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

[marcus]

Of the twenty 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/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain

http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

http://arxiv.org/abs/1205.6166
On the space of generalized fluxes for loop quantum gravity
Bianca Dittrich, Carlos Guedes, Daniele Oriti

http://arxiv.org/abs/1205.5733
Entropy in the Classical and Quantum Polymer Black Hole Models
Etera R. Livine, Daniel R. Terno

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1202.0412
Emission spectra of self-dual black holes
Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

http://arxiv.org/abs/1201.2120
Spinors and Twistors in Loop Gravity and Spin Foams
Maite Dupuis, Simone Speziale, Johannes Tambornino

http://arxiv.org/abs/1201.0525
String Field Theory from Quantum Gravity
Louis Crane

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
Forty different authors are represented this time.

 

[marcus]

Here are the abstracts for the 20 papers listed above.

http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto
(Submitted on 28 Jun 2012)
Loop Quantum Cosmology yields two kinds of quantum corrections to the effective equations of motion for cosmological perturbations. Here we focus on the holonomy kind and we study the problem of the closure of the resulting algebra of constraints. Up to now, tensor, vector and scalar perturbations were studied independently, leading to different algebras of constraints. The structures of the related algebras were imposed by the requirement of anomaly freedom. In this article we show that the algebra can be modified by a very simple quantum correction, holding for all types of perturbations. This demonstrates the consistency of the theory and shows that lessons from the study of scalar perturbations should be taken into account when studying tensor modes. The Mukhanov-Sasaki equations of motion are similarly modified by a simple term.
5 pages

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily
(Submitted on 25 Jun 2012)
Canonical methods allow the derivation of effective gravitational actions from the behavior of space-time deformations reflecting general covariance. With quantum effects, the deformations and correspondingly the effective actions change, revealing dynamical implications of quantum corrections. A new systematic way of expanding these actions is introduced showing as a first result that inverse-triad corrections of loop quantum gravity simplify the asymptotic dynamics near a spacelike collapse singularity. By generic quantum effects, the singularity is removed.
10 pages

http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann
(Submitted on 17 Jun 2012)
In the past, the possibility to employ (scalar) material reference systems in order to describe classical and quantum gravity directly in terms of gauge invariant (Dirac) observables has been emphasised frequently. This idea has been picked up more recently in Loop Quantum Gravity (LQG) with the aim to perform a reduced phase space quantisation of the theory thus possibly avoiding problems with the (Dirac) operator constraint quantisation method for constrained system. In this work, we review the models that have been studied on the classical and/or the quantum level and parametrise the space of theories so far considered. We then describe the quantum theory of a model that, to the best of our knowledge, so far has only been considered classically. This model could arguably called the optimal one in this class of models considered as it displays the simplest possible true Hamiltonian while at the same time reducing all constraints of General Relativity.
28 pages

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain
(Submitted on 7 Jun 2012)
The primordial Universe can be used as a laboratory to set constraints on quantum gravity. In the framework of Loop Quantum Cosmology, we show that such a proposal for quantum gravity not only solves for the big bang singularity issue but also naturally generates inflation. Thanks to a quantitative computation of the amount of gravity waves produced in the loopy early Universe, we show that future cosmological datas on the polarized anisotropies of the Cosmic Microwave Background can be used to probe LQC model of the Universe.
5 pages, ICGC (2011) Goa Conference proceedings

http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise
(Submitted on 4 Jun 2012)
Hamiltonian gravity, relying on arbitrary choices of "space," can obscure spacetime symmetries. We present an alternative, manifestly spacetime covariant formulation that nonetheless distinguishes between "spatial" and "temporal" variables. The key is viewing dynamical fields from the perspective of a field of observers -- a unit timelike vector field that also transforms under local Lorentz transformations. On one hand, all fields are spacetime fields, covariant under spacetime symmeties. On the other, when the observer field is normal to a spatial foliation, the fields automatically fall into Hamiltonian form, recovering the Ashtekar formulation. We argue this provides a bridge between Ashtekar variables and covariant phase space methods. We also outline a framework where the 'space of observers' is fundamental, and spacetime geometry itself may be observer-dependent.
8 pages

http://arxiv.org/abs/1205.6166
On the space of generalized fluxes for loop quantum gravity
Bianca Dittrich, Carlos Guedes, Daniele Oriti
(Submitted on 28 May 2012)
We show that the space of generalized fluxes - momentum space - for loop quantum gravity cannot be constructed by Fourier transforming the projective limit construction of the space of generalized connections - position space - due to the non-abelianess of the gauge group SU(2). From the abelianization of SU(2), U(1)^3, we learn that the space of generalized fluxes turns out to be an inductive limit, and we determine the consistency conditions the fluxes should satisfy under coarse-graining of the underlying graphs. We comment on the applications to loop quantum cosmology, in particular, how the characterization of the Bohr compactification of the real line as a projective limit opens the way for a similar analysis for LQC.
25 pages, 2 figures

http://arxiv.org/abs/1205.5733
Entropy in the Classical and Quantum Polymer Black Hole Models
Etera R. Livine, Daniel R. Terno
(Submitted on 25 May 2012)
We investigate the entropy counting for black hole horizons in loop quantum gravity (LQG). We argue that the space of 3d closed polyhedra is the classical counterpart of the space of SU(2) intertwiners at the quantum level. Then computing the entropy for the boundary horizon amounts to calculating the density of polyhedra or the number of intertwiners at fixed total area. Following the previous work arXiv:1011.5628, we dub these the classical and quantum polymer models for isolated horizons in LQG. We provide exact micro-canonical calculations for both models and we show that the classical counting of polyhedra accounts for most of the features of the intertwiner counting (leading order entropy and log-correction), thus providing us with a simpler model to further investigate correlations and dynamics. To illustrate this, we also produce an exact formula for the dimension of the intertwiner space as a density of "almost-closed polyhedra".
24 pages

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin
(Submitted on 24 May 2012)
Building on recent significant results of Frodden, Ghosh and Perez (FGP) and Bianchi, I present a quantum version of Jacobson's argument that the Einstein equations emerge as the equation of state of a quantum gravitational system. I give three criteria a quantum theory of gravity must satisfy if it is to allow Jacobson's argument to be run. I then show that the results of FGP and Bianchi provide evidence that loop quantum gravity satisfies two of these criteria and argue that the third should also be satisfied in loop quantum gravity. I also show that the energy defined by FGP is the canonical energy associated with the boundary term of the Holst action.
9 pages, 3 figures

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 3 May 2012)
We study time-reversal and parity ---on the physical manifold and in internal space--- in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences.
8 pages

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi
(Submitted on 23 Apr 2012)
We compute the entropy of non-extremal black holes using the quantum dynamics of Loop Gravity. The horizon entropy is finite, scales linearly with the area A, and reproduces the Bekenstein-Hawking expression S = A/4 with the one-fourth coefficient for all values of the Immirzi parameter. The near-horizon geometry of a non-extremal black hole - as seen by a stationary observer - is described by a Rindler horizon. We introduce the notion of a quantum Rindler horizon in the framework of Loop Gravity. The system is described by a quantum surface and the dynamics is generated by the boost Hamiltonion of Lorentzian Spinfoams. We show that the expectation value of the boost Hamiltonian reproduces the local horizon energy of Frodden, Ghosh and Perez. We study the coupling of the geometry of the quantum horizon to a two-level system and show that it thermalizes to the local Unruh temperature. The derived values of the energy and the temperature allow one to compute the thermodynamic entropy of the quantum horizon. The relation with the Spinfoam partition function is discussed.
6 pages, 1 figure

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 5 Apr 2012)
The era of precision cosmology has allowed us to accurately determine many important cosmological parameters, in particular via the CMB. Confronting Loop Quantum Cosmology with these observations provides us with a powerful test of the theory. For this to be possible we need a detailed understanding of the generation and evolution of inhomogeneous perturbations during the early, Quantum Gravity, phase of the universe. Here we describe how Loop Quantum Cosmology provides a completion of the inflationary paradigm, that is consistent with the observed power spectra of the CMB.
4 pages, ICGC (2011) Goa Conference proceedings

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito
(Submitted on 2 Apr 2012)
We achieve a group theoretical quantization of the flat Friedmann-Robertson-Walker model coupled to a massless scalar field adopting the improved dynamics of loop quantum cosmology. Deparameterizing the system using the scalar field as internal time, we first identify a complete set of phase space observables whose Poisson algebra is isomorphic to the su(1,1) Lie algebra. It is generated by the volume observable and the Hamiltonian. These observables describe faithfully the regularized phase space underlying the loop quantization: they account for the polymerization of the variable conjugate to the volume and for the existence of a kinematical non-vanishing minimum volume. Since the Hamiltonian is an element in the su(1,1) Lie algebra, the dynamics is now implemented as SU(1,1) transformations. At the quantum level, the system is quantized as a time-like irreducible representation of the group SU(1,1). These representations are labeled by a half-integer spin, which gives the minimal volume. They provide superselection sectors without quantization anomalies and no factor ordering ambiguity arises when representing the Hamiltonian. We then explicitly construct SU(1,1) coherent states to study the quantum evolution. They not only provide semiclassical states but truly dynamical coherent states. Their use further clarifies the nature of the bounce that resolves the big bang singularity.
33 pages

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta
(Submitted on 7 Mar 2012)
We find all the spin-foams contributing in the first order of the vertex expansion to the transition amplitude of the Bianchi-Rovelli-Vidotto Dipole Cosmology model. Our algorithm is general and provides spin-foams of arbitrarily given, fixed: boundary and, respectively, a number of internal vertices. We use the recently introduced Operator Spin-Network Diagrams framework.
23 pages, 30 figures

http://arxiv.org/abs/1202.0412
Emission spectra of self-dual black holes
Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 2 Feb 2012)
We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. We first estimate the relevant mass and temperature range and find that the masses are below the Planck mass, and the temperature of the black holes is small compared to their mass. In this limit, we then derive the number-density of the primary emission particles, and, by studying the wave-equation of a scalar field in the background metric of the black hole, show that we can use the low energy approximation for the greybody factors. We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.
15 pages, 6 figures

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak
(Submitted on 24 Jan 2012)
The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2-complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which (1) is finite, (2) produces a topological invariant, (3) matches with the properties of the continuum BF theory, (4) corresponds to its loop quantization. These results significantly clarify the foundations - and limitations - of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetry-breaking which reduces BF theory to gravity.
6 pages

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya
(Submitted on 23 Jan 2012)
We numerically investigate the solutions to the effective equations of the Bianchi II model within the "improved" Loop Quantum Cosmology (LQC) dynamics. The matter source is a massless scalar field. We perform a systematic study of the space of solutions, and focus on the behavior of several geometrical observables. We show that the big-bang singularity is replaced by a bounce and the point-like singularities do not saturate the energy density bound. There are up to five directional bounces in the scale factors, one global bounce in the expansion, the shear presents up to four local maxima and can be zero at the bounce. This allows for solutions with density larger than the maximal density for the isotropic and Bianchi I cases. The asymptotic behavior is shown to behave like that of a Bianchi I model, and the effective solutions connect anisotropic solutions even when the shear is zero at the bounce. All known facts of Bianchi I are reproduced. In the "vacuum limit", solutions are such that almost all the dynamics is due to the anisotropies. Since Bianchi II plays an important role in the Bianchi IX model and the the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture, our results can provide an intuitive understanding of the behavior in the vicinity of general space-like singularities.
23 pages, 26 figures

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida
(Submitted on 17 Jan 2012)
We introduce a fully coherent spin network amplitude whose expansion generates all SU(2) spin networks associated with a given graph. We then give an explicit evaluation of this amplitude for an arbitrary graph. We show how this coherent amplitude can be obtained from the specialization of a generating functional obtained by the contraction of parametrized intertwiners a la Schwinger. We finally give the explicit evaluation of this generating functional for arbitrary graphs.
21 pages, 1 figure

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle
(Submitted on 10 Jan 2012)
Spin-foam models are hoped to provide a dynamics for loop quantum gravity. All 4-d spin-foam models of gravity start from the Plebanski formulation, in which gravity is recovered from a topological field theory, BF theory, by the imposition of constraints, which, however, select not only the gravitational sector, but also unphysical sectors. We show that this is the root cause for terms beyond the required Feynman-prescribed exponential of i times the action in the semiclassical limit of the EPRL spin-foam vertex. By quantizing a condition isolating the gravitational sector, we modify the EPRL vertex, yielding what we call the proper EPRL vertex amplitude. This provides at last a vertex amplitude for loop quantum gravity with the correct semiclassical limit.
4 pages

http://arxiv.org/abs/1201.2120
Spinors and Twistors in Loop Gravity and Spin Foams
Maite Dupuis, Simone Speziale, Johannes Tambornino
(Submitted on 10 Jan 2012)
Spinorial tools have recently come back to fashion in loop gravity and spin foams. They provide an elegant tool relating the standard holonomy-flux algebra to the twisted geometry picture of the classical phase space on a fixed graph, and to twistors. In these lectures we provide a brief and technical introduction to the formalism and some of its applications.
16 pages; 3rd Zakopane School proceedings

http://arxiv.org/abs/1201.0525
String Field Theory from Quantum Gravity
Louis Crane
(Submitted on 2 Jan 2012)
Recent work on neutrino oscillations suggests that the three generations of fermions in the standard model are related by representations of the finite group A(4), the group of symmetries of the tetrahedron. Motivated by this, we explore models which extend the EPRL model for quantum gravity by coupling it to a bosonic quantum field of representations of A(4). This coupling is possible because the representation category of A(4) is a module category over the representation categories used to construct the EPRL model. The vertex operators which interchange vacua in the resulting quantum field theory reproduce the bosons and fermions of the standard model, up to issues of symmetry breaking which we do not resolve. We are led to the hypothesis that physical particles in nature represent vacuum changing operators on a sea of invisible excitations which are only observable in the A(4) representation labels which govern the horizontal symmetry revealed in neutrino oscillations. The quantum field theory of the A(4) representations is just the dual model on the extended lattice of the Lie group E6, as explained by the quantum Mckay correspondence of Frenkel Jing and Wang. The coupled model can be thought of as string field theory, but propagating on a discretized quantum spacetime rather than a classical manifold.
15 pages

 

[marcus]

Thanks to Francesca, MTd2, Nonlinearity, and Julian for your valuable input to the poll! It's really interesting to see what other people pick. It shows, for one thing, how we see the future of QG research---which papers will turn out to have been the important ones. I feel I learn a lot from thinking through and understanding other people's choices.

It's definitely no easy task to choose which to vote for! Especially this time when we have so many solid original research papers to compare and rate.

I will summarize the standing so far. Bianchi leads. It's an important landmark paper because it shows that Loop really contains the Bekenstein Hawking S=A/4 Entropy formula, no need to adjust Immirzi parameter. Also leading is the paper by two other young researchers, Steffen Gielen and Derek Wise. Derek is currently a postdoc at Erlangen, in Thiemann's group. They present a novel and interesting idea: the "field of observers" which one actually already finds in Cosmology, in a certain sense---the set of all observers at rest relative to the cosmic microwave background, or with respect to the "Hubble flow" (the expansion process itself). Distances between stationary observers can increase.

Three votes:
http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. WiseTwo votes:
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

One vote:
http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida

 

[marcus]

Thanks to all who've responded so far!

Four votes:
http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

Three votes:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

Two votes:
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

One vote:
http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida[/QUOTE]

 

[marcus]

Eight respondents so far. Thanks all! It just happens that the Erlangen group makes a strong showing in our poll, this time. Thiemann and Giesel are newly appointed professors at Erlangen and Derek Wise is one of the Erlangen postdocs. Erlangen's is the youngest of the half-dozen or so leading centers of LQG research.

Other main groups (all of which have been active longer) are at Marseille, Perimeter, Penn State, Warsaw, Louisiana, Lyon, Morelia, AEI-Golm, Nottingham...

Four votes:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

Three votes:
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

Two votes:
http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

One vote:
http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida

I should also mention that Perimeter Institute authors also made a strong showing in the poll, with papers by Bianchi, Smolin, Freidel, Bonzom and Smerlak. Not unexpectedly, since PI has been an active LQG research center for quite a few years, ever since its founding.

 

[atyy]

I think the Giesel and Thiemann paper is interesting because it points to a way to restricting matter couplings. It's also timely that their paper refers to the Higgs mechanism":

"This can be understood likewise to the case of the Higgs mechanism of the standard model where three out of the four Higgs fields are ’eaten’ by the boson fields yielding to a gauge invariant description of massive vector bosons. The main difference to the standard Higgs mechanism is that here all reference field considered in the models are ’eaten’ by the other degrees of freedom, whereas in the ordinary Higgs mechanism one scalar field remains in the theory and is not absorbed."

"The real physical question is whether scalar fields such as the Higgs field or the dust considered here exist in nature. The answer might quite well be negative."

Their future directions are stated in an earlier paper by Domagala et al:

"Now, with our model, we can consider the same system of fields from the point of view of the full theory, without the symmetry reduction. Similarly, we can also consider the quantum gravitational collapse, quantum black holes, theory entropy. All those cases are manageable within our model, and the only difficulty is of technical nature. Also the Hawking radiation and black hole evaporation process expected from the theory of quantum fields on the classical black hole background are in the range of our model. The next step to obtain progress in this direction is the construction of semiclassical states for full LQG, which are preserved under quantum dynamics generated by the physical Hamiltonian on appropriate time scales."

 

[marcus]

More votes to report, so I'll update the standings. I know I learn from seeing and thinking about other people's ideas of the relative importance of various research efforts and I'm probably not the only one who does. Many thanks to the others (8 besides myself) who have taken part!

Four votes:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

Three votes:
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

Two votes:
http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

One vote:
http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida

 

[marcus]

Starting to sort out the third quarter 2012 Loop-and-allied QG papers.

http://arxiv.org/abs/1207.0637
New ground state for quantum gravity
Joao Magueijo, Laura Bethke
(Submitted on 3 Jul 2012)
In this paper we conjecture the existence of a new "ground" state in quantum gravity, supplying a wave function for the inflationary Universe. We present its explicit perturbative expression in the connection representation, exhibiting the associated inner product. The state is chiral, dependent on the Immirzi parameter, and is the vacuum of a second quantized theory of graviton particles. We identify the physical and unphysical Hilbert sub-spaces. We then contrast this state with the perturbed Kodama state and explain why the latter can never describe gravitons in a de Sitter background. Instead, it describes self-dual excitations, which are composites of the positive frequencies of the right-handed graviton and the negative frequencies of the left-handed graviton. These excitations are shown to be unphysical under the inner product we have identified. Our rejection of the Kodama state has a moral tale to it: the semi-classical limit of quantum gravity can be the wrong path for making contact with reality (which may sometimes be perturbative but nonetheless fully quantum). Our results point towards a non-perturbative extension, and we present some conjectures on the nature of this hypothetical state.

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.

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/1208.6217
A complete hybrid quantization in inhomogeneous cosmology
Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 30 Aug 2012)
A complete quantization of a homogeneous and isotropic spacetime with closed spatial sections coupled to a massive scalar field is provided, within the framework of Loop Quantum Cosmology. We identify solutions with their initial data on the minimum volume section, and from this we construct the physical Hilbert space. Moreover, a perturbative study allows us to introduce small inhomogeneities. After gauge fixing, the inhomogeneous part of the system is reduced to a linear field theory. We then adopt a standard Fock representation to quantize these degrees of freedom. For the considered case of compact spatial topology, the requirements of: i) invariance under the spatial isometries, and ii) unitary implementation of the quantum dynamics, pick up a unique Fock representation and a particular set of canonical fields (up to unitary equivalence).
6 pages

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/1209.0396
Lorentz-covariant Hamiltonian analysis of BF gravity with the Immirzi parameter
Mariano Celada, Merced Montesinos
(Submitted on 3 Sep 2012)
We perform the Lorentz-covariant Hamiltonian analysis of two Lagrangian action principles that describe general relativity as a constrained BF theory and that include the Immirzi parameter. The relation between these two Lagrangian actions has been already studied through a map among the fields involved. The main difference between these is the way the Immirzi parameter is included, since in one of them the Immirzi parameter is included explicitly in the BF terms, whereas in the other (the CMPR action) it is in the constraint on the B fields. In this work we continue the analysis of their relationship but at the Hamiltonian level. Particularly, we are interested in seeing how the above difference appears in the constraint structure of both action principles. We find that they both possesses the same number of first-class and second-class constraints and satisfy a very similar (off-shell) Poisson-bracket algebra on account of the type of canonical variables employed. The two algebras can be transformed into each other by making a suitable change of variables
16 pages

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/1209.3403
Quantum Cosmology: Effective Theory
Martin Bojowald
(Submitted on 15 Sep 2012)
Quantum cosmology has traditionally been studied at the level of symmetry-reduced minisuperspace models, analyzing the behavior of wave functions. However, in the absence of a complete full setting of quantum gravity and detailed knowledge of specific properties of quantum states, it remained difficult to make testable predictions. For quantum cosmology to be part of empirical science, it must allow for a systematic framework in which corrections to well-tested classical equations can be derived, with any ambiguities and ignorance sufficiently parameterized. As in particle and condensed-matter physics, a successful viewpoint is one of effective theories, adapted to specific issues one encounters in quantum cosmology. This review presents such an effective framework of quantum cosmology, taking into account, among other things, space-time structures, covariance, the problem of time and the anomaly issue.
75 pages, 3 figures, Invited Topical Review for Class. Quantum Grav. 29 (2012) 213001

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/1209.4038
Fixed points and infrared completion of quantum gravity
Nicolai Christiansen, Daniel F. Litim, Jan M. Pawlowski, Andreas Rodigast
(Submitted on 18 Sep 2012)
The phase diagram of four-dimensional Einstein-Hilbert gravity is studied using Wilson's renormalization group. Smooth trajectories connecting the ultraviolet fixed point at short distances with attractive infrared fixed points at long distances are derived from the non-perturbative graviton propagator. Implications for the asymptotic safety conjecture and further results are discussed.
4 pages, 4 figures

 

[marcus]

Revised listing of third quarter 2012 Loop-and-allied QG papers. Two new ones which just appeared (Ashtekar et al, Dittrich et al). We have 19 paper, which is a lot, may have to drop some and narrow the list down.

http://arxiv.org/abs/1207.0637
New ground state for quantum gravity
Joao Magueijo, Laura Bethke
(Submitted on 3 Jul 2012)
In this paper we conjecture the existence of a new "ground" state in quantum gravity, supplying a wave function for the inflationary Universe. We present its explicit perturbative expression in the connection representation, exhibiting the associated inner product. The state is chiral, dependent on the Immirzi parameter, and is the vacuum of a second quantized theory of graviton particles. We identify the physical and unphysical Hilbert sub-spaces. We then contrast this state with the perturbed Kodama state and explain why the latter can never describe gravitons in a de Sitter background. Instead, it describes self-dual excitations, which are composites of the positive frequencies of the right-handed graviton and the negative frequencies of the left-handed graviton. These excitations are shown to be unphysical under the inner product we have identified. Our rejection of the Kodama state has a moral tale to it: the semi-classical limit of quantum gravity can be the wrong path for making contact with reality (which may sometimes be perturbative but nonetheless fully quantum). Our results point towards a non-perturbative extension, and we present some conjectures on the nature of this hypothetical state.

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.

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/1208.6217
A complete hybrid quantization in inhomogeneous cosmology
Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 30 Aug 2012)
A complete quantization of a homogeneous and isotropic spacetime with closed spatial sections coupled to a massive scalar field is provided, within the framework of Loop Quantum Cosmology. We identify solutions with their initial data on the minimum volume section, and from this we construct the physical Hilbert space. Moreover, a perturbative study allows us to introduce small inhomogeneities. After gauge fixing, the inhomogeneous part of the system is reduced to a linear field theory. We then adopt a standard Fock representation to quantize these degrees of freedom. For the considered case of compact spatial topology, the requirements of: i) invariance under the spatial isometries, and ii) unitary implementation of the quantum dynamics, pick up a unique Fock representation and a particular set of canonical fields (up to unitary equivalence).
6 pages

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/1209.0396
Lorentz-covariant Hamiltonian analysis of BF gravity with the Immirzi parameter
Mariano Celada, Merced Montesinos
(Submitted on 3 Sep 2012)
We perform the Lorentz-covariant Hamiltonian analysis of two Lagrangian action principles that describe general relativity as a constrained BF theory and that include the Immirzi parameter. The relation between these two Lagrangian actions has been already studied through a map among the fields involved. The main difference between these is the way the Immirzi parameter is included, since in one of them the Immirzi parameter is included explicitly in the BF terms, whereas in the other (the CMPR action) it is in the constraint on the B fields. In this work we continue the analysis of their relationship but at the Hamiltonian level. Particularly, we are interested in seeing how the above difference appears in the constraint structure of both action principles. We find that they both possesses the same number of first-class and second-class constraints and satisfy a very similar (off-shell) Poisson-bracket algebra on account of the type of canonical variables employed. The two algebras can be transformed into each other by making a suitable change of variables
16 pages

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

 

[marcus]

More respondents to the poll now than when I last reported. Thanks all!
I appreciate seeing others' perspective on which research stands out and also it's interesting to check how much these papers have been cited already, even though in some cases it has only been 3 months or so since they appeared---not really time enough for many citations. I've listed them most recent first and only included papers that got more than one citation from someone other than the author. Certainly we must all realize that citations in the professional literature don't tell the whole story and there can be valuable papers that go unnoticed for a time. But seeing what the researchers themselves are finding valuable can jog my attention and alert me to things I may have unconsciously dismissed, so I want to be aware of this aspect as well.

Five cites:
http://arxiv.org/abs/1201.2187
http://arxiv.org/cits/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

Four cites:
http://arxiv.org/abs/1206.6736
http://arxiv.org/cits/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1205.0733
http://arxiv.org/cits/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1205.5529
http://arxiv.org/cits/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

http://arxiv.org/abs/1204.5122
http://arxiv.org/cits/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

http://arxiv.org/abs/1204.1288
http://arxiv.org/cits/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

Three cites:
http://arxiv.org/abs/1201.4996
http://arxiv.org/cits/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

Two cites:
http://arxiv.org/abs/1201.4853
http://arxiv.org/cits/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

 

[Naty1]

Have past poll favorites correlated in any positive way to the subsequent 'importance' of papers, say after a year or two or more??

 

[marcus]

Have past poll favorites correlated in any positive way to the subsequent 'importance' of papers, say after a year or two or more??

I rarely pay attention to what the "favorites" are in the sense of most votes. I'm more interested in understanding the perspectives of individual people. I want to appreciate the PoV of this or that person. So I don't have any definite impression of how the "favorites" correlate with any kind of professional rating. Quite likely there is no significant correlation.

You can look though. Here is a link:
https://www.physicsforums.com/showthread.php?p=3996653#post3996653.
That time when I checked and looked up the cites it turned out that I had voted for 4 out of the top 5 but that was hardly surprising since I liked a lot of the papers that time and had voted for NINE By now the top five may have changed. You can check the current cites numbers (links are provided) if you want.
I don't bother to do that every time.

You got me curious about whether there was any correlation between "most votes" and "most cites" and I looked at 1st Quarter 2009 and there just happened to be a huge correlation in the sense that the poll favorite was Horava's paper, which got a huge number of cites.
https://www.physicsforums.com/showthread.php?t=304081
But I would not expect that, as a rule.

I just checked another, 1st quarter 2010
https://www.physicsforums.com/showthread.php?t=391043
Two favorites stand out with 6 and 7 votes, way more than the rest. I checked their cites numbers, one has 288 and the other 66. So of the whole list they are probably the top two in cites. That's another case where our votes seem to correlate with professional cites. Still, I doubt that is typical.

 

[marcus]

Eleven respondents in all-- I'll update the standings. I appreciate seeing and thinking about other people's ideas of the relative importance of various research efforts and I'm probably not the only one who learns from this. Many thanks to the others (10 besides myself) who have taken part!

Five votes:
http://arxiv.org/abs/1206.0658
Linking Covariant and Canonical General Relativity via Local Observers
Steffen Gielen, Derek K. Wise

Four votes:
http://arxiv.org/abs/1204.5122
Entropy of Non-Extremal Black Holes from Loop Gravity
Eugenio Bianchi

Three votes:
http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann

http://arxiv.org/abs/1205.5529
General relativity as the equation of state of spin foam
Lee Smolin

Two votes:
http://arxiv.org/abs/1204.1288
Perturbations in loop quantum cosmology
Ivan Agullo, Abhay Ashtekar, William Nelson

http://arxiv.org/abs/1206.1511
Loop quantum cosmology in the cosmic microwave background
Julien Grain

http://arxiv.org/abs/1205.0733
Discrete Symmetries in Covariant LQG
Carlo Rovelli, Edward Wilson-Ewing

http://arxiv.org/abs/1204.0539
Group theoretical Quantization of Isotropic Loop Cosmology
Etera R. Livine, Mercedes Martín-Benito

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle

One vote:
http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily

http://arxiv.org/abs/1203.1530
One vertex spin-foams with the Dipole Cosmology boundary
Marcin Kisielowski, Jerzy Lewandowski, Jacek Puchta

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida[/QUOTE]

 

[Naty1]

That's another case where our votes seem to correlate with professional cites. Still, I doubt that is typical.

Very interesting...

I would have expcted that IS fairly 'typical' ..am I now the optimist??{LOL}

It could a subsequent paper developed from an insight from a less 'popular' paper cited here, and that stimulated new [unanticipated] interest in the cited paper...

I guess there is no reason to assume our voting member population here reflects population of researchers in the field...those who would do most of the citing...

 

[marcus]

It's complicated. I think we do remarkably well, all things considered. I wouldn't expect even actively engaged researchers to always be reliable in their first impressions. Nobody is clairvoyant IMHO. Everybody (expert and non) guesses because that is how you decide what to spend time reading and trying to understand. But you only know a year or two later what ideas are paying off. Or three, or four!

Remember that we are guessing often within the first 2 or 3 months that something appears. The verdict of citations only comes later. Nobody (expert or non- ) is clairvoyant.

Take me for example: Right now I'm convinced that the most important QG advance by far this year is what Dittrich's group calls "Holonomy spin foams". Basically it extends well-developed Lattice Gauge Theory to include variation of the geometry of the lattice, that is to include gravity as well as the other stuff that LGT already handles adeptly. And it turns out that when they do this they get a single LGT-spinfoam format that can simulate all or almost all the different versions of spinfoam and do so with remarkably few parameters. So it appears to be a lean versatile format that is based on extending from an already well-developed proven line of physics that already has a lot of mathematical/computational tools available.

So it seems to me an open-and-shut case now BUT obviously I COULD BE WRONG. In fact as a general rule I expect to be wrong--the future course of scientific research is by its very nature unpredictable. Nevertheless frankly this is my hunch. If I had to bet on a single line of development that has appeared this year it would have to be this one.

But because it's so hard to predict I want to keep a half-dozen other themes and topics open, at least for now. Hedge. People should have plenty of room to spread their bets, so to speak. To be "many-minded" if they wish, rather than single-minded. I know what I'd choose if I HAD to choose one, but I don't have to choose only one, and I'm not going to.

These are just some private thoughts but perhaps relevant.

 

[marcus]

Have a look at this package and see if you don't partially share my intuitive feeling about this

http://relativity.phys.lsu.edu/ilqgs/hellmann090412.pdf
http://relativity.phys.lsu.edu/ilqgs/hellmann090412.wav
Holonomy Spin Foam Models: Asymptotic Dynamics
The talk covers material from three papers by various of B. Bahr, B. Dittrich, FH, W. Kaminski. The first two have been posted on arxiv, the third not yet:
Holonomy Spin Foam Models: Definition and coarse graining.
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
Holonomy Spin Foam Models: Asymptotic Dynamics of EPRL TypeFirst there is Frank Hellman's online seminar talk (audio+PDF). He is speaking from the MPI-Potsdam. It's a clear well-organized talk and gets a lot of questions, which he handles informatively. Other participants are speaking from places like Perimeter, PennState, LSU etc.
Bianca Dittrich is scheduled to give another seminar talk over the same network on 27 November. Here is the main ILQGS website (for Hellmann's slides and audio, scroll down to 4 September):
http://relativity.phys.lsu.edu/ilqgs/
For convenience, I'll copy the abstracts for the two papers he covers in his talk which have already appeared:
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.

My comment: "transfer operator" is a lattice analog of a Hamiltonian. It represents time evolution where you're working on a 4D spacetime lattice rather than a continuum. It strikes me as messy but there are advantages to working on a 2-complex or lattice that seem to compensate for the mess.

They tackle the "transfer" or "time-evolution" operators in this September paper:
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

Dittrich will cochair the Loop+Spin Foams session at the big triennial GR conference to be held next year and is one of the 3 organizers in charge of the biennial Loops conference to be held a couple of weeks later.
http://gr20-amaldi10.edu.pl/index.php?id=18 (note session D1)
http://www.perimeterinstitute.ca/en/Events/Loops_13/Loops_13/

 

[marcus]

One more Dittrich paper to add to the list. I'll remove the abstract text of all but the new one to make the list easier to scan. It's too long, one or more are almost sure to be dropped.

http://arxiv.org/abs/1207.0637
New ground state for quantum gravity
Joao Magueijo, Laura Bethke

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/1208.6217
A complete hybrid quantization in inhomogeneous cosmology
Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo

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

http://arxiv.org/abs/1209.0396
Lorentz-covariant Hamiltonian analysis of BF gravity with the Immirzi parameter
Mariano Celada, Merced Montesinos

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

 

[marcus]

Provisional list for the 3rd quarter MIP poll. Draft version of first post would be:
================================
Of the twenty 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 bare list of titles, for constructing the poll itself would be as follows:
Which paper(s) will contribute most to future research?

Emergent perspective of Gravity and Dark Energy
Inflation from non-minimally coupled scalar field in loop quantum cosmology
The Construction of Spin Foam Vertex Amplitudes
Divergences and Orientation in Spinfoams
The twistorial structure of loop-gravity transition amplitudes
Resilience of the Spectral Standard Model
Bohr-Sommerfeld Quantization of Space
Holonomy Spin Foam Models: Definition and Coarse Graining
Asymptotic safety, hypergeometric functions, and the Higgs mass in spectral action models
A possibility to solve the problems with quantizing gravity
General relativistic statistical mechanics
Observational effects from quantum cosmology
A Quantum Gravity Extension of the Inflationary Scenario
Functional renormalization with fermions and tetrads
On the Uniqueness of Kinematics of Loop Quantum Cosmology
Holonomy Spin Foam Models: Boundary Hilbert spaces and Time Evolution Operators
On the role of the Barbero-Immirzi parameter in discrete quantum gravity
C*-algebras of Holonomy-Diffeomorphisms & Quantum Gravity I
The Tensor Track: an Update