done in string-type theories, indeed particle theorists have argued that GR is impossible to quantize---un-renormalizable.
So there is a group of non-string approaches to QG which includes Loop, and also Reuter's QEG ("quantum einstein gravity") and also Hawking's "Euclidean Path Integral" approach which was worked on in 1980s and I thought died in the 1990s, but which seems to be revitalized by the
Causal Dynamical Triangulations approach of AJL (ambjorn jurkiewicz loll).
we have AJL papers (which hark back to Hawking) making progress,
and we have Reuter and his co-workers
and we have Loop research lines also making progress on various fronts
and Gambini-Pullin's work one either thinks of as a close parallel or part of Loop, also making progress.
to me it is beginning to look like a small stampede.
Potentially these parallel efforts can be expected to support each other or even merge.
for instance, if Renate Loll's simplex gravity works or if Martin Reuter can really dispense with both dark energy and dark matter, then they will probably share these features which could also be assimilated by Loop as well. They are all trying to quantize Gen Rel and so are all, in that way, similar enterprises with some analogous mathematics.
This thread that selfAdjoint started is also a good one to keep in the picture
http://physicsforums.com/showthread.php?t=44414
String Gravitons yield GR. NOT
The thread discusses the recent paper of Thanu Padmanabhan
http://www.arxiv.org/abs/gr-qc/0409089
From Gravitons to Gravity: Myths and Reality
This paper (http://www.arxiv.org/abs/gr-qc/0409089) does a lot of testing of different kinds, and concludes that the string theorists assertion that the graviton reproduces the physics of GR in flat spacetime is a myth.
At least from my viewpoint it looks questionable whether stringy approaches have actually caught gravity---the fish may have slipped through their fingers. While on the other hand some non-string approaches are showing signs of overcoming the legendary intractability of Gen Rel, its famous resistence to being quantized. So there is a possible shift of balance under way.
mitchbicpu
Nov28-04, 07:14 PM
Check out two US PTO documents at www.epimedia.com/gravitypush/simple1.htm :surprised
the above post about pushgravity or some new theory of the cosmos seems out of place (i.e. is not relevant to this LQG thread)
here are three new LQG papers that came out today
a common theme seems to be the existence of a good semiclassical limit
in various cases of interest:
http://arxiv.org/abs/hep-th/0411245
Existence of a Semiclassical Approximation in Loop Quantum Gravity
Marco Frasca
5 pages
"We consider a spherical symmetric black hole in the Schwarzschild metric and apply Bohr-Sommerfeld quantization to determine the energy levels. The canonical partition function is then computed and we show that the entropy coincides with the Bekenstein-Hawking formula when the maximal number of states for the black hole is the same as computed in loop quantum gravity, proving in this case the existence of a semiclassical limit and obtaining an independent derivation of the Barbero-Immirzi parameter."
========
http://arxiv.org/gr-qc/0411124
Early Universe Dynamics in Semi-Classical Loop Quantum Cosmology
James E. Lidsey
14 pages
"Within the framework of loop quantum cosmology, there exists a semi-classical regime where spacetime may be approximated in terms of a continuous manifold, but where the standard Friedmann equations of classical Einstein gravity receive non-perturbative quantum corrections. An approximate, analytical approach to studying cosmic dynamics in this regime is developed for both spatially flat and positively-curved isotropic universes sourced by a self-interacting scalar field. In the former case, a direct correspondence between the classical and semi-classical field equations can be established together with a scale factor duality that directly relates different expanding and contracting universes. Some examples of non-singular, bouncing cosmologies are presented together with a scaling, power-law solution."
============
http://arxiv.org/gr-qc/0411125
Inflationary Cosmology and Oscillating Universes in Loop Quantum Cosmology
D. J. Mulryne, N.J. Nunes, R. Tavakol, J.E. Lidsey
7 pages
"We study oscillatory universes within the context of Loop Quantum Cosmology. We make a comparative study of flat and positively curved universes sourced by scalar fields with either positive or negative potentials. We investigate how oscillating universes can set the initial conditions for successful slow-roll inflation, while ensuring that the semi-classical bounds are satisfied. We observe rich oscillatory dynamics with negative potentials, although it is difficult to respect the semi-classical bounds in models of this type."
this is something I must use nearly every day.
it is the loop related papers that come up from an arxiv search
either for a given year or for the last twelve month period (LTM)
I will put the links here and afterwards tabulate how many papers the search comes up with for various years.
I think this bunch of papers about the semiclassical limit signals a bunch of new researchers, so one could expect a growing research output. Also the availability of standard textbooks facilitates entry. Interesting to watch the field grow.
----the Last Twelve Month count is as of 28 November 2004----
Year 1994:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/2003/0/1
Last twelve months (e.g. 28 November 2003 to 28 November 2004):
http://arXiv.org/find/nucl-ex,astro-ph,nucl-th,math-ph,hep-ex,physics,cond-mat,hep-lat,quant-ph,gr-qc,hep-ph,hep-th/1/OR+OR+abs:+AND+AND+loop+quantum+OR+cosmology+gravi ty+abs:+AND+AND+quantum+gravity+OR+OR+discrete+phe nomenology+OR+canonical+nonperturbative+abs:+OR+OR +spinfoam+AND+spin+foam+AND+OR+triply+doubly+speci al/0/1/0/past/0/1
1994 61
1995 83
1996 72
1997 70
1998 67
1999 76
2000 89
2001 98
2002 121
2003 140
LTM 180
---------
some news: Albert Einstein Institute, at Golm, is celebrating the first 10 years of its existence (and the Centennial of Einstein's 1905 relativity papers) this spring by holding a conference
http://www.aei.mpg.de/events/conference/
Abhay Ashtekar will do the invited lecture on Loop Gravity and related stuff.
The ever popular Brian Greene will also be giving one of the invited lectures.
Dates are April 5-8, 2005
the webpage says that the talks will be broadcast on the web from the AEI site.
the Mexico Loop and String conference (November 21-27) should be just finished now!
does anyone have any news to report?
http://www.nuclecu.unam.mx/~gravit/EscuelaVI/english.html
Loop quantum cosmology is experiencing rapid growth and getting a lot of results these days about the early universe, so we should probably collect a few links on the standard view of how matter was generated. Here is a survey article from the Summer 2004 institute at SLAC
http://arxiv.org/abs/hep-ph/0411301
Baryogenesis and Leptogenesis Mark Trodden
22 pages, extended version of lecture delivered at the SLAC 2004 Summer Science Institute
"The energy budget of the universe contains two components, dark matter and dark energy, about which we have much to learn. One should not forget, however, that the baryonic component presents its own questions for particle cosmology. In the context of cosmology, baryons would have annihilated with their antiparticles in the early universe, leaving a negligible abundance of baryons, in disagreement with that observed. In this general lecture, delivered at the SLAC 2004 Summer Science Institute, I provide an overview of the central issue and the general principles behind candidate models. I also briefly discuss some popular examples of models that are firmly rooted in particle physics."
http://arxiv.org/gr-qc/0412004
Physics of Deformed Special Relativity: Relativity Principle revisited
Florian Girelli, Etera R. Livine
24 pages
"In many different ways, Deformed Special Relativity (DSR) has been argued to provide an effective limit of quantum gravity in almost-flat regime. Some experiments will soon be able to test some low energy effects of quantum gravity, and DSR is a very promising candidate to describe these latter. Unfortunately DSR is up to now plagued by many conceptual problems (in particular how it describes macroscopic objects) which forbids a definitive physical interpretation and clear predictions. Here we propose a consistent framework to interpret DSR. We extend the principle of relativity: the same way that Special Relativity showed us that the definition of a reference frame requires to specify its speed, we show that DSR implies that we must also take into account its mass. We further advocate a 5-dimensional point of view on DSR physics and the extension of the kinematical symmetry from the Poincare group to the Poincare-de Sitter group (ISO(4,1)). This leads us to introduce the concept of a pentamomentum and to take into account the renormalization of the DSR deformation parameter kappa. This allows the resolution of the "soccer ball problem" (definition of many-particle-states) and provides a physical interpretation of the non-commutativity and non-associativity of the addition the relativistic quadrimomentum. In particular, the coproduct of the kappa-Poincare algebra is interpreted as defining the law of change of reference frames and not the law of scattering. This point of view places DSR as a theory, half-way between Special Relativity and General Relativity, effectively implementing the Schwarzschild mass bound in a flat relativistic context."
John Baez has alphabetized photos of many of the LQG people
at his website
http://math.ucr.edu/home/baez/marseille/
the last update of this thread was back on page 17, posts 245, 246, and 247.
I should probably update the main list of sources again.
BTW I see Baez is working on a paper with Urs Schreiber
(Urs has been doing category hocuspocus lately at the "coffee table")
wonders never cease in case you hadnt noticed :smile:
Baez Marseille pictures include a lot more than just the people, but
the snapshots of the people are labeled reliably with who they are
so finally, in case you were wondering what Alejandro Corichi looks like...
If you have Windows (with Media Player 10) then you have access to the videos----or else the slides/audio----of several dozen talks given at the October 2004 conference at Perimeter.
I have not seen them but another PF poster has checked several of them out. Here is the URL of a 4 page catalog of video talks:
http://streamer.perimeterinstitute.ca:81/MediasiteLive30/LiveViewer/FrontEnd/Front.aspx?cid=f8fb0405-e71a-4d04-a219-5080e1a8d535
Here is the main page for the conference:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/
here's a one page list of the talks and who gave them:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php
There were over 30 titles, several looked interesting to me and I wish it were convenient to watch the videos. Just to mention a few (not a complete list):
Ashtekar
Physical Ramifications of Quantum Geometry
Conrady
Vacuum State for LQG
Dittrich
Status of the Master Constraint Programme
Reyes
Higgs propagation in loop quantum geometry
Pullin
Semi-discrete solution to the dynamics of LQG
Sahlmann
String Theory with LQG methods
Smolin
Physics from Loop Quantum Gravity
Two interesting QG papers appeared on arxiv today
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
50 pages, 11 figures
Report-no: AEI-2004-129
"We present a pedagogical review of loop quantum gravity, with the aim of enabling a precise but critical assessment of its achievements so far. Special attention is paid to the appearance of a large number of ambiguities in the theory, in particular in the formulation of the Hamiltonian constraint. We emphasise that the off-shell ('strong') closure of the constraint algebra is a crucial test of the consistency of the theory, and should be used as the main tool to select one (if any) of the proposed Hamiltonians. Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge."
Hermann Nicolai directs the relevant part of the Albert Einstein Institute (MPI Potsdam) where Thomas Thiemann and Martin Bojowald and several other Loop gravitists are (like Bianca Dittrich recently). Nicolai has done much of his research in String, but (unlike many US string theorists) he does not favor his own specialty exclusively and he supports research in Loop as well. Nicolai can provide a valuable outside understanding of LQG and what problems need to be worked on. I believe that Nicolai's insight into what he sees are the important features and unresolved questions about LQG should be very helpful to read. This is one i expect I shall print out.
This next one is in an interesting form. A three-expert Dialog, or since there are three, a "Trialog". Ted Jacobson's specialty is TESTING Quantum Gravity theories by astronomical observation---finding vulnerable points where observation may refute certain theories, or versions. Don Marolf has done reserarch in string theory, but is also familiar with LQG. Carlo Rovelli is one of the founders of LQG and contributed an early paper on Black Hole entropy. So this conversation should show contrasts between different expert viewpoints
http://arxiv.org/abs/hep-th/0501103
Black hole entropy: inside or out?
Ted Jacobson, Donald Marolf, Carlo Rovelli
42 pages, contribution to proceedings of Peyresq 9
"A trialogue. Ted, Don, and Carlo consider the nature of black hole entropy. Ted and Carlo support the idea that this entropy measures in some sense "the number of black hole microstates that can communicate with the outside world.'' Don is critical of this approach, and discussion ensues, focusing on the question of whether the first law of black hole thermodynamics can be understood from a statistical mechanics point of view."
I should also include a paper posted by Lee Smolin last week, this argues an important point that (rather in contrast to string theorizing) Smolin's LQG does make predictions that are clear and firm enough to test (with upcoming experiments) and could refute the theory. This is a "coming of age" thing. A theory is "grown up" when it is clear enough to be proven false. So this is potentially a contentious issue. Some people may not wish to acknowledge that LQG is mature enough as a theory to actually make predictions and be tested---to bet its life on future experimental outcomes.
http://arxiv.org/hep-th/0501091
Falsifiable predictions from semiclassical quantum gravity
Lee Smolin
9 pages
"Predictions are derived for the upcoming AUGER and GLAST experiments from a semiclassical approximation to quantum gravity. It is argued that to first order in the Planck length the effect of quantum gravity is to make the low energy effective spacetime metric energy dependent. The diffeomorphism invariance of the semiclassical theory forbids the appearance of a preferred frame of reference, consequently the local symmetry of this energy-dependent effective metric is a non-linear realization of the Lorentz transformations, which renders the Planck energy observer independent. This gives a form of deformed or doubly special relativity (DSR), previously explored with Magueijo, called the rainbow metric. The argument is general, and applies in all dimensions with and without supersymmetry, and is, at least to leading order, universal for all matter couplings. The argument is illustrated in detail in a specific example in loop quantum gravity.
A consequence of DSR realized with an energy dependent effective metric is a helicity independent energy dependence in the speed of light to first order in the Planck length. However, thresholds for Tev photons and GZK protons are unchanged from special relativistic predictions. These predictions of quantum gravity are falsifiable by the upcoming AUGER and GLAST experiments."
selfAdjoint
Jan16-05, 10:52 PM
In my opinion the Nicolai, Peeters, and Zamaklar paper, hep-th/0501114, is a wonderful discussion. I especially appreciated their explication of the constraint algebra closure issue which blew up in public around Thiemann's string quantization paper, but which, as NPZ show, was all the time simmering under the LQG surface. While all the other big names in LQG discretely stayed away from this uncomfortable issue, Thiemann had the guts to attack it head on (and is still attacking it with his new replacement for group averaging, a feature too new to get into LPZ). RTWT.
In this paper there are some references to recent work by Thiemann and Dittrich
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
On page 22, references [79] and [80] are to the papers by Thiemann and Dittrich that came out in November 2004. (about 6 lines from the bottom of the page)
On page 41 reference [110] is to Thiemann's Loop-String paper (three lines from the bottom, where it refers to an "intense debate")
On page 34, where there is a discussion of group averaging and the way the diffeomorphism constraint is implemented, you will find a reference to the paper by Fairbairn and Rovelli
see reference [102] about 9 lines from the bottom of the page.
This is the Separable Hilbert Space in Loop Quantum Gravity which we discussed some at PF last year.
I read this paper as flagging major problems that (Nicolai thinks) would be worthwhile for his postdocs and visiting researchers at the Institute to tackle. See especially the conclusions paragraph on page 45.
As is usually the case with overviews and pedagogicial introductions (as this paper is in part) the paper only briefly touches on recent (2004) work
and gives a picture that is more "as-of-some-point-in-the-past". But even the brief mention of several of 2004 papers is helpful, or so I found, because it shows the authors' perspective on them.
[added in edit: I just saw your post. What does RTWT stand for? Ah! I bet TWT is Time Will Tell. still dont read the R]
selfAdjoint
Jan17-05, 09:12 AM
RTWT is blogger for Read the whole thing. And yes I did notice the discussion of nonseparable and separable Hilbert spaces. They are a little sly here, offering nonspeparability as a potential show stopper and then showing that it ha been handled. Or maybe this is just the result of having most of the paper written when the Fairbairn and Rovelli paper came out.
I am ashamed to say that after I promised to forge ahead on Thiemann's series of papers on his new technique, I sluffed off. With this NPZ insight on the constraint algebra problem I mean to tackle the applications of the technique this week.
no need to rush! what you have picked is a prime and hard objective.
Other than Thiemann, only Gambini (consistent discretization QG) seems to have an alternative way past the hamiltonian constraint difficulty----with him time is discretized and there is an evolution operator that advances things step by step, so there is no hamiltonian constraint
I am looking forward to any insights you have about Thiemann's approach.
On a different topic, here are two new papers Jorge Alfaro just posted on arxiv, which I want to keep tabs on.
http://arxiv.org/abs/hep-th/0501116
Loop Quantum Gravity Effects on the High Energy Cosmic Ray Spectrum
Jorge Alfaro, Gonzalo A. Palma
17 pages. Talk at The XVIIIth International Workshop High Energy Physics and Quantum Field Theory Saint-Petersburg, June 17-23, 2004
"Recent observations on ultra high energy cosmic rays (those cosmic rays with energies greater than about 4 x 10^{18} eV) suggest an abundant flux of incoming particles with energies above 1 x 10^{20} eV. These observations violate the Greisen-Zatsepin-Kuzmin cutoff. To explain this anomaly we argue that quantum-gravitational effects may be playing a decisive role in the propagation of ultra high energy cosmic rays. We consider the loop quantum gravity approach and provide useful techniques to establish and analyze constraints on the loop quantum gravity parameters arising from observational data. In particular, we study the effects on the predicted spectrum for ultra high energy cosmic rays and conclude that is possible to reconcile observations."
http://arxiv.org/abs/hep-th/0501129
LIV Dimensional Regularization and Quantum Gravity effects in the Standard Model
Jorge Alfaro
2 pages
"Recently, we have remarked that the main effect of Quantum Gravity(QG) will be to modify the measure of integration of loop integrals in a renormalizable Quantum Field Theory. In the Standard Model this approach leads to definite predictions, depending on only one arbitrary parameter. In particular, we found that the maximal attainable velocity for particles is not the speed of light, but depends on the specific couplings of the particles within the Standard Model. Also birrefringence occurs for charged leptons, but not for gauge bosons. Our predictions could be tested in the next generation of neutrino detectors such as NUBE. In this paper, we elaborate more on this proposal. In particular, we extend the dimensional regularization prescription to include Lorentz invariance violations(LIV) of the measure, preserving gauge invariance. Then we comment on the consistency of our proposal."
these are about testing for QG effects. I just want to keep track because it seems to be an expanding area of research. Alfaro is at a university in Chile and his co-author is at Cambridge.
Two new papers today of possible interest
http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
31 pages; 4 figures (gif); submitted to Science
"A theory of quantum gravity based on quantum computation is proposed. In this theory, fundamental processes are described in terms of quantum information processing: the geometry of space-time is a construct, derived from the underlying quantum computation. Explicit mechanisms are provided for the back-reaction of the metric to computational `matter,' black-hole evaporation, holography, and quantum cosmology."
Seth Lloyd is at MIT. the main body of the paper is 13 pages, then come references, appendices, and figures. some of the figures are in GIF format with the postscript version, so in PDF you only get placeholders.
Seth Lloyd has published 83 papers, a lot seem to be Physical Review A
and Physical Review Letters
but also two in Science, one in 2004 and another in 1996.
(preprint here---- http://arxiv.org/abs/quant-ph/9604015)
His specialty seems to be Quantum Computing. It is interesting that he thinks quantum theory of spacetime can be derived from theory of quantum computation
THE OTHER PAPER is one that I already flagged with its own thread, and one that John Baez highlighted in his report from the October 2004 Perimeter conference
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."
http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
I had a quick look at this. It doesn't look very enlightening. Where is SetAI when we need him? The idea of building QG with qubits is now well entrenched in certain circles.
Cheers
Kea :smile:
this thread has become a surrogate sticky for Loop (and allied) Quantum Gravity links, so we regularly post useful QG reference links here: new articles, websites and the like, and then every now and then I update the main list. It is getting to be time to do that again.
Today there were a couple of new papers by Etera Livine, solo and with Florian Girelli
http://arxiv.org/gr-qc/0501075
Reconstructing Quantum Geometry from Quantum Information: Spin Networks as Harmonic Oscillators
Florian Girelli, Etera R. Livine
16 pages, 3 figures
"Loop Quantum Gravity defines the quantum states of space geometry as spin networks and describes their evolution in time. We reformulate spin networks in terms of harmonic oscillators and show how the holographic degrees of freedom of the theory are described as matrix models. This allow us to make a link with non-commutative geometry and to look at the issue of the semi-classical limit of LQG from a new angle. This work is thought as part of a bigger project of describing quantum geometry in quantum information terms."
*
http://arxiv.org/gr-qc/0501076
Some Remarks on the Semi-Classical Limit of Quantum Gravity
Etera R. Livine
5 pages, Proceedings of the Second International Workshop DICE2004 (Castello di Piombino, Tuscany) "From Decoherence and Emergent Classicality to Emergent Quantum Mechanics"
"One of the most important issues in quantum gravity is to identify its semi-classical regime. First the issue is to define for we mean by a semi-classical theory of quantum gravity, then we would like to use it to extract physical predictions. Writing an effective theory on a flat background is a way to address this problem and I explain how the non-commutative spacetime of deformed special relativity is the natural arena for such considerations. On the other hand, I discuss how the definition of the semi-classical regime can be formulated in a background independent fashion in terms of quantum information and renormalisation of geometry."
this is one I dont want to lose track of because it may have some pedagogical usefulness
http://arxiv.org/abs/gr-qc/0501082
DISCRETE GRAVITY AS A LOCAL THEORY OF THE POINCARÉ GROUP IN THE FIRST ORDER FORMALISM
It deals in simple terms with the relation of Regge calculus to continuum General Relativity. By a Jesuit named Gabriele Gionti, who belongs to the Vatican Obsersatory Research Group at Seward in Arizona. He has a longstanding interest in quantum gravity and did his thesis in Dynamical Triangulations (the Ambjorn Loll approach but before they made the move to Lorentian (Causal) DT and were still getting pathologies.
G.Gionti, Discrete Approaches Toward the Definition of a Quantum Theory of Gravity, Ph.D. thesis SISSA (1998).
http://arxiv.org/gr-qc/9812080
this is one that Wolram spotted and flagged with its own thread
http://physicsforums.com/showthread.php?p=444123#post444123
http://lanl.arxiv.org/abs/gr-qc/0501053
The Search for Quantum Gravity Signals
G. Amelino-Camelia, C. Lämmerzahl, A. Macias, H. Müller
47 pages, submitted to AIP Conference Proceedings of the 2nd Mexican Meeting on Mathematical and Experimental Physics
"We give an overview of ongoing searches for effects motivated by the study of the quantum-gravity problem. We describe in greater detail approaches which have not been covered in recent "Quantum Gravity Phenomenology'' reviews. In particular, we outline a new framework for describing Lorentz invariance violation in the Maxwell sector. We also discuss the general strategy on the experimental side as well as on the theoretical side for a search for quantum gravity effects. The role of test theories, kinematical and dymamical, in this general context is emphasized. The present status of controlled laboratory experiments is described, and we also summarize some key results obtained on the basis of astrophysical observations.
Lee Smolin replied to Nicolai et al survey of LQG from "outsider" standpoint
Here is the Nicolai paper:
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
50 pages, 11 figures
Report-no: AEI-2004-129
Here is Smolin's response (an email to Nicolai which he copied to Peter Woit)
http://www.math.columbia.edu/~woit/blog/archives/000145.html
An advance in the spinfoam area may have been signaled by this paper
(I am trying to assemble links to background and developments leading up to it. First here is the paper I'm talking about:)
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."
Some helpful background can be found in these two papers from 1998 and 1999
http://arxiv.org/hep-th/9807092
Spin Foam Models and the Classical Action Principle
Freidel and Krasnov
65 pages, many figures (published version)
"We propose a new systematic approach that allows one to derive the spin foam (state sum) model of a theory starting from the corresponding classical action functional. It can be applied to any theory whose action can be written as that of the BF theory plus a functional of the B field. Examples of such theories include BF theories with or without cosmological term, Yang-Mills theories and gravity in various spacetime dimensions. Our main idea is two-fold. First, we propose to take into account in the path integral certain distributional configurations of the B field in which it is concentrated along lower dimensional hypersurfaces in spacetime. Second, using the notion of generating functional we develop perturbation expansion techniques, with the role of the free theory played by the BF theory. We test our approach on various theories for which the corresponding spin foam (state sum) models are known. We find that it exactly reproduces the known models for BF and 2D Yang-Mills theories. For the BF theory with cosmological term in 3 and 4 dimensions we calculate the terms of the transition amplitude that are of the first order in the cosmological constant, and find an agreement with the corresponding first order terms of the known state sum models. We discuss implications of our results for existing quantum gravity models."
http://arxiv.org/gr-qc/9905087
An Introduction to Spin Foam Models of Quantum Gravity and BF Theory
John Baez
55 pages, 31 figures
"In loop quantum gravity we now have a clear picture of the quantum geometry of space, thanks in part to the theory of spin networks. The concept of 'spin foam' is intended to serve as a similar picture for the quantum geometry of spacetime. In general, a spin network is a graph with edges labelled by representations and vertices labelled by intertwining operators. Similarly, a spin foam is a 2-dimensional complex with faces labelled by representations and edges labelled by intertwining operators. In a 'spin foam model' we describe states as linear combinations of spin networks and compute transition amplitudes as sums over spin foams. This paper aims to provide a self-contained introduction to spin foam models of quantum gravity and a simpler field theory called BF theory."
Yesterday Freidel posted another paper. It is number III in a series called "Ponzano-Regge Revisited", the first two of which were co-authored with David Louapre (who sometimes visits PF)
http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages
"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"
Here is a quote from a related passage from Freidel/Livine introduction near top of page 4
---quote from Freidel and Livine---
Then, at G=0, the spin foam amplitudes are to be interpreted as providing the Feynman graph evaluation of particles coupled to quantum gravity. We study the perturbative G expansion of the spin foam amplitudes. Remarkably, this expansion can be re-summed and expressed as the Feynman graphs of a non-commutative braided quantum field theory with deformation parameter G, which thus describes the effective theory for matter in quantum gravity.
Any deformed Poincaré theory usually suffers from a huge ambiguity [5] coming from what should be identify as the physical energy and momenta since the introduction of the Planck scale allows non-linear redefinitions. This ambiguity can also be understood as an ambiguity in the identification of the non-commutative space-time. Our work shows that the Ponzano-Regge model naturally defines a star product and a duality between space and momenta, therefore no ambiguity remains once we identify quantum gravity as being responsible for the effective deformation of the Poincaré symmetry.
This realizes explicitly, for the first time from first principles, the now popular idea that quantum gravity will eventually lead to an effective non-commutative field theory incorporating the principle of doubly special relativity [6].
---end quote---
It looks to me like there are four main contenders any of whom could show up at the QG finish line.
1. spin foam program (connections with perturbation analysis, topological field theory, feynman diagrams, doubly special rel, includes matter)
2. master constraint (Thiemann's program)
3. consistent discretization (Gambini group)
4. dynamical triangulation (Ambjorn-Loll approach)
this does not include considerable work going on in Loop cosmology.
In quantum cosmology symmetry is used to reduce the complexity of the LQG model and impressive results have been achieved.
the links in this thread, that provide a kind of bibliography for Loop-and-allied QG, should IMO be organized under these four headings. will try to do this, in hope it makes the thread more useful
this is a short list of papers in each of the 4 approaches, it is very incomplete---just what I happened to have handy and could bring to it at the moment (remember also this leaves out active lines of research in loop cosmology and QG testing or phenomenology: this is just four approaches
currently making progress to getting a full theory of quantum gravity)
1. spin foam program (connections with perturbation analysis, topological field theory, feynman diagrams, doubly special rel, includes matter)
http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
http://arxiv.org/hep-th/9807092
Spin Foam Models and the Classical Action Principle
http://arxiv.org/gr-qc/9905087
An Introduction to Spin Foam Models of Quantum Gravity and BF Theory
2. master constraint (Thiemann's program)
http://physicsforums.com/showthread.php?t=54711
http://arxiv.org/abs/gr-qc/0411138
Testing the Master Constraint Programme for Loop Quantum Gravity I. General Framework
http://arxiv.org/abs/gr-qc/0411139
Testing the Master Constraint Programme for Loop Quantum Gravity II. Finite Dimensional Systems
http://arxiv.org/abs/gr-qc/0411140
Testing the Master Constraint Programme for Loop Quantum Gravity III. SL(2,R) Models
http://arxiv.org/abs/gr-qc/0411141
Testing the Master Constraint Programme for Loop Quantum Gravity IV. Free Field Theories
http://arxiv.org/abs/gr-qc/0411142
Testing the Master Constraint Programme for Loop Quantum Gravity V. Interacting Field Theories
Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013
Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031
3. consistent discretization (Gambini group)
http://arxiv.org/abs/gr-qc/0409057
Consistent discretization and loop quantum geometry
4. dynamical triangulation (Ambjorn-Loll approach)
http://arxiv.org/abs/hep-th/0404156
Emergence of a 4D World from Causal Quantum Gravity]
http://arxiv.org/abs/hep-th/0411152
Semiclassical Universe from First Principles
Frank Wilczek who just got nobel for work in QCD is interested in QG.
I'm a fan, like a lot of other people I guess. he and Sean Robinson just posted this today:
http://arxiv.org/abs/gr-qc/0502074
A Relationship Between Hawking Radiation and Gravitational Anomalies
5 pages, 1 figure
---exerpt from introduction---
Hawking radiation from black holes is one of the most striking effects that is known, or at least widely agreed, to arise from the combination of quantum mechanics and general relativity. Hawking radiation originates upon quantization of matter in a background spacetime that contains an event horizon—for example, a blackhole.
One finds that the occupation number spectrum of quantum field modes in the vacuum state is that ofa blackbody at a fixed temperature given by the surface gravity of the horizon.
The literature contains several derivations of Hawking radiation, each with strengths and weaknesses. Hawking’s original derivation[1, 2] is very direct and physical, but it relies on hypothetical properties of modes that undergo extreme blueshifts, and specifically assumes that their interactions with matter can be ignored.
Derivations based on Euclidean quantum gravity are quick and elegant, but the formalism lacks a secure microscopic foundation[3].
Derivations based on string theory have a logically consistent foundation, but they only apply to special solutions in unrealistic world models, and they do not explain the simplicity and generality of the results inferred from the other methods[4, 5].
In all these approaches, the Hawking radiation appears as a rather special and isolated phenomenon. Here we discuss another approach, which ties its existence to the cancellation of gravitational anomalies...
---end quote---
sounds to me like Wilczek thinks he might be able to do better than the approaches to Quantum Gravity he's familiar with, including stringy ones.
he's a creative thinker. I like the sense of optimism I get.
Two Loop Quantum Cosmology papers today.
http://arxiv.org/abs/gr-qc/0502082
On the Hamiltonian Constraint of Loop Quantum Cosmology
Kevin Vandersloot
28 pages, 2 figures
"In this paper we construct the Hamiltonian constraint operator of loop quantum cosmology using holonomies defined for arbitrary irreducible SU(2) representations labeled by spin J. We show that modifications to the effective semi-classical equations of motion arise both in the gravitational part of the constraint as well as matter terms. The modifications are important for phenomenological investigations of the cosmological imprints of loop quantum cosmology. We discuss the implications for the early universe evolution."
http://arxiv.org/abs/gr-qc/0502086
Effective State Metamorphosis in Semi-Classical Loop Quantum Cosmology
Parampreet Singh
5 pages, 3 figures
"Modification to the behavior of geometrical density at short scales is a key result of loop quantum cosmology, responsible for an interesting phenomenology in the very early universe. We demonstrate the way a perfect fluid with arbitrary equation of state incorporates this change in its effective dynamics in the loop modified phase. We show that irrespective of the choice of matter component, stress-energy conservation law generically implies that classical equation of state metamorphoses itself to an effective negative equation of state below a critical scale determined by the theory."
Both Kevin Vandersloot and P. Singh are postdocs at Ashtekar's Penn State QG center. BTW Singh gave a set of 3 talks on LQC phenomenology there in Fall 2004 which are downloadable. He has co-authored three papers with Bojowald and an interesting one with Maartens and Tsujikawa about inflation being automatic in LQC, among others. Kevin Vandersloot has co-authored three papers with Bojowald, and one with Perez and Noui as well.
In each case it is the guy's first solo paper.
just out
http://arxiv.org/abs/gr-qc/0502097
Modern tests of Lorentz invariance
David Mattingly
DRAFT copy of a review submitted to Living Reviews in Relativity.
my comment:
clear exposition, written for non-specialists and grad students.
important area. LQG/Foam models appear to need some modification of Lorentz invariance although this still remains undecided. Mattingly (with Ted Jacobson) is a leader among those carrying out or initiating relevant observational tests. Living Reviews articles tend to be definitive at least for several years at a time. This introductory survey of current QG testing may turn out like that.
a new Loop Cosmology paper.
http://arxiv.org/abs/astro-ph/0502589
An emergent universe from a loop
David J. Mulryne, Reza Tavakol, James E. Lidsey, George F. R. Ellis
11 pages, 8 figures
"Closed, singularity-free, inflationary cosmological models have recently been studied in the context of general relativity. Despite their appeal, these so called emergent models suffer from a number of limitations. These include the fact that they rely on an initial Einstein static state to describe the past eternal phase of the universe. Given the instability of such a state within the context of general relativity, this amounts to a very severe fine tuning. Also in order to be able to study the dynamics of the universe within the context of general relativity, they set the initial conditions for the universe in the classical phase. Here we study the existence and stability of such models in the context of Loop Quantum Cosmology and show that both these limitations can be partially remedied, once semi-classical effects are taken into account. An important consequence of these effects is to give rise to a static solution (not present in GR), which dynamically is a centre equilibrium point and located in the more natural semi-classical regime. This allows the construction of emergent models in which the universe oscillates indefinitely about such an initial static state. We construct an explicit emergent model of this type, in which a non-singular past eternal oscillating universe enters a phase where the symmetry of the oscillations is broken, leading to an emergent inflationary epoch, while satisfying all observational and semi-classical constraints. We also discuss emergent models in which the universe possesses both early- and late-time accelerating phases."
new Loop Cosmology paper by Bojowald
http://arxiv.org/abs/gr-qc/0503020
The Early Universe in Loop Quantum Cosmology
Martin Bojowald
10 pages, 3 figures, plenary talk at VI Mexican School on Gravitation and Mathematical Physics, Nov 21-27, 2004
"Loop quantum cosmology applies techniques derived for a background independent quantization of general relativity to cosmological situations and draws conclusions for the very early universe. Direct implications for the singularity problem as well as phenomenology in the context of inflation or bouncing universes result, which will be reviewed here. The discussion focuses on recent new results for structure formation and generalizations of the methods."
just out
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures
"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."
just out
http://arxiv.org/abs/gr-qc/0503065
On Energy Conditions and Stability in Effective Loop Quantum Cosmology
Golam Mortuza Hossain
28 pages
"In isotropic loop quantum cosmology, non-perturbatively modified dynamics of a minimally coupled scalar field violates weak, strong and dominant energy conditions when they are stated in terms of equation of state parameter. The violation of strong energy condition helps to have non-singular evolution by evading singularity theorems thus leading to a generic inflationary phase. However, the violation of weak and dominant energy conditions raises concern, as in general relativity these conditions ensure causality of the system and stability of vacuum via Hawking-Ellis conservation theorem. It is shown here that the non-perturbatively modified kinetic term contributes negative pressure but positive energy density. This crucial feature leads to violation of energy conditions but ensures positivity of energy density, as scalar matter Hamiltonian remains bounded from below. It is also shown that the modified dynamics restricts group velocity for inhomogeneous modes to remain sub-luminal thus ensuring causal propagation across spatial distances."
http://arxiv.org/abs/gr-qc/0503062
Fermionic sectors for the Kodama state
Stephon Alexander, Kristin Schleich, Donald M. Witt
4 pages
SLAC-PUB-10841
"Diffeomorphisms not connected to the identity can act nontrivially on the quantum state space for gravity. However, in stark contrast to the case of nonabelian Yang-Mills field theories, for which the quantum state space is always in 1 dimensional representation of the large gauge transformations, the quantum state space for gravity can have higher dimensional representations. In particular, the Kodama state will have 2 dimensional representations, that is sectors with spin 1/2, for many topologies that admit positive scalar curvature. The existence of these spin 1/2 states are used to point out a possible answer to certain criticisms raised recently in the literature."
Stephon is at Stanford/SLAC and has co-authored with Lee Smolin where they were talking about the Kodama state, after Witten (in 2003) published "certain criticisms" tending to discourage interest in it. the other two authors are a University of British Columbia. It is noticeable that Kodama state keeps coming up.
http://arxiv.org/abs/gr-qc/0503078
New Quantum Gravity Phenomenology
Alejandro Corichi, Daniel Sudarsky
8 pages
"The idea that quantum gravity manifestations would be associated with a violation of Lorentz invariance is very strongly bounded and faces serious theoretical challenges. This leads us to consider an alternative scheme for such phenomenological search. We discuss the underlying viewpoint and briefly mention its possible connections with current theoretical ideas. We also outline the new experimental avenues that would be open along these lines."
http://arxiv.org/abs/hep-th/0503140
A quantization of topological M theory
Lee Smolin
20 pages
"A conjecture is made as to how to quantize topological M theory. We study a Hamiltonian decomposition of Hitchin's 7-dimensional action and propose a formulation for it in terms of 13 first class constraints. The theory has 2 degrees of freedom per point, and hence is diffeomorphism invariant, but not strictly speaking topological. The result is argued to be equivalent to Hitchin's formulation. The theory is quantized using loop quantum gravity methods. An orthonormal basis for the diffeomorphism invariant states is given by diffeomorphism classes of networks of two dimensional surfaces in the six dimensional manifold. The hamiltonian constraint is polynomial and can be regulated by methods similar to those used in LQG.
To connect topological M theory to full M theory, a reduction from 11 dimensional supergravity to Hitchin's 7 dimensional theory is proposed. One important conclusion is that the complex and symplectic structures represent non-commuting degrees of freedom. This may have implications for attempts to construct phenomenologies on Calabi-Yau compactifications."
Today on SPR Thomas Larsson noted this 2004 paper, which is one I missed. I cannot say how essential it is: the problem (which is clearly important) may have been addressed independently by others.
But I want to keep it accessible if only on Larsson's recommendation.
http://www.arxiv.org/abs/gr-qc/0412059
General Relativity Histories Theory
Ntina Savvidou
"The canonical description is based on the prior choice of a
spacelike foliation, hence making a reference to a spacetime metric.
However, the metric is expected to be a dynamical, fluctuating quantity
in quantum gravity. After presenting the developments in the History
Projection Operator histories theory in the last seven years--giving
special emphasis on the novel temporal structure of the formalism--we
show how this problem can be solved in the histories formulation of
general relativity. We implement the 3+1 decomposition using
metric-dependent foliations which remain spacelike with respect to all
possible Lorentzian metrics. This allows us to find an explicit relation
of covariant and canonical quantities which preserves the spacetime
character of the canonical description. In this new construction we have
a coexistence of the spacetime diffeomorphisms group Diff(M) and the
Dirac algebra of constraints."
I want to collect a few recent papers in one post
1.
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures
"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms..."
2.
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."
3.
http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages
"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"
from Freidel/Livine introduction near top of page 4:
"Then, at G=0, the spin foam amplitudes are to be interpreted as providing the Feynman graph evaluation of particles coupled to quantum gravity. We study the perturbative G expansion of the spin foam amplitudes. Remarkably, this expansion can be re-summed and expressed as the Feynman graphs of a non-commutative braided quantum field theory with deformation parameter G, which thus describes the effective theory for matter in quantum gravity.
Any deformed Poincaré theory usually suffers from a huge ambiguity [5] coming from what should be identify as the physical energy and momenta since the introduction of the Planck scale allows non-linear redefinitions. This ambiguity can also be understood as an ambiguity in the identification of the non-commutative space-time. Our work shows that the Ponzano-Regge model naturally defines a star product and a duality between space and momenta, therefore no ambiguity remains once we identify quantum gravity as being responsible for the effective deformation of the Poincaré symmetry.
This realizes explicitly, for the first time from first principles, the now popular idea that quantum gravity will eventually lead to an effective non-commutative field theory incorporating the principle of doubly special relativity [6]..."
4.
http://arxiv.org/abs/hep-th/0503140
A quantization of topological M theory
Lee Smolin
20 pages
"A conjecture is made as to how to quantize topological M theory. We study a Hamiltonian decomposition of Hitchin's 7-dimensional action and propose a formulation for it in terms of 13 first class constraints. The theory has 2 degrees of freedom per point, and hence is diffeomorphism invariant, but not strictly speaking topological. The result is argued to be equivalent to Hitchin's formulation. The theory is quantized using loop quantum gravity methods. An orthonormal basis for the diffeomorphism invariant states is given by diffeomorphism classes of networks of two dimensional surfaces in the six dimensional manifold. The hamiltonian constraint is polynomial and can be regulated by methods similar to those used in LQG.
To connect topological M theory to full M theory, a reduction from 11 dimensional supergravity to Hitchin's 7 dimensional theory is proposed. One important conclusion is that the complex and symplectic structures represent non-commuting degrees of freedom. This may have implications for attempts to construct phenomenologies on Calabi-Yau compactifications."
=============
these are some salient papers from the first three months of 2005.
maybe we can figure out the main directions that LQG research is taking
this year. (when I say LQG I mean the term inclusively, including allied approaches like spin foam and LQC)
1. LQG already got rid of the bigbang singularity in 2001. It has taken a long time to do the same with blackholes but now it seems to be happening. with a simplified model of the matter that is collapsing the authors see that it does not result in a singularity (i.e. LQG survives the collapse and continues to model conditions where the classical theory had a failure). I guess one can expect more results this year that for the first time do a quantum spacetime analysis (not merely semiclassical analysis) of black holes, and the issues around evaporation to be addressed.
2. It seems natural to expect LQG to develop a perturbative sector, Freidel and Starodubstev have several more papers in preparation. Perturbing around a vacuum or ground state of gravity can be expected to expedite calculation.
3. 2+1 gravity has been the focus of a lot of research and it looks nearly solved and judging from Freidel/Livine work it connects well with matter and also with DSR (modified special relativity). so that suggests we are going to see DSR in 3+1 gravity. (this is consistent with Smolin's recent paper http://arxiv.org/hep-th/0501091 "Falsifiable predictions from semiclassical quantum gravity"). It looks like the gammarayburst test (if Glast flies in 2007) will be an arguably valid experimental trial of LQG, which will therefore risk refutation.
this post involves a lot of guesswork on my part and I may revise it.
the papers that stand out for the first quarter of 2005 ought to serve as
signposts and I am trying to see where they point (and obviously could be mistaken several ways)
Spicerack offered this link to a popular article about Martin Bojowald in Nature magazine
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7021/full/433012a_r.html
Kea flagged a Stephon Alexander article
A Quantum Gravitational Relaxation of the Cosmological Constant
http://arxiv.org/hep-th/0503146
this is one that Wolram spotted and flagged with its own thread
http://physicsforums.com/showthread.php?t=69503
Particle scattering in loop quantum gravity
Leonardo Modesto and Carlo Rovelli
http://arxiv.org/gr-qc/0502036
stuff on the Kodama state keeps coming up
even tho Witten warned in 2003 that it was not normalizable (IIRC)
previously the most recent papers were by Stephon Alexander and by Alexander with Kristin Schleich and Donald Witt
today this was posted
http://arxiv.org/abs/gr-qc/0504010
A Generalization of the Kodama State for Arbitrary Values of the Immirzi Parameter
Andrew Randono
16 pages
"The Kodama State for Lorentzian gravity presupposes a particular value for the Immirzi-parameter, namely beta=-i. However, the derivation of black hole entropy in Loop Quantum Gravity suggests that the Immirzi parameter is a fixed value whose magnitude is on the order of unity but larger than one. Since the Kodama state has de-Sitter spacetime as its classical limit, to get the proper radiation temperature, the Kodama state should be extended to incorporate a more physical value for beta. Thus, we present an extension of the Kodama state for arbitrary values of the Immirzi parameter, beta, that reduces to the ordinary Chern-Simons state for the particular value beta=-i. The state for real values of beta is free of several of the outstanding problems that cast doubts on the original Kodama state as a ground state for quantum general relativity. We show that for real values of beta, the state is invariant under large gauge transformations, it is CPT invariant (but not CP invariant), and it is expected to be delta-function normalizable with respect to the kinematical inner product. To aid in the construction, we first present a general method for solving the Hamiltonian constraint for imaginary values of beta that allows one to use the simpler self-dual and anti-self-dual forms of the constraint as an intermediate step."
the guy is at Austen Texas
I dont know whether this paper by itself is so weighty but it impresses me that stuff keeps piling up about Kodama state
here is the Alexander, Schleich, Witt paper
http://arxiv.org/abs/gr-qc/0503062
here is the Alexander solo paper
http://arxiv.org/abs/hep-th/0503146
(see its conclusion section for discussion of Kodama state)
both of these were flagged earlier
selfAdjoint
Apr5-05, 10:05 AM
the guy is at Austen Texas
I dont know whether this paper by itself is so weighty but it impresses me that stuff keeps piling up about Kodama state
I don't know who Andrew Randono is either, but if the paper holds up, it's path-breaking. He has embedded the existing, problematical Kodama state (proposed as the ground state of quantum gravity, the QG vacuum if you will) in a continuum of such states parametrized by the Immirzi parameter. Since the Baez et. al. path integral paper strongly suggests the I.P. as a little bigger than 1 and real, while existing Kodama has Immirzi = -i, he wants to, well, sort of continue through his new continuum to get to the real value. And when he does this he gets a new kind of Kodama state which is nice. I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!
... I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!
I so much hope he is right too, or that something along those lines can be rigorously established. Bringing in a few more string theorists is a nice idea too :smile:
I should post something about this newly arxived paper by Sorkin:
http://arxiv.org/abs/hep-th/0504037
Ten Theses on Black Hole Entropy
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
To appear in the Proceedings of the European Science Foundation Conference on Philosophical and Foundational Issues in Statistical Physics, held Utrecht, the Netherlands, 28-30 November 2003. The proceedings are to be published in a special issue of Studies in History and Philosophy of Modern Physics (2005)
"I present a viewpoint on black hole thermodynamics according to which the entropy: derives from horizon "degrees of freedom''; is finite because the deep structure of spacetime is discrete; is "objective'' thanks to the distinguished coarse graining provided by the horizon; and obeys the second law of thermodynamics precisely because the effective dynamics of the exterior region is not unitary."
I wonder what our long-time PF-member whose handle is "nonunitary" thinks of Sorkin's paper. :wink: Also PF-member Edgar1813, whose friends Gambini and Pullin have argued that time-evolution (told with a realistic quantum clock) must be nonunitary. The idea presses in from several directions.
http://arxiv.org/abs/gr-qc/0504029
Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
18 pages, 4 figures
IGPG04/8-4, AEI-2004-072
"A paradigm describing black hole evaporation in non-perturbative quantum gravity is developed by combining two sets of detailed results: i) resolution of the Schwarzschild singularity in loop quantum gravity; and ii) time-evolution of black holes in the dynamical horizon framework. Quantum geometry effects introduce a major modification in the traditional space-time diagram of black hole evaporation, providing a possible mechanism for recovery of information that is classically lost in the process of black hole formation. The paradigm is developed directly in the Lorentzian regime and necessary conditions for its viability are discussed. If these conditions are met, much of the tension between expectations based on space-time geometry and structure of quantum theory would be resolve."
this is a long-awaited paper. Ashtekar gave a talk at Penn State last year about this (audio and slides are available). the paper has already been cited in some others.
Ashtekar and Bojowald have another BH paper in preparation called
Non-singular quantum geometry of the Schwarzschild black hole interior
that is reference [1] in their paper that just came out.
in the present paper they discuss the results in [1] somewhat, as in the abstract, and on pages 5 and 7:
abstract: " i) resolution of the Schwarzschild singularity in loop quantum gravity[1]; "
page 5:"Since the key issues involve the final black hole singularity and since we expect this singularity to be generically space-like, one can first focus just on the interior of the Schwarzschild horizon. This region is naturally foliated by 3-manifolds ...Using quantum geometry, we can go to the exact quantum theory [1]. The situation is similar but technically more complicated than that encountered in the rigorous treatment of spatially homogeneous and isotropic cosmologies [16]. (See also [15] where the same kind of representation is used, based on ADM variables.)... "
page 7: "... Thus, as in quantum cosmology, one finds that the quantum evolution does not stop at the singularity; one can evolve right through it [1]. The state remains pure. However, in the deep Planck regime around the singularity, the notion of a classical space-time geometry fails to make even an approximate sense. Nonetheless, since there is no longer a final boundary in the interior, the full quantum evolution is quite different from the classical one..."
page 7: "...This calculation was done [1] in the Kantowski-Sachs mini-superspace and |Psi> represents the state of the Schwarzschild black hole interior in loop quantum gravity. This black hole can not evaporate: there is no matter and because of the restriction to spherical symmetry there can not be Hawking radiation of gravitons either. However, since the generic singularity is expected to be space-like, one may hope that the general intuition about the resolution of the Schwarzschild singularity it provides can be taken over to models in which gravity is coupled to scalar fields, where the evaporation does occur. We will assume that the overall, qualitative features of our singularity resolution will continue to be valid in these models."
http://arxiv.org/abs/gr-qc/0504043
Quantum Gravitational Collapse
Leonardo Modesto
13 pages
"We apply the recent results in Loop Quantum Cosmology and in the resolution of Black Hole singularity to the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. The boundary condition identify an unique time inside and outside the region where the matter is present. We consider a star during the collapse in the particular case in which inside the collapsing star we take null pressure, homogeneity and isotropy. The space-time outside the matter is homogeneous and anisotropic. We show that the space time is singularity free and that we can extend dynamically the space-time beyond the classical singularity."
ohwilleke
Apr12-05, 09:36 AM
If Modesto's paper really new? I thought that LQG implies no singularities had been established for some time. Is the new contribution the description of what is going on inside the black hole?
here is post #302 on this thread
just out
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures
"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."
Bojowald removed the cosmological singularity in 2001, assuming isotropy. The result has since been extended to more general cases---post #301 has a link to a recent review.
Removing the black hole singularity is just happening this year, for the first time.
Just because the cosmological (BB) singularity was cured does not mean that the gravitational collapse (BH) singularity was cured.
In any given case the LQG analysis has to be done to see if the theory breaks down (and makes a singularity) or not. Including matter makes for some additional technical complications
========================
a new paper by Ashtekar was posted Tuesday 12 April
http://arxiv.org/abs/gr-qc/0504052
Semiclassical States for Constrained Systems
Abhay Ashtekar, Luca Bombelli, Alejandro Corichi
25 pages, 3 figures
short reading list for "LQG explains the constants" thread:
1.these two papers, and the references therein, discuss the removal of the classical BB ex-singularity in a range of cases that has gradually extended the generality of Bojowald's initial 2001 result.
http://arxiv.org/abs/gr-qc/0402053
Loop Quantum Cosmology: Recent Progress
Martin Bojowald
17 pages, 2 figures, Plenary talk at ICGC 2004
"Aspects of the full theory of loop quantum gravity can be studied in a simpler context by reducing to symmetric models like cosmological ones. This leads to several applications where loop effects play a significant role when one is sensitive to the quantum regime. As a consequence, the structure of and the approach to classical singularities are very different from general relativity: The quantum theory is free of singularities, and there are new phenomenological scenarios for the evolution of the very early universe including inflation. We give an overview of the main effects, focussing on recent results obtained by several different groups."
http://arxiv.org/abs/gr-qc/0503020
The Early Universe in Loop Quantum Cosmology
Martin Bojowald
10 pages, 3 figures, plenary talk at VI Mexican School on Gravitation and Mathematical Physics, Nov 21-27, 2004
"Loop quantum cosmology applies techniques derived for a background independent quantization of general relativity to cosmological situations and draws conclusions for the very early universe. Direct implications for the singularity problem as well as phenomenology in the context of inflation or bouncing universes result, which will be reviewed here. The discussion focuses on recent new results for structure formation and generalizations of the methods."
2. these papers, and references therein, go towards understanding the BH ex-singularity and showing that it evolves into the BB ex-singularity:
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures
"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."
http://arxiv.org/abs/gr-qc/0504043
Quantum Gravitational Collapse
Leonardo Modesto
13 pages
We apply the recent results in Loop Quantum Cosmology and in the resolution of Black Hole singularity to the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. The boundary condition identify an unique time inside and outside the region where the matter is present. We consider a star during the collapse in the particular case in which inside the collapsing star we take null pressure, homogeneity and isotropy. The space-time outside the matter is homogeneous and anisotropic. We show that the space time is singularity free and that we can extend dynamically the space-time beyond the classical singularity."
3. if the models of BH collapse and BB expansion can be joined in theory then one can take seriously the CNS (cosm. nat. selection) hypothesis as a way of understanding why the basic constants in the Standard Models are what they are. The CNS theory can be tested empirically by observation and experiment. This paper discusses CNS:
http://arxiv.org/abs/hep-th/0407213
Scientific alternatives to the anthropic principle
Lee Smolin
for "Universe or Multiverse", ed. by Bernard Carr et. al., to be published by Cambridge University Press.
It is explained in detail why the Anthropic Principle (AP) cannot yield any falsifiable predictions, and therefore cannot be a part of science. Cases which have been claimed as successful predictions from the AP are shown to be not that. Either they are uncontroversial applications of selection principles in one universe (as in Dicke's argument), or the predictions made do not actually logically depend on any assumption about life or intelligence, but instead depend only on arguments from observed facts (as in the case of arguments by Hoyle and Weinberg). The Principle of Mediocrity is also examined and shown to be unreliable, as arguments for factually true conclusions can easily be modified to lead to false conclusions by reasonable changes in the specification of the ensemble in which we are assumed to be typical.
We show however that it is still possible to make falsifiable predictions from theories of multiverses, if the ensemble predicted has certain properties specified here. An example of such a falsifiable multiverse theory is cosmological natural selection. It is reviewed here and it is argued that the theory remains unfalsified. But it is very vulnerable to falsification by current observations, which shows that it is a scientific theory.
The consequences for recent discussions of the AP in the context of string theory are discussed."
wolram found this paper and contributed the link:
http://arxiv.org/gr-qc/0504059
Geometric spin foams, Yang-Mills theory and background-independent models
Florian Conrady (CPT, Marseille & Potsdam, Max Planck Inst.)
28 pages, 27 diagrams
AEI-2005-090
"We review the dual transformation from pure lattice gauge theory to spin foam models with an emphasis on a geometric viewpoint. This allows us to give a simple dual formulation of SU(N) Yang-Mills theory, where spin foam surfaces are weighted with the exponentiated area. In the case of gravity, we introduce a symmetry condition which demands that the amplitude of an individual spin foam depends only on its geometric properties and not on the lattice on which it is defined. For models that have this property, we define a new sum over abstract spin foams that is independent of any choice of lattice or triangulation. We show that a version of the Barrett-Crane model satisfies our symmetry requirement."
a thread for discussing the paper:
http://physicsforums.com/showthread.php?p=530764#post530764
http://arxiv.org/gr-qc/9404011
The fate of black hole singularities and the parameters of the standard models of particle physics and cosmology
Lee Smolin
27 pages
CGPG-94/3-5
"A cosmological scenario which explains the values of the parameters of the standard models of elementary particle physics and cosmology is discussed. In this scenario these parameters are set by a process analogous to natural selection which follows naturally from the assumption that the singularities in black holes are removed by quantum effects leading to the creation of new expanding regions of the universe. The suggestion of J. A. Wheeler that the parameters change randomly at such events leads naturally to the conjecture that the parameters have been selected for values that extremize the production of black holes. This leads directly to a prediction, which is that small changes in any of the parameters should lead to a decrease in the number of black holes produced by the universe. On plausible astrophysical assumptions it is found that changes in many of the parameters do lead to a decrease in the number of black holes produced by spiral galaxies. These include the masses of the proton,neutron, electron and neutrino and the weak, strong and electromagnetic coupling constants. Finally,this scenario predicts a natural time scale for cosmology equal to the time over which spiral galaxies maintain appreciable rates of star formation, which is compatible with current observations that Omega = .1-.2."
A recent review article about BH vibration modes (relevant to LQG)
http://arxiv.org/abs/gr-qc/0411025
A critique of Smolin CNS by Rudy Vaas (in his capacity as Philosophy of Science expert) with a good bibliography of other people's reactions to CNS
http://arxiv.org/gr-qc/0205119
Is there a Darwinian Evolution of the Cosmos? - Some Comments on Lee Smolin's Theory of the Origin of Universes by Means of Natural Selection
Ruediger Vaas
Comments: 20 pages; extended version of a contribution to the MicroCosmos - MacroCosmos conference in Aachen, Germany, September 2-5 1998; finished in late 1998 and published in the conference proceedings
"For Lee Smolin, our universe is only one in a much larger cosmos (the Multiverse) - a member of a growing community of universes, each one being born in a bounce following the formation of a black hole. In the course of this, the values of the free parameters of the physical laws are reprocessed and slightly changed. This leads to an evolutionary picture of the Multiverse, where universes with more black holes have more descendants. Smolin concludes, that due to this kind of Cosmological Natural Selection our own universe is the way it is. The hospitality for life of our universe is seen as an offshoot of this self-organized process. - This paper outlines Smolin's hypothesis, its strength, weakness and limits, its relationship to the anthropic principle and evolutionary biology, and comments on the hypothesis from different points of view: physics, biology, philosophy of science, philosophy of nature, and metaphysics..."
http://arxiv.org/abs/gr-qc/0504125
Large volume quantum correction in loop quantum cosmology: Graviton illusion?
Golam Mortuza Hossain
4 pages
IMSc/2005/04/10
"The leading quantum correction to Einstein-Hilbert Hamiltonian coming from large volume vacuum isotropic loop quantum cosmology, is independent of quantization ambiguity parameters. It is shown here that this correction can be viewed as finite volume gravitational Casimir energy due to one-loop 'graviton' contributions. In vacuum case sub-leading quantum corrections and in non-vacuum case even leading quantum correction depend on ambiguity parameters. It may be recalled that these are in fact analogous features of perturbative quantum gravity where it is well-known that pure gravity (on-shell) is one-loop finite whereas higher-loops contributions are not even renormalizable. These features of the quantum corrections coming from non-perturbative quantization, sheds a new light on a major open issue; how to communicate between non-perturbative and perturbative approaches of quantum gravity."
http://arxiv.org/abs/physics/0504179
Einstein and Hilbert: The Creation of General Relativity
Ivan T. Todorov (Institut fuer Theoretische Physik, Universitaet Goettingen, Germany, and Institute for Nuclear Research and Nuclear Energy, Sofia, Bulgaria)
Colloquium talk; 15 pages
Subj-class: History of Physics
"It took eight years after Einstein announced the basic physical ideas behind the relativistic gravity theory before the proper mathematical formulation of general relativity was mastered. The efforts of the greatest physicist and of the greatest mathematician of the time were involved and reached a breathtaking concentration during the last month of the work.
Recent controversy, raised by a much publicized 1997 reading of Hilbert's proof-sheets of his article of November 1915, is also discussed."
knowing something about the history of how Einstein (and David Hilbert who was working along similar lines) arrived at General Relativity, in several ways a completely new kind of physical theory, can be a help in understanding it and even today's efforts to quantize this geometrical theory of gravity. this account draws on a lot of contemporary stuff, letters, quotes, to make an interesting story.
Rovelli's book Quantum Gravity also has much of this history in chapter 2. Rovelli obviously considered it essential to LQG that one get some perspective on GR and the challenge of finding a common ground with QM.
I dont happen to know other online accounts of Einstein's "Long March" to general relativity----what he went through over about 8 years to get there and the help he got from others. If anyone knows of another worthwhile online history of the years 1907-1915 or thereabout, please let us know.
http://arxiv.org/abs/gr-qc/0504147
Uniqueness of diffeomorphism invariant states on holonomy-flux algebras
Jerzy Lewandowski, Andrzej Okolow, Hanno Sahlmann, Thomas Thiemann
38 pages, one figure
"Loop quantum gravity is an approach to quantum gravity that starts from the Hamiltonian formulation in terms of a connection and its canonical conjugate. Quantization proceeds in the spirit of Dirac: First one defines an algebra of basic kinematical observables and represents it through operators on a suitable Hilbert space. In a second step, one implements the constraints. The main result of the paper concerns the representation theory of the kinematical algebra: We show that there is only one cyclic representation invariant under spatial diffeomorphisms.
While this result is particularly important for loop quantum gravity, we are rather general: The precise definition of the abstract *-algebra of the basic kinematical observables we give could be used for any theory in which the configuration variable is a connection with a compact structure group. The variables are constructed from the holonomy map and from the fluxes of the momentum conjugate to the connection. The uniqueness result is relevant for any such theory invariant under spatial diffeomorphisms or being a part of a diffeomorphism invariant theory."
http://arxiv.org/abs/gr-qc/0505002
Absence of the Kasner singularity in the effective dynamics from loop quantum cosmology
Ghanashyam Date
4 pages
IMSc/2005/4/11
"In classical general relativity, the generic approach to the initial singularity is usually understood in terms of the BKL scenario. In this scenario, along with the Bianchi IX model, the exact, singular, Kasner solution of vacuum Bianchi I model also plays a pivotal role. Using an effective classical Hamiltonian obtained from loop quantization of vacuum Bianchi I model, exact solution is obtained which is non-singular due to a discreteness parameter. The solution is parameterized in exactly the same manner as the usual Kasner solution and reduces to the Kasner solution as discreteness parameter is taken to zero. At the effective Hamiltonian level, the avoidance of Kasner singularity uses a mechanism distinct from the `inverse volume' modifications characteristic of loop quantum cosmology."
Ghanashyam Date is a senior relativist at Chennai Institute, who is an LQC expert. he has co-authored with Bojowald. Golam Hossain, several of whose LQC papers we have seen, is the student of Ghanashyam Date. some of Date's previous papers are Genericity of Inflation in LQC and
another one about the absence of cosmological singularity which showed that a bounce was generic in LQC.
http://arxiv.org/abs/hep-th/0505004
Foliations and 2+1 Causal Dynamical Triangulation Models
Tomasz Konopka
9 pages, 3 figures
"Most models of causal dynamical triangulations construct space-time by arranging a set of simplices in layers separated by a fixed time-like distance. The importance of the foliation structure in the 2+1 dimensional model is studied by considering variations of the model in which this property is relaxed. It turns out that the fixed-lapse condition can be equivalently replaced by a set of global constraints that have geometrical interpretation. On the other hand, the introduction of new types of simplices that puncture the foliating sheets in general leads to different low-energy behavior compared to the original model."
A year ago there was the Marseille conference and Renate Loll delivered the DT paper "Emergence of a 4D World..." which raised a lot of interest in Causal DT. So Smolin and Markopoulou have done a little with DT in less than 4 dimensions, since then, and I guess that Tom Konopka is a grad student or postdoc maybe of Fotini Markopoulou, she said she had someone at perimeter/waterloo who was working on Dynamical Triangulations.
http://arxiv.org/abs/hep-th/0505016
Group Field Theory: An overview
Laurent Freidel (PI, ENS-Lyon)
10 pages
"We give a brief overview of the properties of a higher dimensional generalization of matrix model which arises naturally in the context of a background independent approach to quantum gravity, the so called group field theory. We show that this theory leads to a natural proposal for the physical scalar product of quantum gravity. We also show in which sense this theory provides a third quantization point of view on quantum gravity."
Gambini and Pullin have a unique approach to quantum gravity, not LQG but able to make contact with LQG in some of its results. There is no Hamiltonian constraint in their approach unlike in LQG, and no "problem of time"
Ashtekar considers G&P "consistent discretization" one of 3 or 4 promising approaches to quantizing General Relativity (particularly as to the dynamics) and evidently he is going to include G&P in the landmark book he is putting together called "100 Years of Relativity".
http://arxiv.org/abs/gr-qc/0505023
Discrete space-time
Rodolfo Gambini, Jorge Pullin
16 pages, submitted to the volume "100 Years of Relativity - Space-time Structure: Einstein and Beyond", A. Ashtekar, ed., to be published by World Scientific.
"We review recent efforts to construct gravitational theories on discrete space-times, usually referred to as the "consistent discretization'' approach. The resulting theories are free of constraints at the canonical level and therefore allow to tackle many problems that cannot be currently addressed in continuum quantum gravity. In particular the theories imply a natural method for resolving the big bang (and other types) of singularities and predict a fundamental mechanism for decoherence of quantum states that might be relevant to the black hole information paradox. At a classical level, the theories may provide an attractive new path for the exploration of issues in numerical relativity. Finally, the theories can make direct contact with several kinematical results of continuum loop quantum gravity. We review in broad terms several of these results and present in detail as an illustration the classical treatment with this technique of the simple yet conceptually challenging model of two oscillators with constant energy sum."
A growing number of researchers are involved with improved models of BH that don't have the singularity problem. I want to keep the links to their papers handy. here are some of the people:
Abhay Ashtekar, Viqar Husain, Oliver Winkler, Leonardo Modesto, Martin Bojowald, Roy Maartens, Rituparno Goswami, Parampreet Singh,
and here are some of their recent papers:
http://arxiv.org/abs/gr-qc/0504029
http://arxiv.org/abs/gr-qc/0503041
http://arxiv.org/abs/gr-qc/0504043
http://arxiv.org/abs/gr-qc/0411032
http://arxiv.org/abs/gr-qc/0407097
http://arxiv.org/abs/gr-qc/0412039
http://arxiv.org/abs/gr-qc/0410125
http://arxiv.org/abs/gr-qc/0505002
Absence of the Kasner singularity in the effective dynamics from loop quantum cosmology
Ghanashyam Date
4 pages
IMSc/2005/4/11
... the absence of cosmological singularity which showed that a bounce was generic in LQC.
I just reported a paper by Ghanashyam Date, here is another:
http://arxiv.org/abs/gr-qc/0505030
Pre-classical solutions of the vacuum Bianchi I loop quantum cosmology
Ghanashyam Date
Comments: 4 pages, revtex4, no figures
IMSc/2005/4/12
"Loop quantization of diagonalized Bianchi class A models, leads to a partial difference equation as the Hamiltonian constraint at the quantum level. In the absence of an adequate candidate for a physical inner product and/or physical observables, a criterion for testing a viable semiclassical limit has been formulated in terms of existence of the so-called pre-classical solutions. We demonstrate the existence of pre-classical solutions of the quantum equation for the vacuum Bianchi I model. All these solutions avoid the classical singularity at vanishing volume."
=========================
Here is one in Loop Quantum Cosmology by Thiemann, a new field for him.
http://arxiv.org/abs/gr-qc/0505032
On (Cosmological) Singularity Avoidance in Loop Quantum Gravity
Johannes Brunnemann, Thomas Thiemann
34 pages, 16 figures
AEI-2005-098
"Loop Quantum Cosmology (LQC), mainly due to Bojowald, is not the cosmological sector of Loop Quantum Gravity (LQG). Rather, LQC consists of a truncation of the phase space of classical General Relativity to spatially homogeneous situations which is then quantized by the methods of LQG. Thus, LQC is a quantum mechanical toy model (finite number of degrees of freedom) for LQG(a genuine QFT with an infinite number of degrees of freedom) which provides important consistency checks. However, it is a non trivial question whether the predictions of LQC are robust after switching on the inhomogeneous fluctuations present in full LQG. Two of the most spectacular findings of LQC are that 1. the inverse scale factor is bounded from above on zero volume eigenstates which hints at the avoidance of the local curvature singularity and 2. that the Quantum Einstein Equations are non -- singular which hints at the avoidance of the global initial singularity. We display the result of a calculation for LQG which proves that the (analogon of the) inverse scale factor, while densely defined, is {\it not} bounded from above on zero volume eigenstates. Thus, in full LQG, if curvature singularity avoidance is realized, then not in this simple way. In fact, it turns out that the boundedness of the inverse scale factor is neither necessary nor sufficient for curvature singularity avoidance and that non -- singular evolution equations are neither necessary nor sufficient for initial singularity avoidance because none of these criteria are formulated in terms of observable quantities.After outlining what would be required, we present the results of a calculation for LQG which could be a first indication that our criteria at least for curvature singularity avoidance are satisfied in LQG."
this is a companion, or auxilliary paper by the same authors:
http://arxiv.org/abs/gr-qc/0505033
Unboundedness of Triad -- Like Operators in Loop Quantum Gravity
Johannes Brunnemann, Thomas Thiemann
57 pages, 19 figures
AEI-2005-099
"In this paper we deliver the proofs for the claims, made in a companion paper, concerning the avoidance of cosmological curvature singularities in in full Loop Quantum Gravity (LQG)."
http://arxiv.org/abs/hep-th/0505069
Knot theory and a physical state of quantum gravity
Tomas Liko, Louis H. Kauffman
37 pages, 4 figures; review paper; comments/suggestions welcome
"We discuss the theory of knots, and describe how knot invariants arise naturally in theoretical physics. The focus of this review is to delineate the relationship between topological field theory and conformal field theory at both the classical and quantum levels, and to describe in detail the loop representation of non-perturbative canonical quantum general relativity (loop quantum gravity). This leads naturally to a discussion of the Kodama wavefunction, and a framing of the loop observables. The latter may be important for a background-independent formulation of perturbative string theory. This review can serve as a self-contained introduction to loop quantum gravity and related areas."
http://arxiv.org/abs/gr-qc/0505043
Consistent discretizations: the Gowdy spacetimes
Rodolfo Gambini, Marcelo Ponce, Jorge Pullin
10 pages, 8 figures
"We apply the consistent discretization scheme to general relativity particularized to the Gowdy space-times. This is the first time the framework has been applied in detail in a non-linear generally-covariant gravitational situation with local degrees of freedom. We show that the scheme can be correctly used to numerically evolve the space-times. We show that the resulting numerical schemes are convergent and preserve approximately the constraints as expected."
We now seem to have several distinct but related quantum gravity approaches to cosmology.
1. full LQG
2. LQC (as Bojowald and others do it)
3. the older pre-Loop quantum gravity (various people: Husain, Modesto, Reuter)
using the Wheeler-DeWitt eqn and quantizing the metric.
4. Gambini-Pullin consistent discretizations approach
and more (don't forget Renate Loll, and the semiclassical analysis of Ganashyam Date)
it will be interesting to see who confirms whose results.
Gambini and Pullin have been posting a lot lately. Three papers just this month (May 2005). Here is one from earlier this month.
Gambini and Pullin have a unique approach to quantum gravity, not LQG but able to make contact with LQG in some of its results. There is no Hamiltonian constraint in their approach unlike in LQG, and no "problem of time"
Ashtekar considers G&P "consistent discretization" one of 3 or 4 promising approaches to quantizing General Relativity (particularly as to the dynamics) and evidently he is going to include G&P in the landmark book he is putting together called "100 Years of Relativity".
http://arxiv.org/abs/gr-qc/0505023
Discrete space-time
Rodolfo Gambini, Jorge Pullin
16 pages, submitted to the volume "100 Years of Relativity - Space-time Structure: Einstein and Beyond", A. Ashtekar, ed., to be published by World Scientific.
"We review recent efforts to construct gravitational theories on discrete space-times, usually referred to as the "consistent discretization'' approach. The resulting theories are free of constraints at the canonical level and therefore allow to tackle many problems that cannot be currently addressed in continuum quantum gravity. In particular the theories imply a natural method for resolving the big bang (and other types) of singularities and predict a fundamental mechanism for decoherence of quantum states that might be relevant to the black hole information paradox. At a classical level, the theories may provide an attractive new path for the exploration of issues in numerical relativity. Finally, the theories can make direct contact with several kinematical results of continuum loop quantum gravity. We review in broad terms several of these results and present in detail as an illustration the classical treatment with this technique of the simple yet conceptually challenging model of two oscillators with constant energy sum."
Here is one they posted today:
http://arxiv.org/abs/gr-qc/0505052
Classical and quantum general relativity: a new paradigm
Rodolfo Gambini, Jorge Pullin
Comments: 8 pages, one figure
"We argue that recent developments in discretizations of classical and quantum gravity imply a new paradigm for doing research in these areas. The paradigm consists in discretizing the theory in such a way that the resulting discrete theory has no constraints. This solves many of the hard conceptual problems of quantum gravity. It also appears as a useful tool in some numerical simulations of interest in classical relativity. We outline some of the salient aspects and results of this new framework."
Here is Martin Bojowald's latest survey of LQC
http://arxiv.org/abs/gr-qc/0505057
Elements of Loop Quantum Cosmology
Martin Bojowald
30 pages, 4 figures, Chapter contributed to "100 Years of Relativity - Space-time Structure: Einstein and Beyond", Ed. A. Ashtekar (World Scientific)
Report-no: AEI-2005-025
"The expansion of our universe, when followed backward in time, implies that it emerged from a phase of huge density, the big bang. These stages are so extreme that classical general relativity combined with matter theories is not able to describe them properly, and one has to refer to quantum gravity. A complete quantization of gravity has not yet been developed, but there are many results about key properties to be expected. When applied to cosmology, a consistent picture of the early universe arises which is free of the classical pathologies and has implications for the generation of structure which are potentially observable in the near future."
the style of this survey is very un-headline grabbing.
according to LQC a gravitational collapse preceded the current expansion and that the turnaround from contraction to expansion, sometimes called the bounce, involved a flip in the orientation of the spatial triad or volume element.
also according to LQC inflation is generic, it happens after a bounce automatically without fine tuning or much extra paraphernalia. some Bojo short papers headline these results.
however you can either think of LQC as a testable theory IN ITS OWN RIGHT or you can think of it as a simplified stripped-down version of the LQG FULL THEORY and if you think of it as a simplification of the full LQG theory then all these conclusions have to be checked by more elaborate calculation in the full theory. this is now in progress.
So now I guess that LQC people are being very modest and are politely waiting for the elaborate LQG ceremony of consulting the entrails of the full theory to see in what sense they confirm the LQC results.
I may put together a reading list of a few SHORT Bojowald papers that are more exciting reading than this careful survey.
Here is a recent Ambjorn Jurkiewicz Loll (AJL) paper. They say their long article called Reconstructing the Universe dated May 2005 from the University of Utrecht, is to appear. But it is not yet out on archiv.
http://arxiv.org/abs/hep-th/0505113
Spectral Dimension of the Universe
J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
10 pages, 1 figure
SPIN-05/05, ITP-UU-05/07
"We measure the spectral dimension of universes emerging from nonperturbative quantum gravity, defined through state sums of causal triangulated geometries. While four-dimensional on large scales, the quantum universe appears two-dimensional at short distances. We conclude that quantum gravity may be "self-renormalizing" at the Planck scale, by virtue of a mechanism of dynamical dimensional reduction."
=========
this thread is a substitute for a sticky thread devolted to Loop-and-allied LINKS: a kind of library to store useful online LQG-and-related stuff.
Periodically I try to sort it out and organize the links selectively and by topic. this has not been done for a while.
There are other Renate Loll or AJL links back further in this thread
==========
this present paper, about the spacetime dimension being less than 4D at very small scale, has its own thread for discussion
http://www.physicsforums.com/showthread.php?t=75472
http://arxiv.org/abs/gr-qc/0505081
Physical effects of the Immirzi parameter
Alejandro Perez, Carlo Rovelli
3 pages
"The Immirzi parameter is a constant appearing in the version of the general relativity action utilized as a starting point for the loop quantization of gravity.The parameter is commonly believed not to show up in the equations of motion, because it appears in front of a term in the action that vanishes on shell. We show that in the presence of fermions, instead, the Immirzi term of the action does not vanish on shell, and the Immirzi parameter appears in the equations of motion. It is the coupling constant of a parity violating four-fermion interaction. Therefore the nontriviality of the Immirzi parameter leads to effects that are observables in principle, even independently from nonperturbative quantum gravity."
http://arxiv.org/abs/hep-th/0505154
Reconstructing the Universe
J. Ambjorn (NBI Copenhagen and U. Utrecht), J. Jurkiewicz (U. Krakow), R. Loll (U. Utrecht)
52 pages, 20 postscript figures
"We provide detailed evidence for the claim that nonperturbative quantum gravity, defined through state sums of causal triangulated geometries, possesses a large-scale limit in which the dimension of spacetime is four and the dynamics of the volume of the universe behaves semiclassically. This is a first step in reconstructing the universe from a dynamical principle at the Planck scale, and at the same time provides a nontrivial consistency check of the method of causal dynamical triangulations. A closer look at the quantum geometry reveals a number of highly nonclassical aspects, including a dynamical reduction of spacetime to two dimensions on short scales and a fractal structure of slices of constant time."
Spin_Network
May18-05, 10:15 AM
http://arxiv.org/abs/gr-qc/0505081
Physical effects of the Immirzi parameter
Alejandro Perez, Carlo Rovelli
3 pages
"The Immirzi parameter is a constant appearing in the version of the general relativity action utilized as a starting point for the loop quantization of gravity.The parameter is commonly believed not to show up in the equations of motion, because it appears in front of a term in the action that vanishes on shell. We show that in the presence of fermions, instead, the Immirzi term of the action does not vanish on shell, and the Immirzi parameter appears in the equations of motion. It is the coupling constant of a parity violating four-fermion interaction. Therefore the nontriviality of the Immirzi parameter leads to effects that are observables in principle, even independently from nonperturbative quantum gravity."
Marcus, this is a bu**er!
I spent some time on arxiv, looking for a paper on The Immirzi parameter !..hoping to link it to some recent postings, I came offline and spent three hours going through pre-prints I have myself, this is amazing!
Great, and thanks for this link.
P.S I did not see this paper when browsing arxiv so I guess you have a time advantage over the UK :grumpy:
ohwilleke
May18-05, 01:47 PM
Can anyone connect the dots and explain what sort of context one could observe the Immirzi parameter effects in?
Spin_Network, I'm glad you found the Perez Rovelli paper useful.
Ohwilleke, I started a separate thread about the paper in hopes of some comment or clarification.
Can anyone connect the dots and explain what sort of context one could observe the Immirzi parameter effects in?
thanks to selfAdjoint for noticing another new CDT paper. the rate of posting CDT papers seems to be up this year. here is the one that sA flagged:
http://arxiv.org/hep-th/0505165
A statistical formalism of Causal Dynamical Triangulations
Mohammad H. Ansari, Fotini Markopoulou
20 pages, 19 pictures, 1 graph
"We rewrite the 1+1 Causal Dynamical Triangulations model as a spin system and thus provide a new method of solution of the model."
here's another Loop related paper, by Kirill Krasnov, who has co-authored with Laurent Freidel IIRC
http://arxiv.org/abs/hep-th/0505174
Quantum Gravity with Matter via Group Field Theory
Kirill Krasnov
43 pages, many figures
A generalization of the matrix model idea to quantum gravity in three and higher dimensions is known as group field theory (GFT). In this paper we show how GFT can be used to describe 3D quantum gravity coupled to point particles. This is achieved by a generalization similar to the one used in 2D where multi-matrix models or matrix quantum mechanics are considered. Thus, we replace the group that leads to pure quantum gravity by the twisted product of the group with its dual -the so-called Drinfeld double of the group. The Drinfeld double is a quantum group in that it is an algebra that is both non-commutative and non-cocommutative, and special care is needed to define group field theory for it. We show how this is done, and consider the resulting GFT models. Of special interest is a new topological model that is the "Ponzano-Regge'' model for the Drinfeld double. We also consider a more general class of models that are defined using not GFT, but the so-called chain mail techniques. A general model of this class does not produce 3-manifold invariants, but has an interpretation in terms of point particle Feynman diagrams."
I have not checked this one out, but Krasnov is an old hand and it is probably worth keeping the link within easy reach
http://arxiv.org/abs/gr-qc/0505111
Entropy and Area in Loop Quantum Gravity
John Swain
7 pages, this essay received an Honourable Mention in the Gravity Research Foundation Essay Competition 2005
"Black hole thermodynamics suggests that the maximum entropy that can be contained in a region of space is proportional to the area enclosing it rather than its volume. I argue that this follows naturally from loop quantum gravity and a result of Kolmogorov and Bardzin' on the the realizability of networks in three dimensions. This represents an alternative to other approaches in which some sort of correlation between field configurations helps limit the degrees of freedom within a region. It also provides an approach to thinking about black hole entropy in terms of states inside rather than on its surface. Intuitively, a spin network complicated enough to imbue a region with volume only lets that volume grow as quickly as the area bounding it."
David Poulin investigates relational time and gradual decoherence
I am not sure if his papers are relevant enough to QG for me to flag them, here is one
http://arxiv.org/abs/quant-ph/0505175
Relational time for systems of oscillators
G.J.Milburn, David Poulin
Contribution to the Int. J. of Quant. Info. issue dedicated to the memory of Asher Peres
"Using an elementary example based on two simple harmonic oscillators, we show how a relational time may be defined that leads to an approximate Schrodinger dynamics for subsystems, with corrections leading to an intrinsic decoherence in the energy eigenstates of the subsystem."
here is another
http://arxiv.org/abs/quant-ph/0505081
A Relational Formulation of Quantum Theory
David Poulin
14 pages, comments welcome
"We investigate, with the help of a simple model, how a relational quantum theory can emerge from the combination of the general framework of quantum mechanics with the requirement of background independence of general relativity. More precisely, we argue that any quantum mechanical experiment admits a purely relational description at a fundamental level, from which the original "non-relational" theory emerges in a semi-classical limit. According to this thesis, the non-relational theory is therefore an approximation of the fundamental relational theory. We propose four simple rules that can be used to translate an "orthodox" quantum mechanical description into a relational description, independent of an external spatial reference frame or clock. The techniques used to construct these relational theories are motivated by a Bayesian approach to quantum mechanics, and rely on the noiseless subsystem method of quantum information science used to protect quantum states against undesired noise. The relational theory naturally predicts a fundamental decoherence mechanism, so an arrow of time emerges from a time-symmetric theory. Moreover, there is no need for a "collapse of the wave packet" in this theory: the probability interpretation is only applied to diagonal density operators. Finally, the physical states of the relational theory can be described in terms of "spin networks" introduced by Penrose as a combinatorial description of geometry, and widely studied in the loop formulation of quantum gravity. Thus, our simple bottom-up approach (starting from the semi-classical limit to derive the quantum theory) may offer interesting insights on the low energy limit of quantum gravity."
i cannot evaluate this or vouch for it. just feel a nagging sense that we ought to keep tabs on research in relational time. Gambini and Pullin have some papers about it
http://arxiv.org/abs/gr-qc/0506035
Counting a black hole in Lorentzian product triangulations
B. Dittrich (AEI, Golm), R. Loll (U. Utrecht)
42 pages, 11 figures
"We take a step toward a nonperturbative gravitational path integral for black-hole geometries by deriving an expression for the expansion rate of null geodesic congruences in the approach of causal dynamical triangulations. We propose to use the integrated expansion rate in building a quantum horizon finder in the sum over spacetime geometries. It takes the form of a counting formula for various types of discrete building blocks which differ in how they focus and defocus light rays. In the course of the derivation, we introduce the concept of a Lorentzian dynamical triangulation of product type, whose applicability goes beyond that of describing black-hole configurations."
http://arxiv.org/abs/gr-qc/0506031
Hermann Nicolai's contribution to Abhay Ashtekar's new book
http://arxiv.org/abs/gr-qc/0506024
a new Loop Quantum Cosmology paper
http://arxiv.org/abs/gr-qc/0506021
new MOND paper by Moffat
There are now at least 7 chapters of Ashtekar's book (A Hundred Years of Relativity) available as arxiv preprint. Here is a post about the book giving links to the other chapters:
http://www.physicsforums.com/showpost.php?p=566800&postcount=56
http://arxiv.org/gr-qc/0506075
General Relativity in the Undergraduate Physics Curriculum
James B. Hartle
9 pages, 2 figures
"Einstein's general relativity is increasingly important in contemporary physics on the frontiers of both the very largest distance scales (astrophysics and cosmology) and the very smallest(elementary particle physics). This paper makes the case for a 'physics first' approach to introducing general relativity to undergraduate physics majors."
http://arxiv.org/gr-qc/0506067
A group field theory for 3d quantum gravity coupled to a scalar field
Laurent Freidel, Daniele Oriti, James Ryan
11 pages
"We present a new group field theory model, generalising the Boulatov model, which incorporates both 3-dimensional gravity and matter coupled to gravity. We show that the Feynman diagram amplitudes of this model are given by Riemannian quantum gravity spin foam amplitudes coupled to a scalar matter field. We briefly discuss the features of this model and its possible generalisations."
---------------------------
some comment: I've been watching Freidel's work with the greatest interest for the past couple of years. He made some waves earlier this year with two papers, Freidel/Starodubtsev (that Baez called to our attention) and Freidel/Livine (Ponzano-Regge revisited III).
Freidel is at Uni. Lyon in France (also part time Perimeter in Canada) and the other two authors are at Cambridge in the UK.
Here is an exerpt from the Introduction section of the new Freidel paper:
---quote gr-qc/0506067---
Spin foam models [1, 2] represent a purely combinatorial and algebraic implementation of the sum-over-histories approach to quantum gravity, in any signature and spacetime dimension, with an abstract 2-complex playing the role of a discrete spacetime, and algebraic data from the representation theory of the Lorentz group playing the role of geometric data assigned to it.
This approach has recently been developed to a great extent in the 3-dimensional case. It is now clear that it provides a full quantisation of pure gravity[3], whose relation with the one obtained by other approaches is well understood[4, 5].
Moreover, matter can be consistently included in the picture[3, 6], providing a link between spin foam models and effective field theory[7] living on a non-commutative geometry. This picture allows us to naturally address the semi-classical limit of spin foam models and shows that quantum gravity in dimension 3 effectively follows the principle of the so-called deformed (or doubly) special relativity[8].
The group field theory formalism[9] represents a generalisation of matrix models of 2-dimensional quantum gravity [10]. It is a universal structure lying behind any spin foam model for quantum gravity[11, 12], providing a third quantisation point of view on gravity[9] and allowing us to sum over pure quantum gravity amplitudes associated with different topologies[13].
In this picture, spin foams, and thus spacetime itself, appear as (higher-dimensional analogues of) Feynman diagrams of a field theory defined on a group manifold and spin foam amplitudes are simply the Feynman amplitudes weighting the different graphs in the perturbative expansion of the quantum field theory.
On the other hand, we can construct a noncommutative field theory whose Feynman diagram amplitudes reproduce the coupling of matter fields to 3d quantum gravity for a trivial topology of spacetime[7]. Remarkably, the momenta of the fields are labelled also by group elements.
Moreover, in three dimensions there is a duality between matter and geometry, and the insertion of matter can be understood as the insertion of a topological defect charged under the Poincaré group[3].
This suggests that one should be able to treat the third quantisation of gravity and the second quantisation of matter fields in one stroke (see[14] for an early attempt). The purpose of this paper is to study how the coupling of matter to quantum gravity is realised in the group field theory, and whether it is possible to write down a group field theory for gravity and particles that reproduces the amplitudes derived in [3] coupling quantum matter to quantum geometry. This is what we achieve in the present work.
The way the correct amplitudes are generated as Feynman amplitudes of the group field theory is highly non-trivial. It requires an extension of the usual group field theory (gft) formalism to a higher number of field variables, and produces an interesting intertwining of gravity and matter degrees of freedom, as we are going to discuss in the following...
---endquote---
back in post #339 of this thread there is a link to a related paper that also appeared recently:
http://arxiv.org/abs/hep-th/0505174
Quantum Gravity with Matter via Group Field Theory
Kirill Krasnov
43 pages, many figures
(as one would expect, the Krasnov paper is cited by Freidel et al)
http://arxiv.org/abs/gr-qc/0506082
Quantized Black Holes, Their Spectrum and Radiation
I.B. Khriplovich
----time only to note this to check out later----
http://arxiv.org/abs/gr-qc/0506084
Doubly Special Relativity as a Limit of Gravity
Katarzyna Imilkowska, Jerzy Kowalski-Glikman
26 pages, Submitted to Lecture Notes in Physics
"Doubly Special Relativity (DSR) is a theory with two observer-independent scales, of velocity and mass, which is expected to replace Special Relativity at ultra-high energies. In these notes we first discuss the postulates of DSR, and then turn to presenting arguments supporting the hypothesis that DSR can be regarded as a flat space, semiclassical limit of gravity. The notes are based on the talk presented at the conference Special Relativity -- Will it Survive the Next 100 Years?''
my comment: a significant development this year was the paper by Freidel and Starodubtsev hep-th/0501191 "Quantum Gravity in Terms of Topological Observables". this is some way a follow-up on that.
At PF we have discussed Kowalski-Glikman work on several occasions, he being one of the leading theorists involved in DSR. He organized the Polish Winterschool workshop on QG Phenomenology of February 2004. Often works with QG people.
Now see page 8 of the KG et al paper----they are taking off from the QG formalism of Freidel and Starodubtsev. this is the "BF" approach where it was found there is a possible way to get a BACKGROUND INDEPENDENT, but nevertheless PERTURBATIVE approach, with the cosmological constant and the Barbero-Immirzi parameter playing significant roles.
KG is arguing as generally as he can that the flat limit of QG should be not Minkowski space but the corresponding DSR space (very much like Minkowski but with a second invariant scale)
the exposition is pedagogical, the level is of lecture notes for graduate students, so it is easier reading than usual Kowalski-Glikman. and also kind of an update since it comes after the landmark Freidel-Starodubtsev.
selfAdjoint
Jun15-05, 09:11 PM
http://arxiv.org/abs/gr-qc/0506082
Quantized Black Holes, Their Spectrum and Radiation
I.B. Khriplovich
----time only to note this to check out later----
I read the paper. Here is the abstract:
Under quite natural general assumptions, the following results are obtained. The maximum entropy of a quantized surface is demonstrated to be proportional to the surface area in the classical limit. The general structure of the horizon spectrum is found. The discrete spectrum of thermal radiation of a black hole Under quite natural general assumptions, the following results are obtained. The maximum entropy of a quantized surface is demonstrated to be proportional to the surface area in the classical limit. The general structure of the horizon spectrum is found. The discrete spectrum of thermal radiation of a black hole fits the Wien profile. The natural widths of the lines are much smaller than the distances between them. The total intensity of the thermal radiation is estimated.
In the special case of loop quantum gravity, the value of the Barbero -- Immirzi parameter is found. Different values for this parameter, obtained under additional assumption that the horizon is described by a U(1) Chern -- Simons theory, are demonstrated to be in conflict with the firmly established holographic bound.
His derivation of the "holographic bound", which he uses several times to show other people's calculations are wrong, is particularly intuitive. But the whole argument is really just baby statistical mechanics applied to a surface constructed of patches forming the event horizon of a black hole. Many of us here should be able to follow it.
more about the black hole radiation spectrum
http://arxiv.org/abs/hep-th/0506214
Selection Rules for Black-Hole Quantum Transitions
Shahar Hod, Uri Keshet
4 pages, 2 figures
"We suggest that quantum transitions of black holes comply with selection rules, analogous to those of atomic spectroscopy. In order to identify such rules, we apply Bohr's correspondence principle to the quasinormal ringing frequencies of black holes. In this context, classical ringing frequencies with an asymptotically vanishing real part
\omega_R
correspond to virtual quanta, and may thus be interpreted as forbidden quantum transitions. With this motivation, we calculate the quasinormal spectrum of neutrino fields in spherically symmetric black-hole spacetimes. It is shown that
\omega_R \rightarrow 0
for these resonances, suggesting that the corresponding fermionic transitions are quantum mechanically forbidden."
Shahar Hod was who started the uproar about quasinormal vibration modes of black holes in the first place. He cites his own 1998 paper
http://arxiv.org/abs/gr-qc/0506128
Nonsingular Black Holes and Degrees of Freedom in Quantum Gravity
Martin Bojowald
4 pages
"Spherically symmetric space-times provide many examples for interesting black hole solutions, which classically are all singular. Following a general program, space-like singularities in spherically symmetric quantum geometry, as well as other inhomogeneous models, are shown to be absent. Moreover, one sees how the classical reduction from infinitely many kinematical degrees of freedom to only one physical one, the mass, can arise, where aspects of quantum cosmology such as the problem of initial conditions play a role."
Spin_Network
Jun29-05, 06:01 PM
http://arxiv.org/abs/gr-qc/0506128
Nonsingular Black Holes and Degrees of Freedom in Quantum Gravity
Martin Bojowald
4 pages
"Spherically symmetric space-times provide many examples for interesting black hole solutions, which classically are all singular. Following a general program, space-like singularities in spherically symmetric quantum geometry, as well as other inhomogeneous models, are shown to be absent. Moreover, one sees how the classical reduction from infinitely many kinematical degrees of freedom to only one physical one, the mass, can arise, where aspects of quantum cosmology such as the problem of initial conditions play a role."
Marcus the last two papers are great, Hod's in perticular, very interesting!
...the last two papers are great, Hod's in perticular, very interesting!
Spin Network, I am so glad you found the papers readable and of interest to you! At first sight, I could not understand much of the Hod paper. but it was his intuition (more than 5 years ago now IIRC) that set off that long train of research into BH quasinormal modes (with considerable consequences for quantum gravity, especially Loop). so posting the Hod link was a no brainer.
this next link is about nothing in particular. I just need a place to stash it so as to have it handy.
http://adsabs.harvard.edu/physics_service.html
it is a good search engine, but the database is limited in some way
I'm not certain about
Here's an odd one!
http://arxiv.org/abs/gr-qc/0506129
Quantum evaporation of a naked singularity
Rituparno Goswami, Pankaj S. Joshi, Parampreet Singh
4 pages, 2 figures
I respect Parampreet Singh. He is a postdoc of Ashtekar at Penn State who has coauthored interesting papers with Bojowald and with Roy Maartens.
Several of his seminar talks at Penn State are online---talking about LQC phenomenology: observable signature of Loop gravity in CMB and so forth. He is very focused on observable quantum gravity effects.
I wasnt familiar with the other two authors, but now I see that e.g. Goswami has 15 papers and has co-authored with Bojowald on an interesting one that we discussed earlier at PF
http://arxiv.org/abs/gr-qc/0503041
This present paper talks about something very strange. Not sure what to make of it!
"We investigate here gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before a naked singularity can form. Quantum gravitational effects can thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which can lead to a novel observable signature in astrophysical bursts."
it seems that the authors may have found a quantum reason for the absence of naked glitches ("cosm. censorship") and also they may have may have may have a prediction about gammaray bursts which could provide a way of empirically testing what they are saying.
Spin_Network
Jun29-05, 11:42 PM
more about the black hole radiation spectrum
http://arxiv.org/abs/hep-th/0506214
Selection Rules for Black-Hole Quantum Transitions
Shahar Hod, Uri Keshet
4 pages, 2 figures
"We suggest that quantum transitions of black holes comply with selection rules, analogous to those of atomic spectroscopy. In order to identify such rules, we apply Bohr's correspondence principle to the quasinormal ringing frequencies of black holes. In this context, classical ringing frequencies with an asymptotically vanishing real part
\omega_R
correspond to virtual quanta, and may thus be interpreted as forbidden quantum transitions. With this motivation, we calculate the quasinormal spectrum of neutrino fields in spherically symmetric black-hole spacetimes. It is shown that
\omega_R \rightarrow 0
for these resonances, suggesting that the corresponding fermionic transitions are quantum mechanically forbidden."
Shahar Hod was who started the uproar about quasinormal vibration modes of black holes in the first place. He cites his own 1998 paper
Marcus this paper may be of great interest:http://uk.arxiv.org/abs/quant-ph/0506228
he thanks Rovelli and Smolin to name but two!
and this paper may/will? be of interest to the Hod paper:http://uk.arxiv.org/abs/quant-ph/0506231
selfAdjoint
Jun30-05, 11:12 AM
Marcus this paper may be of great interest:http://uk.arxiv.org/abs/quant-ph/0506228
he thanks Rovelli and Smolin to name but two!
and this paper may/will? be of interest to the Hod paper:http://uk.arxiv.org/abs/quant-ph/0506231
http://uk.arxiv.org/abs/quant-ph/0506228 is certainly a greatly interesting paper! I am going to nring it to the attention of the quantum physics subforum.
http://uk.arxiv.org/abs/quant-ph/0506228 is certainly a greatly interesting paper! I am going to bring it to the attention of the quantum physics subforum.
thanks for fielding that one!
http://arxiv.org/abs/gr-qc/0506139
"...The existence of the Pioneer anomaly is no longer in doubt. Further, after much understandable hesitancy, a steadily growing part of the community has concluded that the anomaly should be subject to interpretation. Our program presents an ordered approach to doing this..."
"...This mission is designed to determine the origin of the discovered anomaly and to characterize its properties to an accuracy of at least three orders of magnitude below its measured value ..."
A MISSION TO EXPLORE THE PIONEER ANOMALY
the list of authors has some 39 names, they call themselves the Pioneer Collaboration.
it is an 8 page paper.
the noises keep getting louder that something about gravity needs adjustment
http://arxiv.org/hep-th/0507012
Taming the cosmological constant in 2D causal quantum gravity with topology change
R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
19 pages, 4 figures
"As shown in previous work, there is a well-defined nonperturbative gravitational path integral including an explicit sum over topologies in the setting of Causal Dynamical Triangulations in two dimensions. In this paper we derive a complete analytical solution of the quantum continuum dynamics of this model, obtained uniquely by means of a double-scaling limit. We show that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant, reminiscent of the suppression mechanism considered by Coleman and others in the four-dimensional Euclidean path integral. Remarkably, in the continuum limit we obtain a finite spacetime density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense of a gravitational path integral which includes a sum over topologies, provided suitable causality restrictions are imposed on the path integral histories."
this is the paper they will present this month in Paris at the Einstein2005 conference
Willem Westra did his Masters at Utrecht working for Loll, on this problem, and they published a paper in 2003 about it, and now he is doing his PhD.
Including topology-change in the path integral is very interesting.
one takes a weighted average not only over all possible spacetime geometries, but also adds up all possible spacetime topologies and all possible geometries of each topology. it could get amusing
Abhay Ashtekar and Martin Bojowald have posted an updated version of this
http://arxiv.org/abs/gr-qc/0504029
Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
21 pages, 4 figures, v2: new references and discussion of relation to other ideas
"A paradigm describing black hole evaporation in non-perturbative quantum gravity is developed by combining two sets of detailed results: i) resolution of the Schwarzschild singularity using quantum geometry methods; and ii) time-evolution of black holes in the trapping and dynamical horizon frameworks. Quantum geometry effects introduce a major modification in the traditional space-time diagram of black hole evaporation, providing a possible mechanism for recovery of information that is classically lost in the process of black hole formation. The paradigm is developed directly in the Lorentzian regime and necessary conditions for its viability are discussed. If these conditions are met, much of the tension between expectations based on space-time geometry and structure of quantum theory would be resolved."
Black hole evaporation seems to be a hot topic in quantum gravity now---thinking of the recent paper by Joshi, Goswami, and P.Singh. Also a couple recent papers by Bojowald solo.
BTW in this one Ashtekar and Bojo cite this highly readable and provocative paper by Sean Hayward
http://arxiv.org/abs/gr-qc/0504038
The disinformation problem for black holes (pop version)
Sean A. Hayward
6 pages
The supposed information paradox for black holes is based on the fundamental misunderstanding that black holes are usefully defined by event horizons. Understood in terms of locally defined trapping horizons, the paradox disappears: information will escape from an evaporating black hole. According to classical properties of trapping horizons, a general scenario is outlined whereby a black hole evaporates completely without singularity, event horizon or loss of energy or information.
as another BTW here are Ruth Williams' papers
http://arxiv.org/find/grp_physics/1/au:+Williams_Ruth/0/1/0/all/0/1
(she co-authored with Tullio Regge around 2000) and here is a new one
http://arxiv.org/abs/gr-qc/0506137
Nonlocal Effective Field Equations for Quantum Cosmology
Herbert W. Hamber, Ruth M. Williams
9 pages
"The possibility that the strength of gravitational interactions might slowly increase with distance, is explored by formulating a set of effective field equations, which incorporate the gravitational, vacuum-polarization induced, running of Newton's constant G. The resulting long distance (or large time) behaviour depends on only one adjustable parameter \xi , and the implications for the Robertson-Walker universe are calculated, predicting an accelerated power-law expansion at later times t \sim \xi \sim 1/H.
This week, Loll and Westra have posted an updated version of their 2003 paper
http://arxiv.org/hep-th/0306183
Sum over topologies and double-scaling limit in 2D Lorentzian quantum gravity
9 pages, 3 Postscript figures; added comments on strip versus bulk partition function
"We construct a combined non-perturbative path integral over geometries and topologies for two-dimensional Lorentzian quantum gravity. The Lorentzian structure is used in an essential way to exclude geometries with unacceptably large causality violations. The remaining sum can be performed analytically and possesses a unique and well-defined double-scaling limit, a property which has eluded similar models of Euclidean quantum gravity in the past."
this was a first. before, the moment you allowed wormholes the sum would blow up and you would get infinities. too many baby universes, too many possibilities. so you had to make a rule against topology-change at the outset.
I am oversimplifying. Anyway in earlier CDT the topology of spacetime had to be restricted to be simple, and then within that you could have all different shape geometries. But this little paper of Loll and Westra is a kind of landmark because at least in 2D they are allowing topology-change and it is not a complete disaster.
now there is a little more progress
http://arxiv.org/hep-th/0507012
Taming the cosmological constant in 2D causal quantum gravity with topology change
They are getting their stuff together for the Paris conference this month.
there is this curious result of a finite density of wormholes.
they are going back and polishing the 2003 paper a little, because it will be a footnote in the 2005 paper they give in Paris. what busy people
Just a week or so ago this one came out, Parmapreet Singh being one of the co-authors.
http://arxiv.org/abs/gr-qc/0506129
Quantum evaporation of a naked singularity
Rituparno Goswami, Pankaj S. Joshi, Parampreet Singh
4 pages, 2 figures
"We investigate here gravitational collapse of a scalar field model which classically leads to a naked singularity. We show that non-perturbative semi-classical modifications near the singularity, based on loop quantum gravity, give rise to a strong outward flux of energy. This leads to the dissolution of the collapsing cloud before a naked singularity can form. Quantum gravitational effects can thus censor naked singularities by avoiding their formation. Further, quantum gravity induced mass flux has a distinct feature which can lead to a novel observable signature in astrophysical bursts."
Today, another P. Singh paper:
http://arxiv.org/abs/gr-qc/0507029
Semi-classical States, Effective Dynamics and Classical Emergence in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot
8 pages, 4 figures
IGPG-05/7-1, AEI-2005-122
"We construct physical semi-classical states annihilated by the Hamiltonian constraint operator in the framework of loop quantum cosmology as a method of systematically determining the regime and validity of the semi-classical limit of the quantum theory. Our results indicate that the evolution can be effectively described using continuous classical equations of motion with non-perturbative corrections down to near the Planck scale below which the universe can only be described by the discrete quantum constraint. These results, for the first time, provide concrete evidence of the emergence of classicality in loop quantum cosmology and also clearly demarcate the domain of validity of different effective theories. We prove that discrete quantum geometry effects may become very significant and lead to various new phenomenological applications. Furthermore the understanding of semi-classical states allows for a framework for interpreting the quantum wavefunctions and understanding questions of a semi-classical nature within the quantum theory of loop quantum cosmology."
mounting evidence that in the cosmology sector LQG is consistent with classical cosmology----that it has the right largescale limit in other words.
http://arxiv.org/abs/gr-qc/0507036
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity I
Kristina Giesel, Thomas Thiemann
20 pages, 5 figures
"The volume operator plays a pivotal role for the quantum dynamics of Loop Quantum Gravity (LQG). It is essential in order to construct Triad operators that enter the Hamiltonian constraint and which become densely defined operators on the full Hilbert space even though in the classical theory the triad becomes singular when classical GR breaks down. The expression for the volume and triad operators derives from the quantisation of the fundamental electric flux operator of LQG by a complicated regularisation procedure. In fact, there are two inequivalent volume operators available in the literature and, moreover, both operators are unique only up to a finite, multiplicative constant which should be viewed as a regularisation ambiguity. Now on the one hand, classical volumes and triads can be expressed directly in terms of fluxes and this fact was used to construct the corresponding volume and triad operators. On the other hand, fluxes can be expressed in terms of triads and therefore one can also view the volume operator as fundamental and consider the flux operator as a derived operator. In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs."
http://arxiv.org/abs/gr-qc/0507037
Consistency Check on Volume and Triad Operator Quantisation in Loop Quantum Gravity II
Kristina Giesel, Thomas Thiemann
67 pages, 6 figures, 36 pages paper, 31 pages appendix
"In this paper we provide the techniques and proofs for the resuls presented in our companion paper concerning the consistency check on volume and triad operator quantisation in Loop Quantum Gravity."
http://arxiv.org/abs/gr-qc/0507038
Loop Quantum Geometry: A primer
Alejandro Corichi
Comments: 25 pages. Contribution for the Proceedings of the VI Mexican School of Gravitation and Mathematical Physics
"This is the written version of a lecture given at the ``VI Mexican School of Gravitation and Mathematical Physics" (Nov 21-27, 2004, Playa del Carmen, Mexico), introducing the basics of Loop Quantum Geometry. The purpose of the written contribution is to provide a Primer version, that is, a first entry into Loop Quantum Gravity and to present at the same time a friendly guide to the existing pedagogical literature on the subject. This account is geared towards graduate students and non-experts interested in learning the basics of the subject."
This is by someone who used to visit here at PF fairly often and made some very useful posts (but under an internet "handle" name, not his own) according to my considered opinion.
I am very glad that there is a new Primer, introduction to the subject for grad students getting into it. for many years the most convenient LQG Primer was the 1998 one of Rovelli Upadhya and it is good to have another, so the beginner can have more choice.
Chronos
Jul11-05, 12:47 AM
Thanks marcus. I'm a pedagogologist, as you probably know.
Thanks marcus. I'm a pedagogologist, as you probably know.
a connoisseur of the art of teaching?
no I didn't know.
but actually Alejandro Corichi is aiming at a narrowly defined level of grad student who has completed a graduate course in General Relativity.
And one or two other substantial prerequisites. He is talking to a definite audience and not going out of his way to reach others.
If he was here i would tell him that on page 2 paragraphs 3 and 4 he misspells heart "hearth" and thought "though".
This is the pitfall of the spell-checker, which doesn't know what word you are trying to spell so just gives you SOME correctly spelled word.
Spin_Network
Jul11-05, 07:05 AM
Bing that you are HERE marcus, I convey that "hearth" was meant to be 'Hearted', as in :not for the faint hearted! ;)
I myself have 'boo**ied' in spelling many times, actually in this very POST!..so I will refrain from altering it :(
selfAdjoint
Jul12-05, 05:45 PM
This is the first paper (http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf), in which they state their results. The other one (http://lanl.arxiv.org/abs/gr-qc/0507037) has the proofs.
From the abstract of gr-qc/0507036:
In this paper we examine whether the volume, triad and flux quantisations are consistent with each other. The results of this consistency analysis are rather surprising. Among other findings we show: 1. The regularisation constant can be uniquely fixed. 2. One of the volume operators can be ruled out as inconsistent. 3. Factor ordering ambiguities in the definition of triad operators are immaterial for the classical limit of the derived flux operator. The results of this paper show that within full LQG triad operators are consistently quantized. In this paper we present ideas and results of the consistency check. In a companion paper we supply detailed proofs.
From the introduction:
First of all, there are in fact two unitarily
inequivalent volume operators [5, 6] which come from two, a priori equally justified background independent regularisation techniques. We will denote them by Rovelli – Smolin (RS) and Ashtekar – Lewandowski (AL) volume respectively for the rest of this paper. Secondly, both volume operators are anyway only determined up to a multiplicative regularisation constant C_reg [12] which remains undetermined when taking the limit, quite similar to finite regularisation constants that appear in counterterms of standard renormaisation of ordinary QFT. The ambiguity is further enhanced by factor ordering ambiguities once we consider triad operators. These ambiguities are parameterized by a spin quantum number ℓ = 1/2, 1, 3/2, ...
In this paper we will be able to remove all those ambiguities by the following consistency check: As we mentioned above, the volume and triad can be considered as functions of the fluxes. But the converse is also true: The fluxes can be written in terms of triads and thus the volume. Is it then true that there exists a regularisation constant for the volume operator and a factor ordering of the flux operator considered as a function of the triad operator or volume operator such that the corresponding alternative flux operator agrees (at least in the correspondence limit of large eigenvalues of the volume operator) with the fundamental flux operator, independent of the choice of ℓ? This better be possible as otherwise the inescapable conclusion would be that the volume operator is inconsistently quantised
Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.
...
Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.
Hi selfAdj, concise and nicely crafted. It's a definite plus to have more than one person scouting and flagging papers. So it looks like Ashtekar's volume wins over Smolin and Rovelli volume.
Here's a snapshot of Kristina Giesel
http://math.ucr.edu/home/baez/marseille/giesel.jpg
Spin_Network
Jul12-05, 10:30 PM
This is the first paper (http://lanl.arxiv.org/PS_cache/gr-qc/pdf/0507/0507036.pdf), in which they state their results. The other one (http://lanl.arxiv.org/abs/gr-qc/0507037) has the proofs.
From the abstract of gr-qc/0507036:
From the introduction:
Thus by essentially running the derivation backward they show that one of the two quantum volume formulations is wrong and the other is consistent.
This is actually going to be very..very interesting, I have not gone through both papers fully, I actually was reading the proof one first, which set alarm bells ringing, and I am indulged in a number of Rovelli's papers, I beleive Rovelli has allready highlighted a specific relevant aspect, but then again I might just go straight to the Volume operator Einstein detailed in an much overlooked correspondance, I believe Rovelli's veiwpoint not be 'over-ruled' just yet!
Measure..measure..measure! :smile:
http://arxiv.org/abs/hep-th/0507171
he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper
this 5-pager may not settle all the questions.
In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.
At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.
=======================
WARNING: this paper may leave you quite frustrated. that's how it left me anyway.
In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."
Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.
http://arxiv.org/abs/hep-th/0507167
From Big Bang to Asymptotic de Sitter: Complete Cosmologies in a Quantum Gravity Framework
M. Reuter, F. Saueressig
47 pages, 17 figures
"Using the Einstein-Hilbert approximation of asymptotically safe quantum gravity we present a consistent renormalization group based framework for the inclusion of quantum gravitational effects into the cosmological field equations. Relating the renormalization group scale to cosmological time via a dynamical cutoff identification this framework applies to all stages of the cosmological evolution. The very early universe is found to contain a period of "oscillatory inflation'' with an infinite sequence of time intervals during which the expansion alternates between acceleration and deceleration. For asymptotically late times we identify a mechanism which prevents the universe from leaving the domain of validity of the Einstein-Hilbert approximation and obtain a classical de Sitter era."
Martin Reuter is one of the invited speakers at the Loops 05 conference in October. He has his own approach to QG, which is different from LQG but when it is applied to cosmology it gets some similar results. Here he gets some results similar to Martin Bojowald's Loop Quantum Cosmology. So there is an interesting convergence. Bojowald derived this "oscillatory inflation" business earlier. Now reuter is getting it by a different method.
Spin_Network
Jul19-05, 02:50 AM
http://arxiv.org/abs/hep-th/0507171
he gave the talk almost exactly one year ago, and everybody said
they'd have to wait to read the paper
this 5-pager may not settle all the questions.
In the paper Hawking says information is not lost down a black hole, however reconstructing the information that fell in from the hawking radiation that comes from the hole as it evaporates is (in some sense) like reconstructing an encyclopedia from the smoke and ashes it leaves when consumed by fire.
At the end Hawking reflects that when he paid off the bet to John Preskill and gave him the Encyclopedia
perhaps he should have burned the Encyclopedia first and given Preskill the ashes.
=======================
WARNING: this paper may leave you quite frustrated. that's how it left me anyway.
In the paper he says:
"I adopt the Euclidean [path integral] approach, the only sane way to do quantum gravity nonperturbatively."
Hawking Euclidean QG path integral is a 1980s and 1990s precursor to Loll Lorentzian QG path integral. Loll papers refer back to Hawking Euclidean sum over histories and related Hawking QG matters. I'm persuaded that path integral sum over geometries is a promising way to do QG and Hawking deserves credit for establishing this research direction. But this paper has not convinced me that Hawking's specific (Euclidean) approach to it is necessarily the right one.
Thanks marcus, I have been waiting for this for some time. On another forum, and at PF under a different name, questions have I asked about the ILP (Information Loss Paradox) now you have yourself read the paper and it frustrates to an extent?
Lets read between the path-integral lines? straight away I see Hawking has confirmed my 'past' question of Blackhole 'Time-dependant' reasoning. Take the last sentence in the paper:I gave John an encyclopedia of baseball, but maybe I should just have given him the ashes.
Is this really cricket? :blushing: ..or is this fact that one can derive an integral of information entering a Blackhole, but cannot derieve the same integral of the information that 'rebounds' , scatters back out?
What this really means is that if you throw an English Encyclopedia into a Blackhole, the only thing you can guarantee, is that an English Encyclopedia will never emerge, the particles that went into the construction of the pre-blackhole Encyclopedia, can never re-construct it , the particles that scatter from a certain blackhole horizon, are 'Time-Stamped', and are thus 'younger', 'older' but never the same 'age' as those that entered the Blackhole.
Into the fire, Out of the ashes ?..you will never get the particles of the Encyclopedia back scattered, but you could theoretically Get the Particles of the Trees that went into 'before' the Paper was created, and thus only re-create an Encyclopedia that has 'no-written-words', a sort of Encyclopedia Template!..not an 'Historical' Documentation of Factual Writings!
"The information loss corresponds to the classical relaxation of black holes according to the no hair theorem. One can not ask when the information gets out of a black hole because that would require the use of a semi-classical metric which has already lost the information"
The Time-Dependant paths of Galactic Blackholes have no Branching off to 'other-universes' . "If you jump into a black hole, your mass energy will
be returned to our universe but in a mangled form which contains the information about what you were like but in a state where it can not be easily recognized. It is like burning an encyclopedia. Information is not lost, if one keeps the smoke and the ashes. But it is difficult to read."
This paper contravines one of the most rigourous time-evolution paramiters of Big-Bang theory, if one rewinds our Galaxy, with the theorized Blackhole at its Core, then our Galaxy has a Time-Stamp 'information' that is Unique to our Galaxy. It is evident that all Galaxies that have theorized Blackhole's at their core's, are thus themselves 'Unique'.
The only way in GR to travel to another 'Time-PAST/FUTURE', is to remain inside you Galaxy, wait for another Galaxy to head your way as a merging process, then to 'jump-ship' when the intertwined Galaxies exchange information during the collision process.
You cannot leave our Galaxy and travel to Andromeda, which observationally is within our 'information-time' locally, but yet according to Hawking, if one waits for the 'future' collision of Andromeda and Milkyway, then this collision harbours a good chance of 'Time-Travel'..infact the ONLY chance.
Thanks marcus, I have been waiting for this for some time. On another forum, and at PF under a different name, questions have I asked about the ILP (Information Loss Paradox) now you have yourself read the paper and it frustrates to an extent?
...
I've been waiting too, for some clarification of his position on "ILP". But I guess I am still waiting. the present paper doesnt resolve my doubts. Although it does have some additional content beyond his talk at GR17, it's generally quite similar.
To avoid overloading this thread, which is mostly a kind of library of links, I will put anything that occurs to me about this in the "new Hawking paper" thread.
http://arxiv.org/abs/gr-qc/0507106
Partial and Complete Observables for Canonical General Relativity
Bianca Dittrich
33 pages
Report-no: AEI-2005-128
"In this work we will consider the concepts of partial and complete observables for canonical general relativity. These concepts provide a method to calculate Dirac observables. The central result of this work is that one can compute Dirac observables for general relativity by dealing with just one constraint. For this we have to introduce spatial diffeomorphism invariant Hamiltonian constraints. It will turn out that these can be made to be Abelian. Furthermore the methods outlined here provide a connection between observables in the space--time picture, i.e. quantities invariant under space--time diffeomorphisms, and Dirac observables in the canonical picture."
http://arxiv.org/abs/hep-th/0507235
The case for background independence
Lee Smolin
46 pages, no figures
"The aim of this paper is to explain carefully the arguments behind the assertion that the correct quantum theory of gravity must be background independent. We begin by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background independent theory. The leading background independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity and dynamical triangulations and their main achievements are summarized along with the problems that remain open. Some first attempts to cast string/M theory into a background independent formulation are also mentioned.
The relational/absolute debate has implications also for other issues such as unification and how the parameters of the standard models of physics and cosmology are to be explained. The recent issues concerning the string theory landscape are reviewed and it is argued that they can only be resolved within the context of a background independent formulation. Finally, we review some recent proposals to make quantum theory more relational."
http://arxiv.org/abs/hep-th/0507235
The case for background independence
Lee Smolin
46 pages, no figures
This should be fun to read, but I gave up after a couple of pages. Although it is true that Descartes deliberated relationalism he ended up espousing absolutism, whereas Newton only reluctantly followed, realising that his mathematics was not sufficient to deal with this issue.
Anyway, must be off ... NCG school in progress
Kea :smile:
http://arxiv.org/abs/hep-th/0507253
Quantum Gravity, Torsion, Parity Violation and all that
Laurent Freidel, Djordje Minic, Tatsu Takeuchi
11 pages
VPI-IPPAP-05-02
"We discuss the issue of parity violation in quantum gravity. In particular, we study the coupling of fermionic degrees of freedom in the presence of torsion and the physical meaning of the Immirzi parameter from the viewpoint of effective field theory. We derive the low-energy effective lagrangian which turns out to involve two parameters, one measuring the non-minimal coupling of fermions in the presence of torsion, the other being the Immirzi parameter. In the case of non-minimal coupling the effective lagrangian contains an axial-vector interaction leading to parity violation. Alternatively, in the case of minimal coupling there is no parity violation and the effective lagrangian contains only the usual axial-axial interaction. In this situation the real values of the Immirzi parameter are not at all constrained. On the other hand, purely imaginary values of the Immirzi parameter lead to violations of unitarity for the case of non-minimal coupling. Finally, the effective lagrangian blows up for the positive and negative unit imaginary values of the Immirzi parameter."
http://arxiv.org/abs/quant-ph/0507262
Fundamental gravitational limitations to quantum computing
Rodolfo Gambini, Rafael A. Porto, Jorge Pullin
3 pages no figures
LSU-REL-072105
"Lloyd has considered the ultimate limitations physics places on quantum computers. He concludes in particular that for an "ultimate laptop'' (a computer of one liter of volume and one kilogram of mass) the maximum number of operations per second is bounded by 10^{51}. The limit is derived considering ordinary quantum mechanics. Here we consider additional limits that are placed by quantum gravity ideas, namely the use of a relational notion of time and fundamental gravitational limits that exist on time measurements. We then particularize for the case of an ultimate laptop and show that the maximum number of operations is further constrained to 10^{47} per second."
Gambini and Pullin should be familiar to anyone watching the QG scene. their approach to QG is called "Consistent Discretizations". Last year they also published a resolution of the BH information paradox using relational time. An occasional poster here at PF, Edgar1813, has collaborated with Gambini and Pullin on QG research in much the same way as, for instance, the grad student Rafael Porto has. Here they are getting over into quantum computing (refering to Seth Lloyd) and applying gravitational limits to the ideal laptop.
In case anyone is interested here are other papers by Gambini Pullin et al.
http://arxiv.org/find/grp_physics/1/au:+Gambini/0/1/0/all/0/1
and a sampling of recent titles:
1. gr-qc/0505052
Classical and quantum general relativity: a new paradigm
2. gr-qc/0505043
Consistent discretizations: the Gowdy spacetimes
3. gr-qc/0505023
Discrete space-time
4. gr-qc/0501027
Fundamental decoherence in quantum gravity
5. gr-qc/0409057
Consistent discretization and loop quantum geometry
6. gr-qc/0409045
Unified model of loop quantum gravity and matter
7. gr-qc/0408050
Fundamental decoherence from relational time in discrete quantum gravity: Galilean covariance
9. hep-th/0406260
Realistic clocks, universal decoherence and the black hole information paradox
10. hep-th/0405183
No black hole information puzzle in a relational universe
15. gr-qc/0306095
Discrete quantum gravity: a mechanism for selecting the value of fundamental constants
John Stachel gave the opening talk at the Paris Einstein Century conference
http://einstein2005.obspm.fr/programmer.php
(Gerard 't Hooft gave the final talk, other speakers included Ashtekar, Rovelli, Brian Greene...)
maybe we should pay attention to what Stachel says.
he is a philosopher and historian of science
a specialist in the Philosophy of Science as relates to the current tectonic merging of the Gen Rel and Quantum Theory plates. Stachel seems to be really interested in quantum gravity and thinking about it at a foundations level
he works with ideas, does not pursue this or that bunch of equations, though may be competent and savy as regards equations.
Rovelli said in his book that there are periods in physics when the hard philosophical questions are not superfluous and when, just in order to make progress in physics, people have to ask really fundamental questions about What is Space and What is Time.
for most of the latter half of the 20th Century physicists mostly thought they didnt have to consider Foundations or Philosophy questions, they just had to charge ahead with "doing physics". Foundations issues were considered a digression, a waste of time, those things will take care of themselves just keep your eye on getting the next formula.
but from time to time, according to Rovelli, it pays to consider these frustrating (to me a bit dumb-sounding) basic philosophical questions. Maybe you never get it right, but at least you could get some fresh ideas.
So I am going to include this one article of John Stachel
http://arxiv.org/abs/gr-qc/0507078
Structure, Individuality and Quantum Gravity
John Stachel
39 pages, to appear in "Structural Foundations of Quantum Gravity," edited by D.P. Rickles, S.R.D. French and J. Saatsi Oxford University Press
part of the abstract: "After reviewing various interpretations of structural realism, I adopt here a definition that allows both relations between things that are already individuated (which I call "relations between things'') and relations that individuate previously un-individuated entities ("things between relations"). Since both space-time points in general relativity and elementary particles in quantum theory fall into the latter category, I propose a principle of maximal permutability as a criterion for the fundamental entities of any future theory of "quantum gravity''; i.e., a theory yielding both general relativity and quantum field theory in appropriate limits..."
table of contents:
"Contents
1 What is Structural Realism? 3
2 Structure and Individuality 5
3 Effective field theory approach and asymptotic quantization 11
4 String Theory 15
5 Quantum general relativity - some preliminary problems 17
- 5.1 States or Processes: Which is primary ? . . . . . . . . . . . . 17
- 5.2 Formalism and measurability . . . . . . . . . . . . . . . . . . . 20
6 Canonical quantization (loop quantum gravity). 25
7 The causal set (causet) approach 29
8 What Structures to Quantize? 31
9 Acknowledgements 33"
http://arxiv.org/astro-ph/0507683
Inflation: A graceful entrance from Loop Quantum Cosmology
N. J. Nunes (U. Minnesota)
9 pages, 8 figures
Abstract: "Dynamical scalar fields in the framework of loop quantum cosmology have recently risen a considerable amount of attention. This interest relates mainly to the natural way the initial conditions for slow roll inflation are set and the avoidance of a big crunch in closed models. In this work, the evolution of a scalar field is explored..."
Start of Introduction: "Currently, the leading background independent and non-perturbative candidate for a quantum theory of gravity is loop quantum gravity [1, 2, 3] which is a canonical quantization of general relativity based in Ashtekar’s variables. ... Loop quantum Cosmology(LQC) is the application of loop quantum gravity to homogeneous and isotropic mini-superspaces [4]. An important featureof LQC is that eigenvalues of the inverse scale factor operator..."
N.J.Nunes is a comparative newcomer to LQC, having earlier done research in string/brane cosmology.
Thru 2004 he was at Queen Mary
and is now at U. Minnesota.
He has 14 papers going back to 1998, several in cosmology-related topics.
The string/brane stuff was back around 2003, and possibly earlier.
EXCELLENT TALK BY SMOLIN ON VIDEO
http://cfa-www.harvard.edu/colloquia/spring05/smolin.html
this was a colloquium at Harvard for astronomy/astrophysics audience given in May 2005
covers relation of QG to predictions and observations
general survey of problem of QG, basics of Loop approach, contrast with string, gives a lot of intuition about LQG. explains where the various predictions come from, which they are still working on to get in final shape for AUGER and GLAST. about 50 minutes talk followed by questions from
astronomers and Smolin answering.
we just got DSL, what a difference! less than a minute to download the video. probably never would have seen this if we still had our older slow connection.
title was Astrophysical Tests of Quantum Gravity
but talk was, as I indicated, much broader and more generally useful, could serve as a good introduction to both the full LQG theory and to Loop Quantum Cosmology.
NEW ROVELLI PAPER
http://www.arxiv.org/abs/gr-qc/0508007
[B]From 3-geometry transition amplitudes to graviton states/B]
Federico Mattei (CPT), Carlo Rovelli (CPT), Simone Speziale (CPT), Massimo Testa (CPT)
18 pages
"In various background independent approaches, quantum gravity is defined in terms of a field propagation kernel: a sum over paths interpreted as a transition amplitude between 3-geometries, expected to project quantum states of the geometry on the solutions of the Wheeler-DeWitt equation. We study the relation between this formalism and conventional quantum field theory methods. We consider the propagation kernel of 4d Lorentzian general relativity in the temporal gauge, defined by a conventional formal Feynman path integral, gauge fixed a' la Fadeev--Popov. If space is compact, this turns out to depend only on the initial and final 3--geometries, while in the asymptotically flat case it depends also on the asymptotic proper time. We compute the explicit form of this kernel at first order around flat space, and show that it projects on the solutions of all quantum constraints, including the Wheeler-DeWitt equation, and yields the correct vacuum and n-graviton states. We also illustrate how the Newtonian interaction is coded into the propagation kernel, a key open issue in the spinfoam approach."
Spin_Network
Aug2-05, 03:54 PM
Marcus this needs slotting in:http://arxiv.org/abs/gr-qc/0508007
Marcus this needs slotting in:http://arxiv.org/abs/gr-qc/0508007
thanks I actually got to it yesterday afternoon and slotted it into the preceding post :smile:
I am glad you are watching out Spin Network, so will be less worried about missing things.
this paper was written a while back---an updated version would be nice to have---but it serves a useful purpose:
it is a kind of "Rosetta Stone" connecting LQG and Spin Foams to Dynamical Triangulations
http://arxiv.org/abs/gr-qc/0110026
Relating Covariant and Canonical Approaches to Triangulated Models of Quantum Gravity
Matthias Arnsdorf
28 pages, 10 figures
Class.Quant.Grav. 19 (2002) 1065-1092
"In this paper explore the relation between covariant and canonical approaches to quantum gravity and $BF$ theory. We will focus on the dynamical triangulation and spin-foam models, which have in common that they can be defined in terms of sums over space-time triangulations. Our aim is to show how we can recover these covariant models from a canonical framework by providing two regularisations of the projector onto the kernel of the Hamiltonian constraint. This link is important for the understanding of the dynamics of quantum gravity. In particular, we will see how in the simplest dynamical triangulations model we can recover the Hamiltonian constraint via our definition of the projector. Our discussion of spin-foam models will show how the elementary spin-network moves in loop quantum gravity, which were originally assumed to describe the Hamiltonian constraint action, are in fact related to the time-evolution generated by the constraint. We also show that the Immirzi parameter is important for the understanding of a continuum limit of the theory."
the first two references cited are papers by Ambjorn Jurkiewicz and Loll.
[1] J. Ambjorn, J. Jurkiewicz, and R. Loll. Lorentzian and euclidean quantum gravity: Analytical and numerical results. 1999, hep-th/0001124.
[2] J. Ambjorn, J. Jurkiewicz, and R. Loll. Dynamically triangulating lorentzian quantum gravity. Nucl. Phys., B610:347–382, 2001, hep-th/0105267.
It is clear that Arnsdorf is trying to put together a picture unifying AJL work with the quantum gravity of Rovelli, Baez, Smolin, Thiemann ...
Chronos has supplied a paper which appears to refute the Smolin CNS-multiverse conjecture
http://www.arxiv.org/abs/astro-ph/0508050
A 2.1 Solar Mass Pulsar Measured by Relativistic Orbital Decay
David J. Nice, Eric M. Splaver (Princeton), Ingrid H. Stairs (UBC), Oliver Loehmer, Axel Jessner (MPIfR), Michael Kramer (Jodrell Bank), James M. Cordes (Cornell)
9 pages, Submitted to ApJ
Here are some supporting articles on the physics of neutron stars which the present one cites
http://www.arxiv.org/abs/astro-ph/0405262
http://www.arxiv.org/abs/astro-ph/0002232
Here is a recent discussion of the CNS conjecture
http://www.arxiv.org/abs/hep-th/0407213
just speaking for myself, this discovery further tips the balance of interest
from Loop towards the Loll Triangulation approach to QG.
Chronos
Aug21-05, 04:41 AM
I totally agree, marcus. Not to claim any great insights, but, I believe the universe will ultimately submit to being described as a quantum computer.
I flagged this of Smolin in post #372 of this thread almost a month ago---on 25 July---and I didn't realize what a lot of fuss it would cause. There has been quite a racket in "the string community" about Smolin urging them to work out a non-perturbative formulation. No other paper, that has appeared in the month since, has had such repercussions. So I will bring this one forward to have the link handy.
http://arxiv.org/abs/hep-th/0507235
The case for background independence
Lee Smolin
46 pages, no figures
"The aim of this paper is to explain carefully the arguments behind the assertion that the correct quantum theory of gravity must be background independent. We begin by recounting how the debate over whether quantum gravity must be background independent is a continuation of a long-standing argument in the history of physics and philosophy over whether space and time are relational or absolute. This leads to a careful statement of what physicists mean when we speak of background independence. Given this we can characterize the precise sense in which general relativity is a background independent theory. The leading background independent approaches to quantum gravity are then discussed, including causal set models, loop quantum gravity and dynamical triangulations and their main achievements are summarized along with the problems that remain open. Some first attempts to cast string/M theory into a background independent formulation are also mentioned... "
Smolin has been discussing background independence in the open blog-place of ideas. for a sample, look at post #8 of this PF thread
http://www.physicsforums.com/showthread.php?t=85306
or scroll down the 100 or so posts in the "Landscape" thread at cosmicvariance.
Post #8 of the "smolin letter" thread is here
http://www.physicsforums.com/showthread.php?p=720463#post720463
and has a good sample of what smolin has been writing on that subject, besides what is already in the paper. It is interesting to see the reactions from the stringy flock, say from Jacques Distler (also on post #8) and also at the "String Coffee Table" blog.
http://golem.ph.utexas.edu/string/archives/000621.html
In that thread, Robert Helling displays a significant reaction to Smolin's paper. He says: "... Smolin’s arguments would appeal especially to outsiders of the field. We should prevent too many people from getting the idea that 'string theory is obviously wrong as it ignores the basic notion of background independence'. So I collected some thoughts that might help you arguing the next time you are envolved in this debate...."
Robert Helling's collected thoughts, to help in arguing against Smolin's suggestion that they put some effort into a nonperturbative string formulation, are online at:
http://www.damtp.cam.ac.uk/user/rch47/background.pdf
http://www.arxiv.org/abs/gr-qc/0508085
Quantum Black Holes: Entropy and Entanglement on the Horizon
Etera R. Livine, Daniel R. Terno
25 pages, 4 figures
"We are interested in black holes in Loop Quantum Gravity (LQG). We study the simple model of static black holes: the horizon is made of a given number of identical elementary surfaces and these small surfaces all behaves as a spin-s system accordingly to LQG. The chosen spin-s defines the area unit or area resolution, which the observer uses to probe the space(time) geometry. For s=1/2, we are actually dealing with the qubit model, where the horizon is made of a certain number of qubits. In this context, we compute the black hole entropy and show that the factor in front of the logarithmic correction to the entropy formula is independent of the unit s. We also compute the entanglement between parts of the horizon. We show that these correlations between parts of the horizon are directly responsible for the asymptotic logarithmic corrections. This leads us to speculate on a relation between the evaporation process and the entanglement between a pair of qubits and the rest of the horizon. Finally, we introduce a concept of renormalisation of areas in LQG."
All I can say right now is that I've watched Livine's research for a couple of years and I'm impressed. If he is doing something with LQG and black holes it is probably worth doing. Livine's thesis came out in 2003, if I remember, and we flagged it at PF and had a look. The name Terno is a new one to me.
I now see that Livine and Terno have already published a paper this year in Physical Review A
and also I see that they have a paper with Girelli in preparation, called
F. Girelli, E. R. Livine, D. R. Terno, Reconstructing Quantum Geometry from Quantum Information: Entanglement as a Measure of Distance
(as Kea once said, "where is SetAI when we need him?")
http://www.arxiv.org/abs/gr-qc/0508088
Finiteness and Dual Variables for Lorentzian Spin Foam Models
Wade Cherrington
"We describe here some new results concerning the Lorentzian Barrett-Crane model, a well-known spin foam formulation of quantum gravity. Generalizing an existing finiteness result, we provide a concise proof of finiteness of the partition function associated to all non-degenerate triangulations of 4-manifolds and for a class of degenerate triangulations not previously shown. This is accomplished by a suitable re-factoring and re-ordering of integration, through which a large set of variables can be eliminated. The resulting formulation can be interpreted as a 'dual variables' model that uses hyperboloid variables associated to spin foam edges in place of representation variables associated to faces. We outline how this method may also be useful for numerical computations, which have so far proven to be very challenging for Lorentzian spin foam models."
The name Cherrington is also a new one. I can only take note of this paper, to evaluate later. Cherrington is at UWO, where Dan Christensen is. He might be a grad student or postdoc working with Dan. Looks like they may collaborate on a paper. At UWO they do computer calculation with spin foams, John Baez worked with them on this at UWO. It is one of the places where advanced computer facilities and techniques is paired with QG.
http://www.arxiv.org/abs/gr-qc/0508091
Background independent quantizations: the scalar field I
Wojciech Kaminski, Jerzy Lewandowski, Marcin Bobienski
13 pages
"We are concerned with the issue of quantization of a scalar field in a diffeomorphism invariant manner. We apply the method used in Loop Quantum Gravity. It relies on the specific choice of scalar field variables referred to as the polymer variables. The quantization, in our formulation, amounts to introducing the 'quantum' polymer *-star algebra and looking for positive linear functionals, called states. The assumed in our paper homeomorphism invariance allows to determine a complete class of the states. Except one, all of them are new. In this letter we outline the main steps and conclusions, and present the results: the GNS representations, characterization of those states which lead to essentially self adjoint momentum operators (unbounded), identification of the equivalence classes of the representations as well as of the irreducible ones. The algebra and topology of the problem, the derivation, all the technical details and more are contained in the paper-part II."
Lewandowski is by now a familiar face. he is the L in the LOST (Lewandowski, Okolow, Sahlmann, Thiemann) uniqueness theorem.
Also a frequent-coauthor with Ashtekar.
Lewandowski cites the recent Smolin paper. Here is the first paragraph
The phrase "background independent theory" means in Physics a theory defined on a bare manifold endowed with no extra structure like geometry or fixed coordinates. A prominent example is the theory of matter fields coupled to Einstein’s gravity. In the case of a background independent classical theory it is natural to assume the background independence in a corresponding quantum theory.
A profound polemic devoted to that issue can be found in recent paper of Smolin [CITES "THE CASE FOR BACKGROUND INDEPENDENCE"]. The canonical formulation of the field theory relies on the 3 + 1 decomposition of space-time into the "space" M and "time" R. Then, the background independence implies invariance with respect to the diffeomorphisms of M. The invariance concerns in particular any matter fields in question: they have to be quantized in an often new, background independent way. In this letter and the accompanying paper [2] we are concerned with the issue of a diffeomorphism invariant quantization of a scalar field.
http://www.arxiv.org/abs/gr-qc/0508100
On the quantum origin of the seeds of cosmic structure
Alejandro Perez, Hanno Sahlmann, Daniel Sudarsky
"The current understanding of the quantum origin of cosmic structure is discussed critically. We point out that in the existing treatments a transition from a symmetric quantum state to an (essentially classical) non-symmetric state is implicitly assumed, but not specified or analyzed in any detail. In facing the issue we are led to conclude that new physics is required to explain the apparent predictive power of the usual schemes. Furthermore we show that the new way of looking at the relevant issues opens new windows from where relevant information might be extracted regarding cosmological issues and perhaps even clues about aspects of quantum gravity."
We know Sahlmann from the LQG uniqueness theorem. here he is taking quite a different direction
------from conclusions section pages 44 through 48----
We have discussed the problematic part of the standard analysis that is supposed to predict the primordial spectrum of fluctuations responsible for the deviation of our universe from perfect homogeneity and isotropy and in particular for the eventual evolution of galaxies stars and our own. We have argued that there is an essential element that is missing in existing proposals. We have argued that the missing element must contain some new physics. We have considered this issue following the line of thought exposed by Penrose, that such new physics might be tied to some quantum aspect of gravitation, and we have employed this idea in what we called the collapse hypothesis, which is reflected concretely in our model in the fact that we take the Newtonian potential to couple to expectation values of the quantum matter degrees of freedom, and have allowed such expectation values to “jump” in association with the so called collapse process in a particular set of states. It should thus be emphasized that this can be justified only if we declare that gravitation is, at the quantum level profoundly different from other degrees of freedom as only such posture would justify the different treatment awarded to both the gravitational and the scalar sectors in the present work. We have shown that a relatively simple proposal concerning a collapse of the wave function induced by some unknown mechanism, possibly tied to Quantum Gravity can in fact account in a transparent way, for the scale invariant spectrum that seems to fit very well with the observations. ....
....
....
We end by noting a paradoxical aspect of the situation in our field of study: On the one hand there is an almost frenetic search for any form of experimental manifestations of any conceivable aspect of quantum gravity, while on the other hand, when faced with one such clear arena for these type studies, as the one we have treated in this work, the prevailing attitude seems to be to hide the mysteries under the rug and declare that everything is fine. It is our hope that this paper contributes to changing this situation.
----end quote----
Spin_Network
Aug24-05, 08:37 PM
Great paper marcus, thanks again, and a great excuse to hit a significant number! :) shuttle re-entry speed !
Glad you approve, Spin_Network! Just to condense this down to the essentials:
http://www.arxiv.org/abs/gr-qc/0508100
On the quantum origin of the seeds of cosmic structure
Alejandro Perez, Hanno Sahlmann, Daniel Sudarsky
---quote conclusions page 48---
We end by noting a paradoxical aspect of the situation in our field of study: On the one hand there is an almost frenetic search for any form of experimental manifestations of any conceivable aspect of quantum gravity, while on the other hand, when faced with one such clear arena for these type studies, as the one we have treated in this work, the prevailing attitude seems to be to hide the mysteries under the rug and declare that everything is fine. It is our hope that this paper contributes to changing this situation.
---end quote---
I normally refrain from complaining about getting my brain fried because I think it is more considerate of other people not to fuss about it. an inevitable side-effect we all have to cope with occasionally. but I will acknowledge that the seeds of structure in quantum fluctuations business is something of an enigma.
http://www.arxiv.org/abs/gr-qc/0508106
On the perturbative expansion of a quantum field theory around a topological sector
Carlo Rovelli, Simone Speziale
7 pages
"The idea of treating general relativistic theories in a perturbative expansion around a topological theory has been recently put forward in the quantum gravity literature. Here we investigate the viability of this idea, by applying it to conventional Yang--Mills theory on flat spacetime. We find that the expansion around the topological theory coincides with the usual expansion around the abelian theory, though the equivalence is non-trivial. In this context, the technique appears therefore to be viable, but not to bring particularly new insights. Some implications for gravity are discussed."
With this next one, Noldus is a postdoc of Loll at Utrecht. I don't know how this fits in. It does not look like CDT.
http://www.arxiv.org/abs/gr-qc/0508104
A fully consistent relativistic quantum mechanics and a change of perspective on quantum gravity
Johan Noldus
16 pages
"This paper can be seen as an exercise in how to adapt quantum mechanics from a strict relativistic perspective while being respectful and critical towards the experimental achievements of the contemporary theory. The result is a fully observer independent relativistic quantum mechanics for N particle systems without tachyonic solutions. A remaining worry for the moment is Bell's theorem."
Reuter and Lauscher are going to be at the October Loops 05
and they have a different approach to quantum gravity where they just flat out quantize classical Einstein equation and renormalize the sucker.
(no loops, no strings, no networks, no foams, no triangles)
this was supposed not to be possible, but Reuter apparently very determined and stubborn---insists on it being possible.
selfAdjoint was reading an earlier Reuter paper and had some comments, maybe we can get some clues from him about this paper too. ohwilleke too.
Reuter is one of the invited speakers at Loops 05 conference..
He says he has "made contact" with Loll triangulation path integral gravity.
that would be news if it is what it sounds like
http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
20 pages
"Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an asymptotically safe theory which is applicable at arbitrarily small distance scales. On sub-Planckian distances it predicts that spacetime is a fractal with an effective dimensionality of 2. The original argument leading to this result was based upon the anomalous dimension of Newton's constant. In the present paper we demonstrate that also the spectral dimension equals 2 microscopically, while it is equal to 4 on macroscopic scales. This result is an exact consequence of asymptotic safety and does not rely on any truncation. Contact is made with recent Monte Carlo simulations."
Interestingly, Loll got something fractally and 2D looking at very small scale in a macro-4D world----measuring the spectral dimension with a diffusion process. looks like Reuter got something like that (very different approach)
Because this looks like a CONVERGENCE of Loll and Reuter approaches to a quantum spacetime dynamics, I have put some discussion of this Reuter paper in the Loll thread "Quantum Graffiti":
http://www.physicsforums.com/showthread.php?p=727388#post727388
selfAdjoint
Aug28-05, 11:17 PM
The business about quantizing GR turns on the existing concept of asymptotic safety, which itself turns on the behavior of the renormalization group fixed point. I don't have my notes and I'm too tired tonight to post on it, but I'll try to put something up tomorrow. Asymptotic safety, if it is accepted, could revolutionize a lot of old abandoned ideas.
http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
I hadn't come across these guys before. Very nice program they have going, called Quantum Einstein Gravity. See the series of papers
http://www.arxiv.org/find/hep-th/1/au:+Lauscher_O/0/1/0/all/0/1
They appear to have put CDT into a slightly more fundamental context (if the results do indeed match up). selfAdjoint, I'm looking forward to you explaining your notes on this.
Cheers
Kea :smile:
...this was supposed not to be possible, but Reuter apparently very determined and stubborn...insists on it being possible.
The family of Lagrangians idea that they work with is very nice. The so-called impossibility stems from not thinking this way, and insisting that something like a standard path integral over geometries should do the trick. They also stress that they study effective theories and make no claims that the full theory should be described in these terms, unlike some of the CDT papers.
:smile:
if anyone is feeling extra interested in Reuter's work we have this thread from around a year ago:
http://www.physicsforums.com/showthread.php?t=47209
and also I think some more recent (e.g. July 18) in this "reference link library" thread
http://www.physicsforums.com/showthread.php?p=682965#post682965
(this is the "Big Bang to asympt. de Sitter" paper, the earlier one from last October might be of more interest, obviating dark energy by running newton G)
http://arxiv.org/gr-qc/0508118
Degenerate Configurations, Singularities and the Non-Abelian Nature of Loop Quantum Gravity
Martin Bojowald
24 pages, 1 figure
Report-no: AEI-2005-133
"Degenerate geometrical configurations in quantum gravity are important to understand if the fate of classical singularities is to be revealed. However, not all degenerate configurations arise on an equal footing, and one must take into account dynamical aspects when interpreting results: While there are many degenerate spatial metrics, not all of them are approached along the dynamical evolution of general relativity or a candidate theory for quantum gravity. For loop quantum gravity, relevant properties and steps in an analysis are summarized and evaluated critically with the currently available information, also elucidating the role of degrees of freedom captured in the sector provided by loop quantum cosmology. This allows an outlook on how singularity removal might be analyzed in a general setting and also in the full theory. The general mechanism of loop quantum cosmology will be shown to be insensitive to recently observed unbounded behavior of inverse volume in the full theory. Moreover, significant features of this unboundedness are not a consequence of inhomogeneities but of non-Abelian effects which can also be included in homogeneous models."
http://arxiv.org/gr-qc/0508120
Quantum Gravity: General Introduction and Recent Developments
Claus Kiefer
Comments: 21 pages, 6 figures, invited contribution for "Annalen der Physik"
"I briefly review the current status of quantum gravity. After giving some general motivations for the need of such a theory, I discuss the main approaches in quantizing general relativity: Covariant approaches (perturbation theory, effective theory, and path integrals) and canonical approaches (quantum geometrodynamics, loop quantum gravity). I then address quantum gravitational aspects of string theory. This is followed by a discussion of black holes and quantum cosmology. I end with some remarks on the observational status of quantum gravity."
http://arxiv.org/gr-qc/0508109
Causal sets and the deep structure of spacetime
Fay Dowker
17 pages. Article contributed to "100 Years of Relativity - Space-time Structure: Einstein and Beyond" ed Abhay Ashtekar (World Scientific)
"The causal set approach to quantum gravity embodies the concepts of causality and discreteness. This article explores some foundational and conceptual issues within causal set theory."
getting back to this, probably important, paper. I will get some Wiki links that could help people interested in reading it
First, here is the paper
http://www.arxiv.org/abs/hep-th/0508202
Fractal Spacetime Structure in Asymptotically Safe Gravity
O. Lauscher, M. Reuter
20 pages
"Four-dimensional Quantum Einstein Gravity (QEG) is likely to be an asymptotically safe theory which is applicable at arbitrarily small distance scales. On sub-Planckian distances it predicts that spacetime is a fractal with an effective dimensionality of 2. The original argument leading to this result was based upon the anomalous dimension of Newton's constant. In the present paper we demonstrate that also the spectral dimension equals 2 microscopically, while it is equal to 4 on macroscopic scales. This result is an exact consequence of asymptotic safety and does not rely on any truncation. Contact is made with recent Monte Carlo simulations."
.... a CONVERGENCE of Loll and Reuter approaches to a quantum spacetime dynamics, I have put some discussion of this Reuter paper in the Loll thread "Quantum Graffiti":
http://www.physicsforums.com/showthread.php?p=727388#post727388
Now here are some relevant Wiki links
http://en.wikipedia.org/wiki/Path_integral_formulation
http://en.wikipedia.org/wiki/Effective_field_theory
http://en.wikipedia.org/wiki/Renormalizable
I should warn that Lubos Motl seems to have inserted string buzz into a number of Wiki entries, which one can sometimes discover by clicking on the entry's "history" tab. In connection with some biased language one may find in the history that it was supplied by "Lumidek" or other signature. As for example in "Renormalizable" entry where one sees Lumidek in connection with this:
"On the other hand, Fermi's interaction or general relativity are non-renormalizable. This problem is a hint that these theories should be replaced by a more complete theory at very short distance. These more complete theories are the electroweak theory and string theory, respectively."
The nonrenormalizability of Gen Rel is discussed in the literature and several conclusions are drawn. The slanted and overly simple inference (that Motl gives here) is challenged, for example by Rovelli in his book. Rovelli gives an argument why the simple analogy with electroweak doesn't work and that a different conclusion can be drawn. But this does not matter, I think----soon one adjusts to the fact that string propaganda has been inserted in parts of the Wiki environment, and one can filter it out.
On the whole Wiki is extremely useful! Here is more Wiki stuff:
http://en.wikipedia.org/wiki/Renormalization
http://en.wikipedia.org/wiki/Renormalization_group
http://en.wikipedia.org/wiki/Asymptotic_freedom
http://www.arxiv.org/abs/gr-qc/0508124
Graviton propagator from background-independent quantum gravity
Carlo Rovelli
6 pages
"We study the graviton propagator in euclidean loop quantum gravity, using the spinfoam formalism. We use boundary-amplitude and group-field-theory techniques, and compute one component of the propagator to first order, under a number of approximations, obtaining the correct spacetime dependence. In the large distance limit, the only term of the vertex amplitude that contributes is the exponential of the Regge action: the other terms, that have raised doubts on the physical viability of the model, are suppressed by the phase of the vacuum state, which is determined by the extrinsic geometry of the boundary."
http://www.arxiv.org/abs/hep-th/0509010
The Universe from Scratch
R. Loll, J. Ambjorn, J. Jurkiewicz
30 pages, 5 figures; review paper commissioned by Contemporary Physics and aimed at a wider physics audience
I started a thread for this, for any comments:
http://www.physicsforums.com/showthread.php?p=732021#post732021
a paper like this has been needed.
the authors discuss the connection with Martin Reuter work ("quantum einstein gravity", assymptotic safety)
http://www.arxiv.org/abs/gr-qc/0509039
Bibliography of Publications related to Classical Self-dual variables and Loop Quantum Gravity
Alejandro Corichi, Alberto Hauser
45 pages
"This bibliography attempts to give a comprehensive overview of all the literature related to what is known as the Ashtekar-Sen connection and the Rovelli-Smolin loop variables, from which the program currently known as Loop Quantum Gravity emerged..."
===================================
Some Loops 05 links:
http://loops05.aei.mpg.de/
http://loops05.aei.mpg.de/index_files/Programme.html
http://loops05.aei.mpg.de/index_files/Participants.html
http://loops05.aei.mpg.de/index_files/abstract_rovelli.html
http://loops05.aei.mpg.de/index_files/abstract_reuter.html
http://loops05.aei.mpg.de/index_files/abstract_loll.html
http://loops05.aei.mpg.de/index_files/abstract_freidel.html
http://loops05.aei.mpg.de/index_files/abstract_starodubtsev.html
..and so on..
the full program was posted Friday 9 September, a month before the start of conference
http://www.arxiv.org/abs/gr-qc/0509049
Eigenvalues of the volume operator in loop quantum gravity
Krzysztof A. Meissner
12 pages
"We present a simple method to calculate certain sums of the eigenvalues of the volume operator in loop quantum gravity. We derive the asymptotic distribution of the eigenvalues in the classical limit of very large spins which turns out to be of a very simple form. The results can be useful for example in the statistical approach to quantum gravity."
Sample from the conclusions section at the end:
"...Besides being mathematically interesting, these results can be important for example in the statistical approach to loop quantum gravity. Several conceptual problems arise in this context by the results of the present paper. The first one is what observables (besides the volume) should be specified to actually describe a macroscopic state in a given region. Since the contribution to the volume from a given 4-valent vertex grows like j1j2j3 it is important to know whether these observables provide suppression of the contribution from very large spins rendering final expectation value of the volume finite. The second problem is connected with the first one and concerns the bulk entropy: there are many sets of graphs with many different assignments of spins on legs that give (macroscopically) the same volume. If we specify both the volume and the other observables we are left with a certain number of graphs. The question is, whether the logarithm of this number is connected with the bulk entropy much as sequences of spins are connected with the black hole surface entropy [7]."
we know Meissner from his earlier work with Lewandowski, calculating BH entropy (http://arxiv.org/gr-qc/0407052 ) and estimating Immirzi parameter.
Popular articles:
One by Rovelli from Physics World (November 2003)
http://cgpg.gravity.psu.edu/people/Ashtekar/articles/rovelli03.pdf
Ashtekar's collection of semipopular QG articles
http://cgpg.gravity.psu.edu/people/Ashtekar/articles.html
Loll's collection of articles for general public:
http://www.phys.uu.nl/~loll/Web/press/press.html
Lee Smolin's review, published in American Scientist, of Harry Collins' book on the search for gravity waves.
http://www.americanscientist.org/template/BookReviewTypeDetail/assetid/45917?&print=yes
also Jaron Lanier's review of Roger Penrose The Road to Reality
http://www.americanscientist.org/template/BookReviewTypeDetail/assetid/45919?&print=yes
===================================
LQG may have taken root at China's Beijing University
http://www.arxiv.org/abs/gr-qc/0509064
Fundamental Structure of Loop Quantum Gravity
Muxin Han, Weiming Huang, Yongge Ma
76 pages, submitted to Review of Modern Physics
"In this review, the fundamental structure of loop quantum gravity is presented pedagogically. Our main aim is to help non-experts to understand the motivations, basic structures, as well as general results. We will focus on the theoretical framework itself, rather than its applications, and do our best to write it in modern and precise langauge while keeping the presentation accessible for beginners. After reviewing the classical connection dynamical formalism of general relativity, as a foundation, the construction of kinematical Ashtekar-Isham-Lewandowski representation is introduced in the content of quantum kinematics. In the content of quantum dynamics, we mainly introduce the construction of a Hamiltonian constraint operator and the master constraint project. It should be noted that this strategy of quantizing gravity can also be extended to obtain other background independent quantum gauge theories. There is no divergence within this background independent and diffeomorphism invariant quantization programme of matter coupled to gravity."
I see that Yong-ge Ma is presenting a paper in October at Loops 05.
http://www.arxiv.org/abs/gr-qc/0509075
Quantum geometry and the Schwarzschild singularity
Abhay Ashtekar, Martin Bojowald
31 pages, 1 figure
IGPG-05-09/01, AEI-2005-132
"In homogeneous cosmologies, quantum geometry effects lead to a resolution of the classical singularity without having to invoke special boundary conditions at the singularity or introduce ad-hoc elements such as unphysical matter. The same effects are shown to lead to a resolution of the Schwarzschild singularity. The resulting quantum extension of space-time is likely to have significant implications to the black hole evaporation process. Similarities and differences with the situation in quantum geometrodynamics are pointed out."
http://www.arxiv.org/abs/gr-qc/0509080
Positivity in Lorentzian Barrett-Crane Models of Quantum Gravity
J. Wade Cherrington, J. Daniel Christensen
13 pages
"The Barrett-Crane models of Lorentzian quantum gravity are a family of spin foam models based on the Lorentz group. We show that for various choices of edge and face amplitudes, including the Perez-Rovelli normalization, the amplitude for every triangulated closed 4-manifold is a non-negative real number. Roughly speaking, this means that if one sums over triangulations, there is no interference between the different triangulations. We prove non-negativity by transforming the model into a "dual variables'' formulation in which the amplitude for a given triangulation is expressed as an integral over three copies of hyperbolic space for each tetrahedron. Then we prove that, expressed in this way, the integrand is non-negative. In addition to implying that the amplitude is non-negative, the non-negativity of the integrand is highly significant from the point of view of numerical computations, as it allows statistical methods such as the Metropolis algorithm to be used for efficient computation of expectation values of observables."
http://www.arxiv.org/abs/gr-qc/0509078
Loop quantum black hole
Leonardo Modesto
11 pages
"In this paper we consider the Kantowski-Sachs space-time in Ashtekar variables and the quantization of this space-time starting from the complete loop quantum gravity theory. The Kanthowski-Sachs space-time coincides with the Schwarzschild black hole solution inside the horizon. By studying this model we can obtain information about the black hole singularity and about the dynamics across the point r=0. We studied this space-time in ADM variables in two previous papers where we showed that the classical black hole singularity disappears in quantum theory. In this work we study the same model in Ashtekar variables and we obtain a regular space-time inside the horizon region and that the dynamics can be extend further the classical singularity."
---quote---
An important consequence of the quantization is that, unlike the classical evolution, the quantum evolution doesn’t stop at the classical singularity and the “other side” of the singularity corresponds to a new domain where the triad reverses its orientation. This work is useful if we want understand what is the mechanism to resolve the problem of the “information loss” in the process of black hole formation [15].
---end quote---
just a little paper on DSR (doubly special relativity) by a new researcher at the perimeter institute, Jonathan Hackett. havent heard of him before. undergraduate physics major University of Waterloo in 2003, maybe a graduate student there now
http://arxiv.org/gr-qc/0509103
Asymptotic Flatness in Rainbow Gravity
Jonathan Hackett
13 pages
"A construction of conformal infinity in null and spatial directions is constructed for the Rainbow-flat space-time corresponding to doubly special relativity. From this construction a definition of asymptotic DSRness is put forward which is compatible with the correspondence principle of Rainbow gravity. Furthermore a result equating asymptotically flat space-times with asymptotically DSR spacetimes is presented."
curious thing is that rainbow-gravity has a RUNNING METRIC which is something we also saw with some Martin Reuter's work.
in the scheme Hackett uses (due to Smolin and Maguiejo IIRC) the metric runs with energy which is how Reuter's QEG does too. so the metric depends on the scale that you are looking at the geometry---what microscope you use to look at the geometry.
so what does this paper signify? maybe just that Lee Smolin has gotten another potential grad student who is interested in the rainbowgravity scheme (energy or scale dependent metric) and in DSR.
i expect it has to do with the upcoming AUGER and GLAST observational tests
footnote: the "rainbow" could be a reference to the fact that in this approach you dont has just one metric on the manifold, you can have a whole SPECTRUM of metrics. corresponding (as often the case with spectra) to different energies or scales. strange idea, odd that at least superficially it resembles something that came up with Reuter QEG
=======================
Ashtekar presentation, lecture notes
http://www.phys.psu.edu/events/index.html
select "this semester" from the menu. Abhay talk is at top of list:
THE OTHER SIDE OF THE BIG BANG An Analytical and Numerical Study
IGPG Seminar by Abhay Ashtekar from Penn State
Monday at 3:00 PM in 318 Osmond (8/29/2005)
click on "presentation" to download
http://www.phys.psu.edu/events/index.html?event_id=1257&event_type_ids=0&span=2005-08-20.2005-12-25
http://arxiv.org/gr-qc/0509118
On the regularization ambiguities in loop quantum gravity
Alejandro Perez
21 pages
http://arxiv.org/hep-th/0509192
Coherent States for 3d Deformed Special Relativity: semi-classical points in a quantum flat spacetime
E.R. Livine, D. Oriti
20 pages
"We analyse the quantum geometry of 3-dimensional deformed special relativity (DSR) and the notion of spacetime points in such a context, identified with coherent states that minimize the uncertainty relations among spacetime coordinates operators. We construct this system of coherent states in both the Riemannian and Lorentzian case, and study their properties and their geometric interpretation."
http://www.arxiv.org/abs/gr-qc/0510011
Quantum Spin Dynamics VIII. The Master Constraint
Thomas Thiemann
19 pages
AEI-2005-152
"Recently the Master Constraint Programme (MCP) for Loop Quantum Gravity (LQG) was launched which replaces the infinite number of Hamiltonian constraints by a single Master constraint. The MCP is designed to overcome the complications associated with the non -- Lie -- algebra structure of the Dirac algebra of Hamiltonian constraints and was successfully tested in various field theory models. For the case of 3+1 gravity itself, so far only a positive quadratic form for the Master Constraint Operator was derived. In this paper we close this gap and prove that the quadratic form is closable and thus stems from a unique self-adjoint Master Constraint Operator. The proof rests on a simple feature of the general pattern according to which Hamiltonian constraints in LQG are constructed and thus extends to arbitrary matter coupling and holds for any metric signature. With this result the existence of a physical Hilbert space for LQG is established by standard spectral analysis."
As Thomald Thiemannegger said: "I'll be back."
selfAdjoint
Oct4-05, 08:41 PM
Huzzah! How ya like THEM apples, Lubos!
:biggrin:
more tonight.
http://www.arxiv.org/abs/gr-qc/0510014
Master Constraint Operator in Loop Quantum Gravity
Muxin Han, Yongge Ma
11 pages, no figures
"We introduce a Master Constraint Operator ˆMˆ densely defined in the diffeomorphism invariant Hilbert space in loop quantum gravity. The corresponding quadratic form coincides with the one proposed by Thiemann in the master constraint programme. It is shown that ˆMˆ is positive and symmetric, and hence has its Friedrichs self-adjoint extension. So the master constraint programme for loop quantum gravity can be carried out in principle by employing ˆMˆ."
taking root at Beijing Normal
Yongge Ma is presenting a paper at Loops 05 in a week or so
=====================
something from Thanu Padmanabhan (Snark Hunting fan)
http://www.arxiv.org/abs/gr-qc/0510015
A new perspective on Gravity and the dynamics of Spacetime
T. Padmanabhan
Based on the Essay selected for Honorable Mention in the Gravity Research Foundation Essay Contest, 2005; to appear in the special issue of IJMPD
"The Einstein-Hilbert action has a bulk term and a surface term (which arises from integrating a four divergence). I show that one can obtain Einstein's equations from the surface term alone. This leads to: (i) a novel, completely self contained, perspective on gravity and (ii) a concrete mathematical framework in which the description of spacetime dynamics by Einstein's equations is similar to the description of a continuum solid in the thermodynamic limit."
Fabien Besnard heard T.P. give this talk in Paris at the Einstein Century conference this summer. Fabien blogged about it. thought it was one of the more interesting things from the conference. I can't evaluate. But Padmanabhan has done interesting stuff in the past.
http://www.arxiv.org/abs/gr-qc/0510022
Correspondence between Loop-inspired and Braneworld Cosmology
Edmund J. Copeland, James E. Lidsey, Shuntaro Mizuno
"Braneworld scenarios are motivated by string/M-theory and can be characterized by the way in which they modify the conventional Friedmann equations of Einstein gravity. An alternative approach to quantum gravity, however, is the loop quantum cosmology program. In the semi-classical limit, the cosmic dynamics in this scenario can also be described by a set of modified Friedmann equations. We demonstrate that a dynamical correspondence can be established between these two paradigms at the level of the effective field equations. This allows qualitatively similar features between the two approaches to be compared and contrasted..."
===============
Jorge Pullin's newsletter
http://www.arxiv.org/abs/gr-qc/0510021
Matters of Gravity
================
John Baez talk at Loops 05
http://math.ucr.edu/home/baez/loops05/
the 18 page lecture notes:
http://math.ucr.edu/home/baez/loops05/loops05.pdf
selfAdjoint
Oct6-05, 09:42 PM
Here's another from tonight Two of the authors are the same as the ones of "Master Constraint M" above. Intended as a pedagogical introduction for beginners.
Fundamental Structure of Loop Quantum Gravity
http://www.arxiv.org/abs/gr-qc/0509064
Muxin Han, Weiming Huang, Yongge Ma
From the abstract:
In this review, the fundamental structure of loop quantum gravity is presented pedagogically. Our main aim is to help non-experts to understand the motivations, basic structures, as well as general results. We will focus on the theoretical framework itself, rather than its applications, and do our best to write it in modern and precise langauge while keeping the presentation accessible for beginners. After reviewing the classical connection dynamical formalism of general relativity, as a foundation, the construction of kinematical Ashtekar-Isham-Lewandowski representation is introduced in the content of quantum kinematics. In the content of quantum dynamics, we mainly introduce the construction of a Hamiltonian constraint operator and the master constraint project. It should be noted that this strategy of quantizing gravity can also be extended to obtain other background independent quantum gauge theories. There is no divergence within this background independent and diffeomorphism invariant quantization programme of matter coupled to gravity
Sergei Alexandrov is one of the Utrecht people
http://www.arxiv.org/abs/gr-qc/0510050
Reality conditions for Ashtekar gravity from Lorentz-covariant formulation
Sergei Alexandrov
14 pages
ITP-UU-05/45, SPIN-05/31
"We show the equivalence of the Lorentz-covariant canonical formulation considered for the Immirzi parameter beta = i to the selfdual Ashtekar gravity. We also propose to deal with the reality conditions in terms of Dirac brackets derived from the covariant formulation and defined on an extended phase space which involves, besides the selfdual variables, also their anti-selfdual counterparts."
If I remember, he was getting his PhD in String at Paris around 2002 or 2003, when he was also sometimes co-authoring in loop/foam with Etera Livine. Then he went to Utrecht for postdoc. All this time he is refusing to go along with anybody and take conventional approaches. He liked to do LQG and spinfoam but only in his own way. He suggests that when Astekar at the begining put the Immirzi equal to square root -1 this was right and that later Ashtekar and the others took the wrong turn by making Immirzi real. To me Sergei seems determined to eventually prove that his way is right and that everybody else took the wrong turn of the road. I admire what I interpret as a polite independence of thought and civilized willingness to take risks.
here's a picture of Sergei
http://www1.phys.uu.nl/spinoza/members/Sergei.htm
In the acknowledgements he appreciates talks with two other Utrecht people Loll and Hanno Sahlmann.
===============
We all know that in a sense Lee Smolin is chained to a rock and the tide is coming in----the tide called GLAST.
From the core version of LQG, as Smolin presents it, there is a prediction that the speed of light should depend slightly on the energy of the photon. Very energetic photons should go just enough faster that after a GRB blast has traveled for a billion years the harder more energetic one should arrive just a little early.
We have Smolin's "Falsifiable Predictions..." paper assuring us. Now John Ellis and Nick Mavromatos would like to constrain or narrow down the amount of that speed-variation. The more they narrow it the more they pinch LQG.
Or think of it as a tide of data, for photons of higher and higher energy, that still inexorably confirms that they are traveling at indistinguishable speed---all seem so far to be traveling same speed----and the energy level at which one has checked this is rising. Anyway that is the cartoon version.
So let's look at the latest in this story, just posted today:
http://www.arxiv.org/abs/astro-ph/0510172
Robust Limits on Lorentz Violation from Gamma-Ray Bursts
John Ellis (CERN), Nick E. Mavromatos (King's Coll., London), Dimitri V. Nanopoulos (Texas A-M & HARC, Woodlands & Athens Academy), Alexander S. Sakharov (CERN & ETHZ), Edward K.G. Sarkisyan (CERN & Univ. Manchester)
18 pages, 4 figures
"We constrain the possibility of a non-trivial refractive index in free space corresponding to an energy-dependent velocity of light: c(E) \simeq c_0 (1 - E/M), where M is a mass scale that might represent effect of quantum-gravitational space-time foam, using the arrival times of sharp features observed in the intensities of radiation with different energies from a large sample of gamma-ray bursters (GRBs) with known redshifts. We use wavelet techniques to identify genuine features, which we confirm in simulations with artificial added noise. Using the weighted averages of the time-lags calculated using correlated features in all the GRB light curves, we find a systematic tendency for more energetic photons to arrive earlier. However, there is a very strong correlation between the parameters characterizing an intrinsic time-lag at the source and a distance-dependent propagation effect. Moreover, the significance of the earlier arrival times is less evident for a subsample of more robust spectral structures. Allowing for intrinsic stochastic time-lags in these features, we establish a statistically robust lower limit: M > 0.9x10^{16} GeV on the scale of violation of Lorentz invariance."
==============
I am very surprised to see this:
"...Using the weighted averages of the time-lags calculated using correlated features in all the GRB light curves, we find a systematic tendency for more energetic photons to arrive earlier. However..."
From Ellis and Mavromatos I only remember seeing observations ruling out any variation in the speed of light with energy----and this is consistent in spirit with their "However..."
It is very surprising to see that this time they allow for a slight possible tendency for the more energetic ones to arrive earlier.
Maybe the tide is destined to rise only so far up the rock where Smolin is chained, and he will survive this test. It only takes just a very tiny variation, so that the more energetic ones (these super hard gammarays) arrive (after all have been traveling a billion years) just a very tiny bit before the others.
============
John Baez student named Derek Wise has posted a 61 page paper:
http://www.arxiv.org/abs/gr-qc/0510033
Lattice p-Form Electromagnetism and Chain Field Theory
"... 'chain field theory' -- a theory analogous to topological quantum field theory, but with chain complexes replacing manifolds..."
Thanks to Spin_Network for pointing out this preprint and providing some background:
In this post
http://www.physicsforums.com/showthread.php?p=782595#post782595
SN recalled this from Baez on SPR
http://groups.google.com/group/sci.physics.research/browse_frm/thread/6d8038e9fd92cf2/efe70e642e41021f?hl=en&lr=&ie=UTF-8&rnum=1&prev=/groups%3Fhl%3Den%26lr%3D%26ie%3DISO-8859-1%26q%3DLattice%2BGauge%2BTheory%26btnG%3DGoogle%2 BSearch%26meta%3Dgroup%253Dsci.physics.research#ef e70e642e41021f[/QUOTE]
today 21 October Parampreet Singh gave a seminar talk on quantum gravitational collapse.
He said that the LIGHTCURVE of a collapsing star could show a distinctive signature that was predicted by the quantum theory (LQG) but not by the classical theory of gravitational collapse.
this seems quite iffy, but still a step in the right direction----looking for observable effects, so as to test LQG
http://www.phys.psu.edu/events/index.html?event_id=1302&event_type_ids=0&span=2005-08-20.2005-12-25
Quantum Gravity Effects and the Fate on Gravitational Collapse
the slides (or else they are his lecture notes) are available for download as well as audio.
PAGE 27:
"this quantum gravitational signature can be in principle observed by an external observer as a slight dimming and subsequent brightening of the collapsing star.
An observer can estimate the loop quantum parameter j by observing the flux profile of the burst based on this mechanism and measuring the variation in luminosity of the collapsing cloud."
P. Singh is a co-author with Bojowald of
http://www.arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Physical Review Letters 95 (2005) 091302
http://arxiv.org/abs/hep-th/0511021
A Minimal Length from the Cutoff Modes in Asymptotically Safe Quantum Gravity
Martin Reuter, Jan-Markus Schwindt
26 pages, 1 figure
MZ-TH/05-23
"Within asymptotically safe Quantum Einstein Gravity (QEG), the quantum 4-sphere is discussed as a specific example of a fractal spacetime manifold. The relation between the infrared cutoff built into the effective average action and the corresponding coarse graining scale is investigated. Analyzing the properties of the pertinent cutoff modes, the possibility that QEG generates a minimal length scale dynamically is explored. While there exists no minimal proper length, the QEG sphere appears to be "fuzzy" in the sense that there is a minimal angular separation below which two points cannot be resolved by the cutoff modes."
http://arxiv.org/abs/gr-qc/0511007
Phenomenological implications of an alternative Hamiltonian constraint for quantum cosmology
Mikhail Kagan
10 pages, 7 figures
"In this paper we review a model based on loop quantum cosmology that arises from a symmetry reduction of the self dual Plebanski action. In this formulation the symmetry reduction leads to a very simple Hamiltonian constraint that can be quantized explicitly in the framework of loop quantum cosmology. We investigate the phenomenological implications of this model in the semi-classical regime and compare those with the known results of the standard Loop Quantum Cosmology."
new Weinberg!
http://arxiv.org/abs/hep-th/0511037
Living in the Multiverse
Steven Weinberg
13 pages
UTTG-12-05
"This is the written version of the opening talk at the symposium "Expectations of a Final Theory," at Trinity College, Cambridge, on September 2, 2005. It is to be published in Universe or Multiverse?, ed. B. Carr (Cambridge University Press)."
IIRC Lee Smolin has an essay in that same book. His is called "Scientific Alternatives to the Anthropic Principle" and we've discussed it some at PF
http://arxiv.org/abs/gr-qc/0511031
Deformed Special Relativity as an effective theory of measurements on quantum gravitational backgrounds
R. Aloisio, A. Galante, A. Grillo, S. Liberati, E. Luzio, F. Mendez
11 pages
"In this article we elaborate on a recently proposed interpretation of DSR as an effective measurement theory in the presence of non-negligible (albeit small) quantum gravitational fluctuations. We provide several heuristic arguments to explain how such a new theory can emerge and discuss the possible observational consequences of this framework."
http://arxiv.org/abs/quant-ph/0511096
A Polynomial Quantum Algorithm for Approximating the Jones Polynomial
Dorit Aharonov, Vaughan Jones, Zeph Landau
26 pages
"The Jones polynmial, discovered in 1984, is an important knot invariant in topology, which is intimately connected to Topological Quantum Field Theory (TQFT). The works of Freedman, Kitaev, Larsen and Wang provide an efficient simulation of TQFT by a quantum computer, and vice versa. These results implicitly imply the existence of an efficient quantum algorithm that provides a certain additive approximation of the Jones polynomial at the fifth root of unity, and moreover, that this problem is BQP-complete. Unfortunately, this important algorithm was never explicitly formulated. Moreover, the results of Freedman et. al are heavily based on deep knowledge of TQFT, which makes the algorithm essentially inaccessible for computer scientists.
We provide an explicit and simple polynomial algorithm to approximate the Jones polynomial of an n strands braid with m crossings at the primitive k'th root of unity, for any k, where the running time of the algorithm is polynomial in m,n and k. Our algorithm does not use TQFT at all. By the results of Freedman et. al, our algorithm solves a BQP complete problem.
The algorithm we provide exhibits a structure which we hope is generalizable to other quantum algorithmic problems. A candidate of particular interest is the approximation of the partition function of the Potts model."
http://arxiv.org/abs/hep-th/0511086
Calabi-Yau Manifolds and the Standard Model
John C. Baez
4 pages
"For any subgroup G of O(n), define a "G-manifold" to be an n-dimensional Riemannian manifold whose holonomy group is contained in G. Then a G-manifold where G is the Standard Model gauge group is precisely a Calabi-Yau manifold of 10 real dimensions whose tangent spaces split into orthogonal 4- and 6-dimensional subspaces...."
quote:"It would be nice to find a use for these results."
either of these two results, of Vaughn Jones and of John Baez, could turn out to be mathematically fertile, and might have consequences for QG
we have a thread about the Baez result
http://www.physicsforums.com/showthread.php?t=99073
selfAdjoint
Nov10-05, 10:01 AM
we have a thread about the Baez resu
And I see you started one on the Jones polynomial too.
NEW BOJOWALD
http://arxiv.org/abs/math-ph/0511043
Effective Equations of Motion for Quantum Systems
Martin Bojowald, Aureliano Skirzewski
29 pages
AEI-2005-169, NI05063
"In many situations, one can approximate the behavior of a quantum system, i.e. a wave function subject to a partial differential equation, by effective classical equations which are ordinary differential equations. A general method and geometrical picture is developed and shown to agree with effective action results, commonly derived through path integration, for perturbations around a harmonic oscillator ground state. The same methods are used to describe dynamical coherent states, which in turn provide means to compute quantum corrections to the symplectic structure of an effective system."
======================================
Interestingly enough although the main Quantum Gravity conference of 2005 was Loops '05 there was another QG conference in September with the abbreviated title of QG '05
that many of us including myself didn't hear about at the time and quite a few Quantum Gravity people attended!
http://www.phy.olemiss.edu/GR/qg05/info.html
There were 101 participants---compared to some 155 at Loops '05 AEI-Potsdam----and it was at a beautiful spot on the island of Sardinia.
Here is a paper that was presented at QG '05---it sounds related to the work of Laurent Freidel and Artem Starodubtsev that we discussed earlier this year.
http://arxiv.org/abs/gr-qc/0511077
Quantum Gravity as a Deformed Topological Quantum Field Theory
A. Mikovic
7 pages, talk presented at the QG05 conference, 12-16 September 2005, Cala Gonone, Italy
"It is known that the Einstein-Hilbert action with a positive cosmological constant can be represented as a perturbation of the SO(4,1) BF theory by a symmetry-breaking term quadratic in the B field. Introducing fermionic matter generates additional terms in the action which are polynomial in the tetrads and the spin connection. We describe how to construct the generating functional in the spin foam formalism for a generic BF theory when the sources for the B and the gauge field are present. This functional can be used to obtain a path integral for General Relativity with matter as a perturbative series whose the lowest order term is a path integral for a topological gravity coupled to matter."
Yeah, Mikovic reference [9] is
[9] L. Freidel and A. Starodubtsev, hep-th/0501191
The odd thing about the naming of the conferences is that John Baez was just saying here at PF that he thought Loops '05 SHOULD HAVE BEEN CALLED QG '05 and that if Renate Loll wanted to host a conference next year at Utrecht she might not want to call it "Loops '06" because that is too specific and CDT is distinct from LQG. But it seems as if some other organization had preempted the tag, so that they could not have used the tag QG '05 anyway regardless of what JB said was better. It doesnt matter, it is just names and abbreviations, who cares about acronyms? But sometimes little things like a grain of sand mess something up.
Well I still hope they have a 2006 quantum gravity conference-----maybe it could be at perimeter and Smolin could call it Loops '06. He interprets LQG very inclusively and actively promotes a many-approaches philosophy.
here is a curious paper, I keep the link because might conceivably turn out useful
http://arxiv.org/abs/hep-th/0511114
Lectures on Fuzzy and Fuzzy SUSY Physics
A. P. Balachandran, S. Kurkcuoglu, S. Vaidya
"fuzzy suzy" what were they thinking? peach-fuzz? hope it was something nice
===============
just stashing this one to check out later:
http://arxiv.org/abs/gr-qc/0511080
Spin_Network
Nov15-05, 03:35 PM
See you got the recent papers marcus! great stuff.
ANOTHER NEW BOJOWALD
http://arxiv.org/abs/gr-qc/0511058
Perturbative Degrees of Freedom in Loop Quantum Gravity: Anisotropies
Martin Bojowald, Hector H. Hernandez, Hugo A Morales-Tecotl
32 pages
AEI-2005-170, NI05064
"The relation between an isotropic and an anisotropic model in loop quantum cosmology is discussed in detail, comparing the strict symmetry reduction with a perturbative implementation of symmetry. While the latter cannot be done in a canonical manner, it allows to consider the dynamics including the role of small non-symmetric degrees of freedom for the symmetric evolution. This serves as a model for the general situation of perturbative degrees of freedom in a background independent quantization such as loop quantum gravity, and for the more complicated addition of perturbative inhomogeneities. While being crucial for cosmological phenomenology, it is shown that perturbative non-symmetric degrees of freedom do not allow definitive conclusions for the singularity issue and in such a situation could even lead to wrong claims."
=================
(viqar husain and oliver winkler gave talks at Loops '05 about what replaces the cosmological singularity, their talks came right after Ashtekar IIRC, not sure what husain is driving at here but want to watch his moves)
http://arxiv.org/abs/hep-th/0511131
Background independent duals of the harmonic oscillator
Viqar Husain
5 pages
"We show that a class of topological field theories are quantum duals of the harmonic oscillator. This is demonstrated by establishing a correspondence between the creation and annihilation operators and non-local gauge invariant observables of the topological field theory. The example is used to discuss some issues concerning background independence and the relation of vacuum energy to the problem of time in quantum gravity."
===========
I can't judge how interesting this next paper is, but list it as a way to keep track of development in LQG research in China
http://arxiv.org/abs/gr-qc/0511084
"Effective Gauge Group of Pure Loop Quantum Gravity is SO(3)"
Chung-Hsien Chou, Yi Ling, Chopin Soo, Hoi-Lai Yu
selfAdjoint
Nov16-05, 08:55 PM
I haven't read the paper but since Bojowald is the author of the idea that LQG cosmology removes the initial singularity (at the big bang), and now he says that pertubative consideration of anisometries may have untold results on that, it is an important evolution in his thinking. No?
...anisometries may have untold results on that, it is an important evolution in his thinking. No?
definitely. we should check it out-----a real-life gravitational collapse seems apt to have significant asymmetries. the issue is whether LQG can handle that or does it break down (IMO nature does not have singularities, it is only theories that break down and give infinities and other physically meaningless results when they are pushed too far)
so this is a way of testing Loop gravity it seems to me. Would you like to start a thread on this, or should I?
BTW I see Bojo cites this:
[27] http://arxiv.org/gr-qc/0506128
and says on page 23
"It is interesting to compare this situation with recent results in string theory [26] where inhomogeneities on a background are seen to prevent the occurence of a bounce instead of a singularity (which would otherwise be possible in the corresponding homogeneous model). This looks similar to our perturbative quantization of anisotropies on an isotropic background, even though the models and techniques are certainly very different. In contrast, non-perturbative background independent models studied so far are non-singular, including inhomogeneous ones which classically have local physical degrees of freedom [27]. There is a further disadvantage of using perturbative or semiclassical degrees of freedom in order to discuss the singularity issue, as for instance suggested in [28]. Such a perturbative treatment is unlikely to remain valid close to a singularity where potentially all degrees of freedom can be excited strongly. Perturbations certainly allow one to include all degrees of freedom which is important for phenomenology, but properties of the singularity can be extremely blurred as we have seen here..."
he seems to be saying that the trouble is not with LQG, but with using it PERTURBATIVELY to deal with a singularity. maybe perturbative approaces don't mix with singularities very well.
the paper he cited dealt with inhomogeneous case, but NON peturbatively
======================
new GAMBINI-PULLIN
http://arxiv.org/abs/gr-qc/0511096
Consistent discretization and canonical classical and quantum Regge calculus
Rodolfo Gambini, Jorge Pullin
4 Pages, 4 figures
"We apply the 'consistent discretization' technique to the Regge action for (Euclidean and Lorentzian) general relativity in arbitrary number of dimensions. The result is a well defined canonical theory that is free of constraints and where the dynamics is implemented as a canonical transformation. This provides a framework for the discussion of topology change in canonical quantum gravity. In the Lorentzian case, the framework appears to be naturally free of the 'spikes' that plague traditional formulations. It also provides a well defined recipe for determining the measure of the path integral."
=================
REUTER-RELATED
http://arxiv.org/abs/hep-th/0511177
Further Evidence for a Gravitational Fixed Point
R. Percacci
4 pages
"A theory of gravity with a generic action functional and minimally coupled to N matter fields has a nontrivial fixed point in the leading large N approximation. At this fixed point, the cosmological constant and Newton's constant are nonzero and UV relevant; the curvature squared terms are asymptotically free with marginal behaviour; all higher order terms are irrelevant and can be set to zero by a suitable choice of cutoff function."
we've been getting a new Bojowald paper every few days. this is the the third that have been posted lately, since 14 november actually----so three in just the past week.
http://arxiv.org/abs/gr-qc/0511108
Spherically Symmetric Quantum Geometry: Hamiltonian Constraint
Martin Bojowald, Rafal Swiderski
33 pages
AEI-2005-171, NI05065
"Variables adapted to the quantum dynamics of spherically symmetric models are introduced, which further simplify the spherically symmetric volume operator and allow an explicit computation of all matrix elements of the Euclidean and Lorentzian Hamiltonian constraints. The construction fits completely into the general scheme available in loop quantum gravity for the quantization of the full theory as well as symmetric models. This then presents a further consistency check of the whole scheme in inhomogeneous situations, lending further credence to the physical results obtained so far mainly in homogeneous models. New applications in particular of the spherically symmetric model in the context of black hole physics are discussed."
Ooops, make that FOUR Bojo papers appearing in the past 7 days, here is another, this time in the Astronomy-Astrophysics department:
http://arxiv.org/abs/astro-ph/0511557
Universe scenarios from loop quantum cosmology
Martin Bojowald
16 pages, 8 figures, plenary talk at "Pomeranian Workshop in Fundamental Cosmology", Pobierowo, Sep 2005
AEI-2005-168
"Loop quantum cosmology is an application of recent developments for a non-perturbative and background independent quantization of gravity to a cosmological setting. Characteristic properties of the quantization such as discreteness of spatial geometry entail physical consequences for the structure of classical singularities as well as the evolution of the very early universe. While the singularity issue in general requires one to use difference equations for a wave function of the universe, phenomenological scenarios for the evolution are based on effective equations implementing the main quantum modifications. These equations show generic bounces as well as inflation in diverse models, which have been combined to more complicated scenarios."
http://arxiv.org/abs/hep-th/0511260
Asymptotic Safety in Quantum Einstein Gravity: nonperturbative renormalizability and fractal spacetime structure
O. Lauscher, M. Reuter
29 pages, latex, 1 figure, invited paper at the Blaubeuren Workshop 2005 on Mathematical and Physical Aspects of Quantum Gravity
MZ-TH/05-26
"The asymptotic safety scenario of Quantum Einstein Gravity, the quantum field theory of the spacetime metric, is reviewed and it is argued that the theory is likely to be nonperturbatively renormalizable. It is also shown that asymptotic safety implies that spacetime is a fractal in general, with a fractal dimension of 2 on sub-Planckian length scales."
=====================
http://arxiv.org/abs/gr-qc/0511120
Clifford bundle formulation of BF gravity generalized to the standard model
A. Garrett Lisi
24 pages
"The structure and dynamics of the standard model and gravity are described by a Clifford valued connection and its curvature."
congratulations.
===============
http://arxiv.org/abs/gr-qc/0511089
Differential Structures - the Geometrization of Quantum Mechanics
Torsten Asselmeyer-Maluga, Helge Rosé
13 pages, 2 figures
"The usual quantization of a classical space-time field does not touch the non-geometrical character of quantum mechanics. We believe that the deep problems of unification of general relativity and quantum mechanics are rooted in this poor understanding of the geometrical character of quantum mechanics. In Einstein's theory gravitation is expressed by geometry of space-time, and the solutions of the field equation are invariant w.r.t. a certain equivalence class of reference frames. This class can be characterized by the differential structure of space-time. We will show that matter is the transition between reference frames that belong to different differential structures, that the set of transitions of the differential structure is given by a Temperley-Lieb algebra which is extensible to a C*-algebra comprising the field operator algebra of quantum mechanics and that the state space of quantum mechanics is the linear space of the differential structures. Furthermore we are able to explain the appearance of the complex numbers in quantum theory. The strong relation to Loop Quantum Gravity is discussed in conclusion."
http://arxiv.org/abs/astro-ph/0511774
Dimensionless constants, cosmology and other dark matters
Max Tegmark (MIT), Anthony Aguirre (UCSC), Martin Rees (Cambridge), Frank Wilczek (MIT)
29 pages, 12 figs
"We identify 31 dimensionless physical constants required by particle physics and cosmology, and emphasize that both microphysical constraints and selection effects might help elucidate their origin. Axion cosmology provides an instructive example, in which these two kinds of arguments must both be taken into account, and work well together. If a Peccei-Quinn phase transition occurred before or during inflation, then the axion dark matter density will vary from place to place with a probability distribution. By calculating the net dark matter halo formation rate as a function of all four relevant cosmological parameters and assessing other constraints, we find that this probability distribution, computed at stable solar systems, is arguably peaked near the observed dark matter density. If cosmologically relevant WIMP dark matter is discovered, then one naturally expects comparable densities of WIMPs and axions, making it important to follow up with precision measurements to determine whether WIMPs account for all of the dark matter or merely part of it."
============
http://arxiv.org/abs/astro-ph/0511780
A Quantitative Occam's Razor
Rafael D. Sorkin (Syracuse University)
16 pages
International Journal of Theoretical Physics, 22:1091-1104 (1983)
"This paper derives an objective Bayesian "prior" based on considerations of entropy/information. By this means, it produces a quantitative measure of goodness of fit (the "H-statistic") that balances higher likelihood against the number of fitting parameters employed. The method is intended for phenomenological applications where the underlying theory is uncertain or unknown.
For example, it can help decide whether the large angle anomalies in the CMB data should be taken seriously.
I am therefore posting it now, even though it was published before the arxiv existed."
================
http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez, Urs Schreiber
10 encapsulated Postscript figures
Differential Geometry; Category Theory
"Just as gauge theory describes the parallel transport of point particles using connections on bundles, higher gauge theory describes the parallel transport of 1-dimensional objects (e.g. strings) using 2-connections on 2-bundles. A 2-bundle is a categorified version of a bundle: that is, one where the fiber is not a manifold but a category with a suitable smooth structure. Where gauge theory uses Lie groups and Lie algebras, higher gauge theory uses their categorified analogues: Lie 2-groups and Lie 2-algebras. We describe a theory of 2-connections on principal 2-bundles and explain how this is related to Breen and Messing's theory of connections on nonabelian gerbes. The distinctive feature of our theory is that a 2-connection allows parallel transport along paths and surfaces in a parametrization-independent way. In terms of Breen and Messing's framework, this requires that the "fake curvature" must vanish. In this paper we summarize the main results of our theory without proofs."
this fall there were TWO major international quantum gravity conferences Loops '05, which was in October at AEI-Golm outside Berlin, and QG '05, which was held in September on the island of Sardinia
the Loops '05 program is here
http://loops05.aei.mpg.de/index_files/Programme.html
and the recorded talks (usually with slides as well) are online
here is the homepage
http://loops05.aei.mpg.de/
there were 156 registered participants of which 11 were from US institutions, by my count.
there is no separate page with all the abstracts assembled together,
but by clicking on the speaker's name in the program you can get the title and abstract of the talk.
This conference has been discussed in several PF threads, including one that John Baez started.
=======================
If only for completeness, we should also compare the other conference QG '05.
http://www.phy.olemiss.edu/GR/qg05/
here is a page listing the conference talks with abstracts:
http://www.phy.olemiss.edu/GR/qg05/abstracts.html
here is the list of participants---it says there were 101:
http://www.phy.olemiss.edu/cgi-bin/qg05/pr_participants.cgi
At this conference, by my count, 72 people gave talks, of whom 7 were from institutions in the USA. A ten percent showing---roughly comparable to what occurred at the other large Quantum Gravity conference: Loops '05.
Here are some samples of the abstracts, to give a taste:
Daniel Terno (dterno@perimeterinstitute.ca)
Thursday, September 15th, 18:10, Parallel session VI: Black holes
Quantum black holes: entropy and entanglement on the horizon
Abstract: Considering a horizon as a surface beyond which no information is accessible we conclude that the spin network states that are associated with it should be globally SU(2) invariant. We derive the Bekenstein-Hawking entropy and the logarithmic correction with the prefactor 3/2, which is independent from the size of the elementary spin that is used in the calculation. The logarithmic correction turns to be equal to the quantum mutual information (total amount of classical and quantum correlations) between parts of the spin network that describes the horizon. We analyze the relation between the microscopic and the macroscopic surface area, when the elementary patches of the surface are coarse-grained. Joint work with Etera Livine.
Charles Wang (c.wang@abdn.ac.uk)
Monday, September 12th, 18:10, Parallel session II: Quantum gravity
Towards conformal loop quantum gravity
Abstract: In a recent publication [C. H.-T. Wang, Phys. Rev. D 71, 124026 (2005)], the author has presented a new canonical formulation of GR by extending the ADM phase space to that consisting of York's mean extrinsic curvature time, conformal three-metric and their momenta. In addition to the Hamiltonian and diffeomorphism constraints, the resulting theory contains a new first class constraint, called the conformal constraint. The extended algebra of constraints has as subalgebra the Lie algebra for the conformorphisms of the spatial hypersurface. The structure of the new constraints suggests that conformal metric may be used to formulate the unitary functional evolution of quantum gravity with respect to the York time. This talk will outline a further enlarged phase space of GR by incorporating spin gauge as well as conformal symmetries. Remarkably, a new set of gauge variables for canonical GR is found that is shown to be free from a parameter of the Barbero- Immirzi type due to the inherent conformal invariance of the formalism. A discussion is then given of the prospect of constructing a theory of conformal loop quantum gravity to address both the conceptual problem of time and technical problem of functional calculus in quantum gravity.
Ruth Williams (rmw7@damtp.cam.ac.uk)
Monday, September 12th, 12:00, Plenary session
Discrete quantum gravity
Abstract: Discrete approaches to quantum gravity, including Regge calculus, dynamical triangulations and spin foam models, will be reviewed briefly. A fuller account will be given of recent progress in quantum Regge calculus.
James Ryan (jpr25@cam.ac.uk)
Tuesday, September 13th, 18:10, Parallel session III: Quantum gravity
A group field theory for 3d quantum gravity coupled to a scalar field
Abstract: We present a new group field theory model, which incorporates both 3-dimensional gravity and matter coupled to gravity. We show that the Feynman diagram amplitudes of this model are given by Riemannian quantum gravity spin foam amplitudes coupled to a scalar matter field. We briefly discuss the features of this model and its possible generalisations.
Matej Pavsic (matej.pavsic@ijs.si)
Thursday, September 15th, 17:45, Parallel session V: Gauge theories and quantisation
Spin gauge theory of gravity in Clifford space
Abstract: A theory in which a 16-dimensional curved Clifford space (C-space) provides a realization of Kaluza-Klein theory is investigated. No extra dimensions of spacetime are needed: "extra dimensions" are in C-space. We explore the spin gauge theory in C-space and show that the the generalized spin connection contains the usual 4-dimensional gravity and Yang-Mills fields of the U(1)xSU(2)xSU(3) gauge group. The representation space for the latter group is provided by 16-component generalized spinors composed of four usual 4-component spinors, defined geometrically as the members of four independent left minimal ideals of Clifford algebra. [my comment: note possible contact with Garrett Lisi work ]
Daniele Oriti (d.oriti@damtp.cam.ac.uk)
Monday, September 12th, 17:20, Parallel session II: Quantum gravity
The group field theory approach to quantum gravity
Abstract: We review the basic ideas of the group field theory approach to non-perturbative quantum gravity, a generalisation of matrix models for 2d gravity, that provides a third quantization of gravity in higher spacetime dimensions. We also discuss several recent developments, including the coupling of matter fields to quantum gravity, the implementation of causality, and the definition of different transition amplitudes for these theories.
Aleksandar Mikovic (amikovic@ulusofona.pt)
Monday, September 12th, 16:55, Parallel session II: Quantum gravity
Quantum gravity as a topological quantum field theory
Abstract: In the discretized approaches to Quantum Gravity, like spin foam models, one needs to perform a sum over the spacetime triangulations, or to define a continious limit, in order to impose the diffeomorphism invariance. If the QG theory was a topological theory, then a single triangulation would suffice. We describe an approach to define quantum gravity theory as a topological quantum field theory by using a BF theory.
Fotini Markopoulou (fmarkopoulou@perimeterinstitute.ca)
Friday, September 16th, 9:15, Plenary session
The low energy problem of background-independent quantum gravity
Abstract: We review the main issue facing background-independent approaches to quantum gravity, the low-energy problem. This is the task of extracting general relativity (and possibly also quantum field theory) from a microscopic Planckian theory. We find that, perhaps not surprisingly, the central issue is dynamics. We then approach this problem from a quantum information theoretic perspective. In any such application, the focus has to be on dynamics. We propose ways to do so.
there were several other interesting titles and abstracts that could have been included in this sample but were dropped because the list was getting too long.
this fall there were TWO major international quantum gravity conferences Loops '05, which was in October at AEI-Golm outside Berlin, and QG '05, which was held in September on the island of Sardinia
...
...
...
Daniele Oriti (d.oriti@damtp.cam.ac.uk)
Monday, September 12th, 17:20, Parallel session II: Quantum gravity
The group field theory approach to quantum gravity
Abstract: We review the basic ideas of the group field theory approach to non-perturbative quantum gravity, a generalisation of matrix models for 2d gravity, that provides a third quantization of gravity in higher spacetime dimensions. We also discuss several recent developments, including the coupling of matter fields to quantum gravity, the implementation of causality, and the definition of different transition amplitudes for these theories.
....
One sees from the Sardinia conference that Daniele Oriti was giving the GFT overview---essentially substituting for Laurent Freidel. Today he and Etera Livine posted another GFT paper:
http://arxiv.org/abs/gr-qc/0512002
Coupling of spacetime atoms and spin foam renormalisation from group field theory
Etera R. Livine, Daniele Oriti
18 pages
"We study the issue of coupling among 4-simplices in the context of spin foam models obtained from a group field theory formalism. We construct a generalisation of the Barrett-Crane model in which an additional coupling between the normals to tetrahedra, as defined in different 4-simplices that share them, is present. This is realised through an extension of the usual field over the group manifold to a five argument one. We define a specific model in which this coupling is parametrised by an additional real parameter that allows to tune the degree of locality of the resulting model, interpolating between the usual Barrett-Crane model and a flat BF-type one. Moreover, we define a further extension of the group field theory formalism in which the coupling parameter enters as a new variable of the field, and the action presents derivative terms that lead to modified classical equations of motion. Finally, we discuss the issue of renormalisation of spin foam models, and how the new coupled model can be of help regarding this."
==============================
Dan Christensen has been a co-author with John Baez, computing with spinfoams.
he is at UWO (western ontario) where they have a supercomputer center and does both theoretical and computational physics----they developed a fast algorithm for 10j symbols---they can do stuff with spinfoams that is sort of like what Loll does with dynamical triangulations---that is, run them. He also does spinfoam theory. Josh Willis, an Ashtekar Penn State PhD, has gone to postdoc at UWO with Christensen. Dan Cherrington, who gave a paper at Loops '05 is another UWO postdoc.
http://arxiv.org/abs/gr-qc/0512004
Finiteness of Lorentzian 10j symbols and partition functions
J. Daniel Christensen
8 pages
"We give a short and simple proof that the Lorentzian 10j symbol, which forms a key part of the Barrett-Crane model of Lorentzian quantum gravity, is finite. The argument is very general, and applies to other integrals. For example, we show that the Lorentzian and Riemannian causal 10j symbols are finite, despite their singularities. Moreover, we show that integrals that arise in Cherrington's work are finite. Cherrington has shown that this implies that the Lorentzian partition function for a single triangulation is finite, even for degenerate triangulations. Finally, we also show how to use these methods to prove finiteness of integrals based on other graphs and other homogeneous domains."
============================
Here is Charles Wang's paper he referred to in his talk at Sardinia QG '05, and a follow-up by the same author:
http://arxiv.org/abs/gr-qc/0501024
Conformal geometrodynamics: True degrees of freedom in a truly canonical structure
8 pages
Phys.Rev. D71 (2005) 124026
"The standard geometrodynamics is transformed into a theory of conformal geometrodynamics by extending the ADM phase space for canonical general relativity to that consisting of York's mean exterior curvature time, conformal three-metric and their momenta. Accordingly, an additional constraint is introduced, called the conformal constraint. In terms of the new canonical variables, a diffeomorphism constraint is derived from the original momentum constraint. The Hamiltonian constraint then takes a new form. It turns out to be the sum of an expression that previously appeared in the literature and extra terms quadratic in the conformal constraint. The complete set of the conformal, diffeomorphism and Hamiltonian constraints are shown to be of first class through the explicit construction of their Poisson brackets. The extended algebra of constraints has as subalgebras the Dirac algebra for the deformations and Lie algebra for the conformorphism transformations of the spatial hypersurface. This is followed by a discussion of potential implications of the presented theory on the Dirac constraint quantization of general relativity. An argument is made to support the use of the York time in formulating the unitary functional evolution of quantum gravity. Finally, the prospect of future work is briefly outlined."
http://arxiv.org/abs/gr-qc/0507044
Unambiguous spin-gauge formulation of canonical general relativity with conformorphism invariance
4 pages
Phys.Rev. D72 (2005) 087501
"We present a parameter-free gauge formulation of general relativity in terms of a new set of real spin connection variables. The theory is constructed by extending the phase space of the recently formulated conformal geometrodynamics for canonical gravity to accommodate a spin gauge description. This leads to a further enlarged set of first class gravitational constraints consisting of a reduced Hamiltonian constraint and the canonical generators for spin gauge and conformorphism transformations. Owing to the incorporated conformal symmetry, the new theory is shown to be free from an ambiguity of the Barbero-Immirzi type."
here is Charles Wang homepage---he has a remarkable set of research interests and accomplishments---check this out:
http://www.lancs.ac.uk/depts/physics/staff/chtw.htm
He is now at Aberdeen---the page was from 2004 when he was at Lancaster
============================
Here is Charles Wang's paper he referred to in his talk at Sardinia QG '05, and a follow-up by the same author:
http://arxiv.org/abs/gr-qc/0501024
Conformal geometrodynamics: True degrees of freedom in a truly canonical structure
8 pages
Phys.Rev. D71 (2005) 124026
"The standard geometrodynamics is transformed into a theory of conformal geometrodynamics by extending the ADM phase space for canonical general relativity to that consisting of York's mean exterior curvature time, conformal three-metric and their momenta. Accordingly, an additional constraint is introduced, called the conformal constraint. In terms of the new canonical variables, a diffeomorphism constraint is derived from the original momentum constraint. The Hamiltonian constraint then takes a new form. It turns out to be the sum of an expression that previously appeared in the literature and extra terms quadratic in the conformal constraint. The complete set of the conformal, diffeomorphism and Hamiltonian constraints are shown to be of first class through the explicit construction of their Poisson brackets. The extended algebra of constraints has as subalgebras the Dirac algebra for the deformations and Lie algebra for the conformorphism transformations of the spatial hypersurface. This is followed by a discussion of potential implications of the presented theory on the Dirac constraint quantization of general relativity. An argument is made to support the use of the York time in formulating the unitary functional evolution of quantum gravity. Finally, the prospect of future work is briefly outlined."
http://arxiv.org/abs/gr-qc/0507044
Unambiguous spin-gauge formulation of canonical general relativity with conformorphism invariance
4 pages
Phys.Rev. D72 (2005) 087501
"We present a parameter-free gauge formulation of general relativity in terms of a new set of real spin connection variables. The theory is constructed by extending the phase space of the recently formulated conformal geometrodynamics for canonical gravity to accommodate a spin gauge description. This leads to a further enlarged set of first class gravitational constraints consisting of a reduced Hamiltonian constraint and the canonical generators for spin gauge and conformorphism transformations. Owing to the incorporated conformal symmetry, the new theory is shown to be free from an ambiguity of the Barbero-Immirzi type."
here is Charles Wang homepage---he has a remarkable set of research interests and accomplishments---check this out:
http://www.lancs.ac.uk/depts/physics/staff/chtw.htm
He is now at Aberdeen---the page was from 2004 when he was at Lancaster
Today Charles H-T Wang posted another paper:
http://arxiv.org/abs/gr-qc/0512023
Towards conformal loop quantum gravity
Charles H.-T. Wang
6 pages, 1 figure, Talk given at Constrained Dynamics and Quantum Gravity 05, Cala Gonone, Sardinia, Italy, 12-16 September 2005
A discussion is given of recent developments in canonical gravity that assimilates the conformal analysis of gravitational degrees of freedom. The work is motivated by the problem of time in quantum gravity and is carried out at the metric and the triad levels. At the metric level, it is shown that by extending the Arnowitt-Deser-Misner (ADM) phase space of general relativity (GR), a conformal form of geometrodynamics can be constructed. In addition to the Hamiltonian and diffeomorphism constraints, an extra first class constraint is introduced to generate conformal transformations. This phase space consists of York's mean extrinsic curvature time, conformal three-metric and their momenta. At the triad level, the phase space of GR is further enlarged by incorporating spin-gauge as well as conformal symmetries. This leads to a canonical formulation of GR using a new set of real spin connection variables. The resulting gravitational constraints are first class, consisting of the Hamiltonian constraint and the canonical generators for spin-gauge and conformorphism transformations. The formulation has a remarkable feature of being parameter-free. Indeed, it is shown that a conformal parameter of the Barbero-Immirzi type can be absorbed by the conformal symmetry of the extended phase space. This gives rise to an alternative approach to loop quantum gravity that addresses both the conceptual problem of time and the technical problem of functional calculus in quantum gravity."
this guy is a dark horse. I would appreciate help evaluating this work if anyone has any ideas.
http://arxiv.org/abs/hep-th/0512033
Thermal gravity, black holes and cosmological entropy
Stephen D. H. Hsu, Brian M. Murray
5 pages, 2 figures
"Taking seriously the interpretation of black hole entropy as the logarithm of the number of microstates, we argue that thermal gravitons may undergo a phase transition to a kind of black hole condensate. The phase transition proceeds via nucleation of black holes at a rate governed by a saddlepoint configuration whose free energy is of order the inverse temperature in Planck units. Whether the universe remains in a low entropy state as opposed to the high entropy black hole condensate depends sensitively on its thermal history. Our results may clarify an old observation of Penrose regarding the very low entropy state of the universe."
Steve Hsu's blog is Information Processing. It is a real good blog.
He also collaborated with Zee on a fun paper called "A Message in the Sky"
New Witten paper
http://arxiv.org/abs/hep-th/0512039
New Arivero paper
http://arxiv.org/abs/hep-ph/0512065
Oriti presented this at the QG '05 conference
http://arxiv.org/abs/gr-qc/0512048
Quantum gravity as a group field theory: a sketch
Daniele Oriti
8 pages, 9 figures; to appear in the Proceedings of the Fourth Meeting on Constrained Dynamics and Quantum Gravity, Cala Gonone, Italy, September 12-16, 2005
DAMTP-2005-123
"We give a very brief introduction to the group field theory approach to quantum gravity, a generalisation of matrix models for 2-dimensional quantum gravity to higher dimension, that has emerged recently from research in spin foam models."
another new paper by Oriti
http://arxiv.org/abs/gr-qc/0512069
Generalised group field theories and quantum gravity transition amplitudes
Daniele Oriti
6 pages, 2 figures
DAMTP-2005-127
"We construct a generalised formalism for group field theories, in which the domain of the field is extended to include additional proper time variables, as well as their conjugate mass variables. This formalism allows for different types of quantum gravity transition amplitudes in perturbative expansion, and we show how both causal spin foam models and the usual a-causal ones can be derived from it, within a sum over triangulations of all topologies. We also highlight the relation of the so-derived causal transition amplitudes with simplicial gravity actions."
Oriti is the editor of a book Cambridge University Press has scheduled to bring out in 2006, and here is one of the chapters (contributed by Gambini and Pullin)
according to Oriti, the title of the new book is:
Towards quantum gravity: different approaches to a new understanding of space and time Cambridge University Press (2006); but Gambini and Pullin mention a trivially different title.
http://arxiv.org/abs/gr-qc/0512065
Consistent discretizations as a road to quantum gravity
Rodolfo Gambini, Jorge Pullin
Comments: 17 Pages, Draft chapter contributed to the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press
LSU-REL-121105
"We present a brief description of the ``consistent discretization'' approach to classical and quantum general relativity. We exhibit a classical simple example to illustrate the approach and summarize current classical and quantum applications. We also discuss the implications for the construction of a well defined quantum theory and in particular how to construct a quantum continuum limit."
a new paper by Freidel and Livine appeared to day:
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory
Laurent Freidel, Etera R. Livine
9 pages, Proceeding of the conference "Quantum Theory and Symmetries 4" 2005 (Varna, Bulgaria)
"We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincaré group."
a new paper by Jerzy Kowalski-Glikman and others
http://arxiv.org/abs/hep-th/0512107
The Free Particle in Deformed Special Relativity
F. Girelli, T. Konopka, J. Kowalski-Glikman, E.R. Livine
15 pages
"The phase space of a classical particle in DSR contains de Sitter space as the space of momenta. We start from the standard relativistic particle in five dimensions with an extra constraint and reduce it to four dimensional DSR by imposing appropriate gauge fixing. We analyze some physical properties of the resulting theories like the equations of motion, the form of Lorentz transformations and the issue of velocity. We also address the problem of the origin and interpretation of different bases in DSR."
http://arxiv.org/abs/gr-qc/0512072
Quantum information in loop quantum gravity
Daniel R. Terno
4 pages. Proceedings of QG'05, Cala Gonone, 2005
"A coarse-graining of spin networks is expressed in terms of partial tracing, thus allowing to use tools of quantum information theory. This is illustrated by the analysis of a simple black hole model, where the logarithmic correction of the Hawking-Bekenstein entropy is shown to be equal to the total amount of correlations on the horizon. Finally other applications of entanglement to quantum gravity are briefly discussed."
http://arxiv.org/abs/quant-ph/0512125
Quantum information and computation
Jeffrey Bub
103 pages, no figures. Forthcoming as a chapter in Handbook of Philosophy of Physics, edited by John Earman and Jeremy Butterfield (Elsevier/NH)
"This article deals with theoretical developments in the subject of quantum information and quantum computation, and includes an overview of classical information and some relevant quantum mechanics. The discussion covers topics in quantum communication, quantum cryptography, and quantum computation, and concludes by considering whether a perspective in terms of quantum information sheds new light on the conceptual problems of quantum mechanics."
given that Daniel Terno is collaborating on LQG with Eteral Livine at Perimeter (where there are a lot of Q. information and computing people as well as QG) we may eventually need reference material in this area. Scott Aaronson, known for his blog among other things, is another QI at Waterloo. At first sight this source seems comprehensive and not too hard. Anyone have comments?
of possible interest
http://arxiv.org/abs/hep-th/0512197
Eric D'Hoker, D.H. Phong
http://arxiv.org/abs/hep-th/0512200
Observables in effective gravity
Steven B. Giddings, Donald Marolf, James B. Hartle
http://arxiv.org/abs/hep-th/0512201
Holography and entropy bounds in the plane wave matrix model
Raphael Bousso, Aleksey L. Mints
http://arxiv.org/abs/hep-th/0512210
2D Ising Model with non-local links - a study of non-locality
Yidun Wan
4 pages, 6 figures
"Markopoulou and Smolin have argued that the low energy limit of LQG may suffer from a conflict between locality, as defined by the connectivity of spin networks, and an averaged notion of locality that emerges at low energy from a superposition of spin network states. This raises the issue of how much non-locality, relative to the coarse grained metric, can be tolerated in the spin network graphs that contribute to the ground state. To address this question we have been studying statistical mechanical systems on lattices decorated randomly with non-local links. These turn out to be related to a class of recently studied systems called small world networks. We show, in the case of the 2D Ising model, that one major effect of non-local links is to raise the Curie temperature. We report also on measurements of the spin-spin correlation functions in this model and show, for the first time, the impact of not only the amount of non-local links but also of their configuration on correlation functions."
Yidun Wan's "Perimeter name" is Eaton Wan. He gave a talk at Loops '05. Smolin in his talk cited Eaton's results
====================
http://arxiv.org/abs/gr-qc/0512103
Quantum Gravity as a quantum field theory of simplicial geometry
Daniele Oriti
23 pages, 13 figures; to be published in 'Mathematical and Physical Aspects of Quantum Gravity', B. Fauser, J. Tolksdorf and E. Zeidler eds, Birkhaeuser, Basel (2006)
"This is an introduction to the group field theory approach to quantum gravity, with emphasis on motivations and basic formalism, more than on recent results; we elaborate on the various ingredients, both conceptual and formal, of the approach, giving some examples, and we discuss some perspectives of future developments."
=======================
http://arxiv.org/abs/gr-qc/0512102
Towards the graviton from spinfoams: the 3d toy model
Simone Speziale
7 pages, 2 figures
"Recently, a proposal has appeared for the extraction of the 2-point function of linearised quantum gravity, within the spinfoam formalism. This relies on the use of a boundary state, which introduces a semi-classical flat geometry on the boundary. In this paper, we investigate this proposal considering a toy model in the (Riemannian) 3d case, where the semi-classical limit is better understood. We show that in this limit the propagation kernel of the model is the one for the harmonic oscillator. This is at the origin of the expected 1/L behaviour of the 2-point function. Furthermore, we numerically study the short scales regime, where deviations from this behaviour occur."
I dont know anything of either author. Navarro is at Cambridge. I will flag this and watch for future papers.
http://arxiv.org/abs/gr-qc/0512109
Modified gravity, Dark Energy and MOND
Ignacio Navarro, Karel Van Acoleyen
24 pages, 2 figures
DAMTP-2005-129, DCPT/05/154, IPPP/05/77
"We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a_0 = c\mu, where c is the speed of light and \mu is a parameter of the model that also determines the late-time Hubble constant: H_0 \sim \mu. In these models, besides the massless spin two graviton, there is a scalar excitation of the spacetime metric whose mass depends on the background curvature. This dependence is such that this scalar, although almost massless in vacuum, becomes massive and effectively decouples when one gets close to any source and we recover an acceptable weak field limit at short distances. There is also a (classical) 'running' of Newton's constant with the distance to the sources and gravity is easily enhanced at large distances by a large ratio. We comment on the possibility of building a model with a MOND-like Newtonian limit that could explain the rotation curves of galaxies without introducing Dark Matter using this kind of actions. We also explore briefly the characteristic gravitational phenomenology that these models imply: besides a long distance modification of gravity they also predict deviations from Newton's law at short distances. This short distance scale depends on the local background curvature of spacetime, and we find that for experiments on the Earth surface it is of order \sim 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower."
They cite REUTER work (renormalizable QG) as their reference [33] in this passage on page 21
"...there is a second effect in these theories: the Planck mass that controls the coupling strength of the massless graviton also undergoes a rescaling or 'running' with the distance to the sources (or the background curvature). This phenomenon, although a purely classical one in our theory, is reminiscent of the quantum renormalisation group running of couplings. So one might wonder if actions of the type (15) could be an effective classical description of strong renormalisation effects in the infrared that might appear in GR (see e.g. [33] and references therein), as happens in QCD. In fact, corrections depending on the logarithm of the renormalisation scale are ubiquitous in quantum field theory,.."
=======================
this Utrecht master's thesis was flagged by John Baez in TWF #224
I like the way it is written----by a person who gets a kick out of writing clearly and finding the simple way to understand something complex. Baez says he's looking forward to this guy's PhD thesis
http://arxiv.org/abs/math.QA/0512103
Categorical Aspects of Topological Quantum Field Theories
Bruce H. Bartlett
M.Sc Thesis, Utrecht University, 2005. 111 pages, numerous pictures. Supervisors : Dr. S. Vandoren, Prof. I. Moerdijk
"This thesis provides an introduction to the various category theory ideas employed in topological quantum field theory. These theories are viewed as symmetric monoidal functors from topological cobordism categories into the category of vector spaces. In two dimensions, they are classified by Frobenius algebras. In three dimensions, and under certain conditions, they are classified by modular categories. These are special kinds of categories in which topological notions such as braidings and twists play a prominent role. There is a powerful graphical calculus available for working in such categories, which may be regarded as a generalization of the Feynman diagrams method familiar in physics. This method is introduced and the necessary algebraic structure is graphically motivated step by step.
A large subclass of two-dimensional topological field theories can be obtained from a lattice gauge theory construction using triangulations. In these theories, the gauge group is finite. This construction is reviewed, from both the original algebraic perspective as well as using the graphical calculus developed in the earlier chapters.
This finite gauge group toy model can be defined in all dimensions, and has a claim to being the simplest non-trivial quantum field theory. We take the opportunity to show explicitly the calculation of the modular category arising from this model in three dimensions, and compare this algebraic data with the corresponding data in two dimensions, computed both geometrically and from triangulations. We use this as an example to introduce the idea of a quantum field theory as producing a tower of algebraic structures, each dimension related to the previous by the process of categorification."
======================
In the current conversation at Woit blog concerning Cosmological Natural Selection (CNS) Smolin cited this paper as a marginal aside in response to someone's question:
http://arxiv.org/gr-qc/0510052
Geometry from quantum particles
David W. Kribs, Fotini Markopoulou
17 pages
"We investigate the possibility that a background independent quantum theory of gravity is not a theory of quantum geometry. We provide a way for global spacetime symmetries to emerge from a background independent theory without geometry. In this, we use a quantum information theoretic formulation of quantum gravity and the method of noiseless subsystems in quantum error correction. This is also a method that can extract particles from a quantum geometric theory such as a spin foam model."
the CNS discussion is transcribed here:
http://www.physicsforums.com/showthread.php?p=862787#post862787
http://arxiv.org/abs/hep-th/0601004
Noncommutative Harmonic Analysis, Sampling Theory and the Duflo Map in 2+1 Quantum Gravity
L. Freidel, S. Majid
"54 pages, 2 figs
We show that the $\star$-product for $U(su_2)$ arising in \cite{EL} in an effective theory for the Ponzano-Regge quantum gravity model is compatible with the noncommutative bicovariant differential calculus previously proposed for 2+1 Euclidean quantum gravity using quantum group methods in \cite{BatMa}. We show that the effective action for this model essentially agrees with the noncommutative scalar field theory coming out of the noncommutative differential geometry. We show that the required Fourier transform essentially agrees with the previous quantum group Fourier transform. In combining these methods we develop practical tools for noncommutative harmonic analysis for the model including radial quantum delta-functions and Gaussians, the Duflo map and elements of `noncommutative sampling theory' applicable to the bounded $SU_2,SO_3$ momentum groups. This allows us to understand the bandwidth limitation in 2+1 quantum gravity arising from the bounded momentum. We also argue that the the anomalous extra `time' dimension seen in the noncommutative differential geometry should be viewed as the renormalisation group flow visible in the coarse graining in going from $SU_2$ to $SO_3$. Our methods also provide a generalised twist operator for the $\star$-product."
http://arxiv.org/abs/hep-th/0601001
The String Landscape, Black Holes and Gravity as the Weakest Force
Nima Arkani-Hamed, Lubos Motl, Alberto Nicolis, Cumrun Vafa
20 pages, 5 figures
http://arxiv.org/abs/math-ph/0601005
Construction of Generalized Connections
Christian Fleischhack
12 pages
"We present a construction method for mappings between generalized connections, comprising, e.g., the action of gauge transformations, diffeomorphisms and Weyl transformations. Moreover, criteria for continuity and measure preservation are stated."
back in early December (post #429) I flagged this
...a new paper by Jerzy Kowalski-Glikman and others
http://arxiv.org/abs/hep-th/0512107
The Free Particle in Deformed Special Relativity
F. Girelli, T. Konopka, J. Kowalski-Glikman, E.R. Livine
15 pages
"The phase space of a classical particle in DSR contains de Sitter space as the space of momenta. We start from the standard relativistic particle in five dimensions with an extra constraint and reduce it to four dimensional DSR by imposing appropriate gauge fixing. We analyze some physical properties of the resulting theories like the equations of motion, the form of Lorentz transformations and the issue of velocity. We also address the problem of the origin and interpretation of different bases in DSR."
now I see that this gives a helpful perspective on the work of Freidel by people who are not Freidel. It is an outside perspective that can begin to sum up how they see his line of research going (and theirs in relation to it.)
---quote from conclusions---
In this paper, we have studied a classical particle in five space-time dimensions subject to two constraints defining two energy scales m and kappa. We have shown that, after gauge fixing, the 5d model can give rise to various DSR models in 4d. The reduction from 5d to 4d selects a set of phase space coordinates (x, p) via the requirement that they should commute with both the kappa-shell constraint H_5d and the gauge fixing function C.
...
...
In three space-time dimensions, the link between DSR and gravity has been clarified in [7]. Indeed, in 3d quantum gravity, particles are identified as conical singularities and their momentum is defined through non-local measurements as (a function of) the holonomy around the particle. This explicit characterization allows to rigorously derive DSR from 3d quantum gravity and unambiguously compute the Feynman diagrams for the resulting non-commutative quantum field theory [7].
There is also a proposal attempting to move the similarity between DSR and GR to the level of an explicit relationship in four dimensions [27]. In that proposal, the choice of coordinates p_mu (and x_mu) correspond to the definition of the measured momenta (and positions) in terms of the tetrad field e^I_mu. The issue then becomes: what are we exactly measuring physically when we talk about the energy-momentum p_mu? The answer to this question will determine the “correct” choice of physical coordinates to use in DSR. Regardless, we expect the physical predictions of DSR to be independent of any gauge fixing choice and propose that the “correctness” of a particular choice of coordinates should be measured by how convenient these coordinates are to express the measurements of a particular observer. For instance, one could try to properly define length measurements using clocks and time-of-flight experiments to define the metric operationally.
At the end of the day, we cannot make concrete predictions using DSR as long as we do not find gauge invariant quantities (commuting with the two constraints of the 5d action) and their physical interpretation, or equivalently an explicit link between the choices of gauge fixing and measurement. This avenue of research seems to be a natural one from the 5d perspective. It is also our view that the 5d perspective should be a used when looking at twoparticle systems and studying their properties. Other related topics to be investigated are free spinning particles.
Finally, an important unresolved issue regards the physical interpretation of the fifth dimension. Written as a 5d theory, DSR appears as a large extra dimension theory. We have proposed to see the coordinates in the fifth dimension as some effective degree of freedom coming from quantum gravity. The reformulation of GR as a SO(4, 1) BF gauge field theory proposed in [21] may prove to be a guide in this direction. It is also very tempting to interpret P_4 as the energy scale in a renormalisation scheme, as some kind of dynamical cut-off. X_4 would then be the generator of scale transformations. Such a speculation is supported by the fact that X_4 is (more or less) the 4d dilatation operator in the Snyder basis, but this is truly little evidence. One could look at the renormalisation equation of a scalar field and try to interpret them as equations of motion in the DSR framework. The potential link between DSR and quantum gravity and the fact that the renormalisation flow of general relativity can be associated to a fifth dimension (with an AdS metric) [29] also points toward such an interpretation.
---endquote---
what is emerging is some interconnected treatment of spacetime dynamics, matter, and DSR----in this Kolwalski-Glikman paper they are dealing with the flat DSR limit, but in the context of Freidel papers on spacetime dynamics. especially his seemingly successful treatment of the 3D case. this paper is evidently part of a combined initiative by several people to proceed to the 4D case.
BTW we should watch for possible observational tests of QG. GLAST has been discussed in this context and is scheduled for orbit next year. Also Auger (OH-ZHAY)
which is now beginning to report
http://arxiv.org/abs/astro-ph/0601035
The First Scientific Results from the Pierre Auger Observatory
T. Yamamoto (for The Pierre Auger Observatory Collaboration)
4 pages, 1 figure, Proceedings of the PANIC 2005 conference
"The southern site of the Pierre Auger Observatory is under the construction near Malargue in Argentina and now more than 60% of the detectors are completed. The observatory has been collecting data for over 1 year and the cumulative exposure is already similar to that of the largest forerunner experiments. The hybrid technique provides model-independent energy measurements from the Fluorescence Detector to calibrate the Surface Detector. Based on this technique, the first estimation of the energy spectrum above 3 EeV has been presented and is discussed in this paper."
Smolin has a paper "Falsifiable..." describing how Auger and GLAST may be able to distinguish between certain approaches to QG, and test some assumptions.
Carlip is an important figure. In case of interest:
http://arxiv.org/gr-qc/0601041
Horizons, Constraints, and Black Hole Entropy
S. Carlip
16 pages, talk given at the "Peyresq Physics 10 Meeting on Micro and Macro structures of spacetime"
"Black hole entropy appears to be "universal''--many independent calculations, involving models with very different microscopic degrees of freedom, all yield the same density of states. I discuss the proposal that this universality comes from the behavior of the underlying symmetries of the classical theory. To impose the condition that a black hole be present, we must partially break the classical symmetries of general relativity, and the resulting Goldstone boson-like degrees of freedom may account for the Bekenstein-Hawking entropy. In particular, I demonstrate that the imposition of a "stretched horizon'' constraint modifies the algebra of symmetries at the horizon, allowing the use of standard conformal field theory techniques to determine the asymptotic density of states. The results reproduce the Bekenstein-Hawking entropy without any need for detailed assumptions about the microscopic theory."
Mattingly takes off from Carlip's result
http://arxiv.org/gr-qc/0601044
On horizon constraints and Hawking radiation
David Mattingly
"Questions about black holes in quantum gravity generally presuppose the presence of a horizon. Recently Carlip has shown that enforcing an initial data surface to be a horizon leads to the correct form for the Bekenstein-Hawking entropy of the black hole. Requiring a horizon also constitutes fixed background geometry, which generically leads to non-conservation of the matter stress tensor at the horizon. In this work, I show that the generated matter energy flux for a Schwarzschild black hole is in agreement with the first law of black hole thermodynamics, 8 pi G Delta Q = kappa Delta A."
In case anyone is wondering whether gravitons can be detected (Freeman Dyson said not)
http://arxiv.org/gr-qc/0601043
Can Gravitons Be Detected?
Tony Rothman, Stephen Boughn
21 pages, no figures. To be submitted to AJP
"Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson's question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson's conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production."
----------------------as an afterthought-------------------
Two others looked as if they might be interesting as well:
http://arxiv.org/astro-ph/0601219
Constraining Lorentz violations with Gamma Ray Bursts
Maria Rodriguez Martinez, Tsvi Piran
16 pages, 4 figures
"Gamma ray bursts are excellent candidates to constrain physical models which break Lorentz symmetry. We consider deformed dispersion relations which break the boost invariance and lead to an energy-dependent speed of light. In these models, simultaneously emitted photons from cosmological sources reach Earth with a spectral time delay that depends on the symmetry breaking scale. We estimate the possible bounds which can be obtained by comparing the spectral time delays with the time resolution of available telescopes. We discuss the best strategy to reach the strongest bounds. We compute the probability of detecting bursts that improve the current bounds. The results are encouraging. Depending on the model, it is possible to build a detector that within several years will improve the present limits of 0.015 m_pl."
http://arxiv.org/astro-ph/0601247
Alternative proposal to modified Newton dynamics (MOND)
Juan M. Romero, Adolfo Zamora
4 pages. Accepted for publication in PRD
"From a study of conserved quantities of the so-called Modified Newtonian Dynamics (MOND) we propose an alternative to this theory. We show that this proposal is consistent with the Tully-Fisher law, has conserved quantities whose Newtonian limit are the energy and angular momentum, and can be useful to explain cosmic acceleration. The dynamics obtained suggests that, when acceleration is very small, time depends on acceleration. This result is analogous to that of special relativity where time depends on velocity."
note that this paper has been accepted for publication in Physical Review Series D.
http://arxiv.org/abs/hep-th/0601129
Loop and spin foam quantum gravity: a brief guide for beginners
Hermann Nicolai, Kasper Peeters
18 pages, 11 figures; Contributed article to "An assessment of current paradigms in theoretical physics"
Report-no: AEI-2006-004
"We review aspects of loop quantum gravity and spin foam models at an introductory level, with special attention to questions frequently asked by non-specialists."
Nicolai earlier co-authored an "outsider's view" of LQG but did not discuss recent work (e.g. in the past 5 years) and omitted spinfoam.
So the view had some problems---Lee Smolin replied to Nicolai politely and Peter Woit published the letter.
I dont know how this one is going to play out. Basically it is great of Nicolai, as a string theorist, to take an interest in alternatives like Loop and Spinfoam. It is potentially really constructive.
====================
the next is by two authors who are not familiar to me:
http://arxiv.org/abs/hep-th/0601127
Intersecting Connes Noncommutative Geometry with Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup
19 pages, 4 figures
NORDITA-2006-1
An intersection of Noncommutative Geometry and Loop Quantum Gravity is proposed. Alain Connes' Noncommutative Geometry provides a framework in which the Standard Model of particle physics coupled to general relativity is formulated as a unified, gravitational theory. However, to this day no quantization procedure compatible with this framework is known. In this paper we consider the noncommutative algebra of holonomy loops on a functional space of certain spin-connections. The construction of a spectral triple is outlined and ideas on interpretation and classical limit are presented.
http://arxiv.org/abs/gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at "Living Reviews":
http://relativity.livingreviews.org/Articles/lrr-2005-11/
AEI-2005-185, IGPG-06/1-6
Journal-ref: Living Rev. Relativity 8 (2005) 11
"Quantum gravity is expected to be necessary in order to understand situations where classical general relativity breaks down. In particular in cosmology one has to deal with initial singularities, i.e. the fact that the backward evolution of a classical space-time inevitably comes to an end after a finite amount of proper time. This presents a breakdown of the classical picture and requires an extended theory for a meaningful description. Since small length scales and high curvatures are involved, quantum effects must play a role. Not only the singularity itself but also the surrounding space-time is then modified. One particular realization is loop quantum cosmology, an application of loop quantum gravity to homogeneous systems, which removes classical singularities. Its implications can be studied at different levels. Main effects are introduced into effective classical equations which allow to avoid interpretational problems of quantum theory. They give rise to new kinds of early universe phenomenology with applications to inflation and cyclic models. To resolve classical singularities and to understand the structure of geometry around them, the quantum description is necessary. Classical evolution is then replaced by a difference equation for a wave function which allows to extend space-time beyond classical singularities. One main question is how these homogeneous scenarios are related to full loop quantum gravity, which can be dealt with at the level of distributional symmetric states. Finally, the new structure of space-time arising in loop quantum gravity and its application to cosmology sheds new light on more general issues such as time."
To get the movies, go to the Living Reviews version
http://relativity.livingreviews.org/Articles/lrr-2005-11/
and scroll down the sidebar menu all the way to the bottom where it says "figures"
========================
Also in today's arxiv postings:
http://arxiv.org/abs/gr-qc/0601082
Quantum Hamiltonian for gravitational collapse
Viqar Husain, Oliver Winkler
17 pages
"Using a Hamiltonian formulation of the spherically symmetric gravity-scalar field theory adapted to flat spatial slicing, we give a construction of the reduced Hamiltonian operator. This Hamiltonian, together with the null expansion operators presented in an earlier work, form a framework for studying gravitational collapse in quantum gravity. We describe a setting for its numerical implementation, and discuss some conceptual issues associated with quantum dynamics in a partial gauge fixing."
============================
Lee Smolin thinks that if MOND is real then it may have an explanation in quantum gravity. We should keep an eye on MOND research, just in case.
Here is an overview for beginners. Good place to start if you want to learn something about MOND.
http://arxiv.org/abs/astro-ph/0601478
Modified Newtonian Dynamics, an Introductory Review
Riccardo Scarpa
"By the time, in 1937, the Swiss astronomer Zwicky measured the velocity dispersion of the Coma cluster of galaxies, astronomers somehow got acquainted with the idea that the universe is filled by some kind of dark matter. After almost a century of investigations, we have learned two things about dark matter, (i) it has to be non-baryonic -- that is, made of something new that interact with normal matter only by gravitation-- and, (ii) that its effects are observed in stellar systems when and only when their internal acceleration of gravity falls below a fix value a0=1.2x10-8 cm s-2. This systematic, more than anything else, tells us we might be facing a failure of the law of gravity in the weak field limit rather then the effects of dark matter. Thus, in an attempt to avoid the need for dark matter, the Modified Newtonian Dynamics. MOND posits a breakdown of Newton's law of gravity (or inertia) below a0, after which the dependence with distance became linear. Despite many attempts, MOND resisted stubbornly to be falsified as an alternative to dark matter and succeeds in explaining the properties of an impressively large number of objects without invoking the presence of non-baryonic dark matter. In this paper, I will review the basics of MOND and its ability to explain observations without the need of dark matter."
=====================
Of possible interest to the category-minded:
http://arxiv.org/abs/math.QA/0601458
Categorified Algebra and Quantum Mechanics
Jeffrey Morton (University of California, Riverside)
67 pages, 25 figures
Jeffrey Morton has studied Quantum Gravity with John Baez. Here is what he says in the acknowledgements section
"This work grew out of the regular Quantum Gravity seminar taught by John Baez at UCR, notes for which are available online as [2]. I would like to acknowledge his work on this subject (some published as [1]), excellent teaching, and helpful advice and discussions in preparing this paper. Other students in the seminar, especially Toby Bartels, Miguel Carrion-Alvarez, Alissa Crans, and Derek Wise also provided many useful discussions."
We know Miguel and Derek as Baez QG students. Miguel recently finished his thesis and Derek gave a paper at Loops '05. So this comes out of that group, although it is not specifically about gravity.
http://arxiv.org/abs/gr-qc/0601095
The spin-foam-representation of loop quantum gravity
Alejandro Perez
Draft chapter contributed to the book "Towards quantum gravity", being prepared by Daniele Oriti for Cambridge University Press. 19 pages
"The problem of background independent quantum gravity is the problem of defining a quantum field theory of matter and gravity in the absence of an underlying background geometry. Loop quantum gravity (LQG) is a promising proposal for addressing this difficult task. Despite the steady progress of the field, dynamics remains to a large extend an open issue in LQG. Here we present the main ideas behind a series of proposals for addressing the issue of dynamics. We refer to these constructions as the spin foam representation of LQG. This set of ideas can be viewed as a systematic attempt at the construction of the path integral representation of LQG.
The spin foam representation is mathematically precise in 2+1 dimensions, so we will start this chapter by showing how it arises in the canonical quantization of this simple theory. This toy model will be used to precisely describe the true geometric meaning of the histories that are summed over in the path integral of generally covariant theories.
In four dimensions similar structures appear. We call these constructions spin foam models as their definition is incomplete in the sense that at least one of the following issues remains unclear: 1) the connection to a canonical formulation, and 2) regularization independence (renormalizability). In the second part of this chapter we will describe the definition of these models emphasizing the importance of these open issues. We also discuss the non standard picture of quantum spacetime that follows from background independence.
several MOND articles recently, in post #437 we saw this
http://arxiv.org/abs/astro-ph/0601478
Modified Newtonian Dynamics, an Introductory Review
Riccardo Scarpa
"...This systematic, more than anything else, tells us we might be facing a failure of the law of gravity in the weak field limit rather then the effects of dark matter..... In this paper, I will review the basics of MOND and its ability to explain observations without the need of dark matter."
now this has appeared
http://arxiv.org/abs/hep-th/0601213
Introduction to Modified Gravity and Gravitational Alternative for Dark Energy
S. Nojiri, S.D. Odintsov
21 pages, lectures for 42nd Karpacz Winter School on Theoretical Physics
"We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of f(R), f(G) or f(R,G) gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modified gravity) is also described."
this paper was prepared for this year's Polish Winterschool. It happens every year in February. Two years ago we got a bunch of interesting papers from the 2004 Winterschool---it was about QG phenomenology, DSR, possible observable effects. Carlo Rovelli was one of the organizers and Lee Smolin and Jerzy K-G gave weeklong lecture courses. Smolin and others discussed MOND.
Now it seems that the 2006 Winterschool is again touching on some of the same topics! The school is held at a ski resort on the Poland Czech border, in south Poland. It has been about all kinds of theoretical physics, not just QG or DSR or MOND. but now for two out of the past three years it will be about these things. We should watch for more papers coming out on arxiv from this year's school.
http://www.ift.uni.wroc.pl/karp42/#Prog
Bojowald is lecturing about Loop Cosmology at the Winterschool this year
mentioned briefly:
http://arxiv.org/abs/physics/0601218
A Theory of Quantum Gravity may not be possible because Quantum Mechanics violates the Equivalence Principle
Mario Rabinowitz
"Easy to follow original proof of the incompatibility of General Relativity and Quantum Mechanics"
Also a recent mathematics PhD thesis at Göttingen:
http://arxiv.org/abs/math.MG/0601744
Coarse geometry and asymptotic dimension
Bernd Grave
Dissertation
Subj-class: Metric Geometry
"We consider asymptotic dimension of coarse spaces. We analyse coarse structures induced by metrisable compactifications. We calculate asymptotic dimension of coarse cell complexes. We calculate the asymptotic dimension of certain negatively curved spaces, e.g. for complete, simply connected manifolds with bounded, strictly negative sectional curvature."
Bernd's thesis advisor was Tom Schick. This is high abstract math, with no obvious connection to QG or other physics. My personal opinion is that it might be interesting to develop a connection. Renate Loll and Hanno Sahlmann gave seminar talks at Göttingen around November-December last year. The physics department there seems to have an interest in QG.
http://arxiv.org/abs/gr-qc/0601121
The causal set approach to quantum gravity
Joe Henson
22 pages, 4 figures.
"Extended version of a review to be published in "Approaches to Quantum Gravity - Towards a new understanding of space and time" (ed. D. Oriti), Cambridge University Press, 2006.... Dedicated to Rafael Sorkin on the occasion of his 60th birthday"
Renate Loll impresses me as a team player. Acting for the good of the QG field as a whole. She has taken Joe Henson on as a postdoc but he seems not to be doing Loll-type CDT research. He seems to be going great guns on Causal Sets---with several collaborations with Fay Dowker in the works and at least one with Rafael Sorkin.
I suppose this is what Lee Smolin was asking for (independence for worthy postdocs, don't tie support to one particular research program) and it seems an idealistic attempt to treat QG as a single field where the principle investigators share the job of hosting the postdocs. Instead of dividing up into separate competing factions---jealously guarded bailiwicks of funding
Well, I don't know how it will work in practice. I am a little disappointed, I thought Joe Henson going to Utrecht as a Loll postdoc would mean he crosses over into CDT research. Is there a common ground?
Christine flagged the Joe Henson paper on her blog yesterday and got some discussion:
http://christinedantas.blogspot.com/2006/01/causal-set-approach-to-quantum-gravity.html
selfAdjoint
Feb1-06, 01:23 PM
Just a note on Thiemann. I'm reading his latest master constraint paper, gr-qc/0510011, where he brings it all home. Unlike the previous papers in the series, this one is based on triangulations, not networks. Influence of CDT?
Later Edit. Reading farther I find he does make some use of spin networks. See my post # 442. However his Master Constraint is defined, as advertised, in terms of triangulations.
Just a note on Thiemann. I'm reading his latest master constraint paper, gr-qc/0510011, where he brings it all home. ...
I am imagining the TOC of Oriti's book ("towards a new understanding of space and time") with all these guys lined up to evaluate and compare.
Perez---Spinfoams
Henson---Causal Sets
Thiemann---Masterconstraint
Gambini---"Gambinistics"
Loll---Dynamic Triangulation
Bojowald---Loop Cosmology
Freidel---Finessing matter feynman diagrams from foam spacetime
...
...
so far we only actually know that Perez and Henson have contributed chapters, the others are guesses with varying degrees of seriousness.
But Thiemann certainly should be there!
EDIT: based on next post by selfAdjoint I deleted a non-essential mention of triangulations
selfAdjoint
Feb1-06, 05:21 PM
Marcus I have to stand corrected. He does use spin networks, or rather diffeomorphic equivalence classes of them, in defining his new inner product. When I finally get my head around it, I'll start a thread describing it; it has some very important consequences, and as you know, was cited along with CDT at the summer meetings as an important step forward in quantum gravity.
http://arxiv.org/abs/gr-qc/0602010
Group field theory formulation of 3d quantum gravity coupled to matter fields
Daniele Oriti, James Ryan
28 pages, 21 figures
"We present a new group field theory describing 3d Riemannian quantum gravity coupled to matter fields for any choice of spin and mass. The perturbative expansion of the partition function produces fat graphs colored with SU(2) algebraic data, from which one can reconstruct at once a 3-dimensional simplicial complex representing spacetime and its geometry, like in the Ponzano-Regge formulation of pure 3d quantum gravity, and the Feynman graphs for the matter fields. The model then assigns quantum amplitudes to these fat graphs given by spin foam models for gravity coupled to interacting massive spinning point particles, whose properties we discuss."
Christine Dantas blog is turning out to be a real valuable resource.
http://christinedantas.blogspot.com/
Her sidebar has some good references. Not just the Smolin Lectures on Intro to LQG, but also links to a READING LIST to go along with the Smolin Lectures.
For instance Smolin is often recommending Dirac's thin book "Lectures on Quantum Mechanics" but that requires a trip to the library or bookstore. So Christine gives an online substitute:
http://www.tech.port.ac.uk/staffweb/seahras/documents/reviews/quantization.pdf
This is by Sanjeev Seahra
"The Classical and Quantum Mechanics of Systems with Constraints"
Christine does onboard satellite computer code for Brazil government. She running what is, it seems, the world's only QG blog. She also has substantial other demands on her time. ...
[EDIT: correction, selfAdjoint points out another QG blog I didnt know about]
selfAdjoint
Feb4-06, 02:22 PM
...what is, it seems, the world's only QG blog.
Not quite, there is also http://lqg.blogspot.com/, but I think Dantas is better.
Recent postings by James Hartle and by Lev Okun---both papers are somewhat on the philosophical side, and have a bit of historical perspective. Both Hartle and Okun should perhaps be revered as elder statesmen. Hartle was born in 1939 and Okun in 1929.
http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
31 pages, 4 figures, latex, contribution to the 23rd Solvay Conference, The Quantum Structure of Space and Time
"Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory allow for the further generalization that would be necessary were spacetime not fundamental. Emergent spacetime phenomena are discussed in general and illustrated with the example of the classical spacetime geometries with large spacelike surfaces that emerge from the 'no-boundary' wave function of the universe. These must be Lorentzian with one, and only one, time direction. The essay concludes by raising the question of whether quantum mechanics itself is emergent."
====================
a key reference, in the Hartle paper, is
http://arxiv.org/abs/hep-th/0512200
Observables in effective gravity
Steven B. Giddings, Donald Marolf, James B. Hartle
43 pages
We address the construction and interpretation of diffeomorphism-invariant observables in a low-energy effective theory of quantum gravity. The observables we consider are constructed as integrals over the space of coordinates, in analogy to the construction of gauge-invariant observables in Yang-Mills theory via traces. As such, they are explicitly non-local. Nevertheless we describe how, in suitable quantum states and in a suitable limit, the familiar physics of local quantum field theory can be recovered from appropriate such observables, which we term `pseudo-local.' We consider measurement of pseudo-local observables, and describe how such measurements are limited by both quantum effects and gravitational interactions. These limitations support suggestions that theories of quantum gravity associated with finite regions of spacetime contain far fewer degrees of freedom than do local field theories."
this paper has half a dozen citations to work by Carlo Rovelli
13, 14, 15, 20, 42, 46
also about the same number of citations to papers by Abhay Ashtekar
I would say that a central theme of these two Hartle papers is BACKGROUND INDEPENDENCE the need for quantum observables to be defined in a diffeomorphism invariant way.
Hartle presents this in a PALATABLE way. To me he comes across as a reformer but with a tactful restrained manner. He is actually saying stuff not very different from Lee Smolin in The Case for Background Independence but he says it in a soothing way that does not step on anyone's toes.
All through Hartle section 7 he is talking generalities about something where Renate Loll has tried specifics-----but instead of CITING Loll and Ambjorn work, he puts a footnote where he says "Regge" and cites a paper of Ruth Williams (previous generation triangulation gravity).
I guess to say "Loll" at the 23rd Solvay conference (select old boys chosen by David Gross) would have sounded a jarring note.
Here is Hartle's page 14 footnote with the Ruth Williams citation:
"9 Perhaps, most naturally by discrete approximations to geometry such as the Regge calculus (see, e.g. [43, 44]) "
What he is essentially describing there, in section 7, is an approach that Loll has worked out. But seems unaware of this.
================
I think the Hartle paper could be an important contribution for DIPLOMATIC reasons.
It articulates a reform position but nicely, avoiding backlash. It is admirably intelligent and well-reasoned. Its faults (not explicitly pointing out developments in the non-string QG community) can be considered to be its merits.
=======================
http://arxiv.org/abs/hep-ph/0602037
The Concept of Mass in the Einstein Year
L.B. Okun
19 pages, Presented at the 12th Lomonosov conference on Elementary Particle Physics, Moscow State University, August 25-31
"Various facets of the concept of mass are discussed. The masses of elementary particles and the search for higgs. The masses of hadrons. The pedagogical virus of relativistic mass."
(another by Lev Okun is http://arxiv.org/abs/hep-ph/0602036)
===============
while I can still edit, I will tack on notice of a new paper by Thanu Padmanabhan
http://arxiv.org/abs/astro-ph/0602117
this is just a pedagogical cosmology paper, but he has written interesting articles on QG, so I am inclined to flag it.
http://arxiv.org/abs/physics/0601218
A Theory of Quantum Gravity may not be possible because Quantum Mechanics violates the Equivalence Principle
Mario Rabinowitz
"Easy to follow original proof of the incompatibility of General Relativity and Quantum Mechanics"
I hve just readed and it is very basic and very false. It is obvious that the ordinary shcröedinger equation in a newtonian potential can´t implement the weak equivalence principle. Nobody clames that.
The question wich arise from these paper is ¿how difficoult is to publish in arxiv? ¿isn´t there any peer to peer revisión?
ZapperZ
Feb10-06, 05:21 AM
I hve just readed and it is very basic and very false. It is obvious that the ordinary shcröedinger equation in a newtonian potential can´t implement the weak equivalence principle. Nobody clames that.
The question wich arise from these paper is ¿how difficoult is to publish in arxiv? ¿isn´t there any peer to peer revisión?
This is why I always caution people who cite papers of arXiv. There is no peer review. There is only a very rudimentary review of submitted papers to make sure it is not pure quackery. But other than that, papers like this can get through especially if a person has posted a paper before the current endorsement system (i.e. you get grandfathered into the new system).
If you check for this author - Mario Rabinowitz - you'll see that he has had a series of equally dubious papers submitted. The alarm bells should ring when you realize that these papers don't appear anywhere else, and especially in peer-reviewed journals.
Always wait (unless it is a proceeding paper or a text of a speech) for an arXiv submission to appear in a peer-reviewed journal. That is your best bet. Unfortunately, arXiv has become a major "citation source" in String theory. I don't know if that's good, or a poor reflection on the field of study itself.
Zz.
I hve just readed and it is very basic and very false...
Glad you agree :-)
I thought it was preposterous.
Title had amusement value for me since my area of interest is QG and he was saying that QG is apriori impossible(!)
Next time I put in a joke citation, if there is a next time, I will attach a SMILEY to make clear that the citation is not to be taken seriously.
finns have great names for people and places sometimes and this denizen of Espoo has a blog called "Small Window"
http://fysix.blogspot.com/
and Risto has reported about a Zhao/Famaey MOND paper
http://fysix.blogspot.com/2006/02/refining-mond-this-is-not-newtons.html
Here is Z/F paper,
http://arxiv.org/abs/astro-ph/0512425
a Chinese Scotch collaboration (sound potent?)
When you survey Quantum Gravity I think you have to keep MOND in your periferal vision, because if it turns out right it will be a test of QG. Because QG will have to EXPLAIN why this particular modification of Newton law happens. Finally a QG will have to predict mondy effects--or darkmattery effects---and then astronomers will measure to see if the QG got it right out to as many decimal places as you can.
I flagged some MOND papers recently in posts #432 and 435----noticed but did not flag the Zhao/Famaey paper. And Risto supplies something more: a "SOFT" discussion, as Risto calls it:
http://www.interactions.org/cms/?pid=1023887
It is a journalistic introduction to the paper. I like their attitude here. they aren't partisans, they just want the best possible MOND so that it can be tested and pitted against Dark Matter models. Let observation decide.
The paper was published this month in Astrophysical Journal Letters. It seems like these days more and more MOND research is passing review and getting published.
these are noted because PCW Davies is a major league cosmologist and because anything by Gerard 't Hooft is likely to be of interest to someone---he has ideas---it is good to keep track of what he is thinking about these days, even if it is not a big breakthrough
http://arxiv.org/abs/gr-qc/0602076
Invariance under complex transformations, and its relevance to the cosmological constant problem
Gerard 't Hooft, Stefan Nobbenhuis
ITP-UU-06/06, SPIN-06/04
"In this paper we study a new symmetry argument that results in a vacuum state with strictly vanishing vacuum energy. This argument exploits the well-known feature that de Sitter and Anti- de Sitter space are related by analytic continuation. When we drop boundary and hermiticity conditions on quantum fields, we get as many negative as positive energy states, which are related by transformations to complex space. The paper does not directly solve the cosmological constant problem, but explores a new direction that appears worthwhile."
this paper makes several references to earlier work by 't Hooft's co-author Nobbenhuis,
http://arxiv.org/gr-qc/0411093
Categorizing Different Approaches to the Cosmological Constant Problem
Stefan Nobbenhuis
Accepted for publication
ITP-UU-04/40, SPIN-04/23
"We have found that proposals addressing the old cosmological constant problem come in various categories. The aim of this paper is to identify as many different, credible mechanisms as possible and to provide them with a code for future reference. We find that they all can be classified into five different schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to imaginary spacetime."
==========
the next is a talk that Davies gave at a Stanford conference to string theorists. Davies has co-authored with Lineweaver. He is a major cosmologist. I don't necessarily recommend the paper but I want to be able to keep tabs on Davies views of current issues like multiverse/anthropics.
http://arxiv.org/abs/astro-ph/0602420
The problem of what exists
P.C.W. Davies
18 pages, one figure
"Popular multiverse models such as the one based on the string theory landscape require an underlying set of unexplained laws containing many specific features and highly restrictive prerequisites. I explore the consequences of relaxing some of these prerequisites with a view to discovering whether any of them might be justified anthropically. Examples considered include integer space dimensionality, the immutable, Platonic nature of the laws of physics and the no-go theorem for strong emergence. The problem of why some physical laws exist, but others which are seemingly possible do not, takes on a new complexion following this analysis, although it remains an unsolved problem in the absence of an additional criterion."
this is a pretty good talk on MOND
Joao Magueijo gave it today at Perimeter and it is already
available as streamer.
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/
where you click on "seminar series" in the sidebar menu on the left
The talk is based on a recent paper Magueijo did with Bekenstein
http://arxiv.org/abs/astro-ph/0602266
MOND habitats within the solar system
Jacob Bekenstein, Joao Magueijo
"MOdified Newtonian Dynamics (MOND) is an interesting alternative to dark matter in extragalactic systems. We here examine the possibility that mild or even strong MOND behavior may become evident well inside the solar system, in particular near saddle points of the total gravitational potential. Whereas in Newtonian theory tidal stresses are finite at saddle points, they are expected to diverge in MOND, and to remain distinctly large inside a sizeable oblate ellipsoid around the saddle point. We work out the MOND effects using the nonrelativistic limit of the TeVeS theory, both in the perturbative nearly Newtonian regime and in the deep MOND regime. While strong MOND behavior would be a spectacular 'backyard'' vindication of the theory, pinpointing the MOND-bubbles in the setting of the realistic solar system may be difficult. Space missions, such as the LISA Pathfinder, equipped with sensitive accelerometers, may be able to explore the larger perturbative region."
in the talk, one or more members of the audience seemed eager to interrupt with comments and questions, there seemed a fair amount of restrained excitement at times
one important thing involves the spaceprobe LISA which, if I understand correctly, will explore the Earth-Sun Lagrange L1 point, and the gravitational field between the earth and L1.
Magueijo explained how LISA can discount radiation pressure----it has balls floating inside the spacecraft----the spacecraft shields the balls from radiation pressure.
Magueijo explained the strategy of going to SADDLE POINTS where the acceleration due to gravity is small, and how (according to him) one could test MOND within the confines of the solar system.
He seemed to have a balanced view---conventional dark matter has strong points---mond has strong points----one should try to test the theories, may the best survive, maybe mond will be disproved by these tests (as with LISA) that he described. he did not seem to have his ego tied up in either competing theory DM or MOND.
I am not sure if it has been posted here or not.
Anyway like it is a "must be linked" website i post the url:
http://relativity.livingreviews.org/Articles/
In particular the articles by Astekhar in isolated horizonts (very readable althought it just trate too many aspects and sometimes doesn´t go as deep as i would like, and the one by bojowald in llop quantum cosmology wich i have just discovered and can say too much more.
I am not sure if it has been posted here or not.
Anyway like it is a "must be linked" website i post the url:
http://relativity.livingreviews.org/Articles/
...
Excellent choice Sauron! In the past we have linked a few selected Living Reviews articles---including the recent one by Bojowald on LQC. But we have never posted a link to the table of contents of the entire collection. It is good to have. Thanks.
http://arxiv.org/abs/gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)
IGPG-06/2-2, AEI-2006-009
"Black Holes have always played a central role in investigations of quantum gravity. This includes both conceptual issues such as the role of classical singularities and information loss, and technical ones to probe the consistency of candidate theories. Lacking a full theory of quantum gravity, such studies had long been restricted to black hole models which include some aspects of quantization. However, it is then not always clear whether the results are consequences of quantum gravity per se or of the particular steps one had undertaken to bring the system into a treatable form. Over a little more than the last decade loop quantum gravity has emerged as a widely studied candidate for quantum gravity, where it is now possible to introduce black hole models within a quantum theory of gravity. This makes it possible to use only quantum effects which are known to arise also in the full theory, but still work in a rather simple and physically interesting context of black holes. Recent developments have now led to the first physical results about non-rotating quantum black holes obtained in this way. Restricting to the interior inside the Schwarzschild horizon, the resulting quantum model is free of the classical singularity, which is a consequence of discrete quantum geometry taking over for the continuous classical space-time picture. This fact results in a change of paradigm concerning the information loss problem. The horizon itself can also be studied in the quantum theory by imposing horizon conditions at the level of states. Thereby one can illustrate the nature of horizon degrees of freedom and horizon fluctuations. All these developments allow us to study the quantum dynamics explicitly and in detail which provides a rich ground to test the consistency of the full theory."
http://arxiv.org/abs/gr-qc/0602119
Matters of Gravity, The Newsletter of The Topical Group in Gravitation of the American Physical Society, Volume 27, Spring 2006
Jorge Pullin (Editor)
30 pages
"GGR News:
GGR program at the APS April meeting in Dallas
We hear that..., 100 years ago, by Jorge Pullin
Research Briefs:
What's new in LIGO, by David Shoemaker
LISA Pathfinder, by Paul McNamara
Recent progress in binary black hole simulations, by Thomas Baumgarte
Conference reports:
Workshop on Emergence of Spacetime, by Olaf Dreyer
Quantum gravity subprogram at the Isaac Newton Institute, by Jorma Louko
Global problems in Math Relativity at the Newton Institute, by Jim Isenberg
Loops '05, by Thomas Thiemann
Numrel 2005, by Scott Hawley and Richard Matzner
Apples With Apples Workshop in Argentina, by Sascha Husa"
====================
http://arxiv.org/abs/gr-qc/0602120
Categorical Geometry and the Mathematical Foundations of Quantum General Relativity
Louis Crane
Contribution to the Oxford University Press volume on Quantum Gravity
"We explore the possibility of replacing point set topology by higher category theory and topos theory as the foundation for quantum general relativity. We discuss the BC model and problems of its interpretation, and connect with the construction of causal sites."
http://arxiv.org/abs/hep-th/0603022
Quantum Gravity and the Standard Model
Sundance O. Bilson-Thompson, Fotini Markopoulou, Lee Smolin
12 pages, 21 figures
"We show that a class of background independent models of quantum spacetime have local excitations that can be mapped to the first generation fermions of the standard model of particle physics. These states propagate coherently as they can be shown to be noiseless subsystems of the microscopic quantum dynamics. These are identified in terms of certain patterns of braiding of graphs, thus giving a quantum gravitational foundation for the topological preon model proposed by one of us.
These results apply to a large class of theories in which the Hilbert space has a basis of states given by ribbon graphs embedded in a three-dimensional manifold up to diffeomorphisms, and the dynamics is given by local moves on the graphs, such as arise in the representation theory of quantum groups. For such models, matter appears to be already included in the microscopic kinematics and dynamics."
==================
Sundance did a seminar talk at Perimeter on this back in November 2005. The video is online in the PI streaming media center. It is impressive to see him responding to questions and working stuff out at the blackboard. At that time Sundance was collaborating with Smolin and Markopoulou on this and they were putting it together----merging Sundance preon model of matter with Spin-network spacetime.
I will get the link to that video, or a link to the link
Here is a link to a PF thread about the Preons of Sundance Bilson-Thompson that we had back in November 2005
http://www.physicsforums.com/showthread.php?t=100183
that will have a link to the seminar talk
also try this:
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?cid=a9b1d20a-efa7-485f-8d5d-3b62fb7d3e4c&shouldResize=False
and flip to page 5 of the seminar series. the date is 16 November and that is currently on page 5------they list latest first
the title is "Braid New World"
==================
Etera Livine must always be kept track of and he posted today as well
http://arxiv.org/abs/gr-qc/0603008
Reconstructing Quantum Geometry from Quantum Information: Area Renormalisation, Coarse-Graining and Entanglement on Spin Networks
Etera R. Livine, Daniel R. Terno
27 pages, 12 figures
"After a brief review of spin networks and their interpretation as wave functions for the (space) geometry, we discuss the renormalisation of the area operator in loop quantum gravity. In such a background independent framework, we propose to probe the structure of a surface through the analysis of the coarse-graining and renormalisation flow(s) of its area. We further introduce a procedure to coarse-grain spin network states and we quantitatively study the decrease in the number of degrees of freedom during this process. Finally, we use these coarse-graining tools to define the correlation and entanglement between parts of a spin network and discuss their potential interpretation as a natural measure of distance in such a state of quantum geometry."
============
briefly noted
Sabine H.
http://arxiv.org/hep-th/0603032
Thanu P.
http://arxiv.org/astro-ph/0603114
http://arxiv.org/abs/gr-qc/0603022
Doubly Special Relativity: facts and prospects
Jerzy Kowalski-Glikman
Submitted to the volume "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", D. Oriti ed
"In this short review of Doubly Special Relativity I describe first the relations between DSR and (quantum) gravity. Then I show how, in the case of a field theory with curved momentum space, the Hopf algebra of symmetries naturally emerges. I conclude with some remarks concerning DSR phenomenology and description of open problems."
http://arxiv.org/abs/hep-th/0603079
Nonperturbative sum over topologies in 2D Lorentzian quantum gravity
R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
10 pages, 4 figures. Talk given by S. Zohren at the Albert Einstein Century International Conference (Paris, July 18-22 2005)
"The recent progress in the Causal Dynamical Triangulations (CDT) approach to quantum gravity indicates that gravitation is nonperturbatively renormalizable. We review some of the latest results in 1+1 and 3+1 dimensions with special emphasis on the 1+1 model. In particular we discuss a nonperturbative implementation of the sum over topologies in the gravitational path integral in 1+1 dimensions. The dynamics of this model shows that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant. Similar ideas have been considered in the past by Coleman and others in the formal setting of 4D Euclidean path integrals. A remarkable property of the model is that in the continuum limit we obtain a finite space-time density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense out of a gravitational path integral including a sum over topologies, provided one imposes suitable kinematical restrictions on the state-space that preserve large scale causality."
this is listed for completeness although I think there is nothing new here that was not already in
http://arxiv.org/abs/hep-th/0507012
and its references
briefly noted:
http://arxiv.org/abs/gr-qc/0603029
Counting of Black Hole Microstates
A. Ghosh, P. Mitra
a couple of these are discussed in http://www.physicsforums.com/showthread.php?t=114140
http://math.ucr.edu/home/baez/where_we_stand/
"Fundamental Physics: Where We Stand Today"
John Baez
===============
http://www.nyas.org/publications/UpdateUnbound.asp?UpdateID=41
"A Crisis in Fundamental Physics"
Lee Smolin
published in the New York Academy of Sciences magazine.
The Houghton-Mifflin publishing house plans to release a new book by Lee Smolin this year---what the book is about is suggested by:
http://www.lse.ac.uk/collections/CPNSS/events/Conferences/AnnualPublicLecture.htm
Abstract for something called "The Annual Lecture" to be given by Smolin at The London School of Economics CPNSS, in June 2006.
"...I will begin by proposing an answer to the question of what science is and why it works. Part of the answer is that scientists form a community that is defined by adherence to a set of ethics which encourages honest reporting of observations and results as well as an awareness that future generations will know far more than us. As such science is based on a tragic understanding of how easily we fool ourselves and others and its success is due to the discovery of techniques whose use helps us discover error. In this and other ways I will discuss science is tied to democracy in that both require members of a community to adhere to ethics designed to allow us to achieve as a community far more than would be possible each on our own..."
[my comment: a healthy scientific community requires ethical behavior and some "checks and balances" because even prestigeous elites can succumb to self-delusion and error. it is not simply an egalitarian democracy, nor is it simply an amoral turf-based aristocratic hierarchy---a key ingredient is the empirical ethic]
=============
Recent contributions from Rovelli are mentioned here:
http://www.physicsforums.com/showthread.php?t=114024
A 59 slide lecture given at Lyon, Introduction to Loop Quantum Gravity
http://www.cpt.univ-mrs.fr/~rovelli/Lyon2006II.pdf
Course outline for a History of Science course that Rovelli is currently teaching is here
http://www.cpt.univ-mrs.fr/~rovelli/
=============================
We recently had a thread at PF about George Ellis finding misinformation in Susskind's book
http://www.physicsforums.com/showthread.php?t=114068
http://arxiv.org/abs/astro-ph/0603266
On horizons and the cosmic landscape
George F R Ellis
"Susskind claims in his recent book The Cosmic Landscape that evidence for the existence and nature of 'pocket universes' in a multiverse would be available via detailed study of the Cosmic Blackbody Background Radiation. I point out that apart from any other queries one might have about the chain of argument involved, this claim is invalid because it rests on a confusion between the nature of a particle horizon and an event horizon in cosmology."
Ellis is co-author, with Stephen Hawking, of a well-known reference "The Large-Scale Structure of Spacetime". He gives this link to an erroneous passage of Susskind's book.
http://www.twbookmark.com/books/28/0316155799/chapter_excerpt22014.html
http://arxiv.org/abs/hep-lat/0603013
Unexpected Spin-Off from Quantum Gravity
D. Benedetti (U. Utrecht), R. Loll (U. Utrecht)
10 pages, 4 figures
"We propose a novel way of investigating the universal properties of spin systems by coupling them to an ensemble of causal dynamically triangulated lattices, instead of studying them on a fixed regular or random lattice. Somewhat surprisingly, graph-counting methods to extract high- or low-temperature series expansions can be adapted to this case. For the two-dimensional Ising model, we present evidence that this ameliorates the singularity structure of thermodynamic functions in the complex plane, and improves the convergence of the power series."
title seems to involve a pun. Benedetti got his Masters from Rome IIRC and joined Loll's group as a PhD student sometime in the past two years---I have the feeling it was about a year ago
http://arxiv.org/abs/gr-qc/0603062
Conformal decomposition in canonical general relativity
Charles H.-T. Wang
6 pages
"A new canonical transformation is found that enables the direct canonical treatment of the conformal factor in general relativity. The resulting formulation significantly simplifies the previously presented conformal geometrodynamics. It provides a further theoretical basis for the conformal approach to loop quantum gravity and offers a generic framework for the conformal analysis of spacetime dynamics."
Charles Wang is physics prof IIRC at Aberdeen, UK. He does LQG his own independent way and is someone to watch out of the corner of one's eye. He gave a paper at last September QG conference on Sardinia. My fear is that if I ignore mavericks that seem disconnected from the rest of the researchers one of them may sneak up and surprise me. So without recommendation, I keep a link on Wang.
===============
Cosmology is a principle feed and motivation for QG. This book by Loeb (Harvard) will be a valuable reference, I suspect. Around 160 pages.
It is being published by Springer in Germany.
http://arxiv.org/abs/astro-ph/0603360
First Light
Abraham Loeb (Harvard)
158 pages, lecture notes for the 36th Swiss (SAAS-Fee) Winter School, April 2006 (to be published by Springer Verlag); http://obswww.unige.ch/saas-fee2006/preannouncement/course_pres/overview_f.html
"The first dwarf galaxies, which constitute the building blocks of the collapsed objects we find today in the Universe, had formed hundreds of millions of years after the big bang. This pedagogical review describes the early growth of their small-amplitude seed fluctuations from the epoch of inflation through dark matter decoupling and matter-radiation equality, to the final collapse and fragmentation of the dark matter on all mass scales above \~10^{-4} solar masses. The condensation of baryons into halos in the mass range of ~10^5-10^{10} solar masses led to the formation of the first stars and the re-ionization of the cold hydrogen gas, left over from the big bang. The production of heavy elements by the first stars started the metal enrichment process that eventually led to the formation of rocky planets and life. A wide variety of instruments currently under design [including large-aperture infrared telescopes on the ground or in space (JWST), and low-frequency arrays for the detection of redshifted 21cm radiation], will establish better understanding of the first sources of light during an epoch in cosmic history that was largely unexplored so far. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficently high resolution to identify the sources of the ionizing radiation. The technological challenges for observations and the computational challenges for numerical simulations, will motivate intense work in this field over the coming decade."
arivero
Mar20-06, 12:02 PM
Just noticed this one:
http://arxiv.org/abs/hep-th/0602160
Hidden symmetries of the Nambu-Goto action
It seems a sort of follow-up of
http://arxiv.org/abs/hep-th/0601134
String triality, black hole entropy and Cayley's hyperdeterminant
thanks! good to have your contributions.
arivero
Mar20-06, 01:32 PM
hmmm and sorry I put it using "new reply" instead of "new thread":uhh:
hmmm and sorry I put it using "new reply" instead of "new thread":uhh:
well actually it might be simpler to keep this thread specialized for non-string QG papers, as a general rule. and there is a sticky for string literature.
but I hope you will keep watch for interesting Loop-and-allied QG papers and contribute links here---I think you have an eye for what research could be important
no-brainer, this has to go on the link list
http://arxiv.org/abs/gr-qc/0603085
Exotic Statistics for Loops in 4d BF Theory
John C. Baez, Derek K. Wise, Alissa S. Crans
40 pages, many figures
"After a review of exotic statistics for point particles in 3d BF theory, and especially 3d quantum gravity, we show that loop-like defects in 4d BF theory obey exotic statistics governed by the 'loop braid group'. This group has a set of generators that switch two loops just as one would normally switch point particles, but also a set of generators that switch two loops by passing one through the other. The first set generates a copy of the symmetric group, while the second generates a copy of the braid group. Thanks to recent work of Xiao-Song Lin, we can give a presentation of the whole loop braid group, which turns out to be isomorphic to the 'braid permutation group' of Fenn, Rimanyi and Rourke. In the context 4d BF theory this group naturally acts on the moduli space of flat G-bundles on the complement of a collection of unlinked unknotted circles in R^3. When G is unimodular, this gives a unitary representation of the loop braid group. We also discuss 'quandle field theory', in which the gauge group G is replaced by a quandle."
here's a discussion thread, with exerpts
http://www.physicsforums.com/showthread.php?t=115082
=============================
f-h called attention to this new paper, also about the interesting (4D) case of BF theory:
http://arxiv.org/abs/gr-qc/0603076
Alternative symplectic structures for SO(3,1) and SO(4) four-dimensional BF theories
Merced Montesinos
13 pages
Class. Quantum Grav. 23 (2006) 2267-2278
"The most general action, quadratic in the B fields as well as in the curvature F, having SO(3,1) or SO(4) as the internal gauge group for a four-dimensional BF theory is presented and its symplectic geometry is displayed. It is shown that the space of solutions to the equations of motion for the BF theory can be endowed with symplectic structures alternative to the usual one. The analysis also includes topological terms and cosmological constant. The implications of this fact for gravity are briefly discussed."
========briefly noted========
http://arxiv.org/abs/gr-qc/0603090
Fundamental decoherence from quantum gravity: a pedagogical review
Rodolfo Gambini, Rafael Porto, Jorge Pullin
9 pages, dedicated to Octavio Obregon on his 60th birthday
I have no reason to recommend the next one. Just want to stay aware of other (particle) approaches to the c.c. problem. here is an example.
http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."
No special reason to recommend this either. philosophical/historical paper suggesting that there's something wrong with the ideas of b.i. and diffeo-invariance. If true, a serious critique of LQG and allied research.
http://arxiv.org/abs/gr-qc/0603087
Some remarks on the notions of general covariance and background independence
Domenico Giulini
26 pages, 3 figures. Contribution to "An assessment of current paradigms in the physics of fundamental interactions'', edited by I.O. Stamatescu (Springer Verlag, to appear)
John Swain is a experimental particle physicist/phenomenologist who works part at CERN and part in the physics department at Northeastern.
He interests me in part because he also writes LQG papers. He is a working nutsbolts high energy physicist who has enough style to also think creatively in Quantum Gravity.
So when he has new-sounding ideas about anything, I try to pay attention. This has been sitting on my desktop since last September.
Maybe someone here at PF could be interested. It is a way of seeing Higgs field by mass shifts, not by actually detecting the particle quantum of the field.
I suspect that the higgs field will ultimately be found built into the quantum geometry of some QG----intrinsic to the quantum state of spacetime. I would bet this thought has crossed Swain's mind too.
http://arxiv.org/hep-ph/0509151
Probing the Higgs Field Using Massive Particles as Sources and Detectors
S. Reucroft, Y.N. Srivastava, J. Swain, A. Widom
6 pages, no figures; Version 2 corrects some typographical errors of factors of 2 in equations 14, 17, 18 and 19 (all of the same origin) and mentions a linear collider as an interesting place to test the results of this paper
"In the Standard Model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higgs bosons. We provide theoretical estimates showing that the mass shift effect can be large and measurable, especially near pair threshold, at both the Tevatron and the LHC."
The general idea (which has some historical validity I believe) is to look for the field instead of for the particle which represents the field in certain situations.
======================
TIMBUQTU AND F-H thanks for your help with this biblio-links thread! About CD's blog as time goes on it is getting more and more active and useful isnt it, contributions from f-h and also from John Baez among others. It is an odd coincidence that we should post a link to Timb.'s professor's paper at the same time as he assigns homework from it.
Timbuqtu
Mar28-06, 12:22 PM
http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."
That's funny. Last week, one day before this paper was posted, Prokopec gave us a homework exercise to derive this result (the mass calculation). Still have to do it. But apparently there is a squareroot missing in the mass-formula in this abstract, because it must be m = 4.3 * 10^15 ((Lambda_0)^(1/2)/10^13GeV)^(1/2) GeV (according to his own paper).
"f-h called attention to this new paper, also about the interesting (4D) case of BF theory"
Actuall I first saw it on CDs blog.
http://arxiv.org/abs/gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153
"Quantum cosmology in general denotes the application of quantum physics to the whole universe and thus gives rise to many realizations and examples, covering problems at different mathematical and conceptual levels. It is related to quantum gravity and more specifically describes the application to cosmological situations rather than the construction and analysis of quantum field equations. As there are several different approaches to quantum gravity, equations for quantum cosmology are not unique. Most investigations have been performed in the context of canonical quantization, where Wheeler--DeWitt like equations are the prime object. Applications are mostly conceptual, ranging from possible resolutions of classical singularities and explanations of the uniqueness of the universe to the origin of seeds for a classical world and its initial conditions."
this is in rather dry handbook style and covers the general subject instead of focusing on Loop cosmology. For a more focused and detailed discussion see Bojowald's Living Reviews article
http://arxiv.org/abs/gr-qc/0601085
or this short survey for non-specialists, also by Bojowald:
http://arxiv.org/abs/astro-ph/0511557
while on the subject, a recent cosmology overview by Ned Wright just came out
http://arxiv.org/abs/astro-ph/0603750
A Century of Cosmology
E. L. Wright (UCLA Astronomy)
GLAST mission reference
http://arxiv.org/abs/astro-ph/0603762
The GLAST mission, LAT and GRBs
Nicola Omodei, for the GLAST/LAT GRB Science Group
6 pages, 3 figures. Proceedings for the 16th Annual October Astrophysics Conference in Maryland: Gamma Ray Bursts in the Swift Era
Mohammad Ansari is at Perimeter and has been sitting in on the Smolin lectures
Several times Smolin has referred to Ansari's work, and they've talked back and forth, but Smolin has the mike and I can't distinguish what Mohammad says.
Here is some recent work by him
http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."
=====================
http://arxiv.org/abs/gr-qc/0601013
Fermions in Ashtekar-Barbero Connections Formalism for Arbitrary Values of the Immirzi Parameter
Simone Mercuri
15 pp., no figures, title changed, version accepted for publication on Phys. Rev. D
"The Ashtekar-Barbero-Immirzi formulation of General Relativity is extended to include spinor matter fields. Our formulation applies to generic values of the Immirzi parameter and reduces to the Ashtekar-Romano-Tate approach when the Immirzi parameter is taken equal to the imaginary unit. The dynamics of the gravity-fermions coupled system is described by the Holst plus Dirac action with a non-minimal coupling term. The non-minimal interaction together with the Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan invariant, so that the effective action coming out is the one of Einstein-Cartan theory with a typical Fermi-like interaction term: in spite of the presence of spinor matter fields, the Immirzi parameter plays no role in the classical effective dynamics and results to be only a multiplicative factor in front of a total divergence.
We reduce the total action of the theory to the sum of dynamically independent Ashtekar-Romano-Tate actions for self and anti-self dual connections, with different weights depending on the Immirzi parameter. This allows to calculate the constraints of the complete theory in a simple way, it is only necessary to realize that the Barbero-Immirzi connection is a weighted sum of the self and anti-self dual Ashtekar connections. Finally the obtained constraints for the separated action result to be polynomial in terms of the self and anti-self dual connections, this could have implications in the inclusion of spinor matter in the framework of non-perturbative quantum gravity."
From page 5, discussion of equation (22):
"This result shows that the Immirzi parameter, which appears in the action for the gravitational field used as starting point in the construction of LQG, not only appears in non-perturbative quantum effects but also in the classical equations of motion, when fermions are present, leading to (independently from the quantum theory) possible observable effects. It plays the role of coupling constant in front of the four fermions interacting term...."
http://arxiv.org/abs/gr-qc/0604013
Quantum Nature of the Big Bang: An Analytical and Numerical Investigation I
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
59 pages, 19 figures
"Analytical and numerical methods are developed to analyze the quantum nature of the big bang in the setting of loop quantum cosmology. They enable one to explore the effects of quantum geometry both on the gravitational and matter sectors and significantly extend the known results on the resolution of the big bang singularity. Specifically, the following results are established for the homogeneous isotropic model with a massless scalar field: i) the scalar field is shown to serve as an internal clock, thereby providing a detailed realization of the `emergent time' idea; ii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously; iii) the Hamiltonian constraint is solved numerically to show that the big bang is replaced by a big bounce. Thanks to the non-perturbative, background independent methods, unlike in other approaches the quantum evolution is deterministic across the deep Planck regime. Our constructions also provide a conceptual framework and technical tools which can be used in more general models. In this sense, they provide foundations for analyzing physical issues associated with the Planck regime of loop quantum cosmology as a whole."
===============
not sure about this. any discussion of making QM deterministic is bound to be over my head. but there was discussion of this paper in a separate thread---some folks might be interested in checking it out
http://arxiv.org/abs/quant-ph/0604008
The mathematical basis for deterministic quantum mechanics
Gerard 't Hooft
15 pages, 3 figures
"If there exists a classical, i.e. deterministic theory underlying quantum mechanics, an explanation must be found of the fact that the Hamiltonian, which is defined to be the operator that generates evolution in time, is bounded from below. The mechanism that can produce exactly such a constraint is identified in this paper. It is the fact that not all classical data are registered in the quantum description. Large sets of values of these data are assumed to be indistinguishable, forming equivalence classes. It is argued that this should be attributed to information loss, such as what one might suspect to happen during the formation and annihilation of virtual black holes.
The nature of the equivalence classes is further elucidated, as it follows from the positivity of the Hamiltonian. Our world is assumed to consist of a very large number of subsystems that may be regarded as approximately independent, or weakly interacting with one another. As long as two (or more) sectors of our world are treated as being independent, they all must be demanded to be restricted to positive energy states only. What follows from these considerations is a unique definition of energy in the quantum system in terms of the periodicity of the limit cycles of the deterministic model."
http://arxiv.org/abs/gr-qc/0604016
Hidden Quantum Gravity in 3d Feynman diagrams
Aristide Baratin, Laurent Freidel
35 pages, 4 figures
"In this work we show that 3d Feynman amplitudes of standard QFT in flat and homogeneous space can be naturally expressed as expectation values of a specific topological spin foam model. The main interest of the paper is to set up a framework which gives a background independent perspective on usual field theories and can also be applied in higher dimensions. We also show that this Feynman graph spin foam model, which encodes the geometry of flat space-time, can be purely expressed in terms of algebraic data associated with the Poincare group. This spin foam model turns out to be the spin foam quantization of a BF theory based on the Poincare group, and as such is related to a quantization of 3d gravity in the limit where the Newton constant G_N goes to 0. We investigate the 4d case in a companion paper where the strategy proposed here leads to similar results."
http://arxiv.org/abs/gr-qc/0604044
Graviton propagator in loop quantum gravity
Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale
41 pages, 6 figures
"We compute some components of the graviton propagator in loop quantum gravity, using the spinfoam formalism, up to some second order terms in the expansion parameter."
===========================
Chapter for Oriti's book "Towards Quantum Gravity"
http://arxiv.org/abs/gr-qc/0604045
Unfinished revolution
Carlo Rovelli
8 pages
"Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti, that will appear with Cambridge University Press."
=================
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
7 pages
"We argue that EPR-type correlations do not entail any form of "non-locality", when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."
=================
http://arxiv.org/abs/gr-qc/0604050
Tachyon Matter in Loop-Inspired Cosmology
A. A. Sen
5 pages
"An analytical approach for studying the cosmological scenario with a homogeneous tachyon field within the framework of loop quantum gravity is developed. Our study is based on the semi-classical regime where space time can be approximated as a continuous manifold, but matter Hamiltonian gets non-perturbative quantum corrections. A formal correspondence between classical and loop quantum cosmology is also established. The Hamilton-Jacobi method for getting exact solutions is constructed and some exact power-law as well as bouncing solutions are presented."
http://arxiv.org/abs/gr-qc/0604040
Macroscopic observables and Lorentz violation in discrete quantum gravity
Joe Henson
15 pages, 1 figure
"This article concerns the fate of local Lorentz invariance in quantum gravity, particularly for approaches in which a discrete structure replaces continuum spacetime. Some features of standard quantum mechanics, presented in a sum-over-histories formulation, are reviewed, and their consequences for such theories are discussed. It is argued that, if the individual histories of a theory give bad approximations to macroscopic continuum properties in some frames, then it is inevitable that the theory violates Lorentz symmetry.
I don't think discussion of anthropics is so important because I expect nonperturbative approaches to Quantum Gravity (the new understanding of spacetime and matter that is emerging, see Oriti's book when it comes out :)) will explain the CC---what its role is and why it is what it is.
And then people will lose interest in the "Anthropic Lack of Principles" as it has been called. But meanwhile, here's something to check out!
http://arxiv.org/abs/astro-ph/0604242
An Observational Test for the Anthropic Origin of the Cosmological Constant
Abraham Loeb (Harvard)
5 pages, submitted to JCAP
"The existence of multiple regions of space beyond the observable Universe (within the so-called "multiverse") where the vacuum energy density takes different values, has been postulated as an explanation for the low non-zero value observed for it in our Universe. It is often argued that our existence pre-selects regions where the cosmological constant is sufficiently small to allow galaxies like the Milky Way to form and intelligent life to emerge. Here we propose a simple empirical test for this anthropic argument within the boundaries of the observable Universe. We make use of the fact that dwarf galaxies formed in our Universe at redshifts as high as z~10 when the mean matter density was larger by a factor of ~10^3 than today. Existing technology enables to check whether planets form in nearby dwarf galaxies and globular clusters by searching for microlensing or transit events of background stars. The oldest of these nearby systems may have formed at z~10. If planets are as common per stellar mass in these descendents as they are in the Milky Way galaxy, then the anthropic argument would be weakened considerably since planets could have formed in our Universe even if the cosmological constant was three orders of magnitude larger than observed. For a flat probability distribution, this would imply that the probability for us to reside in a region where the cosmological constant obtains its observed value is lower than approx. 10^{-3}. A precise version of the anthropic argument could then be ruled-out at a confidence level of ~99.9%, which constitutes a satisfactory measure of a good experimental test."
http://arxiv.org/abs/gr-qc/0604053
Violation of Strong Energy Condition in Effective Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages
must keep tabs on cousins at Beijing Normal----getting quite strong in LQG and LQC.
several presentations from Beijing Normal at Loops '05 last October.
Joao Magueijo is a hero of mine, ever since I watched an online video seminar talk of him from February 2006 at Perimeter.
I really liked a MOND paper he wrote with Bekenstein posted this year.
Now he has a paper with Rafael Sorkin----familiar to everybody too I guess.
I personally could not get much out of this Magueijo Sorkin paper. But because of who they are, and because AXIS OF EVIL is topical in cosmology and because of WMAP3, I figure there might be someone to whom this is interesting. Even though I dont someone else might.
http://arxiv.org/abs/astro-ph/0604410
Occam's razor meets WMAP
Joao Magueijo, Rafael D. Sorkin
"Using a variety of quantitative implementations of Occam's razor we examine the low quadrupole, the ``axis of evil'' effect and other detections recently made appealing to the excellent WMAP data. We find that some razors fully demolish the much lauded claims for departures from scale-invariance. They all reduce to pathetic levels the evidence for a low quadrupole (or any other low ell cut-off), both in the first and third year WMAP releases. The 'axis of evil' effect is the only anomaly examined here that survives the humiliations of Occam's razor, and even then in the category of 'strong' rather than 'decisive' evidence. Statistical considerations aside, differences between the various renditions of the datasets remain worrying."
Hey look again, this shows ago that Magueijo is a lean-mean phenomenologist. Lots of other folks are talking about the axis of evil and believing departure from scale-invariance at large angles and he says NO! the confidence levels for most of that are pitiful. At least he is frank and willing to take contrary stands. It might be a good paper.
Magueijo has QG importance because he co-authors with Lee Smolin and is into QG phenomenology.
=================================
Couple of Shahn Majid QG-related papers came out today:
OTE=marcus]http://arxiv.org/abs/hep-th/0604130
Algebraic approach to quantum gravity II: noncommutative spacetime
S. Majid
26 pages, 2 figures; book chapter to appear in D. Oriti, ed., Cambridge Univ. Press
"We provide a self-contained introduction to the quantum group approach to noncommutative geometry as the next-to-classical effective geometry that might be expected from any successful quantum gravity theory. We focus particularly on a thorough account of the bicrossproduct model noncommutative spacetimes of the form [t,x_i]=i lambda x_i and the correct formulation of predictions for it including a variable speed of light. We also study global issues in the Poincaré group in the model with the 2D case as illustration. We show that any off-shell momentum can be boosted to infinite negative energy by a finite Lorentz transformaton."
http://arxiv.org/abs/hep-th/0604132
Algebraic approach to quantum gravity III: noncommmutative Riemannian geometry
S. Majid
25 pages, 1 figure; to appear in collection B. Fauser and J. Tolksdorf, eds., Birkhauser
"This is a self-contained introduction to quantum Riemannian geometry based on quantum groups as frame groups, and its proposed role in quantum gravity. Much of the article is about the generalisation of classical Riemannian geometry that arises naturally as the classical limit; a theory with nonsymmetric metric and a skew version of metric compatibilty. Meanwhile, in quantum gravity a key ingredient of our approach is the proposal that the differential structure of spacetime is something that itself must be summed over or 'quantise' as a physical degree of freedom. We illustrate such a scheme for quantum gravity on small finite sets."
http://arxiv.org/abs/gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
"Non-minimally coupling a scalar field to gravity introduces an additional curvature term into the action which can change the general behavior in strong curvature regimes, in particular close to classical singularities. While one can conformally transform any non-minimal model to a minimally coupled one, that transformation can itself become singular. It is thus not guaranteed that all qualitative properties are shared by minimal and non-minimal models. This paper addresses the classical singularity issue in isotropic models and extends singularity removal in quantum gravity to non-minimal models."
http://arxiv.org/abs/hep-th/0604181
6J Symbols Duality Relations
L. Freidel, K. Noui, P. Roche
28 pages, 2 figures
"It is known that the Fourier transformation of the square of (6j) symbols has a simple expression in the case of su(2) and U_q(su(2)) when q is a root of unity. The aim of the present work is to unravel the algebraic structure behind these identities. We show that the double crossproduct construction H_1\bowtie H_2 of two Hopf algebras and the bicrossproduct construction
H_2^{*}\bowtie H_1
are the Hopf algebras structures behind these identities by analysing different examples. We study the case where
D= H_1\bowtie H_2
is equal to the group algebra of ISU(2), SL(2,C) and where D is a quantum double of a finite group, of SU(2) and of U_q(su(2)) when q is real."
the year 2006 is turning out to be the wonderyear of Laurent Freidel
http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schrödinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories.
http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vaccuum and gauge invariant variables
Laurent Freidel
35 pages
In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian.
-------------------
the next paper is discussed in Bee's thread about it here
http://www.physicsforums.com/showthread.php?t=118641
http://arxiv.org/abs/gr-qc/0508013
Anti-Gravitation
S. Hossenfelder
Phys. Lett. B 636 (2006) 119-125
http://arxiv.org/abs/gr-qc/0604112
Background independent quantizations: the scalar field II
W. Kaminski, J. Lewandowski, A. Okolow
51 pages
"We are concerned with the issue of quantization of a scalar field in a diffeomorphism invariant manner. We apply the method used in Loop Quantum Gravity. It relies on the specific choice of scalar field variables referred to as the polymer variables. The quantization, in our formulation, amounts to introducing the `quantum' polymer *-star algebra and looking for positive linear functionals, called states. Assumed in our paper homeomorphism invariance allows to derive the complete class of the states. They are determined by the homeomorphism invariant states defined on the CW-complex *-algebra. The corresponding GNS representations of the polymer *-algebra and their self-adjoint extensions are derived, the equivalence classes are found and invariant subspaces characterized. In the preceding letter (the part I) we outlined those results. Here, we present the technical details."
can't evaluate the next one. list it in part to keep tabs on the Karpacz Winterschool, which several times has chosen a topic related to DSR or quantum gravity
http://arxiv.org/abs/gr-qc/0604120
The Chrono-geometrical Structure of Special and General Relativity: a Re-Visitation of Canonical Geometrodynamics
Luca Lusanna (INFN, Firenze)
33 pages, Lectures given at the 42nd Karpacz Winter School of Theoretical Physics, "Current Mathematical Topics in Gravitation and Cosmology", Ladek, Poland, 6-11 February 2006
A modern re-visitation of the consequences of the lack of an intrinsic notion of instantaneous 3-space in relativistic theories leads to a reformulation of their kinematical basis emphasizing the role of non-inertial frames centered on an arbitrary accelerated observer. In special relativity the exigence of predictability implies the adoption of the 3+1 point of view, which leads to a well posed initial value problem for field equations in a framework where the change of the convention of synchronization of distant clocks is realized by means of a gauge transformation. This point of view is also at the heart of the canonical approach to metric and tetrad gravity in globally hyperbolic asymptotically flat space-times, where the use of Shanmugadhasan canonical transformations allows the separation of the physical degrees of freedom of the gravitational field (the tidal effects) from the arbitrary gauge variables. Since a global vision of the equivalence principle implies that only global non-inertial frames can exist in general relativity, the gauge variables are naturally interpreted as generalized relativistic inertial effects, which have to be fixed to get a deterministic evolution in a given non-inertial frame. As a consequence, in each Einstein's space-time in this class the whole chrono-geometrical structure, including also the clock synchronization convention, is dynamically determined and a new approach to the Hole Argument leads to the conclusion that "gravitational field" and "space-time" are two faces of the same entity. This view allows to get a classical scenario for the unification of the four interactions in a scheme suited to the description of the solar system or our galaxy with a deperametrization to special relativity and the subsequent possibility to take the non-relativistic limit."
several former string theorists are publishing mainly in pure Mathematics now,
I don't know about Michael Douglas but he has posted some papers in the math part of arxiv.
the most recent is today
http://arxiv.org/abs/math.AG/0604597
Algebraic Geometry no less! classy!
http://arxiv.org/abs/hep-th/0604212
Quantum Gravity, or The Art of Building Spacetime
J. Ambjorn, J. Jurkiewicz, R. Loll
22 pages, 6 figures. Contribution to the book "Approaches to Quantum Gravity", ed. D. Oriti, Cambridge University Press
"The method of four-dimensional Causal Dynamical Triangulations provides a background-independent definition of the sum over geometries in quantum gravity, in the presence of a positive cosmological constant. We present the evidence accumulated to date that a macroscopic four-dimensional world can emerge from this theory dynamically. Using computer simulations we observe in the Euclidean sector a universe whose scale factor exhibits the same dynamics as that of the simplest mini-superspace models in quantum cosmology, with the distinction that in the case of causal dynamical triangulations the effective action for the scale factor is not put in by hand but obtained by integrating out in the quantum theory the full set of dynamical degrees of freedom except for the scale factor itself."
http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year
"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."
http://arxiv.org/abs/gr-qc/0605006
Discreteness without symmetry breaking: a theorem
Luca Bombelli, Joe Henson, Rafael D. Sorkin
7 pages
"This paper concerns sprinklings into Minkowski space (Poisson processes). It proves that there exists no equivariant measurable map from sprinklings to spacetime directions (even locally). Therefore, if a discrete structure is associated to a sprinkling in an intrinsic manner, then the structure will not pick out a preferred frame, locally or globally. This implies that the discreteness of a sprinkled causal set will not give rise to 'Lorentz breaking'' effects like modified dispersion relations. Another consequence is that there is no way to associate a finite-valency graph to a sprinkling consistently with Lorentz invariance."
http://arxiv.org/abs/gr-qc/0605011
Gravity, Geometry and the Quantum
Abhay Ashtekar
16 pages, 2 figures. To appear in the Proceedings of the `Einstein Century' Conference, 15-22 July, Paris, edited by J-M Alimi et al (American Institute of Physics)
"After a brief introduction, basic ideas of the quantum Riemannian geometry underlying loop quantum gravity are summarized. To illustrate physical ramifications of quantum geometry, the framework is then applied to homogeneous isotropic cosmology. Quantum geometry effects are shown to replace the big bang by a big bounce. Thus, quantum physics does not stop at the big-bang singularity. Rather there is a pre-big-bang branch joined to the current post-big-bang branch by a 'quantum bridge'. Furthermore, thanks to the background independence of loop quantum gravity, evolution is deterministic across the bridge."
http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures
"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A_0 are computed and the statistical entropy, as a function of the area, is obtained for A_0 up to 550 l^2_P. The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to \gamma=0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."
=============
a lot depends on the GLAST mission, I am hoping very much that it flies in 2007 as scheduled
this technical article is a sign that the mission is still going ahead
http://arxiv.org/abs/astro-ph/0605050
GLAST Tracker
Hiroyasu Tajima, for GLAST Tracker Team (Stanford Linear Accelerator Center)
5 pages, 5 figues, Invited talk at Vertex 2005, Chuzenji Lake, Nikko, Japan, November 7-November 11, 2005, To be published in Nuclear Instruments and Methods A
SLAC-PUB-11833
"The Large Area Telescope (LAT) on board the Gamma-ray Large-Area Space Telescope (GLAST) is a pair-conversion gamma-ray detector designed to explore the gamma-ray universe in the 20 MeV-300 GeV energy band. The Tracker subsystem of the LAT will perform tracking of electron and positrons to determine the origin of the gamma-ray. The design and performance of the GLAST LAT Tracker are described in this paper."
http://arxiv.org/abs/hep-th/0605052
Generic predictions of quantum theories of gravity
Lee Smolin
For inclusion in "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
"I discuss generic consequences (sometimes called "soft predictions") of a class of background independent quantum theories of spacetime called causal spin network theories. These are theories whose kinematics and dynamics is based on the evolution of labeled graphs, by local moves, such as in loop quantum gravity and spin foam models. Some generic consequences are well known, including the discreteness of quantum geometry, the elimination of spacetime singularities, the entropy of black hole and cosmological horizons and the fact that positive cosmological constant spacetimes are hot. Within the last few years three possible generic consequences have come to light. These are:
1) Deformed special relativity as the symmetry of the ground state,
2) Elementary particles as coherent excitations of quantum geometry,
3) Locality is disordered.
I discuss some possible experimental consequences of each."
Larry Krauss is a prominent cosmophysicist and neutrinos get into everything including ultimately QG and how we will observe the very early universe. Neutrinos-in-cosmology deserves a wide-audience survey and Krauss has the writing style to give a good one.
http://arxiv.org/abs/astro-ph/0605378
What is the Role of Neutrinos in Shaping the Universe?
Lawrence M. Krauss (Case Western Reserve University)
10 pages. Invited review lecture, International Workshop on NO-VE, Venice, 2006
"I review various aspects of the role neutrinos have played in shaping various cosmological observables: the nature of large scale structure, observed fluctuations in the CMB, the nature of matter, and the shape of things to come. (Invited review lecture III International Workshop on NO-VE, Venice, 2006)"
==========
Back in post 471, in March, I mentioned this paper of Mohammad Ansari, which Danny recently brought up in a post:
"Recently, I read a beautiful paper in which it is proven that ANY surface in LQG contains degeneracy, no matter it being a boundary horizon or whatever else. (http://uk.arxiv.org/abs/gr-qc/0603121) This degeneracy is such that the surface degeneracy is A/4. This is a critial discovery in LQG and can follow up Padmanabhan's idea of associating the lack of information in (http://uk.arxiv.org/abs/gr-qc/0405072) into quantum gravity."
post 471 is here
http://www.physicsforums.com/showpost.php?p=951694&postcount=471
http://www.physicsforums.com/showthread.php?p=951694#post951694
http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."
that post also had an article about the Immirzi parameter by Simone Mercuri which was accepted for
publication by Physical Review D
Danny's thread about the Ansari paper is here
http://www.physicsforums.com/showthread.php?t=120887
called "Ansari's degeneracy in LQG"
=============================
this appeared yesterday
http://arxiv.org/abs/gr-qc/0605087
Quantization of strings and branes coupled to BF theory
John C. Baez, Alejandro Perez
"BF theory is a topological theory that can be seen as a natural generalization of 3-dimensional gravity to arbitrary dimensions. Here we show that the coupling to point particles that is natural in three dimensions generalizes in a direct way to BF theory in d dimensions coupled to (d-3)-branes. In the resulting model, the connection is flat except along the membrane world-sheet, where it has a conical singularity whose strength is proportional to the membrane tension. As a step towards canonically quantizing these models, we show that a basis of kinematical states is given by 'membrane spin networks', which are spin networks equipped with extra data where their edges end on a brane."
it is a followup on the Baez Wise Crans article mentioned earlier in this thread
Exotic statistics for loops in 4d BF theory
http://arxiv.org/gr-qc/0603085
=====================
Ignacio Navarro and Karel Van Acoleyen recently posted two articles on a variant of MOND
http://arxiv.org/abs/astro-ph/0605322
Dark energy, MOND and sub-millimeter tests of gravity
I. Navarro, K. Van Acoleyen
6 pages, to appear in proceedings of the XLIrst Rencontres de Moriond
"We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."
http://arxiv.org/abs/gr-qc/0605091
Long distance modifications of gravity in four dimensions
I. Navarro, K. Van Acoleyen
4 pages, contribution to the proceedings of the Rencontres de Moriond: Contents and Structures of the Universe, March 18-25, 2006, La Thuile
"We discuss some general characteristics of modifications of the 4D Einstein-Hilbert action that become important for low space-time curvatures. In particular we focus on the chameleon-like behaviour of the massive gravitational degrees of freedom. Generically there is at least one extra scalar that is light on cosmic scales, but for certain models it becomes heavy close to any mass source."
an earlier paper from last year by Navarro and Van Acoleyen
http://arxiv.org/abs/gr-qc/0512109
Modified gravity, Dark Energy and MOND
Ignacio Navarro, Karel Van Acoleyen
24 pages, 2 figures.
"We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a_0 = c mu, where c is the speed of light and mu is a parameter of the model that also determines the late-time Hubble constant: H_0 ~mu.
In these models, besides the massless spin two graviton, there is a scalar excitation of the spacetime metric whose mass depends on the background curvature. This dependence is such that this scalar, although almost massless in vacuum, becomes massive and effectively decouples when one gets close to any source and we recover an acceptable weak field limit at short distances. There is also a (classical) 'running' of Newton's constant with the distance to the sources and gravity is easily enhanced at large distances by a large ratio. We comment on the possibility of building a model with a MOND-like Newtonian limit that could explain the rotation curves of galaxies without introducing Dark Matter using this kind of actions. We also explore briefly the characteristic gravitational phenomenology that these models imply: besides a long distance modification of gravity they also predict deviations from Newton's law at short distances. This short distance scale depends on the local background curvature of spacetime, and we find that for experiments on the Earth surface it is of order ~ 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower."
==========
more work on black hole state counting and determining the Immirzi parameter
http://arxiv.org/abs/hep-th/0605125
Counting of isolated horizon states
A. Ghosh, P. Mitra
4 pages
"The entropy of an isolated horizon has been obtained by counting states in loop quantum gravity. We revisit the calculation of the horizon states using statistical methods and find the possibility of additional states, leading to an increase in the entropy. Apart from this, an isolated horizon temperature is introduced in this framework."
the paper of Ghosh and Mitra cites this recent one of Corichi et al
http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures
"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A0 are computed and the statistical entropy, as a function of the area, is obtained for A0 up to 550 L2P The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to gamma = 0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."
The recent paper of Ghosh Mitra also cites one of theirs that was published in Physical Review Letters B, in 2005. It gives the same value of the Immirzi parameter, namely about 0.274.
http://arxiv.org/abs/gr-qc/0411035
An improved estimate of black hole entropy in the quantum geometry approach
A. Ghosh, P. Mitra
5 pages, LaTeX
Journal-ref: Phys.Lett. B616 (2005) 114-117
"A proper counting of states for black holes in the quantum geometry approach shows that the dominant configuration for spins are distributions that include spins exceeding one-half at the punctures. This raises the value of the Immirzi parameter and the black hole entropy. However, the coefficient of the logarithmic correction remains -1/2 as before."
=====================
not sure about this: an odd thing, don't remember ever seeing anything like it
http://arxiv.org/abs/gr-qc/0605098
Area and Entropy: A New Perspective
Jarmo Mäkelä
4 pages. This paper received an honorary mention in the annual Awards for Essays in Gravitation contest
"We consider a spacelike two-plane originally at rest with respect to electromagnetic radiation in equilibrium. We find that if the plane is moved with respect to the radiation, the plane shrinks such that the maximum amount of entropy carried by radiation through the plane is, in natural units, exactly one-half of the decrease in the area of the plane. This result suggests that the equivalence between area and entropy may not be limited in black holes, nor even in the spacetime horizons only, but the equivalence between horizon area and entropy may be a special case of some general and simple, still undiscovered fundamental principle of nature."
http://arxiv.org/abs/gr-qc/0605113
Avoidance of future singularities in loop quantum cosmology
M. Sami, Parampreet Singh, Shinji Tsujikawa
6 pages, 2 figures
"We consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to \rho^2 modification of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density \rho diverges at future singularities of Friedmann dynamics. Such quantum effects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a finite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory."
I list the following since is to appear in Oriti's book
http://arxiv.org/abs/hep-th/0605202
String Field Theory
Washington Taylor (MIT, Stanford)
To appear in "Towards Quantum Gravity", ed. Daniele Oriti, Cambridge University Press; 22 pages
"This elementary introduction to string field theory highlights the features and the limitations of this approach to quantum gravity as it is currently understood. String field theory is a formulation of string theory as a field theory in space-time with an infinite number of massive fields. Although existing constructions of string field theory require expanding around a fixed choice of space-time background, the theory is in principle background-independent, in the sense that different backgrounds can be realized as different field configurations in the theory. String field theory is the only string formalism developed so far which, in principle, has the potential to systematically address questions involving multiple asymptotically distinct string backgrounds. Thus, although it is not yet well defined as a quantum theory, string field theory may eventually be helpful for understanding questions related to cosmology in string theory."
the following really are raw lecture notes, very sketchy, with some spelling mistakes and awkward wording, by someone at Ben-Gurion university who wants to cover all the QM you ever heard of, at an undergraduate level, in 200-some pages. It is an exercise in writing down one or two sentences and half dozen equations, and moving on. Like a stand-up comedian or serial exhibitionist. Frankly I thought it was a good idea---it had more EFFRONTERY than the usual textbook, I thought, so I wanted to include it:
http://arxiv.org/abs/quant-ph/0605180
Lecture Notes in Quantum Mechanics
Doron Cohen
239 pages
"These lecture notes cover undergraduate textbook topics (e.g. as in Sakurai), and also additional advanced topics at the same level of presentation. In particular: EPR and Bell; Basic postulates; The probability matrix; Measurement theory; Entanglement; Quantum computation; Wigner-Weyl formalism; The adiabatic picture; Berry phase; Linear response theory; Kubo formula; Modern approach to scattering theory with mesoscopic orientation; Theory of the resolvent and the Green function; Gauge and Galilei Symmetries; Motion in magnetic field; Quantum Hall effect; Quantization of the electromagnetic field; Fock space formalism."
Let me know if you check it out and think I should not have included it.
http://arxiv.org/abs/gr-qc/0605123
Towards the graviton from spinfoams: higher order corrections in the 3d toy model
Etera R. Livine, Simone Speziale, Joshua L. Willis
24 pages, many figures
"We consider the recent calculation gr-qc/0508124 of the graviton propagator in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102, we test how the spinfoam formalism can be used to construct the perturbative expansion of graviton amplitudes. We compute explicitly the next to leading and next to next to leading orders, corresponding to one-loop and two-loop corrections. We show that while the first arises entirely from the expansion of the Regge action around the flat background, the latter receives contributions from the microscopic, non Regge-like, quantum geometry. Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number of open issues in the rest of the paper. First, we discuss the boundary state ansatz, which is a key ingredient in the whole construction. We propose a way to enhance the ansatz in order to make the edge lengths and dihedral angles conjugate variables in a mathematically well-defined way. Second, we show that the leading order is stable against different choices of the face weights of the spinfoam model; the next to leading order, on the other hand, is changed in a simple way, and we show that the topological face weight minimizes it. Finally, we extend the leading order result to the case of a regular, but not equilateral, tetrahedron."
just keeping the link-basket up to date:
===========
http://arxiv.org/abs/gr-qc/0605141
Time Delay Predictions in a Modified Gravity Theory
J. W. Moffat
5 pages
"The time delay effect for planets and spacecraft is obtained from a fully relativistic modified gravity theory including a fifth force skew symmetric field by fitting to the Pioneer 10/11 anomalous acceleration data. A possible detection of the predicted time delay corrections to general relativity for the outer planets and future spacecraft missions is considered. The time delay correction to GR predicted by the modified gravity is consistent with the observational limit of the Doppler tracking measurement reported by the Cassini spacecraft on its way to Saturn, and the correction increases to a value that could be measured for a spacecraft approaching Neptune and Pluto."
?
========
http://arxiv.org/abs/gr-qc/0605138
Quantization of diffeomorphism invariant theories of connections with a non-compact structure group - an example
Andrzej Okolow
76 pages,
"A simple diffeomorphism invariant theory of connections with the non-compact structure group R of real numbers is quantized. The theory is defined on a four-dimensional 'space-time' by an action resembling closely the self-dual Plebanski action for general relativity. The space of quantum states is constructed by means of projective techniques by Kijowski. Except this point the applied quantization procedure is based on Loop Quantum Gravity methods."
Okolow is the "O" in the L.O.S.T theorem
http://arxiv.org/abs/gr-qc/0606032
Non-Singular Bouncing Universes in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot, G. V. Vereshchagin
12 pages, 9 figures
"Non-perturbative quantum geometric effects in Loop Quantum Cosmology predict a rho^2 modification to the Friedmann equation at high energies. The quadratic term is negative definite and can lead to generic bounces when the matter energy density becomes equal to a critical value of the order of the Planck density. The non-singular bounce is achieved for arbitrary matter without violation of positive energy conditions. By performing a qualitative analysis we explore the nature of the bounce for inflationary and Cyclic model potentials. For the former we show that inflationary trajectories are attractors of the dynamics after the bounce implying that inflation can be harmoniously embedded in LQC. For the latter difficulties associated with singularities in cyclic models can be overcome. We show that non-singular cyclic models can be constructed with a small variation in the original Cyclic model potential by making it slightly positive in the regime where scalar field is negative."
http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year.
"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."
http://arxiv.org/abs/gr-qc/0606026
The black hole horizon as a dynamical system
Gerard 't Hooft
16 pages. Presented at "Einstein's Legacy in the New Millennium", Puri, India, 15-22 Dec. 2005. An extension of gr-qc/0504120 with chapter added and small sign changes
"Interactions between outgoing Hawking particles and ingoing matter are determined by gravitational forces and Standard Model interactions. In particular the gravitational interactions are responsible for the unitarity of the scattering against the horizon, as dictated by the holographic principle, but the Standard Model interactions also contribute, and understanding their effects is an important first step towards a complete understanding of the horizon's dynamics. The relation between in- and outgoing states is described in terms of an operator algebra. In this contribution, in which earlier results are rederived and elaborated upon, we first describe the algebra induced on the horizon by U(1) vector fields and scalar fields, including the case of an Englert-Brout-Higgs mechanism, and a more careful consideration of the transverse vector field components. We demonstrate that, unlike classical black holes, the quantized black hole has on its horizon an imprint of its (recent) past history, i.e., quantum hair. The relation between in- and outgoing states depends on this imprint. As a first step towards the inclusion of non-Abelian interactions, we then compute the effects of magnetic monopoles both in the in-states and in the out-states. They completely modify, and indeed simplify, our algebra."
John Baez has an online video of an interesting talk given 31 May at Perimeter
Higher-Dimensional Algebra: A Language for Quantum Spacetime
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
select "seminar series" from the left margin menu
the slides/lecturenotes for this talk are available at Baez site
http://math.ucr.edu/home/baez/quantum_spacetime/
http://math.ucr.edu/home/baez/quantum_spacetime/qs.pdf
Online videos of 3 talks at last week's "Theory Canada" conference
Gravitational collapse in quantum gravity
Viqar Husain
"I will describe a framework for studying the dynamical collapse of a scalar field in a Hamiltonian formulation of quantum gravity. The approach makes use of operators that test states for trapped surface formation as a state is evolves, and allows the extraction of invariant information about the scalar field and geometry."
==============
Nonlocal remnants of microscopic discreteness?
Rafael Sorkin
"If 'quantum spacetime' is discrete, then any propagation equation compatible with the Lorentz transformations is necessarily nonlocal. I will present evidence that this sort of nonlocality survives on intermediate length scales much greater than that of the Planck length, yielding for example a nonlocal equation of motion for a scalar field."
================
Protected information and emergent locality in background independent quantum gravity
Fotini Markopoulou
"A successful quantum theory of gravity needs to explain spacetime as an effective, approximate description of a more fundamental theory. I describe an approach to quantum gravity in which the pre-spacetime structure is a directed graph of operator algebras. They may also be thought of as a network of quantum information flow. Methods from quantum information theory can be used to extract degrees of freedom protected from the microscopic evolution and which define the causal structure of the emergent spacetime. As a result of the background independence we find that emergent spacetime means emergent locality and hence locality corrections at all scales. This is a new direction in quantum gravity phenomenology currently under investigation."
==========
There are plenty of others to choose from as well. I didnt sample all by any means, and mention only these.
To get the videos, go here
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
and scroll down to "Theory Canada 2" in the menu on the left
The format is split screen so you get to see both the lecturer and the projected slide.
Several of the talks are brief (20-25 minutes). You may find other talks of particular interest to you.
Among the short talks, I was especially impressed by those given by Sorkin and by Husain. Sorkin's abstract does not indicate the full scope of what he presented.
==============
the talk by Robert Leigh is not yet put online, but it may appear later.
The Spectrum of 2+1 Dimensional Yang-Mills Theory (so far this talk is not available on line)
Robert Leigh
"I review our recent work on an analytic solution of Yang-Mills theory in 2+1 dimensions."
It was about http://arxiv.org/abs/hep-th/0604060
and there was a follow-up
http://arxiv.org/abs/hep-th/0604184
of which Laurent Freidel was co-author
======================
now for something completely different :smile:
http://arxiv.org/abs/gr-qc/0606061
Gravity: A New Holographic Perspective
T. Padmanabhan
Plenary talk at the International Conference on Einstein's Legacy in the New Millennium, December 15 - 22, 2005, Puri, India; to appear in the Proceedings to be published in IJMPD; 16 pages
"A general paradigm for describing classical (and semiclassical) gravity is presented. This approach brings to the centre-stage a holographic relationship between the bulk and surface terms in a general class of action functionals and provides a deeper insight into several aspects of classical gravity which have no explanation in the conventional approach. After highlighting a series of unresolved issues in the conventional approach to gravity, I show that (i) principle of equivalence, (ii) general covariance and (iii)a reasonable condition on the variation of the action functional, suggest a generic Lagrangian for semiclassical gravity of the form L=Q_a^{bcd}R^a_{bcd} with \nabla_b Q_a^{bcd}=0 . The expansion of Q_a^{bcd} in terms of the derivatives of the metric tensor determines the structure of the theory uniquely. The zeroth order term gives the Einstein-Hilbert action and the first order correction is given by the Gauss-Bonnet action. Any such Lagrangian can be decomposed into a surface and bulk terms which are related holographically. The equations of motion can be obtained purely from a surface term in the gravity sector. Hence the field equations are invariant under the transformation T_{ab} \to T_{ab} + \lambda g_{ab} and gravity does not respond to the changes in the bulk vacuum energy density. The cosmological constant arises as an integration constant in this approach. The implications are discussed."
http://arxiv.org/abs/hep-th/0606135
Fixed points of quantum gravity in higher dimensions
Peter Fischer, Daniel F. Litim
Talk presented at Einstein Century Meeting, Paris, 15-22 July 2005
SHEP-06-14, CERN-PH-TH-2006/066, PITHA 06/06
"We study quantum gravity in more than four dimensions by means of an exact functional flow. A non-trivial ultraviolet fixed point is found in the Einstein-Hilbert theory. It is shown that our results for the fixed point and universal scaling exponents are stable. If the fixed point persists in extended truncations, quantum gravity in the metric field is asymptotically safe. We indicate physical consequences of this scenario in phenomenological models with low-scale quantum gravity and large extra dimensions."
cites several papers of Martin Reuter and by Ambjorn, Jurkiewicz, Loll.
suggests ways to make higher dimensional models testable, by showing that einstein metric gravity is assympt. safe, so manageable, in higher dimensions.
http://arxiv.org/abs/gr-qc/0606065
Evidence for an entropy bound from fundamentally discrete gravity
D. Rideout, S. Zohren
25 pages, 9 figures
IMPERIAL/TP/06/SZ/01
"The various entropy bounds that exist in the literature suggest that spacetime is fundamentally discrete, and hint at an underlying relationship between geometry and 'information'. The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We present a measure for the maximal entropy of spherically symmetric spacelike regions within the causal set approach to quantum gravity. In terms of the proposal, a bound for the entropy contained in this region can be derived from a counting of potential 'degrees of freedom' associated to the Cauchy horizon of its future domain of dependence. For different spherically symmetric spacelike regions in Minkowski spacetime of arbitrary dimension, we show that this proposal leads, in the continuum approximation, to Susskind's well-known spherical entropy bound."
Stefan Zohren got his masters at Utrecht, in Loll's group, and then went to London Imperial for PhD. London is strong in Causal Sets. Rideout is experienced researcher in that approach. Did his PhD at Syracuse with Rafael Sorkin in 2002. (Dynamics of Causal Sets gr-qc/0212064)
http://arxiv.org/abs/gr-qc/0606067
How often does the Unruh-DeWitt detector click? Regularisation by a spatial profile
Jorma Louko, Alejandro Satz
28 pages, 1 figure
"We analyse within first-order perturbation theory the instantaneous transition rate of an accelerated Unruh-DeWitt particle detector whose coupling to a massless scalar field on four-dimensional Minkowski space is regularised by a spatial profile. For the Lorentzian profile introduced by Schlicht, the zero size limit is computed explicitly and expressed as manifestly finite integral formula that no longer involves regulators or limits. The same transition rate is obtained for an arbitrary profile of compact support under a modified definition of spatial smearing. Consequences for the asymptotic behaviour of the transition rate are discussed. A number of stationary and nonstationary trajectories are analysed, recovering in particular the Planckian spectrum for uniform acceleration."
Alejandro Satz is an Argentine studying for PhD in Nottingham. He has a blog called REALITY CONDITIONS
Jorma Louko has co-authored with Carlo Rovelli, Don Marolf, Rafael Sorkin, Kirill Krasnov. 35 of his papers are here:
http://arxiv.org/find/gr-qc/1/au:+Louko_J/0/1/0/all/0/1
http://arxiv.org/abs/gr-qc/0606074
A semiclassical tetrahedron
Carlo Rovelli, Simone Speziale
10 pages
"We construct a macroscopic semiclassical state state for a quantum tetrahedron. The expectation values of the geometrical operators representing the volume, areas and dihedral angles are peaked around assigned classical values, with vanishing relative uncertainties."
http://arxiv.org/abs/gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
"Non-minimal actions with matter represented by a scalar field coupled to gravity are considered in the context of a homogeneous and isotropic universe. The coupling is of the form -\xi/2 \phi^2 R. The possibility of successful inflation is investigated taking into account features of loop cosmology. For that end a conformal transformation is performed. That brings the theory into the standard minimally coupled form (Einstein frame) with some effective field and its potential. Both analytical and numerical estimates show that a negative coupling constant is preferable for successful inflation. Moreover, provided fixed initial conditions, larger|\xi| leads to a greater number of {\em e}-folds. The latter is obtained for a reasonable range of initial conditions and the coupling parameter and indicates a possibility for successful inflation."
http://arxiv.org/abs/gr-qc/0606078
On obtaining classical mechanics from quantum mechanics
Ghanashyam Date
22 pages
"Constructing a classical mechanical system associated with a given quantum mechanical one, entails construction of a classical phase space and a corresponding Hamiltonian function from the available quantum structures and a notion of coarser observations. The Hilbert space of any quantum mechanical system naturally has the structure of an infinite dimensional symplectic manifold (`quantum phase space'). There is also a systematic, quotienting procedure which imparts a bundle structure to the quantum phase space and extracts a classical phase space as the base space. This works straight forwardly when the Hilbert space carries weakly continuous representation of the Heisenberg group and recovers the linear classical phase space \mathbb{R}^{\mathrm{2N}}. We report on how the procedure also allows extraction of non-linear classical phase spaces and illustrate it for Hilbert spaces being finite dimensional (spin-j systems), infinite dimensional but separable (particle on a circle) and infinite dimensional but non-separable (Bohr quantization). To construct a corresponding classical dynamics, one needs to choose a suitable section and identify an effective Hamiltonian. The effective dynamics mirrors the quantum dynamics provided the section satisfies conditions of semiclassicality and tangentiality. There is also an alternative and a priori distinct notion of effective Hamiltonian used in the context of a WKB approximation. These two notions are compared for the example of loop quantum cosmology."
http://arxiv.org/abs/astro-ph/0606506
Strong gravitational lens probability in TeVeS
Da-Ming Chen, HongSheng Zhao
5 pages, 4 figures, submitted to ApJL
"We calculate the strong lensing probability with the image-separation greater than a given value \Delta\theta and the image flux ratio q_r less than 10 in a TeVeS (tensor-vector-scalar) cosmology, which is a relativistic version of the MOND (modified Newtonian dynamics). The lensing galaxy is modeled by the Hernquist profile. We assume a flat cosmology with \Omega_b=0.04 and the simplest interpolating function \mu(x)
with \mu(x)=x for x<1 and
\mu(x)=1 for x>1. Our calculations show that MOND exhibits a very high lensing efficiency compared with the CDM paradigm. In order to match the well defined sample of the combined radio Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS), the upper limit of the mass of galaxies should be 1.7\times 10^{11} M_\sun. The galaxies produce much more large image-separation lenses in MOND regime than that in CDM regime, while at small image-separation both regimes match the observational data well."
I wish to highlight this because it is Modified Gravity phenomenology----testing the "Dark Matter" hypotheis against the TeVeS relativistic MOND.
I noticed the article initially because of a confusion of names, a different person (Hongbao Zhang) has reported from Beijing about some talks given this week by Carlo Rovelli.
Here is H-B Zhang's post on N.E.W.
http://www.math.columbia.edu/~woit/wordpress/?p=415#comment-12456
Here is H-B Zhang's blog
http://hongbaozhang.blog.edu.cn/user1/3890/
Carlo Rovelli is giving a series of talks about LQG scattering amplitudes at Beijing Normal coinciding with the international String 2006 conference.
In the following paper STUFF if a technical mathematical term
don't knock it :smile: might be interesting------comes from UC Riverside
http://arxiv.org/abs/math.QA/0601458
Categorified Algebra and Quantum Mechanics
Jeffrey Morton (University of California, Riverside)
67 pages, 25 figures
"Interest in combinatorial interpretations of mathematical entities stems from the convenience of the concrete models they provide. Finding a bijective proof of a seemingly obscure identity can reveal unsuspected significance to it. Finding a combinatorial model for some mathematical entity is a particular instance of the process called "categorification''. Examples include the interpretation of N as the Burnside rig of the category of finite sets with product and coproduct, and the interpretation of N[x] as the category of combinatorial species. This has interesting applications to quantum mechanics, and in particular the quantum harmonic oscillator, via Joyal's ``species'', a new generalization called ``stuff types'', and operators between these, which can be represented as rudimentary Feynman diagrams for the oscillator. In quantum mechanics, we want to represent states in an algebra over the complex numbers, and also want our Feynman diagrams to carry more structure than these ``stuff operators'' can do, and these turn out to be closely related. We will show how to construct a combinatorial model for the quantum harmonic oscillator in which the group of phases, U(1), plays a special role. We describe a general notion of "M-Stuff Types'' for any monoid M, and see that the case M=U(1) provides an interpretation of time evolution in the combinatorial setting, as well as other quantum mechanical features of the harmonic oscillator."
http://arxiv.org/abs/hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics "Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006
"Effective equations are often useful to extract physical information from quantum theories without having to face all technical and conceptual difficulties. One can then describe aspects of the quantum system by equations of classical type, which correct the classical equations by modified coefficients and higher derivative terms. In gravity, for instance, one expects terms with higher powers of curvature. Such higher derivative formulations are discussed here with an emphasis on the role of degrees of freedom and on differences between Lagrangian and Hamiltonian treatments. A general scheme is then provided which allows one to compute effective equations perturbatively in a Hamiltonian formalism. Here, one can expand effective equations around any quantum state and not just a perturbative vacuum. This is particularly useful in situations of quantum gravity or cosmology where perturbations only around vacuum states would be too restrictive. The discussion also demonstrates the number of free parameters expected in effective equations, used to determine the physical situation being approximated, as well as the role of classical symmetries such as Lorentz transformation properties in effective equations. An appendix collects information on effective correction terms expected from loop quantum gravity and string theory."
===========
I didn't catch some of these when they first went on arxiv.
http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez, Urs Schreiber
28 pages, 10 figures
"Just as gauge theory describes the parallel transport of point particles using connections on bundles, higher gauge theory describes the parallel transport of 1-dimensional objects (e.g. strings) using 2-connections on 2-bundles. A 2-bundle is a categorified version of a bundle: that is, one where the fiber is not a manifold but a category with a suitable smooth structure. Where gauge theory uses Lie groups and Lie algebras, higher gauge theory uses their categorified analogues: Lie 2-groups and Lie 2-algebras. We describe a theory of 2-connections on principal 2-bundles and explain how this is related to Breen and Messing's theory of connections on nonabelian gerbes. The distinctive feature of our theory is that a 2-connection allows parallel transport along paths and surfaces in a parametrization-independent way. In terms of Breen and Messing's framework, this requires that the "fake curvature" must vanish. In this paper we summarize the main results of our theory without proofs."
this is a pretty good paper, which clears up doubts about why (from viewpoint of someone to whom spin networks and spinfoam make sense) one should know a bit about twogroups and twoholonomy.
You have to listen when somebody says allows parallel transport along paths and surfaces in a parametrization-independent way-----parallel transport along SURFACES---and then comes up with a necessary condition for that which says that "something" has to equal zero. things that have to be zero for what you want to happen---in this case setting it equal to zero apparently gives something that looks like the BF equation, which I was always wondering where it came from. The paper seems to show that serious old fogeys with gravitas can be interested in twogroups---it is not only just fun, it is required.
Here is an earlier, much longer, related paper:
http://arxiv.org/abs/hep-th/0412325
Higher Gauge Theory: 2-Connections on 2-Bundles
John Baez, Urs Schreiber
73 pages, 4 figures
"Connections and curvings on gerbes are beginning to play a vital role in differential geometry and mathematical physics -- first abelian gerbes, and more recently nonabelian gerbes. These concepts can be elegantly understood using the concept of '2-bundle' recently introduced by Bartels. A 2-bundle is a generalization of a bundle in which the fibers are categories rather than sets. Here we introduce the concept of a '2-connection' on a principal 2-bundle. We describe principal 2-bundles with connection in terms of local data, and show that under certain conditions this reduces to the cocycle data for nonabelian gerbes with connection and curving subject to a certain constraint -- namely, the vanishing of the 'fake curvature', as defined by Breen and Messing. This constraint also turns out to guarantee the existence of '2-holonomies': that is, parallel transport over both curves and surfaces, fitting together to define a 2-functor from the `path 2-groupoid' of the base space to the structure 2-group. We give a general theory of 2-holonomies and show how they are related to ordinary parallel transport on the path space of the base manifold."
http://arxiv.org/abs/hep-th/0511086
Calabi-Yau Manifolds and the Standard Model
John C. Baez
4 pages
"For any subgroup G of O(n), define a "G-manifold" to be an n-dimensional Riemannian manifold whose holonomy group is contained in G. Then a G-manifold where G is the Standard Model gauge group is precisely a Calabi-Yau manifold of 10 real dimensions whose tangent spaces split into orthogonal 4- and 6-dimensional subspaces, each preserved by the complex structure and parallel transport. In particular, the product of Calabi-Yau manifolds of dimensions 4 and 6 gives such a G-manifold. Moreover, any such G-manifold is naturally a spin manifold, and Dirac spinors on this manifold transform in the representation of G corresponding to one generation of Standard Model fermions and their antiparticles.
=================
http://arxiv.org/abs/gr-qc/0606104
Numerical evolutions of a black hole-neutron star system in full General Relativity
Frank Löffler, Luciano Rezzolla, Marcus Ansorg
16 pages, 12 figures
"We present the first simulations in full General Relativity of the head-on collision between a neutron star and a black hole of comparable mass. These simulations are performed through the solution of the Einstein equations combined with an accurate solution of the relativistic hydrodynamics equations via high-resolution shock-capturing techniques. The initial data is obtained by following the York-Lichnerowicz conformal decomposition with the assumption of time symmetry. Unlike other relativistic studies of such systems, no limitation is set for the mass ratio between the black hole and the neutron star, nor on the position of the black hole, whose apparent horizon is entirely contained within the computational domain. The latter extends over 400 M and is covered with six levels of fixed mesh refinement. Concentrating on a prototypical binary system with mass ratio ~6, we find that although a tidal disruption is evident the neutron star is accreted promptly and entirely into the black hole. While the collision is completed before ~300 M, the evolution is carried over up to ~1700 M, thus providing time for the extraction of the gravitational-wave signal produced and allowing for a first estimate of the radiative efficiency of processes of this type."
Basically, the question addressed is: "What kind of ripples does it cause when a black hole eats a neutron star?"
But they were considering the head-on case, and I don't think that makes as much ripples as the spiraling-in case. either way, though, somebody has to compute the gravitational waves that ripple off such events, in case the detectors ever get good enough to pick them up.
================
Recalling that Robert Leigh recently co-authored with Laurent Freidel about 4D Yang-Mills
and there were two papers posted which got a fair amount of attention, in case anyone wants to see a video about this, there is one now:
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/
in the left sidebar menu, scroll down and click where it says THEORY CANADA 2
and you get a menu of 40 presentations at the recent Theory Canada conference,
displayed 12 to a page. Robert Leigh is on page 3, right at the end (he is #36)
I was impressed and thought he gave a good 30 minute talk and even that he and Freidel are making real
progress toward understanding Y-M, and it helps to see the guy talking with his slides instead of only reading the paper.
but if you want the paper too, it is
http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
"We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schr\"odinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories."
and the accompanying Freidel solo paper was
http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vacuum and gauge invariant variables
Laurent Freidel
35 pages
"In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian."
So the spinfoam goes from one spin network to another in the fashion of a path integral. And each spin network is the eigenfunction of the QG Hamiltonion, blah, blah, blah.
OK. But by what process is the spin network actually measured? What "interaction" causes the superposition of spin networks to "colapse" to a particular eigenstate? I have to wonder if such a measurement process can exist at that level. And if not, then could it be that by definition spacetime must always be assumed to be in a superposition? And what would be the implications of that?
So the spinfoam goes from one spin network to another in the fashion of a path integral...
this thread is being used purely for bibliography, Mike, discussion is off topic. Please start a separate thread to ask questions like this--I'm sure folks will be happy to respond. I would not want to reply to your question here in this context (and hope no one else would either) because making a practice of that would clutter the link library and make it harder to use.
http://arxiv.org/abs/gr-qc/0606120
A possible experimental test of quantized gravity
P. J. Salzman, S. Carlip
9 pages, 2 figures
"While it is widely believed that gravity should ultimately be treated as a quantum theory, there remains a possibility that general relativity should not be quantized. If this is the case, the coupling of classical gravity to the expectation value of the quantum stress-energy tensor will naturally lead to nonlinearities in the Schrodinger equation. By numerically investigating time evolution in the nonrelativistic 'Schrodinger-Newton' approximation, we show that such nonlinearities may be observable in the next generation of molecular interferometry experiments."
===============
Wolram called attention to this newly posted paper by Floyd Stecker (NASA, Goddard Space Flight Center), an authority on high energy astronomy----cosmic rays, GRB.
http://arxiv.org/abs/astro-ph/0606641
Testing Relativity at High Energies Using Spaceborne Detectors
F. W. Stecker (NASA/GSFC)
13 pages, Text of invited talk at the "From Quantum to Cosmos: Fundamental Physics Studies from Space" meeting
"The Gamma-ray Large Area Space Telescope (GLAST) will measure the spectra of distant extragalactic sources of high energy gamma-rays. GLAST can look for energy dependent propagation effects from such sources as a signal of Lorentz invariance violation (LIV). Such sources should also exhibit high energy spectral cutoffs from pair production interactions with low energy photons. The properties of such cutoffs can also be used to test LIV. ..."
=========
http://arxiv.org/abs/gr-qc/0606121
Uniform discretizations: a quantization procedure for totally constrained systems including gravity
Miguel Campiglia, Cayetano Di Bartolo, Rodolfo Gambini, Jorge Pullin
4 pages
"We present a new method for the quantization of totally constrained systems including general relativity. The method consists in constructing discretized theories that have a well defined and controlled continuum limit. The discrete theories are constraint-free and can be readily quantized. This provides a framework where one can introduce a relational notion of time and that nevertheless approximates in a well defined fashion the theory of interest. The method is equivalent to the group averaging procedure for many systems where the latter makes sense and provides a generalization otherwise. In the continuum limit it can be shown to contain, under certain assumptions, the 'master constraint' of the 'Phoenix project'. It also provides a correspondence principle with the classical theory that does not require to consider the semiclassical limit."
==============
this seems very strange. hovering drops of superfluid helium ?????
maybe they could be good for something, if not necessarily to detect gravity waves
I may have to erase this because it is so crazy.
http://arxiv.org/gr-qc/0606118
Proposed observations of gravity waves from the early Universe via "Millikan oil drops"
Raymond. Y. Chiao
10 pages, 2 figures, NASA "Quantum-to-Cosmos" conference proceedings to be published in IJMPD
"Pairs of Planck-mass drops of superfluid helium coated by electrons (i.e., 'Millikan oil drops'), when levitated in a superconducting magnetic trap, can be efficient quantum transducers between electromagnetic (EM) and gravitational (GR) radiation. This leads to the possibility of a Hertz-like experiment, in which EM waves are converted at the source into GR waves, and then back-converted at the receiver from GR waves back into EM waves. Detection of the gravity-wave analog of the cosmic microwave background using these drops can discriminate between various theories of the early Universe."
Maybe (maybe!) it is not so crazy after all. It seems that Raymond Chiao was awarded the Lamb Medal in 2006 or in any case delivered the Lamb Medal Lecture at a January 2006 Conference in Utah. (Remember the Lamb Shift?) He is at the new UC Merced campus jointly in the Schools of Natural Science and of Engineering. His Lamb Medal talk was also about these charged helium droplets (!)
http://arxiv.org/abs/quant-ph/0601193
The Interface between Quantum Mechanics and General Relativity
Raymond Y. Chiao
22 pages, 7 figures; Lamb medal lecture on January 5, 2006 at the Physics of Quantum Electronics Winter Colloquium at Snowbird, Utah; accepted for publication in J. Mod. Optics
"The generation, as well as the detection, of gravitational radiation by means of charged superfluids is considered. One example of such a 'charged superfluid' consists of a pair of Planck-mass-scale, ultracold 'Millikan oil drops,' each with a single electron on its surface, in which the oil of the drop is replaced by superfluid helium. When levitated in a magnetic trap, and subjected to microwave-frequency electromagnetic radiation, a pair of such "Millikan oil drops" separated by a microwave wavelength can become an efficient quantum transducer between quadrupolar electromagnetic and gravitational radiation. This leads to the possibility of a Hertz-like experiment, in which the source of microwave-frequency gravitational radiation consists of one pair of 'Millikan oil drops' driven by microwaves, and the receiver of such radiation consists of another pair of 'Millikan oil drops' in the far field driven by the gravitational radiation generated by the first pair. The second pair then back-converts the gravitional radiation into detectable microwaves. The enormous enhancement of the conversion efficiency for these quantum transducers over that for electrons arises from the fact that there exists macroscopic quantum phase coherence in these charged superfluid systems."
http://arxiv.org/abs/gr-qc/0607014
Particles as Wilson lines of gravitational field
L. Freidel, J. Kowalski--Glikman, A. Starodubtsev
19 pages
"Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom."
http://arxiv.org/abs/gr-qc/0607032
The group field theory approach to quantum gravity
Daniele Oriti
17 pages, 2 figures; expanded version of a contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
"We give a very concise review of the group field theory formalism for non-perturbative quantum gravity, a higher dimensional generalisation of matrix models. We motivate it as a simplicial and local realisation of the idea of 3rd quantization of the gravitational field, equivalently of a quantum field theory of simplicial geometry, in which also the topology of space is fully dynamical. We highlight the basic structure of the formalism, and discuss briefly various models that are being studied, some recent results and the many open issues that future research should face. Finally, we point out the connections with other approaches to quantum gravity, such as loop quantum gravity, quantum Regge calculus and dynamical triangulations, and causal sets."
http://arxiv.org/abs/gr-qc/0607013
The emergence of background geometry from quantum fluctuations
J. Ambjorn, R. Janik, W. Westra, S. Zohren
12 pages
"We show how the quantization of two-dimensional gravity leads to an (Euclidean) quantum space-time where the average geometry is that of constant negative curvature and where the Hartle-Hawking boundary condition arises naturally."
http://arxiv.org/abs/gr-qc/0607039
Quantum Nature of the Big Bang: Improved dynamics
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
40 pages, 9 figures
"An improved Hamiltonian constraint operator is introduced in loop quantum cosmology. Quantum dynamics of the spatially flat, isotropic model with a massless scalar field is then studied in detail using analytical and numerical methods. The scalar field continues to serve as `emergent time', the big bang is again replaced by a quantum bounce, and quantum evolution remains deterministic across the deep Planck regime. However, while with the Hamiltonian constraint used so far in loop quantum cosmology the quantum bounce can occur even at low matter densities, with the new Hamiltonian constraint it occurs only at a Planck-scale density. Thus, the new quantum dynamics retains the attractive features of current evolutions in loop quantum cosmology but, at the same time, cures their main weakness."
Also posted today:
http://arxiv.org/abs/gr-qc/0607033
Variational Methods in Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0607034
On the Green's function and iterative solutions of Loop Quantum Cosmology
More on Loop Quantum Cosmology:
http://arxiv.org/abs/gr-qc/0607044
Instabilities in numerical loop quantum cosmology
Jessica Rosen, Jae-Hun Jung, Gaurav Khanna
6 pages, 8 figures
"In this article we perform von Neumann analysis of the difference equations that arise as a result of loop quantum gravity being applied to models of cosmology and black holes. In particular, we study the numerical stability of Bianchi I LRS (symmetric and non-symmetric constraint) and Schwarzschild interior (symmetric constraint) models, and find that there exist domains over which there are instabilities, generically. We also present explicit evolutions of wave-packets in these models and clearly demonstrate the presence of these instabilities."
Khanna is in the Physics Department at Dartmouth. His papers go back a ways and he has co-authored with Bojowald. His two co-authors are in the Dartmouth Math Department. E.g. J-H Jung has 26 papers in arxiv grp_physics going back to 1998. Good for Dartmouth.
http://arxiv.org/abs/astro-ph/0607227
Why anthropic reasoning cannot predict Lambda
Glenn D. Starkman (Case Western Reserve University and Oxford U.), Roberto Trotta (Oxford University)
4 pages, 1 figure
"We revisit anthropic arguments purporting to explain the measured value of the cosmological constant. We argue that different ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, we show that weighting different universes by the total number of possible observations leads to an extremely small probability for observing a value of Lambda equal to or greater than what we now measure. We conclude that anthropic reasoning within the framework of probability as frequency is ill-defined and that it cannot be used to explain the value of Lambda, nor, likely, any other physical parameters."
cant evaluate, but anybody's new idea of a way to test QG, I want to know about it.
http://arxiv.org/abs/hep-ph/0607145
GRBs Neutrinos as a Tool to Explore Quantum Gravity induced Lorentz Violation
Uri Jacob, Tsvi Piran
"Lorentz Invariance Violation (LIV) arises in various quantum-gravity theories. As the typical energy for quantum gravity is the Planck mass, M_{pl}, LIV will, most likely, be manifested at very high energies that are not accessible on Earth in the foreseeable future. One has to turn to astronomical observations. Time of flight measurement from different astronomical sources set current limits on the energy scale of possible LIV to > 0.01 M_{pl} (for n=1 models) and > 10^{-9} M_{pl} (for n=2). According to current models Gamma-Ray Bursts (GRBs) are accompanied by bursts of high energy (\gsim 100TeV) neutrinos. At this energy range the background level of currently constructed neutrino detectors is so low that a detection of a single neutrino from the direction of a GRB months or even years after the burst would imply an association of the neutrino with the burst and will establish a measurement of a time of flight delay. Such time of flight measurements provide the best way to observe (or set limits) on LIV. Detection of a single GRB neutrino would open a new window on LIV and would improve current limits by many orders of magnitude."
and Tsvi Piran is a recognized specialist in figuring out how to test QG
he was invited to the February 2004 Polish winter symposium on testing QG
and gave a bunch of talks (at the time not so many people had thought to investigate
observational tests of QG) I can't tell if his ideas are good but other people pay attention.
Maybe synching neutrino detection with gammaray observation makes sense---leastwise we have the tools
to do it.
this thread is non-string QG bibliography, it gives links to recent non-string QG papers. usually I just keep track of postings on arxiv.org.
this time I am going to add the latest edition of John Baez "This Week's Finds".
the reason is I have a hunch that eventually we are all going to want to understand something about SO(4,1)
I think that particular Lie group is going to be the kitchen stool by which humans reach the high shelf, so it is apt to become familiar, the way the poincaré group and lorentz group are familiar.
Curiously enough, I don't know anything online that is a real kindergarten introduction to SO(4,1). Hey, don't knock kindergarten, it is probably the basis of western civilization or something:smile: We all at some point need a basic introduction to things!
So maybe someone can suggest a better ONLINE intuitive intro to DeSitter group?
Until then, the best I know is a few paragraphs in TWF 235, around page 5 out of 8 pages that talks about a lot of other stuff.
http://www.math.ucr.edu/home/baez/week235.html
http://arxiv.org/abs/astro-ph/0607380
Solving the Problem of Time in General Relativity and Cosmology with Phantoms and k -- Essence
Thomas Thiemann
38 pages
"We show that if the Lagrangean for a scalar field coupled to General Relativity only contains derivatives, then it is possible to completely deparametrise the theory. This means that
1.Physical observables, i.e. functions which Poisson commute with the spatial diffeomorphism and Hamiltonian constraints of General Relativity, can be easily constructed.
2. The physical time evolution of those observables is generated by a natural physical Hamiltonian which is (constrained to be) positive.
The mechanism by which this works is due to Brown and Kuchar. In order that the physical Hamiltonian is close to the Hamiltonian of the standard model and the one used in cosmology, the required Lagrangean must be that of a Dirac -- Born -- Infeld type. Such matter has been independently introduced previously by cosmologists in the context of k -- essence due to Armendariz-Picon, Mukhanov and Steinhardt in order to solve the cosmological coincidence (dark energy) problem. We arrive at it by totally unrelated physical considerations originating from quantum gravity. Our manifestly gauge invariant approach leads to important modifications of the interpretation and the the analytical appearance of the standard FRW equations of classical cosmology in the late universe. In particular, our concrete model implies that the universe should recollapse at late times on purely classical grounds."
http://arxiv.org/abs/gr-qc/0607075
Approximating the physical inner product of Loop Quantum Cosmology
Benjamin Bahr, Thomas Thiemann
35 pages
===================
Quantum Theory in Cosmology
The seminar series at Perimeter Institute has a talk given by Martin Bojowald on 28 July
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
It is about Quantum Gravity in Cosmology and is particularly interesting because it discusses
the role QG plays in early structure formation in the universe---in the conversion of quantum fluctuations
into corresponding classcial ones.
========at FH's suggestion===========
http://arxiv.org/gr-qc/0607068
Dirac Quantization of Parametrized Field Theory
Madhavan Varadarajan
33 pages
"Parametrized field theory (PFT) is free field theory on flat spacetime in a diffeomorphism invariant disguise. It describes field evolution on arbitrary foliations of the flat spacetime instead of only the usual flat ones, by treating the `embedding variables' which describe the foliation as dynamical variables to be varied in the action in addition to the scalar field. A formal Dirac quantization turns the constraints of PFT into functional Schrodinger equations which describe evolution of quantum states from an arbitrary Cauchy slice to an infinitesimally nearby one.This formal Schrodinger picture- based quantization is unitarily equivalent to the standard Heisenberg picture based Fock quantization of the free scalar field if scalar field evolution along arbitrary foliations is unitarily implemented on the Fock space. Torre and Varadarajan (TV) showed that for generic foliations emanating from a flat initial slice in spacetimes of dimension greater than 2, evolution is not unitarily implemented, thus implying an obstruction to Dirac quantization.
We construct a Dirac quantization of PFT, unitarily equivalent to the standard Fock quantization, using techniques from Loop Quantum Gravity (LQG) which are powerful enough to supercede the no-go implications of the TV results. The key features of our quantization include an LQG type representation for the embedding variables, embedding dependent Fock spaces for the scalar field, an anomaly free representation of (a generalization of) the finite transformations generated by the constraints and group averaging techniques. The difference between 2 and higher dimensions is that in the latter, only finite gauge transformations are defined in the quantum theory, not the infinitesimal ones."
Mann and Popescu have co-authored with Freidel. This one sounds like it might connect with Freidel papers on the 3D case. Also with Higher Gauge Theory (HGT) discussed by John Baez----several of whose papers they cite.
http://arxiv.org/abs/gr-qc/0607076
Higher Gauge Theory and Gravity in (2+1) Dimensions
R.B. Mann, Eugeniu M. Popescu
22 pages
"Non-abelian higher gauge theory has recently emerged as a generalization of standard gauge theory to higher dimensional (2-dimensional in the present context) connection forms, and as such, it has been successfully applied to the non-abelian generalizations of the Yang-Mills theory and 2-form electrodynamics. (2+1)-dimensional gravity, on the other hand, has been a fertile testing ground for many concepts related to classical and quantum gravity, and it is therefore only natural to investigate whether we can find an application of higher gauge theory in this latter context. In the present paper we investigate the possibility of applying the formalism of higher gauge theory to gravity in (2+1) dimensions, and we show that a nontrivial model of (2+1)-dimensional gravity coupled to scalar and tensorial matter fields - the \Sigma\Phi EA model - can be formulated both as a standard gauge theory and as a higher gauge theory. Since the model has a very rich structure - it admits as solutions black-hole BTZ-like geometries, particle-like geometries as well as Robertson-Friedman-Walker cosmological-like expanding geometries - this opens a wide perspective for higher gauge theory to be tested and understood in a relevant gravitational context. Additionally, it offers the possibility of studying gravity in (2+1) dimensions coupled to matter in an entirely new framework."
selfAdjoint
Jul24-06, 08:24 PM
Krisitna Giesel and Thomas Thiemann have submitted three papers on a new subject in QG: Algebraic Quantum Gravity. Here are the descriptions.
http://www.arxiv.org/abs/gr-qc/0607099
gr-qc/0607099
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:23:01 GMT (40kb)
Algebraic Quantum Gravity (AQG) I. Conceptual Setup
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-058
"We introduce a new top down approach to canonical quantum gravity, called Algebraic Quantum Gravity (AQG):The quantum kinematics of AQG is determined by an abstract $*-$algebra generated by a countable set of elementary operators labelled by an algebraic graph. The quantum dynamics of AQG is governed by a single Master Constraint operator. While AQG is inspired by Loop Quantum Gravity (LQG), it differs drastically from it because in AQG there is fundamentally no topology or differential structure. A natural Hilbert space representation acquires the structure of an infinite tensor product (ITP) whose separable strong equivalence class Hilbert subspaces (sectors) are left invariant by the quantum dynamics. The missing information about the topology and differential structure of the spacetime manifold as well as about the background metric to be approximated is supplied by coherent states. Given such data, the corresponding coherent state defines a sector in the ITP which can be identified with a usual QFT on the given manifold and background. Thus, AQG contains QFT on all curved spacetimes at once, possibly has something to say about topology change and provides the contact with the familiar low energy physics. In particular, in two companion papers we develop semiclassical perturbation theory for AQG and LQG and thereby show that the theory admits a semiclassical limit whose infinitesimal gauge symmetry agrees with that of General Relativity. In AQG everything is computable with sufficient precision and no UV divergences arise due to the background independence of the undamental combinatorial structure. Hence, in contrast to lattice gauge theory on a background metric, no continuum limit has to be taken, there simply is no lattice regulator that must be sent to zero."
http://www.arxiv.org/abs/gr-qc/0607100
gr-qc/0607100
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:27:43 GMT (43kb)
Algebraic Quantum Gravity (AQG) II. Semiclassical Analysis
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-59
"In the previous article a new combinatorial and thus purely algebraical approach to quantum gravity, called Algebraic Quantum Gravity (AQG), was introduced. In the framework of AQG existing semiclassical tools can be applied to operators that encode the dynamics of AQG such as the Master constraint operator. In this article we will analyse the semiclassical limit of the (extended) algebraic Master constraint operator and show that it reproduces the correct infinitesimal generators of General Relativity. Therefore the question whether General Relativity is included in the semiclassical sector of the theory, which is still an open problem in LQG, can be significantly improved in the framework of AQG. For the calculations we will substitute SU(2) by U(1)^3. That this substitution is justified will be demonstrated in the third article of this series "
http://www.arxiv.org/abs/gr-qc/0607101
gr-qc/0607101
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:30:29 GMT (27kb)
Algebraic Quantum Gravity (AQG) III. Semiclassical Perturbation Theory
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-60
"In the two previous papers of this series we defined a new combinatorical approach to quantum gravity, Algebraic Quantum Gravity (AQG). We showed that AQG reproduces the correct infinitesimal dynamics in the semiclassical limit, provided one incorrectly substitutes the non -- Abelean group SU(2) by the Abelean group $U(1)^3$ in the calculations. The mere reason why that substitution was performed at all is that in the non -- Abelean case the volume operator, pivotal for the definition of the dynamics, is not diagonisable by analytical methods. This, in contrast to the Abelean case, so far prohibited semiclassical computations. In this paper we show why this unjustified substitution nevertheless reproduces the correct physical result: Namely, we introduce for the first time semiclassical perturbation theory within AQG (and LQG) which allows to compute expectation values of interesting operators such as the master constraint as a power series in $\hbar$ with error control. That is, in particular matrix elements of fractional powers of the volume operator can be computed with extremely high precision for sufficiently large power of $\hbar$ in the $\hbar$ expansion. With this new tool, the non -- Abelean calculation, although technically more involved, is then exactly analogous to the Abelean calculation, thus justifying the Abelean analysis in retrospect. The results of this paper turn AQG into a calculational discipline."
I can see this new area as being a magnet for young trained theoretical physicsts because it is both Algebraic (and algebraic is so cool!) and Perturbative (and hence at least potentially accessible to people with non-LQG training).
Good find, selfAdjoint! I was just about to post the Thiemann and Giesel work myself.
Here is another that arrived today
http://arxiv.org/abs/gr-qc/0607097
Semiclassical states for quantum cosmology
Viqar Husain, Oliver Winkler
13 pages
"In a metric variable based Hamiltonian quantization, we give a prescription for constructing semiclassical matter-geometry states for homogeneous and isotropic cosmological models. These "collective" states arise as infinite linear combinations of fundamental excitations in an unconventional "polymer" quantization. They satisfy a number of properties characteristic of semiclassicality, such as peaking on classical phase space configurations. We describe how these states can be used to determine quantum corrections to the classical evolution equations, and to compute the initial state of the universe by a backward time evolution."
keeping an eye on the ekpyrotic alternative:
http://arxiv.org/abs/hep-th/0607164
Cosmic Perturbations Through the Cyclic Ages
Mattingly may have an improved way to constrain the causal set approach to QG
http://arxiv.org/abs/astro-ph/0607485
Low energy bounds on Poincare violation in causal set theory
Nemanja Kaloper, David Mattingly
8 pages
"In the causal set approach to quantum gravity, Poincaré symmetry is modified by swerving in spacetime, induced by the random lattice discretization of the space-time structure. The broken translational symmetry at short distances is argued to lead to a residual diffusion in momentum space, whereby a particle can acquire energy and momentum by drift along its mass shell and a system in equilibrium can spontaneously heat up. We consider bounds on the rate of momentum space diffusion coming from astrophysical molecular clouds, nuclear stability and cosmological neutrino background. We find that the strongest limits come from relic neutrinos, which we estimate to constrain the momentum space diffusion constant by k < 10^{-61} {\rm GeV}^3 for neutrinos with masses m_\nu > 0.01 {\rm eV}, improving the previously quoted bounds by roughly 17 orders of magnitude."
On Thursday 3 August, a few days from now, Andy Randono will be giving a talk at Perimeter. Andy is a graduate student at University of Texas. He did his undergraduate work at Tufts. he has been studying the Kodama state---a QG state with connections to DeSitter space. here is the abstract of the talk:
Andy Randono
Generalizing the Kodama State
Thursday August 3, 2006, 1:30 AM
"The Kodama state is unique in being an exact solution to all the constraints of quantum gravity that also has a well defined semi-classical interpretation as a quantum version of a classical spacetime, namely de-Sitter spacetime. In addition, the exact form of the state is known in both the connection and spin network bases, thereby concretely realizing the beautiful connection between spin networks and functionals on the space of connections. However, the state is riddled with difficulties that we will track down to the complexification necessary in its construction. This suggests a generalization to real variables, which, we will argue, can be accomplished. This solves most of the standard problems associated with the state, but in addition opens up a large Hilbert class of states vastly generalizing the de-Sitter/Chern-Simons solution."
Perimeter Streamers website sometimes puts these seminar talks online. I hope they do this one. Andy also has a couple of papers on arxiv:
http://arxiv.org/abs/gr-qc/0504010
A Generalization of the Kodama State for Arbitrary Values of the Immirzi Parameter
Andrew Randono
16 pages
"The Kodama State for Lorentzian gravity presupposes a particular value for the Immirzi-parameter, namely \beta=-i. However, the derivation of black hole entropy in Loop Quantum Gravity suggests that the Immirzi parameter is a fixed value whose magnitude is on the order of unity but larger than one. Since the Kodama state has de-Sitter spacetime as its classical limit, to get the proper radiation temperature, the Kodama state should be extended to incorporate a more physical value for \beta. Thus, we present an extension of the Kodama state for arbitrary values of the Immirzi parameter, \beta, that reduces to the ordinary Chern-Simons state for the particular value \beta=-i. The state for real values of \beta is free of several of the outstanding problems that cast doubts on the original Kodama state as a ground state for quantum general relativity. We show that for real values of \beta, the state is invariant under large gauge transformations, it is CPT invariant (but not CP invariant), and it is expected to be delta-function normalizable with respect to the kinematical inner product. To aid in the construction, we first present a general method for solving the Hamiltonian constraint for imaginary values of \beta that allows one to use the simpler self-dual and anti-self-dual forms of the constraint as an intermediate step."
http://arxiv.org/abs/hep-th/0510001
A Note on Parity Violation and the Immirzi Parameter
Andrew Randono
7 pages
There has been considerable recent interest in the Immirzi parameter as a measure of parity violating effects in the classical theory of gravitation with fermion coupling. Most recently it was shown that the Immirzi parameter together with the non-minimal coupling constant of Dirac spinors provides the measure for parity violating spin-spin interaction terms in the effective field theory. For complex values of the Immirzi parameter, the resulting effective field theory yields complex values for the torsion, and a non-unitary effective field theory that blows up for the special cases \gamma=\pm i where the gravitational kinetic term is the Ashtekar action. We show that by restricting ourselves to real values for the torsion, there is a natural set of choice for the non-minimal coupling constant that yields real and unitary effective field theory that does not blow up for the special cases \gamma=\pm i . We then show that these particular values for the non-minimal coupling coefficients most naturally follow from a non-minimal pseudo-kinetic term in the fermion Lagrangian."
Some pictures of Andy and friends hiking in the Colorado Rockies
http://www.ma.utexas.edu/users/stirling/0508colorado/0508colorado.html
The best one of Andy is scroll halfway down to where the guy snapping the pictures has captioned one:
"I think that Andy looks like a dork with his walking stick."
In fact he does not look dorky. He is dressed and equipped practically for high mountain hiking
and he looks cheerful, which some of the others don't.
It is amusing to think of Randono occupying the same physics department as Jacques (death-to-Loop-Gravity!) Distler. It shows a certain fortitude and independent spirit IMO.
=======================
http://arxiv.org/abs/gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at "Einstein's Legacy in the New Millenium,'' Puri, India, December 2005
"General relativity successfully describes space-times at scales that we can observe and probe today, but it cannot be complete as a consequence of singularity theorems. For a long time there have been indications that quantum gravity will provide a more complete, non-singular extension which, however, was difficult to verify in the absence of a quantum theory of gravity. By now there are several candidates which show essential hints as to what a quantum theory of gravity may look like. In particular, loop quantum gravity is a non-perturbative formulation which is background independent, two properties which are essential close to a classical singularity with strong fields and a degenerate metric. In cosmological and black hole settings one can indeed see explicitly how classical singularities are removed by quantum geometry: there is a well-defined evolution all the way down to, and across, the smallest scales. As for black holes, their horizon dynamics can be studied showing characteristic modifications to the classical behavior. Conceptual and physical issues can also be addressed in this context, providing lessons for quantum gravity in general. Here, we conclude with some comments on the uniqueness issue often linked to quantum gravity in some form or another."
http://arxiv.org/abs/gr-qc/0607136
Quantum Gowdy T3 model: A uniqueness result
Alejandro Corichi, Jeronimo Cortez, Guillermo A. Mena Marugan, Jose M. Velhinho
27 pages
"Modulo a homogeneous degree of freedom and a global constraint, the linearly polarised Gowdy T3 cosmologies are equivalent to a free scalar field propagating in a fixed nonstationary background. Recently, a new field parameterisation was proposed for the metric of the Gowdy spacetimes such that the associated scalar field evolves in a flat background in 1+1 dimensions with the spatial topology of S1, although subject to a time dependent potential. Introducing a suitable Fock quantisation for this scalar field, a quantum theory was constructed for the Gowdy model in which the dynamics is implemented as a unitary transformation. A question that was left open is whether one might adopt a different, nonequivalent Fock representation by selecting a distinct complex structure. The present work proves that the chosen Fock quantisation is in fact unique (up to unitary equivalence) if one demands unitary implementation of the dynamics and invariance under the group of constant S1translations. These translations are precisely those generated by the global constraint that remains on the Gowdy model. It is also shown that the proof of uniqueness in the choice of complex structure can be applied to more general field dynamics than that corresponding to the Gowdy cosmologies."
http://arxiv.org/abs/astro-ph/0608034
The Accelerating Universe and a Limiting Curvature Proposal
Damien A. Easson
21 pages, 7 figures
"We consider the hypothesis of a limiting minimal curvature in gravity as a way to construct a class of theories exhibiting late-time cosmic acceleration. Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set of scalar tensor theories in which the scalar is non-minimally coupled to the matter Lagrangian. The model is compared to the Lambda Cold Dark Matter concordance model and to the observational data using the gold SNeIa sample of Riess et. al. (2004). We present a toy model designed to demonstrate the possibility that such a new, possibly fundamental, principle may be responsible for the recent period of cosmological acceleration."
Damien Easson is a young fellow in UK who has co-authored with Sean Carroll, Mark Trodden, Michael Turner, Robert Brandenburger. this is just a toy model illustrating a preliminary idea. It is an odd idea---though not too different from just admitting a bare CC in the Einstein equation. He proposes a permanent minimal warp as substitute for Dark Energy. IIRC there is a type of relativistic MOND which dispenses with Dark Energy. In any case, since DE is a puzzle, we might keep an eye out for variations of gravity which explain acceleration some other way. even if the idea seems peculiar and preliminary.
==================
http://arxiv.org/abs/gr-qc/0608018
The History and Present Status of Quantum Field Theory in Curved Spacetime
Robert M. Wald
15 pages, no figures; contribution to 7th International Conference on the History of General Relativity
"Quantum field theory in curved spacetime is a theory wherein matter is treated fully in accord with the principles of quantum field theory, but gravity is treated classically in accord with general relativity. It is not expected to be an exact theory of nature, but it should provide a good approximate description when the quantum effects of gravity itself do not play a dominant role. A major impetus to the theory was provided by Hawking's calculation of particle creation by black holes, showing that black holes radiate as perfect black bodies. During the past 30 years, considerable progress has been made in giving a mathematically rigorous formulation of quantum field theory in curved spacetime. Major issues of principle with regard to the formulation of the theory arise from the lack of Poincare symmetry and the absence of a preferred vacuum state or preferred notion of 'particles'. By the mid-1980's, it was understood how all of these difficulties could be overcome for free (i.e., non-self-interacting) quantum fields by formulating the theory via the algebraic approach and focusing attention on the local field observables rather than a notion of 'particles'. However, these ideas, by themselves, were not adequate for the formulation of interacting quantum field theory, even at a perturbative level, since standard renormalization prescriptions in Minkowski spacetime rely heavily on Poincare invariance and the existence of a Poincare invariant vacuum state. However, during the past decade, great progress has been made, mainly due to the importation into the theory of the methods of 'microlocal analysis'. This article will describe the historical development of the subject and describe some of the recent progress."
survey by a senior guy
================
http://arxiv.org/abs/quant-ph/0608035
Quantum measurements without sums
Bob Coecke, Dusko Pavlovic
36 pages and 46 pictures; earlier version circulated since November 2005 with as title 'Quantum Measurements as Coalgebras''. Invited paper to appear in: The Mathematics of Quantum Computation and Technology; Chen, Kauffman and Lomonaco (eds.); Taylor and Francis
"Sums play a prominent role in the formalisms of quantum mechanics, be it for mixing and superposing states, or for composing state spaces. Surprisingly, a conceptual analysis of quantum measurement seems to suggest that quantum mechanics can be done without direct sums, expressed entirely in terms of the tensor product. The corresponding axioms define classical spaces as objects that allow copying and deleting data. Indeed, the information exchange between the quantum and the classical worlds is essentially determined by their distinct capabilities to copy and delete data. The sums turn out to be an implicit implementation of this capabilities. Realizing it through explicit axioms not only dispenses with the unnecessary structural baggage, but also allows a simple and intuitive graphical calculus. In category-theoretic terms, classical data types are dagger-compact Frobenius algebras, and quantum spectra underlying quantum measurements are Eilenberg-Moore coalgebras induced by these Frobenius algebras."
---sample exerpt from page 2 of the article---
A recent research thread, initiated by Abramsky and the first author [2], aims at recasting the quantum mechanical formalism in categorical terms. The upshot of categorical semantics is that it displays concepts in a compositional and typed framework. In the case of quantum mechanics, it uncovers the quantum information-flows [6] which are hidden in the usual formalism. Moreover, while the investigations of quantum structures have so far been predominantly academic, categorical semantics open an alley towards a practical, low-overhead tool for the design and analysis of quantum informatic protocols, versatile enough to capture both quantitative and qualitative aspects of quantum information [2, 7, 10, 13, 31]. In fact, some otherwise complicated quantum informatic protocols become trivial exercises in this framework [8]. On the other hand, compared with the order-theoretic framework for quantum mechanics in terms of Birkhoff-von Neumann’s quantum logic [29], this categorical setting comes with logical derivations, topologically embodied into something as simple as “yanking a rope”. 2 Moreover, in terms of deductive machanism, it turns out to be some kind of “super-logic” as compared to the Birkhoff-von Neumann “non-logic”.
---endquote---
Baez was talking about stretching out a piece of wet spaghetti. curious propositions in quantum theory, seeming paradoxes, become trivial exercises as Coecke says. Baez was trying to get that idea across---basically one of the reasons why one might see categorical semantics infiltrate into physics.
I am guessing that this paper of Coecke et al will be mentioned in the #237 issue of This Weeks Finds.
http://arxiv.org/abs/gr-qc/0608074
A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology
J. W. Moffat
Comments: 15 pages. Talk given at the International Workshop "From Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton, Virginia, USA
"A relativistic modified gravity (MOG) theory leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters of galaxies data and cosmology."
http://arxiv.org/abs/astro-ph/0608344
The rapid formation a large rotating disk galaxy three billion years after the Big Bang
R. Genzel, L. Tacconi, F. Eisenhauer, N. M. Forster Schreiber, A. Cimatti, E. Daddi, N. Bouche, R. Davies, M. D. Lehnert, D. Lutz, N. Nesvadba, A. Verma, R. Abuter, K. Shapiro, A. Sternberg, A. Renzini, X. Kong, N. Arimoto, M. Mignoli
Nature, accepted (Released Aug 17th)
"Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos led to the build up of galaxy mass. A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its age. The superior angular resolution of these data reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole."
http://arxiv.org/abs/astro-ph/0608345
Quark Matter in Neutron Stars: An aperçu
Prashanth Jaikumar, Sanjay Reddy, Andrew W. Steiner
15 pages, 1 figure. Invited review for Modern Physics Letters A
"The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this brief review article, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including color superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations."
http://arxiv.org/abs/math.CT/0608420
Lectures on n-Categories and Cohomology
John C. Baez, Michael Shulman
68 pages, 2 ps and 2 eps figures
"The goal of these talks was to explain how cohomology and other tools of algebraic topology are seen through the lens of n-category theory. Special topics include nonabelian cohomology, Postnikov towers, the theory of "n-stuff", and n-categories for n = -1 and -2. The talks were very informal, and so are these notes. A lengthy appendix clarifies certain puzzles and ventures into deeper waters such as higher topos theory. For readers who want more details, we include an annotated bibliography."
http://arxiv.org/abs/hep-th/0608120
Holography of Gravitational Action Functionals
A. Mukhopadhyay, T. Padmanabhan
17 pages
"Einstein-Hilbert (EH) action can be separated into a bulk and a surface term, with a specific ("holographic") relationship between the two, so that either can be used to extract information about the other. The surface term can also be interpreted as the entropy of the horizon in a wide class of spacetimes. Since EH action is likely to just the first term in the derivative expansion of an effective theory, it is interesting to ask whether these features continue to hold for more general gravitational actions. We provide a comprehensive analysis of lagrangians of the form L=Q_a^{bcd}R^a_{bcd}, in which Q_a^{bcd} is a tensor with the symmetries of the curvature tensor, made from metric and curvature tensor and satisfies the condition \nabla_cQ^{abcd}=0, and show that they share these features. The Lanczos-Lovelock lagrangians are a subset of these in which Q^{abcd} is a homogeneous function of the curvature tensor. They are all holographic, in a specific sense of the term, and -- in all these cases -- the surface term can be interpreted as the horizon entropy. The thermodynamics route to gravity, in which the field equations are interpreted as TdS=dE+pdV, seems to have greater degree of validity than the field equations of Einstein gravity itself. The results suggest that the holographic feature of EH action could also serve as a new symmetry principle in constraining the semiclassical corrections to Einstein gravity. The implications are discussed."
http://arxiv.org/abs/gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
4 pages, 1 figure
"An exactly solvable bounce model in loop quantum cosmology is identified which serves as a perturbative basis for realistic bounce scenarios. Its bouncing solutions are derived analytically, demonstrating why recent numerical simulations robustly led to smooth bounces under the assumption of semiclassicality. Several effects, easily included in a perturbative analysis, can however change this smoothness. The effective theory is not only applicable to such situations where numerical techniques become highly involved but also allows one to discuss conceptual issues. For instance, consequences of the notoriously difficult physical inner product can be implemented at the effective level. This indicates that even physical predictions from full quantum gravity can be obtained from perturbative effective equations."
http://arxiv.org/abs/astro-ph/0608407
A direct empirical proof of the existence of dark matter
Douglas Clowe (1), Marusa Bradac (2), Anthony H. Gonzalez (3), Maxim Markevitch (4), Scott W. Randall (4), Christine Jones (4), Dennis Zaritsky (1) ((1) Steward Observatory, Tucson, (2) KIPAC, Stanford, (3) Department of Astronomy, Gainesville, (4) CfA, Cambridge)
Accepted for publication in ApJL
"We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."
http://arxiv.org/abs/astro-ph/0608408
Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56
Marusa Bradac (1,2), Douglas Clowe (3), Anthony H. Gonzalez (4), Phil Marshall (1), William Forman (5), Christine Jones (5), Maxim Markevitch (5), Scott Randall (5), Tim Schrabback (2), Dennis Zaritsky (3) ((1) KIPAC, Stanford, (2) AIfA, Bonn, (3) Steward Observatory, Tucson, (4) Department of Astronomy, Gainesville, (5) CfA, Cambridge)
Accepted for publication in ApJ; Version with full-resolution figures available at this URL:
http://www.slac.stanford.edu/~marusa/Work/bradac_strong_weak_III.pdf
"The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004,2006a)."
http://arxiv.org/abs/quant-ph/0605070
High Energy Quantum Teleportation Using Neutral Koans
Yu Shi
6 pages, 4 figures. Appearing in Physics Letters B
[sic]
for traveling Zen masters :smile:
http://arxiv.org/abs/astro-ph/0608417
Planetesimals To Brown Dwarfs: What is a Planet?
Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)
23 pages
Annual Reviews of Earth and Planetary Science, 2006, v. 34, pp. 193-216
"The past 15 years have brought about a revolution in our understanding of our Solar System and other planetary systems. During this time, discoveries include the first Kuiper Belt Objects, the first brown dwarfs, and the first extra-solar planets. Although discoveries continue apace, they have called into question our previous perspectives on planets, both here and elsewhere. The result has been a debate about the meaning of the word ''planet'' itself. It became clear that scientists do not have a widely accepted or clear definition of what a planet is, and both scientists and the public are confused (and sometimes annoyed) by its use in various contexts. Because ''planet'' is a very widely used term, it seems worth the attempt to resolve this problem. In this essay, we try to cover all the issues that have come to the fore, and bring clarity (if not resolution) to the debate."
Mike Brown has good sense. I've heard him talk. I'd take his advice on terminology and science+culture issues.
Besides, he found Sedna.
http://arxiv.org/abs/physics/0608156
Threat Mitigation: The Asteroid Tugboat
Russell Schweickart (B612 Foundation), Clark Chapman, Dan Durda, (Southwest Research Institute), Piet Hut (Institute for Advanced Studies)
Submitted to NASA Workshop on Near-Earth Objects, Vail, Colorado, June 2006
"The Asteroid Tugboat (AT) is a fully controlled asteroid deflection concept using a robotic spacecraft powered by a high efficiency, electric propulsion system (ion or plasma) which docks with and attaches to the asteroid, conducts preliminary operations, and then thrusts continuously parallel to the asteroid velocity vector until the desired velocity change is achieved. Based on early warning, provided by ground tracking and orbit prediction, it would be deployed a decade or more prior to a potential impact. On completion of the initial rendezvous with the near-Earth object (NEO) the AT would first reduce the uncertainty in the orbit of the asteroid via Earth tracking of its radio transponder while it is station keeping with the asteroid. If on analysis of tracking data a deflection is required the AT would execute a reconnaissance phase collecting and processing information about the physical characteristics of the asteroid to support subsequent operations. The AT would then dock at the appropriate pole (i.e. on the spin axis), attach to the asteroid surface, and initiate a NEO reorientation maneuver. Following completion of the NEO reorientation the AT would initiate the deflection phase by thrusting continuously parallel to the asteroid velocity vector until the resultant target orbit is achieved. The orbit of the asteroid is continuously monitored throughout the deflection process and the end state is known in real time. If one assumes a nuclear-electric propulsion (NEP) system similar to that formerly under development in the recently canceled Prometheus Program, the AT would be capable of deflecting threatening NEOs up to 800 meters in diameter or more."
http://arxiv.org/abs/hep-th/0608175
Spacetime topology change and black hole information
Stephen D.H. Hsu
5 pages, 5 figures
"Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in planckian regimes. It does not require non-locality or any modification of low-energy physics."
Steve Hsu has 80 papers on arxiv, several co-authored with Tony Zee, or Brian Murray, or Roman Buniy
a smart and original thinker.
http://arxiv.org/find/grp_physics/1/au:+Hsu_S/0/1/0/all/0/1
"We have proposed a solution of the black hole information paradox which depends entirely on details of planckian physics — no modifications of low-energy physics, such as non-locality, are required. The main assumptions are that the endpoint of interior black hole evolution is topology change and that the quantum gravitational dynamics of pinching off are strongly coupled. Thus, small perturbations to the initial state of a black hole lead to different internal state vectors describing the resulting baby universe, even if the semiclassical properties are only slightly changed. Under this assumption, any violation of cluster decomposition will be practically unobservable.
If our scenario is correct, there is no violation of causality or locality at the semiclassical black hole horizon, and no stable planck mass remnant of black hole evaporation. Instead, much as Hawking first proposed, information is lost: to a baby universe, from which it may or may not someday emerge via tunneling. If the information emerges again, evolution within the parent universe is unitary. If information remains in the baby universe, the parent universe appears to evolve from a pure to mixed state, but the evolution of parent and baby together is unitary. There are no dire consequences, such as energy non-conservation."
This resolution of the BH info puzzle is not unfamiliar to LQG researchers, but it is hopeful and refreshing to see it surface like this in a different context---Hsu has been talking to people like Giddings and Strominger, rather than Martin Bojowald, for example.
http://arxiv.org/abs/astro-ph/0608535
Dark matter burners
I. V. Moskalenko (Stanford), L. Wai (SLAC)
4 pages, 3 figures
"We show that a star orbiting close enough to an adiabatically grown supermassive black hole can capture a large number of weakly interacting massive particles (WIMPs) during its lifetime. WIMP annihilation energy release in low- to medium-mass stars is comparable with or even exceeds the luminosity of such stars due to thermonuclear burning. The excessive energy release in the stellar core may result in an evolution scenario different from what is expected for a regular star. The model thus predicts the existence of unusual stars within the central parsec of galactic nuclei. If found, such stars would provide evidence for the existence of particle dark matter. The excess luminosity of such stars attributed to WIMP "burning" can be used to infer the local WIMP matter density. A white dwarf with a highly eccentric orbit around the central black hole may exhibit variations in brightness correlated with the orbital phase. On the other hand, white dwarfs shown to lack such orbital brightness variations can be used to provide constraints on WIMP matter density, WIMP-nucleus scattering and pair annihilation cross sections."
http://arxiv.org/abs/gr-qc/0608116
Causality and matter propagation in 3d spin foam quantum gravity
Daniele Oriti, Tamer Tlas
25 pages, 2 figures
"In this paper we tackle the issue of causality in quantum gravity, in the context of 3d spin foam models. We identify the correct procedure for implementing the causality/orientation dependence restriction that reduces the path integral for BF theory to that of quantum gravity in first order form. We construct explicitly the resulting causal spin foam model. We then add matter degrees of freedom to it and construct a causal spin foam model for 3d quantum gravity coupled to matter fields. Finally, we show that the corresponding spin foam amplitudes admit a natural approximation as the Feynman amplitudes of a non-commutative quantum field theory, with the appropriate Feynman propagators weighting the lines of propagation, and that this effective field theory reduces to usual QFT in flat space in the no-gravity limit."
http://arxiv.org/abs/gr-qc/0608117
The Shallow Waters of the Big-Bang
Pablo Laguna
5 pages, 3 figures
"Loop quantum cosmology homogeneous models with a massless scalar field show that the big-bang singularity can be replaced by a big quantum bounce. To gain further insight on the nature of this bounce, we study the semi-discrete loop quantum gravity Hamiltonian constraint equation from the point of view of numerical analysis. We show that the bounce is closely related to the method for the temporal update of the system and demonstrate that, in particular, explicit time-updates in general yield bounces. These bounces can be understood as spurious reflections in finite difference discretizations of wave equations in nonuniform grids or, equivalently, as spurious reflections found when solving wave equations with varying coefficients, such as the shallow water equations. We present an implicit time-update devoid of bounces and show back-in-time, deterministic evolutions that reach and partially jump over the big-bang singularity."
http://arxiv.org/abs/astro-ph/0608576
Status and recent results of the Pierre Auger Observatory
Serguei Vorobiov (for the Pierre Auger Collaboration)
Talk given at the International Cosmic Ray Workshop "Tien-Shan 2006", held from 25 to 27 August 2006 near Almaty, Kazakhstan; 8 pages, 4 figures
"The Pierre Auger Observatory aims to determine the nature and origin of the ultra-high energy cosmic rays (UHECR). The Auger hybrid detector combines fluorescence observations of extended air showers, initiated in the atmosphere by these most energetic particles, with measures of the shower front at the ground level by its large array of Cherenkov water tanks. This allows to improve considerably the precision on reconstructed primary cosmic ray parameters, and to make important cross-calibrations between two techniques at these energies, unreachable with accelerator experiments. The Southern Auger site in Argentina is close to completion. The first measure of the primary cosmic rays energy spectrum, the anisotropy search results, and the limit on the photon fraction in the UHECR are discussed."
http://arxiv.org/abs/gr-qc/0608121
Constraining Torsion with Gravity Probe B
Yi Mao (MIT), Max Tegmark (MIT), Alan Guth (MIT), Serkan Cabi (MIT)
26 pages, 6 figures, 5 tables
"It is well-entrenched folklore that torsion gravity theories predict observationally negligible torsion in the solar system, since torsion (if it exists) couples only to the intrinsic spin of elementary particles, not to rotational angular momentum. We argue that this assumption has a logical loophole which can and should be tested experimentally. We give an explicit counterexample where a rotating body generates a torsion field in Weitzenbock spacetime with a Hayashi-Shirafuji Lagrangian. More generally, in the spirit of action=reaction, if a rotating mass like a planet can generate torsion, then a gyroscope should also feel torsion.
Using symmetry arguments, we show that to lowest order, the torsion field around a uniformly rotating spherical mass is determined by seven dimensionless parameters. These parameters effectively generalize the PPN formalism and provide a concrete framework for further testing GR. We construct a parametrized Lagrangian that includes both standard torsion-free GR and Hayashi-Shirafuji maximal torsion gravity as special cases. We demonstrate that classic solar system tests rule out the latter and constrain two observable parameters. We show that Gravity Probe B (GPB) is an ideal experiment for further constraining torsion theories, and work out the most general torsion-induced precession of its gyroscope in terms of our torsion parameters."
http://arxiv.org/abs/gr-qc/0608124
Loop Quantum Gravity Modification of the Compton Effect
Kourosh Nozari, S. Davood Sadatian
11 pages, 2 figures
"Modified dispersion relations(MDRs) as a manifestation of Lorentz invariance violation, have been appeared in alternative approaches to quantum gravity problem. Loop quantum gravity is one of these approaches which evidently requires modification of dispersion relations. These MDRs will affect the usual formulation of the Compton effect. The purpose of this letter is to incorporate the effects of loop quantum gravity MDRs on the formulation of Compton scattering. Using limitations imposed on MDRs parameters from Ultra High Energy Cosmic Rays(UHECR), we estimate the quantum gravity-induced wavelength shift of scattered photons in a typical Compton process. Possible experimental detection of this wavelength shift will provide strong support for underlying quantum gravity proposal."
I need to take a critical look at this, or get someone else to comment, before I decide what to think. It would be very nice if they have discovered an additional way to test LQG. I am not familiar with previous work by the lead author K. Nozari.
http://arxiv.org/abs/hep-th/0608210
Loop Quantum Gravity: An Inside View
Thomas Thiemann
58 pages
"This is a (relatively) non -- technical summary of the status of the quantum dynamics in Loop Quantum Gravity (LQG). We explain in detail the historical evolution of the subject and why the results obtained so far are non -- trivial. The present text can be viewed in part as a response to an article by Nicolai, Peeters and Zamaklar [hep-th/0501114]. We also explain why certain no go conclusions drawn from a mathematically correct calculation in a recent paper by Helling et al [hep-th/0409182] are physically incorrect."
Good old Thomas Thiemann! Glad he has responded to the Nicolai et al and Helling et al papers!
http://arxiv.org/abs/astro-ph/0608602
Can Cosmic Structure form without Dark Matter?
Scott Dodelson, Michele Liguori
5 pages, 3 figures
"One of the prime pieces of evidence for dark matter is the observation of large overdense regions in the universe. Since we know from the cosmic microwave background that the regions that contained the most baryons when the universe was ~400,000 years old were overdense by only one part in ten thousand, perturbations had to have grown since then by a factor greater than (1+z_*)\simeq 1180$ where $z_* is the epoch of recombination. This enhanced growth does not happen in general relativity, so dark matter is needed in the standard theory. We show here that enhanced growth can occur in alternatives to general relativity, in particular in Bekenstein's relativistic version of MOdified Newtonian Dynamics (MOND). The vector field introduced in that theory for a completely different reason plays a key role in generating the instability that produces large cosmic structures today."
Looks like Scott Dodelson isn't sure that TeVes is dead after all. And we shouldn't forget this earlier paper:
http://arxiv.org/abs/astro-ph/0606216
Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry
Garry W. Angus, Benoit Famaey, HongSheng Zhao
14 pages, 9 figures, accepted for publication in MNRAS
"A proper test of Modified Newtonian Dynamics (MOND) in systems of non-trivial geometries depends on modelling subtle differences in several versions of its postulated theories. This is especially important for lensing and dynamics of barely virialised galaxy clusters with typical gravity of scale \sim a_0 \sim 1\AA{\rm s}^{-2}. The original MOND formula, the classical single field modification of the Poisson equation, and the multi-field general relativistic theory of Bekenstein (TeVeS) all lead to different predictions as we stray from spherical symmetry. In this paper, we study a class of analytical MONDian models for a system with a semi-Hernquist baryonic profile. After presenting the analytical distribution function of the baryons in spherical limits, we develop orbits and gravitational lensing of the models in non-spherical geometries. In particular, we can generate a multi-centred baryonic system with a weak lensing signal resembling that of the merging galaxy cluster 1E 0657-56 with a bullet-like light distribution. We finally present analytical scale-free highly non-spherical models to show the subtle differences between the single field classical MOND theory and the multi-field TeVeS theory."
check out Figure 7 on page 7. they illustrate that TeVeS can make a DOUBLECENTER lensing map like the one observed by Maxim Markevitch et al----they get TeVes to produce a two-lens lensing map like goggles or spectacles rather like what the CHANDRA people showed us.
http://arxiv.org/abs/gr-qc/0608131
Group Integral Techniques for the Spinfoam Graviton Propagator
Etera R. Livine, Simone Speziale
16 pages
"We consider the proposal of gr-qc/0508124 for the extraction of the graviton propagator from the spinfoam formalism. We propose a new ansatz for the boundary state, using which we can write the propagator as an integral over SU(2). The perturbative expansion in the Planck length can be recast into the saddle point expansion of this integral. We compute the leading order and recover the behavior expected from low--energy physics. In particular, we prove that the degenerate spinfoam configurations are suppressed."
http://arxiv.org/abs/gr-qc/0608135
Towards a Covariant Loop Quantum Gravity
Etera R. Livine
13 pages, review, draft chapter for the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press, comments welcome
"We review the canonical analysis of the Palatini action without going to the time gauge as in the standard derivation of Loop Quantum Gravity. This allows to keep track of the Lorentz gauge symmetry and leads to a theory of Covariant Loop Quantum Gravity. This new formulation does not suffer from the Immirzi ambiguity, it has a continuous area spectrum and uses spin networks for the Lorentz group. Finally, its dynamics can easily be related to Barrett-Crane like spin foam models."
http://arxiv.org/abs/quant-ph/0608243
Relational physics with real rods and clocks and the measurement problem of quantum mechanics
Rodolfo Gambini, Jorge Pullin
19 pages
"The use of real clocks and measuring rods in quantum mechanics implies a natural loss of unitarity in the description of the theory. We briefly review this point and then discuss the implications it has for the measurement problem in quantum mechanics. The intrinsic loss of coherence allows to circumvent some of the usual objections to the measurement process as due to environmental decoherence."
http://arxiv.org/abs/hep-th/0608221
A Lorentzian version of the non-commutative geometry of the standard model of particle physics
John W. Barrett
14 pages
"A formulation of the non-commutative geometry for the standard model of particle physics with a Lorentzian signature metric is presented. The elimination of the fermion doubling in the Lorentzian case is achieved by a modification of Connes' internal space geometry so that it has signature 6 (mod 8) rather than 0. The fermionic part of the Connes-Chamseddine spectral action can be formulated, and it is shown that it allows an extension with right-handed neutrinos and the correct mass terms for the see-saw mechanism of neutrino mass generation."
http://arxiv.org/abs/hep-th/0608226
Noncommutative Geometry and the standard model with neutrino mixing
Alain Connes
17 pages
"We show that allowing the metric dimension of a space to be independent of its KO-dimension and turning the finite noncommutative geometry F-- whose product with classical 4-dimensional space-time gives the standard model coupled with gravity--into a space of KO-dimension 6 by changing the grading on the antiparticle sector into its opposite, allows to solve three problems of the previous noncommutative geometry interpretation of the standard model of particle physics:
The finite geometry F is no longer put in "by hand" but a conceptual understanding of its structure and a classification of its metrics is given.
The fermion doubling problem in the fermionic part of the action is resolved.
The spectral action of our joint work with Chamseddine now automatically generates the full standard model coupled with gravity with neutrino mixing and see-saw mechanism for neutrino masses. The predictions of the Weinberg angle and the Higgs scattering parameter at unification scale are the same as in our joint work but we also find a mass relation (to be imposed at unification scale)."
=======================
those are the two main things: the Barrett and the Connes papers
but since I can still edit I will tack on some other
http://arxiv.org/abs/astro-ph/0609060
First results from the Pierre Auger Observatory
R. C. Shellard
10 pages, 16 figures, Brazilian National Meeting on Particle and Fields 2005
"We review in these notes the status of the construction of the Pierre Auger Observatory and present the first Physics results, based on the data collected during the first year and a half of operation. These results are preliminary, once the work to understand the systematics of the detectors are still underway. We discuss the cosmic ray spectrum above 3 EeV, based on the measurement done using the Surface Detector and the Fluorescence Detector, both, components of the observatory. We discuss, as well, the search for anisotropy near the Galactic Center and the limit on the photon fraction at the highest energies."
here is from the summary:
"... The Pierre Auger Observatory is still under construction but has already the largest integrated exposure to high energy cosmic rays. The combination of fluorescence and the surface detector measurements allow for the reconstruction of the shower geometry and its energy with much greater quality than what could be achieved with either detector standing alone. Each of the detectors have different systematics, allowing for valuable information for cross-checking the results from each of them. The observatory should be finished by mid 2007, accumulating by then a much larger exposure than what was used for the preliminary results presented here. That will allow for the search of anisotropies in the southern sky, as well as the test of the predicted GZK suppression."
http://arxiv.org/abs/gr-qc/0609012
Why Does Gravity Ignore the Vacuum Energy?
T. Padmanabhan
Invited Contribution to the IJMPD Special Issue on Dark Matter and Dark Energy edited by D.Ahluwalia and D. Grumiller. Appendix clarifies several conceptual and pedgogical aspects of surface term in Hilbert action
"The equations of motion for matter fields are invariant under the shift of the matter lagrangian by a constant. Such a shift changes the energy momentum tensor of matter by T^a_b --> T^a_b +\rho \delta^a_b. In the conventional approach, gravity breaks this symmetry and the gravitational field equations are not invariant under such a shift of the energy momentum tensor. I argue that until this symmetry is restored, one cannot obtain a satisfactory solution to the cosmological constant problem. I describe an alternative perspective to gravity in which the gravitational field equations are [G_{ab} -\kappa T_{ab}] n^an^b =0 for all null vectors n^a. This is obviously invariant under the change T^a_b --> T^a_b +\rho \delta^a_b and restores the symmetry under shifting the matter lagrangian by a constant. These equations are equivalent to G_{ab} = \kappa T_{ab} + Cg_{ab} where C is now an integration constant so that the role of the cosmological constant is very different in this approach. The cosmological constant now arises as an integration constant, somewhat like the mass M in the Schwarzschild metric, the value of which can be chosen depending on the physical context. These equations can be obtained from a variational principle which uses the null surfaces of spacetime as local Rindler horizons and can be given a thermodynamic interpretation. This approach turns out to be quite general and can encompass even the higher order corrections to Einstein's gravity and suggests a principle to determine the form of these corrections in a systematic manner."
In the past there has been considerable interest in the papers of T. Padmanabhan. I can't evaluate this one on its merits, but include it on account of the author. Likewise with the next, a Lisa Randall paper:
http://arxiv.org/abs/hep-ph/0607158
Gravitational Waves from Warped Spacetime
Lisa Randall, Geraldine Servant
18 pages, 15 figures
"We argue that the RSI model can provide a strong signature in gravitational waves. This signal is a relic stochastic background generated during the cosmological phase transition from an AdS-Schwarschild phase to the RS1 geometry that should occur at a temperature in the TeV range. We estimate the amplitude of the signal in terms of the parameters of the potential stabilizing the radion and show that over much of the parameter region in which the phase transition completes, a signal should be detectable at the planned space interferometer, LISA."
http://arxiv.org/abs/gr-qc/0609027
A Measurement of Newton's Gravitational Constant
St. Schlamminger, E. Holzschuh, W. Kündig, F. Nolting, R.E. Pixley, J. Schurr, U. Straumann
26 pages, 20 figures, Accepted for publication by Phys. Rev. D
"A precision measurement of the gravitational constant G has been made using a beam balance. Special attention has been given to determining the calibration, the effect of a possible nonlinearity of the balance and the zero-point variation of the balance. The equipment, the measurements and the analysis are described in detail. The value obtained for G is 6.674252(109)(54) 10^{-11} m^3 kg^{-1} s^{-2}. The relative statistical and systematic uncertainties of this result are 16.3 10^{-6} and 8.1 10^{-6}, respectively."
the following is briefly noted in part for the simple reason that I haven't seen a paper by Larry Krauss for some time,
previous one was early 2004
http://arxiv.org/abs/gr-qc/0609024
Observation of Incipient Black Holes and the Information Loss Problem
Tanmay Vachaspati, Dejan Stojkovic, Lawrence M. Krauss
13 pages; 8 figures
http://arxiv.org/abs/gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
IGPG-06/8-1
"Inhomogeneities are introduced in loop quantum cosmology using regular lattice states, with a kinematical arena similar to that in homogeneous models considered earlier. The framework is intended to encapsulate crucial features of background independent quantizations in a setting accessible to explicit calculations of perturbations on a cosmological background. It is used here only for qualitative insights but can be extended with further more detailed input. One can thus see how several parameters occuring in homogeneous models appear from an inhomogeneous point of view. Their physical roles in several cases then become much clearer, often making previously unnatural choices of values look more natural by providing alternative physical roles. This also illustrates general properties of symmetry reduction at the quantum level and the roles played by inhomogeneities. Moreover, the constructions suggest a picture for gravitons and other metric modes as collective excitations in a discrete theory, and lead to the possibility of quantum gravity corrections in large universes."
exerpt from Conclusions section:
"Models of loop quantum gravity are being investigated actively regarding their phenomenological properties, and perturbative inhomogeneities are currently being included. This brings us closer to reliable computations of potentially observable properties such as those of structure formation. It is then important to check all intrinsic details of such models and see how faithfully they incorporate features of the full theory. As we discussed, qualitative effects are realized in homogeneous as well as inhomogeneous lattice models in the same way. We introduced inhomogeneous lattice constructions in a way which allows for a relation to a homogeneous background. The relation to isotropic models is then clear, which provides a new step toward relating isotropic models to the full theory. Although the lattice construction is analogous to that of isotropic models, quantitative aspects can change which has a bearing on which ranges of parameters one considers as natural or unnatural. It also plays a role for which correction terms will be dominant in different regimes which is the most important aspect for phenomenological investigations."
http://arxiv.org/abs/gr-qc/0609032
Exploring the diffeomorphism invariant Hilbert space of a scalar field
Hanno Sahlmann
19 pages
ITP-UU-06/35, SPIN-06/31
"As a toy model for the implementation of the diffeomorphism constraint, the interpretation of the resulting states, and the treatment of ordering ambiguities in loop quantum gravity, we consider the Hilbert space of spatially diffeomorphism invariant states for a scalar field. We give a very explicit formula for the scalar product on this space, and discuss its structure.
Then we turn to the quantization of a certain class of diffeomorphism invariant quantities on that space, and discuss in detail the ordering issues involved. On a technical level these issues bear some similarity to those encountered in full loop quantum gravity."
http://arxiv.org/abs/gr-qc/0609029
Loop Quantum Cosmology in Bianchi I Models: Analytical Investigation
Dah-Wei Chiou
51 pages
"The comprehensive formulation for loop quantum cosmology in the spatially flat, isotropic model was recently constructed. In this paper, the methods are extended to the anisotropic Bianchi I model. Both the precursor and the improved strategies are applied and the expected results are established: i) the scalar field again serves as an internal clock and is treated as emergent time; ii) the total Hamiltonian constraint is derived by imposing the fundamental discreteness and gives the evolution as a difference equation; and iii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously. It is also shown that the states in the kinematical Hilbert space associated with the singularity and planar collapse are decoupled in the difference evolution equation, indicating that the big bounce may take place when any of the length scales undergoes the vanishing behavior and thereby supporting the long-held assertion: the physics below the Planck regime is unattainable. The investigation affirms the robustness of the framework used in the isotropic model by enlarging its domain of validity and provides foundations to conduct the detailed numerical analysis."
Dah-Wei is currently at the UC Berkeley physics department and also at Ashtekar's insitute at Penn State. This work is closely connected to recent Penn State papers studying the quantum bounce (replacing bang and hole singularities).
Dah-Wei has 6 papers on arxiv going back to 2003, all that time at Berkeley/LBL. So he could be a Berkeley PhD student now moving on to Penn State postdoc. something like that. this is his first LQG paper.
http://arxiv.org/find/grp_physics/1/au:+Chiou_Dah_Wei/0/1/0/all/0/1
http://arxiv.org/abs/gr-qc/0609037
Semi-classical States in Homogeneous Loop Quantum Cosmology
Huahai Tan, Yongge Ma
13 pages, submitted to CQG
"Semi-classical states in homogeneous loop quantum cosmology (LQC) are constructed by two different ways. In the first approach, we firstly construct an exponentiated annihilation operator. Then a kind of semi-classical (coherent) state is obtained by solving the eigen-equation of that operator. Moreover, we use these coherent states to analyze the semi-classical limit of the quantum dynamics. It turns out that the Hamiltonian constraint operator employed currently in homogeneous LQC has correct classical limit with respect to the coherent states. In the second approach, the other kind of semi-classical state is derived from the mathematical construction of coherent states for compact Lie groups due to Hall."
Yong-ge Ma is at Beijing Normal. Author of several LQG papers already.
========
string overview, for general audience (off topic here, but might come in handy):
http://arxiv.org/abs/physics/0609062
String Theory and Einstein's Dream
Ashoke Sen
11 figures
http://arxiv.org/abs/gr-qc/0609040
Fermions in three-dimensional spinfoam quantum gravity
Winston Fairbairn (CPT)
40 pages, 3 figures
"We study the coupling of massive fermions to the quantum mechanical dynamics of spacetime emerging from the spinfoam approach in three dimensions. We first recall the classical theory before constructing a spinfoam model of quantum gravity coupled to spinors. The technique used is based on a finite expansion in inverse fermion masses leading to the computation of the vacuum to vacuum transition amplitude of the theory. The path integral is derived as a sum over closed fermionic loops wrapping around the spinfoam. The effects of quantum torsion are realised as a modification of the intertwining operators assigned to the edges of the two-complex, in accordance with loop quantum gravity. The creation of non-trivial curvature is modelled by a modification of the pure gravity vertex amplitudes. The appendix contains a review of the geometrical and algebraic structures underlying the classical coupling of fermions to three dimensional gravity."
we know Fairbairn from a paper co-authored with Rovelli a couple of years ago. this is his first solo paper that I know of.
this next one I cannot evaluate but thought certain other PF members might like to know about
Khovanov is in the math department at Columbia
Lot of pictures reminiscent of Sundance preon model, possibly also some recent J.B. papers.
http://arxiv.org/abs/math.QA/0609335
Braid cobordisms, triangulated categories, and flag varieties
Mikhail Khovanov, Richard Thomas
89 pages, 21 figures
"We argue that various braid group actions on triangulated categories should be extended to projective actions of the category of braid cobordisms and illustrate how this works in examples. We also construct an action of both the affine braid group and the braid cobordism category on the derived category of coherent sheaves on the cotangent bundle to the full flag variety."
==============
the Alain Connes Teheran interview
http://www.ipm.ac.ir/IPM/news/connes-interview.pdf
1 - The language of "braid cobordisms" suggests a possible relationship to knot theory, from my perspective.
'Braid cobordisms, triangulated categories, and flag varieties'
Mikhail Khovanov, Richard Thomas
89 pages, 21 figures
http://arxiv.org/abs/math.QA/0609335
2 - In 3D braids appear to be helices. Some game theorists think that saddle points [found in helicoids] are equivalent to Nash Equilibria from Mathematical Game Theory.
http://ggierz.ucr.edu/Math121/Winter06/LectureNotes/09SaddlePointsNashEqui.pdf#search=%22saddle%20poin ts%20Nash%20Equilibrium%22
3 - Is it possible that two such diverse mathematical representations of objects might somehow be unifiable? [through Nash Equilibrium and Nash embedding theorems]
Are differential geometries essentially manifestations of energy interactions of energy games [attractor v Disipator / braid v unbraid]?
Dcase this is a bibliography thread. It is not for discussion.
If you want to discuss the Khovanov paper, please do not do it here (if people discussed things it would clutter the already loaded thread.
Anyone wanting to discuss Khovanov paper please continue discussion elsewhere.
For your convenience I have started a discussion thread for you here
http://www.physicsforums.com/showthread.php?p=1083810#post1083810
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The Fall 2006 issue of the APS newsletter Matters of Gravity is out.
http://arxiv.org/abs/gr-qc/0609045
A new editor, David Garfinkle, has taken over from Jorge Pullin.
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an interesting article bearing on the Smolin CNS conjecture appeared yesterday
the CNS conjecture involves a prediction is that no modification in the measured values of standard dimensionless parameters would cause a monotone increase in the observed abundance of black holes.
this prediction was made in 1993 or 1994 and is still standing. The conjecture could be discredited by reliably determining the mass of a neutron star to be above a certain value, and in various other ways, but so far this has not happened.
The CNS conjecture could also be discredited by evidence that the universe is spatially infinite. It predicts instead that the universe is spatially closed---as in the so-called "nearly flat" picture, with slight overall positive curvature---Omega > 1.
http://arxiv.org/abs/gr-qc/0609045
Was the universe open or closed before inflation?
Eduard Masso, Subhendra Mohanty, Gabriel Zsembinszki
5 pages, 3 figures
UAB-FT-609
"If the spatial curvature of the universe at the beginning of inflation is negative, there is an enhancement of the temperature anisotropy of the Cosmic Background Radiation at large angles. On the other hand if at the start of inflation the universe was closed with curvature there will be a suppression of temperature anisotropy at the scale of the present horizon. The observation of a low quadrupole anisotropy by WMAP suggests that the universe was closed with (Omega-1) of order unity at the time when the perturbation scales of the size of our present horizon were exiting the inflationary horizon."
Eduard Masso is at Barcelona---his 47 arxiv papers go back to 1993.
http://arxiv.org/find/grp_physics/1/au:+Masso_E/0/1/0/all/0/1
Subhendra Mohanty is at Ahmedabad PRL---his estimated 60-some arxiv papers go back to 1992.
http://arxiv.org/find/grp_physics/1/au:+Mohanty_S/0/1/0/all/0/1
they did a computer simulation of how CMB anisotropies arose with various angular sizes assuming various initial conditions
and found that a natural explanation for the shape of the power spectrum at large angles is to assume spatial finiteness.
If anyone would like to discuss it, the link and abstract are posted on a suitable discussion thread here:
http://www.physicsforums.com/showthread.php?p=1083650#post1083650
http://arxiv.org/abs/quant-ph/0609109
Could quantum mechanics be an approximation to another theory?
Lee Smolin
10 pages
"We consider the hypothesis that quantum mechanics is an approximation to another, cosmological theory, accurate only for the description of subsystems of the universe. Quantum theory is then to be derived from the cosmological theory by averaging over variables which are not internal to the subsystem, which may be considered non-local hidden variables. We find conditions for arriving at quantum mechanics through such a procedure. The key lesson is that the effect of the coupling to the external degrees of freedom introduces noise into the evolution of the system degrees of freedom, while preserving a notion of averaged conserved energy and time reversal invariance.
These conditions imply that the effective description of the subsystem is Nelson's stochastic formulation of quantum theory. We show that Nelson's formulation is not, by itself, a classical stochastic theory as the conserved averaged energy is not a linear function of the probability density. We also investigate an argument of Wallstrom posed against the equivalence of Nelson's stochastic mechanics and quantum mechanics and show that, at least for a simple case, it is in error."
http://arxiv.org/abs/astro-ph/0609417
Tests of general relativity from timing the double pulsar
M. Kramer, I.H. Stairs, R.N. Manchester, M.A. McLaughlin, A.G. Lyne, R.D. Ferdman, M. Burgay, D.R. Lorimer, A. Possenti, N. D'Amico, J.M. Sarkissian, G.B. Hobbs, J.E. Reynolds, P.C.C. Freire, F. Camilo
Appeared in Science Express, Sept. 14, 2006. Includes supporting material
"The double pulsar system, PSR J0737-3039A/B, is unique in that both neutron stars are detectable as radio pulsars. This, combined with significantly higher mean orbital velocities and accelerations when compared to other binary pulsars, suggested that the system would become the best available testbed for general relativity and alternative theories of gravity in the strong-field regime. Here we report on precision timing observations taken over the 2.5 years since its discovery and present four independent strong-field tests of general relativity. Use of the theory-independent mass ratio of the two stars makes these tests uniquely different from earlier studies. By measuring relativistic corrections to the Keplerian description of the orbital motion, we find that the ``post-Keplerian'' parameter s agrees with the value predicted by Einstein's theory of general relativity within an uncertainty of 0.05%, the most precise test yet obtained. We also show that the transverse velocity of the system's center of mass is extremely small. Combined with the system's location near the Sun, this result suggests that future tests of gravitational theories with the double pulsar will supersede the best current Solar-system tests. It also implies that the second-born pulsar may have formed differently to the usually assumed core-collapse of a helium star."
http://arxiv.org/abs/astro-ph/0609416
The Formation of the Double Pulsar PSR J0737-3039A/B
I. H. Stairs, S. E. Thorsett, R. J. Dewey, M. Kramer, C. A. McPhee
To appear in MNRAS Letters
"Recent timing observations of the double pulsar J0737-3039A/B have shown that its transverse velocity is extremely low, only 10 km/s, and nearly in the Plane of the Galaxy. With this new information, we rigorously re-examine the history and formation of this system, determining estimates of the pre-supernova companion mass, supernova kick and misalignment angle between the pre- and post-supernova orbital planes. We find that the progenitor to the recently formed `B' pulsar was probably less than 2 MSun, lending credence to suggestions that this object may not have formed in a normal supernova involving the collapse of an iron core. At the same time, the supernova kick was likely non-zero. A comparison to the history of the double-neutron-star binary B1534+12 suggests a range of possible parameters for the progenitors of these systems, which should be taken into account in future binary population syntheses and in predictions of the rate and spatial distribution of short gamma-ray burst events."
http://arxiv.org/abs/gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
IGPG--06/9--7
"Cosmological perturbation equations are derived systematically in a canonical scheme based on Ashtekar variables. A comparison with the covariant derivation and various subtleties in the calculation and choice of gauges are pointed out. Nevertheless, the treatment is more systematic when correction terms of canonical quantum gravity are to be included. This is done throughout the paper for one characteristic modification expected from loop quantum gravity."
a replacement by the final published version:
http://arxiv.org/abs/gr-qc/0504147
Uniqueness of diffeomorphism invariant states on holonomy-flux algebras
Jerzy Lewandowski, Andrzej Okolow, Hanno Sahlmann, Thomas Thiemann
38 pages, one figure. v2: Minor changes, final version, as published in CMP
AEI-2005-093, CGPG-04/5-3
Comm. Math. Phys., Vol. 267 No. 3 (2006), 703-733
"Loop quantum gravity is an approach to quantum gravity that starts from the Hamiltonian formulation in terms of a connection and its canonical conjugate. Quantization proceeds in the spirit of Dirac: First one defines an algebra of basic kinematical observables and represents it through operators on a suitable Hilbert space. In a second step, one implements the constraints. The main result of the paper concerns the representation theory of the kinematical algebra: We show that there is only one cyclic representation invariant under spatial diffeomorphisms.
While this result is particularly important for loop quantum gravity, we are rather general: The precise definition of the abstract *-algebra of the basic kinematical observables we give could be used for any theory in which the configuration variable is a connection with a compact structure group. The variables are constructed from the holonomy map and from the fluxes of the momentum conjugate to the connection. The uniqueness result is relevant for any such theory invariant under spatial diffeomorphisms or being a part of a diffeomorphism invariant theory."
thanks to arivero for prompting me to add this to the bibliography thread
http://arxiv.org/abs/hep-th/0609140
Noncommutative geometry, topology and the standard model vacuum
R. A. D. Martins
24 pages
"As a ramification of a motivational discussion for previous joint work, in which equations of motion for the finite spectral action of the Standard Model were derived, we provide a new analysis of the results of the calculations herein, switching from the perspective of Spectral triple to that of Fredholm module and thus from the analogy with Riemannian geometry to the pre-metrical structure of the Noncommutative geometry. Using a suggested Noncommutative version of Morse theory together with algebraic K-theory to analyse the vacuum solutions, the first two summands of the algebra for the finite triple of the Standard Model arise up to Morita equivalence. We also demonstrate a new vacuum solution whose features are compatible with the physical mass matrix."
Arivero notes that, as mentioned in the abstract, this developed out of joint work with John Barrett.
http://arxiv.org/abs/gr-qc/0609108
Prolegomena to any future Quantum Gravity
John Stachel
26 pages
"I shall discuss some 'conditions of possibility' of a quantum theory of gravity, stressing the need for solutions to some of fundamental problems confronting any attempt to apply some method of quantization to the field equations of general relativity."
Stachel is a prominent expert in the history and philosophy of physics---and himself a physicist. There could be some interest in what he finds by taking a look at the basic conceptual circumstances and philosophical underpinning that would apply to any QG theory. I can't tell if this paper is likely to be helpful or not.
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Carlo Rovelli is giving a seminar today (Tuesday 26 September) that will be available online:
http://relativity.phys.lsu.edu/ilqgs/
The slides are already posted
http://relativity.phys.lsu.edu/ilqgs/rovelli092606.pdf
The title of the talk is
Graviton propagator from loop quantum gravity
One good thing about this seminar series is that people from half a dozen different places in the world can ASK QUESTIONS.
Last week at the regular (Tuesday) time, Ashtekar gave one of these seminars. He was talking from Penn State and he got questions from various places and at one point he got a question that he wanted to refer to Carlo Rovelli, who was also "participating" in the seminar from Marseille and who was listening to the talk. and Rovelli answered---it came across very clear (his audio quality was even better than the seminar-average, maybe he had a better mike) so it was like having people from several QG centers present in the same room.
Another good thing is the policy to make the slides for the talk available a day or more in advance of the talk, so you can read the slides ahead of time and understand the talk better
The last slide of todays talk gives 3 conclusions
1. Low energy limit:
(One component of) the graviton propagator (or the Newton law) appears to be correct, to first order in lambda.
2. Barrett-Crane vertex:
...The BC vertex works.
3. Scattering amplitudes:
A technique to compute n-point functions within a background independent formalism exists.
These seminars are always supposed to occur at 9AM Central Time, which is 10 AM eastern and 6 AM pacific.
But I can never keep track of "daylight saving time" which makes an hour difference.
I think it means that they happen at 1:00 PM Greenwich.
In his talk, Rovelli says to be sure to hear next week's talk which will be by Simone Speziale on Spinfoam graviton propagator
http://arxiv.org/abs/hep-ph/0609274
The Standard Model: Alchemy and Astrology
Joseph D. Lykken
12 pages, 0 figures, review talk from "Physics at LHC", Krakow, 3-8 July 2006
Report-no: FERMILAB-CONF-06-347-T
"An brief unconventional review of Standard Model physics, containing no plots."
http://arxiv.org/abs/gr-qc/0609122
Black hole entropy quantization
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 4 figures
"Ever since the pioneer works of Bekenstein and Hawking, black hole entropy has been known to have a quantum origin. Furthermore, it has long been argued by Bekenstein that entropy should be quantized in discrete (equidistant) steps given its identification with horizon area in (semi-)classical general relativity and the properties of area as an adiabatic invariant. This lead to the suggestion that black hole area should also be quantized in equidistant steps to account for the discrete black hole entropy. Here we shall show that loop quantum gravity, in which area is not quantized in equidistant steps can nevertheless be consistent with Bekenstein's equidistant entropy proposal in a subtle way. For that we perform a detailed analysis of the number of microstates compatible with a given area and show that an observed oscillatory behavior in the entropy-area relation, when properly interpreted yields an entropy that has discrete, equidistant values that are consistent with the Bekenstein framework."
http://arxiv.org/abs/hep-th/0609177
Analytic Results in 2D Causal Dynamical Triangulations: A Review
Stefan Zohren
66 pages, 17 figures. Based on the author's thesis for the Master of Science in Theoretical Physics, supervised by R. Loll and co-supervised by J. Ambjorn, J. Jersak, July 2005
"We describe the motivation behind the recent formulation of a nonperturbative path integral for Lorentzian quantum gravity defined through Causal Dynamical Triangulations (CDT). In the case of two dimensions the model is analytically solvable, leading to a genuine continuum theory of quantum gravity whose ground state describes a two-dimensional "universe" completely governed by quantum fluctuations. One observes that two-dimensional Lorentzian and Euclidean quantum gravity are distinct. In the second part of the review we address the question of how to incorporate a sum over space-time topologies in the gravitational path integral. It is shown that, provided suitable causality restrictions are imposed on the path integral histories, there exists a well-defined nonperturbative gravitational path integral including an explicit sum over topologies in the setting of CDT. A complete analytical solution of the quantum continuum dynamics is obtained uniquely by means of a double scaling limit. We show that in the continuum limit there is a finite density of infinitesimal wormholes. Remarkably, the presence of wormholes leads to a decrease in the effective cosmological constant, reminiscent of the suppression mechanism considered by Coleman and others in the context of a Euclidean path integral formulation of four-dimensional quantum gravity in the continuum. In the last part of the review universality and certain generalizations of the original model are discussed, providing additional evidence that CDT define a genuine continuum theory of two-dimensional Lorentzian quantum gravity."
http://arxiv.org/abs/gr-qc/0609129
Rainbow universe
Yi Ling
5 pages
"The formalism of rainbow gravity is studied in a cosmological setting. We consider the very early universe which is radiation dominated. A novel treatment in our paper is to look for an ``averaged'' cosmological metric probed by radiation particles themselves. Taking their cosmological evolution into account, we derive the modified Friedmann-Robertson-Walker(FRW) equations which is a generalization of the solution presented by Magueijo and Smolin. Based on this phenomenological cosmological model we argue that the spacetime curvature has an upper bound such that the cosmological singularity is absent. These modified $FRW$ equations can be treated as effective equations in the semi-classical framework of quantum gravity and its analogy with the one recently proposed in loop quantum cosmology is also discussed."
http://arxiv.org/abs/gr-qc/0609130
The kinematics of particles moving in rainbow spacetime
Yi Ling, Song He, Hongbao Zhang
5 pages
"The kinematics of particles moving in rainbow spacetime is studied in this paper. In particular the geodesics of a massive particle in rainbow flat spacetime is obtained when the semi-classical effect of its own energy on the background is taken into account. We show that in general the trajectory of a freely falling particle remains unchanged which is still a straight line as in the flat spacetime. The implication to the Unruh effect in rainbow flat spacetime is also discussed."
http://arxiv.org/abs/hep-th/0607228
Renormalization Group Running of Newton's G: The Static Isotropic Case
Herbert W. Hamber, Ruth M. Williams
61 pages, 3 figures
CERN-PH-TH/2006-145
"Corrections are computed to the classical static isotropic solution of general relativity, arising from non-perturbative quantum gravity effects. A slow rise of the effective gravitational coupling with distance is shown to involve a genuinely non-perturbative scale, closely connected with the gravitational vacuum condensate, and thereby, it is argued, related to the observed effective cosmological constant. Several analogies between the proposed vacuum condensate picture of quantum gravitation, and non-perturbative aspects of vacuum condensation in strongly coupled non-abelian gauge theories are developed. In contrast to phenomenological approaches, the underlying functional integral formulation of the theory severely constrains possible scenarios for the renormalization group evolution of couplings. The expected running of Newton's constant $G$ is compared to known vacuum polarization induced effects in QED and QCD. The general analysis is then extended to a set of covariant non-local effective field equations, intended to incorporate the full scale dependence of $G$, and examined in the case of the static isotropic metric. The existence of vacuum solutions to the effective field equations in general severely restricts the possible values of the scaling exponent $\nu$."
http://arxiv.org/abs/gr-qc/0610018
The Asymptotic Safety Scenario in Quantum Gravity -- An Introduction
M. Niedermaier
72 pages, 1 figure. Introduction to the subject based on several talks and conference contributions. An elaboration on the evidence (c) and (d) (see abstract) can be found in a forthcoming `Living Reviews in Relativity' article with M. Reuter
"The asymptotic safety scenario in quantum gravity is reviewed, according to which a renormalizable quantum theory of the gravitational field is feasible which reconciles asymptotically safe couplings with unitarity.All presently known evidence is surveyed: (a) from the 2+\epsilon expansion,(b) from renormalizable higher derivative gravity theories and a `large N' expansion in the number of matter fields, (c) from the 2-Killing vector reduction, and (d) from truncated flow equations for the effective average action. Special emphasis is given to the role of perturbation theory as a guide to `asymptotic safety'. Further it is argued that as a consequence of the scenario the selfinteractions appear two-dimensional in the extreme ultraviolet. Two appendices discuss the distinct roles of the ultraviolet renormalization in perturbation theory and in the flow equation formalism."
Niedermeyer is a collaborator of Martin Reuter.
The next paper (orig. 2004) was significantly expanded and improved for resubmission January 2006. Because it did not show up as a new paper then, I did not notice it. But actually it is highly relevant current work.
http://arxiv.org/abs/math-ph/0407006
Representations of the Weyl Algebra in Quantum Geometry
Christian Fleischhack
67 pages, 1 figure. Changes v1 to v2: 1) Due to inconsistencies (paths vs. edges), detected by Garth Warner in v1: old Sect. 3.2 replaced by new Sects. 3.2 and 3.3, begin of Sect. 3.1 extended; proof of Lemma 3.27, Sect. 3.6, Acknowledgements updated; overall results, however, not affected; 2) comparison with paper by Lewandowski, Okolow, Sahlmann, Thiemann added (pp. 63--65); 3) Footnote 2 and Refs. [12], [21], [23] added; Footnote 9 corrected; 4) Begin of Sect. 6.3 slightly modified
"The Weyl algebra A of continuous functions and exponentiated fluxes, introduced by Ashtekar, Lewandowski and others, in quantum geometry is studied. It is shown that, in the piecewise analytic category, every regular representation of A having a cyclic and diffeomorphism invariant vector, is already unitarily equivalent to the fundamental representation. Additional assumptions concern the dimension of the underlying analytic manifold (at least three), the finite wide triangulizability of surfaces in it to be used for the fluxes and the naturality of the action of diffeomorphisms -- but neither any domain properties of the represented Weyl operators nor the requirement that the diffeomorphisms act by pull-backs. For this, the general behaviour of C*-algebras generated by continuous functions and pull-backs of homeomorphisms, as well as the properties of stratified analytic diffeomorphisms are studied. Additionally, the paper includes also a short and direct proof of the irreducibility of A."
There is an interesting CHART which compares Fleischhack's result with the "LOST" result of Lewandowski, Okolow, Sahlmann, and Theimann. They wrote up their results around the same time, Fleischhack maybe even a little earlier. (!) The similarities and slight differences are interesting. BOTH use stratified diffeomorphisms. It looks like we may need to add those to our familiar language of terminology.
http://arxiv.org/abs/gr-qc/0610017
Physical Diffeomorphisms in Loop Quantum Gravity
Tim Koslowski
25 pages, 2 figures
"We investigate the action of diffeomorphisms in the context of Hamiltonian Gravity. By considering how the diffeomorphism-invariant Hilbert space of Loop Quantum Gravity should be constructed, we formulate a physical principle by demanding, that the gauge-invariant Hilbert space is a completion of gauge- (i.e. diffeomorphism-)orbits of the classical (configuration) variables, explaining which extensions of the group of diffeomorphisms must be implemented in the quantum theory. It turns out, that these are at least a subgroup of the stratified analytic diffeomorphisms. Factoring these stratified diffeomorphisms out, we obtain that the orbits of graphs under this group are just labelled by their knot classes, which in turn form a countable set. Thus, using a physical argument, we construct a separable Hilbert space for diffeomorphism invariant Loop Quantum Gravity, that has a spin-knot basis, which is labelled by a countable set consisting of the combination of knot-classes and spin quantum numbers. It is important to notice, that this set of diffeomorphism leaves the set of piecewise analytic edges invariant, which ensures, that one can construct flux-operators and the associated Weyl-operators. A note on the implications for the treatment of the Gauss- and the Hamilton-constraint of Loop Quantum Gravity concludes our discussion."
I gather Koslowski is maybe 26, teaching astronomy at Würzburg, and just getting started in QG. No previous papers. He gave a Loop Quantum Cosmology talk at a conference last year. He was a participant at Loops '05, but did not give a paper. I hadnt heard of him before now. This first paper is outstanding quality (in my humble estimation). Besides English, Koslowski appears to be literate in latin and greek, and to have contributed to Wikipedia. associate of martin bojowald. watch this guy. oh yeah, stratified diffeomorphisms looks like they might be a real good idea to know about.
http://www.astro.uni-wuerzburg.de/~koslowski/
I think Vilenkin has failed to refute the conjecture called Cosmological Natural Selection (CNS) but there are several interesting things coming from this next paper:
http://arxiv.org/abs/hep-th/0610051
On cosmic natural selection
Alexander Vilenkin
4 pages
"The rate of black hole formation can be increased by increasing the value of the cosmological constant. This falsifies Smolin's conjecture that the values of all constants of nature are adjusted to maximize black hole production."
Instead of falsifying the CNS as Smolin would state it, Vilenkin seems to me to have modified the conjecture so it can be about black holes formed by quantum fluctuation rather than by the gravitational collaps of matter in the form of stars. Smolin's writings about the CNS have not been about black holes formed by quantum fluctuation---a somewhat hypothetical sort of object. As a reminder one form of CNS can be stated as the following challenge.
CONJECTURE: You cannot show a direction in which to change fundamental dimensionless constants such that the abundance of black holes formed by stellar collapse would initially increase. If it dips first and THEN rises, that doesnt count. (i.e. we are at a local max.)
http://arxiv.org/abs/gr-qc/0610023
Uniform discretizations: a new approach for the quantization of totally constrained systems
Miguel Campiglia, Cayetano Di Bartolo, Rodolfo Gambini, Jorge Pullin
17 pages
"We discuss in detail the uniform discretization approach to the quantization of totally constrained theories. This approach allows to construct the continuum theory of interest as a well defined, controlled, limit of well behaved discrete theories. We work out several finite dimensional examples that exhibit behaviors expected to be of importance in the quantization of gravity. We also work out the case of BF theory. At the time of quantization, one can take two points of view. The technique can be used to define, upon taking the continuum limit, the space of physical states of the continuum constrained theory of interest. In particular we show in models that it agrees with the group averaging procedure when the latter exists. The technique can also be used to compute, at the discrete level, conditional probabilities and the introduction of a relational time. Upon taking the continuum limit one can show that one reproduces results obtained by the use of evolving constants, and therefore recover all physical predictions of the continuum theory. This second point of view can also be used as a paradigm to deal with cases where the continuum limit does not exist. There one would have discrete theories that at least at certain scales reproduce the semiclassical properties of the theory of interest. In this way the approach can be viewed as a generalization of the Dirac quantization procedure that can handle situations where the latter fails."
http://arxiv.org/abs/hep-th/0610064
Scale Dependent Metric and Minimal Length in QEG
Martin Reuter, Jan-Markus Schwindt
10 pages, 1 figure, Talk given by M.R. at IRGAC 2006, Barcelona, Spain, July 11-15, 2006
"The possibility of a minimal physical length in quantum gravity is discussed within the asymptotic safety approach. Using a specific mathematical model for length measurements ("COM microscope") it is shown that the spacetimes of Quantum Einstein Gravity (QEG) based upon a special class of renormalization group trajectories are "fuzzy" in the sense that there is a minimal coordinate separation below which two points cannot be resolved."
http://arxiv.org/abs/gr-qc/0610026
Nieh-Yan Invariant and Fermions in Ashtekar-Barbero-Immirzi Formalism
Simone Mercuri
3 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin
"In order to consistently introduce an interaction between gravity and fermions in the Ashtekar-Barbero-Immirzi formalism a non-minimal term is necessary. The non-minimal term together with the Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan invariant. The Immirzi parameter does not affect the classical dynamics, which is described by the Einstein-Cartan effective action. "
http://arxiv.org/abs/gr-qc/0610060
A perturbative approach to Dirac observables and their space-time algebra
Bianca Dittrich, Johannes Tambornino
23 pages
"We introduce a general approximation scheme in order to calculate gauge invariant observables in the canonical formulation of general relativity. Using this scheme we will show how the observables and the dynamics of field theories on a fixed background or equivalently the observables of the linearized theory can be understood as an approximation to the observables in full general relativity. Gauge invariant corrections can be calculated up to an arbitrary high order and we will explicitly calculate the first non--trivial correction. Furthermore we will make a first investigation into the Poisson algebra between observables corresponding to fields at different space--time points and consider the locality properties of the observables."
http://arxiv.org/abs/gr-qc/0610072
Hamiltonian and physical Hilbert space in polymer quantum mechanics
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
17 pages, 2 figures
"In this paper, a version of polymer quantum mechanics, which is inspired by loop quantum gravity, is considered and shown to be equivalent, in a precise sense, to the standard, experimentally tested, Schroedinger quantum mechanics. The kinematical cornerstone of our framework is the so called polymer representation of the Heisenberg-Weyl (H-W) algebra, which is the starting point of the construction. The dynamics is constructed as a continuum limit of effective theories characterized by a scale, and requires a renormalization of the inner product. The result is a physical Hilbert space in which the continuum Hamiltonian can be represented and that is unitarily equivalent to the Schroedinger representation of quantum mechanics. As a concrete implementation of our formalism, the simple harmonic oscillator is fully developed."
http://arxiv.org/abs/gr-qc/0610074
Gravitational collapse in loop quantum gravity
Leonardo Modesto
16 pages, 2 figures
"In this paper we study the gravitational collapse in loop quantum gravity. We consider the space-time region inside the Schwarzschild black hole event horizon and we divide this region in two parts, the first one where the matter (dust matter) is localized and the other (outside) where the metric is Kantowski-Sachs type. We calculate the state solving Hamiltonian constraint and we obtain a set of three difference equations that give a regular and natural evolution beyond the classical singularity point in "r=0" localized."
http://arxiv.org/abs/hep-th/0610164
Modified gravity as an alternative for Lambda-CDM cosmology
Shin'ichi Nojiri, Sergei D. Odintsov
9 pages, prepared for Special Issue IRGAC 2006, Barcelona
"The reconstruction scheme is developed for modified f(R) gravity with realistic matter (dark matter, baryons, radiation). Two versions of such theory are constructed: the first one describes the sequence of radiation and matter domination, decceleration-acceleration transition and acceleration era and the second one is reconstructed from exact Lambda-CDM cosmology. The asymptotic behaviour of first model at late times coincides with the theory containing positive and negative powers of curvature while second model approaches to General Relativity without singularity at zero curvature."
http://arxiv.org/abs/hep-th/0610193
Lessons from the LQG String
Robert C. Helling (IU Bremen)
"We give a non-technical description of the differences of quantisation of the bosonic string between the usual Fock-space approach and the treatment inspired by methods of loop quantum gravity termed the LCQ string. We point out the role of covariant states with continuous representations of the Weyl operators versus invariant states leading to discontinuous polymer representations. In the example of the harmonic oscillator we compare the optical absorption spectrum for the two quantisations and find that the question of distinguishability depends on the order in which limits are taken: For a fixed UV cut-off restricting the Hilbert space to a finite dimensional subspace the spectra can be made arbitrarily similar by an appropriate choice of state. However, if the states are chosen first, they differ at high frequencies."
http://arxiv.org/abs/hep-th/0610194
Quantum Gravity as a Broken Symmetry Phase of a BF Theory
A. Mikovic
6 pages, talk given at the LOR2006 Symposium, Budapest, 22-24 June 2006
"We explain how General Relativity with a cosmological constant arises as a broken symmetry phase of a BF theory. In particular we show how to treat de Sitter and anti-de Sitter cases simultaneously. This is then used to formulate a quantization of General Relativity through a spin foam perturbation theory. We then briefly discuss how to calculate the effective action in this quantization procedure."
This paper fails to cite the prior work of Freidel Starodubtsev, to which it is rather similar.
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
19 pages
"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."
http://arxiv.org/abs/hep-th/0610241
Gravity and the standard model with neutrino mixing
Ali H. Chamseddine, Alain Connes, Matilde Marcolli
71 pages, 7 figures
"We present an effective unified theory based on noncommutative geometry for the standard model with neutrino mixing, minimally coupled to gravity. The unification is based on the symplectic unitary group in Hilbert space and on the spectral action. It yields all the detailed structure of the standard model with several predictions at unification scale. Besides the familiar predictions for the gauge couplings as for GUT theories, it predicts the Higgs scattering parameter and the sum of the squares of Yukawa couplings. From these relations one can extract predictions at low energy, giving in particular a Higgs mass around 170 GeV and a top mass compatible with present experimental value. The geometric picture that emerges is that space-time is the product of an ordinary spin manifold (for which the theory would deliver Einstein gravity) by a finite noncommutative geometry F. The discrete space F is of KO-dimension 6 modulo 8 and of metric dimension 0, and accounts for all the intricacies of the standard model with its spontaneous symmetry breaking Higgs sector. "
http://arxiv.org/abs/hep-th/0610231
Does string theory predict an open universe?
R. Buniy, S. Hsu, A. Zee
3 pages, 1 figure
"It has been claimed that the string landscape predicts an open universe, with negative curvature. The prediction is a consequence of a large number of metastable string vacua, and the properties of the Coleman--De Luccia instanton which describes vacuum tunneling. We examine the robustness of this claim, which is of particular importance since it seems to be string theory's sole claim to falsifiability. We find that, due to subleading tunneling processes, the prediction is sensitive to unknown properties of the landscape. Under plausible assumptions, universes like ours are as likely to be closed as open."
http://arxiv.org/abs/gr-qc/0610101
The emergence of AdS(2) from quantum fluctuations
J. Ambjorn, R. Janik, W. Westra, S. Zohren
4 pages. Talk given by W. Westra at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006
"We have shown how the quantization of two-dimensional quantum gravity with an action which contains only a positive cosmological constant and boundary cosmological constants leads to the emergence of a spacetime which can be described as a constant negative curvature spacetime with superimposed quantum fluctuations."
===============
http://arxiv.org/abs/hep-th/0610236
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. I. BF Yang-Mills representation
Florian Conrady (Penn State U.)
24 pages, 2 figures
IGPG-06/10-4
"In this series of three papers, we generalize the derivation of photons and monopoles by Polyakov and Banks, Myerson and Kogut, to obtain gluon-monpole representations of SU(2) lattice gauge theory. The papers take three different representations as their starting points: the representation as a BF Yang-Mills theory, the spin foam representation and the plaquette representation. The subsequent derivations are based on semiclassical expansions.
In this first article, we cast d-dimensional SU(2) lattice gauge theory in the form of a lattice BF Yang-Mills theory. In several steps, the expectation value of a Wilson loop is transformed into a path integral over a gluon field and monopole-like degrees of freedom. The action contains the tree-level Coulomb interaction and a nonlinear coupling between gluons, monopoles and current.
At the end, we compare the results from all three papers."
http://arxiv.org/abs/hep-th/0610237
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. II Spin foam representation
http://arxiv.org/abs/hep-th/0610238
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. III Plaquette representation
http://arxiv.org/abs/gr-qc/0610140
Multiple-event probability in general-relativistic quantum mechanics
Frank Hellmann, Mauricio Mondragon, Alejandro Perez, Carlo Rovelli
"We discuss the definition of quantum probability in the context of 'timeless' general--relativistic quantum mechanics. In particular, we study the probability of sequences of events, or multi-event probability. In conventional quantum mechanics this can be obtained by means of the 'wave function collapse' algorithm. We first point out certain difficulties of some natural definitions of multi-event probability, including the conditional probability widely considered in the literature. We then observe that multi-event probability can be reduced to single-event probability, by taking into account the quantum nature of the measuring apparatus. In fact, by exploiting the von-Neumann freedom of moving the quantum classical boundary, one can always trade a sequence of non-commuting quantum measurements at different times, with an ensemble of simultaneous commuting measurements on the joint system+apparatus system. This observation permits a formulation of quantum theory based only on single-event probability, where the results of the 'wave function collapse' algorithm can nevertheless be recovered. The discussion bears also on the nature of the quantum collapse."
the title was too intriguing to pass up. Malcolm Fairbairn is at Perimeter Institute and Stockholm.
http://arxiv.org/abs/astro-ph/0610844
Shining light through the Sun
Malcolm Fairbairn, Timur Rashba, Sergey Troitsky
4 pages
"It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudo-scalar. In particular, if the axion interpretation of the PVLAS result were true then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C279 by EGRET in 1991; 98% C.L. detection of a non-zero flux of gamma rays passing through the Sun is not yet conclusive. Future experiments, e.g. GLAST, are expected to have better sensitivity."
http://arxiv.org/abs/gr-qc/0611005
Problems and hopes in nonsymmetric gravity
Tomas Janssen, Tomislav Prokopec (Utrecht University)
9 pages, 1 figure. Based on two talks by the authors at the 2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC) 2006, Barcelona
SPIN-06-37, ITP-UU-06-47
"We consider the linearized nonsymmetric theory of gravitation (NGT) within the background of an expanding universe and near a Schwarzschild mass. We show that the theory always develops instabilities unless the linearized nonsymmetric lagrangian reduces to a particular simple form. This form contains a gauge invariant kinetic term, a mass term for the antisymmetric metric-field and a coupling with the Ricci curvature scalar. This form cannot be obtained within NGT. Based on the linearized lagrangian we know to be stable, we consider the generation and evolution of quantum fluctuations of the antisymmetric gravitational field (B-field) from inflation up to the present day. We find that a B-field with a mass m ~ 0.03(H_I/10^(13)GeV)^4 eV is an excellent dark matter candidate."
http://arxiv.org/abs/gr-qc/0611004
Universal Bound on Dynamical Relaxation Times and Black-Hole Quasinormal Ringing
Shahar Hod
4 pages
"From information theory and thermodynamic considerations a universal bound on the relaxation time $\tau$ of a perturbed system is inferred, \tau \geq \hbar/\pi T, where T is the system's temperature. We prove that black holes comply with the bound; in fact they actually saturate it. Thus, when judged by their relaxation properties, black holes are the most extreme objects in nature, having the maximum relaxation rate which is allowed by quantum theory."
http://arxiv.org/abs/hep-th/0611017
Phenomenological Quantum Gravity
S. Hossenfelder
To appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 2006
"Planck scale physics represents a future challenge, located between particle physics and general relativity. The Planck scale marks a threshold beyond which the old description of spacetime breaks down and conceptually new phenomena must appear. In the last years, increased efforts have been made to examine the phenomenology of quantum gravity, even if the full theory is still unknown."
http://arxiv.org/abs/hep-th/0611042
Hidden Quantum Gravity in 4d Feynman diagrams: Emergence of spin foams
Aristide Baratin, Laurent Freidel
28 pages, 7 figures
"We show how Feynman amplitudes of standard QFT on flat and homogeneous space can naturally be recast as the evaluation of observables for a specific spin foam model, which provides dynamics for the background geometry. We identify the symmetries of this Feynman graph spin foam model and give the gauge-fixing prescriptions. We also show that the gauge-fixed partition function is invariant under Pachner moves of the triangulation, and thus defines an invariant of four-dimensional manifolds. Finally, we investigate the algebraic structure of the model, and discuss its relation with a quantization of 4d gravity in the limit where the Newton constant goes to zero."
http://arxiv.org/abs/gr-qc/0611024
Phenomenology of Quantum Gravity and Finsler Geometry
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
10 pages
A common feature of all Quantum Gravity (QG) phenomenology approaches is to consider a modification of the mass shell condition of the relativistic particle to take into account quantum gravitational effects. The framework for such approaches is therefore usually set up in the cotangent bundle (phase space). However it was recently proposed that this phenomenology could be associated with an energy dependent geometry that has been coined ``rainbow metric". We show here that the latter actually corresponds to a Finsler Geometry, the natural generalization of Riemannian Geometry. We provide in this way a new and rigorous framework to study the geometrical structure possibly arising in the semiclassical regime of QG. We further investigate the symmetries in this new context and discuss their role in alternative scenarios like Lorentz violation in emergent spacetimes or Deformed Special Relativity-like models."
JB taught us to look out for whatever Bob Coecke is up to. He often has quite strange ideas. Here is something new by him:
http://arxiv.org/abs/physics/0611064
Early Greek Thought and Perspectives for the Interpretation of Quantum Mechanics: Preliminaries to an Ontological Approach
K. Verelst, B. Coecke
18 pages
History of Physics
" It will be shown in this article that an ontological approach for some problems related to the interpretation of Quantum Mechanics could emerge from a re-evaluation of the main paradox of early Greek thought: the paradox of Being and non-Being, and the solutions presented to it by Plato and Aristotle. Plato's and Aristotle's systems are argued here to do on the ontological level essentially the same: to introduce stability in the world by introducing the notion of a separable, stable object, for which a principle of contradiction is valid: an object cannot be and not-be at the same place at the same time. After leaving Aristotelian metaphysics, early modern science had to cope with these problems: it did so by introducing 'space' as the seat of stability, and 'time' as the theater of motion. But the ontological structure present in this solution remained the same. Therefore the fundamental notion 'separable system', related to the notions observation and measurement, themselves related to the modern concepts of space and time, appears to be intrinsically problematic, because it is inextricably connected to classical logic on the ontological level. We see therefore the problems dealt with by quantum logic not as merely formal, and the problem of 'non-locality' as related to it, indicating the need to re-think the notions 'system', 'entity', as well as the implications of the operation 'measurement', which is seen here as an application of classical logic (including its ontological consequences) on the material world. "
http://arxiv.org/abs/gr-qc/0611043
Black hole interior from loop quantum gravity
Leonardo Modesto
11 pages, 7 figures
"In this paper we calculate modifications to the Schwarzschild solution by using a semiclassical analysis of loop quantum black hole. We obtain a metric inside the event horizon that coincides with the Schwarzschild solution near the horizon but that is substantially different at the Planck scale. In particular we obtain a bounce of the two-sphere for a minimum value of the radius and that it is possible to have another event horizon close to the r=0 point."
Francesca was telling us about the DICE 2006 conference which took place in September at Piombino on the Tuscan coast. Here is one of the papers presented:
http://arxiv.org/abs/quant-ph/0611076
Classicality in Quantum Mechanics
Olaf Dreyer
Converence talk presented at DICE 2006 in Piombino. To be published in the Journal of Physics: Conference Series
"In this article we propose a solution to the measurement problem in quantum mechanics. We point out that the measurement problem can be traced to an a priori notion of classicality in the formulation of quantum mechanics. If this notion of classicality is dropped and instead classicality is defined in purely quantum mechanical terms the measurement problem can be avoided. We give such a definition of classicality. It identifies classicality as a property of large quantum system. We show how the probabilistic nature of quantum mechanics is a result of this notion of classicality. We also comment on what the implications of this view are for the search of a quantum theory of gravity."
http://arxiv.org/abs/gr-qc/0611025
Current tests of alternative gravity theories: the Modified Newtonian Dynamics case
Jorge Paramos, Orfeu Bertolami
12 pages. Talk presented by one of us (O.B.) at the Fundamental Physics in Space section of the Comittee on Space Research, 36th. COSPAR Scientific Assembly, Beijing, China, 16-23 July, 2006
"We address the possibility of taking advantage of high accuracy gravitational space experiments in the Solar System and complementary cosmological tests to distinguish between the usual general relativistic theory from the alternative modified Newtonian dynamics paradigm."
Interest persists in modified gravity. Here is another sample:
http://arxiv.org/abs/hep-th/0611071
Modified gravity and its reconstruction from the universe expansion history
Shin'ichi Nojiri, Sergei D. Odintsov
24 pages, prepared for the proceedings of ERE 2006
"We develop the reconstruction program for the number of modified gravities: scalar-tensor theory, f(R), F(G) and string-inspired, scalar-Gauss-Bonnet gravity. The known (classical) universe expansion history is used for the explicit and successful reconstruction of some versions (of special form or with specific potentials) from all above modified gravities. It is demonstrated that cosmological sequence of matter dominance, decceleration-acceleration transition and acceleration era may always emerge as cosmological solutions of such theory. Moreover, the late-time dark energy FRW universe may have the approximate or exact LambdaCDM form consistent with three years WMAP data. The principal possibility to extend this reconstruction scheme to include the radiation dominated era and inflation is briefly mentioned. Finally, it is indicated how even modified gravity which does not describe the matter-dominated epoch may have such a solution before acceleration era at the price of the introduction of compensating dark energy."
http://arxiv.org/abs/gr-qc/0611073
Generalizing the Kodama State I: Construction
Andrew Randono
First part in two part series, 20 pages
The Kodama State is unique in being an exact solution to all the ordinary constraints of canonical quantum gravity that also has a well defined semi-classical interpretation as a quantum version of a classical spacetime, namely (anti)de Sitter space. However, the state is riddled with difficulties which can be tracked down to the complexification of the phase space necessary in its construction. This suggests a generalization of the state to real values of the Immirzi parameter. In this first part of a two paper series we show that one can generalize the state to real variables and the result is surprising in that it appears to open up an infinite class of physical states. We show that these states closely parallel the ordinary momentum eigenstates of non-relativistic quantum mechanics with the Levi-Civita curvature playing the role of the momentum. With this identification, the states inherit many of the familiar properties of the momentum eigenstates including delta-function normalizability. In the companion paper we will discuss the physical interpretation, CPT properties, and an interesting connection between the inner product and the Macdowell-Mansouri formulation of general relativity. "
http://arxiv.org/abs/gr-qc/0611074
Generalizing the Kodama State II: Properties and Physical Interpretation
Andrew Randono
Second paper in two part series. 18 pages
In this second part of a two paper series we discuss the properties and physical interpretation of the generalized Kodama states. We first show that the states are the three dimensional boundary degrees of freedom of two familiar 4-dimensional topological invariants: the second Chern class and the Euler class. Using this, we show that the states have the familiar interpretation as WKB states, in this case corresponding not only to de Sitter space, but also to first order perturbations therein. In an appropriate spatial topology, the de Sitter solution has pure Chern-Simons functional form, and is the unique state in the class that is identically diffeomorphism and SU(2) gauge invariant. The q-deformed loop transform of this state yields evidence of a cosmological horizon when the deformation parameter is a root of unity. We then discuss the behavior of the states under discrete symmetries, showing that the states violate P and T due to the presence of the Immirzi parameter, but they are CPT invariant. We conclude with an interesting connection between the physical inner product and the Macdowell Mansouri formulation of gravity."
http://arxiv.org/abs/gr-qc/0611075
Quantum Gravity and Matter: Counting Graphs on Causal Dynamical Triangulations
D. Benedetti, R. Loll
40 pages, 15 figures, 13 tables
"An outstanding challenge for models of non-perturbative quantum gravity is the consistent formulation and quantitative evaluation of physical phenomena in a regime where geometry and matter are strongly coupled. After developing appropriate technical tools, one is interested in measuring and classifying how the quantum fluctuations of geometry alter the behaviour of matter, compared with that on a fixed background geometry.
In the simplified context of two dimensions, we show how a method invented to analyze the critical behaviour of spin systems on flat lattices can be adapted to the fluctuating ensemble of curved spacetimes underlying the Causal Dynamical Triangulations (CDT) approach to quantum gravity. We develop a systematic counting of embedded graphs to evaluate the thermodynamic functions of the gravity-matter models in a high- and low-temperature expansion. For the case of the Ising model, we compute the series expansions for the magnetic susceptibility on CDT lattices and their duals up to orders 6 and 12, and analyze them by ratio method, Dlog Padé and differential approximants. Apart from providing evidence for a simplification of the model's analytic structure due to the dynamical nature of the geometry, the technique introduced can shed further light on criteria à la Harris and Luck for the influence of random geometry on the critical properties of matter systems.
http://arxiv.org/abs/gr-qc/0611080
A new proposal for group field theory I: the 3d case
James Ryan
"In this series of papers, we propose a new rendition of 3d and 4d state sum models based upon the group field theory (GFT) approach to non-perturbative quantum gravity. We will see that the group field theories investigated in the literature to date are, when judged from the position of quantum field theory, an unusual manifestation of quantum dynamics. They are one in which the Hadamard function for the field theory propagates a-causally the physical degrees of freedom of quantum gravity. This is fine if we wish to define a scalar product on the physical state space, but it is not what we generally think of as originating directly from a field theory. We propose a model in 3d more in line with standard quantum field theory, and therefore the field theory precipitates causal dynamics. Thereafter, we couple the model to point matter, and extract from the GFT the effective non-commutative field theory describing the matter dynamics on a quantum gravity background. We identify the symmetries of our new model and clarify their meaning in the GFT setting. We are aided in this process by identifying the category theory foundations of this GFT which, moreover, propel us towards a categorified version for the 4d case. "
Krasnov is at Nottingham, where fellow-poster fh has joined the QG group
http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov
5 pages, no figures
"We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological 'constant' and may provide a mechanism for naturally small 'dark energy'. "
http://arxiv.org/abs/astro-ph/0611504
Using globular clusters to test gravity in the weak acceleration regime: NGC 7099
Riccardo Scarpa, Gianni Marconi, Roberto Gilmozzi, Giovanni Carraro
Accepted for publication in A&A Letters. Four pages in total
"A test of Newton's law of gravity in the low acceleration regime using globular clusters is presented. New results for the core collapsed globular cluster NGC 7099 are given. The run of the gravitational potential as a function of distance is probed studying the velocity dispersion profile of the cluster, as derived from a set of 125 radial velocities with accuracy better than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the cluster center. The dispersion is found to be maximal at the center, then decrease until 10+-2 pc from the center, well inside the cluster tidal radius of 42 pc. After that the dispersion remains constant with average value 2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8 cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a value of the internal acceleration of gravity fully consistent with a_0=1.2e-8 cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the clusters with velocity dispersion profile probing acceleration below a_0. All four have been found to have a flat dispersion profile at large radii where the acceleration is below a_0, mimicking qualitatively and quantitatively elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in the low acceleration limit or some more conventional dynamical effect (e.g., tidal heating) is still unclear. However, the similarities emerging between very different globular clusters, as well as between globular clusters and elliptical galaxies seem to favor the first of these two possibilities. "
These two Stanford people have a funny title---couldn't resist.
http://arxiv.org/abs/hep-th/0611183
O'KKLT
Renata Kallosh, Andrei Linde
12 pages, 4 figures
SU-ITP-2006-31
"We propose to combine the quantum corrected O'Raifeartaigh model, which has a dS minimum near the origin of the moduli space, with the KKLT model with an AdS minimum. The combined effective N=1 supergravity model, which we call O'KKLT, has a dS minimum with all moduli stabilized. Gravitino in the O'KKLT model tends to be light in the regime of validity of our approximations. We show how one can construct models with a light gravitino and a high barrier protecting vacuum stability during the cosmological evolution. "
selfAdjoint
Nov17-06, 09:27 AM
These two Stanford people have a funny title---couldn't resist.
http://arxiv.org/abs/hep-th/0611183
O'KKLT
Years and years ago I bought O'Raifeartaigh's Group Structure of Gauge Theories at Christmas time, and in the flyleaf I wrote:
O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages
O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages,
Thou tellst us not just what you know
But challenge us with quiz also,
O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages
(Christmas 1988)
great guy
http://www.stp.dias.ie/Lochlainn/lochlainn.html
this affectionate one-page obit mentions that book in its concluding paragraph, chosing it to sum up the man
"In all his work, Lochlainn O'Raifeartaigh's love of physics, his interest in understanding fundamental problems clearly, and his ability to present results with precision and clarity shone through. He was an inspiring teacher for physics students in Dublin as well as abroad. His lectures on the Group Structure of Gauge Theories, published by Cambridge University Press, are very popular among students as well as researchers..."
I may have to take a look at that book.
your inscription is delightful
odious as it is to emend another's verse, it's hard to resist
in the interest of the English second person, and parallelism, I suggest
Thou tellst us not just what you know
But challengest with quiz also,
I particularly like the scansion which makes a stress fall on "not" in the middle of the first line of the couplet.
http://arxiv.org/abs/hep-th/0611197
Quantum Graphity
Tomasz Konopka, Fotini Markopoulou, Lee Smolin
14 pages, 3 figures
"We introduce a new model of background independent physics in which the degrees of freedom live on a complete graph and the physics is invariant under the permutations of all the points. We argue that the model has a low energy phase in which physics on a low dimensional lattice emerges and the permutation symmetry is broken to the translation group of that lattice. In the high temperature, or disordered, phase the permutation symmetry is respected and the average distance between degrees of freedom is small. This may serve as a tractable model for the emergence of classical geometry in background independent models of spacetime. We use this model to argue for a cosmological scenario in which the universe underwent a transition from the high to the low temperature phase, thus avoiding the horizon problem."
The next thing is something NASA called a press conference about last week. The technical article may give some additional details of interest.
http://arxiv.org/abs/astro-ph/0611572
New Hubble Space Telescope Discoveries of Type Ia Supernovae at z > 1: Narrowing Constraints on the Early Behavior of Dark Energy
Adam G. Riess (JHU, STScI), Louis-Gregory Strolger (UWK), Stefano Casertano (STScI), Henry C. Ferguson (STScI), Bahram Mobasher (STScI), Ben Gold (JHU), Peter J. Challis (CfA), Alexei V. Filippenko (UCB), Saurabh Jha (UCB), Weidong Li (UCB), John Tonry (IfA), Ryan Foley (UCB), Robert P. Kirshner (CfA), Mark Dickinson (NOAO), Emily MacDonald (NOAO), Daniel Eisenstein (UofA), Mario Livio (STScI), Josh Younger (CfA), Chun Xu (STScI), Tomas Dahlen (STScI), Daniel Stern (JPL)
82 pages, 17 figures, 6 tables. Data also available at: this http URL Accepted, Astrophysical Journal vol. 656 for Feb 10, 2007
"We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to trace the history of cosmic expansion over the last 10 billion years. These objects, which include 13 spectroscopically confirmed SNe Ia at z > 1, were discovered during 14 epochs of reimaging of the GOODS fields North and South over two years with the Advanced Camera for Surveys on HST. Together with a recalibration of our previous HST-discovered SNe Ia, the full sample of 23 SNe Ia at z > 1 provides the highest-redshift sample known. Combined with previous SN Ia datasets, we measured H(z) at discrete, uncorrelated epochs, reducing the uncertainty of H(z>1) from 50% to under 20%, strengthening the evidence for a cosmic jerk--the transition from deceleration in the past to acceleration in the present. The unique leverage of the HST high-redshift SNe Ia provides the first meaningful constraint on the dark energy equation-of-state parameter at z >1.
The result remains consistent with a cosmological constant (w(z)=-1), and rules out rapidly evolving dark energy (dw/dz >>1). The defining property of dark energy, its negative pressure, appears to be present at z>1, in the epoch preceding acceleration, with ~98% confidence in our primary fit. Moreover, the z>1 sample-averaged spectral energy distribution is consistent with that of the typical SN Ia over the last 10 Gyr, indicating that any spectral evolution of the properties of SNe Ia with redshift is still below our detection threshold. "
The next is listed in case some wish to follow the development of "anthropic predictions".
http://arxiv.org/abs/astro-ph/0611573
Anthropic predictions for vacuum energy and neutrino masses in the light of WMAP-3
Levon Pogosian, Alexander Vilenkin
11 pages, 6 figures
"Anthropic probability distributions for the cosmological constant and for the sum of neutrino masses are updated using the WMAP-3 data release. The new distribution for Lambda is in a better agreement with observation than the earlier one. The typicality of the observed value, defined as the combined probability of all values less likely than the observed, is no less than 22%. We discuss the dependence of our results on the simplifying assumptions used in deriving the distribution for Lambda and show that the agreement of the anthropic prediction with the data is rather robust. The distribution for the sum of the neutrino masses is peaked at 1 eV, suggesting degenerate masses, but a hierarchical mass pattern is still marginally allowed at a 2 sigma level."
Briefly noted:
http://arxiv.org/abs/math.AG/0611524
Langlands duality and G2 spectral curves
Nigel Hitchin
Briefly noted:
http://arxiv.org/abs/math.AG/0611524
Langlands duality and G2 spectral curves
Nigel Hitchin
Thanks for this reference, Marcus. It looks interesting.
selfAdjoint
Nov19-06, 10:17 PM
odious as it is to emend another's verse, it's hard to resist
in the interest of the English second person, and parallelism, I suggest
Thou tellst us not just what you know
But challengest with quiz also,
I particularly like the scansion which makes a stress fall on "not" in the middle of the first line of the couplet.
You are absolutely right. My only excuse is that, as I dimly recall, I was a little squiffed at the time. BTW I had got the book out to check the verse and started to read it again. By comparison with books on Lie Groups and Algebras that I have studied since then, it is a marvel of clarity and kindnness toward the student. I most heartily recommend it!
http://arxiv.org/abs/gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
IGPG-06/11-4, AEI-2006-086
"Within a perturbative cosmological regime of loop quantum gravity corrections to effective constraints are computed. This takes into account all inhomogeneous degrees of freedom relevant for scalar metric modes around flat space and results in explicit expressions for modified coefficients and of higher order terms. It also illustrates the role of different scales determining the relative magnitude of corrections. Our results demonstrate that loop quantum gravity has the correct classical limit, at least in its sector of cosmological perturbations around flat space, in the sense of perturbative effective theory."
http://arxiv.org/abs/astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
IGPG-06/11-3, AEI-2006-085
"Inhomogeneous cosmological perturbation equations are derived in loop quantum gravity, taking into account corrections in particular in gravitational parts. This provides a framework for calculating the evolution of modes in structure formation scenarios related to inflationary or bouncing models. Applications here are corrections to the Newton potential and to the evolution of large scale modes which imply non-conservation of curvature perturbations possibly noticeable in a running spectral index. These effects are sensitive to quantization procedures and test the characteristic behavior of correction terms derived from quantum gravity."
The next was spotted by selfAdjoint, who started a discussion thread here:
http://physicsforums.com/showthread.php?t=144841
http://arxiv.org/hep-ph/0611279
Beyond the Standard Model
Dmitri I. Kazakov
11 pages, 11 figures, Plenary talk at XXXIII ICHEP, Moscow 2006
"Review of recent developments in attempts to go beyond the Standard Model is given. We concentrate on three main unresolved problems: mechanism of electroweak symmetry breaking, expected new physics at the TeV scale (mainly SUSY) and the origin of the Dark matter."
http://arxiv.org/abs/gr-qc/0611141
The Canonical Approach to Quantum Gravity: General Ideas and Geometrodynamics
Domenico Giulini, Claus Kiefer
21 pages, 6 figures. Contribution to E. Seiler and I.-O. Stamatescu (editors): `Approaches To Fundamental Physics -- An Assessment Of Current Theoretical Ideas' (Springer Verlag, to appear)
"We give an introduction to the canonical formalism of Einstein's theory of general relativity. This then serves as the starting point for one approach to quantum gravity called quantum geometrodynamics. The main features and applications of this approach are briefly summarized."
http://arxiv.org/abs/gr-qc/0611135
Quantum causal histories in the light of quantum information
Etera R. Livine, Daniel R. Terno
9 pages, 8 figures
"We use techniques of quantum information theory to analyze the quantum causal histories approach to quantum gravity. We show that while it is consistent to introduce closed timelike curves (CTCs), they cannot generically carry independent degrees of freedom. Moreover, if the effective dynamics of the chronology-respecting part of the system is linear, it should be completely decoupled from the CTCs. In the absence of a CTC not all causal structures admit the introduction of quantum mechanics. It is possible for those and only for those causal structures that can be represented as quantum computational networks. The dynamics of the subsystems should not be unitary or even completely positive. However, we show that other commonly made assumptions ensure the complete positivity of the reduced dynamics."
http://arxiv.org/abs/hep-th/0611294
Scale-dependent metric and causal structures in Quantum Einstein Gravity
Martin Reuter, Jan-Markus Schwindt
52 pages
"Within the asymptotic safety scenario for gravity various conceptual issues related to the scale dependence of the metric are analyzed. The running effective field equations implied by the effective average action of Quantum Einstein Gravity (QEG) and the resulting families of resolution dependent metrics are discussed. The status of scale dependent vs. scale independent diffeomorphisms is clarified, and the difference between isometries implemented by scale dependent and independent Killing vectors is explained. A concept of scale dependent causality is proposed and illustrated by various simple examples. The possibility of assigning an 'intrinsic length' to objects in a QEG spacetime is also discussed."
http://arxiv.org/abs/quant-ph/0611261
Concerning Dice and Divinity
D.M.Appleby
Contribution to proceedings of Foundations of Probability and Physics, Vaxjo, 2006
"Einstein initially objected to the probabilistic aspect of quantum mechanics - the idea that God is playing at dice. Later he changed his ground, and focussed instead on the point that the Copenhagen Interpretation leads to what Einstein saw as the abandonment of physical realism. We argue here that Einstein's initial intuition was perfectly sound, and that it is precisely the fact that quantum mechanics is a fundamentally probabilistic theory which is at the root of all the controversies regarding its interpretation. Probability is an intrinsically logical concept. This means that the quantum state has an essentially logical significance. It is extremely difficult to reconcile that fact with Einstein's belief, that it is the task of physics to give us a vision of the world apprehended sub specie aeternitatis. Quantum mechanics thus presents us with a simple choice: either to follow Einstein in looking for a theory which is not probabilistic at the fundamental level, or else to accept that physics does not in fact put us in the position of God looking down on things from above. There is a widespread fear that the latter alternative must inevitably lead to a greatly impoverished, positivistic view of physical theory. It appears to us, however, that the truth is just the opposite. The Einsteinian vision is much less attractive than it seems at first sight. In particular, it is closely connected with philosophical reductionism."
http://arxiv.org/abs/gr-qc/0611148
Fundamental spatiotemporal decoherence: a key to solving the conceptual problems of black holes, cosmology and quantum mechanics
Rodolfo Gambini, Rafael Porto, Jorge Pullin
6 pages, Honorable Mention GRF 2006, to appear in IJMPD
"Unitarity is a pillar of quantum theory. Nevertheless, it is also a source of several of its conceptual problems. We note that in a world where measurements are relational, as is the case in gravitation, quantum mechanics exhibits a fundamental level of loss of coherence. This can be the key to solving, among others, the puzzles posed by the black hole information paradox, the formation of inhomogeneities in cosmology and the measurement problem in quantum mechanics."
http://arxiv.org/abs/math.CT/0611930
A Double Bicategory of Cobordisms With Corners
Jeffrey Morton
40 pages, 7 figures
"Interest in cobordism categories arises in areas from topology to theoretical physics, and in particular in Topological Quantum Field Theories (TQFT's). These categories have manifolds as objects, and cobordisms between them as morphisms, have - that is, manifolds of one dimension higher whose boundary decomposes into the source and target. Since the boundary of a boundary is empty, this formulation cannot account for cobordisms between manifolds with boundary. This is needed to describe open-closed TQFT's, and more generally, 'extended TQFT's'. We describe a framework for describing these, in the form of what we call a 'Verity Double Bicategory', after Dominic Verity, who introduced them. This is similar to a double category, but with properties holding only up to certain 2-morphisms. We show how a broad class of examples is given by a construction involving spans in suitable settings, and how this gives cobordisms between cobordisms when we start with the category of manifolds."
http://arxiv.org/abs/gr-qc/0611154
MacDowell-Mansouri gravity and Cartan geometry
Derek K. Wise
34 pages, 5 figures
"The geometric content of the MacDowell-Mansouri formulation of general relativity is best understood in terms of Cartan geometry. In particular, Cartan geometry gives clear geometric meaning to the MacDowell-Mansouri trick of combining the Levi-Civita connection and coframe field, or soldering form, into a single physical field. The Cartan perspective allows us to view physical spacetime as tangentially approximated by an arbitrary homogeneous 'model spacetime', including not only the flat Minkowski model, as is implicitly used in standard general relativity, but also de Sitter, anti de Sitter, or other models. A 'Cartan connection' gives a prescription for parallel transport from one 'tangent model spacetime' to another, along any path, giving a natural interpretation of the MacDowell-Mansouri connection as 'rolling' the model spacetime along physical spacetime. I explain Cartan geometry, and 'Cartan gauge theory', in which the gauge field is replaced by a Cartan connection. In particular, I discuss MacDowell-Mansouri gravity, as well as its recent reformulation in terms of BF theory, in the context of Cartan geometry."
http://arxiv.org/abs/gr-qc/0611156
On Loop States in Loop Quantum Gravity
N. D. Hari Dass, Manu Mathur
12 pages, 3 figures
"We explicitly construct and characterize all possible independent loop states in 3+1 dimensional loop quantum gravity by regulating it on a 3-d regular lattice in the Hamiltonian formalism. These loop states, characterized by the (dual) angular momentum quantum numbers, describe SU(2) rigid rotators on the links of the lattice. The loop states are constructed using the Schwinger bosons which are harmonic oscillators in the fundamental (spin half) representation of SU(2). Using generalized Wigner Eckart theorem, we compute the matrix elements of the volume operator in the loop basis. Some simple loop eigenstates of the volume operator are explicitly constructed."
noted in mid-November (post 535)
http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov
5 pages, no figures
"We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological 'constant' and may provide a mechanism for naturally small 'dark energy'. "
there is now a video to go along with it. Krasnov giving a slide presentation at P.I. and lots of questions (from Laurent Freidel and Lee Smolin among others) and discussion.
Krasnov has a version of gravity that departs from usual at large scale and something like a cosmological constant comes out of it.
If you want to jump to the "Dark Energy" slide it is #5 on page 5 of the slide menu---about halfway through. You can start the video there if you want.
The discussion of "interpretation" begins at slide #7 of page 8 of slide menu. Starting there you get a lot of questions by people in audience and Krasnov responding.
the video is #06110041 (dated 30 November) in the P.I. collection
http://www.perimeterinstitute.ca/en/Scientific/Seminars/PIRSA/
one way to get it is just type that PIRSA number in the box
another way, since it is recent, is to click on "catch up" so you get a list
of all the recent video seminars
revised version just posted:
http://arxiv.org/abs/gr-qc/0601043
Can Gravitons Be Detected?
Tony Rothman, Stephen Boughn
This version as appeared in Foundations of Physics
Journal-ref: Foundations of Physics, vol. 36, No. 12, 1801-1825 (2006)
"Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson's question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson's conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production."
a 1981 paper of Bohm, re-published to make it more accessible:
http://arxiv.org/abs/quant-ph/0612002
Algebraic Quantum Mechanics and Pregeometry
D.J. Bohm P.G. Davies B.J. Hiley
This paper was originally written in 1981 and published as a supplement to my Ph.D. thesis. (Davies, P., (1981) The Weyl Algebra and an Algebraic Mechanics. Ph.D thesis, Birkbeck College, University of London.) It is believed to be one of the "lost papers" of David Bohm as it was is not listed among his completed works and is set forth here for historical completeness
"We discuss the relation between the q-number approach to quantum mechanics suggested by Dirac and the notion of "pregeometry" introduced by Wheeler. By associating the q-numbers with the elements of an algebra and regarding the primitive idempotents as "generalized points" we suggest an approach that may make it possible to dispense with an a priori given space manifold. In this approach the algebra itself would carry the symmetries of translation, rotation, etc. Our suggestion is illustrated in a preliminary way by using a particular generalized Clifford Algebra proposed originally by Weyl, which approaches the ordinary Heisenberg algebra in a suitable limit. We thus obtain a certain insight into how quantum mechanics may be regarded as a purely algebraic theory, provided that we further introduce a new set of "neighbourhood operators", which remove an important kind of arbitrariness that has thus far been present in the attempt to treat quantum mechanics solely in terms of a Heisenberg algebra."
http://arxiv.org/abs/math-ph/0612012
An informal introduction to the ideas and concepts of noncommutative geometry
Thierry Masson
53 pages, Lecture given at the 6th Peyresq meeting "Integrable systems and quantum field theory"
"This informal introduction is an extended version of a three hours lecture given at the 6th Peyresq meeting ``Integrable systems and quantum field theory''. In this lecture, we make an overview of some of the mathematical results which motivated the development of what is called noncommutative geometry. The first of these results is the theorem by Gelfand and Neumark about commutative $C^\ast$-algebras; then come some aspects of the $K$-theories, first for topological spaces, then for $C^\ast$-algebras and finally the purely algebraic version. Cyclic homology is introduced, keeping in mind its relation to differential structures. The last result is the construction of the Chern character, which shows how these developments are related to each other."
Some of these entries do not yet have the title and abstract. Note that there are some here from the Young Researchers Conference that took place at Perimeter this week---Willem Westra (PhD student who has co-authored with Loll), Andy Randono (PhD student with research generalizing the Kodama state).
The videos are split screen, showing a larger-format version of the slide or blackboard. They can be watched at the PI site simply by typing in the PIRSA number.
http://www.perimeterinstitute.ca/en/Scientific/Seminars/PIRSA/
or even easier, check the QG box here and click "do search"
http://www.perimeterinstitute.ca/index.php?option=com_content&task=view&id=113&Itemid=167&p=presentations&with_msl=true
then you get a menu---and can just select what you want to watch from the menu
The ones that I starred (***) are talks that I watched and can recommend to anyone interested in the topic. (Benedetti, Randono, Krasnov...)
***PIRSA#: 06110041
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov - University of Nottingham
We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological "constant" and may provide a mechanism for naturally small "dark energy".
30/11/2006
PIRSA#: 06120017
Kappa Deformed Field Theory
Sebastian Nowak (Wroclaw, probably Kowalski-Glikman's group)
The description of noncommutative space will be given. I will show the relation between field theory on kappa-Minkowski space and the one in Minkowski. This construction leads to deformed energy momentum conservation law for energies close to the Planck scale.
04/12/2006
PIRSA#: 06120018
A Cosmological Sector in Loop Quantum Gravity
Tim Koslowski (Würzburg)
A classical Hamiltonian system can be reduced to a subsystem of "relevant observables" using the pull-back under a Poisson embedding of the "relevant phase space" into the "full phase space". Since a quantum theory can be thought of a noncommutative phase space, one encounters the problem of the embedding of noncommutative spaces, when one tries to extend the reduction via a pull-back to a quantum theory. This problem can be solved for a class of physically interesting quantum systems and embeddings using an analogy to finding the base space of an embedded fibre bundle via the projection in the full fibre bundle. The resulting construction is then applied to Loop Quantum Gravity to extract a cosmological sector. This sector turns out to be similar but not equivalent to Loop Quantum Cosmology.
04/12/2006
PIRSA#: 06120019
Emergence of a Background From Background Independent Quantum Gravity
Willem Westra (Utrecht, Loll's group)
04/12/2006
*?*PIRSA#: 06120030
Dario Benedetti (Utrecht, Loll's group)
A Simplicial Path to the Quantum Hamiltonian of Gravity
06/12/2006
[my comment: this talk presents results in work by Loll and Benedetti which has not yet been posted on arxiv or published anywhere. It looks like Loll may have shifted emphasis from doing Monte Carlo simulations back to proving stuff analytically. This research is about the 2+1 dimensional case. Apparently they got an unexpected result. You can skip to the last two slides and start the talk there, near the end. Something problematical apparently came up, that they are trying to explain. I just saw this and can't comment. The talk starts slow, with review of the whole development of CDT, so it helps to look at the slide menu and decide where to jump in and start the talk. If you know some CDT already you don't have to start at the very beginning.]
***PIRSA#: 06120032
Andy Randono (Texas-Austin)
Generalizing the Kodama State
06/12/2006
==================
The menu gives a longer listing of PIRSA numbers for QG seminars (incl. Smolin, Bojowald,... other well-known speakers)
That's where you check the QG box here http://www.perimeterinstitute.ca/index.php?option=com_content&task=view&id=113&Itemid=167&p=presentations&with_msl=true
and click "do search". You don't have to type in anything. It's pretty easy. Here is a sample.
***PIRSA#:05090005
Shahn Majid
Noncommutative geometry and the origin of time
14/09/2005
***PIRSA#: 05110009
Sundance Bilson-Thompson
Topological Preon Models: a Braid New World
01/11/2005
http://arxiv.org/abs/gr-qc/0612071
Plebanski Theory and Covariant Canonical Formulation
Sergei Alexandrov, Eric Buffenoir, Philippe Roche
18 pages
"We establish an equivalence between the Hamiltonian formulation of the Plebanski action for general relativity and the covariant canonical formulation of the Hilbert-Palatini action. This is done by comparing the symplectic structures of the two theories through the computation of Dirac brackets. We also construct a shifted connection with simplified Dirac brackets, playing an important role in the covariant loop quantization program, in the Plebanski framework. Implications for spin foam models are also discussed."
http://arxiv.org/abs/gr-qc/0612070
Loop quantum cosmology and the k = - 1 RW model
Kevin Vandersloot
14 pages, 3 figures
"The loop quantization of the negatively curved k=-1 RW model poses several technical challenges. We show that the issues can be overcome and a successful quantization is possible that extends the results of the k=0,+1 models in a natural fashion. We discuss the resulting dynamics and show that for a universe consisting of a massless scalar field, a bounce is predicted in the backward evolution in accordance with the results of the k=0,+1 models. We also show that the model predicts a vacuum repulsion in the high curvature regime that would lead to a bounce even for matter with vanishing energy density. We finally comment on the inverse volume modifications of loop quantum cosmology and show that, as in the k=0 model, the modifications depend sensitively on the introduction of a length scale which a priori is independent of the curvature scale or a matter energy scale."
http://arxiv.org/abs/gr-qc/0612084
Evaporating loop quantum black hole
Leonardo Modesto
13 pages, 9 figures
"In this paper we obtain the black hole metric from a semiclassical analysis of loop quantum black hole. Our solution and the Schwarzschild one tend to match well at large distances from Planck region. In r=0 the semiclassical metric is regular and singularity free in contrast to the classical one. By using the new metric we calculate the Hawking temperature and the entropy. For the entropy we obtain the logarithmic correction to the classical area law. Finally we study the mass evaporation process and we show the mass and temperature tend to zero at infinitive time."
http://arxiv.org/abs/gr-qc/0612074
Counting entropy in causal set quantum gravity
D. Rideout, S. Zohren
5 pages, 1 figure. Talk given by S. Zohren at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006
"The finiteness of black hole entropy suggest that spacetime is fundamentally discrete, and hints at an underlying relationship between geometry and "information". The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We review recent attempts to define a microscopic measure for black hole entropy and for the maximum entropy of spherically symmetric spacelike regions, within the causal set approach to quantum gravity."
http://arxiv.org/abs/gr-qc/0612093
Can we see gravitational collapse in (quantum) gravity perturbation theory?
J. Kowalski-Glikman, A. Starodubtsev
23 pages
"In this paper, by making use of the perturbative expansion around topological field theory we are trying to understand why the standard perturbation theory for General Relativity, which starts with linearized gravity does not see gravitational collapse. We start with investigating classical equations of motion. For zero Immirzi parameter the ambiguity of the standard perturbative expansion is reproduced. This ambiguity is related to the appearance of the linearized diffeomorphism symmetry, which becomes unlinked from the original diffeomorphism symmetry. Introducing Immirzi parameter makes it possible to restore the link between these two symmetries and thus removes the ambiguity, but at the cost of making classical perturbation theory rather intractable. Then we argue that the two main sources of complexity of perturbation theory, infinite number of degrees of freedom and non-trivial curvature of the phase space of General Relativity could be disentangled when studying quantum amplitudes. As an illustration we consider zero order approximation in quantum perturbation theory. We identify relevant observables, and sketch their quantization. We find some indications that this zero order approximation might be described by Doubly Special Relativity."
http://arxiv.org/abs/gr-qc/0612101
Closed FRW model in Loop Quantum Cosmology
Lukasz Szulc, Wojciech Kaminski, Jerzy Lewandowski
19 pages
"The basic idea of the LQC applies to every spatially homogeneous cosmological model, however only the spatially flat (so called k=0) case has been understood in detail in the literature thus far. In the closed (so called: k=1) case certain technical difficulties have been the obstacle that stopped the development. In this work the difficulties are overcome, and a new LQC model of the spatially closed, homogeneous, isotropic universe is constructed. The topology of the spacelike section of the universe is assumed to be that of SU(2) or SO(3). Surprisingly, according to the results achieved in this work, the two cases can be distinguished from each other just by the local properties of the quantum geometry of the universe. The quantum hamiltonian operator of the gravitational field takes the form of a difference operator, where the elementary step is the quantum of the 3-volume derived in the flat case by Ashtekar, Pawlowski and Singh. The mathematical properties of the operator are studied: it is essentially self-adjoint, bounded from above by 0, the 0 itself is not an eigenvalue, the eigenvectors form a basis. An estimate on the dimension of the spectral projection on any finite interval is provided."
http://arxiv.org/abs/gr-qc/0612104
Loop quantum cosmology of k=1 FRW models
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh, Kevin Vandersloot
43 pages, 11 figures
IGPG-06/12-1
"The closed, k=1, FRW cosmology coupled to a massless scalar field is investigated in the framework of loop quantum cosmology using analytical and numerical methods. As in the k=0 case, the scalar field can be again used as emergent time to construct the physical Hilbert space and introduce Dirac observables. The resulting framework is then used to address a major challenge of quantum cosmology: resolving the big-bang singularity while retaining agreement with general relativity at large scales. It is shown that the framework fulfills this task. In particular, for states which are semi-classical at some late time, the big-bang is replaced by a quantum bounce and a recollapse occurs at the value of the scale factor predicted by classical general relativity. Thus, the `difficulties' pointed out by Green and Unruh in the k=1 case do not arise in a more systematic treatment. As in k=0 models, quantum dynamics is deterministic across the deep Planck regime. However, because it also retains the classical recollapse, in contrast to the k=0 case one is now led to a cyclic model. Finally, we clarify some issues raised by Laguna's recent work addressed to computational physicists."
http://arxiv.org/abs/gr-qc/0612111
Some comments on the universal constant in DSR
Florian Girelli, Etera R. Livine
12 pages, Proceedings of DICE2006 (Piombino, Italy)
"Deformed Special Relativity is usually presented as a deformation of Special Relativity accommodating a new universal constant, the Planck mass, while respecting the relativity principle. In order to avoid some fundamental problems (e.g. soccer ball problem), we argue that we should switch point of view and consider instead the Newton constant G as the universal constant."
http://arxiv.org/abs/hep-th/0612170
From noncommutative kappa-Minkowski to Minkowski space-time
Laurent Freidel, Jerzy Kowalski-Glikman, Sebastian Nowak
6 pages
"We show that free kappa-Minkowski space field theory is equivalent to a relativistically invariant, non local, free field theory on Minkowski space-time. The field theory we obtain has in spectrum a relativistic mode of arbitrary mass m and a Planck mass tachyon. We show that while the energy momentum for the relativistic mode is essentially the standard one, it diverges for the tachyon, so that there are no asymptotic tachyonic states in the theory. It also follows that the dispersion relation is not modified, so that, in particular, in this theory the speed of light is energy-independent."
http://arxiv.org/abs/hep-th/0612185
The status of cosmological natural selection
Lee Smolin
25 pages
"The problem of making predictions from theories that have landscapes of possible low energy parameters is reviewed. Conditions for such a theory to yield falsifiable predictions for doable experiments are given. It is shown that the hypothesis of cosmological natural selection satisfies these conditions, thus showing that it is possible to continue to do physics on a landscape without invoking the anthropic principle. In particular, this is true whether or not the ensemble of universes generated by black holes bouncing is a sub-ensemble of a larger ensemble that might be generated by a random process such as eternal inflation.
A recent criticism of cosmological natural selection made by Vilenkin in hep-th/0610051 is discussed. It is shown to rely on assumptions about both the infrared and ultraviolet behavior of quantum gravity that are very unlikely to be true."
http://arxiv.org/abs/gr-qc/0612144
Three Dimensional Loop Quantum Gravity: Particles and the Quantum Double
Karim Noui
36 pages, published in J. Math. Phys. 47, 102501 (2006)
"It is well known that the quantum double structure plays an important role in three dimensional quantum gravity coupled to matter field. In this paper, we show how this algebraic structure emerges in the context of three dimensional Riemannian loop quantum gravity (LQG) coupled to a finite number of massive spinless point particles. In LQG, physical states are usually constructed from the notion of SU(2) cylindrical functions on a Riemann surface Sigma and the Hilbert structure is defined by the Ashtekar-Lewandowski measure. In the case where Sigma is the sphere S^2, we show that the physical Hilbert space is in fact isomorphic to a tensor product of simple unitary representations of the Drinfeld double DSU(2): the masses of the particles label the simple representations, the physical states are tensor products of vectors of simple representations and the physical scalar product is given by intertwining coefficients between simple representations. This result is generalized to the case of any Riemann surface Sigma.
http://arxiv.org/abs/gr-qc/0612145
Three dimensional Loop Quantum Gravity: towards a self-gravitating Quantum Field Theory
Karim Noui
36 pages, published in Class. Quant. Grav. 24 (2007)
In a companion paper, we have emphasized the role of the Drinfeld double DSU(2) in the context of three dimensional Riemannian Loop Quantum Gravity coupled to massive spinless point particles. We make use of this result to propose a model for a self-gravitating quantum field theory (massive spinless non-causal scalar field) in three dimensional Riemannian space. We start by constructing the Fock space of the free self-gravitating field: the vacuum is the unique DSU(2) invariant state, one-particle states correspond to DSU(2) unitary irreducible simple representations and any multi-particles states is obtained as the symmetrized tensor product between simple representations. The associated quantum field is defined by the usual requirement of covariance under DSU(2). Then, we introduce a DSU(2)-invariant self-interacting potential (the obtained model is a Group Field Theory) and compute explicitely the lowest order terms (in the self-interaction coupling constant lambda) of the propagator and of the three-points function. Finally, we compute the lowest order quantum gravity corrections (in the Newton constant G) to the propagator and to the three-points function.
http://arxiv.org/abs/gr-qc/0612147
Spectral Analysis of the Volume Operator in Loop Quantum Gravity
J. Brunnemann, D. Rideout
5 pages, 1 figure. Talk given by D. Rideout at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006
Imperial/TP/2006/DR/02
"We describe preliminary results of a detailed numerical analysis of the volume operator as formulated by Ashtekar and Lewandowski. Due to a simplified explicit expression for its matrix elements, it is possible for the first time to treat generic vertices of valence greater than four. It is found that the vertex geometry characterizes the volume spectrum."
http://arxiv.org/abs/hep-th/0612280
Doubly Special Relativity at the age of six
Jerzy Kowalski-Glikman
To appear in the Proceedings of 22nd Max Born Symposium
"The current status of Doubly Special Relativity research program is shortly presented.
I dedicate this paper to my teacher and friend Professor Jerzy Lukierski on occasion of his seventieth birthday."
http://arxiv.org/abs/gr-qc/0612167
Deformed Special Relativity in Position Space
S. Hossenfelder
"We investigate how deformations of special relativity in momentum space can be extended to position space in a consistent way, such that the dimensionless contraction between wave-vector and coordinate-vector remains invariant. By using a parametrization in terms of an energy dependent speed of light, and an energy dependent Planck's constant, we are able to formulate simple requirements that completely determine the active transformations in position space. These deviate from the standard transformations for large velocities of the observed object. Some examples are discussed, and it is shown how the relativistic mass gain of a massive particle is affected. We finally study the construction of passive Lorentz-transformations."
http://arxiv.org/abs/gr-qc/0612170
The Ponzano-Regge model and Reidemeister torsion
John W. Barrett, Ileana Naish-Guzman
4 pages. MG11 conference proceedings
"The Ponzano-Regge model of three-dimensional quantum gravity is well-defined when the observables satisfy a certain condition involving the twisted cohomology. In this case, the partition function is defined in terms of the Reidemeister torsion. Some consequences for the special cases of planar graphs and knots are given."
Francesca flagged this one by Lucien Hardy of PI Waterloo
http://arxiv.org/abs/quant-ph/0701019
Quantum gravity computers: On the theory of computation with indefinite causal structure
A quantum gravity computer is one for which the particular effects of quantum gravity are relevant. In general relativity, causal structure is non-fixed. In quantum theory non-fixed quantities are subject to quantum uncertainty. It is therefore likely that, in a theory of quantum gravity, we will have indefinite causal structure. This means that there will be no matter of fact as to whether a particular interval is timelike or not. We study the implications of this for the theory of computation. Classical and quantum computations consist in ivolving the state of the computer through a sequence of time steps. This will, most likely, not be possible for a quantum gravity computer because the notion of a time step makes no sense if we have indefinite causal structure. We show that it is possible to set up a model for computation even in the absence of definite causal structure by using a certain framework (the causaloid formalism) that was developed for the purpose of correlating data taken in this type of situation. Corresponding to a physical theory is a causaloid, Lambda (this is a mathematical object containing information about the causal connections between different spacetime regions). A computer is given by the pair {Lambda, S} where S is a set of gates. Working within the causaloid formalism, we explore the question of whether universal quantum gravity computers are possible. We also examine whether a quantum gravity computer might be more powerful than a quantum (or classical) computer. In particular, we ask whether indefinite causal structure can be used as a computational resource.
http://arxiv.org/abs/quant-ph/0701097
The Free-Will Postulate in Quantum Mechanics
Gerard 't Hooft
8 pages, 1 figure
"The so-called 'free will axiom' is an essential ingredient in many discussions concerning hidden variables in quantum mechanics. In this paper we argue that 'free will' can be defined in different ways. The definition usually employed is clearly invalid in strictly deterministic theories. A different, more precise formulation is proposed here, defining a condition that may well be a more suitable one to impose on theoretical constructions and models. Our axiom, to be referred to as the `unconstrained initial state' condition, has consequences similar to 'free will', but does not clash with determinism, and appears to lead to different conclusions concerning causality and locality in quantum mechanics. Models proposed earlier by this author fall in this category. Imposing our 'unconstrained initial state' condition on a deterministic theory underlying Quantum Mechanics, appears to lead to a restricted free-will condition in the quantum system: an observer has the free will to modify the setting of a measuring device, but has no control over the phase of its wave function. The dismissal of the usual 'free will' concept does not have any consequences for our views and interpretations of human activities in daily life, and the way our minds function, but it requires a more careful discussion on what, in practice, free will actually amounts to."
http://arxiv.org/abs/hep-th/0701113
The Relativistic Particle: Dirac observables and Feynman propagator
Laurent Freidel, Florian Girelli, Etera R. Livine
14 pages
"We analyze the algebra of Dirac observables of the relativistic particle in four space-time dimensions. We show that the position observables become non-commutative and the commutation relations lead to a structure very similar to the non-commutative geometry of Deformed Special Relativity (DSR). In this framework, it appears natural to consider the 4d relativistic particle as a five dimensional massless particle. We study its quantization in terms of wave functions on the 5d light cone. We introduce the corresponding five-dimensional action principle and analyze how it reproduces the physics of the 4d relativistic particle. The formalism is naturally subject to divergences and we show that DSR arises as a natural regularization: the 5d light cone is regularized as the de Sitter space. We interpret the fifth coordinate as the particle's proper time while the fifth moment can be understood as the mass. Finally, we show how to formulate the Feynman propagator and the Feynman amplitudes of quantum field theory in this context in terms of Dirac observables. This provides new insights for the construction of observables and scattering amplitudes in DSR."
http://arxiv.org/abs/gr-qc/0701071
A signature of quantum gravity at the source of the seeds of cosmic structure?
Daniel Sudarsky
Prepared for the proceedings the Third International Workshop DICE 2006, "Quantum Mechanics between Decoherence and Determinism"
"This article reviews a recent work by a couple of colleagues and myself about the shortcomings of the standard explanations of the quantum origin of cosmic structure in the inflationary scenario, and a proposal to address them. The point it that in the usual accounts the inhomogeneity and anisotropy of our universe seem to emerge from an exactly homogeneous and isotropic initial state through processes that do not break those symmetries. We argued that some novel aspect of physics must be called upon to able to address the problem in a fully satisfactory way. The proposed approach is inspired on Penrose's ideas regarding an quantum gravity induced, real and dynamical collapse of the wave function."
It has been announced that this year Loops '07 conference will be in Mexico City (Sudarsky's home base) this year. I believe he is one of the conference organizers. I will keep special track of QG at Uni Mex, to keep prospective participants apprised of their hosts research interests.
CORRECTION:
LOOPS '07, this year's international conference on background independent (i.e. non-string) quantum gravity, will be at Morelia, about 200 miles west of Mexico City. Morelia is the site of the National University campus and several related institutes. Earlier I was under the mistaken impression that the conference was to be at Mexico City. The old city of Morelia has the reputation of being a picturesque place for sight-seeing, as well as having the University.
here is Francesca's post about the conference:
http://physicsforums.com/showthread.php?p=1212084#post1212084
here is the Morelia Math Institute website:
http://www.matmor.unam.mx/
it has this link to some tourist information in whichever language you prefer:
http://www.matmor.unam.mx/Morelia/espanol/default.html
http://www.matmor.unam.mx/Morelia/ingles/default.html
=========
this paper by Jonathan Engle appeared today:
http://arxiv.org/abs/gr-qc/0701132
On the physical interpretation of states in loop quantum cosmology
Jonathan Engle
20 pages
"In this paper we address the physical meaning of states in loop quantum cosmology (LQC). A first step in this is the completion of the program begun in [1], applied to LQC. Specifically, we introduce a family of (what are called) b-embeddings of isotropic loop quantum cosmology (LQC) into full loop quantum gravity. As a side note, we exhibit a large class of operators preserving each of these embeddings, and show their consistency with the LQC quantization. Embedding at the gauge and diffeomorphism invariant level is discussed in the conclusion section."
Engle got his PhD at Penn State in 2006 and is now in Rovelli's group at Marseille (CPT Luminy)
====================
http://arxiv.org/abs/hep-th/0701239
Loop quantum gravity and black hole singularity
Leonardo Modesto
17 pages, 7 figures. Proceedings of the XVII SIGRAV Conference, Turin, September 4-7, 2006
"In this paper we summarize loop quantum gravity (LQG) and we show how ideas developed in LQG can solve the black hole singularity problem when applied to a minisuperspace model."
=====================
The January 2007 KITP workshop on The Quantum Nature of Spacetime Singularities menu of talks online is here:
http://online.kitp.ucsb.edu/online/singular_m07/
Videos of the talks are available for download.
Some of the talks which may be of special interest to people concentrating on background independent (non-string) QG are those of
Ashtekar
http://online.kitp.ucsb.edu/online/singular_m07/ashtekar/
Thiemann (first hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann/
Thiemann (second hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann1/
Bojowald
http://online.kitp.ucsb.edu/online/singular_m07/bojowald/
===================
http://arxiv.org/abs/astro-ph/0701584
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
Edward L. Wright (UCLA)
16 pages, 8 figure
"The luminosity distance vs. redshift law is now measured using supernovae and gamma ray bursts, and the angular size distance is measured at the surface of last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In this paper this data is fit to models for the equation of state with w = -1, w = const, and w(z) = w_0+w_a(1-a). The last model is poorly constrained by the distance data, leading to unphysical solutions where the dark energy dominates at early times unless the large scale structure and acoustic scale constraints are modified to allow for early time dark energy effects. A flat LambdaCDM model is consistent with all the data."
Note discussion section page 14:
"Using all the data together
gives the plot shown in Figure 5. The best fit model is slightly closed with
Omega_tot = 1.011 and M = 0.315. "
===============
http://arxiv.org/abs/gr-qc/0701140
New Energy Definition for Higher Curvature Gravities
S. Deser, Bayram Tekin
4 pages
"We propose a novel but natural definition of conserved quantities for gravity models quadratic and higher in curvature. Based on the spatial asymptotics of curvature rather than of metric, it avoids the GR energy machinery's more egregious problems--such as zero energy 'theorems' and failure in flat backgrounds -- in this fourth-derivative realm. In D>4, the present expression indeed correctly discriminates between second derivative Gauss-Bonnet and generic, fourth derivative, actions."
http://arxiv.org/abs/gr-qc/0701142
Quantum gravity and cosmological observations
Martin Bojowald
8 pages, plenary talk at the VIth Latin American Symposium on High Energy Physics (Puerto Vallarta, Mexico, Nov. 2006)
"Quantum gravity places entirely new challenges on the formulation of a consistent theory as well as on an extraction of potentially observable effects. Quantum corrections due to the gravitational field are commonly expected to be tiny because of the smallness of the Planck length. However, a consistent formulation now shows that key features of quantum gravity imply magnification effects on correction terms which are especially important in cosmology with its long stretches of evolution. After a review of the salient features of recent canonical quantizations of gravity and their implications for the quantum structure of space-time a new example for potentially observable effects is given."
a possibly interesting challenge to LQC:
http://arxiv.org/abs/gr-qc/0701172
On the Onset of Inflation in Loop Quantum Cosmology
Cristiano Germani (SISSA), William Nelson (KCL London), Mairi Sakellariadou (KCL London)
18 pages, 5 figures
"Using a Liouville measure, similar to the one proposed recently by Gibbons and Turok, we investigate the probability that single-field inflation with a polynomial potential can last long enough to solve the shortcomings of the standard hot big bang model, within the semiclassical regime of loop quantum cosmology. We conclude that, for such a class of inflationary models and for natural values of the loop quantum cosmology parameters, a successful inflationary scenario is highly improbable."
http://arxiv.org/abs/astro-ph/0701848
The modified Newtonian dynamics-MOND-and its implications for new physics
Jacob D. Bekenstein
Pedagogical introduction to MOND and review of modern developments. To appear shortly in Contemporary Physics; 6 figures, 15 pages
"No more salient issue exists in contemporary astrophysics and cosmology than that of the elusive 'dark matter'. For many years already Milgrom's paradigm of modified Newtonian dynamics (MOND) has provided an alternative way to interpret observations without appeal to invisible dark matter. MOND had been successful in elucidating economically the dynamics of disk galaxies of all scales, while doing less well for clusters of galaxies; in its original form it could not address gravitational lensing or cosmology. After reviewing some of the evidence in favor of MOND, I recollect the development of relativistic formulations for it to cope with the last deficiency. I comment on recent work by various groups in confronting TeVeS, a relativistic embodiment of MOND, with observational data on gravitational lensing and cosmology. Throughout I ask what sort of physics can be responsible for the efficacy of MOND, and conclude with an appraisal of what theoretical developments are still needed to reach a full description of the world involving no unobserved matter."
http://arxiv.org/abs/gr-qc/0702002
Effect of the Inverse Volume Modification in Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages
"It is known that in loop quantum cosmology the universe avoids the singularity by a bounce when the matter density approaches the critical density \rho_c (the order of Planck density). In the framework of the effective Hamiltonian incorporating the inverse volume modifications both in the gravitational and matter part, we find that two modifications play different roles for the effective dynamics (the modified Friedmann equation). In the case a_{g*}>a_{bounce}, a_{bounce} denotes the bounce scale and the scale a_{g*} marked the different region below which the modification in the gravitational part become notable, the modification in the gravitational part decrease the matter density at the bounce point such that \rho_{bounce}<\rho _c. For the case a_{m*}>a_{bounce} (the role of a_{m*} is same as a_{g*}), the modification in the matter part helps the matter density to approach the critical density \rho_c quickly. Furthermore, based on the physical reason it is suitable to neglect the inverse volume modification in the gravitational part."
http://arxiv.org/abs/gr-qc/0702003
Tachyon field in Loop Quantum Cosmology: inflation and evolution picture
Hua-Hui Xiong, Jian-Yang Zhu
7 pages, 3 figures
"Loop quantum cosmology (LQC) predicts a nonsingular evolution of the universe through a bounce in the high energy region. We show that this is always true in tachyon matter LQC. Different from the classical FRW cosmology, the superinflation can appear in the tachyon matter LQC; furthermore, the inflation can be extended to the region where classical inflation stops. Using numerical method, we give an evolution picture of the tachyon field with an exponential potential in the context of LQC. It indicates that the quantum dynamical solutions have the attractor behavior as the classical solutions does. And, the whole evolution of the tachyon field is that: at the far past, the tachyon field, being in the contracting cosmology, is accelerated to climb up the potential hill with a negative velocity; and then, the tachyon field at the boundary is bounced into an expanding universe with positive velocity rolling down to the bottom of the potential."
http://arxiv.org/abs/astro-ph/0702010
Universe's Skeleton Sketched
Eric V. Linder
News & Views, Nature 445, 273 (2007)
"The deepest and clearest maps yet of the Universe's skeleton of dark matter structure present a picture broadly in concord with favoured models - although puzzling discrepancies remain."
http://arxiv.org/abs/hep-th/0702016
Multi-Particle States in Deformed Special Relativity
Sabine Hossenfelder
"We investigate the properties of multi-particle states in Deformed Special Relativity (DSR). Starting from the Lagrangian formalism with an energy dependent metric, the conserved Noether current is derived and is found to be additive in the usual way. This quantity had previously been discarded because it was correctly realized that it does no longer obey the DSR transformations. We identify the reason for this mismatch in the fact that DSR depends only on the extensive quantity of total four-momentum instead of the energy-momentum densities as would be appropriate for a field theory. We argue that the reason for the failure of DSR to reproduce the standard transformation behavior in the well established limits is due to the missing sensitivity to the volume inside which energy is accumulated. We show that the soccer-ball problem is absent if one formulates DSR instead for the field densities. As a consequence, estimates for predicted effects have to be corrected by many orders of magnitude. Further, we derive that the modified quantum field theory implies a locality bound."
=========================
briefly noted:
http://arxiv.org/abs/gr-qc/0702023
Evolutionary Reformulation of Quantum Gravity
Giovanni Montani
5 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin
"We present a critical analysis of the Canonical approach to quantum gravity, which relies on the ambiguity of implementing a space-time slicing on the quantum level. We emphasize that such a splitting procedure is consistent only if a real matter fluid is involved in the dynamics."
http://arxiv.org/abs/gr-qc/0702025
Electro-weak Model within a 5-dimensional Lorentz group theory
Orchidea Maria Lecian, Giovanni Montani
3 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin
"The Electroweak model will be geometrized in a 5-D Riemann-Cartan framework: U(1) weak hyper-charge group will be worked out in a Klauza-Klein scheme, while SU(2) weak isospin group will be identified to suitable bein projections of the contortion field. The possibility of introducing Ashtekar formalism in 5-D Klauza-Klein theories will be investigated."
http://arxiv.org/abs/gr-qc/0702026
Electro-weak Model within the framework of Lorentz gauge theory: Ashtekar variables?
Orchidea Maria Lecian, Giovanni Montani
5 pages, to appear in the Proceedings of the 1st Stueckelberg Workshop
"The Electroweak (EW) model is geometrized in the framework of a 5D gauge theory of the Lorentz group, after the implementation of the Kaluza-Klein (KK) paradigm. The possibility of introducing Ashtekar variables on a 5D KK manifold is considered on the ground of its geometrical structure."
http://arxiv.org/abs/gr-qc/0702030
An Introduction to Loop Quantum Gravity Through Cosmology
Abhay Ashtekar
20 pages, 4 figures, Introductory Review
"This introductory review is addressed to beginning researchers. Some of the distinguishing features of loop quantum gravity are illustrated through loop quantum cosmology of FRW models. In particular, these examples illustrate: i) how 'emergent time' can arise; ii) how the technical issue of solving the Hamiltonian constraint and constructing the physical sector of the theory can be handled; iii) how questions central to the Planck scale physics can be answered using such a framework; and, iv) how quantum geometry effects can dramatically change physics near singularities and yet naturally turn themselves off and reproduce classical general relativity when space-time curvature is significantly weaker than the Planck scale."
this paper of Ashtekar's grew out of a talk he gave in Summer 2006 at the First Stueckelberg Workshop at Pescara, italy.
http://www.icra.it/ICRA_Networkshops/INw20_Stueckelberg/Welcome.htm
http://arxiv.org/abs/astro-ph/0702146
The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
J. R. Brownstein, J. W. Moffat
25 pages, 14 figures, 5 tables
"A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006)."
http://arxiv.org/abs/math.QA/0702140
Lectures on Noncommutative Geometry
Masoud Khalkhali
112 pages
"This text is an introduction to a few selected areas of Alain Connes' noncommutative geometry written for the volume of the school/conference 'Noncommutative Geometry 2005' held at IPM Tehran. It is an expanded version of my lectures which was directed at graduate students and novice in the subject."
http://arxiv.org/abs/gr-qc/0702044
Disordered locality in loop quantum gravity states
Fotini Markopoulou, Lee Smolin
11 pages, 4 figures
"We show that loop quantum gravity suffers from a potential problem with non-locality, coming from a mismatch between micro-locality, as defined by the combinatorial structures of their microscopic states, and macro-locality, defined by the metric which emerges from the low energy limit. As a result, the low energy limit may suffer from a disordered locality characterized by identifications of far away points. We argue that if such defects in locality are rare enough they will be difficult to detect."
http://arxiv.org/abs/hep-th/0702051
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
M. Reuter, H. Weyer
18 pages, 4 figures. Invited contribution to the Int. J. Mod. Phys. D special issue on dark matter and dark energy
"The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that at large distances there could be strong renormalization effects, including a scale dependence of Newton's constant, which mimic the presence of dark matter at galactic and cosmological scales."
http://arxiv.org/abs/gr-qc/0702036
Phase-space and Black Hole Entropy of Toroidal Horizons in Loop Quantum Gravity
S. Kloster, J. Brannlund, A. DeBenedictis
14 pages, 6 figures
"In the context of loop quantum gravity, we construct the phase-space of the isolated horizon with toroidal topology. Within the loop quantum gravity framework, this horizon is described by a torus with N punctures and the dimension of the corresponding phase-space is calculated including the toroidal cycles as degrees of freedom. From this, the black hole entropy can be calculated by counting the microstates which correspond to a black hole of fixed area. We find that the leading term agrees with the A/4 law and that the sub-leading contribution is modified by the toroidal cycles."
http://arxiv.org/abs/hep-th/0702064
A fine tuning free resolution of the cosmological constant problem
Stephon Alexander, Deepak Vaid
5 pages, 2 figures
"In a recent paper we discovered that a fermionic condensate is formed from gravitational interactions due to the covariant coupling of fermions in the presence of a torsion-fermion contact interaction. The condensate gap gives a negative contribution to the bare cosmological constant. In this letter, we show that the cosmological constant problem can be solved without fine tuning of the bare cosmological constant. We demonstrate how a universe with a large initial cosmological constant undergoes inflation, during which time the energy gap grows as the volume of the universe. Eventually the gap becomes large enough to cancel out the bare cosmological term, inflation ends and we end up in a universe with an almost vanishing cosmological term. We provide a detailed numerical analysis of the system of equations governing the self regulating relaxation of the cosmological constant."
http://arxiv.org/abs/gr-qc/0702049
Canonical quantization and the spectral action, a nice example
Fabien Besnard (CPL)
22 pages. Revised version submitted to Journal of Geometry and Physics
"We study the canonical quantization of the theory given by Chamseddine-Connes spectral action on a particular finite spectral triple with algebra M_2(C)\oplus C. We define a quantization of the natural distance associated with this noncommutative space and show that the quantum distance operator has a discrete spectrum. We also show that it would be the same for any other geometric quantity. Finally we propose a physical Hilbert space for the quantum theory. This spectral triple had been previously considered by Rovelli as a toy model, but with a different action which was not gauge-invariant. The results are similar in both cases, but the gauge-invariance of the spectral action manifests itself by the presence of a non-trivial degeneracy structure for our distance operator."
http://arxiv.org/abs/astro-ph/0702207
It's Never Too Late For Matter
Lawrence M. Krauss (Case Western Reserve University and Vanderbilt University), Robert J. Scherrer (Vanderbilt University)
submitted to PRL
"We demonstrate that in a vacuum-energy-dominated expansion phase, neither the decay of matter nor matter-antimatter annihilation into relativistic particles can ever cause radiation to once again dominate over matter in the future history of the universe."
MeJennifer
Feb8-07, 08:36 PM
http://arxiv.org/abs/quant-ph/0611261
Concerning Dice and Divinity
D.M.Appleby
Contribution to proceedings of Foundations of Probability and Physics, Vaxjo, 2006
"Einstein initially objected to the probabilistic aspect of quantum mechanics - the idea that God is playing at dice. Later he changed his ground, and focussed instead on the point that the Copenhagen Interpretation leads to what Einstein saw as the abandonment of physical realism. We argue here that Einstein's initial intuition was perfectly sound, and that it is precisely the fact that quantum mechanics is a fundamentally probabilistic theory which is at the root of all the controversies regarding its interpretation. Probability is an intrinsically logical concept. This means that the quantum state has an essentially logical significance. It is extremely difficult to reconcile that fact with Einstein's belief, that it is the task of physics to give us a vision of the world apprehended sub specie aeternitatis. Quantum mechanics thus presents us with a simple choice: either to follow Einstein in looking for a theory which is not probabilistic at the fundamental level, or else to accept that physics does not in fact put us in the position of God looking down on things from above. There is a widespread fear that the latter alternative must inevitably lead to a greatly impoverished, positivistic view of physical theory. It appears to us, however, that the truth is just the opposite. The Einsteinian vision is much less attractive than it seems at first sight. In particular, it is closely connected with philosophical reductionism."
It seems to me that there is a third option.
Assuming that a measurement in a given physical reality can be seen as a higher order self-expression, I don't consider it unreasonable that there would be information loss.
It seems to me that there is a third option.
Assuming that a measurement in a given physical reality can be seen as a higher order self-expression, I don't consider it unreasonable that there would be information loss.
Hi Jennifer, this thread is unfortunately not for discussion---I just collect non-string QG links here. We normally start discussion of papers in a separate discussion thread, not to overcrowd this one.
So I will set up a discussion thread for you, so that you and others can comment on Appleby's paper.
MeJennifer
Feb8-07, 09:25 PM
Ok, no problem. Thanks for setting up a new topic. :smile:
You are most welcome! Here is a link to the discussion thread
http://physicsforums.com/showthread.php?t=155348
http://physicsforums.com/showthread.php?p=1238609#post1238609
http://arxiv.org/abs/gr-qc/0702065
Some Implications of the Cosmological Constant to Fundamental Physics
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
15 pages, lecture presented at the "XIIth Brazilian School of Cosmology and Gravitation", Mangaratiba, Rio de Janeiro, September 10-23, 2006
"In the presence of a cosmological constant, ordinary Poincaré special relativity is no longer valid and must be replaced by a de Sitter special relativity, in which Minkowski space is replaced by a de Sitter spacetime. In consequence, the ordinary notions of energy and momentum change, and will satisfy a different kinematic relation. Such a theory is a different kind of a doubly special relativity. Since the only difference between the Poincaré and the de Sitter groups is the replacement of translations by certain linear combinations of translations and proper conformal transformations, the net result of this change is ultimately the breakdown of ordinary translational invariance. From the experimental point of view, therefore, a de Sitter special relativity might be probed by looking for possible violations of translational invariance. If we assume the existence of a connection between the energy scale of an experiment and the local value of the cosmological constant, there would be changes in the kinematics of massive particles which could hopefully be detected in high-energy experiments. Furthermore, due to the presence of a horizon, the usual causal structure of spacetime would be significantly modified at the Planck scale."
http://arxiv.org/abs/gr-qc/0606122
de Sitter special relativity
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
24 pages, to be published in Classical and Quantum Gravity
"A special relativity based on the de Sitter group is introduced, which is the theory that might hold up in the presence of a non-vanishing cosmological constant. Like ordinary special relativity, it retains the quotient character of spacetime, and a notion of homogeneity. As a consequence, the underlying spacetime will be a de Sitter spacetime, whose associated kinematics will differ from that of ordinary special relativity. The corresponding modified notions of energy and momentum are obtained, and the exact relationship between them, which is invariant under a re-scaling of the involved quantities, explicitly exhibited. Since the de Sitter group can be considered a particular deformation of the Poincaré group, this theory turns out to be a specific kind of deformed (or doubly) special relativity. Some experimental consequences, as well as the causal structure of spacetime--modified by the presence of the de Sitter horizon--are briefly discussed."
http://arxiv.org/abs/astro-ph/0702275
Escaping from MOND
Benoit Famaey, Jean-Philippe Bruneton, HongSheng Zhao
4 pages, 1 figure, submitted to MNRAS
"We present a new test of modified Newtonian dynamics (MOND) on galactic scales, based on the escape speed in the solar neighbourhood. This test is independent from other empirical successes of MOND at reproducing the phenomenology of galactic rotation curves. The galactic escape speed in MOND is entirely determined by the baryonic content of the Galaxy and the external field in which it is embedded. We estimate that the external field in which the Milky Way must be embedded to produce the observed local escape speed of 544 km/s is of the order of a_0/100, where a_0 is the dividing acceleration scale below which gravity is boosted in MOND. This is compatible with the external gravitational field actually acting on the Milky Way."
http://arxiv.org/abs/astro-ph/0702298
Missing Pages in Our Photo Album of the Infant Universe
Abraham Loeb (Harvard)
Comments: Popular level review, to appear in "Physica Plus" magazine, Vol. 8; a modified version with original artwork appeared in Scientific American, 295, 46, 2006---see this http URL:
http://cfa-www.harvard.edu/~loeb/sciam.pdf
"Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see. In this popular-level overview, I describe how astronomers plan to observe this nearly-invisible yet crucial period."
http://arxiv.org/abs/hep-th/0702124
Lorentz violation and perpetual motion
Christopher Eling, Brendan Z. Foster, Ted Jacobson, Aron C. Wall
5 pages, 1 figure
"We show that any Lorentz violating theory with two or more propagation speeds is in conflict with the generalized second law of black hole thermodynamics. We do this by identifying a classical energy-extraction method, analogous to the Penrose process, which would decrease the black hole entropy. Although the usual definitions of black hole entropy are ambiguous in this context, we require only very mild assumptions about its dependence on the mass. This extends the result found by Dubovsky and Sibiryakov, which uses the Hawking effect and applies only if the fields with different propagation speeds interact just through gravity. We also point out instabilities that could interfere with their black hole perpetuum mobile, but argue that these can be neglected if the black hole mass is sufficiently large."
Watch out for anything by Ted Jacobson. This is part of a cluster of recent papers dispelling hopes that GLAST might observe gammaray dispersion (energy dependent speed). There was one by Jerzy K-G and one that Laurent Freidel co-authored I think with K-G, and one by Bee Hossenfelder, and now this by Jacobson et al.
http://arxiv.org/abs/gr-qc/0702094
Black Hole Entropy and the Problem of Universality
Steven Carlip
10 pages; talk at DICE 2006, Piombino, Italy
"A key test of any quantum theory of gravity is its ability to reproduce the known thermodynamic properties of black holes. A statistical mechanical description of the Bekenstein-Hawking entropy once seemed remote, but today we suffer an embarrassment of riches: many different approaches to quantum gravity yield the same entropy, despite counting very different states. This 'universality' suggests that some underlying feature of the classical theory may control the quantum density of states. I discuss the possibility that this feature is an approximate two-dimensional conformal symmetry near the horizon."
http://arxiv.org/abs/gr-qc/0702093
Gauge invariant perturbations around symmetry reduced sectors of general relativity: applications to cosmology
Bianca Dittrich, Johannes Tambornino
39 pages, 1 figure
"We develop a gauge invariant canonical perturbation scheme for perturbations around symmetry reduced sectors in generally covariant theories, such as general relativity. The central objects of investigation are gauge invariant observables which encode the dynamics of the system. We apply this scheme to perturbations around a homogeneous and isotropic sector (cosmology) of general relativity. The background variables of this homogeneous and isotropic sector are treated fully dynamically which allows us to approximate the observables to arbitrary high order in a self--consistent and fully gauge invariant manner. Methods to compute these observables are given. The question of backreaction effects of inhomogeneities onto a homogeneous and isotropic background can be addressed in this framework. We illustrate the latter by considering homogeneous but anisotropic Bianchi--I cosmologies as perturbations around a homogeneous and isotropic sector."
Bianca Dittrich has worked with both Thomas Thiemann and Renate Loll (as the junior author) and has clearly aided TT's program. He keeps on citing a solo paper she did which has turned out valuable. I think her method of operation is to do contributory research that she sees needs to be done---to crack some difficult technical obstacle in somebody else's path. So although I don't immediately see where this paper fits in, i guess it's a significant piece is somebody's jigsaw.
http://arxiv.org/abs/gr-qc/0702082
Cosmic clocks, cosmic variance and cosmic averages
David L. Wiltshire
72 pages, 5 figures
"Cosmic acceleration is explained quantitatively, purely in general relativity, as an apparent effect due to quasilocal gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. "Dark energy" is recognised as a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localised, namely gradients in the energy associated with the expansion of space and spatial curvature variations in an inhomogeneous universe, as we observe. Gravitational energy differences between observers in bound systems, such as galaxies, and volume-averaged comoving locations within voids in freely expanding space can be so large that the time dilation between the two significantly affects the parameters of any effective homogeneous isotropic model one fits to the universe. A new approach to cosmological averaging is presented, which implicitly solves the Sandage-de Vaucouleurs paradox. When combined with a nonlinear scheme for cosmological evolution with back-reaction via the Buchert equations, a new observationally viable quantitative model of the universe is obtained. The expansion age is increased, allowing more time for structure formation. The baryon density fraction obtained from primordial nucleosynthesis bounds can be significantly larger, yet consistent with primordial lithium abundance measurements. The angular scale of the first Doppler peak in the CMB anisotropy spectrum fits the new model despite an average negative spatial curvature at late epochs, resolving the anomaly associated with ellipticity in the CMB anisotropies. A number of other testable consequences are discussed, with the potential to profoundly change the whole of theoretical and observational cosmology. [Abridged] "
Special thanks to Kea, who is mentioned in the acknowledgments :-), for calling attention to this new article by David Wiltshire.
http://www.arxiv.org/abs/hep-ex/0702026
Charting the Course for Elementary Particle Physics
Burton Richter
AAAS National Meeting, San Francisco, 2007, Symposium, A New Frontier in Particle Physics, 15 pages, 8 figures
SLAC-PUB-12345
"It was the best of times; it was the worst of times is the way Dickens begins the Tale of Two Cities. The line is appropriate to our time in particle physics. It is the best of times because we are in the midst of a revolution in understanding, the third to occur during my career. It is the worst of times because accelerator facilities are shutting down before new ones are opening, restricting the opportunity for experiments, and because of great uncertainty about future funding. My task today is to give you a view of the most important opportunities for our field under a scenario that is constrained by a tight budget. It is a time when we cannot afford the merely good, but must give first priority to the really important."
Today (20 Feb) Martin Bojowald gave a seminar talk for the ILQGS in which he summarized the 3-week workshop on
The Quantum Nature of Spacetime Singularities at Santa Barbara KITP
The lecture notes are here:
http://relativity.phys.lsu.edu/ilqgs/
http://relativity.phys.lsu.edu/ilqgs/bojowald022007.pdf
I have not yet encountered the audio for the talk. The ILQGS webpage has an error, a different talk was given on 13 Feb.
http://arxiv.org/abs/gr-qc/0702107
Black Hole Thermodynamics from Euclidean Horizon Constraints
S. Carlip
4 pages
"To explain black hole thermodynamics in quantum gravity, one must introduce constraints to ensure that a black hole is actually present. I show that for a large class of black holes, the presence of such 'horizon constraints' makes it possible to use conformal field theory techniques to compute the density of states, reproducing the Bekenstein-Hawking entropy in a nearly model-independent manner. I argue that the relevant degrees of freedom may be Goldstone-boson-like excitations arising from the weak breaking of symmetry by the horizon constraints."
The second paper by Steve Carlip on BH entropy this week.
http://arxiv.org/abs/hep-th/0702159
Deformed Special Relativity in a Canonical Framework
Subir Ghosh (Indian Statistical Institute, India), Probir Pal (Uluberia College, India)
17 pages
"In this paper we have studied the nature of kinematical and dynamical laws in \kappa-Minkowski spacetime from a new perspective: the canonical phase space approach. We have introduced a new form of \kappa-Minkowski phase space algebra from which we recover the \kappa-extended finite Lorentz transformations derived in [13]. This is a particular form of a Deformed Special Relativity model that admits a modified energy-momentum dispersion law as well as noncommutative \kappa-Minkowski phase space. We show that this system can be completely mapped to a set of phase space variables that obey canonical (and not \kappa-Minkowski) phase space algebra and Special Relativity Lorentz transformation (and not \kappa-extended Lorentz transformation). We demonstrate the usefulness and simplicity of this approach through a number of applications both in classical and quantum mechanics. We also construct a Lagrangian for the \kappa-particle."
William Donnelly has started a blog called "Uncommon Information"
and one of the first posts called attention to a 1995 paper by Ted Jacobson and a related 2006 workshop talk he gave at Perimeter
http://arxiv.org/abs/gr-qc/9504004
Thermodynamics of Spacetime: The Einstein Equation of State
Ted Jacobson
8 pages, 1 figure.
Phys.Rev.Lett. 75 (1995) 1260-1263
"The Einstein equation is derived from the proportionality of entropy and horizon area together with the fundamental relation \delta Q=TdS connecting heat, entropy, and temperature. The key idea is to demand that this relation hold for all the local Rindler causal horizons through each spacetime point, with \delta Q and T interpreted as the energy flux and Unruh temperature seen by an accelerated observer just inside the horizon. This requires that gravitational lensing by matter energy distorts the causal structure of spacetime in just such a way that the Einstein equation holds. Viewed in this way, the Einstein equation is an equation of state. This perspective suggests that it may be no more appropriate to canonically quantize the Einstein equation than it would be to quantize the wave equation for sound in air."
Donnelly's blog is here:
http://williamdonnelly.blogspot.com/
and the post about T.J.'s Perimeter workshop talk is here:
http://williamdonnelly.blogspot.com/2006/09/natural-ultraviolet-cutoffs-in_09.html
I couldn't find a PIRSA video of this talk. the date would have been Wednesday 6 September 2006.
Sounds interesting both from Donnelly's summary and from the related 1995 paper----you assume that all causal horizons have the famous S = A/4 relation between horizon area and entropy and from that simple relation you can derive the Einstein equation of GR. The roots of intuition that grew out of must be the most interesting thing about it.
============================
there is a new blog about NonCommutative Geometry
http://noncommutativegeometry.blogspot.com/2007/02/rosetta-stone-of-noncommutative.html
It just started in the past month or so. One of the people posting is Masoud Khalkhali.
Alain Connes submitted a comment to the blog recently.
There is also a new journal called Journal of Non-Commutative Geometry (JNCG)
http://www.ems-ph.org/journals/jncg/jncg.php
I see that Marc Rieffel and Vaughn Jones are on the editorial board. Connes is the journal's top editor.
The first issue of JNCG appeared this month.
NCG and Connes are especially notable now because there is an actual prediction that the higgs boson mass will be about 170 GeV.
http://resonaances.blogspot.com/2007/02/alain-connes-standard-model.html
http://www.physicsforums.com/showthread.php?t=127342
http://www.sciam.com/print_version.cfm?articleID=00039831-4051-14C0-AFE483414B7F4945
Here are PF predictions---Jim Graber set up a poll
http://physicsforums.com/showthread.php?t=147265
briefly noted:
http://arxiv.org/abs/hep-th/0702178
Eternal inflation and its implications
Alan H. Guth
21 pages, 5 figures. Talk presented at the "2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC 2006)," Barcelona, Spain, 11-15 July 2006, to be published in J. Phys. A
http://math.ucr.edu/home/baez/week246.html
John Baez TWF 246 discusses issues raised in
the Smolin book The Trouble with Physics...and What Comes Next and the Woit book Not Even Wrong
Thanks to Christine Dantas for calling attention to the new JB essay!
http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages
"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."
http://arxiv.org/abs/gr-qc/0702125
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
Winston Fairbairn, Etera R. Livine
17 pages, 1 figure
"An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models in hep-th/0512113. In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recasted as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of hep-th/0512113 for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which we propose to interpret as living on curved space-times. We conclude by discussing the possible extension to four-dimensional spinfoam models."
http://arxiv.org/abs/gr-qc/0702132
A generalized Schroedinger equation for loop quantum cosmology
D. C. Salisbury, A. Schmitz
5 pages, to appear in the Proceedings of the Eleventh Marcel Grossmann Meeting
"A temporally discrete Schroedinger time evolution equation is proposed for isotropic quantum cosmology coupled to a massless scalar source. The approach employs dynamically determined intrinsic time and produces the correct semiclassical limit."
http://arxiv.org/abs/astro-ph/0702670
Dynamical Dark Energy or Simply Cosmic Curvature?
Chris Clarkson, Marina Cortes, Bruce A. Bassett
5 pages, 1 figure
"We show that the assumption of a flat universe induces critically large errors in reconstructing the dark energy equation of state at z>~0.9 even if the true cosmic curvature is very small, O(1%) or less. The spuriously reconstructed w(z) shows a range of unusual behaviour, including crossing of the phantom divide and mimicking of standard tracking quintessence models. For 1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching (50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble rate measurements have opposite trends due to the asymmetric influence of the curved geodesics. These results show that including curvature as a free parameter is imperative in any future analyses attempting to pin down the dynamics of dark energy, especially at moderate or high redshifts."
briefly noted:
http://arxiv.org/abs/hep-th/0702192
The Universe as a topological defect
Andres Anabalon, Steven Willison, Jorge Zanelli
http://arxiv.org/abs/hep-th/0702115
Predicting the Cosmological Constant from the Causal Entropic Principle
Raphael Bousso, Roni Harnik, Graham D. Kribs, Gilad Perez
http://arxiv.org/abs/gr-qc/0702144
Singularities and Quantum Gravity
Martin Bojowald
41 pages, lecture course at the XIIth Brazilian School on Cosmology and Gravitation, September 2006
IGPG-07/2-4, NSF-KITP-07-19
"Although there is general agreement that a removal of classical gravitational singularities is not only a crucial conceptual test of any approach to quantum gravity but also a prerequisite for any fundamental theory, the precise criteria for non-singular behavior are often unclear or controversial. Often, only special types of singularities such as the curvature singularities found in isotropic cosmological models are discussed and it is far from clear what this implies for the very general singularities that arise according to the singularity theorems of general relativity. In these lectures we present an overview of the current status of singularities in classical and quantum gravity, starting with a review and interpretation of the classical singularity theorems. This suggests possible routes for quantum gravity to evade the devastating conclusion of the theorems by different means, including modified dynamics or modified geometrical structures underlying quantum gravity. The latter is most clearly present in canonical quantizations which are discussed in more detail. Finally, the results are used to propose a general scheme of singularity removal, quantum hyperbolicity, to show cases where it is realized and to derive intuitive semiclassical pictures of cosmological bounces."
briefly noted:
http://arxiv.org/abs/hep-th/0702219
String Theory: Progress and Problems
John H. Schwarz
14 pages; Presented at the Yukawa--Tomonaga Centennial Symposium
CALT-68-2627
"...This talk will give an overview of some of the progress and some of the unsolved problems that characterize string theory today..."
http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations
Kirill Krasnov
21 pages
"We describe and study a certain class of modified gravity theories. Our starting point is Plebanski formulation of gravity in terms of a triple of 2-forms, a connection A and a 'Lagrange multiplier' field Psi. The generalization we consider stems from presence in the action of an extra term proportional to a scalar function of Psi. As in the usual Plebanski general relativity (GR) case, the equations coming from variations with respect to Psi imply that a certain metric can be introduced. However, unlike in GR, the connection A no longer coincides with the self-dual part of the metric-compatible spin-connection. Field equations of the theory are shown to be relations between derivatives of the metric and components of field Psi, as well as its derivatives, the later being in contrast to the GR case. The equations are of second order in derivatives. An analog of the Bianchi identity is still present in the theory, as well as its contracted version tantamount to energy conservation equation. The arising modifications to the later are possibly of experimental significance."
http://arxiv.org/abs/gr-qc/0703010
Effective Dynamics for the Cosmological Bounces in Bianchi Type I Loop Quantum Cosmology
Dah-Wei Chiou
7 pages, 4 figures
IGPG-07/2-5
"The detailed formulation for loop quantum cosmology (LQC) in Bianchi I models was recently constructed. In this paper, the effective dynamics with the LQC discreteness corrections is studied and exactly solved, showing that the big bang is replaced by big bounces, which take place up to three times, once in each diagonal direction, whenever each of the area scale factors approaches its critical value in the Planck regime measured by the reference of the scalar field momentum."
Dah-Wei has already published the prequel to this paper in Physical Review series D:
http://arxiv.org/abs/gr-qc/0609029
Loop Quantum Cosmology in Bianchi Type I Models: Analytical Investigation
Dah-Wei Chiou
53 pages, 2 figures
Phys.Rev. D75 (2007) 024029
The comprehensive formulation for loop quantum cosmology in the spatially flat, isotropic model was recently constructed. In this paper, the methods are extended to the anisotropic Bianchi I cosmology. Both the precursor and the improved strategies are applied and the expected results are established: (i) the scalar field again serves as an internal clock and is treated as emergent time; (ii) the total Hamiltonian constraint is derived by imposing the fundamental discreteness and gives the evolution as a difference equation; and (iii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously. It is also shown that the state in the kinematical Hilbert space associated with the classical singularity is decoupled in the difference evolution equation, indicating that the big bounce may take place when any of the area scales undergoes the vanishing behavior. The investigation affirms the robustness of the framework used in the isotropic model by enlarging its domain of validity and provides foundations to conduct the detailed numerical analysis.
Dah-Wei Chiou got his PhD in theoretical physics from UC-BERKELEY in IIRC 2005. He was still doing string in 2005, but he escaped out of string and made it to Penn State to work in Ashtekar's group on singularity resolution in quantum cosmology. In my humble private opinion DahWei Chiou made a smart move and he has already done very significant research.
Before, one could handle the big bounce of the universe only in case it was not LOP-SIDED. If it was perfectly symmetrical you could get a collapse turning into an expansion. But what about the lop-sided case? What happens when the space and matter are not perfectly isotropic? Will the collapse get screwed-up and not bounce right? This is the important ANisotropic, or "not-isotropic", asymmetrical case. Dah-Wei has addressed this in one particular version called "Bianchi #1".
Carlo Rovelli and Matteo Smerlak have gotten their Relational EPR accepted for publication and they have made the final revision on it.
They posted the final version yesterday 4 March and it seems actually a little different from the first version, which we discussed here at PF. I will put the link again in case anyone wants to check out the revised version of the paper.
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
Revised, published version
"We study the EPR-type correlations from the perspective of the relational interpretation of quantum mechanics. We argue that these correlations do not entail any form of 'non-locality', when viewed in the context of this interpretation. The abandonment of strict Einstein realism implied by the relational stance permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."
http://arxiv.org/abs/gr-qc/0703044
de Sitter space and the equivalence between f(R) and scalar-tensor gravity
Valerio Faraoni (Bishop's University)
4 pages, to appear in Phys. Rev. D
"It is shown that, when f'' is non-vanishing, metric f(R) gravity is completely equivalent to a scalar-tensor theory (with zero Brans-Dicke parameter) with respect to perturbations of de Sitter space, contrary to previous expectations. Moreover, the stability conditions of de Sitter space with respect to homogeneous and inhomogeneous perturbations coincide in most scalar-tensor theories, as is the case in metric f(R) gravity."
http://arxiv.org/abs/gr-qc/0703050
Vacuum properties of nonsymmetric gravity in de Sitter space
Tomas Janssen, Tomislav Prokopec (ITP & Spinoza Institute, Utrecht University)
32 pages, 2 figures
ITP-UU-07/9, SPIN-07/9
"We consider quantum effects of a massive antisymmetric tensor field on the dynamics of de Sitter space-time. Our starting point is the most general, stable, linearized Lagrangian arising in nonsymmetric gravitational theories (NGTs), where part of the antisymmetric field mass is generated by the cosmological term. We construct a renormalization group (RG) improved effective action by integrating out one loop vacuum fluctuations of the antisymmetric tensor field and show that, in the limit when the RG scale goes to zero, the Hubble parameter -- and thus the effective cosmological constant -- relaxes rapidly to zero. We thus conclude that quantum loop effects in de Sitter space can dramatically change the infrared sector of the on-shell gravity, making the expansion rate insensitive to the original (bare) cosmological constant."
http://arxiv.org/abs/quant-ph/0703060
A Topos Foundation for Theories of Physics: I. Formal Languages for Physics
A. Doering, C.J. Isham
36 pages
"This paper is the first in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper we discuss two different types of language that can be attached to a system, S. The first is a propositional language, PL(S); the second is a higher-order, typed language L(S). Both languages provide deductive systems with an intuitionistic logic. The reason for introducing PL(S) is that, as shown in paper II of the series, it is the easiest way of understanding, and expanding on, the earlier work on topos theory and quantum physics. However, the main thrust of our programme utilises the more powerful language L(S) and its representation in an appropriate topos."
http://arxiv.org/abs/quant-ph/0703062
A Topos Foundation for Theories of Physics: II. Daseinisation and the Liberation of Quantum Theory
A. Doering, C.J. Isham
34 pages
"This paper is the second in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper, we study in depth the topos representation of the propositional language, PL(S), for the case of quantum theory. In doing so, we make a direct link with, and clarify, the earlier work on applying topos theory to quantum physics. The key step is a process we term `daseinisation' by which a projection operator is mapped to a sub-object of the spectral presheaf--the topos quantum analogue of a classical state space. In the second part of the paper we change gear with the introduction of the more sophisticated local language L(S). From this point forward, throughout the rest of the series of papers, our attention will be devoted almost entirely to this language. In the present paper, we use L(S) to study `truth objects' in the topos. These are objects in the topos that play the role of states: a necessary development as the spectral presheaf has no global elements, and hence there are no microstates in the sense of classical physics. Truth objects therefore play a crucial role in our formalism."
http://arxiv.org/abs/quant-ph/0703064
A Topos Foundation for Theories of Physics: III. The Representation of Physical Quantities With Arrows
A. Doering, C.J. Isham
38 pages
"This paper is the third in a series whose goal is to develop a fundamentally new way of viewing theories of physics. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. In paper II, we studied the topos representations of the propositional language PL(S) for the case of quantum theory, and in the present paper we do the same thing for the, more extensive, local language L(S). One of the main achievements is to find a topos representation for self-adjoint operators. This involves showing that, for any physical quantity A, there is an arrow \breve{\delta}^o(A):\Sig\map symbol, where symbol is the quantity-value object for this theory. The construction of \breve{\delta}^o(A) is an extension of the daseinisation of projection operators that was discussed in paper II. The object symbol is a monoid-object only in the topos, \tau_\phi, of the theory, and to enhance the applicability of the formalism, we apply to symbol a topos analogue of the Grothendieck extension of a monoid to a group. The resulting object, symbol, is an abelian group-object in \tau_\phi. We also discuss another candidate, PR, for the quantity-value object. In this presheaf, both inner and outer daseinisation are used in a symmetric way. Finally, there is a brief discussion of the role of unitary operators in the quantum topos scheme."
[Comment: I couldn't get many of the symbols in this abstract to translate into LaTex and eventually left a substantial portion untranslated.]
http://arxiv.org/abs/quant-ph/0703066
A Topos Foundation for Theories of Physics: IV. Categories of Systems
A. Doering, C.J. Isham
38 pages
"This paper is the fourth in a series whose goal is to develop a fundamentally new way of building theories of physics. The motivation comes from a desire to address certain deep issues that arise in the quantum theory of gravity. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. The previous papers in this series are concerned with implementing this programme for a single system. In the present paper, we turn to considering a collection of systems: in particular, we are interested in the relation between the topos representation for a composite system, and the representations for its constituents. We also study this problem for the disjoint sum of two systems. Our approach to these matters is to construct a category of systems and to find a topos representation of the entire category."
http://arxiv.org/abs/gr-qc/0703027
Conserved Quantities in Background Independent Theories
Fotini Markopoulou
11 pages, 3 figures
"We discuss the difficulties that background independent theories based on quantum geometry encounter in deriving general relativity as the low energy limit. We follow a geometrogenesis scenario of a phase transition from a pre-geometric theory to a geometric phase which suggests that a first step towards the low energy limit is searching for the effective collective excitations that will characterize it. Using the correspondence between the pre-geometric background independent theory and a quantum information processor, we are able to use the method of noiseless subsystems to extract such coherent collective excitations. We illustrate this in the case of locally evolving graphs."
http://arxiv.org/abs/gr-qc/0703052
Existence of generalized semiclassical Kodama states. I. The Ashtekar--Klein--Gordon model
Eyo Eyo Ita
32 pages
"This is the first in a series of papers aimed at outlining an algorithm to explicitly construct a finite quantum theory of gravity in Ashtekar variables. The algorithm is based upon extending some properties of a special state, the Kodama state for pure gravity, to more general models. In this paper we analyse a simple case, gravity coupled to a Klein-Gordon scalar field in the minisuperspace Ansatz, in order to derive a criterion for a new semiclassical state and its corresponding semiclassical orbits of spacetime. We then illustrate a presciption for nonperturbatively constructing the analog of the Kodama state for a general case, in preparation for subsequent works in this series."
http://arxiv.org/abs/gr-qc/0703056
Existence of generalized quantum Kodama states. II. The minisuperspace Ashtekar--Klein--Gordon model
Eyo Eyo Ita
41 pages
"This is the second in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. In Part I we constructed a generalized semiclassical Kodama state by solving the classical Hamiltonian constraint under the condition of a broken semiclassical-quantum correspondence due to a Klein-Gordon scalar field. In Part II we will demonstrate a method of restoring this correspondence by generalizing the self-duality condition for the Ashtekar electromagnetic field. The end result will be to establish the existence of a generalized quantum Kodama state devoid of quantum corrections in the minisuperspace model. We also derive the equations needed to solve for the full theory of a finite theory of quantum gravity within the context of this new interpretation."
http://arxiv.org/abs/gr-qc/0703057
Existence of generalized Kodama quantum states. III. A new approach to finite, full quantum gravity
Eyo Eyo Ita
18 pages
"This is the third in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper is a first attempt at the quantization of the full theory coupled to matter, in this case to a spatially inhomogeneous Klein-Gordon scalar field. We delineate the conditions required to construct a solution to the quantum Hamiltonian constraint under the Ansatz of an isotropic, but spatially inhomogeneous, Ashtekar connection, and highlight some differences relative to the minisuperspace case."
briefly mentioned:
http://arxiv.org/abs/gr-qc/0703055
Hawking radiation as tunneling from Gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu
http://arxiv.org/abs/gr-qc/0703058
Asymptotic quasinormal modes of scalar field in a gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu
briefly mentioned:
http://arxiv.org/abs/hep-th/0703055
(reminder about Vaas new book "Beyond the big bang" Springer 2007)
http://arxiv.org/abs/gr-qc/0703078
Snyder's Model -- de Sitter Special Relativity Duality and de Sitter Gravity
Han-Ying Guo, Chao-Guang Huang, Yu Tian, Hong-Tu Wu, Bin Zhou
28 pages
"Between Snyder's quantized space-time model in de Sitter space of momenta and the dS special relativity on dS-spacetime of radius R with Beltrami coordinates, there is a one-to-one dual correspondence supported by a minimum uncertainty-like argument. Together with Planck length \ell_P, R\simeq (3/\Lambda)^{1/2} should be a fundamental constant. They lead to a dimensionless constant g{\sim\ell_PR^{-1}}=(G\hbar c^{-3}\Lambda/3)^{1/2}\sim 10^{-61}. These indicate that physics at these two scales should be dual to each other and there is in between gravity of local dS-invariance characterized by g. A simple model of dS-gravity with a gauge-like action on umbilical manifolds may show these characters. It can pass the observation tests and support the duality."
http://arxiv.org/abs/gr-qc/0703074
Three-geometry and reformulation of the Wheeler-DeWitt equation
Chopin Soo
10 pages
Class. Quantum Grav. 24 (2007) 1547-1555
"A reformulation of the Wheeler-DeWitt equation which highlights the role of gauge-invariant three-geometry elements is presented. It is noted that the classical super-Hamiltonian of four-dimensional gravity as simplified by Ashtekar through the use of gauge potential and densitized triad variables can furthermore be succinctly expressed as a vanishing Poisson bracket involving three-geometry elements. This is discussed in the general setting of the Barbero extension of the theory with arbitrary non-vanishing value of the Immirzi parameter, and when a cosmological constant is also present. A proposed quantum constraint of density weight two which is polynomial in the basic conjugate variables is also demonstrated to correspond to a precise simple ordering of the operators, and may thus help to resolve the factor ordering ambiguity in the extrapolation from classical to quantum gravity. Alternative expression of a density weight one quantum constraint which may be more useful in the spin network context is also discussed, but this constraint is non-polynomial and is not motivated by factor ordering. The article also highlights the fact that while the volume operator has become a preeminient object in the current manifestation of loop quantum gravity, the volume element and the Chern-Simons functional can be of equal significance, and need not be mutually exclusive. Both these fundamental objects appear explicitly in the reformulation of the Wheeler-DeWitt constraint."
http://arxiv.org/abs/astro-ph/0703352
Confrontation of MOND with the rotation curves of early-type disc galaxies
R.H. Sanders, E. Noordermeer
9 pages, 2 figures, submitted MNRAS
brief mention:
http://arxiv.org/abs/hep-th/0703115
Transient Observers and Variable Constants, or Repelling the Invasion of the Boltzmann's Brains
S. Carlip
4 pages
"If the universe expands exponentially without end, 'ordinary observers' like ourselves may be vastly outnumbered by 'Boltzmann's brains,' transient observers who briefly flicker into existence as a result of quantum or thermal fluctuations. One might then wonder why we are so atypical. I show that tiny changes in physics--for instance, extremely slow variations of fundamental constants--can drastically change this result, and argue that one should be wary of conclusions that rely on exact knowledge of the laws of physics in the very distant future."
http://arxiv.org/abs/hep-th/0703116
Quantization in black hole backgrounds
Steven B. Giddings
28 pages, 4 figures
first sentences read in part:
"1. Introduction
Hawking’s discovery of black hole radiance[1] has produced a paradox that may be as important to finding a quantum description of gravity as the paradox of the classical instability of matter was in the foundation of quantum mechanics. There is no commonly accepted explanation for what is wrong with Hawking’s original argument that black holes destroy information[2]. This is despite widespread belief that black holes respect unitary quantum evolution, which is now shared by originator of the paradox himself[3]."
To fill out the picture somewhat, AFAIK Hawking may have conceded his BET at Dublin in 2004, but he did not thereby establish anything to change experts opinion about quantum geometry, information loss, black holes. Hawking notwithstanding it is quite possible that some information that falls into hole is indeed lost from this universe. So Hawking's error may ultimately turn out to be not what he originally proposed, but the fact that he mistakenly conceded.
I wonder why Giddings didn't cite Steve Hsu? And why Hsu didn't cite a bunch of LQG work relevant to his paper that Ted Jacobson could have told him about. It seems this field of inquiry is fragmented, not to say "atomized". Hsu and Giddings know each other personally, so the omission is especially puzzling. Here is Hsu's recent paper on the subject:
http://arxiv.org/abs/hep-th/0608175
Spacetime topology change and black hole information
Stephen D.H. Hsu
5 pages, 5 figures, to appear in Physics Letters B
Phys.Lett. B644 (2007) 67-71
"Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in planckian regimes. It does not require non-locality or any modification of low-energy physics."
http://arxiv.org/abs/astro-ph/0703566
Thermal fluctuations in loop cosmology
Joao Magueijo, Parampreet Singh
10 pages
"Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations. We show that the no-go results for scale invariance in classical thermal models can be evaded even if we just consider modifications to the background (zeroth order) gravitational dynamics. Since a complete and systematic treatment of the perturbed Einstein equations in loop cosmology is still lacking, we simply parameterize their expected modifications. These change quantitatively, but not qualitatively, our conclusions. We thus urge the community to more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe."
http://arxiv.org/abs/gr-qc/0703098
Relativity theory does not imply that the future already exists: a counterexample
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
Comments: plainTeX, 12 pages, no figures. To appear in Vesselin Petkov (editor), Relativity and the Dimensionality of the World (Springer 2007, in press). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/
"It is often said that the relativistic fusion of time with space rules out genuine change or 'becoming'. I offer the classical sequential growth models of causal set theory as counterexamples."
http://arxiv.org/abs/gr-qc/0703099
Does Locality Fail at Intermediate Length-Scales
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
24 pages, 2 figures. To appear in Daniele Oriti (ed.), Towards Quantum Gravity (Cambridge University Press, 2007). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/
"If quantum gravity implies a fundamental spatiotemporal discreteness, and if its 'laws of motion' are compatible with the Lorentz transformations, then physics cannot remain local. One might expect this nonlocality to be confined to the fundamental discreteness scale, but I will present evidence that it survives at much lower energies, yielding for example a nonlocal equation of motion for a scalar field propagating on an underlying causal set."
http://arxiv.org/abs/gr-qc/0703097
New directions in Background Independent Quantum Gravity
Fotini Markopoulou
26 pages. Contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press
"We discuss the meaning of background independence in quantum theories of gravity where geometry and gravity are emergent and illustrate the possibilities using the framework of quantum causal histories."
brief mention:
http://arxiv.org/abs/physics/0703191
The Expansion of the Universe and the Cosmological Constant Problem
R. F. O'Connell
Phys. Lett. A, in press
http://arxiv.org/find/grp_physics/1/au:OConnell_R_F/0/1/0/all/0/1
http://arxiv.org/abs/quant-ph/0703179
Disproof of Bell's Theorem by Clifford Algebra Valued Local Variables
Joy Christian (Perimeter and Oxford)
4 pages
http://arxiv.org/abs/gr-qc/0703114
Note on non-metric gravity
Ingemar Bengtsson
9 pages
We discuss a class of alternative gravity theories that are specific to four dimensions, do not introduce new degrees of freedom, and come with a physical motivation. In particular we sketch their Hamiltonian formulation, and their relation to some earlier constructions."
This refers to two recent papers by Kirill Krasnov
http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations
For anyone not familiar with Bengtsson here are 34 papers in gr-qc, hep-th, and quant-ph going back to 1992
http://arxiv.org/find/grp_physics/1/au:+Bengtsson_I/0/1/0/all/0/1
He picked up fast on Krasnov's idea of a way to make gravity renormalizable,
relating it to the asymptotic safety program.
http://arxiv.org/abs/gr-qc/0703116
Loop quantum gravity and Planck-size black hole entropy
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
21 pages, 5 figures. Contribution to the Proceedings of the NEB XII International Conference
"The Loop Quantum Gravity (LQG) program is briefly reviewed and one of its main applications, namely the counting of black hole entropy within the framework is considered. In particular, recent results for Planck size black holes are reviewed. These results are consistent with an asymptotic linear relation (that fixes uniquely a free parameter of the theory) and a logarithmic correction with a coefficient equal to -1/2. The account is tailored as an introduction to the subject for non-experts."
http://arxiv.org/abs/gr-qc/0703144
Dynamical coherent states and physical solutions of quantum cosmological bounces
Martin Bojowald
30 pages, 3 figures
IGPG-07/3-5, NSF-KITP-07-55
"A new model is studied which describes the quantum behavior of transitions through an isotropic quantum cosmological bounce in loop quantum cosmology sourced by a free and massless scalar field. As an exactly solvable model even at the quantum level, it illustrates properties of dynamical coherent states and provides the basis for a systematic perturbation theory of loop quantum gravity. The detailed analysis is remarkably different from what is known for harmonic oscillator coherent states. Results are evaluated with regard to their implications in cosmology, including a demonstration that in general quantum fluctuations before and after the bounce are unrelated. Thus, even within this solvable model the condition of classicality at late times does not imply classicality at early times before the bounce without further assumptions. Nevertheless, the quantum state does evolve deterministically through the bounce."
http://arxiv.org/abs/gr-qc/0703135
Loop quantization of spherically symmetric midi-superspaces
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
18 pages
"We quantize the exterior of spherically symmetric vacuum space-times using a midi-superspace reduction within the Ashtekar new variables. Through a partial gauge fixing we eliminate the diffeomorphism constraint and are left with a Hamiltonian constraint that is first class. We complete the quantization in the loop representation. We also use the model to discuss the issues that will arise in more general contexts in the 'uniform discretization' approach to the dynamics."
http://arxiv.org/abs/gr-qc/0703137
Decoherence in Quantum Gravity: Issues and Critiques
C. Anastopoulos, B. L. Hu
25 pages, proceedings of DICE06 (Piombino)
"An increasing number of papers have appeared in recent years on decoherence in quantum gravity at the Planck energy. We discuss the meaning of decoherence in quantum gravity starting from the common notion that quantum gravity is a theory for the microscopic structures of spacetime, and invoking some generic features of quantum decoherence from the open systems viewpoint. We dwell on a range of issues bearing on this process including the relation between statistical and quantum, noise from effective field theory, the meaning of stochasticity, the origin of non-unitarity and the nature of nonlocality in this and related contexts. To expound these issues we critique on two representative theories: One claims that decoherence in quantum gravity scale leads to the violation of CPT symmetry at sub-Planckian energy which is used to explain today's particle phenomenology. The other uses this process in place with the Brownian motion model to prove that spacetime foam behaves like a thermal bath."
could be a useful text and reference work:
http://arxiv.org/abs/astro-ph/0703730
Why CMB physics?
Massimo Giovannini
179 pages, 27 figures
CERN-PH-TH/2007-048
"The aim of these lectures is to introduce some basic problems arising in gravitation and modern cosmology. All along the discussion the guiding theme is provided by the phenomenological and theoretical properties of the Cosmic Microwave Background (CMB). These lectures have been prepared for a regular Phd course of the University of Milan-Bicocca."
briefly noted:
http://arxiv.org/abs/astro-ph/0703722
Observable Signatures of a Black Hole Ejected by Gravitational Radiation Recoil in a Galaxy Merger
Abraham Loeb (Harvard)
4 pages, submitted to PRL
"According to recent general-relativistic simulations, the coalescence of two spinning black holes (BHs) could lead to recoil speeds of the BH remnant of up to thousands of km/s as a result of the emission of gravitational radiation. Such speeds would enable the merger product to escape its host galaxy. Here we examine the circumstances resulting from a gas-rich galaxy merger under which the ejected BH would carry an accretion disk with it and be observable. As the initial BH binary emits gravitational radiation and its orbit tightens, a hole is opened around it in the disk which delays the consumption of gas prior to the eventual BH ejection. The punctured disk remains bound to the ejected BH within the region where the gas orbital velocity is larger than the ejection speed. For a ~10^7 solar mass BH the ejected disk has a characteristic size of tens of thousands of Schwarzschild radii and an accretion lifetime of ~10^7 years. During that time, the ejected BH could traverse a considerable distance and appear as an off-center quasar with a feedback trail along the path it left behind. A small fraction of all quasars could be associated with an escaping BH."
briefly noted:
http://arxiv.org/abs/gr-qc/0703150
A gravitational explanation for quantum theory - non-time-orientable manifolds
Mark J Hadley
7 pages Talk given at FFP8 in Madrid 2006
"Spacetime manifolds that are not time orientable play a key role in a gravitational explanation of quantum theory. Such manifolds allow topology change, but also have fascinating additional properties such as net charge from source-free equations and spin half transformation properties. It is shown how the logical structure of propositions and the probabilities of quantum theory arise from such acausal space times."
http://arxiv.org/abs/hep-th/0703265
A new PPN parameter to test Chern-Simons gravity
Stephon Alexander, Nicolas Yunes
4 pages, submitted to PRL
"We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN) framework through weak-field solutions of the modified field equations for a perfect fluid source... This new term encodes the key physical effect of CS gravity in the weak-field limit, leading to a modification of frame dragging and, thus, the Lense-Thirring contribution to gyroscopic precession. We provide a physical interpretation for the new term, as well as an estimate of the size of this effect relative to the general relativistic Lense-Thirring prediction. This correction to frame dragging might be used in experiments,.."
http://arxiv.org/abs/0704.0145
Singularity Resolution in Isotropic Loop Quantum Cosmology: Recent Developments
Ghanashyam Date
(Submitted on 2 Apr 2007)
"Since the past Iarge meeting in December 2004, new developments in loop quantum cosmology have taken place, especially with regards to the resolution of the Big Bang singularity in the isotropic models. The singularity resolution issue has been discussed in terms of physical quantities (expectation values of Dirac observables) and there is also an 'improved' quantization of the Hamiltonian constraint. These developments are briefly discussed. This is an expanded version of the review talk given at the 24-th IAGRG meeting in February 2007."
http://arxiv.org/abs/0704.0007
Polymer Quantum Mechanics and its Continuum Limit
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
(Submitted on 31 Mar 2007)
"A rather non-standard quantum representation of the canonical commutation relations of quantum mechanics systems, known as the polymer representation has gained some attention in recent years, due to its possible relation with Planck scale physics. In particular, this approach has been followed in a symmetric sector of loop quantum gravity known as loop quantum cosmology (LQC). Here we explore different aspects of the relation between the ordinary Schroedinger theory and the polymer description. The paper has two parts. In the first one, we derive the polymer quantum mechanics starting from the ordinary Schroedinger theory and show that the polymer description arises as an appropriate limit. In the second part we consider the continuum limit of this theory, namely, the reverse process in which one starts from the discrete theory and tries to recover back the ordinary Schroedinger quantum mechanics. We consider several examples of interest, including the harmonic oscillator, the free particle and a simple cosmological model."
http://arxiv.org/abs/0704.0221
The Return of a Static Universe and the End of Cosmology
Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
(Submitted on 2 Apr 2007)
"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented."
briefly noted:
http://arxiv.org/abs/0704.0058
Intelligent Life in Cosmology
Frank J. Tipler
http://arxiv.org/abs/0704.0278
q-Deformed spin foam models of quantum gravity
Igor Khavkine, J. Daniel Christensen
(Submitted on 2 Apr 2007)
"We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous."
Dan Christensen accesses the supercomputer at U. West Ontario (UWO) where he has charge of programs in mathematics, computing, and quantum gravity. Co-authored spinfoam papers with John Baez several years back and with others since. Use of numerical techniques notable here---probably a Beowolf cluster.
http://arxiv.org/abs/0704.0367
Existence of generalized Kodama states. IV. The search for a quantization of 4-dimensional gravity
Eyo Eyo Ita III
"This is the fourth in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper continues essentially from where papers II and III left off, treating the kinematic constraints in greater depth and moving on to a higher level of complexity with regard to the Hamiltonian constraint of the full, unrestricted theory. First we identify some of the traditional obstacles to the consistent quantization of four-dimensional gravity, then provide suggestions for how these obstacles may possibly be surmounted within the context of our new approach. This inevitably entails a critical analysis of the relationship of gauge transformations to diffeomorphisms, which in turn leads to the reduced phase space approach to quantization of the kinematic constraints, as well as the implementation of the semiclassical-quantum correspondence. We also compute some more of the terms needed to implement the full quantum Hamiltonian constraint, focusing in this work on its antisymmetric CDJ components. Some of the relationships among generalized Kodama states for the more general model are clearly elucidated due to the calculation of such terms."
33 pages
E.E.Ita is a PhD student at Cambridge.
http://arxiv.org/abs/0704.0299
Parametrized Post-Newtonian Expansion of Chern-Simons Gravity
Stephon Alexander, Nicolas Yunes
"We investigate the weak-field, post-Newtonian expansion to the solution of the field equations in Chern-Simons gravity with a perfect fluid source. In particular, we study the mapping of this solution to the parameterized post-Newtonian formalism to 1 PN order in the metric. We find that the PPN parameters of Chern-Simons gravity are identical to those of general relativity, with the exception of the inclusion of a new term that is proportional to the Chern-Simons coupling parameter and the curl of the PPN vector potentials. We also find that the new term is naturally enhanced by the non-linearity of spacetime and we provide a physical interpretation for it. By mapping this correction to the gravito-electro-magnetic framework, we study the corrections that this new term introduces to the acceleration of point particles and the frame-dragging effect in gyroscopic precession. We find that the Chern-Simons correction to these classical predictions could be used by current and future experiments to place bounds on intrinsic parameters of Chern-Simons gravity and, thus, string theory."
14 pages
http://arxiv.org/abs/0704.1137
Lattice refining loop quantum cosmology, anisotropic models and stability
Martin Bojowald, Daniel Cartin, Gaurav Khanna
24 pages
"A general class of loop quantizations for anisotropic models is introduced and discussed, which enhances loop quantum cosmology by relevant features seen in inhomogeneous situations. The main new effect is an underlying lattice which is being refined during dynamical changes of the volume. In general, this leads to a new feature of dynamical difference equations which may not have constant step-size, posing new mathematical problems. It is discussed how such models can be evaluated and what lattice refinements imply for semiclassical behavior. Two detailed examples illustrate that stability conditions can put strong constraints on suitable refinement models, even in the absence of a fundamental Hamiltonian which defines changes of the underlying lattice. Thus, a large class of consistency tests of loop quantum gravity becomes available. In this context, it will also be seen that quantum corrections due to inverse powers of metric components in a constraint are much larger than they appeared recently in more special treatments of isotropic, free scalar models where they were artificially suppressed."
http://arxiv.org/abs/0704.0992
Compatibility of radial, Lorenz and harmonic gauges
Elena Magliaro, Claudio Perini, Carlo Rovelli
9 pages
"We observe that the radial gauge can be consistently imposed together with the Lorenz gauge in Maxwell theory, and with the harmonic traceless gauge in linearized general relativity. This simple observation has relevance for some recent developments in quantum gravity where the radial gauge is implicitly utilized.
Marcus
Perhaps you could enlighten us as to the LQG program predictions for the MiniBooNE results, which come out this week. I haven't seen any anywhere...
http://arxiv.org/abs/0704.2397
The Quantum Configuration Space of Loop Quantum Cosmology
Jose Manuel Velhinho
14 pages
"The article gives an account of several aspects of the space known as the Bohr compactification of the line, featuring as the quantum configuration space in loop quantum cosmology, as well as of the corresponding configuration space realization of the so-called polymer representation. Analogies with loop quantum gravity are explored, providing an introduction to (part of) the mathematical structure of loop quantum gravity, in a technically simpler context."
http://arxiv.org/abs/0704.2291
Fundamentalist physics: why Dark Energy is bad for Astronomy
Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics. 19 pages including 3 figures
"Astronomers carry out observations to explore the diverse processes and objects which populate our Universe. High-energy physicists carry out experiments to approach the Fundamental Theory underlying space, time and matter. Dark Energy is a unique link between them, reflecting deep aspects of the Fundamental Theory, yet apparently accessible only through astronomical observation. Large sections of the two communities have therefore converged in support of astronomical projects to constrain Dark Energy. In this essay I argue that this convergence can be damaging for astronomy. The two communities have different methodologies and different scientific cultures. By uncritically adopting the values of an alien system, astronomers risk undermining the foundations of their own current success and endangering the future vitality of their field. Dark Energy is undeniably an interesting problem to attack through astronomical observation, but it is one of many and not necessarily the one where significant progress is most likely to follow a major investment of resources."
Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics.
Goodness, Marcus! Do such articles usually use such strong language?
:smile:
ensabah6
Apr24-07, 02:32 PM
Hey Marcus
not sure if you've ever mentioned this (I can't search for this personally)
arXiv:gr-qc/0611156
Title: On Loop States in Loop Quantum Gravity
Authors: N. D. Hari Dass, Manu Mathur
Comments: 12 pages, 3 figures, the version to be published in Classical and Quantum Gravity
We explicitly construct and characterize all possible independent loop states in 3+1 dimensional loop quantum gravity by regulating it on a 3-d regular lattice in the Hamiltonian formalism. These loop states, characterized by the (dual) angular momentum quantum numbers, describe SU(2) rigid rotators on the links of the lattice. The loop states are constructed using the Schwinger bosons which are harmonic oscillators in the fundamental (spin half) representation of SU(2). Using generalized Wigner Eckart theorem, we compute the matrix elements of the volume operator in the loop basis. Some simple loop eigenstates of the volume operator are explicitly constructed.
the preceding post duplicates an entry made last year in #543 of this thread
http://arxiv.org/abs/0704.3214
(2+1)-Dimensional Quantum Gravity as the Continuum Limit of Causal Dynamical Triangulations
D. Benedetti, R. Loll, F. Zamponi
38 pages, 13 figures
"We perform a non-perturbative sum over geometries in a (2+1)-dimensional quantum gravity model given in terms of Causal Dynamical Triangulations. Inspired by the concept of triangulations of product type introduced previously, we impose an additional notion of order on the discrete, causal geometries. This simplifies the combinatorial problem of counting geometries just enough to enable us to calculate the transfer matrix between boundary states labelled by the area of the spatial universe, as well as the corresponding quantum Hamiltonian of the continuum theory. This is the first time in dimension larger than two that a Hamiltonian has been derived from such a model by mainly analytical means, and opens the way for a better understanding of scaling and renormalization issues."
http://arxiv.org/abs/0704.2667
Wormholes as Black Hole Foils
Thibault Damour, Sergey N. Solodukhin
13 pages
"We study to what extent wormholes can mimic the observational features of black holes. It is surprisingly found that many features that could be thought of as 'characteristic' of a black hole (endowed with an event horizon) can be closely mimicked by a globally static wormhole, having no event horizon. This is the case for: the apparently irreversible accretion of matter down a hole, no-hair properties, quasi-normal-mode ringing, and even the dissipative properties of black hole horizons, such as a finite surface resistivity equal to 377 Ohms. The only way to distinguish the two geometries on an observationally reasonable time scale would be through the detection of Hawking's radiation, which is, however, too weak to be of practical relevance for astrophysical black holes. We point out the existence of an interesting spectrum of quantum microstates trapped in the throat of a wormhole which could be relevant for storing the information 'lost' during a gravitational collapse."
http://arxiv.org/abs/0704.1746
Tunnelling from black holes in the Hamilton Jacobi approach
Bhramar Chatterjee, Amit Ghosh, P. Mitra
5 pages
"It has recently been shown that it is possible to understand Hawking radiation as tunnelling across black hole horizons using appropriate Hamilton-Jacobi boundary conditions. The procedure is applied to the non-rotating black hole in different coordinate systems and to the rotating charged black hole. Differences with the earlier literature are pointed out."
ensabah6
Apr24-07, 08:39 PM
Thanks, how do you search that? When I type it in I get wrong hits.
Anyhow did you also mention this?
http://arxiv.org/abs/gr-qc/0606100
A lattice bosonic model as a quantum theory of gravity
Authors: Zheng-Cheng Gu, Xiao-Gang Wen
(Submitted on 23 Jun 2006)
A local quantum bosonic model on a lattice is constructed whose low energy excitations are gravitons described by linearized Einstein action. Thus the bosonic model is a quantum theory of gravity, at least at the linear level. We find that the compactification and the discretization of metric tenor are crucial in obtaining a quantum theory of gravity.
John Baez is at Les Treilles (interesting people at rustic estate in south France, QG discussion this year) to present this talk
http://math.ucr.edu/home/baez/treilles/
the slides are downloadable.
ftp://ftp.alainconnes.org/Inteng.pdf
recent Alain Connes interview with G. Skandalis and C. Goldstein
http://arxiv.org/abs/0704.2630
Are We Typical?
James B. Hartle, Mark Srednicki
6 pages
"Bayesian probability theory is used to analyze the oft-made assumption that humans are typical observers in the universe. Some theoretical calculations make the {\it selection fallacy} that we are randomly chosen from a class of objects by some physical process, despite the absence of any evidence for such a process, or any observational evidence favoring our typicality. It is possible to favor theories in which we are typical by appropriately choosing their prior probabilities, but such assumptions should be made explicit to avoid confusion."
http://arxiv.org/abs/0704.3429
Light Propagation on Quantum Curved Spacetime and Back reaction effects
Carlos Kozameh, Florencia Parisi
"We study the electromagnetic field equations on an arbitrary quantum curved background in the semiclassical approximation of Loop Quantum Gravity. The effective interaction hamiltonian for the Maxwell and gravitational fields is obtained and the corresponding field equations, which can be expressed as a modified wave equation for the Maxwell potential, are derived. We use these results to analyze electromagnetic wave propagation on a quantum Robertson-Walker space time and show that Lorentz Invariance is not preserved. The formalism developed can be applied to the case where back reaction effects on the metric due to the electromagnetic field are taken into account, leading to non-covariant field equations."
http://arxiv.org/abs/0704.3595
Generalized Kodama states. V. Evidence of the normalizability and renormalizability of 4D QGRA
Eyo Eyo Ita III
"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. In this work we transform the pure Kodama state into the metric representation for the Bianchi IX minisuperspace model. Previous such calculations have been carried out to semiclassical order for a particular choice of gauge, revealing the existence of five topologically distinct states in the metric representation. We have performed our calculation to all orders in perturbation theory by maintaining the gauge degrees of freedom explicit, revealing the existence of a sixth state. We propose a resolution to the issue of reality conditions for the Ashtekar variables, and also for the recasting of 4-dimensional general relativity as a renormalizable field theory, stemming from the nonperturbative result obtained from this work. We also address the issue of normalizability of the Kodama state."
briefly noted
http://arxiv.org/abs/0704.3306
Quantum mechanics on Hilbert manifolds: The principle of functional relativity
Alexey A. Kryukov
45 pages, 9 figures
"Quantum mechanics is formulated as a geometric theory on a Hilbert manifold..."
http://arxiv.org/abs/0704.3414
Phantom field dynamics in loop quantum cosmology
Daris Samart, Burin Gumjudpai (TPTP Naresuan U.)
"We consider a dynamical system of phantom scalar field under exponential potential in background of loop quantum cosmology... "
http://arxiv.org/abs/0705.0006
Multiple-event probability in general-relativistic quantum mechanics: a discrete model
Mauricio Mondragon, Alejandro Perez, Carlo Rovelli
(Submitted on 30 Apr 2007)
"We introduce a simple quantum mechanical model in which time and space are discrete and periodic. These features avoid the complications related to continuous-spectrum operators and infinite-norm states. The model provides a tool for discussing the probabilistic interpretation of generally-covariant quantum systems, without the confusion generated by spurious infinities. We use the model to illustrate the formalism of general-relativistic quantum mechanics, and to test the definition of multiple-event probability introduced in a companion paper. We consider a version of the model with unitary time-evolution and a version without unitary time-evolution."
http://arxiv.org/abs/0705.0235
The canonical versus path integral quantization approach to generalized Kodama states (Part II)
Eyo Eyo Ita III
41 pages
"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. A major feature of the quantization programme resides in the inherent ability of the path integral to exhaustively pick out a complete basis of quantum states precisely matching the canonically determined wavefunctions for an arbitrary model coupled to gravity. First we develop the notation and tools necessary to expose this hidden feature of the path integral, and then we apply it to the nonperturbative construction of the pure Kodama state (Part I) and the generalized Kodama state (Part II) for an arbitraty matter coupling, in analogy to the no-boundary proposal. We argue that the well-definedness and the explicit convergence of the path integral is intimately connected to its equivalence to the canonical approach, the principle of the SQC, and in particular to the existence of the generalized Kodama states (GKodS). We then indicate how these features, combined, can ultimately lead to a resolution of the problem of time in quantum gravity. In the end, we recast the equations necessary to eliminate the quantum counterterms for a general model into a standard form suitable for model-specific expansion of the GKodS about the pure Kodama state. The explicit solution of these equations is reserved for future work."
http://arxiv.org/abs/0705.0165
Is Modified Gravity Required by Observations? An Empirical Consistency Test of Dark Energy Models
Sheng Wang (Brookhaven; Columbia), Lam Hui (Columbia; ISCAP), Morgan May (Brookhaven), Zoltan Haiman (Columbia)
"We apply the technique of parameter-splitting to existing cosmological data sets, to check for a generic failure of dark energy models. Given a dark energy parameter, such as the energy density Omega_Lambda or equation of state w, we split it into two meta-parameters with one controlling geometrical distances, and the other controlling the growth of structure. Observational data spanning Type Ia Supernovae, the cosmic microwave background (CMB), galaxy clustering, and weak gravitational lensing statistics are fit without requiring the two meta-parameters to be equal. This technique checks for inconsistency between different data sets, as well as for internal inconsistency within any one data set (e.g., CMB or lensing statistics) that is sensitive to both geometry and growth. We find that the cosmological constant model is consistent with current data. Theories of modified gravity generally predict a relation between growth and geometry that is different from that of general relativity. Parameter-splitting can be viewed as a crude way to parametrize the space of such theories. Our analysis of current data already appears to put sharp limits on these theories: assuming a flat universe, current data constrain the difference Omega_Lambda(geom) - Omega_Lambda(grow) to be -0.0044 +/- 0.0058 (68% C.L.); allowing the equation of state w to vary, the difference w(geom) - w(grow) is constrained to be 0.37 +/- 0.37 (68% C.L.). Interestingly, the region w(grow) > w(geom), which should be generically favored by theories that slow structure formation relative to general relativity, is quite restricted by data already. We find w(grow) < -0.80 at 2 sigma."
briefly noted:
http://arxiv.org/abs/0705.0164
Inflationary Cosmology
Andrei Linde
60 pages, 10 figs, based on a talk given at the 22nd IAP Colloquium, "Inflation+25", Paris, June 2006
http://arxiv.org/abs/0705.0674
A new spinfoam vertex for quantum gravity
Etera R. Livine, Simone Speziale
17+8 pages, 6 figures
"We introduce a new spinfoam vertex to be used in models of 4d quantum gravity based on SU(2) and SO(4) BF theory plus constraints. It can be seen as the conventional vertex of SU(2) BF theory, the 15j symbol, in a particular basis constructed using SU(2) coherent states. This basis makes the geometric interpretation of the variables transparent: they are the vectors normal to the triangles within each tetrahedron. We study the condition under which these states can be considered semiclassical, and we show that the semiclassical ones dominate the evaluation of quantum correlations. Finally, we describe how the constraints reducing BF to gravity can be directly written in terms of the new variables, and how the semiclassicality of the states might improve understanding the correct way to implement the constraints."
.
http://arxiv.org/abs/0705.0991
From semiconductors to quantum gravity: to centenary of Matvei Bronstein
G.E. Volovik
12 pages, 1 figure, draft for proceedings of the conference devoted to centenary of Marvei Bronstein, St. Petersburg, November 2006
"Investigation of the many-body condensed-matter systems allows us to connect the microscopic physics at the atomic energy scale and the macroscopic physics emerging in the low-energy corner. It gives some hints on the mechanisms of the formation of the physical laws which our Universe obeys. The paper is devoted to the centenary of Matvei Petrovich Bronstein."
briefly noted:
http://arxiv.org/abs/0705.1060
Obtaining the spacetime metric from cosmological observations
http://arxiv.org/abs/0705.1029
No Way Back: Maximizing survival time below the Schwarzschild event horizon
http://arxiv.org/abs/0705.1178
A global picture of quantum de Sitter space
Steven B. Giddings, Donald Marolf
17 pages, 1 figure
"Perturbative gravity about a de Sitter background motivates a global picture of quantum dynamics in `eternal de Sitter space,' the theory of states which are asymptotically de Sitter to both future and past. Eternal de Sitter physics is described by a finite dimensional Hilbert space in which each state is precisely invariant under the full de Sitter group. This resolves a previously-noted tension between de Sitter symmetry and finite entropy. Observables, implications for Boltzmann brains, and Poincare recurrences are briefly discussed."
Giddings is known as a string theorist but this is not a string paper. It finds a mistake in a 2002 paper by Leonard Susskind and others called "The Trouble with deSitter Space" http://arxiv.org/hep-th/0212209
Many papers by people in the LQG community are cited---four by Rovelli, several by Ambjorn, by Thiemann, Ashtekar, Gambini and Pullin. Doubtless others, I didnt make a careful count. Since Giddings and Marolf are well-known people at KITP Santa Barbara it suggests a significant direction of interest. At the same time on the nonstring QG side a substantial amoung of research is involved with deSitter space---Baez student Derek Wise was discussing Cartan geometry using the deSitter group instead of Poincaré---DSR (deformed special relativity) deals a lot with deSitter group. What happens when quantum spacetime geometry is "locally deSitter"? This paper will be of interest and will have usefulness outside of string research.
http://arxiv.org/abs/0705.1032
Matter density perturbations and effective gravitational constant in modified gravity models of dark energy
Shinji Tsujikawa
10 pages
"We derive the equation of matter density perturbations on sub-horizon scales for a general Lagrangian density f(R, phi, X) that is a function of a Ricci scalar R, a scalar field phi and a kinetic term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing..."
http://arxiv.org/abs/0705.1158
Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
Wayne Hu, Ignacy Sawicki (KICP, U. Chicago)
13 pages, 10 figures. Submitted to Phys. Rev. D
"We study a class of metric-variation f(R) models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in f(R) models do not have substantially deeper potentials than expected in LCDM, then cosmological field amplitudes |f_R| > 10^{-6} will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of f(R) models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on f(R) are phrased in a nearly model-independent manner."
My impression is that Wayne Hu is very highly regarded in cosmology. It seems significant to me that he is seriously talking about ways to get away from having "Dark Energy" by modifying the law of gravity.
http://arxiv.org/abs/0705.1170
The Collision Between The Milky Way And Andromeda
T.J. Cox, Abraham Loeb (Harvard/CfA)
submitted to MNRAS
"We use a N-body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the first close encounter of the two galaxies, there is a 12% chance that the Sun will be pulled from its present position and reside in the extended tidal material. After the second close encounter, there is a 30% chance that the Sun will reside in the extended tidal material, and a 2.7% chance that our Sun will be more tightly bound to Andromeda than to the Milky Way. Eventually, after the merger has completed, the Sun is likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies."
http://arxiv.org/abs/0705.2222
Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
Abhay Ashtekar
21 pages, to appear in the Proceedings of the 11th Marcel Grossmann Conference
"As per organizers' request, my talk at the 11th Marcel Grossmann Conference consisted of two parts. In the first, I illustrated recent advances in loop quantum gravity through examples. In the second, I presented an overall assessment of the status of the program by addressing some frequently asked questions. This account is addressed primarily to researchers outside the loop quantum gravity community."
http://arxiv.org/abs/0705.2047
Non-Metric Gravity II: Spherically Symmetric Solution, Missing Mass and Redshifts of Quasars
Kirill Krasnov, Yuri Shtanov
37 pages, 2 figures
"We continue the study of the non-metric theory of gravity introduced in hep-th/0611182 and gr-qc/0703002 and obtain its general spherically symmetric vacuum solution. It respects the analog of the Birkhoff theorem, i.e., the vacuum spherically symmetric solution is necessarily static. As in general relativity, the spherically symmetric solution is seen to describe a black hole. The exterior geometry is essentially the same as in the Schwarzschild case, with power-law corrections to the Newtonian potential. The behavior inside the black-hole region is different from the Schwarzschild case in that the usual spacetime singularity gets replaced by a singular of a new type, where all basic fields of the theory remain finite but metric ceases to exist. The theory does not admit arbitrarily small black holes: for small objects, the curvature on the would-be horizon is so strong that non-metric modifications prevent the horizon from being formed. The theory allows for modifications of gravity of very interesting nature. We discuss three physical effects, namely, (i) correction to Newton's law in the neighborhood of the source, (ii) renormalization of effective gravitational and cosmological constants at large distances from the source, and (iii) additional redshift factor between spatial regions of different curvature. The first two effects can be responsible, respectively, for the observed anomaly in the acceleration of the Pioneer spacecraft and for the alleged missing mass in spiral galaxies and other astrophysical objects. The third effect can be used to propose a non-cosmological explanation of high redshifts of quasars."
http://arxiv.org/abs/0705.2197
Black holes, information, and locality
Steven B. Giddings
Essay submitted to the Gravity Research Foundation essay contest; with minor updates
"Thirty years of a deepening information paradox suggest the need to revise our basic physical framework. Multiple indicators point toward reassessment of the principle of locality: lack of a precise definition in quantum gravity, behavior of high-energy scattering, hints from strings and AdS/CFT, conundrums of quantum cosmology, and finally lack of good alternative resolutions of the paradox. A plausible conjecture states that the non-perturbative dynamics of gravity is unitary but nonlocal. String theory may directly address these issues but so far important aspects remain elusive. If this viewpoint is correct, critical questions are to understand the 'correspondence' limit where nonlocal physics reduces to local quantum field theory, and beyond, to unveil principles of an underlying nonlocal theory."
http://arxiv.org/abs/0705.2388
The loop-quantum-gravity vertex-amplitude
Jonathan Engle, Roberto Pereira, Carlo Rovelli
6 pages
"Spinfoam theories are hoped to provide the dynamics of non-perturbative loop quantum gravity. But a number of their features remain elusive. The best studied one -the euclidean Barrett-Crane model- does not have the boundary state space needed for this, and there are recent indications that, consequently, it may fail to yield the correct low-energy n-point functions. These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way certain second class constraints are imposed, arguably incorrectly, strongly. We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly. Its state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network space. The model provides a long sought SO(4)-covariant vertex amplitude for loop quantum gravity."
http://arxiv.org/abs/0705.2357
The inevitable nonlinearity of quantum gravity falsifies the many-worlds interpretation of quantum mechanics
T. P. Singh
6 pages. Honorable Mention in Gravity Research Foundation Essay Competition 2007. This is also a summary of a talk given at the Meeting `Himalayan Relativity Dialogue', Mirik, India, 18-20 April, 2007. Detailed version of this essay is in preparation
"There are fundamental reasons as to why there should exist a reformulation of quantum mechanics which does not refer to a classical spacetime manifold. It follows as a consequence that quantum mechanics as we know it is a limiting case of a more general nonlinear quantum theory, with the nonlinearity becoming significant at the Planck mass/energy scale. This nonlinearity is responsible for a dynamically induced collapse of the wave-function, during a quantum measurement, and it hence falsifies the many-worlds interpretation of quantum mechanics. We illustrate this conclusion using a mathematical model based on a generalized Doebner-Goldin equation. The non-Hermitian part of the Hamiltonian in this norm-preserving, nonlinear, Schrodinger equation dominates during a quantum measurement, and leads to a breakdown of linear superposition."
briefly noted:
http://arxiv.org/abs/0705.2306
Near-Extreme Black Holes and the Universal Relaxation Bound
Shahar Hod
2 Pages. Submitted to PRD
=========
Still able to edit this, I reply to Francesca here so as not to make an extra post:
Francesca, thanks for the two references! Both the Percacci et al, and the Chamseddine-Connes look interesting and relevant as you say.
Ontoplankton
May16-07, 06:58 AM
So does anyone else think "Carlo Rovelli (http://cgpg.gravity.psu.edu/online/Html/Seminars/Fall1998/Rovelli/rovelli.jpg)" is secretly Karl Rove (http://www.nndb.com/people/353/000022287/karl-rove-sm.jpg) with a wig? I think he's trying to destroy string theory as part of the Republican War on Science. That sneaky bastard.
francesca
May16-07, 09:57 AM
Sorry, I didn't want to replay but I can't...
ST is right, QG is left!
That's an Italian play started by the movie maker Nanni Moretti in a movie of him, "Aprile"... the ham steak is right, the Bologna ham is left...
francesca
May16-07, 03:14 PM
Do you remember the great interest for Reuter's lecture "Asymptotic Safety in Quantum Einstein Gravity"? This is a related paper:
http://arxiv.org/abs/0705.1769
Title: Ultraviolet properties of f(R)-Gravity
Authors: Alessandro Codello, Roberto Percacci, Christoph Rahmede
Comments: 4 pages
We discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile.
and BTW:
http://arxiv.org/abs/0705.1786
Title: Quantum Gravity Boundary Terms from Spectral Action
Authors: Ali H. Chamseddine, Alain Connes
Comments: RevTex 4 pages
We study the boundary terms of the spectral action of the noncommutative space, defined by the spectral triple dictated by the physical spectrum of the standard model, unifying gravity with all other fundamental interactions. We prove that the spectral action predicts uniquely the gravitational boundary term required for consistency of quantum gravity with the correct sign and coefficient. This is a remarkable result given the lack of freedom in the spectral action to tune this term.
http://arxiv.org/abs/0705.2440
Quantum Structure of Geometry: Loopy and fuzzy?
Alejandro Corichi, Jose A. Zapata
9 pages
"In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy."
http://arxiv.org/abs/0705.2533
Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures
"I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular 'concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract)."
http://arxiv.org/abs/0705.2525
On the physical meaning of the Unruh effect
Emil T.Akhmedov, Douglas Singleton
7 pages
"We present simple arguments that detectors moving with constant acceleration (even acceleration for a finite time) should detect particles. The effect is seen to be universal. Moreover, detectors undergoing linear acceleration and uniform, circular motion both detect particles for the same physical reason. We show that if one uses a circularly orbiting electron in a constant external magnetic field as the Unruh--DeWitt detector, then the Unruh effect physically coincides with the experimentally verified Sokolov--Ternov effect."
http://arxiv.org/abs/0705.2462
LambdaCDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?
Richard Lieu
14 pages, 3 figures and 3 tables
"Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter and now dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of LambdaCDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind LambdaCDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark."
http://arxiv.org/abs/0705.2462
LambdaCDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?
Richard Lieu
"Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark."
Fighting words indeed. Check out the table on page 6!
:smile:
http://arxiv.org/abs/0705.2656
Inflationary universe in loop quantum cosmology
Xin Zhang, Yi Ling
25 pages, 5 figures
"Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed."
http://arxiv.org/abs/0705.2629
Dual Computations of Non-abelian Yang-Mills on the Lattice
J. Wade Cherrington, Dan Christensen, Igor Khavkine
18 pages, 7 figures
"In the past several decades there have been a number of proposals for computing with dual forms of non-abelian Yang-Mills theories on the lattice. Motivated by the gauge-invariant, geometric picture offered by dual models and successful applications of duality in the U(1) case, we revisit the question of whether it is practical to perform numerical computation using non-abelian dual models. Specifically, we consider three-dimensional SU(2) pure Yang-Mills as an accessible yet non-trivial case in which the gauge group is non-abelian. Using methods developed recently in the context of spin foam quantum gravity, we describe a Metropolis algorithm for sampling the dual ensemble and efficiently computing the dual amplitude. We relate our algorithms to prior work in non-abelian dual computations of Hari Dass and his collaborators, addressing several problems that have (to the best our knowledge) been left open. We report results of spin expectation value computations over a range of lattice sizes and couplings that are in agreement with our conventional lattice computations. We conclude with an outlook on further development of dual methods and their application to problems of current interest."
brief mention:
http://arxiv.org/abs/0705.2643
Symmetries, Singularities and the De-Emergence of Space
Thibault Damour, Hermann Nicolai
10 pages
though in a very different formal context, this paper reminded me of the recent work of Kirill Krasnov and Yuri Shtanov----where the metric disappears near the singularity, but spacetime evolution continues because the theory of gravity is non-metric. you could say that in Krasnov-Shtanov non-metric gravity the conventional idea of space also "de-emerges" near a classical singularity---while the classical singularity is removed. For the Krasnov-Shtanov paper see post #591 or http://arxiv.org/abs/0705.2047
EDITED IN to reply to francesca's next post:
Dear francesca, thanks for helping by supplying the papers of Corichi et al and Padmanabhan. However look back at post #595
http://physicsforums.com/showpost.php?p=1334313&postcount=595
from four days ago. they are the first two papers mentioned in that post, IIRC. :-)
francesca
May21-07, 08:37 AM
http://arxiv.org/abs/0705.2440
Quantum Structure of Geometry: Loopy and fuzzy?
Authors: Alejandro Corichi, Jose A. Zapata
Abstract: In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy.
http://arxiv.org/abs/0705.2533
Dark Energy and Gravity
Authors: T. Padmanabhan
Abstract: I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).
http://arxiv.org/abs/0705.3024
Symmetries, Horizons, and Black Hole Entropy
S. Carlip
6 pages; first prize essay, 2007 Gravity Research Foundation essay contest
"Black holes behave as thermodynamic systems, and a central task of any quantum theory of gravity is to explain these thermal properties. A statistical mechanical description of black hole entropy once seemed remote, but today we suffer an embarrassment of riches: despite counting very different states, many inequivalent approaches to quantum gravity obtain identical results. Such 'universality' may reflect an underlying two-dimensional conformal symmetry near the horizon, which can be powerful enough to control the thermal characteristics independent of other details of the theory. This picture suggests an elegant description of the relevant degrees of freedom as Goldstone-boson-like excitations arising from symmetry breaking by the conformal anomaly."
ensabah6
May23-07, 12:50 AM
Hey Marcus, I do not know how to search this thread but did you ever post this paper?
http://arxiv.org/abs/gr-qc/0612134
From Quantum Hydrodynamics to Quantum Gravity
G.E. Volovik
(Submitted on 21 Dec 2006 (v1), last revised 17 Jan 2007 (this version, v5))
We discuss some lessons from quantum hydrodynamics to quantum gravity.
Comments: 20 pages, 1 figure, rapporteur article for Proceedings of MG11, session `Analog Models of and for General Relativity', references added
Subjects: General Relativity and Quantum Cosmology (gr-qc); Soft Condensed Matter (cond-mat.soft); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:gr-qc/0612134v5
http://arxiv.org/abs/0705.3892
Spin foam model from canonical quantization
Sergei Alexandrov
24 pages
(Submitted on 26 May 2007)
"We suggest a modification of the Barrett-Crane spin foam model of 4-dimensional Lorentzian general relativity motivated by the canonical quantization. The starting point is Lorentz covariant loop quantum gravity. Its kinematical Hilbert space is found as a space of the so-called projected spin networks. These spin networks are identified with the boundary states of a spin foam model and provide a generalization of the unique Barrette-Crane intertwiner. We propose a way to modify the Barrett-Crane quantization procedure to arrive at this generalization: the B field (bi-vectors) should be promoted not to generators of the gauge algebra, but to their certain projection. The modification is also justified by the canonical analysis of Plebanski formulation. Finally, we compare our construction with other proposals to modify the Barret-Crane model."
francesca
Jun3-07, 09:20 AM
http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Authors: Martin Bojowald
Abstract: While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often http://arxiv.org/abs/associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
I always like Bojowald jobs, and this one seems very readable at the first sight...
Loop Quantum Cosmology could be the main issue of the month
(see below how many papers this week :wink: )
and the conference on phenomenology is coming, we have the program now:
http://www.sissa.it/app/QGconference (http://www.sissa.it/app/QGconference/program.htm)
http://arxiv.org/abs/0705.4449
Title: Relic gravitons as the observable for Loop Quantum Cosmology
Authors: Jakub Mielczarek, Marek Szydlowski
http://arxiv.org/abs/0705.3741
Title: Black hole state counting in loop quantum gravity
Authors: P. Mitra
http://arxiv.org/abs/0705.3375
Title: Unsuccessful cosmology with Modified Gravity Models
Authors: Antonio De Felice, Mark Hindmarsh
http://arxiv.org/abs/0705.4002
Title: Dynamics of Quintom and Hessence Energies in Loop Quantum Cosmology
Authors: Hao Wei, Shuang Nan Zhang
thanks for spotting these, francesca, especially the Bojowald.
Here are some more that appeared in the past week:
http://arxiv.org/abs/0706.0471
http://arxiv.org/abs/0706.0469
Properties of the Volume Operator in Loop Quantum Gravity I: Results
Johannes Brunnemann, David Rideout
37 pages, 7 figures
(Submitted on 4 Jun 2007)
"We analyze the spectral properties of the volume operator of Ashtekar and Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the classical volume expression for regions in three dimensional Riemannian space. Our analysis considers for the first time generic graph vertices of valence greater than four. Here we find that the geometry of the underlying vertex characterizes the spectral properties of the volume operator, in particular the presence of a `volume gap' (a smallest non-zero eigenvalue in the spectrum) is found to depend on the vertex embedding. We compute the set of all non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of valence 5--7, and argue that these sets can be used to label spatial diffeomorphism invariant states. We observe how gauge invariance connects vertex geometry and representation properties of the underlying gauge group in a natural way. Analytical results on the spectrum on 4-valent vertices are included, for which the presence of a volume gap is proved. This paper presents our main results; details are provided by a companion paper arXiv:0706.0382v1."
http://arxiv.org/abs/0706.0382
Properties of the Volume Operator in Loop Quantum Gravity II: Detailed Presentation
Authors: Johannes Brunneman, David Rideout
95 pages, 65 figures
(Submitted on 4 Jun 2007)
"The properties of the Volume operator in Loop Quantum Gravity, as constructed by Ashtekar and Lewandowski, are analyzed for the first time at generic vertices of valence greater than four. The present analysis benefits from the general simplified formula for matrix elements of the Volume operator derived in gr-qc/0405060, making it feasible to implement it on a computer as a matrix which is then diagonalized numerically. The resulting eigenvalues serve as a database to investigate the spectral properties of the volume operator. Analytical results on the spectrum at 4-valent vertices are included. This is a companion paper to arXiv:0706.0469, providing details of the analysis presented there."
See also companion paper arXiv:0706.0469
http://arxiv.org/abs/0706.0283
Cosmography in testing loop quantum gravity
Marek Szydlowski, Wlodzimierz Godlowski, Tomasz Stachowiak
19 pages, 1 figure
(Submitted on 2 Jun 2007)
"It was recently suggested by Martin Bojowald that quantum gravity effects give rise to new, potentially observable effects. We check whether this is the case for astronomical tests by trying to constrain the density parameters of the Friedmann equation with a $(-)(1+z)^6$ type of contribution. We describe different interpretations of such an additional term: geometric effects of Loop Quantum Cosmology, effects of braneworld cosmological models, non-standard cosmological models in metric-affine gravity, and models with spinning fluid. Kinematical (or geometrical) tests based on null geodesics are insufficient to separate individual matter components when they behave like perfect fluid and scale in the same way. Still, it is possible to measure their overall effect. We use recent measurements of the coordinate distances from Fanaroff-Riley type IIb (FRIIb) radio galaxy (RG) data, supernovae type Ia (SNIa) data, baryon oscillation peak and cosmic microwave background radiation (CMBR) observations to obtain stronger bounds for the contribution of the considered type. We demonstrate that, while rho^2 corrections are very small, they can be tested by astronomical observations -- at least in principle. Bayesian criteria of model selection (Bayesian factor, AIC, and BIC) are used to check if additional parameters are detectable in the present epoch. As it turns out, the LambdaCDM model is favoured over the bouncing model driven by loop quantum effects. Or, in other words, the bounds obtained from cosmography are very weak, and from the point of view of the present data this model is indistinguishable from the LambdaCDM one."
http://arxiv.org/abs/0706.0174
Entropy signature of the running cosmological constant
Authors: Alfio Bonanno, Martin Reuter
57 pages, 7 figures
(Submitted on 1 Jun 2007 (v1), last revised 3 Jun 2007 (this version, v2))
"Renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values are investigated using a system of cosmological evolution equations which allows for an unrestricted energy exchange between the vacuum and the matter sector. It is demonstrated that the scale dependence of the gravitational parameters, the cosmological constant in particular, leads to an entropy production in the matter system. The picture emerges that the Universe started out from a state of vanishing entropy, and that the radiation entropy observed today is essentially due to the coarse graining (RG flow) in the quantum gravity sector which is related to the expansion of the Universe. Furthermore, the RG improved field equations are shown to possess solutions with an epoch of power law inflation immediately after the initial singularity. The inflation is driven by the cosmological constant and ends automatically once the RG running has reduced the vacuum energy to the level of the matter energy density."
http://arxiv.org/abs/0706.0179
Lattice Refining Loop Quantum Cosmology and Inflation
William Nelson, Mairi Sakellariadou (King's College, London)
12 pages
(Submitted on 1 Jun 2007)
"We study the importance of lattice refinement in achieving a successful inflationary era. We solve, in the continuum limit, the second order difference equation governing the quantum evolution in loop quantun cosmology, assuming both a fixed and a dynamically varying lattice in a suitable refinement model. We thus impose a constraint on the potential of a scalar field, so that the continuum approximation is not broken. Considering that such a scalar field could play the role of the inflaton, we obtain a second constraint on the inflationary potential so that there is consistency with the CMB data on large angular scales. For a $m^2\phi^2/2$ inflationary model, we combine the two constraints on the inflaton potential to impose an upper limit on $m$, which is severely fine-tuned in the case of a fixed lattice. We thus conclude that lattice refinement is necessary to achieve a natural inflationary model."
http://arxiv.org/abs/0706.0142
Quantum gravity phenomenology via Lorentz violations
Stephano Liberati
21 pages, 1 figure
(Submitted on 1 Jun 2007)
"The search for a quantum theory of gravity has been one of the main aims of theoretical physics for many years by now. However the efforts in this direction have been often hampered by the lack of experimental/observational tests able to select among, or at least constrain, the numerous quantum gravity models proposed so far. This situation has changed in the last decade thanks to the realization that some QG inspired violations of Lorentz symmetry could be constrained using current experiments and observations. This study it is not only allowing us to test at higher and higher energies a fundamental symmetry of spacetime but it is also providing us with hints and perspectives about the fundamental nature of gravity."
http://arxiv.org/abs/0706.1057
Effective equations for isotropic quantum cosmology including matter
Martin Bojowald, Hector Hernandez, Aureliano Skirzewski
42 pages
(Submitted on 7 Jun 2007)
"Effective equations often provide powerful tools to develop a systematic understanding of detailed properties of a quantum system. This is especially helpful in quantum cosmology where several conceptual and technical difficulties associated with the full quantum equations can be avoided in this way. Here, effective equations for Wheeler-DeWitt and loop quantizations of spatially flat, isotropic cosmological models sourced by a massive or interacting scalar are derived and studied. The resulting systems are remarkably different from that given for a free, massless scalar. This has implications for the coherence of evolving states and the realization of a bounce in loop quantum cosmology.
http://arxiv.org/abs/0706.0985
Bulk Entropy in Loop Quantum Gravity
Etera R. Livine, Daniel R. Terno
13 pages
(Submitted on 7 Jun 2007)
"In the framework of loop quantum gravity (LQG), having quantum black holes in mind, we generalize the previous boundary state counting (gr-qc/0508085) to a full bulk state counting. After a suitable gauge fixing we are able to compute the bulk entropy of a bounded region (the "black hole") with fixed boundary. This allows us to study the relationship between the entropy and the boundary area in details and we identify a holographic regime of LQG where the leading order of the entropy scales with the area. We show that in this regime we can fine tune the factor between entropy and area without changing the Immirzi parameter."
http://arxiv.org/abs/0706.1534
Coupling gauge theory to spinfoam 3d quantum gravity
Simone Speziale
18 pages
(Submitted on 11 Jun 2007)
"We construct a spinfoam model for Yang-Mills theory coupled to quantum gravity in three dimensional riemannian spacetime. We define the partition function of the coupled system as a power series in g_0^2 G that can be evaluated order by order using grasping rules and the recoupling theory. With respect to previous attempts in the literature, this model assigns the dynamical variables of gravity and Yang-Mills theory to the same simplices of the spinfoam, and it thus provides transition amplitudes for the spin network states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly that the partition function has a semiclassical limit given by the Regge discretization of the classical Yang-Mills action."
http://arxiv.org/abs/0706.1279
The wedding ring of MOND and non-exotic dark matter
B. Famaey, G.W. Angus, G. Gentile, H.S. Zhao
2 pages, research note submitted to A&A
(Submitted on 9 Jun 2007)
"The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND."
http://arxiv.org/abs/0706.1654
Gravity as an emergent phenomenon: A conceptual description
T. Padmanabhan
11 pages; Summary of several plenary talks in the conferences including XXIII Texas Symposium in Relativistic Astrophysics (Melbourne, Dec, 06); IAGRG Meeting (Delhi, Feb, 07); International workshop on theoretical high energy physics (Roorkee, Mar, 2007); to appear in the proceedings of IWTHEP
(Submitted on 12 Jun 2007)
"I describe several broad features of a programme to understand gravity as an emergent, long wavelength, phenomenon (like elasticity) and discuss one concrete framework for realizing this paradigm in the backdrop of several recent results."
http://arxiv.org/abs/0706.1899
Spacetime Lagrangian Formulation of Barbero-Immirzi Gravity
L. Fatibene, M. Francaviglia, C. Rovelli
12 pages
(Submitted on 13 Jun 2007)
"We shall here discuss a new spacetime gauge-covariant Lagrangian formulation of General Relativity by means of the Barbero-Immirzi SU(2)-connection on spacetime. To the best of our knowledge the Lagrangian based on SU(2) spacetime fields seems to appear here for the first time."
the above is a continuation of work posted earlier this year by the same authors:
http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages
(Submitted on 26 Feb 2007)
"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."
http://arxiv.org/abs/0706.1979
Note on black hole radiation spectrum in Loop Quantum Gravity
Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 2 figures
(Submitted on 14 Jun 2007)
"Recent detailed analysis within the Loop Quantum Gravity calculation of black hole entropy show a stair-like structure in the behavior of entropy as a function of horizon area. The non-trivial distribution of the degeneracy of the black hole horizon area eigenstates is at the origin of this behavior. This degeneracy distribution is analyzed and a phenomenological model is put forward to study the possible implications of this distribution in the black hole radiation spectrum."
Accidentally duplicated a post, so I'll edit to avoid repeating content. An important aspect of the papers being posted on arxiv is their rate of publication in peer-review journals. Prime examples of good places to publish Loop cosmology and gravity research are Physical Review D and Physical Review Letters. As a sample publication history: in 2006 Martin Bojowald posted 12 papers on arxiv of which 4 were published in Physical Review D.
For completeness, at the end of the list I'll tally the publication results.
7. arXiv:astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
Phys.Rev.Lett. 98 (2007) 031301
8. arXiv:gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
Phys.Rev. D75 (2007) 064022
9. arXiv:gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
Phys.Rev. D74 (2006) 123512
10. arXiv:gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
Gen.Rel.Grav. 38 (2006) 1771-1795
11. arXiv:gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
5 pages, 1 figure
Phys. Rev. D 75 (2007) 081301(R)
12. arXiv:gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at 'Einstein's Legacy in the New Millenium,' Puri, India, December 2005
Int.J.Mod.Phys. D15 (2006) 1545-1559
13. arXiv:hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics ``Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006
Int.J.Geom.Meth.Mod.Phys. 4 (2007) 25-52
14. arXiv:gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
Phys.Rev. D74 (2006) 044033
15. arXiv:gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
Class.Quant.Grav. 23 (2006) 4983-4990
16. arXiv:gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153
17. arXiv:gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)
18. arXiv:gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at this http URL
Living Rev.Rel. 8 (2005) 11
Of the dozen 2006 papers, those published in Physical Review D (PRD) were 4
in Physical Review Letters (PRL) 1
in Classical and Quantum Gravity (CQG) 1
in General Relativity and Gravitation (GRG) 1
in International Journal of Modern Physics 1
in International Journal of Geometrical Methods in Modern Physics 1
in Living Reviews in Relativity 1
chapters in books 2
http://arxiv.org/abs/0706.2342
Gravitational Wilson Loop and Large Scale Curvature
Herbert W. Hamber, Ruth M. Williams
22 pages, 6 figures
(Submitted on 15 Jun 2007)
"In a quantum theory of gravity the gravitational Wilson loop, defined as a suitable quantum average of a parallel transport operator around a large near-planar loop, provides important information about the large-scale curvature properties of the geometry. Here we shows that such properties can be systematically computed in the strong coupling limit of lattice regularized quantum gravity, by performing a local average over rotations, using an assumed near-uniform measure in group space. We then relate the resulting quantum averages to an expected semi-classical form valid for macroscopic observers, which leads to an identification of the gravitational correlation length appearing in the Wilson loop with an observed large-scale curvature. Our results suggest that strongly coupled gravity leads to a positively curved (De Sitter-like) quantum ground state, implying a positive effective cosmological constant at large distances."
Bojowald also recently showed that a positive effective cosmological constant may not need to be put in "by hand" but could be *derived* from quantum gravity. Is there some connection with Hamber and Williams result? Francesca reported this Bojowald paper a few posts back in this thread, and Jal in a separate thread, but I will repeat it since it seems especially important:
http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
(Submitted on 30 May 2007)
While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.
Meanwhile Michael Turner, apparently unaware of Bojowald's result, or the Hamber Williams paper, surveys the problem.
http://arxiv.org/abs/0706.2186
Cosmic Acceleration, Dark Energy and Fundamental Physics
Michael S. Turner, Dragan Huterer (KICP, University of Chicago)
10 pages, 8 figures, invited review for Journal of the Physical Society of Japan, in press
(Submitted on 14 Jun 2007)
"A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (p\sim -rho), very smooth form of energy called 'dark energy' accounting for about 75% of the Universe. The 'simplest' explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science."
brief mention:
Masters Thesis in LQG by Muxin Han, a student of Jorge Pullin at LSU
http://arxiv.org/abs/0706.2623
Quantum Dyanmics of Loop Quantum Gravity
95 pages
http://arxiv.org/abs/0706.2445
GRB Cosmology
Volker Bromm, Abraham Loeb
24 pages, 9 figures, review to appear in "Gamma-ray Bursts" (CUP)
(Submitted on 18 Jun 2007)
"Current observations are about to open up a direct window into the final frontier of cosmology: the first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium (IGM) up to redshifts of several tens, when the first (Population III) stars had formed. The Swift mission, or future satellites such as EXIST, might be the first observatories to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up observations of the GRB afterglow emission would allow to probe the ionization state and metal enrichment of the IGM as a function of redshift."
http://arxiv.org/abs/0706.3658
Field theory on kappa--Minkowski space revisited: Noether charges and breaking of Lorentz symmetry
L. Freidel, J. Kowalski-Glikman, S. Nowak
22 pages, 1 figure
(Submitted on 25 Jun 2007)
"This paper is devoted to detailed investigations of free scalar field theory on $\kappa$-Minkowski space. After reviewing necessary mathematical tools we discuss in depth the Lagrangian and solutions of field equations. We analyze the spacetime symmetries of the model and construct the conserved charges associated with translational and Lorentz symmetry. We show that the version of the theory usually studied breaks Lorentz invariance in a subtle way: There is an additional trans-Planckian mode present, and an associated conserved charge (the number of such modes) is not a Lorentz scalar."
http://arxiv.org/abs/0706.3688
Why the Standard Model
Ali H. Chamseddine, Alain Connes
13 pages
(Submitted on 25 Jun 2007)
"The Standard Model is based on the gauge invariance principle with gauge group U(1)xSU(2)xSU(3) and suitable representations for fermions and bosons, which are begging for a conceptual understanding. We propose a purely gravitational explanation: space-time has a fine structure given as a product of a four dimensional continuum by a finite noncommutative geometry F. The raison d'etre for F is to correct the K-theoretic dimension from four to ten (modulo eight). We classify the irreducible finite noncommutative geometries of K-theoretic dimension six and show that the dimension (per generation) is a square of an integer k. Under an additional hypothesis of quaternion linearity, the geometry which reproduces the Standard Model is singled out (and one gets k=4)with the correct quantum numbers for all fields. The spectral action applied to the product MxF delivers the full Standard Model,with neutrino mixing, coupled to gravity, and makes predictions(the number of generations is still an input)."
http://arxiv.org/abs/0706.3690
A Dress for SM the Beggar
Ali H. Chamseddine
4 pages
(Submitted on 25 Jun 2007)
"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states."
http://arxiv.org/abs/0706.3586
Graceful exit via polymerization of pre-big bang cosmology
Giuseppe De Risi, Roy Maartens, Parampreet Singh
(Submitted on 25 Jun 2007)
"We consider a phenomenological modification of the Pre Big Bang scenario using ideas from the resolution of curvature singularities in Loop Quantum Cosmology. We show that non-perturbative Loop modifications to the dynamics, arising from the underlying polymer representation, can resolve the graceful exit problem. The curvature and the dilaton energy stay finite at all times, in both the string and Einstein frames. In the string frame, the dilaton tends to a constant value at late times after the bounce."
Subjects: High Energy Physics - Theory (hep-th)
Report number: IGPG-07/6-9http://arxiv.org/abs/0706.3239
Black hole entropy, curved space and monsters
Stephen D.H. Hsu, David Reeb
4 pages
(Submitted on 21 Jun 2007)
"We investigate the microscopic origin of black hole entropy, in particular the gap between the maximum entropy of ordinary matter and that of black holes. Using curved space, we construct configurations with entropy greater than their area in Planck units. These configurations have pathological properties and we refer to them as monsters. When monsters are excluded we recover the entropy bound on ordinary matter S < A3/4. This bound implies that essentially all of the microstates of a semiclassical black hole are associated with the growth of a slightly smaller black hole which absorbs some additional energy. Our results suggest that the area entropy of black holes is the logarithm of the number of distinct ways in which one can form the black hole from ordinary matter and smaller black holes, but only after the exclusion of monster states."
Steve Hsu is a proven researcher with a good publication record. Although this paper sounds very strange, I'll gamble on it being of interest.
http://arxiv.org/abs/0706.4431
The Einstein static universe in Loop Quantum Cosmology
Luca Parisi, Marco Bruni, Roy Maartens, Kevin Vandersloot
6 pages, 7 figures
(Submitted on 29 Jun 2007)
"Loop Quantum Cosmology strongly modifies the high-energy dynamics of Friedman-Robertson-Walker models and removes the big-bang singularity. We investigate how LQC corrections affect the stability properties of the Einstein static universe. In General Relativity, the Einstein static model with positive cosmological constant Lambda is unstable to homogeneous perturbations. We show that LQC modifications can lead to a centre of stability for a large enough positive value of Lambda."
http://arxiv.org/abs/0706.4452
What is the Mathematical Structure of Quantum Spacetime?
Louis Crane
25 pages
(Submitted on 29 Jun 2007)
"We survey indications from different branches of Physics that the fine scale structure of spacetime is not adequately described by a manifold. Based on the hints we accumulate, we propose a new structure, which we call a quantum topos. In the process of constructing a quantum topos for quantum gravity, we propose a new, operational approach to the problem of the obervables in quantum gravity, which leads to a new mathematical point of view on the state sum models."
http://arxiv.org/abs/0706.4481
Isogravity: Toward an Electroweak and Gravitational Unification
Stephon H.S. Alexander
(Submitted on 29 Jun 2007)
"We present a model that unites the electroweak interaction with general relativity without specifying a space-time metric. This is made possible by embedding the kinetic terms for gravity and electroweak theory using one SL connection variable. The gauge theory is specified without relying on a space-time metric. We show that once a symmetry breaking mechanism is implemented that selects a global time-like direction, the electroweak theory and general relativity emerges with their associated massless degrees of freedom; the spin 1 vector boson and the spin 2 graviton."
http://arxiv.org/abs/0707.0588
Lattice Refining LQC and the Matter Hamiltonian
William Nelson, Mairi Sakellariadou (King's College, London)
14 pages, 3 figures
(Submitted on 4 Jul 2007)
"In the context of loop quantum cosmology, we parametrise the lattice refinement by a parameter, A, and the matter Hamiltonian by a parameter, delta. We then solve the Hamiltonian constraint for both a self-adjoint, and a non-self-adjoint Hamiltonian operator. Demanding that the solutions for the wave-functions obey certain physical restrictions, we impose constraints on the two-dimensional, (A,delta), parameter space, thereby restricting the types of matter content that can be supported by a particular lattice refinement model."
brief mention of a curious paper, seemingly off beaten track, which however has been accepted for publication by Classical and Quantum Gravity
http://arxiv.org/abs/0707.0341
Considering boundary conditions for black hole entropy in loop quantum gravity
Takashi Tamaki
4 pages, final version to be published in CQG
(Submitted on 3 Jul 2007)
"We argue for black hole entropy in loop quantum gravity (LQG) by taking into account the interpretation that there is no other side of the horizon..."
John Baez has a useful Derek Wise page
http://math.ucr.edu/home/baez/derek/
Cartanization is something that is likely to happen soon in quantum geometry/gravity.
Derek Wise thesis is a step in that direction and Baez page has links to talks and stuff related to that---preliminary research by him and Wise and others.
Thomas Thiemann's big book comes out in September
http://www.amazon.com/Canonical-Relativity-Cambridge-Monographs-Mathematical/dp/0521842638
Cambridge University Press.
688 pages (!)
there is a very early draft (2001) on arxiv
but that may not bear much relation to what is to appear six years later
Here is the C.U.P. webpage about the book, which gives the table of contents and other information:
http://www.cambridge.org/uk/catalogue/catalogue.asp?isbn=9780521842631
these are some exerpts from the publisher's webpage:
Modern Canonical Quantum General Relativity
Cambridge Monographs on Mathematical Physics
Thomas Thiemann
Max-Planck-Institut für Gravitationsphysik, Germany
...
...In order to construct quantum gravity one must reformulate quantum theory in a background independent way. ... complete treatise of the canonical quantisation of general relativity.... can be read by graduate students with basic knowledge of quantum field theory or general relativity.
...
Contents
Preface; Notation and conventions; Introduction;
Part I. Classical Foundations, Interpretation and the Canonical Quantisation Programme:
1. Classical Hamiltonian formulation of general relativity;
2. The problem of time, locality and the interpretation of quantum mechanics;
3. The programme of canonical quantisation;
4. The new canonical variables of Ashtekar for general relativity;
Part II. Foundations of Modern Canonical Quantum General Relativity:
5. Introduction;
6. Step I: the holonomy-flux algebra [P];
7. Step II: quantum algebra;
8. Step III: representation theory of [A];
9. Step IV: 1. Implementation and solution of the kinematical constraints;
10. Step V: 2. implementation and solution of the Hamiltonian constraint;
11. Step VI: semiclassical analysis;
Part III. Physical Applications:
12. Extension to standard matter;
13. Kinematical geometrical operators;
14. Spin foam models;
15. Quantum black hole physics;
16. Applications to particle physics and quantum cosmology;
17. Loop quantum gravity phenomenology;
Part IV. Mathematical Tools and their Connection to Physics:
18. Tools from general topology;
19. Differential, Riemannian, symplectic and complex geometry;
20. Semianalytical category;
21. Elements of fibre bundle theory;
22. Holonomies on non-trivial fibre bundles;
23. Geometric quantisation;
24. The Dirac algorithm for field theories with constraints;
25. Tools from measure theory;
26. Elementary introduction to Gelfand theory for Abelean C* algebras;
27. Bohr compactification of the real line;
28. Operator algebras and spectral theorem;
29. Refined algebraic quantisation (RAQ) and direct integral decomposition (DID);
30. Basics of harmonic analysis on compact Lie groups;
31. Spin network functions for SU(2);
32. Functional analytical description of classical connection dynamics;
Bibliography;
Index.
Ratzinger
Jul10-07, 06:27 PM
Judging by the toc, that looks extremely nice. Hope it will be really pitched at level of the beginner who has some knowledge in QFT and GR. Thielmann likes it more rigorous, am I right?
to get an audio+slide presentation that gives a condensed Thiemann treatment and could give, in a way, a "taste" of the book, get the slides here:
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL2/Thiemann.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
at the end, he says since he went very fast over it, and was a little sloppy in places, that everybody should buy his book (so they would understand then thoroughly the details)
and the last slide is a picture of his book.
BTW Thiemann mentions this animation sequence in his slides. It is a picture of spinnetwork quantum states of geometry evolving----but for graphic purposes one can replace a spinnetwork picture by a dual triangulation picture---so each colored EDGE is replaced by a colored TRIANGLE, and four-valent NODES are replaced by (foursided) TETRAHEDRA. So then the combinatorial or mathematical content is exactly the same there is simply more color in the picture. One sees chunky colored blocks instead of skinny colored sticks. Perhaps as a demo some animation studio made a short sequence of this model of evolving geometry. I will put the link here and try it to see if it works:
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/index.html
well it takes two minutes (part of which is title and credits). to find the links to the animation you need to scroll down pretty far on the index page.
If you have a Mac then you can skip the index page and click directly on
http://www.einstein-online.info/de/vertiefung/Spinnetzwerke/spinfoam2.mov
to me it looks like an abstract cinematic artwork and not like I could learn from it, but maybe that's just me.
One of the most impressive talks at Loops '07 was probably the one given by Martin Reuter about "asymptotically safe" quantum gravity, specifically the QEG approach.
get the slides here:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.pdf
and then when you are prepared to scroll rapidly down the slides in synch with the talk, click on the audio:
http://www.matmor.unam.mx/eventos/loops07/talks/PL3/Reuter.mp3
if you can stay in synch, slides with audio, it will mesh very well and make good sense----the audio follows the slides closely
==============
I am not familiar with the next author, he is perhaps a new arrival in the LQC field
http://arxiv.org/abs/0707.1816
Open FRW model in Loop Quantum Cosmology
Lukasz Szulc
12 pages
(Submitted on 12 Jul 2007)
"Open FRW model in Loop Quantum Cosmology is under consideration. The left and right invariant vector fields and holonomies along them are studied. It is shown that in the hyperbolic geometry of k=-1 it is possible to construct a suitable loop which provides us with quantum scalar constraint originally introduced by Vandersloot. Such an operator has correct geometrical interpretation. The quantum scalar constraint operator with negative cosmological constant is proven to be essentially self-adjoint."
Penrose talk given on 7 November 2005 at the Isaac Newton Institute at Cambridge describing
some "crazy ideas" about what came before the big bang
http://www.newton.cam.ac.uk/webseminars/pg+ws/2005/gmr/gmrw04/1107/penrose/
audio-and-slides show, great handdrawn pictures by Penrose
===========
http://arxiv.org/abs/0707.2548
The behavior of non-linear anisotropies in bouncing Bianchi I models of loop quantum cosmology
Dah-Wei Chiou, Kevin Vandersloot
15 pages, 10 figures
(Submitted on 17 Jul 2007)
"In homogeneous and isotropic loop quantum cosmology, gravity can behave repulsively at Planckian energy densities leading to the replacement of the big bang singularity with a big bounce. Yet in any bouncing scenario it is important to include non-linear effects from anisotropies which typically grow during the collapsing phase. We investigate the dynamics of a Bianchi I anisotropic model within the framework of loop quantum cosmology. Using effective semi-classical equations of motion to study the dynamics, we show that the big bounce is still predicted with only differences in detail arising from the inclusion of anisotropies. We show that the anisotropic shear term grows during the collapsing phase, but remains finite through the bounce. Immediately following the bounce, the anisotropies decay and with the inclusion of matter with equation of state w < +1, the universe isotropizes in the expanding phase."
=================================
http://arxiv.org/abs/0707.2153
Dark Energy from Structure - A Status Report
Thomas Buchert
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 58 pages, 2 figures
(Submitted on 14 Jul 2007)
"The effective evolution of an inhomogeneous universe model in any theory of gravitation may be described in terms of spatially averaged variables. In Einstein's theory, restricting attention to scalar variables, this evolution can be modeled by solutions of a set of Friedmann equations for an effective volume scale factor, with matter and backreaction source terms. The latter can be represented by an effective scalar field (`morphon field') modeling Dark Energy.
The present work provides an overview over the Dark Energy debate in connection with the impact of inhomogeneities, and formulates strategies for a comprehensive quantitative evaluation of backreaction effects both in theoretical and observational cosmology. We recall the basic steps of a description of backreaction effects in relativistic cosmology that lead to refurnishing the standard cosmological equations, but also lay down a number of challenges and unresolved issues in connection with their observational interpretation.
The present status of this subject is intermediate: we have a good qualitative understanding of backreaction effects pointing to a global instability of the standard model of cosmology; exact solutions and perturbative results modeling this instability lie in the right sector to explain Dark Energy from inhomogeneities. It is fair to say that, even if backreaction effects turn out to be less important than anticipated by some researchers, the concordance high-precision cosmology, the architecture of current N-body simulations, as well as standard perturbative approaches all fall short in correctly describing the Late Universe."
we now have FOUR preprints of articles which are to be included in the SPECIAL GRG ISSUE ON DARK ENERGY
the one that appeared earlier is by Bojowald, describing how the effect of accelerating expansion could be the result of a quantum correction in LQG dynamics---something which, if true, would lead to a distinctive expansion history different from what one would see in a simple cosmological constant model.
So besides Buchert, we have Bojowald, Koyama, and Padmanabhan:
1. arXiv:0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
2. arXiv:0706.1557
The cosmological constant and dark energy in braneworlds
Kazuya Koyama
Invited Review for a special Gen. Rel. Grav. issue on Dark Energy, 22 pages, 13 figures
3. arXiv:0705.2533
Title: Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures
We also know that the team editing the special issue is George Ellis of Capetown, Roy Maartens of Portsmouth UK, and Hermann Nicolai of AEI Potsdam.
============
I am currently in doubt as to what to make of this paper by Philip Mannheim
http://arxiv.org/abs/0707.2283
Conformal Gravity Challenges String Theory
Philip D. Mannheim
8 pages. Proceedings write-up of talk presented at PASCOS-07, Imperial College London, July 2007
(Submitted on 16 Jul 2007)
"The cosmological constant problem and the compatibility of gravity with quantum mechanics are the two most pressing problems in all of gravitational theory. While string theory nicely addresses the latter, it has so far failed to provide any compelling solution to the former. On the other hand, while conformal gravity nicely addresses the cosmological constant problem (by naturally quenching the amount by which the cosmological constant gravitates rather than by quenching the cosmological constant itself), the fourth order derivative conformal theory has long been thought to possess a ghost when quantized. However, it has recently been shown by Bender and Mannheim that not only do theories based on fourth order derivative equations of motion not have ghosts, they actually never had any to begin with, with the apparent presence of ghosts being due entirely to treating operators which were not Hermitian on the real axis as though they were. When this is taken care of via an underlying PT symmetry that such theories are found to possess, there are then no ghosts at all and the S-matrix is fully unitary. Conformal gravity is thus advanced as a fully consistent four-dimensional alternative to ten-dimensional string theory."
An essential reference in this paper is [1] another Mannheim article in Progress in Particle and Nuclear Physics 2005
http://arxiv.org/abs/astro-ph/0505266
Alternatives to Dark Matter and Dark Energy
Philip D. Mannheim (University of Connecticut)
87 pages, 3 figures. To appear in Progress in Particle and Nuclear Physics, 2005
(Submitted on 12 May 2005 (v1), last revised 1 Aug 2005 (this version, v2))
"We review the underpinnings of the standard Newton-Einstein theory of gravity, and identify where it could possibly go wrong. In particular, we discuss the logical independence from each other of the general covariance principle, the equivalence principle and the Einstein equations, and discuss how to constrain the matter energy-momentum tensor which serves as the source of gravity. We identify the a priori assumption of the validity of standard gravity on all distance scales as the root cause of the dark matter and dark energy problems, and discuss how the freedom currently present in gravitational theory can enable us to construct candidate alternatives to the standard theory in which the dark matter and dark energy problems could then be resolved. We identify three generic aspects of these alternate approaches: that it is a universal acceleration scale which determines when a luminous Newtonian expectation is to fail to fit data, that there is a global cosmological effect on local galactic motions which can replace galactic dark matter, and that to solve the cosmological constant problem it is not necessary to quench the cosmological constant itself, but only the amount by which it gravitates."
Another key reference [2] is
http://arxiv.org/abs/0706.0207
No-ghost theorem for the fourth-order derivative Pais-Uhlenbeck oscillator model
Carl M. Bender, Philip D. Mannheim
4 pages
(Submitted on 1 Jun 2007)
"Contrary to common belief, it is shown that theories whose field equations are higher than second order in derivatives need not be stricken with ghosts. In particular, the prototypical fourth-order derivative Pais-Uhlenbeck oscillator model is shown to be free of states of negative energy or negative norm. When correctly formulated (as a PT symmetric theory), the theory determines its own Hilbert space and associated positive-definite inner product. In this Hilbert space the model is found to be a fully acceptable quantum-mechanical theory that exhibits unitary time evolution."
http://arxiv.org/abs/0707.3070
Quantum Gravity from Simplices: Analytical Investigations of Causal Dynamical Triangulations
Dario Benedetti
116 pages, 42 figures; PhD thesis, Utrecht University, Advisor: Prof. Renate Loll
(Submitted on 20 Jul 2007)
"A potentially powerful approach to quantum gravity has been developed over the last few years under the name of Causal Dynamical Triangulations. Numerical simulations have given very interesting results in the cases of two, three and four spacetime dimension. The aim of this thesis is to give an introduction to the subject (Chapter 1), and try to push the analytical understanding of these models further. This is done by first studying (Chapter 2) the case of a (1+1)-dimensional spacetime coupled to matter, in the form of an Ising model, by means of high- and low-temperature expansions. And after (Chapter 3) by studying a specific model in (2+1) dimensions, whose solution and continuum limit are presented."
http://arxiv.org/abs/0707.3064
Water vapour in the atmosphere of a transiting extrasolar planet
Giovanna Tinetti, Alfred Vidal-Madjar, Mao-Chang Liang, Jean-Philippe Beaulieu, Yuk Yung, Sean Carey, Robert J. Barber, Jonathan Tennyson, Ignasi Ribas, Nicole Allard, Gilda E. Ballester, David K. Sing, Franck Selsis
Nature 2007, 448, p 163
(Submitted on 20 Jul 2007)
"Water is predicted to be among, if not the most abundant molecular species after hydrogen in the atmospheres of close-in extrasolar giant planets (hot-Jupiters) Several attempts have been made to detect water on an exoplanet, but have failed to find compelling evidence for it or led to claims that should be taken with caution. Here we report an analysis of recent observations of the hot-Jupiter HD189733b taken during the transit, where the planet passed in front of its parent star. We find that absorption by water vapour is the most likely cause of the wavelength-dependent variations in the effective radius of the planet at the infrared wavelengths 3.6, 5.8 and 8 microns. The larger effective radius observed at visible wavelengths may be due to either star variability or the presence of clouds/hazes. We explain the most recent thermal infrared observations of the planet during secondary transit behind the star, reporting a non-detection of water on HD189733b, as being a consequence of the nearly isothermal vertical profile of the planet.s atmosphere. Our results show that water is detectable on extrasolar planets using the primary transit technique and that the infrared should be a better wavelength region than the visible, for such searches."
http://arxiv.org/abs/0707.4026
Renormalization and black hole entropy in Loop Quantum Gravity
Ted Jacobson
7 pages
(Submitted on 26 Jul 2007)
"Microscopic state counting for a black hole in Loop Quantum Gravity yields a result proportional to horizon area, and inversely proportional to Newton's constant and the Immirzi parameter. It is argued here that before this result can be compared to the Bekenstein-Hawking entropy of a macroscopic black hole, the scale dependence of both Newton's constant and the area must be accounted for. The two entropies could then agree for any value of the Immirzi parameter, if a certain renormalization property holds."
Jacobson's reference [15] is a Martin Reuter paper
[15] M. Reuter and J. M. Schwindt, “Scale-dependent metric and causal
structures in quantum Einstein gravity,” JHEP 0701, 049 (2007)
[arXiv:hep-th/0611294].
http://arxiv.org/abs/0707.4513
Linearized dynamics from the 4-simplex Regge action
Bianca Dittrich, Laurent Freidel, Simone Speziale
16 (+9 Appendix) pages, 1 figure
(Submitted on 31 Jul 2007)
"We study the relation between the hessian matrix of the riemannian Reggae action on a 4-simplex and linearized quantum gravity. We give an explicit formula for the hessian as a function of the geometry, and show that it has a single zero mode. We then use a 3d lattice model to show that (i) the zero mode is a remnant of the continuum diffeomorphism invariance, and (ii) we recover the complete free graviton propagator in the continuum limit. The results help clarify the structure of the boundary state needed in the recent calculations of the graviton propagator in loop quantum gravity, and in particular its role in fixing the gauge."
http://arxiv.org/abs/0708.0037
The Immirzi Parameter as a topological quantization ambiguity
Simone Mercuri
5 pages
(Submitted on 31 Jul 2007)
"The Immirzi ambiguity is traced back to the non-trivial behavior of the state functional under large gauge transformations of the spatial rotations group, emphasizing the role that the Nieh-Yan class plays in gravity when spinor matter is considered."
http://arxiv.org/abs/0707.4568
Emergent Quantum Mechanics and Emergent Symmetries
Gerard 't Hooft
10 pages, 1 figure. Presented at PASCOS 13, Imperial College, London, July 6, 2007
(Submitted on 31 Jul 2007)
"Quantum mechanics is 'emergent' if a statistical treatment of large scale phenomena in a locally deterministic theory requires the use of quantum operators. These quantum operators may allow for symmetry transformations that are not present in the underlying deterministic system. Such theories allow for a natural explanation of the existence of gauge equivalence classes (gauge orbits), including the equivalence classes generated by general coordinate transformations. Thus, local gauge symmetries and general coordinate invariance could be emergent symmetries, and this might lead to new alleys towards understanding the flatness problem of the Universe."
http://arxiv.org/abs/0707.4572
The Grand View of Physics
Gerard 't Hooft
5 pages, 1 figure. Presented at Salam +50, Imperial College, London, July 7, 2007
(Submitted on 31 Jul 2007)
"Abdus Salam was known for his `grand views', grand views of science as well as grand views of society. In this talk the grand view of theoretical physics is put in perspective."
http://arxiv.org/abs/0708.0250
Holography in spherically symmetric loop quantum gravity
Rodolfo Gambini, Jorge Pullin
5 pages
(Submitted on 2 Aug 2007)
"We show that holography arises naturally in the context of spherically symmetric loop quantum gravity. The result is not dependent on detailed assumptions about the dynamics of the theory being considered. It ties strongly the amount of information contained in a region of space to the tight mathematical underpinnings of loop quantum geometry, at least in this particular context."
http://arxiv.org/abs/0708.0062
On Information Theory, Spectral Geometry and Quantum Gravity
Achim Kempf, Robert Martin
4 pages
(Submitted on 1 Aug 2007)
"We show that there exists a deep link between the two disciplines of information theory and spectral geometry. This allows us to obtain new results on a well known quantum gravity motivated natural ultraviolet cutoff which describes an upper bound on the spatial density of information. Concretely, we show that, together with an infrared cutoff, this natural ultraviolet cutoff beautifully reduces the path integral of quantum field theory on curved space to a finite number of ordinary integrations. We then show, in particular, that the subsequent removal of the infrared cutoff is safe."
Although I could not evaluate this paper, it sounded too interesting not to mention.
http://arxiv.org/abs/0708.0573
The Height of a Giraffe
Don N. Page
12 pages
(Submitted on 3 Aug 2007 (v1), last revised 3 Aug 2007 (this version, v2))
"A minor modification of the arguments of Press and Lightman leads to an estimate of the height of the tallest running, breathing organism on a habitable planet as the Bohr radius multiplied by the three-tenths power of the ratio of the electrical to gravitational forces between two protons (rather than the one-quarter power that Press got for the largest animal that would not break in falling over, after making an assumption of unreasonable brittleness). My new estimate gives a height of about 3.6 meters rather than Press's original estimate of about 2.6 cm. It also implies that the number of atoms in the tallest runner is very roughly of the order of the nine-tenths power of the ratio of the electrical to gravitational forces between two protons, which is about 3 x 10^32."
http://arxiv.org/abs/0708.0429
Observing the temperature of the Big Bang through large scale structure
Pedro Ferreira, Joao Magueijo
(Submitted on 2 Aug 2007)
"It is widely accepted that the Universe underwent a period of thermal equilibrium at very early times. One expects a residue of this primordial state to be imprinted on the large scale structure of space time. In this paper we study the morphology of this thermal residue in a universe whose early dynamics is governed by a scalar field. We calculate the amplitude of fluctuations on large scales and compare it to the imprint of vacuum fluctuations. We then use the observed power spectrum of fluctuations on the cosmic microwave background to place a constraint on the temperature of the Universe before and during inflation. We also present an alternative scenario where the fluctuations are predominantly thermal and near scale-invariant."
http://arxiv.org/abs/0708.0750
On the q-quantum gravity loop algebra
Seth Major
(Submitted on 6 Aug 2007)
"A class of deformations of the q-quantum gravity loop algebra is shown to be incompatible with the combinatorics of Temperley-Lieb recoupling theory with deformation parameter at a root of unity. This incompatibility appears to extend to more general deformation parameters."
Alejandro Satz has a series of four posts about the Loops 07 conference. The first had a number of photographs and the other three reported on presentations and discussion. Here's a sample from the last report:
==quote from Reality Conditions blog==
And now the last question. It asked, to all plenary speakers, to say they "dream for Loops '17"; that is, on their most optimistic possible view, what is the title and abstract of the talk they imagine themselves presenting within ten years?
Many of the answers were predictable and variations of a basic template: abstracts saying "we present a complete theory of quantum gravity with testable (or, in the most ambitious cases, confirmed) predictions." ... Reuter had one of the most concrete dreams: "It is shown that LQG is equivalent to Asymptotic Safety, and that that the quantization ambiguities in it are finite in number and equivalent to the dimensionality of the Non-Gaussian Fixed Point." And finally, there was an extremely amusing exchange between...
==endquote==
Loops 17 means the Quantum Geometry/Quantum Gravity conference of 2017, only ten years out from now. Reuter's program is a bold one---joining two fertile lines of QG/QG research. My sense of him doesn't compat with his saying something merely as pleasantry or diplomatic grace-note. there's probably some serious longrange vision in his "dream for 2017"/
You might be interested to read all three of Satz' reports. Garrett Lisi has an interesting one as well. I think for various reasons this conference will be remembered by those who had the good sense and fortune to be there.
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-3.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-2.html
http://realityconditions.blogspot.com/2007/07/loops-07-conference-report-part-1.html
Here's a PF post from Garrett while he was at the conference:
http://physicsforums.com/showpost.php?p=1365902&postcount=4
The paper he delivered at the conference, and a report containing lots more personal impressions is at his website.
Bee Hossenfelder did an outstanding email interview with Garrett
http://backreaction.blogspot.com/2007/08/garrett-lisis-inspiration.html
and for the interview she prepared a valuable footnote which gives a links-thumbnail-bio
containing "All URL need to know" about Garrett Lisi and his E8 ToE (exceptional Liegroup number eight theory-of-basically-everything)
==Bee's links footnote==
http://interstice.com/~aglisi/Physics/CV.html
Garrett Lisi is a wandering surfer-physicist, working on nomothetic unification while searching for the perfect wave. After graduating UCLA at the top of his class and getting his Ph.D. from UC San Diego, Garrett took off for Maui to windsurf and do physics on his own. Last year Garrett won a research grant from http://www.fqxi.org/ FQXi, which he spent on food, a laptop, and a new snowboard.
His work on unifying general relativity and the standard model as an E8 principal bundle was featured as a recent http://math.ucr.edu/home/baez/week253.html
This Week's Find by John Baez . Impatient with the slow progress of technology, Garrett has been manually uploading his brain to the web as an open-source theoretical research wiki: http://deferentialgeometry.org/ Deferential Geometry. He also blogs occasionally at http://www.fqxi.org/community/blogs.php FQXi blogs and has a semi-secret personal journal.
Garrett recently presented his work at conferences in http://www.matmor.unam.mx/eventos/loops07/ Mexico and http://interstice.com/~aglisi/albums/Iceland/Iceland.html Iceland, is currently hopping around California, and is looking forward to visiting the Perimeter Institute in October.
==endquote==
http://arxiv.org/abs/0708.0883
The complete LQG propagator: I. Difficulties with the Barrett-Crane vertex
Emanuele Alesci, Carlo Rovelli
31 pages
(Submitted on 7 Aug 2007)
"Some components of the graviton two-point function have been recently computed in the context of loop quantum gravity, using the spinfoam Barrett-Crane vertex. We complete the calculation of the remaining components. We find that, under our assumptions, the Barrett-Crane vertex does not yield the correct long distance limit. We argue that the problem is general and can be traced to the intertwiner-independence of the Barrett-Crane vertex, and therefore to the well-known mismatch between the Barrett-Crane formalism and the standard canonical spin networks. In a companion paper we illustrate the asymptotic behavior of a vertex amplitude that can correct this difficulty."
Part 2, which is TO APPEAR, is their reference [13]
[13] E. Alesci, C. Rovelli, “The complete LQG propagator: II. Asymptotics of the vertex” to appear.
This paper is actually shorter than you might expect. The main body is only pages 1 - 15 and the rest is a technical appendix and bibliography. On page 15, at the end of the Conclusions section, it says
"In the companion paper [13], we show that, perhaps surprisingly, a vertex with a suitable asymptotic behavior can overcame all these difficulties."
the paper also proposes some possibly interesting topics for young researchers. On page 2 it says "Recently, a vertex amplitude that modifies the BC amplitude, and which addresses precisely the problems that we find here, has been proposed [16, 17], see also [18]. It would be of great interest to repeat the calculation presented here for the new vertex proposed in those papers."
the way I read this is that there still good entry-level research topics in the field. it seems to say "repeat our calcuations but in this other case and you have a PhD thesis" whether you get a positive negative result doesnt matter---it needs to be checked. it's nice---the field is producing plenty of problems and it is still not overcrowded. References [17,18] are to vertex formulas proposed by Livine Speziale and by Alexandrov, not by Rovelli himself et al.
===============
The September 2007 issue of the monthly Notices of the American Mathematical Society has a perceptive book review of Smolin's The Trouble with Physics.
http://www.ams.org/notices/200708/tx070800990p.pdf
Since the exceptional Liegroup E8 plays an important role in Garrett Lisi's unification work that we've been hearing about, I pass on a link that Peter Woit found to an exponsitory piece about E8.
http://www-math.mit.edu/~dav/notices07.pdf
In case anyone wants to check out the whole issue, the TOC link to the current AMS Notices is http://www.ams.org/notices/200708/
http://arxiv.org/abs/0708.1236
Flipped spinfoam vertex and loop gravity
Jonathan Engle, Roberto Pereira, Carlo Rovelli
37 pages, 4 figures
(Submitted on 9 Aug 2007)
"We introduce a vertex amplitude for 4d loop quantum gravity. We derive it from a conventional quantization of a Regge discretization of euclidean general relativity. This yields a spinfoam sum that corrects some difficulties of the Barrett-Crane theory. The second class simplicity constraints are imposed weakly, and not strongly as in Barrett-Crane theory. Thanks to a flip in the quantum algebra, the boundary states turn out to match those of SO(3) loop quantum gravity -- the two can be identified as eigenstates of the same physical quantities -- providing a solution to the problem of connecting the covariant SO(4) spinfoam formalism with the canonical SO(3) spin-network one. The vertex amplitude is SO(3) and SO(4)-covariant. It rectifies the triviality of the intertwiner dependence of the Barrett-Crane vertex, which is responsible for its failure to yield the correct propagator tensorial structure. The construction provides also an independent derivation of the kinematics of loop quantum gravity and of the result that geometry is quantized."
http://arxiv.org/abs/0708.1317
Functional Renormalization Group Equations, Asymptotic Safety, and Quantum Einstein Gravity
Martin Reuter, Frank Saueressig
Based on lectures given by M.R. at the 'First Quantum Geometry and Quantum Gravity School', Zakopane, Poland, March 2007, and the 'Summer School on Geometric and Topological Methods for Quantum Field Theory', Villa de Leyva, Colombia, July 2007, and by F.S. at NIKHEF, Amsterdam, The Netherlands, June 2006
(Submitted on 9 Aug 2007)
"These lecture notes provide a pedagogical introduction to a specific continuum implementation of the Wilsonian renormalization group, the effective average action. Its general properties and, in particular, its functional renormalization group equation are explained in a simple scalar setting. The approach is then applied to Quantum Einstein Gravity (QEG). The possibility of constructing a fundamental theory of quantum gravity in the framework of Asymptotic Safety is discussed and the supporting evidence is summarized."
http://arxiv.org/abs/0708.1261
Super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
10 pages
(Submitted on 9 Aug 2007)
"We investigate the dynamics of super-inflation in two versions of Loop Quantum Cosmology, one in which the Friedmann equation is modified by the presence of inverse volume corrections, and one in which quadratic corrections are important. Computing the tilt of the power spectrum of the perturbed scalar field in terms of fast-roll parameters, we conclude that the first case leads to a power spectrum that is scale invariant for steep power law negative potentials and for the second case, scale invariance is obtained for positive potentials that asymptote to a constant value for large values of the scalar field. It is found that in both cases, the horizon problem is solved with only a few e-folds of super-inflationary evolution."
http://arxiv.org/abs/0708.1264
Testing modified gravity with motion of satellites around galaxies
J. W. Moffat, V. T. Toth
5 pages, 1 figure
(Submitted on 9 Aug 2007)
"A modified gravity (MOG) theory that has been successfully fitted to galaxy rotational velocity data, cluster data and the Bullet Cluster 1E0657-56 is fitted to the motion of satellite galaxies around host galaxies at distances 50-400 kpc providing a new sensitive test to the MOG. We show that observational data from the Sloan Digital Sky Survey strongly favor the MOG, while Milgrom's MOND fails on these scales by predicting nearly constant rms velocities of satellites."
Marcus,
How was the Loop Quantum congres last week between 9 and the 11 of august.
Thanks for asking, Steve. I wasnt at the inaugural IGC conference (9-11 August) but I should post links to the program of speakers.
http://www.gravity.psu.edu/igc/conf_files/prelim_agenda.html
At the moment this has not been updated since 8 August, and there is no indication that there are downloads available of the slides and audio.
Here is the start of a thread on the IGC conference:
http://physicsforums.com/showthread.php?t=179185
The main Loops conference for the year was "Loops 07" which took place in June. Here is the program
http://www.matmor.unam.mx/eventos/loops07/program.html
which has links to the plenary talks page and the contributed talks page
these have links to SLIDES AND AUDIO that you can download
the homepage for Loops 07 is here:
http://www.matmor.unam.mx/eventos/loops07/
=============
the IGC conference at Penn State 9-11 August was not exactly a Loops conference but it was interesting because of what it stands for. It inaugurated a NEW INSTITUTE that will bring together observational cosmologists with quantum gravity people and other theorists. If you refer to that thread or if you download the PDF files listing the names of the talks and speakers you will see that the focus is very broad and inclusive. So both the Institute for Gravitation and the Cosmos (IGC) and its inaugural celebration will serve as a MEETING GROUND for people in quite a range of different specialties
(quantum gravity, gravitational wave observation, cosmic ray observation, mainstream or classical cosmology, quantum cosmology....)
Ashtekar, the director of the new Institute, is a Loop researcher with current emphasis on Loop Quantum Cosmology (LQC) but this does not mean that the IGC will be specializing in LQC! His vision is very broad and inclusive.
There is a lot of action on all fronts in cosmology and the IGC will probably connect with all of it.
Besides the fact that I'm happy about the creation of the new Institute, I can't tell you very much about the Birthday Party. What little I know about it is in that other thread.
here's the homepage for the Institute for Gravitation and the Cosmos conference
http://www.gravity.psu.edu/igc/
http://arxiv.org/abs/0708.1595
A New Spin Foam Model for 4d Gravity
Laurent Freidel, Kirill Krasnov
40 pages
(Submitted on 13 Aug 2007)
"Starting from the Plebanski formulation of gravity as a constrained BF theory we propose a new spin foam model for 4d Riemmanian quantum gravity that generalises the well-known model of Barrett-Crane and resolves the ultralocality problem that this model is known to possess. It is well known that the BF formulation of 4d gravity possesses two sectors: one corresponding to gravity and the other topological. The model presented here is shown to give a quantisation of the gravitational sector. The present model is dual to the recently proposed spin foam model of Engle et al. which, we show, corresponds to the topological sector of the theory. One important outcome of our approach is that it also allow us to introduce the Immirzi parameter into the framework of spin foam quantisation. We generalize some of our considerations to the Lorentzian setting and obtain a new spin foam model in that context as well."
http://arxiv.org/abs/0708.1721
Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?
Bianca Dittrich, Thomas Thiemann
12 pages
(Submitted on 13 Aug 2007)
"One of the celebrated results of Loop Quantum Gravity (LQG) is the discreteness of the spectrum of geometrical operators such as length, area and volume operators. This is an indication that Planck scale geometry in LQG is discontinuous rather than smooth. However, there is no rigorous proof thereof at present, because the afore mentioned operators are not gauge invariant, they do not commute with the quantum constraints. The relational formalism in the incarnation of Rovelli's partial and complete observables provides a possible mechanism for turning a non gauge invariant operator into a gauge invariant one. In this paper we investigate whether the spectrum of such a physical, that is gauge invariant, observable can be predicted from the spectrum of the corresponding gauge variant observables. We will not do this in full LQG but rather consider much simpler examples where field theoretical complications are absent. We find, even in those simpler cases, that kinematical discreteness of the spectrum does not necessarily survive at the gauge invariant level. Whether or not this happens depends crucially on how the gauge invariant completion is performed. This indicates that 'fundamental discreteness at Planck scale in LQG' is an empty statement. To prove it, one must provide the detailed construction of gauge invariant versions of geometrical operators."
http://arxiv.org/abs/0708.1915
Consistently Solving the Simplicity Constraints for Spinfoam Quantum Gravity
Etera R. Livine, Simone Speziale
6 pages, 2 figures
(Submitted on 14 Aug 2007 (v1), last revised 14 Aug 2007 (this version, v2))
"We give an independent derivations of the Engle-Pereira-Rovelli spinfoam model for quantum gravity which appeared in arXiv:0705.2388. Using the coherent state techniques we introduced in arXiv:0705.0674, we show that the EPR model realizes a consistent imposition of the simplicity constraints."
http://arxiv.org/abs/0708.1561
Towards Quantum Noncommutative kappa-deformed Field Theory
M. Daszkiewicz (IFT, Wroclaw University), J. Lukierski (IFT, Wroclaw University), M. Woronowicz (IFT, Wroclaw University)
15 pages
(Submitted on 11 Aug 2007)
"We introduce new quantum kappa-star product describing the multiplication of quantized kappa-deformed free fields. The kappa-deformation of local free quantum fields originates from two sources: noncommutativity of space-time and the kappa-deformation of field oscillators algebra. We demonstrate that for suitable choice of kappa-deformed field oscillators the kappa-deformed version of microcausality condition is satisfied, and it leads to the deformation of the Pauli-Jordan commutation function defined by the kappa-deformed mass shell. We show by constructing the kappa-deformed Fock space that effectively the kappa-deformed oscillator algebra does not change the bosonic statistics of n-particle states. The proposed star product is extended to the product of n fields, which for n=4 defines the interaction vertex in perturbative description of noncommutative quantum lambda phi^4 field theory. It follows that the classical fourmomentum conservation law is satisfied at the interaction vertices."
http://arxiv.org/abs/0708.2481
Comment on "Are the spectra of geometrical operators in Loop Quantum Gravity really discrete?" by B. Dittrich and T. Thiemann
Carlo Rovelli
6 pages, 1 figure
(Submitted on 20 Aug 2007)
"I argue that the prediction of physical discreteness at the Planck scale in loop gravity is a reasonable conclusion that derives from a sensible ensemble of hypotheses, in spite of some contrary arguments considered in an interesting recent paper by Dittrich and Thiemann. The counter-example presented by Dittrich and Thiemann illustrates a pathology which does not seem to be present in gravity. I also point out a common confusion between two distinct frameworks for the interpretation of general-covariant quantum theory, and observe that within one of these, the derivation of physical discreteness is immediate, and not in contradiction with gauge invariance."
brief mention, the following might also be of interest
http://arxiv.org/abs/0708.2738
The Physical Process First Law for Bifurcate Killing Horizons
Aaron J. Amsel, Donald Marolf, Amitabh Virmani
19 pages
(Submitted on 20 Aug 2007)
"The physical process version of the first law for black holes states that the passage of energy and angular momentum through the horizon results in a change in area \frac{\kappa}{8 \pi} \Delta A = \Delta E - \Omega \Delta J, so long as this passage is quasi-stationary. A similar physical process first law can be derived for any bifurcate Killing horizon in any spacetime dimension d >=3 using much the same argument. However, to make this law non-trivial, one must show that sufficiently quasi-stationary processes do in fact occur. In particular, one must show that processes exist for which the shear and expansion remain small, and in which no new generators are added to the horizon. Thorne, MacDonald, and Price considered related issues when an object falls across a d=4 black hole horizon. By generalizing their argument to arbitrary d >=3 and to any bifurcate Killing horizon, we derive a condition under which these effects are controlled and the first law applies. In particular, by providing a non-trivial first law for Rindler horizons, our work completes the parallel between the mechanics of such horizons and those of black holes for d >=3. We also comment on the situation for d=2. "
http://arxiv.org/abs/0708.2639
Black holes in the TeVeS theory of gravity and their thermodynamics
Eva Sagi, Jacob D. Bekenstein
11 pages
(Submitted on 20 Aug 2007)
TeVeS, a relativistic theory of gravity, was designed to provide a basis for the modified Newtonian dynamics. Since TeVeS differs from general relativity (e.g., it has two metrics, an Einstein metric and a physical metric), black hole solutions of it would be valuable for a number of endeavors ranging from astrophysical modeling to investigations into the interrelation between gravity and thermodynamics. Giannios has recently found a TeVeS analogue of the Schwarzschild black hole solution. We proceed further with the program by analytically solving the TeVeS equations for a static spherically symmetric and asymptotically flat system of electromagnetic and gravity fields. We show that one solution is provided by the Reissner-Nordström metric as physical metric, the TeVeS vector field pointing in the time direction, and a TeVeS scalar field positive everywhere (the last feature protects from superluminal propagation of disturbances in the fields). We work out black hole thermodynamics in TeVeS using the physical metric; black hole entropy, temperature and electric potential turn out to be identical to those in general relativity. We find it inconsistent to base thermodynamics on the Einstein metric. Consequently the two temperatures Dubovsky--Sibiryakov scenario for violating the second law of thermodynamics cannot be set up in TeVeS."
http://arxiv.org/abs/0708.2428
A macroscopic test of the Aharonov-Bohm effect
Adam Caprez, Brett Barwick, Herman Batelaan
13 pages, 4 figures
(Submitted on 17 Aug 2007)
http://arxiv.org/abs/0708.3051
Topological Higher Gauge Theory - from BF to BFCG theory
F. Girelli, H. Pfeiffer, E. M. Popescu
15 pages
(Submitted on 22 Aug 2007)
"We study generalizations of 3- and 4-dimensional BF-theory in the context of higher gauge theory. First, we construct topological higher gauge theories as discrete state sum models and explain how they are related to the state sums of Yetter, Mackaay, and Porter. Under certain conditions, we can present their corresponding continuum counterparts in terms of classical Lagrangians. We then explain that two of these models are already familiar from the literature: the SigmaPhiEA-model of 3-dimensional gravity coupled to topological matter, and also a 4-dimensional model of BF-theory coupled to topological matter."
http://arxiv.org/abs/0708.2935
Loss of entanglement in quantum mechanics due to the use of realistic measuring rods
Rodolfo Gambini, Rafael A. Porto, Jorge Pullin
5 pages
(Submitted on 21 Aug 2007)
"We show that the use of real measuring rods in quantum mechanics places a fundamental gravitational limit to the level of entanglement that one can ultimately achieve in quantum systems. The result can be seen as a direct consequence of the fundamental gravitational limitations in the measurements of length and time in realistic physical systems. The effect may have implications for long distance teleportation and the measurement problem in quantum mechanics."
brief mention:
http://arxiv.org/abs/0708.2943
Dark Energy or Apparent Acceleration Due to a Relativistic Cosmological Model More Complex than FLRW?
Mustapha Ishak, James Richardson, Delilah Whittington, David Garred (The University of Texas at Dallas)
5 pages, 1 figure
(Submitted on 22 Aug 2007)
"We use the Szekeres inhomogeneous relativistic models in order to fit supernova combined data sets. We show that with a choice of the spatial curvature function that is guided by current observations, the models fit the supernova data as well as the LCDM model without requiring any dark energy component. The Szekeres models were originally derived as an exact solution to Einstein's equations with a general metric that has no symmetries and are regarded in the field as good candidates to represent the true lumpy universe that we observe. The best fit model found is also consistent with the requirement of spatial flatness at CMB scales. While more work remains, the result presented in this first paper appears to support the possibility of apparent acceleration."
http://arxiv.org/abs/0708.3017
Black Holes and Quantum Gravity at the LHC
Patrick Meade, Lisa Randall
http://arxiv.org/abs/0708.3288
The probability of inflation in Loop Quantum Cosmology
William Nelson
Proceedings for the International Workshop "From Quantum to Emergent Gravity: Theory and Phenomenology'', SISSA, Trieste (Italy), June 11-15 2007
(Submitted on 24 Aug 2007)
"The probability of there being sufficient inflation to solve the fine-tuning associated with the horizon and flatness problems has recently been shown to be exponentially small, within the context of classical general relativity. Here this result is extended by considering loop quantum gravity effects, that are significant at small scales. In addition to accounting for high-energy departures from classicality, it is shown that, in contrast to the classical case, the loop quantum cosmological probability measure is naturally finite, at least in some well defined region. It is also shown that these loop quantum gravity corrections can overcome the classical suppression of the probability only for extremely unnatural choices of ambiguity parameters, implying that single field, slow-roll inflation is exponentially unlikely."
special mention (PF poster)
http://arxiv.org/abs/0708.3563
Renormalization of vacuum energy in linearized quantum gravity
H. Nikolic
5 pages
(Submitted on 27 Aug 2007)
"In linearized quantum gravity, a shift of the average energy-momentum can be compensated by a shift of the average gravitational field. This allows a renormalization scheme that naturally removes the contribution of quantum vacuum fluctuations to the cosmological constant, solving the old cosmological-constant problem for weak gravitational fields."
http://arxiv.org/abs/0708.3813
Non-Commutativity of Effective Space-Time Coordinates and the Minimal Length
Florian Girelli, Etera R. Livine
5 pages
(Submitted on 28 Aug 2007)
"Considering that a position measurement can effectively involve a momentum-dependent shift and rescaling of the "true" space-time coordinates, we construct a set of effective space-time coordinates which are naturally non-commutative. They lead to a minimum length and are shown to be related to Snyder's coordinates and the five-dimensional formulation of Deformed Special Relativity. This effective approach then provides a natural physical interpretation for both the extra fifth dimension and the deformed momenta appearing in this context."
http://arxiv.org/abs/0708.2889
Probing Quantum Gravity using Photons from a Mkn 501 Flare Observed by MAGIC
J. Albert, et al., for the MAGIC Collaboration, John Ellis, N.E. Mavromatos, D.V. Nanopoulos, A.S. Sakharov, E.K.G. Sarkisyan
5 pages, 3 figures, submitted to Phys. Rev. Lett
(Submitted on 21 Aug 2007)
"We use the timing of photons observed by the MAGIC gamma-ray telescope during a flare of the active galaxy Markarian 501 to probe a vacuum refractive index ~ 1-(E/MQGn)n, n = 1,2, that might be induced by quantum gravity. The peaking of the flare is found to maximize for quantum-gravity mass scales MQG1 ~ 0.4x1018 GeV or MQG2 ~ 0.6x1011 GeV, and we establish lower limits MQG1 > 0.26x1018 GeV or MQG2 > 0.39x1011 GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC sensitivity to propagation effects at these levels. Thermal plasma effects in the source are negligible, but we cannot exclude the importance of some other source effect."
http://arxiv.org/abs/0708.2934
Observation of Galactic Sources of Very High Energy Gamma-Rays with the MAGIC Telescope
H. Bartko, for the MAGIC collaboration
Mod. Phys. Lett. A, Vol. 22, No. 29 (2007) pp. 2167-2174
(Submitted on 21 Aug 2007 (v1), last revised 25 Aug 2007 (this version, v2))
"The MAGIC telescope with its 17m diameter mirror is today the largest operating single-dish Imaging Air Cherenkov Telescope (IACT). It is located on the Canary Island La Palma, at an altitude of 2200m above sea level, as part of the Roque de los Muchachos European Northern Observatory. The MAGIC telescope detects celestial very high energy gamma-radiation in the energy band between about 50 GeV and 10 TeV. Since the autumn of 2004 MAGIC has been taking data routinely, observing various objects, like supernova remnants (SNRs), gamma-ray binaries, Pulsars, Active Galactic Nuclei (AGN) and Gamma-ray Bursts (GRB). We briefly describe the observational strategy, the procedure implemented for the data analysis, and discuss the results of observations of Galactic Sources."
http://arxiv.org/abs/0708.3571
MAGIC multiwavelength observations: policy, and some recent results
MAGIC Collaboration: Alessandro De Angelis
5 pages, 5 figures; to be published in the Proceedings The Second Multiwavelength Workshop for Next Generation Gamma-Ray Experiments, Adler Planetarium, Chicago, Illinois, August 9-10, 2007
(Submitted on 27 Aug 2007 (v1), last revised 28 Aug 2007 (this version, v2))
"MAGIC, 17 meters of diameter, is the world's largest single dish Imaging Atmospheric Cherenkov Telescope, and reaches in the analysis the lowest energy threshold (60 GeV) among the VHE gamma detectors. Completed in September 2004, MAGIC started full operation with its first cycle of data taking in February 2005. MAGIC observations in the galaxy cover, among others, supernova remnants, the Galactic Center and binary systems. The low threshold makes of MAGIC the IACT looking deepest in the Universe: the record of extragalactic sources detected includes Active Galactic Nuclei (AGN) at z > 0.2. Here we discuss the present performance of MAGIC and the policy for the use of MAGIC data in multiwavelength campaigns. After a review of some recent highlights from MW studies, including the discovery of the most distant source ever detected (the AGN 3C279 at z = 0.54), we present the expected performance of MAGIC after the inauguration of the second telescope, scheduled for September 21st, 2008. Multiwavelength studies are a key for the study of emission mechanisms from galactic and extragalactic sources, and Very-High Energy photon detectors are becoming crucial as the GLAST era approaches."
http://arxiv.org/abs/astro-ph/0702008
Variable VHE gamma-ray emission from Markarian 501
MAGIC Collaboration, (J. Albert, et al)
51 pages, 28 figures. Submitted to ApJ. Changes with respect to V1: a) More detailed abstract, b)Section 2 (technical details of instrument and analysis) significantly shortened, c) Corrected formula 8, d) Corrected figures 20 and 21, e) Discussion on Quantum Gravity limits is removed f) Corrected some typos in text and added some references
(Submitted on 1 Feb 2007 (v1), last revised 8 Jun 2007 (this version, v2))
"The blazar Markarian 501 (Mrk 501) was observed at energies above 0.10 TeV with the MAGIC telescope from May through July 2005. The high sensitivity of the instrument enabled the determination of the flux and spectrum of the source on a night-by-night basis. Throughout our observational campaign, the flux from Mrk 501 was found to vary by an order of magnitude. Intra-night flux variability with flux-doubling times down to 2 minutes was observed during the two most active nights, namely June 30 and July 9. These are the fastest flux variations ever observed in Mrk 501. The ~20-minute long flare of July 9 showed an indication of a 4 +/- 1 min time delay between the peaks of F(<0.25 TeV) and F(>1.2 TeV), which may indicate a progressive acceleration of electrons in the emitting plasma blob. The flux variability was quantified for several energy ranges, and found to increase with the energy of the gamma-ray photons. The spectra hardened significantly with increasing flux, and during the two most active nights, a spectral peak was clearly detected at 0.43 +/- 0.06 TeV and 0.25 +/- 0.07 TeV, respectively for June 30 and July 9. There is no evidence of such spectral feature for the other nights at energies down to 0.10 TeV, thus suggesting that the spectral peak is correlated with the source luminosity. These observed characteristics could be accommodated in a Synchrotron-Self-Compton (SSC) framework in which the increase in gamma-ray flux is produced by a freshly injected (high energy) electron population."
the next two papers form a pair.
the first, by an eminent European astrophysicist Simon White (director of the Garching MPI for Astrophysics) warned of degradation to his field's creative talent, intelligence and credibility resulting from collaboration with hordes of experimental particle physicists in singleminded quest to detect "dark energy"
the second is a REPLY to Simon White's April paper, from an American astrophysicist Edward W. "Rocky" Kolb.
He is director of Particle Astrophysics Center at Fermilab, and led a joint Dark Energy Task Force made up of astrophysicists and particle physicists commissioned to advise the DOE, NSF, and NASA. Here is the task force report:
http://arxiv.org/abs/astro-ph/0609591
http://arxiv.org/abs/0704.2291
Fundamentalist physics: why Dark Energy is bad for Astronomy
Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics. 19 pages including 3 figures
(Submitted on 18 Apr 2007)
"Astronomers carry out observations to explore the diverse processes and objects which populate our Universe. High-energy physicists carry out experiments to approach the Fundamental Theory underlying space, time and matter. Dark Energy is a unique link between them, reflecting deep aspects of the Fundamental Theory, yet apparently accessible only through astronomical observation. Large sections of the two communities have therefore converged in support of astronomical projects to constrain Dark Energy. In this essay I argue that this convergence can be damaging for astronomy. The two communities have different methodologies and different scientific cultures. By uncritically adopting the values of an alien system, astronomers risk undermining the foundations of their own current success and endangering the future vitality of their field. Dark Energy is undeniably an interesting problem to attack through astronomical observation, but it is one of many and not necessarily the one where significant progress is most likely to follow a major investment of resources."
http://arxiv.org/abs/0708.1199
A Thousand Invisible Cords Binding Astronomy and High-Energy Physics
Rocky Kolb (Department of Astronomy & Astrophysics, The Enrico Fermi Institute, and The Kavli Institute for Cosmological Physics, The University of Chicago)
Why "Fundamentalist" Physics Is Good for Astronomy (in response to the paper of Simon White, http://arxiv.org/abs/0704.2291
(Submitted on 9 Aug 2007)
"The traditional realm of astronomy is the observation and study of the largest objects in the Universe, while the traditional domain of high-energy physics is the study of the smallest things in nature. But these two sciences concerned with opposite ends of the size spectrum are, in Muir's words, bound fast by a thousand invisible cords that cannot be broken. In this essay I propose that collaborations of astronomers and high-energy physicists on common problems are beneficial for both fields, and that both astronomy and high-energy physics can advance by this close and still growing relationship. Dark matter and dark energy are two of the binding cords I will use to illustrate how collaborations of astronomers and high-energy physicists on large astronomical projects can be good for astronomy, and how discoveries in astronomy can guide high-energy physicists in their quest for understanding nature on the smallest scales. Of course, the fields have some different intellectual and collaborative traditions, neither of which is ideal. The cultures of the different fields cannot be judged to be right or wrong; they either work or they don't. When astronomers and high-energy physicists work together, the binding cords can either encourage or choke creativity. The challenge facing the astronomy and high-energy physics communities is to adopt the best traditions of both fields. It is up to us to choose wisely."
http://arxiv.org/abs/0709.0076
Entropy calculation for a toy black hole
Hanno Sahlmann
12 pages, 3 figures
(Submitted on 2 Sep 2007)
"In this note we carry out the counting of states for a black hole in loop quantum gravity, however assuming an equidistant area spectrum. We find that this toy-model is exactly solvable, and we show that its behavior is very similar to that of the correct model. Thus this toy-model can be used as a nice and simplifying `laboratory' for questions about the full theory."
http://arxiv.org/abs/0708.4361
Fundamental Constants
Frank Wilczek
20 pages
(Submitted on 31 Aug 2007)
"The notion of 'fundamental constant' is heavily theory-laden. A natural, fairly precise formulation is possible in the context of the standard model (here defined to include gravity). Some fundamental constants have profound geometric meaning. The ordinary gravitational constant parameterizes the stiffness, or resistance to curvature, of space-time. The cosmological term parameterizes space-time's resistance to expansion -- which may be, and apparently is at present, a negative resistance, i.e. a tendency toward expansion. The three gauge couplings of the strong, electromagnetic, and weak interactions parameterize resistance to curvature in internal spaces. The remaining fundamental couplings, of which there are a few dozen, supply an ungainly accommodation of inertia. The multiplicity and variety of fundamental constants are esthetic and conceptual shortcomings in our present understanding of foundational physics. I discuss some ideas for improving the situation. I then briefly discuss additional constants, primarily cosmological, that enter into our best established present-day world model. Those constants presently appear as macroscopic state parameters, i.e. as empirical 'material constants' of the Universe. I mention a few ideas for how they might become fundamental constants in a future theory. In the course of this essay I've advertised several of my favorite speculations, including a few that might be tested soon."
http://arxiv.org/abs/0708.4236
Anticipating a New Golden Age
Frank Wilczek
25 pages, 15 figures
(Submitted on 31 Aug 2007)
"The standard model of fundamental interactions is remarkably successful, but it leaves an unfinished agenda. Several major questions seem ripe for exploration in the near future. I anticipate that the coming decade will be a Golden Age of discovery in fundamental physics."
http://arxiv.org/abs/0709.0502
Anthropics and Myopics: Conditional Probabilities and the Cosmological Constant
Irit Maor (1), Lawrence Krauss (1,2), Glenn Starkman (1,2) ((1) CERCA, Case Western Reserve University, (2) Dept of Astronomy, CWRU)
13 pages and 3 figures
(Submitted on 4 Sep 2007)
"The Anthropic Principle is claimed by many to provide a possible explanation for the observed smallness of the cosmological constant. However, correlations between the value of the cosmological constant and the existence of life can be demonstrated only under quite restrictive assumptions. Even allowing for a possible correlation, we demonstrate here that suggesting any such correlation is in fact causative is a much more subtle issue, and in general this latter claim will not be implied by the former."
exerpt: "We thank ... Raphael Bousso for lively discussions."
I can believe those were lively discussions :biggrin:
Apparently reduces anthropic "prediction" of the CC to absurdity. Bousso is a Landscape supporter. He wouldn't have liked that!
http://arxiv.org/abs/0709.0539
Causal sets and conservation laws in tests of Lorentz symmetry
David Mattingly
7 pages
(Submitted on 4 Sep 2007)
"Many of the most important astrophysical tests of Lorentz symmetry also assume that energy-momentum of the observed particles is exactly conserved. In the causal set approach to quantum gravity Lorentz symmetry holds but energy-momentum conservation is violated. We show that incorrectly assuming exact conservation can give rise to a spurious signal of Lorentz symmetry violation for a causal set. However, the size of this spurious signal is much smaller than can be currently detected and hence astrophysical Lorentz symmetry tests as currently performed are safe from Lorentz invariant violations of energy-momentum conservation."
http://arxiv.org/abs/0709.0551
Quantum Gravity Phenomenology without Lorentz Invariance Violation: a detailed proposal
Yuri Bonder, Daniel Sudarsky
23 pages
(Submitted on 5 Sep 2007)
"We propose a scheme for quantum gravity phenomenology which could be thought as arising form a granularity of space-time. Such granularity is considered as unspecified but is assumed to respect Lorentz Invariance. The proposal involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. Finally, we present the effective hamiltonian which could be used to analyze concrete experimental situations, some of which are briefly described."
Bojowald's article in the August 2007 issue of NATURE PHYSICS is available to non-subscribers here
http://npg.nature.com/nphys/journal/v3/n8/index.html
this gives the TOC for the August issue.
Scroll halfway down the page to where it says LETTERS and click on the PDF for the Bojo piece. it is called
What happened before the Big Bang?
In this piece Bojowald derives some definite limitations on what can be known about the universe prior to the beginning of expansion. Even though the singularity is no longer there, in the Loop Cosmology model, a Heisenberg-like principle of indeterminacy limits knowledge in some (but not all) respects.
The same August issue of Nature Physics also has a short commentary by Carlo Rovelli which discusses Bojowald's findings.
The next article is not directly related to the publications in Nature Physics. It reports ongoing work which attempts to go beyond the simple "minisuperspace" Loop Cosmology model. Current work is perturbative and goes outside the confines of the earlier models.
http://arxiv.org/abs/0709.0872
Cosmological vector modes and quantum gravity effects
Martin Bojowald, Golam Mortuza Hossain
20 pages
(Submitted on 6 Sep 2007)
"In contrast to scalar and tensor modes, vector modes of linear perturbations around an expanding Friedmann--Robertson--Walker universe decay. This makes them largely irrelevant for late time cosmology, assuming that all modes started out at a similar magnitude at some early stage. By now, however, bouncing models are frequently considered which exhibit a collapsing phase. Before this phase reaches a minimum size and re-expands, vector modes grow. Such modes are thus relevant for the bounce and may even signal the breakdown of perturbation theory if the growth is too strong. Here, a gauge invariant formulation of vector mode perturbations in Hamiltonian cosmology is presented. This lays out a framework for studying possible canonical quantum gravity effects, such as those of loop quantum gravity, at an effective level. As an explicit example, typical quantum corrections, namely those coming from inverse densitized triad components and holonomies, are shown to increase the growth rate of vector perturbations in the contracting phase, but only slightly. Effects at the bounce of the background geometry can, however, be much stronger."
brief mention
http://arxiv.org/abs/0709.0942
Matters of Gravity, The Newsletter of the Topical Group in Gravitation of the American Physical Society, Volume 30, Fall 2007
David Garfinkle, Greg Comer
(Submitted on 6 Sep 2007 (v1), last revised 6 Sep 2007 (this version, v2))
sample exerpt, from Jorge Pullin's report on GRG 18 conference:
"The 18th International Conference on General Relativity and Gravitation (GR18) ... in Sydney, Australia, July 8-14 2007.
Over 600 scientists converged on the Sydney Convention and Exhibition Center at spectacular Darling Harbour. There were 15 plenary talks and 55 parallel sessions...
During the conference the Committee of the International Society of General Relativity met. Among other topics, the results for the election of the president of the society were announced, Abhay Ashtekar was elected. ...
The Basilis Xanthopoulos prize was presented jointly to Martin Bojowald (PennState) and Thomas Thiemann (Albert Einstein Institute) for their seminal contributions in loop quantum gravity..."
Here is some information about the prize
http://www.physics.uoc.gr/Xanthopoulos/index.html
The six previous recipients of the Xantho include Gary Horowitz (1993), Carlo Rovelli (1995), and Juan Maldacena (2001).
http://arxiv.org/abs/0709.1625
Dark energy from corrections to the Wheeler-DeWitt equation
William Nelson, Mairi Sakellariadou (King's College, University of London, U.K.)
4 pages, 1 figure
(Submitted on 11 Sep 2007)
"We present a method for approximating the effective consequence of generic quantum gravity corrections to the Wheeler-DeWitt equation. We show that in many cases these corrections can produce departures from classical physics at large scales and that this behaviour is equivalent to additional matter components. This opens up the possibility that dark energy (and possible dark matter) could be large scale manifestations of quantum gravity corrections to classical general relativity. We examine the first order corrections to the Wheeler-DeWitt equation arising from loop quantum cosmology in the absence of lattice refinement and show how the ultimate breakdown in large scale physics occurs."
http://arxiv.org/abs/0709.1680
Towards new background independent representations for Loop Quantum Gravity
Madhavan Varadarajan
22 pages
(Submitted on 11 Sep 2007 (v1), last revised 11 Sep 2007 (this version, v2))
"Recently, uniqueness theorems were constructed for the representation used in Loop Quantum Gravity. We explore the existence of alternate representations by weakening the assumptions of the so called LOST uniqueness theorem. The weakened assumptions seem physically reasonable and retain the key requirement of explicit background independence. For simplicity, we restrict attention to the case of gauge group U(1)."
http://arxiv.org/abs/0709.1694
Successful ToO triggers on the extragalactic sources with the MAGIC telescope
D. Mazin, E. Lindfors, for the MAGIC Collaboration
Contribution to the 30th ICRC, Merida Mexico, July 2007 on behalf of the MAGIC Collaboration
(Submitted on 11 Sep 2007)
"The MAGIC collaboration has been performing Target of Opportunity (ToO) observations whenever alerted that known or potential very high energy gamma-ray emitting extragalactic sources were in a high flux state in the optical, X-ray band or/and in the TeV energy range. Here we report on MAGIC observations performed after such triggers, results of the analysis, and a possible optical-TeV correlation seen in the data. Detections as well as spectral and temporal characterestics of Mkn 180, PKS 2155-304, and 1ES 1011+496 are reported.
My note: this is a routine report of MAGIC observing AGN flares which describes some of the relevant details. It was their report on one flare, Makarian 501, that caused such a lot of controversy last month. This paper serves to provide background and perspective on the Makarian 501 findings.
http://arxiv.org/abs/0709.1621
On the Configuration Spaces of Homogeneous Loop Quantum Cosmology and Loop Quantum Gravity
Johannes Brunnemann, Christian Fleischhack
11 pages
(Submitted on 11 Sep 2007)
"The set of homogeneous isotropic connections, as used in loop quantum cosmology,forms a line l in the space of all connections \cal A. This embedding, however, does not continuously extend to an embedding of the configuration space \bar l of homogeneous isotropic loop quantum cosmology into that of loop quantum gravity, \bar{\cal A}. This follows from the fact that the parallel transports for general, non-straight paths in the base manifold do not depend almost periodically on l. Analogous results are given for the anisotropic case.
http://arxiv.org/abs/0709.1258
Fermi-point scenario of emergent gravity
G.E. Volovik
10 pages, 6 figures, draft for proceedings of conference "From Quantum to Emergent Gravity: Theory and Phenomenology", Trieste June 11-15 (2007)
(Submitted on 9 Sep 2007)
"Let us assume, that gravity is emergent low-energy phenomenon arising from the topologically stable defect in momentum space -- the Fermi point. What are the consequences? We discuss the natural values of fermion masses and cosmological constant; flatness of the Universe; bounds on the Lorentz violation; etc "
http://arxiv.org/abs/0709.1144
Cosmic Microwave Background Statistics for a Direction-Dependent Primordial Power Spectrum
Anthony R. Pullen, Marc Kamionkowski
8 pages, submitted to Phys Rev D
(Submitted on 7 Sep 2007)
"Statistical isotropy of primordial perturbations is a common assumption in cosmology, but it is an assumption that should be tested. To this end, we develop cosmic microwave background statistics for a primordial power spectrum that depends on the direction, as well as the magnitude, of the Fourier wavevector. We first consider a simple estimator that searches in a model-independent way for anisotropy in the square of the temperature (and/or polarization) fluctuation. We then construct the minimum-variance estimators for the coefficients of a spherical-harmonic expansion of the direction-dependence of the primordial power spectrum. To illustrate, we apply these statistics to an inflation model with a quadrupole dependence of the primordial power spectrum on direction and find that a power quadrupole as small as 2.3% can be detected with the Planck satellite."
http://arxiv.org/abs/0709.2051
The perturbative Regge-calculus regime of Loop Quantum Gravity
Eugenio Bianchi, Leonardo Modesto
40 pages
(Submitted on 13 Sep 2007)
"The relation between Loop Quantum Gravity and Regge calculus has been pointed out many times in the literature. In particular the large spin asymptotics of the Barrett-Crane vertex amplitude is known to be related to the Regge action. In this paper we study a semiclassical regime of Loop Quantum Gravity and show that it admits an effective description in terms of perturbative area-Regge-calculus. The regime of interest is identified by a class of states given by superpositions of four-valent spin networks, peaked on large spins.
As a probe of the dynamics in this regime, we compute explicitly two- and three-area correlation functions at the vertex amplitude level. We find that they match with the ones computed perturbatively in area-Regge-calculus with a single 4-simplex, once a specific perturbative action and measure have been chosen in the Regge-calculus path integral. Correlations of other geometric operators and the existence of this regime for other models for the dynamics are briefly discussed."
see page 33 (section 8) for question of how Regge fits with the new Spinfoam vertex proposed by Rovelli and others this year. The same analysis by Bianchi and Modesto presumably must be performed on the new vertex.
http://arxiv.org/abs/0709.2129
Loop Quantum Dynamics of the Schwarzschild Interior
Christian G. Boehmer, Kevin Vandersloot
15 pages, 13 figures
(Submitted on 13 Sep 2007)
"We examine the Schwarzschild interior of a black hole, incorporating quantum gravitational modifications due to loop quantum gravity. We consider an improved loop quantization using techniques that have proven successful in loop quantum cosmology. The central Schwarzschild singularity is resolved and the implications for the fate of an in-falling test particle in the interior region is discussed. The singularity is replaced by a Nariai type Universe. We discuss the resulting conformal diagram, providing a clear geometrical interpretation of the quantum effects.
brief mention:
http://arxiv.org/abs/0709.1965
Natural extension of the Generalised Uncertainty Principle
C. Bambi, F. R. Urban
4 pages, 1 figure
(Submitted on 13 Sep 2007)
"We discuss a gedanken experiment for the simultaneous measurement of position and momentum of a particle in de Sitter spacetime. We propose a further extension of the so called Generalised Uncertainty Principle (GUP) which suggests the existence of a minimum observable momentum. The new GUP is directly connected to the non--zero cosmological constant, which becomes a necessary ingredient for a more complete picture of the quantum spacetime."
http://arxiv.org/abs/0709.2365
Loop quantum gravity corrections to gravitational wave dispersion
Martin Bojowald, Golam Mortuza Hossain
27 pages
(Submitted on 14 Sep 2007)
"Cosmological tensor perturbations equations are derived for Hamiltonian cosmology based on Ashtekar's formulation of general relativity, including typical quantum gravity effects in the Hamiltonian constraint as they are expected from loop quantum gravity. This translates to corrections of the dispersion relation for gravitational waves. The main application here is the preservation of causality which is shown to be realized due to the absence of anomalies in the effective constraint algebra used."
Johannes Tambornino ILQGS talk, 4 September 2007
Relational observables and cosmological perturbation theory
Slides
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.pdf
Audio (4MB)
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.aif
Audio (38MB)
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.wav
Here is the main page for the International LQG Seminar, which lists upcoming and past semester talks
http://relativity.phys.lsu.edu/ilqgs/
Today Tuesday 18 September there was an important meeting of the ILQGS
http://relativity.phys.lsu.edu/ilqgs/
The scheduled presentation was by Jonathan Engle at Marseille, talking about the new spinfoam vertices. Several people joined in lively discussion (Abhay at Penn State, Carlo at Marseille, Laurent at Perimeter, Jorge at Louisiana, who was hosting the teleseminar.
Engle's notes PDF
http://relativity.phys.lsu.edu/ilqgs/engle091807.pdf
The better of the two audio options
http://relativity.phys.lsu.edu/ilqgs/engle091807.wav
http://arxiv.org/abs/0709.2433
Toward explaining black hole entropy quantization in loop quantum gravity
Hanno Sahlmann
14 pages, 5 figures
(Submitted on 15 Sep 2007)
"In a remarkable numerical analysis of the spectrum of states for a spherically symmetric black hole in loop quantum gravity, Corichi, Diaz-Polo and Fernandez-Borja found that the entropy of the black hole horizon increases in what resembles discrete steps as a function of area. In the present article we reformulate the combinatorial problem of counting horizon states in terms of paths through a certain space. This formulation sheds some light on the origins of this step-like behavior of the entropy. In particular, using a few extra assumptions we arrive at a formula that reproduces the observed step-length to a few tenths of a percent accuracy. However, in our reformulation the periodicity ultimately arises as a property of some complicated process, the properties of which, in turn, depend on the properties of the area spectrum in loop quantum gravity in a rather opaque way. Thus, in some sense, a deep explanation of the observed periodicity is still lacking."
Alain Connes and Matilde Marcolli NCG lectures on YouTube
http://noncommutativegeometry.blogspot.com/2007/09/you-tube-qft.html
I haven't had time to adequately sample these. They are from two years ago. My guess is that they are going to prove quite useful. Matilde gives the first one, racing thru QFT.
http://arxiv.org/abs/0709.2784
Putting a cap on causality violations in CDT
J. Ambjorn, R. Loll, W. Westra, S. Zohren
17 pages, 4 figures
(Submitted on 18 Sep 2007)
"The formalism of causal dynamical triangulations (CDT) provides us with a non-perturbatively defined model of quantum gravity, where the sum over histories includes only causal space-time histories. Path integrals of CDT and their continuum limits have been studied in two, three and four dimensions. Here we investigate a generalization of the two-dimensional CDT model, where the causality constraint is partially lifted by introducing weighted branching points, and demonstrate that the system can be solved analytically in the genus-zero sector."
http://arxiv.org/abs/0709.2905
In Search of Quantum de Sitter Space: Generalizing the Kodama State
Andrew Randono
Ph.D. dissertation, University of Texas at Austin. 150 pages
(Submitted on 18 Sep 2007)
"The Kodama state is unique in being an exact solution to all the constraints of quantum gravity that also has a well defined semi-classical interpretation as the quantum version of a classical spacetime, namely de Sitter or anti-de sitter space. Despite this, the state fails to pass some of the key tests of a physically realistic quantum state. In an attempt to resolve this problem, we track down the root of the problem to a choice for a particular parameter: the Immirzi parameter. The Kodama state takes this parameter to be complex, whereas modern formulations of canonical quantum gravity require that the parameter is real. We generalize the Kodama state to real values of the Immirzi parameter, and find that the generalization opens up a large Hilbert space of states, one of which can be directly interpreted as particular slicing of de Sitter space. We then show that these states resolve, or are expected to resolve many of the problems associated with the original version of the Kodama state. In order to resolve the interpretation of the multitude of states, we develop a new model of covariant classical and quantum gravity where the full Lorentz group is retained as a local symmetry group, and the canonical evolution generated by the constraints has a close relation to a larger group: the de Sitter group. This formalism gives strong evidence that the multitude of generalized Kodama states can be unified into a single quantum state that is quantum de Sitter space."
Thomas Larsson, an occasional poster here at PF Beyond forum, recently put two papers up on arxiv that have been published in past years.
http://arxiv.org/abs/0709.2539
Multi-dimensional Virasoro algebra and quantum gravity
T. A. Larsson
In: Mathematical physics research at the leading edge, ed: Charles V. Benton, pp 91-111, 2004 Nova Science Publishers Inc. ISBN 1-59033-905-3
(Submitted on 17 Sep 2007)
"I review the multi-dimensional generalizations of the Virasoro algebra, i.e. the non-central Lie algebra extensions of the algebra vect(N) of general vector fields in N dimensions, and its Fock representations. Being the Noether symmetry of background independent theories such as N-dimensional general relativity, this algebra is expected to be relevant to the quantization of gravity. To this end, more complicated modules which depend on dynamics in the form of Euler-Lagrange equations are described. These modules can apparently only be interpreted as quantum fields if spacetime has four dimensions and both bosons and fermions are present."
http://arxiv.org/abs/0709.2540
Manifestly covariant canonical quantization of gravity and diffeomorphism anomalies in four dimensions
T. A. Larsson
In: Focus on quantum gravity research, ed: David C. Moore, pp 261-310, 2006 Nova Science Publishers Inc. ISBN 1-59454-660-6
(Submitted on 17 Sep 2007)
"Canonical quantization of gravity requires knowledge about the representation theory of its constraint algebra, which is physically equivalent to the algebra of arbitrary 4-diffeomorphisms. All interesting lowest-energy representations are projective, making the relevant algebra into a four-dimensional generalization of the Virasoro algebra. Such diffeomorphism anomalies are invisible in field theory, because the relevant cocycles are functionals of the observer's trajectory in spacetime. The multi-dimensional Virasoro algebra acts naturally in the phase space of arbitrary histories, with dynamics playing the role of first-class constraints. General relativity is regularized by expanding all fields in Taylor series around the observer's trajectory, and truncating at some fixed order. This regularized but manifestly general-covariant theory is quantized in the history phase space, and dynamics is imposed afterwards, in analogy with BRST quantization. Infinities arise when the regularization is removed; it is presently unclear how these should be dealt with."
http://arxiv.org/abs/0709.3120
The flat FRW model in LQC: the self-adjointness
Wojciech Kamiński, Jerzy Lewandowski
13 pages
(Submitted on 19 Sep 2007)
"The flat FRW model coupled to the massless scalar field according to the improved, background scale independent version of Ashtekar, Pawlowski and Singh is considered. The core of the theory is addressed directly: the APS construction of the quantum Hamiltonian is analyzed under the assumption that cosmological constant Lambda <= 0. We prove the essential self-adjointness of the operator whose square-root defines the quantum Hamiltonian operator and therefore, provide the explicit definition. If Lambda<0, then the spectrum is discrete. In the Lambda=0 case, the essential and absolutely continues spectra of the operator are derived. The latter operator is related in the unitary way to the absolutely continuous part of the Quantum Mechanics operator a(-\frac{\partial^2}{\partial y^2} - \frac{b}{{\rm cosh}^2\hat{y}})
(a,b>0 being some constants) plus a trace class operator."
The following describes what seems like an important new way to get a handle on dark energy and the expansion history of the universe
http://arxiv.org/abs/0709.2955
Baryonic Acoustic Oscillations in 21cm Emission: A Probe of Dark Energy out to High Redshifts
Stuart Wyithe, Abraham Loeb, Paul Geil
16 pages, 9 figures. Submitted to MNRAS
(Submitted on 19 Sep 2007)
"Low-frequency observatories are currently being constructed with the goal of detecting redshifted 21cm emission from the epoch of reionization. These observatories will also be able to detect intensity fluctuations in the cumulative 21cm emission after reionization, from hydrogen in unresolved damped Ly-alpha absorbers (such as gas rich galaxies) down to a redshift z~3.5. The inferred power spectrum of 21cm fluctuations at all redshifts will show acoustic oscillations, whose co-moving scale can be used as a standard ruler to infer the evolution of the equation of state for the dark energy. We find that the first generation of low-frequency experiments (such as MWA or LOFAR) will be able to constrain the acoustic scale to within a few percent in a redshift window just prior to the end of the reionization era, provided that foregrounds can be removed over frequency band-passes of >8MHz. This sensitivity to the acoustic scale is comparable to the best current measurements from galaxy redshift surveys, but at much higher redshifts. Future extensions of the first generation experiments (involving an order of magnitude increase in the antennae number of the MWA) could reach sensitivities below one percent in several redshift windows and could be used to study the dark energy in the unexplored redshift regime of 3.5<z<12. Moreover, new experiments with antennae designed to operate at higher frequencies would allow precision measurements (<1%) of the acoustic peak to be made at more moderate redshifts (1.5<z<3.5), where they would be competitive with ambitious spectroscopic galaxy surveys covering more than 1000 square degrees. Together with other data sets, observations of 21cm fluctuations will allow full coverage of the acoustic scale from the present time out to z~12."
http://arxiv.org/abs/0709.2948
The Contribution of the Cosmological Constant to the Relativistic Bending of Light Revisited
Wolfgang Rindler, Mustapha Ishak (The University of Texas at Dallas)
5 pages, 2 figures
(Submitted on 19 Sep 2007)
"We study the effect of the cosmological constant Lambda on the bending of light by a concentrated spherically symmetric mass. Contrarily to previous claims, we show that when the Schwarzschild-de Sitter geometry is taken into account, Lambda does indeed contribute to the bending."
http://arxiv.org/abs/0709.3851
Asymptotic Safety
R. Percacci
To appear in "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter", ed. D. Oriti, Cambridge University Press
(Submitted on 24 Sep 2007)
"Asymptotic safety is a set of conditions, based on the existence of a nontrivial fixed point for the renormalization group flow, which would make a quantum field theory consistent up to arbitrarily high energies. After introducing the basic ideas of this approach, I review the present evidence in favor of an asymptotically safe quantum field theory of gravity".
I suspect this Percacci paper on Asymptotic Safety is important, so I just started a thread on it:
http://physicsforums.com/showthread.php?t=187046
http://arxiv.org/abs/0709.4225
The status of Quantum Geometry in the dynamical sector of Loop Quantum Cosmology
Wojciech Kaminski, Jerzy Lewandowski, Lukasz Szulc
12 pages
(Submitted on 26 Sep 2007)
"This letter is motivated by the recent papers by Ditrich and Thiemann and, respectively, by Rovelli discussing the status of Quantum Geometry in the dynamical sector of Loop Quantum Geometry. Since the papers consider model examples, we also study the issue in the case of an example, namely on the Loop Quantum Cosmology model of space-isotropic universe. We derive the Rovelli-Thiemann-Ditrich partial observables corresponding to the quantum geometry operators of LQC in both Hilbert spaces: the kinematical one and, respectively, the physical Hilbert space of solutions to the quantum constraints. We find, that Quantum Geometry can be used to characterize the physical solutions, and the operators of quantum geometry preserve many of their kinematical properties."
http://arxiv.org/abs/0709.4235
Extended matter coupled to BF theory
Winston J. Fairbairn (Phys-ENS), Alejandro Perez (CPT)
27 pages, 7 figures
(Submitted on 26 Sep 2007)
"Recently, a topological field theory of membrane-matter coupled to BF theory in arbitrary spacetime dimensions was proposed [1]. In this paper, we discuss various aspects of the four-dimensional theory. Firstly, we study classical solutions leading to an interpretation of the theory in terms of strings propagating on a flat spacetime. We also show that the general classical solutions of the theory are in one-to-one correspondence with solutions of Einstein's equations in the presence of distributional matter (cosmic strings). Secondly, we quantize the theory and present, in particular, a prescription to regularize the physical inner product of the canonical theory. We show how the resulting transition amplitudes are dual to evaluations of Feynman diagrams coupled to three-dimensional quantum gravity. Finally, we remove the regulator by proving the topological invariance of the transition amplitudes.
http://arxiv.org/abs/0709.4157
A combinatorial and field theoretic path to quantum gravity: the new challenges of group field theory
Daniele Oriti
16 pages, 7 figures; RevTex4; to appear in the proceedings of the Conference on Combinatorics and Physics, MPIM Bonn, March 19-23, 2007
(Submitted on 26 Sep 2007)
"Group field theories are a new type of field theories over group manifolds and a generalization of matrix models, that have recently attracted much interest in quantum gravity research. They represent a development of and a possible link between different approaches such as loop quantum gravity and simplicial quantum gravity. After a brief introduction to the GFT formalism we put forward a long but still far from exhaustive list of open issues that this line of research faces, and that could be represent interesting challenges for mathematicians and mathematical physicists alike."
http://arxiv.org/abs/0709.3947
Lorentz transformations in de Sitter relativity
R. Aldrovandi, J. P. Beltran Almeida, C. S. O. Mayor, J. G. Pereira
7 pages
(Submitted on 25 Sep 2007)
"The properties of Lorentz transformations in de Sitter relativity are studied. It is shown that, in addition to leaving invariant the velocity of light, they also leave invariant the length-scale related to the curvature of the de Sitter spacetime. The basic conclusion is that it is possible to have an invariant length parameter without breaking the Lorentz symmetry. This result may have important implications for the study of quantum kinematics, and in particular for quantum gravity."
a Baez student's thesis
http://arxiv.org/abs/0710.0032
Extended TQFT's and Quantum Gravity
Jeffrey Morton
112 pages, 17 figures - Ph.D Thesis (University of California, Riverside - 2007)
(Submitted on 29 Sep 2007)
"This paper gives a definition of an extended topological quantum field theory (TQFT) as a weak 2-functor Z: nCob_2 -> 2Vect, by analogy with the description of a TQFT as a functor Z: nCob -> Vect. We also show how to obtain such a theory from any finite group G. This theory is related to a topological gauge theory, the Dijkgraaf-Witten model. To give this definition rigorously, we first define a bicategory of cobordisms between cobordisms. We also give some explicit description of a higher-categorical version of Vect, denoted 2Vect, a bicategory of "2-vector spaces". Along the way, we prove several results showing how to construct 2-vector spaces of "Vect-valued presheaves" on certain kinds of groupoids. In particular, we use the case when these are groupoids whose objects are connections, and whose morphisms are gauge transformations, on the manifolds on which the extended TQFT is to be defined. On cobordisms between these manifolds, we show how a construction of ``pullback and pushforward'' of presheaves gives both the morphisms and 2-morphisms in 2Vect for the extended TQFT, and that these satisfy the axioms for a weak 2-functor. Finally, we discuss the motivation for this research in terms of Quantum Gravity. If the results can be extended from a finite group G to a Lie group, then for some choices of G this theory will recover an existing theory of Euclidean quantum gravity in 3 dimensions. We suggest extensions of these ideas which may be useful to further this connection and apply it in higher dimensions.
http://arxiv.org/abs/0709.4619
Gauge-invariant coherent states for Loop Quantum Gravity I: Abelian gauge groups
Benjamin Bahr, Thomas Thiemann
36 pages
(Submitted on 28 Sep 2007)
"In this paper we investigate the properties of gauge-invariant coherent states for Loop Quantum Gravity, for the gauge group U(1). This is done by projecting the corresponding complexifier coherent states, which have been applied in numerous occasions to investigate the semiclassical limit of the kinematical sector, to the gauge-invariant Hilbert space. This being the first step to construct physical coherent states, we arrive at a set of gauge-invariant states that approximate well the gauge-invariant degrees of freedom of abelian LQG. Furthermore, these states turn out to encode explicit information about the graph topology, and show the same pleasant peakedness properties known from the gauge-variant complexifier coherent states."
http://arxiv.org/abs/0709.4636
Gauge-invariant coherent states for Loop Quantum Gravity II: Non-abelian gauge groups
Benjamin Bahr, Thomas Thiemann
60 pages, 8 figures
(Submitted on 28 Sep 2007)
"This is the second paper concerning gauge-invariant coherent states for Loop Quantum Gravity. Here, we deal with the gauge group SU(2), this being a significant complication compared to the abelian U(1) case encountered in the previous article. We study gauge-invariant coherent states on certain special graphs by analytical and numerical methods. We find that their overlap is Gauss-peaked in gauge-invariant quantities, as long as states are not labeled by degenerate gauge orbits, i.e. points where the gauge-invariant configuration space has singularities. In these cases the overlaps are still concentrated around these points, but the peak profile exhibits a plateau structure. This shows how the semiclassical properties of the states are influenced by the geometry of the gauge-invariant phase space."
http://arxiv.org/abs/0710.0323
A Dual Algorithm for Non-abelian Yang-Mills coupled to Dynamical Fermions
J. Wade Cherrington
17 pages, 11 figures
(Submitted on 1 Oct 2007)
"We extend the dual algorithm recently described for pure, non-abelian Yang-Mills on the lattice to the case of lattice fermions coupled to Yang-Mills, by constructing an ergodic Metropolis algorithm for dynamic fermions that is local, exact, and built from gauge-invariant boson-fermion coupled configurations. For concreteness, we present in detail the case of three dimensions, for the group SU(2) and staggered fermions, however the algorithm readily generalizes with regard to group and dimension. The treatment of the fermion determinant makes use of a polymer expansion; as with previous proposals making use of the polymer expansion in higher than two dimensions, the critical question for practical applications is whether the presence of negative amplitudes can be managed in the continuum limit."
http://arxiv.org/abs/0710.0617
Numerical evidence of regularized correlations in spin foam gravity
J. Daniel Christensen, Etera R. Livine, Simone Speziale
5 pages, 3 figures
(Submitted on 2 Oct 2007)
"We report on the numerical analysis of the area correlations in spin foam gravity on a single 4-simplex considered by Rovelli in PRL 97 (2006) 151301. We compare the asymptotics and confirm the inverse squared distance leading behaviour at large scales. This supports the recent advances on testing the semiclassical limit of the theory. Furthermore, we show that the microscopic discreteness of the theory dynamically suppresses and regularizes the correlations at the Planck scale."
http://arxiv.org/abs/0710.0416
Effective Dynamics, Big Bounces and Scaling Symmetry in Bianchi Type I Loop Quantum Cosmology
Dah-Wei Chiou
19 pages, 6 figures, 1 table
(Submitted on 2 Oct 2007)
"The detailed formulation for loop quantum cosmology (LQC) in the Bianchi I model with a scalar massless field has been constructed. In this paper, its effective dynamics is studied in two improved strategies for implementing the LQC discreteness corrections. Both schemes show that the big bang is replaced by the big bounces, which take place up to three times, once in each diagonal direction, when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. These two strategies give different evolutions: In one scheme, the effective dynamics is independent of the choice of the finite sized cell prescribed to make Hamiltonian finite; in the other, the effective dynamics reacts to the macroscopic scales introduced by the boundary conditions. Both schemes reveal interesting symmetries of scaling, which are reminiscent of the relational interpretation of quantum mechanics and also suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale."
http://arxiv.org/abs/0710.0610
de Sitter Relativity and Quantum Physics
Authors: R. Aldrovandi, J. P. Beltran Almeida, C. S. O. Mayor, J. G. Pereira
10 pages. To appear in "Quantum Theory: Reconsideration of Foundations 4", 11-16 June 2007, Vaxjo University, Sweden (AIP Conference Proceedings, ed. by G. Adenier, A. Khrennikov and T. Nieuwenhuizen).
(Submitted on 2 Oct 2007)
"In the presence of a cosmological constant, interpreted as a purely geometric entity, absence of matter is represented by a de Sitter spacetime. As a consequence, ordinary Poincare' special relativity is no longer valid and must be replaced by a de Sitter special relativity. By considering the kinematics of a spinless particle in a de Sitter spacetime, we study the geodesics of this spacetime, the ensuing definitions of canonical momenta, and explore possible implications for quantum mechanics."
http://arxiv.org/abs/0710.1548
Propagation and interaction of chiral states in quantum gravity
Lee Smolin, Yidun Wan
34 pages, 30 figures
(Submitted on 5 Oct 2007)
"We study the stability, propagation and interactions of braid states in models of quantum gravity in which the states are four-valent spin networks embedded in a topological three manifold and the evolution moves are given by the dual Pachner moves. There are results for both the framed and unframed case. We study simple braids made up of two nodes which share three edges, which are possibly braided and twisted. We find three classes of such braids, those which both interact and propagate, those that only propagate, and the majority that do neither."
http://arxiv.org/abs/0710.1312
On Braid Excitations in Quantum Gravity
Yidun Wan
24 pages, 16 figures, 5 tables
(Submitted on 5 Oct 2007)
"We propose a new notation for the states in some models of quantum gravity, namely 4-valent spin networks embedded in a topological three manifold. With the help of this notation, equivalence moves, namely translations and rotations, can be defined, which relate the projections of diffeomorphic embeddings of a spin network. Certain types of topological structures, viz 3-strand braids as local excitations of embedded spin networks, are defined and classified by means of the equivalence moves. This paper formulates a mathematical approach to the further research of particle-like excitations in quantum gravity."
http://arxiv.org/abs/0710.1675
Is the cosmological "constant" a nonlocal quantum residue of discreteness of the causal set type?
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
13 pages. To appear in the proceedings of the PASCOS-07 Conference, held July, 2007, London
(Submitted on 9 Oct 2007)
"The evidence for an accelerating Hubble expansion appears to have confirmed the heuristic prediction, from causal set theory, of a fluctuating and 'ever-present' cosmological term in the Einstein equations. A more concrete phenomenological model incorporating this prediction has been devised and tested, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime."
http://arxiv.org/abs/0710.1562
Dark energy and 3-manifold topology
Torsten Asselmeyer-Maluga, Helge Rose
7 pages
(Submitted on 8 Oct 2007)
"We show that the differential-geometric description of matter by differential structures of spacetime leads to a unifying model of the three types of energy in the cosmos: matter, dark matter and dark energy. Using this model we are able to calculate the ratio of dark energy to the total energy of the cosmos."
Torsten and Helge have both posted here at Beyond forum.
There was quite a bit of discussion of some earlier work of theirs around 2005, if I remember right.
http://arxiv.org/abs/0710.2886
kappa-Minkowski space, scalar field, and the issue of Lorentz invariance
Laurent Freidel, Jerzy Kowalski-Glikman
Based on the talk given at the conference "From Quantum to Emergent Gravity", SISSA, June, 2007; to appear in the proceedings
(Submitted on 15 Oct 2007)
"We describe kappa-Minkowski space and its relation to group theory. The group theoretical picture makes it possible to analyze the symmetries of this space. As an application of this analysis we analyze in detail free field theory on kappa-Minkowski space and the Noether charges associated with deformed spacetime symmetries."
http://arxiv.org/abs/0710.2679
A New Class of Group Field Theories for 1st Order Discrete Quantum Gravity
Daniele Oriti, Tamer Tlas
48 pages, 4 figures
(Submitted on 14 Oct 2007)
"Group Field Theories, a generalization of matrix models for 2d gravity, represent a 2nd quantization of both loop quantum gravity and simplicial quantum gravity. In this paper, we construct a new class of Group Field Theory models, for any choice of spacetime dimension and signature, whose Feynman amplitudes are given by path integrals for clearly identified discrete gravity actions, in 1st order variables. In the 3-dimensional case, the corresponding discrete action is that of 1st order Regge calculus for gravity (generalized to include higher order corrections), while in higher dimensions, they correspond to a discrete BF-theory (again, generalized to higher order) with an imposed orientation restriction on hinge volumes, similar to that characterizing discrete gravity. The new models shed also light on the large distance or semi-classical approximation of spin foam models. This new class of group field theories may represent a concrete unifying framework for loop quantum gravity and simplicial quantum gravity approaches."
http://arxiv.org/abs/0710.3565
On the robustness of key features of loop quantum cosmology
Abhay Ashtekar, Alejandro Corichi, Parampreet Singh
(Submitted on 18 Oct 2007)
"A small simplification based on well motivated approximations is shown to make loop quantum cosmology of the k=0 FRW model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler DeWitt theory approximates loop quantum cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying LQC is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity."
http://arxiv.org/abs/0710.3540
N=2 supersymmetric spin foams in three dimensions
Etera R. Livine, James P. Ryan
12 pages
(Submitted on 18 Oct 2007)
"We construct the spin foam model for N=2 supergravity in three dimensions. Classically, it is a BF theory with gauge algebra osp(2|2). This algebra has representations which are not completely reducible. This complicates the procedure when building a state sum. Fortunately, one can and should excise these representations. We show that the restricted subset of representations form a subcategory closed under tensor product. The resulting state-sum is once again a topological invariant. Furthermore, within this framework one can identify positively and negatively charged fermions propagating on the spin foam. These results on osp(2|2) representations and intertwiners apply more generally to spin network states for N=2 loop quantum supergravity (in 3+1 dimensions) where it allows to define a notion of BPS states. "
http://arxiv.org/abs/0710.3276
Group field theory as the microscopic description of the quantum spacetime fluid: a new perspective on the continuum in quantum gravity
Daniele Oriti
38 pages, 6 figures; contribution to the proceedings of the conference 'From quantum to emergent gravity: theory and phenomenology', SISSA, Trieste, Italy, June 11-15 (2007)
(Submitted on 17 Oct 2007)
"We introduce the group field theory (GFT) formalism for non-perturbative quantum gravity, and present it as a potential unifying framework for several other quantum gravity approaches, i.e. loop quantum gravity and simplicial quantum gravity ones. We then argue in favor of and present in detail what we believe is a new GFT perspective on the emergence of continuum spacetime from discrete quantum structures, based on the idea of quantum space as a condensed matter system. We put forward a more specific, albeit still very much tentative, proposal for the relevant phase of the GFT corresponding to the continuum: a Bose-Einstein condensate of GFT quanta. Finally, we sketch how the proposal may be realised and its effective dynamics could be extracted in the GFT setting and compared with continuum gravity theories."
http://arxiv.org/abs/0710.3415
Modified gravity and the origin of inertia
J. W. Moffat, V. T. Toth
4 pages, 2 figures
(Submitted on 18 Oct 2007)
"Modified gravity theory is known to violate Birkhoff's theorem. We explore a key consequence of this violation, the effect of distant matter in an Einstein-de Sitter universe on the motion of test particles. We find that when a particle is accelerated, a force is experienced that is proportional to the particle's mass and acceleration and acts in the direction opposite to that of the acceleration. We identify this force with inertia. At very low accelerations, our inertial law deviates slightly from that of Newton, yielding a testable prediction that may be verified with relatively simple experiments."
http://arxiv.org/abs/0710.3438
Can one detect passage of small black hole through the Earth?
I.B. Khriplovich, A.A. Pomeransky, N. Produit, G.Yu. Ruban
6 pages
(Submitted on 18 Oct 2007)
"The energy losses of a small black hole passing through the Earth are examined. In particular, we investigate the excitations in the frequency range accessible to modern acoustic detectors. The main contribution to the effect is given by the coherent sound radiation of the Cherenkov type."
http://arxiv.org/abs/0710.3187
On the Quantum Origin of Structure in the Inflationary Universe
Daniel Baumann
29 pages; pedagogical lecture notes
(Submitted on 16 Oct 2007)
"In this lecture I give a pedagogical introduction to inflationary cosmology with a special focus on the quantum generation of cosmological perturbations."
http://arxiv.org/abs/0710.4543
Quantum bounce and cosmic recall
Alejandro Corichi, Parampreet Singh
4 pages
(Submitted on 24 Oct 2007)
"Loop quantum cosmology predicts that, in simple models, the big bang singularity of classical general relativity is replaced by a quantum bounce. Because of the extreme physical conditions near the bounce, a natural question is whether the universe can retain, after the bounce, its memory about the previous epoch. More precisely, does the universe recall various properties of the state after evolving unitarily through the bounce or does it suffer from cosmic amnesia as has been recently suggested? Here we show that this issue can be answered unambiguously by means of an exactly solvable model, derived from a small simplification of loop quantum cosmology, for which there is full analytical control on the quantum theory. We show that if there exists a semi-classical state at late times on one side, peaked around a pair of canonically conjugate variables, then there are very strong bounds on the fluctuations on the other side of the bounce, implying semi-classicality. For a model universe which grows to a size of 1 megaparsec at late times, the change in relative fluctuations of the only non-trivial observable of the model across the bounce is less than 10-57 (becoming smaller for universes which grow larger). The universe maintains (an almost) total recall.
http://arxiv.org/abs/0710.4345
A Primer for Black Hole Quantum Physics
Robert Brout, Serge Massar, Renaud Parentani, Philippe Spindel
An old review article on black hole evaporation and black hole thermodynamics, put on the archive following popular demand, 178 pages, 21 figures
Physics Report 260 (6), 329-454 (1995)
(Submitted on 23 Oct 2007)
"The mechanisms which give rise to Hawking radiation are revealed by analyzing in detail pair production in the presence of horizons. In preparation for the black hole problem, three preparatory problems are dwelt with at length: pair production in an external electric field, thermalization of a uniformly accelerated detector and accelerated mirrors. In the light of these examples, the black hole evaporation problem is then presented.
The leitmotif is the singular behavior of modes on the horizon which gives rise to a steady rate of production. Special emphasis is put on how each produced particle contributes to the mean albeit arising from a particular vacuum fluctuation. It is the mean which drives the semiclassical back reaction. This aspect is analyzed in more detail than heretofore and in particular its drawbacks are emphasized. It is the semiclassical theory which gives rise to Hawking's famous equation for the loss of mass of the black hole due to evaporation dM/dt \simeq -1/M^2. Black hole thermodynamics is derived from the evaporation process whereupon the reservoir character of the black hole is manifest. The relation to the thermodynamics of the eternal black hole through the Hartle--Hawking vacuum and the Killing identity are displayed.
It is through the analysis of the fluctuations of the field configurations which give rise to a particular Hawking photon that the dubious character of the semiclassical theory is manifest. The present frontier of research revolves around this problem and is principally concerned with the fact that one calls upon energy scales that are greater than Planckian and the possibility of a non unitary evolution as well. These last subjects are presented in qualitative fashion only, so that this review stops at the threshold of quantum gravity."
===================
The following are included because I've found previous papers by Dreyer and by Girelli interesting. Did not have time today to evaluate the current work.
http://arxiv.org/abs/0710.4350
Why things fall
Olaf Dreyer
Talk given at the conference "From Quantum to Emergent Gravity: Theory and Phenomenology", June 11-15, 2007, Trieste, Italy. To appear in the proceedings
(Submitted on 23 Oct 2007)
http://arxiv.org/abs/0710.4393
Quantum reference frames and deformed symmetries
Florian Girelli, David Poulin
11 pages
(Submitted on 24 Oct 2007)
http://arxiv.org/abs/0710.4919
Harmonic cosmology: How much can we know about a universe before the big bang?
Martin Bojowald
16 pages
(Submitted on 25 Oct 2007)
"Quantum gravity may remove classical space-time singularities and thus reveal what a universe at and before the big bang could be like. In loop quantum cosmology, an exactly solvable model is available which allows one to address precise dynamical coherent states and their evolution in such a setting. It is shown here that quantum fluctuations before the big bang are generically unrelated to those after the big bang. A reliable determination of pre-big bang quantum fluctuations would require exceedingly precise observations."
This paper can be seen as responding to one by Corichi and Singh noted in the previous post:
http://arxiv.org/abs/0710.4543
Quantum bounce and cosmic recall
Alejandro Corichi, Parampreet Singh
4 pages
(Submitted on 24 Oct 2007)
"Loop quantum cosmology predicts that, in simple models, the big bang singularity of classical general relativity is replaced by a quantum bounce. Because of the extreme physical conditions near the bounce, a natural question is whether the universe can retain, after the bounce, its memory about the previous epoch. More precisely, does the universe recall various properties of the state after evolving unitarily through the bounce or does it suffer from cosmic amnesia...?
... The universe maintains (an almost) total recall."
http://arxiv.org/abs/0710.5034
Numerical indications on the semiclassical limit of the flipped vertex
Elena Magliaro, Claudio Perini, Carlo Rovelli
4 pages, 8 figures
(Submitted on 26 Oct 2007 (v1), last revised 27 Oct 2007 (this version, v2))
"We introduce a technique for testing the semiclassical limit of a quantum gravity vertex amplitude. The technique is based on the propagation of a semiclassical wave packet. We apply this technique to the newly introduced "flipped" vertex in loop quantum gravity, in order to test the intertwiner dependence of the vertex. Under some drastic simplifications, we find very preliminary, but surprisingly good numerical evidence for the correct classical limit."
http://arxiv.org/abs/0710.5017
Coherent states, constraint classes, and area operators in the new spin-foam models
Jonathan Engle, Roberto Pereira
21 pages
(Submitted on 26 Oct 2007)
"Recently, two new spin-foam models have appeared in the literature, both motivated by a desire to modify the Barrett-Crane model in such a way that the imposition of certain second class constraints, called cross-simplicity constraints, are weakened. We refer to these two models as the FKLS model, and the flipped model. Both of these models are based on a reformulation of the cross-simplicity constraints. This paper has two main parts. First, we clarify the structure of the reformulated cross-simplicity constraints and the nature of their quantum imposition in the new models. In particular we show that in the FKLS model, quantum cross-simplicity implies no restriction on states. The deeper reason for this is that, with the symplectic structure relevant for FKLS, the reformulated cross-simplicity constraints, in a certain relevant sense, are now first class, and this causes the coherent state method of imposing the constraints, key in the FKLS model, to fail to give any restriction on states. Nevertheless, the cross-simplicity can still be seen as implemented via suppression of intertwiner degrees of freedom in the dynamical propagation. In the second part of the paper, we investigate area spectra in the models. The results of these two investigations will highlight how, in the flipped model, the Hilbert space of states, as well as the spectra of area operators exactly match those of loop quantum gravity, whereas in the FKLS (and Barrett-Crane) models, the boundary Hilbert spaces and area spectra are different."
http://arxiv.org/abs/0710.5043
Lorentzian LQG vertex amplitude
Roberto Pereira
9 pages
(Submitted on 26 Oct 2007)
"We generalize a model recently proposed for Euclidean quantum gravity to the case of Lorentzian signature. The main features of the Euclidean model are preserved in the Lorentzian one. In particular, the boundary Hilbert space matches the one of SU(2) loop quantum gravity. As in the Euclidean case, the model can be obtained from the Lorentzian Barrett-Crane model from a flipping of the Poisson structure, or alternatively, by adding a topological term to the action and taking the small Barbero-Immirzi parameter limit."
http://arxiv.org/abs/0710.4953
Different canonical formulations of Einstein's theory of gravity
V.A. Franke
18 pages. Theor.Math.Phys. 148(1), 2006, 995-1010; Teor.Mat.Fiz. 148(1), 2006, 143-160
(Submitted on 25 Oct 2007)
"We describe the four most famous versions of the classical canonical formalism in the Einstein theory of gravity: the Arnovitt-Deser-Misner formalism, the Faddeev-Popov formalism, the tetrad formalism in the usual form, and the tetrad formalism in the form best suited for constructing the loop theory of gravity, which is now being developed. We present the canonical transformations relating these formalisms. The paper is written mainly for pedagogical purposes."
http://arxiv.org/abs/0710.4959
Outlook from SUSY07
John Ellis
Submitted for the SUSY07 proceedings, 9 pages, 12 figures
(Submitted on 25 Oct 2007)
"Make-or-break time is near for the Higgs boson and supersymmetry. The LHC will soon put to the sword many theoretical ideas, and define the future for collider physics."
http://arxiv.org/abs/0710.5734
Dirac Fields in Loop Quantum Gravity and Big Bang Nucleosynthesis
Martin Bojowald, Rupam Das, Robert J. Scherrer
15 pages, 2 figures
(Submitted on 30 Oct 2007)
"Big Bang nucleosynthesis requires a fine balance between equations of state for photons and relativistic fermions. Several corrections to equation of state parameters arise from classical and quantum physics, which are derived here from a canonical perspective. In particular, loop quantum gravity allows one to compute quantum gravity corrections for Maxwell and Dirac fields. Although the classical actions are very different, quantum corrections to the equation of state are remarkably similar. To lowest order, these corrections take the form of an overall expansion-dependent multiplicative factor in the total density. We use these results, along with the predictions of Big Bang nucleosynthesis, to place bounds on these corrections."
http://arxiv.org/abs/0710.5721
The radiation equation of state and loop quantum gravity corrections
Martin Bojowald, Rupam Das
11 pages, 1 figure
Phys. Rev. D 75 (2007) 123521
(Submitted on 30 Oct 2007)
"The equation of state for radiation is derived in a canonical formulation of the electromagnetic field. This allows one to include correction terms expected from canonical quantum gravity and to infer implications to the universe evolution in radiation dominated epochs. Corrections implied by quantum geometry can be interpreted in physically appealing ways, relating to the conformal invariance of the classical equations."
http://arxiv.org/abs/0710.5722
Canonical Gravity with Fermions
Martin Bojowald, Rupam Das
27 pages
(Submitted on 30 Oct 2007)
"Canonical gravity in real Ashtekar-Barbero variables is generalized to allow for fermionic matter. The resulting torsion changes several expressions in Holst's original vacuum analysis, which are explicitly displayed here. This in turn requires adaptations to the known canonical (loop) quantization of gravity coupled to fermions, which is discussed on the basis of the classical analysis."
http://arxiv.org/abs/0710.5608
Measurement of the space-time interval between two events using the retarded and advanced times of each event with respect to a time-like world-line
Giovanni Amelino-Camelia, John Stachel
17 pages
(Submitted on 30 Oct 2007)
"Several recent studies have been devoted to investigating the limitations that ordinary quantum mechanics and/or quantum gravity might impose on the measurability of space-time observables. These analyses are often confined to the simplified context of two-dimensional flat space-time and rely on a simple procedure for the measurement of space-like distances based on the exchange of light signals. We present a generalization of this measurement procedure applicable to all three types of space-time intervals between two events in space-times of any number of dimensions. We also present some preliminary observations on an alternative measurement procedure that can be applied taking into account the gravitational field of the measuring apparatus, and briefly discuss quantum limitations of measurability in this context.
Alejandro Satz has the blog called Reality Conditions, which had the best blog reporting of the Zakopane school and Loops '07
http://arxiv.org/abs/0710.5671
Transition rate of the Unruh-DeWitt detector in curved spacetime
Jorma Louko, Alejandro Satz
(Submitted on 30 Oct 2007)
"We examine the Unruh-DeWitt particle detector coupled to a scalar field in an arbitrary Hadamard state in four-dimensional curved spacetime. Using smooth switching functions to turn on and off the interaction, we obtain a regulator-free integral formula for the total excitation probability, and we show that an instantaneous transition rate can be recovered in a suitable limit. Previous results in Minkowski space are recovered as a special case. As applications, we consider an inertial detector in the Rindler vacuum and a detector at rest in a static Newtonian gravitational field. Gravitational corrections to decay rates in atomic physics laboratory experiments on the surface of the Earth are estimated to be suppressed by 42 orders of magnitude."
Didn't want to omit this (classical, not quantum geometry) paper, on the chance it might give additional understanding of behavior at the classical singularity which could be of use to quantum gravitists.
http://arxiv.org/abs/0710.5692
Describing general cosmological singularities in Iwasawa variables
Thibault Damour, Sophie de Buyl
50 pages, 4 figures
(Submitted on 30 Oct 2007)
"Belinskii, Khalatnikov, and Lifshîtz (BKL) conjectured that the description of the asymptotic behavior of a generic solution of Einstein equations near a spacelike singularity could be drastically simplified by considering that the time derivatives of the metric asymptotically dominate (except at a sequence of instants, in the `chaotic case') over the spatial derivatives. We present a precise formulation of the BKL conjecture (in the chaotic case) that consists of basically three elements: (i) we parametrize the spatial metric g_{ij} by means of Iwasawa variables (\beta^a, {\cal N}^a{}_i); (ii) we define, at each spatial point, a (chaotic) asymptotic evolution system made of ordinary differential equations for the Iwasawa variables; and (iii) we characterize the exact Einstein solutions \beta, {\cal{N}} whose asymptotic behavior is described by a solution \beta_{[0]}, {\cal N}_{[0]} of the previous evolution system by means of a `generalized Fuchsian system' for the differenced variables \bar \beta = \beta - \beta_{[0]}, \bar {\cal N} = {\cal N} - {\cal N}_{[0]}, and by requiring that \bar \beta and \bar {\cal N} tend to zero on the singularity. We also show that, in spite of the apparently chaotic infinite succession of `Kasner epochs' near the singularity, there exists a well-defined asymptotic geometrical structure on the singularity : it is described by a partially framed flag. Our treatment encompasses Einstein-matter systems (comprising scalar and p-forms), and also shows how the use of Iwasawa variables can simplify the usual (`asymptotically velocity term dominated') description of non-chaotic systems.
http://arxiv.org/abs/0710.5590
Gauguin's questions in particle physics: Where are we coming from? What are we? Where are we going?
John Ellis
16 pages, 11 figures, plenary talk at the International Europhysics Conference on High-Energy Physics (EPS-HEP2007), Manchester, England, July 2007
(Submitted on 30 Oct 2007)
"Within particle physics itself, Gauguin's questions may be interpreted as: P1 - What is the status of the Standard Model? P2 - What physics may lie beyond the Standard Model? P3 - What is the `Theory of Everything'? Gauguin's questions may also asked within a cosmological context: C1 - What were the early stages of the Big Bang? C2 - What is the material content of the Universe today? C3 - What is the future of the Universe? In this talk I preview many of the topics to be discussed in the plenary sessions of this conference, highlighting how they bear on these fundamental questions."
http://arxiv.org/abs/0710.5844
Lessons from (2+1)-dimensional quantum gravity
B J Schroers
17 pages, two figures, based on talk given at the conference ``From Quantum to Emergent Gravity: Theory and Phenomenology'',June 11-15 2007, Trieste, Italy
(Submitted on 31 Oct 2007)
"Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations of Poincare symmetry are examined in the context of (2+1)-dimensional quantum gravity. The results are expressed in five lessons, which summarise how the gravitational constant, Planck's constant and the cosmological constant enter the non-commutative and non-cocommutative structures arising in (2+1)-dimensional quantum gravity. It is emphasised that the much studied bicrossproduct kappa-Poincare algebra does not arise directly in (2+1)-dimensional quantum gravity."
http://arxiv.org/abs/0711.0146
LQG vertex with finite Immirzi parameter
Jonathan Engle, Etera Livine, Roberto Pereira, Carlo Rovelli
(Submitted on 1 Nov 2007)
"We extend the definition of the "flipped" loop-quantum-gravity vertex to the case of a finite Immirzi parameter gamma. We cover the euclidean as well as the lorentzian case. We show that the resulting dynamics is defined on a Hilbert space isomorphic to the one of loop quantum gravity, and that the area operator has the same discrete spectrum as in loop quantum gravity. This includes the correct dependence on gamma, and, remarkably, holds in the lorentzian case as well. The ad hoc flip of the symplectic structure that was required to derive the flipped vertex is not anymore required for finite gamma. These results establish a bridge between canonical loop quantum gravity and the spinfoam formalism in four dimensions."
The above paper is probably important. I put some comment on it as the second post in this discussion thread
http://physicsforums.com/showthread.php?t=194651
Additional comment would be welcome there.
http://arxiv.org/abs/0711.0090
On deformations of Ashtekar's constraint algebra
Kirill Krasnov
3 pages
(Submitted on 1 Nov 2007)
"We show that the constraint algebra of Ashtekar's Hamiltonian formulation of general relativity can be non-trivially deformed by allowing the cosmological constant to become an arbitrary function of the (Weyl) curvature. Our result implies that there is not one but infinitely many (parameterized by an arbitrary function) four-dimensional gravity theories propagating two degrees of freedom."
http://arxiv.org/abs/0711.0115
Manifestly Gauge-Invariant General Relativistic Perturbation Theory: I. Foundations
K. Giesel, S. Hofmann, T. Thiemann, O. Winkler
77 pages
(Submitted on 1 Nov 2007)
"Linear cosmological perturbation theory is pivotal to a theoretical understanding of current cosmological experimental data provided e.g. by cosmic microwave anisotropy probes. A key issue in that theory is to extract the gauge invariant degrees of freedom which allow unambiguous comparison between theory and experiment. When one goes beyond first (linear) order, the task of writing the Einstein equations expanded to n'th order in terms of quantities that are gauge invariant up to terms of higher orders becomes highly non-trivial and cumbersome. This fact has prevented progress for instance on the issue of the stability of linear perturbation theory and is a subject of current debate in the literature. In this series of papers we circumvent these difficulties by passing to a manifestly gauge invariant framework. In other words, we only perturb gauge invariant, i.e. measurable quantities, rather than gauge variant ones. Thus, gauge invariance is preserved non perturbatively while we construct the perturbation theory for the equations of motion for the gauge invariant observables to all orders. In this first paper we develop the general framework which is based on a seminal paper due to Brown and Kuchar as well as the realtional formalism due to Rovelli. In the second, companion, paper we apply our general theory to FRW cosmologies and derive the deviations from the standard treatment in linear order. As it turns out, these deviations are negligible in the late universe, thus our theory is in agreement with the standard treatment. However, the real strength of our formalism is that it admits a straightforward and unambiguous, gauge invariant generalisation to higher orders. This will also allow us to settle the stability issue in a future publication."
http://arxiv.org/abs/0711.0117
Manifestly Gauge-Invariant General Relativistic Perturbation Theory: II. FRW Background and First Order
K. Giesel, S. Hofmann, T. Thiemann, O. Winkler
51 pages
(Submitted on 1 Nov 2007)
"In our companion paper we identified a complete set of manifestly gauge-invariant observables for general relativity. This was possible by coupling the system of gravity and matter to pressureless dust which plays the role of a dynamically coupled observer. The evolution of those observables is governed by a physical Hamiltonian and we derived the corresponding equations of motion. Linear perturbation theory of those equations of motion around a general exact solution in terms of manifestly gauge invariant perturbations was then developed. In this paper we specialise our previous results to an FRW background which is also a solution of our modified equations of motion. We then compare the resulting equations with those derived in standard cosmological perturbation theory (SCPT). We exhibit the precise relation between our manifestly gauge-invariant perturbations and the linearly gauge-invariant variables in SCPT. We find that our equations of motion can be cast into SCPT form plus corrections. These corrections are the trace that the dust leaves on the system in terms of a conserved energy momentum current density. It turns out that these corrections decay, in fact, in the late universe they are negligible whatever the value of the conserved current. We conclude that the addition of dust which serves as a test observer medium, while implying modifications of Einstein's equations without dust, leads to acceptable agreement with known results, while having the advantage that one now talks about manifestly gauge-invariant, that is measurable, quantities, which can be used even in perturbation theory at higher orders."
http://arxiv.org/abs/0711.0119
Algebraic Quantum Gravity (AQG) IV. Reduced Phase Space Quantisation of Loop Quantum Gravity
K. Giesel, T. Thiemann
31 pages
(Submitted on 1 Nov 2007)
"We perform a canonical, reduced phase space quantisation of General Relativity by Loop Quantum Gravity (LQG) methods. The explicit construction of the reduced phase space is made possible by the combination of 1. the Brown -- Kuchar mechanism in the presence of pressure free dust fields which allows to deparametrise the theory and 2. Rovelli's relational formalism in the extended version developed by Dittrich to construct the algebra of gauge invariant observables. Since the resulting algebra of observables is very simple, one can quantise it using the methods of LQG. Basically, the kinematical Hilbert space of non reduced LQG now becomes a physical Hilbert space and the kinematical results of LQG such as discreteness of spectra of geometrical operators now have physical meaning. The constraints have disappeared, however, the dynamics of the observables is driven by a physical Hamiltonian which is related to the Hamiltonian of the standard model (without dust) and which we quantise in this paper."
The following is an overview article for a special issue of General Relativity and Gravitation (Springer) which will be devoted to the topic of Dark Energy. Martin Bojowald contributed one of the articles in that issue---indicating a way that the effect associated with dark energy might arise naturally in LQG from an already present quantum correction, without need for assuming a new type of exotic "dark energy" field.
The co-editors of GRG are George Ellis and Hermann Nicolai. The special edition on Dark Energy should be interesting, with many competing ideas presented.
http://arxiv.org/abs/0711.0077
Dark Energy and Dark Gravity
Ruth Durrer, Roy Maartens
21 pages 10 figures Overview article for the dark energy issue of GRG
(Submitted on 1 Nov 2007)
"Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy. Speculative ideas from string theory may hold some promise, but it is fair to say that no convincing model has yet been proposed. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity (or dark gravity) provides a new angle on the problem, but also faces serious difficulties, including in all known cases severe fine-tuning and the problem of explaining why the vacuum energy does not gravitate. The lack of an adequate theoretical framework for the late-time acceleration of the universe represents a deep crisis for theory -- but also an exciting challenge for theorists. It seems likely that an entirely new paradigm is required to resolve this crisis."
http://arxiv.org/abs/0711.0150
Space-based research in fundamental physics and quantum technologies
S.G. Turyshev, U.E. Israelsson, M. Shao, N. Yu, A. Kusenko, E.L. Wright, C.W.F. Everitt, M.A. Kasevich, J.A. Lipa, J.C. Mester, R.D. Reasenberg, R.L. Walsworth, N. Ashby, H. Gould, H.-J.Paik
a white paper, 27 pages
(Submitted on 1 Nov 2007)
"Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for precision investigations in cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics.
We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency ``Fundamental Physics Task Force'' to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including ``laboratory research in fundamental physics'' as an element in agency's ongoing space research efforts."
http://arxiv.org/abs/0711.0273
The Emergence of Spacetime, or, Quantum Gravity on Your Desktop
R. Loll
21 pages, 11 figures, write-up of plenary talk at GR18, Sydney, July 2007
(Submitted on 2 Nov 2007)
"Is there an approach to quantum gravity which is conceptually simple, relies on very few fundamental physical principles and ingredients, emphasizes geometric (as opposed to algebraic) properties, comes with a definite numerical approximation scheme, and produces robust results, which go beyond showing mere internal consistency of the formalism? The answer is a resounding yes: it is the attempt to construct a nonperturbative theory of quantum gravity, valid on all scales, with the technique of so-called Causal Dynamical Triangulations. Despite its conceptual simplicity, the results obtained up to now are far from trivial. Most remarkable at this stage is perhaps the fully dynamical emergence of a classical background (and solution to the Einstein equations) from a nonperturbative sum over geometries, without putting in any preferred geometric background at the outset. In addition, there is concrete evidence for the presence of a fractal spacetime foam on Planckian distance scales. The availability of a computational framework provides built-in reality checks of the approach, whose importance can hardly be overestimated."
http://arxiv.org/abs/0711.0373
Automorphisms in Loop Quantum Gravity
Benjamin Bahr, Thomas Thiemann
57 pages, 7 figures
(Submitted on 2 Nov 2007)
"We investigate a certain distributional extension of the group of spatial diffeomorphisms in Loop Quantum Gravity. This extension, which is given by the automorphisms Aut(P) of the path groupoid P, was proposed by Velhinho and is inspired by category theory. This group is much larger than the group of piecewise analytic diffeomorphisms. In particular, we will show that graphs with the same combinatorics but different generalized knotting classes can be mapped into each other. We describe the automorphism-invariant Hilbert space and comment on how a combinatorial formulation of LQG might arise."
http://arxiv.org/abs/0711.0697
Non-metric gravity: A status report
Kirill Krasnov
13 pages, no figures, invited review for Modern Physics Letters A
(Submitted on 5 Nov 2007)
"We review the status of a certain (infinite) class of four-dimensional generally covariant theories propagating two degrees of freedom that are formulated without any direct mention of the metric. General relativity itself (in its Plebanski formulation) belongs to the class, so these theories are examples of modified gravity. We summarize the current understanding of the nature of the modification, of the renormalizability properties of these theories, of their coupling to matter fields, and describe some of their physical properties."
http://arxiv.org/abs/0711.0788
On a Continuum Limit for Loop Quantum Cosmology
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
8 pages. Contribution for the Proceedings of the Third Mexican Meeting on Mathematical and Experimental Physics
(Submitted on 6 Nov 2007)
"The use of non-regular representations of the Heisenberg-Weyl commutation relations has proved to be useful for studying conceptual and technical issues in quantum gravity. Of particular relevance is the study of Loop Quantum Cosmology (LQC), a symmetry reduced theory that is related to Loop Quantum Gravity, and that is based on a non-regular, polymeric representation. Recently, a soluble model was used by Ashtekar, Corichi and Singh to study the relation between Loop Quantum Cosmology and the standard Wheeler-DeWitt theory and in particular the passage to the limit in which the auxiliary parameter (interpreted as "quantum geometry discreetness") is sent to zero in hope to get rid of this `regulator ambiguity' in the LQC dynamics. In this note we outline the first steps toward reformulating this question within the program developed by the authors for studying the continuum limit of polymeric theories, which was successfully applied to simple systems such as a Simple Harmonic Oscillator and the Free Particle."
http://arxiv.org/abs/0711.0770
An Exceptionally Simple Theory of Everything
A. Garrett Lisi
31 pages, 7 figures
(Submitted on 6 Nov 2007)
"All fields of the standard model and gravity are unified as an E8 principal bundle connection. A non-compact real form of the E8 Lie algebra has G2 and F4 subalgebras which break down to strong su(3), electroweak su(2) x u(1), gravitational so(3,1), the frame-Higgs, and three generations of fermions related by triality. The interactions and dynamics of these 1-form and Grassmann valued parts of an E8 superconnection are described by the curvature and action over a four dimensional base manifold."
http://arxiv.org/abs/0711.0743
Gravitational Wave Sirens as a Triple Probe of Dark Energy
Eric V. Linder
7 pages, 5 figures
(Submitted on 5 Nov 2007)
"Gravitational wave standard sirens have been considered as precision distance indicators to high redshift; however, at high redshift standard sirens or standard candles such as supernovae suffer from lensing noise. We investigate lensing noise as a signal instead and show how measurements of the maximum demagnification (minimum convergence) probe cosmology in a highly complementary manner to the distance itself. Revisiting the original form for minimum convergence we quantify the bias arising from the commonly used approximation. Furthermore, after presenting a new lensing probability function we discuss how the width of the lensed standard siren amplitude distribution also probes growth of structure. Thus standard sirens and candles can serve as triple probes of dark energy, measuring both the cosmic expansion history and growth history."
http://arxiv.org/abs/0711.0757
Universal quantum mechanics
Steven B. Giddings
15 pages
(Submitted on 5 Nov 2007)
"If gravity respects quantum mechanics, it is important to identify the essential postulates of a quantum framework capable of incorporating gravitational phenomena. Such a construct likely requires elimination or modification of some of the "standard" postulates of quantum mechanics, in particular those involving time and measurement. This paper proposes a framework that appears sufficiently general to incorporate some expected features of quantum gravity. These include the statement that space and time may only emerge approximately and relationally. One perspective on such a framework is as a sort of generalization of the S-matrix approach to dynamics. Within this framework, more dynamical structure is required to fully specify a theory; this structure is expected to lack some of the elements of local quantum field theory. Some aspects of this structure are discussed, both in the context of scattering of perturbations about a flat background, and in the context of cosmology."
http://arxiv.org/abs/0711.1284
The complete LQG propagator: II. Asymptotic behavior of the vertex
Emanuele Alesci, Carlo Rovelli
16 pages
(Submitted on 8 Nov 2007)
"In a previous article we have show that there are difficulties in obtaining the correct graviton propagator from the loop-quantum-gravity dynamics defined by the Barrett-Crane vertex amplitude. Here we show that a vertex amplitude that depends nontrivially on the intertwiners can yield the correct propagator. We give an explicit example of asymptotic behavior of a vertex amplitude that gives the correct full graviton propagator in the large distance limit."
Making a 1978 classic available online
http://arxiv.org/abs/0711.2445
An introduction to quantum gravity
Bryce S. DeWitt, Giampiero Esposito
68 pages. Sections 2 to 17, with kind permission of Springer
(Submitted on 15 Nov 2007)
"After an overview of the physical motivations for studying quantum gravity, we reprint THE FORMAL STRUCTURE OF QUANTUM GRAVITY, i.e. the 1978 Cargese Lectures by Professor B.S. DeWitt, with kind permission of Springer. The reader is therefore introduced, in a pedagogical way, to the functional integral quantization of gravitation and Yang-Mills theory. It is hoped that such a paper will remain useful for all lecturers or Ph.D. students who face the task of introducing (resp. learning) some basic concepts in quantum gravity in a relatively short time. In the second part, we outline selected topics such as the braneworld picture with the same covariant formalism of the first part, and spectral asymptotics of Euclidean quantum gravity with diffeomorphism-invariant boundary conditions. The latter might have implications for singularity avoidance in quantum cosmology."
http://arxiv.org/abs/0711.2455
Physical boundary state for the quantum tetrahedron
Etera R. Livine, Simone Speziale
20 pages, 6 figures
(Submitted on 15 Nov 2007)
"We consider stability under evolution as a criterion to select a physical boundary state for the spinfoam formalism. As an example, we apply it to the simplest spinfoam defined by a single quantum tetrahedron and solve the associated eigenvalue problem at leading order in the large spin limit. We show that this fixes uniquely the free parameters entering the boundary state. Remarkably, the state obtained this way gives a correlation between edges which runs at leading order with the inverse distance between the edges, in agreement with the linearized continuum theory. Finally, we give an argument why this correlator represents the propagation of a pure gauge, consistently with the absence of physical degrees of freedom in 3d general relativity."
http://arxiv.org/abs/0711.2198
Stability, Singularities and Mass Thresholds in Child Universe Production: a concise survey including some recent results and prospects
Stefano Ansoldi, Eduardo I. Guendelman, Idan Shilon
19 pages, including 68 references; to appear in the Proceedings of "BH2, Dynamics and Thermodynamics of Blackholes and Naked Singularities", May 10-12 2007, Milano, Italy; conference website: http://www.mate.polimi.it/bh2/
(Submitted on 14 Nov 2007)
"We present a review of selected topics concerning the creation and evolution of child universes, together with a concise account of some recent progress in the field."
http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)
"The Poincare' group generalizes the Galilei group for high-velocity kinematics. The de Sitter group is here assumed to go one step further, generalizing Poincare' as the group governing high-energy kinematics. Algebraically, this is done by supplementing spacetime translations with proper conformal transformations. This change in special relativity implies concomitant changes in general relativity -- yielding a de Sitter general relativity. The source current turns out to include now, in addition to energy-momentum, the proper conformal current, which appears as the origin of the cosmological constant. In consequence, it is no longer a free parameter, and can be determined in terms of other quantities. When applied to the propagation of ultra-high energy photons, de Sitter general relativity gives a good estimate of the time delay observed in extragalactic gamma-ray flares. It can, for this reason, be considered a new approach to quantum gravity."
The next paper draws conclusions which are unfamiliar and seem radical to me. Not qualified to evaluate, but decided it was safer to include it.
http://arxiv.org/abs/0711.1879
Area, ladder symmetry, degeneracy and fluctuations of a horizon
Mohammad H. Ansari
12 pages, 1 figure
(Submitted on 13 Nov 2007)
"Loop quantum gravity admits a kind of area quantization that is characterized by three quantum numbers. We show the complete spectrum of area is the union of equidistant subsets and a universal reformulation with fewer parameters is possible. Associated with any area there is also another number that determines its degeneracy. One application is that a quantum horizon manifests harmonic modes in vacuum fluctuations. It is discussed the physical fluctuations of a space-time horizon should include all the excluded area eigenvalues, where quantum amplification effect occurs. Due to this effect the uniformity of transition matrix elements between near levels could be assumed. Based on these, a modification to the previous method of analyzing the radiance intensities in hep-th/0607081 is presented that makes the result one step further precise. A few of harmonic modes appear to be extremely amplified on top of the Hawking's radiation. They are expected to form a few brightest lines with the wavelength not larger than the black hole size."
http://arxiv.org/abs/0711.2875
Two-dimensional topological field theories coupled to four-dimensional BF theory
Merced Montesinos, Alejandro Perez
(Submitted on 19 Nov 2007)
"Four dimensional BF theory admits a natural coupling to extended sources supported on two dimensional surfaces or string world-sheets. Solutions of the theory are in one to one correspondence with solutions of Einstein equations with distributional matter (cosmic strings). We study new (topological field) theories that can be constructed by adding extra degrees of freedom to the two dimensional world-sheet. We show how two dimensional Yang-Mills degrees of freedom can be added on the world-sheet, producing in this way, an interactive (topological) theory of Yang-Mills fields with BF fields in four dimensions. We also show how a world-sheet tetrad can be naturally added. As in the previous case the set of solutions of these theories are contained in the set of solutions of Einstein's equations if one allows distributional matter supported on two dimensional surfaces. These theories are argued to be exactly quantizable. In the context of quantum gravity, one important motivation to study these models is to explore the possibility of constructing a background independent quantum field theory where local degrees of freedom at low energies arise from global topological (world-sheet) degrees of freedom at the fundamental level."
http://arxiv.org/abs/0711.3107
The theta parameter in loop quantum gravity: effects on quantum geometry and black hole entropy
Danilo Jimenez Rezende, Alejandro Perez
(Submitted on 20 Nov 2007)
"The precise analog of the theta-quantization ambiguity of Yang-Mills theory exists for the real SU(2) connection formulation of general relativity. As in the former case theta labels representations of large gauge transformations, which are super-selection sectors in loop quantum gravity. We show that unless theta=0, the (kinematical) geometric operators such as area and volume are not well defined on spin network states. More precisely the intersection of their domain with the dense set Cyl in the kinematical Hilbert space H of loop quantum gravity is empty. The absence of a well defined notion of area operator acting on spin network states seems at first in conflict with the expected finite black hole entropy. However, we show that the black hole (isolated) horizon area--which in contrast to kinematical area is a (Dirac) physical observable--is indeed well defined, and quantized so that the black hole entropy is proportional to the area. The effect of theta is negligible in the semiclassical limit where proportionality to area holds."
edit to reply to next:
Thanks. I made the correction.
The links should be switched around.
jal
There is an interesting tie-in between this paper (which I already listed above) and two other new ones from Fulvio Melia, which are listed below:
http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)
"The Poincaré group generalizes the Galilei group for high-velocity kinematics. The de Sitter group is here assumed to go one step further, generalizing Poincaré as the group governing high-energy kinematics. Algebraically, this is done by supplementing spacetime translations with proper conformal transformations. This change in special relativity implies concomitant changes in general relativity -- yielding a de Sitter general relativity. The source current turns out to include now, in addition to energy-momentum, the proper conformal current, which appears as the origin of the cosmological constant. In consequence, it is no longer a free parameter, and can be determined in terms of other quantities. When applied to the propagation of ultra-high energy photons, de Sitter general relativity gives a good estimate of the time delay observed in extragalactic gamma-ray flares. It can, for this reason, be considered a new approach to quantum gravity."
http://arxiv.org/abs/0711.4181
The Cosmic Horizon
Fulvio Melia
Accepted for publication in MNRAS
(Submitted on 27 Nov 2007)
"The cosmological principle, promoting the view that the universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson-Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal properties of the cosmic spacetime manifested in Birkhoff's theorem and its corollary. In this paper, we compare two forms of the metric--written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates--first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler's event horizon--evident in the co-moving system--coincides with what one might call the 'curvature horizon' appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic spacetime is that with the latest WMAP results, we now have a much better determination of the universe's mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change."
http://arxiv.org/abs/0711.4810
Dark Energy in Light of the Cosmic Horizon
Fulvio Melia
Submitted to MNRAS
(Submitted on 29 Nov 2007)
"Based on dramatic observations of the CMB with WMAP and of Type Ia supernovae with the Hubble Space Telescope and ground-based facilities, it is now generally believed that the Universe's expansion is accelerating. Within the context of standard cosmology, the Universe must therefore contain a third 'dark' component of energy, beyond matter and radiation. However, the current data are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the role played by our cosmic horizon R0 in our interrogation of the data, and reach the rather firm conclusion that the existence of a cosmological constant is untenable. The observations are telling us that R0=c t0, where t0 is the perceived current age of the Universe, yet a cosmological constant would drive R0 towards ct (where t is the cosmic time) only once, and that would have to occur right now. In contrast, scaling solutions simultaneously eliminate several conundrums in the standard model, including the 'coincidence' and 'flatness' problems, and account very well for the fact that R0=c t0. We show here that for such dynamical dark energy models, either R0=ct for all time (thus eliminating the apparent coincidence altogether), or that what we believe to be the current age of the universe is actually the horizon time th=R0/c, which is always shorter than t0. Our best fit to the Type Ia supernova data indicates that t0 would then have to be ~16.9 billion years. Though surprising at first, an older universe such as this would actually eliminate several other long-standing problems in cosmology, including the (too) early appearance of supermassive black holes (at a redshift > 6) and the glaring deficit of dwarf halos in the local group."
Thanks to Wolram, Patty144, and Wallace for alerting us to these new papers by Melia, which I suspect may be important and may also clarify some of the issues raised by Pereira and Aldrovandi.
http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)
"The Poincaré group generalizes the Galilei group for high-velocity kinematics. The de Sitter group is here assumed to go one step further, generalizing Poincaré as the group governing high-energy kinematics. Algebraically, this is done by supplementing spacetime translations with proper conformal transformations. This change in special relativity implies concomitant changes in general relativity -- yielding a de Sitter general relativity. The source current turns out to include now, in addition to energy-momentum, the proper conformal current, which appears as the origin of the cosmological constant. In consequence, it is no longer a free parameter, and can be determined in terms of other quantities. When applied to the propagation of ultra-high energy photons, de Sitter general relativity gives a good estimate of the time delay observed in extragalactic gamma-ray flares. It can, for this reason, be considered a new approach to quantum gravity."
So I'm skimming this and I'm trying to understand... they say the de sitter group is SO(4,1)/L, where "L" is the lorentz group?
I'm getting kind of curious exactly which models there are that try to deny poincare invariance or replace it with something different. Ones I can think of include:
- This De Sitter Relativity thing ( SO(4,1)/Lorentz )
- LQG ( SO(3,1), which is the "restricted lorentz subgroup"? or just the lorentz subgroup? or does LQG assert poincare invariance as well as SO(3,1)? )
- Lisi's E8 theory ( SO(3,1) )
Anything else? Do doubly-special relativity or MOND imply any specific spacetime symmetry? Is doubly-special relativity related to this de sitter relativity in any way? (The de sitter relativity paper seems to be using some buzzwords similar to doubly-special relativity near the beginning, but maybe I am imagining this...)
I'm kinda confused here, so maybe everything I ask above is nonsense. In particular, are SO(3,1) and SO(1,3) the same thing?! I'm finding a couple of places using the two interchangeably :O
But basically I am curious whether there are any commonalities between all of these models which reject the poincare group (besides of course the detail that they reject the poincare group...)
Actually I am only just now noticing this thread (http://physicsforums.com/showthread.php?t=126285), perhaps I should read this?
...
But basically I am curious whether there are any commonalities between all of these models which reject the poincare group (besides of course the detail that they reject the poincare group...)
Actually I am only just now noticing this thread (http://physicsforums.com/showthread.php?t=126285), perhaps I should read this?
Hi Coin, the custom is to start discussions in a separate thread. Otherwise this biblio link thread would get overloaded.
You are new and there's no reason you would know. Anyway I thought you raised a bunch of related issues, not merely having to do with the Pereira Aldrovandi paper, so I started a thread in case people want to discuss
I hope the title accurately reflects the cluster of issues you have in mind
How extensions of relativity apply to quantum gravity (Coin's gambit)
http://physicsforums.com/showthread.php?t=202438
If I got the drift wrong (I apologize and) let me know so i can try to correct and get it right.
================================================== =
Continuing the biblio-linkage---this important 4-page june 2007 paper used to have a title that made it difficult to recommend to people because you had to explain the title, but now, as a couple of weeks ago, it has been accepted for publication in PRL and has a new, more descriptive title:
http://arxiv.org/abs/0706.3690
Conceptual Explanation for the Algebra in the Noncommutative Approach to the Standard Model
Ali H. Chamseddine, Alain Connes
Phys. Rev. Lett. 99, 191601 (2007)
(Submitted on 25 Jun 2007)
"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states."
This is just the sort of thing we need more of-----conceptual (hopefully intuitive) explanations of what may seem, like Connes' algebraic/geometric derivation of the Standard Model, like an extremely strange and puzzling coincidence.
==============================
Frank Saueressig and Pedro Machado are both at Utrecht, or were until recently. Saueressig has co-authored some Asymptotic Safety papers with Martin Reuter, including a recent pedagogical survey of A.S. Now, with this new one, Saueressig seems to be going out on his own, without Reuter as senior author.
http://arxiv.org/abs/0712.0445
On the renormalization group flow of f(R)-gravity
Pedro F. Machado, Frank Saueressig
55 pages, 7 figures
(Submitted on 4 Dec 2007)
"We use the functional renormalization group equation for quantum gravity to construct a non-perturbative flow equation for modified gravity theories of the form S = \int d^dx \sqrt{g} f(R). Based on this equation we show that certain gravitational interactions monomials can be consistently decoupled from the renormalization group (RG) flow and reproduce recent results on the asymptotic safety conjecture. The non-perturbative RG flow of non-local extensions of the Einstein-Hilbert truncation including \int d^dx \sqrt{g} \ln(R) and \int d^dx \sqrt{g} R^{-n} interactions is investigated in detail. The inclusion of such interactions resolves the infrared singularities plaguing the RG trajectories with positive cosmological constant in previous truncations. In particular, in some R^{-n}-truncations all physical trajectories emanate from a Non-Gaussian (UV) fixed point and are well-defined on all RG scales. The RG flow of the \ln(R)-truncation contains an infrared attractor which drives a positive cosmological constant to zero, thereby providing a dynamical explanation of the tiny value of Lambda observed today."
Pedro Machado is a PhD student of Loll at Utrecht. Just as a refresher, look at Loll's group
http://www.phys.uu.nl/~loll/Web/group/group.html
She gets quality people and they move on to places like Perimeter, Marseille.
Saueressig is a young guy who has published mainly string research (plus 4 or 5 papers with Reuter), he could easily be pursuing a string carreer, but for some reason he has moved over into nonstring QG at least for the moment.
===========================
Rudy Vaas has been the main popularizer of LQG, LQC, and nonstring QG in general. He writes in German for the SciAm-like Bild der Wissenschaft, Ashtekar has English translations of Vaas QG articles at his personal website, and some are on arxiv as well. Vaas is a Philosophy of Science expert who also writes science for wide audience. Now Springer Press has commissioned Vaas to do a book scheduled to come out in 2008 called BEYOND THE BIG BANG.
We will be seeing preprints of chapters contributed by various people, I expect. Today a preprint chapter by Tony Aguirre showed up:
http://arxiv.org/abs/0712.0571
Eternal Inflation, past and future
Authors: Anthony Aguirre
38 pp., 6 color figures. Contribution to R. Vaas (ed.): Beyond the Big Bang. Springer 2008
(Submitted on 4 Dec 2007)
"Cosmological inflation, if it occurred, radically alters the picture of the 'big bang', which would merely point to reheating at the end of inflation. Moreover, this reheating may be only local, so that inflation continues elsewhere and forever, continually spawning big-bang-like regions. This chapter reviews this idea of 'eternal inflation', then focuses on what this may mean for the ultimate beginning of the universe. In particular, I will argue that given eternal inflation, the universe may be free of a cosmological initial singularity, might be eternal (and eternally inflating) to the past, and might obey an interesting sort of cosmological time-symmetry."
I don't know of any evidence that points to the "Eternal Inflation" picture being real, or that makes it a theoretical necessity. It may be just a fantasy that appeals to certain people. But Vaas is evidently including all kinds of idea of before big bang in his book, which should be quite interesting.
===========================
http://arxiv.org/abs/0712.0315
Very-High Energy Gamma Astrophysics
Alessandro De Angelis, Oriana Mansutti, Massimo Persic
Invited Review Talk at the Sixth International Workshop on New Worlds in Astroparticle Physics, September 6-8, 2007, University of the Algarve, Faro, Portugal
(Submitted on 3 Dec 2007)
"High energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge or the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high energy gamma rays. Observation of gamma rays gives an exciting view of the high energy universe thanks to the current (AGILE) and future (GLAST) satellite-based telescopes and to the current and future ground-based telescopes like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which discovered in the recent years more than 60 new very-high energy sources. The progress achieved with the latest generation of Cherenkov telescopes is comparable to the one drawn by EGRET satellite-borne observatory with respect to the previous gamma-ray satellite detectors. This article reviews the present status of high-energy gamma astrophysics, with emphasis on the recent results and on the experimental developments."
Stephon Alexander has co-authored with Lee Smolin and is currently postdoc at Ashtekar's institute at Penn State
http://arxiv.org/abs/0712.0370
Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae
Stephon Alexander, Tirthabir Biswas, Alessio Notari, Deepak Vaid
26 pages, 11 figures
(Submitted on 3 Dec 2007)
"It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an 'apparent acceleration', even when any form of dark energy is absent. In this paper, we present a 'Minimal Void'' scenario, i.e. a 'void' with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only be consistent with the supernovae data, but also with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter. We also point out possible other observable signatures."
http://arxiv.org/abs/0712.0977
The Plebanski action extended to a unification of gravity and Yang-Mills theory
Lee Smolin
13 pages, one figure
(Submitted on 6 Dec 2007)
"We study a unification of gravity with Yang-Mills fields based on a simple extension of the Plebanski action to a Lie group G which contains the local lorentz group. The Coleman-Mandula theorem is avoided because the theory necessarily has a non-zero cosmological constant and the dynamics has no global spacetime symmetry. This may be applied to Lisi's proposal of an E8 unified theory, giving a fully E8 invariant action. The extended form of the Plebanski action suggests a new class of spin foam models."
http://arxiv.org/abs/0712.0817
Loop quantization of spherically symmetric midi-superspaces : the interior problem
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
12 Pages, to appear in Proceedings of the Third Mexican Meeting on Mathematical and Experimental Physics, A. Macias, C. Laemmerzahl, A. Camacho, editors
(Submitted on 5 Dec 2007)
"We continue the study of spherically symmetric vacuum space-times in loop quantum gravity by treating the interior of a black hole. We start from a midi-superspace approach, but a simple gauge fixing leads to a Kantowski--Sachs form for the variables. We show that one can solve the quantum theory exactly in the (periodic) connection representation, including the inner product. The evolution can be solved exactly by de-parameterizing the theory and can be easily interpreted as a semi-classical evolution plus quantum corrections. A relational evolution can also be introduced in a precise manner, suggesting what may happen in situations where it is not possible to de-parameterize. We show that the singularity is replaced by a bounce at which quantum effects are important and that the extent of the region at the bounce where one departs from classical general relativity depends on the initial data."
http://arxiv.org/abs/0712.0921
Going Beyond Bell's Theorem
Daniel M. Greenberger, Michael A. Horne, Anton Zeilinger
Originally published in: 'Bell's Theorem, Quantum Theory, and Conceptions of the Universe', M. Kafatos (Ed.), Kluwer, Dordrecht, 69-72 (1989)
This original suggestion of GHZ-states and of what is today called multi-qubit entanglement is not generally accessible, so here we make it available electronically.
(Submitted on 6 Dec 2007)
"Bell's Theorem proved that one cannot in general reproduce the results of quantum theory with a classical, deterministic local model. However, Einstein originally considered the case where one could define an 'element of reality', namely for the much simpler case where one could predict with certainty a definite outcome for an experiment. For this simple case, Bell's Theorem says nothing. But by using a slightly more complicated model than Bell, one can show that even in this simple case where one can make definite predictions, one still cannot generally introduce deterministic, local models to explain the results."
http://arxiv.org/abs/0712.0683
Loop Quantization of Polarized Gowdy Model on T3: Classical Theory
Kinjal Banerjee, Ghanashyam Date
20 pages
(Submitted on 5 Dec 2007)
"The vacuum Gowdy models provide much studied, non-trivial midi-superspace examples. Various technical issues within Loop Quantum Gravity can be studied in these models as well as one can hope to understand singularities and their resolution in the loop quantization. The first step in this program is to reformulate the model in real connection variables in a manner that is amenable to loop quantization. We begin with the unpolarized model and carry out a consistent reduction to the polarized case. Carrying out complete gauge fixing, the known solutions are recovered."
http://arxiv.org/abs/0712.0687
Loop Quantization of Polarized Gowdy Model on T3: Quantum Theory
Kinjal Banerjee, Ghanashyam Date
24 pages
(Submitted on 5 Dec 2007)
"In this second paper on loop quantization of Gowdy model, we introduce the kinematical Hilbert space on which appropriate holonomies and fluxes are well represented. The quantization of the volume operator and the Gauss constraint is straight forward. Imposition of the Gauss constraint can be done on the kinematical Hilbert space to select subspace of gauge invariant states. We carry out the quantization of the Hamiltonian constraint making specific choices. Alternative choices are briefly discussed. It appears that to get spatial correlations reflected in the Hamiltonian constraint, one may have to adopt the so called 'effective operator viewpoint'."
http://arxiv.org/abs/0712.2485
Planckian Birth of the Quantum de Sitter Universe
J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll
10 pages, 3 figures
(Submitted on 17 Dec 2007)
"We show that the quantum universe emerging from a nonperturbative, Lorentzian sum-over-geometries can be described with high accuracy by a four-dimensional de Sitter spacetime. By a scaling analysis involving Newton's constant, we establish that the linear size of the quantum universes under study is in between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum fluctuations around the de Sitter universe in this regime are to good approximation still describable semiclassically. The numerical evidence presented comes from a regularization of quantum gravity in terms of causal dynamical triangulations."
http://arxiv.org/abs/0712.2811
A Note on Quantum Field Theories with a Minimal Length Scale
S. Hossenfelder
(Submitted on 17 Dec 2007)
"The aim of this note is to address the low energy limit of quantum field theories with a minimal length scale. The essential feature of these models is that the minimal length acts as a regulator in the asymptotic high energy limit which is incorporated through an infinite series of higher order derivatives. If one investigates a perturbative expansion in inverse powers of the Planck mass, one generically obtains extra poles in the propagator, and instabilities connected with the higher order derivative Lagrangian, that are however artifacts of truncating the series."
http://arxiv.org/abs/0712.3242
Unspeakables and the Epistemological path towards Quantum Gravity
Daniel Sudarsky
Invited article for "GRF2007 Special Issue" of the IJMPD . In press
(Submitted on 19 Dec 2007)
"We offer a critical assessment of some generic features of various of the current approaches towards the construction of a Theory of Quantum Gravity. We will argue that there is a need for further conceptual clarifications before such an enterprise can be launched on a truly well grounded setting, and that one of the guiding principles that can be viewed as part of the reasons for successes of the past theoretical developments is the identification of Unspeakables: Concepts that should not only play no role in the formulation of the theories, but ones that the formalism of the theory itself should prevent from ever been spoken about."
http://arxiv.org/abs/0712.2843
Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei
The Pierre Auger Collaboration
33 pages, 8 figures, submitted to Astropart. phys
(Submitted on 17 Dec 2007)
"Data collected by the Pierre Auger Observatory provide evidence for anisotropy in the arrival directions of the cosmic rays with the highest energies, which are correlated with the positions of relatively nearby active galactic nuclei (AGN) [1]. The correlation has maximum significance for cosmic rays with energy greater than ~ 6x10^{19} eV and AGN at a distance less than ~ 75 Mpc. We have confirmed the anisotropy at a confidence level of more than 99% through a test with parameters specified a priori, using an independent data set. The observed correlation is compatible with the hypothesis that cosmic rays with the highest energies originate from extra-galactic sources close enough so that their flux is not significantly attenuated by interaction with the cosmic background radiation (the Greisen-Zatsepin-Kuz'min effect). The angular scale of the correlation observed is a few degrees, which suggests a predominantly light composition unless the magnetic fields are very weak outside the thin disk of our galaxy. Our present data do not identify AGN as the sources of cosmic rays unambiguously, and other candidate sources which are distributed as nearby AGN are not ruled out. We discuss the prospect of unequivocal identification of individual sources of the highest-energy cosmic rays within a few years of continued operation of the Pierre Auger Observatory."
http://arxiv.org/abs/0712.3099
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Chas A. Egan, Charles H. Lineweaver
16 pages, 8 figures, Submitted to Phys. Rev. D
(Submitted on 19 Dec 2007)
"Dynamic dark energy (DDE) models are often designed to solve the cosmic coincidence (why, just now, is the dark energy density \rho_{de}, the same order of magnitude as the matter density \rho_m?) by guaranteeing \rho_{de} \sim \rho_m for significant fractions of the age of the universe. This typically entails ad-hoc tracking or oscillatory behaviour in the model. However, such behaviour is neither sufficient nor necessary to solve the coincidence problem. What must be shown is that a significant fraction of observers see \rho_{de} \sim \rho_m. Precisely when, and for how long, must a DDE model have \rho_{de} \sim \rho_{m} in order to solve the coincidence? We explore the coincidence problem in dynamic dark energy models using the temporal distribution of terrestrial-planet-bound observers. We find that any dark energy model fitting current observational constraints on \rho_{de} and the equation of state parameters w_0 and w_a, does have \rho_{de} \sim \rho_m for a large fraction of observers in the universe. This demotivates DDE models specifically designed to solve the coincidence using long or repeated periods of \rho_{de} \sim \rho_m."
http://arxiv.org/abs/0712.3219
Habitability of Super-Earths: Gliese 581c and 581d
W. von Bloh, C. Bounama, M. Kuntz, S. Franck
3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)
(Submitted on 19 Dec 2007)
"The unexpected diversity of exoplanets includes a growing number of super-Earth planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogical composition as Earth. We present a thermal evolution model for super-Earth planets to identify the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply our model to calculate the habitability of the two super-Earths in the Gliese 581 system. The super-Earth Gl 581c is clearly outside the pHZ, while Gl 581d is at the outer edge of the pHZ, and therefore could at least harbor some primitive forms of life."
http://arxiv.org/abs/0712.2865
New Frontiers in Cosmology and Galaxy Formation: Challenges for the Future
Richard Ellis (Caltech), Joseph Silk (Oxford)
29 pages. To appear in "Structure Formation in the Universe", ed. Chabrier, G., Cambridge University Press.
"(Abridged) Cosmology faces three distinct challenges in the next decade. (1) The dark sector, both dark matter and dark energy, dominates the Universe. Key questions include determining the nature of both. Improved observational probes are crucial. (2) Galaxy formation was initiated at around the epoch of reionization: we need to understand how and when as well as to develop probes of earlier epochs. (3) Our simple dark matter-driven picture of galaxy assembly is seemingly at odds with several observational results, including the presence of ULIRGs at high z, the `downsizing' signature, chemical signatures of alpha-element ratios and suggestions that merging may not be important in defining the Hubble sequence. Understanding the physical implications is a major challenge for theorists and refiniing the observational uncertainties a major goal for observers."
An ULIRG is an ultra luminous infra-red galaxy http://www.daviddarling.info/encyclopedia/U/ULIRG.html
thought to be the site of intense star formation activity
http://arxiv.org/abs/0712.3457
A general test of the Copernican Principle
Chris Clarkson, Bruce A. Bassett, Teresa Hui-Ching Lu (UCT & SAAO, Cape Town)
4 pages
(Submitted on 20 Dec 2007)
"Here we present an observational test for the Copernican assumption which can be automatically implemented while we search for dark energy in the coming decade. Our test, which relies on the constant curvature of FLRW models, is entirely independent of any model for dark energy or theory of gravity and thereby represents a model-independent test of the Copernican Principle."
http://arxiv.org/abs/0712.4143
Cosmological Plebanski theory
Karim Noui, Alejandro Perez, Kevin Vandersloot
20 pages
(Submitted on 26 Dec 2007)
"We consider the cosmological symmetry reduction of the Plebanski action as a toy-model to explore, in this simple framework, some issues related to loop quantum gravity and spin-foam models. We make the classical analysis of the model and perform both path integral and canonical quantizations. As for the full theory, the reduced model admits two types of classical solutions: topological and gravitational ones. The quantization mixes these two solutions, which prevents the model to be equivalent to standard quantum cosmology. Furthermore, the topological solution dominates at the classical limit. We also study the effect of an Immirzi parameter in the model."
http://arxiv.org/abs/0712.3545
The Higgs Phenomenon in Quantum Gravity
R. Percacci
This paper was published in 1991 but was not previously available in the archive. Some updates have been added in a postscript
(Submitted on 20 Dec 2007)
"The Higgs phenomenon occurs in theories of gravity in which the connection is an independent dynamical variable. The role of order parameters is played by the soldering form and a fiber metric. The breaking of the original gauge symmetry is linked to the appearance of geometrical structures on spacetime. These facts suggest certain modifications and generalizations of the theory. We propose a Higgs-like model which provides a dynamical explanation for the nondegeneracy of the metric and a framework for the unification of gravity with the other interactions."
The next paper might be of pedagogical value.
http://arxiv.org/abs/0712.3709
Complete calculations of the perihelion precession of Mercury and the deflection of light by the Sun in General Relativity
17 pages, 3 figures
Christian Magnan
(Submitted on 21 Dec 2007)
"Taking up a method devised by Taylor and Wheeler and collecting pieces of their work we offer a self-contained derivation of the formulae giving both the precession of the orbit of a planet around the Sun and the deflection angle of a light pulse passing near the Sun in the framework of General Relativity. The demonstration uses only elementary algebra without resorting to tensor formalism. No prior knowledge in relativity is needed to follow the presentation."
http://arxiv.org/abs/0801.0502
Exact solutions for Big Bounce in loop quantum cosmology
Jakub Mielczarek, Tomasz Stachowiak, Marek Szydlowski
12 pages, 12 figures
(Submitted on 3 Jan 2008)
"In this paper we study the cosmological FRW model k=0 with holonomy corrections of Loop Quantum Gravity. The considered universe contains a massless scalar field and cosmological constant Lambda. We find analytical solutions for this model in different configurations and investigate its dynamical behaviour in the whole phase space. We show the explicit influence of Lambda on the qualitative and quantitative character of solutions. For the case of positive Lambda the oscillating solutions without the initial and final singularity appear as a generic case for some quantisation schemes."
My comment is that I don't know the authors from their previous work. Ashtekar has a close collaborator at the jagellonian University, and so also do Ambjorn and Loll. These authors are also Jagellonians. Last year there was the international QG school in Poland.
The senior author in this case has 64 preprints on arxiv going back to 1997
http://arxiv.org/find/grp_physics/1/au:+Szydlowski_M/0/1/0/all/0/1
CODATA has the accepted collection of recommended values of the fundamental physical constants. They just brought out the 2006 revised edition on arxiv.
http://arxiv.org/abs/0801.0028
CODATA Recommended Values of the Fundamental Physical Constants: 2006
Peter J. Mohr, Barry N. Taylor, David B. Newell
(Submitted on 29 Dec 2007)
"This paper gives the 2006 self-consistent set of the basic constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA) for international use..."
You can always get the latest CODATA values of the constants online from the NIST.gov/constants website. But it's sometimes more convenient to have it all in a few pages of hardcopy. The whole batch can be printed off in about 10 pages: you can either print off pages 94-105-----or the short one-page list on page 94
Thanks to Garth for pointing out the following paper by George Ellis et al.
http://arxiv.org/abs/0801.0068
Time drift of cosmological redshifts as a test of the Copernican principle
Jean-Philippe Uzan, Chris Clarkson, George F.R. Ellis
4 pages
(Submitted on 29 Dec 2007)
"The time drift of the cosmological redshift in a general spherically symmetric spacetime is derived. It is shown that its observation would offer the possibility to construct a test of the Copernican principle. In particular, it allows to close the reconstruction problem of a Lemaitre-Tolman-Bondi spacetime from background observations."
http://arxiv.org/abs/0801.0861
Quantum Graphity: a model of emergent locality
Tomasz Konopka, Fotini Markopoulou, Simone Severini
25 pages
(Submitted on 6 Jan 2008)
"Quantum graphity is a background independent model for emergent locality, spatial geometry and matter. The states of the system correspond to dynamical graphs on N vertices. At high energy, the graph describing the system is highly connected and the physics is invariant under the full symmetric group acting on the vertices. We present evidence that the model also has a low-energy phase in which the graph describing the system breaks permutation symmetry and appears to be ordered, low-dimensional and local. Consideration of the free energy associated with the dominant terms in the dynamics shows that this low-energy state is thermodynamically stable under local perturbations. The model can also give rise to an emergent U(1) gauge theory in the ground state by the string-net condensation mechanism of Levin and Wen. We also reformulate the model in graph-theoretic terms and compare its dynamics to some common graph processes."
http://arxiv.org/abs/0801.0705
Realistic Equations of State for the Primeval Universe
R. Aldrovandi, R.R. Cuzinatto, L. G. Medeiros
29 pages, 5 figures
(Submitted on 4 Jan 2008)
"Early universe equations of state including realistic interactions between constituents are built up. Under certain reasonable assumptions, these equations are able to generate an inflationary regime prior to the nucleosynthesis period. The resulting accelerated expansion is intense enough to solve the flatness and horizon problems. In the cases of curvature parameter kappa equal to 0 or +1, the model is able to avoid the initial singularity and offers a natural explanation for why the universe is in expansion."
http://arxiv.org/abs/0801.1073
\mathcal{O}(\mu^4) corrections from holonomies in Loop Quantum Gravity and its effect on flat FRW models
Jakub Mielczarek, Marek Szydlowski
6 pages, 1 figure
(Submitted on 7 Jan 2008)
"In this paper we calculate \mathcal{O}(\mu^4) corrections from holonomies in Loop Quantum Gravity. We apply these corrections to the flat FRW cosmological model and calculate the modified Friedmann equation. We show that the bounce appeared for in the models with lowest \mathcal{O}(\mu^2) order corrections is shifted to the higher energies \rho_{\text{bounce}} = 3 \rho_{\text{c}}. Also a pole in the Hubble parameter appear for \rho_{\text{pole}} = {3/2} \rho_{\text{c}}. This result suggests that an ordinary bouncing solution appears only when quantum corrections in the lowest order are considered. Higher order corrections can lead to the nonperturbative effects.
The next paper happens to cite a Derek Wise' paper
http://arxiv.org/abs/0801.0905
Cosmological Solutions with Torsion in a Model of de Sitter Gauge Theory of Gravity
Chao-Guang Huang, Hai-Qing Zhang, Han-Ying Guo
16 pages, 2 figures
(Submitted on 7 Jan 2008)
"The torsion is shown to be vitally important in the explanation of the evolution of the universe in a large class of gravitational theories containing quadratic terms of curvature and torsion. The cosmological solutions with homogeneous and isotropic torsion in a model of de Sitter gauge theory of gravity are presented, which may explain the observation data for SN Ia when parameters are suitably chosen and supply a natural transit from decelerating expansion to accelerating expansion without the help of the introduction of other strange fields in the theory."
[22] D.K. Wise, MacDowell-Mansouri, Gravity and Cartan Geometry, arXiv: gr-qc/0611154.
http://arxiv.org/abs/0801.1547
Einstein-aether gravity: a status report
Ted Jacobson
18 pages, for the proceedings of the conference "From Quantum to Emergent Gravity: Theory and Phenomenology", June 11-15 2007, SISSA; Trieste, Italy
(Submitted on 10 Jan 2008)
"This paper reviews the theory, phenomenology, and observational constraints on the coupling parameters of Einstein-aether gravity, i.e. General Relativity coupled to a dynamical unit timelike vector field. A discussion of open questions concerning both phenomenology and fundamental issues is included."
http://arxiv.org/abs/0801.1565
Dynamics of dark energy with a coupling to dark matter
Christian G. Boehmer, Gabriela Caldera-Cabral, Ruth Lazkoz, Roy Maartens
11 pages, 4 figures
(Submitted on 10 Jan 2008)
"Dark energy and dark matter are the dominant sources in the evolution of the late universe. They are currently only indirectly detected via their gravitational effects, and there could be a coupling between them without violating observational constraints. We investigate the background dynamics when dark energy is modelled as exponential quintessence, and is coupled to dark matter via simple models of energy exchange. We introduce a new form of dark sector coupling, which leads to a more complicated dynamical phase space and has a better physical motivation than previous mathematically similar couplings."
http://arxiv.org/abs/0801.1847
What is the entropy of the universe?
Paul Frampton, Stephen D.H. Hsu, Thomas W. Kephart, David Reeb
4 pages, 2 figures
(Submitted on 11 Jan 2008)
"Standard calculations suggest that the entropy of the universe is dominated by black holes, although they comprise only a tiny fraction of its total energy. We give a physical interpretation of this result. Statistical entropy is the logarithm of the number of microstates consistent with the observed macroscopic properties of a system, hence a measure of uncertainty about its precise state. The largest uncertainty in the present and future state of the universe is due to the (unknown) internal microstates of its black holes. We also discuss the qualitative gap between the entropies of black holes and ordinary matter."
The following is presented as a pedagogical paper
http://arxiv.org/abs/0801.1734
The Volume Inside a Black Hole
Brandon S. DiNunno, Richard A. Matzner
17 pages, 5 figures
(Submitted on 11 Jan 2008)
"The horizon (the surface) of a black hole is a null surface, defined by those hypothetical "outgoing" light rays that just hover under the influence of the strong gravity at the surface. Because the light rays are orthogonal to the spatial 2-dimensional surface at one instant of time, the surface of the black hole is the same for all observers (i.e. the same for all coordinate definitions of "instant of time"). This value is 4*(pi)* (2Gm/c^2)^2 for nonspinning black holes, with G= Newton's constant, c= speed of light, and m= mass of the black hole.
The 3-dimensional spatial volume inside a black hole, in contrast, depends explicitly on the definition of time, and can even be time dependent, or zero. We give examples of the volume found inside a standard, nonspinning spherical black hole, for several different standard time-coordinate definitions. Elucidating these results for the volume provides a new pedagogical resource of facts already known in principle to the relativity community, but rarely worked out."
http://arxiv.org/abs/0801.1811
Information is Not Lost in the Evaporation of 2-dimensional Black Holes
Abhay Ashtekar, Victor Taveras, Madhavan Varadarajan
4 pages, 2 figures
(Submitted on 11 Jan 2008)
"We analyze Hawking evaporation of the Callen-Giddings-Harvey-Strominger (CGHS) black holes from a quantum geometry perspective and show that information is not lost, primarily because the quantum space-time is sufficiently larger than the classical. Using suitable approximations to extract physics from quantum space-times we establish that: i)future null infinity of the quantum space-time is sufficiently long for the the past vacuum to evolve to a pure state in the future; ii) this state has a finite norm in the future Fock space; and iii) all the information comes out at future infinity; there are no remnants."
http://arxiv.org/abs/0801.1928
Supersymmetry in Elementary Particle Physics
Michael E. Peskin
75 pages, 36 figures
(Submitted on 13 Jan 2008)
"These lectures, presented at the 2006 TASI summer school, give a general introduction to supersymmetry, emphasizing its application to models of elementary particle physics at the 100 GeV energy scale. I discuss the following topics: the construction of supersymmetric Lagrangians with scalars, fermions, and gauge bosons, the structure and mass spectrum of the Minimal Supersymmetric Standard Model (MSSM), the measurement of the parameters of the MSSM at high-energy colliders, and the solutions that the MSSM gives to the problems of electroweak symmetry breaking and dark matter."
http://arxiv.org/abs/0801.1852
Detecting the Glint of Starlight on the Oceans of Distant Planets
D.M. Williams, E. Gaidos
41 pages, 7 figures. Icarus in press
(Submitted on 11 Jan 2008)
"We propose that astronomers will be eventually be able to discriminate between extrasolar Earth-like planets with surface oceans and those without using the shape of phase light curves in the visible and near-IR spectrum. We model the visible light curves of planets having Earth-like surfaces, seasons, and optically-thin atmospheres with idealized diffuse-scattering clouds. We show that planets partially covered by water will appear measurably brighter near crescent phase (relative to Lambertian planets) because of the efficient specular reflection (i.e., glint) of starlight incident on their surfaces at a highly oblique angle. Planets on orbits within 30 degrees of edge-on orientation (half of all planets) will show pronounced glint over a sizeable range of orbital longitudes, from quadrature to crescent, all outside the glare of their parent stars. Also, water-covered planets will appear darker than a Lambertian disk near full illumination. Finally, we show that planets with a mixed land/water surface will polarize the reflected signal by as much as 30-70 percent. These results suggest several new ways of directly identifying water on distant planets."
I didn't realize that this was somehow a special thread. I'm sorry. I removed my post to shorten this. Thanks for the new thread.
/Fredrik
Fra thanks for moving the discussion to
http://physicsforums.com/showthread.php?p=1572779#post1572779
By custom this thread is not for discussion. I am grateful for this courtesy, because discussion would interfere with its function as a list of abstracts with links. When questions like this come up, we can start a separate thread.
============================================
http://arxiv.org/abs/0801.2564
Modern space-time and undecidability
Rodolfo Gambini, Jorge Pullin
8 pages, contribution to the volume "Minkowski spacetime: a hundred years later", edited by Vesselin Petkov
(Submitted on 16 Jan 2008)
"The picture of space-time that Minkowski created in 1907 has been followed by two important developments in physics not contained in the original picture: general relativity and quantum mechanics. We will argue that the use of concepts of those theories to construct space-time implies conceptual modifications in quantum mechanics. In particular one can construct a viable picture of quantum mechanics without a reduction process that has outcomes equivalent to a picture with a reduction process. One therefore has two theories that are entirely equivalent experimentally but profoundly different in the description of reality they give. This introduces a fundamental level of undecidability in physics of a kind that has not been present before. We discuss some of the implications.
This is a nice invited review paper by a top cosmologist
http://arxiv.org/abs/0801.2968
Mapping the Cosmological Expansion
Eric V. Linder
49 pages, 29 figures; Review invited for Reports on Progress in Physics
(Submitted on 18 Jan 2008)
"The ability to map the cosmological expansion has developed enormously, spurred by the turning point one decade ago of the discovery of cosmic acceleration. The standard model of cosmology has shifted from a matter dominated, standard gravity, decelerating expansion to the present search for the origin of acceleration in the cosmic expansion. We present a wide ranging review of the tools, challenges, and physical interpretations. The tools include direct measures of cosmic scales through Type Ia supernova luminosity distances, and angular distance scales of baryon acoustic oscillation and cosmic microwave background density perturbations, as well as indirect probes such as the effect of cosmic expansion on the growth of matter density fluctuations. Accurate mapping of the expansion requires understanding of systematic uncertainties in both the measurements and the theoretical framework, but the result will give important clues to the nature of the physics behind accelerating expansion and to the fate of the universe."
He put some work into it. It's thorough and well-written. And it has the perspective of an old hand.
http://arxiv.org/abs/0801.3287
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
Martin Reuter, Holger Weyer
4 figures
(Submitted on 21 Jan 2008)
"We analyze the conceptual role of background independence in the application of the effective average action to quantum gravity. Insisting on a background independent renormalization group (RG) flow the coarse graining operation must be defined in terms of an unspecified variable metric since no rigid metric of a fixed background spacetime is available. This leads to an extra field dependence in the functional RG equation and a significantly different RG flow in comparison to the standard flow equation with a rigid metric in the mode cutoff. The background independent RG flow can possess a non-Gaussian fixed point, for instance, even though the corresponding standard one does not. We demonstrate the importance of this universal, essentially kinematical effect by computing the RG flow of Quantum Einstein Gravity in the 'conformally reduced' Einstein--Hilbert approximation which discards all degrees of freedom contained in the metric except the conformal one. Without the extra field dependence the resulting RG flow is that of a simple \phi^4-theory. Including it one obtains a flow with exactly the same qualitative properties as in the full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian fixed point which is necessary for asymptotic safety."
http://arxiv.org/abs/0801.4001
Quantum nature of cosmological bounces
Martin Bojowald
26 pages
(Submitted on 25 Jan 2008)
"Several examples are known where quantum gravity effects resolve the classical big bang singularity by a bounce. The most detailed analysis has probably occurred for loop quantum cosmology of isotropic models sourced by a free, massless scalar. Once a bounce has been realized under fairly general conditions, the central questions are how strongly quantum it behaves, what influence quantum effects can have on its appearance, and what quantum space-time beyond the bounce may look like. This, then, has to be taken into account for effective equations which describe the evolution properly and can be used for further phenomenological investigations. Here, we provide the first analysis with interacting matter with new effective equations valid for weak self-interactions or small masses. They differ from the free scalar equations by crucial terms and have an important influence on the bounce and the space-time around it. Especially the role of squeezed states, which have often been overlooked in this context, is highlighted. The presence of a bounce is proven for uncorrelated states, but as squeezing is a dynamical property and may change in time, further work is required for a general conclusion."
http://arxiv.org/abs/0801.4241
A Immirzi-like parameter for 3d quantum gravity
Valentin Bonzom, Etera R. Livine
14 pages
(Submitted on 28 Jan 2008)
"We study an Immirzi-like ambiguity in three-dimensional quantum gravity. It shares some features with the Immirzi parameter of four-dimensional loop quantum gravity: it does not affect the equations of motion, but modifies the Poisson brackets and the constraint algebra at the canonical level. We focus on the length operator and show how to define it through non-commuting fluxes. We compute its spectrum and show the effect of this Immirzi-like ambiguity. Finally, we extend these considerations to 4d gravity and show how the different topological modifications of the action affect the canonical structure of loop quantum gravity."
http://arxiv.org/abs/0802.0896
Topology change in causal quantum gravity
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
4 pages, proceedings of the workshop JGRG 17 (Nagoya, Japan, December 2007)
(Submitted on 6 Feb 2008)
"The role of topology change in a fundamental theory of quantum gravity is still a matter of debate. However, when regarding string theory as two-dimensional quantum gravity, topological fluctuations are essential. Here we present a third quantization of two-dimensional surfaces based on the method of causal dynamical triangulation (CDT). Formally, our construction is similar to the c = 0 non-critical string field theory developed by Ishibashi, Kawai and others, but physically it is quite distinct. Unlike in non-critical string theory the topology change of spatial slices is well controlled and regulated by Newton's constant."
http://arxiv.org/abs/0802.0864
Area-angle variables for general relativity
Bianca Dittrich, Simone Speziale
7 pages, 1 figure
(Submitted on 6 Feb 2008)
"We introduce a modified Regge calculus for general relativity on a triangulated four dimensional Riemannian manifold where the fundamental variables are areas and a certain class of angles. These variables satisfy constraints which are local in the triangulation. We expect the formulation to have applications to classical discrete gravity and non-perturbative approaches to quantum gravity."
http://arxiv.org/abs/0802.0880
Entanglement Entropy in Loop Quantum Gravity
William Donnelly
4 pages
(Submitted on 6 Feb 2008)
The entanglement entropy between quantum fields inside and outside a black hole horizon is a promising candidate for the microscopic origin of black hole entropy. We show that the entanglement entropy may be defined in loop quantum gravity, and compute its value for spin network states. The entanglement entropy for an arbitrary region of space is expressed as a sum over punctures where the spin network intersects the region's boundary. Our result agrees asymptotically with results previously obtained from the isolated horizon framework, and we give a justification for this agreement. We conclude by proposing a new method for studying corrections to the area law and its implications for quantum corrections to the gravitational action."
http://arxiv.org/abs/0802.0719
A String Field Theory based on Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
29 pages, 4 figures
(Submitted on 5 Feb 2008)
"We formulate the string field theory in zero-dimensional target space corresponding to the two-dimensional quantum gravity theory defined through Causal Dynamical Triangulations. This third quantization of the quantum gravity theory allows us in principle to calculate the transition amplitudes of processes in which the topology of space changes in time, and to include non-trivial topologies of space-time. We formulate the corresponding Dyson-Schwinger equations and illustrate how they can be solved iteratively."
http://arxiv.org/abs/0802.0767
A fundamental length as a candidate for dark energy: a DSR inspired FRW spacetime
N. Khosravi, H. R. Sepangi
8 pages, to appear in PLA
(Submitted on 6 Feb 2008)
"We show that the existence of a fundamental length, introduced in Deformed Special Relativity (DSR) inspired minisuper (phase-) space, causes the behavior of the scale factor of the universe to change from that of a universe filled with dust to an accelerating universe driven by a cosmological constant."
http://arxiv.org/abs/0802.0702
Strategies for Determining the Nature of Dark Matter
Dan Hooper, Edward A. Baltz
25 pages, 5 figures, Review intended for the Annual Review of Nuclear and Particle Science
(Submitted on 5 Feb 2008)
"In this review, we discuss the role of the various experimental programs taking part in the broader effort to identify the particle nature of dark matter. In particular, we focus on electroweak scale dark matter particles and discuss a wide range of search strategies being carried out and developed to detect them. These efforts include direct detection experiments, which attempt to observe the elastic scattering of dark matter particles with nuclei, indirect detection experiments, which search for photons, antimatter and neutrinos produced as a result of dark matter annihilations, and collider searches for new TeV-scale physics. Each of these techniques could potentially provide a different and complementary set of information related to the mass, interactions and distribution of dark matter. Ultimately, it is hoped that these many different tools will be used together to conclusively identify the particle or particles that constitute the dark matter of our universe."
http://arxiv.org/abs/0802.0013
Cosmic Neutrinos
Chris Quigg
27 pages, 16 figures, lecture at 2007 SLAC Summer Institute
(Submitted on 31 Jan 2008)
"I recall the place of neutrinos in the electroweak theory and summarize what we know about neutrino mass and flavor change. I next review the essential characteristics expected for relic neutrinos and survey what we can say about the neutrino contribution to the dark matter of the Universe. Then I discuss the standard-model interactions of ultrahigh-energy neutrinos, paying attention to the consequences of neutrino oscillations, and illustrate a few topics of interest to neutrino observatories. I conclude with short comments on the remote possibility of detecting relic neutrinos through annihilations of ultrahigh-energy neutrinos at the Z resonance."
http://arxiv.org/abs/0802.1201
Tensorial Structure of the LQG graviton propagator
Emanuele Alesci
4 pages,; to appear in the proceedings of the II Stueckelberg Workshop, Int.J.Mod.Phys.A
(Submitted on 8 Feb 2008)
"We review the construction of the tensorial structure of the graviton propagator in the context of loop quantum gravity and spinfoam formalism. The main result of this analysis is that applying the same strategy used to compute the diagonal terms, the Barrett-Crane vertex is unable to yield the correct propagator in the long distance limit. The problem is in the intertwiner-independence of the Barrett-Crane vertex. We also review the asymptotic behavior of an alternative vertex that is able to give the correct propagator."
http://arxiv.org/abs/0802.1129
Doubly special relativity in de Sitter spacetime
Salvatore Mignemi
18 pages
(Submitted on 8 Feb 2008)
"We discuss the generalization of Doubly Special Relativity to a curved de Sitter background. The model has three observer-independent scales, the velocity of light c, the radius of curvature of the geometry alpha, and the Planck energy kappa, and can be realized in a noncommutative position space. It is possible to construct a model exhibiting a duality for the interchange of positions and momenta together with the exchange of alpha and kappa."
http://arxiv.org/abs/0802.1221
Immirzi parameter and fermions with non-minimal coupling
Sergei Alexandrov
4 pages
(Submitted on 8 Feb 2008)
"We clarify the role played by the Immirzi parameter in classical gravity coupled to fermions. Considering the general non-minimal coupling, we show that, although the torsion depends explicitly on the Immirzi parameter, in a suitable parametrization the effective action obtained by integrating out the spin-connection is independent of it. Thus the Immirzi parameter is not detectable in classical theory even after coupling of fermions."
http://arxiv.org/abs/0802.1215
TeVeS gets caught on caustics
Carlo R. Contaldi, Toby Wiseman, Benjamin Withers
21 pages,12 figures
(Submitted on 10 Feb 2008)
"TeVeS uses a dynamical vector field with timelike unit norm constraint to specify a preferred local frame. When matter moves slowly in this frame - the so-called quasi-static regime - Modified Newtonian Dynamics (MoND) results. Theories with such vectors (such as Einstein-aether) are prone to the vector dynamics forming singularities which render their classical evolution problematic. Here we analyse the dynamics of the vector in TeVeS in various situations. We find that, quite generically, the vector field develops caustic singularities on time scales of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein's original formulation of TeVeS appears dynamically problematic. We argue that TeVeS might be saved as a relativistic theory with a MoND limit by modifying the vector field kinetic terms to the more general form used by Einstein-aether."
http://arxiv.org/abs/0802.1783
On Spectral Triples in Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
84 pages, 8 figures
(Submitted on 13 Feb 2008)
"This paper establishes a link between Noncommutative Geometry and canonical quantum gravity. A semi-finite spectral triple over a space of connections is presented. The triple involves an algebra of holonomy loops and a Dirac type operator which resembles a global functional derivation operator. The interaction between the Dirac operator and the algebra reproduces the Poisson structure of General Relativity. Moreover, the associated Hilbert space corresponds, up to a discrete symmetry group, to the Hilbert space of diffeomorphism invariant states known from Loop Quantum Gravity. Correspondingly, the square of the Dirac operator has, in terms of canonical quantum gravity, the form of a global area-squared operator. Furthermore, the spectral action resembles a partition function of Quantum Gravity. The construction is background independent and is based on an inductive system of triangulations. This paper is the first of two papers on the subject."
http://arxiv.org/abs/0802.1784
On Spectral Triples in Quantum Gravity II
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
43 pages, 1 figure
(Submitted on 13 Feb 2008)
"A semifinite spectral triple for an algebra canonically associated to canonical quantum gravity is constructed. The algebra is generated by based loops in a triangulation and its barycentric subdivisions. The underlying space can be seen as a gauge fixing of the unconstrained state space of Loop Quantum Gravity. This paper is the second of two papers on the subject."
http://arxiv.org/abs/0802.1561
Have we tested Lorentz invariance enough?
David Mattingly
17 pages, Talk given at 'From Quantum to Emergent Gravity: Theory and Phenomenology', SISSA, June 2007
(Submitted on 12 Feb 2008)
"Motivated by ideas from quantum gravity, Lorentz invariance has undergone many stringent tests over the past decade and passed every one. Since there is no conclusive reason from quantum gravity that the symmetry \textit{must} be violated at some point we should ask the questions: a) are the existing tests sufficient that the symmetry is already likely exact at the Planck scale? b) Are further tests simply blind searches for new physics without reasonable expectation of a positive signal? Here we argue that the existing tests are not quite sufficient and describe some theoretically interesting areas of existing parameterizations for Lorentz violation in the infrared that are not yet ruled out but are accessible (or almost accessible) by current experiments. We illustrate this point using a vector field model for Lorentz violation containing operators up to mass dimension six and analyzing how terrestrial experiments, neutrino observatories, and Auger results on ultra-high energy cosmic rays limit this model."
http://arxiv.org/abs/0802.1798
Emergent gravity and Dark Energy
T. Padmanabhan
26 pages
(Submitted on 13 Feb 2008)
"This is an invited contribution to be included in a multi-authored book on 'Dark Energy', to be edited by Pilar Ruiz-Lapuente and published by Cambridge University Press."
http://arxiv.org/abs/0802.2527
Asymptotics and Hamiltonians in a First order formalism
Abhay Ashtekar, Jonathan Engle, David Sloan
18 pages
(Submitted on 18 Feb 2008)
"We consider 4-dimensional space-times which are asymptotically flat at spatial infinity and show that, in the first order framework, action principle is well-defined without the need of infinite counter terms. It naturally leads to a covariant phase space in which the Hamiltonians generating asymptotic symmetries provide the total energy-momentum and angular momentum of the space-time. We address the subtle but important problems that arise because of logarithmic translations and super-translations both in the Langrangian and Hamiltonian frameworks. As a forthcoming paper will show, the treatment of higher dimensions is considerably simpler. Our first order framework also suggests a new direction for generalizing the spectral action of non-commutative geometry."
http://arxiv.org/abs/0802.2230
Canonical Lagrangian Dynamics and General Relativity
Andrew Randono
(Submitted on 15 Feb 2008)
"Building towards a more covariant approach to canonical classical and quantum gravity we outline an approach to constrained dynamics that de-emphasizes the role of the Hamiltonian phase space and highlights the role of the Lagrangian phase space. We identify a 'Lagrangian one-form' to replace the standard symplectic one-form, which we use to construct the canonical constraints and an associated constraint algebra. The method is particularly useful for generally covariant systems and systems with a degenerate canonical symplectic form, such as Einstein Cartan gravity, to which we apply the method explicitly. We find that one can demonstrate the closure of the constraints without gauge fixing the Lorentz group or introducing primary constraints on the phase space variables. Finally, using geometric quantization techniques, we briefly discuss implications of the formalism for the quantum theory."
http://arxiv.org/abs/0802.3188
Black hole state degeneracy in Loop Quantum Gravity
Ivan Agullo, Jacobo Diaz-Polo, Enrique Fernandez-Borja
22 pages, 7 figures
(Submitted on 21 Feb 2008)
"The combinatorial problem of counting the black hole quantum states within the Isolated Horizon framework in Loop Quantum Gravity is analyzed. A qualitative understanding of the origin of the band structure shown by the degeneracy spectrum, which is responsible for the black hole entropy quantization, is reached. Even when motivated by simple considerations, this picture allows to obtain analytical expressions for the most relevant quantities associated to this effect."
http://arxiv.org/abs/0802.3389
Simplicity and closure constraints in spin foam models of gravity
Sergei Alexandrov
16 pages
(Submitted on 23 Feb 2008)
"We revise imposition of various constraints in spin foam models of 4-dimensional general relativity. We argue that the usual simplicity constraint must be supplemented by a constraint on holonomies and together they must be inserted explicitly into the discretized path integral. At the same time, the closure constraint must be relaxed so that the new constraint expresses covariance of intertwiners assigned to tetrahedra by spin foam quantization. As a result, the spin foam boundary states are shown to be realized in terms of projected spin networks of the covariant loop approach to quantum gravity."
http://arxiv.org/abs/0802.3422
Black Holes without Event Horizons
Alex B. Nielsen
Talk at APCTP Winter School, Daejeon, Korea, 2008. 7 pages
(Submitted on 23 Feb 2008)
"We discuss some of the drawbacks of using event horizons to define black holes. The reasons are both practical, physical and theoretical. We argue that locally defined trapping horizons can remedy many of these drawbacks. We examine of the question of whether black hole thermodynamics should be associated with event horizons or trapping horizons. To this end we discuss what role trapping horizons may play in black hole thermodynamics. In addition, we show how trapping horizons may give rise to Hawking radiation and discuss the issue of gravitational entropy."
http://arxiv.org/abs/0802.3983
Towards the graviton from spinfoams: the complete perturbative expansion of the 3d toy model
Valentin Bonzom, Etera R. Livine, Matteo Smerlak, Simone Speziale
16 pages, 3 figs
(Submitted on 27 Feb 2008)
"We consider an exact expression for the 6j-symbol for the isosceles tetrahedron, involving integrals over SU(2), and use it to write the two-point function of 3d gravity on a single tetrahedron as a group integral. The perturbative expansion of this expression is then performed with respect to the boundary geometry using a simple saddle-point analysis. We derive the complete expansion in inverse powers of the length scale and evaluate explicitly the quantum corrections up to second order. Finally, we use the same method to provide the complete expansion of the isosceles 6j-symbol with the explicit phases at all orders and the next-to-leading correction to the Ponzano-Regge asymptotics."
http://arxiv.org/abs/0802.4077
Black hole state counting in LQG: A number theoretical approach
Ivan Agullo, J. Fernando Barbero G., Jacobo Diaz-Polo, Enrique Fernandez-Borja, Eduardo J. S. Villaseñor
4 pages
(Submitted on 27 Feb 2008)
"We give a practical method to exactly compute black hole entropy in the framework of Loop Quantum Gravity. Along the way we provide a complete characterization of the relevant sector of the spectrum of the area operator, including degeneracies, and determine the number of solutions to the projection constraint analytically. We use a computer implementation of the proposed algorithm to confirm and extend previous results on the detailed structure of the black hole degeneracy spectrum."
http://arxiv.org/abs/0802.4274
Loop Quantum Cosmology: Effective theories and oscillating universes
Martin Bojowald, Reza Tavakol
24 pages, 3 figures, Chapter contributed to: Beyond the Big Bang, edited by R. Vaas (Springer Verlag, 2008)
"Despite its great successes in accounting for the current observations, the so called 'standard' model of cosmology faces a number of fundamental unresolved questions. Paramount among these are those relating to the nature of the origin of the universe and its early evolution. Regarding the question of origin, the main difficulty has been the fact that within the classical general relativistic framework, the 'origin' is almost always a singular event at which the laws of physics break down, thus making it impossible for such an event, or epochs prior to it, to be studied. Recent studies have shown that Loop Quantum Cosmology may provide a non-singular framework where these questions can be addressed. The crucial role here is played by quantum effects, i.e. corrections to the classical equations of motion, which are incorporated in effective equations employed to develop cosmological scenarios.
In this chapter we shall consider the three main types of quantum effects expected to be present within such a framework and discuss some of their consequences for the effective equations. In particular we discuss how such corrections can allow the construction of non-singular emergent scenarios for the origin of the universe, which are past-eternal, oscillating and naturally emerge into an inflationary phase. These scenarios provide a physically plausible picture for the origin and early phases of the universe, which is in principle testable. We pay special attention to the interplay between these different types of correction terms. Given the absence, so far, of a complete derivation of such corrections in general settings, it is important to bear in mind the questions of consistency and robustness of scenarios based on partial inclusion of such effects."
Amazon is now taking advance orders on the new book Beyond the Big Bang, edited by R. Vaas
http://www.amazon.com/Beyond-Big-Bang-Prospects-Collection/dp/3540714227
The book is 600 pages, several different experts in quantum cosmology have contributed chapters.
The publication date is 1 July 2008.
http://arxiv.org/abs/0803.0982
Dark Energy and the Accelerating Universe
Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan)
Invited review for Annual Reviews of Astronomy and Astrophysics; 53 pages, 18 figures
(Submitted on 7 Mar 2008)
"The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years."
http://arxiv.org/abs/0803.2546
Primordial Entropy Production and Lambda-driven Inflation from Quantum Einstein Gravity
Alfio Bonanno, Martin Reuter
12 pages, 4 figures, IGCG-07 Pune
(Submitted on 17 Mar 2008)
"We review recent work on renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values. In particular we argue that QEG effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. This phase is a pure quantum effect and requires no classical inflaton field."
This is the paper Martin Reuter gave at an international conference that was held in December 2007 in India.
Reuter is also scheduled to give a talk at the QG2 conference at Nottingham, first week of July.
Sets of slides for two important talks at the Zakopane workshop. More PDF files from the workshop are available at Kostecki's website:
* Abhay Ashtekar - An Overview of Loop Quantum Cosmology of FRW Models
http://cift.fuw.edu.pl/users/kostecki/zakopane08/ashtekar.pdf
* Carlo Rovelli - Looppy & Foammy: at long last, falling in love
http://cift.fuw.edu.pl/users/kostecki/zakopane08/rovelli.pdf
http://arxiv.org/abs/0803.2926
Matrix universality of gauge field and gravitational dynamics
Lee Smolin
21 pages,
(Submitted on 20 Mar 2008)
"A simple cubic matrix model is presented, which has truncations that, it is argued, lead at the classical level to a variety of theories of gauge fields and gravity. These include Yang-Mills theories and background independent theories of connections. The latter includes Chern-Simons theory in d=3, and BF theory and general relativity in d=4. General relativity coupled to Yang-mills theory for any SU(N) may also arise from quantum corrections.
On the basis of these results we conjecture that there are large universality classes of cut-off gauge and gravity theories, connected by transformations that mix up local and spacetime symmetries. If our universe is described by one of these theories then the question of the choice of the laws of physics is to a large extent subsumed in the problem of the choice of initial conditions in cosmology."
I don't mean to suggest that the next one is a big deal, only a reminder that things can be more complicated than at first suspected and require correction. Linder has a solid track record---perhaps I'm biased but I tend to pay extra attention.
http://arxiv.org/abs/0803.2877
Shifting the Universe: Early Dark Energy and Standard Rulers
Eric V. Linder, Georg Robbers
6 pages, 3 figures
(Submitted on 19 Mar 2008)
"The presence of dark energy at high redshift influences both the cosmic sound horizon and the distance to last scattering of the cosmic microwave background. We demonstrate that through the degeneracy in their ratio, early dark energy can lie hidden in the CMB temperature and polarization spectra, leading to an unrecognized shift in the sound horizon. If the sound horizon is then used as a standard ruler, as in baryon acoustic oscillations, then the derived cosmological parameters can be nontrivially biased. Fitting for the absolute ruler scale (just as supernovae must be fit for the absolute candle magnitude) removes the bias but decreases the leverage of the BAO technique by a factor 2."
http://arxiv.org/abs/0803.3203
Conserved Quantities for Interacting Four Valent Braids in Quantum Gravity
Jonathan Hackett, Yidun Wan
18 pages, 7 figures
(Submitted on 21 Mar 2008)
"We derive conservation laws from interactions of actively-interacting braid-like excitations of embedded framed spin networks in Quantum Gravity. Additionally we demonstrate that actively-interacting braid-like excitations interact in such a way that the product of interactions involving two actively-interacting braid-like excitations produces a resulting actively-interacting form."
http://arxiv.org/abs/0803.3456
Is Quantum Gravity Necessary?
S. Carlip
based on a talk given at Peyresq Physics 11, to appear in Class. Quant. Grav
(Submitted on 24 Mar 2008)
"In view of the enormous difficulties we seem to face in quantizing general relativity, we should perhaps consider the possibility that gravity is a fundamentally classical interaction. Theoretical arguments against such mixed classical-quantum models are strong, but not conclusive, and the question is ultimately one for experiment. I review some work in progress on the possibility of experimental tests, exploiting the nonlinearity of the classical-quantum coupling, that could help settle this question."
http://arxiv.org/abs/0803.2309
Cosmic Microwave Weak lensing data as a test for the dark universe
Erminia Calabrese, Anze Slosar, Alessandro Melchiorri, George F. Smoot, Oliver Zahn
7 Pages, 3 Figures
(Submitted on 17 Mar 2008)
"Combined analyses of WMAP 3-year and ACBAR Cosmic Microwave Anisotropies angular power spectra have presented evidence for gravitational lensing >3 sigma level. This signal could provide a relevant test for cosmology. After evaluating and confirming the statistical significance of the detection in light of the new WMAP 5-year data, we constrain a new parameter A_L that scales the lensing potential such that A_L=0 corresponds to unlensed while A_L=1 is the expected lensed result. We find from WMAP5+ACBAR a 2.5 sigma indication for a lensing contribution larger than expected, with A_L=3.1_{-1.5}^{+1.8} at 95% c.l.. The result is stable under the assumption of different templates for an additional Sunyaev-Zel'dovich foreground component or the inclusion of an extra background of cosmic strings. We find negligible correlation with other cosmological parameters as, for example, the energy density in massive neutrinos. While unknown systematics may be present, dark energy or modified gravity models could be responsible for the over-smoothness of the power spectrum. Near future data, most notably from the Planck satellite mission, will scrutinize this interesting possibility."
http://arxiv.org/abs/0803.3559
Loop Quantum Cosmology of Diagonal Bianchi Type I model: simplified theory
Lukasz Szulc
10 pages, 10 figures
(Submitted on 25 Mar 2008)
"A simplified theory of diagonal Bianchi type I model coupled with a massless scalar field in Loop Quantum Cosmology is constructed. The quantum constraint operator and physical sector are under good analytical control. The problem of finding semi-classical states is reduced to the following one: how to compute an ordinary, continuous, three-dimensional Fourier Transform with an amplitude given analytically? Moreover, the evolution of the three gravitational degrees of freedom is numerically shown to be non-singular."
http://arxiv.org/abs/0803.3659
Phenomenological dynamics of loop quantum cosmology in Kantowski-Sachs spacetime
Dah-Wei Chiou
36 pages, 4 figures, 1 table
(Submitted on 26 Mar 2008)
"The full theory and the semiclassical description of loop quantum cosmology (LQC) have been studied in the Friedmann-Robertson-Walker and Bianchi I models. As an extension to include both anisotropy and intrinsic curvature, this paper investigates the cosmological model of Kantowski-Sachs spacetime with a free massless scalar field at the level of phenomenological dynamics with the LQC discreteness corrections. The LQC corrections are implemented in two different improved quantization schemes. In both schemes, the big bang and big crunch singularities of the classical solution are resolved and replaced by the big bounces when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. Symmetries of scaling are also noted and suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale. The bouncing scenarios are in an analogous fashion of the Bianchi I model, naturally extending the observations obtained earlier."
http://arxiv.org/abs/0803.4484
Recollapsing quantum cosmologies and the question of entropy
Martin Bojowald, Reza Tavakol
23 pages, 2 figures
(Submitted on 31 Mar 2008)
"Recollapsing homogeneous and isotropic models present one of the key ingredients for cyclic scenarios. This is considered here within a quantum cosmological framework in presence of a free scalar field with, in turn, a negative cosmological constant and spatial curvature. Effective equations shed light on the quantum dynamics around a recollapsing phase and the evolution of state parameters such as fluctuations and correlations through such a turn around. In the models considered here, the squeezing of an initial state is found to be strictly monotonic in time during the expansion, turn around and contraction phases. The presence of such monotonicity is of potential importance in relation to a long standing intensive debate concerning the (a)symmetry between the expanding and contracting phases in a recollapsing universe. Furthermore, together with recent analogous results concerning a bounce one can extend this monotonicity throughout an entire cycle. This provides a strong motivation for employing the degree of squeezing as an alternative measure of (quantum) entropy. It may also serve as a new concept of emergent time described by a variable without classical analog. The evolution of the squeezing in emergent oscillating scenarios can in principle provide constraints on the viability of such models."
It is an important issue. Penrose has made it central to all his discussion of cosmology, including opposition to the bounce explanation of the big bang. They may have answered Penrose. London is getting to be an important place for studying quantum cosmology. Tavakol is at Queen Mary London, but there is also a group at King's College.
http://arxiv.org/abs/0803.4483
Numerical techniques for solving the quantum constraint equation of generic lattice-refined models in loop quantum cosmology
William Nelson, Mairi Sakellariadou (King's College London)
17 pages, 14 figures
(Submitted on 31 Mar 2008)
"To avoid instabilities in the continuum semi-classical limit of loop quantum cosmology models, refinement of the underlying lattice is necessary. The lattice refinement leads to new dynamical difference equations which, in general, do not have a uniform step-size, implying complications in their analysis and solutions. We propose a numerical method based on Taylor expansions, which can give us the necessary information to calculate the wave-function at any given lattice point. The method we propose can be applied in any lattice-refined model, while in addition the accuracy of the method can be estimated. Moreover, we confirm numerically the stability criterion which was earlier found following a von Neumann analysis. Finally, the `motion' of the wave-function due to the underlying discreteness of the space-time is investigated, for both a constant lattice, as well as lattice refinement models."
http://arxiv.org/abs/0803.4446
Anti-deSitter universe dynamics in LQC
Eloisa Bentivegna, Tomasz Pawlowski
29 pages, 20 figures
(Submitted on 31 Mar 2008)
"A model for a flat isotropic universe with a negative cosmological constant $\Lambda$ and a massless scalar field as sole matter content is studied within the framework of Loop Quantum Cosmology. By application of the methods introduced for the model with Lambda=0, the physical Hilbert space and the set of Dirac observables are constructed. As in that case, the scalar field plays here the role of an emergent time. The properties of the system are found to be similar to those of the k=1 FRW model: for small energy densities, the quantum dynamics reproduces the classical one, whereas, due to modifications at near-Planckian densities, the big bang and big crunch singularities are replaced by a quantum bounce connecting deterministically the large semiclassical epochs. Thus in Loop Quantum Cosmology the evolution is qualitatively cyclic."
http://arxiv.org/abs/0804.0030
A Pointless Model for the Continuum as the Foundation for Quantum Gravity
Louis Crane
Submitted to GRF
(Submitted on 31 Mar 2008)
"In this paper, we outline a new approach to quantum gravity; describing states for a bounded region of spacetime as eigenstates for two classes of physically plausible gedanken experiments. We end up with two complementary descriptions in which the point set continuum disappears.
The first replaces the continuum of events with a handlebody decomposition of loop space. We conjecture that techniques fron algebraic topology will allow us to extend state sum models on spacetime to loop space.
The second picture replaces the continuum with a nondistributive lattice; the classical limit seems more tractible in this picture."
http://arxiv.org/abs/0804.0054
Formalism Locality in Quantum Theory and Quantum Gravity
Lucien Hardy
To appear in "Philosophy of Quantum Information and Entanglement" Eds A. Bokulich and G. Jaeger (CUP)
(Submitted on 1 Apr 2008)
"We expect a theory of Quantum Gravity to be both probabilistic and have indefinite causal structure. Indefinite causal structure poses particular problems for theory formulation since many of the core ideas used in the usual approaches to theory construction depend on having definite causal structure. For example, the notion of a state across space evolving in time requires that we have some definite causal structure so we can define a state on a space-like hypersurface. We will see that many of these problems are mitigated if we are able to formulate the theory in a "formalism local" (or F-local) fashion. A formulation of a physical theory is said to be F-local if, in making predictions for any given arbitrary space-time region, we need only refer to mathematical objects pertaining to that region. This is a desirable property both on the grounds of efficiency and since, if we have indefinite causal structure, it is not clear how to select some other space-time region on which our calculations may depend. The usual ways of formulating physical theories (the time evolving state picture, the histories approach, and the local equations approach) are not F-local.
We set up a framework for probabilistic theories with indefinite causal structure. This, the causaloid framework, is F-local. We describe how Quantum Theory can be formulated in the causaloid framework (in an F-local fashion). This provides yet another formulation of Quantum Theory. This formulation, however, may be particularly relevant to the problem of finding a theory of Quantum Gravity."
http://arxiv.org/abs/0804.0037
Particle Identifications from Symmetries of Braided Ribbon Network Invariants
Sundance Bilson-Thompson, Jonathan Hackett, Lou Kauffman, Lee Smolin
9 pages, 7 figures
(Submitted on 1 Apr 2008)
"We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge."
Here is a quote from the Conclusions, section 5 on page 8:
We have presented an embedding of the fermion and weak vector boson states of the standard model in a class of loop quantum gravity models. These are models in which the states are based on embeddings of framed trivalent spin networks, with possibly arbitrary labellings, whose dynamics is given by the standard dual Pachner trivalent moves, plus additional moves consistent with the conservation of the topological invariants (a; b; c).
There are a number of interrelated questions that remain open before the promise of this development can be fully understood...
http://arxiv.org/abs/0804.0328
A locally finite model for gravity
Gerard 't Hooft
26 pages, 9 figures
(Submitted on 2 Apr 2008)
"Matter interacting classically with gravity in 3+1 dimensions usually gives rise to a continuum of degrees of freedom, so that, in any attempt to quantize the theory, ultraviolet divergences are nearly inevitable. Here, we investigate matter of a form that only displays a finite number of degrees of freedom in compact sections of space-time. In finite domains, one has only exact, analytic solutions. This is achieved by limiting ourselves to straight pieces of string, surrounded by locally flat sections of space-time. Globally, however, the model is not finite, because solutions tend to generate infinite fractals. The model is not (yet) quantized, but could serve as an interesting setting for analytical approaches to classical general relativity, as well as a possible stepping stone for quantum models. Details of its properties are explained, but some problems remain unsolved, such as a complete description of the most violent interactions, which can become quite complex."
http://arxiv.org/abs/0804.0252
A Matrix Model for 2D Quantum Gravity defined by Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
13 pages, 1 figure
(Submitted on 1 Apr 2008)
"A novel continuum theory of two-dimensional quantum gravity, based on a version of Causal Dynamical Triangulations which incorporates topology change, has recently been formulated as a genuine string field theory in zero-dimensional target space (arXiv:0802.0719). Here we show that the Dyson-Schwinger equations of this string field theory are reproduced by a cubic matrix model. This matrix model also appears in the so-called Dijkgraaf-Vafa correspondence if the superpotential there is required to be renormalizable. In the spirit of this model, as well as the original large-N expansion by 't Hooft, we need no special double-scaling limit involving a fine tuning of coupling constants to obtain the continuum quantum-gravitational theory. Our result also implies a matrix model representation of the original, strictly causal quantum gravity model."
http://arxiv.org/abs/0804.0279
A Discrete Representation of Einstein's Geometric Theory of Gravitation: The Fundamental Role of Dual Tessellations in Regge Calculus
Jonathan R. McDonald, Warner A. Miller
25 pages, 12 figures, submitted to "Tessellations in the Science: Virtues, Techniques and Applications of Geometric Tilings," ed. R. van de Weijgaert, G. Vegter, J. Ritzerveld and V. Icke
(Submitted on 2 Apr 2008)
"In 1961 Tullio Regge provided us with a beautiful lattice representation of Einstein's geometric theory of gravity. This Regge Calculus (RC) is strikingly different from the more usual finite difference and finite element discretizations of gravity. In RC the fundamental principles of General Relativity are applied directly to a tessellated spacetime geometry. In this manuscript, and in the spirit of this conference, we reexamine the foundations of RC and emphasize the central role that the Voronoi and Delaunay lattices play in this discrete theory. In particular we describe, for the first time, a geometric construction of the scalar curvature invariant at a vertex. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding and more familiar hinge-based expression in RC (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas. What is most striking to us is how naturally spacetime is represented by Voronoi and Delaunay structures and that the laws of gravity appear to be encoded locally on the lattice spacetime with less complexity than in the continuum, yet the continuum is recovered by convergence in mean. Perhaps these prominent features may enable us to transcend the details of any particular discrete model gravitation and yield clues to help us discover how we may begin to quantize this fundamental interaction."
http://arxiv.org/abs/0804.0632
Reconstructing AdS/CFT
Laurent Freidel
34 pages
(Submitted on 4 Apr 2008)
"In this note we clarify the dictionary between pure quantum gravity on the bulk in the presence of a cosmological constant and a CFT on the boundary. We show for instance that there is a general correspondence between quantum gravity 'radial states' and a pair of CFT's. Restricting to one CFT is argued to correspond to states possessing an asymptotic infinity. This point of view allows us to address the problem of reconstructing the bulk from the boundary. And in the second part of this paper we present an explicit formula which gives, from the partition function of any 2 dimensional conformal field theory, a wave functional solution to the 3-dimensional Wheeler-DeWitt equation. This establishes at the quantum level a precise dictionary between 2d CFT and pure gravity."
http://arxiv.org/abs/0804.0672
Quantum Cosmology
Claus Kiefer, Barbara Sandhoefer
29 pages, 9 figures, contribution to "Beyond the Big Bang", ed. by R. Vaas (Springer 2008)
(Submitted on 4 Apr 2008)
"We give an introduction into quantum cosmology with emphasis on its conceptual parts. After a general motivation we review the formalism of canonical quantum gravity on which discussions of quantum cosmology are usually based. We then present the minisuperspace Wheeler--DeWitt equation and elaborate on the problem of time, the imposition of boundary conditions, the semiclassical approximation, the origin of irreversibility, and singularity avoidance. Restriction is made to quantum geometrodynamics; loop quantum gravity and string theory are discussed in other contributions to this volume."
http://arxiv.org/abs/0804.0619
A model for delayed emission in a very-high energy gamma-ray flare in Markarian 501
W. Bednarek (1), R. M. Wagner (2) ((1) University of Lodz, Poland (2) MPI für Physik, München, Germany)
3 pages, no figures, submitted to Astronomy & Astrophysics
(Submitted on 4 Apr 2008)
"Recently the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a gamma-ray flare from the blazar Markarian 501 on 2005 July 9. We aim at describing the observed delayed high-energy emission. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. Such a modified SSC model predicts the appearance of the gamma-ray flare first at lower energies and subsequently at higher energies. Moreover, we argue that the time delay between the flare observed at different energies depends on the gamma-ray energy. Based on the reported time delay of approx. 240 s between the flare observed at 190 GeV and 2.7 TeV, we predict it should be on the order of 1 h if it was observed between 10 GeV and 100 GeV. Such delay time scales can be tested in the future by simultaneous observations of flares with GLAST and Cherenkov telescopes."
http://arxiv.org/abs/0804.1098
Hybrid Quantum Gowdy Cosmology: Combining Loop and Fock Quantizations
Mercedes Martin-Benito, Luis J. Garay, Guillermo A. Mena Marugan
4 pages
(Submitted on 7 Apr 2008)
"We quantize the linearly polarized Gowdy T^3 family of cosmologies by combining loop and Fock techniques. The loop quantization of the degrees of freedom that describe the subfamily of homogeneous cosmologies proves sufficient to solve the initial singularity. We obtain the general expression of the solutions to the quantum constraints. These solutions are determined by their value on an initial spatial section, where they arise from nothing. They can be provided with a Hilbert structure that reproduces the conventional Fock quantization of the inhomogeneities."
http://arxiv.org/abs/0804.1686
On the origin of the particles in black hole evaporation
Ralf Schützhold, William G. Unruh
4 pages
(Submitted on 10 Apr 2008)
"We present an analytic derivation of Hawking radiation for an arbitrary (spatial) dispersion relation \omega(k) as a model for ultra-high energy deviations from general covariance. It turns out that the Hawking temperature is proportional to the product of the group d\omega/dk and phase \omega/k velocities evaluated at the frequency \omega of the outgoing radiation far away, which suggests that Hawking radiation is basically a low-energy phenomenon. Nevertheless, a group velocity growing too fast at ultra-short distances would generate Hawking radiation at ultra-high energies ('ultra-violet catastrophe') and hence should not be a realistic model for the microscopic structure of quantum gravity."
http://arxiv.org/abs/0804.1771
The cosmic variance of Omega
T. P. Waterhouse, J. P. Zibin
10 pages, 2 figures
(Submitted on 10 Apr 2008)
"How much can we know about our Universe? All of our observations are restricted to a finite volume, and therefore our estimates of presumably global cosmological parameters are necessarily based on incomplete information. Even assuming that the Standard Model of cosmology is correct, this means that some cosmological questions may be unanswerable. For example, is the curvature parameter Omega_K positive, negative, or identically zero? If its magnitude is sufficiently small, then due to cosmic variance no causal observation can ever answer that question. In this article, we first describe the gauge problems associated with defining the cosmic variance of cosmological parameters, then describe a solution involving the use of parameters defined on the surface of last scattering, and finally calculate the statistical variance of ideal measurements of the matter, radiation, and curvature density parameters. We find that Omega_K cannot be measured to better than about 1.5x10^(-5) (1 sigma), and that this limit has already begun to decrease due to the flattening effect of dark energy. Proposed 21 cm hydrogen experiments, for example, make this limit more than just a theoretical curiosity."
The thread has a very interesting beginning where there was an attempt at a introductory level discussion of the concepts of loop quantum gravity. Would it be possible to start a new thread doing that? Where someone could start explaining the ideas of LQG at a level appropriate for, say, beginning graduate students.
That would be awesome.
Thanks nrqed, that's a good suggestion! There are people who post here who are graduate students in QG who, if not too busy, could do a great tutorial.
Maybe if I get inspired I'll start one and count on someone else taking over if it catches on.
http://arxiv.org/abs/0804.1797
Chern-Simons Modified Gravity as a Torsion Theory and its Interaction with Fermions
Stephon Alexander, Nicolas Yunes
11 pages, submitted to Phys. Rev. D
(Submitted on 10 Apr 2008)
"We study the tetrad formulation of Chern-Simons (CS) modified gravity, which adds a Pontryagin term to the Einstein-Hilbert action with a spacetime-dependent coupling field. We first verify that CS modified gravity leads to a theory with torsion, where this tensor is given by an antisymmetric product of the Riemann tensor and derivatives of the CS coupling. We then calculate the torsion in the far field of a weakly gravitating source within the parameterized post-Newtonian formalism, and specialize the result to Earth. We find that CS torsion vanishes only if the coupling vanishes, thus generically leading to a modification of gyroscopic precession, irrespective of the coupling choice. Perhaps most interestingly, we couple fermions to CS modified gravity via the standard Dirac action and find that these further correct the torsion tensor. Such a correction leads to two new results: (i) a generic enhancement of CS modified gravity by the Dirac equation and axial fermion currents; (ii) a new two-fermion interactions, mediated by an axial current and the CS correction. We conclude with a discussion of the consequences of these results in particle detectors and realistic astrophysical systems."
http://arxiv.org/abs/0804.2541
Phase space quantization and Loop Quantum Cosmology: A Wigner function for the Bohr-compactified real line
Christopher J. Fewster, Hanno Sahlmann
26 pages, 3 figures
(Submitted on 16 Apr 2008)
"We give a definition for the Wigner function for quantum mechanics on the Bohr compactification of the real line and prove a number of simple consequences of this definition. We then discuss how this formalism can be applied to loop quantum cosmology. As an example, we use the Wigner function to give a new quantization of an important building block of the Hamiltonian constraint."
http://arxiv.org/abs/0804.2475
The Void Phenomenon Explained
Jeremy L. Tinker (KICP, UChicago), Charlie Conroy (Princeton)
8 pages, 5 figures, submitted to ApJ
(Submitted on 16 Apr 2008)
"We use high-resolution N-body simulations, combined with a halo occupation model of galaxy bias, to investigate voids in the galaxy distribution. Our goal is to address the 'void phenomenon' of Peebles (2001), which presents the observed dearth of faint galaxies in voids as a challenge to the current cosmology. In our model, galaxy luminosity is determined only as a function of dark matter halo mass. With this simple assumption, we demonstrate that large, empty voids of ~15 Mpc/h in diameter are expected even for galaxies seven magnitudes fainter than L*. The predictions of our model are in excellent agreement with several statistical measures; (i) the luminosity function of galaxies in underdense regions, (ii) nearest neighbor statistics of dwarf galaxies, (iii) the void probability function of faint galaxies. In the transition between filaments and voids in the dark matter, the halo mass function changes abruptly, causing the maximum galaxy luminosity to decrease by ~5 magnitudes over a range of ~1 Mpc/h. Thus the boundary between filaments and voids in the galaxy distribution is nearly as sharp for dwarfs as for ~L* objects. These results support a picture in which galaxy formation is driven predominantly by the mass of the host dark matter halo, and is nearly independent of the larger-scale halo environment. Further, they demonstrate that LCDM, combined with a straightforward bias model, naturally explains the existence of the void phenomenon."
http://arxiv.org/abs/0804.2082
BTZ Black Hole Entropy: A spin foam model description
J.Manuel Garcia-Islas
9 pages, 2 figures
(Submitted on 13 Apr 2008)
"We present a microscopical explanation of the entropy of the BTZ black hole using discrete spin foam models of quantum gravity. The entropy of a black hole is given in geometrical terms which lead us to think that its statistical description must be given in terms of a quantum geometry. In this paper we present it in terms of spin foam geometrical observables at the horizon of the black hole."
http://arxiv.org/abs/0804.2720
Black Hole Thermodynamics and Lorentz Symmetry
Ted Jacobson, Aron C. Wall
4 pages, prepared for the 2008 Gravity Research Foundation Essay Competition
(Submitted on 17 Apr 2008)
"Recent developments point to a breakdown in the generalized second law of thermodynamics for theories with Lorentz symmetry violation. It appears possible to construct a perpetual motion machine of the second kind in such theories, using a black hole to catalyze the conversion of heat to work. Here we describe the arguments leading to that conclusion. We suggest the implication that Lorentz symmetry should be viewed as an emergent property of the macroscopic world, required by the second law of black hole thermodynamics."
http://arxiv.org/abs/0804.2778
Effects of the quantisation ambiguities on the Big Bounce dynamics
Orest Hrycyna, Jakub Mielczarek, Marek Szydlowski
26 pages, 10 figs
(Submitted on 17 Apr 2008)
"In this paper we investigate dynamics of the modified loop quantum cosmology models using dynamical systems methods. Modifications considered come from the choice of the different field strength operator \hat{F} and result in different forms of the effective Hamiltonian. Such an ambiguity of the choice of this expression from some class of functions is allowed in the framework of loop quantisation. Our main goal is to show how such modifications can influence the bouncing universe scenario in the loop quantum cosmology. In effective models considered we classify all evolutional paths for all admissible initial conditions. The dynamics is reduced to the form of a dynamical system of the Newtonian type on a 2-dimensional phase plane. These models are equivalent dynamically to the FRW models with the decaying effective cosmological term parametrised by the canonical variable p (or by the scale factor a). We find that for the positive cosmological constant there is a class of oscillating models without the initial and final singularities. The new phenomenon is the appearance of curvature singularities for the finite values of the scale factor, but we find that for the positive cosmological constant these singularities can be avoided. For the positive cosmological constant the evolution begins at the asymptotic state in the past represented by the deSitter contracting (deS-) spacetime or the static Einstein universe H=0 or H=- infinity state and reaches the deSitter expanding state (deS+) , the state H=0 or H=+infinity state. In the case of the negative cosmological constant we obtain the past and future asymptotic states as the Einstein static universes.
http://arxiv.org/abs/0804.2811
Spin Foam Perturbation Theory for Three-Dimensional Quantum Gravity
Joao Faria Martins, Aleksandar Mikovic
34 pages, 17 figures
(Submitted on 17 Apr 2008)
"We develop the spin foam perturbation theory for three-dimensional Euclidean Quantum Gravity. We analyse the perturbative expansion of the partition function in the dilute gas limit and argue that a conjecture due to Baez does not hold for arbitrary triangulations. However, the conjecture holds for a special class of triangulations which are based on subdivisions of certain 3-manifold cubulations. In this case we calculate the partition function."
http://arxiv.org/abs/0804.3157
Loop Quantization of Vacuum Bianchi I Cosmology
M. Martin-Benito, G. A. Mena Marugan, T. Pawlowski
10 pages
(Submitted on 19 Apr 2008)
"We analyze the loop quantization of the family of vacuum Bianchi I spacetimes, a gravitational system whose classical solutions describe homogeneous anisotropic cosmologies. We rigorously construct the operator that represents the Hamiltonian constraint, showing that the states of zero volume completely decouple from the rest of quantum states. This fact ensures that the classical cosmological singularity is resolved in the quantum theory. In addition, this allows us to adopt an equivalent quantum description in terms of a well defined densitized Hamiltonian constraint. This latter constraint can be regarded in a certain sense as a difference evolution equation in an internal time provided by one of the triad components, which is polymerically quantized. Generically, this evolution equation is a relation between the projection of the quantum states in three different sections of constant internal time. Nevertheless, around the initial singularity the equation involves only the two closest sections with the same orientation of the triad. This has a double effect: on the one hand, physical states are determined just by the data on one section, on the other hand, the evolution defined in this way never crosses the singularity, without the need of any special boundary condition. Finally, we provide these physical states with a Hilbert structure, completing the quantization."
http://arxiv.org/abs/0804.3365
Effective Constraints for Quantum Systems
Martin Bojowald, Barbara Sandhoefer, Aureliano Skirzewski, Artur Tsobanjan
40 pages
(Submitted on 21 Apr 2008)
"An effective formalism for quantum constrained systems is presented which allows manageable derivations of solutions and observables, including a treatment of physical reality conditions without requiring full knowledge of the physical inner product. Instead of a state equation from a constraint operator, an infinite system of constraint functions on the quantum phase space of expectation values and moments of states is used. The examples of linear constraints as well as the free non-relativistic particle in parameterized form illustrate how standard problems of constrained systems can be dealt with in this framework."
http://arxiv.org/abs/0804.3726
Quantum Geometry and Quantum Gravity
J. Fernando Barbero G.
To appear in the AIP Conference Proceedings of the XVI International Fall Workshop on Geometry and Physics, Lisbon - Portugal, 5-8 September 2007
(Submitted on 23 Apr 2008)
"The purpose of this contribution is to give an introduction to quantum geometry and loop quantum gravity for a wide audience of both physicists and mathematicians. From a physical point of view the emphasis will be on conceptual issues concerning the relationship of the formalism with other more traditional approaches inspired in the treatment of the fundamental interactions in the standard model. Mathematically I will pay special attention to functional analytic issues, the construction of the relevant Hilbert spaces and the definition and properties of geometric operators: areas and volumes."
http://arxiv.org/abs/0804.3765
Groups of generalized flux transformations in loop quantum gravity
J. M. Velhinho
6 pages
(Submitted on 23 Apr 2008)
"We present a group of transformations in the space of generalized connections that contains the set of transformations generated by the flux variables of loop quantum gravity. This group is labelled by certain SU(2)-valued functions on the bundle of directions in the spatial manifold. A further generalization is obtained by considering functions that depend on germs of analytic curves, rather than just on directions."
I can't evaluate this paper adequately (taking Brans-Dicke modified gravity seriously strikes me as highly innovative at this point). However two of the authors, Schuller and Wohlfarth, have good publication track records and have co-authored with well-knowns such as Paul Townsend and Mark Trodden. This is not the first paper of theirs developing this new approach. It's not safe to ignore it.
http://arxiv.org/abs/0804.4067
Brans-Dicke geometry
Raffaele Punzi, Frederic P. Schuller, Mattias N.R. Wohlfarth
8 pages
(Submitted on 25 Apr 2008)
"We reveal the non-metric geometry underlying omega-->0 Brans-Dicke theory by unifying the metric and scalar field into a single geometric structure. Taking this structure seriously as the geometry to which matter universally couples, we show that the theory is fully consistent with solar system tests. This is in striking constrast with the standard metric coupling, which grossly violates post-Newtonian experimental constraints."
Brief mention:
http://arxiv.org/abs/0804.4161
Discrete Quantum Gravity I
P. Kramer (University of Tuebingen) M. Lorente (University of Oviedo)
24 pages
(Submitted on 25 Apr 2008)
"...The crucial step for the Barrett-Crane model in Quantum Gravity is the description of the amplitude for the quantum 4-simplex that is used in the state sum partition function. We obtain the zonal spherical functions for the construction of the SO(4,R) invariant weight and associate them to the triangular faces of the 4-simplices."
http://arxiv.org/abs/0804.4162
Discrete Quantum Gravity II
P. Kramer (University of Tuebingen), M. Lorete (University of Oviedo)
19 pages
(Submitted on 25 Apr 2008)
"In part I of [1] we have developed the tensor and spin representation of SO(4) in order to apply it to the simplicial decomposition of the Barrett-Crane model. We attach to each face of a triangle the spherical function constructed from the Dolginov-Biedenharn function.
In part II we apply the same technique to the Lorentz invariant state sum model. We need three new ingredients: the classification of the edges and the corresponding subspaces that arises in the simplicial decomposition, the irreps of SL(2,C) and its isomorphism to the bivectors appearing in the 4-simplices, the need of a zonal spherical function from the intertwining condition of the tensor product for the simple representations attached to the faces of the simplicial decomposition."
Bill Unruh (remember the Unruh effect?) is a prominent figure in Quantum Gravity. A recent paper by Ashtekar et al was devoted to disposing of an objection to Loop Quantum Cosmology that Unruh had raised. Here is a new one from Unruh and Bojan Losic.
http://arxiv.org/abs/0804.4296
Cosmological perturbation theory near de Sitter spacetime
B. Losic, W.G. Unruh
Submitted to PRL
(Submitted on 27 Apr 2008)
"We present an argument that a nonlocal measure of second order metric and matter perturbations dominates that of linear fluctuations in its effect on the gravitational field in spacetimes close to de Sitter spacetime."
There seems to be growing interest in the quantum geometry/gravity of (near) deSitter space. The next paper is by lesser-known authors and already has been accepted for publication in the International Journal of Modern Physics D
http://arxiv.org/abs/0804.4326
Thermodynamics of noncommutative de Sitter spacetime
B. Vakili, N. Khosravi, H. R. Sepangi
11 pages, accepted for publication in IJMPD
(Submitted on 28 Apr 2008)
"We study the effects of noncommutativity of spacetime geometry on the thermodynamical properties of the de Sitter horizon. We show that noncommutativity results in modifications in temperature, entropy and vacuum energy and that these modifications are of order of the Planck scale, suggesting that the size of the noncommutative parameter should be close to that of the Planck. In an alternative way to deal with noncommutativity, we obtain a quantization rule for the entropy. Since noncommutativity in spacetime geometry modifies the Heisenberg algebra and introduces the general uncertainty principle, we also investigate the above problem in this framework."
http://arxiv.org/abs/0804.4784
Generating functions for black hole entropy in Loop Quantum Gravity
J. Fernando Barbero G., Eduardo J. S. Villaseñor
(Submitted on 30 Apr 2008)
"We introduce, in a systematic way, a set of generating functions that solve all the different combinatorial problems that crop up in the study of black hole entropy in Loop Quantum Gravity. Specifically we give generating functions for: The different sources of degeneracy related to the spectrum of the area operator, the solutions to the projection constraint, and the black hole degeneracy spectrum. Our methods are capable of handling the different countings proposed and discussed in the literature. The generating functions presented here provide the appropriate starting point to extend the results already obtained for microscopic black holes to the macroscopic regime --in particular those concerning the area law and the appearance of an effectively equidistant area spectrum."
http://arxiv.org/abs/0805.0136
Is loop quantization in cosmology unique?
Alejandro Corichi, Parampreet Singh
(Submitted on 1 May 2008)
"We re-examine the process of loop quantization for flat isotropic models in cosmology. In particular, we contrast different inequivalent 'loop quantizations' of these simple models through their respective successes and limitations and asses whether they can lead to any viable physical description. We propose three simple requirements which any such admissible quantum model should satisfy: i) independence from any auxiliary structure, such as a fiducial interval/cell introduced to define the phase space when integrating over non-compact manifolds; ii) existence of a well defined classical limit and iii) provide a sensible 'Planck scale' where quantum gravitational effects become manifest. We show that even when it may seem that one can have several possible loop quantizations, these physical requirements considerably narrow down the consistent choices. Apart for the so called improved dynamics of LQC, none of the other available inequivalent loop quantizations pass above tests, showing the limitations of lattice refinement models to approximate the homogeneous sector and loop modified quantum geometrodynamics. We conclude that amongst a large class of loop quantizations in isotropic cosmology, there is a unique consistent choice."
http://arxiv.org/abs/0805.0453
Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity
Song He, Yidun Wan
25 pages, 2 figures
(Submitted on 5 May 2008)
"We derive conservation laws from interactions of braid-like excitations of embedded framed spin networks in Quantum Gravity. We also demonstrate that the set of stable braid-like excitations form a noncommutative algebra under braid interaction, in which the set of actively-interacting braids is a subalgebra."
http://arxiv.org/abs/0805.0543
"So what will you do if string theory is wrong?"
Moataz H. Emam
Americal Journal of Physics, July 2008
(Submitted on 5 May 2008)
"I briefly discuss the accomplishments of string theory that would survive a complete falsification of the theory as a model of nature and argue the possibility that such a survival may necessarily mean that string theory would become its own discipline, independently of both physics and mathematics."
http://arxiv.org/abs/0805.1265
C, P, and T of Braid Excitations in Quantum Gravity
Song He, Yidun Wan
28 pages, 5 figures
(Submitted on 9 May 2008)
"We study the discrete transformations of four-valent braid excitations of framed spin networks embedded in a topological three-manifold. We show that four-valent braids allow seven and only seven discrete transformations. These transformations can be uniquely mapped to C, P, T, and their products. Each CPT multiplet of actively-interacting braids is found to be uniquely characterized by a non-negative integer. Finally, braid interactions turn out to be invariant under C, P, and T."
http://arxiv.org/abs/0805.1187
Black holes in loop quantum gravity: the complete space-time
Rodolfo Gambini, Jorge Pullin
4 pages, 2 figures
(Submitted on 8 May 2008)
"We consider the quantization of the complete extension of the Schwarzschild space-time using spherically symmetric loop quantum gravity. We find an exact solution corresponding to the semi-classical theory. The singularity is eliminated but the space-time still contains a horizon. Although the solution is known partially numerically and therefore a proper global analysis is not possible, a global structure akin to a singularity-free Reissner--Nordstrom space-time including a Cauchy horizon is suggested."
http://arxiv.org/abs/0805.1178
Loop and braneworlds cosmologies from a deformed Heisenberg algebra
Marco Valerio Battisti
9 pages, submitted to PRD
(Submitted on 8 May 2008)
"The implications of a deformed Heisenberg algebra on the Friedmann-Robertson-Walker cosmological models are investigated. In particular, we consider generalized commutation relations which leave undeformed the translation group and preserve the rotational invariance. The resulting algebra is related to the kappa-Poincaré one and no sign in the deformation term is selected at all. The analysis of the models is performed at classical level by studying the modifications induced on the symplectic geometry by the deformed algebra. We show that this framework leads to a modified Friedmann equation which coincide with that one found in loop quantum cosmology as well as in the Randall-Sundrum braneworlds scenario. In fact, the complementary sign of the loop and brane term, in the effective cosmological dynamics, naturally emerges from the free sign of the deformed algebra. This way, a common phenomenological description for both these theories is obtained and a relation with the low energy quantum gravity framework established."
http://arxiv.org/abs/0805.1192
How quantum is the big bang?
Martin Bojowald
4 pages
(Submitted on 8 May 2008)
"When quantum gravity is used to discuss the big bang singularity, the most important, though rarely addressed, question is what role genuine quantum degrees of freedom play. Here, complete effective equations are derived for isotropic models with an interacting scalar to all orders in the expansions involved. The resulting coupling terms show that quantum fluctuations do not affect the bounce much. Quantum correlations, however, do have an important role and could even eliminate the bounce. How quantum gravity regularizes the big bang depends crucially on properties of the quantum state."
http://arxiv.org/abs/0805.1219
GLAST and Lorentz violation
Raphael Lamon
11 pages, 7 figures and 2 tables
(Submitted on 8 May 2008)
"We study possible Lorentz violations by means of gamma-ray bursts (GRB) with special focus on the Large Array Telescope (LAT) of GLAST. We simulate bursts with gtobssim and introduce a Lorentz violating term in the arrival times of the photons. We further perturb these arrival times and energies with a Gaussian distribution corresponding to the time resp. energy resolution of GLAST. We then vary the photon flux in gtobssim in order to derive a relation between the photon number and the standard deviation of the Lorentz violating term. We conclude with the fact that our maximum likelihood method as first developed in [1] is able to make a statement whether Nature breaks the Lorentz symmetry if the number of bursts with known redshifts is of the order of 100."
http://arxiv.org/abs/0805.2124
Noncommutative gravity, a 'no strings attached' quantum-classical duality, and the cosmological constant puzzle
Tejinder P. Singh
7 pages. Second Prize in Gravity Research Foundation Essay Competition, 2008...To appear in Gen. Rel. Grav.
(Submitted on 14 May 2008)
"There ought to exist a reformulation of quantum mechanics which does not refer to an external classical spacetime manifold. Such a reformulation can be achieved using the language of noncommutative differential geometry. A consequence which follows is that the 'weakly quantum, strongly gravitational' dynamics of a relativistic particle whose mass is much greater than Planck mass is dual to the 'strongly quantum, weakly gravitational' dynamics of another particle whose mass is much less than Planck mass. The masses of the two particles are inversely related to each other, and the product of their masses is equal to the square of Planck mass. This duality explains the observed value of the cosmological constant, and also why this value is nonzero but extremely small in Planck units."
http://arxiv.org/abs/0805.2373
Exotic Statistics for Ordinary Particles in Quantum Gravity
John Swain
Awarded an honourable mention in the 2008 Gravity Research Foundation Essay Competition
(Submitted on 15 May 2008)
"Objects exhibiting statistics other than the familiar Bose and Fermi ones are natural in theories with topologically nontrivial objects including geons, strings, and black holes. It is argued here from several viewpoints that the statistics of ordinary particles with which we are already familiar are likely to be modified due to quantum gravity effects. In particular, such modifications are argued to be present in loop quantum gravity and in any theory which represents spacetime in a fundamentally piecewise-linear fashion. The appearance of unusual statistics may be a generic feature (such as the deformed position-momentum uncertainty relations and the appearance of a fundamental length scale) which are to be expected in any theory of quantum gravity, and which could be testable."
http://arxiv.org/abs/0805.2183
Gravity and its Mysteries: Some Thoughts and Speculations
A. Zee
18 pages, conference talk
(Submitted on 14 May 2008)
"I gave a rambling talk about gravity and its many mysteries at Chen-Ning Yang's 85th Birthday Celebration held in November 2007. I don't have any answers."
Some people will know Tony Zee from his textbook Quantum Field Theory in a Nutshell
or from his popular book Fearful Symmetry
http://www.kitp.ucsb.edu/~zee/
http://arxiv.org/abs/0805.2503
Fundamentals and recent developments in non-perturbative canonical Quantum Gravity
F. Cianfrani, O.M. Lecian, G. Montani
94 pages
(Submitted on 16 May 2008)
"The aim of this review is to provide a detailed account of the physical content emerging from this story of the canonical approach to Quantum Gravity. All the crucial steps in our presentation have a pedagogical character, providing the reader with the necessary tools to become involved in the field. Such a pedagogical aspect is then balanced and completed by subtle discussions on specific topics which we regard as relevant for the physical insight they outline on the treated questions. Our analysis is not aimed at convincing the reader about a pre-constituted point of view, bu instead our principal goal is to review the picture of Canonical Quantum Gravity on the basis of the concrete facts at the ground of its clear successes, but also of its striking shortcomings."
[Giovanni Montani is a senior guy at Rome. 87 papers on arxiv. This pedagogical review of LQG looks like it may prove useful.]
http://arxiv.org/abs/0805.2536
The entropic boundary law in BF theory
Etera R. Livine, Daniel R. Terno
17 pages
(Submitted on 16 May 2008)
"We compute the entropy of a closed bounded region of space for pure 3d Riemannian gravity formulated as a topological BF theory for the gauge group SU(2) and show its holographic behavior. More precisely, we consider a fixed graph embedded in space and study the flat connection spin network state without and with particle-like topological defects. We regularize and compute exactly the entanglement for a bipartite splitting of the graph and show it scales at leading order with the number of vertices on the boundary (or equivalently with the number of loops crossing the boundary). More generally these results apply to BF theory with any compact gauge group in any space-time dimension."
http://arxiv.org/abs/0805.2411
Measuring the Scalar Curvature with Clocks and Photons: Voronoi-Delaunay Lattices in Regge Calculus
Jonathan R. McDonald, Warner A. Miller
7 pages, 2 figures, submitted to Classical and Quantum Gravity
(Submitted on 15 May 2008)
"The Riemann scalar curvature plays a central role in Einstein's geometric theory of gravity. We describe a new geometric construction of this scalar curvature invariant at an event (vertex) in a discrete spacetime geometry. This allows one to constructively measure the scalar curvature using only clocks and photons. Given recent interest in discrete pre-geometric models of quantum gravity, we believe is it ever so important to reconstruct the curvature scalar with respect to a finite number of communicating observers. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the original simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding hinge-based expression in Regge calculus (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas."
http://arxiv.org/abs/0805.2584
Is Physics Asking for a New Kinematics?
R. Aldrovandi, J. G. Pereira
8 pages. Honorable Mention in the Gravity Research Foundation essay contest, 2008
(Submitted on 16 May 2008)
"It is discussed whether some of the consistency problems of present-day physics could be solved by replacing special relativity, whose underlying kinematics is ruled by the Poincare' group, by de Sitter relativity, with underlying kinematics ruled by the de Sitter group. In contrast to ordinary special relativity, which seems to fail at the Planck scale, this new relativity is "universal" in the sense that it holds at all energy scales."
http://arxiv.org/abs/0805.2555
Quantum black-hole information missing in the semiclassical treatment
H. Nikolic
7 pages
(Submitted on 16 May 2008)
"In the semiclassical treatment of gravity, an external observer can measure only the mean (not the exact) mass of the black hole (BH). By contrast, in fully quantum gravity the exact (not only mean) BH mass is measurable by the external observer. This additional information (missing in the semiclassical treatment) available to the external observer significantly helps to understand how information leaks out during the BH evaporation."
[Harvey Nikolic is a PF Beyond regular---Demystifier. Might like to discuss/explain the paper.]
http://arxiv.org/abs/0805.2429
Galactic Neutrino Communication
John G. Learned, Sandip Pakvasa, A. Zee
6 pages, 2 figures
(Submitted on 16 May 2008)
"We examine the possibility to employ neutrinos to communicate within the galaxy. We discuss various issues associated with transmission and reception, and suggest that the resonant neutrino energy near 6.3 PeV may be most appropriate. In one scheme we propose to make Z^o particles in an overtaking e^+ - e^- collider such that the resulting decay neutrinos are near the W^- resonance on electrons in the laboratory. Information is encoded via time structure of the beam. In another scheme we propose to use a 30 PeV pion accelerator to create neutrino or anti-neutrino beams. The latter encodes information via the particle/anti-particle content of the beam, as well as timing. Moreover, the latter beam requires far less power, and can be accomplished with presently foreseeable technology. Such signals from an advanced civilization, should they exist, will be eminently detectable in neutrino detectors now under construction."
http://arxiv.org/abs/0805.2668
Halos of Modified Gravity
Kirill Krasnov, Yuri Shtanov
Honorable Mention in the 2008 Essay Competition of the Gravity Research Foundation; 9 pages, 2 figures
(Submitted on 17 May 2008)
"We describe how a certain simple modification of general relativity, in which the local cosmological constant is allowed to depend on the space-time curvature, predicts the existence of halos of modified gravity surrounding spherically-symmetric objects. We show that the gravitational mass of an object weighed together with its halo can be much larger than its gravitational mass as seen from inside the halo. This effect could provide an alternative explanation of the dark-matter phenomenon in galaxies. In this case, the local cosmological constant in the solar system must be some six orders of magnitude larger than its cosmic value obtained in the supernovae type Ia experiments. This is well within the current experimental bounds, but may be directly observable in the future high-precision experiments."
Interesting collaboration. I think of Krasnov as one of the central people in nonstring QG who has co-authored with Ashtekar, Baez, Freidel, and Rovelli. On the other hand, I think of Shtanov's background as string---braneworld cosmology, at least until recently. Now they are both working on what is a classical (nonquantum) gravity modification. It's an intriguing modification that Krasnov has written already maybe 6 papers about. Another one of these longshot gambles that you don't necessarily hear about.
I can't evaluate or vouch for this paper. Randono was at UTex Austin and is now postdoc at Penn State in Ashtekar's group. The conclusion here is unexpected and unlike anything else I have seen. It opens a possible phenomenology for QG at intermediate scale---well before Planck scale. this sounds incredible. But he has been talking with Ashtekar and Golam Hossain about this and we know from past papers that he is a capable and original researcher. So I have to include the link.
http://arxiv.org/abs/0805.2955
A Mesoscopic Quantum Gravity Effect
Andrew Randono
10 pages, 2 figures
(Submitted on 19 May 2008)
Yesterday there was that mesoscopic QG effects Randono.
Today another Randono
http://arxiv.org/abs/0805.3169
A New Perspective on Covariant Canonical Gravity
Andrew Randono
25 pages
(Submitted on 20 May 2008)
"We present a new approach to the covariant canonical formulation of Einstein-Cartan gravity that preserves the full Lorentz group as the local gauge group. The method exploits lessons learned from gravity in 2+1 dimensions regarding the relation between gravity and a general gauge theory. The dynamical variables are simply the frame field and the spin-connection pulled-back to the hypersurface, thereby eliminating the need for simplicity constraints on the momenta. A consequence of this is a degenerate (pre)symplectic form, which appears to be a necessary feature of the Einstein-Cartan formulation. A new feature unique to this approach arises when the constraint algebra is computed: the algebra is a deformation of the de Sitter, anti-de Sitter, or Poincaré algebra (depending on the value of the cosmological constant) with the deformation parameter being the conformal Weyl tensor."
http://arxiv.org/abs/0805.3175
Conserved Topological Defects in Non-Embedded Graphs in Quantum Gravity
Fotini Markopoulou, Isabeau Prémont-Schwarz
42 pages, 34 figures
(Submitted on 20 May 2008)
"We follow up on previous work which found that commonly used graph evolution moves lead to conserved quantities that can be expressed in terms of the braiding of the graph in its embedding space. We study non-embedded graphs under three distinct sets of dynamical rules and find non-trivial conserved quantities that can be expressed in terms of topological defects in the dual geometry. For graphs dual to 2-dimensional simplicial complexes we identify all the conserved quantities of the evolution. We also indicate expected results for graphs dual to 3-dimensional simplicial complexes."
http://arxiv.org/abs/0805.3511
The covariant entropy bound and loop quantum cosmology
Abhay Ashtekar, Edward Wilson-Ewing
15 pages, 3 figures
(Submitted on 22 May 2008)
"We examine Bousso's covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances."
I believe this is an important paper supporting the existence of a positive cosmological constant (aka "dark energy") and tending to dispell alternative explanations of accelerated expansion
http://arxiv.org/abs/0805.3695
An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect
Benjamin R. Granett, Mark C. Neyrinck, István Szapudi (IfA, Hawaii)
5 pages, 1 figure
(Submitted on 25 May 2008)
"We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey Luminous Red Galaxy catalog. The mean temperature deviation is 9.6 +/- 2.2 microK. We interpret this as a detection of the late-time Integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy. The statistical significance of our detection is over 4 sigma, making it the clearest detection to date using a single galaxy dataset. Moreover, our method produces a compelling visual image of the effect."
http://arxiv.org/abs/0805.3750
Direct Detection of Gravity Waves from Neutron Stars
Redouane Al Fakir, William G. Unruh
19 pages, 2 figures
(Submitted on 24 May 2008)
"In light of the discovery of the first-ever double pulsar system, PSR J0737-3039, we re-examine an earlier proposal to directly detect gravity waves from neutron stars, which was predicated on a hypothetical system almost identical to the later discovered double pulsar. We re-derive the effect in more detail, and confirm the initial estimate--sometimes doubted in the literature--that it includes a 1/b dependence, where b is the impact parameter of a pulsar with respect to its foreground, gravity-wave emitting, neutron star companion. A coherent modulation in pulsar time-of-arrival measurements of 10 nano-sec/sec is possible. A one-year intermittent experiment on an instrument comparable to the SKA could thus detect the exceedingly faint gravity waves from individual neutron stars."
This is our PF Francesca. Great going! Homogeneity needs to be relaxed in LQC, and they have found a way to do it in a stepwise gradual fashion. Rovelli has delivered an online seminar talk about this at ILQGS.
http://arxiv.org/abs/0805.4585
Stepping out of Homogeneity in Loop Quantum Cosmology
Carlo Rovelli, Francesca Vidotto
16 pages
(Submitted on 29 May 2008)
"We explore the extension of quantum cosmology outside the homogeneous approximation, using the formalism of loop quantum gravity. We introduce a model where some of the inhomogeneous degrees of freedom are present, providing a tool for describing general fluctuations of quantum geometry near the initial singularity. We show that the dynamical structure of the model reduces to that of loop quantum cosmology in the Born-Oppenheimer approximation. This result corroborates the assumptions that ground loop cosmology, sheds light on the physical and mathematical relation between loop cosmology and full loop quantum gravity, and on the nature of the cosmological approximation. Finally, we show that the non-graph-changing Hamiltonian constraint considered in the context of algebraic quantum gravity provides a viable effective dynamics within this approximation."
Song He is familiar to us as the co-author of braid-matter research with Yidun Wan. While he was at Perimeter Institute working on braid-matter, Song He took over Sabine Hossenfelder's office (she was back in Germany). I am happy about this paper. It seems very clever. Extend Bousso covariant entropy bound, apply it to the standard cosmological model, take account of black holes, and bingo! you get a nice upper bound on the cosmological constant.
http://arxiv.org/abs/0805.4614
A covariant entropy bound conjecture on the dynamical horizon
Song He, Hongbao Zhang
JHEP style, 9 pages, 1 figure, honorable mention award received from Gravity Research Foundation for 2008 Essay Competition
(Submitted on 29 May 2008 (v1), last revised 29 May 2008 (this version, v2))
"As a compelling pattern for the holographic principle, our covariant entropy bound conjecture is proposed for more general dynamical horizons. Then we apply our conjecture to LambdaCDM cosmological models, where we find it imposes a novel upper bound 10^-90 on the cosmological constant for our own universe by taking into account the dominant entropy contribution from super-massive black holes, which thus provides an alternative macroscopic perspective to understand the longstanding cosmological constant problem. As an intriguing implication of this conjecture, we also discuss the possible profound relation between the present cosmological constant, the origin of mass, and the anthropic principle."
http://arxiv.org/abs/0805.4545
Not so non-renormalizable gravity
Dirk Kreimer
7 pages
(Submitted on 29 May 2008)
"We review recent progress with the understanding of quantum fields, including ideas how gravity might turn out to be a renormalizable theory after all."
http://arxiv.org/abs/0805.2909
Investigating the Ultraviolet Properties of Gravity with a Wilsonian Renormalization Group Equation
Alessandro Codello, Roberto Percacci, Christoph Rahmede
(Submitted on 19 May 2008)
"We review and extend in several directions recent results on the asymptotic safety approach to quantum gravity. The central issue in this approach is the search of a Fixed Point having suitable properties, and the tool that is used is a type of Wilsonian renormalization group equation. We begin by discussing various cutoff schemes, i.e. ways of implementing the Wilsonian cutoff procedure. We compare the beta functions of the gravitational couplings obtained with different schemes, studying first the contribution of matter fields and then the so-called Einstein-Hilbert truncation, where only the cosmological constant and Newton's constant are retained. In this context we make connection with old results, in particular we reproduce the results of the epsilon expansion and the perturbative one loop divergences. We then apply the Renormalization Group to higher derivative gravity. In the case of a general action quadratic in curvature we recover, within certain approximations, the known asymptotic freedom of the four-derivative terms, while Newton's constant and the cosmological constant have a nontrivial fixed point. In the case of actions that are polynomials in the scalar curvature of degree up to eight we find that the theory has a fixed point with three UV-attractive directions, so that the requirement of having a continuum limit constrains the couplings to lie in a three-dimensional subspace, whose equation is explicitly given. We emphasize throughout the difference between scheme-dependent and scheme-independent results, and provide several examples of the fact that only dimensionless couplings can have 'universal' behavior.
http://arxiv.org/abs/0805.4696
Regularization and finiteness of the Lorentzian LQG vertices
Jonathan Engle, Roberto Pereira
13 pages
(Submitted on 30 May 2008)
"We give an explicit form for the Lorentzian vertices recently introduced for possibly defining the dynamics of loop quantum gravity. As a result of so doing, a natural regularization of the vertices is found and suggested. The regularized vertices are then proven to be finite."
http://arxiv.org/abs/0806.0339
Quantum Gravity Phenomenology
Giovanni Amelino-Camelia
82 pages
(Submitted on 2 Jun 2008)
"I review the present status of the development of Quantum Gravity Phenomenology. Among the accomplishments of this young research area I stress in particular the significance of studies which established that some appropriate data analyses provide sensitivity to effects introduced genuinely at the Planck scale. The objective of testing/falsifying theories that provide comprehensive solutions to the quantum-gravity problem appears to be still rather far, but we might soon be in a position to investigate some 'falsifiable quantum-gravity theories of not everything' ".
http://arxiv.org/abs/0806.0397
The Self-Organized de Sitter Universe
J. Ambjorn, J. Jurkiewicz, R. Loll
7 pp, 1 figure, entry to 2008 GRF essay competition, honourable mention
(Submitted on 2 Jun 2008)
"We propose a theory of quantum gravity which formulates the quantum theory as a nonperturbative path integral, where each spacetime history appears with a weight given by the exponentiated Einstein-Hilbert action of the corresponding causal geometry. The path integral is diffeomorphism-invariant (only geometries appear) and background-independent. The theory can be investigated by computer simulations, which show that a de Sitter universe emerges on large scales. This emergence is of an entropic, self-organizing nature, with the weight of the Einstein-Hilbert action playing a minor role. Also the quantum fluctuations around this de Sitter universe can be studied quantitatively and remain small until one gets close to the Planck scale. The structures found to describe Planck-scale gravity are reminiscent of certain aspects of condensed-matter systems."
http://arxiv.org/abs/0806.0595
Unique factor ordering in the continuum limit of LQC
William Nelson, Mairi Sakellariadou (KCL London)
12 pages
(Submitted on 3 Jun 2008)
"We show that the factor ordering ambiguities associated with the loop quantisation of the gravitational part of the cosmological Hamiltonian constraint, disappear at the level of Wheeler-DeWitt equation only for a particular choice of lattice refinement model, which coincides with constraints imposed from phenomenological and consistency arguments."
http://arxiv.org/abs/0806.2593
Lemaitre-Tolman-Bondi collapse from the perspective of loop quantum gravity
Martin Bojowald, Tomohiro Harada, Rakesh Tibrewala
56 pages, 42 figures
(Submitted on 16 Jun 2008)
"Lemaitre-Tolman-Bondi models as specific spherically symmetric solutions of general relativity simplify in their reduced form some of the mathematical ingredients of black hole or cosmological applications. The conditions imposed in addition to spherical symmetry turn out to take a simple form at the kinematical level of loop quantum gravity, which allows a discussion of their implications at the quantum level. Moreover, the spherically symmetric setting of inhomogeneity illustrates several non-trivial properties of lattice refinements of discrete quantum gravity. Nevertheless, the situation at the dynamical level is quite non-trivial and thus provides insights to the anomaly problem. At an effective level, consistent versions of the dynamics are presented which implement the conditions together with the dynamical constraints of gravity in an anomaly-free manner. These are then used for analytical as well as numerical investigations of the fate of classical singularities, including non-spacelike ones, as they generically develop in these models. None of the corrections used here resolve those singularities by regular effective geometries. However, there are numerical indications that the collapse ends in a tamer shell-crossing singularity prior to the formation of central singularities for mass functions giving a regular conserved mass density. Moreover, we find quantum gravitational obstructions to the existence of exactly homogeneous solutions within this class of models. This indicates that homogeneous models must be seen in a wider context of inhomogeneous solutions and their reduction in order to provide reliable dynamical conclusions."
http://arxiv.org/abs/0806.2783
Effective Action for Loop Quantum Cosmology à la Palatini
Gonzalo J. Olmo, Parampreet Singh
4 pages, 2 figures
(Submitted on 17 Jun 2008)
"The resolution of the big bang singularity in loop quantum cosmology can be described by quantum gravitational modifications to the Friedman dynamics without introducing any new degrees of freedom. A challenging question is to construct a covariant effective action able to reproduce that new physics emergent at the Planck scale. By enlarging our scope to the realm of non-metric theories, we show that apparent tensions with conventional approaches can be overcome. We provide an algorithm to construct an effective action for loop quantum cosmology in the Palatini framework and obtain an action which involves an infinite series in the scalar curvature of the independent connection. This effective action successfully captures non-local quantum geometric effects and the non-singular bounce at the Planck scale and leads to general relativity at low curvatures."
http://arxiv.org/abs/0806.2821
Fermions in Loop Quantum Cosmology and the Role of Parity
Martin Bojowald, Rupam Das
17 pages
(Submitted on 17 Jun 2008)
"Fermions play a special role in homogeneous models of quantum cosmology because the exclusion principle prevents them from forming sizable matter contributions. They can thus describe the matter ingredients only truly microscopically and it is not possible to avoid strong quantum regimes by positing a large matter content. Moreover, possible parity violating effects are important especially in loop quantum cosmology whose basic object is a difference equation for the wave function of the universe defined on a discrete space of triads. The two orientations of a triad are interchanged by a parity transformation, which leaves the difference equation invariant for ordinary matter. Here, we revisit and extend loop quantum cosmology by introducing fermions and the gravitational torsion they imply, which renders the parity issue non-trivial. A treatable locally rotationally symmetric Bianchi model is introduced which clearly shows the role of parity. General wave functions cannot be parity-even or odd, and parity violating effects in matter influence the microscopic big bang transition which replaces the classical singularity in loop quantum cosmology."
francesca
Jun20-08, 10:29 AM
http://arxiv.org/abs/0806.3082
Anomalous CMB polarization and gravitational chirality
Carlo R. Contaldi, Joao Magueijo, Lee Smolin
5 pages (Submitted on 18 Jun 2008)
We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation.
Francesca, thanks for catching that one by Smolin, Magueijo, Contaldi
good luck on your presentation at QG2!
The next listing is by a relatively new author on the scene, whose previous LQG paper was published by CGG in 2007.
http://arxiv.org/abs/0806.3356
Spontaneous symmetry breaking in Loop Quantum Gravity
Gabor Helesfai
24 pages
(Submitted on 20 Jun 2008)
"In this paper we investigate the question how spontaneous symmetry breaking works in the framework of Loop Quantum Gravity and we compare it to the results obtained in the case of the Proca field. We obtained that the Hamiltonian of the two systems are very similar, the only difference is an extra scalar field in the case of spontaneous symmetry breaking. This field can be identified as the field that carries the mass of the vector field. In the quantum regime this becomes a well defined operator, which turns out to be a self adjoint operator with continuous spectrum. To calculate the spectrum we used a new representation in the case of the scalar fields, which in addition enabled us to rewrite the constraint equations to a finite system of linear partial differential equations. This made it possible to solve part of the constraints explicitly."
ccdantas
Jun23-08, 07:57 AM
http://arxiv.org/abs/0806.3082
Anomalous CMB polarization and gravitational chirality
Carlo R. Contaldi, Joao Magueijo, Lee Smolin
5 pages (Submitted on 18 Jun 2008)
We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation.
Hmm. I think "Uncle Al" will like this one.
George Jones
Jun23-08, 08:50 AM
Hmm. I think "Uncle Al" will like this one.
:rofl:
http://arxiv.org/abs/0806.3506
Shaken, but not stirred - Potts model coupled to quantum gravity
J. Ambjorn, K.N. Anagnostopoulos, R. Loll, I. Pushkina
19 pages, 9 figures
(Submitted on 21 Jun 2008)
"We investigate the critical behaviour of both matter and geometry of the three-state Potts model coupled to two-dimensional Lorentzian quantum gravity in the framework of causal dynamical triangulations. Contrary to what general arguments of the effects of disorder suggest, we find strong numerical evidence that the critical exponents of the matter are not changed under the influence of quantum fluctuations in the geometry, compared to their values on fixed, regular lattices. This lends further support to previous findings that quantum gravity models based on causal dynamical triangulations are in many ways better behaved than their Euclidean counterparts."
http://arxiv.org/abs/0806.3713
Cosmic Rays and the Search for a Lorentz Invariance Violation
Wolfgang Bietenholz
76 pages, 15 figures
(Submitted on 23 Jun 2008)
"This is an introductory review about the on-going search for a signal of Lorentz Invariance Violation (LIV) in cosmic rays. We first summarise basic aspects of cosmic rays, focusing on rays of ultra high energy (UHECRs). We discuss the Greisen-Zatsepin-Kuz'min (GZK) energy cutoff for cosmic protons, which is predicted due to photopion production in the Cosmic Microwave Background (CMB). This is a process of modest energy in the proton rest frame. It can be investigated to a high precision in the laboratory, if Lorentz transformations apply even at factors \gamma \sim O(10^{11}). For heavier nuclei the energy attenuation is even faster due to photo-disintegration, again if this process is Lorentz invariant. Hence the viability of Lorentz symmetry up to tremendous gamma-factors - far beyond accelerator tests - is a central issue. Next we comment on conceptual aspects of Lorentz Invariance and the possibility of its spontaneous breaking. This could lead to slightly particle dependent 'Maximal Attainable Velocities'. We discuss their effect in decays, Cerenkov radiation, the GZK cutoff and neutrino oscillation in cosmic rays. We also review the search for LIV in cosmic gamma-rays. For multi TeV gamma-rays we possibly encounter another puzzle related to the transparency of the CMB, similar to the GZK cutoff. The photons emitted in a Gamma Ray Burst occur at lower energies, but their very long path provides access to information not far from the Planck scale. No LIV has been observed so far. However, even extremely tiny LIV effects could change the predictions for cosmic ray physics drastically. An Appendix is devoted to the recent hypothesis by the Pierre Auger Collaboration, which identifies nearby Active Galactic Nuclei - or objects next to them - as probable UHECR sources."
http://arxiv.org/abs/0806.3929
Anomaly freedom in perturbative loop quantum gravity
Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, S. Shankaranarayanan
54 pages
(Submitted on 24 Jun 2008)
"A fully consistent linear perturbation theory for cosmology is derived in the presence of quantum corrections as they are suggested by properties of inverse volume operators in loop quantum gravity. The underlying constraints present a consistent deformation of the classical system, which shows that the discreteness in loop quantum gravity can be implemented in effective equations without spoiling space-time covariance. Nevertheless, non-trivial quantum corrections do arise in the constraint algebra. Since correction terms must appear in tightly controlled forms to avoid anomalies, detailed insights for the correct implementation of constraint operators can be gained. The procedures of this article thus provide a clear link between fundamental quantum gravity and phenomenology."
http://arxiv.org/abs/0806.3776
The quasiclassical realms of this quantum universe
James B. Hartle (University of California, Santa Barbara)
24 pages
(Submitted on 23 Jun 2008)
"The most striking observable feature of our indeterministic quantum universe is the wide range of time, place, and scale on which the deterministic laws of classical physics hold to an excellent approximation. This essay describes how this domain of classical predictability of every day experience emerges from a quantum theory of the universe's state and dynamics."
http://arxiv.org/abs/0806.4239
On the emergence of time and gravity
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
10 pages
(Submitted on 26 Jun 2008)
"In recent years, a growing momentum has been gained by the emergent gravity framework. Within the latter, the very concepts of geometry and gravitational interaction are not seen as elementary aspects of Nature but rather as collective phenomena associated to the dynamics of more fundamental objects. In this paper we want to further explore this possibility by proposing a model of emergent time and scalar gravity. Assuming that the dynamics of the fundamental objects can give rise in first place to a Riemannian manifold and a set of scalar fields we show how time (in the sense of hyperbolic equations) can emerge as a property of perturbations dynamics around some specific class of solutions of the field equations. Moreover, we show that these perturbations can give rise to a full theory of gravity via a suitable redefinition of the fields that identifies the relevant degrees of freedom. In particular, we find that our model gives rise to Nordström gravity. Since this theory is invariant under general coordinate transformations, this also shows how diffeomorphism invariance can emerge from much simpler systems."
since tests of QG may come out gammaray astronomy, we should occasionally check up on that field. here is a recent survey perspective on it
http://arxiv.org/abs/0806.4363
From MAGIC to CTA: the INAF participation to Cherenkov Telescopes experiments for Very High Energy Astrophysics
L. Angelo Antonelli (INAF MAGIC Collaboration)
5 pages, 2 figures, to appear in Mem. SAIt, Proceedings of the "LII congresso della Societa' Astronomica Italiana", Teramo, May 4-8 2008
(Submitted on 26 Jun 2008)
"The next decade can be considered the 'golden age' of the Gamma Ray Astronomy with the two satellites for Gamma Ray Astronomy (AGILE and GLAST) in orbit. Therefore, thanks to many other X-ray experiments already in orbit (e.g. Swift, Chandra, NewtonXMM, etc.) it will be possible to image the Universe for the first time all over the electromagnetic spectrum almost contemporarily. The new generations of ground-based very high gamma-ray instruments are ready to extend the observed band also to the very high frequencies. Scientists from the Italian National Institute for Astrophysics (INAF) are involved in many, both space- and ground- based gamma ray experiments, and recently such an involvement has been largely improved in the field of the Imaging Atmospheric Cherenkov Telescopes (IACT). INAF is now member of the MAGIC collaboration and is participating to the realization of the second MAGIC telescope. MAGIC, as well other IACT experiments, is not operated as an observatory so a proper guest observer program does not exist. A consortium of European scientists (including INAF scientists) is thus now thinking to the design of a new research infrastructure: the Cherenkov Telescope Array (CTA). CTA is conceived to provide 10 times the sensitivity of current instruments, combined with increased flexibility and increased coverage from some 10 GeV to some 100 TeV. CTA will be operated as an observatory to serve a wider community of astronomer and astroparticle physicists."
http://arxiv.org/abs/0806.4382
Superconducting loop quantum gravity and the cosmological constant
Stephon H.S. Alexander, Gianluca Calcagni
5 pages
(Submitted on 26 Jun 2008)
"We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen--Cooper--Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary."
http://arxiv.org/abs/0806.4640
Path integral representation of spin foam models of 4d gravity
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
29 pages, 6 figures
(Submitted on 28 Jun 2008)
"We give a unified description of all recent spin foam models introduced by Engle, Livine, Pereira and Rovelli (ELPR) and by Freidel and Krasnov (FK). We show that the FK models are, for all values of the Immirzi parameter, equivalent to path integrals of a discrete theory and we provide an explicit formula for the associated actions. We discuss the relation between the FK and ELPR models and also study the corresponding boundary states. For general Immirzi parameter, these are given by Alexandrov's and Livine's SO(4) projected states. For 0 <= gamma < 1, the states can be restricted to SU(2) spin networks."
http://arxiv.org/abs/0806.4710
The length operator in Loop Quantum Gravity
Eugenio Bianchi
33 pages
(Submitted on 28 Jun 2008)
"The dual picture of quantum geometry provided by a spin network state is discussed. From this perspective, we introduce a new operator in Loop Quantum Gravity - the length operator. We describe its quantum geometrical meaning and derive some of its properties. In particular we show that the operator has a discrete spectrum and is diagonalized by appropriate superpositions of spin network states. A series of eigenstates and eigenvalues is presented and an explicit check of its semiclassical properties is discussed."
http://arxiv.org/abs/0807.0160
Loop Quantum Cosmology corrections to inflationary models
Michal Artymowski, Zygmunt Lalak, Lukasz Szulc
16 pages, 1 figure
(Submitted on 1 Jul 2008)
"In the recent years the quantization methods of Loop Quantum Gravity have been successfully applied to the homogeneous and isotropic Friedmann-Robertson-Walker space-times. The resulting theory, called Loop Quantum Cosmology (LQC), resolves the Big Bang singularity by replacing it with the Big Bounce. We argue that LQC generates also certain corrections to field theoretical inflationary scenarios. These corrections imply that in the LQC the effective sonic horizon becomes infinite at some point after the bounce and that the scale of the inflationary potential implied by the COBE normalisation increases. The evolution of scalar fields immediately after the Bounce becomes modified in an interesting way. We point out that one can use COBE normalisation to establish an upper bound on the quantum of length of LQG."
Zygmunt Lalak is a new author in LQC. He has some 71 publications going back to 1986, in various other research areas including inflation scenarios, braneworld models, string-related topics. But this seems to be his first venture into Loop Quantum Cosmology. Lukasz Szulc has co-authored several papers with Jerzy Lewandowski (a longtime LQG and LQC expert and collaborator of Ashtekar's)
The conclusion that Cosmic Microwave Background data might be used to bound the smallest eigenvalue of the LQG length operator surprised me initially, but the bound one finds in the paper is not very tight. Still it is a beginning and may suggest ways to get improved results (assuming both LQC and some particular inflation parameters).
http://arxiv.org/abs/0807.0225
Quantum gravity as a Fermi liquid
Stephon H.S. Alexander, Gianluca Calcagni
43 pages, 1 figure
(Submitted on 1 Jul 2008)
"We present a reformulation of loop quantum gravity with a cosmological constant and no matter as a Fermi-liquid theory. When the topological sector is deformed and large gauge symmetry is broken, we show that the Chern-Simons state reduces to Jacobson's degenerate sector describing 1+1 dimensional propagating fermions with nonlocal interactions. The Hamiltonian admits a dual description which we realize in the simple BCS model of superconductivity. On one hand, Cooper pairs are interpreted as wormhole correlations at the de Sitter horizon; their number yields the de Sitter entropy. On the other hand, BCS is mapped into a deformed conformal field theory reproducing the structure of quantum spin networks. When area measurements are performed, Cooper-pair insertions are activated on those edges of the spin network intersecting the given area, thus providing a description of quantum measurements in terms of excitations of a Fermi sea to superconducting levels. The cosmological constant problem is naturally addressed as a nonperturbative mass-gap effect of the true Fermi-liquid vacuum."
http://arxiv.org/abs/0807.0712
Gravitational waves from the Big Bounce
Jakub Mielczarek
19 pages, 9 figures
(Submitted on 4 Jul 2008)
"In this paper we investigate gravitational waves production during the Big Bounce phase inspired by the Loop Quantum Cosmology. We consider the influence of the holonomy corrections to the equation for tensor modes. We show that they act like additional effective graviton mass, suppressing gravitational waves creation. However, this effects can be treated perturbatively. We investigate the simplified model without these corrections and find its exact analytical solution. For this model we calculate a spectrum of the gravitational waves from the Big Bounce phase. The obtained spectrum decreases to zero for the low energy modes. Based on this observation we indicate that this effect can lead to the low CMB multipoles suppression and gives a potential way to test Loop Quantum Cosmology models. We also consider a scenario with a post-bounce inflationary phase. The obtained power spectrum gives qualitative explanation of the CMB spectra, including low multipoles suppression. This result is a challenge to construct a consistent bounce+inflation model in the Loop Quantum Cosmology."
Jakub Mielczarek has co-authored seven papers with Marek Szydlowski
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SZYDLOWSKI&FORMAT=www&SEQUENCE=citecount%28d%29
this is J.M.'s first solo paper. Someone it might be good to watch.
http://arxiv.org/abs/0807.0665
Phenomenological loop quantum geometry of the Schwarzschild black hole
Dah-Wei Chiou
26 pages, 6 figures, 1 table
(Submitted on 4 Jul 2008)
"The interior of a Schwarzschild black hole is investigated at the level of phenomenological dynamics with the discreteness corrections of loop quantum geometry implemented in two different improved quantization schemes. In one scheme, the classical black hole singularity is resolved by the quantum bounce, which bridges the black hole interior with a white hole interior..."
Briefly noted as part of an ongoing effort to understand the black hole interior in terms of LQG. D-W Chiou is a postdoc in Ashtekar's group at Penn State who has already several papers on the LQG black hole. I don't claim ability to judge but I think at this point the results are interesting but not conclusive.
http://arxiv.org/abs/0807.1481
Is gravitational entropy quantized?
Dawood Kothawala, T. Padmanabhan, Sudipta Sarkar
4 pages
(Submitted on 9 Jul 2008)
"In Einstein's gravity, the entropy of horizons is proportional to their area. Several arguments given in the literature suggest that, in this context, both area and entropy should be quantized with an equally spaced spectrum for large quantum numbers. But in more general theories (like, for e.g, in the black hole solutions of Gauss-Bonnet or Lanczos-Lovelock gravity) the horizon entropy is not proportional to area and the question arises as to which of the two (if at all) will