Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1307.0256
Quantum astrometric observables II: time delay in linearized quantum gravity
Béatrice Bonga, Igor Khavkine
(Submitted on 1 Jul 2013)
A clock synchronization thought experiment is modeled by a diffeomorphism invariant "time delay" observable. In a sense, this observable probes the causal structure of the ambient Lorentzian spacetime. Thus, upon quantization, it is sensitive to the long expected smearing of the light cone by vacuum fluctuations in quantum gravity. After perturbative linearization, its mean and variance are computed in the Minkowski Fock vacuum of linearized gravity. The na\"ive divergence of the variance is meaningfully regularized by a length scale $\mu$, the physical detector resolution. This is the first time vacuum fluctuations have been fully taken into account in a similar calculation. Despite some drawbacks this calculation provides a useful template for the study of a large class of similar observables in quantum gravity. Due to their large volume, intermediate calculations were performed using computer algebra software. The resulting variance scales like $(s \ell_p/\mu)^2$, where $\ell_p$ is the Planck length and $s$ is the distance scale separating the ("lab" and "probe") clocks. Additionally, the variance depends on the relative velocity of the lab and the probe, diverging for low velocities. This puzzling behavior may be due to an oversimplified detector resolution model or a neglected second order term in the time delay.

http://arxiv.org/abs/1307.0745
Rainbow gravity and scale-invariant fluctuations
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 2 Jul 2013)
We re-examine a recently proposed scenario where the deformed dispersion relations associated with a flow of the spectral dimension to a UV value of 2 leads to a scale-invariant spectrum of cosmological fluctuations, without the need for inflation. In that scenario Einstein gravity was assumed. The theory displays a wavelength-dependent speed of light but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. We find that the ensuing rainbow gravity theory is such that gravity switches off at high energy (or at least leads to a universal conformal coupling). This explains why the fluctuations are scale-invariant on all scales: there is no horizon scale as such. For dispersion relations that do not lead to exact scale invariance we find instead esoteric inflation in the rainbow frame. We argue that these results shed light on the behaviour of gravity under the phenomenon of dimensional reduction

http://arxiv.org/abs/1307.0722
Linear lattice gauge theory
C.Wetterich
(Submitted on 2 Jul 2013)
Linear lattice gauge theory is based on link variables that are arbitrary complex or real $N\times N$ matrices. This contrasts with the usual (non-linear) formulation with unitary or orthogonal matrices. The additional degrees of freedom correspond to massive particles. We discuss a limit in parameter space where linear lattice gauge theory becomes equivalent to the standard formulation. We argue that the continuum limit of linear lattice gauge theory may be a useful setting for an analytic description of confinement. The running gauge coupling corresponds to the flow of the minimum of a "link potential". This minimum occurs for nonzero values $l_0$ in the perturbative regime, while $l_0$ vanishes in the confinement regime.

http://arxiv.org/abs/1307.0246
Noisy soccer balls
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
(Submitted on 30 Jun 2013)
In her Comment arXiv:1202.4066 [hep-th] Hossenfelder proposes a generalization of the results we reported in Phys. Rev. D84 (2011) 087702 and argues that thermal fluctuations introduce incurable pathologies for the description of macroscopic bodies in the relative-locality framework. We here show that Hossenfelder's analysis, while raising a very interesting point, is incomplete and leads to incorrect conclusions. Her estimate for the fluctuations did not take into account some contributions from the geometry of momentum space which must be included at the relevant order of approximation. Using the full expression here derived one finds that thermal fluctuations are not in general large for macroscopic bodies in the relative-locality framework. We find that such corrections can be unexpectedly large only for some choices of momentum-space geometry, and we comment on the possibility of developing a phenomenology suitable for possibly ruling out such geometries of momentum space.

 

[MTd2]

Relative Locality in $κ$-Poincaré

http://arxiv.org/abs/1106.5710

Giulia Gubitosi, Flavio Mercati
(Submitted on 28 Jun 2011 (v1), last revised 3 Jul 2013 (this version, v2))
We show that the $\kappa$-Poincar\'e Hopf algebra can be interpreted in the framework of curved momentum space leading to the relativity of locality \cite{AFKS}. We study the geometric properties of the momentum space described by $\kappa$-Poincar\'e, and derive the consequences for particles propagation and energy-momentum conservation laws in interaction vertices, obtaining for the first time a coherent and fully workable model of the deformed relativistic kinematics implied by $\kappa$-Poincar\'e. We describe the action of boost transformations on multi-particles systems, showing that in order to keep covariant the composed momenta it is necessary to introduce a dependence of the rapidity parameter on the particles momenta themselves. Finally, we show that this particular form of the boost transformations keeps the validity of the relativity principle, demonstrating the invariance of the equations of motion under boost transformations.

 

[marcus]

http://arxiv.org/abs/1307.1420
Approximation methods in Loop Quantum Cosmology: From Gowdy cosmologies to inhomogeneous models in Friedmann-Robertson-Walker geometries
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
(Submitted on 4 Jul 2013)
We develop approximation methods in the hybrid quantization of the Gowdy model with linear polarization and a massless scalar field, for the case of three-torus spatial topology. The loop quantization of the homogeneous gravitational sector of the Gowdy model (according to the improved dynamics prescription) and the presence of inhomogeneities lead to a very complicated Hamiltonian constraint. Therefore, the extraction of physical results calls for the introduction of well justified approximations. We first show how to approximate the homogeneous part of the Hamiltonian constraint, corresponding to Bianchi I geometries, as if it described a Friedmann-Robertson-Walker (FRW) model corrected with anisotropies. This approximation is valid in the high-energy sector of the FRW geometry (concerning its contribution to the constraint) and for anisotropy profiles that are sufficiently smooth. In addition, for certain families of states associated to regimes of physical interest, with negligible effects of the anisotropies and small inhomogeneities, one can approximate the Hamiltonian constraint of the inhomogeneous system by that of an FRW geometry with a relatively simple matter content, and then obtain its solutions.
20 pages, 3 figures

not QG, however possibly of general interest:
http://arxiv.org/abs/1307.1331
A new perspective on CP and T violation
Abhay Ashtekar
(Submitted on 4 Jul 2013)
It is shown that the results of CP and T violation experiments can be interpreted using a very general framework that does not require a Hilbert space of states or linear operators to represent the symmetries or dynamics. Analysis within this general framework brings out the aspects of quantum mechanics that are essential for this interpretation. More importantly, should quantum mechanics be eventually replaced by a new paradigm, this framework could still be used to establish violation of CP and T invariance from the already known experimental results.
This work arose as a 'formal response' to a talk "Three Merry Roads to T-Violation" by Dr. Bryan Roberts [1]. Both talks were given at a "Workshop on Cosmology and Time" held at Penn State in April 2013, which brought together physicists and philosophers of science.
10 pages; 'Formal Response' to the talk "Three Merry Roads to T-Violation" by Bryan Roberts at the Workshop on Cosmology and Time, held at the Institute for Gravitation and the Cosmos, Penn State, in April 2013

 

[marcus]

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski
(Submitted on 5 Jul 2013)
We study the asymptotic geometry of the spin foam partition function for a large class of models, including the models of Barrett and Crane, Engle, Pereira, Rovelli and Livine, and, Freidel and Krasnov.
The asymptotics is taken with respect to the boundary spins only, no assumption of large spins is made in the interior. We give a sufficient criterion for the existence of the partition function. We find that geometric boundary data is suppressed unless its interior continuation satisfies certain accidental curvature constraints. This means in particular that most Regge manifolds are suppressed in the asymptotic regime. We discuss this explicitly for the case of the configurations arising in the 3-3 Pachner move. We identify the origin of these accidental curvature constraints as an incorrect twisting of the face amplitude upon introduction of the Immirzi parameter and propose a way to resolve this problem, albeit at the price of losing the connection to the SU(2) boundary Hilbert space.
The key methodological innovation that enables these results is the introduction of the notion of wave front sets, and the adaptation of tools for their study from micro local analysis to the case of spin foam partition functions.
63 pages, 5 figures
Note: Frank Hellmann is giving a plenary talk at Loops 2013 later this month, quite possibly the topic will be holonomy spin foams, but i haven't yet seen the title of his talk listed.

 

[marcus]

http://arxiv.org/abs/1307.1848
High Energy Physics - Theory
Local Conformal Symmetry in Physics and Cosmology
Itzhak Bars, Paul Steinhardt, Neil Turok
(Submitted on 7 Jul 2013)
We review some of the arguments for why scale symmetry may be a fundamental principle in nature and, if so, why it is likely to be manifest as a local conformal symmetry including gravity. We show how to lift a generic non-scale invariant theory in Einstein frame into a Weyl-invariant theory and present the general form for Lagrangians consistent with Weyl symmetry. Various applications to cosmology are then discussed: the construction of classically geodesically complete cosmologies, the determination of initial conditions after the big bang, inflation, the metastability of the Higgs and cyclic cosmology. As examples, we focus on the standard model and Higgs cosmology, exploring the notion that the Higgs alone could be sufficient to explain the stages of cosmic evolution after (and perhaps before) the big bang and the large-scale features of the universe.
28 pages, 1 figure

brief mention:
http://arxiv.org/abs/1307.2060
Pseudo Weyl invariance is still anomalous
Enrique Alvarez, Mario Herrero-Valea
(Submitted on 8 Jul 2013)
A Weyl invariant extension of Einstein gravity is studied. It simply consists in the group averaging of Einstein's action under Weyl transformations. Contradicting cherished beliefs, a conformal anomaly is found in the trace of the equations of motion ...It is however possible to keep Weyl invariance as a bona fide symmetry at the price of losing full diffeomorphism invariance. This is what happens in unimodular gravity, a closely related theory.
19 pages

 

[marcus]

http://arxiv.org/abs/1307.2227
The two faces of Hawking radiation
Matteo Smerlak
(Submitted on 5 Jul 2013)
What happens when Alice falls into a black hole? In spite of recent challenges by Almheiri et al. -- the ""firewall" hypothesis -- the consensus on this question tends to remain "nothing special". Here I argue that something rather special can happen near the horizon, already at the semiclassical level: besides the standard Hawking outgoing modes, Alice can record a quasi-thermal spectrum of ingoing modes, whose temperature and intensity diverges as Alice's Killing energy E goes to zero. I suggest that this effect can be thought of in terms a "horizon-infinity duality", which relates the perception of near-horizon and asymptotic geodesic observers -- the two faces of Hawking radiation.
7 pages Honorable Mention in the Gravity Research Foundation 2013 Essay Competition

 

[marcus]

http://arxiv.org/abs/1307.2719
Deformations of Polyhedra and Polygons by the Unitary Group
Etera R. Livine
(Submitted on 10 Jul 2013)
We introduce the set of framed (convex) polyhedra with N faces as the symplectic quotient C^2N //SU(2). A framed polyhedron is then parametrized by N spinors living in C² satisfying suitable closure constraints and defines a usual convex polyhedron plus extra U(1) phases attached to each face. We show that there is a natural action of the unitary group U(N) on this phase space, which changes the shape of faces and allows to map any (framed) polyhedron onto any other with the same total (boundary) area. This identifies the space of framed polyhedra to the Grassmannian space U(N )/ (SU(2)×U(N −2)). We show how to write averages of geometrical observables (polynomials in the faces’ area and the angles between them) over the ensemble of polyhedra (distributed uniformly with respect to the Haar measure on U(N)) as polynomial integrals over the unitary group and we provide a few methods to compute these integrals systematically. We also use the Itzykson-Zuber formula from matrix models as the generating function for these averages and correlations.
In the quantum case, a canonical quantization of the framed polyhedron phase space leads to the Hilbert space of SU(2) intertwiners (or, in other words, SU(2)-invariant states in tensor products of irreducible representations). The total boundary area as well as the individual face areas are quantized as half-integers (spins), and the Hilbert spaces for fixed total area form irreducible representations of U(N). We define semi-classical coherent intertwiner states peaked on classical framed polyhedra and transforming consistently under U(N) transformations. And we show how the U(N) character formula for unitary transformations is to be considered as an extension of the Itzykson-Zuber to the quantum level and generates the traces of all polynomial observables over the Hilbert space of intertwiners.
We finally apply the same formalism to two dimensions and show that classical (convex) polygons can be described in a similar fashion trading the unitary group for the orthogonal group. We conclude with a discussion of the possible (deformation) dynamics that one can define on the space of polygons or polyhedra. This work is a priori useful in the context of discrete geometry but it should hopefully also be relevant to (loop) quantum gravity in 2+1 and 3+1 dimensions when the quantum geometry is defined in terms of gluing of (quantized) polygons and polyhedra.
33 pages

 

[marcus]

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto
(Submitted on 11 Jul 2013)
A simple argument indicates that covariant loop gravity (spinfoam theory) predicts a maximal acceleration, and hence forbids the development of curvature singularities. This supports the results obtained for cosmology and black holes using canonical methods.
4 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1102.1592

From Dimensional Reduction of 4d Spin Foam Model to Adding Non-Gravitational Fields to 3d Spin Foam Model
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 14 Jul 2013 (this version, v5))
A Kaluza-Klein like approach for a 4d spin foam model is considered. By applying this approach to a model based on group field theory in 4d (TOCY model), and using the Peter-Weyl expansion of the gravitational field, reconstruction of new non gravitational fields and interactions in the action are found. The perturbative expansion of the partition function produces graphs colored with su(2) algebraic data, from which one can reconstruct a 3d simplicial complex representing space-time and its geometry; (like in the Ponzano-Regge formulation of pure 3d quantum gravity), as well as the Feynman graph for typical matter fields. Thus a mechanism for generation of matter and construction of new dimensions are found from pure gravity.

 

[marcus]

http://arxiv.org/abs/1307.4747
Asymptotic of Lorentzian Polyhedra Propagator
Jacek Puchta
(Submitted on 17 Jul 2013)
A certain operator T=\int_{SL(2,C)}dgY^{\dagger}gY can be found in various Lorentzian EPRL calculations. The properties of this operator has been studied here in large j limit. The leading order of T is proportional to the identity operator.
Knowing the operator T one can renormalize spin-foam's edge self-energy by computing the amplitude of sum of a series of edges with increasing number of vertices and bubbles. This amplitude is calculated and is shown to be convergent.
Moreover some technical tools useful in Lorentzian Spin-Foam calculation have been developed.
28 pages, 1 figure

http://arxiv.org/abs/1307.4687
Comment on "Time and a Physical Hamiltonian for Quantum Gravity"
Jedrzej Świeżewski
(Submitted on 17 Jul 2013)
The aim of this comment is to present a simple argument showing that in the irrotational dust model the Hamilotnian constraint is indeed free of the square root if the time-gauge is chosen. This feature requires no additional assumptions, namely no choices of signs.
2 pages (comment on Husain Pawlowski http://arxiv.org/abs/1108.1145 )

brief mention:
http://arxiv.org/abs/1307.4706
Gauge/Gravity Duality and the Black Hole Interior
Donald Marolf, Joseph Polchinski
(Submitted on 17 Jul 2013)
We present a further argument that typical black holes with field theory duals have firewalls at the horizon...
... We also address the ER=EPR conjecture of Maldacena and Susskind, arguing that the correlations in generic highly entangled states cannot be geometrized as a smooth wormhole.
5 pages
[my comment: Ashtekar pointed out at GR20 that LQG BH eliminates singularity and necessity of "firewall"]

http://gr20-amaldi10.edu.pl/userfiles/book_07_07_2013.pdf
Quantum space-times and unitarity of black hole evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.
page 216 of the book of abstracts of the July 2013 GR20 conference.

There is also a paper by Don Marolf presented at the same joint session

Ads/cft, unitary black hole evaporation, and firewalls
Marolf D
We review arguments that black hole evaporation is unitary in AdS/CFT. As a result, the physics expe- rienced by infalling observers at the horizon of at least sufficiently old black holes described by AdS/CFT must be dramatically different from that described by familiar field theory in a smooth spacetime.
page 217 of the GR20 abstracts.

 

[MTd2]

http://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379

http://arxiv.org/abs/1303.6878

A Universe without expansion
C. Wetterich
(Submitted on 27 Mar 2013 (v1), last revised 5 Jul 2013 (this version, v2))
We discuss a cosmological model where the universe shrinks rather than expands during the radiation and matter dominated periods. Instead, the Planck mass and all particle masses grow exponentially. Together with a preceding inflationary phase and a late dark energy dominated epoch this model is compatible with all observations. The curvature is almost constant during all epochs. Cosmology has no big bang singularity. There exist other, equivalent choices of field variables for which the universe shows the usual expansion or is static during the radiation or matter dominated epochs. For those ``field coordinates`` the big bang is singular. Thus the big bang singularity turns out to be related to a singular choice of field coordinates.

 

[marcus]

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer
(Submitted on 18 Jul 2013)
We propose a derivation for computing black hole entropy for spherical non-rotating isolated horizons from loop quantum gravity in four and higher dimensions. The state counting problem effectively reduces to the well studied 3+1-dimensional one based on an SU(2)-Chern-Simons theory, differing only in the precise form of the area spectrum and the restriction to integer spins.
5 pages

http://arxiv.org/abs/1307.5026
Melonic phase transition in group field theory
Aristide Baratin, Sylvain Carrozza, Daniele Oriti, James P. Ryan, Matteo Smerlak
(Submitted on 18 Jul 2013)
Group field theories have recently been shown to admit a 1/N expansion dominated by so-called `melonic graphs', dual to triangulated spheres. In this note, we deepen the analysis of this melonic sector. We obtain a combinatorial formula for the melonic amplitudes in terms of a graph polynomial related to a higher dimensional generalization of the Kirchhoff tree-matrix theorem. Simple bounds on these amplitudes show the existence of a phase transition driven by melonic interaction processes. We restrict our study to the Boulatov-Ooguri models, which describe topological BF theories and are the basis for the construction of four dimensional models of quantum gravity.
8 pages, 4 figures

 

[marcus]

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau
(Submitted on 19 Jul 2013)
This article addresses the issue of the closure of the algebra of constraints for generic (cosmological) perturbations when taking into account simultaneously the two main corrections of effective loop quantum cosmology, namely the holonomy and the inverse-volume terms. Previous works on either the holonomy or the inverse volume case are reviewed and generalized. In the inverse-volume case, we point out new possibilities. An anomaly-free solution including both corrections is found for perturbations, and the corresponding equations of motion are derived.
19 pages.

http://arxiv.org/abs/1307.5281
Double Scaling in Tensor Models with a Quartic Interaction
Stephane Dartois, Razvan Gurau, Vincent Rivasseau
(Submitted on 19 Jul 2013)
In this paper we identify and analyze in detail the subleading contributions in the 1/N expansion of random tensors, in the simple case of a quartically interacting model. The leading order for this 1/N expansion is made of graphs, called melons, which are dual to particular triangulations of the D-dimensional sphere, closely related to the "stacked" triangulations. For D<6 the subleading behavior is governed by a larger family of graphs, hereafter called cherry trees, which are also dual to the D-dimensional sphere. They can be resummed explicitly through a double scaling limit. In sharp contrast with random matrix models, this double scaling limit is stable. Apart from its unexpected upper critical dimension 6, it displays a singularity at fixed distance from the origin and is clearly the first step in a richer set of yet to be discovered multi-scaling limits.
40 pages.

brief mention:
http://arxiv.org/abs/1307.5303
Invariant Connections in Loop Quantum Gravity
Maximilian Hanusch
(Submitted on 19 Jul 2013)
38 pages

 

[marcus]

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli
(Submitted on 20 Jul 2013)
Loop Quantum Gravity (LQG) is an attempt to describe the quantum gravity regime. Introducing a non-zero cosmological constant Λ in this context has been a withstanding problem. Other approaches, such as Chern-Simons gravity, suggest that quantum groups can be used to introduce Λ in the game. Not much is known when defining LQG with a quantum group. Tensor operators can be used to construct observables in any type of discrete quantum gauge theory with a classical/quantum gauge group. We illustrate this by constructing explicitly geometric observables for LQG defined with a quantum group and show for the first time that they encode a quantized hyperbolic geometry. This is a novel argument pointing out the usefulness of quantum groups as encoding a non-zero cosmological constant. We conclude by discussing how tensor operators provide the right formalism to unlock the LQG formulation with a non-zero cosmological constant.
6 pages, 1 figure

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll
(Submitted on 20 Jul 2013)
We present a detailed analysis of a recently introduced version of Causal Dynamical Triangulations (CDT) that does not rely on a distinguished time slicing. Focussing on the case of 2+1 spacetime dimensions, we analyze its geometric and causal properties, present details of the numerical set-up and explain how to extract "volume profiles". Extensive Monte Carlo measurements of the system show the emergence of a de Sitter universe on large scales from the underlying quantum ensemble, similar to what was observed previously in standard CDT quantum gravity. This provides evidence that the distinguished time slicing of the latter is not an essential part of its kinematical set-up.
44 pages, 29 figures

http://arxiv.org/abs/1307.5527
Loop Quantum Cosmology and the Fine Structure Constant
David Sloan
(Submitted on 21 Jul 2013)
The cosmological implications of introducing a variation to the fine structure 'constant', α are examined within the context of Loop Quantum Cosmology. The evolution of α is described using the model introduced by Bekenstein, Sandvik, Barrow and Magueijo, in which a ghost scalar field produces the variation. The dynamics of the system are examined in flat and closed cosmological settings. Matter consisting of the scalar field and radiation are examined with a thermodynamically motivated coupling between the two, which can lead to a series of bounces induced by both the negative density effects of the ghost field and the loop effects.
10 pages, 5 figures

http://arxiv.org/abs/1307.5352
Cosine problem in EPRL/FK spinfoam model
Marko Vojinovic
(Submitted on 19 Jul 2013)
We calculate the classical limit effective action of the EPRL/FK spinfoam model of quantum gravity coupled to matter fields. By employing the standard QFT background field method adapted to the spinfoam setting, we find that the model has many different classical effective actions. Most notably, these include the ordinary Einstein-Hilbert action coupled to matter, but also an action which describes antigravity. All those multiple classical limits appear as a consequence of of the fact that the EPRL/FK vertex amplitude has cosine-like large spin asymptotics. We discuss some possible ways to eliminate the unwanted classical limits.
20 pages

possibly of general interest:
http://arxiv.org/abs/1307.5737
Structures in the Planck map of the CMB
Daniel An, Krzysztof A. Meissner, Pawel Nurowski
(Submitted on 22 Jul 2013)
We present the results of the quest for ring-type structures on the maps observed by the Planck satellite.

 

[marcus]

http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin
(Submitted on 23 Jul 2013)
We describe a new class of models of quantum space-time based on energetic causal sets and show that under natural conditions space-time emerges from them. These are causal sets whose causal links are labelled by energy and momentum and conservation laws are applied at events. The models are motivated by principles we propose govern microscopic physics which posit a fundamental irreversibility of time. One consequence is that each event in the history of the universe has a distinct causal relationship to the rest; this requires a novel form of dynamics which an be applied to uniquely distinctive events. We hence introduce a new kind of deterministic dynamics for a causal set in which new events are generated from pairs of progenitor events by a rule which is based on extremizing the distinctions between causal past sets of events. This dynamics is asymmetric in time, but we find evidence from numerical simulations of a 1+1 dimensional model, that an effective dynamics emerges which restores approximate time reversal symmetry. Finally we also present a natural twistorial representation of energetic causal sets.
26 pages, 5 figures

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel
(Submitted on 22 Jul 2013)
In a seminal paper, Kaminski, Kisielowski an Lewandowski for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of Loop Quantum Gravity (LQG) and allows to investigate the question whether any of the presently considered spin foam models yield a rigging map for any of the presently defined Hamiltonian constraint operators. The KKL extension cannot be described in terms of Group Field Theory (GFT) since arbitrary foams are involved while GFT is tied to simplicial complexes. Therefore one has to define the sum over spin foams with given boundary spin networks in an independent fashion using natural axioms, most importantly a gluing property for 2-complexes. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. This is achieved by constructing a spin foam operator based on abstract 2-complexes that acts on the kinematical Hilbert space of Loop Quantum Gravity. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
62 pages, 14 figures

http://arxiv.org/abs/1307.5979
The Large-Volume Limit of a Quantum Tetrahedron is a Quantum Harmonic Oscillator
John Schliemann
(Submitted on 23 Jul 2013)
It is shown that the volume operator of a quantum tetrahedron is, in the sector of large eigenvalues, accurately described by a quantum harmonic oscillator. This result relies on the fact that (i) the volume operator couples only neighboring states of its standard basis, and (ii) its matrix elements show a unique maximum as a function of internal angular momentum quantum numbers. These quantum numbers, considered as a continuous variable, are the coordinate of the oscillator describing its quadratic potential, while the corresponding derivative defines a momentum operator. We also analyze the scaling properties of the oscillator parameters as a function of the size of the tetrahedron, and the role of different angular momentum coupling schemes.
10 pages, 3 figures; comments welcome

not directly QG-related, but possibly of interest:
http://arxiv.org/abs/1307.6122
Black-hole entropy and minimal diffusion
Michele Arzano, Gianluca Calcagni
(Submitted on 23 Jul 2013)
The density of states reproducing the Bekenstein-Hawking entropy-area scaling can be modeled via a nonlocal field theory. We define a diffusion process based on the kinematics of this theory and find a spectral dimension whose flow exhibits surprising properties. While it asymptotes the infrared value of four from above, in the ultraviolet the spectral dimension diverges at a finite (Planckian) value of the diffusion length, signalling a breakdown of the notion of diffusion on a continuum spacetime below that scale. The correlation length remains finite throughout the flow. We comment on the implications of this minimal diffusion scale for the entropy bound in a holographic and field-theoretic context.
4 pages, 1 figure

http://arxiv.org/abs/1307.6169
The world is discrete
Olaf Dreyer
(Submitted on 23 Jul 2013)
We argue that the scale-free spectrum that is observed in the cosmic microwave background is the result of a phase transition in the early universe. The observed tilt of the spectrum, which has been measured to be 0.04, is shown to be equal to the anomalous scaling dimension of the correlation function. The phase transition replaces inflation as the mechanism that produces this spectrum. The tilt further indicates that there is a fundamental small length scale in nature that we have not yet observed in any other way.
12 pages, 1 figure

 

[John86]

Maybe Freidel is trying to close the gap between strings and loops ?

http://arxiv.org/abs/1307.7080
Born Reciprocity in String Theory and the Nature of Spacetime
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 26 Jul 2013)
After many years, the deep nature of spacetime in string theory remains an enigma. In this letter we incorporate the concept of Born reciprocity in order to provide a new point of view on string theory in which spacetime is a derived dynamical concept. This viewpoint may be thought of as a dynamical chiral phase space formulation of string theory, in which Born reciprocity is implemented as a choice of a Lagrangian submanifold of the phase space, and amounts to a generalization of T-duality. In this approach the fundamental symmetry of string theory contains phase space diffeomorphism invariance and the underlying string geometry should be understood in terms of dynamical bi-Lagrangian manifolds and an apparently new geometric structure, somewhat reminiscent of para-quaternionic geometry, which we call Born geometry.

 

[John86]

http://arxiv.org/abs/1307.7376
Multiversality
Frank Wilczek
(Submitted on 28 Jul 2013)
Valid ideas that physical reality is vastly larger than human perception of it, and that the perceived part may not be representative of the whole, exist on many levels and have a long history. After a brief general inventory of those ideas and their implications, I consider the cosmological "multiverse" much discussed in recent scientific literature. I review its theoretical and (broadly) empirical motivations, and its disruptive implications for the traditional program of fundamental physics. I discuss the inflationary axion cosmology, which provides an example where firmly rooted, plausible ideas from microphysics lead to a well-characterized "mini-multiverse" scenario, with testable phenomenological consequences.

 

[marcus]

not directly Loop-or-allied gravity but of general interest:
http://arxiv.org/abs/1307.8106
Cyclic Cosmology, Conformal Symmetry and the Metastability of the Higgs
Itzhak Bars, Paul J. Steinhardt, Neil Turok
(Submitted on 30 Jul 2013)
Recent measurements at the LHC suggest that the current Higgs vacuum could be metastable with a modest barrier (height 10^10-12GeV⁴) separating it from a ground state with negative vacuum density of order the Planck scale. We note that metastability is problematic for big bang to end one cycle, bounce, and begin the next. In this paper, motivated by the approximate scaling symmetry of the standard model of particle physics and the primordial large-scale structure of the universe, we use our recent formulation of the Weyl-invariant version of the standard model coupled to gravity to track the evolution of the Higgs in a regularly bouncing cosmology. We find a band of solutions in which the Higgs field escapes from the metastable phase during each big crunch, passes through the bang into an expanding phase, and returns to the metastable vacuum, cycle after cycle after cycle. We show that, due to the effect of the Higgs, the infinitely cycling universe is geodesically complete, in contrast to inflation.
16 pages, 4 figures

http://arxiv.org/abs/1307.7988
Pathways to relativistic curved momentum spaces: de Sitter case study
Giovanni Amelino-Camelia, Giulia Gubitosi, Giovanni Palmisano
(Submitted on 30 Jul 2013)
Several arguments suggest that the Planck scale could be the characteristic scale of curvature of momentum space. As other recent studies we assume that the metric of momentum space determines the condition of on-shellness while the momentum-space affine connection governs the form of the law of composition of momenta. We show that the possible choices of laws of composition of momenta are more numerous than the possible choices of affine connection on a momentum space. This motivates us to propose a new prescription for associating an affine connection to momentum composition, which we compare to the one most used in the recent literature. We find that the two prescriptions lead to the same picture of the so-called κ-momentum space, with de Sitter metric and κ-Poincaré connection. We also examine in greater detail than ever before the DSR-relativistic properties of κ-momentum space, particularly in relation to its noncommutative law of composition of momenta. We then show that in the case of "proper de Sitter momentum space", with the de Sitter metric and its Levi-Civita connection, the two prescriptions are inequivalent. Our novel prescription leads to a picture of proper de Sitter momentum space which is DSR-relativistic and is characterized by a commutative law of composition of momenta, a possibility for which no explicit curved-momentum-space picture had been previously found. We argue that our construction provides a natural test case for the study of momentum spaces with commutative, and yet deformed, laws of composition of momenta. Moreover, it can serve as laboratory for the exploration of the properties of DSR-relativistic theories which are not connected to group-manifold momentum spaces and Hopf algebras.
36 pages, 1 figure

Footnote: one of the authors gives a 20 minute talk on this here (start at minute 49:00 of the recording):
http://pirsa.org/13070052/
Phenomenology - 1
Jonathan Granot, Julien Bolmont, Giulia Gubitosi, Giovanni Palmisano, Linqing Chen
21:00, 49:00, 75:00, 95:00
===========
For the complete list of parallel session talks, with minute-marks, go here:
https://www.physicsforums.com/showthread.php?p=4458326#post4458326

 

[atyy]

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick
(Submitted on 31 Jul 2013)
It is well known that the $\SU(2)$-gauge invariant phase space of loop gravity can be represented in terms of twisted geometries. These are piecewise-linear-flat geometries obtained by gluing together polyhedra, but the resulting geometries are not continuous across the faces. Here we show that this phase space can also be represented by continuous, piecewise-flat three-geometries called spinning geometries. These are composed of metric-flat three-cells glued together consistently. The geometry of each cell and the manner in which they are glued is compatible with the choice of fluxes and holonomies.
We first remark that the fluxes provide each edge with an angular momentum. By studying the piecewise-flat geometries which minimize edge lengths, we show that these angular momenta can be literally interpreted as the spin of the edges: the geometries of all edges are necessarily helices. We also show that the compatibility of the gluing maps with the holonomy data results in the same conclusion. This shows that a spinning geometry represents a way to glue together the three-cells of a twisted geometry to form a continuous geometry which represents a point in the loop gravity phase space.

http://arxiv.org/abs/1308.0300
Snyder Momentum Space in Relative Locality
Andrzej Banburski, Laurent Freidel
(Submitted on 1 Aug 2013)
The standard approaches of phenomenology of Quantum Gravity have usually explicitly violated Lorentz invariance, either in the dispersion relation or in the addition rule for momenta. We investigate whether it is possible in 3+1 dimensions to have a non local deformation that preserves fully Lorentz invariance, as it is the case in 2+1D Quantum Gravity. We answer positively to this question and show for the first time how to construct a homogeneously curved momentum space preserving the full action of the Lorentz group in dimension 4 and higher, despite relaxing locality. We study the property of this relative locality deformation and show that this space leads to a noncommutativity related to Snyder spacetime.

http://arxiv.org/abs/1212.5233
Causal loop in the theory of Relative Locality
Lin-Qing Chen
(Submitted on 20 Dec 2012 (v1), last revised 3 Feb 2013 (this version, v2))
We find that Relative Locality, a recently proposed Planck-scale deformation of Special Relativity, suffers from the existence of causal loops. A simple and general construction of such on-shell loop processes is studied. We then show that even in one of the weakest deformations of the Poincar\'e group in Relative Locality, causality can be violated.

http://arxiv.org/abs/1308.0318
Orientability of loop processes in Relative Locality
Lin-Qing Chen
(Submitted on 1 Aug 2013)
We introduce a way to classify loop processes in relative locality in the case of Kappa-Poincare momentum space. We show that orientability is connected to a few essential properties in loop processes. Non-orientable loops have "effective curvature", which explicitly breaks translation symmetry, and can lead to breaking of causality and global momentum conservation. Orientable loops are "flat". Causality and global momentum conservation are all preserved in this kind of loops. We argue that the non-trivial classical loops in relative locality might be understood as dual effects from general relativity.

 

[marcus]

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 3 Aug 2013)
The dynamics of a homogeneous, anisotropic universe are investigated within the context of spinfoam cosmology. Transition amplitudes are calculated for a graph consisting of a single node and three links - the `Daisy graph' - probing the behaviour a classical Bianchi I spacetime. It is shown further how the use of such single node graphs gives rise to a simplification of states such that all orders in the spin expansion can be calculated, indicating that it is the vertex expansion that contains information about quantum dynamics.
28 pages, 1 figure

http://arxiv.org/abs/1308.1012
A look at area Regge calculus
Yasha Neiman
(Submitted on 5 Aug 2013)
Area Regge calculus is a candidate theory of simplicial gravity, based on the Regge action with triangle areas as the dynamical variables. It is characterized by metric discontinuities and vanishing deficit angles. Area Regge calculus arises in the large-spin limit of the Barrett-Crane spinfoam model, but not in the newer EPRL/FK model. We address the viability of area Regge calculus as a discretization of General Relativity. We argue that when all triangles are spacelike and all tetrahedra have the same signature, non-trivial solutions of the area calculus are associated with a nonzero Ricci scalar. Our argument rests on a seemingly natural regularization of the metric discontinuities. It rules out the Euclidean area calculus, as well as the Lorentzian sector with all tetrahedra spacelike - the two setups usually considered in spinfoam models. On the other hand, we argue that the area calculus has attractive properties from the point of view of finite-region observables in quantum gravity.
15 pages, 2 figures

general interest (brief mention):
http://arxiv.org/abs/1308.1007
The Fate of the Quantum
Gerard 't Hooft
(Submitted on 5 Aug 2013)
Although the suspicion that quantum mechanics is emergent has been lingering for a long time, only now we begin to understand how a bridge between classical and quantum mechanics might be squared with Bell's inequalities and other conceptual obstacles...
14 pages, 1 figure, talk given at the "Time and Matter" conference, Venice March 2013.

 

[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.