Loop-and-allied QG bibliography

[selfAdjoint]

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.

 

[Mike2]

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]

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.

 

[marcus]

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

 

[Mike2]

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.

 

[marcus]

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?

 

[marcus]

flagging the Causal Sites paper

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 don't 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]

 

[Mike2]

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 don't 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?

 

[marcus]

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

 

[marcus]

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

 

[marcus]

a new Quantum Gravity paper

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).

 

[Mike2]

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?

 

[marcus]

... 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---

 

[marcus]

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 doesn't 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.

 

[marcus]

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)

 

[marcus]

Baez talk at PI Friday 29 October

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]

Has anyone had problems with the Perimiter Seminar/Lectures links?..I have tried every single link, and not a single lecture can be viewed?

 

[marcus]

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 don't know about video.
I can't recall having seen any Perimeter talks that have even the audio available! Maybe I just haven't 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 haven't gotten around to it yet.

 

[marcus]

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.

 

[marcus]

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---

 

[marcus]

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

 

[maddy]

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?

 

[marcus]

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 don't 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

 

[marcus]

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 don't 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."

 

[marcus]

Recent Highlights

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 don't 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

 

[marcus]

We should also keep track of some parallel developments, as noted in this thread
https://www.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
https://www.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
https://www.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 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]

Simple Monopole theory of the Cosmos

Check out two US PTO documents at www.epimedia.com/gravitypush/simple1.htm[/URL]

 

[marcus]

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."

 

[marcus]

automatic bibliography from Arxiv search

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...m+AND+OR+triply+doubly+special/0/1/0/1994/0/1
Year 1995:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1995/0/1
Year 1996:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1996/0/1
Year 1997:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1997/0/1
Year 1998:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1998/0/1
Year 1999:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/1999/0/1
Year 2000:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2000/0/1
Year 2001:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2001/0/1
Year 2002:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/0/1/0/2002/0/1
Year 2003:
http://arXiv.org/find/nucl-ex,astro...m+AND+OR+triply+doubly+special/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...m+AND+OR+triply+doubly+special/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

 

[marcus]

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."