Loop-and-allied QG bibliography

[marcus]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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]

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

 

[selfAdjoint]

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

I don't know who Andrew Randono is either, but if the paper holds up, it's path-breaking. He has embedded the existing, problematical Kodama state (proposed as the ground state of quantum gravity, the QG vacuum if you will) in a continuum of such states parametrized by the Immirzi parameter. Since the Baez et. al. path integral paper strongly suggests the I.P. as a little bigger than 1 and real, while existing Kodama has Immirzi = -i, he wants to, well, sort of continue through his new continuum to get to the real value. And when he does this he gets a new kind of Kodama state which is nice. I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!

 

[marcus]

... I hope he's right, because it could really simplify things for QG, and might even convert some more string theorists!

I so much hope he is right too, or that something along those lines can be rigorously established. Bringing in a few more string theorists is a nice idea too

I should post something about this newly arxived paper by Sorkin:

http://arxiv.org/abs/hep-th/0504037
Ten Theses on Black Hole Entropy
Rafael D. Sorkin (Perimeter Institute and Syracuse University)

To appear in the Proceedings of the European Science Foundation Conference on Philosophical and Foundational Issues in Statistical Physics, held Utrecht, the Netherlands, 28-30 November 2003. The proceedings are to be published in a special issue of Studies in History and Philosophy of Modern Physics (2005)

"I present a viewpoint on black hole thermodynamics according to which the entropy: derives from horizon "degrees of freedom''; is finite because the deep structure of spacetime is discrete; is "objective'' thanks to the distinguished coarse graining provided by the horizon; and obeys the second law of thermodynamics precisely because the effective dynamics of the exterior region is not unitary."

I wonder what our long-time PF-member whose handle is "nonunitary" thinks of Sorkin's paper. Also PF-member Edgar1813, whose friends Gambini and Pullin have argued that time-evolution (told with a realistic quantum clock) must be nonunitary. The idea presses in from several directions.

 

[marcus]

http://arxiv.org/abs/gr-qc/0504029

Black hole evaporation: A paradigm
Abhay Ashtekar, Martin Bojowald
18 pages, 4 figures
IGPG04/8-4, AEI-2004-072

"A paradigm describing black hole evaporation in non-perturbative quantum gravity is developed by combining two sets of detailed results: i) resolution of the Schwarzschild singularity in loop quantum gravity; and ii) time-evolution of black holes in the dynamical horizon framework. Quantum geometry effects introduce a major modification in the traditional space-time diagram of black hole evaporation, providing a possible mechanism for recovery of information that is classically lost in the process of black hole formation. The paradigm is developed directly in the Lorentzian regime and necessary conditions for its viability are discussed. If these conditions are met, much of the tension between expectations based on space-time geometry and structure of quantum theory would be resolve."

this is a long-awaited paper. Ashtekar gave a talk at Penn State last year about this (audio and slides are available). the paper has already been cited in some others.

 

[marcus]

Ashtekar and Bojowald have another BH paper in preparation called
Non-singular quantum geometry of the Schwarzschild black hole interior

that is reference [1] in their paper that just came out.

in the present paper they discuss the results in [1] somewhat, as in the abstract, and on pages 5 and 7:

abstract: " i) resolution of the Schwarzschild singularity in loop quantum gravity[1]; "

page 5:"Since the key issues involve the final black hole singularity and since we expect this singularity to be generically space-like, one can first focus just on the interior of the Schwarzschild horizon. This region is naturally foliated by 3-manifolds ...Using quantum geometry, we can go to the exact quantum theory [1]. The situation is similar but technically more complicated than that encountered in the rigorous treatment of spatially homogeneous and isotropic cosmologies [16]. (See also [15] where the same kind of representation is used, based on ADM variables.)... "

page 7: "... Thus, as in quantum cosmology, one finds that the quantum evolution does not stop at the singularity; one can evolve right through it [1]. The state remains pure. However, in the deep Planck regime around the singularity, the notion of a classical space-time geometry fails to make even an approximate sense. Nonetheless, since there is no longer a final boundary in the interior, the full quantum evolution is quite different from the classical one..."

page 7: "...This calculation was done [1] in the Kantowski-Sachs mini-superspace and |Psi> represents the state of the Schwarzschild black hole interior in loop quantum gravity. This black hole can not evaporate: there is no matter and because of the restriction to spherical symmetry there can not be Hawking radiation of gravitons either. However, since the generic singularity is expected to be space-like, one may hope that the general intuition about the resolution of the Schwarzschild singularity it provides can be taken over to models in which gravity is coupled to scalar fields, where the evaporation does occur. We will assume that the overall, qualitative features of our singularity resolution will continue to be valid in these models."

 

[marcus]

http://arxiv.org/abs/gr-qc/0504043
Quantum Gravitational Collapse
Leonardo Modesto
13 pages

"We apply the recent results in Loop Quantum Cosmology and in the resolution of Black Hole singularity to the gravitational collapse of a star. We study the dynamic of the space time in the interior of the Schwarzschild radius. In particular in our simple model we obtain the evolution of the matter inside the star and of the gravity outside the region where the matter is present. The boundary condition identify an unique time inside and outside the region where the matter is present. We consider a star during the collapse in the particular case in which inside the collapsing star we take null pressure, homogeneity and isotropy. The space-time outside the matter is homogeneous and anisotropic. We show that the space time is singularity free and that we can extend dynamically the space-time beyond the classical singularity."

 

[ohwilleke]

If Modesto's paper really new? I thought that LQG implies no singularities had been established for some time. Is the new contribution the description of what is going on inside the black hole?