Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1404.3696
Lorentz Invariance in Shape Dynamics
S. Carlip, H. Gomes
(Submitted on 14 Apr 2014)
Shape dynamics is a reframing of canonical general relativity in which time reparametrization invariance is "traded" for a local conformal invariance. We explore the emergence of Lorentz invariance in this model in three contexts: as a maximal symmetry, an asymptotic symmetry, and a local invariance.
13 pages

 

[marcus]

http://arxiv.org/abs/1404.4036
Loop quantum cosmology of a radiation-dominated flat FLRW universe
Tomasz Pawlowski, Roberto Pierini, Edward Wilson-Ewing
(Submitted on 15 Apr 2014)
We study the loop quantum cosmology of a flat Friedmann-Lemaitre-Robertson-Walker space-time with a Maxwell field. We show that many of the qualitative properties derived for the case of a massless scalar field also hold for a Maxwell field. In particular, the big-bang singularity is replaced by a quantum bounce, and the operator corresponding to the matter energy density is bounded above by the same critical energy density. We also numerically study the evolution of wave functions that are sharply peaked in the low energy regime, and derive effective equations which very closely approximate the full quantum dynamics of sharply peaked states at all times, including the near-bounce epoch.
27 pages, 6 figures

not QG, but of possible interest:
http://arxiv.org/abs/1404.3729
3.5 keV X-rays as the "21 cm line" of dark atoms, and a link to light sterile neutrinos
James M. Cline, Yasaman Farzan, Zuowei Liu, Wei Xue
(Submitted on 14 Apr 2014)
The recently discovered 3.5 keV X-ray line from extragalactic sources may be evidence of dark matter scatterings or decays. We show that dark atoms can be the source of the emission, through their hyperfine transitions, which would be the analog of 21 cm radiation from a dark sector. We identify two families of dark atom models that match the X-ray observations and are consistent with other constraints. In the first, the hyperfine excited state is long-lived compared to the age of the universe, and the dark atom mass is relatively unconstrained; dark atoms could be strongly self-interacting in this case. In the second, the excited state is short-lived and viable models are parameterized by the value of the dark proton-to-electron mass ratio R: for R=10²−10⁴, the dark atom mass is predicted be in the range 350−1300 GeV, with fine structure constant α′≅0.1−0.6. In either class of models, the dark photon must be massive with mγ′≳ 1 MeV and decay into e+e−. Evidence for the model could come from direct detection of the dark atoms. In a natural extension of this framework, the dark photon could decay predominantly into invisible particles, for example ∼0.5 eV sterile neutrinos, explaining the extra radiation degree of freedom recently suggested by data from BICEP2.
5 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1404.4167
Loop quantum gravity, twistors, and some perspectives on the problem of time
Simone Speziale
(Submitted on 16 Apr 2014)
I give a brief introduction to the relation between loop quantum gravity and twistor theory, and comment on some perspectives on the problem of time.
10 pages, invited lecture to the "2nd International Conference on New Frontiers in Physics 2013" (ICNFP 2013), to be published in EPJ Web of Conferences vol. 71

 

[MTd2]

As Marcus said, general interest, but not QG(or is it?):

http://arxiv.org/abs/1404.4044

Breakdown of the equivalence between gravitational mass and energy for a composite quantum body

Andrei G. Lebed
(Submitted on 14 Apr 2014)
The simplest quantum composite body, a hydrogen atom, is considered in the presence of a weak external gravitational field. We define an operator for the passive gravitational mass of the atom in the post-Newtonian approximation of the general relativity and show that it does not commute with its energy operator. Nevertheless, the equivalence between the expectation values of the mass and energy is shown to survive at a macroscopic level for stationary quantum states. Breakdown of the equivalence between passive gravitational mass and energy at a microscopic level for stationary quantum states can be experimentally detected by studying unusual electromagnetic radiation, emitted by the atoms, supported by and moving in the Earth's gravitational field with constant velocity, using spacecraft or satellite.

http://arxiv.org/abs/1404.3765

Does the Equivalence between Gravitational Mass and Energy Survive for a Composite Quantum Body?

Andrei G. Lebed
(Submitted on 14 Apr 2014)
We define passive and active gravitational mass operators of the simplest composite quantum body - a hydrogen atom. Although they do not commute with its energy operator, the equivalence between the expectation values of passive and active gravitational masses and energy is shown to survive for stationary quantum states. In our calculations of passive gravitational mass operator, we take into account not only kinetic and Coulomb potential energies but also the so-called relativistic corrections to electron motion in a hydrogen atom. Inequivalence between passive and active gravitational masses and energy at a macroscopic level is demonstrated to reveal itself as time dependent oscillations of the expectation values of the gravitational masses for superpositions of stationary quantum states. Breakdown of the equivalence between passive gravitational mass and energy at a microscopic level reveals itself as unusual electromagnetic radiation, emitted by macroscopic ensemble of hydrogen atoms, moved by small spacecraft with constant velocity in the Earth's gravitational field. We suggest the corresponding experiment on the Earth's orbit to detect this radiation, which would be the first direct experiment where quantum effects in general relativity are observed.

 

[MTd2]

Non QG:

http://arxiv.org/abs/1404.4252

The Riemann zeros as energy levels of a Dirac fermion in a potential built from the prime numbers in Rindler spacetime

German Sierra
(Submitted on 16 Apr 2014)
We construct a Hamiltonian H whose discrete spectrum contains, in a certain limit, the Riemann zeros. H is derived from the action of a massless Dirac fermion living in a domain of Rindler spacetime, in 1+1 dimensions, that has a boundary given by the world line of a uniformly accelerated observer.
The action contains a sum of delta function potentials that can be viewed as partially reflecting moving mirrors. An appropriate choice of the accelerations of the mirrors, provide primitive periodic orbits associated to the prime numbers p, whose periods, measured by the observer's clock, are log p. Acting on the chiral components of the fermion, H becomes the Berry-Keating Hamiltonian (x p + p x)/2, where x is identified with the Rindler spatial coordinate and p with the conjugate momentum.
The delta function potentials give the matching conditions of the fermion wave functions on both sides of the mirrors. There is also a phase shift for the reflection of the fermions at the boundary where the observer sits. The eigenvalue problem is solved by transfer matrix methods in the limit where the reflection amplitudes become infinitesimally small. We find that for generic values of the phase shift the spectrum is a continuum, where the Riemann zeros are missing, as in the adelic Connes model. However, for some values of phase shift, related to the phase of the zeta function, the Riemann zeros appear as discrete eigenvalues immersed in the continuum.
We generalize this result to the zeros of Dirichlet L-functions, associated to primitive characters, that are encoded in the reflection coefficients of the mirrors. Finally, we show that the Hamiltonian associated to the Riemann zeros belongs to class AIII, or chiral GUE, of Random Matrix Theory.

 

[marcus]

http://arxiv.org/abs/1404.4364
Nonsingular bouncing cosmologies in light of BICEP2
Yi-Fu Cai, Jerome Quintin, Emmanuel N. Saridakis, Edward Wilson-Ewing
(Submitted on 16 Apr 2014)
We confront various nonsingular bouncing cosmologies with the recently released BICEP2 data and investigate the observational constraints on their parameter space. In particular, within the context of the effective field approach, we analyze the constraints on the matter bounce curvaton scenario with a light scalar field, and the new matter bounce cosmology model in which the universe successively experiences a period of matter contraction and an ekpyrotic phase. Additionally, we consider three nonsingular bouncing cosmologies obtained in the framework of modified gravity theories, namely the Horava-Lifshitz bounce model, the f(T) bounce model, and loop quantum cosmology.
8 pages, 3 figures

Refers to this February paper by two of the authors, Yi-Fu Cai and Edward Wilson-Ewing:
http://arxiv.org/abs/1402.3009
which in turn refers to this by Wilson-Ewing:
http://arxiv.org/abs/1306.6582

very brief mention:
http://arxiv.org/abs/1404.4614
Negative running prevents eternal inflation
William H. Kinney (Univ. at Buffalo, SUNY), Katherine Freese (Univ. of Michigan)
(Submitted on 17 Apr 2014)
Current data from the Planck satellite and the BICEP2 telescope favor, at around the 2σ level, negative running of the spectral index of curvature perturbations...

 

[atyy]

http://arxiv.org/abs/1404.4537
En route to Background Independence: Broken split-symmetry, and how to restore it with bi-metric average actions
Daniel Becker, Martin Reuter
(Submitted on 17 Apr 2014)
The most momentous requirement a quantum theory of gravity must satisfy is Background Independence, necessitating in particular an ab initio derivation of the arena all non-gravitational physics takes place in, namely spacetime. Using the background field technique, this requirement translates into the condition of an unbroken split-symmetry connecting the (quantized) metric fluctuations to the (classical) background metric. If the regularization scheme used violates split-symmetry during the quantization process it is mandatory to restore it in the end at the level of observable physics. In this paper we present a detailed investigation of split-symmetry breaking and restoration within the Effective Average Action (EAA) approach to Quantum Einstein Gravity (QEG) with a special emphasis on the Asymptotic Safety conjecture. In particular we demonstrate for the first time in a non-trivial setting that the two key requirements of Background Independence and Asymptotic Safety can be satisfied simultaneously. Carefully disentangling fluctuation and background fields, we employ a `bi-metric' ansatz for the EAA and project the flow generated by its functional renormalization group equation on a truncated theory space spanned by two separate Einstein-Hilbert actions for the dynamical and the background metric, respectively. A new powerful method is used to derive the corresponding renormalization group (RG) equations for the Newton- and cosmological constant, both in the dynamical and the background sector. We classify and analyze their solutions in detail, determine their fixed point structure, and identify an attractor mechanism which turns out instrumental in the split-symmetry restoration. We show that there exists a subset of RG trajectories which are both asymptotically safe and split-symmetry restoring. We conclude that the next generation of truncations must be bi-metric.

 

[marcus]

http://arxiv.org/abs/1404.5288
Discretization independence implies non-locality in 4D discrete quantum gravity
Bianca Dittrich, Wojciech Kaminski, Sebastian Steinhaus
(Submitted on 21 Apr 2014)
The 4D Regge action is invariant under 5--1 and 4--2 Pachner moves, which define a subset of (local) changes of the triangulation. Given this fact one might hope to find a local path integral measure that makes the quantum theory invariant under these moves and hence makes the theory partially triangulation invariant. We show that such a local invariant path integral measure does not exist for the 4D linearized Regge theory.
To this end we uncover an interesting geometric interpretation for the Hessian of the 4D Regge action. This geometric interpretation will allow us to prove that the determinant of the Hessian of the 4D Regge action does not factorize over 4--simplices or subsimplices. It furthermore allows to determine configurations where this Hessian vanishes, which only appears to be the case in degenerate backgrounds or if one allows for different orientations of the simplices.
We suggest a non--local measure factor that absorbs the non--local part of the determinant of the Hessian under 5--1 moves as well as a local measure factor that is preserved for very special configurations.
18 pages, 6 figures

http://arxiv.org/abs/1404.5284
Fluctuation energies in quantum cosmology
Martin Bojowald
(Submitted on 21 Apr 2014)
Quantum fluctuations or other moments of a state contribute to energy expectation values and can imply interesting physical effects. In quantum cosmology, they turn out to be important for a discussion of density bounds and instabilities of initial-value problems in the presence of signature change in loop-quantized models. This article provides an effective description of these issues, accompanied by a comparison with existing numerical results and an extension to squeezed states. The comparison confirms that canonical effective methods are well-suited for computations of properties of physical states. As a side product, an example is found for a simple state in which quantum fluctuations can cancel holonomy modifications of loop quantum cosmology.
35 pages, 6 figures

 

[MTd2]

http://arxiv.org/abs/1404.5821

Planck star phenomenology

Aurelien Barrau, Carlo Rovelli
(Submitted on 23 Apr 2014)
It is possible that black holes hide a core of Planckian density, sustained by quantum-gravitational pressure. As a black hole evaporates, the core remembers the initial mass and the final explosion occurs at macroscopic scale. We investigate possible phenomenological consequences of this idea. Under several rough assumptions, we estimate that up to several short gamma-ray bursts per day, around 10 MeV, with isotropic distribution, can be expected coming from a region of a few hundred light years around us.

Related:

http://arxiv.org/abs/1404.5635

Proof of a Quantum Bousso Bound

Raphael Bousso, Horacio Casini, Zachary Fisher, Juan Maldacena
(Submitted on 22 Apr 2014)
We prove the generalized Covariant Entropy Bound, ΔS≤(A−A′)/4Gℏ, for light-sheets with initial area A and final area A′. The entropy ΔS is defined as a difference of von Neumann entropies of an arbitrary state and the vacuum, with both states restricted to the light-sheet under consideration. The proof applies to free fields, in the limit where gravitational backreaction is small. We do not assume the null energy condition. In regions where it is violated, we find that the bound is protected by the defining property of light-sheets: that their null generators are nowhere expanding.

 

[marcus]

http://arxiv.org/abs/1404.6797
Quantum Gravity models - brief conceptual summary
Jerzy Lukierski
(Submitted on 27 Apr 2014)
After short historical overview we describe the difficulties with application of standard QFT methods in quantum gravity (QG). The incompatibility of QG with the use of classical continuous space-time required conceptually new approach. We present briefly three proposals:

• loop quantum gravity (LQG),
• the field-theoretic framework on noncommutative space-time and
• QG models formulated on discretized (triangularized) space-time.

We evaluate these models as realizing expected important properties of QG: background independence, consistent quantum diffeomorphisms, noncommutative or discrete structure of space-time at very short distances, finite/renormalizable QG corrections. We only briefly outline an important issue of embedding QG into larger geometric and dynamical frameworks (e.g. supergravity, (super)strings, p-branes, M-theory), with the aim to achieve full unification of all fundamental interactions.
24 pages; invited article for the book "Mathematical Structure of the Universe", publ. Copernicus Center Press, Copernicus Center for Interdisciplinary Studies, Cracow 2014

 

[marcus]

general interest (observational cosmology/astrophysics are potential windows on QG):
http://arxiv.org/abs/1404.7266
Cosmological constraints on dark energy
Tamara M. Davis
(Submitted on 29 Apr 2014)
23 pages, 8 figures.

 

[marcus]

http://arxiv.org/abs/1404.7517
The double scaling limit of random tensor models
Valentin Bonzom, Razvan Gurau, James P. Ryan, Adrian Tanasa
(Submitted on 29 Apr 2014)
Tensor models generalize matrix models and generate colored triangulations of pseudo-manifolds in dimensions D≥3. The free energies of some models have been recently shown to admit a double scaling limit, i.e. large tensor size N while tuning to criticality, which turns out to be summable in dimension less than six. This double scaling limit is here extended to arbitrary models. This is done by means of the Schwinger--Dyson equations, which generalize the loop equations of random matrix models, coupled to a double scale analysis of the cumulants.
36 pages

 

[MTd2]

http://arxiv.org/abs/1405.0249

GR 20 Parallel Session A3: Modified Gravity

Petr Horava, Arif Mohd, Charles M. Melby-Thompson, Peter Shawhan
(Submitted on 1 May 2014)
This is the contribution representing Parallel Session A3, on Modified Gravity, in the Proceedings of the GR 20 Conference (July 2013, Warszawa, Poland). It consists of three invited chapters, selected by the Session Chair (P.H.) to represent the broad spectrum of topics discussed in the Session, which ranged from theoretical and phenomenological, to experimental, observational and numerical aspects of gravity. The three chapters are "Einstein-Aether Theory: Thermodynamics of Universal Horizons" by Arif Mohd, "The Curious Case of Conformal Anomalies in Horava-Lifshitz Gravity" by Charles M. Melby-Thompson, and "Detectability of Scalar Gravitational-Wave Bursts with LIGO and Virgo" by Peter Shawhan.

 

[MTd2]

When they talk about stochastic inflation, I cannot stop thinking that they might also be gluing different Penrose's aeons.

http://arxiv.org/abs/1405.0298

De Sitter Space Without Quantum Fluctuations

Kimberly K. Boddy, Sean M. Carroll, Jason Pollack
(Submitted on 1 May 2014)
We argue that, under certain plausible assumptions, de Sitter space settles into a quiescent vacuum in which there are no quantum fluctuations. Quantum fluctuations require time-dependent histories of out-of-equilibrium recording devices, which are absent in stationary states. For a massive scalar field in a fixed de Sitter background, the cosmic no-hair theorem implies that the state of the patch approaches the vacuum, where there are no fluctuations. We argue that an analogous conclusion holds whenever a patch of de Sitter is embedded in a larger theory with an infinite-dimensional Hilbert space, including semiclassical quantum gravity with false vacua or complementarity in theories with at least one Minkowski vacuum. This reasoning provides an escape from the Boltzmann brain problem in such theories. It also implies that vacuum states do not uptunnel to higher-energy vacua and that perturbations do not decohere while slow-roll inflation occurs, suggesting that eternal inflation is much less common than often supposed. On the other hand, if a de Sitter patch is a closed system with a finite-dimensional Hilbert space, there will be Poincare recurrences and Boltzmann fluctuations into lower-entropy states. Our analysis does not alter the conventional understanding of the origin of density fluctuations from primordial inflation, since reheating naturally generates a high-entropy environment and leads to decoherence.

 

[MTd2]

Just interesting:

arxiv.org/abs/1405.1310

bEvidence of the Big Fix

Yuta Hamada, Hikaru Kawai, Kiyoharu Kawana
(Submitted on 6 May 2014)
We give an evidence of the Big Fix. The theory of wormholes and multiverse suggests that the parameters of the Standard Model are fixed in such a way that the total entropy at the late stage of the universe is maximized, which we call the maximum entropy principle. In this paper, we discuss how it can be confirmed by the experimental data, and we show that it is indeed true for the Higgs vacuum expectation value vh. We assume that the baryon number is produced by the sphaleron process, and that the current quark masses, the gauge couplings and the Higgs self coupling are fixed when we vary vh. It turns out that the existence of the atomic nuclei plays a crucial role to maximize the entropy. This is reminiscent of the anthropic principle, however it is required by the fundamental low in our case.

http://arxiv.org/abs/1405.1188

Pre-inflationary genesis with CMB B-mode polarization

Zhi-Guo Liu, Hong Li, Yun-Song Piao
(Submitted on 6 May 2014)
The observation of the CMB B-mode polarization at large angular scales will probably tell us what occurs before inflation. We show that for a primordial universe, which is in a slowly expanding genesis phase before the slow-roll inflation, the primordial tensor spectrum will get a large-scale cutoff, i.e. nT≳1 at large scales while nT≃0 at small scale. We find that this inflationary scenario not only may reconcile the observations of BICEP2 and Planck well, but also predicts a large-scale anomaly in BB power spectrum, i.e. due to the large suppression of tensor perturbation amplitude we will hardly see the reionization bump at low l, which may be falsified by the Planck polarization data.

 

[marcus]

may be of general interest, quantum foundations:
http://arxiv.org/abs/1405.1548
The Cellular Automaton Interpretation of Quantum Mechanics. A View on the Quantum Nature of our Universe, Compulsory or Impossible?
Gerard 't Hooft
(Submitted on 7 May 2014)
When investigating theories at the tiniest conceivable scales in nature, "quantum logic" is taking over from "classical logic" in the minds of almost all researchers today. Dissatisfied, the author investigated how one can look at things differently. This report is an overview of older material, but also contains many new observations and calculations. Quantum mechanics is looked upon as a tool, not as a theory. Examples are displayed of models that are classical in essence, but can be analysed by the use of quantum techniques, and we argue that even the Standard Model, together with gravitational interactions, may be viewed as a quantum mechanical approach to analyse a system that could be classical at its core. We then explain how these apparently heretic thoughts can be reconciled with Bell's theorem and the usual objections voiced against the notion of 'super determinism'. Our proposal would eradicate the collapse problem and the measurement problem.
202 pages, 20 figures

 

[marcus]

http://arxiv.org/abs/arXiv:1405.1753
Exhaustive investigation of the duration of inflation in effective anisotropic loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 7 May 2014)
This article addresses the issue of estimating the duration in inflation in bouncing cosmology when anisotropies, inevitably playing and important role, are taken into account. It is shown that in Bianchi-I loop quantum cosmology, the higher the shear, the shorter the period of inflation. For a wide range of parameters, the probability distribution function of the duration of inflation is however peaked at values compatible with data, but not much higher. This makes the whole bounce/inflationary scenario consistent and phenomenologically appealing as all the information from the bounce might then not have been fully washed out.
7pages, 5 figures

http://arxiv.org/abs/arXiv:1405.1830
Non-commutative quantum geometric data in group field theories
Daniele Oriti
(Submitted on 8 May 2014)
We review briefly the motivations for introducing additional group-theoretic data in tensor models, leading to the richer framework of group field theories, themselves a field theory formulation of loop quantum gravity. We discuss how these data give to the GFT amplitudes the structure of lattice gauge theories and simplicial gravity path integrals, and make their quantum geometry manifest. We focus in particular on the non-commutative flux/algebra representation of these models.
10 pages; to appear in the proceedings of the workshop "Non-commutative field theory and gravity", Corfu', Greece, EU, September 2013

 

[David Horgan]

Looking forward to reading this.

 

[David Horgan]

Thanks for this post.Gerard 't Hooft is always worth reading.

 

[marcus]

http://arxiv.org/abs/1405.2808
Bubbling Group Field Theory
Aristide Baratin, Laurent Freidel, Razvan Gurau
(Submitted on 12 May 2014)
Group field theories (GFT) are higher dimensional generalizations of matrix models whose Feynman diagrams are dual to triangulations. Here we propose a modification of GFT models that includes extra field indices keeping track of the bubbles of the graphs in the Feynman evaluations. In dimension three, our model exhibits new symmetries, interpreted as the action of the vertex translations of the triangulation. The extra field indices have an elegant algebraic interpretation: they encode the structure of a semi-simple algebra. Remarkably, when the algebra is chosen to be associative, the new structure contributes a topological invariant from each bubble of the graph to the Feynman amplitudes.
19 pages, 6 figures

possible interest:
http://arxiv.org/abs/1405.2776
See Saw Inflation / Dark Energy
George F. Smoot
(Submitted on 12 May 2014)
10 pages

 

[marcus]

http://arxiv.org/abs/1405.3483
Quantum Mechanics Without State Vectors
Steven Weinberg
(Submitted on 14 May 2014)
It is proposed to give up the description of physical states in terms of ensembles of state vectors with various probabilities, relying instead solely on the density matrix as the description of reality. With this definition of a physical state, even in entangled states nothing that is done in one isolated system can instantaneously effect the physical state of a distant isolated system. This change in the description of physical states opens up a large variety of new ways that the density matrix may transform under various symmetries, different from the unitary transformations of ordinary quantum mechanics. Such new transformation properties have been explored before, but so far only for the symmetry of time translations into the future, treated as a semi-group. Here new transformation properties are studied for general symmetry transformations forming groups, rather than semi-groups. Arguments are given that such symmetries should act on the density matrix as in ordinary quantum mechanics, but loopholes are found for all of these arguments.
28 pages

http://arxiv.org/abs/1405.3492
The birth of spacetime atoms as the passage of time
Fay Dowker
(Submitted on 14 May 2014)
The view that the passage of time is physical finds expression in the classical sequential growth models of Rideout and Sorkin in which a discrete spacetime grows by the partially ordered accretion of new spacetime atoms.
15 pages, 1 figure. Article based on an invited talk at the conference "Do we need a physics of passage?" Cape Town, South Africa, 10-14 Dec 2012. Submitted for publication in Annals of the New York Academy of Sciences (2014)

 

[marcus]

http://arxiv.org/abs/1405.4585
Renormalization Group Flow in CDT
J. Ambjorn, A. Goerlich, J. Jurkiewicz, A. Kreienbuehl, R. Loll
(Submitted on 19 May 2014)
We perform a first investigation of the coupling constant flow of the nonperturbative lattice model of four-dimensional quantum gravity given in terms of Causal Dynamical Triangulations (CDT). After explaining how standard concepts of lattice field theory can be adapted to the case of this background-independent theory, we define a notion of "lines of constant physics" in coupling constant space in terms of certain semiclassical properties of the dynamically generated quantum universe. Determining flow lines with the help of Monte Carlo simulations, we find that the second-order phase transition line present in this theory can be interpreted as a UV phase transition line if we allow for an anisotropic scaling of space and time.
20 pages

 

[MTd2]

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!

 

[marcus]

http://arxiv.org/abs/1405.4881
Non-equilibrium thermodynamics of gravitational screens
Laurent Freidel, Yuki Yokokura
(Submitted on 19 May 2014)
We study the Einstein gravity equations projected on a timelike surface, which represents the time evolution of what we call a gravitational screen. We show that such a screen possesses a surface tension and an internal energy, and that the Einstein equations reduce to the thermodynamic equations of a viscous bubble. We also provide a complete dictionary between gravitational and thermodynamical variables. In the non-viscous cases there are three thermodynamic equations which characterise a bubble dynamics: These are the first law, the Marangoni flow equation and the Young-Laplace equation. In all three equations the surface tension plays a central role: In the first law it appears as a work term per unit area, in the Marangoni flow its gradient drives a force, and in the Young-Laplace equation it contributes to a pressure proportional to the surface curvature. The gravity equations appear as a natural generalization of these bubble equations when the bubble itself is viscous and dynamical. In particular, it shows that the mechanism of entropy production for the viscous bubble is mapped onto the production of gravitational waves. We also review the relationship between surface tension and temperature, and discuss the usual black-hole thermodynamics from this point of view.
27 pages, 3 figures

http://arxiv.org/abs/1405.4967
Grin of the Cheshire cat: Entropy density of spacetime as a relic from quantum gravity
Dawood Kothawala, T. Padmanabhan
(Submitted on 20 May 2014)
There is considerable evidence to suggest that the field equations of gravity have the same status as, say, the equations describing an emergent phenomenon like elasticity. In fact, it is possible to derive the field equations from a thermodynamic variational principle in which a set of normalized vector fields are varied rather than the metric. We show that this variational principle can arise as a low energy (L_P=(Gℏ/c³)^1/2→0) relic of a plausible nonperturbative effect of quantum gravity, viz. the existence of a zero-point-length in the spacetime. Our result is nonperturbative in the following sense: If we modify the geodesic distance in a spacetime by introducing a zero-point-length, to incorporate some effects of quantum gravity, and take the limit L_P→0 of the Ricci scalar of the modified metric, we end up getting a nontrivial, leading order (L_P - independent) term. This term is identical to the expression for entropy density of spacetime used previously in the emergent gravity approach. This reconfirms the idea that the microscopic degrees of freedom of the spacetime, when properly described in the full theory, could lead to an effective description of geometry in terms of a thermodynamic variational principle. This is conceptually similar to the emergence of thermodynamics from mechanics of, say, molecules. The approach also has important implications for cosmological constant which are briefly discussed.
21+9 pages

http://arxiv.org/abs/1405.5174
The Hafele-Keating experiment, velocity and length interval transformations and resolution of the Ehrenfest paradox
J.H. Field
(Submitted on 23 Apr 2014)
6 pages

 

[marcus]

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
(Submitted on 16 May 2014)
We show within the usual two-dimensional approximation that unitarity and the restoration of Minkowski vacuum correlations at the end of black hole evaporation impose unexpected constraints on its mass loss rate: before disappearing the black hole emits one or more negative energy burst, leading to a temporary increase of its mass.
8 pages; nontechnical version of http://arxiv.org/abs/1404.0602 E. Bianchi, M.Smerlak
"Entanglement entropy and negative-energy fluxes in two-dimensional space times"

http://arxiv.org/abs/1405.5219
How General Relativity and Lorentz Covariance Arise from the Spatially Covariant Effective Field Theory of the Transverse, Traceless Graviton
Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley
(Submitted on 20 May 2014)
Traditional derivations of general relativity from the graviton degrees of freedom assume space-time Lorentz covariance as an axiom. In this essay, we survey recent evidence that general relativity is the unique spatially-covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector. From this point of view, Lorentz covariance is a necessary feature of low-energy graviton dynamics, not a property of space-time. This result has revolutionary implications for fundamental physics.
5 pages. Essay received honorable mention in the Gravity Research Foundation 2014 essay competition. Some overlap with http://arxiv.org/abs/1305.0822
J. Khoury, G. E. J. Miller and A. J. Tolley, “On the Origin of Gravitational Lorentz Covariance,”

 

[MTd2]

http://pirsa.org/14050134/

Vacuogenesis in Discrete Cosomology
Speaker(s): Ted Jacobson
Abstract:
Date: 22/05/2014 - 5:40 pmThis looks like devastating to BICEP2:

http://arxiv.org/abs/1405.5857v1

A joint analysis of Planck and BICEP2 B modes including dust polarization uncertainty

Michael J. Mortonson, Uroš Seljak
(Submitted on 22 May 2014)
We analyze BICEP2 and Planck data using a model that includes CMB lensing, gravity waves, and polarized dust. Recently published Planck dust polarization maps have highlighted the difficulty of estimating the amount of dust polarization in low intensity regions, suggesting that the polarization fractions have considerable uncertainties and may be significantly higher than previous predictions. In this paper we do not assume anything about the dust polarization, except for the power spectrum shape, which we take to be CBB,dustl∝l−2.3. The resulting joint BICEP2+Planck analysis favors solutions without gravity waves, and the upper limit on the tensor-to-scalar ratio is r<0.11, a slight improvement relative to the Planck analysis alone which gives r<0.13 (95% c.l.). The estimated amplitude of the dust polarization power spectrum is in rough agreement with expectations for this field based on HI column density. We address the cross-correlation analysis of BICEP2 at 150 GHz with BICEP1 at 100 GHz as a test of foreground contamination. We find that the null hypothesis of dust and lensing with r=0 gives Δχ2<2 relative to the hypothesis of no dust, so the frequency analysis does not strongly favor either model over the other. We also discuss how Planck dust polarization maps may improve our constraints. If the dust polarization is measured perfectly, the limit can reach r<0.05 (or the corresponding detection significance if the observed dust signal plus the expected lensing signal is below the BICEP2 observations), but this degrades quickly to almost no improvement if the dust calibration error is 20% or larger or if the Planck dust maps are not processed through the BICEP2 pipeline, inducing sampling variance noise.

 

[marcus]

Perimeter held a series of eighteen talks called Quantum Gravity Day 2014 on 21 and 22 May.
Links to all the videos are here:
http://pirsa.org/C14026
The majority were short (20 minute) talks, covering a wide variety of approaches to QG.
The previous post by MTd2 already gave a link to the QG-day talk by Ted Jacobson, and I'll list a sampling of the others to give an idea of the scope:

http://pirsa.org/14050119/
New Vacuum and Representation For Loop Quantum Gravity
Bianca Dittrich

http://pirsa.org/14050122
Entanglement and Edge Modes in Gauge Theory and Gravity
William Donnelly

http://pirsa.org/14050124/
Phases for (analogue) spin foam models
Sebastian Steinhaus

http://pirsa.org/14050125
Asymptotic safety – a quantum theory of gravity and matter
Astrid Eichhorn

http://pirsa.org/14050126
Positive Energy in Quantum Gravity
Lee Smolin

http://pirsa.org/14050127
What About Gravitizing Quantum Mechanics?
Laurent Freidel

http://pirsa.org/14050138
Reality Check for Quantum Gravity
Renate Loll

http://pirsa.org/14050133
Elements of Quantum Theory from limited Information and complementarity
Philipp Hoehn

http://pirsa.org/14050135
Cosmological Dynamics of Space As a Quantum Gravity Condensate
Steffen Gielen

 

[MTd2]

This a VERY IMPORTANT 2 page paper!

http://arxiv.org/abs/1405.6351v1

Comment on "Scalar Einstein-Aether theory"

Ted Jacobson, Antony J. Speranza
(Submitted on 25 May 2014)
A recent paper studies a modification of Einstein-aether theory in which the aether vector is restricted, at the level of the action, to be the gradient of a scalar. In this comment we note that this scalar version of Einstein-aether theory is equivalent to the projectable version of the IR limit of Ho\v{r}ava gravity when the potential for the scalar is constant. This provides a generally covariant formulation for projectable Ho\v{r}ava gravity.

 

[marcus]

http://arxiv.org/abs/1405.7287
Statistical and entanglement entropy for black holes in quantum geometry
Alejandro Perez
(Submitted on 28 May 2014)
We analyze the relationship between entanglement (or geometric) entropy with statistical mechanical entropy of horizon degrees of freedom when described in the framework of isolated horizons in loop quantum gravity. We show that, once the relevant degrees of freedom are identified, the two notions coincide. The key ingredient linking the two notions is the structure of quantum geometry at Planck scale implied by loop quantum gravity, where correlations between the inside and outside of the black hole are mediated by eigenstates of the horizon area operator.

http://arxiv.org/abs/1405.7249
Singularity avoidance in quantum-inspired inhomogeneous dust collapse
Yue Liu, Daniele Malafarina, Leonardo Modesto, Cosimo Bambi
(Submitted on 28 May 2014)
In a previous paper, some of us studied general relativistic homogeneous gravitational collapses for dust and radiation, in which the density profile was replaced by an effective density justified by some quantum gravity models. It was found that the effective density introduces an effective pressure that becomes negative and dominant in the strong-field regime. With this set-up, the central singularity is replaced by a bounce, after which the cloud starts expanding. Motivated by the fact that in the classical case homogeneous and inhomogeneous collapse models have different properties, here we extend our previous work to the inhomogeneous case. As in the quantum-inspired homogeneous collapse model, the classical central singularity is replaced by a bounce, but the inhomogeneities strongly affect the structure of the bounce curve and of the trapped region.
8 pages, 5 figures
[BTW: Planck star paper cited in reference 14]

 

[atyy]

http://arxiv.org/abs/1405.7056
CFT/Gravity Correspondence on the Isolated Horizon
Amit Ghosh, Daniele Pranzetti
(Submitted on 27 May 2014)
A quantum isolated horizon can be modeled by an SU(2) Chern-Simons theory on a punctured 2-sphere. We show how a local 2-dimensional conformal symmetry arises at each puncture inducing an infinite set of new observables localized at the horizon which satisfy a Kac-Moody algebra. By means of the isolated horizon boundary conditions, we represent the gravitational fluxes degrees of freedom in terms of the zero modes of the Kac-Moody algebra defined on the boundary of a punctured disk. In this way, our construction encodes a precise notion of CFT/gravity correspondence. The higher modes in the algebra represent new nongeometric charges which can be represented in terms of free matter field degrees of freedom. When computing the CFT partition function of the system, these new states induce an extra degeneracy factor, representing the density of horizon states at a given energy level, which reproduces the Bekenstein's holographic bound for an imaginary Immirzi parameter. This allows us to recover the Bekenstein-Hawking entropy formula without the large quantum gravity corrections associated with the number of punctures.

 

[marcus]

Helpful 70-minute PIRSA video of talk by Smerlak explaining the 16 May paper:
http://pirsa.org/14050027
Last gasp ‎of a black hole: why unitary evaporation must be non-monotonic
Abstract: I will describe the relationship between radiated energy and entanglement entropy of massless fields at future null infinity (the "Page curve") in two-dimensional models of black hole evaporation. ‎I will use this connection to derive a general feature of any unitary-preserving evaporation scenario: the Bondi mass of the hole must be non-monotonic. Time permitting, I will comment on time scales in such scenarios.
29/05/2014

http://arxiv.org/abs/1405.5235
Last gasp of a black hole: unitary evaporation implies non-monotonic mass loss
Eugenio Bianchi, Matteo Smerlak
(Submitted on 16 May 2014)
We show within the usual two-dimensional approximation that unitarity and the restoration of Minkowski vacuum correlations at the end of black hole evaporation impose unexpected constraints on its mass loss rate: before disappearing the black hole emits one or more negative energy burst, leading to a temporary increase of its mass.
8 pages; nontechnical version of http://arxiv.org/abs/1404.0602 E. Bianchi, M.Smerlak
"Entanglement entropy and negative-energy fluxes in two-dimensional space times"
...

Comment: problems which keep arising with BH evaporation may have to do with the assumption of unitarity.

 

[marcus]

http://arxiv.org/abs/1406.0369
Viability of the matter bounce scenario in Loop Quantum Cosmology for general potentials
Jaume Haro, Jaume Amorós
(Submitted on 2 Jun 2014)
We consider the matter bounce scenario in Loop Quantum Cosmology (LQC) for physical potentials that at early times provide a nearly matter dominated Universe in the contracting phase, having a reheating mechanism in the expanding phase, i.e., being able to release the energy of the scalar field creating particles that thermalize in order to match with the hot Friedmann Universe, and finally at late times leading to the current cosmic acceleration. For these models, numerically solving the dynamical equations we have seen that the teleparallel version of LQC leads to theoretical results that fit well with current observational data. More precisely, in teleparallel LQC there is a set of solutions which leads to theoretical results that match correctly with last BICEP2 data, and there is another set whose theoretical results fit well with Planck's experimental data. On the other hand, in holonomy corrected LQC the theoretical value of the tensor/scalar ratio is smaller than in teleparallel LQC, which means that there is always a set of solutions that matches with Planck's data, but for some potentials BICEP2 experimental results disfavours holonomy corrected LQC.
29 pages, 8 figures

 

[marcus]

http://arxiv.org/abs/1406.0579
The Koslowski-Sahlmann representation: Quantum Configuration Space
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 3 Jun 2014)
The Koslowski-Sahlmann (KS) representation is a generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity (LQG), to accommodate states labelled by smooth spatial geometries. As shown recently, the KS representation supports, in addition to the action of the holonomy and flux operators, the action of operators which are the quantum counterparts of certain connection dependent functions known as "background exponentials".
Here we show that the KS representation displays the following properties which are the exact counterparts of LQG ones:
(i) the abelian ∗ algebra of SU(2) holonomies and 'U(1)' background exponentials can be completed to a C* algebra
(ii) the space of semianalytic SU(2) connections is topologically dense in the spectrum of this algebra
(iii) there exists a measure on this spectrum for which the KS Hilbert space is realized as the space of square integrable functions on the spectrum
(iv) the spectrum admits a characterization as a projective limit of finite numbers of copies of SU(2) and U(1)
(v) the algebra underlying the KS representation is constructed from cylindrical functions and their derivations in exactly the same way as the LQG (holonomy-flux) algebra except that the KS cylindrical functions depend on the holonomies and the background exponentials, this extra dependence being responsible for the differences between the KS and LQG algebras.

While these results are obtained for compact spaces, they are expected to be of use for the construction of the KS representation in the asymptotically flat case.
33 pages

brief mention:
http://arxiv.org/abs/1405.7860
What is the distance to the CMB? How relativistic corrections remove the tension with local H₀ measurements
Chris Clarkson, Obinna Umeh, Roy Maartens, Ruth Durrer
(Submitted on 30 May 2014)
.. removes the tension between local measurements of H₀ and those measured through the CMB and favours a closed universe.
9 pages, 6 figures

brief mention (relevance to the cosmological constant problem):
http://arxiv.org/abs/1406.0711
Vacuum Energy Sequestering: The Framework and Its Cosmological Consequences
Nemanja Kaloper, Antonio Padilla
(Submitted on 3 Jun 2014)
38 pages, 1 figure

 

[MTd2]

Important to Flying Saucers builders:

http://arxiv.org/abs/1406.1105

Can a Casimir cavity fly - and other connected questions

Massimo Cerdonio, Carlo Rovelli
(Submitted on 4 Jun 2014)
In the attempt to extract from the current vast literature about Casimir cavities if an experiment based on them can actually weight the vacuum, and if one of a suitably small mass could ever float, we discuss a simple gedanken experiment, based on minimal assumptions, Special Relativity, the Equivalence Principle, conservation of local energy and the limit of weak gravitational field. It appears to show i) that the Casimir energy negative contribution to the total mass-energy of the cavity behaves as an ordinary mass defects, due the Casimir binding force, ii) that such a cavity cannot fly on first principles, and iii) that the vacuum is weighted only as far as its contribution comes from the interaction with the matter of the plates. We conclude by considering a possible relation of our findings with the role of vacuum energy of the electromagnetic field in cosmology.

 

[marcus]

http://arxiv.org/abs/1406.1456
Fermi-bounce cosmology and the fermion curvaton mechanism
Stephon Alexander, Yi-Fu Cai, Antonino Marciano
(Submitted on 5 Jun 2014)
A nonsingular bouncing cosmology can be achieved by introducing a fermion field with BCS condensation occurring at high energy scales. In this paper we are able to dilute the anisotropic stress near the bounce by means of releasing the gap energy density near the phase transition between the radiation and condensate states. In order to explain the nearly scale-invariant CMB spectrum, another fermion field is required. We investigate one possible curvaton mechanism by involving one another fermion field without condensation where the mass is lighter than the background field. We show that, by virtue of the fermion curvaton mechanism, our model can satisfy the latest cosmological observations very well, and that the fermion species involved may realize a cosmological see-saw mechanism after one finely tunes model parameters.
9 pages, 4 figures

brief mention, unrelated but of possible interest:
http://arxiv.org/abs/1406.0711
Vacuum Energy Sequestering: The Framework and Its Cosmological Consequences
Nemanja Kaloper, Antonio Padilla
(Submitted on 3 Jun 2014)
Recently we suggested a reformulation of General Relativity which completely sequesters from gravity all of the vacuum energy from a protected matter sector, assumed to contain the Standard Model. …We also consider cosmological consequences in more detail and show that the mechanism is consistent with a variety of inflationary models ... We also note that since the universe should be compact in spacetime, and so will collapse in the future, the current phase of acceleration with w_DE≈−1 is just a transient. This could be tested by future cosmological observations.
38 pages, 1 figure

http://arxiv.org/abs/1406.1136
Velocities hasten to tell us about the Universe
Yin-Zhe Ma, Douglas Scott
(Submitted on 4 Jun 2014)
The peculiar velocities of galaxies are driven by gravity, and hence hold the promise of probing details of how gravity forms structures. ...we appear to be entering an era when velocity measurements may finally be living up to their promise.
6 pages, 8 figures. Review article.

 

[marcus]

http://arxiv.org/abs/1406.1486
Numerical evolution of squeezed and non-Gaussian states in loop quantum cosmology
Peter Diener, Brajesh Gupt, Miguel Megevand, Parampreet Singh
(Submitted on 5 Jun 2014)
In recent years, numerical simulations with Gaussian initial states have demonstrated the existence of a quantum bounce in loop quantum cosmology in various models. A key issue pertaining to the robustness of the bounce and the associated physics is to understand the quantum evolution for more general initial states which may depart significantly from Gaussianity and may have no well defined peakedness properties. The analysis of such states, including squeezed and highly non-Gaussian states, has been computationally challenging until now. In this manuscript, we overcome these challenges by using the Chimera scheme for the spatially flat, homogeneous and isotropic model sourced with a massless scalar field. We demonstrate that the quantum bounce in loop quantum cosmology occurs even for states which are highly squeezed or are non-Gaussian with multiple peaks and with little resemblance to semi-classical states. The existence of the bounce is found to be robust, and does not depend on the properties of the states. The evolution of squeezed and non-Gaussian states turns out to be qualitatively similar to the Gaussian states and satisfies strong constraints on the growth of the relative fluctuations across the bounce. We also compare the results from the effective dynamics and find that, though it captures the qualitative aspects of the evolution for squeezed and highly non-Gaussian states, it always underestimates the bounce volume. We show that various properties of the evolution, such as the energy density at the bounce, are in excellent agreement with the predictions from an exactly solvable loop quantum cosmological model for arbitrary states.
26 pages, 16 figures

 

[marcus]

Correction

I overlooked this March 2014 paper by Arzano when first posted. It might turn out to be useful in connection with the LQG black hole, so I'll add a link to it even though out of sequential order:

http://arxiv.org/abs/1403.6457
Purity is not eternal at the Planck scale
Michele Arzano
(Submitted on 25 Mar 2014)
Theories with Planck-scale deformed symmetries exhibit quantum time evolution in which purity of the density matrix is not preserved. In particular we show that the non-trivial structure of momentum space of these models is reflected in a deformed action of translation generators on operators. Such action in the case of time translation generators leads to a Lindblad-like evolution equation for density matrices when expanded at leading order in the Planckian deformation parameter. This evolution equation is covariant under the deformed realization of Lorentz symmetries characterizing these models.
6 pages.

Here is a 2010 online video that gives an introduction to the kind of thing he is talking about in the March 2014 paper
http://pirsa.org/10050017/
Fun from none: deformed Fock space and hidden entanglement
Michele Arzano
Attempts to go beyond the framework of local quantum field theory include scenarios in which the action of external symmetries on the quantum fields Hilbert space is deformed. A common feature of these models is that the quantum group symmetry of their Hilbert spaces induces additional structure in the multiparticle states which in turns reflects a non-trivial momentum-dependent statistics. In certain particular models which might be relevant for quantum gravity the richer structure of the deformed Fock space allows for the possibility of entanglement between the field modes and certain ''planckian'' degrees of freedom invisible to an observer that cannot probe the Planck scale.
19/05/2010

 

[marcus]

http://arxiv.org/abs/1406.2610
Emergence of string-like physics from Lorentz invariance in loop quantum gravity
Rodolfo Gambini, Jorge Pullin
(Submitted on 10 Jun 2014)
We consider a quantum field theory on a spherically symmetric quantum space time described by loop quantum gravity. The spin network description of space time in such a theory leads to equations for the quantum field that are discrete. We show that to avoid significant violations of Lorentz invariance one needs to consider specific non-local interactions in the quantum field theory similar to those that appear in string theory. This is the first sign that loop quantum gravity places restrictions on the type of matter considered, and points to a connection with string theory physics.
7 pages. Honorable mention Gravity Research Foundation 2014.


http://arxiv.org/abs/1406.2611
Positive energy in quantum gravity
Lee Smolin
(Submitted on 10 Jun 2014)
This paper addresses the question of whether Witten's proof of positive ADM energy for classical general relativity can be extended to give a proof of positive energy for a non-perturbative quantization of general relativity. To address this question, a set of conditions is shown to be sufficient for showing the positivity of a Hamiltonian operator corresponding to the ADM energy. One of these conditions is a particular factor ordering for the constraints of general relativity, in a representation where the states are functionals of the Ashtekar connection, and the auxiliary, Witten spinor.
These developments are partly based on results derived with Artem Starodubtsev (unpublished notes, 2004).
14 pages.

http://arxiv.org/abs/1406.2337
Asymptotic safety guaranteed
Daniel F. Litim, Francesco Sannino
(Submitted on 9 Jun 2014)
We study the ultraviolet behaviour of four-dimensional quantum field theories involving non-abelian gauge fields, fermions and scalars in the Veneziano limit. In a regime where asymptotic freedom is lost, we explain how the three types of fields cooperate to develop fully interacting ultraviolet fixed points, strictly controlled by perturbation theory. Extensions towards strong coupling and beyond the large-N limit are discussed.
31 pages, 7 figures

brief mention:
http://arxiv.org/abs/1406.2609
Superluminal Gravitational Waves
J. W. Moffat
(Submitted on 10 Jun 2014)
The quantum gravity effects of vacuum polarization of gravitons propagating in a curved spacetime cause the quantum vacuum to act as a dispersive medium with a refractive index. Due to this dispersive medium gravitons acquire superluminal velocities. ... It is shown that in a Friedmann-Lemaître-Robertson-Walker spacetime in the early universe near the Planck time..., the speed of gravitational waves c_g ≫ c_g0=c₀, where c_g0 and c₀ are the speeds of gravitational waves and light today. The large speed of gravitational waves stretches their wavelengths to super-horizon sizes, allowing them to be observed in B-polarization experiments.
5 pages

 

[marcus]

http://arxiv.org/abs/1406.3706
Our Universe from the cosmological constant
Aurelien Barrau, Linda Linsefors
(Submitted on 14 Jun 2014)
In this article, we consider a bouncing Universe, as described for example by Loop Quantum Cosmology. If the current acceleration is due to a true cosmological constant, this constant is naturally conserved through the bounce and the Universe should also be in a (contracting) de Sitter phase in the remote past. We investigate here the possibility that the de Sitter temperature in the contracting branch fills the Universe with radiation and causes the bounce and the subsequent inflation and reheating. We also consider the possibility that this gives rise to a cyclic model of the Universe and suggest some possible tests.
5 pages

http://arxiv.org/abs/1406.3635
Tunneling across the quantum horizon does not resolve the information paradox
Avik Roy, Moinul Hossain Rahat
(Submitted on 13 Jun 2014)
Hawking radiation has been demonstrated as quantum tunneling across the event horizon and the spectrum has been proved to be nonthermal. These nonthermality factors emerging due to back reaction effects have been claimed to be responsible for correlations among the emitted quanta. It has been claimed by several authors in literature that these correlations actually carry out information locked in a black hole and hence provide a resolution to the long debated black hole information paradox. This paper demonstrates that this is a fallacious proposition.
13 pages

http://arxiv.org/abs/1406.3829
Operational quantum theory without predefined time
Ognyan Oreshkov, Nicolas J. Cerf
(Submitted on 15 Jun 2014)
The current operational formulation of quantum theory is based on the concept of operation with an input and an output system, which assumes a prior notion of time and is asymmetric under time reversal. But in certain contexts, such as those involving gravity, time is expected to be dynamical and not predefined. Here, we propose an operational formulation of quantum theory without any predefined notion of time. It is based on a generalization of the concept of operation motivated by an epistemic approach: an operation is a description of knowledge about the events in a given region, which can be updated conditionally on information obtained from that region. Each such region has a set of boundary systems which by definition provide the sole means of information exchange between the events in the region and the events in other regions. Separate operations can be connected in networks through their boundary systems with no directionality assumed for the connections, which generalizes the standard circuit picture. The events associated with an operation are described by positive semidefinite operators on the Hilbert spaces of the boundary systems, while the connections between regions are described by entangled states that encode a non-trivial physical symmetry. A simple rule provides the joint probabilities for the events in a network of operations. We discuss how it may be possible to understand the emergence of a causal structure in space-time from properties of the operators on the boundaries of compact space-time regions. The framework allows for indefinite causal order, timelike loops, and other acausal structures. As part of this work, we obtain a generalization of Wigner's theorem, which is based on the preservation of probabilities of actual events and thus puts the concept of time reversal symmetry on operational grounds.
29 pages, 10 figures

 

[marcus]

http://arxiv.org/abs/1406.4531
Renormalization of lattice-regularized quantum gravity models II. The case of causal dynamical triangulations
Joshua H. Cooperman
(Submitted on 17 Jun 2014)
The causal dynamical triangulations approach aims to construct a quantum theory of gravity as the continuum limit of a lattice-regularized model of dynamical geometry. A renormalization group scheme--in concert with finite size scaling analysis--is essential to this aim. Formulating and implementing such a scheme in the present context raises novel and notable conceptual and technical problems. I explored these problems, and, building on standard techniques, suggested potential solutions in the first paper of this two-part series. As an application of these solutions, I now propose a renormalization group scheme for causal dynamical triangulations. This scheme differs significantly from that studied recently by Ambjorn, Gorlich, Jurkiewicz, Kreienbuehl, and Loll.
26 pages, 11 figures. The first paper in the two-part series will appear shortly. The second paper is sufficiently self-contained to be read on its own.

http://arxiv.org/abs/1406.4532
Renormalization for Philosophers
Jeremy Butterfield, Nazim Bouatta
(Submitted on 17 Jun 2014)
We have two aims. The main one is to expound the idea of renormalization in quantum field theory, with no technical prerequisites (Sections 2 and 3). Our motivation is that renormalization is undoubtedly one of the great ideas, and great successes, of twentieth-century physics. Also it has strongly influenced in diverse ways, how physicists conceive of physical theories. So it is of considerable philosophical interest. Second, we will briefly relate renormalization to Ernest Nagel's account of inter-theoretic relations, especially reduction (Section 4).
One theme will be a contrast between two approaches to renormalization. The old approach, which prevailed from ca. 1945 to 1970, treated renormalizability as a necessary condition for being an acceptable quantum field theory. On this approach, it is a piece of great good fortune that high energy physicists can formulate renormalizable quantum field theories that are so empirically successful. But the new approach to renormalization (from 1970 onwards) explains why the phenomena we see, at the energies we can access in our particle accelerators, are described by a renormalizable quantum field theory. For whatever non-renormalizable interactions may occur at yet higher energies, they are insignificant at accessible energies. Thus the new approach explains why our best fundamental theories have a feature, viz. renormalizability, which the old approach treated as a selection principle for theories.
We also maintain that universality, a concept stressed in renormalization theory, is essentially the familiar philosophical idea of multiple realizability; and that it causes no problems for reductions of a Nagelian kind.
Comments: 48 pages, 1 figure, Forthcoming in Metaphysics in Contemporary Physics: a volume of Poznan Studies in Philosophy of Science, eds. T. Bigaj and C. Wuethrich; 2015.

briefly noted as possibly of side interest:
http://arxiv.org/abs/1406.4354
Reduction, Emergence and Renormalization
Jeremy Butterfield
(Submitted on 17 Jun 2014)
43 pages, The Journal of Philosophy, volume 111 (2014), pp. 5-49

http://arxiv.org/abs/1406.4745
On Time in Quantum Physics
Jeremy Butterfield
(Submitted on 18 Jun 2014)
First, I briefly review the different conceptions of time held by three rival interpretations of quantum theory: the collapse of the wave-packet, the pilot-wave interpretation, and the Everett interpretation (Section 2).
Then I turn to a much less controversial task: to expound the recent understanding of the time-energy uncertainty principle, and indeed of uncertainty principles in general, that has been established by such authors as Busch, Hilgevoord and Uffink.
Although this may at first seem a narrow topic, I point out connections to other conceptual topics about time in quantum theory: for example, the question under what circumstances there is a time operator (Section 4.3).
26 pages, 'The Blackwell Companion to the Philosophy of Time', edited by A. Bardon and H. Dyke, Wiley-Blackwell, 2013; pp. 220-241

http://arxiv.org/abs/1406.4747
On Under-determination in cosmology
Jeremy Butterfield
(Submitted on 18 Jun 2014)
31 pages, Studies in the History and Philosophy of Modern Physics, vol 46, (2014), pp. 57-69

http://arxiv.org/abs/1406.4732
Laws, Causation and Dynamics at Different Levels
Jeremy Butterfield
(Submitted on 18 Jun 2014)
29 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1406.6021
Analytic Continuation of Black Hole Entropy in Loop Quantum Gravity
Jibril Ben Achour (APC - UMR 7164), Amaury Mouchet (LMPT), Karim Noui (APC - UMR 7164, LMPT)
(Submitted on 23 Jun 2014)
We define the analytic continuation of the number of black hole microstates in Loop Quantum Gravity to complex values of the Barbero-Immirzi parameter γ. This construction deeply relies on the link between black holes and Chern-Simons theory. Technically, the key point consists in writing the number of microstates as an integral in the complex plane of a holomorphic function, and to make use of complex analysis techniques to perform the analytic continuation. Then, we study the thermodynamical properties of the corresponding system (the black hole is viewed as a gas of indistinguishable punctures) in the framework of the grand canonical ensemble where the energy is defined à la Frodden-Gosh-Perez from the point of view of an observer located close to the horizon. The semi-classical limit occurs at the Unruh temperature T_U associated to this local observer. When γ=±i, the entropy reproduces at the semi-classical limit the area law with quantum corrections. Furthermore, the quantum corrections are logarithmic provided that the chemical potential is fixed to the simple value μ=2T_U.
31 pages, 2 figures

http://arxiv.org/abs/1406.5524
Black hole entropy with and without log correction in loop quantum gravity
P. Mitra
(Submitted on 2 Jun 2014)
Earlier calculations of black hole entropy in loop quantum gravity have given a term proportional to the area with a correction involving the logarithm of the area when the area eigenvalue is close to the classical area. However the calculations yield an entropy proportional to the area eigenvalue with no such correction when the area eigenvalue is large compared to the classical area.
6 pages; talk delivered at Light Cone conference, Delhi, 2012, to appear in Nucl. Phys. B, 251-252 (2014)

http://arxiv.org/abs/1406.5518
A 3.55 keV line from DM→a→γ: predictions for cool-core and non-cool-core clusters
Joseph P. Conlon, Andrew J. Powell

http://arxiv.org/abs/1406.5808
3.55 keV X-ray Line Interpretation in Radiative Neutrino Model
Hiroyuki Ishida, Hiroshi Okada

 

[atyy]

http://arxiv.org/abs/1406.7159
Gravitons and a complex of differential operators
Kirill Krasnov
(Submitted on 27 Jun 2014)
Gravity is now understood to become simple on-shell. We sketch how it becomes simple also off-shell, when reformulated appropriately. Thus, we describe a simple Lagrangian for gravitons that makes use of a certain complex of differential operators. The Lagrangian is constructed analogously to that of Maxwell's theory, just using a different complex. The complex, and therefore also our description of gravitons, makes sense on any half-conformally flat four-dimensional manifold.

 

[marcus]

http://arxiv.org/abs/1406.7304
Entanglement entropy and nonabelian gauge symmetry
William Donnelly
(Submitted on 27 Jun 2014)
Entanglement entropy has proven to be an extremely useful concept in quantum field theory. Gauge theories are of particular interest, but for these systems the entanglement entropy is not clearly defined because the physical Hilbert space does not factor as a tensor product according to regions of space. Here we review a definition of entanglement entropy that applies to abelian and nonabelian lattice gauge theories. This entanglement entropy is obtained by embedding the physical Hilbert space into a product of Hilbert spaces associated to regions with boundary. The latter Hilbert spaces include degrees of freedom on the entangling surface that transform like surface charges under the gauge symmetry. These degrees of freedom are shown to contribute to the entanglement entropy, and the form of this contribution is determined by the gauge symmetry. We test our definition using the example of two-dimensional Yang-Mills theory, and find that it agrees with the thermal entropy in de Sitter space, and with the results of the Euclidean replica trick. We discuss the possible implications of this result for more complicated gauge theories, including quantum gravity.
12 pages. Invited article for Classical and Quantum Gravity special issue on Entanglement and Quantum Gravity


brief mention:
http://arxiv.org/abs/1406.7318
Cosmological Perturbations in Antigravity
Marius Oltean, Robert Brandenberger
We compute the evolution of cosmological perturbations in a recently proposed Weyl-symmetric theory of two scalar fields with oppositely-signed conformal couplings to Einstein gravity. It is motivated from the minimal conformal extension of the Standard Model, such that one of these scalar fields is the Higgs while the other is a new particle, the dilaton, introduced to make the Higgs mass conformally symmetric. At the background level, the theory admits novel geodesically-complete cyclic cosmological solutions characterized by a brief period of repulsive gravity, or "antigravity", during each successive transition from a Big Crunch to a Big Bang. We show that despite the necessarily wrong-signed kinetic term of the dilaton in the full action, its cosmological solutions are stable at the perturbative level.
15 pages.

http://arxiv.org/abs/1406.7713
Absence of cosmological constant problem in special relativistic field theory of gravity
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
The principles of quantum field theory in flat spacetime suggest that gravity is mediated by a massless particle with helicity ±2, the so-called graviton. It is regarded as textbook knowledge that, when the self-coupling of a particle with these properties is considered, the long-wavelength structure of such a nonlinear theory is fixed to be that of general relativity. However, …
...This special relativistic field theory of gravity implies the decoupling of vacuum zero-point energies of matter and passes all the known experimental tests in gravitation.
10 pages.

 

[marcus]

http://arxiv.org/abs/1407.0025
New action for simplicial gravity in four dimensions
Wolfgang M. Wieland
(Submitted on 30 Jun 2014)
We develop a proposal for a theory of simplicial gravity with spinors as the fundamental configuration variables. The underlying action describes a mechanical system with finitely many degrees of freedom, the system has a Hamiltonian and local gauge symmetries. We will close with some comments on the resulting quantum theory, and explain the relation to loop quantum gravity and twisted geometries. The paper appears in parallel with an article by Cortês and Smolin, who study the relevance of the model for energetic causal sets and various other approaches to quantum gravity.
26 pages, 2 figures.

http://arxiv.org/abs/1407.0032
Spin foam models as energetic causal sets
Marina Cortês, Lee Smolin
(Submitted on 30 Jun 2014)
Energetic causal sets are causal sets endowed by a flow of energy-momentum between causally related events. These incorporate a novel mechanism for the emergence of space-time from causal relations. Here we construct a spin foam model which is also an energetic causal set model. This model is closely related to the model introduced by Wieland, and this construction makes use of results used there. What makes a spin foam model also an energetic causal set is Wieland's identification of new momenta, conserved at events (or four-simplices), whose norms are not mass, but the volume of tetrahedra. This realizes the torsion constraints, which are missing in previous spin foam models, and are needed to relate the connection dynamics to those of the metric, as in general relativity. This identification makes it possible to apply the new mechanism for the emergence of space-time to a spin foam model.
16 pages, 4 figures.

http://arxiv.org/abs/1407.0284
The Tensor Theory Space
Vincent Rivasseau
(Submitted on 1 Jul 2014)
The tensor track is a background-independent discretization of quantum gravity which includes a sum over all topologies. We discuss how to define a functional renormalization group flow and the Wetterich equation in the corresponding theory space. This space is different from the Einsteinian theory space of asymptotic safety. It includes all fixed-rank tensor-invariant interactions, hence generalizes matrix models and the (Moyal) non-commutative field theory space.
12 pages. This short note is intended as a complement to arXiv:1311.1461, to appear in the Proceedings of the Workshop on Noncommutative Field Theory and Gravity in Corfu September 2013, Fortshritt. Phys. 2014

 

[MTd2]

http://arxiv.org/abs/1406.7237

Cosmic Jerk and Snap in Penrose's CCC model

Maciej Dunajski
(Submitted on 27 Jun 2014)
We obtain a constraint on cosmological scalars for the FRW metric with a pure radiation fluid source and positive cosmological constant. We demonstrate that this constraint is conformally invariant in the context of Penrose's Conformal Cyclic Cosmology proposal, where the metrics of the late stages of the previous aeon and the early stages of the present aeon are described by FRW cosmologies.

Jerk: http://en.wikipedia.org/wiki/Jerk_(physics )
Snap: http://en.wikipedia.org/wiki/Jounce

http://arxiv.org/abs/1407.0130

Probing Beyond Standard Model via Hawking Radiated Gravitational Waves

Tomohiro Fujita
(Submitted on 1 Jul 2014)
We propose a novel technique to probe the beyond standard model (BSM) of particle physics. The mass spectrum of unknown BSM particles can be scanned by observing gravitational waves (GWs) emitted by Hawking radiation of black holes. This is because information on the radiation of the BSM particles is imprinted in the spectrum of the GWs. We fully calculate the GW spectrum from evaporating black holes taking into account the greybody factor. As an observationally interesting application, we consider primordial black holes which evaporate in the very early universe. In that case, since the frequencies of GWs are substantially redshifted, the GWs emitted with the BSM energy scales become accessible by observations.

 

[marcus]

http://arxiv.org/abs/1407.0989
Black hole fireworks: quantum-gravity effects outside the horizon spark black to white hole tunneling
Hal M. Haggard, Carlo Rovelli
(Submitted on 3 Jul 2014)
We show that there is a classical metric satisfying the Einstein equations outside a finite spacetime region where matter collapses into a black hole and then emerges from a white hole. We compute this metric explicitly. We show how quantum theory determines the (long) time for the process to happen. A black hole can thus quantum-tunnel into a white hole. For this to happen, quantum gravity should affect the metric also in a small region outside the horizon: we show that contrary to what is commonly assumed, this is not forbidden by causality or by the semiclassical approximation, because quantum effects can pile up over a long time. This scenario alters radically the discussion on the black hole information puzzle.
10 pages, 5 figures

http://arxiv.org/abs/1407.0998
Cosmological perturbations in Hybrid Loop Quantum Cosmology: Mukhanov-Sasaki variables
Laura Castelló Gomar, Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 3 Jul 2014)
We study cosmological perturbations in the framework of Loop Quantum Cosmology, using a hybrid quantization approach and Mukhanov-Sasaki variables. The formulation in terms of these gauge invariants allows one to clarify the independence of the results on choices of gauge and facilitates the comparison with other approaches proposed to deal with cosmological perturbations in the context of Loop Quantum Theory. A kind of Born-Oppenheimer ansatz is employed to extract the dynamics of the inhomogeneous perturbations, separating them from the degrees of freedom of the Friedmann-Robertson-Walker geometry. With this ansatz, we derive an approximate Schrödinger equation for the cosmological perturbations and study its range of validity. We also prove that, with an alternate factor ordering, the dynamics deduced for the perturbations is similar to the one found in the so-called "dressed metric approach", apart from a possible scaling of the matter field in order to preserve its unitary evolution in the regime of Quantum Field Theory in a curved background and some quantization prescription issues. Finally, we obtain the effective equations that are naturally associated with the Mukhanov-Sasaki variables, both with and without introducing the Born-Oppenheimer ansatz, and with the different factor orderings that we have studied.
45 pages

 

[atyy]

http://arxiv.org/abs/1407.1124
Cosmological Constant in a Regge State-sum Model of Quantum Gravity
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 4 Jul 2014)
We study the quantum contributions to the classical cosmological constant in a Regge state-sum model of quantum gravity in the effective action approach. We use a special path-integral measure and we include matter, in the form of a massive scalar field. The effective cosmological constant is given as a sum of 3 terms: the classical CC, the quantum gravity CC and the matter CC. Since observations can only measure the sum of these 3 terms, we can choose the classical CC to be equal to the negative value of the matter CC. Hence the effective CC is given by the quantum gravity CC, which is determined by the path-integral measure only. Since the path-integral measure depends on a free parameter, this parameter can be chosen such that the effective CC gives the observed value.

 

[marcus]

http://arxiv.org/abs/1407.1391
Mutiny at the white-hole district
Carlos Barceló, Raúl Carballo-Rubio, Luis J. Garay
(Submitted on 5 Jul 2014)
The white-hole sector of Kruskal's solution is almost never used in physical applications. However, it might contain the solution to many of the problems associated with gravitational collapse and evaporation. This essay tries to draw attention to some bouncing geometries that make a democratic use of the black- and white-hole sectors. We will argue that these types of behaviour could be perfectly natural in some approaches to the next physical level beyond classical general relativity.
8 pages, 1 figure. Essay awarded a honorable mention in the 2014 Gravity Research Foundation essay competition

http://arxiv.org/abs/1407.1394
Regge Quantum Gravity Solution to the Cosmological Constant Problem
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 5 Jul 2014)
We show that it is possible to solve the cosmological constant (CC) problem in a discrete quantum gravity theory based on Regge calculus by using the effective action approach and a special path-integral measure. The effective cosmological constant is a sum of 3 terms: the classical CC, the quantum gravity CC and the matter CC. Since observations can only measure the sum of these 3 terms, we can choose the classical CC to be equal to the negative value of the matter CC. Hence the effective CC is given by the quantum gravity CC, which is determined by the path-integral measure. Since the path-integral measure depends on a free parameter, this parameter can be chosen such that the effective CC gives the observed value.
5 pages

http://arxiv.org/abs/1407.1784
Neutrinos from the Early Universe and Physics Beyond Standard Models
Daniela Kirilova
(Submitted on 7 Jul 2014)
Neutrino oscillations present the only robust example of experimentally detected physics beyond the standard model. This review discusses the established and several hypothetical beyond standard models neutrino characteristics and their cosmological effects and constraints. Particularly, the contemporary cosmological constraints on the number of neutrino families, neutrino mass differences and mixing, lepton asymmetry in the neutrino sector, neutrino masses, light sterile neutrino are briefly reviewed.
20 pages, invited review

 

[marcus]

http://arxiv.org/abs/1407.2428
Kantowski-Sachs spacetime in loop quantum cosmology: geometric scalars and the viability of quantization prescriptions
Anton Joe, Parampreet Singh
(Submitted on 9 Jul 2014)
Using effective dynamics, we investigate the behavior of expansion and shear scalars in different proposed quantizations of the Kantowski-Sachs spacetime with matter in loop quantum cosmology. We find that out of the various proposed choices, there is only one known prescription which leads to the generic bounded behavior of these scalars. The bounds turn out to be universal and are determined by the underlying quantum geometry. This quantization is analogous to the so called `improved dynamics' in the isotropic loop quantum cosmology, which is also the only one to respect the freedom of the rescaling of the fiducial cell. Other proposed quantization prescriptions yield geometric scalars which may not be bounded for certain initial conditions within the validity of effective spacetime description. These prescriptions also have a limitation that the "quantum geometric effects" can occur at an arbitrary scale. We show that the 'improved dynamics' of Kantowski-Sachs spacetime turns out to be a unique choice in a general class of possible quantization prescriptions, in the sense of leading to generic bounds on geometric scalars and the associated physics being free from fiducial cell dependence. The behavior of the energy density in the 'improved dynamics' reveals some interesting features. Even without considering any details of the dynamical evolution, it is possible to rule out pancake singularities in this spacetime. The energy density is found to be dynamically bounded. These results show that the Planck scale physics of the loop quantized Kantowski-Sachs spacetime has key features common with the loop quantization of isotropic and Bianchi-I spacetimes.
18 pages, 2 figures

 

[MTd2]

General interest:

http://arxiv.org/abs/1407.2528

Faster than light motion does not imply time travel

H. Andréka, J. X. Madarász, I. Németi, M. Stannett, G. Székely
(Submitted on 9 Jul 2014)
Seeing the many examples in the literature of causality violations based on faster-than- light (FTL) signals one naturally thinks that FTL motion leads inevitably to the possibility of time travel. We show that this logical inference is invalid by demonstrating a model, based on (3+1)-dimensional Minkowski spacetime, in which FTL motion is permitted (in every direction without any limitation on speed) yet which does not admit time travel. Moreover, the Principle of Relativity is true in this model in the sense that all observers are equivalent. In short, FTL motion does not imply time travel after all.