Notable Loop gravity papers this quarter

[marcus]

The fourth quarter 2013 has already seen an interesting bunch of Loop gravity research papers. I'll list a few and say why I think they are remarkable. Some make meaningful progress along established lines, while one or more others take an unexpected direction and are clearly exceptional. Highlights in the authors' abstracts are in blue, my comments on why the paper seems especially important are in green.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

Progress in showing that the Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes further generalization possible.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain proceedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://arxiv.org/abs/1310.1290
Singularity avoidance in the hybrid quantization of the Gowdy model
Paula Tarrío, Mikel Fernández Méndez, Guillermo A. Mena Marugán
(Submitted on 4 Oct 2013)
One of the most remarkable phenomena in Loop Quantum Cosmology is that, at least for homogeneous cosmological models, the Big Bang is replaced with a Big Bounce that connects our universe with a previous branch without passing through a cosmological singularity. The goal of this work is to study the existence of singularities in Loop Quantum Cosmology including inhomogeneities and check whether the behavior obtained in the purely homogeneous setting continues to be valid. With this aim, we focus our attention on the three-torus Gowdy cosmologies with linearly polarized gravitational waves and use effective dynamics to carry out the analysis. For this model, we prove that all the potential cosmological singularities are avoided, generalizing the results about resolution of singularities to this scenario with inhomogeneities. We also demonstrate that, if a bounce in the (Bianchi background) volume occurs, the inhomogeneities increase the value of this volume at the bounce with respect to its counterpart in the homogeneous case.
11 pages, 2 figures

Important to relax the requirement of inhomogeneity and extend the cosmological Big Bounce result to increasingly general cases.

 

[marcus]

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. They make a comparative study of the main alternatives and arrive at a careful choice.

 

[marcus]

http://arxiv.org/abs/1310.8654
Why are the effective equations of loop quantum cosmology so accurate?
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 31 Oct 2013)
We point out that the Heisenberg uncertainty relations vanish for non-compact spaces in loop quantum cosmology, thus explaining the surprising accuracy of the effective equations in describing the dynamics of sharply peaked wave packets. This underlines the fact that minisuperspace models ---where it is global variables that are quantized--- do not capture the local quantum fluctuations of the geometry.
5 pages

Underscores the need for Loop cosmology models that are not homogeneous and isotropic. Research on inhomogeneous and especially anisotropic models has recently become more common. The paper motivates treating cosmology in the context of the full quantum theory.

http://arxiv.org/abs/1310.7786
Group field theory as the 2nd quantization of Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Oct 2013)
We construct a 2nd quantized reformulation of canonical Loop Quantum Gravity at both kinematical and dynamical level, in terms of a Fock space of spin networks, and show in full generality that it leads directly to the Group Field Theory formalism. In particular, we show the correspondence between canonical LQG dynamics and GFT dynamics leading to a specific GFT model from any definition of quantum canonical dynamics of spin networks. We exemplify the correspondence of dynamics in the specific example of 3d quantum gravity. The correspondence between canonical LQG and covariant spin foam models is obtained via the GFT definition of the latter.
23 pages, 5 figures

GFT offers a convenient alternative way to look at Loop gravity that has already played a significant part in its development. The paper provides review, summary, pointers to new research directions, and outlook for an increased role.

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. The authors make a comparative study of the main alternatives and arrive at a reasoned choice.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

Progress in showing that the Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes possible further generalization to a broader range of cases.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain procedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://arxiv.org/abs/1310.1290
Singularity avoidance in the hybrid quantization of the Gowdy model
Paula Tarrío, Mikel Fernández Méndez, Guillermo A. Mena Marugán
(Submitted on 4 Oct 2013)
One of the most remarkable phenomena in Loop Quantum Cosmology is that, at least for homogeneous cosmological models, the Big Bang is replaced with a Big Bounce that connects our universe with a previous branch without passing through a cosmological singularity. The goal of this work is to study the existence of singularities in Loop Quantum Cosmology including inhomogeneities and check whether the behavior obtained in the purely homogeneous setting continues to be valid. With this aim, we focus our attention on the three-torus Gowdy cosmologies with linearly polarized gravitational waves and use effective dynamics to carry out the analysis. For this model, we prove that all the potential cosmological singularities are avoided, generalizing the results about resolution of singularities to this scenario with inhomogeneities. We also demonstrate that, if a bounce in the (Bianchi background) volume occurs, the inhomogeneities increase the value of this volume at the bounce with respect to its counterpart in the homogeneous case.
11 pages, 2 figures

Important to relax the requirement of inhomogeneity and extend the cosmological Big Bounce result to increasingly general cases.

 

[marcus]

http://arxiv.org/abs/1311.0186
Twistor relative locality
Lee Smolin
(Submitted on 1 Nov 2013)
We present a version of relative locality based on the geometry of twistor space. This can also be thought of as a new kind of deformation of twistor theory based on the construction of a bundle of twistor spaces over momentum space. Locality in space-time is emergent and is deformed in a precise way when a connection on that bundle is non-flat. This gives a precise and controlled meaning to Penrose's hypothesis that quantum gravity effects will deform twistor space in such a way as to maintain causality and relativistic invariance while weakening the notion that interactions take place at points in spacetime.
10 pages

Spacetime is an abstraction and one that Nature might conceivably disagree with. Objectively, we know what happens in our immediate vicinity, which includes exchanging messages with observers elsewhere. Unlikely as it seems, spacetime as a collective abstraction reconciling everybody's local experiences could be vulnerable. Also QG seems to be morphing in the direction of spinor/twistor formulations, see 2013 papers by e.g. Livine, Wieland. I am in doubt about this and want to hear others' opinions.

 

[marcus]

Updated listing. These 4th quarter papers may be on the upcoming poll.

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalims are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

"... step towards our goal of understanding the dynamics of null surfaces in LQG. .. From the possibility of including dynamical effects in black hole physics, describing the near horizon quantum geometry, to the use in the constraint-free formulation of GR on null hypersurfaces. To that end, many nontrivial steps are needed. First of all, our analysis needs to be complemented with a continuum canonical analysis of the Plebanski action on a null hypersurface [27]. ... one should also investigate what type of spin foams can support the boundary data here studied. We expect this line of research to bring new tools and results to LQG, and to show us how deep the connection with twistors goes."

Reference [27] is to work said "to appear" by Speziale and Alexandrov

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

Strongly motivated in the paper's introduction (which see.) The brief excerpts here can't do it justice: "...The models, and the investigations of their fixed point structure under coarse graining, are motivated by a research program to understand the phase diagram and continuum limit of spin foam models [1],... There are however several additional tantalizing connections to quantum gravity as well as other areas of physics. One is the theory of anyon condensation [4, 5], for which we provide Hamiltonians. The condensate states appear as ground states of these Hamiltonians and are given by the topological models. ...
Besides these topics there are other reasons why these models and its fixed points are of interest for quantum gravity:
• For the construction of spin foam models itself, in particular the intertwiners defining these models. The fixed points of our models define naturally intertwiners for spin foam vertices of arbitrary valency...
• ...spin net models,..
• ...geometrical interpretation of the underlying variables…"

http://arxiv.org/abs/1310.7786
Group field theory as the 2nd quantization of Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Oct 2013)
We construct a 2nd quantized reformulation of canonical Loop Quantum Gravity at both kinematical and dynamical level, in terms of a Fock space of spin networks, and show in full generality that it leads directly to the Group Field Theory formalism. In particular, we show the correspondence between canonical LQG dynamics and GFT dynamics leading to a specific GFT model from any definition of quantum canonical dynamics of spin networks. We exemplify the correspondence of dynamics in the specific example of 3d quantum gravity. The correspondence between canonical LQG and covariant spin foam models is obtained via the GFT definition of the latter.
23 pages, 5 figures

GFT offers a convenient alternative way to look at Loop gravity that has already played a significant part in its development. The paper provides review, summary, pointers to new research directions, and outlook for an increased role.

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. The authors make a comparative study of the main alternatives and arrive at a reasoned choice.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

The Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes possible further generalization to a broader range of cases.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain procedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://inspirehep.net/search?ln=en&...search=Search&sf=&so=d&rm=citation&rg=25&sc=0

 

[marcus]

This is a third quarter paper that fell through the cracks. We failed to spot it in September when it came out. Noticed it belatedly thanks to a question Skydivephil asked in Cosmology forum. The paper deviates from mainstream but need not be wrong, in my view. It explores an alternative to inflation that makes use of both the Loop cosmology bounce and the teleparallel variant of GR.

http://arxiv.org/abs/arXiv:1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013 (v1), last revised 18 Nov 2013 (this version, v3))
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
18 pages.

The author is at UPC-Barcelona (Universitat Politècnica de Catalunya = Catalonia Polytechnic University).


http://inspirehep.net/author/profile/J.Haro.1
http://inspirehep.net/author/profile/J.de.Haro.1

 

[marcus]

Another interesting 4th quarter paper came to my attention through yesterday's ILQGS talk (26 Nov) by Johannes Thürigen on DIMENSIONALITY in quantum geometry.
http://relativity.phys.lsu.edu/ilqgs/thueringen112613.pdf (typo, ingen might sometime be corrected to igen)
http://relativity.phys.lsu.edu/ilqgs/thueringen112613.wav

This is based on work at AEI Potsdam with Calcagni and Oriti. JT says there is work in progress and also there's a paper that just appeared this month

The idea dimensionality seems to get more sophisticated in QG. Maybe it isn't even meaningful at very small scale. If the geometry is quantum mechanical how do you even define it?
One method we've been hearing about since around 2005 and have discussed here at PF on occasion is spectral dimension where the dimensionality around a given point in space or spacetime is explored by conducting a diffusion process or random walk starting at that point.

The higher the dimension of the space the less likely the wanderer is to accidentally return to his starting point. It is well-adapted to spaces with vague indeterminate geometry, and it can assume non-integer values. The dimension of the space does not have to be a whole number.

Looking at JT's seminar slides makes me think they might be getting fairly deep into the idea of dimension--may be finding some new stuff. I want to withhold judgment but watch closely. Here's the November paper:

http://arxiv.org/abs/1311.3340
Spectral dimension of quantum geometries
Gianluca Calcagni, Daniele Oriti, Johannes Thürigen
(Submitted on 13 Nov 2013)
The spectral dimension is an indicator of geometry and topology of spacetime and a tool to compare the description of quantum geometry in various approaches to quantum gravity. This is possible because it can be defined not only on smooth geometries but also on discrete (e.g., simplicial) ones. In this paper, we consider the spectral dimension of quantum states of spatial geometry defined on combinatorial complexes endowed with additional algebraic data: the kinematical quantum states of loop quantum gravity (LQG). Preliminarily, the effects of topology and discreteness of classical discrete geometries are studied in a systematic manner. We look for states reproducing the spectral dimension of a classical space in the appropriate regime. We also test the hypothesis that in LQG, as in other approaches, there is a scale dependence of the spectral dimension, which runs from the topological dimension at large scales to a smaller one at short distances. While our results do not give any strong support to this hypothesis, we can however pinpoint when the topological dimension is reproduced by LQG quantum states. Overall, by exploring the interplay of combinatorial, topological and geometrical effects, and by considering various kinds of quantum states such as coherent states and their superpositions, we find that the spectral dimension of discrete quantum geometries is more sensitive to the underlying combinatorial structures than to the details of the additional data associated with them.
38 pages, 18 multiple figures

 

[marcus]

Updated listing. Comments in green.

http://arxiv.org/abs/1311.6942
A note on the spinor construction of Spin Foam amplitudes
Giorgio Immirzi
(Submitted on 27 Nov 2013)
I discuss the use of spinors in the construction of spin-foam models, in particular the form of the closure and simplicity constraints for triangles that are space-like, i.e. with (area)² = 1/2 S^IJ S_IJ > 0, regardless of whether they belong the tetrahedra with a space-like or time-like normal, emphasizing the role of the light-like 4-vector u_tσ^I u ̄_t. In the quantization of the model, with the representations of SL(2,C) acting on spaces of functions of light-like vectors, one may use the canonical basis of SU(2) representations, or the pseudobasis limited to the discrete representations of SU(1,1); in alternative it is proposed to use instead a basis of eigenstates of (L₃,K₃), which might give matrix elements and vertex functions with the same classical limit. A detailed example of a small triangulation is presented, which among other things indicates, on the basis of a classical calculation, that it would be impractical to limit oneself to tetrahedra with time-like normals.
20 pages, 1 figure.

Immirzi!

http://arxiv.org/abs/1311.6841
Observables in Loop Quantum Gravity with a cosmological constant
Maïté Dupuis, Florian Girelli
(Submitted on 26 Nov 2013)
An open issue in loop quantum gravity (LQG) is the introduction of a non-vanishing cosmological constant Λ. In 3d, Chern-Simons theory provides some guiding lines: Λ appears in the quantum deformation of the gauge group. The Turaev-Viro model, which is an example of spin foam model is also defined in terms of a quantum group. By extension, it is believed that in 4d, a quantum group structure could encode the presence of Λ≠0. In this article, we introduce by hand the quantum group U_q(su(2)) into the LQG framework, that is we deal with U_q(su(2))-spin networks. We explore some of the consequences, focusing in particular on the structure of the observables. Our fundamental tools are tensor operators for U_q(su(2)). We review their properties and give an explicit realization of the spinorial and vectorial ones. We construct the generalization of the U(n) formalism in this deformed case, which is given by the quantum group U_q(u(n)). We are then able to build geometrical observables, such as the length, area or angle operators ... We show that these operators characterize a quantum discrete hyperbolic geometry in the 3d LQG case. Our results confirm that the use of quantum group in LQG can be a tool to introduce a non-zero cosmological constant into the theory.
29 pages, 2 figures

Important to work out the consequences for the theory as a whole when the cosmological constant is included this way.

http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non-degenerate at infinity but not necessarily so in the interior, we initiate a generalisation of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification.
45 pages.

Earlier paper by MV http://arxiv.org/1306.6126 and two in progress by MC and MV, refs 24 and 25 on pages 35 and 36.]

http://arxiv.org/abs/1311.3340
Spectral dimension of quantum geometries
Gianluca Calcagni, Daniele Oriti, Johannes Thürigen
(Submitted on 13 Nov 2013)
The spectral dimension is an indicator of geometry and topology of spacetime and a tool to compare the description of quantum geometry in various approaches to quantum gravity. This is possible because it can be defined not only on smooth geometries but also on discrete (e.g., simplicial) ones. In this paper, we consider the spectral dimension of quantum states of spatial geometry defined on combinatorial complexes endowed with additional algebraic data: the kinematical quantum states of loop quantum gravity (LQG). Preliminarily, the effects of topology and discreteness of classical discrete geometries are studied in a systematic manner. We look for states reproducing the spectral dimension of a classical space in the appropriate regime. We also test the hypothesis that in LQG, as in other approaches, there is a scale dependence of the spectral dimension, which runs from the topological dimension at large scales to a smaller one at short distances. While our results do not give any strong support to this hypothesis, we can however pinpoint when the topological dimension is reproduced by LQG quantum states. Overall, by exploring the interplay of combinatorial, topological and geometrical effects, and by considering various kinds of quantum states such as coherent states and their superpositions, we find that the spectral dimension of discrete quantum geometries is more sensitive to the underlying combinatorial structures than to the details of the additional data associated with them.
38 pages, 18 multiple figures

The spectral dimension around a given point is explored by conducting a diffusion process or random walk starting at that point. Higher dimensionality makes it less likely the wanderer will accidentally return to his starting point. Spectral dimension is well-adapted to spaces with vague indeterminate geometry and can assume non-integer values.

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalims are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

"... step towards our goal of understanding the dynamics of null surfaces in LQG. .. From the possibility of including dynamical effects in black hole physics, describing the near horizon quantum geometry, to the use in the constraint-free formulation of GR on null hypersurfaces. To that end, many nontrivial steps are needed. First of all, our analysis needs to be complemented with a continuum canonical analysis of the Plebanski action on a null hypersurface [27]. ... one should also investigate what type of spin foams can support the boundary data here studied. We expect this line of research to bring new tools and results to LQG, and to show us how deep the connection with twistors goes."
Reference [27] is to work said "to appear" by Speziale and Alexandrov

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

Strongly motivated in the paper's introduction (which see.) The brief excerpts here can't do it justice: "...The models, and the investigations of their fixed point structure under coarse graining, are motivated by a research program to understand the phase diagram and continuum limit of spin foam models [1],... There are however several additional tantalizing connections to quantum gravity as well as other areas of physics. One is the theory of anyon condensation [4, 5], for which we provide Hamiltonians. The condensate states appear as ground states of these Hamiltonians and are given by the topological models. ...
Besides these topics there are other reasons why these models and its fixed points are of interest for quantum gravity:
• For the construction of spin foam models itself, in particular the intertwiners defining these models. The fixed points of our models define naturally intertwiners for spin foam vertices of arbitrary valency...
• ...spin net models,..
• ...geometrical interpretation of the underlying variables…"

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. The authors make a comparative study of the main alternatives and arrive at a reasoned choice.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

The Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes possible further generalization to a broader range of cases.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain procedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://arxiv.org/abs/arXiv:1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013)
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
18 pages.

I missed this 3rd quarter paper when it came out, so I'm including it in the 4th quarter list. It explores an alternative to inflation making use of both the Loop cosmology bounce and the teleparallel variant of GR.

http://inspirehep.net/search?ln=en&...search=Search&sf=&so=d&rm=citation&rg=25&sc=0

 

[marcus]

Here's a selection of 4th quarter papers taken from this Inspire search:
http://inspirehep.net/search?ln=en&...&action_search=Search&sf=&so=d&rm=&rg=25&sc=0
They number thirteen so far. In some cases I've added comment or used highlighting to indicate why I think the paper is especially interesting.

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus
(Submitted on 3 Dec 2013)
So far spin foam models are hardly understood beyond a few of their basic building blocks. To make progress on this question, we define analogue spin foam models, so called spin nets, for quantum groups SU(2)_k and examine their effective continuum dynamics via tensor network renormalization. In the refinement limit of this coarse graining procedure, we find a vast non-trivial fixed point structure beyond the degenerate and the BF phase. In comparison to previous work, we use fixed point intertwiners, inspired by Reisenberger's construction principle [1] and the recent work [2], as the initial parametrization. In this new parametrization fine tuning is not required in order to flow to these new fixed points. Encouragingly, each fixed point has an associated extended phase, which allows for the study of phase transitions in the future. Finally we also present an interpretation of spin nets in terms of melonic spin foams. The coarse graining flow of spin nets can thus be interpreted as describing the effective coupling between two spin foam vertices or space time atoms.
30+5 pages, many figures

The reference [2] is to 1311.1798 which appears further down in this list. Foams with a finite label set facilitate numerical investigation and the full treatment can in principle be recovered in the k→∞ limit.
==excerpt from page 2==
To be more precise, we introduce here spin net models with structure group SU(2)_k, the quantum deformation (at the root of unity) of the group SU(2). This makes a numerical investigation of the models possible, as in the quantum group only finitely many representations appear. On the technical level this requires the introduction of the Haar projector for the quantum group which we construct in this work. Models based on the quantum deformation of the rotation group describe gravitational systems with a cosmological constant [19]. For the 4D systems one would need SU(2)_k × SU(2)_k, however also an effective description in terms of SU(2)_k alone might be possible [20]. Thus we lift the simplification, considered in [12, 21], of the structure group almost completely. Moreover considering the behaviour of the models for growing level k allows to make conjectures on the limit k → ∞, which gives back the classical group SU(2).
As in previous work [12, 21] we will employ tensor network techniques [17, 18] to derive a coarse graining flow equation, which is then investigated numerically...
==endquote==

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
We argue that refining, coarse graining and entangling operators can be obtained from time evolution operators. This applies in particular to geometric theories, such as spin foams. We point out that this provides a construction principle for the physical vacuum in quantum gravity theories and more generally allows to construct a (cylindrically) consistent continuum limit of the theory.
33 pages, 9 figures

http://arxiv.org/abs/1311.6942
A note on the spinor construction of Spin Foam amplitudes
Giorgio Immirzi
(Submitted on 27 Nov 2013)
I discuss the use of spinors in the construction of spin-foam models, in particular the form of the closure and simplicity constraints for triangles that are space-like, i.e. with (area)² = 1/2 S^IJ S_IJ > 0, regardless of whether they belong the tetrahedra with a space-like or time-like normal, emphasizing the role of the light-like 4-vector u_tσ^I u ̄_t. In the quantization of the model, with the representations of SL(2,C) acting on spaces of functions of light-like vectors, one may use the canonical basis of SU(2) representations, or the pseudobasis limited to the discrete representations of SU(1,1); in alternative it is proposed to use instead a basis of eigenstates of (L₃,K₃), which might give matrix elements and vertex functions with the same classical limit. A detailed example of a small triangulation is presented, which among other things indicates, on the basis of a classical calculation, that it would be impractical to limit oneself to tetrahedra with time-like normals.
20 pages, 1 figure.

http://arxiv.org/abs/1311.6841
Observables in Loop Quantum Gravity with a cosmological constant
Maïté Dupuis, Florian Girelli
(Submitted on 26 Nov 2013)
An open issue in loop quantum gravity (LQG) is the introduction of a non-vanishing cosmological constant Λ. In 3d, Chern-Simons theory provides some guiding lines: Λ appears in the quantum deformation of the gauge group. The Turaev-Viro model, which is an example of spin foam model is also defined in terms of a quantum group. By extension, it is believed that in 4d, a quantum group structure could encode the presence of Λ≠0. In this article, we introduce by hand the quantum group U_q(su(2)) into the LQG framework, that is we deal with U_q(su(2))-spin networks. We explore some of the consequences, focusing in particular on the structure of the observables. Our fundamental tools are tensor operators for U_q(su(2)). We review their properties and give an explicit realization of the spinorial and vectorial ones. We construct the generalization of the U(n) formalism in this deformed case, which is given by the quantum group U_q(u(n)). We are then able to build geometrical observables, such as the length, area or angle operators ... We show that these operators characterize a quantum discrete hyperbolic geometry in the 3d LQG case. Our results confirm that the use of quantum group in LQG can be a tool to introduce a non-zero cosmological constant into the theory.
29 pages, 2 figures

Important to work out the consequences for the theory as a whole when the cosmological constant is included this way. BTW note parallel with with 1312.0905 by Perimeter authors, listed above.

http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non-degenerate at infinity but not necessarily so in the interior, we initiate a generalisation of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification.
45 pages.

Earlier paper by MV http://arxiv.org/1306.6126 and two in progress by MC and MV, refs 24 and 25 on pages 35 and 36.]

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalism are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

"... step towards our goal of understanding the dynamics of null surfaces in LQG. .. From the possibility of including dynamical effects in black hole physics, describing the near horizon quantum geometry, to the use in the constraint-free formulation of GR on null hypersurfaces. To that end, many nontrivial steps are needed. First of all, our analysis needs to be complemented with a continuum canonical analysis of the Plebanski action on a null hypersurface [27]. ... one should also investigate what type of spin foams can support the boundary data here studied. We expect this line of research to bring new tools and results to LQG, and to show us how deep the connection with twistors goes."
Reference [27] is to work said "to appear" by Speziale and Alexandrov

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

Strongly motivated in the paper's introduction (which see.) The brief excerpts here can't do it justice: "...The models, and the investigations of their fixed point structure under coarse graining, are motivated by a research program to understand the phase diagram and continuum limit of spin foam models [1],... There are however several additional tantalizing connections to quantum gravity as well as other areas of physics. One is the theory of anyon condensation [4, 5], for which we provide Hamiltonians. The condensate states appear as ground states of these Hamiltonians and are given by the topological models. ...
Besides these topics there are other reasons why these models and its fixed points are of interest for quantum gravity:
• For the construction of spin foam models itself, in particular the intertwiners defining these models. The fixed points of our models define naturally intertwiners for spin foam vertices of arbitrary valency...
• ...spin net models,..
• ...geometrical interpretation of the underlying variables…"

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. The authors make a comparative study of the main alternatives and arrive at a reasoned choice.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

The Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes possible further generalization to a broader range of cases.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain procedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://arxiv.org/abs/arXiv:1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013)
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
18 pages.

I missed this 3rd quarter paper when it came out, so I'm including it in the 4th quarter list. It explores an alternative to inflation making use of both the Loop cosmology bounce and the teleparallel variant of GR.

http://inspirehep.net/search?ln=en&...&action_search=Search&sf=&so=d&rm=&rg=25&sc=0

 

[marcus]

This Brunetti Fredenhagen Rejzner paper risks being lost track of because it doesn't belong to any of the usual categories. It seems to say that QG is an easy problem if you realize that below a certain scale (above a certain energy density) geometry doesn't make operational sense. You can't measure lengths, areas, angles. So we just want an "effective" theory after all. So it appears to contradict the need for a radical departure in QG. The much heralded "non-renormalizableness" of gravity is shrugged off (for what they argue are good reasons) and one just plows ahead with the usual perturbative treatment. Somehow they manage to include diffeomorphism invariance ("general covariance") and background independence to their own satisfaction. It's a "new/old" approach to QG.

http://arxiv.org/abs/1306.1058
Quantum gravity from the point of view of locally covariant quantum field theory
Romeo Brunetti, Klaus Fredenhagen, Katarzyna Rejzner
(Submitted on 5 Jun 2013)
We construct perturbative quantum gravity in a generally covariant way. In particular our construction is background independent. It is based on the locally covariant approach to quantum field theory and the renormalized Batalin-Vilkovisky formalism. We do not touch the problem of nonrenormalizability and interpret the theory as an effective theory at large length scales.
51 pages, in this version: proof of the background independence corrected

...The introduction of Brunetti Fredenhagen Rejzner gives a clear perspective on Quantum Gravity which I haven't heard much about lately. It's worth quoting in part.

==excerpts from introduction http://arxiv.org/pdf/1306.1058v3.pdf ==
The incorporation of gravity into quantum theory is one of the great challenges of physics. The last decades were dominated by attempts to reach this goal by rather radical new concepts, the best known being string theory and loop quantum gravity. A more conservative approach via quantum field theory was originally considered to be hopeless because of severe conceptual and technical problems. In the meantime it became clear that also the other attempts meet enormous problems, and it might be worthwhile to reconsider the quantum field theoretical approach. Actually, there are indications that the obstacles in this approach are less heavy than originally expected.

One of these obstacles is perturbative non-renormalisability [66, 73] which actually means that the counter terms arising in higher order of perturbation theory cannot be taken into account by readjusting the parameters in the Lagrangian. Nevertheless, theories with this property can be considered as effective theories with the property that only finitely many parameters have to be considered below a fixed energy scale [76]. Moreover, it may be that the theory is actually asymptotically safe in the sense that there is an ultraviolet fixed point of the renormalisation group flow with only finitely many relevant directions [75]. Results supporting this perspective have been obtained by Reuter et al. [64, 65].

Another obstacle is the incorporation of the principle of general covariance. Quantum field theory is traditionally based on the symmetry group of Minkowski space, the Poincaré group. In particular, the concept of particles with the associated notions of a vacuum (absence of particles) and scattering states heavily relies on Poincaré symmetry. Quantum field theory on curved spacetime which might be considered as an intermediate step towards quantum gravity already has no distinguished particle interpretation. In fact, one of the most spectacular results of quantum field theory on curved space times is Hawking’s prediction of black hole evaporation [43], a result which may be understood as a consequence of different particle interpretations in different regions of spacetime. (For a field theoretical derivation of the Hawking effect see [32].)

Quantum field theory on curved spacetime is nowadays well understood. This success is based on a consequent use of appropriate concepts. First of all, one has to base the theory on the principles of algebraic quantum field theory since there does not exist a distinguished Hilbert space of states. In particular, all structures are formulated in terms of local quantities. Global properties of spacetime do not enter the construction of the algebra of observables. They become relevant in the analysis of the space of states whose interpretation up to now is less well understood. It is at this point where the concept of particles becomes important if the spacetime under consideration has asymptotic regions similar to Minkowski space. Renormalization can be done without invoking any regularization by the methods of causal perturbation theory [28]. Originally these methods made use of properties of a Fock space representation, but could be generalized to a formalism based on algebraic structures on a space of functionals of classical field configurations where the problem of singularities can be treated by methods of microlocal analysis [13, 11, 45]. The lack of isometries in the generic case could be a problem for a comparison of renormalisation conditions at different points of spacetime. But this problem could be overcome by requiring local covariance, a principle, which relates theories at different spacetimes. The arising theory is already generally covariant and includes all typical quantum field theoretical models with the exception of supersymmetric theories (since supersymmetry implies the existence of a large group of isometries (Poincaré group or Anti de Sitter group)). See [14, 16] for more details.

It is the aim of this paper to extend this approach to gravity. But here there seems to be a conceptual obstacle. As discussed above, a successful treatment of quantum field theory on generic spacetimes requires the use of local observables, but unfortunately there are no diffeomorphism invariant localized functionals of the dynamical degrees of freedom (the metric in pure gravity). The way out is to replace the requirement of invariance by covariance which amounts to consider partial observables in the sense of [67, 22, 70].

Because of its huge group of symmetries the quantization of gravity is plagued by problems known from gauge theories, and a construction seems to require the introduction of redundant quantities which at the end have to be removed. In perturbation theory the Batalin-Vilkovisky (BV) approach [3, 4] has turned out to be the most systematic method, generalizing the BRST approach [5, 6, 72]. In the BV approach one constructs at the end the algebra of observables as a cohomology of a certain differential…
==endquote==

 

[marcus]

Here's a selection of 4th quarter QG papers, numbering seventeen in all. In some cases I've added comment or used highlighting to indicate why I think the paper is especially interesting.

http://arxiv.org/abs/1312.3595
Hawking radiation from a spherical loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 12 Dec 2013)
We introduce quantum field theory on quantum space-times techniques to characterize the quantum vacua as a first step towards studying black hole evaporation in spherical symmetry in loop quantum gravity and compute the Hawking radiation. We use as quantum space time the recently introduced exact solution of the quantum Einstein equations in vacuum with spherical symmetry and consider a spherically symmetric test scalar field propagating on it. The use of loop quantum gravity techniques in the background space-time naturally regularizes the matter content, solving one of the main obstacles to back reaction calculations in more traditional treatments. The discreteness of area leads to modifications of the quantum vacua, eliminating the trans-Planckian modes close to the horizon, which in turn eliminates all singularities from physical quantities, like the expectation value of the stress energy tensor. Apart from this, the Boulware, Hartle--Hawking and Unruh vacua differ little from the treatment on a classical space-time. The asymptotic modes near scri are reproduced very well. We show that the Hawking radiation can be computed, leading to an expression similar to the conventional one but with a high frequency cutoff. Since many of the conclusions concern asymptotic behavior, where the spherical mode of the field behaves in a similar way as higher multipole modes do, the results can be readily generalized to non spherically symmetric fields.
13 pages

"We have studied the quantization of a scalar field on a quantum space time that approximates well the geometry of a Schwarzschild black hole. The treatment reproduces the results of quantum field theory on a classical space-time well, with some interesting differences. The presence of a discrete structure for the space-time eliminates the divergences…" Based on paper #1310.5996 below.

]http://arxiv.org/abs/1312.3253
General Relativity from a Thermodynamic Perspective
T. Padmanabhan
(Submitted on 11 Dec 2013)
Several recent results suggest that gravity is an emergent phenomenon with its field equations having the same status as, say, the equations of fluid dynamics. I describe several additional results, supporting this paradigm and connecting the gravitational dynamics in a bulk region of space with a thermodynamic description in the boundary of that region:
(1) The Noether charge contained in a bulk region, associated with a specific time evolution vector field, has a direct thermodynamic interpretation as the gravitational heat content of the boundary surface.
(2) This result, in turn, shows that all static spacetimes maintain holographic equipartition; in these spacetimes, the number of degrees of freedom in the boundary is equal to the number of degrees of freedom in the bulk.
(3) In a general, evolving spacetime, the rate of change of gravitational momentum is related to the difference between the number of bulk and boundary degrees of freedom. It is this departure from the holographic equipartition which drives the time evolution of the spacetime.
(4) When the equations of motion hold, the (naturally defined) total energy of the gravity plus matter within a bulk region, will be equal to the boundary heat content.
(5) After motivating the need for an alternate description of gravity (if we have to solve the cosmological constant problem), I describe a thermodynamic variational principle based on null surfaces to achieve this goal. The concept of gravitational heat density of the null surfaces arises naturally from the Noether charge associated with the null congruence. The null surface variational principle, in fact, extremises the total heat content of the matter plus gravity system. Several variations on this theme and implications are described.
53 pages
See also Freidel paper below.

http://arxiv.org/abs/1312.3220
Multisymplectic effective General Boundary Field Theory
Mona Arjang, José A. Zapata
(Submitted on 11 Dec 2013)
The transfer matrix in lattice field theory connects the covariant and the initial data frameworks; in spin foam models, it can be written as a composition of elementary cellular amplitudes/propagators. We present a framework for discrete spacetime classical field theory in which solutions to the field equations over elementary spacetime cells may be amalgamated if they satisfy simple gluing conditions matching the composition rules of cellular amplitudes in spin foam models. Furthermore, the formalism is endowed with a multisymplectic structure responsible for local conservation laws.
Some models within our framework are effective theories modeling a system at a given scale. Our framework allows us to study coarse graining and the continuum limit.
47 pages

http://arxiv.org/abs/1312.1538
Gravitational Energy, Local Holography and Non-equilibrium Thermodynamics
Laurent Freidel
(Submitted on 5 Dec 2013)
We study the properties of gravitational system in finite regions bounded by gravitational screens. We present the detail construction of the total energy of such regions and of the energy and momentum balance equations due to the flow of matter and gravitational radiation through the screen. We establish that the gravitational screen possesses analogs of surface tension, internal energy and viscous stress tensor, while the conservations are analogs of non-equilibrium balance equations for a viscous system. This gives a precise correspondence between gravity in finite regions and non-equilibrium thermodynamics.
41 pages, 3 figures
See above.

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus
(Submitted on 3 Dec 2013)
So far spin foam models are hardly understood beyond a few of their basic building blocks. To make progress on this question, we define analogue spin foam models, so called spin nets, for quantum groups SU(2)_k and examine their effective continuum dynamics via tensor network renormalization. In the refinement limit of this coarse graining procedure, we find a vast non-trivial fixed point structure beyond the degenerate and the BF phase. In comparison to previous work, we use fixed point intertwiners, inspired by Reisenberger's construction principle [1] and the recent work [2], as the initial parametrization. In this new parametrization fine tuning is not required in order to flow to these new fixed points. Encouragingly, each fixed point has an associated extended phase, which allows for the study of phase transitions in the future. Finally we also present an interpretation of spin nets in terms of melonic spin foams. The coarse graining flow of spin nets can thus be interpreted as describing the effective coupling between two spin foam vertices or space time atoms.
30+5 pages, many figures

The reference [2] is to 1311.1798 which appears further down in this list. Foams with a finite label set (e.g. derived from SU(2)_k) facilitate numerical investigation and the full treatment can in principle be recovered in the k→∞ limit.

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
We argue that refining, coarse graining and entangling operators can be obtained from time evolution operators. This applies in particular to geometric theories, such as spin foams. We point out that this provides a construction principle for the physical vacuum in quantum gravity theories and more generally allows to construct a (cylindrically) consistent continuum limit of the theory.
33 pages, 9 figures

http://arxiv.org/abs/1311.6942
A note on the spinor construction of Spin Foam amplitudes
Giorgio Immirzi
(Submitted on 27 Nov 2013)
I discuss the use of spinors in the construction of spin-foam models, in particular the form of the closure and simplicity constraints for triangles that are space-like, i.e. with (area)² = 1/2 S^IJ S_IJ > 0, regardless of whether they belong the tetrahedra with a space-like or time-like normal, emphasizing the role of the light-like 4-vector u_tσ^I u ̄_t. In the quantization of the model, with the representations of SL(2,C) acting on spaces of functions of light-like vectors, one may use the canonical basis of SU(2) representations, or the pseudobasis limited to the discrete representations of SU(1,1); in alternative it is proposed to use instead a basis of eigenstates of (L₃,K₃), which might give matrix elements and vertex functions with the same classical limit. A detailed example of a small triangulation is presented, which among other things indicates, on the basis of a classical calculation, that it would be impractical to limit oneself to tetrahedra with time-like normals.
20 pages, 1 figure.

http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non-degenerate at infinity but not necessarily so in the interior, we initiate a generalisation of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification.
45 pages.

Earlier paper by MV http://arxiv.org/1306.6126 and two in progress by MC and MV, refs 24 and 25 on pages 35 and 36.]

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalism are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

"... step towards our goal of understanding the dynamics of null surfaces in LQG. .. from the possibility of including dynamical effects in black hole physics, describing the near horizon quantum geometry, to the use in the constraint-free formulation of GR on null hyper surfaces... We expect this line of research to bring new tools and results to LQG, and to show us how deep the connection with twistors goes."
Reference [27] is to work by Speziale and Alexandrov to appear.

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

"...The models, and the investigations of their fixed point structure under coarse graining, are motivated by a research program to understand the phase diagram and continuum limit of spin foam models [1],..
... the theory of anyon condensation [4, 5], for which we provide Hamiltonians. The condensate states appear as ground states of these Hamiltonians…
… the construction of spin foam models itself, in particular the intertwiners defining these models. The fixed points of our models define naturally intertwiners for spin foam vertices of arbitrary valency…"

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

A technically valuable paper. There are several ways to carry out the quantization in Loop cosmology. The authors make a comparative study of the main alternatives and arrive at a reasoned choice.

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

The Loop black hole does not develop a singularity. The authors published a result earlier this year in PRL which is now strengthened by showing that when one takes an alternate quantization route it again goes through and gives equivalent physics.

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

Computer simulations of have played an important role in Loop cosmology especially since 2006---having been extensively used to check and confirm solvable equation models and verify the Big Bounce under increasingly general assumptions. More efficient code makes possible further generalization to a broader range of cases.

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

The paper explains a new coarsegrain procedure to collapse a compact region of a spin(or) network with multiple vertices and edges down to a single point. A Fock-style Hilbert space is constructed at the remaining vertex which represents information condensed by the coarsening move. The reverse process of refinement is studied.

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

Riello's earlier paper this year dealt with spinfoam amplitude divergences which turned out to be at most logarithmic. He continues to make headway in studying details of the covariant Loop gravity path integral under refinement

http://arxiv.org/abs/arXiv:1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013)
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
18 pages.

I missed this 3rd quarter paper when it came out, so I'm including it in the 4th quarter list. It explores an alternative to inflation making use of both the Loop cosmology bounce and the teleparallel variant of GR.

http://arxiv.org/abs/1306.1058
Quantum gravity from the point of view of locally covariant quantum field theory
Romeo Brunetti, Klaus Fredenhagen, Katarzyna Rejzner
(Submitted on 5 Jun 2013)
We construct perturbative quantum gravity in a generally covariant way. In particular our construction is background independent. It is based on the locally covariant approach to quantum field theory and the renormalized Batalin-Vilkovisky formalism. We do not touch the problem of nonrenormalizability and interpret the theory as an effective theory at large length scales.
51 pages, in this version: proof of the background independence corrected

A noteworthy 2nd quarter paper I missed earlier and so include with the 4th quarter titles. Arnold Neumaier called this to our attention: a signal advance in a "new/old" approach to QG. https://www.physicsforums.com/showthread.php?p=4596143#post4596143

 

[marcus]

Another one that just came out:

http://arxiv.org/abs/1312.3657
Structural aspects of loop quantum gravity and loop quantum cosmology from an algebraic perspective
Alexander Stottmeister, Thomas Thiemann
(Submitted on 12 Dec 2013)
We comment on structural properties of the algebras A_LQG/LQC underlying loop quantum gravity and loop quantum cosmology, especially the representation theory, relating the appearance of the (dynamically induced) superselection structure (θ-sectors) in loop quantum cosmology to recently proposed representations with non-degenerate background geometries in loop quantum gravity with Abelian structure group. To this end, we review and employ the concept of extending a given (observable) algebra with possibly non-trivial centre to a (charged) field algebra with (global) gauge group.We also interpret the results in terms of the geometry of the structure group G. Furthermore, we analyze the Koslowski-Sahlmann representations with non-degenerate background in the case of a non-Abelian structure group. We find that these representations can be interpreted from two different, though related, points view: Either, the standard algebras of loop quantum gravity need to be extended by a (possibly) central term, or the elementary flux vector fields need to acquire a shift related to the (classical) background to make these representations well-defined. Both perspectives are linked by the fact that the background shift is not an automorphism of the algebras, but rather an affine transformation. Finally, we show how similar algebraic mechanisms, which are used to explain the breaking of chiral symmetry and the occurrence of θ-vacua in quantum field theory, extend to loop quantum gravity. Thus, opening a path for the discussion of these questions in loop quantum gravity.
45 pages

Have to think about this. It got my attention that they chose to study the Koslowski-Sahlmann representation, which was also the subject of Madhavan Varadarajan's paper listed above.
http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods...

 

[marcus]

Rough draft of 4th quarter poll listing. I've included two papers from earlier, which were overlooked when they first appeared.

http://arxiv.org/abs/1312.7273
On How Neutrino Protects the Axion
Gia Dvali, Sarah Folkerts, Andre Franca
(Submitted on 27 Dec 2013)
9 pages

http://arxiv.org/abs/1312.3657
Structural aspects of loop quantum gravity and loop quantum cosmology from an algebraic perspective
Alexander Stottmeister, Thomas Thiemann
(Submitted on 12 Dec 2013)
45 pages

http://arxiv.org/abs/1312.3595
Hawking radiation from a spherical loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 12 Dec 2013)
13 pages

http://arxiv.org/abs/1312.3253
General Relativity from a Thermodynamic Perspective
Thanu Padmanabhan
(Submitted on 11 Dec 2013)
53 pages

http://arxiv.org/abs/1312.1538
Gravitational Energy, Local Holography and Non-equilibrium Thermodynamics
Laurent Freidel
(Submitted on 5 Dec 2013)
41 pages, 3 figures

http://arxiv.org/abs/arXiv:1312.0905
Quantum group spin nets: refinement limit and relation to spin foams
Bianca Dittrich, Mercedes Martin-Benito, Sebastian Steinhaus
(Submitted on 3 Dec 2013)
30+5 pages, many figures

http://arxiv.org/abs/1311.7565
Time evolution as refining, coarse graining and entangling
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 29 Nov 2013)
33 pages, 9 figures

http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
45 pages.

http://arxiv.org/abs/1311.5325
Note on the super inflation in loop quantum cosmology
Kui Xiao, Xiao-Kai He, Jian-Yang Zhu
(Submitted on 21 Nov 2013)
9 pages, 4 figures. Physics Letters B

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
22 pages, 3 figures

http://arxiv.org/abs/1311.2898
Matter matters in asymptotically safe quantum gravity
Pietro Donà, Astrid Eichhorn, Roberto Percacci
(Submitted on 12 Nov 2013)
22 pages, 18 figures, 4 tables

http://arxiv.org/abs/1311.0186
Twistor relative locality
Lee Smolin
(Submitted on 1 Nov 2013)
10 pages

http://arxiv.org/abs/1311.0054
Relative information at the foundation of physics
Carlo Rovelli
(Submitted on 31 Oct 2013)
3 pages. Second prize in the 2013 FQXi contest "It From Bit or Bit From It?"

http://arxiv.org/abs/1310.7786
Group field theory as the 2nd quantization of Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Oct 2013)
23 pages, 5 figures

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
34 pages

http://arxiv.org/abs/1310.5167
A Gravitational Origin of the Arrows of Time
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 18 Oct 2013)
44+14 pages, 8 figures, 1 table

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
24 pages

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

http://arxiv.org/abs/arXiv:1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013)
Cosmological perturbations in Loop Quantum Cosmology (LQC) are usually studied incorporating either holonomy corrections, where the Ashtekar connection is replaced by a suitable sinus function in order to have a well-defined quantum analogue, or inverse-volume corrections coming from the eigenvalues of the inverse-volume operator.
In this paper we will develop an alternative approach to calculate cosmological perturbations in LQC based on the fact that, holonomy corrected LQC in the flat Friedmann-Lemaître-Robertson-Walker (FLRW) geometry could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). The main idea of our approach is to mix the simple bounce provided by holonomy corrections in LQC with the non-singular perturbation equations given by F(T) gravity, in order to obtain a matter bounce scenario as a viable alternative to slow-roll inflation.
In our study, we have obtained an scale invariant power spectrum of cosmological perturbations. However, the ratio of tensor to scalar perturbations is of order 1, which does not agree with the current observations. For this reason, we suggest a model where a transition from the matter domination to a quasi de Sitter phase is produced in order to enhance the scalar power spectrum.
18 pages. Journal of Cosmology and Astroparticle Physics

http://arxiv.org/abs/1306.1058
Quantum gravity from the point of view of locally covariant quantum field theory
Romeo Brunetti, Klaus Fredenhagen, Katarzyna Rejzner
(Submitted on 5 Jun 2013)
51 pages