Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1407.2909
Linking shape dynamics and loop quantum gravity
Lee Smolin
(Submitted on 10 Jul 2014)
Shape dynamics is a reformulation of general relativity, locally equivalent to Einstein's theory, in which the refoliation invariance of the older theory is traded for local scale invariance. Shape dynamics is here derived in a formulation related to the Ashtekar variables by beginning with a modification of the Plebanski action. The constraints of shape dynamics and their algebra are reproduced in terms of these new variables.
12 pages,

 

[marcus]

http://arxiv.org/abs/1407.3027
Convergent Y ̃-Map for a new covariant Loop Quantum Gravity formulation and Implicit Reality Condition
Leonid Perlov
(Submitted on 11 Jul 2014)
One of the most important elements in a new spin-foam loop quantum gravity formulation is the map Y: H^SU(2)→H^SL(2,C). In this paper we provide an alternative improved map Y ̃ . The image of a new map Y ̃ contains the weighted infinite sums of SL(2,C) matrix coefficients. The sums are convergent and their limits are the square integrable functions of SL(2,C) with the measure L²(g,e^−|Y|2/ℏη(g)dudY) according to the recently proved Holomorphic Peter-Weyl theorem [2]. We also discuss the consequence of a choice of a unitary principal series instead of the general principal series (sometimes called non-unitary) in EPRL model. The general principal series contains the unitary principal series as a sub-representation and becomes unitary when its parameter ν is real rather than complex. The solution of the simplicity constraint then implicitly makes the Barbero-Immirizi parameter real instead of complex. We call this - an implicit reality condition.
7 pages

 

[marcus]

http://arxiv.org/abs/1407.3768
Loop Quantum Cosmology with Complex Ashtekar Variables
Jibril Ben Achour, Julien Grain, Karim Noui
(Submitted on 14 Jul 2014)
We construct and study Loop Quantum Cosmology (LQC) when the Barbero-Immirzi parameter takes the complex value γ=±i. We refer to this new quantum cosmology as complex Loop Quantum Cosmology. We proceed in making an analytic continuation of the Hamiltonian constraint (with no inverse volume corrections) from real γ to γ=±i in the simple case of a flat FLRW Universe coupled to a massless scalar field with no cosmological constant. For that purpose, we first compute the non-local curvature operator (defined by the trace of the holonomy of the connection around a fundamental plaquette) evaluated in any spin j representation and we find a new close formula for it. This allows to define explicitly a one parameter family of regularizations of the Hamiltonian constraint in LQC, parametrized by the spin j. It is immediate to see that any spin j regularization leads to a bounce scenario. Then, motivated particularly by previous results on black hole thermodynamics, we perform the analytic continuation of the Hamiltonian constraint defined by γ=±i and j=−1/2+is where s is real. Even if the area spectrum is now continuous, we show that the so-defined complex LQC removes also the original singularity which is replaced by a quantum bounce. In addition, the maximal density and the minimal volume of the Universe are obviously independent of γ. Furthermore, the dynamics before and after the bounce are no more symmetric, which makes a clear distinction between these two phases of the evolution of the Universe.
22 pages

http://arxiv.org/abs/1407.3384
Why do we remember the past and not the future? The 'time oriented coarse graining' hypothesis
Carlo Rovelli
(Submitted on 12 Jul 2014)
Phenomenological arrows of time can be traced to a past low-entropy state. Does this imply the universe was in an improbable state in the past? I suggest a different possibility: past low-entropy depends on the coarse-graining implicit in our definition of entropy. This, in turn depends on our physical coupling to the rest of the world. I conjecture that any generic motion of a sufficiently rich system satisfies the second law of thermodynamics, in either direction of time, for some choice of macroscopic observables. The low entropy of the past could then be due to the way we couple to the universe (a way needed for us doing what we do), hence to our natural macroscopic variables, rather than to a strange past microstate of the world at large.
5 pages. Few equations. An idea

http://arxiv.org/abs/1407.3577
Astrophysical Black Hole horizons in a cosmological context: Nature and possible consequences on Hawking Radiation
George F R Ellis, Rituparno Goswami, Aymen I. M. Hamid, Sunil D. Maharaj
(Submitted on 14 Jul 2014)
This paper considers the nature of apparent horizons for astrophysical black hole situated in a realistic cosmological context. Using semi-tetrad covariant methods we study the local evolutions of the boundaries of the trapped region in the spacetime. For a collapsing massive star immersed in a cosmology with Cosmic Microwave Background Radiation, we show that the initial 2 dimensional marginally trapped surface bifurcates into inner and outer horizons. The inner horizon is timelike while the continuous CMBR influx into the black hole makes the outer horizon spacelike. We discuss the possible consequences of these features for Hawking radiation in realistic astrophysical contexts.
13 pages, 4 figures

http://arxiv.org/abs/1407.3457
Multiple choices of time in quantum cosmology
Przemyslaw Malkiewicz
(Submitted on 13 Jul 2014)
It is often conjectured that a choice of time function merely sets up a frame for the quantum evolution of gravitational field, meaning that all choices should be in some sense compatible. In order to explore this conjecture (and the meaning of compatibility), we develop suitable tools for determining the relation between quantum theories based on different time functions. First, we discuss how a time function fixes a canonical structure on the constraint surface. The presentation includes both the kinematical and the reduced perspective, and the relation between them. Second, we formulate twin theorems about the existence of two inequivalent maps between any two deparameterizations, a {\it formal canonical} and a {\it coordinate} one. They are used to separate the effect of choice of clock from other effects. We show, in an example, how the spectra of quantum observables are transformed under the change of clock and prove, via a general argument, the existence of choice-of-time-induced semiclassical effects. Finally, we study an example, in which we find that the semiclassical discrepancies can in fact be arbitrarily large for dynamical observables. We conclude that the values of critical energy density or critical volume in the bouncing scenarios of quantum cosmology cannot in general be at the Planck scale and always need to be given with reference to a specific time function.
21 pages, 7 figures

 

[marcus]

http://arxiv.org/abs/1407.3977
Minimal Coupling and Attractors
David Sloan
(Submitted on 15 Jul 2014)
The effects of minimally coupling a gravity to matter on a flat Robertson-Walker geometry are explored. Particular attention is paid to the evolution of the symplectic structure and the Liouville measure it defines. We show that the rescaling freedom introduced by choice of fiducial cell leads to a symmetry between dynamical trajectories, which together with the Liouville measure provides a natural volume weighting explanation for the generic existence of attractors.
11 pages

"...from the perspective of an observer who only has access to homogeneous local field configurations there is no physical observation of the fields from which this length scale can be deduced. This freedom to rescale leads to a symmetry on the space of solutions. This has been discussed in the context of inflation in Loop Quantum Cosmology [1, 2] and its result in explaining inflationary attractors in [3]…"

refs [1-3] are to LQC papers by Ashtekar, Sloan, and others

 

[marcus]

http://arxiv.org/abs/1407.4444
Effective constraint algebras with structure functions
Martin Bojowald, Suddhasattwa Brahma
(Submitted on 16 Jul 2014)
This article presents the result that fluctuations and higher moments of a state do not imply quantum corrections in structure functions of constrained systems. Consequences for canonical quantum gravity, whose structure functions encode space-time structure, are discussed. In particular, deformed algebras found in models of loop quantum gravity provide reliable information even in the Planck regime.
16 pages

To see what Bojowald is driving at, note this final passage in the conclusions:
==quote==
...Such ordering terms may affect the details of concrete models based on constraint algebras with deformed structure functions, especially in strong quantum regimes. Nevertheless, our main result that moment-dependent quantum back-reaction terms do not affect the leading structure functions has an important consequence also in this context: In all consistent effective models found so far in the presence of holonomy corrections, the deformed structure functions change sign around any local maximum of holonomies as functions of the connection or extrinsic curvature. (The structure functions are proportional to the second derivative of holonomy modification functions [24, 28, 33].) This change of sign can be interpreted as signature change, with a quantum version of 4-dimensional Euclidean space replacing Lorentzian space-time when the structure functions take the opposite sign [6, 7, 8]. Even if holonomy modification functions are subjected to ℏ-corrections from factor ordering terms, this general conclusion about signature change remains unaltered. (The only assumption in its derivation is that the classical quadratic dependence of the Hamiltonian constraint on the connection or extrinsic curvature is replaced by some function of the basic expectation values, not necessarily of any specific form such as a sine function often used in this context.) Our results therefore show that moment terms do not affect the central statements about signature change.
As we have discussed in Section 2.2, results about effective constraint algebras are reliable in regimes in which semiclassical approximations of the dynamics may be expected to be poor. One can therefore trust implications of deformed algebras even in the Planck regime. The main such result is signature change at high curvature or density.
==endquote==

 

[marcus]

http://arxiv.org/abs/1407.4792
On the Second Law of Thermodynamics: The Significance of Coarse-Graining and the Role of Decoherence
Mahdiyar Noorbala
(Submitted on 17 Jul 2014)
We take up the question why the initial entropy in the universe was small, in the context of evolution of the entropy of a classical system. We note that coarse-graining is a an important aspect of entropy evaluation which can reverse the direction of the increase in entropy, i.e., the direction of thermodynamic arrow of time. Then we investigate the role of decoherence in the selection of coarse-graining and explain how to compute entropy for a decohered classical system. Finally, we argue that the requirement of low initial entropy imposes constraints on the decoherence process.
4 pages
==quote from conclusions==
Note added. At the time of completion of this work, an independent paper by Rovelli with similar ideas appeared on arXiv [5]. While some of the basic ideas are shared in our works, there are differences as well. The notion of coarse-graining employed in Ref. [5] is different from the one used here. It is related to coarse-graining with fixing macroscopic variables, as discussed in the first paragraph of Section II here. Then the macroscopic observables that are to be used for coarse-graining are picked by interactions among subsystems. Thus in Ref. [5], interactions play the role that decoherence plays here. As we discussed at the end of Section III, decoherence plays this role through interactions as well as system/environment splitting and the quantum state. So there is quite a similarity here.

[5] C. Rovelli, “Why do we remember the past and not the future? The ‘time oriented coarse graining’ hypothesis,” arXiv:1407.3384 [hep-th].
==endquote==
see post #2212 of this thread:
https://www.physicsforums.com/showthread.php?p=4797616#post4797616

http://arxiv.org/abs/1407.4748
Perturbative Quantum Gravity Comes of Age
R. P. Woodard (University of Florida)
(Submitted on 17 Jul 2014)
I argue that cosmological data from the epoch of primordial inflation is catalyzing the maturation of quantum gravity from speculation into a hard science. I explain why quantum gravitational effects from primordial inflation are observable. I then review what has been done, both theoretically and observationally, and what the future holds. I also discuss what this tells us about quantum gravity.
64 pages, 6 figures, review article written for "One Hundred Years of General Relativity"

http://arxiv.org/abs/1407.4615
Discrete Renormalization Group for SU(2) Tensorial Group Field Theory
Sylvain Carrozza
(Submitted on 17 Jul 2014)
This article provides a Wilsonian description of the perturbatively renormalizable Tensorial Group Field Theory introduced in arXiv:1303.6772 [hep-th] (Commun. Math. Phys. 330, 581-637). It is a rank-3 model based on the gauge group SU(2), and as such is expected to be related to Euclidean quantum gravity in three dimensions. By means of a power-counting argument, we introduce a notion of dimensionality of the free parameters defining the action. General flow equations for the dimensionless bare coupling constants can then be derived, in terms of a discretely varying cut-off, and in which all the so-called melonic Feynman diagrams contribute. Linearizing around the Gaussian fixed point allows to recover the splitting between relevant, irrelevant, and marginal coupling constants. Pushing the perturbative expansion to second order for the marginal parameters, we are able to determine their behaviour in the vicinity of the Gaussian fixed point. Along the way, several technical tools are reviewed, including a discussion of combinatorial factors and of the Laplace approximation, which reduces the evaluation of the amplitudes in the UV limit to that of Gaussian integrals.
36 pages, 20 figures

 

[John86]

http://arxiv.org/abs/1407.5848
Propagating gravitons vs. dark matter in asymptotically safe quantum gravity
Daniel Becker, Martin Reuter
(Submitted on 22 Jul 2014)
Within the Asymptotic Safety scenario, we discuss whether Quantum Einstein Gravity (QEG) can give rise to a semi-classical regime of propagating physical gravitons (gravitational waves) governed by an effective theory which complies with the standard rules of local quantum field theory. According to earlier investigations based on single-metric truncations there is a tension between this requirement and the condition of Asymptotic Safety since the former (latter) requires a positive (negative) anomalous dimension of Newton's constant. We show that the problem disappears using the bi-metric renormalization group flows that became available recently: They admit an asymptotically safe UV limit and, at the same time, a genuine semi-classical regime with a positive anomalous dimension. This brings the gravitons of QEG on a par with arbitrary (standard model, etc.) particles which exist as asymptotic states. We also argue that metric perturbations on almost Planckian scales might not be propagating, and we propose an interpretation as a form of `dark matter'.

http://arxiv.org/abs/1407.5678
Super-radiance and flux conservation
Petarpa Boonserm (Chulalongkorn University), Tritos Ngampitipan (Chulalongkorn University), Matt Visser (Victoria University of Wellington)
(Submitted on 21 Jul 2014)
The theoretical foundations of the phenomenon known as super-radiance still continues to attract considerable attention. Despite many valiant attempts at pedagogically clear presentations, the effect nevertheless still continues to generate some significant confusion. Part of the confusion arises from the fact that super-radiance in a quantum field theory [QFT] context is not the same as super-radiance (super-fluorescence) in some condensed matter contexts; part of the confusion arises from traditional but sometimes awkward normalization conventions, and part is due to sometimes unnecessary confusion between fluxes and probabilities. We shall argue that the key point underlying the effect is flux conservation, (and, in the presence of dissipation, a controlled amount of flux non-conservation), and that attempting to phrase things in terms of reflection and transmission probabilities only works in the absence of super-radiance. To help clarify the situation we present a simple exactly solvable toy model exhibiting both super-radiance and damping.

 

[atyy]

http://arxiv.org/abs/1407.5977
Is Quantum Gravity a Chern-Simons Theory?
R. Bonezzi, O. Corradini, A. Waldron
(Submitted on 22 Jul 2014)
We propose a model of quantum gravity in arbitrary dimensions defined in terms of the BV quantization of a supersymmetric, infinite dimensional matrix model. This gives an (AKSZ-type) Chern-Simons theory with gauge algebra the space of observables of a quantum mechanical Hilbert space H. The model is motivated by previous attempts to formulate gravity in terms of non-commutative, phase space, field theories as well as the Fefferman-Graham curved analog of Dirac spaces for conformally invariant wave equations. The field equations are flat connection conditions amounting to zero curvature and parallel conditions on operators acting on H. This matrix-type model may give a better defined setting for a quantum gravity path integral. We demonstrate that its underlying physics is a summation over Hamiltonians labeled by a conformal class of metrics and thus a sum over causal structures. This gives in turn a model summing over fluctuating metrics plus a tower of additional modes-we speculate that these could yield improved UV behavior

 

[marcus]

http://arxiv.org/abs/1407.6010
Inflation Driven by Unification Energy
Mark P. Hertzberg, Frank Wilczek
(Submitted on 22 Jul 2014)
We examine the hypothesis that inflation is primarily driven by vacuum energy at a scale indicated by gauge coupling unification. Concretely, we consider a class of hybrid inflation models wherein the vacuum energy associated with a grand unified theory condensate provides the dominant energy during inflation, while a second "inflaton" scalar slow-rolls. We show that it is possible to obtain significant tensor-to-scalar ratios while fitting the observed spectral index.
5 double column pages, 1 figure

 

[John86]

http://arxiv.org/abs/1407.6928
Quantum Cellular Automaton Theory of Light
Authors: Alessandro Bisio, Giacomo Mauro D'Ariano, Paolo Perinotti
(Submitted on 25 Jul 2014)
Abstract: We present a quantum theory of light based on quantum cellular automata (QCA). This approach allows us to have a thorough quantum theory of free electrodynamics encompassing an hypothetical discrete Planck scale. The theory is particularly relevant because it provides predictions at the macroscopic scale that can be experimentally tested. We show how, in the limit of small wave-vector k, the free Maxwell's equations emerge from two Weyl QCAs derived from informational principles in Ref. [1]. Within this framework the photon is introduced as a composite particle made of a pair of correlated massless Fermions, and the usual Bosonic statistics is recovered in the low photon density limit. We derive the main phenomenological features of the theory, consisting in dispersive propagation in vacuum, the occurrence of a small longitudinal polarization, and a saturation effect originated by the Fermionic nature of the photon. We then discuss whether these effects can be experimentally tested, and observe that only the dispersive effects are accessible with current technology, from observations of arrival times of pulses originated at cosmological distances.

 

[marcus]

http://arxiv.org/abs/1407.7149
Self-gravitating Interferometry and Intrinsic Decoherence
Cisco Gooding, William G. Unruh
(Submitted on 26 Jul 2014)
To investigate the possibility that intrinsic gravitational decoherence can be theoretically demonstrated within canonical quantum gravity, we develop a model of a self-gravitating interferometer. We search for evidence in the resulting interference pattern that would indicate coherence is fundamentally limited due to general relativistic effects. To eliminate the occurence of gravitational waves, we work in spherical symmetry, and construct the "beam" of the interferometer out of WKB states for an infinitesimally thin shell of matter. For internal consistency, we encode information about the beam optics within the dynamics of the shell itself, by arranging an ideal fluid on the surface of the shell with an equation of state that enforces beam-splitting and reflections. We then determine sufficient conditions for (interferometric) coherence to be fully present even after general relativistic corrections are introduced, test whether or not they can be satisfied, and remark on the implications of the results.
16 pages, 6 figures

http://arxiv.org/abs/1407.7295
Physical observability of horizons

Matt Visser (Victoria University of Wellington)
(Submitted on 28 Jul 2014)
Event horizons are (generically) not physically observable. In contrast, apparent horizons (and the closely related trapping horizons) are generically physically observable --- in the sense that they can be detected by observers working in finite-size regions of spacetime. Consequently event horizons are inappropriate tools for defining astrophysical black holes, or indeed for defining any notion of evolving}black hole, (evolving either due to accretion or Hawking radiation). The only situation in which an event horizon becomes physically observable is for the very highly idealized stationary or static black holes, when the event horizon is a Killing horizon which is degenerate with the apparent and trapping horizons; and then it is the physical observability of the apparent/trapping horizons that is fundamental --- the event horizon merely comes along for the ride.
4 pages

http://arxiv.org/abs/1407.7243
The Evolving Block Universe and the Meshing Together of Times
George F R Ellis
(Submitted on 27 Jul 2014)
It is proposed that spacetime should be regarded as an evolving block universe, bounded to the future by the present time, which continually extends to the future. This future boundary is defined at each time by measuring proper time along Ricci eigenlines from the start of the universe. A key point is that physical reality can be represented at many different scales: hence the passage of times may be seen as different at different scales, with quantum gravity determining the evolution of space time itself but quantum field theory determining the evolution of events within spacetime .The fundamental issue then arises as to how the effective times at different scales mesh together, leading to the concept so global and local times.
23 pages,4 figures

 

[MTd2]

http://arxiv.org/abs/1407.6993

Asymptotically Safe Higgs Inflation

Zhong-Zhi Xianyu, Hong-Jian He

We construct a new inflation model in which the standard model Higgs boson couples minimally to gravity and acts as the inflaton. Our construction of Higgs inflation incorporates the standard model with Einstein gravity which exhibits asymptotic safety in the ultraviolet region. The slow roll condition is satisfied at large field value due to the asymptotically safe behavior of Higgs self-coupling at high energies. We find that this minimal construction is highly predictive, and is consistent with both cosmological observations and collider experiments.

 

[marcus]

http://arxiv.org/abs/1407.7746
On background-independent renormalization of spin foam models
Benjamin Bahr
(Submitted on 29 Jul 2014)
In this article we discuss an implementation of renormalization group ideas to spin foam models, where there is no a priori length scale with which to define the flow. In the context of the continuum limit of these models, we show how the notion of cylindrical consistency of path integral measures gives a natural analogue of Wilson's RG flow equations for background-independent systems. We discuss the conditions for the continuum measures to be diffeomorphism-invariant, and consider both exact and approximate examples.
23 pages, 12 figures

http://arxiv.org/abs/1407.7766
Quantum collapse rules from maximum relative entropy principle
Frank Hellmann, Wojciech Kamiński, Ryszard Paweł Kostecki
(Submitted on 29 Jul 2014)
We show that the von Neumann--Lueders collapse rules in quantum mechanics always select the unique state that maximises the quantum relative entropy with respect to the premeasurement state, subject to the constraint that the postmeasurement state has to be compatible with the knowledge gained in the measurement. This way we provide an information theoretic characterisation of quantum collapse rules by means of the maximum relative entropy principle.
5 pages.
[my comment: Kostecki has an interest in quantum geometry, with work in preparation provisionally titled Towards quantum information geometric foundations. All three authors have published papers in QG]

http://arxiv.org/abs/1407.7544
Re-Examining Astrophysical Constraints on the Dark Matter Model
Alyson Brooks
(Submitted on 28 Jul 2014)
Recent high-resolution simulations that include Cold Dark Matter (CDM) and baryons have shown that baryonic physics can dramatically alter the dark matter structure of galaxies. These results modify our predictions for observed galaxy evolution and structure. Given these updated expectations, it is timely to re-examine observational constraints on the dark matter model. A few observations are reviewed that may indirectly trace dark matter, and may help confirm or deny possible dark matter models. Warm Dark Matter (WDM) and Self-Interacting Dark Matter (SIDM) are currently the favorite alternative models to CDM. Constraints on the WDM particle mass require it to be so heavy that WDM is nearly indistinguishable from CDM. The best observational test of SIDM is likely to be in the dark matter distribution of faint dwarf galaxies, but there is a lack of theoretical predictions for galaxy structure in SIDM that account for the role of baryons.
10 pages. Invited review article, accepted for publication in Annalen der Physik.

 

[marcus]

http://arxiv.org/abs/1407.8166
Loop quantum cosmology from group field theory
Gianluca Calcagni
(Submitted on 30 Jul 2014)
We show that the effective dynamics of the recently proposed isotropic condensate state of group field theory (GFT) with Laplacian kinetic operator can be equivalent to that of homogeneous and isotropic loop quantum cosmology (LQC) in the improved dynamics quantization scheme, where the area of elementary holonomy plaquettes is constant. This constitutes a somewhat surprising example of a cosmological model of quantum gravity where the operations of minisuperspace symmetry reduction and quantization can actually commute.
5 pages

http://arxiv.org/abs/1407.8167
Quantum cosmology from quantum gravity condensates: cosmological variables and lattice-refined dynamics
Steffen Gielen, Daniele Oriti
(Submitted on 30 Jul 2014)
In the context of group field theory condensate cosmology, we clarify the extraction of cosmological variables from the microscopic quantum gravity degrees of freedom. We show that an important implication of the second quantized formalism is the dependence of cosmological variables and equations on the quantum gravitational atomic number N (number of spin network vertices/elementary simplices). We clarify the relation of the effective cosmological equations with loop quantum cosmology, understood as an effective (hydrodynamic-like) approximation of a more fundamental quantum gravity theory. By doing so, we provide a fundamental basis to the idea of lattice refinement, showing the dependence of the effective cosmological connection on N, and hence indirectly on the scale factor. Our results open a new arena for exploring effective cosmological dynamics, as this depends crucially on the new observable N, which is entirely of quantum gravitational origin.
6 pages

http://arxiv.org/abs/1407.8143
Realization of DSR-relativistic symmetries in Finsler geometries
Giovanni Amelino-Camelia, Leonardo Barcaroli, Giulia Gubitosi, Stefano Liberati, Niccoló Loret
(Submitted on 30 Jul 2014)
Finsler geometry is a well known generalization of Riemannian geometry which allows to account for a possibly non trivial structure of the space of configurations of relativistic particles. We here establish a link between Finsler geometry and the sort of models with curved momentum space and DSR-relativistic symmetries which have been recently of interest in the quantum-gravity literature. We use as case study the much-studied scenario which is inspired by the κ-Poincaré quantum group, and show that the relevant deformation of relativistic symmetries can be implemented within a Finsler geometry.
26 pages.

http://arxiv.org/abs/1407.8084
How well is our universe described by an FLRW model?
Stephen R. Green, Robert M. Wald
(Submitted on 30 Jul 2014)
Extremely well! The spacetime metric, g_ab, of our universe is approximated by an FLRW metric, g⁽⁰⁾_ab, to about 1 part in 10⁴ or better on both large and small scales, except in the immediate vicinity of very strong field objects, such as black holes. However, derivatives of g_ab are not close to derivatives of g⁽⁰⁾_ab, so there can be significant differences in the behavior of geodesics and huge differences in curvature. Consequently, observable quantities in the actual universe may differ significantly from the corresponding observables in the FLRW model. Nevertheless, as we shall review here, we have proven general results showing that the large matter inhomogeneities that occur on small scales cannot produce significant backreaction effects on large scales, so g⁽⁰⁾_ab satisfies Einstein's equation with the averaged stress-energy tensor of matter as its source. We discuss the flaws in some other approaches that have suggested that large backreaction effects may occur. As we also will review here, with a suitable "dictionary," Newtonian cosmologies provide excellent approximations to cosmological solutions to Einstein's equation (with dust and a cosmological constant) on all scales.
18 pages, 2 figures.

http://arxiv.org/abs/1407.8058
If time is a local observable, then Hawking radiation is unitary
H. Nikolic
(Submitted on 28 Jul 2014)
In the usual formulation of quantum theory, time is a global classical evolution parameter, not a local quantum observable. On the other hand, both canonical quantum gravity (which lacks fundamental time-evolution parameter) and the principle of spacetime covariance (which insists that time should be treated on an equal footing with space) suggest that quantum theory should be slightly reformulated, in a manner that promotes time to a local observable. Such a reformulated quantum theory is unitary in a more general sense than the usual quantum theory. In particular, this promotes the non-unitary Hawking radiation to a unitary phenomenon, which avoids the black-hole information paradox.
11 pages, accepted for publication in Int. J. Quantum Inf.

http://arxiv.org/abs/1407.8028
The trivial solution of the gravitational energy-momentum tensor problem
H. Nikolic
(Submitted on 30 Jul 2014)
In the literature one often finds the claim that there is no such thing as an energy-momentum tensor for the gravitational field, and consequently, that the total energy-momentum conservation can only be defined in terms of a gravitational energy-momentum pseudo-tensor. I make a trivial observation that such a conclusion can be avoided by relaxing the assumption that gravitational energy-momentum tensor should only depend on first derivatives of the metric. With such a relaxation, the Einstein equation directly leads to the result that gravitational energy-momentum tensor is essentially the Einstein tensor.
5 pages

http://arxiv.org/abs/1407.7896
Emergent gravitational dynamics in relativistic Bose--Einstein condensate
Alessio Belenchia, Stefano Liberati, Arif Mohd
(Submitted on 29 Jul 2014)
22 pages.

http://arxiv.org/abs/1407.7891
Planck-scale soccer-ball problem: a case of mistaken identity
Giovanni Amelino-Camelia
(Submitted on 29 Jul 2014)
5 pages.

 

[marcus]

http://arxiv.org/abs/1408.0121
Thermally correlated states in Loop Quantum Gravity
Goffredo Chirco, Carlo Rovelli, Paola Ruggiero
(Submitted on 1 Aug 2014)
We study a class of loop-quantum-gravity states characterized by (ultra-local) thermal correlations that reproduce some features of the ultraviolet structure of the perturbative quantum field theory vacuum. In particular, they satisfy an analog of the Bisognano-Wichmann theorem. These states are peaked on the intrinsic geometry and admit a semiclassical interpretation. We study how the correlations extend on the spin-network beyond the ultra local limit.
11 pages, 4 figures

http://arxiv.org/abs/1408.0276
Asymptotic safety and the cosmological constant
Kevin Falls
(Submitted on 1 Aug 2014)
We study the non-perturbative renormalisation of quantum gravity in four dimensions. Taking care to disentangle physical degrees of freedom, we observe the topological nature of conformal fluctuations arising from the functional measure. The resulting beta functions possesses an asymptotically safe fixed point with a global phase structure leading to classical general relativity for positive, negative or vanishing cosmological constant. If only the conformal fluctuations are quantised we find an asymptotically safe fixed point predicting a vanishing cosmological constant on all scales. At this fixed point we reproduce the critical exponent, ν=1/3, found in numerical lattice studies by Hamber. This suggests the fixed point may be physical while solving the cosmological constant problem.
34 pages, 9 figures

brief mention:
http://arxiv.org/abs/1408.0156
Inflation, quintessence, and the origin of mass
C.Wetterich
(Submitted on 1 Aug 2014)
In a unified picture both inflation and present dynamical dark energy arise from the same scalar field. The history of the Universe describes a crossover from a scale invariant "past fixed point" where all particles are massless, to a "future fixed point" where spontaneous breaking of the exact scale symmetry generates the particle masses. The cosmological solution can be extrapolated to the infinite past in physical time - the universe has no beginning. This is seen most easily in a frame where particle masses and the Planck mass are field-dependent and increase with time…
...A simple model with no more free parameters than ΛCDM is compatible with all present observational tests.
21 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1408.0710
Exact solutions of the Wheeler-DeWitt equation and the Yamabe construction
Eyo Ita, Chopin Soo
(Submitted on 30 Jul 2014)
Exact solutions of the Wheeler-DeWitt equation of the full theory of four dimensional gravity of Lorentzian signature are obtained. They are characterized by Schrödinger wavefunctionals having support on 3-metrics of constant spatial scalar curvature, and thus contain two full physical field degrees of freedom in accordance with the Yamabe construction. These solutions are moreover Gaussians of minimum uncertainty and they are naturally associated with a rigged Hilbert space. In addition, in the limit the regulator is removed, exact 3-dimensional diffeomorphism and local gauge invariance of the solutions are recovered.
13 Pages

http://arxiv.org/abs/1408.0778
Cosmic Matter Flux May Turn Hawking Radiation Off
Javad T. Firouzjaee, George F. R. Ellis
(Submitted on 4 Aug 2014)
An astrophysical (cosmological) black hole forming in a cosmological context will be subject to a flux of infalling matter and radiation, which will cause the outer apparent horizon (a marginal trapping surface) to be spacelike [5]. As a consequence the radiation emitted close to the apparent horizon no longer arrives at infinity with a diverging redshift. Standard calculations of the emission of Hawking radiation then indicate that no blackbody radiation is emitted to infinity by the black hole in these circumstances, hence there will also then be no black hole evaporation process due to emission of such radiation as long as the matter flux is significant. The essential adiabatic condition (eikonal approximation) for black hole radiation gives a strong limit to the black holes that can emit Hawking radiation. We give the mass range for the black holes that can radiate, according to their cosmological redshift, for the special case of the cosmic blackbody radiation (CBR) influx (which exists everywhere in the universe). At a very late stage of black hole formation when the CBR influx decays away, the black hole horizon becomes first a slowly evolving horizon and then an isolated horizon; at that stage, black hole radiation will start. This study suggests that the primordial black hole evaporation scenario should be revised to take these considerations into account.
21 pages.

 

[marcus]

http://arxiv.org/abs/1408.1238
Non-singular Cosmology from Evolutionary Quantum Gravity
Francesco Cianfrani, Giovanni Montani, Fabrizio Pittorino
(Submitted on 6 Aug 2014)
We provide a cosmological implementation of the evolutionary quantum gravity, describing an isotropic Universe, in the presence of a negative cosmological constant and a massive (pre-inflationary) scalar field. We demonstrate that the considered Universe has a non-singular quantum behavior, associated to a primordial bounce, whose ground state has a high occupation number. Furthermore, in such a vacuum state, the super-Hamiltonian eigenvalue is negative, corresponding to a positive emerging dust energy density. The regularization of the model is performed via a polymer quantum approach to the Universe scale factor and the proper classical limit is then recovered, in agreement with a pre-inflationary state of the Universe. Since the dust energy density is red-shifted by the Universe de-Sitter phase and the cosmological constant does not enter the ground state eigenvalue, we get a late time cosmology, at all compatible with the present observations, endowed with a turning point in the far future.
16 pages, 8 figures

 

[atyy]

http://arxiv.org/abs/1408.1449
Topological quantum field theory and quantum gravity
Steven Kerr
(Submitted on 7 Aug 2014)
This thesis is broadly split into two parts. In the first part, simple state sum models for minimally coupled fermion and scalar fields are constructed on a 1-manifold. The models are independent of the triangulation and give the same result as the continuum partition functions evaluated using zeta-function regularisation. Some implications for more physical models are discussed.
In the second part, the gauge gravity action is written using a particularly simple matrix technique. The coupling to scalar, fermion and Yang-Mills fields is reviewed, with some small additions. A sum over histories quantisation of the gauge gravity theory in 2+1 dimensions is then carried out for a particular class of triangulations of the three-sphere. The preliminary stage of the Hamiltonian analysis for the (3+1)-dimensional gauge gravity theory is undertaken.

 

[marcus]

http://arxiv.org/abs/1408.1444
Modelling the Evaporation of Non-singular Black Holes
Tim Taves, Gabor Kunstatter
(Submitted on 7 Aug 2014)
We present a model for studying the formation and evaporation of non-singular (quantum corrected) black holes. The model is based on a generalized form of the dimensionally reduced, spherically symmetric Einstein-Hilbert action and includes a suitably generalized Polyakov action to provide a mechanism for radiation back-reaction. The equations of motion describing self-gravitating scalar field collapse are derived in local form both in null coordinates and in Painleve-Gullstrand (flat slice) co-ordinates. They provide the starting point for numerical studies of complete spacetimes containing dynamical horizons that bound a compact trapped region. Such spacetimes have been proposed in the past as solutions to the information loss problem because they possesses neither an event horizon nor a singularity. Since the equations of motion in our model are derived from a diffeomorphism invariant action, they preserve the constraint algebra and the resulting energy momentum tensor is manifestly conserved.
14 pages

 

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http://arxiv.org/abs/1408.1994
Gauge theory of gravity and matter
Steven Kerr
(Submitted on 8 Aug 2014)
It is shown how to write the first order action for gravity in a gauge theoretic formalism where the spin connection and frame field degrees of freedom are assimilated together into a gauge connection. It is then shown how to couple the theory to spin-0, 1/2, 1 and 3/2 fields in a gauge invariant fashion. The results hold in any number of spacetime dimensions.
20 pages.

http://arxiv.org/abs/1408.2503
Checking the dark matter origin of 3.53~keV line with the Milky Way center
Alexey Boyarsky, Jeroen Franse, Dmytro Iakubovskyi, Oleg Ruchayskiy
(Submitted on 11 Aug 2014)
We detect a line at 3.539±0.011 keV in the deep exposure dataset of the Galactic Center region, observed with the XMM-Newton. Although it is hard to exclude completely astrophysical origin of this line in the Galactic Center data alone, the dark matter interpretation of the signal observed in Perseus galaxy cluster and Andromeda galaxy [1402.4119] and in the stacked spectra of galaxy clusters [1402.2301] is fully consistent with these data. Moreover, the Galactic Center data support this interpretation as the line is observed at the same energy and has flux consistent with the expectations about the Galactic dark matter distribution for a class of the Milky Way mass models.
10 pages, 4 figures, 3 tables

 

[marcus]

http://arxiv.org/abs/1408.3050
A scenario for black hole evaporation on a quantum geometry
Rodolfo Gambini, Jorge Pullin
(Submitted on 13 Aug 2014)
We incorporate elements of the recently discovered exact solutions of the quantum constraints of loop quantum gravity for vacuum spherically symmetric space-times into the paradigm of black hole evaporation due to Ashtekar and Bojowald. The quantization of the area of the surfaces of symmetry of the solutions implies that the number of nice slices that can be fit inside the black hole is finite. The foliation eventually moves through the region where the singularity in the classical theory used to be and all the particles that fell into the black hole due to Hawking radiation emerge finally as a white hole. This yields a variant of a scenario advocated by Arkani-Hamed et al. Fluctuations in the horizon that naturally arise in the quantum space time allow radiation to emerge during the evaporation process due to stimulated emission allowing evaporation to proceed beyond Page time without reaching the maximum entanglement limit until the formation of the white hole. No firewalls nor remnants arise in this scenario.
5 pages.

http://arxiv.org/abs/1408.2959
Localization and diffusion in polymer quantum field theory
Michele Arzano, Marco Letizia
(Submitted on 13 Aug 2014)
Polymer quantization is a non-standard approach to quantizing a classical system inspired by background independent approaches to quantum gravity such as loop quantum gravity. When applied to field theory it introduces a characteristic polymer scale at the level of the fields classical configuration space. Compared with models with space-time discreteness or non-commutativity this is an alternative way in which a characteristic scale can be introduced in a field theoretic context. Motivated by this comparison we study here localization and diffusion properties associated with polymer field observables and dispersion relation in order to shed some light on the novel physical features introduced by polymer quantization. While localization processes seems to be only mildly affected by polymer effects, we find that polymer diffusion differs significantly from the "dimensional reduction" picture emerging in other Planck-scale models beyond local quantum field theory.
16 pages, 5 figures

 

[atyy]

http://arxiv.org/abs/1408.3260
Lorentzian Connes Distance, Spectral Graph Distance and Loop Gravity
Carlo Rovelli
(Submitted on 14 Aug 2014)
Connes' formula defines a distance in loop quantum gravity, via the spinfoam Dirac operator. A simple notion of spectral distance on a graph can be extended do the discrete Lorentzian context, providing a physically natural example of Lorentzian spectral geometry, with a neat space of Dirac operators. The Hilbert structure of the fermion space is Lorentz covariant rather than invariant.

 

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http://arxiv.org/abs/1408.3420
Entanglement in curved spacetimes and cosmology
Eduardo Martin-Martinez, Nicolas C. Menicucci
(Submitted on 14 Aug 2014)
We review recent results regarding entanglement in quantum fields in cosmological spacetimes and related phenomena in flat spacetime such as the Unruh effect. We being with a summary of important results about field entanglement and the mathematics of Bogoliubov transformations that is very often used to describe it. We then discuss the Unruh-DeWitt detector model, which is a useful model of a generic local particle detector. This detector model has been successfully used as a tool to obtain many important results. In this context we discuss two specific types of these detectors: a qubit and a harmonic oscillator. The latter has recently been shown to have important applications when one wants to probe nonperturbative physics of detectors interacting with quantum fields. We then detail several recent advances in the study and application of these ideas, including echoes of the early universe, entanglement harvesting, and a nascent proposal for quantum seismology.
29 pages, 7 figures. Accepted in Classical and Quantum Gravity. Currently in press. (Special Issue focused on the topic of "Entanglement and Quantum Gravity")

 

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http://arxiv.org/abs/1408.3989
Observing Shape in Spacetime
Sean Gryb
(Submitted on 18 Aug 2014)
The notion of "reference frame" is a central theoretical construct for interpreting the physical implications of spacetime diffeomorphism invariance in General Relativity. However, the alternative formulation of classical General Relativity known as Shape Dynamics suggest that a subset of spacetime diffeomorphisms - namely hypersurface deformations - are, in a certain sense, dual to spatial conformal (or Weyl) invariance. Moreover, holographic gauge/gravity dualities suggest that bulk spacetime diffeomorphism invariance can be replaced by the properties of boundary CFTs. How can these new frameworks be compatible with the traditional notion of reference frame so fundamental to our interpretation of General Relativity? In this paper, we address this question by investigating the classical case of maximally symmetric spacetimes with a positive cosmological constant. We find that it is possible to define a notion of "Shape Observer" that represents a conformal reference frame that is dual to the notion of inertial reference frame in spacetime. We then provide a precise dictionary relating the two notions. These Shape Observers are holographic in the sense that they are defined on the asymptotic conformal boundaries of spacetime but know about bulk physics. This leads to a first principles derivation of an exact classical holographic correspondence that can easily be generalized to more complicated situations and may lead to insights regarding the interpretation of the conformal invariance manifest in Shape Dynamics.
23 pages, 3 figures.

http://arxiv.org/abs/1408.4023
Black holes and Boyle's law -- the thermodynamics of the cosmological constant
Brian P. Dolan
(Submitted on 18 Aug 2014)
When the cosmological constant, Λ, is interpreted as a thermodynamic variable in the study of black hole thermodynamics a very rich structure emerges. It is natural to interpret Λ as a pressure and define the thermodynamically conjugate variable to be the thermodynamic volume of the black hole (which need not bear any relation to the geometric volume). Recent progress in this new direction for black hole thermodynamics is reviewed.
31 pages, 3 figures. Review article

 

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http://arxiv.org/abs/1408.4336
From General Relativity to Quantum Gravity
Abhay Ashtekar, Martin Reuter, Carlo Rovelli
(Submitted on 19 Aug 2014)
In general relativity (GR), spacetime geometry is no longer just a background arena but a physical and dynamical entity with its own degrees of freedom. We present an overview of approaches to quantum gravity in which this central feature of GR is at the forefront. However, the short distance dynamics in the quantum theory are quite different from those of GR and classical spacetimes and gravitons emerge only in a suitable limit. Our emphasis is on communicating the key strategies, the main results and open issues. In the spirit of this volume, we focus on a few avenues that have led to the most significant advances over the past 2-3 decades.
54 pages, 5 figures. To appear in General Relativity and Gravitation: A Centennial Survey, commissioned by the International Society for General Relativity and Gravitation and to be published by Cambridge University Press. Abhay Ashtekar served as the 'coordinating author' and combined the three contributions

http://arxiv.org/abs/1408.4127
Towards phase transitions between discrete and continuum quantum spacetime from the Renormalization Group
Astrid Eichhorn, Tim Koslowski
(Submitted on 18 Aug 2014)
We establish the functional Renormalization Group as an exploratory tool to investigate a possible phase transition between a pre-geometric discrete phase and a geometric continuum phase in quantum gravity. In this paper, based on the analysis of [1], we study three new aspects of the double-scaling limit of matrix models as Renormalization Group fixed points: Firstly, we investigate multicritical fixed points, which are associated with quantum gravity coupled to conformal matter. Secondly, we discuss an approximation that reduces the scheme dependence of our results as well as computational effort while giving good numerical results. This is a consequence of the approximation being a solution to the unitary Ward-identity associated to the U(N) symmetry of the hermitian matrix model. Thirdly, we discuss a scenario that relates the double scaling limit to fixed points of continuum quantum gravity.
14 pages, 1 figure

http://arxiv.org/abs/1408.4118
Dynamics of Gauge Field Inflation
Stephon Alexander, Dhrubo Jyoti, Arthur Kosowsky, Antonino Marciano
(Submitted on 18 Aug 2014)
We analyze the existence and stability of dynamical attractor solutions for cosmological inflation driven by the coupling between fermions and a gauge field. Assuming a spatially homogeneous and isotropic gauge field and fermion current,... We prove the existence of exactly one stable solution, and demonstrate the stability numerically. Inflation arises without fine tuning, and does not require postulating any effective potential or non-standard coupling.
7 pages, 2 figures

http://arxiv.org/abs/1408.4427
Dark matter, dark energy and the time evolution of masses in the Universe
Joan Sola
(Submitted on 19 Aug 2014)
...
12 pages, 2 tables, accepted for publication in Int. J. of Mod. Phys.
http://inspirehep.net/author/profile/J.Sola.1

 

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http://arxiv.org/abs/1408.4635
Quantum shells in a quantum space-time
Rodolfo Gambini, Jorge Pullin
(Submitted on 20 Aug 2014)
We study the quantum motion of null shells in the quantum space-time of a black hole in loop quantum gravity. We treat the shells as test fields and use an effective dynamics for the propagation equations. The shells propagate through the region where the singularity was present in the classical black hole space-time, but is absent in the quantum space-time, eventually emerging through a white hole to a new asymptotic region of the quantum space-time. The profiles of the shells get distorted due to the quantum fluctuations in the Planckian region that replaces the singularity. The evolution of the shells is unitary throughout the whole process.
5 pages, 3 figures

brief mention, of possible interest:
http://arxiv.org/abs/1408.4770
Holographic Holes and Differential Entropy
Matthew Headrick, Robert C. Myers, Jason Wien
(Submitted on 20 Aug 2014)
61 pages

 

[marcus]

http://arxiv.org/abs/1408.4791
Geometrical variables with direct thermodynamic significance in Lanczos-Lovelock gravity
Sumanta Chakraborty, T. Padmanabhan
(Submitted on 19 Aug 2014)
It has been shown in an earlier work [arXiv:1303.1535] that there exists a pair of canonically conjugate variables (f^bc,N^a_bc) in general relativity which also act as thermodynamically conjugate variables on any horizon. In particular their variations (f^bc∂N^a_bc, N^a_bc∂f^bc), which occur in the surface term of the Einstein-Hilbert action, when integrated over a null surface, have direct correspondence with (S∂T, T∂S ) where (T,S) are the temperature and entropy. We generalize these results to Lanczos-Lovelock models in this paper. We identify two such variables in Lanczos-Lovelock models such that (a) our results reduce to that of general relativity in the appropriate limit and (b) the variation of surface term in the action, when evaluated on a null surface, has direct thermodynamic interpretation as in the case of general relativity. The variations again correspond to S∂T and T∂S where S is now the appropriate Wald entropy for the Lanczos-Lovelock model. The implications are discussed.
22 pages

 

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http://arxiv.org/abs/1408.5367
Rethinking Connes' approach to the standard model of particle physics via non-commutative geometry
Shane Farnsworth, Latham Boyle
(Submitted on 22 Aug 2014)
Connes' non-commutative geometry (NCG) is a generalization of Riemannian geometry that is particularly apt for expressing the standard model of particle physics coupled to Einstein gravity. In a previous paper, we suggested a reformulation of this framework that is: (i) simpler and more unified in its axioms, and (ii) allows the Lagrangian for the standard model of particle physics (coupled to Einstein gravity) to be specified in a way that is tighter and more explanatory than the traditional algorithm based on effective field theory. Here we explain how this same reformulation yields a new perspective on the symmetries of a given NCG. Applying this perspective to the NCG traditionally used to describe the standard model we find, instead, an extension of the standard model by an extra U(1)_B−L gauge symmetry, and a single extra complex scalar field σ, which is a singlet under SU(3)_C×SU(2)_L×U(1)_Y, but has B−L=2. This field has cosmological implications, and offers a new solution to the discrepancy between the observed Higgs mass and the NCG prediction.
5 pages

 

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http://arxiv.org/abs/1408.6646
Classical and Quantum Polyhedra
John Schliemann
(Submitted on 28 Aug 2014)
Quantum polyhedra constructed from angular momentum operators are the building blocks of space in its quantum description as advocated by Loop Quantum Gravity. Here we extend previous results on the semiclassical properties of quantum polyhedra. Regarding tetrahedra, we compare the results from a canonical quantization of the classical system with a recent wave function based approach to the large-volume sector of the quantum system. Both methods agree in the leading order of the resulting effective operator (given by an harmonic oscillator), while minor differences occur in higher corrections. Perturbative inclusion of such corrections improves the approximation to the eigenstates. Moreover, the comparison of both methods leads also to a full wave function description of the eigenstates of the (square of the) volume operator at negative eigenvalues of large modulus.
For the case of general quantum polyhedra described by discrete angular momentum quantum numbers we formulate a set of quantum operators fulfilling in the semiclassical regime the standard commutation relations between momentum and position. Differently from previous formulations, the position variable here is chosen to have dimension of (Planck) length squared which facilitates the identification of quantum corrections.

 

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http://arxiv.org/abs/1408.7112
Group Field Theory and Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Aug 2014)
We introduce the group field theory formalism for quantum gravity, mainly from the point of view of loop quantum gravity, stressing its promising aspects. We outline the foundations of the formalism, survey recent results and offer a perspective on future developments.

 

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http://arxiv.org/abs/1409.0144
Entanglement entropy production in gravitational collapse: covariant regularization and solvable models
Eugenio Bianchi, Tommaso De Lorenzo, Matteo Smerlak
(Submitted on 30 Aug 2014)
We study the dynamics of vacuum entanglement in the process of gravitational collapse and subsequent black hole evaporation. In the first part of the paper, we introduce a covariant regularization of entanglement entropy tailored to curved spacetimes; this regularization allows us to propose precise definitions for the concepts of black hole "exterior entropy" and "radiation entropy." For a Vaidya model of collapse we find results consistent with the standard thermodynamic properties of Hawking radiation. In the second part of the paper, we compute the vacuum entanglement entropy of various spherically-symmetric spacetimes of interest, including the nonsingular black hole model of Bardeen, Hayward, Frolov and Rovelli-Vidotto and the "black hole fireworks" model of Haggard-Rovelli. We discuss specifically the role of event and trapping horizons in connection with the behavior of the radiation entropy at future null infinity. We observe in particular that (i) in the presence of an event horizon the radiation entropy diverges at the end of the evaporation process, (ii) in models of nonsingular evaporation (with a trapped region but no event horizon) the generalized second law holds only at early times and is violated in the "purifying" phase, (iii) at late times the radiation entropy can become negative (i.e. the radiation can be less correlated than the vacuum) before going back to zero leading to an up-down-up behavior for the Page curve of a unitarily evaporating black hole.
35 pages, 14 figures

side interest:
http://arxiv.org/abs/1409.0469
On tidal capture of primordial black holes by neutron stars
Guillaume Defillon, Etienne Granet, Petr Tinyakov, Michel H.G. Tytgat
(Submitted on 1 Sep 2014)
7 pages

 

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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.

 

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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)

 

[atyy]

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.