# Loop-and-allied QG bibliography

Spin_Network
Great paper marcus, thanks again, and a great excuse to hit a significant number! :) shuttle re-entry speed !

Gold Member
Dearly Missed
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.

Last edited:
Gold Member
Dearly Missed
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."

Last edited:
Gold Member
Dearly Missed
Reuter confirms Loll---or "makes contact" anyway

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.

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":

Last edited:
Staff Emeritus
Gold Member
Dearly Missed
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.

Kea
marcus said:
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

Kea
marcus said:
...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.

Gold Member
Dearly Missed
Last edited:
Gold Member
Dearly Missed
new Bojowald, new Kiefer, new Dowker

http://arxiv.org/gr-qc/0508118 [Broken]
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 [Broken]
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 [Broken]
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."

Last edited by a moderator:
Gold Member
Dearly Missed
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":

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

Last edited:
Gold Member
Dearly Missed
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."

Gold Member
Dearly Missed
new Loll paper, CDT for wider physics audience

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

a paper like this has been needed.

the authors discuss the connection with Martin Reuter work ("quantum einstein gravity", assymptotic safety)

Gold Member
Dearly Missed
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..."

===================================

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

Last edited:
Gold Member
Dearly Missed
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 [Broken] ) and estimating Immirzi parameter.

Last edited by a moderator:
Gold Member
Dearly Missed
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 language 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.

Last edited:
Gold Member
Dearly Missed
Ashtekar and Bojowald Black Hole paper (long awaited)

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."

Gold Member
Dearly Missed
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---

Last edited:
Gold Member
Dearly Missed
just a little paper on DSR (doubly special relativity) by a new researcher at the perimeter institute, Jonathan Hackett. haven't heard of him before. undergraduate physics major University of Waterloo in 2003, maybe a graduate student there now

http://arxiv.org/gr-qc/0509103 [Broken]
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 don't 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 [Broken]
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)

http://www.phys.psu.edu/events/index.html?event_id=1257&event_type_ids=0&span=2005-08-20.2005-12-25 [Broken]

Last edited by a moderator:
Gold Member
Dearly Missed
new papers from Perez and from Livine/Oriti

http://arxiv.org/gr-qc/0509118 [Broken]
On the regularization ambiguities in loop quantum gravity
Alejandro Perez
21 pages

http://arxiv.org/hep-th/0509192 [Broken]
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."

Last edited by a moderator:
Gold Member
Dearly Missed
Thomas Thiemann---Master Constraint paper

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."

Last edited:
Staff Emeritus
Gold Member
Dearly Missed
Huzzah! How you like THEM apples, Lubos!

Gold Member
Dearly Missed

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

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.

Last edited:
Gold Member
Dearly Missed
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..."

===============

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

Last edited:
Staff Emeritus
Gold Member
Dearly Missed
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 language 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

Gold Member
Dearly Missed
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 beginning 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 [Broken]

In the acknowledgments 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
SN recalled this from Baez on SPR

Last edited by a moderator:
Gold Member
Dearly Missed
Parampreet Singh seminar talk today at Penn State

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 [Broken]

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

Last edited by a moderator:
Gold Member
Dearly Missed
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

Last edited:
Gold Member
Dearly Missed
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."

Last edited:
Gold Member
Dearly Missed
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

Staff Emeritus
Gold Member
Dearly Missed
Marcus said:

And I see you started one on the Jones polynomial too.

Gold Member
Dearly Missed
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 [Broken]

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 doesn't 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

Last edited by a moderator:
Spin_Network
See you got the recent papers marcus! great stuff.

Gold Member
Dearly Missed
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

Last edited:
Staff Emeritus
Gold Member
Dearly Missed
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?

Gold Member
Dearly Missed
...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 [Broken]
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."

Last edited by a moderator: