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

In summary: CarlipThe Small Scale Structure of Spacetimehttp://arxiv.org/abs/1009.1136http://arxiv.org/cits/1009.1136(Submitted on 14 Sep 2010)This paper is about the small scale structure of spacetime. It starts by talking about the way space is represented in different theories, and then goes on to talk about how general relativity handles small scales. It talks about how general relativity doesn't have a good way of describing the geometry of small scales, and how quantum field theory can help fix this problem. It also talks about the way that different scales can interact with each other, and how this can lead to strange

Which papers (see titles below) will prove most valuable to future research?


  • Total voters
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  • #1
marcus
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Of these twelve candidates, please select 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 choose several if you wish.

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

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

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

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

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

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

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

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

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

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

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

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

Included are some which could have a significant bearing on QG even though not restricted to the field itself. As usual with these quarterly polls, the focus is on non-string QG research. The papers' abstract summaries follow in the next post.
 
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  • #2
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 items added to the bibliography or brought up in a discussion thread. Special thanks to MTd2, Atyy, John, and Francesca.
Since multiple choice is possible in the poll, you may wish to check off three or four favorites. It's too early to be looking for citations since many have only been out a month or two, but for future convenience I've included the cites links.

Tennie Wohlfarth
Consistent matter couplings for Plebanski gravity
http://arxiv.org/abs/1009.5595
http://arxiv.org/cits/1009.5595
(Submitted on 28 Sep 2010)
We develop a scheme for the minimal coupling of all standard types of tensor and spinor field matter to Plebanski gravity. This theory is a geometric reformulation of vacuum general relativity in terms of two-form frames and connection one-forms, and provides a covariant basis for various quantization approaches. Using the spinor formalism we prove the consistency of the newly proposed matter coupling by demonstrating the full equivalence of Plebanski gravity plus matter to Einstein--Cartan gravity. As a byproduct we also show the consistency of some previous suggestions for matter actions.

Alexandrov Roche
Critical Overview of Loops and Foams
http://arxiv.org/abs/1009.4475
http://arxiv.org/cits/1009.4475
(Submitted on 22 Sep 2010)
This is a review of the present status of loop and spin foam approaches to quantization of four-dimensional general relativity. It aims at raising various issues which seem to challenge some of the methods and the results often taken as granted in these domains. A particular emphasis is given to the issue of diffeomorphism and local Lorentz symmetries at the quantum level and to the discussion of new spin foam models. We also describe modifications of these two approaches which may overcome their problems and speculate on other promising research directions.

Gambini Garcia-Pintos Pullin
Undecidability as solution to the problem of measurement
http://arxiv.org/abs/1009.3817
http://arxiv.org/cits/1009.3817
(Submitted on 20 Sep 2010)
In recent papers we put forth a new interpretation of quantum mechanics, colloquially known as "the Montevideo interpretation''. This interpretation is based on taking into account fundamental limits that gravity imposes on the measurement process. As a consequence one has that situations develop where a reduction process is undecidable from an evolution operator. When such a situation is achieved, an event has taken place. In this paper we sharpen the definition of when and how events occur, more precisely we give sufficient conditions for the occurrence of events. We probe the new definition in an example. In particular we show that the concept of undecidability used is not "FAPP'' (for all practical purposes), but fundamental.

Barbour Murchadha
Conformal Superspace: the configuration space of general relativity
http://arxiv.org/abs/1009.3559
http://arxiv.org/cits/1009.3559
(Submitted on 18 Sep 2010)
It has long been considered that conformal superspace is the natural configuration space for canonical general relativity. However, this was never definitively demonstrated. We have found that the standard conformal method of solving the Einstein constraints has an unexpected extra symmetry. This allows us to complete the project. We show that given a point and a velocity in conformal superspace, the Einstein equations generate a unique curve in conformal superspace.

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

Domagala Giesel Kaminski Lewandowski
Gravity quantized
http://arxiv.org/abs/1009.2445
http://arxiv.org/cits/1009.2445
(Submitted on 13 Sep 2010)
..."but we do not have quantum gravity." This phrase is often used when analysis of a physical problem enters the regime in which quantum gravity effects should be taken into account. In fact, there are several models of the gravitational field coupled to (scalar) fields for which the quantization procedure can be completed using loop quantum gravity techniques. The model we present in this paper consist of the gravitational field coupled to a scalar field. The result has similar structure to the loop quantum cosmology models, except for that it involves all the local degrees of freedom because no symmetry reduction has been performed at the classical level.

Carlip
The Small Scale Structure of Spacetime
http://arxiv.org/abs/1009.1136
http://arxiv.org/cits/1009.1136
(Submitted on 6 Sep 2010)
Several lines of evidence hint that quantum gravity at very small distances may be effectively two-dimensional. I summarize the evidence for such "spontaneous dimensional reduction," and suggest an additional argument coming from the strong-coupling limit of the Wheeler-DeWitt equation. If this description proves to be correct, it suggests a fascinating relationship between small-scale quantum spacetime and the behavior of cosmologies near an asymptotically silent singularity.

Sakellariadou
Noncommutative Geometry Spectral Action as a framework for unification
http://arxiv.org/abs/1008.5348
http://arxiv.org/cits/1008.5348
(Submitted on 31 Aug 2010)
I will summarize Noncommutative Geometry Spectral Action, an elegant geometrical model valid at unification scale, which offers a purely gravitational explanation of the Standard Model, the most successful phenomenological model of particle physics. Noncommutative geometry states that close to the Planck energy scale, space-time has a fine structure and proposes that it is given as the product of a four-dimensional continuum compact Riemaniann manifold by a tiny discrete finite noncommutative space. The spectral action principle, a universal action functional on spectral triples which depends only on the spectrum of the Dirac operator, applied to this almost commutative product geometry, leads to the full Standard Model, including neutrino mixing which has Majorana mass terms and a see-saw mechanism, minimally coupled to gravity. It also makes various predictions at unification scale. I will review some of the phenomenological and cosmological consequences of this beautiful and purely geometrical approach to unification.

Dupuis Livine
Lifting SU(2) Spin Networks to Projected Spin Networks
http://arxiv.org/abs/1008.4093
http://arxiv.org/cits/1008.4093
(Submitted on 24 Aug 2010)
Projected spin network states are the canonical basis of quantum states of geometry for the most recent EPR-FK spinfoam models for quantum gravity. They are functionals of both the Lorentz connection and the time normal field. We analyze in details the map from these projected spin networks to the standard SU(2) spin networks of loop quantum gravity. We show that this map is not one-to-one and that the corresponding ambiguity is parameterized by the Immirzi parameter. We conclude with a comparison of the scalar products between projected spin networks and SU(2) spin network states.

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

Geloun Gurau Rivasseau
EPRL/FK Group Field Theory
http://arxiv.org/abs/1008.0354
http://arxiv.org/cits/1008.0354
(Submitted on 2 Aug 2010)
The purpose of this short note is to clarify the Group Field Theory vertex and propagators corresponding to the EPRL/FK spin foam models and to detail the subtraction of leading divergences of the model.

Campiglia Henderson Nelson
Vertex Expansion for the Bianchi I model
http://arxiv.org/abs/1007.3723
http://arxiv.org/cits/1007.3723
(Submitted on 21 Jul 2010)
A perturbative expansion of Loop Quantum Cosmological transitions amplitudes of Bianchi I models is performed. Following the procedure outlined in [1,2] for isotropic models, it is shown that the resulting expansion can be written in the form of a series of amplitudes each with a fixed number of transitions mimicking a spin foam expansion. This analogy is more complete than in the isotropic case, since there are now the additional anisotropic degrees of freedom which play the role of 'colouring' of the spin foams. Furthermore, the isotropic expansion is recovered by integrating out the anisotropies.
 
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  • #3
Oops, I missed seeing Alexandrov and Roche - would have registered a vote there if I hadn't been blind.
 
  • #4
That was one you called to my attention! We discussed it! I know how that is. My eyes get tired and miss things, just skip over stuff. It's a nuisance.

In any case I will count one extra from you for Alexandrov Roche just as if you had voted it.
 
  • #5
Five people have already registered their picks--thanks to everyone who has responded! It's interesting to see other people's choices of the potentially most significant papers. Also I'm curious (and perhaps others are) to see which, so far, of this year's Loop-and-allied QG papers have been most cited..

Here are the 8 most cited, of those posted on arxiv in 2010, using this 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 [Broken]

1) Newtonian gravity in loop quantum gravity.
Lee Smolin. Jan 2010. 16pp.
http://arxiv.org/abs/1001.3668
http://arxiv.org/cits/1001.3668
Cited 46 times

2) Entropic corrections to Newton's law.
Leonardo Modesto, Andrew Randono. Mar 2010. 7pp.
http://arxiv.org/abs/1003.1998
http://arxiv.org/cits/1003.1998
Cited 21 times

3) Twisted geometries: A geometric parametrisation of SU(2) phase space.
Laurent Freidel, Simone Speziale. Jan 2010. 28pp.
http://arxiv.org/abs/1001.2748
http://arxiv.org/cits/1001.2748
Cited 14 times

4) A new look at loop quantum gravity.
Carlo Rovelli, (Marseille, CPT) . Apr 2010. 15pp.
http://arxiv.org/abs/1004.1780
http://arxiv.org/cits/1004.1780
Cited 13 times

5) Towards Spinfoam Cosmology.
Eugenio Bianchi, Carlo Rovelli, Francesca Vidotto, . Mar 2010. 8pp.
http://arxiv.org/abs/1003.3483
http://arxiv.org/cits/1003.3483
Cited 11 times

6) 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.
http://arxiv.org/abs/1001.5147
http://arxiv.org/cits/1001.5147
Cited 10 times

7) 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.
http://arxiv.org/abs/1002.3592
http://arxiv.org/cits/1002.3592
Cited 10 times

8) Group field theory with non-commutative metric variables.
Aristide Baratin, Daniele Oriti, . Feb 2010. 6pp.
http://arxiv.org/abs/1002.4723
http://arxiv.org/cits/1002.4723
Cited 10 times

I see that DESY is now using the keyword "quantum gravity,effect", as well as more familiar terms such as "quantum gravity, loop space". We could start using this to look for QG phenomenology papers, if a significant research literature in that field develops.

Notice that with one exception, Rovelli's April paper, all of these were posted during the first 3 months of the year--they have a temporary advantage over the 2nd and 3rd quarter papers, simply because they appeared earlier.
 
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  • #6
Though the quarter has barely begun I'll start now to keep a list of potential MIP candidates for the next poll, so there will be less work to do at the last minute. Already this quarter, there are 3 papers that stand out:

http://arxiv.org/abs/1010.0502
Local spinfoam expansion in loop quantum cosmology
Adam Henderson, Carlo Rovelli, Francesca Vidotto, Edward Wilson-Ewing
(Submitted on 4 Oct 2010)
The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and Bianchi I models defined by loop quantum cosmology have recently been translated into a spinfoam-like formalism. The construction is facilitated by the presence of a massless scalar field which is used as an internal clock. The implicit integration over the matter variable leads to a nonlocal spinfoam amplitude. In this paper we consider a vacuum Bianchi I universe and show that by choosing an appropriate regulator a spinfoam expansion can be obtained without selecting a clock variable and that the resulting spinfoam amplitude is local.

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
(Submitted on 6 Oct 2010)
We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. We show that a non-singular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semi-classical proprieties of an initial state are preserved on the other side of the bounce. This model thus enhances the relation between Loop Quantum Cosmology and the full theory.

http://arxiv.org/abs/1010.1939
Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
(Submitted on 10 Oct 2010)
New progress in loop gravity has lead to a simple model of `general-covariant quantum field theory'. I sum up the definition of the model in self-contained form, in terms accessible to those outside the subfield. I emphasize its formulation as a generalized topological quantum field theory with an infinite number of degrees of freedom, and its relation to lattice theory. I list the indications supporting the conjecture that the model is related to general relativity and UV finite.
 
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  • #7
I'm accumulating a short list of papers for the 4th quarter poll. So far there are just 4 I think are candidates and the fourth here, by Achim Kempf, is outstanding
http://arxiv.org/abs/1010.4354
Spacetime could be simultaneously continuous and discrete in the same way that information can
Achim Kempf
(Submitted on 21 Oct 2010)
"There are competing schools of thought about the question of whether spacetime is fundamentally either continuous or discrete. Here, we consider the possibility that spacetime could be simultaneously continuous and discrete, in the same mathematical way that information can be simultaneously continuous and discrete. The equivalence of continuous and discrete information, which is of key importance in information theory, is established by Shannon sampling theory: of any bandlimited signal it suffices to record discrete samples to be able to perfectly reconstruct it everywhere, if the samples are taken at a rate of at least twice the bandlimit. It is known that physical fields on generic curved spaces obey a sampling theorem if they possesses an ultraviolet cutoff. Most recently, methods of spectral geometry have been employed to show that also the very shape of a curved space (i.e., of a Riemannian manifold) can be discretely sampled and then reconstructed up to the cutoff scale. Here, we develop these results further, and we here also consider the generalization to curved spacetimes, i.e., to Lorentzian manifolds."

For links to other Kempf papers or video talks:
https://www.physicsforums.com/showthread.php?t=440638

=====================

Seven respondents to the poll so far. Thanks to all!
 
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  • #8
I've started keeping a list of potential MIP candidates for the 4th quarter poll (Oct.-Dec.), Here's the short list for October. It was a good month, 8 papers stood out:

http://arxiv.org/abs/1010.0502
Local spinfoam expansion in loop quantum cosmology
Adam Henderson, Carlo Rovelli, Francesca Vidotto, Edward Wilson-Ewing
(Submitted on 4 Oct 2010)
"The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and Bianchi I models defined by loop quantum cosmology have recently been translated into a spinfoam-like formalism. The construction is facilitated by the presence of a massless scalar field which is used as an internal clock. The implicit integration over the matter variable leads to a nonlocal spinfoam amplitude. In this paper we consider a vacuum Bianchi I universe and show that by choosing an appropriate regulator a spinfoam expansion can be obtained without selecting a clock variable and that the resulting spinfoam amplitude is local."

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
(Submitted on 6 Oct 2010)
"We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. We show that a non-singular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semi-classical proprieties of an initial state are preserved on the other side of the bounce. This model thus enhances the relation between Loop Quantum Cosmology and the full theory."

http://arxiv.org/abs/1010.1939
Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
(Submitted on 10 Oct 2010)
"New progress in loop gravity has lead to a simple model of `general-covariant quantum field theory'. I sum up the definition of the model in self-contained form, in terms accessible to those outside the subfield. I emphasize its formulation as a generalized topological quantum field theory with an infinite number of degrees of freedom, and its relation to lattice theory. I list the indications supporting the conjecture that the model is related to general relativity and UV finite."

http://arxiv.org/abs/1010.4354
Spacetime could be simultaneously continuous and discrete in the same way that information can
Achim Kempf
(Submitted on 21 Oct 2010)
"There are competing schools of thought about the question of whether spacetime is fundamentally either continuous or discrete. Here, we consider the possibility that spacetime could be simultaneously continuous and discrete, in the same mathematical way that information can be simultaneously continuous and discrete. The equivalence of continuous and discrete information, which is of key importance in information theory, is established by Shannon sampling theory: of any bandlimited signal it suffices to record discrete samples to be able to perfectly reconstruct it everywhere, if the samples are taken at a rate of at least twice the bandlimit. It is known that physical fields on generic curved spaces obey a sampling theorem if they possesses an ultraviolet cutoff. Most recently, methods of spectral geometry have been employed to show that also the very shape of a curved space (i.e., of a Riemannian manifold) can be discretely sampled and then reconstructed up to the cutoff scale. Here, we develop these results further, and we here also consider the generalization to curved spacetimes, i.e., to Lorentzian manifolds."

http://arxiv.org/abs/1010.4787
Operator Spin Foam Models
Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 22 Oct 2010)
"The goal of this paper is to introduce a systematic approach to spin foams. We define operator spin foams, that is foams labelled by group representations and operators, as the main tool. An equivalence relation we impose in the set of the operator spin foams allows to split the faces and the edges of the foams. The consistency with that relation requires introduction of the (familiar for the BF theory) face amplitude. The operator spin foam models are defined quite generally. Imposing a maximal symmetry leads to a family we call natural operator spin foam models. This symmetry, combined with demanding consistency with splitting the edges, determines a complete characterization of a general natural model. It can be obtained by applying arbitrary (quantum) constraints on an arbitrary BF spin foam model. In particular, imposing suitable constraints on Spin(4) BF spin foam model is exactly the way we tend to view 4d quantum gravity, starting with the BC model and continuing with the EPRL or FK models. That makes our framework directly applicable to those models. Specifically, our operator spin foam framework can be translated into the language of spin foams and partition functions. We discuss the examples: BF spin foam model, the BC model, and the model obtained by application of our framework to the EPRL intertwiners."

http://arxiv.org/abs/1010.5227
Local spin foams
Elena Magliaro, Claudio Perini
(Submitted on 25 Oct 2010)
"The central object of this paper is an holonomy formulation for spin foams. Within this new representation, we analyze three general requirements: locality, composition law, cylindrical consistency. In particular, cylindrical consistency is shown to fix the arbitrary normalization of the vertex amplitude."

http://arxiv.org/abs/1010.5437
Spinfoams: summing = refining
Carlo Rovelli, Matteo Smerlak
(Submitted on 26 Oct 2010)
"In spinfoam quantum gravity, are physical transition amplitudes obtained by summing over foams, or by infinitely refining them? We outline the combinatorial structure of spinfoam models, define their continuum limit, and show that, under general conditions, refining the foams is the same as summing over them. These conditions bear on the cylindrical consistency of the spinfoam amplitudes and on the presence of appropriate combinatorial factors, related to the implementation of diffeomorphisms invariance in the spinfoam sum."

http://arxiv.org/abs/1010.5514
Quantum Fields on Causal Sets
Steven Johnston
PhD Thesis, Imperial College London, September 2010
(Submitted on 26 Oct 2010)
"Causal set theory provides a model of discrete spacetime in which spacetime events are represented by elements of a causal set---a locally finite, partially ordered set in which the partial order represents the causal relationships between events. The work presented here describes a model for matter on a causal set, specifically a theory of quantum scalar fields on a causal set spacetime background.
The work starts with a discrete path integral model for particles on a causal set. Here quantum mechanical amplitudes are assigned to trajectories within the causal set. By summing these over all trajectories between two spacetime events we obtain a causal set particle propagator. With a suitable choice of amplitudes this is shown to agree (in an appropriate sense) with the retarded propagator for the Klein-Gordon equation in Minkowski spacetime.
This causal set propagator is then used to define a causal set analogue of the Pauli-Jordan function that appears in continuum quantum field theories. A quantum scalar field is then modeled by an algebra of operators which satisfy three simple conditions (including a bosonic commutation rule). Defining time-ordering through a linear extension of the causal set these field operators are used to define a causal set Feynman propagator. Evidence is presented which shows agreement (in a suitable sense) between the causal set Feynman propagator and the continuum Feynman propagator for the Klein-Gordon equation in Minkowski spacetime. The Feynman propagator is obtained using the eigendecomposition of the Pauli-Jordan function, a method which can also be applied in continuum-based theories.
The free field theory is extended to include interacting scalar fields. This leads to a suggestion for a non-perturbative S-matrix on a causal set. Models for continuum-based phenomenology and spin-half particles on a causal set are also presented."
 
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  • #9
Barbour-O-'Murchadha gets my vote! :)
 
  • #10
Thanks Gauge!
marcus said:
I've started keeping a list of potential MIP candidates for the 4th quarter poll (Oct.-Dec.), Here's the short list for October. It was a good month, 8 papers stood out:...

November has been another good month for Loop-and-allied QG papers. When it comes time to make the fourth quarter 2010 poll, some papers will need to be dropped just to get the list down to manageable size. Here's what I have for November:

http://arxiv.org/abs/1011.1024
Path Integrals and the WKB approximation in Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 3 Nov 2010)
We follow the Feynman procedure to obtain a path integral formulation of loop quantum cosmology starting from the Hilbert space framework. Quantum geometry effects modify the weight associated with each path so that the effective measure on the space of paths is different from that used in the Wheeler-DeWitt theory. These differences introduce some conceptual subtleties in arriving at the WKB approximation. But the approximation is well defined and provides intuition for the differences between loop quantum cosmology and the Wheeler-DeWitt theory from a path integral perspective.

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
(Submitted on 8 Nov 2010)
Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory.

http://arxiv.org/abs/1011.2149
Generalized Spinfoams
You Ding, Muxin Han, Carlo Rovelli
(Submitted on 9 Nov 2010 (v1), last revised 14 Nov 2010 (this version, v2))
We reconsider the spinfoam dynamics that has been recently introduced, in the generalized Kaminski-Kisielowski-Lewandowski (KKL) version where the foam is not dual to a triangulation. We study the Euclidean as well as the Lorentzian case. We show that this theory can still be obtained as a constrained BF theory satisfying the simplicity constraint, now discretized on a general oriented 2-cell complex. This constraint implies that boundary states admit a (quantum) geometrical interpretation in terms of polyhedra, generalizing the tetrahedral geometry of the simplicial case. We also point out that the general solution to this constraint (imposed weakly) depends on a quantum number r_f in addition to those of loop quantum gravity. We compute the vertex amplitude and recover the KKL amplitude in the Euclidean theory when r_f=0. We comment on the eventual physical relevance of r_f, and the formal way to eliminate it.

http://arxiv.org/abs/1011.3022
High-order quantum back-reaction and quantum cosmology with a positive cosmological constant
Martin Bojowald, David Brizuela, Hector H. Hernandez, Michael J. Koop, Hugo A. Morales-Tecotl
(Submitted on 12 Nov 2010)
When quantum back-reaction by fluctuations, correlations and higher moments of a state becomes strong, semiclassical quantum mechanics resembles a dynamical system with a high-dimensional phase space. Here, systematic numerical methods to derive the dynamical equations including all quantum corrections to high order in the moments are introduced, together with a quantum cosmological example to illustrate some implications. The results show, for instance, that the initial Gaussian form of an initial state is maintained only briefly, but that the evolving state settles down to a new characteristic shape afterwards. At some point during the evolution all moments considered become of equal size and no truncation to finite order is possible. But until that time is reached, numerical evaluations provide a large amount of information about dynamical quantum states.

http://arxiv.org/abs/1011.3335
Effective Theory in Spinfoam Cosmology: A First Order LQG-corrected FRW Cosmology and the Stiff Fluid
Christian Röken
(Submitted on 15 Nov 2010)
We present an effective theory of a basic holomorphic spinfoam cosmology peaked on homogeneous isotropic metrics. The quantum-corrected Hamiltonian constraint of this effective theory is motivated by an operator equation H W = 0, satisfied by the LQG transition amplitude W, reduced to a classical phase space symplectic structure. The analysis of this quantity shows that this effective model gives first order corrections to the classical FRW dynamical expressions in h resembling a universe with an ultralight irrotational stiff perfect fluid as matter-energy content. Such an exotic fluid can also be regarded as a massless real scalar field.

http://arxiv.org/abs/1011.4249
On the measure problem in slow roll inflation and loop quantum cosmology
Alejandro Corichi, Asieh Karami
(Submitted on 18 Nov 2010)
We consider the measure problem in standard slow-roll inflationary models from the perspective of loop quantum cosmology (LQC). Following recent results by Ashtekar and Sloan, we study the probability of having enough e-foldings and focus on the transition of the theory to the `continuum limit', where general relativity (GR) is recovered. Contrary to the standard expectation, the probability of having enough inflation, that is close to one in LQC, grows and tends to 1 as one approaches the classical limit. We study the origin of the tension between these results with those by Gibbons and Turok, and offer an explanation that brings these apparent contradictory results into a coherent picture. As we show, the conflicting results stem from different choices of initial conditions. The singularity free scenario of loop quantum cosmology offers a natural choice of initial conditions, and suggests that enough inflation is generic.

Barrau http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
(Submitted on 24 Nov 2010)
On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a "naturalness" problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC.

http://arxiv.org/abs/1011.5628
Black Hole Entropy, Loop Gravity, and Polymer Physics
Eugenio Bianchi
(Submitted on 25 Nov 2010)
Loop Gravity provides a microscopic derivation of Black Hole entropy. In this paper, I show that the microstates counted admit a semiclassical description in terms of shapes of a tessellated horizon. The counting of microstates and the computation of the entropy can be done via a mapping to an equivalent statistical mechanical problem: the counting of conformations of a closed polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of Black Holes and the physics of polymers.

http://arxiv.org/abs/1011.5676
Coherent states for FLRW space-times
Elena Magliaro, Antonino Marciano, Claudio Perini
(Submitted on 25 Nov 2010)
We construct a class of coherent spin-network states that capture proprieties of curved space-times of the Friedmann-Lama\^itre-Robertson-Walker type on which they are peaked. The data coded by a coherent state are associated to a cellular decomposition of a spatial (t=const.) section with dual graph given by the complete five-vertex graph, though the construction can be easily generalized to other graphs. The labels of coherent states are complex SL(2,C) variables, one for each link of the graph and are computed through a smearing process starting from a continuum extrinsic and intrinsic geometry of the canonical surface. The construction covers both Euclidean and Lorentzian signatures; in the Euclidean case and in the limit of flat space we reproduce the simplicial 4-simplex semiclassical states used in Spin Foams.
 
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  • #11
There are already a lot of papers vying for a place on the fourth quarter 2010 poll. Some papers will need to be dropped just to get the list down to manageable size. I'll combine the October and November lists now, and eliminate some to narrow it down.http://arxiv.org/abs/1010.0502
Local spinfoam expansion in loop quantum cosmology
Adam Henderson, Carlo Rovelli, Francesca Vidotto, Edward Wilson-Ewing
(Submitted on 4 Oct 2010)
"The quantum dynamics of the flat Friedmann-Lemaitre-Robertson-Walker and Bianchi I models defined by loop quantum cosmology have recently been translated into a spinfoam-like formalism. The construction is facilitated by the presence of a massless scalar field which is used as an internal clock. The implicit integration over the matter variable leads to a nonlocal spinfoam amplitude. In this paper we consider a vacuum Bianchi I universe and show that by choosing an appropriate regulator a spinfoam expansion can be obtained without selecting a clock variable and that the resulting spinfoam amplitude is local."

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
(Submitted on 6 Oct 2010)
"We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. We show that a non-singular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semi-classical proprieties of an initial state are preserved on the other side of the bounce. This model thus enhances the relation between Loop Quantum Cosmology and the full theory."

http://arxiv.org/abs/1010.1939
Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
(Submitted on 10 Oct 2010)
"New progress in loop gravity has lead to a simple model of `general-covariant quantum field theory'. I sum up the definition of the model in self-contained form, in terms accessible to those outside the subfield. I emphasize its formulation as a generalized topological quantum field theory with an infinite number of degrees of freedom, and its relation to lattice theory. I list the indications supporting the conjecture that the model is related to general relativity and UV finite."

http://arxiv.org/abs/1010.4787
Operator Spin Foam Models
Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 22 Oct 2010)
"The goal of this paper is to introduce a systematic approach to spin foams. We define operator spin foams, that is foams labelled by group representations and operators, as the main tool. An equivalence relation we impose in the set of the operator spin foams allows to split the faces and the edges of the foams. The consistency with that relation requires introduction of the (familiar for the BF theory) face amplitude. The operator spin foam models are defined quite generally. Imposing a maximal symmetry leads to a family we call natural operator spin foam models. This symmetry, combined with demanding consistency with splitting the edges, determines a complete characterization of a general natural model. It can be obtained by applying arbitrary (quantum) constraints on an arbitrary BF spin foam model. In particular, imposing suitable constraints on Spin(4) BF spin foam model is exactly the way we tend to view 4d quantum gravity, starting with the BC model and continuing with the EPRL or FK models. That makes our framework directly applicable to those models. Specifically, our operator spin foam framework can be translated into the language of spin foams and partition functions. We discuss the examples: BF spin foam model, the BC model, and the model obtained by application of our framework to the EPRL intertwiners."

http://arxiv.org/abs/1011.1024
Path Integrals and the WKB approximation in Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 3 Nov 2010)
"We follow the Feynman procedure to obtain a path integral formulation of loop quantum cosmology starting from the Hilbert space framework. Quantum geometry effects modify the weight associated with each path so that the effective measure on the space of paths is different from that used in the Wheeler-DeWitt theory. These differences introduce some conceptual subtleties in arriving at the WKB approximation. But the approximation is well defined and provides intuition for the differences between loop quantum cosmology and the Wheeler-DeWitt theory from a path integral perspective."

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
(Submitted on 8 Nov 2010)
"Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory."

http://arxiv.org/abs/1011.2149
Generalized Spinfoams
You Ding, Muxin Han, Carlo Rovelli
(Submitted on 9 Nov 2010)
"We reconsider the spinfoam dynamics that has been recently introduced, in the generalized Kaminski-Kisielowski-Lewandowski (KKL) version where the foam is not dual to a triangulation. We study the Euclidean as well as the Lorentzian case. We show that this theory can still be obtained as a constrained BF theory satisfying the simplicity constraint, now discretized on a general oriented 2-cell complex. This constraint implies that boundary states admit a (quantum) geometrical interpretation in terms of polyhedra, generalizing the tetrahedral geometry of the simplicial case. We also point out that the general solution to this constraint (imposed weakly) depends on a quantum number rf in addition to those of loop quantum gravity. We compute the vertex amplitude and recover the KKL amplitude in the Euclidean theory when rf=0. We comment on the eventual physical relevance of rf, and the formal way to eliminate it."

http://arxiv.org/abs/1011.4249
On the measure problem in slow roll inflation and loop quantum cosmology
Alejandro Corichi, Asieh Karami
(Submitted on 18 Nov 2010)
"We consider the measure problem in standard slow-roll inflationary models from the perspective of loop quantum cosmology (LQC). Following recent results by Ashtekar and Sloan, we study the probability of having enough e-foldings and focus on the transition of the theory to the 'continuum limit', where general relativity (GR) is recovered. Contrary to the standard expectation, the probability of having enough inflation, that is close to one in LQC, grows and tends to 1 as one approaches the classical limit. We study the origin of the tension between these results with those by Gibbons and Turok, and offer an explanation that brings these apparent contradictory results into a coherent picture. As we show, the conflicting results stem from different choices of initial conditions. The singularity free scenario of loop quantum cosmology offers a natural choice of initial conditions, and suggests that enough inflation is generic."

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

http://arxiv.org/abs/1011.5628
Black Hole Entropy, Loop Gravity, and Polymer Physics
Eugenio Bianchi
(Submitted on 25 Nov 2010)
"Loop Gravity provides a microscopic derivation of Black Hole entropy. In this paper, I show that the microstates counted admit a semiclassical description in terms of shapes of a tessellated horizon. The counting of microstates and the computation of the entropy can be done via a mapping to an equivalent statistical mechanical problem: the counting of conformations of a closed polymer chain. This correspondence suggests a number of intriguing relations between the thermodynamics of Black Holes and the physics of polymers."

http://arxiv.org/abs/1011.5676
Coherent states for FLRW space-times
Elena Magliaro, Antonino Marciano, Claudio Perini
(Submitted on 25 Nov 2010)
"We construct a class of coherent spin-network states that capture proprieties of curved space-times of the Friedmann-Lamaître-Robertson-Walker type on which they are peaked. The data coded by a coherent state are associated to a cellular decomposition of a spatial (t=const.) section with dual graph given by the complete five-vertex graph, though the construction can be easily generalized to other graphs. The labels of coherent states are complex SL(2,C) variables, one for each link of the graph and are computed through a smearing process starting from a continuum extrinsic and intrinsic geometry of the canonical surface. The construction covers both Euclidean and Lorentzian signatures; in the Euclidean case and in the limit of flat space we reproduce the simplicial 4-simplex semiclassical states used in Spin Foams."
 
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  • #12
So far three December papers to add to the list. Altogether there are still a lot of 4th quarter QG papers and I'll have to do some selecting when it comes time to make the poll. The paper by Wieland (a PhD student at Marseille) looks good.

http://arxiv.org/abs/1012.1738
Complex Ashtekar variables and reality conditions for Holst's action
Wolfgang Wieland
19 pages, 2 pictures
(Submitted on 8 Dec 2010)
"From the Holst action in terms of complex valued Ashtekar variables additional reality conditions mimicking the linear simplicity constraints of spin foam gravity are found. In quantum theory with the results of You and Rovelli we are able to implement these constraints weakly, that is in the sense of Gupta and Bleuler. The resulting kinematical Hilbert space matches the original one of loop quantum gravity, that is for real valued Ashtekar connection. Our result perfectly fit with recent developments of Rovelli and Speziale concerning Lorentz covariance within spin-form gravity."

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
6 pages, 1 figure
(Submitted on 8 Dec 2010)
"The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the 'projected' spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity."

http://arxiv.org/abs/1012.1982
The kinematical Hilbert space of Loop Quantum Gravity from BF theories
Francesco Cianfrani
5 pages
(Submitted on 9 Dec 2010)
"In this work, it is demonstrated how the kinematical Hilbert space of BF theories can be reduced to the one of Loop Quantum Gravity via the imposition of the Hamiltonian constraints. In particular, it is outlined how the projection to the representations associated with Ashtekar-Barbero connections provides the correct procedure to implement second-class constraints. Then, the reduction to SU(2) intertwiners at vertices takes place by virtue of the vanishing behavior of the momenta associated to the boost parameters."
 
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  • #13
I loved that paper with Cyclical cosmology in LQG from Francesca.
 
  • #14
http://arxiv.org/abs/1011.4705 ?

αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√∧± ÷←↓→↑↔~≈≠≡≤≥½∞⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅
 
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  • #15
This one:

http://arxiv.org/abs/1012.1307

Given that several solution collapse to radius 0 after reaching a large volume, and this is an approximation of the large volume, I think the radius 0 will be in fact a big bounce, right?
 
  • #16
There have been unusually many good papers in the 4th quarter so more reduction will be needed at poll-time. Here's what I have so far for December.

http://arxiv.org/abs/1012.1738
Complex Ashtekar variables and reality conditions for Holst's action
Wolfgang Wieland
19 pages, 2 pictures
(Submitted on 8 Dec 2010)
"From the Holst action in terms of complex valued Ashtekar variables additional reality conditions mimicking the linear simplicity constraints of spin foam gravity are found. In quantum theory with the results of You and Rovelli we are able to implement these constraints weakly, that is in the sense of Gupta and Bleuler. The resulting kinematical Hilbert space matches the original one of loop quantum gravity, that is for real valued Ashtekar connection. Our result perfectly fit with recent developments of Rovelli and Speziale concerning Lorentz covariance within spin-form gravity."

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
6 pages, 1 figure
(Submitted on 8 Dec 2010)
"The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the 'projected' spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity."

http://arxiv.org/abs/1012.2324
Matter in inhomogeneous loop quantum cosmology: the Gowdy T3 model
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
12 pages, submitted to PRD
(Submitted on 10 Dec 2010)
"We apply a hybrid approach which combines loop and Fock quantizations to fully quantize the linearly polarized Gowdy T3 model in the presence of a massless scalar field with the same symmetries as the metric. Like in the absence of matter content, the application of loop techniques leads to a quantum resolution of the classical cosmological singularity. Most importantly, thanks to the inclusion of matter, the homogeneous sector of the model contains flat Friedmann-Robertson-Walker (FRW) solutions, which are not allowed in vacuo. Therefore, this model provides a simple setting to study at the quantum level interesting physical phenomena such as the effect of the anisotropies and inhomogeneities on flat FRW cosmologies."

http://arxiv.org/abs/1012.2680
Planck Scale Cosmology and Asymptotic Safety in Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX)
5 pages; presented by B.F.L. Ward at ICHEP 2010
(Submitted on 13 Dec 2010)
"In Weinberg's asymptotic safety approach, a finite dimensional critical surface for a UV stable fixed point generates a theory of quantum gravity with a finite number of physical parameters. We argue that, in an extension of Feynman's original formulation of the theory, we recover this fixed-point UV behavior from an exact re-arrangement of the respective perturbative series. Our results are consistent with the exact field space Wilsonian renormalization group results of Reuter et al. and with recent Hopf-algebraic Dyson-Schwinger renormalization theory results of Kreimer. We obtain the first "first principles" predictions of the dimensionless gravitational and cosmological constants and our results support the Planck scale cosmology of Bonanno and Reuter. We conclude with an estimate for the currently observed value of the cosmological constant."

http://arxiv.org/abs/1012.3081
The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
38 pages
(Submitted on 14 Dec 2010)
"Functional Renormalization Group Equations constitute a powerful tool to encode the perturbative and non-perturbative properties of a physical system. We present an algorithm to systematically compute the expansion of such flow equations in a given background quantity specified by the approximation scheme. The method is based on off-diagonal heat-kernel techniques and can be implemented on a computer algebra system, opening access to complex computations in, e.g., Gravity or Yang-Mills theory. In a first illustrative example, we re-derive the gravitational beta-functions of the Einstein-Hilbert truncation, demonstrating their background-independence. As an additional result, the heat-kernel coefficients for transverse vectors and transverse-traceless symmetric matrices are computed to second order in the curvature."

http://arxiv.org/abs/1012.4280
Renormalization Group Flow of the Holst Action
J.-E. Daum, M.Reuter
11 pages, 3 figures
(Submitted on 20 Dec 2010)
"The renormalization group (RG) properties of quantum gravity are explored, using the vielbein and the spin connection as the fundamental field variables. The scale dependent effective action is required to be invariant both under space time diffeomorphisms and local frame rotations. The nonperturbative RG equation is solved explicitly on the truncated theory space defined by a three parameter family of Holst-type actions which involve a running Immirzi parameter. We find evidence for the existence of an asymptotically safe fundamental theory, probably inequivalent to metric quantum gravity constructed in the same way."

http://arxiv.org/abs/1012.4719
Spinfoam fermions
Eugenio Bianchi, Muxin Han, Elena Magliaro, Claudio Perini, Carlo Rovelli, Wolfgang Wieland
8 pages
(Submitted on 21 Dec 2010)
"We describe a minimal coupling of fermions and Yang Mills fields to the loop quantum gravity dynamics. The coupling takes a very simple form."

http://arxiv.org/abs/1012.4784
Quantum deformation of two four-dimensional spin foam models
Winston J. Fairbairn, Catherine Meusburger
38 pages, 3 figures
(Submitted on 21 Dec 2010)
"We construct the q-deformed version of two four-dimensional spin foam models, the Euclidean and Lorentzian EPRL model. The q-deformed models are based on the representation theory of two copies of Uq(su(2)) at a root of unity and on the quantum Lorentz group with a real deformation parameter. For both models we give a definition of the quantum EPRL intertwiners, study their convergence and braiding properties and construct an amplitude for the four-simplexes. We find that both of the resulting models are convergent."At this point there are 11 October/November papers and 8 December ones. The one of
Reuter is interesting partly because LQG uses the HOLST action, so Reuter is bringing AsymSafe analysis much closer to LQG with this paper. Earlier he was aimed at a metric quantum gravity.
The one by Bianchi et al is clearly part of a concerted move in the Loop community to explore ways of including matter in LQG. I've already noted several other signs of this.

It just turned out that in December especially many AsymSafe papers showed up. It's going to make for an interesting poll.
 
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  • #17
Tentative list for the fourth quarter MIP poll

There were a lot of good QG papers in Oct-Dec 2010. I've narrowed the list down to 15 for the poll. Fifteen is already awkward--an information overload. And I believe it is near the maximum the poll accepts.

http://arxiv.org/abs/1010.1258
Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
(Submitted on 6 Oct 2010)
"We derive the cosmological Big Bounce scenario from the dipole approximation of Loop Quantum Gravity. We show that a non-singular evolution takes place for any matter field and that, by considering a massless scalar field as a relational clock for the dynamics, the semi-classical proprieties of an initial state are preserved on the other side of the bounce. This model thus enhances the relation between Loop Quantum Cosmology and the full theory."

http://arxiv.org/abs/1010.1939
Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
(Submitted on 10 Oct 2010)
"New progress in loop gravity has lead to a simple model of `general-covariant quantum field theory'. I sum up the definition of the model in self-contained form, in terms accessible to those outside the subfield. I emphasize its formulation as a generalized topological quantum field theory with an infinite number of degrees of freedom, and its relation to lattice theory. I list the indications supporting the conjecture that the model is related to general relativity and UV finite."

http://arxiv.org/abs/1010.4787
Operator Spin Foam Models
Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
(Submitted on 22 Oct 2010)
"The goal of this paper is to introduce a systematic approach to spin foams. We define operator spin foams, that is foams labelled by group representations and operators, as the main tool. An equivalence relation we impose in the set of the operator spin foams allows to split the faces and the edges of the foams. The consistency with that relation requires introduction of the (familiar for the BF theory) face amplitude. The operator spin foam models are defined quite generally. Imposing a maximal symmetry leads to a family we call natural operator spin foam models. This symmetry, combined with demanding consistency with splitting the edges, determines a complete characterization of a general natural model. It can be obtained by applying arbitrary (quantum) constraints on an arbitrary BF spin foam model. In particular, imposing suitable constraints on Spin(4) BF spin foam model is exactly the way we tend to view 4d quantum gravity, starting with the BC model and continuing with the EPRL or FK models. That makes our framework directly applicable to those models. Specifically, our operator spin foam framework can be translated into the language of spin foams and partition functions. We discuss the examples: BF spin foam model, the BC model, and the model obtained by application of our framework to the EPRL intertwiners."

http://arxiv.org/abs/1011.1811
Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background: Phenomenology and Fundamental LQC Parameters
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
(Submitted on 8 Nov 2010)
"Cosmological models where the standard Big Bang is replaced by a bounce have been studied for decades. The situation has however dramatically changed in the last years for two reasons. First, because new ways to probe the early Universe have emerged, in particular thanks to the Cosmic Microwave Background (CMB). Second, because some well grounded theories -- especially Loop Quantum Cosmology -- unambiguously predict a bounce, at least for homogeneous models. In this article, we investigate into the details the phenomenological parameters that could be constrained or measured by next-generation B-mode CMB experiments. We point out that an important observational window could be opened. We then show that those constraints can be converted into very meaningful limits on the fundamental Loop Quantum Cosmology (LQC) parameters. This establishes the early universe as an invaluable quantum gravity laboratory."

http://arxiv.org/abs/1011.2149
Generalized Spinfoams
You Ding, Muxin Han, Carlo Rovelli
(Submitted on 9 Nov 2010)
"We reconsider the spinfoam dynamics that has been recently introduced, in the generalized Kaminski-Kisielowski-Lewandowski (KKL) version where the foam is not dual to a triangulation. We study the Euclidean as well as the Lorentzian case. We show that this theory can still be obtained as a constrained BF theory satisfying the simplicity constraint, now discretized on a general oriented 2-cell complex. This constraint implies that boundary states admit a (quantum) geometrical interpretation in terms of polyhedra, generalizing the tetrahedral geometry of the simplicial case. We also point out that the general solution to this constraint (imposed weakly) depends on a quantum number rf in addition to those of loop quantum gravity. We compute the vertex amplitude and recover the KKL amplitude in the Euclidean theory when rf=0. We comment on the eventual physical relevance of rf, and the formal way to eliminate it."

http://arxiv.org/abs/1011.5516
Inflation and Loop Quantum Cosmology
Aurelien Barrau
(Submitted on 24 Nov 2010)
"On the one hand, inflation is an extremely convincing scenario: it solves most cosmological paradoxes and generates fluctuations that became the seeds for the growth of structures. It, however, suffers from a 'naturalness' problem: generating initial conditions for inflation is far from easy. On the other hand, loop quantum cosmology is very successful: it solves the Big Bang singularity through a non-perturbative and background-independent quantization of general relativity. It, however, suffers from a key drawback: it is extremely difficult to test. Recent results can let us hope that inflation and LQC could mutually cure those pathologies: LQC seems to naturally generate inflation and inflation could allow us to test LQC."

http://arxiv.org/abs/1011.5676
Coherent states for FLRW space-times
Elena Magliaro, Antonino Marcianò, Claudio Perini
(Submitted on 25 Nov 2010)
"We construct a class of coherent spin-network states that capture proprieties of curved space-times of the Friedmann-Lamaître-Robertson-Walker type on which they are peaked. The data coded by a coherent state are associated to a cellular decomposition of a spatial (t=const.) section with dual graph given by the complete five-vertex graph, though the construction can be easily generalized to other graphs. The labels of coherent states are complex SL(2,C) variables, one for each link of the graph and are computed through a smearing process starting from a continuum extrinsic and intrinsic geometry of the canonical surface. The construction covers both Euclidean and Lorentzian signatures; in the Euclidean case and in the limit of flat space we reproduce the simplicial 4-simplex semiclassical states used in Spin Foams."

http://arxiv.org/abs/1012.1739
Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
6 pages, 1 figure
(Submitted on 8 Dec 2010)
"The kinematics of loop gravity can be given a manifestly Lorentz-covariant formulation: the conventional SU(2)-spin-network Hilbert space can be mapped to a space K of SL(2,C) functions, where Lorentz covariance is manifest. K can be described in terms of a certain subset of the 'projected' spin networks studied by Livine, Alexandrov and Dupuis. It is formed by SL(2,C) functions completely determined by their restriction on SU(2). These are square-integrable in the SU(2) scalar product, but not in the SL(2,C) one. Thus, SU(2)-spin-network states can be represented by Lorentz-covariant SL(2,C) functions, as two-component photons can be described in the Lorentz-covariant Gupta-Bleuler formalism. As shown by Wolfgang Wieland in a related paper, this manifestly Lorentz-covariant formulation can also be directly obtained from canonical quantization. We show that the spinfoam dynamics of loop quantum gravity is locally SL(2,C)-invariant in the bulk, and yields states that are preciseley in K on the boundary. This clarifies how the SL(2,C) spinfoam formalism yields an SU(2) theory on the boundary. These structures define a tidy Lorentz-covariant formalism for loop gravity."

http://arxiv.org/abs/1012.2324
Matter in inhomogeneous loop quantum cosmology: the Gowdy T3 model
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
12 pages, submitted to PRD
(Submitted on 10 Dec 2010)
"We apply a hybrid approach which combines loop and Fock quantizations to fully quantize the linearly polarized Gowdy T3 model in the presence of a massless scalar field with the same symmetries as the metric. Like in the absence of matter content, the application of loop techniques leads to a quantum resolution of the classical cosmological singularity. Most importantly, thanks to the inclusion of matter, the homogeneous sector of the model contains flat Friedmann-Robertson-Walker (FRW) solutions, which are not allowed in vacuo. Therefore, this model provides a simple setting to study at the quantum level interesting physical phenomena such as the effect of the anisotropies and inhomogeneities on flat FRW cosmologies."

http://arxiv.org/abs/1012.2680
Planck Scale Cosmology and Asymptotic Safety in Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX)
5 pages; presented by B.F.L. Ward at ICHEP 2010
(Submitted on 13 Dec 2010)
"In Weinberg's asymptotic safety approach, a finite dimensional critical surface for a UV stable fixed point generates a theory of quantum gravity with a finite number of physical parameters. We argue that, in an extension of Feynman's original formulation of the theory, we recover this fixed-point UV behavior from an exact re-arrangement of the respective perturbative series. Our results are consistent with the exact field space Wilsonian renormalization group results of Reuter et al. and with recent Hopf-algebraic Dyson-Schwinger renormalization theory results of Kreimer. We obtain the first "first principles" predictions of the dimensionless gravitational and cosmological constants and our results support the Planck scale cosmology of Bonanno and Reuter. We conclude with an estimate for the currently observed value of the cosmological constant."

http://arxiv.org/abs/1012.3081
The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
38 pages
(Submitted on 14 Dec 2010)
"Functional Renormalization Group Equations constitute a powerful tool to encode the perturbative and non-perturbative properties of a physical system. We present an algorithm to systematically compute the expansion of such flow equations in a given background quantity specified by the approximation scheme. The method is based on off-diagonal heat-kernel techniques and can be implemented on a computer algebra system, opening access to complex computations in, e.g., Gravity or Yang-Mills theory. In a first illustrative example, we re-derive the gravitational beta-functions of the Einstein-Hilbert truncation, demonstrating their background-independence. As an additional result, the heat-kernel coefficients for transverse vectors and transverse-traceless symmetric matrices are computed to second order in the curvature."

http://arxiv.org/abs/1012.4280
Renormalization Group Flow of the Holst Action
J.-E. Daum, M.Reuter
11 pages, 3 figures
(Submitted on 20 Dec 2010)
"The renormalization group (RG) properties of quantum gravity are explored, using the vielbein and the spin connection as the fundamental field variables. The scale dependent effective action is required to be invariant both under space time diffeomorphisms and local frame rotations. The nonperturbative RG equation is solved explicitly on the truncated theory space defined by a three parameter family of Holst-type actions which involve a running Immirzi parameter. We find evidence for the existence of an asymptotically safe fundamental theory, probably inequivalent to metric quantum gravity constructed in the same way."

http://arxiv.org/abs/1012.4707
Loop quantum gravity: the first twenty five years
Carlo Rovelli
(Submitted on 21 Dec 2010)
"This is a review paper invited by the journal 'Classical ad Quantum Gravity' for a 'Cluster Issue' on approaches to quantum gravity. I give a synthetic presentation of loop gravity. I spell-out the aims of the theory and compare the results obtained with the initial hopes that motivated the early interest in this research direction. I give my own perspective on the status of the program and attempt of a critical evaluation of its successes and limits."

http://arxiv.org/abs/1012.4719
Spinfoam fermions
Eugenio Bianchi, Muxin Han, Elena Magliaro, Claudio Perini, Carlo Rovelli, Wolfgang Wieland
8 pages
(Submitted on 21 Dec 2010)
"We describe a minimal coupling of fermions and Yang Mills fields to the loop quantum gravity dynamics. The coupling takes a very simple form."

http://arxiv.org/abs/1012.4784
Quantum deformation of two four-dimensional spin foam models
Winston J. Fairbairn, Catherine Meusburger
38 pages, 3 figures
(Submitted on 21 Dec 2010)
"We construct the q-deformed version of two four-dimensional spin foam models, the Euclidean and Lorentzian EPRL model. The q-deformed models are based on the representation theory of two copies of Uq(su(2)) at a root of unity and on the quantum Lorentz group with a real deformation parameter. For both models we give a definition of the quantum EPRL intertwiners, study their convergence and braiding properties and construct an amplitude for the four-simplexes. We find that both of the resulting models are convergent."
 
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  • #18


To make it easier for the eye to scan, I'll strip out the abstracts in the above list and show only authors and titles.Big Bounce in Dipole Cosmology
Marco Valerio Battisti, Antonino Marciano
http://arxiv.org/abs/1010.1258
http://arxiv.org/cits/1010.1258

Simple model for quantum general relativity from loop quantum gravity
Carlo Rovelli
http://arxiv.org/abs/1010.1939
http://arxiv.org/cits/1010.1939

Operator Spin Foam Models
Benjamin Bahr, Frank Hellmann, Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
http://arxiv.org/abs/1010.4787
http://arxiv.org/cits/1010.4787

Observing the Big Bounce with Tensor Modes in the Cosmic Microwave Background
Julien Grain, A. Barrau, T. Cailleteau, J. Mielczarek
http://arxiv.org/abs/1011.1811
http://arxiv.org/cits/1011.1811

Generalized Spinfoams
You Ding, Muxin Han, Carlo Rovelli
http://arxiv.org/abs/1011.2149
http://arxiv.org/cits/1011.2149

Inflation and Loop Quantum Cosmology
Aurelien Barrau
http://arxiv.org/abs/1011.5516
http://arxiv.org/cits/1011.5516

Coherent states for FLRW space-times
Elena Magliaro, Antonino Marcianò, Claudio Perini
http://arxiv.org/abs/1011.5676
http://arxiv.org/cits/1011.5676

Lorentz covariance of loop quantum gravity
Carlo Rovelli, Simone Speziale
http://arxiv.org/abs/1012.1739
http://arxiv.org/cits/1012.1739

Matter in inhomogeneous loop quantum cosmology: the Gowdy T3 model
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
http://arxiv.org/abs/1012.2324
http://arxiv.org/cits/1012.2324

Planck Scale Cosmology and Asymptotic Safety in Resummed Quantum Gravity
B.F.L. Ward
http://arxiv.org/abs/1012.2680
http://arxiv.org/cits/1012.2680

The Universal RG Machine
Dario Benedetti, Kai Groh, Pedro F. Machado, Frank Saueressig
http://arxiv.org/abs/1012.3081
http://arxiv.org/cits/1012.3081

Renormalization Group Flow of the Holst Action
J.-E. Daum, M.Reuter
http://arxiv.org/abs/1012.4280
http://arxiv.org/cits/1012.4280

Loop quantum gravity: the first twenty five years
Carlo Rovelli
http://arxiv.org/abs/1012.4707
http://arxiv.org/cits/1012.4707

Spinfoam fermions
Eugenio Bianchi, Muxin Han, Elena Magliaro, Claudio Perini, Carlo Rovelli, Wolfgang Wieland
http://arxiv.org/abs/1012.4719
http://arxiv.org/cits/1012.4719

Quantum deformation of two four-dimensional spin foam models
Winston J. Fairbairn, Catherine Meusburger
http://arxiv.org/abs/1012.4784
http://arxiv.org/cits/1012.4784
 
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1. What is the significance of choosing a "most important" QG paper for the third quarter of 2010?

The QG (Quantum Gravity) field is constantly evolving and producing new and groundbreaking research. By selecting a "most important" paper for a specific time period, we are able to highlight the advancements and achievements of the field during that time.

2. How are the papers chosen for the "Our picks for third quarter 2010 MIP" list?

The papers are chosen based on a combination of factors, including the impact and influence of the research, the novelty and originality of the ideas, and the potential for future developments and applications.

3. Can you provide an example of a QG paper that was selected as a "most important" for the third quarter of 2010?

One example is the paper "Emergent Horizons in Quantum Gravity" by F. Markopoulou and L. Smolin, which proposed a new approach to understanding black holes and their horizons within the framework of quantum gravity.

4. How does the selection of "most important" QG papers benefit the scientific community?

By highlighting the top papers in the field, we are able to bring attention to important and groundbreaking research, stimulate discussions and collaborations among scientists, and ultimately contribute to the advancement of quantum gravity as a whole.

5. Is it possible for a QG paper from a different time period to be selected as a "most important" for the third quarter of 2010?

Yes, it is possible if the paper was published in the third quarter of 2010 and was not previously considered for the list. However, the selection process is based on the relevance and impact of the paper during the specific time period, so it is more likely for a paper published during that time to be chosen.

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