Loop-and-allied QG bibliography

marcus · Oct 25, 2004

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 · Oct 26, 2004

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 · Oct 26, 2004

Mike2 said:

... 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 · Oct 26, 2004

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 · Oct 28, 2004

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 · Oct 29, 2004

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 · Oct 29, 2004

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 · Oct 29, 2004

Wave's_Hand_Particle said:

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 · Oct 31, 2004

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 · Nov 1, 2004

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 · Nov 8, 2004

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 · Nov 17, 2004

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 · Nov 17, 2004

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.

maddy said:

...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 · Nov 17, 2004

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 · Nov 21, 2004

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 · Nov 21, 2004

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

selfAdjoint said:

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 · Nov 28, 2004

Simple Monopole theory of the Cosmos

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

marcus · Nov 28, 2004

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 · Nov 28, 2004

Code:

---------

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 · Nov 29, 2004

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."

marcus · Dec 2, 2004

a new paper by Girelli and Livine, physics of DSR

http://arxiv.org/gr-qc/0412004

Physics of Deformed Special Relativity: Relativity Principle revisited
Florian Girelli, Etera R. Livine
24 pages

"In many different ways, Deformed Special Relativity (DSR) has been argued to provide an effective limit of quantum gravity in almost-flat regime. Some experiments will soon be able to test some low energy effects of quantum gravity, and DSR is a very promising candidate to describe these latter. Unfortunately DSR is up to now plagued by many conceptual problems (in particular how it describes macroscopic objects) which forbids a definitive physical interpretation and clear predictions. Here we propose a consistent framework to interpret DSR. We extend the principle of relativity: the same way that Special Relativity showed us that the definition of a reference frame requires to specify its speed, we show that DSR implies that we must also take into account its mass. We further advocate a 5-dimensional point of view on DSR physics and the extension of the kinematical symmetry from the Poincare group to the Poincare-de Sitter group (ISO(4,1)). This leads us to introduce the concept of a pentamomentum and to take into account the renormalization of the DSR deformation parameter kappa. This allows the resolution of the "soccer ball problem" (definition of many-particle-states) and provides a physical interpretation of the non-commutativity and non-associativity of the addition the relativistic quadrimomentum. In particular, the coproduct of the kappa-Poincare algebra is interpreted as defining the law of change of reference frames and not the law of scattering. This point of view places DSR as a theory, half-way between Special Relativity and General Relativity, effectively implementing the Schwarzschild mass bound in a flat relativistic context."

marcus · Dec 24, 2004

John Baez has alphabetized photos of many of the LQG people
at his website
http://math.ucr.edu/home/baez/marseille/

the last update of this thread was back on page 17, posts 245, 246, and 247.
I should probably update the main list of sources again.

BTW I see Baez is working on a paper with Urs Schreiber
(Urs has been doing category hocuspocus lately at the "coffee table")
wonders never cease in case you hadnt noticed

Baez Marseille pictures include a lot more than just the people, but
the snapshots of the people are labeled reliably with who they are
so finally, in case you were wondering what Alejandro Corichi looks like...

marcus · Dec 24, 2004

If you have Windows (with Media Player 10) then you have access to the videos----or else the slides/audio----of several dozen talks given at the October 2004 conference at Perimeter.

I have not seen them but another PF poster has checked several of them out. Here is the URL of a 4 page catalog of video talks:
http://streamer.perimeterinstitute....aspx?cid=f8fb0405-e71a-4d04-a219-5080e1a8d535

Here is the main page for the conference:

http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/

here's a one page list of the talks and who gave them:
http://www.perimeterinstitute.ca/activities/scientific/PI-WORK-2/participants.php

There were over 30 titles, several looked interesting to me and I wish it were convenient to watch the videos. Just to mention a few (not a complete list):

Ashtekar
Physical Ramifications of Quantum Geometry

Conrady
Vacuum State for LQG

Dittrich
Status of the Master Constraint Programme

Reyes
Higgs propagation in loop quantum geometry

Pullin
Semi-discrete solution to the dynamics of LQG

Sahlmann
String Theory with LQG methods

Smolin
Physics from Loop Quantum Gravity

marcus · Jan 16, 2005

Hermann Nicolai takes a look at LQG

Two interesting QG papers appeared on arxiv today

http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
50 pages, 11 figures
Report-no: AEI-2004-129

"We present a pedagogical review of loop quantum gravity, with the aim of enabling a precise but critical assessment of its achievements so far. Special attention is paid to the appearance of a large number of ambiguities in the theory, in particular in the formulation of the Hamiltonian constraint. We emphasise that the off-shell ('strong') closure of the constraint algebra is a crucial test of the consistency of the theory, and should be used as the main tool to select one (if any) of the proposed Hamiltonians. Developing suitable approximation methods to establish a connection with classical gravity on the one hand, and with the physics of elementary particles on the other, remains a major challenge."

Hermann Nicolai directs the relevant part of the Albert Einstein Institute (MPI Potsdam) where Thomas Thiemann and Martin Bojowald and several other Loop gravitists are (like Bianca Dittrich recently). Nicolai has done much of his research in String, but (unlike many US string theorists) he does not favor his own specialty exclusively and he supports research in Loop as well. Nicolai can provide a valuable outside understanding of LQG and what problems need to be worked on. I believe that Nicolai's insight into what he sees are the important features and unresolved questions about LQG should be very helpful to read. This is one i expect I shall print out.

This next one is in an interesting form. A three-expert Dialog, or since there are three, a "Trialog". Ted Jacobson's specialty is TESTING Quantum Gravity theories by astronomical observation---finding vulnerable points where observation may refute certain theories, or versions. Don Marolf has done reserarch in string theory, but is also familiar with LQG. Carlo Rovelli is one of the founders of LQG and contributed an early paper on Black Hole entropy. So this conversation should show contrasts between different expert viewpoints

http://arxiv.org/abs/hep-th/0501103
Black hole entropy: inside or out?
Ted Jacobson, Donald Marolf, Carlo Rovelli
42 pages, contribution to proceedings of Peyresq 9

"A trialogue. Ted, Don, and Carlo consider the nature of black hole entropy. Ted and Carlo support the idea that this entropy measures in some sense "the number of black hole microstates that can communicate with the outside world.'' Don is critical of this approach, and discussion ensues, focusing on the question of whether the first law of black hole thermodynamics can be understood from a statistical mechanics point of view."

I should also include a paper posted by Lee Smolin last week, this argues an important point that (rather in contrast to string theorizing) Smolin's LQG does make predictions that are clear and firm enough to test (with upcoming experiments) and could refute the theory. This is a "coming of age" thing. A theory is "grown up" when it is clear enough to be proven false. So this is potentially a contentious issue. Some people may not wish to acknowledge that LQG is mature enough as a theory to actually make predictions and be tested---to bet its life on future experimental outcomes.

http://arxiv.org/hep-th/0501091
Falsifiable predictions from semiclassical quantum gravity
Lee Smolin
9 pages

"Predictions are derived for the upcoming AUGER and GLAST experiments from a semiclassical approximation to quantum gravity. It is argued that to first order in the Planck length the effect of quantum gravity is to make the low energy effective spacetime metric energy dependent. The diffeomorphism invariance of the semiclassical theory forbids the appearance of a preferred frame of reference, consequently the local symmetry of this energy-dependent effective metric is a non-linear realization of the Lorentz transformations, which renders the Planck energy observer independent. This gives a form of deformed or doubly special relativity (DSR), previously explored with Magueijo, called the rainbow metric. The argument is general, and applies in all dimensions with and without supersymmetry, and is, at least to leading order, universal for all matter couplings. The argument is illustrated in detail in a specific example in loop quantum gravity.
A consequence of DSR realized with an energy dependent effective metric is a helicity independent energy dependence in the speed of light to first order in the Planck length. However, thresholds for Tev photons and GZK protons are unchanged from special relativistic predictions. These predictions of quantum gravity are falsifiable by the upcoming AUGER and GLAST experiments."

selfAdjoint · Jan 16, 2005

In my opinion the Nicolai, Peeters, and Zamaklar paper, hep-th/0501114, is a wonderful discussion. I especially appreciated their explication of the constraint algebra closure issue which blew up in public around Thiemann's string quantization paper, but which, as NPZ show, was all the time simmering under the LQG surface. While all the other big names in LQG discretely stayed away from this uncomfortable issue, Thiemann had the guts to attack it head on (and is still attacking it with his new replacement for group averaging, a feature too new to get into LPZ). RTWT.

marcus · Jan 16, 2005

footnotes on Nicolai's paper

In this paper there are some references to recent work by Thiemann and Dittrich
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar

On page 22, references [79] and [80] are to the papers by Thiemann and Dittrich that came out in November 2004. (about 6 lines from the bottom of the page)

On page 41 reference [110] is to Thiemann's Loop-String paper (three lines from the bottom, where it refers to an "intense debate")

On page 34, where there is a discussion of group averaging and the way the diffeomorphism constraint is implemented, you will find a reference to the paper by Fairbairn and Rovelli
see reference [102] about 9 lines from the bottom of the page.
This is the Separable Hilbert Space in Loop Quantum Gravity which we discussed some at PF last year.

I read this paper as flagging major problems that (Nicolai thinks) would be worthwhile for his postdocs and visiting researchers at the Institute to tackle. See especially the conclusions paragraph on page 45.
As is usually the case with overviews and pedagogicial introductions (as this paper is in part) the paper only briefly touches on recent (2004) work
and gives a picture that is more "as-of-some-point-in-the-past". But even the brief mention of several of 2004 papers is helpful, or so I found, because it shows the authors' perspective on them.

[added in edit: I just saw your post. What does RTWT stand for? Ah! I bet TWT is Time Will Tell. still don't read the R]

selfAdjoint · Jan 17, 2005

RTWT is blogger for Read the whole thing. And yes I did notice the discussion of nonseparable and separable Hilbert spaces. They are a little sly here, offering nonspeparability as a potential show stopper and then showing that it ha been handled. Or maybe this is just the result of having most of the paper written when the Fairbairn and Rovelli paper came out.

I am ashamed to say that after I promised to forge ahead on Thiemann's series of papers on his new technique, I sluffed off. With this NPZ insight on the constraint algebra problem I mean to tackle the applications of the technique this week.

marcus · Jan 17, 2005

no need to rush! what you have picked is a prime and hard objective.
Other than Thiemann, only Gambini (consistent discretization QG) seems to have an alternative way past the hamiltonian constraint difficulty----with him time is discretized and there is an evolution operator that advances things step by step, so there is no hamiltonian constraint
I am looking forward to any insights you have about Thiemann's approach.

On a different topic, here are two new papers Jorge Alfaro just posted on arxiv, which I want to keep tabs on.

http://arxiv.org/abs/hep-th/0501116
Loop Quantum Gravity Effects on the High Energy Cosmic Ray Spectrum
Jorge Alfaro, Gonzalo A. Palma
17 pages. Talk at The XVIIIth International Workshop High Energy Physics and Quantum Field Theory Saint-Petersburg, June 17-23, 2004

"Recent observations on ultra high energy cosmic rays (those cosmic rays with energies greater than about 4 x 10^{18} eV) suggest an abundant flux of incoming particles with energies above 1 x 10^{20} eV. These observations violate the Greisen-Zatsepin-Kuzmin cutoff. To explain this anomaly we argue that quantum-gravitational effects may be playing a decisive role in the propagation of ultra high energy cosmic rays. We consider the loop quantum gravity approach and provide useful techniques to establish and analyze constraints on the loop quantum gravity parameters arising from observational data. In particular, we study the effects on the predicted spectrum for ultra high energy cosmic rays and conclude that is possible to reconcile observations."

http://arxiv.org/abs/hep-th/0501129
LIV Dimensional Regularization and Quantum Gravity effects in the Standard Model
Jorge Alfaro
2 pages
"Recently, we have remarked that the main effect of Quantum Gravity(QG) will be to modify the measure of integration of loop integrals in a renormalizable Quantum Field Theory. In the Standard Model this approach leads to definite predictions, depending on only one arbitrary parameter. In particular, we found that the maximal attainable velocity for particles is not the speed of light, but depends on the specific couplings of the particles within the Standard Model. Also birrefringence occurs for charged leptons, but not for gauge bosons. Our predictions could be tested in the next generation of neutrino detectors such as NUBE. In this paper, we elaborate more on this proposal. In particular, we extend the dimensional regularization prescription to include Lorentz invariance violations(LIV) of the measure, preserving gauge invariance. Then we comment on the consistency of our proposal."

these are about testing for QG effects. I just want to keep track because it seems to be an expanding area of research. Alfaro is at a university in Chile and his co-author is at Cambridge.

marcus · Jan 24, 2005

Two new papers today of possible interest

http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd
31 pages; 4 figures (gif); submitted to Science

"A theory of quantum gravity based on quantum computation is proposed. In this theory, fundamental processes are described in terms of quantum information processing: the geometry of space-time is a construct, derived from the underlying quantum computation. Explicit mechanisms are provided for the back-reaction of the metric to computational `matter,' black-hole evaporation, holography, and quantum cosmology."

Seth Lloyd is at MIT. the main body of the paper is 13 pages, then come references, appendices, and figures. some of the figures are in GIF format with the postscript version, so in PDF you only get placeholders.

Seth Lloyd has published 83 papers, a lot seem to be Physical Review A
and Physical Review Letters
but also two in Science, one in 2004 and another in 1996.
(preprint here---- http://arxiv.org/abs/quant-ph/9604015)
His specialty seems to be Quantum Computing. It is interesting that he thinks quantum theory of spacetime can be derived from theory of quantum computation

THE OTHER PAPER is one that I already flagged with its own thread, and one that John Baez highlighted in his report from the October 2004 Perimeter conference

http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev

"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."

Kea · Jan 24, 2005

marcus said:

http://arxiv.org/abs/quant-ph/0501135
The Computational Universe: Quantum gravity from quantum computation
Seth Lloyd

I had a quick look at this. It doesn't look very enlightening. Where is SetAI when we need him? The idea of building QG with qubits is now well entrenched in certain circles.

Cheers
Kea

marcus · Jan 25, 2005

this thread has become a surrogate sticky for Loop (and allied) Quantum Gravity links, so we regularly post useful QG reference links here: new articles, websites and the like, and then every now and then I update the main list. It is getting to be time to do that again.

Today there were a couple of new papers by Etera Livine, solo and with Florian Girelli
http://arxiv.org/gr-qc/0501075
Reconstructing Quantum Geometry from Quantum Information: Spin Networks as Harmonic Oscillators
Florian Girelli, Etera R. Livine
16 pages, 3 figures

"Loop Quantum Gravity defines the quantum states of space geometry as spin networks and describes their evolution in time. We reformulate spin networks in terms of harmonic oscillators and show how the holographic degrees of freedom of the theory are described as matrix models. This allow us to make a link with non-commutative geometry and to look at the issue of the semi-classical limit of LQG from a new angle. This work is thought as part of a bigger project of describing quantum geometry in quantum information terms."
*

http://arxiv.org/gr-qc/0501076
Some Remarks on the Semi-Classical Limit of Quantum Gravity
Etera R. Livine
5 pages, Proceedings of the Second International Workshop DICE2004 (Castello di Piombino, Tuscany) "From Decoherence and Emergent Classicality to Emergent Quantum Mechanics"

"One of the most important issues in quantum gravity is to identify its semi-classical regime. First the issue is to define for we mean by a semi-classical theory of quantum gravity, then we would like to use it to extract physical predictions. Writing an effective theory on a flat background is a way to address this problem and I explain how the non-commutative spacetime of deformed special relativity is the natural arena for such considerations. On the other hand, I discuss how the definition of the semi-classical regime can be formulated in a background independent fashion in terms of quantum information and renormalisation of geometry."

marcus · Jan 27, 2005

this is one I don't want to lose track of because it may have some pedagogical usefulness
http://arxiv.org/abs/gr-qc/0501082
DISCRETE GRAVITY AS A LOCAL THEORY OF THE POINCARÉ GROUP IN THE FIRST ORDER FORMALISM

It deals in simple terms with the relation of Regge calculus to continuum General Relativity. By a Jesuit named Gabriele Gionti, who belongs to the Vatican Obsersatory Research Group at Seward in Arizona. He has a longstanding interest in quantum gravity and did his thesis in Dynamical Triangulations (the Ambjorn Loll approach but before they made the move to Lorentian (Causal) DT and were still getting pathologies.

G.Gionti, Discrete Approaches Toward the Definition of a Quantum Theory of Gravity, Ph.D. thesis SISSA (1998).
http://arxiv.org/gr-qc/9812080

marcus · Jan 28, 2005

this is one that Wolram spotted and flagged with its own thread
https://www.physicsforums.com/showthread.php?p=444123#post444123

http://lanl.arxiv.org/abs/gr-qc/0501053

The Search for Quantum Gravity Signals
G. Amelino-Camelia, C. Lämmerzahl, A. Macias, H. Müller
47 pages, submitted to AIP Conference Proceedings of the 2nd Mexican Meeting on Mathematical and Experimental Physics

"We give an overview of ongoing searches for effects motivated by the study of the quantum-gravity problem. We describe in greater detail approaches which have not been covered in recent "Quantum Gravity Phenomenology'' reviews. In particular, we outline a new framework for describing Lorentz invariance violation in the Maxwell sector. We also discuss the general strategy on the experimental side as well as on the theoretical side for a search for quantum gravity effects. The role of test theories, kinematical and dymamical, in this general context is emphasized. The present status of controlled laboratory experiments is described, and we also summarize some key results obtained on the basis of astrophysical observations.

marcus · Jan 29, 2005

Lee Smolin replied to Nicolai et al survey of LQG from "outsider" standpoint
Here is the Nicolai paper:
http://arxiv.org/abs/hep-th/0501114
Loop quantum gravity: an outside view
Hermann Nicolai, Kasper Peeters, Marija Zamaklar
50 pages, 11 figures
Report-no: AEI-2004-129

Here is Smolin's response (an email to Nicolai which he copied to Peter Woit)
http://www.math.columbia.edu/~woit/blog/archives/000145.html

marcus · Feb 14, 2005

An advance in the spinfoam area may have been signaled by this paper
(I am trying to assemble links to background and developments leading up to it. First here is the paper I'm talking about:)

http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev

"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."

Some helpful background can be found in these two papers from 1998 and 1999

http://arxiv.org/hep-th/9807092
Spin Foam Models and the Classical Action Principle
Freidel and Krasnov
65 pages, many figures (published version)

"We propose a new systematic approach that allows one to derive the spin foam (state sum) model of a theory starting from the corresponding classical action functional. It can be applied to any theory whose action can be written as that of the BF theory plus a functional of the B field. Examples of such theories include BF theories with or without cosmological term, Yang-Mills theories and gravity in various spacetime dimensions. Our main idea is two-fold. First, we propose to take into account in the path integral certain distributional configurations of the B field in which it is concentrated along lower dimensional hypersurfaces in spacetime. Second, using the notion of generating functional we develop perturbation expansion techniques, with the role of the free theory played by the BF theory. We test our approach on various theories for which the corresponding spin foam (state sum) models are known. We find that it exactly reproduces the known models for BF and 2D Yang-Mills theories. For the BF theory with cosmological term in 3 and 4 dimensions we calculate the terms of the transition amplitude that are of the first order in the cosmological constant, and find an agreement with the corresponding first order terms of the known state sum models. We discuss implications of our results for existing quantum gravity models."

http://arxiv.org/gr-qc/9905087
An Introduction to Spin Foam Models of Quantum Gravity and BF Theory
John Baez
55 pages, 31 figures

"In loop quantum gravity we now have a clear picture of the quantum geometry of space, thanks in part to the theory of spin networks. The concept of 'spin foam' is intended to serve as a similar picture for the quantum geometry of spacetime. In general, a spin network is a graph with edges labelled by representations and vertices labelled by intertwining operators. Similarly, a spin foam is a 2-dimensional complex with faces labelled by representations and edges labelled by intertwining operators. In a 'spin foam model' we describe states as linear combinations of spin networks and compute transition amplitudes as sums over spin foams. This paper aims to provide a self-contained introduction to spin foam models of quantum gravity and a simpler field theory called BF theory."

Yesterday Freidel posted another paper. It is number III in a series called "Ponzano-Regge Revisited", the first two of which were co-authored with David Louapre (who sometimes visits PF)

http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages

"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"

Here is a quote from a related passage from Freidel/Livine introduction near top of page 4

---quote from Freidel and Livine---
Then, at G=0, the spin foam amplitudes are to be interpreted as providing the Feynman graph evaluation of particles coupled to quantum gravity. We study the perturbative G expansion of the spin foam amplitudes. Remarkably, this expansion can be re-summed and expressed as the Feynman graphs of a non-commutative braided quantum field theory with deformation parameter G, which thus describes the effective theory for matter in quantum gravity.

Any deformed Poincaré theory usually suffers from a huge ambiguity [5] coming from what should be identify as the physical energy and momenta since the introduction of the Planck scale allows non-linear redefinitions. This ambiguity can also be understood as an ambiguity in the identification of the non-commutative space-time. Our work shows that the Ponzano-Regge model naturally defines a star product and a duality between space and momenta, therefore no ambiguity remains once we identify quantum gravity as being responsible for the effective deformation of the Poincaré symmetry.

This realizes explicitly, for the first time from first principles, the now popular idea that quantum gravity will eventually lead to an effective non-commutative field theory incorporating the principle of doubly special relativity [6].
---end quote---

marcus · Feb 15, 2005

It looks to me like there are four main contenders any of whom could show up at the QG finish line.

1. spin foam program (connections with perturbation analysis, topological field theory, feynman diagrams, doubly special rel, includes matter)

2. master constraint (Thiemann's program)

3. consistent discretization (Gambini group)

4. dynamical triangulation (Ambjorn-Loll approach)

this does not include considerable work going on in Loop cosmology.
In quantum cosmology symmetry is used to reduce the complexity of the LQG model and impressive results have been achieved.

the links in this thread, that provide a kind of bibliography for Loop-and-allied QG, should IMO be organized under these four headings. will try to do this, in hope it makes the thread more useful

marcus · Feb 15, 2005

this is a short list of papers in each of the 4 approaches, it is very incomplete---just what I happened to have handy and could bring to it at the moment (remember also this leaves out active lines of research in loop cosmology and QG testing or phenomenology: this is just four approaches
currently making progress to getting a full theory of quantum gravity)

1. spin foam program (connections with perturbation analysis, topological field theory, feynman diagrams, doubly special rel, includes matter)

http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory

http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables

http://arxiv.org/hep-th/9807092
Spin Foam Models and the Classical Action Principle

http://arxiv.org/gr-qc/9905087
An Introduction to Spin Foam Models of Quantum Gravity and BF Theory

2. master constraint (Thiemann's program)

https://www.physicsforums.com/showthread.php?t=54711

http://arxiv.org/abs/gr-qc/0411138
Testing the Master Constraint Programme for Loop Quantum Gravity I. General Framework

http://arxiv.org/abs/gr-qc/0411139
Testing the Master Constraint Programme for Loop Quantum Gravity II. Finite Dimensional Systems

http://arxiv.org/abs/gr-qc/0411140
Testing the Master Constraint Programme for Loop Quantum Gravity III. SL(2,R) Models

http://arxiv.org/abs/gr-qc/0411141
Testing the Master Constraint Programme for Loop Quantum Gravity IV. Free Field Theories

http://arxiv.org/abs/gr-qc/0411142
Testing the Master Constraint Programme for Loop Quantum Gravity V. Interacting Field Theories

Partial and Complete Observables for Hamiltonian Constrained Systems
http://arxiv.org/abs/gr-qc/0411013

Reduced Phase Space Quantization and Dirac Observables
http://arxiv.org/abs/gr-qc/0411031

3. consistent discretization (Gambini group)

http://arxiv.org/abs/gr-qc/0409057
Consistent discretization and loop quantum geometry

4. dynamical triangulation (Ambjorn-Loll approach)

http://arxiv.org/abs/hep-th/0404156
Emergence of a 4D World from Causal Quantum Gravity]

http://arxiv.org/abs/hep-th/0411152
Semiclassical Universe from First Principles

marcus · Feb 15, 2005

Frank Wilczek who just got nobel for work in QCD is interested in QG.
I'm a fan, like a lot of other people I guess. he and Sean Robinson just posted this today:

http://arxiv.org/abs/gr-qc/0502074
A Relationship Between Hawking Radiation and Gravitational Anomalies
5 pages, 1 figure

---exerpt from introduction---

Hawking radiation from black holes is one of the most striking effects that is known, or at least widely agreed, to arise from the combination of quantum mechanics and general relativity. Hawking radiation originates upon quantization of matter in a background spacetime that contains an event horizon—for example, a black hole.

One finds that the occupation number spectrum of quantum field modes in the vacuum state is that ofa blackbody at a fixed temperature given by the surface gravity of the horizon.

The literature contains several derivations of Hawking radiation, each with strengths and weaknesses. Hawking’s original derivation[1, 2] is very direct and physical, but it relies on hypothetical properties of modes that undergo extreme blueshifts, and specifically assumes that their interactions with matter can be ignored.

Derivations based on Euclidean quantum gravity are quick and elegant, but the formalism lacks a secure microscopic foundation[3].

Derivations based on string theory have a logically consistent foundation, but they only apply to special solutions in unrealistic world models, and they do not explain the simplicity and generality of the results inferred from the other methods[4, 5].

In all these approaches, the Hawking radiation appears as a rather special and isolated phenomenon. Here we discuss another approach, which ties its existence to the cancellation of gravitational anomalies...
---end quote---

sounds to me like Wilczek thinks he might be able to do better than the approaches to Quantum Gravity he's familiar with, including stringy ones.
he's a creative thinker. I like the sense of optimism I get.

marcus · Feb 21, 2005

Two Loop Quantum Cosmology papers today.

http://arxiv.org/abs/gr-qc/0502082
On the Hamiltonian Constraint of Loop Quantum Cosmology
Kevin Vandersloot
28 pages, 2 figures

"In this paper we construct the Hamiltonian constraint operator of loop quantum cosmology using holonomies defined for arbitrary irreducible SU(2) representations labeled by spin J. We show that modifications to the effective semi-classical equations of motion arise both in the gravitational part of the constraint as well as matter terms. The modifications are important for phenomenological investigations of the cosmological imprints of loop quantum cosmology. We discuss the implications for the early universe evolution."

http://arxiv.org/abs/gr-qc/0502086
Effective State Metamorphosis in Semi-Classical Loop Quantum Cosmology
Parampreet Singh
5 pages, 3 figures

"Modification to the behavior of geometrical density at short scales is a key result of loop quantum cosmology, responsible for an interesting phenomenology in the very early universe. We demonstrate the way a perfect fluid with arbitrary equation of state incorporates this change in its effective dynamics in the loop modified phase. We show that irrespective of the choice of matter component, stress-energy conservation law generically implies that classical equation of state metamorphoses itself to an effective negative equation of state below a critical scale determined by the theory."

Both Kevin Vandersloot and P. Singh are postdocs at Ashtekar's Penn State QG center. BTW Singh gave a set of 3 talks on LQC phenomenology there in Fall 2004 which are downloadable. He has co-authored three papers with Bojowald and an interesting one with Maartens and Tsujikawa about inflation being automatic in LQC, among others. Kevin Vandersloot has co-authored three papers with Bojowald, and one with Perez and Noui as well.
In each case it is the guy's first solo paper.

marcus · Feb 22, 2005

Mattingly: Tests of Lorentz invariance

just out

http://arxiv.org/abs/gr-qc/0502097
Modern tests of Lorentz invariance
David Mattingly
DRAFT copy of a review submitted to Living Reviews in Relativity.

my comment:
clear exposition, written for non-specialists and grad students.
important area. LQG/Foam models appear to need some modification of Lorentz invariance although this still remains undecided. Mattingly (with Ted Jacobson) is a leader among those carrying out or initiating relevant observational tests. Living Reviews articles tend to be definitive at least for several years at a time. This introductory survey of current QG testing may turn out like that.

marcus · Feb 28, 2005

a new Loop Cosmology paper.

http://arxiv.org/abs/astro-ph/0502589
An emergent universe from a loop
David J. Mulryne, Reza Tavakol, James E. Lidsey, George F. R. Ellis
11 pages, 8 figures

"Closed, singularity-free, inflationary cosmological models have recently been studied in the context of general relativity. Despite their appeal, these so called emergent models suffer from a number of limitations. These include the fact that they rely on an initial Einstein static state to describe the past eternal phase of the universe. Given the instability of such a state within the context of general relativity, this amounts to a very severe fine tuning. Also in order to be able to study the dynamics of the universe within the context of general relativity, they set the initial conditions for the universe in the classical phase. Here we study the existence and stability of such models in the context of Loop Quantum Cosmology and show that both these limitations can be partially remedied, once semi-classical effects are taken into account. An important consequence of these effects is to give rise to a static solution (not present in GR), which dynamically is a centre equilibrium point and located in the more natural semi-classical regime. This allows the construction of emergent models in which the universe oscillates indefinitely about such an initial static state. We construct an explicit emergent model of this type, in which a non-singular past eternal oscillating universe enters a phase where the symmetry of the oscillations is broken, leading to an emergent inflationary epoch, while satisfying all observational and semi-classical constraints. We also discuss emergent models in which the universe possesses both early- and late-time accelerating phases."

marcus · Mar 7, 2005

new Loop Cosmology paper by Bojowald

http://arxiv.org/abs/gr-qc/0503020
The Early Universe in Loop Quantum Cosmology
Martin Bojowald
10 pages, 3 figures, plenary talk at VI Mexican School on Gravitation and Mathematical Physics, Nov 21-27, 2004

"Loop quantum cosmology applies techniques derived for a background independent quantization of general relativity to cosmological situations and draws conclusions for the very early universe. Direct implications for the singularity problem as well as phenomenology in the context of inflation or bouncing universes result, which will be reviewed here. The discussion focuses on recent new results for structure formation and generalizations of the methods."

marcus · Mar 9, 2005

Black Hole paper by Bojowald et al

just out
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures

"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms -- quantum gravity may exclude very small astrophysical black holes."

marcus · Mar 15, 2005

just out
http://arxiv.org/abs/gr-qc/0503065
On Energy Conditions and Stability in Effective Loop Quantum Cosmology
Golam Mortuza Hossain
28 pages

"In isotropic loop quantum cosmology, non-perturbatively modified dynamics of a minimally coupled scalar field violates weak, strong and dominant energy conditions when they are stated in terms of equation of state parameter. The violation of strong energy condition helps to have non-singular evolution by evading singularity theorems thus leading to a generic inflationary phase. However, the violation of weak and dominant energy conditions raises concern, as in general relativity these conditions ensure causality of the system and stability of vacuum via Hawking-Ellis conservation theorem. It is shown here that the non-perturbatively modified kinetic term contributes negative pressure but positive energy density. This crucial feature leads to violation of energy conditions but ensures positivity of energy density, as scalar matter Hamiltonian remains bounded from below. It is also shown that the modified dynamics restricts group velocity for inhomogeneous modes to remain sub-luminal thus ensuring causal propagation across spatial distances."

http://arxiv.org/abs/gr-qc/0503062
Fermionic sectors for the Kodama state
Stephon Alexander, Kristin Schleich, Donald M. Witt
4 pages
SLAC-PUB-10841

"Diffeomorphisms not connected to the identity can act nontrivially on the quantum state space for gravity. However, in stark contrast to the case of nonabelian Yang-Mills field theories, for which the quantum state space is always in 1 dimensional representation of the large gauge transformations, the quantum state space for gravity can have higher dimensional representations. In particular, the Kodama state will have 2 dimensional representations, that is sectors with spin 1/2, for many topologies that admit positive scalar curvature. The existence of these spin 1/2 states are used to point out a possible answer to certain criticisms raised recently in the literature."

Stephon is at Stanford/SLAC and has co-authored with Lee Smolin where they were talking about the Kodama state, after Witten (in 2003) published "certain criticisms" tending to discourage interest in it. the other two authors are a University of British Columbia. It is noticeable that Kodama state keeps coming up.

marcus · Mar 17, 2005

http://arxiv.org/abs/gr-qc/0503078

New Quantum Gravity Phenomenology
Alejandro Corichi, Daniel Sudarsky
8 pages

"The idea that quantum gravity manifestations would be associated with a violation of Lorentz invariance is very strongly bounded and faces serious theoretical challenges. This leads us to consider an alternative scheme for such phenomenological search. We discuss the underlying viewpoint and briefly mention its possible connections with current theoretical ideas. We also outline the new experimental avenues that would be open along these lines."

http://arxiv.org/abs/hep-th/0503140
A quantization of topological M theory
Lee Smolin
20 pages

"A conjecture is made as to how to quantize topological M theory. We study a Hamiltonian decomposition of Hitchin's 7-dimensional action and propose a formulation for it in terms of 13 first class constraints. The theory has 2 degrees of freedom per point, and hence is diffeomorphism invariant, but not strictly speaking topological. The result is argued to be equivalent to Hitchin's formulation. The theory is quantized using loop quantum gravity methods. An orthonormal basis for the diffeomorphism invariant states is given by diffeomorphism classes of networks of two dimensional surfaces in the six dimensional manifold. The hamiltonian constraint is polynomial and can be regulated by methods similar to those used in LQG.
To connect topological M theory to full M theory, a reduction from 11 dimensional supergravity to Hitchin's 7 dimensional theory is proposed. One important conclusion is that the complex and symplectic structures represent non-commuting degrees of freedom. This may have implications for attempts to construct phenomenologies on Calabi-Yau compactifications."

marcus · Mar 20, 2005

Today on SPR Thomas Larsson noted this 2004 paper, which is one I missed. I cannot say how essential it is: the problem (which is clearly important) may have been addressed independently by others.
But I want to keep it accessible if only on Larsson's recommendation.
http://www.arxiv.org/abs/gr-qc/0412059
General Relativity Histories Theory
Ntina Savvidou

"The canonical description is based on the prior choice of a
spacelike foliation, hence making a reference to a spacetime metric.
However, the metric is expected to be a dynamical, fluctuating quantity
in quantum gravity. After presenting the developments in the History
Projection Operator histories theory in the last seven years--giving
special emphasis on the novel temporal structure of the formalism--we
show how this problem can be solved in the histories formulation of
general relativity. We implement the 3+1 decomposition using
metric-dependent foliations which remain spacelike with respect to all
possible Lorentzian metrics. This allows us to find an explicit relation
of covariant and canonical quantities which preserves the spacetime
character of the canonical description. In this new construction we have
a coexistence of the spacetime diffeomorphisms group Diff(M) and the
Dirac algebra of constraints."

marcus · Mar 21, 2005

I want to collect a few recent papers in one post
1.
http://arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Martin Bojowald, Rituparno Goswami, Roy Maartens, Parampreet Singh
4 pages, 3 figures

"Quantum gravity is expected to remove the classical singularity that arises as the end-state of gravitational collapse. To investigate this, we work with a simple toy model of a collapsing homogeneous scalar field. We show that non-perturbative semi-classical effects of Loop Quantum Gravity cause a bounce and remove the classical black hole singularity. Furthermore, we find a critical threshold scale, below which no horizon forms..."

2.
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev

"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."

3.
http://arxiv.org/hep-th/0502106
Ponzano-Regge model revisited III: Feynman diagrams and Effective field theory
Laurent Freidel, Etera R. Livine
46 pages

"We study the no gravity limit G_{N}-> 0 of the Ponzano-Regge amplitudes with massive particles and show that we recover in this limit Feynman graph amplitudes (with Hadamard propagator) expressed as an abelian spin foam model. We show how the G_{N} expansion of the Ponzano-Regge amplitudes can be resummed. This leads to the conclusion that the dynamics of quantum particles coupled to quantum 3d gravity can be expressed in terms of an effective new non commutative field theory which respects the principles of doubly special relativity. We discuss the construction of Lorentzian spin foam models including Feynman propagators"

from Freidel/Livine introduction near top of page 4:

"Then, at G=0, the spin foam amplitudes are to be interpreted as providing the Feynman graph evaluation of particles coupled to quantum gravity. We study the perturbative G expansion of the spin foam amplitudes. Remarkably, this expansion can be re-summed and expressed as the Feynman graphs of a non-commutative braided quantum field theory with deformation parameter G, which thus describes the effective theory for matter in quantum gravity.

Any deformed Poincaré theory usually suffers from a huge ambiguity [5] coming from what should be identify as the physical energy and momenta since the introduction of the Planck scale allows non-linear redefinitions. This ambiguity can also be understood as an ambiguity in the identification of the non-commutative space-time. Our work shows that the Ponzano-Regge model naturally defines a star product and a duality between space and momenta, therefore no ambiguity remains once we identify quantum gravity as being responsible for the effective deformation of the Poincaré symmetry.

This realizes explicitly, for the first time from first principles, the now popular idea that quantum gravity will eventually lead to an effective non-commutative field theory incorporating the principle of doubly special relativity [6]..."

4.
http://arxiv.org/abs/hep-th/0503140
A quantization of topological M theory
Lee Smolin
20 pages

"A conjecture is made as to how to quantize topological M theory. We study a Hamiltonian decomposition of Hitchin's 7-dimensional action and propose a formulation for it in terms of 13 first class constraints. The theory has 2 degrees of freedom per point, and hence is diffeomorphism invariant, but not strictly speaking topological. The result is argued to be equivalent to Hitchin's formulation. The theory is quantized using loop quantum gravity methods. An orthonormal basis for the diffeomorphism invariant states is given by diffeomorphism classes of networks of two dimensional surfaces in the six dimensional manifold. The hamiltonian constraint is polynomial and can be regulated by methods similar to those used in LQG.
To connect topological M theory to full M theory, a reduction from 11 dimensional supergravity to Hitchin's 7 dimensional theory is proposed. One important conclusion is that the complex and symplectic structures represent non-commuting degrees of freedom. This may have implications for attempts to construct phenomenologies on Calabi-Yau compactifications."

=============
these are some salient papers from the first three months of 2005.
maybe we can figure out the main directions that LQG research is taking
this year. (when I say LQG I mean the term inclusively, including allied approaches like spin foam and LQC)

1. LQG already got rid of the bigbang singularity in 2001. It has taken a long time to do the same with black holes but now it seems to be happening. with a simplified model of the matter that is collapsing the authors see that it does not result in a singularity (i.e. LQG survives the collapse and continues to model conditions where the classical theory had a failure). I guess one can expect more results this year that for the first time do a quantum spacetime analysis (not merely semiclassical analysis) of black holes, and the issues around evaporation to be addressed.

2. It seems natural to expect LQG to develop a perturbative sector, Freidel and Starodubstev have several more papers in preparation. Perturbing around a vacuum or ground state of gravity can be expected to expedite calculation.

3. 2+1 gravity has been the focus of a lot of research and it looks nearly solved and judging from Freidel/Livine work it connects well with matter and also with DSR (modified special relativity). so that suggests we are going to see DSR in 3+1 gravity. (this is consistent with Smolin's recent paper http://arxiv.org/hep-th/0501091 "Falsifiable predictions from semiclassical quantum gravity"). It looks like the gammarayburst test (if Glast flies in 2007) will be an arguably valid experimental trial of LQG, which will therefore risk refutation.

this post involves a lot of guesswork on my part and I may revise it.
the papers that stand out for the first quarter of 2005 ought to serve as
signposts and I am trying to see where they point (and obviously could be mistaken several ways)

marcus · Mar 27, 2005

Spicerack offered this link to a popular article about Martin Bojowald in Nature magazine
http://www.nature.com/cgi-taf/DynaPage.taf?file=/nature/journal/v433/n7021/full/433012a_r.html

Kea flagged a Stephon Alexander article
A Quantum Gravitational Relaxation of the Cosmological Constant
http://arxiv.org/hep-th/0503146[/URL]

marcus · Apr 1, 2005

new Rovelli posting

this is one that Wolram spotted and flagged with its own thread
https://www.physicsforums.com/showthread.php?t=69503

Particle scattering in loop quantum gravity
Leonardo Modesto and Carlo Rovelli
http://arxiv.org/gr-qc/0502036

marcus · Apr 4, 2005

more on the Kodama state

stuff on the Kodama state keeps coming up
even tho Witten warned in 2003 that it was not normalizable (IIRC)
previously the most recent papers were by Stephon Alexander and by Alexander with Kristin Schleich and Donald Witt
today this was posted
http://arxiv.org/abs/gr-qc/0504010
A Generalization of the Kodama State for Arbitrary Values of the Immirzi Parameter
Andrew Randono
16 pages

"The Kodama State for Lorentzian gravity presupposes a particular value for the Immirzi-parameter, namely beta=-i. However, the derivation of black hole entropy in Loop Quantum Gravity suggests that the Immirzi parameter is a fixed value whose magnitude is on the order of unity but larger than one. Since the Kodama state has de-Sitter spacetime as its classical limit, to get the proper radiation temperature, the Kodama state should be extended to incorporate a more physical value for beta. Thus, we present an extension of the Kodama state for arbitrary values of the Immirzi parameter, beta, that reduces to the ordinary Chern-Simons state for the particular value beta=-i. The state for real values of beta is free of several of the outstanding problems that cast doubts on the original Kodama state as a ground state for quantum general relativity. We show that for real values of beta, the state is invariant under large gauge transformations, it is CPT invariant (but not CP invariant), and it is expected to be delta-function normalizable with respect to the kinematical inner product. To aid in the construction, we first present a general method for solving the Hamiltonian constraint for imaginary values of beta that allows one to use the simpler self-dual and anti-self-dual forms of the constraint as an intermediate step."

the guy is at Austen Texas
I don't know whether this paper by itself is so weighty but it impresses me that stuff keeps piling up about Kodama state

here is the Alexander, Schleich, Witt paper
http://arxiv.org/abs/gr-qc/0503062

here is the Alexander solo paper
http://arxiv.org/abs/hep-th/0503146
(see its conclusion section for discussion of Kodama state)

both of these were flagged earlier

Loop-and-allied QG bibliography

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