Loop-and-allied QG bibliography

[atyy]

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

 

[marcus]

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

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

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

 

[marcus]

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

 

[MTd2]

General interest:

http://arxiv.org/abs/1407.2528

Faster than light motion does not imply time travel

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

 

[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.

 

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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

 

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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.

 

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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

 

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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.

 

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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

 

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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

 

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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

 

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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