Our picks for second quarter 2010 MIP (most important QG paper)

[marcus]

Of these fifteen candidates, please indicate the paper or papers which you think will contribute most significantly to future quantum gravity research. Multiple choice is possible in the poll, so please select several if you wish.

Chamseddine Connes
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
http://arxiv.org/abs/1004.0464

Rovelli
A new look at loop quantum gravity
http://arxiv.org/abs/1004.1780

Bianchi Magliaro Perini
Spinfoams in the holomorphic representation
http://arxiv.org/abs/1004.4550

Lisi Smolin Speziale
Unification of gravity, gauge fields, and Higgs bosons
http://arxiv.org/abs/1004.4866

Alesci Rovelli
A regularization of the hamiltonian constraint compatible with the spinfoam dynamics
http://arxiv.org/abs/1005.0817

Denicola Marcolli al-Yasry
Spin Foams and Noncommutative Geometry
http://arxiv.org/abs/1005.1057

Mercuri Randono
The Immirzi Parameter as an Instanton Angle
http://arxiv.org/abs/1005.1291

Randono
Gravity from a fermionic condensate of a gauge theory
http://arxiv.org/abs/1005.1294

Freidel Livine
U(N) Coherent States for Loop Quantum Gravity
http://arxiv.org/abs/1005.2090

Rovelli Smerlak
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time"
http://arxiv.org/abs/1005.2985

Bonanno Contillo Percacci
Inflationary solutions in asymptotically safe f(R) gravity
http://arxiv.org/abs/1006.0192

Freidel Speziale
From twistors to twisted geometries
http://arxiv.org/abs/1006.0199

Borja Diaz-Polo Garay Livine
Dynamics for a 2-vertex Quantum Gravity Model
http://arxiv.org/abs/1006.2451

Dittrich Ryan
Simplicity in simplicial phase space
http://arxiv.org/abs/1006.4295

Lisi
An Explicit Embedding of Gravity and the Standard Model in E8
http://arxiv.org/abs/1006.4908

 

[marcus]

Here are the abstracts, as a reminder of what each paper is about. These were spotted by several PF members and called to our attention either as an item added to the bibliography or in a discussion thread. Special thanks as usual to MTd2, Francesca, and Atyy for keeping track of new research output.
Since multiple choice is possible in the poll, you may wish to check off a half-dozen or so favorites. There were a lot of potentially important papers this quarter making it difficult to narrow down the list.Chamseddine Connes
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
http://arxiv.org/abs/1004.0464
(Submitted on 3 Apr 2010)
We examine the hypothesis that space-time is a product of a continuous four-dimensional manifold times a finite space. A new tensorial notation is developed to present the various constructs of noncommutative geometry. In particular, this notation is used to determine the spectral data of the standard model. The particle spectrum with all of its symmetries is derived, almost uniquely, under the assumption of irreducibility and of dimension 6 modulo 8 for the finite space. The reduction from the natural symmetry group SU(2)xSU(2)xSU(4) to U(1)xSU(2)xSU(3) is a consequence of the hypothesis that the two layers of space-time are finite distance apart but is non-dynamical. The square of the Dirac operator, and all geometrical invariants that appear in the calculation of the heat kernel expansion are evaluated. We re-derive the leading order terms in the spectral action. The geometrical action yields unification of all fundamental interactions including gravity at very high energies. We make the following predictions: (i) The number of fermions per family is 16. (ii) The symmetry group is U(1)xSU(2)xSU(3). (iii) There are quarks and leptons in the correct representations. (iv) There is a doublet Higgs that breaks the electroweak symmetry to U(1). (v) Top quark mass of 170-175 Gev. (v) There is a right-handed neutrino with a see-saw mechanism. Moreover, the zeroth order spectral action obtained with a cut-off function is consistent with experimental data up to few percent. We discuss a number of open issues. We prepare the ground for computing higher order corrections since the predicted mass of the Higgs field is quite sensitive to the higher order corrections. We speculate on the nature of the noncommutative space at Planckian energies and the possible role of the fundamental group for the problem of generations.

Rovelli
A new look at loop quantum gravity
http://arxiv.org/abs/1004.1780
(Submitted on 11 Apr 2010)
I describe a possible perspective on the current state of loop quantum gravity, at the light of the developments of the last years. I point out that a theory is now available, having a well-defined background-independent kinematics and a dynamics allowing transition amplitudes to be computed explicitly in different regimes. I underline the fact that the dynamics can be given in terms of a simple vertex function, largely determined by locality, diffeomorphism invariance and local Lorentz invariance. I emphasize the importance of approximations. I list open problems.

Bianchi Magliaro Perini
Spinfoams in the holomorphic representation
http://arxiv.org/abs/1004.4550
(Submitted on 26 Apr 2010)
We study a holomorphic representation for spinfoams. The representation is obtained via the Ashtekar-Lewandowski-Marolf-Mourão-Thiemann coherent state transform. We derive the expression of the 4d spinfoam vertex for Euclidean and for Lorentzian gravity in the holomorphic representation. The advantage of this representation rests on the fact that the variables used have a clear interpretation in terms of a classical intrinsic and extrinsic geometry of space. We show how the peakedness on the extrinsic geometry selects a single exponential of the Regge action in the semiclassical large-scale asymptotics of the spinfoam vertex.

Lisi Smolin Speziale
Unification of gravity, gauge fields, and Higgs bosons
http://arxiv.org/abs/1004.4866
(Submitted on 27 Apr 2010)
We consider a diffeomorphism invariant theory of a gauge field valued in a Lie algebra that breaks spontaneously to the direct sum of the spacetime Lorentz algebra, a Yang-Mills algebra, and their complement. Beginning with a fully gauge invariant action -- an extension of the Plebanski action for general relativity -- we recover the action for gravity, Yang-Mills, and Higgs fields. The low-energy coupling constants, obtained after symmetry breaking, are all functions of the single parameter present in the initial action and the vacuum expectation value of the Higgs.

Alesci Rovelli
A regularization of the hamiltonian constraint compatible with the spinfoam dynamics
http://arxiv.org/abs/1005.0817
(Submitted on 5 May 2010)
We introduce a new regularization for Thiemann's Hamiltonian constraint. The resulting constraint can generate the 1-4 Pachner moves and is therefore more compatible with the dynamics defined by the spinfoam formalism. We calculate its matrix elements and observe the appearence of the 15j Wigner symbol in these.

Denicola Marcolli al-Yasry
Spin Foams and Noncommutative Geometry
http://arxiv.org/abs/1005.1057
(Submitted on 6 May 2010)
We extend the formalism of embedded spin networks and spin foams to include topological data that encode the underlying three-manifold or four-manifold as a branched cover. These data are expressed as monodromies, in a way similar to the encoding of the gravitational field via holonomies. We then describe convolution algebras of spin networks and spin foams, based on the different ways in which the same topology can be realized as a branched covering via covering moves, and on possible composition operations on spin foams. We illustrate the case of the groupoid algebra of the equivalence relation determined by covering moves and a 2-semigroupoid algebra arising from a 2-category of spin foams with composition operations corresponding to a fibered product of the branched coverings and the gluing of cobordisms. The spin foam amplitudes then give rise to dynamical flows on these algebras, and the existence of low temperature equilibrium states of Gibbs form is related to questions on the existence of topological invariants of embedded graphs and embedded two-complexes with given properties. We end by sketching a possible approach to combining the spin network and spin foam formalism with matter within the framework of spectral triples in noncommutative geometry.

Mercuri Randono
The Immirzi Parameter as an Instanton Angle
http://arxiv.org/abs/1005.1291
(Submitted on 7 May 2010)
The Barbero-Immirzi parameter is a one parameter quantization ambiguity underpinning the loop approach to quantum gravity that bears tantalizing similarities to the theta parameter of gauge theories such as Yang-Mills and QCD. Despite the apparent semblance, the Barbero-Immirzi field has resisted a direct topological interpretation along the same lines as the theta-parameter. Here we offer such an interpretation. Our approach begins from the perspective of Einstein-Cartan gravity as the symmetry broken phase of a de Sitter gauge theory. From this angle, just as in ordinary gauge theories, a theta-term emerges from the requirement that the vacuum is stable against quantum mechanical tunneling. The Immirzi parameter is then identified as a combination of Newton's constant, the cosmological constant, and the theta-parameter.

Randono
Gravity from a fermionic condensate of a gauge theory
http://arxiv.org/abs/1005.1294
(Submitted on 7 May 2010)
The most prominent realization of gravity as a gauge theory similar to the gauge theories of the standard model comes from enlarging the gauge group from the Lorentz group to the de Sitter group. To regain ordinary Einstein-Cartan gravity the symmetry must be broken, which can be accomplished by known quasi-dynamic mechanisms. Motivated by symmetry breaking models in particle physics and condensed matter systems, we propose that the symmetry can naturally be broken by a homogenous and isotropic fermionic condensate of ordinary spinors. We demonstrate that the condensate is compatible with the Einstein-Cartan equations and can be imposed in a fully de Sitter invariant manner. This lends support, and provides a physically realistic mechanism for understanding gravity as a gauge theory with a spontaneously broken local de Sitter symmetry.

Freidel Livine
U(N) Coherent States for Loop Quantum Gravity
http://arxiv.org/abs/1005.2090
(Submitted on 12 May 2010)
We investigate the geometry of the space of N-valent SU(2)-intertwiners. We propose a new set of holomorphic operators acting on this space and a new set of coherent states which are covariant under U(N) transformations. These states are labeled by elements of the Grassmannian Gr(N,2), they possesses a direct geometrical interpretation in terms of framed polyhedra and are shown to be related to the well-known coherent intertwiners.

Rovelli Smerlak
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time"
http://arxiv.org/abs/1005.2985
(Submitted on 17 May 2010)
The thermal time hypothesis has been introduced as a possible basis for a fully general-relativistic thermodynamics. Here we use the notion of thermal time to study thermal equilibrium on stationary spacetimes. Notably, we show that the Tolman-Ehrenfest effect (the variation of temperature in space so that $$T\sqrt{g_{00}}$$ remains constant) can be reappraised as a manifestation of this fact: at thermal equilibrium, temperature is locally the rate of flow of thermal time with respect to proper time - pictorially, "the speed of (thermal) time". Our derivation of the Tolman-Ehrenfest effect makes no reference to the physical mechanisms underlying thermalization, thus illustrating the import of the notion of thermal time.

Bonanno Contillo Percacci
Inflationary solutions in asymptotically safe f(R) gravity
http://arxiv.org/abs/1006.0192
(Submitted on 1 Jun 2010)
We discuss the existence of inflationary solutions in a class of renormalization group improved polynomial f(R) theories, which have been studied recently in the context of the asymptotic safety scenario for quantum gravity. These theories seem to possesses a nontrivial ultraviolet fixed point, where the dimensionful couplings scale according to their canonical dimensionality. Assuming that the cutoff is proportional to the Hubble parameter, we obtain modified Friedmann equations which admit both power law and exponential solutions. We establish that for sufficiently high order polynomial the solutions are reliable, in the sense that considering still higher order polynomials is very unlikely to change the solution.

Freidel Speziale
From twistors to twisted geometries
http://arxiv.org/abs/1006.0199
(Submitted on 1 Jun 2010)
In a previous paper we showed that the phase space of loop quantum gravity on a fixed graph can be parametrized in terms of twisted geometries, quantities describing the intrinsic and extrinsic discrete geometry of a cellular decomposition dual to the graph. Here we unravel the origin of the phase space from a geometric interpretation of twistors.

Borja Diaz-Polo Garay Livine
Dynamics for a 2-vertex Quantum Gravity Model
http://arxiv.org/abs/1006.2451
(Submitted on 12 Jun 2010)
We use the recently introduced U(N) framework for loop quantum gravity to study the dynamics of spin network states on the simplest class of graphs: two vertices linked with an arbitrary number N of edges. Such graphs represent two regions, in and out, separated by a boundary surface. We study the algebraic structure of the Hilbert space of spin networks from the U(N) perspective. In particular, we describe the algebra of operators acting on that space and discuss their relation to the standard holonomy operator of loop quantum gravity. Furthermore, we show that it is possible to make the restriction to the isotropic/homogeneous sector of the model by imposing the invariance under a global U(N) symmetry. We then propose a U(N) invariant Hamiltonian operator and study the induced dynamics. Finally, we explore the analogies between this model and loop quantum cosmology and sketch some possible generalizations of it.

Dittrich Ryan
Simplicity in simplicial phase space
http://arxiv.org/abs/1006.4295
(Submitted on 22 Jun 2010)
A key point in the spin foam approach to quantum gravity is the implementation of simplicity constraints in the partition functions of the models. Here, we discuss the imposition of these constraints in a phase space setting corresponding to simplicial geometries. On the one hand, this could serve as a starting point for a derivation of spin foam models by canonical quantisation. On the other, it elucidates the interpretation of the boundary Hilbert space that arises in spin foam models.
More precisely, we discuss different versions of the simplicity constraints, namely gauge-variant and gauge-invariant versions. In the gauge-variant version, the primary and secondary simplicity constraints take a similar form to the reality conditions known already in the context of (complex) Ashtekar variables. Subsequently, we describe the effect of these primary and secondary simplicity constraints on gauge-invariant variables. This allows us to illustrate their equivalence to the so-called diagonal, cross and edge simplicity constraints, which are the gauge-invariant versions of the simplicity constraints. In particular, we clarify how the so-called gluing conditions arise from the secondary simplicity constraints. Finally, we discuss the significance of degenerate configurations, and the ramifications of our work in a broader setting.

Lisi
An Explicit Embedding of Gravity and the Standard Model in E8
http://arxiv.org/abs/1006.4908
(Submitted on 25 Jun 2010)
The algebraic elements of gravitational and Standard Model gauge fields acting on a generation of fermions may be represented using real matrices. These elements match a subalgebra of spin(11,3) acting on a Majorana-Weyl spinor, consistent with GraviGUT unification. This entire structure embeds in the quaternionic real form of the largest exceptional Lie algebra, E8. These embeddings are presented explicitly and their implications discussed.

 

[MTd2]

Hey Marcus, you should let us think a bit longer about suggestions for Q2 papers!

 

[marcus]

Thanks for the suggestion. I will try to remember that for next time. The main problem this time was that there were so many potentially valuable/important papers. I could have used some help eliminating some, not necessarily adding any to the list.

The MIP poll has always focused on 4D QG, normally background independent. What did you think of including Connes' paper? It is not strictly QG, although it does use a 4D differential manifold to construct the spacetime. I decided to include it because as we see from Marcolli's paper (Spinfoam QG + NCG) the Noncommutative Geometery standard model does offer LQG a path to unification with particle physics.

I see 4D Asymptotic Safety as bearing on QG as well.

Let me know if you would have suggested a replacement. Is there one you would have dropped from the list and replaced with another paper?

 

[marcus]

Thanks to everyone who has responded to the poll so far!

Tom Stoer, your vote for the new LQG hamiltonian paper called it to my attention and I've been periodically dipping into it for the past couple of days.

I'm trying to understand how the tetrahedron-based (instead of triangle-based) curvature works, and to visualize the moves that make up the new hamiltonian. The new paper could indeed prove important---by giving Thiemann's canonical approach a version of dynamics which exactly fits the dynamics in the spinfoam approach.

You are the only one so far who voted for the Alesci-Rovelli paper. As I'm beginning to appreciate its possible significance, I'm glad you put the spotlight on it.

 

[tom.stoer]

Marcus,

I was insisting for months (years?) on paying more attention to this missing link between SF and "old fashioned" LQG.

I have some experience with QCD / SU(N) gauge theories where it became clear about 20 years ago that QCD cannot be fully understood by ignoring non-perturbative phenomena like Gribov copies. The path integral formalism and especially the Faddeev-Popov trick was the first successful quantization method for QCD (about 40 years ago). It was claimed for a rather long time (even in textbooks) that the canonical approach (like Gupta-Bleuler) does not work and one must use the PI for non-abelian gauge theories. Decades later it became clear that it was not the PI that was successful, but rather that the problem of inverting certain operators had been simplified in the PI by using the "perturbative FP gauge fixing". So the original problem had not been solved but ignored. Years later studying the Gribov structure and use a "physical gauge fixing" is one essential way to understand confinement, especially in the Coulomb gauge which has "been ruled out decades ago because it violates Poincare symmetry" - which is wrong! So it became clear that to fully understand the PI one must remember that originally (Feynman) the PI had been derived via the Hamiltonian formulation and that it is by no means clear that simply writing down a PI does mean anything. It is deeply rooted in the canonical approach; therefore carrying out a full constraint analysis a la Dirac is relevant for both the canonical appraoch and the PI.

My feeling was that ignoring the problems with the Hamiltonian constraint / canonical approach in LQG (which had been improved over the years but which seemed to be a dead end even after Thiemann's trick) cannot be the end of the story. In order to understand the deep relation of SFs to GR the old fashioned LQG approach was a necessary ingredient.

The Alesci-Rovelli paper is rather technical and "only" tries to fill this gap. But this is exactly what LQG must do: provide a consistent and coherent framework where all relevant results are derived and understood from different perspectives. Comparing this paper to Connes' Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions ... and Marcolli's Spin Foams and Noncommutative Geometry it is clearly not as ambitious, but it may provide the cornerstone for completing the LQG building.

 

[marcus]

Marcus,

I was insisting for months (years?) on paying more attention to this missing link between SF and "old fashioned" LQG...

Tom, I know---I've been hearing the message but not responding. It would be great if that missing link could be solidly filled in! Have to go out to a family 4th July picnic now. I hope to have more to say later.

Also joining these diverse lines of development tends to confirm the whole program.

 

[tom.stoer]

Marcus, I know you know ... ever

 

[marcus]

QG research? What does that mean to you?

The only international funding agency that I know of specifically aimed at QG research (not String/M theory) is this:

http://www.maths.nottingham.ac.uk/qg/AboutQG.html

it is a branch of the ESF (European Science Foundation) and is simply called QG

which stands for Quantum Geometry and Quantum Gravity.

==quote from the QG website==
The research programme will study several approaches to quantum gravity, namely loop quantum gravity, spin foam models, dynamical triangulations and matrix models. The common theme is the occurrence of quantum geometry in all these approaches. The research programme will study mathematical tools and techniques in non-commutative geometry and quantum groups and their applications to quantum gravity. For more information, see the Research Area.
==endquote==

 

[MTd2]

Well, on the publications page:

José Miguel Figueroa-O'Farrill, "**M2-branes***, Einstein Manifolds and Triple Systems," Proceedings of The Planck Scale, XXV Max Born symposium, Wroclaw, Poland 29 June-3 July 2009, AIP Conference Proceedings 1196, p36-43, eds. J.Kowalski-Glikman, R. Durka, M. Szczachor, AIP Melville, New York 2009.
Acknowledgement: Workshop funded by QG network.
Journal Article
Preprint
Video
Conference Website

 

[marcus]

QG research? What does that mean to you?

... The Planck Scale, XXV Max Born symposium, Wroclaw, Poland 29 June-3 July 2009, AIP Conference Proceedings 1196, p36-43, eds. J.Kowalski-Glikman, R. Durka, M. Szczachor, AIP Melville, New York 2009.
Acknowledgement: Workshop funded by QG network.
...

The Planck Scale conference, last year, is a good example of the kind of international conference that the QG organization supports. The focus was on LQG, AsymSafety, Causal Dynamical Triangulations, extensions of QFT, and didn't they also have some Noncommutative Geometry? In any case in the 2008 international QG conference they had NCG and all these other research lines (plus one string/M guy, Alex Maloney.)

QG is primarily about geometric approaches to quantum gravity. Gravity as geometry. So a mathematical object representing the geometry of the universe appears in whatever theory, as a general rule. String/M participation in QG workshops tends to be token or marginal.

There is no other category name for this bundle of research directions, that I know of.

 

[marcus]

Just to be real clear, our focus in these MIP polls is on 4-dimensional QG, the kind of research that the QG branch of the European Science Foundation supports, and allied approaches. This is definitely non-string QG. I've often made that clear in other contexts but it doesn't hurt to say it again, especially for the benefit of new members.

The only international funding agency that I know of specifically aimed at QG research (not String/M theory) is this:

http://www.maths.nottingham.ac.uk/qg/AboutQG.html

it is a branch of the ESF (European Science Foundation) and is simply called QG

which stands for Quantum Geometry and Quantum Gravity.

==quote from the QG website==
The research programme will study several approaches to quantum gravity, namely loop quantum gravity, spin foam models, dynamical triangulations and matrix models. The common theme is the occurrence of quantum geometry in all these approaches. The research programme will study mathematical tools and techniques in non-commutative geometry and quantum groups and their applications to quantum gravity. For more information, see the Research Area.
==endquote==

On the first quarter MIP poll, we already checked to see if any of the papers had been cited. It is still too early for this second quarter list. But I will put the links out there for future use. It has been said many times: citation counts don't tell the whole story (but after a couple of years they can be suggestive---and sometimes have something to teach us even earlier than that.)

Chamseddine Connes
Noncommutative Geometry as a Framework for Unification of all Fundamental Interactions including Gravity. Part I
http://arxiv.org/abs/1004.0464
http://arxiv.org/cits/1004.0464

Rovelli
A new look at loop quantum gravity
http://arxiv.org/abs/1004.1780
http://arxiv.org/cits/1004.1780

Bianchi Magliaro Perini
Spinfoams in the holomorphic representation
http://arxiv.org/abs/1004.4550
http://arxiv.org/cits/1004.4550

Lisi Smolin Speziale
Unification of gravity, gauge fields, and Higgs bosons
http://arxiv.org/abs/1004.4866
http://arxiv.org/cits/1004.4866

Alesci Rovelli
A regularization of the hamiltonian constraint compatible with the spinfoam dynamics
http://arxiv.org/abs/1005.0817
http://arxiv.org/cits/1005.0817

Denicola Marcolli al-Yasry
Spin Foams and Noncommutative Geometry
http://arxiv.org/abs/1005.1057
http://www.slac.stanford.edu/spires/find/hep?c=ARXIV:1005.1057 (link may not work)
(still looking for way to find cites to this one)

Mercuri Randono
The Immirzi Parameter as an Instanton Angle
http://arxiv.org/abs/1005.1291
http://arxiv.org/cits/1005.1291

Randono
Gravity from a fermionic condensate of a gauge theory
http://arxiv.org/abs/1005.1294
http://arxiv.org/cits/1005.1294

Freidel Livine
U(N) Coherent States for Loop Quantum Gravity
http://arxiv.org/abs/1005.2090
http://arxiv.org/cits/1005.2090

Rovelli Smerlak
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time"
http://arxiv.org/abs/1005.2985
http://arxiv.org/cits/1005.2985

Bonanno Contillo Percacci
Inflationary solutions in asymptotically safe f(R) gravity
http://arxiv.org/abs/1006.0192
http://arxiv.org/cits/1006.0192

Freidel Speziale
From twistors to twisted geometries
http://arxiv.org/abs/1006.0199
http://arxiv.org/cits/1006.0199

Borja Diaz-Polo Garay Livine
Dynamics for a 2-vertex Quantum Gravity Model
http://arxiv.org/abs/1006.2451
http://arxiv.org/cits/1006.2451

Dittrich Ryan
Simplicity in simplicial phase space
http://arxiv.org/abs/1006.4295
http://arxiv.org/cits/1006.4295

Lisi
An Explicit Embedding of Gravity and the Standard Model in E8
http://arxiv.org/abs/1006.4908
http://arxiv.org/cits/1006.4908

 

[atyy]

Just to be real clear, our focus in these MIP polls is on 4-dimensional QG, the kind of research that the QG branch of the European Science Foundation supports, and allied approaches.

The ESF does support work on non-4D quantum gravity. Among organizers and speakers at http://hep.itp.tuwien.ac.at/~miw/bzell2010/ ,Steinacker, Blaschke, and Chatzistavrakidis study non-4D constructions:

http://arxiv.org/abs/1005.0499
http://hep.itp.tuwien.ac.at/~miw/bzell2010/Chatzistavrakidis-2010.pdf

 

[marcus]

Noncommutative field theory. Noncommutative geometry. Matrix models. And allied approaches.

Here's the program. It has a good many familiar names/topics we have listed in non-string QG. I don't see it as especially stringy. But you may detect a different focus or emphasis:

Bayrischzell Workshop May 14 - 17, 2010
Noncommutativity and Physics: Spacetime Quantum Geometry

Program

Saturday, May 15

9.00-9.40 Rainer Verch Dirac field on NC Moyal Minkowski spacetime, NC potential scattering
9.40-10.20 Michael Wohlgenannt NC gauge models
10.20-10.40 --- coffee break ---
10.40-11.20 Raimar Wulkenhaar The two-point function of NC Φ44 theory
11.20-12.00 Carmelo P. Martin NC GUTs: Anomalies, renormalizability and SUSY, where do we stand?

16.20-17.00 Daniel Blaschke Schwarzschild geometry emerging from matrix models
17.00-17.40 Jens Hoppe NC curvature and classical geometry
17.40-18.00 --- break ---
18.00-18.40 Paolo Aschieri Twist deformation quantization, gravity and Einstein spaces
18.40-19.10 Alexander Schenkel Field theory on curved NC spacetimes
--- Dinner ---

Sunday, May 16

9.40-10.20 Stefan Waldmann Quantized phase space reduction and Morita theory
10.20-10.40 --- coffee break ---
10.40-11.20 Daniele Oriti The microscopic dynamics of quantum space as a group field theory
11.20-12.00 Catherine Meusburger Observables for quantum gravity from higher categories

16.20-17.00 Richard Szabo tba.
17.00-17.40 John Barrett State sum models, induced gravity and the spectral action
17.40-18.00 --- break ---
18.00-18.30 Athanasios Chatzistavrakidis Fuzzy extra dimensions and particle physics models
18.30-18.50 Pierre Martinetti Geometrical action of the modular group for disjoint intervals in 2D boundary conformal field theory
18.50-19.10 Frank Pfäffle The spectral action for Dirac operators with skew-symmetric torsion
--- Dinner ---

Monday, May 17

9.00-9.30 Gherardo Piacitelli κ-Minkowski: topology, symmetries and uncertainty relations
9.30-10.10 Sasa Kresic-Juric Differential structure on κ-Minkowski space and κ-Poincare algebra
10.10-10.30 --- coffee break ---
10.30-11.10 Pierre Bieliavsky Symmetry of universal deformation formulae
11.10-11.30 Gandalf Lechner Star products on Minkowski space
11.30-12.10 Fedele Lizzi Bosonic spectral action induced from anomaly cancellation

 

[marcus]

I think the QG website gives a pretty clear brief account of their main focus. I tend to see a broad range of interrelated research efforts. Why I said "and allied..."
I'll highlight what I quoted before:

The only international funding agency that I know of specifically aimed at QG research (not String/M theory) is this:

http://www.maths.nottingham.ac.uk/qg/AboutQG.html

it is a branch of the ESF (European Science Foundation) and is simply called QG

which stands for Quantum Geometry and Quantum Gravity.

==quote from the QG website==
The research programme will study several approaches to quantum gravity, namely loop quantum gravity, spin foam models, dynamical triangulations and matrix models. The common theme is the occurrence of quantum geometry in all these approaches. The research programme will study mathematical tools and techniques in non-commutative geometry and quantum groups and their applications to quantum gravity. For more information, see the Research Area.
==endquote==

 

[marcus]

Thanks to the 9 people, so far, who have responded to the new poll!

With the caveat that, especially so early in the game, citations can't be assumed to mean too much, I will list the five most cited papers.

Rovelli 10
Freidel Livine 4
Freidel Speziale 2
Bianchi Magliaro Perini 2
Lisi Smolin Speziale 2
====
In case anyone wants to see which researchers have cited some of these, here are the links.

Rovelli: A new look at loop quantum gravity 10
http://arxiv.org/cits/1004.1780

Freidel Livine: U(N) Coherent States for Loop Quantum Gravity 4
http://arxiv.org/cits/1005.2090

Freidel Speziale: From twistors to twisted geometries 2
http://arxiv.org/cits/1006.0199

Bianchi Magliaro Perini: Spinfoams in the holomorphic representation 2
http://arxiv.org/cits/1004.4550

Lisi Smolin Speziale: Unification of gravity, gauge fields, and Higgs bosons 2
http://arxiv.org/cits/1004.4866

 

[marcus]

Hello and special thanks to the three (comparatively new) people who, along with humanino, picked out the Rovelli Smerlak paper as especially interesting/important.

Belated welcome to dx, Prathyush, and qsa.

Note that the first reference in that May 2010 paper is to a 1994 paper of Connes and Rovelli, which introduced the "thermal time" idea.

You might be interested in a thread on the 1994 Connes Rovelli paper which we started a few months back, in April 2010.
https://www.physicsforums.com/showthread.php?t=392819

 

[marcus]

...
On the first quarter MIP poll, we already checked to see if any of the papers had been cited. It is still too early for this second quarter list. But I will put the links out there for future use. It has been said many times: citation counts don't tell the whole story (but after a couple of years they can be suggestive---and sometimes have something to teach us even earlier than that.)...

It's still early days but we can check the citation counts on our second quarter MIP picks.
The Rovelli paper (April 2010) already has 11 cites.
The Freidel Livine has 5 cites.

Here is a short list of links for keeping track of a half-dozen second-quarter 2010 papers that now have multiple citations:



Rovelli
A new look at loop quantum gravity (11)
http://arxiv.org/abs/1004.1780
http://arxiv.org/cits/1004.1780

Freidel Livine
U(N) Coherent States for Loop Quantum Gravity (5)
http://arxiv.org/abs/1005.2090
http://arxiv.org/cits/1005.2090

Bianchi Magliaro Perini
Spinfoams in the holomorphic representation (2)
http://arxiv.org/abs/1004.4550
http://arxiv.org/cits/1004.4550

Lisi Smolin Speziale
Unification of gravity, gauge fields, and Higgs bosons (2)
http://arxiv.org/abs/1004.4866
http://arxiv.org/cits/1004.4866

Rovelli Smerlak
Thermal time and the Tolman-Ehrenfest effect: temperature as the "speed of time" (2)
http://arxiv.org/abs/1005.2985
http://arxiv.org/cits/1005.2985

Freidel Speziale
From twistors to twisted geometries (2)
http://arxiv.org/abs/1006.0199
http://arxiv.org/cits/1006.0199

===============
This link to get cites for the Marcolli et al works now:
Denicola Marcolli al-Yasry
Spin Foams and Noncommutative Geometry
http://www.slac.stanford.edu/spires/find/hep?c=ARXIV:1005.1057

 

[marcus]

It might be interesting to look back and see how some of the leading 2009 papers are doing cite-wise. Keeping in mind that citations aren't the whole story.
GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2009&FORMAT=www&SEQUENCE=citecount%28d%29

Many of these papers and authors are ones you will remember from our quarterly 2009 polls. The citation numbers given here are as of 12 August 2010. So there has been some time (but not a lot) for researchers to build on the 2009 results and generate references to the 2009 papers.

In some sense these are 15 papers that the Loop/Spinfoam/GroupFieldTheory community has hinted that it, the relevant research community, thinks are especially interesting or valuable. This might not agree with our judgement as observers, or with what eventually turns out longterm. But in any case it's interesting to check out and try to understand.

1) Asymptotic analysis of the EPRL four-simplex amplitude.
John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Henrique Gomes, Frank Hellmann, (Nottingham U.) . Feb 2009. 26pp.
Published in J.Math.Phys.50:112504,2009.
e-Print: arXiv:0902.1170 [gr-qc]
Cited 32 times

2) Spin foam quantization and anomalies.
Martin Bojowald, Alejandro Perez, (Penn State U.) . CGPG-03-3-1, ESI-1288, 2009. 32pp.
Published in Gen.Rel.Grav.42:877-907,2010.
e-Print: gr-qc/0303026
Cited 30 times

3) Cosmological footprints of loop quantum gravity.
J. Grain, (APC, Paris & Paris, Inst. Astrophys.) , A. Barrau, (LPSC, Grenoble & IHES, Bures-sur-Yvette) . Feb 2009. (Published Feb 27, 2009). 7pp.
Published in Phys.Rev.Lett.102:081301,2009.
e-Print: arXiv:0902.0145 [gr-qc]
Cited 25 times

4) Quantum geometry from phase space reduction.
Florian Conrady, Laurent Freidel, (Perimeter Inst. Theor. Phys.) . PI-QG-118, Feb 2009. 33pp.
Published in J.Math.Phys.50:123510,2009.
e-Print: arXiv:0902.0351 [gr-qc]
Cited 23 times

5) Loop quantum cosmology of Bianchi I models.
Abhay Ashtekar, Edward Wilson-Ewing, (Penn State U.) . Mar 2009. (Published Mar 2009). 33pp.
Published in Phys.Rev.D79:083535,2009.
e-Print: arXiv:0903.3397 [gr-qc]
Cited 22 times

6) LQG propagator from the new spin foams.
Eugenio Bianchi, (Marseille, CPT) , Elena Magliaro, Claudio Perini, (Marseille, CPT & Rome III U.) . May 2009. 28pp.
Published in Nucl.Phys.B822:245-269,2009.
e-Print: arXiv:0905.4082 [gr-qc]
Cited 19 times

7) Entropy-Corrected Holographic Dark Energy.
Hao Wei, (Beijing, Inst. Tech.) . Feb 2009. 12pp.
Published in Commun.Theor.Phys.52:743-749,2009.
e-Print: arXiv:0902.0129 [gr-qc]
Cited 18 times

8) Interaction of the Barbero-Immirzi Field with Matter and Pseudo-Scalar Perturbations.
Simone Mercuri, Victor Taveras, (Penn State U.) . Mar 2009. (Published Nov 15, 2009). 13pp.
Published in Phys.Rev.D80:104007,2009.
e-Print: arXiv:0903.4407 [gr-qc]
Cited 18 times

9) Group field theory renormalization - the 3d case: Power counting of divergences.
Laurent Freidel, Razvan Gurau, (Perimeter Inst. Theor. Phys.) , Daniele Oriti, (Perimeter Inst. Theor. Phys. & Potsdam, Max Planck Inst.) . AEI-2009-031-PI-QG-135, May 2009. (Published May 2009). 25pp.
Published in Phys.Rev.D80:044007,2009.
e-Print: arXiv:0905.3772 [hep-th]
Cited 18 times

10) Scaling behaviour of three-dimensional group field theory.
Jacques Magnen, (Ecole Polytechnique, CPHT) , Karim Noui, (Tours U., CNRS) , Vincent Rivasseau, (Orsay, LPT) , Matteo Smerlak, (Marseille, CPT) . Jul 2009. 20pp.
Published in Class.Quant.Grav.26:185012,2009.
e-Print: arXiv:0906.5477 [hep-th]
Cited 17 times

11) Black hole entropy and SU(2) Chern-Simons theory.
Jonathan Engle, Alejandro Perez, (Marseille, CPT) , Karim Noui, (Tours U., CNRS) . May 2009. (Published Jul 16, 2010). 4pp.
Published in Phys.Rev.Lett.105:031302,2010.
e-Print: arXiv:0905.3168 [gr-qc]
Cited 15 times

12) Colored Group Field Theory.
Razvan Gurau, (Perimeter Inst. Theor. Phys.) . PI-QG-138, Jul 2009. 21pp.
e-Print: arXiv:0907.2582 [hep-th]
Cited 15 times

13) 4d Deformed Special Relativity from Group Field Theories.
Florian Girelli, (SISSA, Trieste & INFN, Trieste & Sydney U.) , Etera R. Livine, (Lyon, Ecole Normale Superieure) , Daniele Oriti, (Perimeter Inst. Theor. Phys. & Utrecht U. & Potsdam, Max Planck Inst.) . ITP-UU-08-57, SPIN-08-44, AEI-2009-029, Mar 2009. (Published Jan 15, 2010). 23pp.
Published in Phys.Rev.D81:024015,2010.
e-Print: arXiv:0903.3475 [gr-qc]
Cited 14 times

14) Possible observational effects of loop quantum cosmology.
Jakub Mielczarek, (Jagiellonian U., Astron. Observ. & LPSC, Grenoble) . Aug 2009. (Published Mar 15, 2010). 11pp.
Published in Phys.Rev.D81:063503,2010.
e-Print: arXiv:0908.4329 [gr-qc]
Cited 14 times

15) Loop Quantum Cosmology and Spin Foams.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson, (Penn State U.) . IGC-09-9-1, Sep 2009. 11pp.
Published in Phys.Lett.B681:347-352,2009.
e-Print: arXiv:0909.4221 [gr-qc]
Cited 13 times

 

[marcus]

I got curious and looked to see which of these 15 I had voted for in the quarterly polls.
It's not as if the citation winners are the right ones. But I was interested to know how the ones I picked (near the time they first appeared) eventually fared over the course of the year.

I see I picked 1, 4, and 5 (in the first quarter poll) that is Barrett et al, Conrady Freidel, and Ashtekar Wilson-Ewing
https://www.physicsforums.com/poll.php?do=showresults&pollid=1568
If you want to see what your choices were, and how they did cite-wise in the subsequent months, here are the links.
https://www.physicsforums.com/poll.php?do=showresults&pollid=1614
https://www.physicsforums.com/poll.php?do=showresults&pollid=1664
https://www.physicsforums.com/poll.php?do=showresults&pollid=1735

I see I also picked papers listed here as 10 and 11 in the second quarter poll. I expect some others of us did "better" than I did (if you count it good to anticipate how the research community is going to go) but I didn't check others' choices, only my own.

 

[marcus]

Now that the year is 2/3 over we could take a look and see how the 2010 LQG papers are doing citewise. It's not a perfect measure, but does give some indication of value as seen by the rest of the community. I did a similar listing for 2009 paper a couple of posts back. Here are the TOP EIGHT from 2010:

1) Newtonian gravity in loop quantum gravity.
Lee Smolin. Jan 2010. 16pp.
e-Print: arXiv:1001.3668
Cited 43 times

2) Twisted geometries: A geometric parametrisation of SU(2) phase space.
Laurent Freidel, Simone Speziale. Jan 2010. 28pp.
e-Print: arXiv:1001.2748 [gr-qc]
Cited 12 times

3) Holomorphic Factorization for a Quantum Tetrahedron.
Laurent Freidel, (Perimeter Inst. Theor. Phys.) , Kirill Krasnov, (Nottingham U.) , Etera R. Livine, (Lyon, Ecole Normale Superieure) . 2010. 45pp.
Published in Commun.Math.Phys.297:45-93,2010.
e-Print: arXiv:0905.3627
Cited 11 times

4) A new look at loop quantum gravity.
Carlo Rovelli, (Marseille, CPT) . Apr 2010. 15pp.
e-Print: arXiv:1004.1780
Cited 11 times

5) Topological Graph Polynomials in Colored Group Field Theory.
Razvan Gurau, (Perimeter Inst. Theor. Phys.) . PI-QG-165, 2010. 21pp.
Published in Annales Henri Poincare 11:565-584,2010.
e-Print: arXiv:0911.1945
Cited 9 times

6) Group field theory with non-commutative metric variables.
Aristide Baratin, Daniele Oriti, . Feb 2010. 6pp.
e-Print: arXiv:1002.4723
Cited 9 times

7) Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli, Francesca Vidotto, . Mar 2010. 8pp.
e-Print: arXiv:1003.3483
Cited 9 times

8) Casting Loop Quantum Cosmology in the Spin Foam Paradigm.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson, . IGC-10-1-1, Jan 2010. 36pp.
Published in Class.Quant.Grav.27:135020,2010.
e-Print: arXiv:1001.5147 [gr-qc]
Cited 8 times
================

There is a slight discrepancy in the way the date is determined here. In our polls the date of a paper is consistently taken as its preprint date, which is also when the paper normally starts getting cited by other researchers. However to obtain this list I used Spires setting "DATE = 2010", and Spires, when it realizes something has been published, resets the preprint date to be the final publication date. Numbers 3 and 5 on this Spires list have a preprint date of some time in 2009, so you wouldn't find them in our first and second quarter 2010 polls. You can think of #3 and #5 as having a slight "head-start advantage" here because they started acquiring cites back in 2009, before the other papers were posted on arxiv. But this is only meant to give a rough casual glimpse, no need to worry about petty details.

 

[marcus]

I see that now 14 of us have responded to the poll! Thanks to the most recent and to everybody---the more people's different viewpoints it reflects the more useful I think it becomes. I noticed the large interest in the Rovelli Smerlak paper about ("the speed of") time, which motivated me to re-read and think about it more.

By some glitch I can't explain, in the previous post when listing topcited 2010 papers (including both quarters) I missed the Modesto Randono paper. M&R are postdocs at Perimeter. Their paper got the second highest number of cites. I will re-do the list, limited to papers which appeared on Arxiv this year, including the one I missed earlier.
Here is the search URL
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+K+SPIN%2CFOAM+OR+DK+FIELD+THEORY%2C+GROUP+OR+DK+QUANTUM+GRAVITY%2C+LOOP+SPACE+OR+QUANTUM+COSMOLOGY%2C+LOOP+SPACE+AND+DATE+%3D+2010&FORMAT=www&SEQUENCE=citecount%28d%29

Here are the 8 top-cited Loop-and-allied papers that appeared in 2010:

1) Newtonian gravity in loop quantum gravity.
Lee Smolin. Jan 2010. 16pp.
e-Print: arXiv:1001.3668
Cited 43 times

2) Entropic corrections to Newton's law.
Leonardo Modesto, Andrew Randono. Mar 2010. 7pp.
e-Print: arXiv:1003.1998
Cited 17 times

3) Twisted geometries: A geometric parametrisation of SU(2) phase space.
Laurent Freidel, Simone Speziale. Jan 2010. 28pp.
e-Print: arXiv:1001.2748
Cited 12 times

4) A new look at loop quantum gravity.
Carlo Rovelli, (Marseille, CPT) . Apr 2010. 15pp.
e-Print: arXiv:1004.1780
Cited 11 times

5) Group field theory with non-commutative metric variables.
Aristide Baratin, Daniele Oriti, . Feb 2010. 6pp.
e-Print: arXiv:1002.4723
Cited 9 times

6) Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli, Francesca Vidotto, . Mar 2010. 8pp.
e-Print: arXiv:1003.3483
Cited 9 times

7) Casting Loop Quantum Cosmology in the Spin Foam Paradigm.
Abhay Ashtekar, Miguel Campiglia, Adam Henderson. IGC-10-1-1, Jan 2010. 36pp.
Published in Class.Quant.Grav.27:135020,2010.
e-Print: arXiv:1001.5147
Cited 8 times

8) Linearized Group Field Theory and Power Counting Theorems.
Joseph Ben Geloun, Thomas Krajewski, Jacques Magnen, Vincent Rivasseau. Feb 2010. 16pp.
Published in Class.Quant.Grav.27:155012,2010.
e-Print: arXiv:1002.3592
Cited 8 times

Speaking of the Modesto Randono paper, I see that DESY is using the keyword "quantum gravity, effect". DESY librarians tagged the M&R paper with more than half-dozen keywords, including this one which I had not noticed before. Our search picked it up because they also tagged it "quantum gravity, loop space". I may have missed the paper earlier because DESY librarians had not finished putting all the keyword tags on the paper.

To see which of us voted for which papers in the poll, click on the number of votes over on the right. Like for example click on the numeral "5" that gives the number of votes for the Rovelli Smerlak paper.

 

[marcus]

It's time to begin gathering candidates for the 3rd quarter MIP poll. Loop-and-allied QG papers that we think will turn out to be especially valuable/important to future research. I'll list some links to start examining and selecting from. These are QG papers that appeared on Arxiv during the months of July, August, and September 2010:

http://arxiv.org/abs/1007.0735
http://arxiv.org/cits/1007.0735
Unimodular Loop Quantum Cosmology
Dah-Wei Chiou, Marc Geiller

http://arxiv.org/abs/1007.2768
http://arxiv.org/cits/1007.2768
Generic isolated horizons in loop quantum gravity
Christopher Beetle, Jonathan Engle

http://arxiv.org/abs/1007.3723
http://arxiv.org/cits/1007.3723
Vertex Expansion for the Bianchi I model
Miguel Campiglia, Adam Henderson, William Nelson

http://arxiv.org/abs/1008.0354
http://arxiv.org/cits/1008.0354
EPRL/FK Group Field Theory
Joseph Ben Geloun, Razvan Gurau, Vincent Rivasseau

http://arxiv.org/abs/1008.1759
http://arxiv.org/cits/1008.1759
Unimodular loop quantum gravity and the problems of time
Lee Smolin

http://arxiv.org/abs/1008.4093
http://arxiv.org/cits/1008.4093
Lifting SU(2) Spin Networks to Projected Spin Networks
Maité Dupuis, Etera R. Livine

http://arxiv.org/abs/1008.5348
http://arxiv.org/cits/1008.5348
Noncommutative Geometry Spectral Action as a framework for unification: Introduction and phenomenological/cosmological consequences
Mairi Sakellariadou

http://arxiv.org/abs/1009.1136
http://arxiv.org/cits/1009.1136
The Small Scale Structure of Spacetime
Steven Carlip

http://arxiv.org/abs/1009.2445
http://arxiv.org/cits/1009.2445
Gravity quantized
Marcin Domagala, Kristina Giesel, Wojciech Kaminski, Jerzy Lewandowski

 

[marcus]

Bianchi Doná Speziale

This one for sure gets added to the list:
http://arxiv.org/abs/1009.3402
Polyhedra in loop quantum gravity
Eugenio Bianchi, Pietro Doná, Simone Speziale
32 pages, many figures
(Submitted on 17 Sep 2010)
"Interwiners are the building blocks of spin-network states. The space of intertwiners is the quantization of a classical symplectic manifold introduced by Kapovich and Millson. Here we show that a theorem by Minkowski allows us to interpret generic configurations in this space as bounded convex polyhedra in Euclidean space: a polyhedron is uniquely described by the areas and normals to its faces. We provide a reconstruction of the geometry of the polyhedron: we give formulas for the edge lengths, the volume and the adjacency of its faces. At the quantum level, this correspondence allows us to identify an intertwiner with the state of a quantum polyhedron, thus generalizing the notion of quantum tetrahedron familiar in the loop quantum gravity literature. Moreover, coherent intertwiners result to be peaked on the classical geometry of a polyhedron. We discuss the relevance of this result for loop quantum gravity. In particular, coherent spin-network states with nodes of arbitrary valence represent a collection of semiclassical polyhedra. Furthermore, we introduce an operator that measures the volume of a quantum polyhedron and examine its relation with the standard volume operator of loop quantum gravity. We also comment on the semiclassical limit of spinfoams with non-simplicial graphs."

MTd2 spotted this one today---the Bianchi Doná Speziale paper. Important step forward, I think.

Also note that two papers on our list so far deal with quantization of unimodular GR. It is a topic that has been of growing interest for the past year or two.

 

[MTd2]

I found this paper

http://arxiv.org/PS_cache/arxiv/pdf/1009/1009.3267v1.pdf

More fundamental than the "The Small Scale Structure of Spacetime", since it is shows a much more generic character is shared between different quantum gravity theories, and goes deeper, with several different scale transitions. So, I choose this one.

BTW, marcus, would you mind opening a thread to discuss and explain a bit of the paper you cited in the last post?

 

[atyy]

How about http://arxiv.org/abs/1009.4475 ?

 

[marcus]

Have you noticed how out-of-date the treatment of dynamics is?

I'm not sure the paper, long as it is (75 pages) will have much influence. Although it should be fairly easy for someone to reply to most critical points. This has happened before with him. In some cases his criticisms missed the boat, in others cases they connected and proved constructive. Either way they were treated responsively.

By way of illustration, Alexandrov posted a short paper already in April 2010 similarly pointing to perceived flaws the relation of canonical LQG to SF. http://arxiv.org/abs/1004.2260 It has provided at least one helpful suggestion, and perhaps in other instances has served as a useful foil, although as I don't think it accurately present the way SF transition amplitudes are currently defined----see e.g. Lewandowski 2009 or Rovelli April 2010 papers.

Alexandrov's April paper has gotten 4 citations, 3 of the 4 are by Rovelli, answering. Rovelli's responses were as follows:

''Also, it has been pointed out [by Alexandrov] that imposing second class constraints weakly might lead to inconsistencies in some cases [27]. In the present case, however, these worries are not relevant, since the space obtained is directly related to the one of the canonical theory, which we can trust to capture the degrees of freedom of gravity correctly."

"However, doubts can be raised against this argument. For instance, Alexandrov[17] has stressed the fact that the implementation of second class constraints into a Feynman path integral in general requires a modification of the measure, andherethefaceamplitudeplays precisely the role of such measure, since Av∼eiAction. Do we have an independent way of fixing the face amplitude?
Here we argue that the face amplitude is uniquely determined fo rany spinfoam sum of the form (1) by three inputs: ..."

"The model we construct contains in fact a slight modification with respect to the one in [14, 15, 16, 17] (corresponding to a slightly different factor ordering of the constraints). The same modification was already considered by Alexandrov in [23]. We show that with the modification the matrix elements vanish exactly, and not just in the large quantum number limit, as in previous constructions. ..."

These responses to Alexandrov by Rovelli et al can be found here:
page 6 of http://arxiv.org/pdf/0911.0543
page 1 of http://arxiv.org/pdf/1005.0764
page 2 of http://arxiv.org/pdf/1006.1294

I don't know that this matters but Alexandrov also takes a rather stiff position that whatever LQG must be arrived at by quantizing GR, but that is not the current program's aim. The aim (see page 1 of the April 2010 paper) is to find a backgroundless quantum theory which has GR as the appropriate limit---in other words a pragmatic approach, find something that works, regardless of what steps were taken to arrive at it.

Alexandrov proposed a variation of LQG back around 2003, called CLQG, which he argued was much better. But no one took him up on it. He wrote a number of papers on CLQG but they were largely ignored, and it now seems to have been a dead end.

 

[atyy]

Have you noticed how out-of-date the treatment of dynamics is?

I'm not sure the paper, long as it is (75 pages) will have much influence. Although it should be fairly easy for someone to reply to most critical points. We'll see.

Not for the dynamics, I think the discussion of emergence is good. Yes, I'm sure it will not have much influence!

 

[marcus]

http://arxiv.org/abs/1008.1196
"What about experiments? The experimental predictions of the two theories are the same, so no experiment can tell the difference between them, except for one fundamental feature: the EFE (confirmed in the solar system and by binary pulsar measurements to high accuracy) together with the QFT prediction for the vacuum energy density (confirmed by Casimir force measurements) give the wrong answer by many orders of magnitude; the TFE does not suffer this problem. In this respect, the TFE are strongly preferred by experiment."...

Atyy, that paper you spotted by George Ellis, Jeff Murugan, Henk van Elst has got to be on our 3rd quarter poll. It is major. There seems to be a revival of interest in unimodular gravity and this paper takes the bold step of saying right out that we should be using UG instead of conventional GR. I highlighted the specially relevant portion of your post.

As you are well aware, Ellis is probably now the best known most influential GR-cosmology guy. Former co-author with Stephen Hawking of the classic text "The Large Scale Structure of Space Time". He just last year hosted a Quantum Gravity conference at Cape Town to celebrate his 70th. Brought together ALL approaches: string, loop, triangulations,...

To say what you already pointed out to me: he is basically arguing that quantum gravity should give UNIMODULAR AS THE LIMIT. Not just ordinary GR. He wants the QG people to develop unimodular quantum gravity. More precisely he says TFE (tracefree + a conservation conditon). It's flat-out remarkable. I will quote the abstract and then the conclusions (which are more explicit and forthright.)http://arxiv.org/abs/1008.1196
The gravitational effect of the vacuum
George F. R. Ellis, Jeff Murugan, Henk van Elst
(Submitted on 6 Aug 2010)
"The quantum field theoretic prediction for the vacuum energy density leads to a value for the effective cosmological constant that is incorrect by between 60 to 120 orders of magnitude. We review an old proposal of replacing Einstein's Field Equations by their trace-free part (the Trace-Free Einstein Equations), together with an independent assumption of energy--momentum conservation by matter fields. We confirm that while this does not solve the fundamental issue of why the cosmological constant has the value it has, it is indeed a viable theory that resolves the problem of the discrepancy between the vacuum energy density and the observed value of the cosmological constant. We also point out that this proposal may have a valid quantum field theory basis in terms of a spin-2 field theory for the graviton interaction with matter."

 

[marcus]

I've narrowed the 3rd quarter list down to 12. Included with potentially important QG papers are some from outside the field which could have significant implications for quantum gravity and influence future QG research

Tennie Wohlfarth
Consistent matter couplings for Plebanski gravity
http://arxiv.org/abs/1009.5595
http://arxiv.org/cits/1009.5595

Alexandrov Roche
Critical Overview of Loops and Foams
http://arxiv.org/abs/1009.4475
http://arxiv.org/cits/1009.4475

Gambini Garcia-Pintos Pullin
Undecidability as solution to the problem of measurement
http://arxiv.org/abs/1009.3817
http://arxiv.org/cits/1009.3817

Barbour Murchadha
Conformal Superspace: the configuration space of general relativity
http://arxiv.org/abs/1009.3559
http://arxiv.org/cits/1009.3559

Bianchi Doná Speziale
Polyhedra in loop quantum gravity
http://arxiv.org/abs/1009.3402
http://arxiv.org/cits/1009.3402

Domagala Giesel Kaminski Lewandowski
Gravity quantized
http://arxiv.org/abs/1009.2445
http://arxiv.org/cits/1009.2445

Carlip
The Small Scale Structure of Spacetime
http://arxiv.org/abs/1009.1136
http://arxiv.org/cits/1009.1136

Sakellariadou
Noncommutative Geometry Spectral Action as a framework for unification
http://arxiv.org/abs/1008.5348
http://arxiv.org/cits/1008.5348

Dupuis Livine
Lifting SU(2) Spin Networks to Projected Spin Networks
http://arxiv.org/abs/1008.4093
http://arxiv.org/cits/1008.4093

Ellis Murugan van Elst
The gravitational effect of the vacuum
http://arxiv.org/abs/1008.1196
http://arxiv.org/cits/1008.1196

Geloun Gurau Rivasseau
EPRL/FK Group Field Theory
http://arxiv.org/abs/1008.0354
http://arxiv.org/cits/1008.0354

Campiglia Henderson Nelson
Vertex Expansion for the Bianchi I model
http://arxiv.org/abs/1007.3723
http://arxiv.org/cits/1007.3723