Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/1308.1278

Second Order Standard Model
Johnny Espin, Kirill Krasnov
(Submitted on 6 Aug 2013)
We rewrite the Lagrangian of the fermionic sector of the Standard Model in a novel compact form. The new Lagrangian is second order in derivatives, and is obtained from the usual first order Lagrangian by integrating out all primed (or dotted) 2-component spinors. The Higgs field enters the new Lagrangian non-polynomially, very much like the metric enters the Einstein-Hilbert Lagrangian of General Relativity. We also discuss unification in the second order formalism, and describe a natural in this framework SU(2)xSU(4) unified theory.

 

[marcus]

http://arxiv.org/abs/1308.1982
Polyhedra in spacetime from null vectors
Yasha Neiman
(Submitted on 8 Aug 2013)
We consider convex spacelike polyhedra oriented in Minkowski space. These are the classical analogues of spinfoam intertwiners. We point out a parametrization of these shapes using null face normals, with no constraints or redundancies. Our construction is dimension-independent. In 3+1d, it provides the spacetime picture behind a well-known property of the loop quantum gravity intertwiner space in spinor form, namely that the closure constraint is always satisfied after some SL(2,C) rotation. As a simple application of our variables, we incorporate them in a 4-simplex action that reproduces the large-spin behavior of the Barrett-Crane vertex amplitude.
12 pages, 1 figure

http://arxiv.org/abs/1308.2206
Energetic Causal Sets
Marina Cortês, Lee Smolin
(Submitted on 9 Aug 2013)
We propose an approach to quantum theory based on the energetic causal sets, introduced in Cortês and Smolin (2013). Fundamental processes are causal sets whose events carry momentum and energy, which are transmitted along causal links and conserved at each event. Fundamentally there are amplitudes for such causal processes, but no space-time. An embedding of the causal processes in an emergent space-time arises only at the semiclassical level. Hence, fundamentally there are no commutation relations, no uncertainty principle and, indeed, no hbar. All that remains of quantum theory is the relationship between the absolute value squared of complex amplitudes and probabilities. Consequently, we find that neither locality, nor non locality, are primary concepts, only causality exists at the fundamental level.
9 pages. Article companion to http://arxiv.org/abs/1307.6167

general interest:
http://arxiv.org/abs/1308.1977
Holography without strings?
Donald Marolf
(Submitted on 8 Aug 2013)
A defining feature of holographic dualities is that, along with the bulk equations of motion, boundary correlators at any given time t determine those of observables deep in the bulk. We argue that this property emerges from the bulk gravitational Gauss law together with bulk quantum entanglement as embodied in the Reeh-Schlieder theorem. Stringy bulk degrees of freedom are not required and play little role even when they exist. As an example we study a toy model whose matter sector is a free scalar field. The energy density (ρ) sources what we call a pseudo-Newtonian potential (Φ) through Poisson's equation on each constant time surface, but there is no back-reaction on the matter. We show the Hamiltonian to be essentially self-adjoint on the domain generated from the vacuum by acting with boundary observables localized in an arbitrarily small neighborhood of the chosen time t. Since the Gauss law represents the Hamiltonian as a boundary term, the model is holographic in the sense stated above.
13 pages

 

[John86]

http://arxiv.org/abs/1308.2746
On the renormalization of the Gibbons-Hawking boundary term
Ted Jacobson, Alejandro Satz
(Submitted on 13 Aug 2013)
The bulk (Einstein-Hilbert) and boundary (Gibbons-Hawking) terms in the gravitational action are generally renormalized differently when integrating out quantum fluctuations. The former is affected by nonminimal couplings, while the latter is affected by boundary conditions. We use the heat kernel method to analyze this behavior for a nonminimally coupled scalar field, the Maxwell field, and the graviton field. Allowing for Robin boundary conditions, we examine in which cases the renormalization preserves the ratio of boundary and bulk terms required for the effective action to possesses a stationary point. The implications for field theory and black hole entropy computations are discussed.

http://arxiv.org/abs/1308.2946
Purely geometric path integral for spin foams
Atousa Chaharsough Shirazi, Jonathan Engle
(Submitted on 13 Aug 2013)
Spin-foams are a proposal for defining the dynamics of loop quantum gravity via path integral. In order for a path integral to be at least formally equivalent to the corresponding canonical quantization, at each point in the space of histories it is important that the integrand have not only the correct phase -- a topic of recent focus in spin-foams -- but also the correct modulus, usually referred to as the measure factor. The correct measure factor descends from the Liouville measure on the reduced phase space, and its calculation is a task of canonical analysis.
The covariant formulation of gravity from which spin-foams are derived is the Plebanski-Holst formulation, in which the basic variables are a Lorentz connection and a Lorentz-algebra valued two-form, called the Plebanski two-form. However, in the final spin-foam sum, one sums over only spins and intertwiners, which label eigenstates of the Plebanski two-form alone. The spin-foam sum is therefore a discretized version of a Plebanski-Holst path integral in which only the Plebanski two-form appears, and in which the connection degrees of freedom have been integrated out. We call this a purely geometric Plebanski-Holst path integral.
In prior work in which one of the authors was involved, the measure factor for the Plebanski-Holst path integral with both connection and two-form variables was calculated. Before one discretizes this measure and incorporates it into a spin-foam sum, however, one must integrate out the connection in order to obtain the purely geometric version of the path integral. To calculate this purely geometric path integral is the principal task of the present paper, and it is done in two independent ways. Gauge-fixing and the background independence of the resulting path integral are discussed in the appendices.

 

[John86]

Distant related, but interesting

http://arxiv.org/abs/1308.3044
Nonperturbative analysis of the evolution of cosmological perturbations through a nonsingular bounce
BingKan Xue, David Garfinkle, Frans Pretorius, Paul J. Steinhardt
(Submitted on 14 Aug 2013)
In bouncing cosmology, the primordial fluctuations are generated in a cosmic contraction phase before the bounce into the current expansion phase. For a nonsingular bounce, curvature and anisotropy grow rapidly during the bouncing phase, raising questions about the reliability of perturbative analysis. In this paper, we study the evolution of adiabatic perturbations in a nonsingular bounce by nonperturbative methods including numerical simulations of the nonsingular bounce and the covariant formalism for calculating nonlinear perturbations. We show that the bounce is disrupted in regions of the universe with significant inhomogeneity and anisotropy over the background energy density, but is achieved in regions that are relatively homogeneous and isotropic. Sufficiently small perturbations, consistent with observational constraints, can pass through the nonsingular bounce with negligible alteration from nonlinearity. We follow scale invariant perturbations generated in a matter-like contraction phase through the bounce. Their amplitude in the expansion phase is determined by the growing mode in the contraction phase, and the scale invariance is well preserved across the bounce.

 

[John86]

http://arxiv.org/abs/1308.3337
Information-Based Physics and the Influence Network
Kevin H. Knuth
(Submitted on 15 Aug 2013)
This essay considers a simple model of observers that are influenced by the world around them. Consistent quantification of information about such influences results in a great deal of familiar physics. The end result is a new perspective on relativistic quantum mechanics, which includes both a way of conceiving of spacetime as well as particle "properties" that may be amenable to a unification of quantum mechanics and gravity. Rather than thinking about the universe as a computer, perhaps it is more accurate to think about it as a network of influences where the laws of physics derive from both consistent descriptions and optimal information-based inferences made by embedded observers.

http://arxiv.org/abs/1308.3398
Higher Derivative Gravity and Asymptotic Safety in Diverse Dimensions
Nobuyoshi Ohta, Roberto Percacci
(Submitted on 15 Aug 2013)
We derive the one-loop beta functions for a theory of gravity with generic action containing up to four derivatives. The calculation is done in arbitrary dimension and on an arbitrary background. The special cases of three, four, near four, five and six dimensions are discussed in some detail. We find that the theories have nontrivial UV fixed points and are asymptotically safe in all dimensions we study. We also find an indication that Weyl-invariant fixed point exists in four dimensions. The new massive gravity in three dimensions does not correspond to any fixed point.

http://arxiv.org/abs/1308.3488
Statistical physics of black holes as quantum-mechanical systems
Steven B. Giddings
(Submitted on 15 Aug 2013)
Some basic features of black-hole statistical mechanics are investigated, assuming that black holes respect the principles of quantum mechanics. Care is needed in defining an entropy S_bh corresponding to the number of microstates of a black hole, given that the black hole interacts with its surroundings. An open question is then the relationship between this entropy and the Bekenstein-Hawking entropy S_BH. For a wide class of models with interactions needed to ensure unitary quantum evolution, these interactions produce extra energy flux beyond that predicted by Hawking. Arguments are then presented that this results in an entropy S_bh that is smaller than S_BH. Correspondingly, in such scenarios equilibrium properties of black holes are modified. We examine questions of consistency of such an inequality; if it is not consistent, that provides significant constraints on models for quantum-mechanical black hole evolution.

 

[marcus]

http://arxiv.org/abs/1308.4063
Covariant Loop Quantum Gravity, Low Energy Perturbation Theory, and Einstein Gravity
Muxin Han
(Submitted on 19 Aug 2013)
A low-energy perturbation theory is developed from the nonperturbative framework of covariant Loop Quantum Gravity (LQG) by employing the background field method. The resulting perturbation theory is a 2-parameter expansion in the semiclassical and low-energy regime. The two expansion parameters are the large spin and small curvature. The leading order effective action coincides with the Einstein-Hilbert action. The subleading corrections organized by the two expansion parameters give the modifications of Einstein gravity in quantum and high-energy regime from LQG. The result of the paper may be viewed as the first step toward understanding the UV completeness of LQG.
4 pages, 1 figure

brief mention:
http://arxiv.org/abs/1308.4099
Null tests of the cosmological constant using supernovae
Sahba Yahya, Marina Seikel, Chris Clarkson, Roy Maartens, Mathew Smith
(Submitted on 19 Aug 2013)
7 pages, 5 figures.

 

[marcus]

http://arxiv.org/pdf/1308.4348
The Echo of the Quantum Bounce
Luis J. Garay, Mercedes Martin-Benito, Eduardo Martin-Martinez
(Submitted on 20 Aug 2013)
We identify a signature of quantum gravitational effects that survives from the early universe to the current era: Fluctuations of quantum fields as seen by comoving observers are significantly influenced by the history of the early universe. In particular we will show how the existence (or not) of a quantum bounce leaves a trace in the background quantum noise that is not damped and would be non-negligible even nowadays. Furthermore, we will estimate an upper bound to the typical energy and length scales where quantum effects are relevant. We will discuss how this signature might be observed and therefore used to build falsifiability tests of quantum gravity theories.
5 pages, 3 figures

 

[MTd2]

http://arxiv.org/abs/1308.4044

Notes on several phenomenological laws of quantum gravity

Jean-Philippe Bruneton
(Submitted on 19 Aug 2013)
Phenomenological approaches to quantum gravity try to infer model-independent laws by analyzing thought experiments and combining both quantum, relativistic, and gravitational ingredients. We first review these ingredients -three basic inequalities- and discuss their relationships with the nature of fundamental constants. In particular, we argue for a covariant mass bound conjecture: in a spacetime free of horizon, the mass inside a surface $A$ cannot exceed $16 \pi G^2 m^2< A $, while the reverse holds in a spacetime with horizons. This is given a precise definition using the formalism of light-sheets. We show that $\hbar/c$ may be also given a geometrical interpretation, namely $4 \pi \hbar^2/m^2< A$. We then combine these inequalities and find/review the following: (1) Any system must have a size greater than the Planck length, in the sense that there exists a minimal area (2) We comment on the Minimal Length Scenarios and the fate of Lorentz symmetry near the Planck scale (3) Quanta with transplanckian frequencies are allowed in a large enough boxes (4) There exists a mass-dependent maximal acceleration given by $m c^3/\hbar$ if $m<m_p$ and by $c^4/G m$ if $m>m_p$ (5) There exists a mass dependent maximal force and power (6) There exists a maximal energy density and pressure (7) Physical systems must obey the Holographic Principle (8) Holographic bounds can only be saturated by systems with $m>m_p$; systems lying on the ``Compton line'' $l \sim 1/m$ are fundamental objects without substructures (9) We speculate on a new bound from above for the action. In passing, we note that the maximal acceleration is of the order of Milgrom's acceleration $a_0$ for ultra-light particles ($m\sim H_0)$ that could be associated to the Dark Energy fluid. This suggests designing toy-models in which modified gravity in galaxies is driven by the DE field, via the maximal acceleration principle.

 

[John86]

http://arxiv.org/abs/1308.4667
Conformal Symmetry, Rindler Space and The AdS/CFT Correspondence
Prasant Samantray, T. Padmanabhan
(Submitted on 21 Aug 2013)
Field theories in black hole spacetimes undergo dimensional reduction near horizon (in the Rindler limit) to two dimensional conformal field theories. We investigate this enhancement of symmetries in the context of gauge/gravity duality by considering Rindler space as boundary of Anti-de Sitter space in three spacetime dimensions. We show that the loxodromy conjugacy class of the SO(2,2) isometry group is responsible for generating the special conformal transformations on the boundary under RG flow. We use this approach to present an alternative derivation of the two-point function in Rindler space using AdS/CFT correspondence.

 

[John86]

http://arxiv.org/abs/1308.4976
Evolution of quantum field, particle content and classicality in the three stage universe
Suprit Singh, Sujoy Kumar Modak, T. Padmanabhan
(Submitted on 22 Aug 2013)
We study the evolution of a quantum scalar field in a toy universe which has three stages of evolution, viz., (i) an early (inflationary) de Sitter phase (ii) radiation dominated phase and (iii) late-time (cosmological constant dominated) de Sitter phase. Using Schr\"odinger picture, the scalar field equations are solved separately for the three stages and matched at the transition points. The boundary conditions are chosen so that field modes in the early de Sitter evolves from Bunch-Davies vacuum state. We determine the (time-dependent) particle content of this quantum state for the entire evolution of the universe and describe the various features both numerically and analytically. We also describe the quantum to classical transition in terms of a {\it classicality parameter} which tracks the particle creation and its effect on phase space correlation of the quantum field.

http://arxiv.org/abs/1308.5009
Quantum nonlocal correlations are not dominated
Adrian Kent (Centre for Quantum Information and Foundations, DAMTP, University of Cambridge and Perimeter Institute)
(Submitted on 22 Aug 2013)
We show that no probability distribution of spin measurement outcomes on pairs of spin 1/2 particles is unambiguously more nonlocal than the quantum correlations. That is, any distribution that produces a CHSH violation larger than the quantum violation for some axis choices also produces a smaller CHSH violation for some other axis choices. In this sense, it is not possible for nature to be strictly more nonlocal than quantum theory allows.

this is quite an interesting paper

http://arxiv.org/abs/1308.5097
Foundations of Quantum Gravity : The Role of Principles Grounded in Empirical Reality
M. Holman
(Submitted on 23 Aug 2013)
When attempting to assess the strengths and weaknesses of various principles in their potential role of guiding the formulation of a theory of quantum gravity, it is crucial to distinguish between principles which are strongly supported by empirical data - either directly or indirectly - and principles which instead (merely) rely heavily on theoretical arguments for their justification. These remarks are illustrated in terms of the current standard models of cosmology and particle physics, as well as their respective underlying theories, viz. general relativity and quantum (field) theory. It is argued that if history is to be of any guidance, the best chance to obtain the key structural features of a putative quantum gravity theory is by deducing them, in some form, from the appropriate empirical principles (analogous to the manner in which, say, the idea that gravitation is a curved spacetime phenomenon is arguably implied by the equivalence principle). It is subsequently argued that the appropriate empirical principles for quantum gravity should at least include (i) quantum nonlocality, (ii) irreducible indeterminacy, (iii) the thermodynamic arrow of time, (iv) homogeneity and isotropy of the observable universe on the largest scales. In each case, it is explained - when appropriate - how the principle in question could be implemented mathematically in a theory of quantum gravity, why it is considered to be of fundamental significance and also why contemporary accounts of it are insufficient.

 

[marcus]

http://arxiv.org/abs/1308.5599
Why Gauge?
Carlo Rovelli
(Submitted on 26 Aug 2013)
The world appears to be well described by gauge theories; why? I suggest that gauge is more than mathematical redundancy. Gauge variables describe handles though which systems couple. Gauge-dependent quantities can not be predicted, but there is a sense in which they can be measured. This observation leads to a physical interpretation for the ubiquity of gauge: it is a consequence of a relational structure of the physical quantities.
7 pages

http://arxiv.org/abs/1308.5648
Semiclassical states in quantum gravity: Curvature associated to a Voronoi graph
Jacobo Diaz-Polo, Iñaki Garay
(Submitted on 26 Aug 2013)
The building blocks of a quantum theory of general relativity are expected to be discrete structures. Loop quantum gravity is formulated using a basis of spin networks (wave functions over oriented graphs with coloured edges), thus realizing the aforementioned expectation. Semiclassical states should, however, reproduce the classical smooth geometry in the appropriate limits. The question of how to recover a continuous geometry from these discrete structures is, therefore, relevant in this context. Following previous works by Bombelli et al. we explore this problem from a rather general mathematical perspective using, in particular, properties of Voronoi graphs to search for their compatible continuous geometries. We test the previously proposed methods for computing the curvature associated to such graphs and analyse the framework in detail, in the light of the results obtained.
16 pages

possible interest, no time to evaluate:
http://arxiv.org/abs/1308.5290

 

[marcus]

http://arxiv.org/abs/1308.6289
Indistinguishability of thermal and quantum fluctuations
Sanved Kolekar, T. Padmanabhan
(Submitted on 28 Aug 2013)
The existence of Davies-Unruh temperature in a uniformly accelerated frame shows that quantum fluctuations of the inertial vacuum state appears as thermal fluctuations in the accelerated frame. Hence thermodynamic experiments cannot distinguish between phenomena occurring in a thermal bath of temperature T in the inertial frame from those in a frame accelerating through inertial vacuum with the acceleration a=2π T. We show that this indisguishability between quantum fluctuations and thermal fluctuations goes far beyond the fluctuations in the vacuum state. We show by an exact calculation, that the reduced density matrix for a uniformly accelerated observer when the quantum field is in a thermal state of temperature $T^\prime$ is symmetric between acceleration temperature T = a/(2π) and the thermal bath temperature $T^\prime$. Thus thermal phenomena cannot distinguish whether (i) one is accelerating with $a = 2\pi T$ through a bath of temperature $T^\prime$ or (ii) accelerating with $a=2\pi T^\prime$ through a bath of temperature T. This shows that thermal and quantum fluctuations in an accelerated frame affect the observer in a symmetric manner. The implications are discussed.
4 pages

 

[marcus]

http://arxiv.org/abs/1308.6586
Canonical structure of Tetrad Bimetric Gravity
Sergei Alexandrov
(Submitted on 29 Aug 2013)
We perform the complete canonical analysis of the tetrad formulation of bimetric gravity and confirm that it is ghost-free describing the seven degrees of freedom of a massless and a massive gravitons. In particular, we find explicit expressions for secondary constraints, one of which is responsible for removing the ghost, whereas the other ensures the equivalence with the metric formulation. Both of them have a remarkably simple form and, being combined with conditions on Lagrange multipliers, can be written in a covariant way.
18 pages

brief mention:
http://arxiv.org/abs/1308.6773
Quantum field theory on curved spacetime and the standard cosmological model
Klaus Fredenhagen, Thomas-Paul Hack
(Submitted on 30 Aug 2013)
The aim of this review is to outline a full route from the fundamental principles of algebraic quantum field theory on curved spacetime in its present-day form to explicit phenomenological applications which allow for comparison with experimental data. We give a brief account on the quantization of the free scalar field and its Wick powers in terms of an algebra of functionals on configuration space. Afterwards we demonstrate that there exist states on this algebra in which the energy momentum tensor is qualitatively and quantitatively of the perfect fluid form assumed in the standard model of cosmology up to small corrections. We indicate the potential relevance of one of these corrections for the actively debated phenomenon of Dark Radiation.
18 pages, 1 figure.

 

[marcus]

http://arxiv.org/abs/1309.0311
Phenomenology of Space-time Imperfection I: Nonlocal Defects
Sabine Hossenfelder
(Submitted on 2 Sep 2013)
If space-time is emergent from a fundamentally non-geometric theory it will generically be left with defects. Such defects need not respect the locality that emerges with the background. Here, we develop a phenomenological model that parameterizes the effects of nonlocal defects on the propagation of particles. In this model, Lorentz-invariance is preserved on the average. We derive constraints on the density of defects from various experiments.
25 pages, 7 figures

http://arxiv.org/abs/1309.0314
Phenomenology of Space-time Imperfection II: Local Defects
Sabine Hossenfelder
(Submitted on 2 Sep 2013)
We propose a phenomenological model for the scattering of particles on space-time defects in a treatment that maintains Lorentz-invariance on the average. The local defects considered here cause a stochastic violation of momentum conservation. The scattering probability is parameterized in the density of defects and the distribution of the momentum that a particle can obtain when scattering on the defect. We identify the most promising observable consequences and derive constraints from existing data.
18 pages, 5 figures

http://arxiv.org/abs/1309.0352
Cosmological perturbations in teleparallel Loop Quantum Cosmology
Jaime Haro
(Submitted on 2 Sep 2013)
Cosmological perturbations in Loop Quantum Cosmology (LQC) could be studied from two totally different ways. The first one, called holonomy corrected LQC, is performed in the Hamiltonian framework, where the Asthekar connection is replaced by a suitable sinus function (holonomy correction), in order to have a well-defined quantum analogue. The alternative approach is based in the fact that isotropic LQC could be also obtained as a particular case of teleparallel F(T) gravity (teleparallel LQC). Then, working in the Lagrangian framework and using the well-know perturbation equations in F(T) gravity, we have obtained, in teleparallel LQC, the equations for scalar and tensor perturbations, and the corresponding Mukhanov-Sasaki equations. For scalar perturbations, our equation only differs from the one obtained by holonomy corrections in the velocity of sound, leading both formulations, essentially to the same scale invariant power spectrum when a matter-dominated universe is considered. However for tensor perturbations our equation is completely different from the one obtained using the other approach. In fact, in holonomy corrected LQC, since the equation for tensor perturbations contains two singular points, the corresponding power spectrum is "mode dependent", that is, it is not unambiguously defined. This problem does not appear in teleparallel LQC where,for a matter-dominated universe, we have obtained a ratio of tensor to scalar perturbations of the order 1.
4 pages

 

[MTd2]

http://arxiv.org/abs/1309.0132

Rovelli' s relational quantum mechanics, monism and quantum becoming
Mauro Dorato
(Submitted on 31 Aug 2013)
In this paper I present and defend Rovelli's relation quantum mechanics from some foreseeable objections, so as to clarify its philosophical implications vis a vis rival interpretations. In particular I ask whether RQM presupposes a hidden recourse to both a duality of evolutions and of ontology (the relationality of quantum world and the intrinsicness of the classical world, which in the limit must be recovered from the former). I then concentrate on the pluralistic, antimonistic metaphysical consequences of the theory, due to the impossibility of assigning a state to the quantum universe. Finally, in the last section I note interesting consequences of RQM with respect to the possibility of defining a local, quantum relativistic becoming (in flat spacetimes).Given the difficulties of having the cosmic form of becoming that would be appropriate for priority monism, RQM seems to present an important advantage with respect to monistic views, at least as far as the possibility of explaining our experience of time is concerned.

 

[marcus]

http://arxiv.org/abs/1309.0777
Coupling and thermal equilibrium in general-covariant systems
Goffredo Chirco, Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Sep 2013)
A fully general-covariant formulation of statistical mechanics is still lacking. We take a step toward this theory by studying the meaning of statistical equilibrium for coupled, parametrized systems. We discuss how to couple parametrized systems. We express the thermalization hypothesis in a general-covariant context. This takes the form of vanishing of information flux. An interesting relation emerges between thermal equilibrium and gauge.
8 pages, 3 figures

http://arxiv.org/abs/1309.0652
Non-abelian Gauge Fields from Defects in Spin-Networks
Deepak Vaid
(Submitted on 3 Sep 2013)
Effective gauge fields arise in the description of the dynamics of defects in lattices of graphene in condensed matter. The interactions between neighboring nodes of a lattice/spin-network are described by the Hubbard model whose effective field theory at long distances is given by the Dirac equation for an emergent gauge field. The spin-networks in question can be used to describe the geometry experienced by a non-inertial observer in flat spacetime moving at a constant acceleration in a given direction. We expect such spin-networks to describe the structure of quantum horizons of black holes in loop quantum gravity. We argue that the abelian and non-abelian gauge fields of the Standard Model can be identified with the emergent degrees of freedom required to describe the dynamics of defects in symmetry reduced spin-networks.
6 pages.

http://arxiv.org/abs/1309.0804
On-shell Techniques and Universal Results in Quantum Gravity
N.E.J Bjerrum-Bohr, John F. Donoghue, Pierre Vanhove
(Submitted on 3 Sep 2013)
We compute the leading post-Newtonian and quantum corrections to the Coulomb and Newtonian potentials using the full modern arsenal of on-shell techniques; we employ spinor-helicity variables everywhere, use the Kawai-Lewellen-Tye (KLT) relations to derive gravity amplitudes from gauge theory and use unitarity methods to extract the terms needed at one-loop order. We stress that our results are universal and thus will hold in any quantum theory of gravity with the same low-energy degrees of freedom as we are considering. Previous results for the corrections to the same potentials, derived historically using Feynman graphs, are verified explicitly, but our approach presents a huge simplification, since starting points for the computations are compact and tedious index contractions and various complicated integral reductions are eliminated from the onset, streamlining the derivations. We also analyze the spin dependence of the results using the KLT factorization, and show how the spinless correction in the framework are easily seen to be independent of the interacting matter considered.
34 pages, 7 figures

http://arxiv.org/abs/1309.0713
Projective Structures in Loop Quantum Cosmology
Maximilian Hanusch
(Submitted on 3 Sep 2013)
Projective structures have successfully been used for the construction of measures in the framework of loop quantum gravity. In the present paper we establish such a structure for the space R ⊔ R_Bohr recently constructed in the context of homogeneous isotropic loop quantum cosmology. This space has the advantage to be canonically embedded into the quantum configuration space of the full theory, but, in contrast to the traditional space R_Bohr there exists no Haar measure on R ⊔ R_Bohr. The introduced projective structure, however, allows to construct a family of canonical measures on R ⊔ R_Bohr whose corresponding Hilbert spaces of square integrable functions we finally investigate.
29 pages

brief mention, not Loop-and-allied QG but of general interest:
http://arxiv.org/abs/1309.0773
Quantum Weak Measurements and Cosmology
Paul Davies
(Submitted on 3 Sep 2013)
The indeterminism of quantum mechanics generally permits the independent specification of both an initial and a final condition on the state. Quantum pre-and-post-selection of states opens up a new, experimentally testable, sector of quantum mechanics, when combined with statistical averages of identical weak measurements. In this paper I apply the theory of weak quantum measurements combined with pre-and-post-selection to cosmology. Here, pre-selection means specifying the wave function of the universe or, in a popular semi-classical approximation, the initial quantum state of a subset of quantum fields propagating in a classical back-ground spacetime. The novel feature is post-selection: the additional specification of a condition on the quantum state in the far future. I discuss "natural" final conditions, and show how they may lead to potentially large and observable effects at the present cosmological epoch. I also discuss how pre-and-post-selected quantum contrast to the expectation value of the stress-energy-momentum tensor, resolving a vigorous debate from the 1970's. The paper thus provides a framework for computing large-scale cosmological effects arising from this new sector of quantum mechanics. A simple experimental test is proposed.
15 pages.

http://arxiv.org/abs/1309.0792
Measurements according to "Consistent Quantum Theory"
Elias Okon, Daniel Sudarsky
(Submitted on 3 Sep 2013)
We critically evaluate the treatment of the notion of measurement in the Consistent Histories approach to quantum mechanics. We find such treatment unsatisfactory because it relies, often implicitly, on elements external to the provided formalism. In particular, when dealing with measurement scenarios, the formalism, in order to be informative, needs to assume that after measurements measuring apparatuses are always in states of well defined pointer positions. The problem is that there is nothing in the formalism to justify this assumption. We conclude that the Consistent Histories approach, contrary to what is claimed by its proponents, fails to provide a truly satisfactory resolution to the measurement problem of quantum mechanics.
15 pages

 

[John86]

http://arxiv.org/abs/1309.1090
Sustainable entanglement farming from a quantum field
Eduardo Martin-Martinez, Eric G. Brown, William Donnelly, Achim Kempf
(Submitted on 4 Sep 2013)
We propose a protocol by which entanglement can be extracted repeatedly from a quantum field. In analogy with prior work on entanglement harvesting, we call this protocol entanglement farming. It consists of successively sending pairs of unentangled particles through an optical cavity. Using non-perturbative Gaussian methods, we show that in certain generic circumstances this protocol drives the cavity field towards a non-thermal metastable state. This state of the cavity is such that successive pairs of unentangled particles sent through the cavity will reliably emerge significantly entangled. We calculate thermodynamic aspects of the harvesting process, such as energies and entropies, and also the long-term behavior beyond the few-mode approximation. Significant for possible experimental realizations is the fact that this entangling fixed point state of the cavity is reached largely independently of the initial state in which the cavity was prepared. Our results suggest that reliable entanglement farming on the basis of such a fixed point state should be possible also in various other experimental settings, namely with the to-be-entangled particles replaced by arbitrary qudits and with the cavity replaced by a suitable reservoir system.

http://arxiv.org/abs/1309.1119
A first look at Weyl anomalies in shape dynamics
Henrique Gomes
(Submitted on 4 Sep 2013 (v1), last revised 5 Sep 2013 (this version, v2))
One of the more popular objections towards shape dynamics is the suspicion that anomalies in the spatial Weyl symmetry will arise upon quantization. The purpose of this short paper is to establish the tools required for an investigation of the sort of anomalies that can possibly arise. The first step is to adapt to our setting Barnich and Henneaux's formulation of gauge cohomology in the Hamiltonian setting, which serve to decompose the anomaly into a spatial component and time component. The spatial part of the anomaly, i.e. the anomaly in the symmetry algebra itself ($[\Omega, \Omega]\propto \hbar$ instead of vanishing) is given by a projection of the second ghost cohomology of the Hamiltonian BRST differential associated to $\Omega$, modulo spatial derivatives. The temporal part, $[\Omega, H]\propto\hbar$ is given by a different projection of the first ghost cohomology and an extra piece arising from a solution to a functional differential equation. Assuming locality of the gauge cohomology groups involved, this part is always local. Assuming locality for the gauge cohomology groups, using Barnich and Henneaux's results, the classification of Weyl cohomology for higher ghost numbers performed by Boulanger, and following the descent equations, we find a complete characterizations of anomalies in 3+1 dimensions. The spatial part of the anomaly and the first component of the temporal anomaly are always local given these assumptions even in shape dynamics. The part emerging from the solution of the functional differential equations explicitly involves the shape dynamics Hamiltonian, and thus might be non-local. If one restricts this extra piece of the temporal anomaly to be also local, then overall no \emph{local} Weyl anomalies, either temporal or spatial, emerge in the 3+1 case.

http://arxiv.org/abs/1309.1660
Gauge gravity and discrete quantum models
John W. Barrett, Steven Kerr
(Submitted on 6 Sep 2013)
The gauge gravity action for general relativity in any dimension using a connection for the Euclidean or Poincar\'e group and a symmetry-breaking scalar field is written using a particularly simple matrix technique. A discrete version of the gauge gravity action for variables on a triangulated 3-manifold is given and it is shown how, for a certain class of triangulations of the three-sphere, the discrete quantum model this defines is equivalent to the Ponzano-Regge model of quantum gravity.

http://arxiv.org/abs/1309.1403
Atomism and Relationalism as guiding principles for Quantum Gravity
Francesca Vidotto
(Submitted on 5 Sep 2013)
The research in quantum gravity has jauntily grown in the recent years, intersecting with conceptual and philosophical issues that have a long history. In this paper I analyze the conceptual basis on which Loop Quantum Gravity has grown, the way it deals with some classical problems of philosophy of science and the main methodological and philosophical assumptions on which it is based. In particular, I emphasize the importance that atomism (in the broadest sense) and relationalism have had in the construction of the theory.

http://arxiv.org/abs/1309.1690
Continuum limit in matrix models for quantum gravity from the Functional Renormalization Group
Astrid Eichhorn, Tim Koslowski
(Submitted on 6 Sep 2013)
We consider the double-scaling limit in matrix models for two-dimensional quantum gravity, and establish the nonperturbative functional Renormalization Group as a novel technique to compute the corresponding interacting fixed point of the Renormalization Group flow. We explicitly evaluate critical exponents and compare to the exact results. The functional Renormalization Group method allows a generalization to tensor models for higher-dimensional quantum gravity and to group field theories. As a simple example how this method works for such models, we compute the leading-order beta function for a colored matrix model that is inspired by recent developments in tensor models.

 

[MTd2]

http://arxiv.org/abs/1303.0195

Living in Curved Momentum Space

J. Kowalski-Glikman
(Submitted on 1 Mar 2013 (v1), last revised 10 Sep 2013 (this version, v2))
In this paper we review some aspects of relativistic particles' mechanics in the case of a non-trivial geometry of momentum space. We start with showing how the curved momentum space arises in the theory of gravity in 2+1 dimensions coupled to particles, when (topological) degrees of freedom of gravity are solved for. We argue that there might exist a similar topological phase of quantum gravity in 3+1 dimensions. Then we characterize the main properties of the theory of interacting particles with curved momentum space and the symmetries of the action. We discuss the spacetime picture and the emergence of the principle of relative locality, according to which locality of events is not absolute but becomes observer dependent, in the controllable, relativistic way. We conclude with the detailed review of the most studied kappa-Poincare framework, which corresponds to the de Sitter momentum space.

 

[MTd2]

General curiosity:

http://arxiv.org/abs/1309.2396

Lorenz, Gödel and Penrose: New perspectives on geometry and determinism in fundamental physics
T.N.Palmer
(Submitted on 10 Sep 2013)
Meteorologist Ed Lorenz, pioneer of chaos theory, is well known for his demonstration of `the butterfly effect'. More fundamentally, however, Lorenz's research established a profound link between space-time calculus and state-space fractal geometry. Amazingly, properties of Lorenz's fractal invariant set can be shown to relate space-time calculus to deep areas of mathematics associated with Wiles' proof of Fermat's Last Theorem and G\"{o}del's Incompleteness Theorem. Motivated by this, it is proposed that our theories of fundamental physics should also be framed in terms of state-space geometry rather than the traditional space-time calculus. To develop these ideas more concretely, it is supposed that the universe U is itself a deterministic dynamical system evolving on a fractal invariant set I_U in its state space. Symbolic representations of I_U are constructed explicitly based on permutation representations of quaternions. The resulting `Invariant Set Theory' provides a conspiracy-free causal perspective on fundamental physics from which some key concepts in quantum theory are emergent: incompatible observables, wave-particle duality, Planck's constant and apparent nonlocality; Bell's theorem being nullified by exploiting a finite-precision loophole arising from the fractal geometry of I_U. The complex Hilbert Space of quantum theory emerges as a singular limit of this topological representation of I_U. The primacy of geometry as embodied in the proposed theory extends the principles underpinning general relativity. As a result, the physical basis for contemporary programmes which seek a `quantum theory of gravity' is questioned. Based on the geometry of I_U, an alternative `gravitational theory of the quantum' is proposed. Some meat is put on the bones of Penrose's suggestion that the correct theory of quantum gravity might be a deterministic but non-computable theory.

 

[MTd2]

http://arxiv.org/abs/1209.0881

The Physics of Events: A Potential Foundation for Emergent Space-Time

Kevin H. Knuth, Newshaw Bahreyni
(Submitted on 5 Sep 2012 (v1), last revised 12 Sep 2013 (this version, v2))
Everything that is detected or measured is the direct result of something influencing something else. This is the essence of the concept of force, which has become central to physics. By considering both the act of influencing and the response to such influence as a pair of events, we can describe a universe of interactions as a partially-ordered set of events. In this paper, we take the partially-ordered set of events as a fundamental picture of influence and aim to determine what interesting physics can be recovered. This is accomplished by identifying a means by which events in a partially-ordered set can be aptly and consistently quantified. Since, in general, a partially-ordered set lacks symmetries to constraint any quantification, we propose to distinguish a chain of events, which represents an observer, and quantify some subset of events with respect to the observer chain. We demonstrate that consistent quantification with respect to pairs of observer chains exhibiting a constant relationship with one another results in a metric analogous to the Minkowski metric and that transformation of the quantification with respect to one pair of chains to quantification with respect to another pair of chains results in the Bondi k-calculus, which represents a Lorentz transformation under a simple change of variables. We further demonstrate that chain projection induces geometric structure in the partially-ordered set, which itself is inherently both non-geometric and non-dimensional. Collectively, these results suggest that the concept of space-time geometry may emerge as a unique way for an embedded observer to aptly and consistently quantify a partially-ordered set of events. In addition to having potential implications for space-time physics, this also may serve as a foundation for understanding analogous space-time in condensed matter systems.

 

[marcus]

http://arxiv.org/abs/1309.3261
Uniformly accelerated observer in a thermal bath
Sanved Kolekar
(Submitted on 12 Sep 2013)
We investigate the quantum field aspects in flat spacetime for an uniformly accelerated observer moving in a thermal bath. In particular, we obtain an exact closed expression of the reduced density matrix for an uniformly accelerated observer with acceleration a = 2πT when the state of the quantum field is a thermal bath at temperature T′. We find that the density matrix has a simple form with an effective partition function Z being a product, Z = ZT ZT′ , of two thermal partition functions corresponding to temperatures T and T′ and hence is not thermal, even when T = T′. We show that, even though the partition function has a product structure, the two thermal baths are, in fact, interacting systems; although in the high frequency limit ω_k ≫ T and ω_k ≫ T′, the interactions are found to become sub-dominant. We further demonstrate that the resulting spectrum of the Rindler particles can be interpreted in terms of spontaneous and stimulated emission due to the background thermal bath. The density matrix is also found to be symmetric in the acceleration temperature T and the thermal bath temperature T′ indicating that thermodynamic experiments alone cannot distinguish between the thermal effects due to T and those due to T′. The entanglement entropy associated with the reduced density matrix (with the background contribution of the Davies-Unruh bath removed) is shown to satisfy, in the ωk ≫ T′ limit, a first law of thermodynamics relation of the form TδS = δE where δE is the difference in the energies corresponding to the reduced density matrix and the background Davies-Unruh bath. The implications are discussed.
16 pages

 

[John86]

http://arxiv.org/abs/1309.3999
The measure matters
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 16 Sep 2013)
We adopt a framework where quantum-gravity's dynamical dimensional reduction of spacetime at short distances is described in terms of modified dispersion relations. We observe that by subjecting such models to a momentum-space diffeomorphism one obtains a "dual picture" with unmodified dispersion relations, but a modified measure of integration over momenta. We then find that the UV {\it Hausdorff} dimension of momentum space which can be inferred from this modified integration measure coincides with the short-distance {\it spectral} dimension of spacetime. This result sheds light into why scale-invariant fluctuations are obtained if the original model for two UV spectral dimensions is combined with Einstein gravity. By studying the properties of the inner product we derive the result that it is only in 2 energy-momentum dimensions that microphysical vacuum fluctuations are scale-invariant. This is true ignoring gravity, but then we find that if Einstein gravity is postulated in the original frame, in the dual picture gravity switches off, since all matter becomes conformally coupled. We argue that our findings imply that the following concepts are closely connected: scale-invariance of vacuum quantum fluctuations, conformal invariance of the gravitational coupling, UV reduction to spectral dimension 2 in position space and UV reduction to Hausdorff dimension 2 in energy-momentum space.

 

[John86]

http://arxiv.org/abs/1309.4424
Quantum field theory in the Rindler-Rindler spacetime
Sanved Kolekar, T. Padmanabhan
(Submitted on 17 Sep 2013)
It is well known that Minkowski vacuum appears as a thermal bath in the Rindler spacetime when the modes on the left wedge are traced out. We introduce the concept of a Rindler-Rindler spacetime, obtained by a further coordinate transformation from the Rindler spacetime, in a manner similar to the transformation from inertial to Rindler frame. We show that the Rindler vacuum appears as a thermal state in the Rindler-Rindler frame. Further, the spectrum of particles seen by the Rindler-Rindler observers in the original Minkowski vacuum state is shown to be identical to that seen by detector accelerating through a real thermal bath. Thus the Davies-Unruh effect acts as a proxy for a real thermal bath, for a certain class of observers in the Rindler-Rindler spacetime. We interpret this similarity as indicating further evidence of the indistinguishablity between thermal and quantum fluctuations along the lines of the recent work in arXiv:1308.6289. The implications are briefly discussed.

http://arxiv.org/abs/1309.4084
Unified meta-theory of information, consciousness, time and the classical-quantum universe
Martin A. Green
(Submitted on 16 Sep 2013)
As time advances in our perceived real world, existing information is preserved and new information is added to history. All the information that may ever be encoded in history must be about some fundamental, unique, atemporal and pre-physical structure: the bare world. Scientists invent model worlds to efficiently explain aspects of the real world. This paper explores the features of and relationships between the bare, real, and model worlds. Time -- past, present and future -- is naturally explained. Both quantum uncertainty and state reduction are needed for time to progress, since unpredictable new information must be added to history. Deterministic evolution preserves existing information. Finite, but steadily increasing, information about the bare world is jointly encoded in equally uncertain spacetime geometry and quantum matter. Because geometry holds no information independent of matter, there is no need to quantize gravity. At the origin of time, information goes to zero and geometry and matter fade away.


This gives some food for thought...

 

[atyy]

http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez
(Submitted on 18 Sep 2013)
In loop quantum gravity the quantum states of a black hole horizon are produced by point-like discrete quantum geometry excitations (or punctures) labelled by spin $j$. The excitations possibly carry other internal degrees of freedom also, and the associated quantum states are eigenstates of the area $A$ operator. On the other hand, the appropriately scaled area operator $A/(8\pi\ell)$ is also the physical Hamiltonian associated with the quasilocal stationary observers located at a small distance $\ell$ from the horizon. Thus, the local energy is entirely accounted for by the geometric operator $A$.
We assume that: In a suitable vacuum state with regular energy momentum tensor at and close to the horizon the local temperature measured by stationary observers is the Unruh temperature and the degeneracy of `matter' states is exponential with the area $\exp{(\lambda A/\ell_p^2)}$---this is supported by the well established results of QFT in curved spacetimes, which do not determine $\lambda$ but asserts an exponential behaviour. The geometric excitations of the horizon (punctures) are indistinguishable. In the semiclassical limit the area of the black hole horizon is large in Planck units.
It follows that: Up to quantum corrections, matter degrees of freedom saturate the holographic bound, viz. $\lambda=\frac{1}{4}$. Up to quantum corrections, the statistical black hole entropy coincides with Bekenstein-Hawking entropy $S={A}/({4\ell_p^2})$. The number of horizon punctures goes like $N\propto \sqrt{A/\ell_p^2}$, i.e the number of punctures $N$ remains large in the semiclassical limit. Fluctuations of the horizon area are small while fluctuations of the area of an individual puncture are large. A precise notion of local conformal invariance of the thermal state is recovered in the $A\to\infty$ limit where the near horizon geometry becomes Rindler.

 

[John86]

http://arxiv.org/abs/1309.4480
Scale-free primordial cosmology
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 17 Sep 2013)
The large-scale structure of the universe suggests that the physics underlying its early evolution is scale-free. This was the historic motivation for the Harrison-Zel'dovich-Peebles spectrum and for inflation. Based on a hydrodynamical approach, we identify scale-free forms for the background equation-of-state for both inflationary and cyclic scenarios and use these forms to derive predictions for the spectral tilt and tensor-to-scalar ratio of primordial density perturbations. For the case of inflation, we find three classes of scale-free models with distinct predictions. Including all classes, we show that scale-free inflation predicts tensor-to-scalar ratio $r > 10^{-4}$. We show that the observationally favored class is theoretically disfavored because it suffers from an initial conditions problem and the hydrodynamical form of an unlikeliness problem similar to that identified recently for certain inflaton potentials. We contrast these results with those for scale-free cyclic models.

 

[marcus]

http://arxiv.org/abs/1309.5343
Using Cosmology to Establish the Quantization of Gravity
Lawrence M. Krauss (1,2), Frank Wilczek (3) ((1) Arizona State University, (2) Australian National Univeresity, (3) MIT)
(Submitted on 20 Sep 2013)
While many aspects of general relativity have been tested, and general principles of quantum dynamics demand its quantization, there is no direct evidence for that. It has been argued that development of detectors sensitive to individual gravitons is unlikely, and perhaps impossible. We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity.
3 pages

 

[marcus]

http://arxiv.org/abs/1309.6304
Quantum-Reduced Loop-Gravity: Relation with the Full Theory
Emanuele Alesci, Francesco Cianfrani, Carlo Rovelli
(Submitted on 24 Sep 2013)
The quantum-reduced loop-gravity technique has been introduced for dealing with cosmological models. We show that it can be applied rather generically: anytime the spatial metric can be gauge-fixed to a diagonal form. The technique selects states based on reduced graphs with Livine-Speziale coherent intertwiners and could simplify the analysis of the dynamics in the full theory.
5 pages

 

[John86]

http://arxiv.org/abs/1309.5266
Asymptotic Flatness and Quantum Geometry
Sandipan Sengupta
(Submitted on 20 Sep 2013)
We construct a canonical quantization of the two dimensional theory of a parametrized scalar field on noncompact spatial slices. The kinematics is built upon states that carry a label corresponding to continuous embeddings, in addition to discrete embedding and matter labels. The resulting quantum geometry admits a nondegenerate vacuum metric, which allows a consistent realization of the asymptotic conditions on the canonical fields. However, out of the full classical symmetry group of conformal isometries, only the subgroup of continuous global translations can be implemented. The quantum spacetime as characterised by a gauge invariant state is shown to be made up of discrete strips at the interior, and smooth at asymptotia. The analysis here is expected to be particularly relevant for a loop quantization of asymptotically flat gravity.

 

[marcus]

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013)
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.
75 pages, invited topical review for Classical and Quantum Gravity

 

[marcus]

http://arxiv.org/abs/1309.7296
Astrophysical constraints on Planck scale dissipative phenomena
Stefano Liberati (SISSA and INFN, Trieste), Luca Maccione (LMU and MPP, Munich)
(Submitted on 27 Sep 2013)
The emergence of a classical spacetime from any quantum gravity model is still a subtle and only partially understood issue. If indeed space-time is arising as some sort of large scale condensate of more fundamental objects then it is natural to expect that matter, being a collective excitations of the spacetime constituents, will present modified kinematics at sufficiently high energies. We consider here the phenomenology of the dissipative effects necessarily arising in such a picture. Adopting dissipative hydrodynamics as a general framework for the description of the energy exchange between collective excitations and the spacetime fundamental degrees of freedom, we discuss how rates of decays for elementary particles can be derived from dispersion relations and used to provide strong constraints on the base of current astrophysical observations of high energy particles.
5 pages, 1 figure

http://arxiv.org/abs/1309.7273
Renormalization group flow of Hořava-Lifshitz gravity at low energies
Adriano Contillo, Stefan Rechenberger, Frank Saueressig
(Submitted on 27 Sep 2013)
The functional renormalization group equation for projectable Horava-Lifshitz gravity is used to derive the non-perturbative beta functions for the Newton's constant, cosmological constant and anisotropy parameter. The resulting coupled differential equations are studied in detail and exemplary RG trajectories are constructed numerically. The beta functions possesses a non-Gaussian fixed point and a one-parameter family of Gaussian fixed points. One of the Gaussian fixed points corresponds to the Einstein-Hilbert action with vanishing cosmological constant and constitutes a saddle point with one IR-attractive direction. For RG trajectories dragged into this fixed point at low energies diffeomorphism invariance is restored. The emergence of general relativity from Horava-Lifshitz gravity can thus be understood as a crossover-phenomenon where the IR behavior of the theory is controlled by this Gaussian fixed point. In particular RG trajectories with a tiny positive cosmological constant also come with an anisotropy parameter which is compatible with experimental constraints, providing a mechanism for the approximate restoration of diffeomorphism invariance in the IR. The non-Gaussian fixed point is UV-attractive in all three coupling constants. Most likely, this fixed point is the imprint of Asymptotic Safety at the level of Horava-Lifshitz gravity.
32 pages, 6 figures

http://arxiv.org/abs/1309.7206
Decoherence in quantum cosmology and the cosmological constant
T. Asselmeyer-Maluga, J. Krol
(Submitted on 27 Sep 2013)
We discuss a spacetime having the topology of S³ x R but with a different smoothness structure. The initial state of the cosmos in our model is identified with a wildly embedded 3-sphere (or a fractal space). In previous work we showed that a wild embedding is obtained by a quantization of a usual (or tame) embedding. Then a wild embedding can be identified with a (geometrical) quantum state. During a decoherence process this wild 3-sphere is changed to a homology 3-sphere. We are able to calculate the decoherence time for this process. After the formation of the homology 3-sphere, we obtain a spacetime with an accelerated expansion enforced by a cosmological constant. The calculation of this cosmological constant gives a qualitative agreement with the current measured value.
8 pages, 1 figure, accepted in MPLA

http://arxiv.org/abs/1309.7248
The equations of CCC
Paul Tod
(Submitted on 27 Sep 2013)
I review the equations of Conformal Cyclic Cosmology given by Penrose. I suggest a slight modification to Penrose's prescription and show how this works out for FRW cosmologies and for Class A Bianchi cosmologies.
9 pages. Corrected version of a talk given at the conference `The Mathematics of CCC: Mathematical Physics with Positive Lambda' organised by the Clay Mathematics Institute at the Mathematical Institute, University of Oxford from September 11-13, 2013

http://arxiv.org/abs/1309.7271
A Fundamental Solution to the CCC equation
Ezra Newman
(Submitted on 27 Sep 2013)
We display a simple solution to the Penrose CCC scenario. For this solution we chose for the late stages of the previous aeon a FRW, k=0, universe with a both a cosmological constant and radiation (no mass) while for the early stages of the 'present' aeon we have again a FRW universe, k=0, with the same cosmological constant and again with radiation but with mass not yet present. The Penrose conditions force the parameters describing the radiation of the former and present aeons to be equal and the transition metric in the overlap region turns out to be flat. We further study how different rest-mass zero fields transition between the different conformally related regions. These (test) fields appears to easily allow perturbations of the geometry within the CCC scenario.
13 pages

http://arxiv.org/abs/1309.7267
Cosmology and the Dark Matter Frontier
L. Bergstrom
(Submitted on 27 Sep 2013)
A brief overview is given about some issues in current astroparticle physics, focusing on the dark matter (DM) problem, where the connection to LHC physics is particularly strong. New data from the Planck satellite has made the evidence in favour of the existence of DM even stronger. The favourite, though not the only, candidates for cosmological DM, weakly interacting massive particles (WIMPs), are being probed by a variety of experiments - direct detection through scattering in terrestrial detectors, indirect detection by observing products of annihilation of DM in the Galaxy, and finally searches at accelerators such as the LHC. The field is in the interesting situation that all of these search methods are reaching sensitivities where signals of DM may plausibly soon be found, and a vast array of models will be probed in the next few years. Of course, expectations for a positive signature are high, which calls for caution regarding 'false alarms'. Some of the presently puzzling and partly conflicting pieces of evidence for DM detection are discussed as well as expectations for the future.
15 pages. Invited talk at the Nobel Symposium on LHC Physics, Krusenberg, Sweden, May 13-17, 2013

 

[John86]

http://arxiv.org/abs/1310.0558
A Quantum Model of Exaptation: Incorporating Potentiality into Evolutionary Theory
Liane Gabora, Eric O. Scott, Stuart Kauffman
(Submitted on 2 Oct 2013)
The phenomenon of preadaptation, or exaptation (wherein a trait that originally evolved to solve one problem is co-opted to solve a new problem) presents a formidable challenge to efforts to describe biological phenomena using a classical (Kolmogorovian) mathematical framework. We develop a quantum framework for exaptation with examples from both biological and cultural evolution. The state of a trait is written as a linear superposition of a set of basis states, or possible forms the trait could evolve into, in a complex Hilbert space. These basis states are represented by mutually orthogonal unit vectors, each weighted by an amplitude term. The choice of possible forms (basis states) depends on the adaptive function of interest (e.g., ability to metabolize lactose or thermoregulate), which plays the role of the observable. Observables are represented by self-adjoint operators on the Hilbert space. The possible forms (basis states) corresponding to this adaptive function (observable) are called eigenstates. The framework incorporates key features of exaptation: potentiality, contextuality, nonseparability, and emergence of new features. However, since it requires that one enumerate all possible contexts, its predictive value is limited, consistent with the assertion that there exists no biological equivalent to "laws of motion" by which we can predict the evolution of the biosphere.

http://arxiv.org/abs/1310.0693
Constraining f(R) gravity with PLANCK data on galaxy cluster profiles
I. De Martino, M. De Laurentis, F. Atrio-Barandela, S. Capozziello
(Submitted on 2 Oct 2013)
Models of $f(R)$ gravity that introduce corrections to the Newtonian potential in the weak field limit are tested at the scale of galaxy clusters. These models can explain the dynamics of spiral and elliptical galaxies without resorting to dark matter. We compute the pressure profiles of 579 galaxy clusters assuming that the gas is in hydrostatic equilibrium within the potential well of the modified gravitational field. The predicted profiles are compared with the average profile obtained by stacking the data of our cluster sample in the Planck foreground clean map SMICA. We find that the resulting profiles of these systems fit the data without requiring a dominant dark matter component, with model parameters similar to those required to explain the dynamics of galaxies. Our results do not rule out that clusters are dynamically dominated by Dark Matter but support the idea that Extended Theories of Gravity could provide an explanation to the dynamics of self-gravitating systems and to the present period of accelerated expansion, alternative to the concordance cosmological model.

http://arxiv.org/abs/1310.0675
Can a non-local model of gravity reproduce Dark Matter effects in agreement with MOND?
Ivan Arraut
(Submitted on 2 Oct 2013)
I analyze the possibility of reproducing MONDian Dark Matter effects by using a non-local model of gravity. The model was used before in order to recreate screening effects for the Cosmological Constant ($\Lambda$) value. Although the model in the weak-field approximation (in static coordinates) can reproduce the field equations in agreement with the AQUAL Lagrangian, the solutions are scale dependent and cannot reproduce the same dynamics in agreement with MOND.

 

[marcus]

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

http://arxiv.org/abs/1310.1088
Geometrodynamics and Lorentz symmetry
Derek K. Wise
(Submitted on 3 Oct 2013)
We study the dynamics of gauge theory and general relativity using fields of local observers, thus maintaining local Lorentz symmetry despite a space/time splitting of fields. We start with Yang--Mills theory, where observer fields are defined as normalized future-timelike vector fields. We then define observers without a fixed geometry, and find these play two related roles in general relativity: splitting fields into spatial and temporal parts, and "breaking" gauge symmetry, effectively reducing the spacetime SO(n,1) connection to an observer-dependent spatial SO(n) connection. In both gauge theory and gravity, the observer field reduces the action to canonical form, without using gauge fixing. In the 4d gravity case, the result is a manifestly Lorentz covariant counterpart of the Ashtekar-Barbero formulation. We also explain how this leads geometrically to a picture of general relativity in terms of "observer space" rather than spacetime---a setting where both spacetime symmetry and the dynamical description are simultaneously available.
Comments: 11 pages. Submission for the proceedings of "3Quantum: Algebra, Geometry, Information", Tallinn, July 2012

 

[marcus]

http://arxiv.org/abs/1310.1839
Action and entanglement in gravity and field theory
Yasha Neiman
(Submitted on 7 Oct 2013)
In non-gravitational quantum field theory, the entanglement entropy across a surface depends on the short-distance regularization. Quantum gravity should not require such regularization, and it's been conjectured that the entanglement entropy there is always given by the black hole entropy formula evaluated on the entangling surface. We show that these statements have precise classical counterparts at the level of the action. Specifically, we point out that the action can have a non-additive imaginary part. In gravity, the latter is fixed by the black hole entropy formula, while in non-gravitating theories, it is arbitrary. From these classical facts, the entanglement entropy conjecture follows by heuristically applying the relation between actions and wavefunctions.
4 pages, 2 figures

brief mention:
http://arxiv.org/abs/1310.1524
On Background Independence
Edward Anderson
(Submitted on 5 Oct 2013)
This paper concerns what Background Independence itself is (as opposed to some particular physical theory that is background independent). The notions presented mostly arose from a layer-by-layer analysis of the facets of the Problem of Time in Quantum Gravity... Other aspects of Background Independence include the algebraic closure of these constraints,.., foliation independence as implemented by refoliation invariance, the reconstruction of spacetime from space... Thus these arise naturally and are problems to be resolved, as opposed to avoided `by making one's physics background-dependent in order not to have these problems'...
14 pages including 2 figures

not Loop-and-allied QG but possibly of wider interest:
http://arxiv.org/abs/1310.1605
Limits on anisotropic inflation from the Planck data
Jaiseung Kim, Eiichiro Komatsu
(Submitted on 6 Oct 2013)
[my comment: Komatsu is a top cosmologist and it tends to be worth paying attention to what he has to say IMHO. Plus this has to do with several active controversies.]

 

[John86]

http://arxiv.org/abs/1310.1667
Information-Based Physics: An Observer-Centric Foundation
Kevin H. Knuth
(Submitted on 7 Oct 2013)
It is generally believed that physical laws, reflecting an inherent order in the universe, are ordained by nature. However, in modern physics the observer plays a central role raising questions about how an observer-centric physics can result in laws apparently worthy of a universal nature-centric physics. Over the last decade, we have found that the consistent apt quantification of algebraic and order-theoretic structures results in calculi that possesses constraint equations taking the form of what are often considered to be physical laws. I review recent derivations of the formal relations among relevant variables central to special relativity, probability theory and quantum mechanics in this context by considering a problem where two observers form consistent descriptions of and make optimal inferences about a free particle that simply influences them. I show that this approach to describing such a particle based only on available information leads to the mathematics of relativistic quantum mechanics as well as a description of a free particle that reproduces many of the basic properties of a fermion. The result is an approach to foundational physics where laws derive from both consistent descriptions and optimal information-based inferences made by embedded observers.

http://arxiv.org/abs/1310.1699
Conformal geometrodynamics regained: gravity from duality
Henrique Gomes
(Submitted on 7 Oct 2013)
I propose the following conjecture: the conformal reduction of Hamiltonian general relativity is the sole reduced theory that allows description in the canonical metric phase space by \emph{dual} spatially covariant theories, each possessing different symmetry content than the other. One of the symmetries is the usual refoliation symmetry of general relativity in 3+1, and the other, its dual, is spatial Weyl symmetry. I prove the conjecture under mild extra assumptions.

 

[marcus]

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

brief mention:
http://arxiv.org/abs/1310.1920
Where the world stands still: turnaround as a strong test of ΛCDM cosmology
Vasiliki Pavlidou, Theodore N. Tomaras
(Submitted on 7 Oct 2013)
Structure formation in ΛCDM cosmology is a cosmic battle between...[expansion and gravity]... Our intuitive understanding of the process works best in scales small enough so that bound, relaxed gravitating systems are no longer adjusting their radius; and large enough so that space and matter follow the average expansion of the Universe. Yet one of the most robust predictions of ΛCDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from the center of a bound structure. The maximum possible value of the turnaround radius within the framework of the ΛCDM model is, for a given mass M, equal to (3GM/Λc²)^1/3, ... independently of cosmic epoch, detailed assumptions, exact nature of dark matter, or baryonic effects. We discuss the possible use of this prediction as an observational test for ΛCDM cosmology.
5 pages, 1 figure

 

[marcus]

brief mention (not Loop-and-allied QG, but possibly of general interest):
http://arxiv.org/abs/1310.3225
A Turing test for free will
Seth Lloyd
(Submitted on 11 Oct 2013)
Before Alan Turing made his crucial contributions to the theory of computation, he studied the question of whether quantum mechanics could throw light on the nature of free will. This article investigates the roles of quantum mechanics and computation in free will. Although quantum mechanics implies that events are intrinsically unpredictable, the 'pure stochasticity' of quantum mechanics adds only randomness to decision making processes, not freedom. By contrast, the theory of computation implies that even when our decisions arise from a completely deterministic decision-making process, the outcomes of that process can be intrinsically unpredictable, even to -- especially to -- ourselves. I argue that this intrinsic computational unpredictability of the decision making process is what give rise to our impression that we possesses free will. Finally, I propose a 'Turing test' for free will: a decision maker who passes this test will tend to believe that he, she, or it possesses free will, whether the world is deterministic or not.
20 pages, published in Philosophical Transactions of the Royal Society, Series A.

 

[marcus]

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

http://arxiv.org/abs/1310.3736
Tensorial methods and renormalization in Group Field Theories
Sylvain Carrozza
(Submitted on 14 Oct 2013)
In this thesis, we study the structure of Group Field Theories (GFTs) from the point of view of renormalization theory. Such quantum field theories are found in approaches to quantum gravity related to Loop Quantum Gravity (LQG) on the one hand, and to matrix models and tensor models on the other hand. They model quantum space-time, in the sense that their Feynman amplitudes label triangulations, which can be understood as transition amplitudes between LQG spin network states. The question of renormalizability is crucial if one wants to establish interesting GFTs as well-defined (perturbative) quantum field theories, and in a second step connect them to known infrared gravitational physics. Relying on recently developed tensorial tools, this thesis explores the GFT formalism in two complementary directions. First, new results on the large cut-off expansion of the colored Boulatov-Ooguri models allow to explore further a non-perturbative regime in which infinitely many degrees of freedom contribute. The second set of results provide a new rigorous framework for the renormalization of so-called Tensorial GFTs (TGFTs) with gauge invariance condition. In particular, a non-trivial 3d TGFT with gauge group SU(2) is proven just-renormalizable at the perturbative level, hence opening the way to applications of the formalism to (3d Euclidean) quantum gravity.
229 pages, many figures. PhD thesis, partly based on arXiv:1104.5158, arXiv:1203.5082, arXiv:1207.6734 and arXiv:1303.6772

http://arxiv.org/abs/1310.3759
A pure Dirac's method for Husain-Kuchar theory
Alberto Escalante, J. Berra
(Submitted on 27 Sep 2013)
A pure Dirac's canonical analysis, defined in the full phase space for the Husain-Kuchar model is discussed in detail. This approach allows us to determine the extended action, the extended Hamiltonian, the complete constraint algebra and the gauge transformations for all variables that occur in the action principle. The complete set of constraints defined on the full phase space allow us to calculate the Dirac algebra structure of the theory and a local weighted measure for the path integral quantization method. Finally, we discuss briefly the necessary mathematical structure to perform the canonical quantization program within the framework of the loop quantum gravity approach.
12 pages.

 

[John86]

http://arxiv.org/abs/1310.4143
Dynamical Emergence of Universal Horizons during the formation of Black Holes
Mehdi Saravani, Niayesh Afshordi, Robert B. Mann
(Submitted on 15 Oct 2013)
Motivations for the existence of a fundamental preferred frame range from pure phenomenology to attempts to solve the non-renormalizability of quantum gravity, the problem of time (and scale), and the cosmological constant problem(s). In many explicit constructions, such as Einstein-Aether or Gravitational Aether theories, K-essence, Cuscuton theory, Shape Dynamics, or (non-projectable) Horava-Lifshitz gravity, the low energy theory contains a fluid (which defines a preferred frame) with superluminal or incompressible excitations. We study here the formation of black holes in the presence of such a fluid. In particular, we focus on the incompressible limit of the fluid (or Constant Mean Curvature foliation) in the space-time of a spherically collapsing shell within an asymptotically cosmological space-time. In this case, ignoring the fluid back reaction, we can analytically show that an observer inside 3/4 of the Schwarzschild radius cannot send a signal outside, after a stage in collapse, even using signals that propagate infinitely fast in the preferred frame. This confirms the dynamical emergence of universal horizons that have been previously found in static solutions. We argue that this universal horizon should be considered as the future boundary of the classical space-time.

 

[John86]

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013)
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.

http://arxiv.org/abs/1310.4180
Holographic Charged Renyi Entropies
Alexandre Belin, Ling-Yan Hung, Alexander Maloney, Shunji Matsuura, Robert C. Myers, Todd Sierens
(Submitted on 15 Oct 2013)
We construct a new class of entanglement measures by extending the usual definition of Renyi entropy to include a chemical potential. These charged Renyi entropies measure the degree of entanglement in different charge sectors of the theory and are given by Euclidean path integrals with the insertion of a Wilson line encircling the entangling surface. We compute these entropies for a spherical entangling surface in CFT's with holographic duals, where they are related to entropies of charged black holes with hyperbolic horizons. We also compute charged Renyi entropies in free field theories.

 

[marcus]

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

http://arxiv.org/abs/1310.4771
Astrophysical black holes may radiate, but they do not evaporate
George F R Ellis
(Submitted on 17 Oct 2013)
This paper argues that the effect of Hawking radiation on an astrophysical black hole is not total evaporation of the black hole; rather there will always be a remnant. The key point is that the locus of emission of Hawking radiation is not the globally defined event horizon, but rather is just outside a timelike Marginal Outer Trapped Surface that is locally defined and lies inside the event horizon, close to the collapsing star. A spacelike singularity forms behind the event horizon, and most of the Hawking radiation ends up at this singularity rather than at infinity. Whether any Hawking radiation reaches infinity depends on the relation between the dynamic radiation emission surface and the event horizon, which is affected both by Cosmic Background Radiation and by back-reaction due to the Hawking radiation. From the outside view, even if radiation is seen as always being emitted, the black hole never evaporates away, rather its mass and entropy asymptote to finite non-zero limits. The argument is based on the broad nature of the processes at work, plus a careful delineation of the relevant causal domains; detailed calculations of back reaction effects are necessary in order to confirm this model and determine details of the outcome.
52 pages, 9 Figures, 4 Tables

 

[John86]

http://arxiv.org/abs/1308.5210
Consistent Probabilities in Perfect Fluid Quantum Universes
Clécio R. Bom, Nelson Pinto-Neto, Grasiele B. Santos
(Submitted on 23 Aug 2013 (v1), last revised 26 Aug 2013 (this version, v2))
Recently it has been claimed that the Wheeler-DeWitt quantization of gravity is unable to avoid cosmological singularities. However, in order to make this assertion, one must specify the underlying interpretation of quantum mechanics which has been adopted. For instance, several nonsingular models were obtained in Wheeler-DeWitt quantum cosmology in the framework of the de Broglie-Bohm quantum theory. Conversely, there are specific situations where the singularity cannot be avoided in the framework of the Consistent Histories approach to quantum mechanics. In these specific situations, the matter content is described by a scalar field, and the Wheeler-DeWitt equation looks-like a Klein-Gordon equation. The aim of this work is to study the Wheeler-DeWitt quantization of cosmological models where the matter content is described by an hydrodynamical perfect fluid, where the Wheeler-DeWitt equation reduces to a genuine Schr\"odinger equation. In this case, it is shown that the conclusions of the Consistent Histories and the de Broglie-Bohm approaches coincide in the quantum cosmological models where the curvature of the spatial sections is not positive definite, namely, that the cosmological singularities are eliminated. In the case of positive spatial curvature, the family of histories is no longer consistent, and no conclusion can be given in this framework.

http://arxiv.org/abs/1310.1600
Universality of Phase Transition Dynamics: Topological Defects from Symmetry Breaking
Adolfo del Campo, Wojciech H. Zurek
(Submitted on 6 Oct 2013 (v1), last revised 15 Oct 2013 (this version, v2))
In the course of a non-equilibrium continuous phase transition, the dynamics ceases to be adiabatic in the vicinity of the critical point as a result of the critical slowing down (the divergence of the relaxation time in the neighborhood of the critical point). This enforces a local choice of the broken symmetry and can lead to the formation of topological defects. The Kibble-Zurek mechanism (KZM) was developed to describe the associated nonequilibrium dynamics and to estimate the density of defects as a function of the quench rate through the transition. During recent years, several new experiments investigating formation of defects in phase transitions induced by a quench both in classical and quantum mechanical systems were carried out. At the same time, some established results were called into question. We review and analyze the Kibble-Zurek mechanism focusing in particular on this surge of activity, and suggest possible directions for further progress.

http://arxiv.org/abs/1310.4691
Time from quantum entanglement: an experimental illustration
Ekaterina Moreva, Giorgio Brida, Marco Gramegna, Vittorio Giovannetti, Lorenzo Maccone, Marco Genovese
(Submitted on 17 Oct 2013)
In the last years several theoretical papers discussed if time can be an emergent propertiy deriving from quantum correlations. Here, to provide an insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters' mechanism of "static" time, and Gambini et al. subsequent refinements. A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: an "internal" observer that becomes correlated with the clock photon sees the other system evolve, while an "external" observer that only observes global properties of the two photons can prove it is static.

 

[MTd2]

Because of the author:

http://arxiv.org/abs/1310.4957

Nonlocality in string theory

Gianluca Calcagni, Leonardo Modesto
(Submitted on 18 Oct 2013)
We discuss an aspect of string theory which has been tackled under many different perspectives, but incompletely: the role of nonlocality in the theory and its relation with the geometric shape of the string. In particular, we will describe in quantitative terms how one can zoom out an extended object such as the string so that, at sufficiently large scales, it appears structureless. Since there are no free parameters in free string theory, the notion of large scales will be univocally determined. In other words, we will be able to answer the question: How and at which scale can the string be seen as a particle? In doing so, we will employ the concept of spectral dimension in a new way with respect to its usual applications in quantum gravity. The operational notions of worldsheet and target spacetime dimension in string theory are also clarified and found to be in mutual agreement.

 

[marcus]

http://arxiv.org/abs/1310.5167
A Gravitational Origin of the Arrows of Time
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 18 Oct 2013)
The only widely accepted explanation for the various arrows of time that everywhere and at all epochs point in the same direction is the `past hypothesis': the Universe had a very special low-entropy initial state. We present the first evidence for an alternative conjecture: the arrows exist in all solutions of the gravitational law that governs the Universe and arise because the space of its true degrees of freedom (shape space) is asymmetric. We prove our conjecture for arrows of complexity and information in the Newtonian N-body problem. Except for a set of measure zero, all of its solutions for non-negative energy divide at a uniquely defined point into two halves. In each a well-defined measure of complexity fluctuates but grows irreversibly between rising bounds from that point. Structures that store dynamical information are created as the complexity grows. Recognition of the division is a key novelty of our approach. Each solution can be viewed as having a single past and two distinct futures emerging from it. Any internal observer must be in one half of the solution and will only be aware of one past and one future. The 'paradox' of a time-symmetric law that leads to observationally irreversible behaviour is fully resolved. General Relativity shares enough architectonic structure with the N-body problem for us to prove the existence of analogous complexity arrows in the vacuum Bianchi IX model. In the absence of non-trivial solutions with matter we cannot prove that arrows of dynamical information will arise in GR, though they have in our Universe. Finally, we indicate how the other arrows of time could arise.
44 pages + 14 pages appendices and references. 8 figures and 1 table

general interest:
http://arxiv.org/abs/1310.5217
Dark Matter 2013
Marc Schumann
(Submitted on 19 Oct 2013)
This article reviews the status of the exciting and fastly evolving field of dark matter research as of summer 2013, when it was discussed at ICRC 2013 in Rio de Janeiro. It focuses on the three main avenues to detect WIMP dark matter: direct detection, indirect detection and collider searches. The article is based on the dark matter rapporteur talk summarizing the presentations given at the conference, filling some gaps for completeness.
9 pages, 7 figures. To appear in the proceedings of ICRC 2013

 

[John86]

http://arxiv.org/abs/1310.5412
Transient Weak-Lensing by Cosmological Dark Matter Microhaloes
Sohrab Rahvar, Shant Baghram, Niayesh Afshordi
(Submitted on 21 Oct 2013)
We study the time variation of the apparent flux of cosmological point sources due to the transient weak lensing by dark matter microhaloes. Assuming a transverse motion of microhaloes with respect to our line of sight, we derive the correspondence between the temporal power spectrum of the weak lensing magnification, and the spatial power spectrum of density on small scales. Considering different approximations for the small scale structure of dark matter, we predict the apparent magnitude of cosmological point sources to vary by as much as , due to this effect, within a period of a few months. This red photometric noise has an almost perfect gaussian statistics, to one part in . We also compare the transient weak lensing power spectrum with the background effects such as the stellar microlensing on cosmological scales. A quasar lensed by a galaxy or cluster like SDSSJ1004+4112 strong lensing system, with multiple images, is a suitable system for this study as: (i) using the time-delay method between different images, we can remove the intrinsic variations of the quasar, and (ii) strong lensing enhances signals from the transient weak lensing. We also require the images to form at large angular separations from the center of the lensing structure, in order to minimize contamination by the stellar microlensing. With long-term monitoring of quasar strong lensing systems with a 10-meter class telescope, we can examine the existence of dark microhaloes as the building blocks of dark matter structures. Failure to detect this signal may either be caused by a breakdown of cold dark matter (CDM) hierarchy on small scales, or rather interpreted as evidence against CDM paradigm, e.g. in favor of modified gravity models.

http://arxiv.org/abs/1310.5115
Undoing the twist: the Hořava limit of Einstein-aether
Ted Jacobson
(Submitted on 18 Oct 2013)
Ho\v{r}ava gravity can be obtained from Einstein-aether theory in the limit that the twist coupling constant goes to infinity, while holding fixed the expansion, shear and acceleration couplings. This limit helps to clarify the relation between the two theories, and allows Ho\v{r}ava results to be obtained from Einstein-aether ones. This is illustrated with several examples, including rotating black hole equations, PPN parameters, and radiation rates from binary systems.

 

[marcus]

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

 

[marcus]

http://arxiv.org/abs/1310.6095
A Rotating Black Hole Solution for Shape Dynamics
Henrique Gomes, Gabriel Herczeg
(Submitted on 23 Oct 2013)
Shape dynamics is a classical theory of gravity which agrees with general relativity in many important aspects, but which possesses different gauge symmetries and can present some fundamental global differences with respect to Einstein spacetimes. Here, we present a general procedure for (locally) mapping stationary, axisymmetric general relativity solutions onto their shape dynamic counterparts. We focus in particular on the rotating black hole solution for shape dynamics and show that many of the properties of the spherically symmetric solution are preserved in the extension to the axisymmetric case: it is also free of physical singularities, it does not form a space-time at the horizon, and it possesses an inversion symmetry about the horizon.
13 pages

 

[John86]

http://arxiv.org/abs/1310.5700
Effective field theory models for nonviolent information transfer from black holes
Steven B. Giddings, Yinbo Shi
(Submitted on 21 Oct 2013)
Transfer of quantum information from the interior of a black hole to its atmosphere is described, in models based on effective field theory. This description illustrates that such transfer need not be violent to the semiclassical geometry or to infalling observers, and in particular can avoid producing a singular horizon or "firewall." One can specifically quantify the rate of information transfer, and show that a rate necessary to unitarize black hole evaporation produces a relatively mild modification to the stress tensor near the horizon. In an exterior description of the transfer, the new interactions responsible for it are approximated by "effective sources" acting on fields in the black hole atmosphere. If the necessary interactions couple to general modes in the black hole atmosphere, one also finds a straightforward mechanism for information transfer rates to increase when a black hole is mined, avoiding paradoxical behavior. Correspondence limits are discussed, in the presence of such new interactions, for both small black holes and large ones; the near-horizon description of the latter is approximately that of Rindler space.

http://arxiv.org/abs/1310.6052
Holographic Space-time and Newton's Law
Tom Banks, Willy Fischler
(Submitted on 22 Oct 2013)
We derive Newton's Law from the formalism of Holographic Space-Time (HST). More precisely, we show that for a large class of Hamiltonians of the type proposed previously for the HST description of a geodesic in Minkowski space, the eikonal for scattering of two massless particles at large impact parameter scales as expected with the impact parameter and the energies of the particles in the center of mass (CM) frame. We also discuss the criteria for black hole production in this collision, and find an estimate, purely within the HST framework, for the impact parameter at which it sets in, which coincides with the estimate based on general relativity.

 

[marcus]

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

http://arxiv.org/abs/1310.6399
Inflationary Attractors and their Measures
Alejandro Corichi, David Sloan
(Submitted on 23 Oct 2013)
Several recent misconceptions about the measure problem in inflation and the nature of inflationary attractors are addressed. We show that within the Hamiltonian system of flat Friedmann-Lemaître-Robertson-Walker cosmology coupled to a massive scalar field, the focussing of the Liouville measure on attractor solutions is brought about by a spread in a gauge degree of freedom - the spatial volume. Using this we show how the Liouville measure formulated on a surface of constant Hubble rate induces a probability distribution function on surfaces of other Hubble rates, and the attractor behaviour is seen through the focussing of this function on a narrow range of physical observables. One can conclude then that standard techniques from Hamiltonian dynamics suffice to provide a satisfactory description of attractor solutions and the measure problem.
6 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1310.7426
Nonequivalence of equivalence principles
Eolo Di Casola, Stefano Liberati, Sebastiano Sonego
(Submitted on 28 Oct 2013)
Equivalence principles played a central role in the development of general relativity. Furthermore, they have provided operative procedures for testing the validity of general relativity, or constraining competing theories of gravitation. This has led to a flourishing of different, and inequivalent, formulations of these principles, with the undesired consequence that often the same name, "equivalence principle", is associated with statements having a quite different physical meaning. In this paper we provide a precise formulation of the several incarnations of the equivalence principle, clarifying their uses and reciprocal relations. We also discuss their possible role as selecting principles in the design and classification of viable theories of gravitation.
10 pages; submitted to Am. J. Phys

remote interest:
http://arxiv.org/abs/1310.7121

 

[marcus]

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