Our picks for third quarter 2012 MIP (most important QG paper)

[marcus]

Of the nineteen candidates, please choose those you think will prove most significant for future research in Loop-and-allied quantum gravity. Since the poll is multiple choice, it's possible to vote for several papers. Abstract summaries follow in the next post.

http://arxiv.org/abs/1207.0505
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan

http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski

http://arxiv.org/abs/1207.4596
The Construction of Spin Foam Vertex Amplitudes
Eugenio Bianchi, Frank Hellmann

http://arxiv.org/abs/1207.5156
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli

http://arxiv.org/abs/1207.6348
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland

http://arxiv.org/abs/1208.1030
Resilience of the Spectral Standard Model
Ali H. Chamseddine, Alain Connes

http://arxiv.org/abs/1208.2228
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard

http://arxiv.org/abs/1208.3388
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1208.5023
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
Christopher Estrada, Matilde Marcolli

http://arxiv.org/abs/1208.5874
A possibility to solve the problems with quantizing gravity
S. Hossenfelder

http://arxiv.org/abs/1209.0065
General relativistic statistical mechanics
Carlo Rovelli

http://arxiv.org/abs/1209.0473
Observational effects from quantum cosmology
Gianluca Calcagni

http://arxiv.org/abs/1209.1609
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1209.3649
Functional renormalization with fermions and tetrads
Pietro Donà, Roberto Percacci

http://arxiv.org/abs/1209.4374
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia

http://arxiv.org/abs/1209.4539
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1209.4892
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
Bianca Dittrich, James P. Ryan

http://arxiv.org/abs/1209.5060
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup

http://arxiv.org/abs/1209.5284
The Tensor Track: an Update
Vincent Rivasseau

The poll follows the same general format as MIP polls in the past, for example
third quarter 2011:
https://www.physicsforums.com/showthread.php?t=535170
fourth quarter 2011:
https://www.physicsforums.com/showthread.php?t=563724
second quarter 2012:
https://www.physicsforums.com/showthread.php?t=617250

 

[marcus]

http://arxiv.org/abs/1207.0505
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan
(Submitted on 2 Jul 2012)
There is sufficient amount of internal evidence in the nature of gravitational theories to indicate that gravity is an emergent phenomenon like, e.g, elasticity. Such an emergent nature is most apparent in the structure of gravitational dynamics. It is, however, possible to go beyond the field equations and study the space itself as emergent in a well-defined manner in (and possibly only in) the context of cosmology. In the first part of this review, I describe various pieces of evidence which show that gravitational field equations are emergent. In the second part, I describe a novel way of studying cosmology in which I interpret the expansion of the universe as equivalent to the emergence of space itself. In such an approach, the dynamics evolves towards a state of holographic equipartition, characterized by the equality of number of bulk and surface degrees of freedom in a region bounded by the Hubble radius. This principle correctly reproduces the standard evolution of a Friedmann universe. Further, (a) it demands the existence of an early inflationary phase as well as late time acceleration for its successful implementation and (b) allows us to link the value of late time cosmological constant to the e-folding factor during inflation.
38 pages; 5 figures

http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures

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

http://arxiv.org/abs/1207.5156
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli
(Submitted on 21 Jul 2012)
We suggest that large radiative corrections appearing in the spinfoam framework might be tied to the implicit sum over orientations. Specifically, we show that in a suitably simplified context the characteristic "spike" divergence of the Ponzano-Regge model disappears when restricting the theory to just one of the two orientations appearing in the asymptotic limit of the vertex amplitude.
10 pages, 5 figures

http://arxiv.org/abs/1207.6348
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland
(Submitted on 26 Jul 2012)
The spin foam formalism provides transition amplitudes for loop quantum gravity. Important aspects of the dynamics are understood, but many open questions are pressing on. In this paper we address some of them using a twistorial description, which brings new light on both classical and quantum aspects of the theory. At the classical level, we clarify the covariant properties of the discrete geometries involved, and the role of the simplicity constraints in leading to SU(2) Ashtekar-Barbero variables. We identify areas and Lorentzian dihedral angles in twistor space, and show that they form a canonical pair. The primary simplicity constraints are solved by simple twistors, parametrized by SU(2) spinors and the dihedral angles. We construct an SU(2) holonomy and prove it to correspond to the Ashtekar-Barbero connection. We argue that the role of secondary constraints is to provide a non trivial embedding of the cotangent bundle of SU(2) in the space of simple twistors. At the quantum level, a Schroedinger representation leads to a spinorial version of simple projected spin networks, where the argument of the wave functions is a spinor instead of a group element. We rewrite the Liouville measure on the cotangent bundle of SL(2,C) as an integral in twistor space. Using these tools, we show that the Engle-Pereira-Rovelli-Livine transition amplitudes can be derived from a path integral in twistor space. We construct a curvature tensor, show that it carries torsion off-shell, and that its Riemann part is of Petrov type D. Finally, we make contact between the semiclassical asymptotic behaviour of the model and our construction, clarifying the relation of the Regge geometries with the original phase space.
39 pages

http://arxiv.org/abs/1208.1030
Resilience of the Spectral Standard Model
Ali H. Chamseddine, Alain Connes
(Submitted on 5 Aug 2012)
We show that the inconsistency between the spectral Standard Model and the experimental value of the Higgs mass is resolved by the presence of a real scalar field strongly coupled to the Higgs field. This scalar field was already present in the spectral model and we wrongly neglected it in our previous computations. It was shown recently by several authors, independently of the spectral approach, that such a strongly coupled scalar field stabilizes the Standard Model up to unification scale in spite of the low value of the Higgs mass. In this letter we show that the noncommutative neutral singlet modifies substantially the RG analysis, invalidates our previous prediction of Higgs mass in the range 160--180 Gev, and restores the consistency of the noncommutative geometric model with the low Higgs mass.
13 pages.

http://arxiv.org/abs/1208.2228
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard
(Submitted on 10 Aug 2012)
We introduce semiclassical methods into the study of the volume spectrum in loop gravity. The classical system behind a 4-valent spinnetwork node is a Euclidean tetrahedron. We investigate the tetrahedral volume dynamics on phase space and apply Bohr-Sommerfeld quantization to find the volume spectrum. The analysis shows a remarkable quantitative agreement with the volume spectrum computed in loop gravity. Moreover, it provides new geometrical insights into the degeneracy of this spectrum and the maximum and minimum eigenvalues of the volume on intertwiner space.
32 pages, 10 figures

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

http://arxiv.org/abs/1208.5023
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
Christopher Estrada, Matilde Marcolli
(Submitted on 24 Aug 2012)
We study the renormalization group flow for the Higgs self coupling in the presence of gravitational correction terms. We show that the resulting equation is equivalent to a singular linear ODE, which has explicit solutions in terms of hypergeometric functions. We discuss the implications of this model with gravitational corrections on the Higgs mass estimates in particle physics models based on the spectral action functional.
25 pages

http://arxiv.org/abs/1208.5874
A possibility to solve the problems with quantizing gravity
S. Hossenfelder
(Submitted on 29 Aug 2012)
It is generally believed that quantum gravity is necessary to resolve the known tensions between general relativity and the quantum field theories of the standard model. Since perturbatively quantized gravity is non-renormalizable, the problem how to unify all interactions in a common framework has been open since the 1930s. Here, I propose a possibility to circumvent the known problems with quantizing gravity, as well as the known problems with leaving it unquantized: By changing the prescription for second quantization, a perturbative quantization of gravity is sufficient as an effective theory because matter becomes classical before the perturbative expansion breaks down. This is achieved by considering the vanishing commutator between a field and its conjugated momentum as a symmetry that is broken at low temperatures, and by this generates the quantum phase that we currently live in, while at high temperatures Planck's constant goes to zero.
4 pages, 1 figure

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

http://arxiv.org/abs/1209.0473
Observational effects from quantum cosmology
Gianluca Calcagni
(Submitted on 3 Sep 2012)
The status of quantum cosmologies as testable models of the early universe is assessed in the context of inflation. While traditional Wheeler-DeWitt quantization is unable to produce sizable effects in the cosmic microwave background, the more recent loop quantum cosmology can generate potentially detectable departures from the standard cosmic spectrum. Thus, present observations constrain the parameter space of the model, which could be made falsifiable by near-future experiments.
14 pages, 3 figures. Invited review article also containing original material

http://arxiv.org/abs/1209.1609
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 7 Sep 2012)
Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the paradigm is extended to a self-consistent theory from the Planck scale to the onset of slow roll inflation, covering some 11 orders of magnitude in energy density and curvature. This pre-inflationary dynamics also opens a small window for novel effects, e.g. a source for non-Gaussianities, which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
4 pages, 2 figures

http://arxiv.org/abs/1209.3649
Functional renormalization with fermions and tetrads
Pietro Donà, Roberto Percacci
(Submitted on 17 Sep 2012)
We investigate some aspects of the renormalization group flow of gravity in the presence of fermions, which have remained somewhat puzzling so far. The first is the sign of the fermionic contribution to the running of Newton's constant, which depends on details of the cutoff. We argue that only one of the previously used schemes correctly implements the cutoff on eigenvalues of the Dirac operator, and it acts in the sense of screening Newton's constant. We also show that Kähler fermions give the same contribution to the running of the cosmological and Newton constant as four Dirac spinors. We then calculate the graviton contributions to the beta functions by imposing the cutoffs on the irreducible spin components of the tetrad. In this way we can probe the gauge dependence of the off-shell flow. The results resemble closely those of the metric formalism, except for an increased scheme-- and (off shell) gauge--dependence.
28 pages, 4 figures

http://arxiv.org/abs/1209.4374
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia
(Submitted on 19 Sep 2012)
The holonomy-flux algebra A of loop quantum gravity is known to admit a natural representation that is uniquely singled out by the requirement of covariance under spatial diffeomorphisms. In the cosmological context, the requirement of spatial homogeneity naturally reduces A to a much smaller algebra, A_Red, used in loop quantum cosmology. In Bianchi I models, it is shown that the requirement of covariance under residual diffeomorphism symmetries again uniquely selects the representation of A_Red that has been commonly used. We discuss the close parallel between the two uniqueness results and also point out a difference.
9 pages

http://arxiv.org/abs/1209.4539
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski
(Submitted on 20 Sep 2012)
In this and the companion paper a novel holonomy formulation of so called Spin Foam models of lattice gauge gravity are explored. After giving a natural basis for the space of simplicity constraints we define a universal boundary Hilbert space, on which the imposition of different forms of the simplicity constraints can be studied. We detail under which conditions this Hilbert space can be mapped to a Hilbert space of projected spin networks or an ordinary spin network space.
These considerations allow to derive the general form of the transfer operators which generates discrete time evolution. We will describe the transfer operators for some current models on the different boundary Hilbert spaces and highlight the role of the simplicity constraints determining the concrete form of the time evolution operators.
51 pages, 18 figures

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

http://arxiv.org/abs/1209.5060
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 23 Sep 2012)
A new approach to a unified theory of quantum gravity based on noncommutative geometry and canonical quantum gravity is presented. The approach is built around a *-algebra generated by local holonomy-diffeomorphisms on a 3-manifold and a quantized Dirac type operator; the two capturing the kinematics of quantum gravity formulated in terms of Ashtekar variables. We prove that the separable part of the spectrum of the algebra is contained in the space of measurable connections modulo gauge transformations and we give limitations to the non-separable part. The construction of the Dirac type operator -- and thus the application of noncommutative geometry -- is motivated by the requirement of diffeomorphism invariance. We conjecture that a semi-finite spectral triple, which is invariant under volume-preserving diffeomorphisms, arise from a GNS construction of a semi-classical state. Key elements of quantum field theory emerge from the construction in a semi-classical limit, as does an almost commutative algebra. Finally, we note that the spectrum of loop quantum gravity emerges from a discretization of our construction. Certain convergence issues are left unresolved. This paper is the first of two where the second paper is concerned with mathematical details and proofs concerning the spectrum of the holonomy-diffeomorphism algebra.
43 pages, 2 figures.

http://arxiv.org/abs/1209.5284
The Tensor Track: an Update
Vincent Rivasseau
(Submitted on 24 Sep 2012 (v1), last revised 27 Sep 2012 (this version, v2))
The tensor track approach to quantum gravity is based on a new class of quantum field theories, called tensor group field theories (TGFTs). We provide a brief review of recent progress and list some desirable properties of TGFTs. In order to narrow the search for interesting models, we also propose a set of guidelines for TGFT's loosely inspired by the Osterwalder-Schrader axioms of ordinary Euclidean QFT.
12 pages, 1 figure. This paper is based on a talk given at the XXIX International Colloquium on Group-Theoretical Methods in Physics in Tian-Jin (China).

 

[tom.stoer]

as usual I voted for a paper that sheds light on a (new) approach towards a mathematical consistent construction of LQG

 

[marcus]

You chose a very interesting one! Wolfgang Wieland is first rate. He gave quite a convincing (I thought) presentation of this same material in a Perimeter seminar earlier this year:
http://pirsa.org/12020129/

You might enjoy seeing the video. You get to see everybody asking him questions, and how he answers--I recall him as being quick, poised, and humorous---a likeable intelligent style. He stood up well considering that he was a young postdoc, just visiting (as Eugenio Bianchi's guest) and he was being questioned energetically by senior Perimeter people.

I put that Wolfgang Wieland video on the first quarter 2012 poll
https://www.physicsforums.com/showthread.php?t=591299

The term twistor doesn't appear in the title (he is working with pairs of spinors and alternates terminology) but does in the abstract:

Spinor Quantisation for Complex Ashtekar Variables
Speaker(s): Wolfgang Wieland
Abstract: During the last couple of years Dupuis, Freidel, Livine, Speziale and Tambornino developed a twistorial formulation for loop quantum gravity.
Constructed from Ashtekar--Barbero variables, the formalism is restricted to SU(2) gauge transformations.
In this talk, I perform the generalisation to the full Lorentzian case, that is the group SL(2,C).
The phase space of SL(2,C) (i.e. complex or selfdual) Ashtekar variables on a spinnetwork graph is decomposed in terms of twistorial variables. To every link there are two twistors---one to each boundary point---attached. The formalism provides a clean derivation of the solution space of the reality conditions of loop quantum gravity.
Key features of the EPRL spinfoam model are perfectly recovered.
If there is still time, I'll scatch my current project concerning a twistorial path integral for spinfoam gravity as well.
Date: 29/02/2012

 

[marcus]

Thanks to those who have already responded, getting the poll off to a good start! Most of these papers have only been out for a month or two so it's early to be looking for citations, but wherever available I'll insert links to their "Inspire" records which give a bit more information up front than "Arxiv" does, including cite numbers. Inspire is Stanford's new HEP database that replaces "Spires". We can see if the Inspire links are useful and omit them in the next poll if they are not.

While getting the links I was pleased and a bit surprised to notice how many of the papers had already been cited in other research. One picked by Tom Stoer already has 2 cites, one that Chronos voted for (but I didn't) already has 4 cites, another of Chronos choices already shows 2, one that both Atyy and I picked has 3 cites...

http://arxiv.org/abs/1207.0505
http://inspirehep.net/record/1120932?ln=en
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan

http://arxiv.org/abs/1207.4353
http://inspirehep.net/record/1122962?ln=en
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski

http://arxiv.org/abs/1207.4596
http://inspirehep.net/record/1123095?ln=en
The Construction of Spin Foam Vertex Amplitudes
Eugenio Bianchi, Frank Hellmann

http://arxiv.org/abs/1207.5156
http://inspirehep.net/record/1123482?ln=en
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli

http://arxiv.org/abs/1207.6348
http://inspirehep.net/record/1123906?ln=en
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland

http://arxiv.org/abs/1208.1030
http://inspirehep.net/record/1125991?ln=en
Resilience of the Spectral Standard Model
Ali H. Chamseddine, Alain Connes

http://arxiv.org/abs/1208.2228
http://inspirehep.net/record/1127239?ln=en
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard

http://arxiv.org/abs/1208.3388
http://inspirehep.net/record/1127713?ln=en
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1208.5023
http://inspirehep.net/record/1181787?ln=en
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
Christopher Estrada, Matilde Marcolli

http://arxiv.org/abs/1208.5874
http://inspirehep.net/record/1182510?ln=en
A possibility to solve the problems with quantizing gravity
S. Hossenfelder

http://arxiv.org/abs/1209.0065
http://inspirehep.net/record/1184153?ln=en
General relativistic statistical mechanics
Carlo Rovelli

http://arxiv.org/abs/1209.0473
http://inspirehep.net/record/1184281?ln=en
Observational effects from quantum cosmology
Gianluca Calcagni

http://arxiv.org/abs/1209.1609
http://inspirehep.net/record/1184789?ln=en
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1209.3649
http://inspirehep.net/record/1185819?ln=en
Functional renormalization with fermions and tetrads
Pietro Donà, Roberto Percacci

http://arxiv.org/abs/1209.4374
http://inspirehep.net/record/1186716?ln=en
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia

http://arxiv.org/abs/1209.4539
http://inspirehep.net/record/1186723?ln=en
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1209.4892
http://inspirehep.net/record/1187613?ln=en
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
Bianca Dittrich, James P. Ryan

http://arxiv.org/abs/1209.5060
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup

http://arxiv.org/abs/1209.5284
http://inspirehep.net/record/1187780?ln=en
The Tensor Track: an Update
Vincent Rivasseau
=========================
EDIT: Sorry about this. I just realized that I omitted a paper that should have been on the poll. There should have been 20 instead of 19. The paper I somehow neglected to include is by Carrozza, Oriti, Rivasseau
http://arxiv.org/abs/arXiv:1207.6734
http://inspirehep.net/record/1124138
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau.
(Submitted on 28 Jul 2012)
We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.
33 pages, 8 figures.

If anyone wants to register a vote for this COR paper just let me know---by a post or PM, and I will tally up those votes along with the rest. The paper already has one vote, mine

 

[marcus]

as usual I voted for a paper that sheds light on a (new) approach towards a mathematical consistent construction of LQG

Tom, since you are interested in this possibly consistent construction (using twistor labels on the spin networks describing quantum states of geometry) as in the paper of Speziale and Wieland, and others may also be, I will mention the upcoming SEMINAR TALK by Speziale on this subject.

The talk will be on Tuesday 13 November and the link, to get the slides and audio online, will be http://relativity.phys.lsu.edu/ilqgs/

The title of the talk is:
Twistorial structure of loop quantum gravity transition amplitudes
This is almost word-for-word the same as the title of the paper by them in the poll, so it is probably going to be a good chance to get some more intuition of how the paper works, what the outlook for future is, and to hear discussion of it by other researchers. I'm glad we have these online ILQGS and PIRSA seminars--I feel I benefit a lot from hearing the talk and discussion.

 

[tom.stoer]

thanks for the hint ;-)

 

[marcus]

Looking back at the Loop-and-related papers that have come out this year, I got curious to see which ones were already cited in the literature. As was said earlier it is really too early to count cites a paper has to be out for 6 months to a year to give enough time for other researchers to use its results in their own work. But still we might learn something from taking a look.
Here are Loop and Spinfoam papers ranked by cites where it wasn't merely a lone author citing his own papers. Rightly or not I deducted lone self-citations, but kept them if they occurred through co-authorship. Where there was no self-citation, even as part of a collaboration, I marked the count "clear". After a while I stopped distinguishing.
Adapted from: http://inspirehep.net/search?ln=en&...2y=2012&sf=&so=a&rm=citation&rg=25&sc=0&of=hb

1. (11 clear)
The Spin Foam Approach to Quantum Gravity.
Alejandro Perez (Marseille, CPT). May 2012. 121 pp.
e-Print: arXiv:1205.2019 [gr-qc] PDF

2. (8 clear)
Entropy of quantum black holes.
Romesh K. Kaul (IMSc, Chennai). Jan 2012. 30 pp.
Published in SIGMA 8 (2012) 005
e-Print: arXiv:1201.6102 [gr-qc] PDF

3. (7 clear + 1 collaborative)
Entropy of Non-Extremal Black Holes from Loop Gravity.
Eugenio Bianchi (Perimeter Inst. Theor. Phys.). Apr 2012. 6 pp.
e-Print: arXiv:1204.5122 [gr-qc] PDF

4. (6 clear)
Constraint algebra in LQG reloaded : Toy model of a U(1)^{3} Gauge Theory I.
Adam Henderson, Alok Laddha, Casey Tomlin. Apr 2012. 60 pp.
e-Print: arXiv:1204.0211 [gr-qc] PDF

5. (5 clear)
A spin-foam vertex amplitude with the correct semiclassical limit.
Jonathan Engle (Florida U.). Jan 2012. 4 pp.
e-Print: arXiv:1201.2187 [gr-qc] PDF

6. (4 clear)
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology.
Thomas Cailleteau, Aurelien Barrau (LPSC, Grenoble), Julien Grain (Orsay, IAS), Francesca Vidotto (Marseille, CPT). Jun 2012. 5 pp.
e-Print: arXiv:1206.6736 [gr-qc] PDF

7. (4 clear)
General relativity as the equation of state of spin foam.
Lee Smolin (Perimeter Inst. Theor. Phys.). May 2012. 9 pp.
e-Print: arXiv:1205.5529 [gr-qc] PDF

8. (4 clear)
Hybrid quantization of an inflationary universe.
Mikel Fernandez-Mendez, Guillermo A. Mena Marugan, Javier Olmedo (Madrid, Inst. Estructura Materia). May 2012. 16 pp.
Published in Phys.Rev. D86 (2012) 024003
DOI: 10.1103/PhysRevD.86.024003
e-Print: arXiv:1205.1917 [gr-qc] PDF

9. (4 clear)
Perturbations in loop quantum cosmology.
William Nelson, Ivan Agullo, Abhay Ashtekar (Penn State U.). Apr 2012. 4 pp.
e-Print: arXiv:1204.1288 [gr-qc] PDF

10. (4 clear)
Radiation from quantum weakly dynamical horizons in LQG.
Daniele Pranzetti (Potsdam, Max Planck Inst.). Apr 2012. 9 pp.
Published in Phys.Rev.Lett. 109 (2012) 011301
e-Print: arXiv:1204.0702 [gr-qc] PDF

11. (4 clear)
Loop quantum gravity without the Hamiltonian constraint.
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn (Erlangen - Nuremberg U., Theorie III). Mar 2012. 5 pp.
e-Print: arXiv:1203.6525 [gr-qc] PDF

12. (4)
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau. Jul 2012. 33 pp.
e-Print: arXiv:1207.6734 [hep-th] PDF

13. (4)
Horizon energy as the boost boundary term in general relativity and loop gravity.
Eugenio Bianchi, Wolfgang Wieland (Perimeter Inst. Theor. Phys. and Marseille, CPT). May 2012. 3 pp.
e-Print: arXiv:1205.5325 [gr-qc] PDF

14. (4)
Discrete Symmetries in Covariant LQG.
Carlo Rovelli, Edward Wilson-Ewing (Marseille, CPT). May 2012. 8 pp.
Published in Phys.Rev. D86 (2012) 064002
e-Print: arXiv:1205.0733 [gr-qc] PDF

15. (4)
Group field cosmology: a cosmological field theory of quantum geometry.
Gianluca Calcagni, Steffen Gielen, Daniele Oriti. Jan 2012. 14 pp.
Published in Class.Quant.Grav. 29 (2012) 105005
e-Print: arXiv:1201.4151 [gr-qc] PDF

16. (4)
Problem of Time in Quantum Gravity.
Edward Anderson (APC, Paris). Jun 2012. 26 pp.
e-Print: arXiv:1206.2403 [gr-qc] PDF

 

[marcus]

Thanks to everyone who has joined in the poll so far!
I think two of those responding are professional researchers familiar with (and at least one directly involved in) LQG. It's helpful to know their choices, along with those of everyone else.

Eugenio Bianchi did an excellent job today, presenting his Black Hole Horizon Entropy talk.
This fall semester set of ILQGS talks is the best of any semester I can remember.
Holonomy Spinfoams (i.e. foams carrying group element labels)
BH Horizon Entropy
Tensor GFT (30 October)
Twistorial LQG (13 November)
Dittrich's TBA
GR in Observer Space
http://relativity.phys.lsu.edu/ilqgs/

all the talks this fall season are by young people coming into their own as strong contributors to the field.

Bianchi had some new things to say, referring to work in preparation (with various co-authors) and drawing connections with the papers on time reversal in LQG (e.g. "divergences and orientation" paper)---in fact with several of the papers on this MIP poll
If I remember right he also drew a connection with the paper on General Relativistic Statistical Mechanics, which also deals with Thermodynamics in a GR setting. There was 20 minutes of lively discussion at the end of the hour.

 

[marcus]

This is a reminder that the poll has one additional paper (COR = Carrozza Oriti Rivasseau) which I added the next day, October 1, after posting the initial list of 19 papers. See the note at the bottom of this list. Anyone can vote for the COR paper, even if you already responded to the poll, because it was accidentally omitted from the original list.

It has only one vote so far, possibly because people didn't notice that it was one of the options.

...
http://arxiv.org/abs/1207.0505
http://inspirehep.net/record/1120932?ln=en
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan

http://arxiv.org/abs/1207.4353
http://inspirehep.net/record/1122962?ln=en
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski

http://arxiv.org/abs/1207.4596
http://inspirehep.net/record/1123095?ln=en
The Construction of Spin Foam Vertex Amplitudes
Eugenio Bianchi, Frank Hellmann

http://arxiv.org/abs/1207.5156
http://inspirehep.net/record/1123482?ln=en
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli

http://arxiv.org/abs/1207.6348
http://inspirehep.net/record/1123906?ln=en
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland

http://arxiv.org/abs/1208.1030
http://inspirehep.net/record/1125991?ln=en
Resilience of the Spectral Standard Model
Ali H. Chamseddine, Alain Connes

http://arxiv.org/abs/1208.2228
http://inspirehep.net/record/1127239?ln=en
Bohr-Sommerfeld Quantization of Space
Eugenio Bianchi, Hal M. Haggard

http://arxiv.org/abs/1208.3388
http://inspirehep.net/record/1127713?ln=en
Holonomy Spin Foam Models: Definition and Coarse Graining
Benjamin Bahr, Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1208.5023
http://inspirehep.net/record/1181787?ln=en
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
Christopher Estrada, Matilde Marcolli

http://arxiv.org/abs/1208.5874
http://inspirehep.net/record/1182510?ln=en
A possibility to solve the problems with quantizing gravity
S. Hossenfelder

http://arxiv.org/abs/1209.0065
http://inspirehep.net/record/1184153?ln=en
General relativistic statistical mechanics
Carlo Rovelli

http://arxiv.org/abs/1209.0473
http://inspirehep.net/record/1184281?ln=en
Observational effects from quantum cosmology
Gianluca Calcagni

http://arxiv.org/abs/1209.1609
http://inspirehep.net/record/1184789?ln=en
A Quantum Gravity Extension of the Inflationary Scenario
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1209.3649
http://inspirehep.net/record/1185819?ln=en
Functional renormalization with fermions and tetrads
Pietro Donà, Roberto Percacci

http://arxiv.org/abs/1209.4374
http://inspirehep.net/record/1186716?ln=en
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Abhay Ashtekar, Miguel Campiglia

http://arxiv.org/abs/1209.4539
http://inspirehep.net/record/1186723?ln=en
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Bianca Dittrich, Frank Hellmann, Wojciech Kaminski

http://arxiv.org/abs/1209.4892
http://inspirehep.net/record/1187613?ln=en
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
Bianca Dittrich, James P. Ryan

http://arxiv.org/abs/1209.5060
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup

http://arxiv.org/abs/1209.5284
http://inspirehep.net/record/1187780?ln=en
The Tensor Track: an Update
Vincent Rivasseau
=========================
EDIT: Sorry about this. I just realized that I omitted a paper that should have been on the poll. There should have been 20 instead of 19. The paper I somehow neglected to include is by Carrozza, Oriti, Rivasseau
http://arxiv.org/abs/arXiv:1207.6734
http://inspirehep.net/record/1124138
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau.
(Submitted on 28 Jul 2012)
We tackle the issue of renormalizability for Tensorial Group Field Theories (TGFT) including gauge invariance conditions, with the rigorous tool of multi-scale analysis, to prepare the ground for applications to quantum gravity models. In the process, we define the appropriate generalization of some key QFT notions, including: connectedness, locality and contraction of (high) subgraphs. We also define a new notion of Wick ordering, corresponding to the subtraction of (maximal) melonic tadpoles. We then consider the simplest examples of dynamical 4-dimensional TGFT with gauge invariance conditions for the Abelian U(1) case. We prove that they are super-renormalizable for any polynomial interaction.
33 pages, 8 figures.

If anyone wants to register a vote for this COR paper just let me know---by a post or PM, and I will tally up those votes along with the rest. The paper already has one vote, mine

The above was posted 1 October, a day after posting the poll itself---when I realized I had overlooked an important paper. Sorry about the oversight.

 

[marcus]

Thanks to everyone who has taken part in the poll so far! Seven people besides myself. I'll tally up our votes.

4 votes:
Resilience of the Spectral Standard Model

3 votes:
Holonomy Spin Foam Models: Definition and Coarse Graining

General relativistic statistical mechanics

2 votes:
Emergent perspective of Gravity and Dark Energy

The twistorial structure of loop-gravity transition amplitudes

A possibility to solve the problems with quantizing gravity

A Quantum Gravity Extension of the Inflationary Scenario

Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators

And among those papers that just got ONE vote there was a "sleeper" that may turn out to be the most important of the whole lot. It was a "write-in"--you had to post or send me a PM to get your choice counted--by an oversight I only added it to the list the day after posting the poll. That "sleeper" was:
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
http://arxiv.org/abs/1207.6734
http://inspirehep.net/record/1124138
If you check out the inspirehep record you'll see that it has 6 cites already--not bad for a paper that's only been out 3 months.

 

[marcus]

This is turning into a really interesting poll, partly because of the 9 who have responded so far (thoughtfully and with wide differences) but also because many of papers this quarter show directions of change in LQG---research lines along which a reformulation of the theory may be in progress. In addition, other (non-Loop) approaches to quantum gravity are represented in this quarter's poll as well.

Again thanks for your responses: Tom, Francesca, Chronos, Atyy, Nonlinearity, FalseVacuum, Devils, and VodkaCannon!

Here's the tally so far:

4 votes:
Resilience of the Spectral Standard Model

3 votes:
Holonomy Spin Foam Models: Definition and Coarse Graining

General relativistic statistical mechanics

The twistorial structure of loop-gravity transition amplitudes

Emergent perspective of Gravity and Dark Energy

2 votes:
A possibility to solve the problems with quantizing gravity

A Quantum Gravity Extension of the Inflationary Scenario

Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators

=========
Just as a reminder, among the papers that got only one vote there was a "write-in"--you had to post or send me a PM to get your choice counted--because by oversight I neglected to include it until the day after posting the list.
Renormalization of Tensorial Group Field Theories: Abelian U(1) Models in Four Dimensions.
http://arxiv.org/abs/1207.6734
=========

In the present situation with LQG it helps to keep in mind a short checklist of five research fronts where the theory may be developing or changing. I'm using short abbreviated names to make them easy to remember:

GR Thermo (see Bianchi's black hole entropy paper, and forthcoming *entanglement entropy and graviton coupling* paper, also the GR Statistical Mechanics paper by Rovelli on the current poll)
TGFT (tensorial group field theory)
HSF (holonomy spinfoam models)
twistorLQG
dust (and field of observers/clocks--ways to recover a real Hamiltonian--see Derek Wise's ILQGS talk, or the Gielen Wise paper on the 2nd quarter MIP poll)

 

[marcus]

Since the last time we checked, two more people have entered their choices in the poll---Oftigus and Pureinterest. So we are 11 in total. Thanks all!
I'll tabulate the votes in the next few days but now, since it is near the end of the 4th quarter, we should start narrowing down the list of candidates for the year-end MIP poll.http://arxiv.org/abs/1212.5246
Gravitational origin of the weak interaction's chirality
Stephon Alexander, Antonino Marciano, Lee Smolin
(Submitted on 20 Dec 2012)
We present a new unification of the electro-weak and gravitational interactions based on the joining the weak SU(2) gauge fields with the left handed part of the space-time connection, into a single gauge field valued in the complexification of the local Lorentz group. Hence, the weak interactions emerge as the right handed chiral half of the space-time connection, which explains the chirality of the weak interaction. This is possible, because, as shown by Plebanski, Ashtekar, and others, the other chiral half of the space-time connection is enough to code the dynamics of the gravitational degrees of freedom.
This unification is achieved within an extension of the Plebanski action previously proposed by one of us. The theory has two phases. A parity symmetric phase yields, as shown by Speziale, a bi-metric theory with eight degrees of freedom: the massless graviton, a massive spin two field and a scalar ghost. Because of the latter this phase is unstable. Parity is broken in a stable phase where the eight degrees of freedom arrange themselves as the massless graviton coupled to an SU(2) triplet of chirally coupled Yang-Mills fields. It is also shown that under this breaking a Dirac fermion expresses itself as a chiral neutrino paired with a scalar field with the quantum numbers of the Higgs.
21 pages

http://arxiv.org/abs/1212.5166
Modeling black holes with angular momentum in loop quantum gravity
Ernesto Frodden, Alejandro Perez, Daniele Pranzetti, Christian Roeken
(Submitted on 20 Dec 2012)
We construct a SU(2) connection formulation of Kerr isolated horizons. As in the non-rotating case, the model is based on a SU(2) Chern-Simons theory describing the degrees of freedom on the horizon. The presence of a non-vanishing angular momentum modifies the admissibility conditions for spin network states. Physical states of the system are in correspondence with open intertwiners with total spin matching the angular momentum of the spacetime.
18 pages.

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers
(Submitted on 20 Dec 2012)
We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein-Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity and loop quantum gravity, as well as the AdS/CFT correspondence.
8 pages, 1 figure

http://arxiv.org/abs/1212.4473
Statistical Entropy of a BTZ Black Hole from Loop Quantum Gravity
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 18 Dec 2012)
We compute the statistical entropy of a BTZ black hole in the context of three-dimensional Euclidean loop quantum gravity with a cosmological constant Λ. As in the four-dimensional case, a quantum state of the black hole is characterized by a spin network state. Now however, the underlying colored graph Γ lives in a two-dimensional spacelike surface Σ, and some of its links cross the black hole horizon, which is viewed as a circular boundary of Σ. Each link l crossing the horizon is colored by a spin j_l (at the kinematical level), and the length L of the horizon is given by the sum L=Ʃ_lL_l of the fundamental length contributions L_l carried by the spins j_l of the links l. We propose an estimation for the number N^BTZ_Γ(L,Λ) of the Euclidean BTZ black hole microstates (defined on a fixed graph Γ) based on an analytic continuation from the case Λ>0 to the case Λ<0. In our model, we show that N^BTZ_Γ(L,Λ) reproduces the Bekenstein-Hawking entropy in the classical limit. This asymptotic behavior is independent of the choice of the graph Γ provided that the condition L=Ʃ_lL_l is satisfied, as it should be in three-dimensional quantum gravity.

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 17 Dec 2012)
In loop quantum gravity, the number N_Γ(a_H, γ) of microstates of a black hole for a given discrete geometry Γ depends on the so-called Barbero-Immirzi parameter γ. Using a suitable analytic continuation of γ to complex values, we show that the number N_Γ(a_H, ±i) of microstates behaves as exp(a_H/(4 l_p²)) for large area a_H in the large spin semiclassical limit. Such a correspondence with the semiclassical Bekenstein-Hawking entropy law points towards an unanticipated and remarkable feature of the original complex Ashtekar variables for quantum gravity.
5 pages

http://arxiv.org/abs/1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek
(Submitted on 14 Dec 2012)
The state of asymptotic silence, characterized by causal disconnection of the space points, emerges from various approaches aiming to describe gravitational phenomena in the limit of large curvatures. In particular, such behavior was anticipated by Belinsky, Khalatnikov and Lifgarbagez (BKL) in their famous conjecture put forward in the early seventies of the last century. While the BKL conjecture is based on purely classical considerations, one can expect that asymptotic silence should have its quantum counterpart at the level of a more fundamental theory of quantum gravity, which is the relevant description of gravitational phenomena in the limit of large energy densities. Here, we summarize some recent results which give support to such a possibility. More precisely, we discuss occurrence of the asymptotic silence due to polymerization of space at the Planck scale, in the framework of loop quantum cosmology. In the discussed model, the state of asymptotic silence is realized at the energy density ρ = ρ_c/2, where ρ_c is the maximal allowed energy density, being of the order of the Planck energy density. At energy densities ρ > ρ_c/2, the universe becomes 4D Euclidean space without causal structure. Therefore, the asymptotic silence appears to be an intermediate state of space between the Lorentzian and Euclidean phases.
4 pages, 3 figures, talk presented at the Multiverse and Fundamental Cosmology Conference, 10-14 September, 2012, Szczecin, Poland

http://arxiv.org/abs/1212.2852
Primordial tensor power spectrum in holonomy corrected Omega-LQC
Linda Linsefors, Thomas Cailleteau, Aurelien Barrau, Julien Grain
(Submitted on 12 Dec 2012)
The holonomy correction is one of the main terms arising when implementing loop quantum gravity ideas at an effective level in cosmology. The recent construction of an anomaly free algebra has shown that the formalism used, up to now, to derive the primordial spectrum of fluctuations was not correct. This article aims at computing the tensor spectrum in a fully consistent way within this deformed and closed algebra.
5 pages, 6 figures

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller
(Submitted on 9 Dec 2012)
We present an analysis of the dynamics of the equifacial pentahedron on the Kapovich-Millson phase space under a volume preserving Hamiltonian. The classical dynamics of polyhedra under such a Hamiltonian may arise from the classical limit of the node volume operators in loop quantum gravity. The pentahedron is the simplest nontrivial polyhedron for which the dynamics may be chaotic. We consider the distribution of polyhedral configurations throughout the space and find indications that the borders between certain configurations act as separatrices. We examine the local stability of trajectories within this phase space and find that locally unstable regions dominate although extended stable regions are present. Canonical and microcanonical estimates of the Kolmogorov-Sinai entropy suggest that the pentahedron is a strongly chaotic system. The presence of chaos is further suggested by calculations of intermediate time Lyapunov exponents which saturate to non zero values.
20 Pages, 19 Figures

http://arxiv.org/abs/1212.2204
Electric Time in Quantum Cosmology
Stephon Alexander, Martin Bojowald, Antonino Marciano, David Simpson
(Submitted on 10 Dec 2012)
Effective quantum cosmology is formulated with a realistic global internal time given by the electric vector potential. New possibilities for the quantum behavior of space-time are found, and the high-density regime is shown to be very sensitive to the specific form of state realized.
19 pages, 5 figures.

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard
(Submitted on 30 Nov 2012)
We show that chaotic classical dynamics associated to the volume of discrete grains of space leads to quantal spectra that are gapped between zero and nonzero volume. This strengthens the connection between spectral discreteness in the quantum geometry of gravity and tame ultraviolet behavior. We complete a detailed analysis of the geometry of a pentahedron, providing new insights into the volume operator and evidence of classical chaos in the dynamics it generates. These results reveal an unexplored realm of application for chaos in quantum gravity.
5 pages, 4 figures

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing
(Submitted on 27 Nov 2012)
In the matter bounce scenario, a dust-dominated contracting space-time generates scale-invariant perturbations that, assuming a nonsingular bouncing cosmology, propagate to the expanding branch and set appropriate initial conditions for the radiation-dominated era. Since this scenario depends on the presence of a bounce, it seems appropriate to consider it in the context of loop quantum cosmology where a bouncing universe naturally arises. It turns out that quantum gravity effects play an important role beyond simply providing the bounce. Indeed, quantum gravity corrections to the Mukhanov-Sasaki equations significantly modify some of the results obtained in a purely classical setting: while the predicted spectra of scalar and tensor perturbations are both almost scale-invariant with identical small red tilts in agreement with previous results, the tensor to scalar ratio is now expected to be r≈ 9 x 10^-4, which is much smaller than the original classical prediction. Finally, for the predicted amplitude of the scalar perturbations to agree with observations, the critical density in loop quantum cosmology must be of the order ρ_crit ~ 10^-9 ρ_Planck.
8 pages

http://arxiv.org/abs/1211.4807
Holonomy-flux spinfoam amplitude
Claudio Perini
(Submitted on 20 Nov 2012)
We introduce a holomorphic representation for the Lorentzian EPRL spinfoam on arbitrary 2-complexes. The representation is obtained via the Ashtekar-Lewandowski-Marolf-Mourao-Thiemann heat kernel coherent state transform. The new variables are classical holonomy-flux phase space variables (h,X) ≈ T*SU(2) of Hamiltonian loop quantum gravity prescribing the holonomies of the Ashtekar connection A = Γ + γK, and their conjugate gravitational fluxes. For small heat kernel 'time' the spinfoam amplitude is peaked on classical space-time geometries, where at most countably many curvatures are allowed for non-zero Barbero-Immirzi parameter. We briefly comment on the possibility to use the alternative flipped classical limit.
33 pages

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland
(Submitted on 9 Nov 2012)
Twisted geometry is a piecewise-flat geometry less rigid than Regge geometry. In Loop Gravity, it provides the classical limit for each step of the truncation utilized in the definition of the quantum theory. We define the torsionless spin-connection of a twisted geometry. The difficulty given by the discontinuity of the triad is addressed by interpolating between triads. The curvature of the resulting spin connection reduces to the Regge curvature in the case of a Regge geometry.
5 pages, 2 figures

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 6 Nov 2012)
Cosmological perturbations are generally described by quantum fields on (curved but) classical space-times. While this strategy has a large domain of validity, it can not be justified in the quantum gravity era where curvature and matter densities are of Planck scale. Using techniques from loop quantum gravity, the standard theory of cosmological perturbations is extended to overcome this limitation. The new framework sharpens conceptual issues by distinguishing between the true and apparent trans-Planckian difficulties and provides sufficient conditions under which the true difficulties can be overcome within a quantum gravity theory. In a companion paper, this framework is applied to the standard inflationary model, with interesting implications to theory as well as observations.
50 pages. This is first of the two detailed papers on which arXiv 1209.1609 (PRL at press) is based

http://arxiv.org/abs/1211.1244
Schwinger-Dyson Equations in Group Field Theories of Quantum Gravity
Thomas Krajewski
(Submitted on 6 Nov 2012)
In this talk, we elaborate on the operation of graph contraction introduced by Gurau in his study of the Schwinger-Dyson equations. After a brief review of colored tensor models, we identify the Lie algebra appearing in the Schwinger-Dyson equations as a Lie algebra associated to a Hopf algebra of the Connes-Kreimer type. Then, we show how this operation also leads to an analogue of the Wilsonian flow for the effective action. Finally, we sketch how this formalism may be adapted to group field theories.
6 pages. Talk given at "The XXIX International Colloquium on Group-Theoretical Methods in Physics", Chern Institute of Mathematics August 2012, submitted to the conference proceedings

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi
(Submitted on 2 Nov 2012)
We compute the low-energy variation of the horizon entanglement entropy for matter fields and gravitons in Minkowski space. While the entropy is divergent, the variation under a perturbation of the vacuum state is finite and proportional to the energy flux through the Rindler horizon. Due to the universal coupling of gravitons to the energy-momentum tensor, the variation of the entanglement entropy is universal and equal to the change in area of the event horizon divided by 4 times Newton's constant - independently from the number and type of matter fields. The physical mechanism presented provides an explanation of the microscopic origin of the Bekenstein-Hawking entropy in terms of entanglement entropy.
7 pages

http://arxiv.org/abs/1211.0161
Emergent Isotropy-Breaking in Quantum Cosmology
Andrea Dapor, Jerzy Lewandowski
(Submitted on 1 Nov 2012)
We consider a massive quantum test Klein-Gordon field probing an isotropic quantum cosmological space-time in the background. The result obtained is surprising. It turns out, that despite the isotropy of the quantum gravitational field, the semi-classical metric
experienced by a mode of the K-G field is non-isotropic. The anisotropy depends on the direction of the momentum of the mode. Specifically, what we do is to derive a semi-classical space-time which emerges to a mode of the field. The method amounts to a comparison between QFT on a quantum background and QFT on a classical curved space-time, giving rise to an emergent metric tensor. The components of the semi-classical metric tensor are calculated from the equation of propagation of the quantum K-G field in the test field approximation. The anisotropies are of a quantum nature: they are proportional to Planck constant and "dress" the isotropic classical space-time obtained in the classical limit.
6 pages

http://arxiv.org/abs/1210.6215
Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering
Gianluca Delfino, Kirill Krasnov, Carlos Scarinci
(Submitted on 23 Oct 2012)
We continue to develop the pure connection formalism for gravity. We derive the Feynman rules for computing the connection correlation functions, as well as the prescription for obtaining the Minkowski space graviton scattering amplitudes from the latter. The present formalism turns out to be significantly simpler than the one based on the metric in many aspects. The most drastic difference with the usual approach is that the conformal factor of the metric, which is a source of difficulties in the metric treatment, does not propagate in the connection formulation even off-shell. This simplifies both the linearized theory and the interactions. For comparison, in our approach the complete off-shell cubic GR interaction contains just 3 terms, with only a single term relevant at tree level. This should be compared to at least a dozen terms in the metric formalism. We put the technology developed to use and compute the simplest graviton-graviton scattering amplitudes...
... This serves as a good illustration of the type of parity violation present in these theories. We find that the parity-violating amplitudes are important at high energies, and that a general parity-violating member of our class of theories "likes" one helicity (negative in our conventions) more than the other in the sense that at high energies it tends to convert all present gravitons into those of negative helicity.
46 pages.

http://arxiv.org/abs/1210.5276
Geometric asymptotics for spin foam lattice gauge gravity on arbitrary triangulations
Frank Hellmann, Wojciech Kaminski
(Submitted on 18 Oct 2012)
We study the behavior of holonomy spin foam partition functions, a form of lattice gauge gravity, on generic 4d-triangulations using micro local analysis. To do so we adapt tools from the renormalization theory of quantum field theory on curved space times. This allows us, for the first time, to study the partition function without taking any limits on the interior of the triangulation.
We establish that for many of the most widely used models the geometricity constraints, which reduce the gauge theory to a geometric one, introduce strong accidental curvature constraints. These limit the curvature around each triangle of the triangulation to a finite set of values. We demonstrate how to modify the partition function to avoid this problem. Finally the new methods introduced provide a starting point for studying the regularization ambiguities and renormalization of the partition function.
4+6 pages, 1 figure

http://arxiv.org/abs/1210.4504
A new perspective on cosmology in Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 16 Oct 2012)
We present a new cosmological model derived from Loop Quantum Gravity. The formulation is based on a projection of the kinematical Hilbert space of the full theory down to a subspace representing the proper arena for an inhomogeneous Bianchi I model. This procedure gives a direct link between the full theory and its cosmological sector. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could also shed light on Loop Quantum Cosmology and its relation with the full theory.
5 pages

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell
(Submitted on 1 Oct 2012)
Loop quantum cosmology allows for arbitrary superpositions of the triad variable. We show here how these superpositions can become indistinguishable from a classical mixture by the interaction with fermions. We calculate the reduced density matrix for a locally rotationally symmetric Bianchi I model and show that the purity factor for the triads decreases by decoherence. In this way, the Universe assumes a definite orientation.
12 pages, 1 figure

That's 21 in all, I'll have to edit some.

 

[marcus]

Time to revise the list of candidates. This gets the list down to 19.

http://arxiv.org/abs/1212.5571
A positive formalism for quantum theory in the generalized boundary formulation
Robert Oeckl
(Submitted on 21 Dec 2012)
We introduce a new "positive formalism" for encoding quantum theories in the general boundary formulation, somewhat analogous to the mixed state formalism of the standard formulation. This makes the probability interpretation more natural and elegant, eliminates operationally irrelevant structure and opens the general boundary formulation to quantum information theory.
28 pages

http://arxiv.org/abs/1212.5246
Gravitational origin of the weak interaction's chirality
Stephon Alexander, Antonino Marciano, Lee Smolin
(Submitted on 20 Dec 2012)
We present a new unification of the electro-weak and gravitational interactions based on the joining the weak SU(2) gauge fields with the left handed part of the space-time connection, into a single gauge field valued in the complexification of the local Lorentz group. Hence, the weak interactions emerge as the right handed chiral half of the space-time connection, which explains the chirality of the weak interaction. This is possible, because, as shown by Plebanski, Ashtekar, and others, the other chiral half of the space-time connection is enough to code the dynamics of the gravitational degrees of freedom.
This unification is achieved within an extension of the Plebanski action previously proposed by one of us. The theory has two phases. A parity symmetric phase yields, as shown by Speziale, a bi-metric theory with eight degrees of freedom: the massless graviton, a massive spin two field and a scalar ghost. Because of the latter this phase is unstable. Parity is broken in a stable phase where the eight degrees of freedom arrange themselves as the massless graviton coupled to an SU(2) triplet of chirally coupled Yang-Mills fields. It is also shown that under this breaking a Dirac fermion expresses itself as a chiral neutrino paired with a scalar field with the quantum numbers of the Higgs.
21 pages

http://arxiv.org/abs/1212.5183
On the Architecture of Spacetime Geometry
Eugenio Bianchi, Robert C. Myers
(Submitted on 20 Dec 2012)
We propose entanglement entropy as a probe of the architecture of spacetime in quantum gravity. We argue that the leading contribution to this entropy satisfies an area law for any sufficiently large region in a smooth spacetime, which, in fact, is given by the Bekenstein-Hawking formula. This conjecture is supported by various lines of evidence from perturbative quantum gravity, simplified models of induced gravity and loop quantum gravity, as well as the AdS/CFT correspondence.
8 pages, 1 figure

http://arxiv.org/abs/1212.4060
Black Hole Entropy from complex Ashtekar variables
Ernesto Frodden, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 17 Dec 2012)
In loop quantum gravity, the number N_Γ(a_H, γ) of microstates of a black hole for a given discrete geometry Γ depends on the so-called Barbero-Immirzi parameter γ. Using a suitable analytic continuation of γ to complex values, we show that the number N_Γ(a_H, ±i) of microstates behaves as exp(a_H/(4 l_p²)) for large area a_H in the large spin semiclassical limit. Such a correspondence with the semiclassical Bekenstein-Hawking entropy law points towards an unanticipated and remarkable feature of the original complex Ashtekar variables for quantum gravity.
5 pages

http://arxiv.org/abs/1212.3527
Asymptotic silence in loop quantum cosmology
Jakub Mielczarek
(Submitted on 14 Dec 2012)
The state of asymptotic silence, characterized by causal disconnection of the space points, emerges from various approaches aiming to describe gravitational phenomena in the limit of large curvatures. In particular, such behavior was anticipated by Belinsky, Khalatnikov and Lifgarbagez (BKL) in their famous conjecture put forward in the early seventies of the last century. While the BKL conjecture is based on purely classical considerations, one can expect that asymptotic silence should have its quantum counterpart at the level of a more fundamental theory of quantum gravity, which is the relevant description of gravitational phenomena in the limit of large energy densities. Here, we summarize some recent results which give support to such a possibility. More precisely, we discuss occurrence of the asymptotic silence due to polymerization of space at the Planck scale, in the framework of loop quantum cosmology. In the discussed model, the state of asymptotic silence is realized at the energy density ρ = ρ_c/2, where ρ_c is the maximal allowed energy density, being of the order of the Planck energy density. At energy densities ρ > ρ_c/2, the universe becomes 4D Euclidean space without causal structure. Therefore, the asymptotic silence appears to be an intermediate state of space between the Lorentzian and Euclidean phases.
4 pages, 3 figures, talk presented at the Multiverse and Fundamental Cosmology Conference, 10-14 September, 2012, Szczecin, Poland

http://arxiv.org/abs/1212.2852
Primordial tensor power spectrum in holonomy corrected Omega-LQC
Linda Linsefors, Thomas Cailleteau, Aurelien Barrau, Julien Grain
(Submitted on 12 Dec 2012)
The holonomy correction is one of the main terms arising when implementing loop quantum gravity ideas at an effective level in cosmology. The recent construction of an anomaly free algebra has shown that the formalism used, up to now, to derive the primordial spectrum of fluctuations was not correct. This article aims at computing the tensor spectrum in a fully consistent way within this deformed and closed algebra.
5 pages, 6 figures

http://lanl.arxiv.org/abs/1212.1930
A "Helium Atom" of Space: Dynamical Instability of the Isochoric Pentahedron
Christopher E. Coleman-Smith, Berndt Müller
(Submitted on 9 Dec 2012)
We present an analysis of the dynamics of the equifacial pentahedron on the Kapovich-Millson phase space under a volume preserving Hamiltonian. The classical dynamics of polyhedra under such a Hamiltonian may arise from the classical limit of the node volume operators in loop quantum gravity. The pentahedron is the simplest nontrivial polyhedron for which the dynamics may be chaotic. We consider the distribution of polyhedral configurations throughout the space and find indications that the borders between certain configurations act as separatrices. We examine the local stability of trajectories within this phase space and find that locally unstable regions dominate although extended stable regions are present. Canonical and microcanonical estimates of the Kolmogorov-Sinai entropy suggest that the pentahedron is a strongly chaotic system. The presence of chaos is further suggested by calculations of intermediate time Lyapunov exponents which saturate to non zero values.
20 Pages, 19 Figures

http://arxiv.org/abs/1211.7311
Pentahedral volume, chaos, and quantum gravity
Hal M. Haggard
(Submitted on 30 Nov 2012)
We show that chaotic classical dynamics associated to the volume of discrete grains of space leads to quantal spectra that are gapped between zero and nonzero volume. This strengthens the connection between spectral discreteness in the quantum geometry of gravity and tame ultraviolet behavior. We complete a detailed analysis of the geometry of a pentahedron, providing new insights into the volume operator and evidence of classical chaos in the dynamics it generates. These results reveal an unexplored realm of application for chaos in quantum gravity.
5 pages, 4 figures

http://arxiv.org/abs/1211.6269
The Matter Bounce Scenario in Loop Quantum Cosmology
Edward Wilson-Ewing
(Submitted on 27 Nov 2012)
In the matter bounce scenario, a dust-dominated contracting space-time generates scale-invariant perturbations that, assuming a nonsingular bouncing cosmology, propagate to the expanding branch and set appropriate initial conditions for the radiation-dominated era. Since this scenario depends on the presence of a bounce, it seems appropriate to consider it in the context of loop quantum cosmology where a bouncing universe naturally arises. It turns out that quantum gravity effects play an important role beyond simply providing the bounce. Indeed, quantum gravity corrections to the Mukhanov-Sasaki equations significantly modify some of the results obtained in a purely classical setting: while the predicted spectra of scalar and tensor perturbations are both almost scale-invariant with identical small red tilts in agreement with previous results, the tensor to scalar ratio is now expected to be r≈ 9 x 10^-4, which is much smaller than the original classical prediction. Finally, for the predicted amplitude of the scalar perturbations to agree with observations, the critical density in loop quantum cosmology must be of the order ρ_crit ~ 10^-9 ρ_Planck.
8 pages

http://arxiv.org/abs/1211.4807
Holonomy-flux spinfoam amplitude
Claudio Perini
(Submitted on 20 Nov 2012)
We introduce a holomorphic representation for the Lorentzian EPRL spinfoam on arbitrary 2-complexes. The representation is obtained via the Ashtekar-Lewandowski-Marolf-Mourao-Thiemann heat kernel coherent state transform. The new variables are classical holonomy-flux phase space variables (h,X) ≈ T*SU(2) of Hamiltonian loop quantum gravity prescribing the holonomies of the Ashtekar connection A = Γ + γK, and their conjugate gravitational fluxes. For small heat kernel 'time' the spinfoam amplitude is peaked on classical space-time geometries, where at most countably many curvatures are allowed for non-zero Barbero-Immirzi parameter. We briefly comment on the possibility to use the alternative flipped classical limit.
33 pages

http://arxiv.org/abs/1211.2166
The spin connection of twisted geometry
Hal M. Haggard, Carlo Rovelli, Francesca Vidotto, Wolfgang Wieland
(Submitted on 9 Nov 2012)
Twisted geometry is a piecewise-flat geometry less rigid than Regge geometry. In Loop Gravity, it provides the classical limit for each step of the truncation utilized in the definition of the quantum theory. We define the torsionless spin-connection of a twisted geometry. The difficulty given by the discontinuity of the triad is addressed by interpolating between triads. The curvature of the resulting spin connection reduces to the Regge curvature in the case of a Regge geometry.
5 pages, 2 figures

http://arxiv.org/abs/1211.1354
An Extension of the Quantum Theory of Cosmological Perturbations to the Planck Era
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 6 Nov 2012)
Cosmological perturbations are generally described by quantum fields on (curved but) classical space-times. While this strategy has a large domain of validity, it can not be justified in the quantum gravity era where curvature and matter densities are of Planck scale. Using techniques from loop quantum gravity, the standard theory of cosmological perturbations is extended to overcome this limitation. The new framework sharpens conceptual issues by distinguishing between the true and apparent trans-Planckian difficulties and provides sufficient conditions under which the true difficulties can be overcome within a quantum gravity theory. In a companion paper, this framework is applied to the standard inflationary model, with interesting implications to theory as well as observations.
50 pages. This is first of the two detailed papers on which arXiv 1209.1609 (PRL at press) is based

http://arxiv.org/abs/1211.1244
Schwinger-Dyson Equations in Group Field Theories of Quantum Gravity
Thomas Krajewski
(Submitted on 6 Nov 2012)
In this talk, we elaborate on the operation of graph contraction introduced by Gurau in his study of the Schwinger-Dyson equations. After a brief review of colored tensor models, we identify the Lie algebra appearing in the Schwinger-Dyson equations as a Lie algebra associated to a Hopf algebra of the Connes-Kreimer type. Then, we show how this operation also leads to an analogue of the Wilsonian flow for the effective action. Finally, we sketch how this formalism may be adapted to group field theories.
6 pages. Talk given at "The XXIX International Colloquium on Group-Theoretical Methods in Physics", Chern Institute of Mathematics August 2012, submitted to the conference proceedings

http://arxiv.org/abs/1211.0522
Horizon entanglement entropy and universality of the graviton coupling
Eugenio Bianchi
(Submitted on 2 Nov 2012)
We compute the low-energy variation of the horizon entanglement entropy for matter fields and gravitons in Minkowski space. While the entropy is divergent, the variation under a perturbation of the vacuum state is finite and proportional to the energy flux through the Rindler horizon. Due to the universal coupling of gravitons to the energy-momentum tensor, the variation of the entanglement entropy is universal and equal to the change in area of the event horizon divided by 4 times Newton's constant - independently from the number and type of matter fields. The physical mechanism presented provides an explanation of the microscopic origin of the Bekenstein-Hawking entropy in terms of entanglement entropy.
7 pages

http://arxiv.org/abs/1211.0161
Emergent Isotropy-Breaking in Quantum Cosmology
Andrea Dapor, Jerzy Lewandowski
(Submitted on 1 Nov 2012)
We consider a massive quantum test Klein-Gordon field probing an isotropic quantum cosmological space-time in the background. The result obtained is surprising. It turns out, that despite the isotropy of the quantum gravitational field, the semi-classical metric experienced by a mode of the K-G field is non-isotropic. The anisotropy depends on the direction of the momentum of the mode. Specifically, what we do is to derive a semi-classical space-time which emerges to a mode of the field. The method amounts to a comparison between QFT on a quantum background and QFT on a classical curved space-time, giving rise to an emergent metric tensor. The components of the semi-classical metric tensor are calculated from the equation of propagation of the quantum K-G field in the test field approximation. The anisotropies are of a quantum nature: they are proportional to Planck constant and "dress" the isotropic classical space-time obtained in the classical limit.
6 pages

http://arxiv.org/abs/1210.6215
Pure connection formalism for gravity: Feynman rules and the graviton-graviton scattering
Gianluca Delfino, Kirill Krasnov, Carlos Scarinci
(Submitted on 23 Oct 2012)
We continue to develop the pure connection formalism for gravity. We derive the Feynman rules for computing the connection correlation functions, as well as the prescription for obtaining the Minkowski space graviton scattering amplitudes from the latter. The present formalism turns out to be significantly simpler than the one based on the metric in many aspects. The most drastic difference with the usual approach is that the conformal factor of the metric, which is a source of difficulties in the metric treatment, does not propagate in the connection formulation even off-shell. This simplifies both the linearized theory and the interactions. For comparison, in our approach the complete off-shell cubic GR interaction contains just 3 terms, with only a single term relevant at tree level. This should be compared to at least a dozen terms in the metric formalism. We put the technology developed to use and compute the simplest graviton-graviton scattering amplitudes...
... This serves as a good illustration of the type of parity violation present in these theories. We find that the parity-violating amplitudes are important at high energies, and that a general parity-violating member of our class of theories "likes" one helicity (negative in our conventions) more than the other in the sense that at high energies it tends to convert all present gravitons into those of negative helicity.
46 pages.

http://arxiv.org/abs/1210.5276
Geometric asymptotics for spin foam lattice gauge gravity on arbitrary triangulations
Frank Hellmann, Wojciech Kaminski
(Submitted on 18 Oct 2012)
We study the behavior of holonomy spin foam partition functions, a form of lattice gauge gravity, on generic 4d-triangulations using micro local analysis. To do so we adapt tools from the renormalization theory of quantum field theory on curved space times. This allows us, for the first time, to study the partition function without taking any limits on the interior of the triangulation.
We establish that for many of the most widely used models the geometricity constraints, which reduce the gauge theory to a geometric one, introduce strong accidental curvature constraints. These limit the curvature around each triangle of the triangulation to a finite set of values. We demonstrate how to modify the partition function to avoid this problem. Finally the new methods introduced provide a starting point for studying the regularization ambiguities and renormalization of the partition function.
4+6 pages, 1 figure

http://arxiv.org/abs/1210.4504
A new perspective on cosmology in Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 16 Oct 2012)
We present a new cosmological model derived from Loop Quantum Gravity. The formulation is based on a projection of the kinematical Hilbert space of the full theory down to a subspace representing the proper arena for an inhomogeneous Bianchi I model. This procedure gives a direct link between the full theory and its cosmological sector. The emerging quantum cosmological model represents a simplified arena on which the complete canonical quantization program can be tested. The achievements of this analysis could also shed light on Loop Quantum Cosmology and its relation with the full theory.
5 pages

http://arxiv.org/abs/1210.0418
Interpretation of the triad orientations in loop quantum cosmology
Claus Kiefer, Christian Schell
(Submitted on 1 Oct 2012)
Loop quantum cosmology allows for arbitrary superpositions of the triad variable. We show here how these superpositions can become indistinguishable from a classical mixture by the interaction with fermions. We calculate the reduced density matrix for a locally rotationally symmetric Bianchi I model and show that the purity factor for the triads decreases by decoherence. In this way, the Universe assumes a definite orientation.
12 pages, 1 figure