Loop-and-allied QG bibliography

[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

 

[marcus]

http://arxiv.org/abs/1501.06270
Matter Bounce Scenario in F(T) gravity
Jaume Haro, Jaume Amorós
(Submitted on 26 Jan 2015)
It is shown that teleparallel F(T) theories of gravity combined with holonomy corrected Loop Quantum Cosmology (LQC) support a Matter Bounce Scenario (MBS) which is a potential alternative to the inflationary paradigm. The Matter Bounce Scenario is reviewed and, according to the current observational data provided by PLANCK's team, we have summarized all the conditions that it has to satisfy in order to be a viable alternative to inflation, such as to provide a theoretical value of the spectral index and its running compatible with the latest PLANCK data, to have a reheating process via gravitational particle production, or to predict some signatures in the non-gaussianities of the power spectrum. The calculation of the power spectrum for scalar perturbations and the ratio of tensor to scalar perturbations has been done, in the simplest case of an exact matter dominated background, for both holonomy corrected LQC and teleparallel F(T) gravity. Finally, we have discussed the challenges (essentially, dealing with non-gaussianities, the calculation of the 3-point function in flat spatial geometries for theories beyond General Relativity) and problems (Jeans instabilities in the case of holonomy corrected LQC or local Lorentz dependence in teleparallelism) that arise in either bouncing scenario.
6 pages. Communication to the FFP2014 (Frontiers in Fundamental Physics, Marseille 2014). To appear in Proceedings of Science

http://arxiv.org/abs/1501.06591
Superbounce and Loop Quantum Ekpyrotic Cosmologies from Modified Gravity: F(R), F(G) and F(T) Theories
S.D. Odintsov, V.K. Oikonomou, Emmanuel N. Saridakis
(Submitted on 26 Jan 2015)
We investigate the realization of two bouncing paradigms, namely of the superbounce and the loop quantum cosmological ekpyrosis, in the framework of various modified gravities. In particular, we focus on the F(R), F(G) and F(T) gravities, and we reconstruct their specific subclasses which lead to such universe evolutions. These subclasses constitute from power laws, polynomials, or hypergeometric ansatzes, which can be approximated by power laws. The qualitative similarity of different effective gravities which realize the above two bouncing cosmologies, indicates to some universality lying behind such a bounce. Finally, performing a linear perturbation analysis, we show that the obtained solutions are conditionally or fully stable.
31 pages.http://arxiv.org/abs/1501.05682
The Quantum Echo of the Early Universe
Ana Blasco, Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 22 Jan 2015)
We show that the fluctuations of quantum fields as seen by late comoving observers are significantly influenced by the history of the early Universe, and therefore they transmit information about the nature of spacetime in timescales when quantum gravitational effects were non-negligible. We discuss how this may be observable even nowadays, and thus used to build falsifiability tests of quantum gravity theories.
3 pages. 2 Figures. Proceedings of Theory Canada 9. Published in Canadian Journal of Physics.

http://arxiv.org/abs/1501.06282
Intrinsic Time Quantum Geometrodynamics
Eyo Eyo Ita III, Chopin Soo, Hoi-Lai Yu
(Submitted on 26 Jan 2015)
Quantum Geometrodynamics with intrinsic time development and momentric variables is presented. An underlying SU(3) group structure at each spatial point regulates the theory. The intrinsic time behavior of the theory is analyzed, together with its ground state and primordial quantum fluctuations. Cotton-York potential dominates at early times when the universe was small; the ground state naturally resolves Penrose's Weyl Curvature Hypothesis, and thermodynamic and gravitational `arrows of time' point in the same direction. Ricci scalar potential corresponding to Einstein's General Relativity emerges as a zero-point energy contribution. A new set of fundamental canonical commutation relations without Planck's constant emerges from the unification of Gravitation and Quantum Mechanics.
6 pages

possible general interest:
http://arxiv.org/abs/1501.05969
Quantum Superpositions Cannot be Epistemic
John-Mark A. Allen
(Submitted on 23 Jan 2015)
Quantum superposition states are behind many of the curious phenomena exhibited by quantum systems, including Bell non-locality, quantum interference, quantum computational speed-up, and the measurement problem. However, many qualitative properties of quantum superpositions can also be observed in classical probability distributions leading to a suspicion that superpositions may be explicable as probability distributions over less problematic states; that is, a suspicion that superpositions are epistemic. Here, it is proved that, for any quantum system of dimension d > 3, this cannot be the case for almost all superpositions. Equivalently, any underlying ontology must contain ontic superposition states. A related question concerns general possibility of non-orthogonal quantum states |ψ⟩,|ϕ⟩ being ontologically indistinct. A similar method proves that if |⟨ϕ|ψ⟩|²∈(0,14) then |ψ⟩,|ϕ⟩ must approach ontological distinctness as d→∞. The robustness of these results to small experimental error is also discussed.
5 + 7 pages

 

[marcus]

http://arxiv.org/abs/1502.00278
Compact phase space, cosmological constant, discrete time
Carlo Rovelli, Francesca Vidotto
(Submitted on 1 Feb 2015)
We study the quantization of geometry in the presence of a cosmological constant, using a discretization with constant-curvature simplices. Phase space turns out to be compact and the Hilbert space finite dimensional for each link. Not only the intrinsic, but also the extrinsic geometry turns out to be discrete, pointing to discreetness of time, in addition to space. We work in 2+1 dimensions, but these results may be relevant also for the physical 3+1 case.
6 pages

General interest, the tenuous possibility of a theory with a becoming, a "now". In contrast to the static block universe, which has no representation of the present moment.
http://arxiv.org/abs/1502.00018
What becomes of a causal set
Christian Wuthrich, Craig Callender
(Submitted on 30 Jan 2015)
Unlike the relativity theory it seeks to replace, causal set theory has been interpreted to leave space for a substantive, though perhaps 'localized', form of 'becoming'. The possibility of fundamental becoming is nourished by the fact that the analogue of Stein's theorem from special relativity does not hold in causal set theory. Despite this, we find that in many ways, the debate concerning becoming parallels the well-rehearsed lines it follows in the domain of relativity. We present, however, some new twists and challenges. In particular, we show that a novel and exotic notion of becoming is compatible with causal sets. In contrast to the 'localized' becoming considered compatible with the dynamics of causal set theory by its advocates, our novel kind of becoming, while not answering to the typical A-theoretic demands, is 'global' and objective.
22 pages, 3 figures; forthcoming in the British Journal for the Philosophy of Science

http://arxiv.org/abs/1502.01225
Schrodinger Evolution for the Universe: Reparametrization
Sean Gryb, Karim Thebault
(Submitted on 4 Feb 2015)
Starting from a generalized Hamilton-Jacobi formalism, we develop a new framework for constructing observables and their evolution in theories invariant under global time reparametrizations. Our proposal relaxes the usual Dirac prescription for the observables of a totally constrained system (`perennials') and allows one to recover the influential partial and complete observables approach in a particular limit. Difficulties such as the non-unitary evolution of the complete observables in terms of certain partial observables are explained as a breakdown of this limit. Identification of our observables (`mutables') relies upon a physical distinction between gauge symmetries that exist at the level of histories and states (`Type 1'), and those that exist at the level of histories and not states (`Type 2'). This distinction resolves a tension in the literature concerning the physical interpretation of the partial observables and allows for a richer class of observables in the quantum theory. There is the potential for the application of our proposal to the quantization of gravity when understood in terms of the Shape Dynamics formalism.
25 pages, 1 figure

http://arxiv.org/abs/1502.01907
Black holes: Their large interiors
Ingemar Bengtsson, Emma Jakobsson
(Submitted on 6 Feb 2015)
Christodoulou and Rovelli have remarked on the large interiors possessed by static black holes. We amplify their remarks, and extend them to the spinning case.
6 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1502.02431
Comparison of primordial tensor power spectra from the deformed algebra and dressed metric approaches in loop quantum cosmology
B. Bolliet, J. Grain, C. Stahl, L. Linsefors, A. Barrau
(Submitted on 9 Feb 2015)
Loop quantum cosmology tries to capture the main ideas of loop quantum gravity and to apply them to the Universe as a whole. Two main approaches within this framework have been considered to date for the study of cosmological perturbations: the dressed metric approach and the deformed algebra approach. They both have advantages and drawbacks. In this article, we accurately compare their predictions. In particular, we compute the associated primordial tensor power spectra. We show -- numerically and analytically -- that the large scale behavior is similar for both approaches and compatible with the usual prediction of general relativity. The small scale behavior is, the other way round, drastically different. Most importantly, we show that in a range of wavenumbers explicitly calculated, both approaches do agree on predictions that, in addition, differ from standard general relativity and do not depend on unknown parameters. These features of the power spectrum at intermediate scales might constitute a universal loop quantum cosmology prediction that can hopefully lead to observational tests and constraints. We also present a complete analytical study of the background evolution for the bouncing universe that can be used for other purposes.
14 pages, 4 figures

http://arxiv.org/abs/1502.02342
Loop quantum cosmology of Bianchi IX: Effective dynamics
Alejandro Corichi, Edison Montoya
(Submitted on 9 Feb 2015)
We study numerically the solutions to the effective equations of Bianchi IX spacetimes within Loop Quantum Cosmology. We consider Bianchi IX models with and without inverse triad corrections whose matter content is a scalar field without mass. The solutions are classified using the classical observables. We show that both effective theories --with lapse N=V and N=1-- solve the big bang singularity and reproduce the classical dynamics far from the bounce. Moreover, due to the spatial compactness, there is an infinity number of bounces and recollapses. We study the limit of large volume and show that both effective theories reproduce the same dynamics, thus recovering general relativity. We implement a procedure to identify amongst the Bianchi IX solutions, those that behave like k=0,1 FLRW as well as Bianchi I, II, and VII_0 models. The effective solutions exhibit Bianchi I phases with Bianchi II transitions and also Bianchi VII_0 phases, which had not been studied before, at the quantum nor effective level. We comment on the possible implications of these results for a quantum modification to the classical BKL behaviour.
24 pages, 7 figures

http://arxiv.org/abs/1502.02919
Noncommutative spectral geometry, Bogoliubov transformations and neutrino oscillations
Maria Vittoria Gargiulo, Mairi Sakellariadou, Giuseppe Vitiello
(Submitted on 10 Feb 2015)
In this report we show that neutrino mixing is intrinsically contained in Connes' noncommutative spectral geometry construction, thanks to the introduction of the doubling of algebra, which is connected to the Bogoliubov transformation. It is known indeed that these transformations are responsible for the mixing, turning the mass vacuum state into the flavor vacuum state, in such a way that mass and flavor vacuum states are not unitary equivalent. There is thus a red thread that binds the doubling of algebra of Connes' model to the neutrino mixing.
9 pages, DICE 14 proceeding

Not QG but possibly of general interest:
http://arxiv.org/abs/1502.02480
Entangled History
Jordan Cotler, Frank Wilczek
(Submitted on 9 Feb 2015)
We introduce quantum history states and their mathematical framework, thereby reinterpreting and extending the consistent histories approach to quantum theory. Through thought experiments, we demonstrate that our formalism allows us to analyze a quantum version of history in which we reconstruct the past by observations. In particular, we can pass from measurements to inferences about "what happened" in a way that is sensible and free of paradox. Our framework allows for a richer understanding of the temporal structure of quantum theory, and we construct history states that embody peculiar, non-classical correlations in time.
16 pages, 1 figure

http://arxiv.org/abs/1502.02087
How to include fermions into General relativity by exotic smoothness
T. Asselmeyer-Maluga, C.H. Brans
(Submitted on 7 Feb 2015)
This paper is two-fold. At first we will discuss the generation of source terms in the Einstein-Hilbert action by using (topologically complicated) compact 3-manifolds. There is a large class of compact 3-manifolds with boundary: a torus given as the complement of a (thickened) knot admitting a hyperbolic geometry, denoted as hyperbolic knot complements in the following. We will discuss the fermionic properties of this class of 3-manifolds, i.e. we are able to identify a fermion with a hyperbolic knot complement. Secondly we will construct a large class of space-times, the exotic ℝ4, containing this class of 3-manifolds naturally. We begin with a topological trivial space, the ℝ4, and change only the differential structure to obtain many nontrivial 3-manifolds. It is known for a long time that exotic ℝ4's generate extra sources of gravity (Brans conjecture) but here we will analyze the structure of these source terms more carefully. Finally we will state that adding a hyperbolic knot complement will result in the appearance of a fermion as source term in the Einstein-Hilbert action.
27 pages, 4 figures, accepted in Gen. Rel. Grav

Oddity:
http://arxiv.org/abs/1502.02429
Brian Josephson

 

[marcus]

http://arxiv.org/abs/1502.03230
An Extended Matter Bounce Scenario: current status and challenges
Jaume de Haro, Yi-Fu Cai
(Submitted on 11 Feb 2015)
As an alternative to the paradigm of slow roll inflation, we propose an extended scenario of the matter bounce cosmology in which the Universe has experienced a quasi-matter contracting phase with a variable background equation of state parameter. This extended matter bounce scenario can be realized by considering a single scalar field evolving along an approximately exponential potential. Our result reveals that the rolling of the scalar field in general leads to a running behavior on the spectral index of primordial cosmological perturbations and a negative running can be realized in this model. We constrain the corresponding parameter space by using the newly released Planck data. To apply this scenario, we revisit bouncing cosmologies within the context of modified gravity theories, in particular, the holonomy corrected loop quantum cosmology and teleparallel F(T) gravity. A gravitational process of reheating is presented in such a matter bounce scenario to demonstrate the condition of satisfying current observations. We also comment on several unresolved issues that often appear in matter bounce models.
31 pages, 2 figures.

http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages

brief mention:
http://arxiv.org/abs/1502.03292
Is there a C-function in 4D Quantum Einstein Gravity?
Daniel Becker, Martin Reuter

http://arxiv.org/abs/1502.03129
Firewalls as artefacts of inconsistent truncations of quantum geometries
Cristiano Germani, Debajyoti Sarkar

 

[John86]

http://arxiv.org/abs/1502.03424
Is dark energy an artifact of decoherence?
Chris Fields
(Submitted on 11 Feb 2015)
Within the quantum Darwinist framework introduced by W. H. Zurek ({\em Nat. Phys.}, 5:181-188, 2009), observers obtain pointer-state information about quantum systems by interacting with the surrounding environment, e.g. the ambient photon field. This framework is applied to the observation of stellar center-of-mass positions, which are assumed to be encoded in a way that is uniformly accessible to all observers regardless of their location. Assuming Landauer's Principle, constructing such environmental encodings requires ∼ kT per bit. For 1025 stars and a binary encoding of center-of-mass positions into 10 km3 voxels, the free energy required at T = 2.7 K is ∼ 5 ⋅ 10−27 kg ⋅ m−3, in striking agreement with the observed value of ΩΛρc. Decreasing the voxel size to l3P results in a free energy requirement 10117 times larger.

http://arxiv.org/abs/1502.02198
Emergence of Four Dimensions in the Causal Set Approach to Discrete Quantum Gravity
Stan Gudder
(Submitted on 8 Feb 2015)
One could begin a study like the present one by simply postulating that our universe is four-dimensional. There are ample reasons for doing this. Experience, observation and experiment all point to the fact that we inhabit a four-dimensional universe. Another approach would be to show that four-dimensions arise naturally from a reasonable model of the universe or multiverse. After reviewing the causal set approach to discrete quantum gravity in Section~1, we shall discuss the emergence of four-dimensions in Section~2. We shall see that certain patterns of four arise that suggest the introduction of a 4-dimensional discrete manifold. In the later sections we shall discuss some consequences of this introduced framework. In particular, we will show that quantum amplitudes can be employed to describe a multiverse dynamics. Moreover, a natural unitary operator together with energy, position and momentum operators will be introduced and their properties studied.

http://arxiv.org/abs/1502.02833
On the initial singularity problem in rainbow cosmology
Grasiele dos Santos, Giulia Gubitosi, Giovanni Amelino-Camelia
(Submitted on 10 Feb 2015)
It has been recently claimed that the initial singularity might be avoided in the context of rainbow cosmology, where one attempts to account for quantum-gravitational corrections through an effective-theory description based on an energy-dependent ("rainbow") spacetime metric. We here scrutinize this exciting hypothesis much more in depth than ever done before. In particular, we take into account all requirements for singularity avoidance, while previously only a subset of these requirements had been considered. Moreover, we show that the implications of a rainbow metric for thermodynamics are more significant than previously appreciated. Through the analysis of two particularly meaningful examples of rainbow metrics we find that our concerns are not merely important conceptually, but actually change in quantitatively significant manner the outcome of the analysis. Notably we only find examples where the singularity is not avoided, though one can have that in the regime where our semi-classical picture is still reliable the approach to the singularity is slowed down when compared to the standard classical scenario. We conclude that the study of rainbow metrics provides tantalizing hints of singularity avoidance but is inconclusive, since some key questions remain to be addressed just when the scale factor is very small, a regime which, as here argued, cannot be reliably described by an effective rainbow-metric picture.

http://arxiv.org/abs/1502.02758
Conformally Friedmann-Lemaitre-Robertson-Walker cosmologies
Matt Visser (Victoria University of Wellington)
(Submitted on 10 Feb 2015)
In a universe where, according to the standard cosmological models, some 97% of the total mass-energy is still "missing in action" it behooves us to spend at least a little effort critically assessing and exploring radical alternatives. Among possible, (dare we say plausible), nonstandard but superficially viable models, those spacetimes conformal to the standard Friedmann-Lemaitre-Robertson-Walker class of cosmological models play a very special role --- these models have the unique and important property of permitting large non-perturbative geometric deviations from Friedmann-Lemaitre-Robertson-Walker cosmology without unacceptably distorting the cosmic microwave background. Performing a "cosmographic" analysis, (that is, temporarily setting aside the Einstein equations, since the question of whether or not the Einstein equations are valid on galactic and cosmological scales is essentially the same question as whether or not dark matter/dark energy actually exist), and using both supernova data and information about galactic structure, one can nevertheless place some quite significant observational constraints on any possible conformal mode --- however there is still an extremely rich range of phenomenological possibilities for both cosmologists and astrophysicists to explore.

 

[marcus]

Not Loop-and-allied quantum gravity, but possibly of general interest:
http://arxiv.org/abs/1502.03410
The Montevideo Interpretation of Quantum Mechanics: a short review
Rodolfo Gambini, Jorge Pullin
(Submitted on 11 Feb 2015)
The Montevideo interpretation of quantum mechanics, which consists in supplementing environmental decoherence with fundamental limitations in measurement stemming from gravity, has been described in several publications. However, some of them appeared before the full picture provided by the interpretation was developed. As such it can be difficult to get a good understanding via the published literature. Here we summarize it in a self contained brief presentation including all its principal elements.
10 pages,

http://arxiv.org/abs/1502.03831
Quantum mechanics, strong emergence and ontological non-reducibility
Rodolfo Gambini, Lucia Lewowicz, Jorge Pullin
(Submitted on 12 Feb 2015)
We show that a new interpretation of quantum mechanics, in which the notion of event is defined without reference to measurement or observers, allows to construct a quantum general ontology based on systems, states and events. Unlike the Copenhagen interpretation, it does not resort to elements of a classical ontology. The quantum ontology in turn allows us to recognize that a typical behavior of quantum systems exhibits strong emergence and ontological non-reducibility. Such phenomena are not exceptional but natural, and are rooted in the basic mathematical structure of quantum mechanics.
8 pages, to appear in Foundations of Chemistry

http://arxiv.org/abs/1502.04640
The Lorentzian proper vertex amplitude: Classical analysis and quantum derivation
Jonathan Engle, Antonia Zipfel
(Submitted on 16 Feb 2015)
Spin foam models, an approach to defining the dynamics of loop quantum gravity, make use of the Plebanski formulation of gravity, in which gravity is recovered from a topological field theory via certain constraints called simplicity constraints. However, the simplicity constraints in their usual form select more than just one gravitational sector as well as a degenerate sector. This was shown, in previous work, to be the reason for the "extra" terms appearing in the semiclassical limit of the Euclidean EPRL amplitude. In this previous work, a way to eliminate the extra sectors, and hence terms, was developed, leading to the what was called the Euclidean proper vertex amplitude. In the present work, these results are extended to the Lorentzian signature, establishing what is called the Lorentzian proper vertex amplitude. This extension is non-trivial and involves a number of new elements since, for Lorentzian bivectors, the split into self-dual and anti-self-dual parts, on which the Euclidean derivation was based, is no longer available. In fact, the classical parts of the present derivation provide not only an extension to the Lorentzian case, but also, with minor modifications, provide a new, more four dimensionally covariant derivation for the Euclidean case. The new elements in the quantum part of the derivation are due to the different structure of unitary representations of the Lorentz group.
36 pages

http://arxiv.org/abs/1502.05619
Gravitational Lensing by Self-Dual Black Holes in Loop Quantum Gravity
Satyabrata Sahu, Kinjalk Lochan, D. Narasimha
(Submitted on 19 Feb 2015)
We study gravitational lensing by a recently proposed black hole solution in Loop Quantum Gravity. We highlight the fact that the quantum gravity corrections to the Schwarzschild metric in this model evade the `mass suppression' effects (that the usual quantum gravity corrections are susceptible to) by virtue of one of the parameters in the model being dimensionless, which is unlike any other quantum gravity motivated parameter. Gravitational lensing in the strong and weak deflection regimes is studied and a sample consistency relation is presented which could serve as a test of this model. We discuss that though the consistency relation for this model is qualitatively similar to what would have been in Brans-Dicke, in general it can be a good discriminator between many alternative theories. Although the observational prospects do not seem to be very optimistic even for a galactic supermassive black hole case, time delay between relativistic images for billion solar mass black holes in other galaxies might be within reach of future relativistic lensing observations.
13 pages; 4 figures; accepted for publication in Physical Review D

http://arxiv.org/abs/1502.05388
Boundary Terms for Causal Sets
Michel Buck, Fay Dowker, Ian Jubb, Sumati Surya
(Submitted on 18 Feb 2015)
We propose a family of boundary terms for the action of a causal set with a spacelike boundary. We show that in the continuum limit one recovers the Gibbons-Hawking-York boundary term in the mean. We also calculate the continuum limit of the mean causal set action for an Alexandrov interval in flat spacetime. We find that it is equal to the volume of the codimension-2 intersection of the two light-cone boundaries of the interval.
23 pages, 4 figures

http://arxiv.org/abs/1502.05296
Lectures on the Cosmological Constant Problem
Antonio Padilla
(Submitted on 18 Feb 2015)
These lectures on the cosmological constant problem were prepared for the X Mexican School on Gravitation and Mathematical Physics. The problem itself is explained in detail, emphasising the importance of radiative instability and the need to repeatedly fine tune as we change our effective description. Weinberg's no go theorem is worked through in detail. I review a number of proposals including Linde's universe multiplication, Coleman's wormholes, the fat graviton, and SLED, to name a few. Large distance modifications of gravity are also discussed, with causality considerations pointing towards a global modification as being the most sensible option. The global nature of the cosmological constant problem is also emphasized, and as a result, the sequestering scenario is reviewed in some detail, demonstrating the cancellation of the Standard Model vacuum energy through a global modification of General Relativity.
31 pages

 

[marcus]

http://arxiv.org/abs/1502.06125
ΛCDM Bounce Cosmology without ΛCDM: the case of modified gravity
S.D. Odintsov, V.K. Oikonomou
(Submitted on 21 Feb 2015)
We provide an F(R) gravity description of a ΛCDM bouncing model, without the need for matter fluids or for cosmological constant. As we explicitly demonstrate, the two cosmological eras that constitute the ΛCDM bouncing model, can be generated by F(R) gravity which can lead to accelerating cosmologies. The resulting F(R) gravity has Einstein frame inflationary properties that have concordance to the latest Planck observational data. Both the F(R) gravity stability properties are thoroughly investigated and also, the gravitational particle production, a feature necessary for the viability of the ΛCDM bounce scenario, is also addressed. As we will show, the ΛCDM bounce model can be successfully described by pure F(R) gravity, with appealing phenomenological attributes, which we extensively discuss.
31 pages, accepted by PRD

http://arxiv.org/abs/1502.05875
Perturbations in Bouncing and Cyclic Models, a General Study
Tirthabir Biswas, Riley Mayes, Colleen Lattyak
(Submitted on 18 Feb 2015)
Being able to reliably track perturbations across bounces and turnarounds in cyclic and bouncing cosmology lies at the heart of being able to compare the predictions of these models with the Cosmic Microwave Background observations. This has been a challenging task due to the unknown nature of the physics involved during the bounce as well as the technical challenge of matching perturbations precisely between the expansion and contraction phases. In this paper, we will present general techniques (analytical and numerical) that can be applied to understand the physics of the fluctuations, especially those with "long" wavelengths, and test its validity in some simple bouncing/cyclic toy models where the physics is well understood. We will then apply our techniques to more interesting cosmological models such as the bounce inflation and cyclic inflation.
21 pages, 12 figures

possible wider interest:
http://arxiv.org/abs/1502.06539
Fields as Bodies: a unified presentation of spacetime and internal gauge symmetry
David Wallace
(Submitted on 23 Feb 2015)
Using the parametrised representation of field theory (in which the location in spacetime of a part of a field is itself represented by a map from the base manifold to Minkowski spacetime) I demonstrate that in both local and global cases, internal (Yang-Mills-type) and spacetime (Poincaré) symmetries can be treated precisely on a par, so that gravitational theories may be regarded as gauge theories in a completely standard sense.
10 pages

http://arxiv.org/abs/1502.06141
ℏ as a Physical Constant of Classical Optics and Electrodynamics
Real Tremblay, Nicolas Doyon, Claudine Ni Allen
(Submitted on 21 Feb 2015)
The Planck constant (ℏ) plays a pivotal role in quantum physics. Historically, it has been proposed as postulate, part of a genius empirical relationship E=ℏω in order to explain the intensity spectrum of the blackbody radiation for which classical electrodynamic theory led to an unacceptable prediction: The ultraviolet catastrophe. While the usefulness of the Planck constant in various fields of physics is undisputed, its derivation (or lack of) remains unsatisfactory from a fundamental point of view. In this paper, the analysis of the blackbody problem is performed with a series expansion of the electromagnetic field in terms of TE, TM modes in a metallic cavity with small losses, that leads to developing the electromagnetic fields in a \textit{complete set of orthonormal functions}. This expansion, based on coupled power theory, maintains both space and time together enabling modeling of the blackbody's evolution toward equilibrium. Reaching equilibrium with a multimodal waveguide analysis brings into consideration the coupling between modes in addition to absorption and emission of radiation. The properties of the modes, such as spectral broadening, losses and lifetime, then progressively become independent of frequency and explains how equilibrium is allowed in good conductor metallic cavities. Based on the free electron relaxation time in gold, a value of ℏ=1.02×10⁻³⁴ J⋅s for the reduced Planck constant is found and the uncertainty principle is also emerging from this a priori classical study. The Planck constant is then obtained no longer as an ad hoc addition but as a natural consequence of the analysis taking boundary conditions into account as into optical resonators. That analysis based on finite-spacetime paradigm, also shine new light on the notion of decoherence in classical optics and electrodynamics.
26 pages, 4 figures.

 

[wabbit]

Hi marcus interesting pointers as always, just wanted to let you know your very first link is misdirected.

 

[marcus]

Thanks for catching that, wabbit! Fixed.

http://arxiv.org/abs/1502.06770
Quantum Transitions Between Classical Histories: Bouncing Cosmologies
James Hartle, Thomas Hertog
(Submitted on 24 Feb 2015)
In a quantum theory of gravity spacetime behaves classically when quantum probabilities are high for histories of geometry and field that are correlated in time by the Einstein equation. Probabilities follow from the quantum state. This quantum perspective on classicality has important implications:
(a) Classical histories are generally available only in limited patches of the configuration space on which the state lives.
(b) In a given patch states generally predict relative probabilities for an ensemble of possible classical histories.
(c) In between patches classical predictability breaks down and is replaced by quantum evolution connecting classical histories in different patches.
(d) Classical predictability can break down on scales well below the Planck scale, and with no breakdown in the classical equations of motion.
We support and illustrate (a)-(d) by calculating the quantum transition across the de Sitter like throat connecting asymptotically classical, inflating histories in the no-boundary quantum state. This supplies probabilities for how a classical history on one side transitions and branches into a range of classical histories on the opposite side. We also comment on the implications of (a)-(d) for the dynamics of black holes and eternal inflation.
36 pages, 6 figures

 

[John86]

http://arxiv.org/abs/1502.08005
Metastring Theory and Modular Space-time
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 27 Feb 2015)
String theory is canonically accompanied with a space-time interpretation which determines S-matrix-like observables, and connects to the standard physics at low energies in the guise of local effective field theory. Recently, we have introduced a reformulation of string theory which does not rely on an {\it a priori} space-time interpretation or a pre-assumption of locality. This \hlt{metastring theory} is formulated in such a way that stringy symmetries (such as T-duality) are realized linearly. In this paper, we study metastring theory on a flat background and develop a variety of technical and interpretational ideas. These include a formulation of the moduli space of Lorentzian worldsheets, a careful study of the symplectic structure and consequently consistent closed and open boundary conditions, and the string spectrum and operator algebra. What emerges from these studies is a new quantum notion of space-time that we refer to as a quantum Lagrangian or equivalently a \hlt{modular space-time}. This concept embodies the standard tenets of quantum theory and implements in a precise way a notion of {relative locality}. The usual string backgrounds (non-compact space-time along with some toroidally compactified spatial directions) are obtained from modular space-time by a limiting procedure that can be thought of as a correspondence limit.

 

[marcus]

http://arxiv.org/abs/1502.07789
Uniqueness of Measures in Loop Quantum Cosmology
Maximilian Hanusch
(Submitted on 26 Feb 2015)
In a paper of Ashtekar and Campiglia, residual diffeomorphisms have been used to single out the standard representation of the reduced holonomy-flux algebra in homogeneous loop quantum cosmology (LQC). We show that, in the homogeneous isotropic case, unitarity of the translations w.r.t. the extended ℝ-action (exponentiated reduced fluxes in the standard approach) singles out the Bohr measure on both the standard quantum configuration space ℝBohr as well as on the Fleischhack one. Thus, leads to the standard kinematical Hilbert space of LQC in both situations.
4 pages

http://arxiv.org/abs/1503.00442
Inflationary cosmology in modified gravity theories
Kazuharu Bamba, Sergei D. Odintsov
(Submitted on 2 Mar 2015)
We review inflationary cosmology in modified gravity such as R² gravity with its extensions in order to generalize the Starobinsky inflation model. In particular, we explore inflation realized by three kinds of effects: modification of gravity, the quantum anomaly, and the R² term in loop quantum cosmology. It is explicitly demonstrated that in these inflationary models, the spectral index of scalar modes of the density perturbations and the tensor-to-scalar ratio can be consistent with the Planck results. Bounce cosmology in F(R) gravity is also explained.
24 pages, invited review to appear in Symmetry

http://arxiv.org/abs/1503.01671
Aspects of the Bosonic Spectral Action
Mairi Sakellariadou (King's College London)
(Submitted on 5 Mar 2015)
A brief description of the elements of noncommutative spectral geometry as an approach to unification is presented. The physical implications of the doubling of the algebra are discussed. Some high energy phenomenological as well as various cosmological consequences are presented. A constraint in one of the three free parameters, namely the one related to the coupling constants at unification, is obtained, and the possible role of scalar fields is highlighted. A novel spectral action approach based upon zeta function regularisation, in order to address some of the issues of the traditional bosonic spectral action based on a cutoff function and a cutoff scale, is discussed.
16 pages, Invited talk in the Fourth Symposium on Prospects in the Physics of Discrete Symmetries, DISCRETE 2014, King's College London,2-6 December 2014

http://arxiv.org/abs/1503.01636
The microscopic structure of 2D CDT coupled to matter
J. Ambjorn, A. Goerlich, J. Jurkiewicz, H. Zhang
(Submitted on 5 Mar 2015)
We show that for 1+1 dimensional Causal Dynamical Triangulations (CDT) coupled to 4 massive scalar fields one can construct an effective transfer matrix if the masses squared is larger than or equal to 0.05. The properties of this transfer matrix can explain why CDT coupled to matter can behave completely different from "pure" CDT. We identify the important critical exponent in the effective action, which may determine the universality class of the model.
14 pages,lot of figures

http://arxiv.org/abs/1503.00359
Creation of quantized particles, gravitons and scalar perturbations by the expanding universe
Leonard Parker
(Submitted on 1 Mar 2015)
Quantum creation processes during the very rapid early expansion of the universe are believed to give rise to temperature anisotropies and polarization patterns in the CMB radiation. These have been observed by satellites such as COBE, WMAP, and PLANCK, and by bolometric instruments placed near the South Pole by the BICEP collaborations. The expected temperature anisotropies are well-confirmed. The B-mode polarization patterns in the CMB are currently under measurement jointly by the PLANCK and BICEP groups to determine the extent to which the B-modes can be attributed to gravitational waves from the creation of gravitons in the earliest universe. It was during 1962 that I proved that quanta of the minimally-coupled scalar field were created by the general expanding FLRW universe. This was relevant also to the creation of quantized perturbations of the gravitational field, since these perturbations satisfied linear field equations that could be quantized in the same way as the minimally-coupled scalar field equation. In fact, in 1946, E.M. Lifshitz had considered the classical Einstein gravitational field in FLRW expanding universes and had shown that the classical linearized Einstein field equations reduced, in what is now known as the Lifshitz gauge, to two separate classical minimally-coupled massless scalar field equations. These field equations of Lifshitz, when quantized, correspond to the field equations for massless gravitons, one equation for each of the two independent polarization components of the spin-2 massless graviton. I will discuss this further in this article.1 1 Plenary Lecture given September 2, 2014 at the ERE2014 Conference in Valencia, Spain
Plenary Lecture given September 2, 2014 at the ERE2014 Conference in Valencia, Spain To appear in the Proceedings of the ERE2014 Conference

http://arxiv.org/abs/1503.01567
Coherent Quantum Dynamics: What Fluctuations Can Tell
John Schliemann
(Submitted on 5 Mar 2015)
Coherent states provide a natural connection of quantum systems to their classical limit and are employed in various fields of physics. Here we derive general systematic expansions, with respect to quantum parameters, of expectation values of products of arbitrary operators within both oscillator coherent states and SU(2) coherent states. In particular, we generally prove that the energy fluctuations of an arbitrary Hamiltonian are in leading order entirely due to the time dependence of the classical variables. These results add to the list of wellknown properties of coherent states and are applied here to the Lipkin-Meshkov-Glick model, the Dicke model, and to coherent intertwiners in spin networks as considered in Loop Quantum Gravity.
13 pages.

possible interest:
http://arxiv.org/abs/1503.02366
Generating Luminous and Dark Matter During Inflation
Neil D. Barrie, Archil Kobakhidze
(Submitted on 9 Mar 2015)
We propose a new mechanism for generating both luminous and dark matter during cosmic inflation. ...
14 pages

 

[marcus]

http://arxiv.org/abs/1503.02916
Inhomogeneous Dark Fluid and Dark Matter, Leading to a Bounce Cosmology
Iver Brevik, Alexander Timoshkin
(Submitted on 10 Mar 2015)
The purpose of this short review is to describe cosmological models with a linear inhomogeneous time-dependent equation of state (EoS) for the dark energy, when the dark fluid is coupled with dark matter. This may lead to a bounce cosmology. We consider equivalent descriptions in terms of the EoS parameters for an exponential, a power-law, or a double-exponential law for the scale factor a. Stability issues are discussed by considering small perturbations around the critical points for the bounce, in the early as well as in the late, universe. The latter part of the paper is concerned with dark energy coupled with dark matter in viscous fluid cosmology. We allow the bulk viscosity ζ=ζ(H,t) to be a function of the Hubble parameter and the time, and consider the Little Rip, the Pseudo Rip, and the bounce universe. Analytic expressions for characteristic properties of these cosmological models are obtained.
13 pages. Mini-review, to appear in the MDPI journal Universe

http://arxiv.org/abs/1503.02981
Four-Dimensional Entropy from Three-Dimensional Gravity
S. Carlip
(Submitted on 10 Mar 2015)
At the horizon of a black hole, the action of (3+1)-dimensional loop quantum gravity acquires a boundary term that is formally identical to an action for three-dimensional gravity. I show how to use this correspondence to obtain the entropy of the (3+1)-dimensional black hole from well-understood conformal field theory computations of the entropy in (2+1)-dimensional de Sitter space.
8 pages

http://arxiv.org/abs/1503.03030
Energy in first order 2+1 gravity
Alejandro Corichi, Irais Rubalcava-Garcia
(Submitted on 10 Mar 2015)
We consider Lambda=0 three dimensional gravity with asymptotically flat boundary conditions. This system was studied by Ashtekar and Varadarajan within the second order formalism -with metric variables- who showed that the Regge-Teitelboim formalism yields a consistent Hamiltonian description where, surprisingly, the energy is bounded from below and from above. The energy of the spacetime is, however, determined up to an arbitrary constant. The natural choice was to fix that freedom such that Minkowski spacetime has zero energy. More recently, Marolf and Patiño started from the Einstein-Hilbert action supplemented with the Gibbons-Hawking term and showed that, in the 2+1 decomposition of the theory, the energy is shifted from the Ashtekar-Varadarajan analysis in such a way that Minkowski spacetime possesses a negative energy. In this contribution we consider the first order formalism, where the fundamental variables are a so(2,1) connection ωaIJ and a triad eIa. We consider two actions. A natural extension to 3 dimensions of the consistent action in 4D Palatini gravity is shown to be finite and differentiable. For this action, the 2+1 decomposition (that we perform using two methods) yields a Hamiltonian boundary term that corresponds to energy. It assigns zero energy to Minkowski spacetime. We then put forward a totally gauge invariant action, and show that it is also well defined and differentiable. Interestingly, it turns out to be related, on shell, to the 3D Palatini action by an additive constant in such a way that its associated energy is given by the Marolf-Patiño expression. Thus, we conclude that, from the perspective of the first order formalism, Minkowski spacetime can consistently have either, zero, or a negative energy equal to -1/4G, depending on the choice of consistent action employed as starting point.
36 pages

 

[John86]

http://arxiv.org/abs/1503.01774
A quantum peek inside the black hole event horizon
Sumanta Chakraborty, Suprit Singh, T. Padmanabhan
(Submitted on 5 Mar 2015)
We solve the Klein-Gordon equation for a scalar field, in the background geometry of a dust cloud collapsing to form a black hole, everywhere in the (1+1) spacetime: that is, both inside and outside the event horizon and arbitrarily close to the curvature singularity. This allows us to determine the regularized stress tensor expectation value, everywhere in the appropriate quantum state (viz., the Unruh vacuum) of the field. We use this to study the behaviour of energy density and the flux measured in local inertial frames for the radially freely falling observer at any given event. Outside the black hole, energy density and flux lead to the standard results expected from the Hawking radiation emanating from the black hole, as the collapse proceeds. Inside the collapsing dust ball, the energy densities of both matter and scalar field diverge near the singularity in both (1+1) and (1+3) spacetime dimensions; but the energy density of the field dominates over that of classical matter. In the (1+3) dimensions, the total energy (of both scalar field and classical matter) inside a small spatial volume around the singularity is finite (and goes to zero as the size of the region goes to zero) but the total energy of the quantum field still dominates over that of the classical matter. Inside the event horizon, but \textit{outside} the collapsing matter, freely falling observers find that the energy density and the flux diverge close to the singularity. In this region, even the integrated energy inside a small spatial volume enclosing the singularity diverges. This result holds in both (1+1) and (1+3) spacetime dimensions with a \emph{milder} divergence for the total energy inside a small region in (1+3) dimensions. These results suggest that the back-reaction effects are significant even in the region \emph{outside the matter but inside the event horizon}, close to the singularity.

 

[marcus]

http://arxiv.org/abs/1503.03407
Unitarity and ultraviolet regularity in cosmology
Ivan Agullo, Abhay Ashtekar
(Submitted on 11 Mar 2015)
Quantum field theory in curved space-times is a well developed area in mathematical physics which has had important phenomenological applications to the very early universe. However, it is not commonly appreciated that on time dependent space-times ---including the simplest cosmological models--- dynamics of quantum fields is not unitary in the standard sense. This issue is first explained with an explicit example and it is then shown that a generalized notion of unitarity does hold. The generalized notion allows one to correctly pass to the Schrödinger picture starting from the Heisenberg picture used in the textbook treatments. Finally, we indicate how these considerations can be extended from simple cosmological models to general globally hyperbolic space-times
30 pages

http://arxiv.org/abs/1503.03907
Gauge-Invariant Perturbations in Hybrid Quantum Cosmology
Laura Castelló Gomar, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 12 Mar 2015)
We consider cosmological perturbations around homogeneous and isotropic spacetimes minimally coupled to a scalar field and present a formulation which is designed to preserve covariance. We truncate the action at quadratic perturbative order and particularize our analysis to flat compact spatial sections and a field potential given by a mass term, although the formalism can be extended to other topologies and potentials. The perturbations are described in terms of Mukhanov-Sasaki gauge invariants, linear perturbative constraints, and variables canonically conjugate to them. This set is completed into a canonical one for the entire system, including the homogeneous degrees of freedom. We find the global Hamiltonian constraint of the model, in which the contribution of the homogeneous sector is corrected with a term quadratic in the perturbations, that can be identified as the Mukhanov-Sasaki Hamiltonian in our formulation. We then adopt a hybrid approach to quantize the model, combining a quantum representation of the homogeneous sector with a more standard field quantization of the perturbations. Covariance is guaranteed in this approach inasmuch as no gauge fixing is adopted. Next, we adopt a Born-Oppenheimer ansatz for physical states and show how to obtain a Schrödinger-like equation for the quantum evolution of the perturbations. This evolution is governed by the Mukhanov-Sasaki Hamiltonian, with the dependence on the homogeneous geometry evaluated at quantum expectation values, and with a time parameter defined also in terms of suitable expectation values on that geometry. Finally, we derive effective equations for the dynamics of the Mukhanov-Sasaki gauge invariants, that include quantum contributions, but have the same ultraviolet limit as the classical equations. They provide the master equation to extract predictions about the power spectrum of primordial scalar perturbations.
35 pages, prepared for submission to JCAP

possible general interest:
http://arxiv.org/abs/1503.05270
Efficiently Controllable Graphs
Can Gokler, Kevin Thompson, Peter Shor, Seth Lloyd
(Submitted on 18 Mar 2015)
We show that universal quantum computation can be performed efficiently on quantum networks while the fraction of controlled subsystems vanishes as the network grows larger. We provide examples of quantum spin network families admitting polynomial quantum gate complexity with a vanishing fraction of controlled spins. We define a new family of graphs, the efficiently controllable family, which admits efficient computation with vanishing fraction of controls. We explore generalizations to percolation clusters, fractals and random graphs. We show that the classical computational complexity of estimating the ground state of Hamiltonians described by controllable graphs is polynomial in the number of subsystems/qubits.
5 pages.
My comment is it sounds interesting but I don't understand quantum computation at all well so I can't judge.

http://arxiv.org/abs/1503.05007
The Evolution of Quantum Field Theory, From QED to Grand Unification
Gerard 't Hooft
(Submitted on 17 Mar 2015)
In the early 1970s, after a slow start, and lots of hurdles, Quantum Field Theory emerged as the superior doctrine for understanding the interactions between relativistic sub-atomic particles. After the conditions for a relativistic field theoretical model to be renormalizable were established, there were two other developments that quickly accelerated acceptance of this approach: first the Brout-Englert-Higgs mechanism, and then asymptotic freedom. Together, these gave us a complete understanding of the perturbative sector of the theory, enough to give us a detailed picture of what is now usually called the Standard Model. Crucial for this understanding were the strong indications and encouragements provided by numerous experimental findings. Subsequently, non-perturbative features of the quantum field theories were addressed, and the first proposals for completely unified quantum field theories were launched. Since the use of continuous symmetries of all sorts, together with other topics of advanced mathematics, were recognised to be of crucial importance, many new predictions were pointed out, such as the Higgs particle, supersymmetry and baryon number violation. There are still many challenges ahead.
25 pages in total. A contribution to: The Standard Theory up to the Higgs discovery - 60 years of CERN - L. Maiani and G. Rolandi, eds

My comment: might reviewing the history of QFT help increase one's general understanding?

 

[marcus]

http://arxiv.org/abs/1503.05943
Curvatures and discrete Gauss-Codazzi equation in (2+1)-dimensional loop quantum gravity
Seramika Ariwahjoedi, Jusak Sali Kosasih, Carlo Rovelli, Freddy P. Zen
(Submitted on 19 Mar 2015)
We derive the Gauss-Codazzi equation in the holonomy and plane-angle representations and we use the result to write a Gauss-Codazzi equation for a discrete (2+1)-dimensional manifold, triangulated by isosceles tetrahedra. This allows us to write operators acting on spin network states in (2+1)-dimensional loop quantum gravity, representing the 3-dimensional intrinsic, 2-dimensional intrinsic, and 2-dimensional extrinsic curvatures.
16 pages, 10 figures

http://arxiv.org/abs/1503.06085
Time asymmetric extensions of general relativity
Marina Cortes, Henrique Gomes, Lee Smolin
(Submitted on 20 Mar 2015)
We describe a class of modified gravity theories that deform general relativity in a way that breaks time reversal invariance and, very mildly, locality. The algebra of constraints, local physical degrees of freedom, and their linearized equations of motion, are unchanged, yet observable effects may be present on cosmological scales, which have implications for the early history of the universe.
This is achieved in the Hamiltonian framework, in a way that requires the constant mean curvature gauge conditions and is, hence, inspired by shape dynamics.
19 pages.

http://arxiv.org/abs/1503.06761
Detecting quantum gravitational effects of loop quantum cosmology in the early universe
Tao Zhu, Anzhong Wang, Gerald Cleaver, Klaus Kirsten, Qin Sheng, Qiang Wu
(Submitted on 23 Mar 2015)
We derive the primordial power spectra and spectral indexes of the density fluctuations and gravitational waves in the framework of loop quantum cosmology (LQC) with holonomy and inverse-volume corrections, by using the uniform asymptotic approximation method to its third-order, at which the upper error bounds are ≲0.15%, accurate enough for the current and forthcoming cosmological observations. Then, using the Planck, BAO and SN data we obtain new constraints on quantum gravitational effects from LQC corrections, and find that such effects could be well within the detection of the current and forthcoming experiments.
5 pages, 2 figures and 1 table

http://arxiv.org/abs/1503.06294
Midisuperspace quantization: possibilities for fractional and emergent spacetime dimensions
Rakesh Tibrewala
(Submitted on 21 Mar 2015)
Recently, motivated by certain loop quantum gravity (LQG) inspired corrections, it was shown that for spherically symmetric midisuperspace models infinitely many second derivative theories of gravity exist (as revealed by the presence of three arbitrary functions in the corresponding Lagrangian/Hamiltonian) and not just those allowed by spherically symmetric general relativity. This freedom can be interpreted as the freedom to accommodate certain quantum gravity corrections in these models even in the absence of higher curvature terms (at a semi-classical level, at least). For a particular choice of the arbitrary functions it is shown that the new theories map to spherically symmetric general relativity in arbitrary number of (integer) dimensions thus explicitly demonstrating that when working with midisuperspace models, one loses the information about the dimensionality of the full spacetime. In addition, it is shown that these new theories can accommodate scenarios of fractional spacetime dimensions as well as those of emergent spacetime dimensions - a possibility suggested by various approaches to quantum gravity.
13 pages.

http://arxiv.org/abs/1503.06233
Critical scaling in quantum gravity from the renormalisation group
Kevin Falls
(Submitted on 20 Mar 2015)
The scaling behaviour of euclidean quantum gravity at an asymptotically safe critical point is studied by means of the exact renormalisation group. Gauge independence is ensured via a specific parameterisation of metric fluctuations introduced in a recent paper. Within a non-perturbative approximation the beta function for Newton's constant takes a simple form to all orders in ℏ. A UV fixed point is found to exist for d≤7 spacetime dimensions at which the critical scaling can be assessed. The critical exponent for the Newton's constant ν is found to be regulator independent close to two dimensions. Applying Litim's optimisation criteria we find ν≈1/3 in four spacetime dimensions. This value is in agreement with lattice studies supporting the existence of a second order phase transition between strongly and weakly coupled phases.
5 pages, 1 figure

http://arxiv.org/abs/1503.06472
Black holes in Asymptotically Safe Gravity
Frank Saueressig, Natalia Alkofer, Giulio D'Odorico, Francesca Vidotto
(Submitted on 22 Mar 2015)
Black holes are among the most fascinating objects populating our universe. Their characteristic features, encompassing spacetime singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. In this note we observe that the renormalization group improved Schwarzschild black holes constructed by Bonanno and Reuter within Weinberg's asymptotic safety program constitute a prototypical example of a Hayward geometry used to model non-singular black holes within quantum gravity phenomenology. Moreover, they share many features of a Planck star: their effective geometry naturally incorporates the one-loop corrections found in the effective field theory framework, their Kretschmann scalar is bounded, and the black hole singularity is replaced by a regular de Sitter patch. The role of the cosmological constant in the renormalization group improvement process is briefly discussed.
6 pages, 3 figures; prepared for the proceedings of the conference "Frontiers of Fundamental Physics 14"

http://arxiv.org/abs/1503.07438
Addendum: Observables for General Relativity related to geometry
Paweł Duch, Wojciech Kamiński, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 24 Mar 2015)
In this addendum we clarify a point which strengthens one of the results from [the original paper]. Namely, we show that the algebra of the observables F(r,θ) is yet simpler then it was described in [the original paper]. This is an important point, because with this simplification an important subalgebra becomes canonical, allowing for a natural reduction of the phase space.
4 pages, addendum to http://arxiv.org/abs/1403.8062

briefly noted, possible side interest:
http://arxiv.org/abs/1503.06109
On the partner particles for moving mirror radiation and black hole evaporation
M. Hotta, R. Schützhold, W. G. Unruh
(Submitted on 20 Mar 2015)
...
...The idea that black holes emit huge amounts of energy in their last stages because of all the information which must be emitted under the assumption of black-hole unitarity is found not necessarily to be the case.
10 pages.

http://arxiv.org/abs/1503.06254
What Chern-Simons theory assigns to a point
Andre Henriques
(Submitted on 21 Mar 2015)
In this note, we answer the questions "What does Chern-Simons theory assign to a point?" and "What kind of mathematical object does Chern-Simons theory assign to a point?".
Our answer to the first question is representations of the based loop group. More precisely, we identify a certain class of projective unitary representations of the based loop group that we call positive energy representations...
...
Our answer to the second question is bicommutant categories. The latter are a sort of categorification of the notion a von Neumann algebras: they are tensor categories that are equivalent to their bicommutant inside a certain fixed tensor category...

http://arxiv.org/abs/1503.07548
Twisted spectral geometry for the standard model
Pierre Martinetti
(Submitted on 25 Mar 2015)
The Higgs field is a connection one-form as the other bosonic fields, provided one describes space no more as a manifold M but as a slightly non-commutative generalization of it. ...
...Applied to the standard model, a similar twist yields in addition the extra scalar field needed to stabilize the electroweak vacuum, and to make the computation of the Higgs mass in noncommutative geometry compatible with its experimental value.

http://arxiv.org/abs/1503.08794
From Causal Dynamical Triangulations To Astronomical Observations
Jakub Mielczarek
(Submitted on 30 Mar 2015)
This essay discusses phenomenological aspects of the diffusion time dependence of the spectral dimension predicted by the Causal Dynamical Triangulations (CDT) approach to quantum gravity. The deformed form of the dispersion relation for the fields defined on the CDT space-time is reconstructed. Using the Fermi satellite observations of the GRB 090510 source we find that the energy scale of the dimensional reduction is E∗>6.7⋅10¹⁰ GeV at (95 % CL).
By applying the deformed dispersion relation to the cosmological perturbations it is shown that, for a scenario when the primordial perturbations are formed in the UV region, the scalar power spectrum P_S∝k^n_S−1 where n_S−1≈3r(d_UV−2)/r+48(d_UV−3). Here, d_UV≈2 is obtained from the CDT value of the spectral dimension in the UV limit and r is the tensor-to-scalar ratio. We find that, the predicted deviation from the scale-invariance (n_S=1) is in contradiction with the up to date Planck and BICEP2 results.
Comments: 10 pages, 1 figure

 

[marcus]

http://inspirehep.net/record/1356275
http://arxiv.org/abs/1503.07855
Loop quantum cosmology with self-dual variables
Edward Wilson-Ewing
(Submitted on 26 Mar 2015)
Using the complex-valued self-dual connection variables, the loop quantum cosmology of a closed Friedmann universe coupled to a massless scalar field is studied. It is shown how the reality conditions can be imposed in the quantum theory by choosing a particular measure for the inner product in the kinematical Hilbert space. While holonomies of the self-dual Ashtekar connection are not well-defined in the kinematical Hilbert space, it is possible to introduce a family of generalized holonomy-like operators, some of which are well-defined; these operators in turn are used in the definition of a Hamiltonian constraint operator where the scalar field can be used as a relational clock. The resulting quantum dynamics are similar, although not identical, to standard loop quantum cosmology constructed from the Ashtekar-Barbero variables with a real Immirzi parameter. Effective Friedmann equations are derived, which provide a good approximation to the full quantum dynamics for sharply-peaked states whose volume remains much larger than the Planck volume, and they show that for these states quantum gravity effects resolve the big-bang and big-crunch singularities and replace them by a non-singular bounce. Finally, the loop quantization in self-dual variables of a flat Friedmann space-time is recovered in the limit of zero spatial curvature and is identical to the standard loop quantization in terms of the real-valued Ashtekar-Barbero variables.
10 pages

http://arxiv.org/abs/1503.09154
Some implications of signature-change in cosmological models of loop quantum gravity
Martin Bojowald, Jakub Mielczarek
(Submitted on 31 Mar 2015)
Signature change at high density has been obtained as a possible consequence of deformed space-time structures in models of loop quantum gravity. This article provides a conceptual discussion of implications for cosmological scenarios, based on an application of mathematical results for mixed-type partial differential equations (the Tricomi problem). While the effective equations from which signature change has been derived are shown to be locally regular and therefore reliable, the underlying theory of loop quantum gravity may face several global problems in its semiclassical solutions.
35 pages, 5 figures

http://arxiv.org/abs/1504.00867
Can Back-Reaction Prevent Eternal Inflation?
Robert Brandenberger, Renato Costa, Guilherme Franzmann (McGill University)
(Submitted on 3 Apr 2015)
We study the effects which the back-reaction of long wavelength fluctuations exert on stochastic inflation. In the cases of power-law and Starobinsky inflation these effects are too weak to terminate the stochastic growth of the inflaton field. However, in the case of the cyclic Ekpyrotic scenario, the back-reaction effects prevent the unlimited growth of the scalar field.
8 pages, 2 figures

 

[John86]

http://arxiv.org/abs/1503.08814
Long-lived resonances at mirrors
Friedemann Queisser, William G. Unruh
(Submitted on 30 Mar 2015)
Motivated by realistic scattering processes of composite systems, we study the dynamics of a two-particle bound system which is scattered at a mirror. The physics of the scattering process will be discussed in the cases when only one particle interacts directly with the mirror and when both particles are scattered directly. It is shown that the coherence between the transmitted and the reflected wave-packet becomes reduced due to the scattering process. When both particles interact directly with the mirror, the system exhibits long-lived resonances. The results should be of interest for interference experiments with composite systems.

http://arxiv.org/abs/1503.08207
Hilbert space structure in quantum gravity: an algebraic perspective
Steven B. Giddings
(Submitted on 27 Mar 2015)
If quantum gravity respects the principles of quantum mechanics, suitably generalized, it may be that a more viable approach to the theory is through identifying the relevant quantum structures rather than by quantizing classical spacetime. This viewpoint is supported by difficulties of such quantization, and by the apparent lack of a fundamental role for locality. In finite or discrete quantum systems, important structure is provided by tensor factorizations of the Hilbert space. However, even in local quantum field theory properties of the generic type III von Neumann algebras and of long range gauge fields indicate that factorization of the Hilbert space is problematic. Instead it is better to focus on on the structure of the algebra of observables, and in particular on its subalgebras corresponding to regions. This paper suggests that study of analogous algebraic structure in gravity gives an important perspective on the nature of the quantum theory. Significant departures from the subalgebra structure of local quantum field theory are found, working in the correspondence limit of long-distances/low-energies. Particularly, there are obstacles to identifying commuting algebras of localized operators. In addition to suggesting important properties of the algebraic structure, this and related observations pose challenges to proposals of a fundamental role for entanglement.

http://arxiv.org/abs/1503.08580
A Spontaneous Signature Change of the Metric in CDT Quantum Gravity?
Jan Ambjørn, Daniel N. Coumbe, Jakub Gizbert-Studnicki, Jerzy Jurkiewicz
(Submitted on 30 Mar 2015 (v1), last revised 1 Apr 2015 (this version, v2))
We study the effective transfer matrix within the semiclassical and bifurcation phases of CDT quantum gravity. We find that for sufficiently large lattice volumes the kinetic term of the effective transfer matrix has a different sign in each of the two phases. We argue that this sign change can be viewed as a Wick rotation of the metric. We discuss the likely microscopic mechanism responsible for the bifurcation phase transition, and propose an order parameter that can potentially be used to determine the precise location and order of the transition. Using the effective transfer matrix we approximately locate the position of the bifurcation transition in some region of coupling constant space, allowing us to present an updated version of the CDT phase diagram.

http://arxiv.org/abs/1503.08911
Variations on an aethereal theme
Ted Jacobson, Antony J. Speranza
(Submitted on 31 Mar 2015)
We consider a class of Lorentz-violating theories of gravity involving a timelike unit vector field (the aether) coupled to a metric, two examples being Einstein-aether theory and Ho\v{r}ava gravity. The action always includes the Ricci scalar of the metric and the invariants quadratic in covariant derivatives of the aether, but the theories differ in how the aether is constructed from other fields, and whether those fields are varied in the action. Fields that are not varied define background structures breaking diffeomorphsim invariance, including threadings, folations, and clocks, which generally produce novel degrees of freedom arising from the violation of what would otherwise be initial value constraints. The principal aims of this paper are to survey the nature of the theories that arise, and to understand the consequences of breaking diffeomorphism invariance in this setting. In a companion paper [1], we address some of the phenomenology of the "ponderable aether" case in which the presence of a background clock endows the aether with a variable internal energy density that behaves in some respects like dark matter.

http://arxiv.org/abs/1503.08794
From Causal Dynamical Triangulations To Astronomical Observations
Jakub Mielczarek
(Submitted on 30 Mar 2015)
This essay discusses phenomenological aspects of the diffusion time dependence of the spectral dimension predicted by the Causal Dynamical Triangulations (CDT) approach to quantum gravity. The deformed form of the dispersion relation for the fields defined on the CDT space-time is reconstructed. Using the \emph{Fermi} satellite observations of the GRB 090510 source we find that the energy scale of the dimensional reduction is E∗>6.7⋅1010 GeV at (95 % CL).
By applying the deformed dispersion relation to the cosmological perturbations it is shown that, for a scenario when the primordial perturbations are formed in the UV region, the scalar power spectrum PS∝knS−1 where nS−1≈3r(dUV−2)r+48(dUV−3). Here, dUV≈2 is obtained from the CDT value of the spectral dimension in the UV limit and r is the tensor-to-scalar ratio. We find that, the predicted deviation from the scale-invariance (nS=1) is in contradiction with the up to date \emph{Planck} and \emph{BICEP2} results.

http://arxiv.org/abs/1503.08770
What can quantum cosmology say about the inflationary universe?
Gianluca Calcagni, Claus Kiefer, Christian F. Steinwachs
(Submitted on 30 Mar 2015)
We propose a method to extract predictions from quantum cosmology for inflation that can be confronted with observations. Employing the tunneling boundary condition in quantum geometrodynamics, we derive a probability distribution for the inflaton field. A sharp peak in this distribution can be interpreted as setting the initial conditions for the subsequent phase of inflation. In this way, the peak sets the energy scale at which the inflationary phase has started. This energy scale must be consistent with the energy scale found from the inflationary potential and with the scale found from a potential observation of primordial gravitational waves. Demanding a consistent history of the universe from its quantum origin to its present state, which includes decoherence, we derive a condition that allows one to constrain the parameter space of the underlying model of inflation. We demonstrate our method by applying it to two models: Higgs inflation and natural inflation.

http://arxiv.org/abs/1503.08725
Statistical mechanics of reparametrization invariant systems. Takes Three to Tango
Thibaut Josset, Goffredo Chirco, Carlo Rovelli
(Submitted on 30 Mar 2015)
It is notoriously difficult to apply statistical mechanics to generally covariant systems, because the notions of time, energy and equilibrium are seriously modified in this context. We discuss the conditions under which weaker versions of these notions can be defined, sufficient for statistical mechanics. We focus on reparametrization invariant systems without additional gauges. The key idea is to reconstruct statistical mechanics from the ergodic theorem. We find that a suitable split of the system into two non-interacting components is sufficient for generalizing statistical mechanics. While equilibrium acquires sense only when the system admits a suitable split into three weakly interacting components ---roughly: a clock and two systems among which a generalization of energy is equi-partitioned. The key property that allows the application of statistical mechanics and thermodynamics is an additivity condition of such generalized energy.

http://arxiv.org/abs/1504.00187
Autonomous quantum thermal machine for generating steady-state entanglement
Jonatan Bohr Brask, Nicolas Brunner, Géraldine Haack, Marcus Huber
(Submitted on 1 Apr 2015)
We discuss a simple quantum thermal machine for the generation of steady-state entanglement between two interacting qubits. The machine is autonomous in the sense that it uses only incoherent interactions with thermal baths, but no source of coherence or external control. By weakly coupling the qubits to thermal baths at different temperatures, inducing a heat current through the system, steady-state entanglement is generated far from thermal equilibrium. Finally, we discuss two possible implementations, using superconducting flux qubits or a semiconductor double quantum dot. Experimental prospects for steady-state entanglement are promising in both systems.

 

[marcus]

http://arxiv.org/abs/1504.01065
Wilson loops in CDT quantum gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 4 Apr 2015)
By explicit construction, we show that one can in a simple way introduce and measure gravitational holonomies and Wilson loops in lattice formulations of nonperturbative quantum gravity based on (Causal) Dynamical Triangulations. We use this set-up to investigate a class of Wilson line observables associated with the world line of a point particle coupled to quantum gravity, and deduce from their expectation values that the underlying holonomies cover the group manifold of SO(4) uniformly.
30 pages, 5 figures

http://arxiv.org/abs/1504.02068
Hamiltonian operator for loop quantum gravity coupled to a scalar field
E. Alesci, M. Assanioussi, J. Lewandowski, I. Mäkinen
(Submitted on 8 Apr 2015)
We present the construction of a physical Hamiltonian operator in the deparametrized model of loop quantum gravity coupled to a free scalar field. This construction is based on the use of the recently introduced curvature operator, and on the idea of so-called "special loops". We discuss in detail the regularization procedure and the assignment of the loops, along with the properties of the resulting operator. We compute the action of the squared Hamiltonian operator on spin network states, and close with some comments and outlooks.
31 pages, numerous graph diagrams

 

[atyy]

http://arxiv.org/abs/1504.02169
Coherent states, quantum gravity and the Born-Oppenheimer approximation, I: General considerations
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
This article, as the first of three, aims at establishing the (time-dependent) Born-Oppenheimer approximation, in the sense of space adiabatic perturbation theory, for quantum systems constructed by techniques of the loop quantum gravity framework, especially the canonical formulation of the latter. The analysis presented here fits into a rather general framework, and offers a solution to the problem of applying the usual Born-Oppenheimer ansatz for molecular (or structurally analogous) systems to more general quantum systems (e.g. spin-orbit models) by means of space adiabatic perturbation theory. The proposed solution is applied to a simple, finite dimensional model of interacting spin systems, which serves as a non-trivial, minimal model of the aforesaid problem. Furthermore, it is explained how the content of this article, and its companion, affect the possible extraction of quantum field theory on curved spacetime from loop quantum gravity (including matter fields).

http://arxiv.org/abs/1504.02170
Coherent states, quantum gravity and the Born-Oppenheimer approximation, II: Compact Lie Groups
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the second of three, we discuss and develop the basis of a Weyl quantisation for compact Lie groups aiming at loop quantum gravity-type models. This Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity. Additionally, we conjecture the existence of a new form of the Segal-Bargmann-Hall "coherent state" transform for compact Lie groups $G$, which we prove for $G=U(1)^{n}$ and support by numerical evidence for $G=SU(2)$. The reason for conjoining this conjecture with the main topic of this article originates in the observation, that the coherent state transform can be used as a basic building block of a coherent state quantisation (Berezin quantisation) for compact Lie groups $G$. But, as Weyl and Berezin quantisation for $\mathbb{R}^{2d}$ are intimately related by heat kernel evolution, it is natural to ask, whether a similar connection exists for compact Lie groups, as well. Moreover, since the formulation of space adiabatic perturbation theory requires a (deformation) quantisation as minimal input, we analyse the question to what extent the coherent state quantisation, defined by the Segal-Bargmann-Hall transform, can serve as basis of the former.

http://arxiv.org/abs/1504.02171
Coherent states, quantum gravity and the Born-Oppenheimer approximation, III: Applications to loop quantum gravity
Alexander Stottmeister, Thomas Thiemann
(Submitted on 9 Apr 2015)
In this article, the third of three, we analyse how the Weyl quantisation for compact Lie groups presented in the second article of this series fits with the projective-phase space structure of loop quantum gravity-type models. Thus, the proposed Weyl quantisation may serve as the main mathematical tool to implement the program of space adiabatic perturbation theory in such models. As we already argued in our first article, space adiabatic perturbation theory offers an ideal framework to overcome the obstacles that hinder the direct implementation of the conventional Born-Oppenheimer approach in the canonical formulation of loop quantum gravity.

 

[marcus]

http://arxiv.org/abs/1504.02822
Duality between Spin networks and the 2D Ising model
Valentin Bonzom, Francesco Costantino, Etera R. Livine
(Submitted on 11 Apr 2015)
The goal of this paper is to exhibit a deep relation between the partition function of the Ising model on a planar trivalent graph and the generating series of the spin network evaluations on the same graph. We provide respectively a fermionic and a bosonic Gaussian integral formulation for each of these functions and we show that they are the inverse of each other (up to some explicit constants) by exhibiting a supersymmetry relating the two formulations. We investigate three aspects and applications of this duality. First, we propose higher order supersymmetric theories which couple the geometry of the spin networks to the Ising model and for which supersymmetric localization still holds. Secondly, after interpreting the generating function of spin network evaluations as the projection of a coherent state of loop quantum gravity onto the flat connection state, we find the probability distribution induced by that coherent state on the edge spins and study its stationary phase approximation. It is found that the stationary points correspond to the critical values of the couplings of the 2D Ising model, at least for isoradial graphs. Third, we analyze the mapping of the correlations of the Ising model to spin network observables, and describe the phase transition on those observables on the hexagonal lattice. This opens the door to many new possibilities, especially for the study of the coarse-graining and continuum limit of spin networks in the context of quantum gravity.
35 pages

possible wider interest:
http://arxiv.org/abs/1504.02818
Path integrals in configuration space and the emergence of classical behavior for closed systems
Henrique Gomes
(Submitted on 11 Apr 2015)
Traditionally, the field of quantum foundations has been preoccupied with different forms of the question "How can an observer be in a state of quantum superposition?". In this paper, I approach this question by exploring a timeless interpretation of quantum mechanics of closed systems, solely in terms of path integrals in non-relativistic timeless configuration space. What prompts a fresh look at the foundational problems in this context, is the advent of multiple gravitational models in which Lorentz symmetry is only emergent. In this setting, I propose a new understanding of records as certain relations between two configurations, the recorded one and the record-holding one. These relations are formalized through a factorization of the amplitude kernel, which forbids unwanted `recoherence' of branches. On this basis, I show that in simple cases the Born rule emerges from counting the relative density of observers with the same records. Furthermore, unlike what occurs in consistent histories, in this context there is indeed a preferred notion of coarse-grainings: those centered around piece-wise classical paths in configuration space. Thus, this new understanding claims to resolve aspects of the measurement problem which are still deemed controversial in the standard approaches.
26 pages, 10 page appendix, 1 figure

http://arxiv.org/abs/1504.02797
Foundations of statistical mechanics from symmetries of entanglement
Sebastian Deffner, Wojciech H. Zurek
(Submitted on 10 Apr 2015)
Envariance - entanglement assisted invariance - is a recently discovered symmetry of composite quantum systems. In this work, it is shown that thermodynamic equilibrium states are fully characterized by their envariance. In particular, the microcanonical equilibrium of a system S with Hamiltonian H_S can be described as an "even", i.e., envariant under any unitary transformation, fully energetically degenerate quantum state. The representation of the canonical equilibrium then follows from simply counting degenerate energy states. Our conceptually novel approach is free of mathematically ambiguous notions such as probability, ensemble, randomness, etc. .
15 pages

http://arxiv.org/abs/1504.04005
Symbolic Dynamics, Modular Curves, and Bianchi IX Cosmologies
Yuri Manin, Matilde Marcolli
(Submitted on 15 Apr 2015)
It is well known that the so called Bianchi IX spacetimes with SO(3)-symmetry in a neighbourhood of the Big Bang exhibit a chaotic behaviour of typical trajectories in the backward movement of time. This behaviour (Mixmaster Model of the Universe) can be encoded by the shift of two-sided continued fractions. Exactly the same shift encodes the sequences of intersections of hyperbolic geodesics with purely imaginary axis in the upper complex half-plane, that is geodesic flow on an appropriate modular surface. A physical interpretation of this coincidence was suggested in arXiv:1402.2158: namely, that Mixmaster chaos is an approximate description of the passage from a hot quantum Universe at the Big Bang moment to the cooling classical Universe. Here we discuss and elaborate this suggestion, looking at the Mixmaster Model from the perspective of the second class of Bianchi IX spacetimes: those with SU(2)-symmetry (self-dual Einstein metrics). We also extend it to the more general context related to Painlevé VI equations.
26 pages

since this paper continues the development begun in a February 2014 paper which I don't recall getting logged on this bibliography, I'll add the abstract to that one here as well:
http://arxiv.org/abs/1402.2158
Big Bang, Blowup, and Modular Curves: Algebraic Geometry in Cosmology
Yuri I. Manin, Matilde Marcolli
(Submitted on 10 Feb 2014)
We introduce some algebraic geometric models in cosmology related to the "boundaries" of space-time: Big Bang, Mixmaster Universe, Penrose's crossovers between aeons. We suggest to model the kinematics of Big Bang using the algebraic geometric (or analytic) blow up of a point x. This creates a boundary which consists of the projective space of tangent directions to x and possibly of the light cone of x. We argue that time on the boundary undergoes the Wick rotation and becomes purely imaginary. The Mixmaster (Bianchi IX) model of the early history of the universe is neatly explained in this picture by postulating that the reverse Wick rotation follows a hyperbolic geodesic connecting imaginary time axis to the real one. Penrose's idea to see the Big Bang as a sign of crossover from "the end of previous aeon" of the expanding and cooling Universe to the "beginning of the next aeon" is interpreted as an identification of a natural boundary of Minkowski space at infinity with the Big Bang boundary.
20 pages. Published in SIGMA (2014)

 

[marcus]

http://arxiv.org/abs/1504.05352
Black hole spectroscopy from Loop Quantum Gravity models
Aurelien Barrau, Xiangyu Cao, Karim Noui, Alejandro Perez
(Submitted on 21 Apr 2015)
Using Monte Carlo simulations, we compute the integrated emission spectra of black holes in the framework of Loop Quantum Gravity (LQG). The black hole emission rates are governed by the entropy whose value, in recent holographic loop quantum gravity models, was shown to agree at leading order with the Bekenstein-Hawking entropy. Quantum corrections depend on the Barbero-Immirzi parameter γ. Starting with black holes of initial horizon area A∼10² in Planck units, we present the spectra for different values of γ. Each spectrum clearly decomposes in two distinct parts: a continuous background which corresponds to the semi-classical stages of the evaporation and a series of discrete peaks which constitutes a signature of the deep quantum structure of the black hole. We show that γ has an effect on both parts that we analyze in details. Finally, we estimate the number of black holes and the instrumental resolution required to experimentally distinguish between the considered models.
11 pages, 9 figures

http://arxiv.org/abs/1503.08640
New first order Lagrangian for General Relativity
Yannick Herfray, Kirill Krasnov
(Submitted on 30 Mar 2015)
We describe a new BF-type first-order in derivatives Lagrangian for General Relativity. The Lagrangian depends on a connection field as well as a Lie-algebra valued two-form field, with no other fields present. There are two free parameters, which translate into the cosmological constant and the coefficient in front of a topological term. When one of the parameters is set to zero, the theory becomes topological. When the other parameter is zero, the theory reduces to the (anti-) self-dual gravity. Thus, our new Lagrangian interpolates between the topological and anti-self-dual gravities. It also interprets GR as the (anti-) self-dual gravity with an extra quadratic in the auxiliary two-form field term added to the Lagrangian, precisely paralleling the situation in Yang-Mills theory.
4 pages

possibly of 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 (v1), last revised 23 Apr 2015 (this version, v2))
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 orthochronous 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. This result also hints at a novel information-theoretic perspective on spacetime.
Comments: 37 pages, 6 figures, added two new sections, additional explanations and motivations, updated references, corrected typos