Loop-and-allied QG bibliography

[marcus]

arXiv:1410.4479
Casimir effect in a quantum space-time
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 16 Oct 2014)
We apply quantum field theory in quantum space-time techniques to study the Casimir effect for large spherical shells. As background we use the recently constructed exact quantum solution for spherically symmetric vacuum space-time in loop quantum gravity. All calculations are finite and one recovers the usual results without the need of regularization or renormalization. This is an example of how loop quantum gravity provides a natural resolution to the infinities of quantum field theories.
4 pages.

http://arxiv.org/abs/1410.4411
Consistency of matter models with asymptotically safe quantum gravity
P. Donà, Astrid Eichhorn, Roberto Percacci
(Submitted on 16 Oct 2014)
We discuss the compatibility of quantum gravity with dynamical matter degrees of freedom. Specifically, we present bounds we obtained in [1] on the allowed number and type of matter fields within asymptotically safe quantum gravity. As a novel result, we show bounds on the allowed number of spin-3/2 (Rarita-Schwinger) fields, e.g., the gravitino. These bounds, obtained within truncated Renormalization Group flows, indicate the compatibility of asymptotic safety with the matter fields of the standard model. Further, they suggest that extensions of the matter content of the standard model are severely restricted in asymptotic safety. This means that searches for new particles at colliders could provide experimental tests for this particular approach to quantum gravity.
8 pages, 1 figure, 1 table. Proceedings of Theory Canada 9; new results on the gravitino,

briefly noted:
http://arxiv.org/abs/1410.4248
Towards Black Hole Entropy in Shape Dynamics
Gabriel Herczeg, Vasudev Shyam
(Submitted on 15 Oct 2014)
Shape dynamics is classical theory of gravity which agrees with general relativity in many important cases, but possesses different gauge symmetries and constraints. Rather than spacetime diffeomorphism invariance, shape dynamics takes spatial diffeomorphism invariance and spatial Weyl invariance as the fundamental gauge symmetries associated with the gravitational field. Since the area of the event horizon of a black hole transforms under a generic spatial Weyl transformation, there has been some doubt that one can speak sensibly about the thermodynamics of black holes in shape dynamics. The purpose of this paper is to show that by treating the event horizon of a black hole as an interior boundary, one can recover familiar notions of black hole thermodynamics in shape dynamics and define a gauge invariant entropy that agrees with general relativity.
9 pages

http://arxiv.org/abs/1410.3881
Universe in a black hole with spin and torsion
Nikodem J. Poplawski
(Submitted on 14 Oct 2014)
The conservation law for the angular momentum in curved spacetime requires that the antisymmetric part of the affine connection (the torsion tensor) is a variable in the principle of least action. The coupling between spin and torsion generates gravitational repulsion in fermionic matter at extremely high densities and avoids the formation of singularities in black holes. We show that every black hole in the presence of torsion forms a nonsingular, closed, nearly flat, homogeneous, and isotropic universe on the other side of its event horizon. Quantum particle production in such a universe can generate a period of exponential expansion which creates an enormous amount of matter in that universe. Accordingly, our Universe may have originated from the interior of a black hole existing in another universe.
10 pages

 

[marcus]

http://arxiv.org/abs/1410.4788
Loop Quantum Cosmology from Loop Quantum Gravity
Emanuele Alesci, Francesco Cianfrani
(Submitted on 17 Oct 2014)
We show how Loop Quantum Cosmology can be derived as an effective semiclassical description of Loop Quantum Gravity. Using the tools of QRLG, a gauge fixed version of LQG, we take the coherent states of the fundamental microscopic theory suitable to describe a Bianchi I Universe and we find a mapping between the expectation value of the Hamiltonian and the dynamics of LQC. Our results are in agreement with a lattice refinement framework for LQC, thus the so called ``old'' and ``improved-dynamics'' regularization schemes can be reproduced. These amount to different choices of relations between local variables and the smeared ones entering the definition of the coherent states. The leading order of the fundamental theory corresponds to LQC, but we also find different inverse volume corrections, that depend on a purely quantum observable, namely the number of nodes of the states.
5 pages

http://arxiv.org/abs/1410.4815
Further evidence for asymptotic safety of quantum gravity
Kevin Falls, Daniel F. Litim, Konstantinos Nikolakopoulos, Christoph Rahmede
(Submitted on 17 Oct 2014)
The asymptotic safety conjecture is examined for quantum gravity in four dimensions. Using the renormalisation group, we find evidence for an interacting UV fixed point for polynomial actions up to the 34th power in the Ricci scalar. The extrapolation to infinite polynomial order is given, and the self-consistency of the fixed point is established using a bootstrap test. All details of our analysis are provided. We also clarify further aspects such as stability, convergence, the role of boundary conditions, and a partial degeneracy of eigenvalues. Within this setting we find strong support for the conjecture.
43 pages, 17 figures

http://arxiv.org/abs/1410.5276
A symmetric scalar constraint for loop quantum gravity
Jerzy Lewandowski, Hanno Sahlmann
(Submitted on 20 Oct 2014)
In the framework of loop quantum gravity, we define a new Hilbert space of states which are solutions of a large number of components of the diffeomorphism constraint. On this Hilbert space, using the methods of Thiemann, we obtain a family of gravitational scalar constraints. They preserve the Hilbert space for every choice of lapse function. Thus adjointness and commutator properties of the constraint can be investigated in a straightforward manner. We show how the space of solutions of the symmetrized constraint can be defined by spectral decomposition, and the Hilbert space of physical states by subsequently fully implementing the diffeomorphism constraint.
9 pages.

 

[marcus]

http://arxiv.org/abs/1410.5608
A quantum reduction to Bianchi I models in loop quantum gravity
Norbert Bodendorfer
(Submitted on 21 Oct 2014)
We propose a quantum symmetry reduction of loop quantum gravity to Bianchi I spacetimes. To this end, we choose the diagonal metric gauge for the spatial diffeomorphism constraint at the classical level, leading to a U(1) gauge theory, and quantise the resulting theory via loop quantum gravity methods. Constraints which lead classically to a suitable reduction are imposed at the quantum level. The dynamics of the resulting model turn out to be very simple and manifestly coincide with those of a polymer quantisation of a Bianchi I model for the simplest choice of full theory quantum states compatible with the Bianchi I reduction.
5 pages.

http://arxiv.org/abs/1410.5609
A quantum reduction to spherical symmetry in loop quantum gravity
Norbert Bodendorfer, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 21 Oct 2014)
Based on a recent purely geometric construction of observables for the spatial diffeomorphism constraint, we propose two distinct quantum reductions to spherical symmetry within full 3+1-dimensional loop quantum gravity. The construction of observables corresponds to using the radial gauge for the spatial metric and allows to identify rotations around a central observer as unitary transformations in the quantum theory. Group averaging over these rotations yields our first proposal for spherical symmetry. Hamiltonians of the full theory with angle-independent lapse preserve this spherically symmetric subsector of the full Hilbert space. A second proposal consists in implementing the vanishing of a certain vector field in spherical symmetry as a constraint on the full Hilbert space, leading to a close analogue of diffeomorphisms invariant states. While this second set of spherically symmetric states does not allow for using the full Hamiltonian, it is naturally suited to implement the spherically symmetric midisuperspace Hamiltonian, as an operator in the full theory, on it. Due to the canonical structure of the reduced variables, the holonomy-flux algebra behaves effectively as a one parameter family of 2+1-dimensional algebras along the radial coordinate, leading to a diagonal non-vanishing volume operator on 3-valent vertices. The quantum dynamics thus becomes tractable, including scenarios like spherically symmetric dust collapse.
5 pages

http://arxiv.org/abs/1410.5763
A Note on Black Hole Entropy in Loop Quantum Gravity
S. Carlip
(Submitted on 21 Oct 2014)
Several recent results have hinted that black hole thermodynamics in loop quantum gravity simplifies if one chooses an imaginary Barbero-Immirzi parameter γ=i. This suggests a connection with SL(2,ℂ) or SL(2,ℝ) conformal field theories at the "boundaries" formed by spin network edges intersecting the horizon. I present a bit of background regarding the relevant conformal field theories, along with some speculations about how they might be used to count black hole states. I show, in particular, that a set of unproven but plausible assumptions can lead to a boundary conformal field theory whose density of states matches the Bekenstein-Hawking entropy.
12 pages.

 

[castles]

General interest.

http://arxiv.org/abs/1405.4601
Generations: Three Prints, in Colour

Cohl Furey
(Submitted on 19 May 2014)
We point out a somewhat mysterious appearance of SUc(3) representations, which exhibit the behaviour of three full generations of standard model particles. These representations are found in the Clifford algebra Cl(6), arising from the complex octonions. In this paper, we explain how this 64-complex-dimensional space comes about. With the algebra in place, we then identify generators of SU(3) within it. These SU(3) generators then act to partition the remaining part of the 64-dimensional Clifford algebra into six triplets, six singlets, and their antiparticles. That is, the algebra mirrors the chromodynamic structure of exactly three generations of the standard model's fermions. Passing from particle to antiparticle, or vice versa, requires nothing more than effecting the complex conjugate, ∗: i↦−i. The entire result is achieved using only the eight-dimensional complex octonions as a single ingredient.It's a very interesting paper. But I don't know what the hell the people from arxiv put it on general physics!

Good question, especially now since it's been published in JHEP.

 

[marcus]

http://arxiv.org/abs/1410.6285
Emergent Gravity Paradigm: Recent Progress
T. Padmanabhan
(Submitted on 23 Oct 2014)
Research during the last one decade or so suggests that the gravitational field equations in a large class of theories (including, but not limited to, general relativity) have the same status as the equations of, say, gas dynamics or elasticity. This paradigm provides a refreshingly different way of interpreting spacetime dynamics and highlights the fact that several features of classical gravitational theories have direct thermodynamic interpretation. I review the recent progress in this approach, achieved during the last few years.
22 pages. Invited Review for the MPLA Special Issue on "New trends on theory of gravity''; edited by A. Mazumdar

http://arxiv.org/abs/1410.6163
On the UV structure of quantum unimodular gravity
Ippocratis D. Saltas
(Submitted on 22 Oct 2014)
It is a well known result that any formulation of unimodular gravity is classically equivalent to General Relativity (GR), however a debate exists in the literature about this equivalence at the quantum level. In this work, we investigate the UV quantum structure of a diffeomorphism invariant formulation of unimodular gravity using functional renormalisation group methods in a Wilsonian context. We show that the effective action of the unimodular theory acquires essentially the same form with that of GR in the UV, as well as that both theories share similar UV completions within the framework of the asymptotic safety scenario for quantum gravity. Furthermore, we find that in this context the unimodular theory can appear to be non--predictive due to an increasing number of relevant couplings at high energies, and explain how this unwanted feature is in the end avoided.
13 pages.

In September there was a QG conference at Rome organized by Amelino-Camelia's group.
http://ctcqg2014.relativerest.org/plenary-talks/ Lee Smolin gave a talk remotely and the slides are interesting:
http://ctcqg2014.relativerest.org/wp-content/docs/presentazioni/ctcqg2014Fri/Smolin_CTCQG2014.pdf
Here are some other slide sets:
http://ctcqg2014.relativerest.org/wp-content/docs/presentazioni/ctcqg2014Mon/
http://ctcqg2014.relativerest.org/wp-content/docs/presentazioni/ctcqg2014Tue/
http://ctcqg2014.relativerest.org/wp-content/docs/presentazioni/ctcqg2014Wed/
http://ctcqg2014.relativerest.org/wp-content/docs/presentazioni/ctcqg2014Fri/

http://arxiv.org/abs/1410.6675
Local Conformal Symmetry: the Missing Symmetry Component for Space and Time
Gerard T. Hooft
(Submitted on 24 Oct 2014)
Local conformal symmetry is usually considered to be an approximate symmetry of nature, which is explicitly and badly broken. Arguments are brought forward here why it has to be turned into an exact symmetry that is spontaneously broken. As in the B.E.H. mechanism in Yang-Mills theories, we then will have a mechanism for disclosing the small-distance structure of the gravitational force. The symmetry could be as fundamental as Lorentz invariance, and guide us towards a complete understanding of physics at the ultra short distance scale.
5 pages

possible general interest, non-technical:
http://arxiv.org/abs/1410.6753
The symmetry and simplicity of the laws of physics and the Higgs boson
Juan Maldacena
(Submitted on 24 Oct 2014)
We describe the theoretical ideas, developed between the 1950s-1970s, which led to the prediction of the Higgs boson, the particle that was discovered in 2012. The forces of nature are based on symmetry principles. We explain the nature of these symmetries through an economic analogy. We also discuss the Higgs mechanism, which is necessary to avoid some of the naive consequences of these symmetries, and to explain various features of elementary particles.
23+8 pages. 18 figures

Interesting marginal note: Rovelli's new book that came out 22 October is #1 physics bestseller on Amazon.it, and #44 among all the books that Amazon italia sells. http://www.amazon.it/dp/8845929256/

• Posizione nella classifica Bestseller di Amazon: n. 44 in Libri

n.1 in Libri > Scienze, tecnologia e medicina > Fisica​

His other new book, that came out January 2014 was #5 on the physics bestseller list today,
that would be http://www.amazon.it/dp/8860306418/

• Posizione nella classifica Bestseller di Amazon: n. 300 in Libri
  • n.5 in Libri > Scienze, tecnologia e medicina > Fisica

 

[marcus]

http://arxiv.org/abs/1410.7003
A new functional flow equation for Einstein-Cartan quantum gravity
Ulrich Harst, Martin Reuter
(Submitted on 26 Oct 2014)
We construct a special-purpose functional flow equation which facilitates non-perturbative renormalization group (RG) studies on theory spaces involving a large number of independent field components that are prohibitively complicated using standard methods. Its main motivation are quantum gravity theories in which the gravitational degrees of freedom are carried by a complex system of tensor fields, a prime example being Einstein-Cartan theory, possibly coupled to matter. We describe a sequence of approximation steps leading from the functional RG equation of the Effective Average Action to the new flow equation which, as a consequence, is no longer fully exact on the untruncated theory space. However, it is by far more "user friendly" when it comes to projecting the abstract equation on a concrete (truncated) theory space and computing explicit beta-functions. The necessary amount of (tensor) algebra reduces drastically, and the usually very hard problem of diagonalizing the pertinent Hessian operator is sidestepped completely. In this paper we demonstrate the reliability of the simplified equation by applying it to a truncation of the Einstein-Cartan theory space. It is parametrized by a scale dependent Holst action, depending on a O(4) spin-connection and the tetrad as the independent field variables. We compute the resulting RG flow, focusing in particular on the running of the Immirzi parameter, and compare it to the results of an earlier computation where the exact equation had been applied to the same truncation. We find consistency between the two approaches and provide further evidence for the conjectured non-perturbative renormalizability (asymptotic safety) of quantum Einstein-Cartan gravity. We also investigate a duality symmetry relating small and large values of the Immirzi parameter which is displayed by the beta-functions in absence of a cosmological constant.
111 pages, 27 figures

http://arxiv.org/abs/1410.7062
No firewalls in quantum gravity: the role of discreteness of quantum geometry in resolving the information loss paradox
Alejandro Perez
(Submitted on 26 Oct 2014)
In an approach to quantum gravity where space-time arises from coarse graining of fundamentally discrete structures, black hole formation and subsequent evaporation can be described by a unitary evolution without the problems encountered by the standard remnant scenario or the schemes where information is assumed to come out with the radiation while evaporation (firewalls and complementarity). The final state is purified by correlations with the fundamental pre-geometric structures (in the sense of Wheeler) which are available in such approaches, and, like defects in the underlying space-time weave, can carry zero energy.
12 pages, 7 figures.

 

[atyy]

http://arxiv.org/abs/1410.7816
Field Parametrization Dependence in Asymptotically Safe Quantum Gravity
Andreas Nink
(Submitted on 28 Oct 2014)
Motivated by conformal field theory studies we investigate Quantum Einstein Gravity with a new field parametrization where the dynamical metric is basically given by the exponential of a matrix-valued fluctuating field, $g_{μν}=\bar{g}_{μρ}(e^{h})_{ρν}$. In this way, we aim to reproduce the critical value of the central charge when considering $2+ϵ$ dimensional spacetimes. With regard to the Asymptotic Safety program, we take special care of possible fixed points and new structures of the corresponding RG flow in $d=4$ for both single- and bi-metric truncations. Finally, we discuss the issue of restoring background independence in the bi-metric setting.

http://arxiv.org/abs/1410.8006
GR uniqueness and deformations
Kirill Krasnov
(Submitted on 29 Oct 2014)
In the metric formulation gravitons are described with the parity symmetric $S^{2}_{+ }\otimes S^{2}_{−}$ representation of Lorentz group. General Relativity is then the unique theory of interacting gravitons with second order field equations. We show that if a chiral $S^{3}_{+} \otimes S_{−}$ representation is used instead, the uniqueness is lost, and there is an infinite-parametric family of theories of interacting gravitons with second order field equations. We use the language of graviton scattering amplitudes, and show how the uniqueness of GR is avoided using simple dimensional analysis. The resulting distinct from GR gravity theories are all parity asymmetric, but share the GR MHV amplitudes. They have new all same helicity graviton scattering amplitudes at every graviton order. The amplitudes with at least one graviton of opposite helicity continue to be determinable by the BCFW recursion.

 

[marcus]

http://arxiv.org/abs/1410.8183
Matter Bounce Loop Quantum Cosmology from F(R) Gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 29 Oct 2014)
Using the reconstruction method, we investigate which F(R) theories, with or without the presence of matter fluids, can produce the matter bounce scenario of holonomy corrected Loop Quantum Cosmology. We focus our study in two limits of the cosmic time, the large cosmic time limit and the small cosmic time limit. For the former, we found that, in the presence of non-interacting and non-relativistic matter, the F(R) gravity that reproduces the late time limit of the matter bounce solution is actually the Einstein-Hilbert gravity plus a power law term. In the early time limit, since it corresponds to large spacetime curvatures, assuming that the Jordan frame is described by a general metric that when it is conformally transformed to the Einstein frame, produces an accelerating Friedmann-Robertson-Walker metric, we found explicitly the scalar field dependence on time. After demonstrating that the solution in the Einstein frame is indeed accelerating, we calculate the spectral index derived from the Einstein frame scalar-tensor counterpart theory of the F(R) theory and compare it with the Planck experiment data. In order to implement the resulting picture, we embed the F(R) gravity explicitly in a Loop Quantum Cosmology framework by introducing holonomy corrections to the F(R) gravity. In this way, the resulting inflation picture corresponding to the F(R) gravity can be corrected in order it coincides to some extent with the current experimental data.
28 pages.

http://arxiv.org/abs/1411.0190
The Entropy of Nonrotating Isolated Horizons in Lovelock Theory from Loop Quantum Gravity
Jingbo Wang, Chao-Guang Huang
(Submitted on 2 Nov 2014)
In this paper, we apply the method developed in loop quantum gravity to the nonrotating isolated horizons in Lovelock theory. We get the entropy that match the Wald entropy formula for this theory. We also confirm the conclusion got by Bodendorfer et al that the entropy is related to the flux operator rather than the area operator in general diffeomorphic-invariant theory.
8 pages.

http://arxiv.org/abs/1411.0272
Silent initial conditions for cosmological perturbations with a change of space-time signature
Jakub Mielczarek, Linda Linsefors, Aurelien Barrau
(Submitted on 2 Nov 2014)
Recent calculations in loop quantum cosmology suggest that a transition from a Lorentzian to an Euclidean space-time might take place in the very early Universe. The transition point leads to a state of silence, characterized by a vanishing speed of light. This behavior can be interpreted as a decoupling of different space points, similar to the one characterizing the BKL phase.
In this study, we address the issue of imposing initial conditions for the cosmological perturbations at the transition point between the Lorentzian and Euclidean phases. Motivated by the decoupling of space points, initial conditions characterized by a lack of correlations are investigated. We show that the "white noise" initial conditions are supported by the analysis of the vacuum state in the Euclidean regime adjacent to the state of silence.
Furthermore, the possibility of imposing the silent initial conditions at the trans-Planckian surface, characterized by a vanishing speed for the propagation of modes with wavelengths of the order of the Planck length, is studied. Such initial conditions might result from a loop-deformations of the Poincaré algebra. The conversion of the silent initial power spectrum to a scale-invariant one is also examined.
12 pages, 8 figures.

http://arxiv.org/abs/1411.0323
Observations on interfacing loop quantum gravity with cosmology
Tomasz Pawłowski
(Submitted on 2 Nov 2014)
A simple idea of relating the LQG and LQC degrees of freedom is discussed in context of toroidal Bianchi I model. The idea is an expansion of the construction originally introduced by Ashtekar and Wilson-Ewing and relies on explicit averaging of certain sub-class of spin-networks over the subgroup of the diffeomorphisms remaining after the gauge fixing used in homogeneous LQC. It is based on the set of clearly defined principles, thus is a convenient tool to control the emergence and behavior of the cosmological degrees of freedom in studies of dynamics in canonical LQG. Relating the proposed LQG-LQC interface with some results on black hole entropy suggests a modification to the area gap value currently used in LQC.
20 pages.

general interest, a memoir of collaboration with R. Feynman
http://arxiv.org/abs/1411.0509
How I got to work with Feynman on the covariant quark model
Finn Ravndal
(Submitted on 3 Nov 2014)
In the period 1968 - 1974 I was a graduate student and then a postdoc at Caltech and was involved with the developments of the quark and parton models. Most of this time I worked in close contact with Richard Feynman and thus was present from the parton model was proposed until QCD was formulated. A personal account is presented how the collaboration took place and how the various stages of this development looked like from the inside until QCD was established as a theory for strong interactions with the partons being quarks and gluons.
20 pages, 2 figures. Contribution to "50 Years of Quarks", to be published by World Scientific

 

[marcus]

http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
(Submitted on 4 Nov 2014)
Motivated by the construction of spectral manifolds in noncommutative geometry, we introduce a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of scalar fields. This commutation relation appears in two versions, one sided and two sided. It implies the quantization of the volume. In the one-sided case it implies that the manifold decomposes into a disconnected sum of spheres which will represent quanta of geometry. The two sided version in dimension 4 predicts the two algebras M₂(H) and M₄(C) which are the algebraic constituents of the Standard Model of particle physics. This taken together with the non-commutative algebra of functions allows one to reconstruct, using the spectral action, the Lagrangian of gravity coupled with the Standard Model. We show that any connected Riemannian Spin 4-manifold with quantized volume >4 (in suitable units) appears as an irreducible representation of the two-sided commutation relations in dimension 4 and that these representations give a seductive model of the "particle picture" for a theory of quantum gravity in which both the Einstein geometric standpoint and the Standard Model emerge from Quantum Mechanics. Physical applications of this quantization scheme will follow in a separate publication.
33 pages, 2 figures

Interesting video:
http://pirsa.org/14110114/
Slides: http://pirsa.org/14110114.pdf
Equivalence of wave-particle duality to entropic uncertainty
Speaker(s): Patrick Coles
Abstract: Interferometers capture a basic mystery of quantum mechanics: a single particle can exhibit wave behavior, yet that wave behavior disappears when one tries to determine the particle's path inside the interferometer. This idea has been formulated quantitatively as an inequality, e.g., by Englert and Jaeger, Shimony, and Vaidman, which upper bounds the sum of the interference visibility and the path distinguishability. Such wave-particle duality relations (WPDRs) are often thought to be conceptually inequivalent to Heisenberg's uncertainty principle, although this has been debated. Here we show that WPDRs correspond precisely to a modern formulation of the uncertainty principle in terms of entropies, namely the min- and max-entropies. This observation unifies two fundamental concepts in quantum mechanics. Furthermore, it leads to a robust framework for deriving novel WPDRs by applying entropic uncertainty relations to interferometric models (arXiv reference: 1403.4687).
Date: 04/11/2014 - 3:30 pm
talk is only 35 minutes but followed by lively 20 minute discussion, the most actively engaged being Robert Spekkens and Rafael Sorkin.
Rafael Sorkin (not for the first time) asks a series of questions starting around minute 41.
Here is the paper (with almost identical abstract) that the talk is based on:

http://arxiv.org/abs/1403.4687
Equivalence of wave-particle duality to entropic uncertainty
Patrick J. Coles, Jędrzej Kaniewski, Stephanie Wehner
(Submitted on 19 Mar 2014 (v1), last revised 16 Sep 2014 (this version, v2))
Interferometers capture a basic mystery of quantum mechanics: a single particle can exhibit wave behavior, yet that wave behavior disappears when one tries to determine the particle's path inside the interferometer. This idea has been formulated quantitively as an inequality, e.g., by Englert and Jaeger, Shimony, and Vaidman, which upper bounds the sum of the interference visibility and the path distinguishability. Such wave-particle duality relations (WPDRs) are often thought to be conceptually inequivalent to Heisenberg's uncertainty principle, although this has been debated. Here we show that WPDRs correspond precisely to a modern formulation of the uncertainty principle in terms of entropies, namely the min- and max-entropies. This observation unifies two fundamental concepts in quantum mechanics. Furthermore, it leads to a robust framework for deriving novel WPDRs by applying entropic uncertainty relations to interferometric models. As an illustration, we derive a novel relation that captures the coherence in a quantum beam splitter.
Comments: 9 + 16 pages, 8 figures. v2 presents a more complete and more general framework for wave-particle duality relations, as well as a more detailed analysis of the literature.

http://arxiv.org/abs/1411.1077
Perturbing a quantum gravity condensate
Steffen Gielen
(Submitted on 4 Nov 2014)
In a recent proposal using the group field theory (GFT) approach, a spatially homogeneous (generally anisotropic) universe is described as a quantum gravity condensate of 'atoms of space', which allows the derivation of an effective cosmological Friedmann equation from the microscopic quantum gravity dynamics. Here we take a first step towards the study of cosmological perturbations over the homogeneous background. We consider a state in which a single 'atom' is added to an otherwise homogeneous condensate. Backreaction of the perturbation on the background is negligible and the background dynamics can be solved separately. The dynamics for the perturbation takes the form of a quantum cosmology Hamiltonian for a 'wavefunction', depending on background and perturbations, of the product form usually assumed in a Born-Oppenheimer approximation. The perturbation we consider can then be interpreted as a spatially homogeneous metric perturbation. For this case, our results show how perturbations can be added to condensate states in quantum gravity, deriving the usual procedures in quantum cosmology from fundamental quantum gravity.
9 pages.

http://arxiv.org/abs/1411.1679
An introduction to the physics of Cartan gravity
H.F. Westman, T.G. Zlosnik
(Submitted on 6 Nov 2014)
A distance can be measured by monitoring how much a wheel has rotated when rolled without slipping. This simple idea underlies the mathematics of Cartan geometry. The Cartan-geometric description of gravity consists of a SO(1,4) gauge connection A^AB(x) and a symmetry-breaking field V^A(x). The clear similarity with symmetry-broken Yang-Mills theory suggests strongly the existence of a new field in nature: the gravitational Higgs field V^A. By treating V^A as a genuine dynamical field we arrive at a natural generalization of General Relativity with a wealth of new phenomenology and with General Relativity reproduced exactly in the limit where V² tends to a positive constant. We show that in regions wherein V²(x) varies, but has a definite sign, the Cartan-geometric formulation is a form of a scalar-tensor theory. A specific choice of action yields the Peebles-Ratra quintessence model whilst more general actions are shown to exhibit propagation of torsion. Regions where the sign of V² changes correspond to a change in signature of the geometry; a simple choice of action with FRW symmetry yields, without any additional ad hoc assumptions, a classical analogue of the Hartle-Hawking no-boundary proposal. Solutions from more general actions are discussed. A gauge prescription for coupling matter to gravity is described and matter actions are presented which reduce to standard ones in the limit V²→const. It becomes clear that Cartan geometry may function as a novel platform for inspiring and exploring modified theories of gravity with applications to dark energy, black holes, and early-universe cosmology. We end by listing a set of open problems.
44 pages, 9 figures.

http://arxiv.org/abs/1411.1935
Absence of Unruh effect in polymer quantization
Golam Mortuza Hossain, Gopal Sardar
(Submitted on 7 Nov 2014)
Unruh effect is a landmark prediction of standard quantum field theory in which Fock vacuum state appears as a thermal state with respect to an uniformly accelerating observer. Given its dependence on trans-Planckian modes, Unruh effect is often considered as an arena for exploring a candidate theory of quantum gravity. Here we show that Unruh effect disappears if, instead of using Fock quantization, one uses polymer quantization or loop quantization, the quantization method used in loop quantum gravity. Secondly, the polymer vacuum state remains a vacuum state even for the accelerating observer in the sense that expectation value of number density operator in it remains zero. Finally, if experimental measurement of Unruh effect is ever possible then it may be used either to verify or rule out a theory of quantum gravity.
5 pages, 1 figure.

 

[marcus]

http://arxiv.org/abs/arXiv:1411.2049
Generating Functionals for Spin Foam Amplitudes
Jeff Hnybida
(Submitted on 7 Nov 2014)
We construct a generating functional for the exact evalutation of a coherent representation of spin network amplitudes. This generating functional is defined for arbitrary graphs and depends only on a pair of spinors for each edge. The generating functional is a meromorphic polynomial in the spinor invariants which is determined by the cycle structure of the graph.
The expansion of the spin network generating function is given in terms of a newly recognized basis of SU(2) intertwiners consisting of the monomials of the holomorphic spinor invariants. This basis is labelled by the degrees of the monomials and is thus discrete. It is also overcomplete, but contains the precise amount of data to specify points in the classical space of closed polyhedra, and is in this sense coherent. We call this new basis the discrete-coherent basis.
We focus our study on the 4-valent basis, which is the first non-trivial dimension, and is also the case of interest for Quantum Gravity. We find simple relations between the new basis, the orthonormal basis, and the coherent basis.
Finally we discuss the process of coarse graining moves at the level of the generating functionals and give a general prescription for arbitrary graphs. A direct relation between the polynomial of cycles in the spin network generating functional and the high temperature loop expansion of the 2d Ising model is found.
128 pages. PhD Thesis.

http://arxiv.org/abs/1411.2072
The Emergence of Spacetime: Transactions and Causal Sets
Ruth E. Kastner
(Submitted on 8 Nov 2014)
This paper discusses how the transactional interpretation of quantum mechanics can provide for a natural account of the emergence of spacetime events from a quantum substratum. In this account, spacetime is not a substantive manifold that becomes occupied with events; rather, spacetime itself exists only in virtue of specific actualized events. This implies that spacetime is discrete rather than continuous, and that properties attributed to spacetime based on the notion of a continuum are idealizations that do not apply to the real physical world. It is further noted that the transactional picture of the emergence of spacetime can provide the quantum dynamics that underlie the causal set approach as proposed by Sorkin and others.
14 pages, 4 figures. Contribution to The Algebraic Way, edited by Ignazio Licata.

My comment: author is new to me so I include some background info.
http://carnap.umd.edu/philphysics/kastner.html
http://experts.excelsior.edu/experts/rkastner/
http://transactionalinterpretation.org
https://www.amazon.com/dp/0521764157/?tag=pfamazon01-20

http://arxiv.org/abs/1411.2854
How big is a black hole?
Marios Christodoulou, Carlo Rovelli
(Submitted on 11 Nov 2014)
The 3d volume inside a spherical black hole can be defined by extending an intrinsic flat-spacetime characterization of the volume inside a 2-sphere. For a collapsed object, the volume grows with time since the collapse, reaching a simple asymptotic form, which has a compelling geometrical interpretation. Perhaps surprising, it is large. The result may have relevance for the discussion on the information paradox.
7 pages, 6 figures.

http://arxiv.org/abs/1411.3180
Functional Renormalisation Group Approach for Tensorial Group Field Theory: a Rank-3 Model
Dario Benedetti, Joseph Ben Geloun, Daniele Oriti
(Submitted on 12 Nov 2014)
We set up the Functional Renormalisation Group formalism for Tensorial Group Field Theory in full generality. We then apply it to a rank-3 model over U(1) x U(1) x U(1), endowed with a linear kinetic term and nonlocal interactions. The system of FRG equations turns out to be non-autonomous in the RG flow parameter. This feature is explained by the existence of a hidden scale, the radius of the group manifold. We investigate in detail the opposite regimes of large cut-off (UV) and small cut-off (IR) of the FRG equations, where the system becomes autonomous, and we find, in both case, Gaussian and non-Gaussian fixed points. We derive and interpret the critical exponents and flow diagrams associated with these fixed points, and discuss how the UV and IR regimes are matched at finite N. Finally, we discuss the evidence for a phase transition from a symmetric phase to a broken or condensed phase, from an RG perspective, finding that this seems to exist only in the approximate regime of very large radius of the group manifold, as to be expected for systems on compact manifolds.
28 pages, 14 figures

 

[John86]

http://arxiv.org/abs/1411.2812
Level spacing distribution for the prototype of the Bianchi IX model
Jakub Mielczarek, Wlodzimierz Piechocki
(Submitted on 11 Nov 2014)
Our results concern quantum chaos of the vacuum Bianchi IX model. We apply the equilateral triangle potential well approximation to the potential of the Bianchi IX model to solve the eigenvalue problem for the physical Hamiltonian. Such approximation is well satisfied in vicinity of the cosmic singularity. Level spacing distribution of the eigenvalues is studied with and without applying the unfolding procedure. In both cases, the obtained distributions are qualitatively described by Brody's distribution with the parameter β≈0.3, revealing some sort of the level repulsion. The observed repulsion may reflect chaotic nature of the classical dynamics of the Bianchi IX universe. However, full understanding of this effects will require examination of the Bianchi IX model with the exact potential.

http://arxiv.org/abs/1411.1854
The Problem of Motion: The Statistical Mechanics of Zitterbewegung
Kevin H. Knuth
(Submitted on 7 Nov 2014)
Around 1930, both Gregory Breit and Erwin Schroedinger showed that the eigenvalues of the velocity of a particle described by wavepacket solutions to the Dirac equation are simply ±c, the speed of light. This led Schroedinger to coin the term Zitterbewegung, which is German for "trembling motion", where all particles of matter (fermions) zig-zag back-and-forth at only the speed of light. The result is that any finite speed less than c, including the state of rest, only makes sense as a long-term average that can be thought of as a drift velocity. In this paper, we seriously consider this idea that the observed velocities of particles are time-averages of motion at the speed of light and demonstrate how the relativistic velocity addition rule in one spatial dimension is readily derived by considering the probabilities that a particle is observed to move either to the left or to the right at the speed of light.

http://arxiv.org/abs/1411.2163
Information-Based Physics, Influence, and Forces
James Lyons Walsh, Kevin H. Knuth
(Submitted on 8 Nov 2014)
In recent works, Knuth and Bahreyni have demonstrated that the concepts of space and time are emergent in a coarse-grained model of direct particle-particle influence. In addition, Knuth demonstrated that observer-made inferences regarding the free particle, which is defined as a particle that influences others, but is not itself influenced, result in a situation identical to the Feynman checkerboard model of the Dirac equation. This suggests that the same theoretical framework that gives rise to an emergent spacetime is consistent with quantum mechanics. In this paper, we begin to explore the effect of influence on the emergent properties of a particle. This initial study suggests that when a particle is influenced, it is interpreted as accelerating in a manner consistent with special relativity implying that, at least in this situation, influence can be conceived of as a force.

http://arxiv.org/abs/1411.3013
Bayesian Evidence and Model Selection
Kevin H. Knuth, Michael Habeck, Nabin K. Malakar, Asim M. Mubeen, Ben Placek
(Submitted on 11 Nov 2014)
In this paper we review the concept of the Bayesian evidence and its application to model selection. The theory is presented along with a discussion of analytic, approximate and numerical techniques. Application to several practical examples within the context of signal processing are discussed.

http://arxiv.org/abs/1411.2642
State disturbance and pointer shift in protective quantum measurements
Maximilian Schlosshauer
(Submitted on 10 Nov 2014)
We investigate the disturbance of the state of a quantum system in a protective measurement for finite measurement times and different choices of the time-dependent system-apparatus coupling function. The ability to minimize this state disturbance is essential to protective measurement. We show that for a coupling strength that remains constant during the measurement interaction of duration T, the state disturbance scales as T−2, while a simple smoothing of the coupling function significantly improves the scaling behavior to T−6. We also prove that the shift of the apparatus pointer in the course of a protective measurement is independent of the particular time dependence of the coupling function, suggesting that the guiding principle for choosing the coupling function should be the minimization of the state disturbance. Our results illuminate the dynamics of protective measurement under realistic circumstances and may aid in the experimental realization of such measurements.

http://arxiv.org/abs/1411.2957
Lorentzian Quantum Reality: Postulates and Toy Models
Adrian Kent
(Submitted on 11 Nov 2014)
We describe postulates for a novel realist version of relativistic quantum theory or quantum field theory in Minkowski space or other background spacetimes with suitable asymptotic properties. We illustrate their application in toy models.

http://arxiv.org/abs/1411.3139
Spinning squashed extra dimensions, chiral gauge theory and hierarchy from N=4 SYM
Harold C. Steinacker
(Submitted on 12 Nov 2014)
New solutions of SU(N) N=4 SYM on R4 interpreted as spinning self-intersecting extra dimensions are discussed. Remarkably, these backgrounds lead to a low-energy sector with 3 generations of chiral fermions coupled to scalar and gauge fields, with standard Lorentz-invariant kinematics. This sector arises from zero modes localized in the extra dimensions, which are oblivious to the background rotation at low energies. In addition there is a sector of "heavy" excitations which is not described by a Lorentz-invariant field theory, but is argued to be suppressed at low energies assuming that resonances can be avoided. Depending on the rotation frequencies, some of the low-energy scalar fields acquire a VEV, and large hierarchies can naturally be stabilized by the background. We identify configurations which may lead to a low-energy physics not far from the broken phase of the standard model.

 

[marcus]

http://arxiv.org/abs/1411.3589
Projective Limits of State Spaces I. Classical Formalism
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum (field) theory as projective families of density matrices. The present first paper aims at clarifying the classical structures that underlies this formalism, namely projective limits of symplectic manifolds. In particular, this allows us to discuss accurately the issues hindering an easy implementation of the dynamics in this context, and to formulate a strategy for overcoming them.
51 pages, many figures

http://arxiv.org/abs/1411.3590
Projective Limits of State Spaces II. Quantum Formalism
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum theory as projective families of density matrices. After discussing the formalism at the classical level in a first paper, the present second paper is devoted to the quantum theory. In particular, we inspect in detail how such quantum projective state spaces relate to inductive limit Hilbert spaces and to infinite tensor product constructions. Regarding the quantization of classical projective structures into quantum ones, we extend the results by Okolów [arXiv:1304.6330], that were set up in the context of linear configuration spaces, to configuration spaces given by simply-connected Lie groups, and to holomorphic quantization of complex phase spaces.
56 pages, 2 figures

http://arxiv.org/abs/1411.3591
Projective Limits of State Spaces III. Toy-Models
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
In this series of papers, we investigate the projective framework initiated by Jerzy Kijowski and Andrzej Okolów, which describes the states of a quantum theory as projective families of density matrices. A strategy to implement the dynamics in this formalism was presented in our first paper, which we now test in two simple toy-models. The first one is a very basic linear model, meant as an illustration of the general procedure, and we will only discuss it at the classical level. In the second one, we reformulate the Schrödinger equation, treated as a classical field theory, within this projective framework, and proceed to its (non-relativistic) second quantization. We are then able to reproduce the physical content of the usual Fock quantization.
40 pages

http://arxiv.org/abs/1411.3592
Projective Loop Quantum Gravity I. State Space
Suzanne Lanéry, Thomas Thiemann
(Submitted on 11 Nov 2014)
Instead of formulating the state space of a quantum field theory over one big Hilbert space, it has been proposed by Kijowski to describe quantum states as projective families of density matrices over a collection of smaller, simpler Hilbert spaces. Beside the physical motivations for this approach, it could help designing a quantum state space holding the states we need. In [Okolów 2013, arXiv:1304.6330] the description of a theory of Abelian connections within this framework was developed, an important insight being to use building blocks labeled by combinations of edges and surfaces. The present work generalizes this construction to an arbitrary gauge group G (in particular, G is neither assumed to be Abelian nor compact). This involves refining the definition of the label set, as well as deriving explicit formulas to relate the Hilbert spaces attached to different labels.
If the gauge group happens to be compact, we also have at our disposal the well-established Ashtekar-Lewandowski Hilbert space, which is defined as an inductive limit using building blocks labeled by edges only. We then show that the quantum state space presented here can be thought as a natural extension of the space of density matrices over this Hilbert space. In addition, it is manifest from the classical counterparts of both formalisms that the projective approach allows for a more balanced treatment of the holonomy and flux variables, so it might pave the way for the development of more satisfactory coherent states.
81 pages, many figures

http://arxiv.org/abs/1411.3661
Spherically symmetric canonical quantum gravity
Suddhasattwa Brahma
(Submitted on 13 Nov 2014)
Canonical quantization of spherically symmetric space-times is carried out, using real-valued densitized triads and extrinsic curvature components, with specific factor ordering choices. Comparison with previous work [1] reveals that the resulting physical Hilbert space has the same form, although the basic canonical variables are different in the two approaches. As an extension, holonomy modifications from Loop Quantum Gravity are shown to deform the Dirac space-time algebra, while going beyond 'effective' calculations.
20 pages

http://arxiv.org/abs/1411.3475
Quasi-matter domination parameters in bouncing cosmologies
Emili Elizalde, Jaume Haro, Sergei D. Odintsov
(Submitted on 13 Nov 2014)
For bouncing cosmologies, a fine set of parameters is introduced in order to describe the nearly matter dominated phase, and which play the same role that the usual slow-roll parameters play in inflationary cosmology. It is shown that, as in the inflation case, the spectral index and the running parameter for scalar perturbations in bouncing cosmologies can be best expressed in terms of these small parameters. Further, they explicitly exhibit the duality which exists between a nearly matter dominated Universe in its contracting phase and the quasi de Sitter regime in the expanding one. The results obtained also confirm and extend the known evidence that the spectral index for a matter dominated Universe in the contracting phase is, in fact, the same as the spectral index for an exact Sitter regime in the expanding phase. Finally, in both the inflationary and the matter bounce scenarios, the theoretical values of the spectral index and of the running parameter are compared with their experimental counterparts, obtained from the most recent PLANCK data, with the result that the bouncing models here discussed do fit well accurate astronomical observations.
14 pages.

http://arxiv.org/abs/1411.4383
Entropic Motion in Loop Quantum Gravity
J.Manuel Garcia-Islas
(Submitted on 17 Nov 2014)
Entropic forces result from an increase of the entropy of a thermodynamical physical system. It has been proposed that gravity is such a phenomenon and many articles have appeared on the literature concerning this problem. Loop quantum gravity has also considered such possibility. We propose a new method in loop quantum gravity which reproduces an entropic force. By considering the interaction between a fixed gravity state space and a particle state in loop quantum gravity, we show that it leads to a mathematical description of a random walk of such particle. The random walk in special situations, can be seen as an entropic motion in such a way that the particle will move towards a location where entropy increases. This may prove that such theory can reproduce gravity as it is expected.
10 pages, 3 figures

http://arxiv.org/abs/1411.5201
Loop quantum cosmology in 2+1 dimension
Xiangdong Zhang
(Submitted on 19 Nov 2014)
As a first step to generalize the structure of loop quantum cosmology to the theories with the spacetime dimension other than four, the isotropic model of loop quantum cosmology in 2+1 dimension is studied in this paper. We find that the classical big bang singularity is again replaced by a quantum bounce in the model. The similarities and differences between the 2+1 dimensional model and the 3+1 dimensional one are also discussed.
13 pages, PRD in press

possible general interest:
http://arxiv.org/abs/1411.5022
The Fastest Unbound Stars in the Universe
James Guillochon (1), Abraham Loeb (1) ((1) Harvard ITC)
(Submitted on 18 Nov 2014)
The discovery of hypervelocity stars (HVS) leaving our galaxy with speeds of nearly 10³ km s⁻¹ has provided strong evidence towards the existence of a massive compact object at the galaxy's center. HVS ejected via the disruption of stellar binaries can occasionally yield a star with v_∞≲10⁴ km s⁻¹, here we show that this mechanism can be extended to massive black hole (MBH) mergers, where the secondary star is replaced by a MBH with mass M₂≳10⁵M⊙. We find that stars that are originally bound to the secondary MBH are frequently ejected with v_∞>10⁴ km s⁻¹, and occasionally with velocities ∼10⁵ km s⁻¹ (one third the speed of light), for this reason we refer to stars ejected from these systems as "semi-relativistic" hypervelocity stars (SHS). Bound to no galaxy, the velocities of these stars are so great that they can cross a significant fraction of the observable universe in the time since their ejection (several Gpc). We demonstrate that if a significant fraction of MBH mergers undergo a phase in which their orbital eccentricity is ≳0.5 and their periapse distance is tens of the primary's Schwarzschild radius, the space density of fast-moving (v_∞>10⁴ km s⁻¹) SHS may be as large as 10³ Mpc⁻³. Hundreds of the SHS will be giant stars that could be detected by future all-sky infrared surveys such as WFIRST or Euclid and proper motion surveys such as LSST, with spectroscopic follow-up being possible with JWST.
20 pages, 18 figures. Submitted to ApJ

 

[John86]

http://arxiv.org/abs/1411.3589
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
(Submitted on 4 Nov 2014)
Motivated by the construction of spectral manifolds in noncommutative geometry, we introduce a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of scalar fields. This commutation relation appears in two versions, one sided and two sided. It implies the quantization of the volume. In the one-sided case it implies that the manifold decomposes into a disconnected sum of spheres which will represent quanta of geometry. The two sided version in dimension 4 predicts the two algebras M₂(H) and M₄(C) which are the algebraic constituents of the Standard Model of particle physics. This taken together with the non-commutative algebra of functions allows one to reconstruct, using the spectral action, the Lagrangian of gravity coupled with the Standard Model. We show that any connected Riemannian Spin 4-manifold with quantized volume >4 (in suitable units) appears as an irreducible representation of the two-sided commutation relations in dimension 4 and that these representations give a seductive model of the "particle picture" for a theory of quantum gravity in which both the Einstein geometric standpoint and the Standard Model emerge from Quantum Mechanics. Physical applications of this quantization scheme will follow in a separate publication.
33 pages, 2 figures
5 pages, 1 figure.

Nice layman non-technical explanation by Allain Connes of his latest co written paper!

http://noncommutativegeometry.blogspot.nl/

 

[marcus]

Thanks John86! It's great to have a concise, more intuitive explanation of the new paper, in ordinary words (for the most part) by Connes himself. Particles in Quantum Gravity (noncommutative aka spectral geometry style). I want to keep tabs on that!
http://noncommutativegeometry.blogspot.nl/2014/11/particles-in-quantum-gravity.html

A paper by Philipp Hoehn came out today which seems to dovetail both with the line of research pursued by Bianca Dittrich and also that by Cortes Smolin and by Wolfgang Wieland. The idea of a geometric process based on Pachner moves:
http://arxiv.org/abs/1411.5672
Canonical linearized Regge Calculus: counting lattice gravitons with Pachner moves
Philipp A. Hoehn
(Submitted on 20 Nov 2014)
We afford a systematic and comprehensive account of the canonical dynamics of 4D Regge Calculus perturbatively expanded to linear order around a flat background. To this end, we consider the Pachner moves which generate the most basic and general simplicial evolution scheme. The linearized regime features a vertex displacement (`diffeomorphism') symmetry for which we derive an abelian constraint algebra. This permits to identify gauge invariant `lattice gravitons' as propagating curvature degrees of freedom. The Pachner moves admit a simple method to explicitly count the gauge and `graviton' degrees of freedom on an evolving triangulated hypersurface and we clarify the distinct role of each move in the dynamics. It is shown that the 1-4 move generates four `lapse and shift' variables and four conjugate vertex displacement generators; the 2-3 move generates a `graviton'; the 3-2 move removes one `graviton' and produces the only non-trivial equation of motion; and the 4-1 move removes four `lapse and shift' variables and trivializes the four conjugate symmetry generators. It is further shown that the Pachner moves preserve the vertex displacement generators. These results may provide new impetus for exploring `graviton dynamics' in discrete quantum gravity models.
26+12 pages, 2 appendices, many figures. This article is fairly self-contained

http://arxiv.org/abs/1411.5385
Group Field Theory in dimension four minus epsilon
Sylvain Carrozza
(Submitted on 19 Nov 2014)
Building on an analogy with ordinary scalar field theories, an epsilon expansion for rank-3 tensorial group field theories with gauge invariance condition is introduced. This allows to continuously interpolate between the dimension four group SU(2) X U(1) and the dimension three SU(2). In the first situation, there is a unique marginal 4-valent coupling constant, but in contrast to ordinary scalar field theory this model is asymptotically free. In the SU(2) case, the presence of two marginally relevant 6-valent coupling constants and one 4-valent super-renormalizable interaction spoils this interesting property. However, the existence of a non-trivial fixed point is established in dimension four minus epsilon, hence suggesting that the SU(2) theory might be asymptotically safe. To pave the way to future non-perturbative calculations, the present perturbative results are discussed in the framework of the effective average action.
14 pages, 6 figures
==quote introduction==
Group Field Theory (GFT) [1–4] is a general formalism aiming at completing the definition of the dynamics of Loop Quantum Gravity (LQG) [5–9], either from a covariant perspective as was historically proposed and since then has been the main line of investigation [10, 11], or directly from the canonical picture as was more recently suggested [12, 13]. An alternative but related approach to the same question relies on lattice gauge theory methods [14–17]. In both Wilson’s renormalization group is central, first to consistently define the theory, and at a later stage to explore its phase structure. In the long run, we hope to understand the effective, low energy limit of LQG, and be in a position to check whether Einstein’s gravity is reproduced or not.
==endquote==

Not Loop-and-allied QG, but of related interest:
http://arxiv.org/abs/1411.5675
Discreteness of time in the evolution of the universe
Mir Faizal, Ahmed Farag Ali, Saurya Das
(Submitted on 20 Nov 2014)
In this paper, we use a deformed second quantized commutation relation to quantize the Hamiltonian constraint of general relativity. The deformed Wheeler-DeWitt equation thus constructed is solved in the minisuperspace approximation. We demonstrate that in this model, the universe evolves by taking discrete jumps rather than in a continuous manner. Thus, the deformation of the second quantized commutation relation naturally gives rise to time crystals in our universe.
5 pages

http://arxiv.org/abs/1411.5708
Constructing black hole entropy from gravitational collapse
Giovanni Acquaviva, George F. R. Ellis, Rituparno Goswami, Aymen I. M. Hamid
(Submitted on 20 Nov 2014)
Based on a recent proposal for the gravitational entropy of free gravitational fields, we investigate the thermodynamic properties of black hole formation through gravitational collapse in the framework of the semitetrad 1+1+2 covariant formalism. In the simplest case of an Oppenheimer-Snyder-Datt collapse we prove that the change in gravitational entropy outside a collapsing body is related to the variation of the surface area of the body itself, even before the formation of horizons. As a result, we are able to relate the Bekenstein-Hawking entropy of the black hole endstate to the variation of the vacuum gravitational entropy outside the collapsing body.
8 pages, 1 figure

http://arxiv.org/abs/1411.7010
Unraveling the nature of Gravity through our clumpy Universe
Shant Baghram, Saeed Tavasoli, Farhang Habibi, Roya Mohayaee, Joseph Silk
(Submitted on 25 Nov 2014)
We propose a new probe to test the nature of gravity at various redshifts through large-scale cosmological observations. We use our void catalog, extracted from the Sloan Digital Sky Survey (SDSS, DR10), to trace the distribution of matter along the lines of sight to SNe Ia that are selected from the Union 2 catalog. We study the relation between SNe Ia luminosities and convergence and also the peculiar velocities of the sources. We show that the effects, on SNe Ia luminosities, of convergence and of peculiar velocities predicted by the theory of general relativity and theories of modified gravities are different and hence provide a new probe of gravity at various redshifts. We show that the present sparse large-scale data does not allow us to determine any statistically- significant deviation from the theory of general relativity but future more comprehensive surveys should provide us with means for such an exploration.
10 pages, 2 figures. Essay received honorable mention in the Gravity Research Foundation 2014 essay contest. To appear in IJMPD.

 

[marcus]

http://arxiv.org/abs/1411.7258
Restrictions on curved cosmologies in modified gravity from metric considerations
Linda Linsefors, Aurelien Barrau
(Submitted on 26 Nov 2014)
This study uses very simple symmetry and consistency considerations to put constraints on possible Friedmann equations for modified gravity models in curved spaces. As an example, it is applied to loop quantum cosmology.
7 pages, 2 figures

 

[atyy]

http://arxiv.org/abs/1411.7712
Evidence for Asymptotic Safety from Dimensional Reduction in Causal Dynamical Triangulations
D.N. Coumbe, J. Jurkiewicz
(Submitted on 27 Nov 2014)
We calculate the spectral dimension for a nonperturbative lattice approach to quantum gravity, known as causal dynamical triangulations (CDT), showing that the dimension of spacetime smoothly decreases from a value consistent with 4 on large distance scales to a value consistent with 3/2 on small distance scales. This novel result may provide a possible resolution to a long-standing argument against the asymptotic safety scenario. A method for determining the relative lattice spacing within the physical phase of the CDT parameter space is also outlined, which might prove useful when studying renormalization group flow in models of lattice quantum gravity.

 

[marcus]

http://arxiv.org/abs/1412.0775
Creation of particles in a cyclic universe driven by loop quantum cosmology
Yaser Tavakoli, Julio C. Fabris
(Submitted on 2 Dec 2014)
We consider an isotropic and homogeneous universe in loop quantum cosmology. We assume that the matter content of the universe is dominated by dust matter in early time and a phantom matter at late time which constitutes the dark energy component. The quantum gravity modifications to the Friedmann equation in this model indicate that the classical big bang singularity and the future big rip singularity are resolved and are replaced by quantum bounce. It turns out that the big bounce and recollapse in the herein model contribute a cyclic scenario for the universe. We then investigate the effects of quantum fields propagating on this cosmological background. By solving the Klein-Gordon equation for a massive and non-minimally coupled scalar field in the primordial region, we study the quantum theory of fields undergoing cosmological evolution towards the late time bounce. By using the exact solutions to describe the quantum fields at early and late time phases we obtain the density of created particles at late time. We find that the density of created particles is negligible comparing with the quantum background density at Planck era, hence, the effects of the quantum particle production do not lead to modification of the future bounce.
8 pages, 2 figures

brief mention, not Loop-and-allied QG but possibly of broader interest:
http://arxiv.org/abs/1412.1723
Communication complexity and the reality of the wave-function
Alberto Montina
(Submitted on 4 Dec 2014)
In this review, we discuss a relation between quantum communication complexity and a long-standing debate in quantum foundation concerning the interpretation of the quantum state. Is the quantum state a physical element of reality as originally interpreted by Schrodinger? Or is it an abstract mathematical object containing statistical information about the outcome of measurements as interpreted by Born? Although these questions sound philosophical and pointless, they can be made precise in the framework of what we call classical theories of quantum processes, which are a reword of quantum phenomena in the language of classical probability theory. In 2012, Pusey, Barrett and Rudolph (PBR) proved, under an assumption of preparation independence, a theorem supporting the original interpretation of Schrodinger in the classical framework. Recently, we showed that these questions are related to a practical problem in quantum communication complexity, namely, quantifying the minimal amount of classical communication required in the classical simulation of a two-party quantum communication process. In particular, we argued that the statement of the PBR theorem can be proved if the classical communication cost of simulating the communication of n qubits grows more than exponentially in 'n'. Our argument is based on an assumption that we call probability equipartition property. This property is somehow weaker than the preparation independence property used in the PBR theorem, as the former can be justified by the latter and the asymptotic equipartition property of independent stochastic sources. The equipartition property is a general and natural hypothesis that can be assumed even if the preparation independence hypothesis is dropped. In this review, we further develop our argument into the form of a theorem.
9 pages.
===new additions===
general interest:

http://arxiv.org/abs/1412.1895
Entanglement entropy of electromagnetic edge modes
William Donnelly, Aron C. Wall
(Submitted on 5 Dec 2014)
The vacuum entanglement entropy of Maxwell theory, when evaluated by standard methods, contains an unexpected term with no known statistical interpretation. We resolve this two-decades old puzzle by showing that this term is the entanglement entropy of edge modes: classical solutions determined by the electric field normal to the entangling surface. We explain how the heat kernel regularization applied to this term leads to the negative divergent expression found by Kabat. This calculation also resolves a recent puzzle concerning the logarithmic divergences of gauge fields in 3+1 dimensions.
7 pages.

http://arxiv.org/abs/1412.2040
How fundamental are fundamental constants?
M. J. Duff
(Submitted on 5 Dec 2014)
I argue that the laws of physics should be independent of one's choice of units or measuring apparatus. This is the case if they are framed in terms of dimensionless numbers such as the fine structure constant, alpha. For example, the Standard Model of particle physics has 19 such dimensionless parameters whose values all observers can agree on, irrespective of what clock, rulers, scales... they use to measure them. Dimensional constants, on the other hand, such as h, c, G, e, k..., are merely human constructs whose number and values differ from one choice of units to the next. In this sense only dimensionless constants are "fundamental". Similarly, the possible time variation of dimensionless fundamental "constants" of nature is operationally well-defined and a legitimate subject of physical enquiry. By contrast, the time variation of dimensional constants such as c or G on which a good many (in my opinion, confusing) papers have been written, is a unit-dependent phenomenon on which different observers might disagree depending on their apparatus. All these confusions disappear if one asks only unit-independent questions.
We provide a selection of opposing opinions in the literature and respond accordingly.
30 pages, 7 figures.

brief mention:
http://arxiv.org/abs/1412.2054
Planck-scale phenomenology with anti-de Sitter momentum space
Michele Arzano, Giulia Gubitosi, Joao Magueijo, Giovanni Amelino-Camelia
(Submitted on 5 Dec 2014)
We investigate the anti-de Sitter (AdS) counterpart to the well studied de Sitter (dS) model for energy-momentum space, viz "κ-momentum space" space (with a structure based on the properties of the κ-Poincaré Hopf algebra). On the basis of previous preliminary results one might expect the two models to be "dual": dS exhibiting an invariant maximal spatial momentum but unbounded energy, AdS a maximal energy but unbounded momentum. If that were the case AdS momentum space could be used to implement a principle of maximal Planck-scale energy, just as several studies use dS momentum space to postulate of maximal Planck-scale spatial momentum. However several unexpected features are uncovered in this paper, which limit the scope of the expected duality, and interestingly they take different forms in different coordinatizations of AdS momentum space...
...
11 pages, 5 figures.

http://arxiv.org/abs/1409.2335
Inflation without Selfreproduction
Viatcheslav Mukhanov
(Submitted on 8 Sep 2014)
We find a rather unique extension of inflationary scenario which avoids selfreproduction and thus resolves the problems of multiverse, predictability and initial conditions. In this theory the amplitude of the cosmological perturbations is expressed entirely in terms of the total duration of inflation.
11 pages.

http://arxiv.org/abs/1412.2518
Inflation without self-reproduction in F(R) gravity
Shin'ichi Nojiri, Sergei D. Odintsov
(Submitted on 8 Dec 2014)
We investigate inflation in frames of two classes of F(R) gravity and check its consistency with Planck data. It is shown that F(R) inflation without self-reproduction may be constructed in close analogy with the corresponding scalar example proposed by Mukhanov for the resolution the problems of multiverse, predictability and initial conditions.
6 pages.

 

[marcus]

http://arxiv.org/abs/1412.2914
A ΛCDM bounce scenario
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 9 Dec 2014)
We study a contracting universe composed of cold dark matter and radiation, and with a positive cosmological constant. As is well known from standard cosmological perturbation theory, under the assumption of initial quantum vacuum fluctuations the Fourier modes of the comoving curvature perturbation that exit the (sound) Hubble radius in such a contracting universe at a time of matter-domination will be nearly scale-invariant. Furthermore, the modes that exit the (sound) Hubble radius when the effective equation of state is slightly negative due to the cosmological constant will have a slight red tilt, in agreement with observations. We assume that loop quantum cosmology captures the correct high-curvature dynamics of the space-time, and this ensures that the big-bang singularity is resolved and is replaced by a bounce. We calculate the evolution of the perturbations through the bounce and find that they remain nearly scale-invariant. We also show that the amplitude of the scalar perturbations in this cosmology depends on a combination of the sound speed of cold dark matter, the Hubble rate in the contracting branch at the time of equality of the energy densities of cold dark matter and radiation, and the curvature scale that the loop quantum cosmology bounce occurs at. Finally, for a small sound speed of cold dark matter, this scenario predicts a small tensor-to-scalar ratio.
14 pages, 8 figures
google [LambdaCDM bounce]

http://arxiv.org/abs/1412.3524
Preferred instantaneous vacuum for linear scalar fields in cosmological space-times
Ivan Agullo, William Nelson, Abhay Ashtekar
(Submitted on 11 Dec 2014)
We discuss the problem of defining a preferred vacuum state at a given time for a quantized scalar field in Friedmann, Lema\^itre, Robertson Walker (FLRW) space-time. Among the infinitely many homogeneous, isotropic vacua available in the theory, we show that there exists at most one for which every Fourier mode makes vanishing contribution to the adiabatically renormalized energy-momentum tensor at any given instant. For massive fields such a state exists in the most commonly used backgrounds in cosmology, and provides a natural candidate for the ground state at that instant of time. The extension to the massless and the conformally coupled case are also discussed.
14 pages.
==excerpt from conclusions==
...The definition of preferred vacua for quantized fields in cosmological space-times is an interesting problem, not only for its conceptual importance but also for its relevance in the computation of primordial cosmic perturbations in the early universe. In those computations one needs to specify the quantum state for perturbations at some “initial” time η₀. ...
...
...
By contrast, the instantaneous vacuum introduced in this paper is free of these limitations. In the most widely used FLRW models, it provides a natural avenue to select a preferred vacuum at any given instant of time. ... it is the state with the least possible back-reaction at η = η₀. In this sense, it can be thought of as the analog of the standard vacuum in Minkowski space-time, albeit only at a given instant of time. The background time dependence is reflected in the fact that in (even the Heisenberg picture) the state so selected changes from one instant to another. The preferred instantaneous vacuum has been applied satisfactorily in the study of cosmological perturbation in loop quantum cosmology, where initial conditions are specified at or near the bounce time [18, 26]. We expect it will be also useful in other scenarios to select “initial conditions” for cosmological perturbations.
==endquote==

http://arxiv.org/abs/1412.3752
Flux formulation of loop quantum gravity: Classical framework
Bianca Dittrich, Marc Geiller
(Submitted on 11 Dec 2014)
We recently introduced a new representation for loop quantum gravity, which is based on the BF vacuum and is in this sense much nearer to the spirit of spin foam dynamics. In the present paper we lay out the classical framework underlying this new formulation. The central objects in our construction are the so-called integrated fluxes, which are defined as the integral of the electric field variable over surfaces of codimension one, and related in turn to Wilson surface operators. These integrated flux observables will play an important role in the coarse graining of states in loop quantum gravity, and can be used to encode in this context the notion of curvature-induced torsion. We furthermore define a continuum phase space as the modified projective limit of a family of discrete phase spaces based on triangulations. This continuum phase space yields a continuum (holonomy-flux) algebra of observables. We show that the corresponding Poisson algebra is closed by computing the Poisson brackets between the integrated fluxes, which have the novel property of being allowed to intersect each other.
60 pages, 13 figures
google [flux formulation LQG]

http://arxiv.org/abs/1412.4362
Loop Quantum Gravity
Dah-Wei Chiou
(Submitted on 14 Dec 2014)
This article presents an "in-a-nutshell" yet self-contained introductory review on loop quantum gravity (LQG) -- a background-independent, nonperturbative approach to a consistent quantum theory of gravity. Instead of rigorous and systematic derivations, it aims to provide a general picture of LQG, placing emphasis on the fundamental ideas and their significance. The canonical formulation of LQG, as the central topic of the article, is presented in a logically orderly fashion with moderate details, while the spin foam theory, black hole thermodynamics, and loop quantum cosmology are covered briefly. Current directions and open issues are also summarized.
87 pages. Invited review article. A large part based on arXiv:gr-qc/0404018 by A. Ashtekar and J. Lewandowski and "Quantum Gravity" by C. Rovelli. To appear in Int. J. Mod. Phys. D and in "One Hundred Years of General Relativity: Cosmology and Gravity," edited by Wei-Tou Ni (World Scientific, Singapore, 2015)

http://arxiv.org/abs/1412.5527
A quantum kinematics for asymptotically flat spacetimes
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 17 Dec 2014)
We construct a quantum kinematics for asymptotically flat spacetimes based on the Koslowski-Sahlmann (KS) representation. The KS representation is a generalization of the representation underlying Loop Quantum Gravity (LQG) which supports, in addition to the usual LQG operators, the action of `background exponential operators' which are connection dependent operators labelled by `background' su(2) electric fields. KS states have, in addition to the LQG state label corresponding to 1 dimensional excitations of the triad, a label corresponding to a `background' electric field which describes 3 dimensional excitations of the triad. Asymptotic behaviour in quantum theory is controlled through asymptotic conditions on the background electric fields which label the {\em states} and the background electric fields which label the {\em operators}. Asymptotic conditions on the triad are imposed as conditions on the background electric field state label while confining the LQG spin net graph labels to compact sets. We show that KS states can be realized as wave functions on a quantum configuration space of generalized connections and that the asymptotic behaviour of each such generalized connection is determined by that of the background electric fields which label the background exponential operators. Similar to the spatially compact case, the Gauss Law and diffeomorphism constraints are then imposed through Group Averaging techniques to obtain a large sector of gauge invariant states. It is shown that this sector supports a unitary action of the group of asymptotic rotations and translations and that, as anticipated by Friedman and Sorkin, for appropriate spatial topology, this sector contains states which display fermionic behaviour under 2π rotations.
64 pages

briefly noted:
http://arxiv.org/abs/1412.5206
Quantum Darwinism, Classical Reality, and the Randomness of Quantum Jumps
Wojciech H. Zurek

http://arxiv.org/abs/1412.4343
Superbounce and Loop Quantum Cosmology...
V.K. Oikonomou
(my comment: explores LQC in a SUSY "supergravity" context)

 

[marcus]

http://arxiv.org/abs/1412.5851
Black holes as gases of punctures with a chemical potential: Bose-Einstein condensation and logarithmic corrections to the entropy
Olivier Asin, Jibril Ben Achour, Marc Geiller, Karim Noui, Alejandro Perez
(Submitted on 18 Dec 2014)
We study the thermodynamical properties of black holes when described as gases of indistinguishable punctures with a chemical potential. In this picture, which arises from loop quantum gravity, the black hole microstates are defined by finite families of half-integers spins coloring the punctures, and the near-horizon energy measured by quasi-local stationary observers defines the various thermodynamical ensembles. The punctures carry excitations of quantum geometry in the form of quanta of area, and the total horizon area a_H is given by the sum of these microscopic contributions. We assume here that the system satisfies the Bose-Einstein statistics, and that each microstate is degenerate with a holographic degeneracy given by exp(λa_H/ℓ_Pl²) and λ>0. We analyze in detail the thermodynamical properties resulting from these inputs, and in particular compute the grand canonical entropy. We explain why the requirements that the temperature be fixed to the Unruh temperature and that the chemical potential vanishes do not specify completely the semi-classical regime of large horizon area, and classify in turn what the various regimes can be. When the degeneracy saturates the holographic bound (λ=1/4), there exists a semi-classical regime in which the subleading corrections to the entropy are logarithmic. Furthermore, this regime corresponds to a Bose-Einstein condensation, in the sense that it is dominated by punctures carrying the minimal (or ground state) spin value 1/2.
22 pages

http://arxiv.org/abs/1412.6000
Rainbow metric from quantum gravity
Mehdi Assaniousssi, Andrea Dapor, Jerzy Lewandowski
(Submitted on 18 Dec 2014)
In this letter, we describe a general mechanism for emergence of a rainbow metric from a quantum cosmological model. This idea is based on QFT on a quantum space-time. Under general assumptions, we discover that the quantum space-time on which the field propagates can be replaced by a classical space-time, whose metric depends explicitly on the energy of the field: as shown by an analysis of dispersion relations, quanta of different energy propagate on different metrics, similar to photons in a refractive material (hence the name "rainbow" used in the literature). In deriving this result, we do not consider any specific theory of quantum gravity: the qualitative behavior of high-energy particles on quantum space-time relies only on the assumption that the quantum space-time is described by a wave-function Ψ_o in a Hilbert space H_G.
4 pages, 2 figures

http://arxiv.org/abs/1412.6015
On the Effective Metric of a Planck Star
Tommaso De Lorenzo, Costantino Pacilio, Carlo Rovelli, Simone Speziale
(Submitted on 18 Dec 2014)
Spacetime metrics describing `non-singular' black holes are commonly studied in the literature as effective modification to the Schwarzschild solution that mimic quantum gravity effects removing the central singularity. Here we point out that to be physically plausible, such metrics should also incorporate the 1-loop quantum corrections to the Newton potential and a non-trivial time delay between an observer at infinity and an observer in the regular center. We present a modification of the well-known Hayward metric that features these two properties. We discuss bounds on the maximal time delay imposed by conditions on the curvature, and the consequences for the weak energy condition, in general violated by the large transversal pressures introduced by the time delay.
10 pages, many figures

http://arxiv.org/abs/1412.6055
Quantum space-time of a charged black hole
Rodolfo Gambini, Esteban Mato Capurro, Jorge Pullin
(Submitted on 18 Dec 2014)
We quantize spherically symmetric electrovacuum gravity. The algebra of Hamiltonian constraints can be made Abelian via a rescaling and linear combination with the diffeomorphism constraint. As a result the constraint algebra is a true Lie algebra. We complete the Dirac quantization procedure using loop quantum gravity techniques. We present explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. The resulting quantum space-times resolve the singularity present in the classical theory inside charged black holes and allows to extend the space-time through where the singularity used to be into new regions. We show that quantum discreteness of space-time may also play a role in stabilizing the Cauchy horizons, though back reaction calculations are needed to confirm this point.
6 pages, one figure

http://arxiv.org/abs/1412.6057
First order gravity on the light front
Sergei Alexandrov, Simone Speziale
(Submitted on 18 Dec 2014)
We study the canonical structure of the real first order formulation of general relativity on a null foliation. We use a tetrad decomposition which allows to elegantly encode the nature of the foliation in the norm of a vector in the fibre bundle. The resulting constraint structure shows some peculiarities. In particular, the dynamical Einstein equations propagating the physical degrees of freedom appear in this formalism as second class tertiary constraints, which puts them on the same footing as the Hamiltonian constraint of the Ashtekar's connection formulation. We also provide a framework to address the issue of zero modes in gravity, in particular, to study the non-perturbative fate of the zero modes of the linearized theory. Our results give a new angle on the dynamics of general relativity and can be used to quantize null hypersurfaces in the formalism of loop quantum gravity or spin foams.
35 pages, 1 figure

 

[nicoo]

http://arxiv.org/abs/1411.0977
Geometry and the Quantum: Basics
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
(Submitted on 4 Nov 2014)
Motivated by the construction of spectral manifolds in noncommutative geometry, we introduce a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of scalar fields. This commutation relation appears in two versions, one sided and two sided. It implies the quantization of the volume. In the one-sided case it implies that the manifold decomposes into a disconnected sum of spheres which will represent quanta of geometry. The two sided version in dimension 4 predicts the two algebras M2(H) and M4(C) which are the algebraic constituents of the Standard Model of particle physics. This taken together with the non-commutative algebra of functions allows one to reconstruct, using the spectral action, the Lagrangian of gravity coupled with the Standard Model. We show that any connected Riemannian Spin 4-manifold with quantized volume >4 (in suitable units) appears as an irreducible representation of the two-sided commutation relations in dimension 4 and that these representations give a seductive model of the "particle picture" for a theory of quantum gravity in which both the Einstein geometric standpoint and the Standard Model emerge from Quantum Mechanics. Physical applications of this quantization scheme will follow in a separate publication.
33 pages, 2 figures

Connes gave a 2-hour lecture about those recent papers with Chamseddine and Mukhanov to the HIM this week.

The video is already on Youtube:

 

[marcus]

http://arxiv.org/abs/1412.7435
Horizon entropy with loop quantum gravity methods
Daniele Pranzetti, Hanno Sahlmann
(Submitted on 23 Dec 2014)
We show that the spherically symmetric isolated horizon can be described in terms of an SU(2) connection and a su(2) valued one form, obeying certain constraints. The horizon symplectic structure is precisely the one of 3d gravity in a first order formulation. We quantize the horizon degrees of freedom in the framework of loop quantum gravity, with methods recently developed for 3d gravity with non-vanishing cosmological constant. Bulk excitations ending on the horizon act very similar to particles in 3d gravity. The Bekenstein-Hawking law is recovered in the limit of imaginary Barbero-Immirzi parameter. Alternative methods of quantization are also discussed.
17 pages, 2 figures

http://arxiv.org/abs/1412.7256
Categorical Operator Algebraic Foundations of Relational Quantum Theory
Paolo Bertozzini
(Submitted on 23 Dec 2014)
We provide an algebraic formulation of C.Rovelli's relational quantum theory that is based on suitable notions of "non-commutative" higher operator categories, originally developed in the study of categorical non-commutative geometry. As a way to implement C.Rovelli's original intuition on the relational origin of space-time, in the context of our proposed algebraic approach to quantum gravity via Tomita-Takesaki modular theory, we tentatively suggest to use this categorical formalism in order to spectrally reconstruct non-commutative relational space-time geometries from categories of correlation bimodules between operator algebras of observables. Parts of this work are joint collaborations with...
6 pages, for the PoS proceedings of the "Frontiers of Fundamental Physics 14" symposium (FFP14) held 15-18 July 2014 at the University of Marseille

http://arxiv.org/abs/1412.7207
RG flows of Quantum Einstein Gravity in the linear-geometric approximation
Maximilian Demmel, Frank Saueressig, Omar Zanusso
(Submitted on 22 Dec 2014)
We construct a novel Wetterich-type functional renormalization group equation for gravity which encodes the gravitational degrees of freedom in terms of gauge-invariant fluctuation fields. Applying a linear-geometric approximation the structure of the new flow equation is considerably simpler than the standard Quantum Einstein Gravity construction since only transverse-traceless and trace part of the metric fluctuations propagate in loops. The geometric flow reproduces the phase-diagram of the Einstein-Hilbert truncation including the non-Gaussian fixed point essential for Asymptotic Safety. Extending the analysis to the polynomial f(R)-approximation establishes that this fixed point comes with similar properties as the one found in metric Quantum Einstein Gravity; in particular it possesses three UV-relevant directions and is stable with respect to deformations of the regulator functions by endomorphisms. In a companion paper we will establish that our flow equation also admits complete fixed functions f∗(R), indicating that the fixed point identified here remains robust when an infinite number of coupling constants is included.
32 pages, 5 figues

http://arxiv.org/abs/1412.7352
Testing the Everett Interpretation of Quantum Mechanics with Cosmology
Aurelien Barrau
(Submitted on 23 Dec 2014)
In this brief note, we argue that contrarily to what is still often stated, the Everett many-worlds interpretation of quantum mechanics is not in principle impossible to test. It is actually not more difficult (but not easier either) to test than most other kinds of multiverse theories. We also remind why multiverse scenarios can be falsified.
5 pages

 

[marcus]

http://arxiv.org/abs/1412.7546
SL(2,C) Chern-Simons Theory, a non-Planar Graph Operator, and 4D Loop Quantum Gravity with a Cosmological Constant: Semiclassical Geometry
Hal M. Haggard, Muxin Han, Wojciech Kamiński, Aldo Riello
(Submitted on 23 Dec 2014)
We study the expectation value of a nonplanar Wilson graph operator in SL(2,C) Chern-Simons theory on S³. In particular we analyze its asymptotic behaviour in the double-scaling limit in which both the representation labels and the Chern-Simons coupling are taken to be large, but with fixed ratio. When the Wilson graph operator has a specific form, motivated by loop quantum gravity, the critical point equations obtained in this double-scaling limit describe a very specific class of flat connection on the graph complement manifold. We find that flat connections in this class are in correspondence with the geometries of constant curvature 4-simplices. The result is fully non-perturbative from the perspective of the reconstructed geometry. We also show that the asymptotic behavior of the amplitude contains at the leading order an oscillatory part proportional to the Regge action for the single 4-simplex in the presence of a cosmological constant. In particular, the cosmological term contains the full-fledged curved volume of the 4-simplex. Interestingly, the volume term stems from the asymptotics of the Chern-Simons action. This can be understood as arising from the relation between Chern-Simons theory on the boundary of a region, and a theory defined by an F² action in the bulk. Another peculiarity of our approach is that the sign of the curvature of the reconstructed geometry, and hence of the cosmological constant in the Regge action, is not fixed a priori, but rather emerges semiclassically and dynamically from the solution of the equations of motion. In other words, this work suggests a relation between 4-dimensional loop quantum gravity with a cosmological constant and SL(2,C) Chern-Simons theory in 3-dimensions with knotted graph defects.
54+11 pages, 9 figures

general interest, summary of the latest Planck mission results, from the December 2014 Ferrara conference, talk by Efstathiou, slides:
http://www.cosmos.esa.int/documents/387566/387653/Ferrara_Dec1_16h30_Efstathiou_Cosmology.pdf

brief mention:
http://arxiv.org/abs/1412.7576
Interpretations of Quantum Theory in the Light of Modern Cosmology
Mario Castagnino, Sebastian Fortin, Roberto Laura, Daniel Sudarsky
(Submitted on 24 Dec 2014)
The difficult issues related to the interpretation of quantum mechanics and, in particular, the "measurement problem" are revisited using as motivation the process of generation of structure from quantum fluctuations in inflationary cosmology. ...
25 pages

http://arxiv.org/abs/1412.7561
Topological aspects of generalized gravitational entropy
Felix M. Haehl, Thomas Hartman, Donald Marolf, Henry Maxfield, Mukund Rangamani
(Submitted on 23 Dec 2014)
28 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1412.7827
LQG predicts the Unruh Effect. Comment to the paper "Absence of Unruh effect in polymer quantization" by Hossain and Sardar
Carlo Rovelli
(Submitted on 25 Dec 2014)
A recent paper claims that loop quantum gravity predicts the absence of the Unruh effect. I show that this is not the case, and take advantage of this opportunity to shed some light on some related issues.
3 pages

http://arxiv.org/abs/1412.8195
Loop Quantum Cosmology Matter Bounce Reconstruction from F(R) Gravity Using an Auxiliary Field
V.K. Oikonomou
(Submitted on 28 Dec 2014)
Using the reconstruction technique with an auxiliary field, we investigate which F(R) gravities can produce the matter bounce cosmological solutions. Owing to the specific functional form of the matter bounce Hubble parameter, the reconstruction technique leads, after some simplifications, to the same Hubble parameter as in the matter bounce scenario. Focusing the study to the large and small cosmic time t limits, we were able to find which F(R) gravities can generate the matter bounce Hubble parameter. In the case of small cosmic time limit, which corresponds to large curvature values, the F(R) gravity is F(R)∼R+αR^2, which is an inflation generating gravity, and at small curvature, or equivalently, large cosmic time, the F(R) gravity generating the corresponding limit of the matter bounce Hubble parameter, is F(R)∼1/R, a gravity known to produce late-time acceleration. Thus we have the physically appealing picture in which a Jordan frame F(R) gravity that imitates the matter bounce solution at large and small curvatures, can generate Starobinsky inflation and late-time acceleration. Moreover, the scale factor corresponding to the reconstruction technique coincides almost completely to the matter bounce scenario scale factor, when considered in the aforementioned limiting curvature cases. This is scrutinized in detail, in order to examine the validity of the reconstruction method in these limiting cases, and according to our analysis, exact agreement is achieved.

http://arxiv.org/abs/1412.8247
Pachner moves in a 4d Riemannian holomorphic Spin Foam model
Andrzej Banburski, Lin-Qing Chen, Laurent Freidel, Jeff Hnybida
(Submitted on 29 Dec 2014)
In this work we study a Spin Foam model for 4d Riemannian gravity, and propose a new way of imposing the simplicity constraints that uses the recently developed holomorphic representation. Using the power of the holomorphic integration techniques, and with the introduction of two new tools: the homogeneity map and the loop identity, for the first time we give the analytic expressions for the behaviour of the Spin Foam amplitudes under 4-dimensional Pachner moves. It turns out that this behaviour is controlled by an insertion of nonlocal mixing operators. In the case of the 5-1 move, the expression governing the change of the amplitude can be interpreted as a vertex renormalisation equation. We find a natural truncation scheme that allows us to get an invariance up to an overall factor for the 4-2 and 5-1 moves, but not for the 3-3 move. The study of the divergences shows that there is a range of parameter space for which the 4-2 move is finite while the 5-1 move diverges. This opens up the possibility to recover diffeomorphism invariance in the continuum limit of Spin Foam models for 4D Quantum Gravity.
48 pages, 30 figures

http://arxiv.org/abs/1412.8390
Dimensional flow in discrete quantum geometries
Gianluca Calcagni, Daniele Oriti, Johannes Thürigen
(Submitted on 29 Dec 2014)
In various theories of quantum gravity, one observes a change in the spectral dimension from the topological spatial dimension d at large length scales to some smaller value at small, Planckian scales. While the origin of such a flow is well understood in continuum approaches, in theories built on discrete structures a firm control of the underlying mechanism is still missing. We shed some light on the issue by presenting a particular class of quantum geometries with a flow in the spectral dimension, given by superpositions of states defined on regular complexes. For particular superposition coefficients parametrized by a real number 0<α<d, we find that the spatial spectral dimension reduces to d_s≃α at small scales. The spatial Hausdorff dimension of such class of states varies between 1 and d, while the walk dimension takes the usual value d_w=2. Therefore, these quantum geometries may be considered as fractal only when α=1, where the "magic number" d_s^spacetime≃2 for the spectral dimension of spacetime, appearing so often in quantum gravity, is reproduced as well. These results apply, in particular, to special superpositions of spin-network states in loop quantum gravity, and they provide more solid indications of dimensional flow in this approach.
10 pages, 6 figures

http://arxiv.org/abs/1412.8452
Spacetime defects and group momentum space
Michele Arzano, Tomasz Trzesniewski
(Submitted on 29 Dec 2014)
We study massive and massless conical defects in Minkowski and de Sitter space in various spacetime dimensions. The energy-momentum of such defects, seen as extended relativistic topological objects, is completely characterized by the holonomy of the connection associated with their spacetime metric. These holonomies are given by rotations and null rotations for massive and massless defects respectively. We observe that in the case of a massless conical defect in five dimensional Minkowski space its restricted momentum space can be parametrized by a subgroup of the five-dimensional Lorentz group, the AN(3) group, corresponding to the well known momentum space associated with the deformed κ-Poincaré algebra and κ-Minkowski noncommutative spacetime. We further argue that massless conical defects in four dimensional de Sitter space can be analogously described by holonomies belonging to the same group. We thus provide the first example of how group-valued momenta related to four-dimensional deformations of relativistic symmetries can arise in the description of the motion of spacetime defects.
11 pages

possible general interest:
http://arxiv.org/abs/1412.8462
An operational approach to spacetime symmetries: Lorentz transformations from quantum communication
Philipp A Hoehn, Markus P Mueller
(Submitted on 29 Dec 2014)
In most approaches to fundamental physics, spacetime symmetries are postulated a priori and then explicitly implemented in the theory. This includes Lorentz covariance in quantum field theory and diffeomorphism invariance in quantum gravity, which are seen as fundamental principles to which the final theory has to be adjusted. In this paper, we suggest within a much simpler setting that this kind of reasoning can actually be reversed, by taking an operational approach inspired by quantum information theory. We consider observers in distant laboratories, with local physics described by the laws of abstract quantum theory, and without presupposing a particular spacetime structure. We ask what information-theoretic effort the observers have to spend to synchronize their descriptions of local physics. If there are "enough" observables that can be measured jointly on different types of systems, we show that the observers' descriptions are related by an element of the Lorentz group O^+(3,1), together with a global scaling factor. This operational derivation of the Lorentz transformations correctly describes the physics of relativistic Stern-Gerlach measurements in the WKB approximation, and predicts representations of different spin and Wigner little groups.
32 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1501.00855
Closure constraints for hyperbolic tetrahedra
Christoph Charles, Etera R. Livine
(Submitted on 5 Jan 2015)
We investigate the generalization of loop gravity's twisted geometries to a q-deformed gauge group. In the standard undeformed case, loop gravity is a formulation of general relativity as a diffeomorphism-invariant SU(2) gauge theory. Its classical states are graphs provided with algebraic data. In particular closure constraints at every node of the graph ensure their interpretation as twisted geometries. Dual to each node, one has a polyhedron embedded in flat space R³. One then glues them allowing for both curvature and torsion. It was recently conjectured that q-deforming the gauge group SU(2) would allow to account for a non-vanishing cosmological constant Lambda, and in particular that deforming the loop gravity phase space with real parameter q>0 would lead to a generalization of twisted geometries to a hyperbolic curvature. Following this insight, we look for generalization of the closure constraints to the hyperbolic case. In particular, we introduce two new closure constraints for hyperbolic tetrahedra. One is compact and expressed in terms of normal rotations (group elements in SU(2) associated to the triangles) and the second is non-compact and expressed in terms of triangular matrices (group elements in SB(2,C)). We show that these closure constraints both define a unique dual tetrahedron (up to global translations on the three-dimensional one-sheet hyperboloid) and are thus ultimately equivalent.
24 pages

http://arxiv.org/abs/1501.00486
Spherical Top-Hat Collapse of Viscous Modified Chaplygin Gas in Einstein's Gravity and Loop Quantum Cosmology
Ujjal Debnath, Mubasher Jamil
(Submitted on 3 Jan 2015)
In this work, we focus on the collapse of a spherically symmetric perturbation, with a classical top-hat profile, to study the nonlinear evolution of only viscous modified Chaplygin gas (VMCG) perturbations in Einstein's gravity as well as in loop quantum Cosmology (LQC). In the perturbed region, we have investigated the natures of equation of state parameter, square speed of sound and another perturbed quantities. The results have been analyzed by numerical and graphical investigations.
7 pages, 14 figures. arXiv admin note: text overlap with arXiv:1401.1270,...

brief mention:
http://arxiv.org/abs/1501.00391
The Structurally Dynamic Cellular Network and Quantum Graphity Approaches to Quantum Gravity - A Review and Comparison
Manfred Requardt, Saeed Rastgoo
(Submitted on 2 Jan 2015)
Starting from the working hypothesis that both physics and the corresponding mathematics have to be described by means of discrete concepts on the Planck-scale, one of the many problems one has to face in this enterprise is to find the discrete protoforms of the building blocks of our ordinary continuum physics and mathematics. We regard these continuum concepts and continuum spacetime in particular as being emergent, coarse-grained and derived relative to an underlying erratic and disordered microscopic substratum which is expected to play by quite different rules. A central role in our analysis is played by a geometric renormalization group which creates (among other things) a kind of sparse translocal network of correlations between the points in classical continuous space-time and underlies, in our view, such mysterious phenomena as holography and the black hole entropy-area law. The same point of view holds for quantum theory which we also regard as a low-energy, coarse-grained continuum theory, being emergent from something more fundamental. In this paper we review our approach and compare it to the quantum graphity framework.
30 pages, 2 tables

http://arxiv.org/abs/1501.00708
Instability of Quantum de Sitter Spacetime
Chiu Man Ho, Stephen D. H. Hsu
(Submitted on 4 Jan 2015)
Quantized fields (e.g., the graviton itself) in de Sitter (dS) spacetime lead to particle production: specifically, we consider a thermal spectrum resulting from the dS (horizon) temperature. The energy required to excite these particles reduces slightly the rate of expansion and eventually modifies the semiclassical spacetime geometry. The resulting manifold no longer has constant curvature nor time reversal invariance, and back-reaction renders the classical dS background unstable to perturbations. In the case of AdS, there exists a global static vacuum state; in this state there is no particle production and the analogous instability does not arise.
Comments: 3 pages

http://arxiv.org/abs/1501.00059
More Is Different: Reconciling eV Sterile Neutrinos and Cosmological Mass Bounds
Yong Tang
(Submitted on 31 Dec 2014)
It is generally expected that adding light sterile species would increase the effective number of neutrinos, N_eff. In this paper we discuss a scenario that Neff can actually decrease due to the neutrino oscillation effect if sterile neutrinos have self-interactions. We specifically focus on the eV mass range, as suggested by the neutrino anomalies. With large self-interactions, sterile neutrinos are not fully thermalized in the early Universe because of the suppressed effective mixing angle or matter effect. As the Universe cools down, flavor equilibrium between active and sterile species can be reached after big bang nucleosynthesis (BBN) epoch, but leading to a decrease of N_eff. In such a scenario, we also show that the conflict with cosmological mass bounds on the additional sterile neutrinos can be relaxed further when more light species are introduced.
13 pages, 4 figures

http://arxiv.org/abs/1501.00119
Unruh effect without Rindler horizon
Nistor Nicolaevici
(Submitted on 31 Dec 2014)
We investigate the Unruh effect for a massless scalar field in the two dimensional Minkowski space in the presence of a uniformly accelerated perfect mirror, with the trajectory of the mirror chosen in such a way that the mirror completely masks the Rindler horizon from the space-time region of interest. We find that the characteristic thermodynamical properties of the effect remain unchanged, i.e. the response of a uniformly co-accelerated Unruh detector and the distribution of the Rindler particles retain their thermal form. However, since in this setup there are no unobserved degrees of freedom of the field the thermal statistics of the Rindler particles is inconsistent with an initial pure vacuum, which leads us to reconsider the problem for the more physical case when the mirror is inertial in the past. In these conditions we find that the distribution of the Rindler particles is non-thermal even in the limit of infinite acceleration times, but an effective thermal statistics can be recovered provided that one restricts to the expectation values of smeared operators associated to finite norm Rindler states. We explain how the thermal statistics in our problem can be understood in analogy with that in the conventional version of the effect.
49 pages, 12 figures

http://arxiv.org/abs/1501.00996
Using Atomic Clocks to Detect Gravitational Waves
Abraham Loeb, Dan Maoz
(Submitted on 5 Jan 2015)
Atomic clocks have recently reached a fractional timing precision of <10⁻¹⁸. We point out that an array of atomic clocks, distributed along the Earth's orbit around the Sun, will have the sensitivity needed to detect the time dilation effect of mHz gravitational waves (GWs), such as those emitted by supermassive black hole binaries at cosmological distances. Simultaneous measurement of clock-rates at different phases of a passing GW provides an attractive alternative to the interferometric detection of temporal variations in distance between test masses separated by less than a GW wavelength, currently envisioned for the eLISA mission.
2 pages, 1 figure, submitted to Phys. Rev. D

 

[marcus]

http://arxiv.org/abs/1501.01650
A glimpse of the early universe without real light
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 7 Jan 2015)
We analyze the implications of the violations of the strong Huygens principle in the transmission of information from the early universe to the current era via massless fields. We show that much more information reaches us through timelike channels (not mediated by real photons) than it is carried by rays of light, which are usually regarded as the only carriers of information.
5 pages, 2 figures.

http://arxiv.org/abs/1501.01408
Quantum Gravity as an Information Network: Self-Organization of a 4D Universe
Carlo A. Trugenberger
(Submitted on 7 Jan 2015)
I propose a quantum gravity model in which the fundamental degrees of freedom are pure information bits. The Hamiltonian is a very simple network model consisting of a ferromagnetic Ising model for space-time vertices and an antiferromagnetic Ising model for the links between them. As a result of the frustration arising between these two terms, the ground state self-organizes as a new type of low-clustering, lattice-like graph with finite Hausdorff dimension. The model has three quantum phases: a mean field phase in which the spectral and Hausdorff dimensions coincide and are larger then 4. A fluctuations-dominated phase in which the Hausdorff dimension can only be 4 and the spectral dimension is lower than the Hausdorff dimension and a disordered phase in which there is no space-time interpretation. The large-scale dimension 4 of the universe is related to the upper critical dimension 4 of the Ising model. An ultraviolet fixed point at the lower critical dimension of the Ising model is conjectured to imply the absence of space-time at very small scales. At finite temperatures the universe emerges without big bang and without singularities from a ferromagnetic phase transition in which space-time itself forms out of a hot soup of information bits. When the temperature is lowered the universe unfolds by lowering its connectivity, a mechanism I have called topological expansion. Topological expansion is associated with one emerging dimension describing the unfolding process. Quantum fluctuations about this semiclassical universes are elementary black holes and wormholes. The model admits, however, also macroscopic black hole configurations corresponding to graphs containing holes with no space time inside and around which there are Schwarzschild-like horizons with a lower spectral dimension and an entropy proportional to their area.
12 pages, several tables.

http://arxiv.org/abs/1501.01610
Analysis of a work of quantum art
Seth Lloyd
(Submitted on 9 Dec 2014)
This paper provides a quantum-mechanical analysis of an artwork, `Wigner's friends,' by Diemut Strebe. The work consists of two telescopes, one on earth, one launched into space, and explores ideas of quantum correlations and quantum measurement. This paper examines the scientific basis of the work and analyzes the form of quantum correlation between the two telescope systems.
7 pages,

 

[John86]

http://arxiv.org/abs/1501.01619
Energy conditions in the epoch of galaxy formation
Matt Visser
(Submitted on 7 Jan 2015)
The energy conditions of Einstein gravity (classical general relativity) do not require one to fix a specific equation of state. In a Friedmann-Robertson-Walker universe where the equation of state for the cosmological fluid is uncertain, the energy conditions provide simple, model-independent, and robust bounds on the behaviour of the density and look-back time as a function of red-shift. Current observations suggest that the "strong energy condition" is violated sometime between the epoch of galaxy formation and the present. This implies that no possible combination of "normal" matter is capable of fitting the observational data.

http://arxiv.org/abs/1501.01053
Null Surfaces: Counter-term for the Action Principle and the Characterization of the Gravitational Degrees of Freedom
Krishnamohan Parattu, Sumanta Chakraborty, Bibhas Ranjan Majhi, T. Padmanabhan
(Submitted on 6 Jan 2015)
Constructing a well-posed variational principle and characterizing the appropriate degrees of freedom that need to be fixed at the boundary are non-trivial issues in general relativity. For spacelike and timelike boundaries, one knows that (i) the addition of a counter-term [like the Gibbons-Hawking-York (GHY) counter-term] will make the variational principle well-defined and (ii) the degrees of freedom to be fixed on the boundary are contained in the induced 3-metric. These results, however, do not directly generalize to null boundaries on which the 3-metric becomes degenerate. In this work, we address the following questions: (i) What is the counter-term that needs to be added on a null boundary to make the variational principle well-defined? (ii) How do we characterize the degrees of freedom which need to be fixed at the boundary? We show that the counter-term to be added is 2q√(Θ+κ) and that the degrees of freedom to be fixed on the surface are in the induced 2-metric on a null surface, qab, and the tangent vector ℓa to the null congruence on the surface. We also demonstrate that the degrees of freedom in ℓa can be eliminated by choosing suitable coordinates. This allows one to identify the physical degrees of freedom of the gravitational field with components qab of the 2-metric in a suitable (1+1+2) double null parametrization of the spacetime. The implications are discussed.

 

[Ilja]

I hope it is ok to post here a critical note about one of the papers? Or would it be better to have started a new thread for this?

http://arxiv.org/abs/1412.7827
LQG predicts the Unruh Effect. Comment to the paper "Absence of Unruh effect in polymer quantization" by Hossain and Sardar
Carlo Rovelli
(Submitted on 25 Dec 2014)
A recent paper claims that loop quantum gravity predicts the absence of the Unruh effect. I show that this is not the case, and take advantage of this opportunity to shed some light on some related issues.

Hm, I see a trajectory of the detector (\frac{1}{a}\sinh(as),\frac{1}{a}\cosh(as),0,0), then assumptions about time invariance and integrals from -\infty to \infty. But with an UV cutoff, there would be no time invariance along that trajectory, and no possibility to compute integrals from -\infty to \infty.

To obtain some Unruh-like radiation for detector acceleration only during a finite time seems possible, see Raval, A., Hu, B. L., Koks, D., Near-thermal radiation in detectors, mirrors, and black holes: A stochastic approach, Phys Rev D 55 (8):4795-4812 (1997), so that finally Rovelli may be right in his claim, but what is presented in this paper does not seem sufficient to prove this.

 

[marcus]

I hope it is ok to post here a critical note about one of the papers? Or would it be better to have started a new thread for this?
...

Better to have started a new thread, I think, Ilja. This thread is a new papers bibliography. Would get overloaded if we included discussion of new papers.
I can't guarantee that you will get a satisfactory discussion of "Unruh effect in Lqg" if you start a thread. But at least it would be visible. some people, like Demystifier, know enough and could usefully render an opinion if they wanted. You would stand a good chance, I think. It's not guaranteed but I encourage you to give it a try.

Continuing now with the new QG papers biblio:
http://arxiv.org/abs/1501.02086
Renormalization of an Abelian Tensor Group Field Theory: Solution at Leading Order
Vincent Lahoche, Daniele Oriti, Vincent Rivasseau
(Submitted on 9 Jan 2015)
We study a just renormalizable tensorial group field theory of rank six with quartic melonic interactions and Abelian group U(1). We introduce the formalism of the intermediate field, which allows a precise characterization of the leading order Feynman graphs. We define the renormalization of the model, compute its (perturbative) renormalization group flow and write its expansion in terms of effective couplings. We then establish closed equations for the two point and four point functions at leading (melonic) order. Using the effective expansion and its uniform exponential bounds we prove that these equations admit a unique solution at small renormalized coupling.
37 pages, 14 figures

http://arxiv.org/abs/1501.02443
Origin of Structure in the Universe: Quantum Cosmology Reconsidered
Edward Anderson
(Submitted on 11 Jan 2015)
Based on a more careful canonical analysis, we motivate a reduced quantization of slightly inhomogeneous cosmology in place of the Dirac quantization in the existing literature, and provide it in the vacuum case. This is attained via consideration of configuration space geometries at various levels of reduction. Some of these have the good fortunate of being flat. Geometrically natural coordinates thereupon are interpreted in terms of the original redundant formulation's well-known mode expansion coefficients.
5 pages, 1 figure

http://arxiv.org/abs/1501.02671
Is Spacetime Countable?
Sean Gryb
(Submitted on 12 Jan 2015)
Is there a number for every bit of spacetime, or is spacetime smooth like the real line? The ultimate fate of a quantum theory of gravity might depend on it. The troublesome infinities of quantum gravity can be cured by assuming that spacetime comes in countable, discrete pieces which one could simulate on a computer. But, perhaps there is another way? In this essay, we propose a picture where scale is meaningless so that there can be no minimum length and, hence, no fundamental discreteness. In this picture, Einstein's Special Relativity, suitably modified to accommodate an expanding Universe, can be reinterpreted as a theory where only the instantaneous shapes of configurations count.
13 pages, 7 figures. Illustrations by Marc Ngui. To appear in Foundations of Physics Frontiers Collections: It From Bit or Bit From It? Winner of 4th prize in FQXi Essay contest

brief mention:
http://arxiv.org/abs/1501.02681
Holographic Inflation and the Low Entropy of the Early Universe
Tom Banks
(Submitted on 12 Jan 2015)
This is a completely rewritten version of the talk I gave at the Philosophy of Cosmology conference in Tenerife, September 2014, which incorporates elements of my IFT Madrid Anthropics Conference talk...
22 pages

 

[marcus]

http://arxiv.org/abs/1501.02963
Quantum Geometry and Black Holes
J. Fernando Barbero G., Alejandro Perez
(Submitted on 13 Jan 2015)
We present an overall picture of the advances in the description of black hole physics from the perspective of loop quantum gravity. After an introduction that discusses the main conceptual issues we present some details about the classical and quantum geometry of isolated horizons and their quantum geometry and then use this scheme to give a natural definition of the entropy of black holes. The entropy computations can be neatly expressed in the form of combinatorial problems solvable with the help of methods based on number theory and the use of generating functions. The recovery of the Bekenstein-Hawking law and corrections to it is explained in some detail. After this, due attention is paid to the discussion of semiclassical issues. An important point in this respect is the proper interpretation of the horizon area as the energy that should appear in the statistical-mechanical treatment of the black hole model presented here. The chapter ends with a comparison between the microscopic and semiclassical approaches to the computation of the entropy and discusses a number of issues regarding the relation between entanglement and statistical entropy and the possibility of comparing the subdominant (logarithmic) corrections to the entropy obtained with the help of the Euclidean path integral with the ones obtained in the present framework.
39 pages. Contribution to appear in the World Scientific series "100 Years of General Relativity" edited by A. Ashtekar and J. Pullin

http://arxiv.org/abs/1501.03007
The shape dynamics description of gravity
Tim Koslowski
(Submitted on 13 Jan 2015)
Classical gravity can be described as a relational dynamical system without ever appealing to spacetime or its geometry. This description is the so-called shape dynamics description of gravity. The existence of relational first principles from which the shape dynamics description of gravity can be derived is a motivation to consider shape dynamics (rather than GR) as the fundamental description of gravity. Adopting this point of view leads to the question: What is the role of spacetime in the shape dynamics description of gravity? This question contains many aspects: Compatibility of shape dynamics with the description of gravity in terms of spacetime geometry, the role of local Minkowski space, universality of spacetime geometry and the nature of quantum particles, which can no longer be assumed to be irreducible representations of the Poincare group. In this contribution I derive effective spacetime structures by considering how matter fluctuations evolve along with shape dynamics. This evolution reveals an "experienced spacetime geometry." This leads (in an idealized approximation) to local Minkowski space and causal relations. The small scale structure of the emergent geometric picture depends on the specific probes used to experience spacetime, which limits the applicability of effective spacetime to describe shape dynamics. I conclude with discussing the nature of quantum fluctuations (particles) in shape dynamics and how local Minkowski spacetime emerges from the evolution of quantum particles.
16 pages, a submission to the proceedings of Theory Canada 9

http://arxiv.org/abs/1501.03491
Is this the end of dark energy?
Edésio M. Barboza Jr., Rafael C. Nunes, Éverton M. C. Abreu, Jorge Ananias Neto
(Submitted on 13 Jan 2015)
In this paper we investigate the limits imposed by thermodynamics to a dark energy fluid. We obtain the heat capacities and the compressibilities for a dark energy fluid. These thermodynamical variables are easily accessible experimentally for any terrestrial fluid. The thermal and mechanical stabilities require these quantities to be positive. We show that such requirements forbid the existence of a cosmic fluid with negative constant EoS parameter which excludes vacuum energy as a candidate to explain the cosmic acceleration. We also show that the current observational data from SN Ia, BAO and H(z) are in conflict with the physical constraints that a general dark energy fluid with a time-dependent EoS parameter must obey which can be interpreted as an evidence against the dark energy hypothesis. Although our result excludes the vacuum energy, a geometrical cosmological term as originally introduced by Einstein in the field equations remains untouched.
6 pages, 1 figure, 1 table

http://arxiv.org/abs/1501.04170
Quantum cosmology with scalar fields: self-adjointness and cosmological scenarios
C.R. Almeida, A.B. Batista, J.C. Fabris, P.R.L.V. Moniz
(Submitted on 17 Jan 2015)
We discuss the issue of unitarity in particular quantum cosmological models with scalar field. The time variable is recovered, in this context, by using the Schutz's formalism for a radiative fluid. Two cases are considered: a phantom scalar field and an ordinary scalar field. For the first case, it is shown that the evolution is unitary provided a convenient factor ordering and inner product measure are chosen; the same happens for the ordinary scalar field, except for some special cases for which the Hamiltonian is not self-adjoint but admits a self-adjoint extension. In all cases, even for those cases not exhibiting unitary evolution, the formal computation of the expectation value of the scale factor indicates a non-singular bounce. The importance of the unitary evolution in quantum cosmology is briefly discussed.
17 pages

http://arxiv.org/abs/1501.04181
Wheeler-DeWitt quantization and singularities
Felipe Tovar Falciano, Nelson Pinto-Neto, Ward Struyve
(Submitted on 17 Jan 2015)
We consider a Bohmian approach to the Wheeler-DeWitt quantization of the Friedmann-Lemaitre-Robertson-Walker model and investigate the question whether or not there are singularities, in the sense that the universe reaches zero volume. We find that for generic wave functions (i.e., non-classical wave functions), there is a non-zero probability for a trajectory to be non-singular. This should be contrasted to the consistent histories approach for which it was recently shown by Craig and Singh that there is always a singularity. This result illustrates that the question of singularities depends much on which version of quantum theory one adopts. This was already pointed out by Pinto-Neto et al., albeit with a different Bohmian approach. Our current Bohmian approach agrees with the consistent histories approach by Craig and Singh for single-time histories, unlike the one studied earlier by Pinto-Neto et al. Although the trajectories are usually different in the two Bohmian approach, their qualitative behavior is the same for generic wave functions.
10 pages, 3 figures,

http://arxiv.org/abs/1501.03054
Cyclic Entropy: An Alternative to Inflationary Cosmology
Paul Howard Frampton
(Submitted on 9 Jan 2015)
We address how to construct an infinitely cyclic universe model. A major consideration is to make the entropy cyclic which requires the entropy to be reset to zero...
14 pages

 

[marcus]

http://arxiv.org/abs/1501.05523
Analytic continuation of real Loop Quantum Gravity : Lessons from black hole thermodynamics
Jibril Ben Achour, Karim Noui
(Submitted on 22 Jan 2015)
This contribution is devoted to summarize the recent results obtained in the construction of an "analytic continuation" of Loop Quantum Gravity (LQG). By this, we mean that we construct analytic continuation of physical quantities in LQG from real values of the Barbero-Immirzi parameter γ to the purely imaginary value γ=±i. This should allow us to define a quantization of gravity with self-dual Ashtekar variables. We first realized in [1] that this procedure, when applied to compute the entropy of a spherical black hole in LQG for γ=±i, allows to reproduce exactly the Bekenstein-Hawking area law at the semi-classical limit. The rigorous construction of the analytic continuation of spherical black hole entropy has been done in [2]. Here, we start with a review of the main steps of this construction: we recall that our prescription turns out to be unique (under natural assumptions) and leads to the right semi-classical limit with its logarithmic quantum corrections. Furthermore, the discrete and γ-dependent area spectrum of the black hole horizon becomes continuous and obviously γ-independent. Then, we review how this analytic continuation could be interpreted in terms of an analytic continuation from the compact gauge group SU(2) to the non-compact gauge group SU(1,1) relying on an analysis of three dimensional quantum gravity.
Comments: 8 pages, 1 figure, Proceedings of Frontiers of Fundamental Physics 2014 - Proceedings of Science (PoS)

http://arxiv.org/abs/1501.04899
Quantum cosmology: a review
Martin Bojowald
(Submitted on 20 Jan 2015)
In quantum cosmology, one applies quantum physics to the whole universe. While no unique version and no completely well-defined theory is available yet, the framework gives rise to interesting conceptual, mathematical and physical questions. This review presents quantum cosmology in a new picture that tries to incorporate the importance of inhomogeneity: De-emphasizing the traditional minisuperspace view, the dynamics is rather formulated in terms of the interplay of many interacting "microscopic" degrees of freedom that describe the space-time geometry. There is thus a close relationship with more-established systems in condensed-matter and particle physics even while the large set of space-time symmetries (general covariance) requires some adaptations and new developments. These extensions of standard methods are needed both at the fundamental level and at the stage of evaluating the theory by effective descriptions.
45 pages, invited review

http://arxiv.org/abs/1501.05358
The Merger of Small and Large Black Holes
P. Mösta, L. Andersson, J. Metzger, B. Szilágyi, J. Winicour
(Submitted on 22 Jan 2015)
We present simulations of binary black holes mergers in which, after the common outer horizon has formed, the marginally outer trapped surfaces (MOTSs) corresponding to the individual black holes continue to approach and eventually penetrate each other. This has very interesting consequences according to recent results in the theory of MOTSs. Uniqueness and stability theorems imply that two MOTSs which touch with a common outer normal must be identical. This suggests a possible dramatic consequence of the collision between a small and large black hole. If the penetration were to continue to completion then the two MOTSs would have to coalesce, by some combination of the small one growing and the big one shrinking. Here we explore the relationship between theory and numerical simulations, in which a small black hole has halfway penetrated a large one.
Comments: 17 pages, 11 figures