Reformulation of Loop gravity in progress, comment?

[negativzero]

From: http://backreaction.blogspot.com/201...spacetime.html

"...It is a peculiar, but well established, property of the quantum vacuum that what happens at one point is not entirely independent from what happens at another point because the quantum vacuum is a spatially entangled state..."

and:
"What does this have to do with quantum gravity? It is a way to rewrite an old problem. Instead of trying to quantize space-time, one could discretize it by sprinkling the points and encode its properties in the eigenvalues of the Greensfunctions. And once one can describe the curvature of space-time by these eigenvalues, which are invariant properties of space-time, one is in a promising new starting position for quantizing space-time."
.
Thus, a well constructed set of measurements would "vibrate" the space between selected pairs of points and reveal various modes of vibration, which would in turn reveal the physical geometry. This is a different way of measuring the geometry of space. The old-fashioned way to determine sphere shape is to measure a uniform distance from a center or, to measure from point-to-point on the surface. If i think i have a sphere using the old method, i can check my theory against the data i get by "thumping" selected points with small perturbations, making space ring like a musical instrument. This would reveal shape of space and presumably discrete spectra would reveal the quantum nature of gravity.
.
(Question mark.)
-0

 

[DimReg]

Why doesn't Time zero create a "fixed background geometry?" And why not pick the particle sphere, event sphere, or "known universe" of some arbitrary particle to establish background?
-0

That depends on what you mean by "Time zero". But more importantly, the issue isn't that you can't pick a background, because you certainly can, but that the background you choose is arbitrary. For example, you may choose "Time zero", and I choose "Time one", and our calculations will still result in the same answer. However, the details of how to get there will be different, for example if we are counting the number of gravitons required to build the final state out of our chosen backgrounds, we will disagree on how many there are. This is because some of the gravitons that appear as perturbations to your background will be part of my background. A third observer with a third background choice will disagree with both of us. This is very similar to the fact that a choice of coordinate system is also arbitrary, but definitely not the same.

This doesn't mean you should never choose a background, or that you can't make significant progress in formulating gravity on a fixed background. The straightforward attempt to quantize gravity, using gravitons on minkowski space, can actually compute quantum gravity corrections, but the theory is non renormalizable which for technical reasons makes it incomplete. The issue is associated with high energies, where curvature is going to increase significantly (in which case considering flat space as background, and the curvature as perturbation doesn't make sense). LQG people see this as saying that the perturbative approach is simply not the right approach to quantum gravity. If you're a relativist at heart, this might seem obvious, but from the particle physics perspective other approaches seem preferable (I won't go into that though, it's really off topic).

 

[marcus]

I'm still trying to figure out where LQG research is going. Bojowald's seminar talk today seems to put Loop cosmology in a new light. If one takes inhomogeneity seriously it seems that different regions of a collapsing universe would bounce at different times, and become causally separate from the rest. A collapsing universe would fragment. I'm not sure what practical effect this could have since each individual nearly homogeneous piece is causally isolated and can be studied using the same LQC model that people are already working on. The difference seems mainly philosophical.
Bojowald's slides and audio are already online from today's talk.
http://relativity.phys.lsu.edu/ilqgs/
http://relativity.phys.lsu.edu/ilqgs/bojowald042313.pdf
http://relativity.phys.lsu.edu/ilqgs/bojowald042313.wav

The talk was based on a December 2012 paper:
http://arxiv.org/abs/1212.5150
A loop quantum multiverse?
Martin Bojowald
(Submitted on 20 Dec 2012)
Inhomogeneous space-times in loop quantum cosmology have come under better control with recent advances in effective methods. Even highly inhomogeneous situations, for which multiverse scenarios provide extreme examples, can now be considered at least qualitatively.
10 pages, 9 figures, based on a plenary talk given at Multicosmofun '12, Szeczin, Poland

The ILQGS spring schedule has been revised so that now the 7 May talk will be by Yasha Neiman
The imaginary part of the GR action and the large-spin 4-simplex amplitude

Here are the three most recent papers by Yasha, who recently joined the Penn State group as a postdoc.
1. arXiv:1304.3025
The Wald entropy formula and loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
16 pages

2. arXiv:1303.4752
Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
22 pages, 5 figures

3. arXiv:1301.7041
The imaginary part of the gravity action and black hole entropy
Yasha Neiman
37 pages, 8 figures
===============================
EDIT: in addition to this interesting new work by Bojowald and by Yasha Neiman (and others) there are also three new papers by Muxin Han that just appeared, I listed them in biblio thread yesterday. Atyy mentions this one by Han and Krajewski:
http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski
(Submitted on 20 Apr 2013)
A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
36 pages

 

[atyy]

http://arxiv.org/abs/1304.5626

They give conditions under which the "critical points" are classical geometries. So I think they need to see if they can get conditions in which the critical points give almost everything - if I understand correctly, the equivalent condition in AdS/CFT is "large N" - ie. away from large N the critical points are bad approximations, and even though the duality is conjectured to still hold, the bulk geometry is no longer classical.

 

[marcus]

http://arxiv.org/abs/1304.5626

They give conditions under which the "critical points" are classical geometries. So I think they need to see if they can get conditions in which the critical points give almost everything - if I understand correctly, the equivalent condition in AdS/CFT is "large N" - ie. away from large N the critical points are bad approximations, and even though the duality is conjectured to still hold, the bulk geometry is no longer classical.

They don't seem to be talking about critical "points". what they mean by critical spinfoam configurations are defined by conditions on the labelings of vertices, edges, faces...
This was discussed already in the 2011 paper that is this paper's reference [10]

==quote ref. [10] page 2==
The present work analyzes the large-j asymptotic analysis of the Lorentzian EPRL spinfoam amplitude to the general situation of a 4d simplicial manifold with or without boundary, with an arbitrary number of simplices. The analysis for the Euclidean EPRL model is presented in [21]. The asymptotics of the spinfoam amplitude is determined by the critical configurations of the “spinfoam action”, and is given by a sum of the amplitudes evaluated at the critical configurations. Therefore the large-j asymptotics is clarified once we find all the critical configurations and clarify their geometrical implications. Here for the Lorentzian EPRL spinfoam amplitude, a critical configuration in general is given by the data (j_f , g_ve, ξ_ef , z_vf ) that solves the critical point equations, where j_f is an SU(2) spin assigned to each triangle, g_ve is an SL(2, C) group variable, and ξ_ef, z_vf are two types of spinors. Here in this work we show that given a general critical configuration, there exists a partition of the simplicial complex K into three types of regions R_Nondeg, R_Deg-A, R_Deg-B, where the three regions are simplicial sub-complexes with boundaries, and they may be disconnected regions. The critical configuration implies different types of geometries in different types of regions:
==endquote==

 

[marcus]

It appears to be quite an interesting paper! They make substantial progress towards showing the correct limit rigorously. There is a difference from the approach used in reference [10] which makes this more elegant, as they describe here:
==quote Han Krajewski 2013 page 2==
The present work focuses on the large spin asymptotic analysis of the Lorentzian EPRL (partial) amplitude, but the analysis starts from a new “spinfoam action” for the stationary phase approximation. The new spinfoam action and the corresponding path integral representation is derived from top to down from the group-representation-theoretic definition of the model in [12], it is more elegant and economic than the one employed in [10] because it has [fewer]less integration variables. Here we still focus on the discussion of spinfoam partial amplitude. When the sum over spin is taking[en] into account, the semiclassical behavior of the spinfoam model is investigated in the companion papers [13].
In the present paper we develop a systematic analysis of the spinfoam large spin asymptotics. We make the discussion pedagogical and self-contained in this paper. Here we clarify the relation between the spinfoam model and classical simplicial geometry via the large spin asymptotic expansion. More precisely, in the large spin regime, there is an equivalence between the spinfoam critical configuration (with [a] certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry (discussed in Section 8). Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations...
==endquote==

While I was reading this I nit-picked some typos. None of us are perfect

 

[marcus]

A kind of punch line occurs at the end of page 2
==quote==
also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration (in Section 10). It is shown (in Section 12) that only at a globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity.
==endquote==

So critical configurations are systems of labelings of a spinfoam's vertices edges faces. And one can classify them. Certain of them are oriented (globally and time-wise). It is these "good" configurations which make the right leading order contribution (agreeing with Regge action of GR.)

Off-hand I'd say this could turn out to be quite a useful result. Anyone else think so? or disagree?

 

[atyy]

They don't seem to be talking about critical "points". what they mean by critical spinfoam configurations are defined by conditions on the labelings of vertices, edges, faces...
This was discussed already in the 2011 paper that is this paper's reference [10]

Yes, critical configurations is their term. To me at looks like a "saddle point approximation" - ie. configurations which extremize the action? In AdS/CFT, the gravity is classical when the saddle point approximation becomes very good, which is the large N condition, which is why I expect there should be a similar condition in spin foams - just saying that the saddle point configuration is correct is necessary but not sufficient, I think.

It appears to be quite an interesting paper! They make substantial progress towards showing the correct limit rigorously. There is a difference from the approach used in reference [10] which makes this more elegant, as they describe here:
==quote Han Krajewski 2013 page 2==
The present work focuses on the large spin asymptotic analysis of the Lorentzian EPRL (partial) amplitude, but the analysis starts from a new “spinfoam action” for the stationary phase approximation. The new spinfoam action and the corresponding path integral representation is derived from top to down from the group-representation-theoretic definition of the model in [12], it is more elegant and economic than the one employed in [10] because it has [fewer]less integration variables. Here we still focus on the discussion of spinfoam partial amplitude. When the sum over spin is taking[en] into account, the semiclassical behavior of the spinfoam model is investigated in the companion papers [13].
In the present paper we develop a systematic analysis of the spinfoam large spin asymptotics. We make the discussion pedagogical and self-contained in this paper. Here we clarify the relation between the spinfoam model and classical simplicial geometry via the large spin asymptotic expansion. More precisely, in the large spin regime, there is an equivalence between the spinfoam critical configuration (with [a] certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry (discussed in Section 8). Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations...
==endquote==

While I was reading this I nit-picked some typos. None of us are perfect

 

[atyy]

Yes, critical configurations is their term. To me at looks like a "saddle point approximation" - ie. configurations which extremize the action? In AdS/CFT, the gravity is classical when the saddle point approximation becomes very good, which is the large N condition, which is why I expect there should be a similar condition in spin foams - just saying that the saddle point configuration is correct is necessary but not sufficient, I think.

The next two papers following http://arxiv.org/abs/1304.5626 start to follow up on this.
http://arxiv.org/abs/1304.5627
http://arxiv.org/abs/1304.5628
"The semiclassical analysis is carried out by taking into account the sum over spins in the regime where all the spins are uniformly large. Such an analysis is a natural continuation of the previous studies of large spin asymptotics [6–9], which don’t take into account the sum over spins."

References [6-9] include the first in Muxin Han's new series http://arxiv.org/abs/1304.5626, and the most important papers on the semiclassical limit before this.
http://arxiv.org/abs/0809.2280
http://arxiv.org/abs/0907.2440
http://arxiv.org/abs/0902.1170

Good job! With almost 5 years since the first of those, I thought they'd abandoned ship for relative locality or a reformulation. I guess the Muxin Han and Mingyi Zhang papers were preparation for this. Now what do they find ...

 

[tom.stoer]

One should not overrate the semiclassical analysis. It is an important consistency check and a calculational tool for quantum corrections, but not more. The quantization ambiguities we still face in LQG need not be visible in these approximations. The deep QG regime is beyond this semiclassical analysis.

So this is an important research program, but not the one that will tell us the ultimate truth about LQG.

 

[atyy]

One should not overrate the semiclassical analysis. It is an important consistency check and a calculational tool for quantum corrections, but not more. The quantization ambiguities we still face in LQG need not be visible in these approximations. The deep QG regime is beyond this semiclassical analysis.

So this is an important research program, but not the one that will tell us the ultimate truth about LQG.

Well, if this fails, that would tell us the ultimate truth about LQG;)

But yes, I agree if this works we still need to know whether the infinite sums implied in Eq 30 of http://arxiv.org/abs/1303.4636 work out.

 

[marcus]

Although the Han Krajewski paper (and other recent ones by Han) are very interesting and in my view contribute to a sense that LQG may be on the right track, this post is about something else. I continue to be surprised by the ecumenical breadth of the upcoming Loops conference. Not only are several allied (also in a sense rival) background independent QG approaches are represented but also continuing observational efforts to constrain energy-dependence of speed of light. For instance among the invited plenary speakers I see Henrique Gomes, Fay Dowker, Dafne Guetta.

Henrique Gomes has done research in spinfoam asymptotics and more recently on shape dynamics.
http://inspirehep.net/author/H.Gomes.1/
Fay Dowker, as we know, is one of the main researchers in Causal Sets
Dafne Guetta http://inspirehep.net/author/D.Guetta.1/ is an expert on Gammaray Bursts (GRB) with 80 citable papers of which the two most recent are
http://inspirehep.net/record/1223049?ln=en
http://inspirehep.net/record/1222810?ln=en
She has been a frequent collaborator with Tsvi Piran.

Vincent Rivasseau and Razvan Gurau, two of those most active in tensor model QG, are also among the plenary speakers. Also Steve Carlip and Bill Unruh. It's a speakers list drawn from a wide range of research interests. I wonder if this will establish a pattern to be followed in subsequent Loops conferences.

I see also David Skinner, whose most recent papers have been about gravity in twistor space and about N=8 supergravity:
http://inspirehep.net/search?p=author:"D.Skinner.1" AND collection:citeable

Frank Hellmann is also one of the plenary speakers.

 

[marcus]

It's interesting to see how the Loops 2013 organizers are allocating the plenary talks. 19 invited speakers are listed so far. The conference is scheduled for 5 full days and unless they break with tradition they will have to save most afternoons for parallel sessions of contributed talks. So my rough guess is there's time for somewhere around 25 plenary speakers---just a really rough guess.

A lot of the 19 announced so far are younger--rising generation people. Some of the names are not all that familiar to me. Some that are familiar (such as Frank Hellmann) have been working on new variants of LQG. Maybe I shouldn't say "reformulation"---the new versions might turn out to be largely equivalent: the same theory couched in a different mathematical language. Or might not. I'll continue to call these efforts reformulation. And there are close relatives that aren't LQG but connect with it, like shape dynamics and tensor models.

Here are some of the younger speakers and some (including senior folk) whose talks seem to indicate a thematic branching out. I've indicated my non-expert guesses as to topics their talks might cover.

Ivan Agullo, DAMPT Cambridge (pre-inflationary, bounce) cosmology
Aurelien Barrau, Universite Joseph Fourier observational tests of loop cosmology
Eugenio Bianchi, Perimeter Institute (several including) loop black holes and thermodynamics
Fay Dowker, Imperial College, London causal sets
Henrique Gomes, University of California, Davis shape dynamics
Dafne Guetta, Braude College constraints from GRB and neutrino astronomy
Razvan Gurau, Université Paris-Sud tensor models
Frank Hellmann, Max Planck Institute for Gravitational Physics holonomy spinfoams
Etera Livine, Ens de Lyon (several possibilities including) spinorial LQG
Alejandro Perez, Centre de Physique Theorique (several including) loop BH and thermodynamics
Vincent Rivasseau, Universite Paris-Sud XI Orsay tensor models
David Skinner, DAMPT Cambridge, IAS N=8 supergravity?
Bill Unruh, University of British Columbia analog models of QG?
Madhavan Varadarajan, Raman Research Institute completing the LQG Hamiltonian approach

Bill Unruh is certainly no youngster, but I've included his name in this list because he might be talking about research outside of what has normally been covered at Loop conferences. Likewise Rivasseau. I find myself unable to predict with any assurance what some of these people will be talking about.

Madhavan Varadarajan is an interesting speaker because his recent papers (solo, with Casey Tomlin, or with Alok Laddha) show progress towards completing the original LQG program involving a satisfactory Hamiltonian constraint operator: e.g. http://arxiv.org/abs/1210.6877
Casey Tomlin gave a [video] lecture on some of this work today:
http://pirsa.org/13040104/

For clarification about recently developed spinorial LQG among the work by Etera Livine see e.g. http://arxiv.org/abs/1302.7142

 

[marcus]

Someone who is not on this list is Yasha Neiman (complex part of GR action, and entropy) but I want to remind us of his papers and also mention a video talk that was given just last week:
http://pirsa.org/13040106
The imaginary part of the gravitational action and black hole entropy
Speaker(s): Yasha Neiman
Abstract: I present a candidate for a new derivation of black hole entropy. The key observation is that the action of General Relativity in bounded regions has an imaginary part, arising from the boundary term. The formula for this imaginary part is closely related to the Bekenstein-Hawking entropy formula, and coincides with it for certain classes of regions. This remains true in the presence of matter, and generalizes appropriately to Lovelock gravity. The imaginary part of the action is a versatile notion, requiring neither stationarity nor any knowledge about asymptotic infinity. Thus, it may provide a handle on quantum gravity in finite and dynamical regions. I derive the above results, make connections with standard approaches to black hole entropy, and speculate on the meaning of it all. Implications for loop quantum gravity are also discussed.
Date: 18/04/2013

The papers I mentioned earlier. Some are with Norbert Bodendorfer. It is conceivable that either Norbert or Yasha could be talking about this at Loops 2013.

Here are the three most recent papers by Yasha, who recently joined the Penn State group as a postdoc.
http://arxiv.org/abs/1304.3025
The Wald entropy formula and loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
16 pages

http://arxiv.org/abs/1303.4752
Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
22 pages, 5 figures

http://arxiv.org/abs/1301.7041
The imaginary part of the gravity action and black hole entropy
Yasha Neiman
37 pages, 8 figures

You can see that the January solo paper by Yasha has almost the same title and the PIRSA video talk that he gave last week.
Also it is noteworthy that the ILQGS schedule was recently revised to give him the 7 May timeslot.
His ILQGS online talk will be:
The imaginary part of the GR action and the large-spin 4-simplex amplitude
I'm not sure but this 7 May talk may turn out to be related to one he gave at Perimeter in 2011:
http://pirsa.org/11110111/
Parity and the Immirzi Parameter in Lorentzian Spinfoams
Yasha Neiman
The parity invariance of spinfoam gravity is an open question. Naively, parity breaking should reside in the sign of the Immirzi parameter. I show that the new Lorentzian vertex formula is in fact independent of this sign, suggesting that the dynamics is parity-invariant. The situation with boundary states and operators is more complicated. I discuss parity-related pieces of the transition amplitude and graviton propagator in the large-spin 4-simplex limit. Numerical results indicate patterns similar to those in the Euclidean case. In particular, parity-related components of the graviton propagator differ by a phase. I discuss possible resolutions of this issue.
02/11/2011

 

[marcus]

I focused earlier on only 14 of the 19 invited speakers listed so far by the Loops organizers and so didn't properly consider what the talks by the following major people might be about.
Abhay Ashtekar, Pennsylvania State University
Steve Carlip, University of California, Davis
Viqar Husain, University of New Brunswick
Kirill Krasnov, University of Nottingham
Carlo Rovelli, Le Centre de Physique Théorique
That will have to wait until more information is available.

Meanwhile here's a short list of the themes identified in the previous post#193. The project of completing LQG Hamiltonian dynamics, pursued by Varadarajan and by Tomlin among others could also be called "closing the quantum constraint algebra" off shell, I suspect. The quantum constraint algebra corresponds classically to the hypersurface deformation algebra, which closes in GR. The snag which the Hamiltonian approach hit in the late 1990s seems essentially to have been that the quantum operator version of HD algebra did not close off shell. Correct me, anyone, if this is isn't clear. I will omit a couple of themes I'm not at all sure about (mere guesses in connection with talks by Unruh and Skinner) and highlight the last four, because less familiar.

cosmology/observational tests
black holes and thermodynamics
causal sets
shape dynamics
tensor models
holonomy spinfoams
spinorial LQG
closing constraint algebra

It should be remembered (I should remind myself frequently) that the main themes of a conference do not necessarily all have to be reflected in the list of plenary talks by invited speakers. Presumably there will be contributed talks in parallel sessions and some of these will arouse significant interest. I'm guessing that something mentioned in post#194, "the imaginary part of the gravitational action" will figure in what the conferees take away. In line with that it could be recommended to watch last week's PIRSA video ( http://pirsa.org/13040106 ) and listen to the 7 May ILQGS talk ( http://relativity.phys.lsu.edu/ilqgs/ )

 

[marcus]

As a way of keeping track of possible changes occurring in LQG, here are the Loops 2013 speakers announced so far and some rough guesses as to their possible topics. Talk titles have not been announced so these really are mere guesses.
Ivan Agullo, DAMPT Cambridge, pre-inflationary bounce cosmology
Abhay Ashtekar, Penn State, overview+pre-inflationary bounce cosmology
Aurelien Barrau, Universite Joseph Fourier, observational tests of loop cosmology
Eugenio Bianchi, Perimeter Institute, several including loop black holes and thermodynamics
Steve Carlip, UC Davis, several e.g. CDT quantiz'n Horava gravity? Shape Dynamics? dimensional reduction?
Fay Dowker, Imperial College London, causal sets
Henrique Gomes, UC Davis, shape dynamics
Dafne Guetta, Braude College, observational constraints from GRB and neutrino astronomy
Razvan Gurau, Université Paris-Sud, tensor models
Frank Hellmann, MPI for Gravitational Physics Potsdam, holonomy spinfoams
Viqar Husain, University of New Brunswick, computable LQG framework
Kirill Krasnov, University of Nottingham, pure connection gravity see http://arxiv.org/abs/1304.6946
Etera Livine, Ens de Lyon, several possibilities including spinorial LQG
Alejandro Perez, Centre de Physique Theorique, several including loop BH thermodynamics
Vincent Rivasseau, Universite Paris-Sud Orsay, tensor models
Carlo Rovelli, Centre de Physique Théorique, overview + QG stat mech/thermodynamics?
David Skinner, DAMPT Cambridge+IAS, N=8 supergravity?
Bill Unruh, University of British Columbia, analog models of QG?
Madhavan Varadarajan, Raman Research Institute, completing the LQG Hamiltonian approachNotes: About recently developed spinorial LQG among the work by Etera Livine see e.g. http://arxiv.org/abs/1302.7142
About computable LQG framework developed by Husain et al, http://arxiv.org/abs/1305.5203
Varadarajan's recent papers (solo, with Casey Tomlin, or with Alok Laddha) show progress towards completing the canonical LQG program: e.g. http://arxiv.org/abs/1210.6877 See also a recent video lecture http://pirsa.org/13040104/
The project of completing LQG Hamiltonian dynamics could also be called "closing the quantum constraint algebra". This corresponds classically to the hypersurface deformation algebra, which closes in GR. The snag which the Hamiltonian approach hit in the late 1990s seems essentially to have been that the quantum operator version of HD algebra did not close off shell.

As a reminder, here are some themes listed earlier:
cosmology/observational tests
black holes and thermodynamics
causal sets
shape dynamics
tensor models
holonomy spinfoams
spinorial LQG
closing constraint algebra

The most interesting recent papers which are not reflected in the announced speaker list are by
Freidel Hnybida (new basis for the intertwiners)
Daniele Pranzetti (broad synthesis of ideas from Connes Rovelli Perez Bianchi Wieland and others.)

The intertwiners are the "atoms" of spatial geometry in both canonical LQG and Spinfoams. It looks to me as if the Freidel et al paper http://arxiv.org/abs/1305.3326 takes a significant step forward in making sense of the intertwiners.
Pranzetti's http://arxiv.org/abs/1305.6714 "BH entropy from KMS states of QIH" puts all this stuff together in a remarkably cogent way. To me it is the closest thing we have, this season, to a LQG overview paper. But though it gathers many lines of development, of course it brings it all to bear on the BH entropy issue.

 

[marcus]

As I said Pranzetti's paper puts a lot of different LQG research threads together in a coherent way. Wieland's working with the selfdual Ashtekar variables (Immirzi = i, pure imaginary Immirzi parameter). Connes-Rovelli concept of thermal time---Tomita flow on *algebra---applied to Ashtekar-Lewandowski holonomy flux algebra, Bianchi's work on BH entropy, also Perez, Frodden, with whom Pranzetti has collaborated extensively. For a 10 page paper this is remarkably deep comprehensive and solid.

It is easier to follow if one also watches the Pirsa video talk (skip minutes 26-39 which is all audience hubbub with almost no Pranzetti input):
http://pirsa.org/12110064/
You can get this simply by googling "pirsa pranzetti".
Dynamical evaporation of quantum horizons
Speaker(s): Daniele Pranzetti
Abstract: We describe of the evaporation process as driven by the dynamical evolution of the quantum gravitational degrees of freedom resident at the horizon, as identified by the Loop Quantum Gravity kinematics. Using a parallel with the Brownian motion, we interpret the first law of quantum dynamical horizon in terms of a fluctuation-dissipation relation applied to this fundamental discrete structure. In this way, the horizon evolution is described in terms of relaxation to an equilibrium state balanced by the excitation of Planck scale constituents of the horizon. We show how from this setting the emergence of several conservative scenarios for the final stage of the evaporation process can be microscopically derived. Namely, the leakage of part of the horizon quantum geometry information prior to the Planckian phase and the stabilization of the hole surface shrinkage forming a massive remnant, which can eventually decay, are shown to take place.
8 November 2012

There are some 30 people in audience (Freidel, Sorkin, Smolin, Dittrich, Bianchi, Geloun, Bonzom,...) From minute 26 thru 39 there is intensive discussion by a number of people in the audience, with Pranzetti hardly able to get in a word edgewise. The microphone does not pick up the voices in the audience distinctly so one cannot follow their discussion. So one loses nothing by skipping over that segment. Another hubbub starts around minute 70. The presentation concludes at minute 79. But then there is a lively discussion by people in audience, Freidel and Sorkin especially, that continues until minute 96. Most of the audience is visible at minute 51:58--you can pause there. And also later e.g. 79:42 during the questions period.

One reason watching the talk helps is because he gives historical development and analogies. A real attempt is made to communicate to the Perimeter audience. The paper contains a lot more, possibly because it comes 6 months later and Pranzetti's work has advanced consderably in the interim, but also because he limited what he covered in the November talk to make an understandable presentation.
http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti
(Submitted on 29 May 2013)
By reintroducing Lorentz invariance via a complex connection formulation in canonical loop quantum gravity, we define a geometrical notion of temperature for quantum isolated horizons. Upon imposition of the reality conditions in the form of the linear simplicity constraints for an imaginary Barbero-Immirzi parameter, the exact formula for the temperature can be derived by demanding that the horizon state satisfying the boundary conditions be a KMS-state. In this way, our analysis reveals the connection between the passage to the Ashtekar self-dual variables and the thermality of the horizon. The horizon equilibrium state can then be used to compute both the von Neumann and the Boltzmann entropies. By means of a natural cut-off introduced by the topological theory on the boundary, we show that the two provide the same finite answer which allows us to recover the Bekenstein-Hawking formula in the semi-classical limit. The connection with Connes-Rovelli thermal time proposal for a general relativistic statistical mechanics is worked out.
10 pages, 1 figure

For me, since there has been no "LQG status report" or "survey/review paper" in the past 18 months, that I know of, this remarkable 10 page paper comes closest to meeting that need. It gives a coherent idea of various strands of Loop and Spinfoam research that have gotten interesting results during the past year and a half.

 

[marcus]

It's not easy for me to evaluate Pranzetti's paper. It is the most comprehensive synthesis of the new directions in LQG-Spinfoam research that I have seen, if it is right that's great, if there is some flaw then it still shows the kind of synthesis that must be made and that others can try to achieve. Either way this is an important paper and exemplifies what this thread has been about all along:

a new formulation of Loop and Spinfoam QG

in particular the new formulation should include the idea of temperature, should embody new insight into time, should re-envision the connection between Spinfoam and Hamiltonian approaches. I like Wolfgang Wieland's ideas about this last topic, so I automatically think of any new formulation as incorporating them, but I could be wrong (we also have alternative and in some way parallel developments by others).

To get a sense of perspective I should also note that Pranzetti just got his PhD in 2011 (at Marseille). He is only this year going into 2nd postdoc. His first PD was at Potsdam MPI and this Fall he moves to Erlangen. So the conventional indications are, I think, at odds with my judgment. Why do I think this paper is important when it is just a 10-pager by a youngster in his first postdoc? Nevertheless I do: it opens up interesting prospects.

 

[marcus]

Here are what I think are some core papers regarding a possible new formulation of LQG+SF that have appeared so far in 2nd quarter 2013. I have condensed the abstracts to facilitate overview:
http://arxiv.org/abs/1306.0861
Matrix Elements of Lorentzian Hamiltonian Constraint in LQG
Emanuele Alesci, Klaus Liegener, Antonia Zipfel
(Submitted on 4 Jun 2013)
... Here we evaluate the action of the full constraint, including the Lorentzian part. The computation requires... heavy use of SU(2) recoupling theory...
... these identities, together with the graphical calculus used to derive them, also simplify the Euclidean constraint and are of general interest in LQG computations.
36 pages.

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti
(Submitted on 29 May 2013)
By reintroducing Lorentz invariance via a complex connection formulation...we define a geometrical notion of temperature ... the exact formula ... can be derived by demanding that the horizon state ... be a KMS-state.
...reveals the connection between ... the Ashtekar self-dual variables and the thermality of the horizon.

The horizon equilibrium state ... used to compute both the von Neumann and the Boltzmann entropies. ...the two provide the same finite answer

which allows us to recover the Bekenstein-Hawking formula in the semi-classical limit.

The connection with Connes-Rovelli thermal time proposal for a general relativistic statistical mechanics is worked out.
10 pages, 1 figure

http://arxiv.org/abs/1305.3326
A Discrete and Coherent Basis of Intertwiners
Laurent Freidel, Jeff Hnybida
(Submitted on 15 May 2013)
We construct a new discrete basis of 4-valent SU(2) intertwiners. This basis...[is]... discrete, while at the same time representing accurately the classical degrees of freedom; hence ...[also] coherent. The closed spin network amplitude obtained from these intertwiners ... can be evaluated... The asymptotic limit of these amplitudes is found. ... Remarkably it gives a generalization of the Regge action to twisted geometries.
31 pages.

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski
(Submitted on 20 Apr 2013)
A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed...
... in the large spin regime, there is an equivalence between the spinfoam critical configuration... and a classical Lorentzian simplicial geometry. Such ... equivalence ... allows us to classify the ... critical configurations...
The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented ... critical configuration. It is shown that only at the globally oriented and time-oriented ... configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge... At all other (unphysical) critical configurations, ...large spin asymptotics modifies the Regge action...
36 pages

 

[marcus]

Invited plenary talks at next month's Loops 2013 conference can help give some idea of changes occurring in Loop gravity. Some of the abstracts are posted here:
http://www.perimeterinstitute.ca/conferences/loops-13
The abstract of Abhay Ashtekar's talk has a reference to lines from the Spanish poet Antonio Machado (1875-1939)
Caminante, son tus huellas
el camino, y nada más;
caminante, no hay camino,
se hace camino al andar.

Here is the abstract for Aurelien Barrau's talk:
Some possible ways to observe consequences of loop quantum gravity

In this talk, I'll briefly review some possible observational consequences of loop quantum gravity. I will first address the issue of the closure of the algebra of constraints in holonomy-corrected effective loop quantum cosmology for tensor, vector, and scalar modes. I will underline some unexpected features like a possible change of signature. The associated primordial power spectrum and the basics of the related CMB analysis will be presented. The "asymptotic silence" hypothesis will be mentioned as a promising alternative. Then, I'll address the issue of the probability for inflation and the prediction of its duration from a new perspective. Finally, I'll present some prospect about the evaporation of black holes in LQG.

In connection with Barrau's results on inflation, here's a recent paper:
http://arxiv.org/abs/1301.1264
Duration of inflation and conditions at the bounce as a prediction of effective isotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Revised 3 Jun 2013 (this version, v2))
Loop quantum cosmology with a scalar field is known to be closely linked with an inflationary phase. In this article, we study probabilistic predictions for the duration of slow-roll inflation, by assuming a minimalist massive scalar field as the main content of the universe. The phase of the field in its "prebounce" oscillatory state is taken as a natural random parameter. We find that the probability for a given number of inflationary e-folds is quite sharply peaked around 145, which is consistent with the most favored minimum values. In this precise sense, a satisfactory inflation is therefore a clear prediction of loop gravity. In addition, we derive an original and stringent upper limit on the Barbero-Immirzi parameter. The general picture of inflation, superinflation, deflation, and superdeflation is also much clarified in the framework of bouncing cosmologies.
7 pages, 7 figures

 

[marcus]

Abstracts have been posted for some of the Loops 2013 talks. In other cases a guess is offered as to the possible topic.

First, here are the talks for which abstracts are already posted:

Carlo Rovelli, Centre de Physique Théorique, overview What have we learned so far about quantum gravity?*
Aurelien Barrau, Universite Joseph Fourier, LQC observational consequences incl. adequate inflation*
Dafne Guetta, Braude College, LQG observational constraints Phenomenology with Neutrinos and high energy photons.*
Madhavan Varadarajan, Raman Research Institute, towards completing canonical LQG*
Abhay Ashtekar, Penn State, outlook. Promising paths for future research*.

*For the abstracts see menu at bottom of http://www.perimeterinstitute.ca/conferences/loops-13

And here are other listed speakers, with some guesses as to topic:
Ivan Agullo, DAMPT Cambridge, LQC bounce cosmology incl. pre-inflation.
Eugenio Bianchi, Perimeter Institute, LQG black hole thermodynamics
Steve Carlip, UC Davis, TBA
Fay Dowker, Imperial College London, causal sets
Henrique Gomes, UC Davis, shape dynamics
Razvan Gurau, Université Paris-Sud, tensor models
Frank Hellmann, MPI for Gravitational Physics Potsdam, holonomy spinfoams
Viqar Husain, University of New Brunswick, computable LQG framework
Kirill Krasnov, University of Nottingham, pure connection gravity
Etera Livine, Ens de Lyon, several possibilities including spinorial LQG
Alejandro Perez, Centre de Physique Theorique, several including loop BH thermodynamics
Vincent Rivasseau, Universite Paris-Sud Orsay, tensor models
David Skinner, DAMPT Cambridge+IAS, N=8 supergravity?
Bill Unruh, University of British Columbia, analog models of QG?Notes:
Krasnov recent pure connection formulation, http://arxiv.org/abs/1304.6946
Etera Livine recent spinorial LQG, http://arxiv.org/abs/1302.7142
Husain computable LQG framework,http://arxiv.org/abs/1305.5203
Varadarajan recent papers progress towards completing canonical LQG: e.g. http://arxiv.org/abs/1210.6877 http://pirsa.org/13040104/
The project of completing LQG Hamiltonian dynamics could also be called "closing the quantum constraint algebra". This corresponds classically to the hypersurface deformation algebra, which closes in GR. The snag which the Hamiltonian approach hit in the late 1990s seems essentially to have been that the quantum operator version of HD algebra did not close off shell. Dittrich and Bonzom have a paper about the HD algebra.
Alesci et al have also made recent progress towards completing LQG Hamiltonian dynamics. http://arxiv.org/abs/1306.0861

Reminders to myself:
cosmology/observational tests
black holes and thermodynamics
causal sets
shape dynamics
tensor models
holonomy spinfoams
spinorial LQG
spin nets (simpler analogs of spin foams using finite group instead of full SU(2) )
closing constraint algebra

Some things I don't see listed:
Marcolli Suijlekom (beautiful work on "gauge networks": graphs labeled with chunks of spectral geometry)
Freidel Hnybida (new basis for the intertwiners http://arxiv.org/abs/1305.3326)
Daniele Pranzetti (broad synthesis of ideas from Connes Rovelli Perez Bianchi Wieland and others. http://arxiv.org/abs/1305.6714 But I think his work may be covered in survey by Alejandro Perez with whom he has co-authored.)
Bianca Dittrich (http://arxiv.org/abs/1306.2987 "Coarse graining of spin net models: dynamics of intertwiners")

The intertwiners are the "atoms" of spatial geometry in both canonical LQG and Spinfoams.
Marcolli Suijlekom also has intertwiners at the nodes but based on spectral triples of NCG
The spin nets that Dittrich et al use are simplified (e.g. finite group) analogs of spin foams not meant to replace them but to facilitate progress understanding them. Dittrich is one of the conference organizers. I don't know what conference etiquette requires. The research on spin nets as "toy version" spinfoams seems potentially important and appropriate for plenary session. Maybe Dittrich can hand it off to one of her co-authors.

 

[marcus]

Talks given at a major conference can give an idea of what areas are active in LQG research. My main focus has been to identify themes emerging as important at Loops 2013 that runs 22-26 July at Perimeter. That's what the preceding post is about. However in a couple of weeks we also have a Loops session of the Warsaw GR20 conference coming up as a kind of opener for the main conference.
Here's a list of 36 talks scheduled for Session D1, organized/chaired by Dittrich and Pawlowski. You can see many of the same lines of investigation as appeared in the Perimeter conference lineup:

Dr. Ivan Agullo A quantum gravity extension of the inflationary scenario
Dr. Emanuele Alesci Quantum Reduced Loop Gravity
Prof. Abhay Ashtekar Loop Quantum Cosmology: Fundamentals and Phenomenology
Mehdi Assanioussi A quantum Ricci operator for LQG
Dr. Fernando Barbero Geometric Constraint algorithm for field theories with boundaries.
Dr. Eugenio Bianchi Black hole entropy and entanglement in spinfoam gravity
Dr. Francesco Cianfrani Introduction to Quantum Reduced Loop Gravity for cosmology
MSc. Andrea Dapor QFT on quantum spacetime
Dr. Jonathan Engle Quantum isotropy and dynamical quantum symmetry reduction
Mikel Fernández-Méndez An Inflationary Model in Loop Quantum Cosmology
Dr. Ernesto Frodden On the Quasilocal First Law for Isolated Horizon and its uses in the Euclidean Partition Function
Prof. Kristina Giesel Scalar Material Reference Systems and Loop Quantum Gravity
Brajesh Gupt Quantum gravitational inflationary scenario in Bianchi-I spacetime
Brajesh Gupt Chimera: A hybrid numerical scheme for isotropic loop quantum cosmology
Dr. Hal Haggard Pentahedral volume, chaos, and quantum gravity
Prof. Viqar Husain Time and a physical hamiltonian for quantum gravity
Dr. Wojciech Kaminski Curvature constraints in spin foam models
MSc. Marcin Kisielowski The Dipole Cosmology transition amplitude: first-order contributions
Dr. Tim Koslowski Shape Dynamics and Quantum Gravity
Linda Linsefors Duration of inflation as a prediction of effective LQC
Prof. Yongge Ma Connection dynamics of a gauge theory of gravity coupled with matter
Abhishek Majhi Microcanonical Entropy of Isolated Horizon and fixation of the Barbero-Immirzi parameter
Seth Major On Loop Quantization of Plane Gravitational Waves
Dr. Jakub Mielczarek Asymptotic silence in quantum gravity
MSc. Edison Montoya Qualitative Effective Dynamics in Bianchi IX Loop Quantum Cosmology
MSc. Jacek Puchta Asymptotic behaviour of lorentzian polyhedra propagator
Prof. Jorge Pullin Complete quantization of vacuum spherically symmetric gravity
Dr. li qin Coherent state functional integrals in quantum cosmology
Dr. Saeed Rastgoo An analysis of the CGHS model in new variables
Prof. Carlo Rovelli Radiative corrections in covariant Loop Quantum Gravity
Jędrzej Świeżewski Construction of Dirac observables for General Relativity with the use of geometry
MSc. Sara Tavares Observables in two-dimensional BF theory
Madhavan Varadarajan Anomaly free constraint algebra for a weak coupling limit of gravity
Dr. jingbo wang The entropy of BTZ black hole from loop quantum gravity
MSc. Wolfgang Wieland Hamiltonian Spinfoam Gravity
Antonia Zipfel On the relation between canonical and covariant Quantum Gravity
========================
List of parallel sessions:
http://gr20-amaldi10.edu.pl/index.php?id=18
List of talks in each parallel session:
http://gr20-amaldi10.edu.pl/index.php?id=32
Loop folk have branched out and besides session D1 (their main session) are also giving talks in sessions A3, A4, D3, D4, and the special joint session D1+2+4

 

[marcus]

Gravi-weak unification at ILQGS (online talk by Marciano)

Loop gravity is making a strong showing at Warsaw G20. This is a conference with over 900 registered participants covering many types of research: gravity wave, dark matter search, numerical GR, observational cosmology, and of course quantum gravity. The public lecture this time will be given by Carlo Rovelli and will be about major questions and developments in QG. Loop folks have gotten into a bunch of other sessions besides their main one D1. They are also giving talks in A3, A4, D3, D4, and a joint session on BH evaporation (e.g. the "firewall" issue) that brings together people from D1+D2+D4 (loop+string+QFTcurvedspacetime...). I will try to identify the themes emerging from just the talks in session D1.
Here's a list of 36 talks scheduled for Session D1.
3 I see engaged in joining LQC and LQG, and
3 focused on BH thermodynamics.
5 talks can be expected to focus on the LQC bounce and inflation. The bounce turns out to be a straightforward model ensuring sufficient inflation (without resort to fine-tunes and multiverses).
7 I see as focused on completing the program--getting a real Hamiltonian, combining the canonical and spinfoam approaches, showing convergence and wellness of definition. This seems to be a time when a lot of people are making progress on that.

Dr. Ivan Agullo A quantum gravity extension of the inflationary scenario bounce inflation
Dr. Emanuele Alesci Quantum Reduced Loop Gravity join LQC to LQG
Prof. Abhay Ashtekar Loop Quantum Cosmology: Fundamentals and Phenomenology bounce inflation + testing + overview
Mehdi Assanioussi A quantum Ricci operator for LQG
Dr. Fernando Barbero Geometric Constraint algorithm for field theories with boundaries.
Dr. Eugenio Bianchi Black hole entropy and entanglement in spinfoam gravity BH thermo
Dr. Francesco Cianfrani Introduction to Quantum Reduced Loop Gravity for cosmology join LQC to LQG
MSc. Andrea Dapor QFT on quantum spacetime completing the program, with matter
Dr. Jonathan Engle Quantum isotropy and dynamical quantum symmetry reduction join LQC to LQG
Mikel Fernández-Méndez An Inflationary Model in Loop Quantum Cosmology bounce inflation
Dr. Ernesto Frodden On the Quasilocal First Law for Isolated Horizon and its uses in the Euclidean Partition Function BH thermo
Prof. Kristina Giesel Scalar Material Reference Systems and Loop Quantum Gravity complete the program with dust
Brajesh Gupt Quantum gravitational inflationary scenario in Bianchi-I spacetime bounce inflation
Brajesh Gupt Chimera: A hybrid numerical scheme for isotropic loop quantum cosmology
Dr. Hal Haggard Pentahedral volume, chaos, and quantum gravity validation
Prof. Viqar Husain Time and a physical hamiltonian for quantum gravity completing the program, with dust
Dr. Wojciech Kaminski Curvature constraints in spin foam models
MSc. Marcin Kisielowski The Dipole Cosmology transition amplitude: first-order contributions
Dr. Tim Koslowski Shape Dynamics and Quantum Gravity
Linda Linsefors Duration of inflation as a prediction of effective LQC bounce inflation, no multiverse, coauthor Barrau
Prof. Yongge Ma Connection dynamics of a gauge theory of gravity coupled with matter
Abhishek Majhi Microcanonical Entropy of Isolated Horizon and fixation of the Barbero-Immirzi parameter BH thermo
Seth Major On Loop Quantization of Plane Gravitational Waves
Dr. Jakub Mielczarek Asymptotic silence in quantum gravity
MSc. Edison Montoya Qualitative Effective Dynamics in Bianchi IX Loop Quantum Cosmology
MSc. Jacek Puchta Asymptotic behaviour of lorentzian polyhedra propagator
Prof. Jorge Pullin Complete quantization of vacuum spherically symmetric gravity
Dr. Li Qin Coherent state functional integrals in quantum cosmology
Dr. Saeed Rastgoo An analysis of the CGHS model in new variables
Prof. Carlo Rovelli Radiative corrections in covariant Loop Quantum Gravity completing the program
Jędrzej Świeżewski Construction of Dirac observables for General Relativity with the use of geometry
MSc. Sara Tavares Observables in two-dimensional BF theory
Madhavan Varadarajan Anomaly free constraint algebra for a weak coupling limit of gravity completing the program
Dr. Jingbo Wang The entropy of BTZ black hole from loop quantum gravity BH thermo
MSc. Wolfgang Wieland Hamiltonian Spinfoam Gravity completing the program
Antonia Zipfel On the relation between canonical and covariant Quantum Gravity completing the program, co-author Alesci
========================
List of parallel sessions:
http://gr20-amaldi10.edu.pl/index.php?id=18
List of talks in each parallel session:
http://gr20-amaldi10.edu.pl/index.php?id=32