What is the recent development of Loop Quantum Gravity

marcus

Earlier I was trying to find a write up that would preview the content of Agullo's invited talk at the April meeting of the American Physical Society. The best source, I now realize, is this set of ILQGS slides by William Nelson:
http://relativity.phys.lsu.edu/ilqgs/nelson101811.pdf
and the corresponding audio
http://relativity.phys.lsu.edu/ilqgs/nelson101811.wav
or
http://relativity.phys.lsu.edu/ilqgs/nelson101811.aif

And scroll down to October 2011 for Nelson's

William Nelson's talk is a "must-hear". It's some very good work (as Lee Smolin comments at the end) and is going to change how we view cosmology. It is joint work by Agullo, Ashteker, Nelson, and it just happens that Nelson gave the ILQGS presentation and Agullo will present it at the April APS in Atlanta.

ILQGS also has an interesting blog where various presentations are discussed by OTHER researchers, who often give more basic intuitive explanations of what the talk is about. Brizuela (AEI) comments on Nelson's talk, Julian Barbour (!) comments on Tim Koslowski's talk about shape dynamics, Frank Hellmann (AEI) on Jacek Puchta's about an extenstion of Spinfoam...
Check out the blog, pedagogically it complements the seminar talks and makes them more accessible.
http://ilqgs.blogspot.com/
Some future talks listed here:
http://relativity.phys.lsu.edu/ilqgs/schedulesp12.html
Note Diaz-Polo upcoming talk on Loop BH evaporation (there's a relevance to obs. testing):
http://arxiv.org/abs/1109.4239

marcus

Something I've heard a lot in talks recently is "the Loop hypothesis".

It gives a useful perspective for understanding what the various QG formulations called Loop have in common.
It is the hypothesis that you can TRUNCATE the dynamic geometry of GR to finite degrees of freedom and then recover the continuous for all practical purposes.

The Loop truncation is to consider making geometric measurements at only a FINITE SET OF POINTS. So you naturally get a graph of places where you measured some volumes and some face areas between adjacent chunk volumes. The details of what constitute possible geometric measurements are not important---angles and lengths are also allowed.

What matters is the observer can only make a finite number of measurements, and that defines the state.

I'm thinking that's what science is about:

The aim of quantitative science is to explain what we can observe and to predict thereabout .

And we never get to make more than a finite number of observations.

So a state of nature (nature's geometry) is naturally going to be truncated to a finite set of points with adjacency relations and whatever labels.
The Loop hypothesis is that this is sufficient to explain and predict what we can observe. It's minimalist. The hypothesis (a kind of gamble) is that this will prove to be sufficient to recover the continuous classical picture by taking more and more points (more elaborate networks of observation).

atyy

I think there are two different truncations.

In Rovelli's Zakopane, he talks about a truncation which is a good approximation.

The FGZ idea is that the full space can be split into nice parts and the continuum recovered exactly (not just for all practical purposes) by joining them together. The idea is that is one can do that, one just needs to quantize each part separately.

marcus

Perhaps I don't understand, or I see FGZ doing something different from you. Piecewise flat decomposition with all the curvature concentrated at the joints---approximating the full range of continuously curving geometries, but not reproducing the full range exactly.

For me, what FGZ does is one further step in the process that goes back to the 1990s of finding the most mathematically convenient way to implement the Loop hypothesis*---truncating geometry to a finite number of degrees of freedom, truncating to N degrees of freedom and then letting N→∞.

There are several, many, ways this has been tried. It is all the same quest. You may wish to focus on just two initiatives: Zakopane and FGZ. But I do not wish to restrict my view that way.

Remember the Lewand--Asht measure, the holonomy-flux algebra, the Lewand--Okol--Sahl--Thiemann theorem? I see it as all part of the same journey.

I suppose that history could replace the Zako formulation and keep some features of it.

I particularly like the Hilbertspace of squareintegrable functions defined on a cartesian product of G where the Lie Group G can refer to either the rotations or the full Lorentz. And I like the gamma map from SU(2) reps to SL(2,C) reps. I hope those features are kept, but who can say?
Suppose the group G becomes somehow twistorial? It would still be like Zako, square integrable functions on G^E=#edges in a way, but it would also be different. Or suppose the hilbertspace of functions on the group manifold become not complex-valued but somethingelse valued?
I really liked Wolfgang Wieland's pirsa talk. It gave me a glimpse of where the further evolution of this "loop hypothesis" could go.

Maybe neither Zako or FQZ is final or exactly right. It would be a shock if it were
The important thing is to have something simple definite and clear--mathematically well-defined--at each step along the way. Zako served as that last year and perhaps also this year. We have to keep our eyes open for what will take shape by spring of 2013, when another Loops conference is coming due.

*I was just listening to Marc Geiller talk about the FGZ work:
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
http://relativity.phys.lsu.edu/ilqgs/geiller022812.wav
He calls it the "Loop assumption" instead of the Loop hypothesis, and he says concretely what he means on slide #8 early in the talk. I have the impression now that I hear many people using this idea, which has entered the shared vocabulary of the Loop community. Perhaps it was always one of the shared concepts but I didn't notice it until recently.

atyy

So are FGZ talking about what Rovelli calls the graph expansion, which he distinguishes from the vertex expansion? http://arxiv.org/abs/1102.3660 (p19).

marcus

In a rough sense I think that's right. Very generally they are both talking about the same thing, namely truncating down to finite d.o.f. by restricting attention to a (finite) graph.

At the level of detail I think what they are talking about is different. I wouldn't use the word "expansion" in the FGZ context.

I think they have found a different way to look at the truncation which allows them to reconstruct a class of continuum states from a discrete one. So they "de-truncate" in a sense. They go back to the continuum picture without having to take a grand limit.

I think this FGZ approach is still embryonic. It might eventually augment or replace the earlier "continuum limit" part of the program. These are work-in-progress areas--parts of the program that are under development and could take new directions.

Yesterday I listened to Marc Geiller's ILQGS talk, with the slides. It's a good talk that covers the main ideas of the FGZ paper. I'd recommend it to anyone who wants to understand their approach better. I already gave the links but will do so again:
http://relativity.phys.lsu.edu/ilqgs/geiller022812.pdf
http://relativity.phys.lsu.edu/ilqgs/geiller022812.wav

marcus

A new development in QG that hasn't been reported yet or discussed here is an initiative by the Warsaw group where they develop a systematic way to enumerate all bulk spinfoams which a given spin network boundary. This is how one calculates transition amplitudes in spinfoam QG dynamics. The boundary spin network can, for instance, be thought of as representing initial and final states of geometry and the spinfoam bulk as a process transitioning from one to the other.

The Warsaw group has introduced a new OSN (operator spin network) formalism---graphs labeled by operators instead of spin numbers.

This paper just came out:
http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

Happily enough much of the content was already presented earlier in a recorded seminar talk at ILQGS by Puchta! Having the audio and slides presentation in parallel with the paper can make it easier to understand both.
ILQGS: The Feynman diagramatics for the spin foam models
Jacek Puchta
SLIDES: http://relativity.phys.lsu.edu/ilqgs/puchta092011.pdf
AUDIO: http://relativity.phys.lsu.edu/ilqgs/puchta092011.wav
(alternative audio http://relativity.phys.lsu.edu/ilqgs/puchta092011.aif )

Nominally the recorded seminar talk was based on this paper, but there is considerable overlap with what just appeared:
http://arxiv.org/abs/1107.5185
Feynman diagrammatic approach to spin foams
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 26 Jul 2011)
"The Spin Foams for People Without the 3d/4d Imagination" could be an alternative title of our work. We derive spin foams from operator spin network diagrams} we introduce. Our diagrams are the spin network analogy of the Feynman diagrams. Their framework is compatible with the framework of Loop Quantum Gravity. For every operator spin network diagram we construct a corresponding operator spin foam. Admitting all the spin networks of LQG and all possible diagrams leads to a clearly defined large class of operator spin foams. In this way our framework provides a proposal for a class of 2-cell complexes that should be used in the spin foam theories of LQG. Within this class, our diagrams are just equivalent to the spin foams. The advantage, however, in the diagram framework is, that it is self contained, all the amplitudes can be calculated directly from the diagrams without explicit visualization of the corresponding spin foams. The spin network diagram operators and amplitudes are consistently defined on their own. Each diagram encodes all the combinatorial information. We illustrate applications of our diagrams: we introduce a diagram definition of Rovelli's surface amplitudes as well as of the canonical transition amplitudes. Importantly, our operator spin network diagrams are defined in a sufficiently general way to accommodate all the versions of the EPRL or the FK model, as well as other possible models. The diagrams are also compatible with the structure of the LQG Hamiltonian operators, what is an additional advantage. Finally, a scheme for a complete definition of a spin foam theory by declaring a set of interaction vertices emerges from the examples presented at the end of the paper.
36 pages, 23 figures

marcus

Karmerlo's original question, starting the thread, was about recent Loop developments. My take on that is based on the idea that this is a fast moving field and that at each stage there should be a clear definite testable formulation. In 2010 we got a new version of Loop that culminated in the Zakopane 2011 version ( http://arxiv.org/abs/1102.3660 ). Now I'm looking ahead to see what the 2013 formulation might be like.

This is a quiet period now when IMHO people are getting new thoughts in order.

I expect quantum relativists to construct a QG which resolves the cosmo ("bang") singularity, is testable with early universe data, and recovers a good approximation of usual GR. As long we don't have SEVERAL alternatives that do this, and therefore do not need to choose between them, I am not going to quibble about the "pedigree" or quantization ritual that was used to arrive at the theory. That comes later when we have more than one satisfactory alternative.

This is simply my (pragmatic) philosophy---other people of course assume other intellectual stances.

So thinking ahead to Loops 2013 (to be held at Perimeter Institute) what is the most important paper we should now be looking at? What has the seeds of a new formulation?
I think it is page 5 of the January 2012 paper of Bonzom and Smerlak. Merely an observant bystander's nonprofessional guess--but maybe sometimes that's OK to offer. In the excerpt that follows I used curly brackets to distinguish the moduli space {M} from the ordinary manifold M, while the authors used a special font.

==quote from page 5 of http://arxiv.org/1201.4996 ==
Relation to the loop formalism. The above method naturally gives rise to the loop quantization of BF theory. In the loop approach, one quantizes before restricting to flat gauge fields. Given an embedded, closed graph γ, cylindrical wave functions are functions of the Wilson lines along the lines of γ. For each graph there is a Hilbert space whose measure is given by the Haar measure of G on each line, ∏_e dg_e. The Hilbert spaces of two different graphs are orthogonal. The standard gauge symmetry requires invariance under G-translation on the source and end nodes of the lines.

Heuristically, the transition amplitudes in the continuum (7) suggest that they can be formulated in the loop approach by taking as boundary states cylindrical functions restricted to the moduli space {M}, the torsion still providing the measure. Assume M has two disconnected boundaries N₁,N₂, with two closed, embedded graphs γ₁, γ₂ associated with two cylindrical functions Ψ_γ1 , Ψ_γ2 . The transition is regularized by choosing a cell decomposition K of M such that γ₁,γ₂ are included into the 1-skeleton. The ungauge-fixed transition amplitude reads

⟨Ψ_γ2|Z_BF|Ψ_γ1⟩=∫∏_edg_e Ψ^∗_γ2(g_e)Ψ_γ1(g_e)∏_fδ(H_f).

As the shift symmetry does not act on Wilson lines, the process of the previous section applies. The wave-functions are evaluated on {M} because there are no fluctuations around flat connections, yielding [eqn (31)]:

⟨Ψ_γ2 |Z'_BF|Ψ_γ1⟩ = Ʃ_[φ]∈MΨ^∗_γ2([φ]) Tor[φ] Ψ_γ1([φ]).

Finally, the regulator K can be removed thanks to the topological invariance of the torsion, which makes the continuum limit result into the above formula. Let us mention an outcome of this result: the loop quantization of the BF model does not distinguish knottings of the graphs γ_1,2.

Conclusion. We have performed a topological quantization of discrete BF theory, proving its equivalence to the usual quantization in the continuum. This result solves several open problems of the field and extends previous results obtained in dimension 3 to arbitrary dimensions:
(1) transition amplitudes are finite, answering the issue of bubble divergences [11, 28];
(2) the gauge symmetries in the discrete setting exist, generalizing [11, 12], and
(3) they can be gauge-fixed to derive the loop quantization, generalizing [13];
(4) as a result, one gets a topological invariant, which proves that the classical gauge symmetries are correctly promoted to the quantum level.

The crucial steps of our quantization require to take into account cells of all dimensions in the cell complex, and not just its 2-skeleton like in the “spinfoam quantization”. A challenge for future investigations is to find a representation of (31) as a state-sum, as is done in the latter approach.

The last issue we mentioned in the introduction is the major difficulty in quantum gravity: understanding the quantum version of diffeomorphism-invariance. It is well-known that diffeomorphism-invariance in the BF model is contained within its shift symmetry [20]. Hence the substance of general relativity is to break the topological invariance while preserving diffeomorphism-invariance. Spinfoam models for quantum gravity are very much in line with this idea, as they start by quantizing BF theory and then introduce some breaking of the shift symmetry to restore the local degrees of freedom. It is also known that discrete models of gravity generically break diffeomorphism-invariance [17]. Showing that it is restored in the continuum limit (after some coarse-graining, or summing over spinfoams appropriately) is one of the main programs in the spinfoam approach. Now that the shift symmetry is correctly controlled in the discrete setting, we feel that the noose is tightening around diffeomorphisms.
==endquote==

atyy

So they don't agree with spinfoam quantization?

marcus

My guess is that this is the new spinfoam. The old 2-skeleton approach was started back in the late 1990s by Reisenberger and Rovelli. It is over 20 years old and probably needs to be updated. Of course I could be wrong

marcus

Matteo Smerlak's PhD thesis is a useful source of background for the 6-page Bonzom-Smerlak letter.
I should give the latter's abstract--didn't do that yet.

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak
(Submitted on 24 Jan 2012)
The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2-complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which
(1) is finite,
(2) produces a topological invariant,
(3) matches with the properties of the continuum BF theory,
(4) corresponds to its loop quantization. These results significantly clarify the foundations - and limitations - of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetry-breaking which reduces BF theory to gravity.
6 pages

A concise excerpt from page 1:
==quote 1201.4996==
The purpose of this letter is to argue that there is a good reason for this: when dealing with 2-complexes only, as in the spinfoam formalism, there is no shift symmetry. To identify this symmetry, one must instead resort to an extension of the spinfoam formalism including higher-dimensional cells. This realization paves the way to what we call cellular quantization. This cellular quantization solves problems 1 to 4, and sheds interesting new light on problem 5.
The letter is organized as follows. We start by reviewing the basic properties of the continuum BF theory, emphasizing its gauge symmetries and relationship to analytic torsion. We then describe the “spinfoam quantization” of BF theory, as described e.g. in Baez’s reference paper [5]. We show how to identify the gauge symmetries in a discrete setting and perform a quantization which does preserve the topological features of the continuum theory. Finally we establish that this cellular quantization corresponds to the loop canonical quantization.
==endquote==

Problems 1 through 5, mentioned in the above excerpt, are as follows:
==quote==
1. Bubble divergences. The original PRO [Ponzano-Regge-Oguri] partition functions are in general divergent. How should one regularize them?

2. Topological invariance. The PRO partition functions are formally invariant under changes of triangulations, up to divergent factors. How can one turn them into finite topological invariants?

3. Relationship to the canonical theory. The connection between the Ponzano-Regge model and loop quantum gravity in 3 dimensions was established in [13]. Can this connection be extended to 4 dimensions and higher?

4. Relationship to the continuum theory. BF theory was quantized in the continuum in [21, 22], and was showed to be related to the Ray-Singer torsion. Are the PRO models similarly related to torsion? (See [14] for a positive answer in certain three-dimensional cases.)

5. Diffeomorphism symmetry. Both the continuum BF action and the Einstein-Hilbert action are diffeomorphism-invariant. What is the fate of this symmetry in the PRO models?
==endquote==

For completeness, here is the abstract of Smerlak's thesis. It doesn't overlap in results, but shares some concepts---therefore is helpful in part simply because it is longer (over 100 pages instead of only 6) and more deliberate. Goes thru some definitions in a less condensed way.
http://arxiv.org/abs/1201.4874
Divergences in spinfoam quantum gravity
Matteo Smerlak
(Submitted on 23 Jan 2012)
In this thesis we study the flat model, the main buidling block for the spinfoam approach to quantum gravity, with an emphasis on its divergences. Besides a personal introduction to the problem of quantum gravity, the manuscript consists in two part. In the first one, we establish an exact powercounting formula for the bubble divergences of the flat model, using tools from discrete gauge theory and twisted cohomology. In the second one, we address the issue of spinfoam continuum limit, both from the lattice field theory and the group field theory perspectives. In particular, we put forward a new proof of the Borel summability of the Boulatov-Freidel-Louapre model, with an improved control over the large-spin scaling behaviour. We conclude with an outlook of the renormalization program in spinfoam quantum gravity.
113 pages. PhD thesis, introduction and conclusion in French, main text in English.


Paper by Ileana Naish-Guzman and John Barrett cited on page 4 ref [14] in connection with the discrete exterior derivative on a cell complex. http://arxiv.org/abs/0803.3319
Similarly cited was [26] an earlier paper by Bonzom and Smerlak http://arxiv.org/abs/1103.3961
Additional webstuff about de Rham complex
http://en.wikipedia.org/wiki/De_Rham_cohomology
http://www.vttoth.com/CMS/pahysics-n...e-rham-complex

marcus

A major international conference like the triennial Marcel Grossmann meeting can give a snapshot of "recent development".

Here is the brief statement summing up the situation from the chair of one of the LQG sessions at MG13 (Stockholm July 2012):

http://www.icra.it/MG/mg13/par_sessi...tm#lewandowski

Jerzy LEWANDOWSKI

Parallel Session: SQG2 - Loop Quantum Gravity, Quantum Geometry, Spin Foams

Description: Loop Quantum Gravity (LQG), a framework suited to quantize general relativity, has seen rapid progress in the last three years. The results achieved strongly suggest that the goal of finding a working and predictive quantum theory of gravity is within reach. For specific kinds of matter couplings, a way to drastically simplify the dynamics and its physical interpretation has been discovered. It gives rise to a set of examples of theories of gravity coupled to the fields in which the canonical quantization scheme can be completed. Independently, there have been important breakthroughs in the path integral formulation of the theory related to the so called Spin Foam Models. The session will review the results of canonical Loop Quantum Gravity and Spin Foam Models with the emphasis on the models admitting local degrees of freedom without the symmetry (or any other) reduction. Related approaches to quantum gravity will be also welcome. The common theme is the background independent quantization of Einstein's gravity and the occurrence of quantum geometry.

Lewandowski heads the LQG group at Warsaw (one of a handful of leading groups: Marseille, Perimeter, PennState, Warsaw, LSU, Erlangen, AEI...)
I gather that another triennial meeting, the International Conference on General Relativity and Gravitation, will have its 2013 venue in Warsaw. Lewandowski will doubtless be the main organizer of GR20. It should have an interesting lineup in QG: background independent quantum geometry.

The next biennial Loops conference, Loops 2013, will be held at Perimeter. One way to gauge progress and follow developments is to keep an eye on the topics featured in the programmes of the main conferences as they take shape.
========================
Along the same lines, we already know the LQG talks to be presented at the April 2012 meeting of the APS (American Physical Society). I listed links and abstracts here:
http://physicsforums.com/showthread....64#post3784064
and here:
http://physicsforums.com/showthread....86#post3788486

Loop is in course of achieving parity with String, visibility-wise at major conferences. One can get an idea of which recent directions and results in Loop research are considered important by seeing what the main conference talks, especially those invited by the organizers, are about.

marcus

Another snapshot of the current definition of Loop and the problems to be worked on will be Rovelli's upcoming (23 April) talk at the Princeton Institute of Advanced Studies.
http://www.princeton.edu/physics/eve...ent.xml?id=347

High Energy Theory Seminar - IAS - Carlo Rovelli, Aix-Marseille University, France - Loop quantum Gravity: Recent Results and Open Problems
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.

Location: Bloomberg Lecture Hall
Date/Time: 04/23/12 at 2:30 - 3:30 pm
============================

My comment. This talk may be substantially similar to the one at Perimeter on 4 April, which I believe will subsequently be available video online. I imagine there might be an eventual write-up covering the same material.
Loop is fast moving and is reformulated from time to time. It has been having small "revolutions" on roughly a 3-5 year basis, so Rovelli's wording should be noted "present the current definition...results that have been proven...what I think is still missing..."

"Tentative" here I think means an attempt: to be tested by observation---to be put on trial in other words.
Any theory can only be tentative until its predictions are tested and either confirmed or not. Over the past few years it's been mainly up to Rovelli to give a clear precise definition of the current Loop theory, write the survey papers, list the open problems.

So these two talks will serve as a significant landmark. Will it be essentially a restatement of the February 2011 formulation (Zakopane lectures http://arxiv.org/abs/1102.3660 ) or will there be some new features? Here's the PIRSA link for the Wednesday 4 April one at Perimeter.
http://pirsa.org/12040059

marcus

The University of Vienna and Vienna Tech are holding a 5-day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravity-related research.

A nice feature is how applied it is.
2 out of the 4 main lecturers are discussing applications (linked to astrophysical observation) rather than pure QG theory.

http://www.coqus.at/events/summerschool2012/

The title of the School is Quantum physics meets Gravity
Here's the poster:
http://www.coqus.at/fileadmin/user_u...s/CoQuS_a3.pdf

Here are the more applied, hardware-oriented topics that two of the lecture series will be about:

"Experimental gravitation and geophysics with matter-wave sensors"

"Gravitational wave detection and quantum control"
===============================

While I think of it, the journal SIGMA is publishing a special issue devoted to Loop gravity and cosmology assembled by a group of guest editors. They now have a dozen articles in final form having gone thru the peer review and editorial process. These could give some clues as to what the editors see as significant current developments in the field.
http://www.emis.de/journals/SIGMA/LQGC.html
==quote==
Papers in this Issue:

Introduction to Loop Quantum Cosmology
Kinjal Banerjee, Gianluca Calcagni and Mercedes Martín-Benito
SIGMA 8 (2012), 016, 73 pages [ abs pdf ]
Learning about Quantum Gravity with a Couple of Nodes
Enrique F. Borja, Iñaki Garay and Francesca Vidotto
SIGMA 8 (2012), 015, 44 pages [ abs pdf ]
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman and Yidun Wan
SIGMA 8 (2012), 014, 43 pages [ abs pdf ]
Matter in Loop Quantum Gravity
Ghanashyam Date and Golam Mortuza Hossain
SIGMA 8 (2012), 010, 26 pages [ abs pdf ]
Lessons from Toy-Models for the Dynamics of Loop Quantum Gravity
Valentin Bonzom and Alok Laddha
SIGMA 8 (2012), 009, 50 pages [ abs pdf ]
Entropy of Quantum Black Holes
Romesh K. Kaul
SIGMA 8 (2012), 005, 30 pages [ abs pdf ]
Discretisations, Constraints and Diffeomorphisms in Quantum Gravity
Benjamin Bahr, Rodolfo Gambini and Jorge Pullin
SIGMA 8 (2012), 002, 29 pages [ abs pdf ]
Numerical Techniques in Loop Quantum Cosmology
David Brizuela, Daniel Cartin and Gaurav Khanna
SIGMA 8 (2012), 001, 26 pages [ abs pdf ]
Statistical Thermodynamics of Polymer Quantum Systems
Guillermo Chacón-Acosta, Elisa Manrique, Leonardo Dagdug and Hugo A. Morales-Técotl
SIGMA 7 (2011), 110, 23 pages [ abs pdf ]
The Space of Connections as the Arena for (Quantum) Gravity
Steffen Gielen
SIGMA 7 (2011), 104, 12 pages [ abs pdf ]
Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter
Merced Montesinos and Mercedes Velázquez
SIGMA 7 (2011), 103, 13 pages [ abs pdf ]
A Lorentz-Covariant Connection for Canonical Gravity
Marc Geiller, Marc Lachièze-Rey, Karim Noui and Francesco Sardelli
SIGMA 7 (2011), 083, 10 pages [ abs pdf ]
==endquote==

marcus

Judging from past performance, Rovelli can be relied on for a comprehensive insightful account of the current state of development of Loop gravity and the remaining problems to be worked out. The abstract of the talk he is to give at Princeton IAS was posted earlier.

Today I see that the abstract for the Perimeter Institute colloquium talk he is giving next Wednesday (4 April) has also been posted. It's approximately the same summary description, probably much the same talk. So if things go as expected, we'll soon have an online video of an up-to-date firsthand view of Loop with Perimeter audience Q&A.

http://pirsa.org/12040059/
Transition Amplitudes in Quantum Gravity
Speaker(s): Carlo Rovelli
Abstract: The covariant formulation of loop quantum gravity has developed strongly during the last few years. I summarize the current definition of the theory and the results that have been proven. I discuss what is missing towards of the goal of defining a consistent quantum theory whose classical limit is general relativity.
Date: 04/04/2012 - 2:00 pm

For comparison here is the announcement of the Princeton talk:
http://www.princeton.edu/physics/eve...ent.xml?id=347
High Energy Theory Seminar - IAS - Carlo Rovelli, Aix-Marseille University, France - Loop quantum Gravity: Recent Results and Open Problems
Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit.
Location: Bloomberg Lecture Hall
Date/Time: 04/23/12 at 2:30 - 3:30 pm

Rovelli will also be giving a series of lectures on QG during the first week of September at the Vienna "Quantum Physics meets Gravity" School that I mentioned in the preceding post:
http://www.coqus.at/events/summerschool2012/
=================================

The April meeting of the American Physical Society also starts next week in Atlanta. There will be several invited and contributed Loop talks. Links and abstracts are listed here:
http://physicsforums.com/showthread....64#post3784064
and here:
http://physicsforums.com/showthread....86#post3788486
Bianchi and Agullo give invited talks Monday 2 April
http://meetings.aps.org/Meeting/APR12/Event/170161
http://meetings.aps.org/Meeting/APR12/Event/170160

marcus

There will be a QG School in Beijing in August. I was interested to see who the lecturers are going to be:
http://physics.bnu.edu.cn/summerschool/en/index.php
==quote==
The 2nd BNU International Summer School on Quantum Gravity: 12-18 August 2012, Beijing
Beijing Normal University (BNU), China
The BNU International Summer School on Quantum Gravity is intended to provide a pedagogical introduction for graduate students and young post-docs to the main fields closely related to loop quantum gravity.

Topics include: Loop quantum gravity, Loop quantum cosmology, Spin foams, Group field theory, Regge calculus

Lecturers:
Abhay Ashtekar (Penn State Univ, USA)
Benjamin Bahr (Cambridge Univ, UK)
John Barrett (Univ of Nottingham, UK)
Jonathan Engle (Florida Atlantic Univ, USA)
Thomas Krajewski (Univ of Provence & CPT Marseille, France)
Jerzy Lewandowski (Univ of Warsaw, Poland)
Etera Livine (ENS de Lyon, France)
==endquote==

The University of Vienna and Vienna Tech are holding a 5-day Quantum physics + Gravity school in early September, intended for PhD students and other young researchers wanting to get into gravity-related research.
http://www.coqus.at/events/summerschool2012/
The title of the School is Quantum physics meets Gravity
Here's the poster:
http://www.coqus.at/fileadmin/user_u...s/CoQuS_a3.pdf
Lecturers:
· Philippe Bouyer (University of Bordeaux, Institut d'optique and CNRS, France)
· Michèle Heurs (Max Planck Institute for Gravitational Physics, Hannover, Germany)
· Ulf Leonhardt (University of St. Andrews, UK)
· Carlo Rovelli (Centre de Physique Theorique de Luminy, Marseille, France
===============================
Update on the journal SIGMA's special issue on Loop Gravity and Cosmology, being assembled by a group of guest editors. So far they have fifteen articles in final form which have passed peer review and been posted online. I was interested to see the lineup since it gives an idea of what the editors see as significant current research directions.
http://www.emis.de/journals/SIGMA/LQGC.html
Here's an updated listing of the articles. They are free online.

Colored Tensor Models - a Review
Razvan Gurau and James P. Ryan
SIGMA 8 (2012), 020, 78 pages [ abs pdf ]
Intersecting Quantum Gravity with Noncommutative Geometry - a Review
Johannes Aastrup and Jesper Møller Grimstrup
SIGMA 8 (2012), 018, 25 pages [ abs pdf ]
Relational Observables in Gravity: a Review
Johannes Tambornino
SIGMA 8 (2012), 017, 30 pages [ abs pdf ]
Introduction to Loop Quantum Cosmology
Kinjal Banerjee, Gianluca Calcagni and Mercedes Martín-Benito
SIGMA 8 (2012), 016, 73 pages [ abs pdf ]
Learning about Quantum Gravity with a Couple of Nodes
Enrique F. Borja, Iñaki Garay and Francesca Vidotto
SIGMA 8 (2012), 015, 44 pages [ abs pdf ]
Emergent Braided Matter of Quantum Geometry
Sundance Bilson-Thompson, Jonathan Hackett, Louis Kauffman and Yidun Wan
SIGMA 8 (2012), 014, 43 pages [ abs pdf ]
Matter in Loop Quantum Gravity
Ghanashyam Date and Golam Mortuza Hossain
SIGMA 8 (2012), 010, 26 pages [ abs pdf ]
Lessons from Toy-Models for the Dynamics of Loop Quantum Gravity
Valentin Bonzom and Alok Laddha
SIGMA 8 (2012), 009, 50 pages [ abs pdf ]
Entropy of Quantum Black Holes
Romesh K. Kaul
SIGMA 8 (2012), 005, 30 pages [ abs pdf ]
Discretisations, Constraints and Diffeomorphisms in Quantum Gravity
Benjamin Bahr, Rodolfo Gambini and Jorge Pullin
SIGMA 8 (2012), 002, 29 pages [ abs pdf ]
Numerical Techniques in Loop Quantum Cosmology
David Brizuela, Daniel Cartin and Gaurav Khanna
SIGMA 8 (2012), 001, 26 pages [ abs pdf ]
Statistical Thermodynamics of Polymer Quantum Systems
Guillermo Chacón-Acosta, Elisa Manrique, Leonardo Dagdug and Hugo A. Morales-Técotl
SIGMA 7 (2011), 110, 23 pages [ abs pdf ]
The Space of Connections as the Arena for (Quantum) Gravity
Steffen Gielen
SIGMA 7 (2011), 104, 12 pages [ abs pdf ]
Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter
Merced Montesinos and Mercedes Velázquez
SIGMA 7 (2011), 103, 13 pages [ abs pdf ]
A Lorentz-Covariant Connection for Canonical Gravity
Marc Geiller, Marc Lachièze-Rey, Karim Noui and Francesco Sardelli
SIGMA 7 (2011), 083, 10 pages [ abs pdf ]

marcus

We can start looking ahead to the next biennial Loops conference: Loops 2013, to be held at Perimeter Institute. I'm told that planning for the conference has already started. Loop gravity is constantly evolving as a theory and a paper suggesting a new classical action sets the stage, I think, for the 2013 version of the quantum theory.

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)
We study time-reversal and parity ---on the physical manifold and in internal space--- in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences.
8 pages

The classical basis for the theory is the Holst action. A 4D manifold M equipped with internal Minkowski space M at each point plus a tetrad e (one-form valued in M) and a connection ω. The conventional Holst action:

S[e,ω]=∫e^IΛe^JΛ(* + 1/γ) F_{I J}

The * denotes the Hodge dual. A proposed new action S' uses the signum of det e: s = sign(det e) defined to be zero if det e = 0 and otherwise ±1.

S'[e,ω]=∫e^IΛe^JΛ(s* + 1/γ) F_{I J}

There is also a closely related alternative action S" discussed in the paper.

It looks to me as if either S' or S", suitably quantized, takes care of the semiclassical limit of the theory. See equation (43) of the paper. Or in any case represents a major step. The problem addressed was that the original EPRL looked at in the limit exhibited both spacetime and "anti-spacetime" evolution. Both time-forward and time-reversed evolution appeared. Otherwise everything was properly Regge as expected. Then there were papers by Yasha Neiman and by Jon Engle that studied this bi-directional time mixup. Rovelli and Wilson-Ewing (RWE) built on their results. So I expect this RWE paper to provide a basis for a new Loop initiative leading up to the conference about a year from now. Of course it is risky (even foolhardy) to forecast research trends. But that's what I think after reading the paper. It addresses several of the remaining problems in the theory---and it's quite interesting as well.