Loop-and-allied QG bibliography

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http://arxiv.org/abs/1409.0836
A note on the secondary simplicity constraints in loop quantum gravity
Fabio Anzà, Simone Speziale
(Submitted on 2 Sep 2014)
A debate has appeared in the literature on loop quantum gravity and spin foams, over whether secondary simplicity constraints should imply the shape matching conditions reducing twisted geometries to Regge geometries. We address the question using a model in which secondary simplicity constraints arise from a dynamical preservation of the primary ones, and answer it in the affirmative. The origin of the extra condition is to be found in the different graph localisations of the various constraints. Our results are consistent with previous claims by Dittrich and Ryan, and extend their validity to Lorentzian signature and a priori arbitrary cellular decompositions. Finally, we show how the (gauge-invariant version of the) twist angle ξ featuring in twisted geometries equals on-shell the Regge dihedral angle multiplied by the Immirzi parameter, thus recovering the discrete extrinsic geometry from the Ashtekar-Barbero holonomy.
15 pages

 

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http://arxiv.org/abs/1409.0917
Identification of a gravitational arrow of time
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 2 Sep 2014)
It is widely believed that special initial conditions must be imposed on any time-symmetric law if its solutions are to exhibit behavior of any kind that defines an `arrow of time'. We show that this is not so. The simplest non-trivial time-symmetric law that can be used to model a dynamically closed universe is the Newtonian N-body problem with vanishing total energy and angular momentum. Because of special properties of this system (likely to be shared by any law of the Universe), its typical solutions all divide at a uniquely defined point into two halves. In each a well-defined measure of shape complexity fluctuates but grows irreversibly between rising bounds from that point. Structures that store dynamical information are created as the complexity grows and act as `records'. 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 the records of one branch and deduce a unique past and future direction from inspection of the available records.
6 pages. To appear in Physical Review Letters

http://arxiv.org/abs/1409.0985
The Entropy of Higher Dimensional Nonrotating Isolated Horizons from Loop Quantum Gravity
Jingbo Wang, Chao-Guang Huang
(Submitted on 3 Sep 2014)
In this paper, we extend the calculation of the entropy of the nonrotating isolated horizons in 4 dimensional spacetime to that in a higher dimensional spacetime. We show that the boundary degrees of freedom on an isolated horizon can be described effectively by a punctured SO(1,1) BF theory. Then the entropy of the nonrotating isolated horizon can be calculated out by counting the microstates. It satisfies the Bekenstein-Hawking law.
13 pages.

 

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http://arxiv.org/abs/1409.1450
The continuum limit of loop quantum gravity - a framework for solving the theory
Bianca Dittrich
(Submitted on 4 Sep 2014)
The construction of a continuum limit for the dynamics of loop quantum gravity is unavoidable to complete the theory. We explain that such a construction is equivalent to obtaining the continuum physical Hilbert space, which encodes the solutions of the theory. We present iterative coarse graining methods to construct physical states in a truncation scheme and explain in which sense this scheme represents a renormalization flow. We comment on the role of diffeomorphism symmetry as an indicator for the continuum limit.

 

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possibly of interest although not in line with usual QG assumptions:
http://arxiv.org/abs/1409.1501
The lifetime problem of evaporating black holes: Mutiny or resignation
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay, Gil Jannes
(Submitted on 4 Sep 2014)
It is logically possible that regularly evaporating black holes exist in Nature. In fact, the prevalent theoretical view is that these are indeed the real objects behind the curtain in astrophysical scenarios. There are several proposals for regularizing the classical singularity of black holes so that their formation and evaporation do not lead to information-loss problems. One characteristic is shared by most of these proposals: these regularly evaporating black holes present long-lived trapping horizons, with absolutely enormous evaporation lifetimes in whatever measure. Guided by the discomfort with these enormous and thus inaccessible lifetimes, we elaborate here on an alternative regularization of the classical singularity, previously proposed by the authors in an emergent gravity framework, which leads to a completely different scenario. In our scheme the collapse of a stellar object would result in a genuine time-symmetric bounce, which in geometrical terms amounts to the connection of a black-hole geometry with a white-hole geometry in a regular manner. The two most differential characteristics of this proposal are: i) The complete bouncing geometry is a solution of standard classical General Relativity everywhere except in a transient region that necessarily extends beyond the gravitational radius associated with the total mass of the collapsing object. ii) The duration of the bounce as seen by external observers is very brief (fractions of milliseconds for neutron-star-like collapses). This scenario motivates the search for new forms of stellar equilibrium different from black holes. In a brief epilogue we compare our proposal with a similar geometrical setting recently proposed by Haggard and Rovelli.
20 pages, 2 figures

 

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Bill Unruh just gave a talk at Perimeter which may be of interest. video and slides PDF are online:
http://pirsa.org/14090066/
Firewalls- A GR perspective
Speaker(s): Bill Unruh
Abstract: This talk will examine the Firewall argument and a number of possible approaches to it, with a variety of simple examples to try to clarify various aspects of the arguments.
Date: 04/09/2014 - 3:00 pm

James Bardeen just gave a talk at Perimeter based on this June 2014 paper:
http://arxiv.org/abs/1406.4098
Black hole evaporation without an event horizon
James M. Bardeen
(Submitted on 16 Jun 2014)
A reformulation of the calculation of the semi-classical energy-momentum tensor on a Schwarzschild background, the Bousso covariant entropy bound, and the ER=EPR conjecture of Maldacena and Susskind taken together suggest a scenario for the evaporation of a large spherically symmetric black hole formed in gravitational collapse in which 1) the classical r = 0 singularity is replaced by an initially small non-singular core inside an inner apparent horizon, 2) the radius of the core grows with time due to the increasing entanglement between Hawking radiation quanta outside the black hole and the Hawking partner quanta in the core contributing to the quantum back-reaction, and 3) by the Page time the trapped surfaces disappear and all quantum information stored in the interior is free to escape. The scenario preserves unitarity without any need for a "firewall" in the vicinity of the outer apparent horizon. Qbits in the Hawking radiation are never mutually entangled, and their number never exceeds the Bekenstein-Hawking entropy of the black hole. The quantum back-reaction, while it must be very large in the deep interior of the black hole, can be described semi-classically in the vicinity of the outer apparent horizon up until close to the Page time. An explicit toy model for the metric in the interior of the black hole, and how its associated energy-momentum tensor can be continued to the exterior in a semi-classical approximation, is discussed.
28 pages.

The PIRSA link for the talk is:
http://pirsa.org/14090001/
Black hole evaporation without firewalls
Speaker(s): James Bardeen
Abstract: There need not be any conflict between unitarity, locality, and regularity of the horizon in black hole evaporation. I discuss a scenario in which the initial collapse that forms the black hole results in a small non-singular core inside an inner event horizon. This core grows as the result of quantum back-reaction associated with the increasing entanglement entropy of Hawking radiation quanta and their partners trapped inside the core. By the Page time the inner and outer apparent horizons either merge into a degenerate horizon, shutting off the Hawking radiation and leaving a massive remnant, or they disappear completely, allowing the trapped quantum information to escape. The scenario is justified by appeals to the Bousso covariant entropy bound and the ER=EPR conjecture. The talk is largely based on arxiv.org/1406.4098.
Date: 02/09/2014 - 11:00 am

 

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http://arxiv.org/abs/1409.1800
Geometry and Physics of Null Infinity
Abhay Ashtekar
(Submitted on 5 Sep 2014)
In asymptotically Minkowski space-times, one finds a surprisingly rich interplay between geometry and physics in both the classical and quantum regimes. On the mathematical side it involves null geometry, infinite dimensional groups, symplectic geometry on the space of gravitational connections and geometric quantization via Kähler structures. On the physical side, null infinity provides a natural home to study gravitational radiation and its structure leads to several interesting effects such as an infinite dimensional enlargement of the Poincarè group, geometrical expressions of energy and momentum carried by gravitational waves, emergence of non-trivial `vacuum configurations' and an unforeseen interplay between infrared properties of the quantum gravitational field and the enlargement of the asymptotic symmetry group. The goal of this article is to present a succinct summary of this subtle and beautiful interplay.
20 pages. Invited article for the volume "Surveys in Differential Geometry", a Jubilee Volume on General Relativity and Mathematics celebrating 100 Years of General Relativity, edited by L. Bieri and S.T. Yau

http://arxiv.org/abs/1409.1902
Fourth order deformed general relativity
Peter D. Cuttell, Mairi Sakellariadou
(Submitted on 5 Sep 2014)
Whenever the condition of anomaly freedom is imposed within the framework of effective approaches to loop quantum cosmology, one seems to conclude that a deformation of general covariance is required. Here, starting from a general deformation we regain an effective gravitational Lagrangian including terms up to fourth order in extrinsic curvature. We subsequently constrain the form of the corrections, and then investigate the conditions for the occurrence of a big bounce and the realisation of an inflationary era, in the presence of a perfect fluid or scalar field.
29 pages, 2 figures

http://arxiv.org/abs/1409.1751
Canonical Quantum Gravity on Noncommutative Spacetime
Martin Kober
(Submitted on 4 Sep 2014)
In this paper canonical quantum gravity on noncommutative space-time is considered. The corresponding generalized classical theory is formulated by using the moyal star product, which enables the representation of the field quantities depending on noncommuting coordinates by generalized quantities depending on usual coordinates. But not only the classical theory has to be generalized in analogy to other field theories. Besides, the necessity arises to replace the commutator between the gravitational field operator and its canonical conjugated quantity by a corresponding generalized expression on noncommutative space-time. Accordingly the transition to the quantum theory has also to be performed in a generalized way and leads to extended representations of the quantum theoretical operators. If the generalized representations of the operators are inserted to the generalized constraints, one obtains the corresponding generalized quantum constraints including the Hamiltonian constraint as dynamical constraint. After considering quantum geometrodynamics under incorporation of a coupling to matter fields, the theory is transferred to the Ashtekar formalism. The holonomy representation of the gravitational field as it is used in loop quantum gravity opens the possibility to calculate the corresponding generalized area operator.
17 pages

side interest:
http://arxiv.org/abs/1409.1818
The twin paradox in the presence of gravity
M. Gasperini
(Submitted on 5 Sep 2014)
Conventional wisdom, based on kinematic (flat-space) intuition, tell us that a static twin is aging faster than his traveling twin brother. However, such a situation could be exactly inverted if the two twins are embedded in an external gravitational field, and if the (dynamical) distortion of the space-time geometry, due to gravity, is strong enough to compensate the kinematic effect of the relative twin motion.
5 pages, 2 figures. Published in Mod. Phys. Lett. A 29 (2014)

huge monograph on ψ-ontic versus epistemic Q-interpretation by Perimeter guy
http://arxiv.org/abs/1409.1570
Is the quantum state real? A review of ψ-ontology theorems
M. S. Leifer
(Submitted on 4 Sep 2014)
116 pages

brief mention:
http://arxiv.org/abs/1409.1837
Back-reaction of the Hawking radiation flux on a gravitationally collapsing star II: Fireworks instead of firewalls
Laura Mersini-Houghton, Harald P. Pfeiffer
(Submitted on 5 Sep 2014)
9 pages, 6 figures.

 

[atyy]

http://arxiv.org/abs/1409.2407
Decorated tensor network renormalization for lattice gauge theories and spin foam models
Bianca Dittrich, Sebastian Mizera, Sebastian Steinhaus
(Submitted on 8 Sep 2014)
Tensor network techniques have proved to be powerful tools that can be employed to explore the large scale dynamics of lattice systems. Nonetheless, the redundancy of degrees of freedom in lattice gauge theories (and related models) poses a challenge for standard tensor network algorithms. We accommodate for such systems by introducing an additional structure decorating the tensor network. This allows to explicitly preserve the gauge symmetry of the system under coarse graining and straightforwardly interpret the fixed point tensors. Using this novel information encoded in the decoration might eventually lead to new methods incorporating both analytical and numerical techniques.

http://arxiv.org/abs/1409.2471
Quanta of Geometry
Ali H. Chamseddine, Alain Connes, Viatcheslav Mukhanov
(Submitted on 8 Sep 2014)
In the construction of spectral manifolds in noncommutative geometry, a higher degree Heisenberg commutation relation involving the Dirac operator and the Feynman slash of real scalar fields naturally appears and implies, by equality with the index formula, the quantization of the volume. We first show that this condition implies that the manifold decomposes into disconnected spheres which will represent quanta of geometry. We then refine the condition by involving the real structure and two types of geometric quanta, and show that connected manifolds with large quantized volume are then obtained as solutions. When this condition is adopted in the gravitational action it leads to the quantization of the four volume with the cosmological constant obtained as an integration constant. Restricting the condition to a three dimensional hypersurface implies quantization of the three volume and the possible appearance of mimetic dark matter. When restricting to a two dimensional hypersurface, under appropriate boundary conditions, this results in the quantization of area and has many interesting applications to black hole physics.

 

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

 

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http://arxiv.org/abs/1409.3150
Group field theories for all loop quantum gravity
Daniele Oriti, James P. Ryan, Johannes Thürigen
(Submitted on 10 Sep 2014)
Group field theories represent a 2nd quantized reformulation of the loop quantum gravity state space and a completion of the spin foam formalism. States of the canonical theory, in the traditional continuum setting, have support on graphs of arbitrary valence. On the other hand, group field theories have usually been defined in a simplicial context, thus dealing with a restricted set of graphs. In this paper, we generalize the combinatorics of group field theories to cover all the loop quantum gravity state space. As an explicit example, we describe the GFT formulation of the KKL spin foam model, as well as a particular modified version. We show that the use of tensor model tools allows for the most effective construction. In order to clarify the mathematical basis of our construction and of the formalisms with which we deal, we also give an exhaustive description of the combinatorial structures entering spin foam models and group field theories, both at the level of the boundary states and of the quantum amplitudes.
49 pages, 24 figures.

http://arxiv.org/abs/1409.3157
Information loss, made worse by quantum gravity
Martin Bojowald
(Submitted on 10 Sep 2014)
Quantum gravity is often expected to solve both the singularity problem and the information-loss problem of black holes. This article presents an example from loop quantum gravity in which the singularity problem is solved in such a way that the information-loss problem is made worse. Quantum effects in this scenario, in contrast to previous non-singular models, do not eliminate the event horizon and introduce a new Cauchy horizon where determinism breaks down. Although infinities are avoided, for all practical purposes the core of the black hole plays the role of a naked singularity. Recent developments in loop quantum gravity indicate that this aggravated information loss problem is likely to be the generic outcome, putting strong conceptual pressure on the theory.
12 pages, 1 figure.

 

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This appeared in open-access form today.
http://iopscience.iop.org/1742-6596/532/1/012020/
http://iopscience.iop.org/1742-6596/532/1/012020/pdf/1742-6596_532_1_012020.pdf
Categorical generalization of spinfoam models
A Mikovic and M Vojinovic
Departamento de Matematica, Universidade Lusofona de Humanidades e Tecnologias
Grupo de Fisica Matematica, Universidade de Lisboa
We give a brief review of the problem of quantum gravity. After the discussion of the nonrenormalizability of general relativity, we briefly mention the main research directions which aim to resolve this problem. Our attention then focuses on the approach of Loop Quantum Gravity, specifically spinfoam models. These models have some issues concerning the semiclassical limit and coupling of matter fields. The recent developments in category theory provide us with the necessary formalism to introduce a new action for general relativity and perform covariant quantization so that the issues of spinfoam models are successfully resolved.
10 pages.

My comment: the approach uses 2-groups. John Baez advocated applying 2-groups to spinfoam QG. We haven't heard much about that in recent years.

This paper is not available on arxiv, but a related one from 2011 is available and may help provide context:
http://arxiv.org/abs/1110.4694
Poincare 2-group and quantum gravity
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 21 Oct 2011)
We show that General Relativity can be formulated as a constrained topological theory for flat 2-connections associated to the Poincaré 2-group. Matter can be consistently coupled to gravity in this formulation. We also show that the edge lengths of the spacetime manifold triangulation arise as the basic variables in the path-integral quantization, while the state-sum amplitude is an evaluation of a colored 3-complex, in agreement with the category theory results. A 3-complex amplitude for Euclidean quantum gravity is proposed.
12 pages, published in Class. Quant. Grav. 29 (2012)

 

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http://arxiv.org/abs/1409.3526
A 2-categorical state sum model
Aristide Baratin, Laurent Freidel
(Submitted on 11 Sep 2014)
It has long been argued that higher categories provide the proper algebraic structure underlying state sum invariants of 4-manifolds. This idea has been refined recently, by proposing to use 2-groups and their representations as specific examples of 2-categories. The challenge has been to make these proposals fully explicit. Here we give a concrete realization of this program. Building upon our earlier work with Baez and Wise on the representation theory of 2-groups, we construct a four-dimensional state sum model based on a categorified version of the Euclidean group. We define and explicitly compute the simplex weights, which may be viewed a categorified analogue of Racah-Wigner 6j-symbols. These weights solve an hexagon equation that encodes the formal invariance of the state sum under the Pachner moves of the triangulation. This result unravels the combinatorial formulation of the Feynman amplitudes of quantum field theory on flat spacetime proposed in [1], which was shown to lead after gauge-fixing to Korepanov's invariant of 4-manifolds.
13 pages

http://arxiv.org/abs/1409.3751
Canonical formulation of Poincare BFCG theory and its quantization
Aleksandar Mikovic, Miguel A. Oliveira
(Submitted on 12 Sep 2014)
We find the canonical formulation of the Poincare BFCG theory in terms of the spatial 2-connection and its canonically conjugate momenta. We show that the Poincare BFCG action is dynamically equivalent to the BF action for the Poincare group and we find the canonical transformation relating the two. We study the canonical quantization of the Poincare BFCG theory by passing to the Poincare-connection basis. The quantization in the 2-connection basis can be then achieved by performing a Fourier transform. We also briefly discuss how to approach the problem of constructing a basis of spin-foam states, which are the categorical generalization of the spin-network states from Loop Quantum Gravity.
15 pages

http://arxiv.org/abs/1409.3770
An Isometric Dynamics for a Causal Set Approach to Discrete Quantum Gravity
Stan Gudder
(Submitted on 12 Sep 2014)
We consider a covariant causal set approach to discrete quantum gravity. We first review the microscopic picture of this approach. In this picture a universe grows one element at a time and its geometry is determined by a sequence of integers called the shell sequence. We next present the macroscopic picture which is described by a sequential growth process. We introduce a model in which the dynamics is governed by a quantum transition amplitude. The amplitude satisfies a stochastic and unitary condition and the resulting dynamics becomes isometric. We show that the dynamics preserves stochastic states. By "doubling down" on the dynamics we obtain a unitary group representation and a natural energy operator. These unitary operators are employed to define canonical position and momentum operators.
18 pages, 1 figure

http://arxiv.org/abs/1409.3816
Asymptotics with a positive cosmological constant: I. Basic framework
Abhay Ashtekar, Beatrice Bonga, Aruna Kesavan
(Submitted on 12 Sep 2014)
The asymptotic structure of the gravitational field of isolated systems has been analyzed in great detail in the case when the cosmological constant Λ is zero. The resulting framework lies at the foundation of research in diverse areas in gravitational science. Examples include: i) positive energy theorems in geometric analysis; ii) the coordinate invariant characterization of gravitational waves in full, non-linear general relativity; iii) computations of the energy-momentum emission in gravitational collapse and binary mergers in numerical relativity and relativistic astrophysics; and iv) constructions of asymptotic Hilbert spaces to calculate S-matrices and analyze the issue of information loss in the quantum evaporation of black holes. However, by now observations have established that Λ is positive in our universe. In this paper we show that, unfortunately, the standard framework does not extend from the Λ=0 case to the Λ>0 case in a physically useful manner. In particular, we do not have positive energy theorems, nor an invariant notion of gravitational waves in the non-linear regime, nor asymptotic Hilbert spaces in dynamical situations of semi-classical gravity. A suitable framework to address these conceptual issues of direct physical importance is developed in subsequent papers.
41 pges, 5 figures

 

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http://arxiv.org/abs/1409.4031
Fast Radio Bursts and White Hole Signals
Aurélien Barrau, Carlo Rovelli, Francesca Vidotto
(Submitted on 14 Sep 2014)
We estimate the size of a primordial black hole exploding today via a white hole transition, and the power in the resulting explosion, using a simple model. We point out that Fast Radio Bursts, strong signals with millisecond duration, probably extragalactic and having unknown source, have wavelength not far from the expected size of the exploding hole. We also discuss the possible higher energy components of the signal.
5 pages

http://arxiv.org/abs/1409.4117
Problem of Time and Background Independence: the Individual Facets
Edward Anderson
(Submitted on 14 Sep 2014)
I lay out the problem of time facets as arising piecemeal from a number of aspects of background independence. Almost all of these already have simpler classical counterparts. This approach can be viewed as a facet by facet completion of the observation that Barbour-type relationalism is a background independent precursor to 2 of the 9 facets. That completion proceeds in an order dictated by the additional layers of mathematical structure required to support each. Moreover, the 'nonlinear nature' of the interactions between the Problem of Time facets renders a joint study of them mandatory. The current article is none the less a useful prequel via gaining a conceptual understanding of each facet, prior to embarking on rendering some combinations of facets consistent and what further obstructions arise in attempting such joint considerations. See [20, 21, 26] for up to date studies of this more complicated joint version. I also identify new facets (threading based), subfacets (of observables and of reconstructions) and further source of variety from how far down the levels of mathematical structure these facets extend.
40 pages, 13 figures

possible side-interest:
http://arxiv.org/abs/1409.4143
Comment on "Dark matter searches going bananas: the contribution of Potassium (and Chlorine) to the 3.5 keV line"
Esra Bulbul, Maxim Markevitch, Adam R. Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(Submitted on 15 Sep 2014)
The recent paper by Jeltema & Profumo(2014) claims that contributions from K XVIII and Cl XVII lines can explain the unidentified emission line found by Bulbul et al 2014 and also by Boyarsky et al, 2014a, 2014b. We show that their analysis relies upon incorrect atomic data and inconsistent spectroscopic modeling. We address these points and summarize in the appendix the correct values for the relevant atomic data from AtomDB.

 

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http://arxiv.org/abs/1409.5984
Supersymmetry and noncommutative geometry Part III: The noncommutative supersymmetric Standard Model
Wim Beenakker, Walter D. van Suijlekom, Thijs van den Broek
(Submitted on 21 Sep 2014)
In a previous paper we developed a formalism to construct (potentially) supersymmetric theories in the context of noncommutative geometry. We apply this formalism to explore the existence of a noncommutative version of the minimal supersymmetric Standard Model (MSSM). We obtain the exact particle content of the MSSM and identify (in form) its interactions but conclude that their coefficients are such that the standard action functional used in noncommutative geometry is in fact not supersymmetric.
19 pages, 3 figures

http://arxiv.org/abs/1409.5823
General Relativity and Gravitation: A Centennial Perspective
Abhay Ashtekar, Beverly K. Berger, James Isenberg, Malcolm A. H. MacCallum
(Submitted on 19 Sep 2014)
To commemorate the 100th anniversary of general relativity, the International Society on General Relativity and Gravitation (ISGRG) commissioned a Centennial Volume, edited by the authors of this article. We jointly wrote introductions to the four Parts of the Volume which are collected here. Our goal is to provide a bird's eye view of the advances that have been made especially during the last 35 years, i.e., since the publication of volumes commemorating Einstein's 100th birthday. The article also serves as a brief preview of the 12 invited chapters that contain in-depth reviews of these advances. The volume will be published by Cambridge University Press and released in June 2015 at a Centennial conference sponsored by ISGRG and the Topical Group of Gravitation of the American Physical Society.
37 pages

http://arxiv.org/abs/1409.6218
Polymer inflation
Syed Moeez Hassan, Viqar Husain, Sanjeev S. Seahra
(Submitted on 22 Sep 2014)
We consider the semi-classical dynamics of a free massive scalar field in a homogeneous and isotropic cosmological spacetime. The scalar field is quantized using the polymer quantization method assuming that it is described by a gaussian coherent state. For quadratic potentials, the semi-classical equations of motion yield a universe that has an early "polymer inflation" phase which is generic and almost exactly de Sitter, followed by a epoch of slow-roll inflation. We compute polymer corrections to the slow roll formalism, and discuss the probability of inflation in this model using a physical Hamiltonian arising from time gauge fixing. These results show the extent to which a quantum gravity motivated quantization method affects early universe dynamics.
12 pages, 5 figures

 

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http://arxiv.org/abs/1409.6311
Production of Sterile Neutrino Dark Matter and the 3.5 keV line
Alexander Merle, Aurel Schneider
(Submitted on 22 Sep 2014)
The recent observation of an X-ray line at an energy of 3.5 keV mainly from galaxy clusters has generated a buzz in the Dark Matter community. If confirmed, this signal could stem from a decaying sterile neutrino of a mass of 7.1 keV. Such a particle could make up all the Dark Matter, but it is not clear how it was produced in the early Universe. In this paper we show that it is possible to discriminate between different production mechanisms with present-day astronomical data. The most stringent constraint comes from the Lyman-α forest and seems to disfavor all but one of the main production mechanisms proposed in the literature, which is the production via decay of heavy scalar singlets. Pinning down the production mechanism will help to decide whether the X-ray signal indeed comprises an indirect detection of Dark Matter.
7 pages, 3 figures

http://arxiv.org/abs/1409.6753
Horizon complementarity in elliptic de Sitter space
Lucas Fabian Hackl, Yasha Neiman
(Submitted on 23 Sep 2014)
We study a quantum field in elliptic de Sitter space dS₄/Z₂ - the spacetime obtained from identifying antipodal points in dS₄. We find that the operator algebra and Hilbert space cannot be defined for the entire space, but only for observable causal patches. This makes the system into an explicit realization of the horizon complementarity principle. In the absence of a global quantum theory, we propose a recipe for translating operators and states between observers. This translation involves information loss, in accordance with the fact that two observers see different patches of the spacetime. As a check, we recover the thermal state at the de Sitter temperature as a state that appears the same to all observers. This thermal state arises from the same functional that, in ordinary dS₄, describes the Bunch-Davies vacuum.
23 pages + 5 pages of appendices, 6 figures

 

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http://arxiv.org/abs/arXiv:1409.7117[/PLAIN]
Symplectic and Semiclassical Aspects of the Schläfli Identity
Hal M. Haggard, Austin Hedeman, Eugene Kur, Robert G. Littlejohn
(Submitted[/PLAIN] on 24 Sep 2014)
The Schläfli identity, which is important in Regge calculus and loop quantum gravity, is examined from a symplectic and semiclassical standpoint in the special case of flat, 3-dimensional space. In this case a proof is given, based on symplectic geometry. A series of symplectic and Lagrangian manifolds related to the Schläfli identity, including several versions of a Lagrangian manifold of tetrahedra, are discussed. Semiclassical interpretations of the various steps are provided. Possible generalizations to 3-dimensional spaces of constant (nonzero) curvature, involving Poisson-Lie groups and q-deformed spin networks, are discussed.
40 pages, 8 figures

http://arxiv.org/abs/1409.7073
'The End'
Nemanja Kaloper, Antonio Padilla
(Submitted on 24 Sep 2014)
Recently we proposed a mechanism for sequestering the Standard Model vacuum energy that predicts that the universe will collapse. Here we present a simple mechanism for bringing about this collapse, employing a scalar field whose potential is linear and becomes negative, providing the negative energy density required to end the expansion. The slope of the potential is chosen to allow for the expansion to last until the current Hubble time, about 1010 years, to accommodate our universe. Crucially, this choice is technically natural due to a shift symmetry. Moreover, vacuum energy sequestering selects radiatively stable initial conditions for the collapse, which guarantee that immediately before the turnaround the universe is dominated by the linear potential which drives an epoch of accelerated expansion for at least an efold. Thus a single, technically natural choice for the slope ensures that the collapse is imminent and is preceded by the current stage of cosmic acceleration, giving a new answer to the 'Why Now?' problem.
8 pages

 

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http://arxiv.org/abs/1409.7455
Point particles in 2+1 dimensions: general relativity and loop gravity descriptions
Jonathan Ziprick
(Submitted on 26 Sep 2014)
We develop a Hamiltonian description of point particles in (2+1)-dimensions using connection and frame-field variables for general relativity. The topology of each spatial hypersurface is that of a punctured two-sphere with particles residing at the punctures. We describe this topology with a CW complex (a collection of two-cells glued together along edges), and use this to fix a gauge and reduce the Hamiltonian. The equations of motion for the fields describe a dynamical triangulation where each vertex moves according to the equation of motion for a free relativistic particle. The evolution is continuous except for when triangles collapse (i.e. the edges become parallel) causing discrete, topological changes in the underlying CW complex.
We then introduce the loop gravity phase space parameterized by holonomy-flux variables on a graph (a network of one-dimensional links). By embedding a graph within the CW complex, we find a description of this system in terms of loop variables. The resulting equations of motion describe the same dynamical triangulation as the connection and frame-field variables. In this framework, the collapse of a triangle causes a discrete change in the underlying graph, giving a concrete realization of the graph-changing moves that many expect to feature in full loop quantum gravity. The main result is a dynamical model of loop gravity which agrees with general relativity and is well-suited for quantization using existing methods.
31 pages, 14 figures

http://arxiv.org/abs/1409.7871
Was Einstein Right? A Centenary Assessment
Clifford M. Will
(Submitted on 28 Sep 2014)
This article is an overview of 100 years of testing general relativity, to be published in the book General Relativity and Gravitation: A Centennial Perspective, to commemorate the 100th anniversary of general relativity. It is effectively an abridged version of the recent update of the author's Living Review in Relativity (http://arxiv.org/abs/1403.7377)
33 pages, 8 figures, to be published in General Relativity and Gravitation: A Centennial Perspective, eds. A. Ashtekar, B. Berger, J. Isenberg and M. A. H. MacCallum (Cambridge University Press), 2015.

brief mention:
http://arxiv.org/abs/1409.7726
A Correction to the Immirzi Parameter of SU(2) Spin Networks
Muhammad Sadiq
(Submitted on 26 Sep 2014)
7 pages.

 

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http://arxiv.org/abs/1410.0026
Renormalization of lattice-regularized quantum gravity models I. General considerations
Joshua H. Cooperman
(Submitted on 30 Sep 2014)
Lattice regularization is a standard technique for the nonperturbative definition of a quantum theory of fields. Several approaches to the construction of a quantum theory of gravity adopt this technique either explicitly or implicitly. A crucial complement to lattice regularization is the process of renormalization through which a continuous description of the quantum theory arises. I provide a comprehensive conceptual discussion of the renormalization of lattice-regularized quantum gravity models. I begin with a presentation of the renormalization group from the Wilsonian perspective. I then consider the application of the renormalization group in four contexts: quantum field theory on a continuous nondynamical spacetime, quantum field theory on a lattice-regularized nondynamical spacetime, quantum field theory of continuous dynamical spacetime, and quantum field theory of lattice-regularized dynamical spacetime. The first three contexts serve to identify successively the particular issues that arise in the fourth context. These issues originate in the inescability of establishing all scales solely on the basis of the dynamics. While most of this discussion rehearses established knowledge, the attention that I pay to these issues, especially the previously underappreciated role of standard units of measure, is largely novel. I conclude by briefly reviewing past studies of renormalization of lattice-regularized quantum gravity models. In the second paper of this two-part series, I illustrate the ideas presented here by proposing a renormalization group scheme for causal dynamical triangulations.
26 pages. This is the much delayed second paper in the two-part series

http://arxiv.org/abs/1410.0203
Signatures of primordial black hole dark matter
K. M. Belotsky, A. D. Dmitriev, E. A. Esipova, V. A. Gani, A. V. Grobov, M. Yu. Khlopov, A. A. Kirillov, S. G. Rubin, I. V. Svadkovsky
(Submitted on 1 Oct 2014)
The nonbaryonic dark matter of the Universe is assumed to consist of new stable forms of matter. Their stability reflects symmetry of micro world and mechanisms of its symmetry breaking. In the early Universe heavy metastable particles can dominate, leaving primordial black holes (PBHs) after their decay, as well as the structure of particle symmetry breaking gives rise to cosmological phase transitions, from which massive black holes and/or their clusters can originate. PBHs can be formed in such transitions within a narrow interval of masses about 10¹⁷g and, avoiding severe observational constraints on PBHs, can be a candidate for the dominant form of dark matter. PBHs in this range of mass can give solution of the problem of reionization in the Universe at the redshift z∼5...10. Clusters of massive PBHs can serve as a nonlinear seeds for galaxy formation, while PBHs evaporating in such clusters can provide an interesting interpretation for the observations of point-like gamma-ray sources. Analysis of possible PBH signatures represents a universal probe for super-high energy physics in the early Universe in studies of indirect effects of the dark matter.

of possible side interest:
http://arxiv.org/abs/1410.0355
Dark matter voids in the SDSS galaxy survey
Florent Leclercq, Jens Jasche, P.M. Sutter, Nico Hamaus, Benjamin Wandelt
(Submitted on 1 Oct 2014)
12 pages, 6 figures, sub

brief mention, PhD thesis from Philosophy department of U. Sydney:
http://arxiv.org/abs/1410.0345
Appearing Out of Nowhere: The Emergence of Spacetime in Quantum Gravity
Karen Crowther
(Submitted on 1 Oct 2014)
=================added by editing to save a post==========================
http://arxiv.org/abs/1410.0632
Scale-dependent homogeneity measures for causal dynamical triangulations
Joshua H. Cooperman
(Submitted on 2 Oct 2014)
I propose two scale-dependent measures of the homogeneity of the quantum geometry determined by an ensemble of causal triangulations. The first measure is volumetric, probing the growth of volume with graph geodesic distance. The second measure is spectral, probing the return probability of a random walk with diffusion time. Both of these measures, particularly the first, are closely related to those used to assess the homogeneity of our own universe on the basis of galaxy redshift surveys. I employ these measures to quantify the quantum spacetime homogeneity as well as the temporal evolution of quantum spatial homogeneity of ensembles of causal triangulations in the well-known physical phase. According to these measures, the quantum spacetime geometry exhibits some degree of inhomogeneity on sufficiently small scales and a high degree of homogeneity on sufficiently large scales. This inhomogeneity appears unrelated to the phenomenon of dynamical dimensional reduction. I also uncover evidence for power-law scaling of both the typical scale on which inhomogeneity occurs and the magnitude of inhomogeneity on this scale with the ensemble average spatial volume of the quantum spatial geometries.
25 pages, 19 figures

http://arxiv.org/abs/1410.0670
Making the case for causal dynamical triangulations
Joshua H. Cooperman
(Submitted on 2 Oct 2014)
The aim of the causal dynamical triangulations approach is to define nonperturbatively a quantum theory of gravity as the continuum limit of a lattice-regularized model of dynamical geometry. My aim in this paper is to give a concise yet comprehensive, impartial yet personal presentation of the causal dynamical triangulations approach.
8.5 pages plus references, 5 figures

 

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http://arxiv.org/abs/1410.1058
Loop quantization of a 3D Abelian BF model with sigma-model matter
Diego C.M. Mendonça, Olivier Piguet
(Submitted on 4 Oct 2014)
The main goal of this work is to explore the symmetries and develop the dynamics associated to a 3D Abelian BF model coupled to scalar fields submitted to a sigma model like constraint, at the classical and quantum levels. We apply to the present model the techniques of Loop Quantum Gravity, construct its physical Hilbert space and its observables.
14 pages, 3 figures

http://arxiv.org/abs/1410.1486
Black Hole Thermodynamics
S. Carlip
(Submitted on 6 Oct 2014)
The discovery in the early 1970s that black holes radiate as black bodies has radically affected our understanding of general relativity, and offered us some early hints about the nature of quantum gravity. In this chapter I will review the discovery of black hole thermodynamics and summarize the many independent ways of obtaining the thermodynamic and (perhaps) statistical mechanical properties of black holes. I will then describe some of the remaining puzzles, including the nature of the quantum microstates, the problem of universality, and the information loss paradox.
Invited review article. A few parts based on an earlier review, arXiv:0807.4520.
47 pages, 7 figures. To appear in Int. J. Mod. Phys. D and in "One Hundred Years of General Relativity: Cosmology and Gravity," edited by Wei-Tou Ni (World Scientific, Singapore, 2015)

 

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http://arxiv.org/abs/1410.1714
Loop quantum gravity and observations
A. Barrau, J. Grain
(Submitted on 7 Oct 2014)
Quantum gravity has long been thought to be completely decoupled from experiments or observations. Although it is true that smoking guns are still missing, there are now serious hopes that quantum gravity phenomena might be tested. We review here some possible ways to observe loop quantum gravity effects either in the framework of cosmology or in astroparticle physics.

 

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

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

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

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

 

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

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

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

 

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

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

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

 

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General interest.

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

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

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

 

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

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

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

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

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

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

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

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

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

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

 

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

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

 

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

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

 

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

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

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

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

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

 

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

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

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

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

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

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

 

[marcus]

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

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

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

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

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

 

[John86]

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

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

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

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

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

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

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

 

[marcus]

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

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

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

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

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

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

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

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

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

 

[John86]

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

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

http://noncommutativegeometry.blogspot.nl/

 

[marcus]

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

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

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

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

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

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

 

[marcus]

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

 

[atyy]

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