Nominations for 2nd quarter MIP poll: Loop+allied QG

[marcus]

We have regular quarterly polls where you pick what you consider the most important research that has come out recently. Before 2012 it was limited to research PAPERS. So it was a MIP (most important paper) poll. This year we're trying a different system where we alternate papers (over the past 6 months) and RECORDED SEMINAR talks (abbreviated RS), again over the past half year--- a MIRS poll in other words.

The first quarter 2012 poll happened to be MIRS---it was posted around end of March and concerned a bunch of exceptionally interesting video and audio talks, with slides PDF files to go with them.

Because we alternate, the next poll, to be posted around 1 June, will be evaluating the most significant Loop-and-allied research papers that appeared during the past 6 months.

 

[marcus]

Possible nominations:

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. Proceeding of the ICGC international conference, Goa, December 2011

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; Essay written for the 2012 Gravity Research Foundation Awards for Essays on Gravitation

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.

http://arxiv.org/abs/1205.5677
Generating Functions for Coherent Intertwiners
Valentin Bonzom, Etera R. Livine
(Submitted on 25 May 2012)
We study generating functions for the scalar products of SU(2) coherent intertwiners, which can be interpreted as coherent spin network evaluations on a 2-vertex graph. We show that these generating functions are exactly summable for different choices of combinatorial weights. Moreover, we identify one choice of weight distinguished thanks to its geometric interpretation. As an example of dynamics, we consider the simple case of SU(2) flatness and describe the corresponding Hamiltonian constraint whose quantization on coherent intertwiners leads to partial differential equations that we solve. Furthermore, we generalize explicitly these Wheeler-DeWitt equations for SU(2) flatness on coherent spin networks for arbitrary graphs.
31 pages

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/1205.5325
Horizon energy as the boost boundary term in general relativity and loop gravity
Eugenio Bianchi, Wolfgang Wieland
(Submitted on 24 May 2012)
We show that the near-horizon energy introduced by Frodden, Ghosh and Perez arises from the action for general relativity as a horizon boundary term. Spin foam variables are used in the analysis. The result provides a derivation of the horizon boost Hamiltonian introduced by one of us to define the dynamics of the horizon degrees of freedom, and shows that loop gravity provides a realization of the horizon Schrodinger equation proposed by Carlip and Teitelboim.
3 pages, 1 figure

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/1205.2019
The Spin Foam Approach to Quantum Gravity
Alejandro Perez
(Submitted on 9 May 2012)
This article reviews the present status of the spin foam approach to the quantization of gravity. Special attention is paid to the pedagogical presentation of the recently introduced new models for four dimensional quantum gravity. The models are motivated by a suitable implementation of the path integral quantization of the Plebanski formulation of gravity on a simplicial regularization. The article also includes a self-contained treatment of the 2+1 gravity. The simple nature of the latter provides the basis and a perspective for the analysis of both conceptual and technical issues that remain open in four dimensions.
121 pages. To appear in Living Reviews in Relativity

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. Deparemeterizing 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

 

[marcus]

I'm going to have to narrow this down a lot. Sift through these and discard about half, I guess. It doesn't work to have too many papers on the poll. But for now we just continue making a first pass through the material.

http://arxiv.org/abs/1203.6525
Loop quantum gravity without the Hamiltonian constraint
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
We show that under certain technical assumptions, including a generalisation of CMC foliability and strict positivity of the scalar field, general relativity conformally coupled to a scalar field can be quantised on a partially reduced phase space, meaning reduced only with respect to the Hamiltonian constraint and a proper gauge fixing. More precisely, we introduce, in close analogy to shape dynamics, the generator of a local conformal transformation acting on both, the metric and the scalar field. A new metric, which is invariant under this transformation, is constructed and used to define connection variables which can be quantised by standard loop quantum gravity methods. Since this connection is invariant under the local conformal transformation, the generator of which is shown to be a good gauge fixing for the Hamiltonian constraint, the Dirac bracket associated with implementing these constraints coincides with the Poisson bracket for the connection. Thus, the well developed kinematical quantisation techniques for loop quantum gravity are available, while the Hamiltonian constraint has been solved (more precisely, gauge fixed) classically. The physical interpretation of this system is that of general relativity on a fixed spatial slice, the associated "time" of which is given by the value of the generator of local conformal transformations. While it is hard to address dynamical problems in this framework (due to the complicated "time" function), it seems, due to good accessibility properties of the gauge in certain situations, to be well suited for problems such as the computation of black hole entropy, where actual physical states can be counted and the dynamics is only of indirect importance. Also, the interpretation of the geometric operators gets an interesting twist, which exemplifies the deep relationship between observables and the choice of a time function.
5 pages

http://arxiv.org/abs/1203.6164
Intersecting Quantum Gravity with Noncommutative Geometry - a Review
Johannes Aastrup, Jesper M. Grimstrup
(Submitted on 28 Mar 2012)
We review applications of noncommutative geometry in canonical quantum gravity. First, we show that the framework of loop quantum gravity includes natural noncommutative structures which have, hitherto, not been explored. Next, we present the construction of a spectral triple over an algebra of holonomy loops. The spectral triple, which encodes the kinematics of quantum gravity, gives rise to a natural class of semiclassical states which entail emerging fermionic degrees of freedom. In the particular semiclassical approximation where all gravitational degrees of freedom are turned off, a free fermionic quantum field theory emerges. We end the paper with an extended outlook section.

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/1202.3637
Random tensor models in the large N limit: Uncoloring the colored tensor models
Valentin Bonzom, Razvan Gurau, Vincent Rivasseau
(Submitted on 16 Feb 2012)
Tensor models generalize random matrix models in yielding a theory of dynamical triangulations in arbitrary dimensions. Colored tensor models have been shown to admit a 1/N expansion and a continuum limit accessible analytically. In this paper we prove that these results extend to the most general tensor model for a single generic, i.e. non-symmetric, complex tensor. Colors appear in this setting as a canonical book-keeping device and not as a fundamental feature. In the large N limit, we exhibit a set of Virasoro constraints satisfied by the free energy and an infinite family of multicritical behaviors with entropy exponents γm=1-1/m.
15 pages

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/1201.5470
New tools for Loop Quantum Gravity with applications to a simple model
Enrique F. Borja, Jacobo Díaz-Polo, Laurent Freidel, Iñaki Garay, Etera R. Livine
(Submitted on 26 Jan 2012)
Loop Quantum Gravity is now a well established approach to quantum gravity. One of the main challenges still faced by the theory is constructing a consistent dynamics which would lead back to the standard dynamics of the gravitational field at large scales. Here we will review the recent U(N) framework for Loop Quantum Gravity and the new spinor representation (that provides a classical setting for the U(N) framework). Then, we will apply these techniques to a simple model in order to propose a dynamics for a symmetry reduced sector of the theory. Furthermore, we will explore certain analogies of this model with Loop Quantum Cosmology.
4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011 (ERE 2011) held in Madrid, Spain


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/1201.4543
Aspects of Quantum Gravity in Cosmology
Massimiliano Rinaldi
(Submitted on 22 Jan 2012)
We review some aspects of quantum gravity in the context of cosmology. In particular, we focus on models with a phenomenology accessible to current and near-future observations, as the early Universe might be our only chance to peep through the quantum gravity realm.
15 pages, 1 figure. Invited review for Modern Physics Letter A

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.

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; to appear in the Proceedings of the 3rd Quantum Gravity and Quantum Geometry School, February 28 - March 13, 2011 Zakopane, Poland


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]

Several of the above papers relate to the issue of the Dirac Hamiltonian constraint. In particular see the one by Bodendorfer and Thurn, two of Thiemann's grad students at Erlangen who co-authored a series of papers with him that were delivered at last year's Loops 2011 conference. Now Bodendorfer and Thurn seem to be on a different tack. But first this one by Gielen and Wise.

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; Essay written for the 2012 Gravity Research Foundation Awards for Essays on Gravitation

http://arxiv.org/abs/1203.6525
Loop quantum gravity without the Hamiltonian constraint
Norbert Bodendorfer, Alexander Stottmeister, Andreas Thurn
(Submitted on 29 Mar 2012)
We show that under certain technical assumptions, including a generalisation of CMC foliability and strict positivity of the scalar field, general relativity conformally coupled to a scalar field can be quantised on a partially reduced phase space, meaning reduced only with respect to the Hamiltonian constraint and a proper gauge fixing. More precisely, we introduce, in close analogy to shape dynamics, the generator of a local conformal transformation acting on both, the metric and the scalar field. A new metric, which is invariant under this transformation, is constructed and used to define connection variables which can be quantised by standard loop quantum gravity methods. Since this connection is invariant under the local conformal transformation, the generator of which is shown to be a good gauge fixing for the Hamiltonian constraint, the Dirac bracket associated with implementing these constraints coincides with the Poisson bracket for the connection. Thus, the well developed kinematical quantisation techniques for loop quantum gravity are available, while the Hamiltonian constraint has been solved (more precisely, gauge fixed) classically. The physical interpretation of this system is that of general relativity on a fixed spatial slice, the associated "time" of which is given by the value of the generator of local conformal transformations. While it is hard to address dynamical problems in this framework (due to the complicated "time" function), it seems, due to good accessibility properties of the gauge in certain situations, to be well suited for problems such as the computation of black hole entropy, where actual physical states can be counted and the dynamics is only of indirect importance. Also, the interpretation of the geometric operators gets an interesting twist, which exemplifies the deep relationship between observables and the choice of a time function.
5 pages

In addition there is a paper by Thiemann himself that was not here when I posted earlier and should be added to the list of possible MIP poll nominees:

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

One by Rovelli recently posted as well:
http://arxiv.org/abs/1206.3903
How to detect an anti-spacetime
Marios Christodoulou, Aldo Riello, Carlo Rovelli
(Submitted on 18 Jun 2012)
Is it possible, in principle, to measure the sign of the Lapse? We show that fermion dynamics distinguishes spacetimes having the same metric but different tetrads, for instance a Lapse with opposite sign. This sign might be a physical quantity not captured by the metric. We discuss its possible role in quantum gravity.
6 pages, 8 figures. Article awarded with an "Honorable Mention" from the 2012 Gravity Foundation Award.

 

[sigma.alpha]

Possible nominations:

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. Proceeding of the ICGC international conference, Goa, December 2011

good approach in cosmology, good work.

 

[marcus]

Narrowing the list. 20 options is all that will fit on one poll.

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/1205.5325
Horizon energy as the boost boundary term in general relativity and loop gravity
Eugenio Bianchi, Wolfgang Wieland
(Submitted on 24 May 2012)
We show that the near-horizon energy introduced by Frodden, Ghosh and Perez arises from the action for general relativity as a horizon boundary term. Spin foam variables are used in the analysis. The result provides a derivation of the horizon boost Hamiltonian introduced by one of us to define the dynamics of the horizon degrees of freedom, and shows that loop gravity provides a realization of the horizon Schrodinger equation proposed by Carlip and Teitelboim.
3 pages, 1 figure

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]

Arxiv may post papers today or tomorrow that change the lineup, but otherwise (following the pattern, for example, of the third quarter 2011 MIP poll:https://www.physicsforums.com/showthread.php?t=535170 ) the second quarter 2012 will probably go like this:

Title: Our picks for second quarter 2012 MIP (most important QG paper)
Poll question: Which paper(s) will contribute most to future research?
Post #1 heading: 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/1206.5765
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/1205.5325
Horizon energy as the boost boundary term in general relativity and loop gravity
Eugenio Bianchi, Wolfgang Wieland

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

Condensed list of titles:

A no-singularity scenario in loop quantum gravity
Scalar Material Reference Systems and Loop Quantum Gravity
Loop quantum cosmology in the cosmic microwave background
Linking Covariant and Canonical General Relativity via Local Observers
On the space of generalized fluxes for loop quantum gravity
Entropy in the Classical and Quantum Polymer Black Hole Models
General relativity as the equation of state of spin foam
Horizon energy as the boost boundary term in general relativity and loop gravity
Discrete Symmetries in Covariant LQG
Entropy of Non-Extremal Black Holes from Loop Gravity
Perturbations in loop quantum cosmology
Group theoretical Quantization of Isotropic Loop Cosmology
One vertex spin-foams with the Dipole Cosmology boundary
Emission spectra of self-dual black holes
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
On the exact evaluation of spin networks
A spin-foam vertex amplitude with the correct semiclassical limit
Spinors and Twistors in Loop Gravity and Spin Foams
String Field Theory from Quantum Gravity