Reformulation of Loop gravity in progress, comment?

marcus

For an up-to-date summary of current definitions, main results, and important open problems see Rovelli's July 2012 Stockholm slides:
http://www.cpt.univ-mrs.fr/~rovelli/...lmSpinFoam.pdf

A specialized talk about LQG black hole thermodynamics, clarifying how to think about the recent results and identifying open questions:
http://www.cpt.univ-mrs.fr/~rovelli/...kholmTermo.pdf

marcus

Incidentally at the science level, the organizers of Loops 2013 are Dittrich, Freidel, Smolin. It will be enlightening to see how they sort out the topics and evalutate directions in current Loop research, as they construct the program and arrange the plenary talks and parallel sessions.
http://www.perimeterinstitute.ca/en/...s_13/Loops_13/
Also I would suggest anyone who hasn't seen it check out Dittrich Freidel Smolin's listing of their International Advisory Committee at the Loops 13 webpage.
Since I haven't done this in a while, I'll update the list of potential reformulation topics I'm watching.

PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.
http://www-library.desy.de/cgi-bin/s...tecount%28d%29
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1209.1609 (A Quantum Gravity Extension of the Inflationary Scenario)
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1204.1288 (Perturbations in loop quantum cosmology)
Artymowski, Dapor, Pawlowski http://arxiv.org/abs/1207.4353 (Inflation from non-minimally coupled scalar field in loop quantum cosmology)
By various of the following: Barrau, Grain, Cailleteau, Vidotto, Mielczarek
http://arxiv.org/abs/1206.6736 (Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology)
http://arxiv.org/abs/1206.1511 (Loop quantum cosmology in the cosmic microwave background)
http://arxiv.org/abs/1111.3535 (Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology)
http://arxiv.org/abs/1011.1811 (Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background)
http://arxiv.org/abs/1003.4660 (Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves)

Holonomy Spin Foam Models Spinfoam labels can be group elements. Finite groups are introduced. Some sample papers, some out, some in preparation:
Bahr, Dittrich,Hellmann, Kaminski http://arxiv.org/abs/1208.3388 (Holonomy Spin Foam Models: Definition and Coarse Graining)
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two-complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods, which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
[28] Same authors (Holonomy Spin Foam Models: Boundary Hilbert spaces and canonical dynamics, 2012, in prep) .
[29]Hellmann, Kaminski (Holonomy Spin Foam Models: Asymptotic dynamics of EPRL type models, 2012, in prep) .
For background, e.g. the transfer operator concept in spinfoam context:
Bahr, Dittrich, Ryan http://arxiv.org/abs/1103.6264 (Spin foam models with finite groups)
In what I think may be a related development Lewandowski's Warsaw group has a way to systematically ENUMERATE and compute spinfoam histories. They stack up successive spin network states of geometry and join them into a single history.
http://arxiv.org/abs/1107.5185

TwistorLQG Papers by Freidel, Livine, Dupuis, Speziale, Wieland... For example Speziale and Wieland http://arxiv.org/abs/1207.6348(The twistorial structure of loop-gravity transition amplitudes)

FreeImmirzi and Geometric Operator Spectra
Bianchi, Haggard http://arxiv.org/abs/1208.2228 (Bohr-Sommerfeld Quantization of Space)
We introduce semiclassical methods into the study of the volume spectrum in loop gravity. ... The analysis shows a remarkable quantitative agreement with the volume spectrum computed in loop gravity.
Also http://arxiv.org/abs/1204.5122

Tetrad-handedness The Tetrad's sign could start to be included both in the classical theory upon which Loop gravity is based and in the quantum theory. Papers by Rovelli and others raise the issue: should the sign be included? If so, in which of two possible ways? How would this affect the quantum theory?
http://arxiv.org/abs/1205.0733
http://arxiv.org/abs/1207.5156

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes observers and measurement--focusing on things we care about and want to predict or bet on happening. Histories are partitioned according to these concerns and a decoherence functional is defined on the partitions telling when sets are sufficiently independent to have ordinary probabilities.
http://arxiv.org/abs/gr-qc/0602013

Relativity and Thermodynamics/Statistical Mechanics of geometry is the theme of some recent papers by Rovelli, Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) the equation of state describing overall behavior of microscopic variables (like the vast number of gas molecules whose collective behavior is summarized by PV = NkT.) If GR is the equation of state, what are the underlying degrees of freedom? Do spinfoams describe the underlying degrees of freedom for which EFE is the EoS?
http://arxiv.org/abs/1204.6349 http://arxiv.org/abs/1205.5529 http://arxiv.org/abs/1207.0505
Rovelli has a new paper in this connection. Just came out.
http://arxiv.org/abs/1209.0065
General relativistic statistical mechanics
(Submitted on 1 Sep 2012)
Understanding thermodynamics and statistical mechanics in the full general relativistic context is an open problem. I give tentative definitions of equilibrium state, mean values, mean geometry, entropy and temperature, which reduce to the conventional ones in the non-relativistic limit, but remain valid for a general covariant theory. The formalism extends to quantum theory. The construction builds on the idea of thermal time, on a notion of locality for this time, and on the distinction between global and local temperature. The last is the temperature measured by a local thermometer, and is given by kT = hbar dτ/ds, with k the Boltzmann constant, hbar the Planck constant, ds proper time and d tau the equilibrium thermal time.
Comments: A tentative second step in the thermal time direction, 10 years after the paper with Connes. The aim is the full thermodynamics of gravity. The language of the paper is a bit technical: look at the Appendix first

Dust is shorthand for the various approaches being used to recover a real physical Hamiltonian. Members of both the Erlangen and Warsaw groups have research along several related lines. This is familiar from cosmology and I think it's of considerable practical value.
http://arxiv.org/abs/1206.3807 http://arxiv.org/abs/1206.0658

marcus

In the preceding I listed 8 frontier categories, look back to see definitions and links to sample research. The third was too narrow and should be made to include Tensorial GFT. I have enlarged it here and tentatively call it Algebraic generalizations. Here is the revised stripped down list.

PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.

Holonomy Spin Foam Models Spinfoam labels can be group elements. Finite groups are introduced. http://arxiv.org/abs/1208.3388 and references.

Algebraic generalizations: tensorGFT and twistorLQG See twistorLQG papers by Freidel, Livine, Dupuis, Speziale, Wieland... For example Speziale and Wieland http://arxiv.org/abs/1207.6348(The twistorial structure of loop-gravity transition amplitudes) For tensorial GFT see review by Razvan Gurau and references therein. http://arxiv.org/abs/1209.3252 (A review of the 1/N expansion in random tensor models)

FreeImmirzi and Geometric Operator Spectra Remarkable agreement of Loop with Bohr-Somerfeld quantization of geometry. immirzi provisionally let free to vary, which could have unforeseen consequences.

Tetrad-handedness The Tetrad's sign could start to be included both in the classical theory upon which Loop gravity is based and in the quantum theory.

Histories refers primarily to Hartle's treatment of quantum mechanics which de-emphasizes the classical observer and measuring device. Might be applicable to spinfoam dynamics.

Relativity and Thermodynamics/Statistical Mechanics of geometry is the theme of some recent papers by Rovelli, Jacobson, Smolin, Padmanabhan and others. Could the Einstein GR equation be (like PV = NkT) an equation of state?

Dust here is shorthand for the various approaches being used to recover time-evolution and a real physical Hamiltonian.

Progress occurring on some or all of these fronts could be expected to show up in the program at Loops 2013:
http://www.perimeterinstitute.ca/en/...s_13/Loops_13/
or possibly earlier at the GR20 meeting in Warsaw.
http://gr20-amaldi10.edu.pl

atyy

There's a direction I'd like to see, but that doesn't seem represented: what is the relationship between the two "visions" of classical general relativity emerging from LQG? The most definite sign is that the large j and small immirzi limit seems to give the Regge action. This is still dangerous because we know that depending on how the Regge action is "interpreted", results could be terrible (DT) or promising (CDT). The question is what interpretation does EPRL imply?

Dittrich is using tensor-network tools to try to coarse grain and get classical GR. Intuitively, that is more sensible. However, that does seem to ignore the clues from the large j limit - is she thinking that's a red herring, or is she secretly keeping that in mind?

marcus

As you know both DT and CDT deal with only a severely limited space of geometries which are constructed by sticking identical blocks together, either all the same shape and size or of two different types. This does not recover the full Regge theory. The Triangulation people HOPE that this limited space of geometries is somehow representative of the full range found in nature.

In assembling identical block they have found that problems can develop---on the one hand feathery structures form, not compact enough, and on the other hand structures that are too compact can form: a kind of dog-pile with too many blocklets adjacent to each other.

The full Regge calculus does not use identical blocklets. It uses what amounts to a lattice of variable length rods. The dynamics involves the lengths of the rods. I never heard of Regges type of GR having the pathologies of DT, and don't see how it even could have (though of course I might be wrong.)

Anyway, Atyy, I do not consider DT and CDT as being alternate "interpretations" of Regge. So of course I did not understand your post. I do not think that Loop faces a fork in the road between two "interpretations": a terrible (DT) and a promising (CDT).

However if you think it does face such a fateful fork in the road, perhaps you should write an email and tell one of the experts about it. Dittrich, Rovelli, Thiemann. Surely if there is some "danger" which they have overlooked they should be warned about it.
==============

My view is that the community is a substantial body of highly intelligent people who are alert to just about every potential problem connected to the approaches they are working on. I've watched the Loop approach evolve for almost 10 years now and they have repeatedly broken down roadblocks and surmounted obstacles, or found a way around them. There are certainly a lot of open problems---as Ashtekar recently said there are enough problems to keep the young researchers happily occupied for years to come. I think the community is always on the lookout for problems, and habitually goes after them vigorously. So it's exciting to watch. I don't expect Loop to stay vintage 2010 EPRL, I think it's already changing. But I do not know what the new shape will be and so am in suspense.
===============

Convergence is probably a central problem that several of the developments I listed are addressing. Certainly the "tetrad-handedness" work is aimed at that. But also "dust" because everything is much simpler when you have time and a real Hamiltonian. And I vaguely suspect that Dittrich group's "holonomy spin foam" enterprise is going to take a swipe at the convergence problem, or at least the classical limit via coarse-graining. I'm not clear about this, it's just my two-bit hunch. You may have some ideas about the holonomy spinfoam business.

marcus

Revised list of categories, with one omitted, look back to see definitions and links to sample research.
PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.

Holonomy Spin Foam Models Spinfoam labels can be group elements. Finite groups. Kinship with lattice gauge theory. http://arxiv.org/abs/1208.3388 and references therein. New paper this week: http://arxiv.org/abs/1209.4539 .

Algebraic generalizations: tensorGFT and twistorLQG

FreeImmirzi and Geometric Operator Spectra

Tetrad-handedness

Relativity and Thermodynamics/Statistical Mechanics GR could be the equation of state and LQG the "molecules" (microscopic degrees of freedom).

Dust various means to recover time-evolution and real physical Hamiltonian.

Progress on any of these fronts could show up in the program at Loops 2013 and Warsaw GR20:
http://www.perimeterinstitute.ca/en/...s_13/Loops_13/
http://gr20-amaldi10.edu.pl
===================
Perhaps the most notable development in this connection this week was the appearance of a new Holonomy Spin Foam paper by Dittrich et al. Ill get the link.
http://arxiv.org/abs/1209.4539
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 20 Sep 2012)
In this and the companion paper a novel holonomy formulation of so called Spin Foam models of lattice gauge gravity are explored. After giving a natural basis for the space of simplicity constraints we define a universal boundary Hilbert space, on which the imposition of different forms of the simplicity constraints can be studied. We detail under which conditions this Hilbert space can be mapped to a Hilbert space of projected spin networks or an ordinary spin network space.
These considerations allow to derive the general form of the transfer operators which generates discrete time evolution. We will describe the transfer operators for some current models on the different boundary Hilbert spaces and highlight the role of the simplicity constraints determining the concrete form of the time evolution operators.
51 pages, 18 figures

The companion paper referred to here appeared last month:
http://arxiv.org/abs/1208.3388
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 16 Aug 2012)
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two-complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods, which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
36 pages, 12 figures.

atyy

They are well aware of it. http://arxiv.org/abs/1204.5394

(I corrected the link.)

marcus

==quote Atyy post #106==
There's a direction I'd like to see, but that doesn't seem represented: what is the relationship between the two "visions" of classical general relativity emerging from LQG? The most definite sign is that the large j and small immirzi limit seems to give the Regge action. This is still dangerous because we know that depending on how the Regge action is "interpreted", results could be terrible (DT) or promising (CDT). The question is what interpretation does EPRL imply?

Dittrich is using tensor-network tools to try to coarse grain and get classical GR. Intuitively, that is more sensible. However, that does seem to ignore the clues from the large j limit - is she thinking that's a red herring, or is she secretly keeping that in mind?
==endquote==

==quote me, post #107==
As you know both DT and CDT deal with only a severely limited space of geometries which are constructed by sticking identical blocks together, either all the same shape and size or of two different types. This does not recover the full Regge theory. The Triangulation people HOPE that this limited space of geometries is somehow representative of the full range found in nature.

In assembling identical block they have found that problems can develop---on the one hand feathery structures form, not compact enough, and on the other hand structures that are too compact can form: a kind of dog-pile with too many blocklets adjacent to each other.

The full Regge calculus does not use identical blocklets. It uses what amounts to a lattice of variable length rods. The dynamics involves the lengths of the rods. I never heard of Regges type of GR having the pathologies of DT, and don't see how it even could have (though of course I might be wrong.)

Anyway, Atyy, I do not consider DT and CDT as being alternate "interpretations" of Regge. So of course I did not understand your post. I do not think that Loop faces a fork in the road between two "interpretations": a terrible (DT) and a promising (CDT).

However if you think it does face such a fateful fork in the road, perhaps you should write an email and tell one of the experts about it. Dittrich, Rovelli, Thiemann. Surely if there is some "danger" which they have overlooked they should be warned about it.

My view is that the community is a substantial body of highly intelligent people who are alert to just about every potential problem connected to the approaches they are working on. I've watched the Loop approach evolve for almost 10 years now and they have repeatedly broken down roadblocks and surmounted obstacles, or found a way around them. There are certainly a lot of open problems---as Ashtekar recently said there are enough problems to keep the young researchers happily occupied for years to come. I think the community is always on the lookout for problems, and habitually goes after them vigorously. So it's exciting to watch. I don't expect Loop to stay vintage 2010 EPRL, I think it's already changing. But I do not know what the new shape will be and so am in suspense.
==endquote==

==quote Atyy, post#109 ===
They are well aware of it. http://arxiv.org/abs/1207.4596
==endquote==
http://arxiv.org/abs/1207.4596
The Construction of Spin Foam Vertex Amplitudes
Eugenio Bianchi, Frank Hellmann
(Submitted on 19 Jul 2012 (v1), last revised 21 Jul 2012 (this version, v2))
Spin foam vertex amplitudes are the key ingredient of spin foam models for quantum gravity. They fall into the realm of discretized path integral, and can be seen as generalized lattice gauge theories. They can be seen as an attempt at a 4 dimensional generalization of the Ponzano-Regge model for 3d quantum gravity. We motivate and review the construction of the vertex amplitudes of recent spin foam models, giving two different and complementary perspectives of this construction. The first proceeds by extracting geometric configurations from a topological theory of the BF type, and can be seen to be in the tradition of the work of Barrett and Crane and Freidel and Krasnov. The second keeps closer contact to the structure of Loop Quantum Gravity and tries to identify an appropriate set of constraints to define a Lorentz-invariant interaction of its quanta of space. This approach is in the tradition of the work of Smolin, Markopoulous, Engle, Pereira, Rovelli and Livine.
22 Pages. 1 Figure. I

==quote Atyy, post#109 ===
They are well aware of it. http://arxiv.org/abs/1204.5394

(I corrected the link.)
==endquote==
http://arxiv.org/abs/1204.5394
Discrete Gravity Models and Loop Quantum Gravity: a Short Review
Maite Dupuis, James P. Ryan, Simone Speziale
(Submitted on 24 Apr 2012 (v1), last revised 13 Aug 2012 (this version, v2))
We review the relation between Loop Quantum Gravity on a fixed graph and discrete models of gravity. We compare Regge and twisted geometries, and discuss discrete actions based on twisted geometries and on the discretization of the Plebanski action. We discuss the role of discrete geometries in the spin foam formalism, with particular attention to the definition of the simplicity constraints.
31 pages.

marcus

I've listed 7 or so lines of development that bear watching but if I had to focus on one to see where it is going over the next 10 months, I believe I would pick Holonomy Spin Foam models. My hunch is it will produce the most by way of unexpected new stuff. I'm talking about the short term: results that will show up in the talks at Loops 2013 and Warsaw GR20:
http://www.perimeterinstitute.ca/en/...s_13/Loops_13/
http://gr20-amaldi10.edu.pl

A good brief introduction to HSF models is contained in the first part of this online seminar talk by Frank Hellmann:

http://relativity.phys.lsu.edu/ilqgs/hellmann090412.pdf
http://relativity.phys.lsu.edu/ilqgs/hellmann090412.wav
Holonomy Spin Foam Models: Asymptotic Dynamics

And here are the papers I was discussing a few posts back:
Incidentally here is a YouTube in which one of the authors is singing the young theoretical physicist song:
http://www.youtube.com/watch?v=FPw-q8RYucE

A point to make is that Lattice Gauge Theory is a large well-developed and established body of mathematical methods, and they are extending LGT in a way that the lattice geometry can vary so as to include gravity.
And moreover it looks like their generalized or extended LGT is able to contain many of the spinfoam models which have been defined by Quantum Relativists, including EPRL, as special cases within a single group-labeled 2-complex format.
This is somehow the way that mathematical evolution ought to go. From having several comparatively ad hoc and partially successful theories, evolution moves towards a single less ad hoc more comprehensive theory that contains them---also evolution is towards more structural assumptions and fewer adjustable parameters (which incidentally makes a theory more firmly testable). This feels right as a direction to move in. And also it feels right to connect up with an already well-developed body of method like LGT. And that means having the 2-complexes be labeled by GROUP elements, rather than with spin or representation labels. I think. If they can make all this work then it seems (to my dim eyes) like the way to go.

For additional light on this, I think we should also check the 2011 (and perhaps earlier) posts by longtime PF member "f-h" in case there is anything relevant to the present situation. My impression is the posts are informative, coolly objective, and to the point regarding the QG research picture.
http://physicsforums.com/search.php?searchid=3391085
Francesca sometimes takes part in the same threads and gives a valuable second perspective.

marcus

A Dittrich Ryan paper just went on arxiv that will probably turn out to be quite important.
I wouldn't be surprised if Bianca Dittrich gives a seminar talk about it at Perimeter or discusses it when she has her ILQGS talk on 27 November. The title of that November seminar talk is still TBA.

This paper could have consequences, or so it seems to me. I'd like to hear others' comments. Here's the abstract:

http://arxiv.org/abs/1209.4892
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
Bianca Dittrich, James P. Ryan
(Submitted on 21 Sep 2012)
The 1-parameter family of transformations identified by Barbero and Immirzi plays a significant role in non-perturbative approaches to quantum gravity, among them Loop Quantum Gravity and Spin Foams. It facilitates the loop quantization programme and subsequently the Barbero-Immirzi parameter (gamma) arises in both the spectra of geometrical operators and in the dynamics provided by Spin Foams. However, the debate continues as to whether quantum physics should be Barbero-Immirzi parameter dependent. Starting from a discrete SO(4)-BF theory phase space, we find two possible reductions with respect to a discrete form of the simplicity constraints. The first reduces to a phase space with gamma-dependent symplectic structure and more generally in agreement with the phase space underlying Loop Quantum Gravity restricted to a single graph - a.k.a. Twisted Geometries. The second, fuller reduction leads to a gamma-independent symplectic structure on the phase space of piecewise-flat-linear geometries - a.k.a. Regge geometries. Thus, the gamma-dependence of physical predictions is related to the choice of phase space underlying the quantization.
16 + 12 pages

marcus

Anyone following the research of Dittrich and her co-authors probably has already watched or might wish to watch this February 2012 Perimeter video lecture.
http://pirsa.org/12020142/
Coarse graining spin nets with tensor networks

The first slide gives motivation:
Spin foam models--candidates for quantum gravity--give (very) small scale physics.
Most important question: what do they describe at large scales?
Spin foams can be understood as lattice systems:
--Use coarse graining to construct effective models for larger scales.
--Problem: spin foam models for gravity have amazingly complicated amplitudes. No coarse graining methods available. Simplify models drastically, keep "spin foam construction principle", develop and test coarse graining methods.

A "spin net" is analogous to a spin foam but dimensionally reduced. Edges take the place of 2D plaquettes.
This is part of the drastic simplification used in this exploratory research. Subsequently, as we have seen, they work back up to "holonomy spin foam" models. Which are spin foams where the labels are elements g of a group G instead of spins and the like.

refers to http://arxiv.org/abs/1109.4927
http://arxiv.org/abs/1111.0967 (shorter version)
==quote page 3 of 1109.4927==
Spin foams are a particular class of lattice gauge models (see e.g. [63] for a recent review and [11] for a review emphasizing the relation to lattice gauge and statistical physics models). Such models are specified by variables, taking values in some group G, associated to the edges of a lattice (or more generally an oriented 2–complex) and weights associated to the plaquettes. They can thus also be termed plaquette models.
A related class of models, which will be introduced below, are so called edge or spin net models [11]. Here group variables are associated to the vertices of a lattice (or more generically an oriented graph or 1–complex) and weights to the edges. This class includes the well–known Ising models, based on the group Z2. Indeed it will turn out that the structures involved in a spin net model are very similar to those involved in spin foam models – just that where, for instance, weights are associated to 2D plaquettes for spin foams, weights are associated to 1D edges in spin nets, similarly for the group variables and so on. In this sense spin nets are a simpler or dimensionally reduced form of spin foams...
==endquote==

marcus

Narrowing down the areas we are watching will require taking some good and important work off the list, but I want to focus on just a few fronts where I think the development occurring could significantly change LQG in the near term. With some exceptions I will, for brevity, mention only one or two key papers in each category. Others could have been cited and were mentioned earlier in the thread.

PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.
http://www-library.desy.de/cgi-bin/s...tecount%28d%29
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1209.1609 (A Quantum Gravity Extension of the Inflationary Scenario)
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1204.1288 (Perturbations in loop quantum cosmology)
Artymowski, Dapor, Pawlowski http://arxiv.org/abs/1207.4353 (Inflation from non-minimally coupled scalar field in loop quantum cosmology)
Many more papers identifying observable effects, by various author: Barrau, Grain, Cailleteau, Vidotto, Mielczarek, and others.

Group-labeled Spinfoams Spinfoam labels can be group elements. Finite groups are introduced. This could change the way LQG is formulated.
Bahr, Dittrich,Hellmann, Kaminski http://arxiv.org/abs/1208.3388 (Holonomy Spin Foam Models: Definition and Coarse Graining)
We propose a new holonomy formulation for spin foams, which naturally extends the theory space of lattice gauge theories. This allows current spin foam models to be defined on arbitrary two-complexes as well as to generalize current spin foam models to arbitrary, in particular finite groups. The similarity with standard lattice gauge theories allows to apply standard coarse graining methods, which for finite groups can now be easily considered numerically. We will summarize other holonomy and spin network formulations of spin foams and group field theories and explain how the different representations arise through variable transformations in the partition function. A companion paper will provide a description of boundary Hilbert spaces as well as a canonical dynamic encoded in transfer operators.
Same authors http://arxiv.org/abs/1209.4539 (Holonomy Spin Foam Models: Boundary Hilbert spaces and canonical dynamics) .
Hellmann, Kaminski (Holonomy Spin Foam Models: Asymptotic dynamics of EPRL type models, in prep) .
For background, e.g. the transfer operator concept in spinfoam context:
Bahr, Dittrich, Ryan http://arxiv.org/abs/1103.6264 (Spin foam models with finite groups)

Immirzi Issues
Bianchi http://arxiv.org/abs/1204.5122 (Entropy of Non-Extremal Black Holes from Loop Gravity)
Dittrich, Ryan http://arxiv.org/abs/1209.4892 (On the role of the Barbero-Immirzi parameter in discrete quantum gravity)

Relativistic Thermodynamics/Statistical Mechanics of Geometry
Rovelli has a new paper out.
http://arxiv.org/abs/1209.0065
General relativistic statistical mechanics
(Submitted on 1 Sep 2012)
Understanding thermodynamics and statistical mechanics in the full general relativistic context is an open problem. I give tentative definitions of equilibrium state, mean values, mean geometry, entropy and temperature, which reduce to the conventional ones in the non-relativistic limit, but remain valid for a general covariant theory. The formalism extends to quantum theory. The construction builds on the idea of thermal time, on a notion of locality for this time, and on the distinction between global and local temperature. The last is the temperature measured by a local thermometer, and is given by kT = hbar dτ/ds, with k the Boltzmann constant, hbar the Planck constant, ds proper time and d tau the equilibrium thermal time.
9 pages. A tentative second step in the thermal time direction, 10 years after the paper with Connes. The aim is the full thermodynamics of gravity. The language of the paper is a bit technical: look at the Appendix first

Observer Space
Gielen, Wise http://arxiv.org/abs/1206.0658 (Linking Covariant and Canonical General Relativity via Local Observers)
See Derek Wise's ILQGS talk Tuesday 2 October:
http://relativity.phys.lsu.edu/ilqgs/
"Lifting General Relativity to Observer Space".

marcus

The June 2012 paper was published in General Relativity and Gravitation.

Derek Wise has a new way to do canonical LQG and link it with Spinfoam QG. Anyone who wants to read up on this before Wise's talk on Tuesday can get additional intuition and more explanation of the notation from this slightly longer 2011 paper also by Gielen and Wise:

http://arxiv.org/abs/1111.7195
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
In Ashtekar's Hamiltonian formulation of general relativity, and in loop quantum gravity, Lorentz covariance is a subtle issue that has been strongly debated. Maintaining manifest Lorentz covariance seems to require introducing either complex-valued fields, presenting a significant obstacle to quantization, or additional (usually second class) constraints whose solution renders the resulting phase space variables harder to interpret in a spacetime picture. After reviewing the sources of difficulty, we present a Lorentz covariant, real formulation in which second class constraints never arise. Rather than a foliation of spacetime, we use a gauge field y, interpreted as a field of observers, to break the SO(3,1) symmetry down to a subgroup SO(3)_y. This symmetry breaking plays a role analogous to that in MacDowell-Mansouri gravity, which is based on Cartan geometry, leading us to a picture of gravity as 'Cartan geometrodynamics.' We study both Lorentz gauge transformations and transformations of the observer field to show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance.
10 pages. Published in Physical Review D.

I also found this 2009 solo paper by Wise helpful:
http://arxiv.org/abs/0904.1738
Symmetric Space Cartan Connections and Gravity in Three and Four Dimensions
Derek K. Wise
Einstein gravity in both 3 and 4 dimensions, as well as some interesting generalizations, can be written as gauge theories in which the connection is a Cartan connection for geometry modeled on a symmetric space. The relevant models in 3 dimensions include Einstein gravity in Chern-Simons form, as well as a new formulation of topologically massive gravity, with arbitrary cosmological constant, as a single constrained Chern-Simons action. In 4 dimensions the main model of interest is MacDowell-Mansouri gravity, generalized to include the Immirzi parameter in a natural way. I formulate these theories in Cartan geometric language, emphasizing also the role played by the symmetric space structure of the model. I also explain how, from the perspective of these Cartan-geometric formulations, both the topological mass in 3d and the Immirzi parameter in 4d are the result of non-simplicity of the Lorentz Lie algebra so(3,1) and its relatives. Finally, I suggest how the language of Cartan geometry provides a guiding principle for elegantly reformulating any 'gauge theory of geometry'.

Incidentally this was written for a special issue of the journal SIGMA which was dedicated to Élie Cartan.

marcus

I completely overlooked an important paper! It feeds into a potential near term reformulation of LQG. It is by Carrozza, Oriti, and Rivasseau about Tensorial GFT.
I should have had this paper on the 3rd quarter MIP poll (it came out in July) and somehow missed it.

Anyway, herewith another strand of current development, one of the halfdozen important lines of investigation that are part of the picture which I'm watching and trying to keep track of.

http://arxiv.org/abs/arXiv:1207.6734
http://inspirehep.net/record/1124138
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau.
(Submitted on 28 Jul 2012)
We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.
33 pages, 8 figures.

If anyone wants to register a vote for this COR paper on the MIP poll just let me know--I will tally up those votes along with the rest.

This paper has only been out a couple of days more than 2 months and it already has 5 cites.

Carrozza will be giving an ILQGS online seminar on it soon, so if anybody is interested in Tensor QFT they can listen and get the audio+slides version, with the other participants asking questions. The talk is scheduled for Tuesday 30 October.

marcus

November conference on Experimental Search for QG. The scheduled list of talks is out:
http://www.perimeterinstitute.ca/en/...he_hard_facts/
http://www.perimeterinstitute.ca/Eve...r_QG/Schedule/

Revised update on some lines of investigation I think we should be keeping track of. For brevity only a few key papers are mentioned in each category. Others were mentioned earlier in the thread. Several of the themes here are covered in online seminar talks, e.g. ILQGS at http://relativity.phys.lsu.edu/ilqgs/

PhenoCosmo Observable effects of the Loop cosmology bounce and of bounce-triggered inflation.
http://www-library.desy.de/cgi-bin/s...tecount%28d%29 (the link gave 64 papers today, won't always work though)
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1209.1609 (A Quantum Gravity Extension of the Inflationary Scenario)
Ashtekar, Agullo, Nelson http://arxiv.org/abs/1204.1288 (Perturbations in loop quantum cosmology)
Artymowski, Dapor, Pawlowski http://arxiv.org/abs/1207.4353 (Inflation from non-minimally coupled scalar field in loop quantum cosmology)
Many more papers identifying observable effects, by various author: Barrau, Grain, Cailleteau, Vidotto, Mielczarek, and others. Claus Kiefer's recent paper should be mentioned http://arxiv.org/abs/1210.0418 (Interpretation of the
Triad Orientations in Loop Quantum Cosmology) though being concerned with basic concepts rather than observable effects it does not fit in with these phenomenology papErs.

Group-labeled Spinfoams Spinfoam labels can be group elements. Finite groups are introduced. This could change the way LQG is formulated. Check out Hellmann's recent ILQGS talk 2 September.
Bahr, Dittrich,Hellmann, Kaminski http://arxiv.org/abs/1208.3388 (Holonomy Spin Foam Models: Definition and Coarse Graining)
Same authors http://arxiv.org/abs/1209.4539 (Holonomy Spin Foam Models: Boundary Hilbert spaces and canonical dynamics) .
Hellmann, Kaminski (Holonomy Spin Foam Models: Asymptotic dynamics of EPRL type models, in prep)
Bahr, Dittrich, Ryan http://arxiv.org/abs/1103.6264 (Spin foam models with finite groups)

Running G and gamma: black hole issues Bianchi's online ILQGS talk will be 16 October. I think his result helped set things up for yesterday's Ghosh and Perez paper.
Bianchi http://arxiv.org/abs/1204.5122 (Entropy of Non-Extremal Black Holes from Loop Gravity)
Dittrich, Ryan http://arxiv.org/abs/1209.4892 (On the role of the Barbero-Immirzi parameter in discrete quantum gravity)
Ghosh, Perez http://arxiv.org/abs/1210.2252 (The scaling of black hole entropy in loop quantum gravity)
They have G and the Immirzi γ run--going to G_* and γ_* in the UV limit. In the IR limit G=G_Newton

We discuss some general properties of black hole entropy in loop quantum gravity from the perspective of local stationary observers at distance £ from the horizon. The present status of the theory indicates that black hole entropy differs from the low energy (IR) expected value A/(4G) (in natural units) in the deep Planckian regime (UV). The partition function is well defined if the number of non-geometric degrees of freedom g_M(encoding the degeneracy of the area a_p eigenvalue at a puncture p) satisfy the holographic bound g_M < exp(a_p/(4G)). Our framework provides a natural renormalization mechanism such that S_UV ---> S_IR=A/(4G_Newton) as the scale £ flows.

For the scale they use a lowercase script L, which I can't type, so I use £ here.

Relativistic Thermodynamics/Statistical Mechanics of Geometry
Rovelli http://arxiv.org/abs/1209.0065 (General relativistic statistical mechanics)

Understanding thermodynamics and statistical mechanics in the full general relativistic context is an open problem. I give tentative definitions of equilibrium state, mean values, mean geometry, entropy and temperature, which reduce to the conventional ones in the non-relativistic limit, but remain valid for a general covariant theory. The formalism extends to quantum theory. The construction builds on the idea of thermal time, on a notion of locality for this time, and on the distinction between global and local temperature. The last is the temperature measured by a local thermometer, and is given by kT = hbar dτ/ds, with k the Boltzmann constant, hbar the Planck constant, ds proper time and d tau the equilibrium thermal time.

Tensorial GFT Carroza will give an online talk on this at ILQGS on 30 October. Numerous others involved--I won't try to list.
Carrozza, Oriti, Rivasseau.http://arxiv.org/abs/arXiv:1207.6734 (Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.)

We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.

Dittrich's ILQGS talk set for 27 November is still "TBA". It could be on Group-valued Spinfoam models or might be concerned with understanding the Immirzi parameter (Dittrich and Ryan have a recent paper on that.)

marcus

I'm trying to keep the list at no more than 5 topics. The last one should read (not simply "Tensorial GFT" but) Twistor LQG and Tensor GFT.

Twistor LQG and Tensor GFT Speziale will give an online ILQGS talk 13 November on a major Twistorial LQG paper he wrote with Wieland. I'll get the abstract in a moment. Carroza will talk about Tensorial GFT renormalization at ILQGS on 30 October. Several other researchers are involved on these two fronts--I won't try to list them.
Speziale, Wieland http://arxiv.org/abs/1207.6348 (The twistorial structure of loop-gravity transition amplitudes)

The spin foam formalism provides transition amplitudes for loop quantum gravity. Important aspects of the dynamics are understood, but many open questions are pressing on. In this paper we address some of them using a twistorial description, which brings new light on both classical and quantum aspects of the theory. At the classical level, we clarify the covariant properties of the discrete geometries involved, and the role of the simplicity constraints in leading to SU(2) Ashtekar-Barbero variables. We identify areas and Lorentzian dihedral angles in twistor space, and show that they form a canonical pair. The primary simplicity constraints are solved by simple twistors, parametrized by SU(2) spinors and the dihedral angles. We construct an SU(2) holonomy and prove it to correspond to the (lattice version of the) Ashtekar-Barbero connection. We argue that the role of secondary constraints is to provide a non trivial embedding of the cotangent bundle of SU(2) in the space of simple twistors. At the quantum level, a Schroedinger representation leads to a spinorial version of simple projected spin networks, where the argument of the wave functions is a spinor instead of a group element. We rewrite the Liouville measure on the cotangent bundle of SL(2,C) as an integral in twistor space. Using these tools, we show that the Engle-Pereira-Rovelli-Livine transition amplitudes can be derived from a path integral in twistor space. We construct a curvature tensor, show that it carries torsion off-shell, and that its Riemann part is of Petrov type D. Finally, we make contact between the semiclassical asymptotic behaviour of the model and our construction, clarifying the relation of the Regge geometries with the original phase space.

Carrozza, Oriti, Rivasseau http://arxiv.org/abs/1207.6734 (Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.)

We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.

Chronos

Pardon my ignorance, but, how does this work under lorentzian transforms? I have no idea, but, would appreciate help.