Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1309.3999
The measure matters
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 16 Sep 2013)
We adopt a framework where quantum-gravity's dynamical dimensional reduction of spacetime at short distances is described in terms of modified dispersion relations. We observe that by subjecting such models to a momentum-space diffeomorphism one obtains a "dual picture" with unmodified dispersion relations, but a modified measure of integration over momenta. We then find that the UV {\it Hausdorff} dimension of momentum space which can be inferred from this modified integration measure coincides with the short-distance {\it spectral} dimension of spacetime. This result sheds light into why scale-invariant fluctuations are obtained if the original model for two UV spectral dimensions is combined with Einstein gravity. By studying the properties of the inner product we derive the result that it is only in 2 energy-momentum dimensions that microphysical vacuum fluctuations are scale-invariant. This is true ignoring gravity, but then we find that if Einstein gravity is postulated in the original frame, in the dual picture gravity switches off, since all matter becomes conformally coupled. We argue that our findings imply that the following concepts are closely connected: scale-invariance of vacuum quantum fluctuations, conformal invariance of the gravitational coupling, UV reduction to spectral dimension 2 in position space and UV reduction to Hausdorff dimension 2 in energy-momentum space.

 

[John86]

http://arxiv.org/abs/1309.4424
Quantum field theory in the Rindler-Rindler spacetime
Sanved Kolekar, T. Padmanabhan
(Submitted on 17 Sep 2013)
It is well known that Minkowski vacuum appears as a thermal bath in the Rindler spacetime when the modes on the left wedge are traced out. We introduce the concept of a Rindler-Rindler spacetime, obtained by a further coordinate transformation from the Rindler spacetime, in a manner similar to the transformation from inertial to Rindler frame. We show that the Rindler vacuum appears as a thermal state in the Rindler-Rindler frame. Further, the spectrum of particles seen by the Rindler-Rindler observers in the original Minkowski vacuum state is shown to be identical to that seen by detector accelerating through a real thermal bath. Thus the Davies-Unruh effect acts as a proxy for a real thermal bath, for a certain class of observers in the Rindler-Rindler spacetime. We interpret this similarity as indicating further evidence of the indistinguishablity between thermal and quantum fluctuations along the lines of the recent work in arXiv:1308.6289. The implications are briefly discussed.

http://arxiv.org/abs/1309.4084
Unified meta-theory of information, consciousness, time and the classical-quantum universe
Martin A. Green
(Submitted on 16 Sep 2013)
As time advances in our perceived real world, existing information is preserved and new information is added to history. All the information that may ever be encoded in history must be about some fundamental, unique, atemporal and pre-physical structure: the bare world. Scientists invent model worlds to efficiently explain aspects of the real world. This paper explores the features of and relationships between the bare, real, and model worlds. Time -- past, present and future -- is naturally explained. Both quantum uncertainty and state reduction are needed for time to progress, since unpredictable new information must be added to history. Deterministic evolution preserves existing information. Finite, but steadily increasing, information about the bare world is jointly encoded in equally uncertain spacetime geometry and quantum matter. Because geometry holds no information independent of matter, there is no need to quantize gravity. At the origin of time, information goes to zero and geometry and matter fade away.


This gives some food for thought...

 

[atyy]

http://arxiv.org/abs/1309.4563
Statistics, holography, and black hole entropy in loop quantum gravity
Amit Ghosh, Karim Noui, Alejandro Perez
(Submitted on 18 Sep 2013)
In loop quantum gravity the quantum states of a black hole horizon are produced by point-like discrete quantum geometry excitations (or punctures) labelled by spin $j$. The excitations possibly carry other internal degrees of freedom also, and the associated quantum states are eigenstates of the area $A$ operator. On the other hand, the appropriately scaled area operator $A/(8\pi\ell)$ is also the physical Hamiltonian associated with the quasilocal stationary observers located at a small distance $\ell$ from the horizon. Thus, the local energy is entirely accounted for by the geometric operator $A$.
We assume that: In a suitable vacuum state with regular energy momentum tensor at and close to the horizon the local temperature measured by stationary observers is the Unruh temperature and the degeneracy of `matter' states is exponential with the area $\exp{(\lambda A/\ell_p^2)}$---this is supported by the well established results of QFT in curved spacetimes, which do not determine $\lambda$ but asserts an exponential behaviour. The geometric excitations of the horizon (punctures) are indistinguishable. In the semiclassical limit the area of the black hole horizon is large in Planck units.
It follows that: Up to quantum corrections, matter degrees of freedom saturate the holographic bound, viz. $\lambda=\frac{1}{4}$. Up to quantum corrections, the statistical black hole entropy coincides with Bekenstein-Hawking entropy $S={A}/({4\ell_p^2})$. The number of horizon punctures goes like $N\propto \sqrt{A/\ell_p^2}$, i.e the number of punctures $N$ remains large in the semiclassical limit. Fluctuations of the horizon area are small while fluctuations of the area of an individual puncture are large. A precise notion of local conformal invariance of the thermal state is recovered in the $A\to\infty$ limit where the near horizon geometry becomes Rindler.

 

[John86]

http://arxiv.org/abs/1309.4480
Scale-free primordial cosmology
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 17 Sep 2013)
The large-scale structure of the universe suggests that the physics underlying its early evolution is scale-free. This was the historic motivation for the Harrison-Zel'dovich-Peebles spectrum and for inflation. Based on a hydrodynamical approach, we identify scale-free forms for the background equation-of-state for both inflationary and cyclic scenarios and use these forms to derive predictions for the spectral tilt and tensor-to-scalar ratio of primordial density perturbations. For the case of inflation, we find three classes of scale-free models with distinct predictions. Including all classes, we show that scale-free inflation predicts tensor-to-scalar ratio $r > 10^{-4}$. We show that the observationally favored class is theoretically disfavored because it suffers from an initial conditions problem and the hydrodynamical form of an unlikeliness problem similar to that identified recently for certain inflaton potentials. We contrast these results with those for scale-free cyclic models.

 

[marcus]

http://arxiv.org/abs/1309.5343
Using Cosmology to Establish the Quantization of Gravity
Lawrence M. Krauss (1,2), Frank Wilczek (3) ((1) Arizona State University, (2) Australian National Univeresity, (3) MIT)
(Submitted on 20 Sep 2013)
While many aspects of general relativity have been tested, and general principles of quantum dynamics demand its quantization, there is no direct evidence for that. It has been argued that development of detectors sensitive to individual gravitons is unlikely, and perhaps impossible. We argue here, however, that measurement of polarization of the Cosmic Microwave Background due to a long wavelength stochastic background of gravitational waves from Inflation in the Early Universe would firmly establish the quantization of gravity.
3 pages

 

[marcus]

http://arxiv.org/abs/1309.6304
Quantum-Reduced Loop-Gravity: Relation with the Full Theory
Emanuele Alesci, Francesco Cianfrani, Carlo Rovelli
(Submitted on 24 Sep 2013)
The quantum-reduced loop-gravity technique has been introduced for dealing with cosmological models. We show that it can be applied rather generically: anytime the spatial metric can be gauge-fixed to a diagonal form. The technique selects states based on reduced graphs with Livine-Speziale coherent intertwiners and could simplify the analysis of the dynamics in the full theory.
5 pages

 

[John86]

http://arxiv.org/abs/1309.5266
Asymptotic Flatness and Quantum Geometry
Sandipan Sengupta
(Submitted on 20 Sep 2013)
We construct a canonical quantization of the two dimensional theory of a parametrized scalar field on noncompact spatial slices. The kinematics is built upon states that carry a label corresponding to continuous embeddings, in addition to discrete embedding and matter labels. The resulting quantum geometry admits a nondegenerate vacuum metric, which allows a consistent realization of the asymptotic conditions on the canonical fields. However, out of the full classical symmetry group of conformal isometries, only the subgroup of continuous global translations can be implemented. The quantum spacetime as characterised by a gauge invariant state is shown to be made up of discrete strips at the interior, and smooth at asymptotia. The analysis here is expected to be particularly relevant for a loop quantization of asymptotically flat gravity.

 

[marcus]

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013)
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.
75 pages, invited topical review for Classical and Quantum Gravity

 

[marcus]

http://arxiv.org/abs/1309.7296
Astrophysical constraints on Planck scale dissipative phenomena
Stefano Liberati (SISSA and INFN, Trieste), Luca Maccione (LMU and MPP, Munich)
(Submitted on 27 Sep 2013)
The emergence of a classical spacetime from any quantum gravity model is still a subtle and only partially understood issue. If indeed space-time is arising as some sort of large scale condensate of more fundamental objects then it is natural to expect that matter, being a collective excitations of the spacetime constituents, will present modified kinematics at sufficiently high energies. We consider here the phenomenology of the dissipative effects necessarily arising in such a picture. Adopting dissipative hydrodynamics as a general framework for the description of the energy exchange between collective excitations and the spacetime fundamental degrees of freedom, we discuss how rates of decays for elementary particles can be derived from dispersion relations and used to provide strong constraints on the base of current astrophysical observations of high energy particles.
5 pages, 1 figure

http://arxiv.org/abs/1309.7273
Renormalization group flow of Hořava-Lifshitz gravity at low energies
Adriano Contillo, Stefan Rechenberger, Frank Saueressig
(Submitted on 27 Sep 2013)
The functional renormalization group equation for projectable Horava-Lifshitz gravity is used to derive the non-perturbative beta functions for the Newton's constant, cosmological constant and anisotropy parameter. The resulting coupled differential equations are studied in detail and exemplary RG trajectories are constructed numerically. The beta functions possesses a non-Gaussian fixed point and a one-parameter family of Gaussian fixed points. One of the Gaussian fixed points corresponds to the Einstein-Hilbert action with vanishing cosmological constant and constitutes a saddle point with one IR-attractive direction. For RG trajectories dragged into this fixed point at low energies diffeomorphism invariance is restored. The emergence of general relativity from Horava-Lifshitz gravity can thus be understood as a crossover-phenomenon where the IR behavior of the theory is controlled by this Gaussian fixed point. In particular RG trajectories with a tiny positive cosmological constant also come with an anisotropy parameter which is compatible with experimental constraints, providing a mechanism for the approximate restoration of diffeomorphism invariance in the IR. The non-Gaussian fixed point is UV-attractive in all three coupling constants. Most likely, this fixed point is the imprint of Asymptotic Safety at the level of Horava-Lifshitz gravity.
32 pages, 6 figures

http://arxiv.org/abs/1309.7206
Decoherence in quantum cosmology and the cosmological constant
T. Asselmeyer-Maluga, J. Krol
(Submitted on 27 Sep 2013)
We discuss a spacetime having the topology of S³ x R but with a different smoothness structure. The initial state of the cosmos in our model is identified with a wildly embedded 3-sphere (or a fractal space). In previous work we showed that a wild embedding is obtained by a quantization of a usual (or tame) embedding. Then a wild embedding can be identified with a (geometrical) quantum state. During a decoherence process this wild 3-sphere is changed to a homology 3-sphere. We are able to calculate the decoherence time for this process. After the formation of the homology 3-sphere, we obtain a spacetime with an accelerated expansion enforced by a cosmological constant. The calculation of this cosmological constant gives a qualitative agreement with the current measured value.
8 pages, 1 figure, accepted in MPLA

http://arxiv.org/abs/1309.7248
The equations of CCC
Paul Tod
(Submitted on 27 Sep 2013)
I review the equations of Conformal Cyclic Cosmology given by Penrose. I suggest a slight modification to Penrose's prescription and show how this works out for FRW cosmologies and for Class A Bianchi cosmologies.
9 pages. Corrected version of a talk given at the conference `The Mathematics of CCC: Mathematical Physics with Positive Lambda' organised by the Clay Mathematics Institute at the Mathematical Institute, University of Oxford from September 11-13, 2013

http://arxiv.org/abs/1309.7271
A Fundamental Solution to the CCC equation
Ezra Newman
(Submitted on 27 Sep 2013)
We display a simple solution to the Penrose CCC scenario. For this solution we chose for the late stages of the previous aeon a FRW, k=0, universe with a both a cosmological constant and radiation (no mass) while for the early stages of the 'present' aeon we have again a FRW universe, k=0, with the same cosmological constant and again with radiation but with mass not yet present. The Penrose conditions force the parameters describing the radiation of the former and present aeons to be equal and the transition metric in the overlap region turns out to be flat. We further study how different rest-mass zero fields transition between the different conformally related regions. These (test) fields appears to easily allow perturbations of the geometry within the CCC scenario.
13 pages

http://arxiv.org/abs/1309.7267
Cosmology and the Dark Matter Frontier
L. Bergstrom
(Submitted on 27 Sep 2013)
A brief overview is given about some issues in current astroparticle physics, focusing on the dark matter (DM) problem, where the connection to LHC physics is particularly strong. New data from the Planck satellite has made the evidence in favour of the existence of DM even stronger. The favourite, though not the only, candidates for cosmological DM, weakly interacting massive particles (WIMPs), are being probed by a variety of experiments - direct detection through scattering in terrestrial detectors, indirect detection by observing products of annihilation of DM in the Galaxy, and finally searches at accelerators such as the LHC. The field is in the interesting situation that all of these search methods are reaching sensitivities where signals of DM may plausibly soon be found, and a vast array of models will be probed in the next few years. Of course, expectations for a positive signature are high, which calls for caution regarding 'false alarms'. Some of the presently puzzling and partly conflicting pieces of evidence for DM detection are discussed as well as expectations for the future.
15 pages. Invited talk at the Nobel Symposium on LHC Physics, Krusenberg, Sweden, May 13-17, 2013

 

[John86]

http://arxiv.org/abs/1310.0558
A Quantum Model of Exaptation: Incorporating Potentiality into Evolutionary Theory
Liane Gabora, Eric O. Scott, Stuart Kauffman
(Submitted on 2 Oct 2013)
The phenomenon of preadaptation, or exaptation (wherein a trait that originally evolved to solve one problem is co-opted to solve a new problem) presents a formidable challenge to efforts to describe biological phenomena using a classical (Kolmogorovian) mathematical framework. We develop a quantum framework for exaptation with examples from both biological and cultural evolution. The state of a trait is written as a linear superposition of a set of basis states, or possible forms the trait could evolve into, in a complex Hilbert space. These basis states are represented by mutually orthogonal unit vectors, each weighted by an amplitude term. The choice of possible forms (basis states) depends on the adaptive function of interest (e.g., ability to metabolize lactose or thermoregulate), which plays the role of the observable. Observables are represented by self-adjoint operators on the Hilbert space. The possible forms (basis states) corresponding to this adaptive function (observable) are called eigenstates. The framework incorporates key features of exaptation: potentiality, contextuality, nonseparability, and emergence of new features. However, since it requires that one enumerate all possible contexts, its predictive value is limited, consistent with the assertion that there exists no biological equivalent to "laws of motion" by which we can predict the evolution of the biosphere.

http://arxiv.org/abs/1310.0693
Constraining f(R) gravity with PLANCK data on galaxy cluster profiles
I. De Martino, M. De Laurentis, F. Atrio-Barandela, S. Capozziello
(Submitted on 2 Oct 2013)
Models of $f(R)$ gravity that introduce corrections to the Newtonian potential in the weak field limit are tested at the scale of galaxy clusters. These models can explain the dynamics of spiral and elliptical galaxies without resorting to dark matter. We compute the pressure profiles of 579 galaxy clusters assuming that the gas is in hydrostatic equilibrium within the potential well of the modified gravitational field. The predicted profiles are compared with the average profile obtained by stacking the data of our cluster sample in the Planck foreground clean map SMICA. We find that the resulting profiles of these systems fit the data without requiring a dominant dark matter component, with model parameters similar to those required to explain the dynamics of galaxies. Our results do not rule out that clusters are dynamically dominated by Dark Matter but support the idea that Extended Theories of Gravity could provide an explanation to the dynamics of self-gravitating systems and to the present period of accelerated expansion, alternative to the concordance cosmological model.

http://arxiv.org/abs/1310.0675
Can a non-local model of gravity reproduce Dark Matter effects in agreement with MOND?
Ivan Arraut
(Submitted on 2 Oct 2013)
I analyze the possibility of reproducing MONDian Dark Matter effects by using a non-local model of gravity. The model was used before in order to recreate screening effects for the Cosmological Constant ($\Lambda$) value. Although the model in the weak-field approximation (in static coordinates) can reproduce the field equations in agreement with the AQUAL Lagrangian, the solutions are scale dependent and cannot reproduce the same dynamics in agreement with MOND.

 

[marcus]

http://arxiv.org/abs/1310.1290
Singularity avoidance in the hybrid quantization of the Gowdy model
Paula Tarrío, Mikel Fernández Méndez, Guillermo A. Mena Marugán
(Submitted on 4 Oct 2013)
One of the most remarkable phenomena in Loop Quantum Cosmology is that, at least for homogeneous cosmological models, the Big Bang is replaced with a Big Bounce that connects our universe with a previous branch without passing through a cosmological singularity. The goal of this work is to study the existence of singularities in Loop Quantum Cosmology including inhomogeneities and check whether the behavior obtained in the purely homogeneous setting continues to be valid. With this aim, we focus our attention on the three-torus Gowdy cosmologies with linearly polarized gravitational waves and use effective dynamics to carry out the analysis. For this model, we prove that all the potential cosmological singularities are avoided, generalizing the results about resolution of singularities to this scenario with inhomogeneities. We also demonstrate that, if a bounce in the (Bianchi background) volume occurs, the inhomogeneities increase the value of this volume at the bounce with respect to its counterpart in the homogeneous case.
11 pages, 2 figures

http://arxiv.org/abs/1310.1088
Geometrodynamics and Lorentz symmetry
Derek K. Wise
(Submitted on 3 Oct 2013)
We study the dynamics of gauge theory and general relativity using fields of local observers, thus maintaining local Lorentz symmetry despite a space/time splitting of fields. We start with Yang--Mills theory, where observer fields are defined as normalized future-timelike vector fields. We then define observers without a fixed geometry, and find these play two related roles in general relativity: splitting fields into spatial and temporal parts, and "breaking" gauge symmetry, effectively reducing the spacetime SO(n,1) connection to an observer-dependent spatial SO(n) connection. In both gauge theory and gravity, the observer field reduces the action to canonical form, without using gauge fixing. In the 4d gravity case, the result is a manifestly Lorentz covariant counterpart of the Ashtekar-Barbero formulation. We also explain how this leads geometrically to a picture of general relativity in terms of "observer space" rather than spacetime---a setting where both spacetime symmetry and the dynamical description are simultaneously available.
Comments: 11 pages. Submission for the proceedings of "3Quantum: Algebra, Geometry, Information", Tallinn, July 2012

 

[marcus]

http://arxiv.org/abs/1310.1839
Action and entanglement in gravity and field theory
Yasha Neiman
(Submitted on 7 Oct 2013)
In non-gravitational quantum field theory, the entanglement entropy across a surface depends on the short-distance regularization. Quantum gravity should not require such regularization, and it's been conjectured that the entanglement entropy there is always given by the black hole entropy formula evaluated on the entangling surface. We show that these statements have precise classical counterparts at the level of the action. Specifically, we point out that the action can have a non-additive imaginary part. In gravity, the latter is fixed by the black hole entropy formula, while in non-gravitating theories, it is arbitrary. From these classical facts, the entanglement entropy conjecture follows by heuristically applying the relation between actions and wavefunctions.
4 pages, 2 figures

brief mention:
http://arxiv.org/abs/1310.1524
On Background Independence
Edward Anderson
(Submitted on 5 Oct 2013)
This paper concerns what Background Independence itself is (as opposed to some particular physical theory that is background independent). The notions presented mostly arose from a layer-by-layer analysis of the facets of the Problem of Time in Quantum Gravity... Other aspects of Background Independence include the algebraic closure of these constraints,.., foliation independence as implemented by refoliation invariance, the reconstruction of spacetime from space... Thus these arise naturally and are problems to be resolved, as opposed to avoided `by making one's physics background-dependent in order not to have these problems'...
14 pages including 2 figures

not Loop-and-allied QG but possibly of wider interest:
http://arxiv.org/abs/1310.1605
Limits on anisotropic inflation from the Planck data
Jaiseung Kim, Eiichiro Komatsu
(Submitted on 6 Oct 2013)
[my comment: Komatsu is a top cosmologist and it tends to be worth paying attention to what he has to say IMHO. Plus this has to do with several active controversies.]

 

[John86]

http://arxiv.org/abs/1310.1667
Information-Based Physics: An Observer-Centric Foundation
Kevin H. Knuth
(Submitted on 7 Oct 2013)
It is generally believed that physical laws, reflecting an inherent order in the universe, are ordained by nature. However, in modern physics the observer plays a central role raising questions about how an observer-centric physics can result in laws apparently worthy of a universal nature-centric physics. Over the last decade, we have found that the consistent apt quantification of algebraic and order-theoretic structures results in calculi that possesses constraint equations taking the form of what are often considered to be physical laws. I review recent derivations of the formal relations among relevant variables central to special relativity, probability theory and quantum mechanics in this context by considering a problem where two observers form consistent descriptions of and make optimal inferences about a free particle that simply influences them. I show that this approach to describing such a particle based only on available information leads to the mathematics of relativistic quantum mechanics as well as a description of a free particle that reproduces many of the basic properties of a fermion. The result is an approach to foundational physics where laws derive from both consistent descriptions and optimal information-based inferences made by embedded observers.

http://arxiv.org/abs/1310.1699
Conformal geometrodynamics regained: gravity from duality
Henrique Gomes
(Submitted on 7 Oct 2013)
I propose the following conjecture: the conformal reduction of Hamiltonian general relativity is the sole reduced theory that allows description in the canonical metric phase space by \emph{dual} spatially covariant theories, each possessing different symmetry content than the other. One of the symmetries is the usual refoliation symmetry of general relativity in 3+1, and the other, its dual, is spatial Weyl symmetry. I prove the conjecture under mild extra assumptions.

 

[marcus]

http://arxiv.org/abs/1310.2174
Radiative corrections to the EPRL-FK spinfoam graviton
Aldo Riello
(Submitted on 8 Oct 2013)
I study the corrections engendered by the insertion of a "melon" graph in the bulk of the first-order spinfoam used for the graviton propagator. I find that these corrections are highly non-trivial and, in particular, that they concern those terms which disappear in the Bojowald-Bianchi-Magliaro-Perini limit of vanishing Barbero-Immirzi parameter at fixed area. This fact is the first realization of the often cited idea that the spinfoam amplitude receives higher order corrections under the refinement of the underlying two-complex.
13 pages, 4 figures

brief mention:
http://arxiv.org/abs/1310.1920
Where the world stands still: turnaround as a strong test of ΛCDM cosmology
Vasiliki Pavlidou, Theodore N. Tomaras
(Submitted on 7 Oct 2013)
Structure formation in ΛCDM cosmology is a cosmic battle between...[expansion and gravity]... Our intuitive understanding of the process works best in scales small enough so that bound, relaxed gravitating systems are no longer adjusting their radius; and large enough so that space and matter follow the average expansion of the Universe. Yet one of the most robust predictions of ΛCDM cosmology concerns the scale that separates these limits: the turnaround radius, which is the non-expanding shell furthest away from the center of a bound structure. The maximum possible value of the turnaround radius within the framework of the ΛCDM model is, for a given mass M, equal to (3GM/Λc²)^1/3, ... independently of cosmic epoch, detailed assumptions, exact nature of dark matter, or baryonic effects. We discuss the possible use of this prediction as an observational test for ΛCDM cosmology.
5 pages, 1 figure

 

[marcus]

brief mention (not Loop-and-allied QG, but possibly of general interest):
http://arxiv.org/abs/1310.3225
A Turing test for free will
Seth Lloyd
(Submitted on 11 Oct 2013)
Before Alan Turing made his crucial contributions to the theory of computation, he studied the question of whether quantum mechanics could throw light on the nature of free will. This article investigates the roles of quantum mechanics and computation in free will. Although quantum mechanics implies that events are intrinsically unpredictable, the 'pure stochasticity' of quantum mechanics adds only randomness to decision making processes, not freedom. By contrast, the theory of computation implies that even when our decisions arise from a completely deterministic decision-making process, the outcomes of that process can be intrinsically unpredictable, even to -- especially to -- ourselves. I argue that this intrinsic computational unpredictability of the decision making process is what give rise to our impression that we possesses free will. Finally, I propose a 'Turing test' for free will: a decision maker who passes this test will tend to believe that he, she, or it possesses free will, whether the world is deterministic or not.
20 pages, published in Philosophical Transactions of the Royal Society, Series A.

 

[marcus]

http://arxiv.org/abs/1310.3362
Deformation Operators of Spin Networks and Coarse-Graining
Etera R. Livine
(Submitted on 12 Oct 2013)
In the context of loop quantum gravity, quantum states of geometry are mathematically defined as spin networks living on graphs embedded in the canonical space-like hypersurface. In the effort to study the renormalisation flow of loop gravity, a necessary step is to understand the coarse-graining of these states in order to describe their relevant structure at various scales. Using the spinor network formalism to describe the phase space of loop gravity on a given graph, we focus on a bounded (connected) region of the graph and coarse-grain it to a single vertex using a gauge-fixing procedure. We discuss the ambiguities in the gauge-fixing procedure and their consequences for coarse-graining spin(or) networks. This allows to define the boundary deformations of that region in a gauge-invariant fashion and to identify the area preserving deformations as U(N) transformations similarly to the already well-studied case of a single intertwiner. The novelty is that the closure constraint is now relaxed and the closure defect interpreted as a local measure of the curvature inside the coarse-grained region. It is nevertheless possible to cancel the closure defect by a Lorentz boost. We further identify a Lorentz-invariant observable related to the area and closure defect, which we name "rest area". Its physical meaning remains an open issue.
24 pages

http://arxiv.org/abs/1310.3736
Tensorial methods and renormalization in Group Field Theories
Sylvain Carrozza
(Submitted on 14 Oct 2013)
In this thesis, we study the structure of Group Field Theories (GFTs) from the point of view of renormalization theory. Such quantum field theories are found in approaches to quantum gravity related to Loop Quantum Gravity (LQG) on the one hand, and to matrix models and tensor models on the other hand. They model quantum space-time, in the sense that their Feynman amplitudes label triangulations, which can be understood as transition amplitudes between LQG spin network states. The question of renormalizability is crucial if one wants to establish interesting GFTs as well-defined (perturbative) quantum field theories, and in a second step connect them to known infrared gravitational physics. Relying on recently developed tensorial tools, this thesis explores the GFT formalism in two complementary directions. First, new results on the large cut-off expansion of the colored Boulatov-Ooguri models allow to explore further a non-perturbative regime in which infinitely many degrees of freedom contribute. The second set of results provide a new rigorous framework for the renormalization of so-called Tensorial GFTs (TGFTs) with gauge invariance condition. In particular, a non-trivial 3d TGFT with gauge group SU(2) is proven just-renormalizable at the perturbative level, hence opening the way to applications of the formalism to (3d Euclidean) quantum gravity.
229 pages, many figures. PhD thesis, partly based on arXiv:1104.5158, arXiv:1203.5082, arXiv:1207.6734 and arXiv:1303.6772

http://arxiv.org/abs/1310.3759
A pure Dirac's method for Husain-Kuchar theory
Alberto Escalante, J. Berra
(Submitted on 27 Sep 2013)
A pure Dirac's canonical analysis, defined in the full phase space for the Husain-Kuchar model is discussed in detail. This approach allows us to determine the extended action, the extended Hamiltonian, the complete constraint algebra and the gauge transformations for all variables that occur in the action principle. The complete set of constraints defined on the full phase space allow us to calculate the Dirac algebra structure of the theory and a local weighted measure for the path integral quantization method. Finally, we discuss briefly the necessary mathematical structure to perform the canonical quantization program within the framework of the loop quantum gravity approach.
12 pages.

 

[John86]

http://arxiv.org/abs/1310.4143
Dynamical Emergence of Universal Horizons during the formation of Black Holes
Mehdi Saravani, Niayesh Afshordi, Robert B. Mann
(Submitted on 15 Oct 2013)
Motivations for the existence of a fundamental preferred frame range from pure phenomenology to attempts to solve the non-renormalizability of quantum gravity, the problem of time (and scale), and the cosmological constant problem(s). In many explicit constructions, such as Einstein-Aether or Gravitational Aether theories, K-essence, Cuscuton theory, Shape Dynamics, or (non-projectable) Horava-Lifshitz gravity, the low energy theory contains a fluid (which defines a preferred frame) with superluminal or incompressible excitations. We study here the formation of black holes in the presence of such a fluid. In particular, we focus on the incompressible limit of the fluid (or Constant Mean Curvature foliation) in the space-time of a spherically collapsing shell within an asymptotically cosmological space-time. In this case, ignoring the fluid back reaction, we can analytically show that an observer inside 3/4 of the Schwarzschild radius cannot send a signal outside, after a stage in collapse, even using signals that propagate infinitely fast in the preferred frame. This confirms the dynamical emergence of universal horizons that have been previously found in static solutions. We argue that this universal horizon should be considered as the future boundary of the classical space-time.

 

[John86]

http://arxiv.org/abs/1309.6896
Observational issues in loop quantum cosmology
A. Barrau, T. Cailleteau, J. Grain, J. Mielczarek
(Submitted on 26 Sep 2013)
Quantum gravity is sometimes considered as a kind of metaphysical speculation. In this review, we show that, although still extremely difficult to reach, observational signatures can in fact be expected. The early universe is an invaluable laboratory to probe "Planck scale physics". Focusing on Loop Quantum Gravity as one of the best candidate for a non-perturbative and background-independant quantization of gravity, we detail some expected features.

http://arxiv.org/abs/1310.4180
Holographic Charged Renyi Entropies
Alexandre Belin, Ling-Yan Hung, Alexander Maloney, Shunji Matsuura, Robert C. Myers, Todd Sierens
(Submitted on 15 Oct 2013)
We construct a new class of entanglement measures by extending the usual definition of Renyi entropy to include a chemical potential. These charged Renyi entropies measure the degree of entanglement in different charge sectors of the theory and are given by Euclidean path integrals with the insertion of a Wilson line encircling the entangling surface. We compute these entropies for a spherical entangling surface in CFT's with holographic duals, where they are related to entropies of charged black holes with hyperbolic horizons. We also compute charged Renyi entropies in free field theories.

 

[marcus]

http://arxiv.org/abs/1310.4795
Chimera: A hybrid approach to numerical loop quantum cosmology
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 17 Oct 2013)
The existence of a quantum bounce in isotropic spacetimes is a key result in loop quantum cosmology (LQC), which has been demonstrated to arise in all the models studied so far. In most of the models, the bounce has been studied using numerical simulations involving states which are sharply peaked and which bounce at volumes much larger than the Planck volume. An important issue is to confirm the existence of the bounce for states which have a wide spread, or which bounce closer to the Planck volume. Numerical simulations with such states demand large computational domains, making them very expensive and practically infeasible with the techniques which have been implemented so far. To overcome these difficulties, we present an efficient hybrid numerical scheme using the property that at the small spacetime curvature, the quantum Hamiltonian constraint in LQC, which is a difference equation with uniform discretization in volume, can be approximated by a Wheeler-DeWitt differential equation. By carefully choosing a hybrid spatial grid allowing the use of partial differential equations at large volumes, and with a simple change of geometrical coordinate, we obtain a surprising reduction in the computational cost. This scheme enables us to explore regimes which were so far unachievable for the isotropic model in LQC. Our approach also promises to significantly reduce the computational cost for numerical simulations in anisotropic LQC using high performance computing.
39 pages, 15 figures

http://arxiv.org/abs/1310.4771
Astrophysical black holes may radiate, but they do not evaporate
George F R Ellis
(Submitted on 17 Oct 2013)
This paper argues that the effect of Hawking radiation on an astrophysical black hole is not total evaporation of the black hole; rather there will always be a remnant. The key point is that the locus of emission of Hawking radiation is not the globally defined event horizon, but rather is just outside a timelike Marginal Outer Trapped Surface that is locally defined and lies inside the event horizon, close to the collapsing star. A spacelike singularity forms behind the event horizon, and most of the Hawking radiation ends up at this singularity rather than at infinity. Whether any Hawking radiation reaches infinity depends on the relation between the dynamic radiation emission surface and the event horizon, which is affected both by Cosmic Background Radiation and by back-reaction due to the Hawking radiation. From the outside view, even if radiation is seen as always being emitted, the black hole never evaporates away, rather its mass and entropy asymptote to finite non-zero limits. The argument is based on the broad nature of the processes at work, plus a careful delineation of the relevant causal domains; detailed calculations of back reaction effects are necessary in order to confirm this model and determine details of the outcome.
52 pages, 9 Figures, 4 Tables

 

[John86]

http://arxiv.org/abs/1308.5210
Consistent Probabilities in Perfect Fluid Quantum Universes
Clécio R. Bom, Nelson Pinto-Neto, Grasiele B. Santos
(Submitted on 23 Aug 2013 (v1), last revised 26 Aug 2013 (this version, v2))
Recently it has been claimed that the Wheeler-DeWitt quantization of gravity is unable to avoid cosmological singularities. However, in order to make this assertion, one must specify the underlying interpretation of quantum mechanics which has been adopted. For instance, several nonsingular models were obtained in Wheeler-DeWitt quantum cosmology in the framework of the de Broglie-Bohm quantum theory. Conversely, there are specific situations where the singularity cannot be avoided in the framework of the Consistent Histories approach to quantum mechanics. In these specific situations, the matter content is described by a scalar field, and the Wheeler-DeWitt equation looks-like a Klein-Gordon equation. The aim of this work is to study the Wheeler-DeWitt quantization of cosmological models where the matter content is described by an hydrodynamical perfect fluid, where the Wheeler-DeWitt equation reduces to a genuine Schr\"odinger equation. In this case, it is shown that the conclusions of the Consistent Histories and the de Broglie-Bohm approaches coincide in the quantum cosmological models where the curvature of the spatial sections is not positive definite, namely, that the cosmological singularities are eliminated. In the case of positive spatial curvature, the family of histories is no longer consistent, and no conclusion can be given in this framework.

http://arxiv.org/abs/1310.1600
Universality of Phase Transition Dynamics: Topological Defects from Symmetry Breaking
Adolfo del Campo, Wojciech H. Zurek
(Submitted on 6 Oct 2013 (v1), last revised 15 Oct 2013 (this version, v2))
In the course of a non-equilibrium continuous phase transition, the dynamics ceases to be adiabatic in the vicinity of the critical point as a result of the critical slowing down (the divergence of the relaxation time in the neighborhood of the critical point). This enforces a local choice of the broken symmetry and can lead to the formation of topological defects. The Kibble-Zurek mechanism (KZM) was developed to describe the associated nonequilibrium dynamics and to estimate the density of defects as a function of the quench rate through the transition. During recent years, several new experiments investigating formation of defects in phase transitions induced by a quench both in classical and quantum mechanical systems were carried out. At the same time, some established results were called into question. We review and analyze the Kibble-Zurek mechanism focusing in particular on this surge of activity, and suggest possible directions for further progress.

http://arxiv.org/abs/1310.4691
Time from quantum entanglement: an experimental illustration
Ekaterina Moreva, Giorgio Brida, Marco Gramegna, Vittorio Giovannetti, Lorenzo Maccone, Marco Genovese
(Submitted on 17 Oct 2013)
In the last years several theoretical papers discussed if time can be an emergent propertiy deriving from quantum correlations. Here, to provide an insight into how this phenomenon can occur, we present an experiment that illustrates Page and Wootters' mechanism of "static" time, and Gambini et al. subsequent refinements. A static, entangled state between a clock system and the rest of the universe is perceived as evolving by internal observers that test the correlations between the two subsystems. We implement this mechanism using an entangled state of the polarization of two photons, one of which is used as a clock to gauge the evolution of the second: an "internal" observer that becomes correlated with the clock photon sees the other system evolve, while an "external" observer that only observes global properties of the two photons can prove it is static.

 

[MTd2]

Because of the author:

http://arxiv.org/abs/1310.4957

Nonlocality in string theory

Gianluca Calcagni, Leonardo Modesto
(Submitted on 18 Oct 2013)
We discuss an aspect of string theory which has been tackled under many different perspectives, but incompletely: the role of nonlocality in the theory and its relation with the geometric shape of the string. In particular, we will describe in quantitative terms how one can zoom out an extended object such as the string so that, at sufficiently large scales, it appears structureless. Since there are no free parameters in free string theory, the notion of large scales will be univocally determined. In other words, we will be able to answer the question: How and at which scale can the string be seen as a particle? In doing so, we will employ the concept of spectral dimension in a new way with respect to its usual applications in quantum gravity. The operational notions of worldsheet and target spacetime dimension in string theory are also clarified and found to be in mutual agreement.

 

[marcus]

http://arxiv.org/abs/1310.5167
A Gravitational Origin of the Arrows of Time
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 18 Oct 2013)
The only widely accepted explanation for the various arrows of time that everywhere and at all epochs point in the same direction is the `past hypothesis': the Universe had a very special low-entropy initial state. We present the first evidence for an alternative conjecture: the arrows exist in all solutions of the gravitational law that governs the Universe and arise because the space of its true degrees of freedom (shape space) is asymmetric. We prove our conjecture for arrows of complexity and information in the Newtonian N-body problem. Except for a set of measure zero, all of its solutions for non-negative energy divide at a uniquely defined point into two halves. In each a well-defined measure of complexity fluctuates but grows irreversibly between rising bounds from that point. Structures that store dynamical information are created as the complexity grows. Recognition of the division is a key novelty of our approach. 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 one past and one future. The 'paradox' of a time-symmetric law that leads to observationally irreversible behaviour is fully resolved. General Relativity shares enough architectonic structure with the N-body problem for us to prove the existence of analogous complexity arrows in the vacuum Bianchi IX model. In the absence of non-trivial solutions with matter we cannot prove that arrows of dynamical information will arise in GR, though they have in our Universe. Finally, we indicate how the other arrows of time could arise.
44 pages + 14 pages appendices and references. 8 figures and 1 table

general interest:
http://arxiv.org/abs/1310.5217
Dark Matter 2013
Marc Schumann
(Submitted on 19 Oct 2013)
This article reviews the status of the exciting and fastly evolving field of dark matter research as of summer 2013, when it was discussed at ICRC 2013 in Rio de Janeiro. It focuses on the three main avenues to detect WIMP dark matter: direct detection, indirect detection and collider searches. The article is based on the dark matter rapporteur talk summarizing the presentations given at the conference, filling some gaps for completeness.
9 pages, 7 figures. To appear in the proceedings of ICRC 2013

 

[John86]

http://arxiv.org/abs/1310.5412
Transient Weak-Lensing by Cosmological Dark Matter Microhaloes
Sohrab Rahvar, Shant Baghram, Niayesh Afshordi
(Submitted on 21 Oct 2013)
We study the time variation of the apparent flux of cosmological point sources due to the transient weak lensing by dark matter microhaloes. Assuming a transverse motion of microhaloes with respect to our line of sight, we derive the correspondence between the temporal power spectrum of the weak lensing magnification, and the spatial power spectrum of density on small scales. Considering different approximations for the small scale structure of dark matter, we predict the apparent magnitude of cosmological point sources to vary by as much as , due to this effect, within a period of a few months. This red photometric noise has an almost perfect gaussian statistics, to one part in . We also compare the transient weak lensing power spectrum with the background effects such as the stellar microlensing on cosmological scales. A quasar lensed by a galaxy or cluster like SDSSJ1004+4112 strong lensing system, with multiple images, is a suitable system for this study as: (i) using the time-delay method between different images, we can remove the intrinsic variations of the quasar, and (ii) strong lensing enhances signals from the transient weak lensing. We also require the images to form at large angular separations from the center of the lensing structure, in order to minimize contamination by the stellar microlensing. With long-term monitoring of quasar strong lensing systems with a 10-meter class telescope, we can examine the existence of dark microhaloes as the building blocks of dark matter structures. Failure to detect this signal may either be caused by a breakdown of cold dark matter (CDM) hierarchy on small scales, or rather interpreted as evidence against CDM paradigm, e.g. in favor of modified gravity models.

http://arxiv.org/abs/1310.5115
Undoing the twist: the Hořava limit of Einstein-aether
Ted Jacobson
(Submitted on 18 Oct 2013)
Ho\v{r}ava gravity can be obtained from Einstein-aether theory in the limit that the twist coupling constant goes to infinity, while holding fixed the expansion, shear and acceleration couplings. This limit helps to clarify the relation between the two theories, and allows Ho\v{r}ava results to be obtained from Einstein-aether ones. This is illustrated with several examples, including rotating black hole equations, PPN parameters, and radiation rates from binary systems.

 

[marcus]

http://arxiv.org/abs/1310.5996
Quantum black holes in Loop Quantum Gravity
Rodolfo Gambini, Javier Olmedo, Jorge Pullin
(Submitted on 22 Oct 2013)
We study the quantization of spherically symmetric vacuum spacetimes within loop quantum gravity. In particular, we give additional details about our previous work in which we showed that one could complete the quantization the model and that the singularity inside black holes is resolved. Moreover, we consider an alternative quantization based on a slightly different kinematical Hilbert space. The ambiguity in kinematical spaces stems from how one treats the periodicity of one of the classical variables in these models. The corresponding physical Hilbert spaces solve the diffeomorphism and Hamiltonian constraint but their intrinsic structure is radically different depending on the kinematical Hilbert space one started from. In both cases there are quantum observables that do not have a classical counterpart. However, one can show that at the end of the day, by examining Dirac observables, both quantizations lead to the same physical predictions.
20 pages

 

[marcus]

http://arxiv.org/abs/1310.6095
A Rotating Black Hole Solution for Shape Dynamics
Henrique Gomes, Gabriel Herczeg
(Submitted on 23 Oct 2013)
Shape dynamics is a classical theory of gravity which agrees with general relativity in many important aspects, but which possesses different gauge symmetries and can present some fundamental global differences with respect to Einstein spacetimes. Here, we present a general procedure for (locally) mapping stationary, axisymmetric general relativity solutions onto their shape dynamic counterparts. We focus in particular on the rotating black hole solution for shape dynamics and show that many of the properties of the spherically symmetric solution are preserved in the extension to the axisymmetric case: it is also free of physical singularities, it does not form a space-time at the horizon, and it possesses an inversion symmetry about the horizon.
13 pages

 

[John86]

http://arxiv.org/abs/1310.5700
Effective field theory models for nonviolent information transfer from black holes
Steven B. Giddings, Yinbo Shi
(Submitted on 21 Oct 2013)
Transfer of quantum information from the interior of a black hole to its atmosphere is described, in models based on effective field theory. This description illustrates that such transfer need not be violent to the semiclassical geometry or to infalling observers, and in particular can avoid producing a singular horizon or "firewall." One can specifically quantify the rate of information transfer, and show that a rate necessary to unitarize black hole evaporation produces a relatively mild modification to the stress tensor near the horizon. In an exterior description of the transfer, the new interactions responsible for it are approximated by "effective sources" acting on fields in the black hole atmosphere. If the necessary interactions couple to general modes in the black hole atmosphere, one also finds a straightforward mechanism for information transfer rates to increase when a black hole is mined, avoiding paradoxical behavior. Correspondence limits are discussed, in the presence of such new interactions, for both small black holes and large ones; the near-horizon description of the latter is approximately that of Rindler space.

http://arxiv.org/abs/1310.6052
Holographic Space-time and Newton's Law
Tom Banks, Willy Fischler
(Submitted on 22 Oct 2013)
We derive Newton's Law from the formalism of Holographic Space-Time (HST). More precisely, we show that for a large class of Hamiltonians of the type proposed previously for the HST description of a geodesic in Minkowski space, the eikonal for scattering of two massless particles at large impact parameter scales as expected with the impact parameter and the energies of the particles in the center of mass (CM) frame. We also discuss the criteria for black hole production in this collision, and find an estimate, purely within the HST framework, for the impact parameter at which it sets in, which coincides with the estimate based on general relativity.

 

[marcus]

http://arxiv.org/abs/1310.6728
Quantization ambiguities and bounds on geometric scalars in anisotropic loop quantum cosmology
Parampreet Singh, Edward Wilson-Ewing
(Submitted on 24 Oct 2013)
We study quantization ambiguities in loop quantum cosmology that arise for space-times with non-zero spatial curvature and anisotropies. Motivated by lessons from different possible loop quantizations of the closed Friedmann-Lemaitre-Robertson-Walker cosmology, we find that using open holonomies of the extrinsic curvature, which due to gauge-fixing can be treated as a connection, leads to the same quantum geometry effects that are found in spatially flat cosmologies. More specifically, in contrast to the quantization based on open holonomies of the Ashtekar-Barbero connection, the expansion and shear scalars in the effective theories of the Bianchi type II and Bianchi type IX models have upper bounds, and these are in exact agreement with the bounds found in the effective theories of the Friedmann-Lemaitre-Robertson-Walker and Bianchi type I models in loop quantum cosmology. We also comment on some ambiguities present in the definition of inverse triad operators and their role.
34 pages

http://arxiv.org/abs/1310.6399
Inflationary Attractors and their Measures
Alejandro Corichi, David Sloan
(Submitted on 23 Oct 2013)
Several recent misconceptions about the measure problem in inflation and the nature of inflationary attractors are addressed. We show that within the Hamiltonian system of flat Friedmann-Lemaître-Robertson-Walker cosmology coupled to a massive scalar field, the focussing of the Liouville measure on attractor solutions is brought about by a spread in a gauge degree of freedom - the spatial volume. Using this we show how the Liouville measure formulated on a surface of constant Hubble rate induces a probability distribution function on surfaces of other Hubble rates, and the attractor behaviour is seen through the focussing of this function on a narrow range of physical observables. One can conclude then that standard techniques from Hamiltonian dynamics suffice to provide a satisfactory description of attractor solutions and the measure problem.
6 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1310.7426
Nonequivalence of equivalence principles
Eolo Di Casola, Stefano Liberati, Sebastiano Sonego
(Submitted on 28 Oct 2013)
Equivalence principles played a central role in the development of general relativity. Furthermore, they have provided operative procedures for testing the validity of general relativity, or constraining competing theories of gravitation. This has led to a flourishing of different, and inequivalent, formulations of these principles, with the undesired consequence that often the same name, "equivalence principle", is associated with statements having a quite different physical meaning. In this paper we provide a precise formulation of the several incarnations of the equivalence principle, clarifying their uses and reciprocal relations. We also discuss their possible role as selecting principles in the design and classification of viable theories of gravitation.
10 pages; submitted to Am. J. Phys

remote interest:
http://arxiv.org/abs/1310.7121

 

[marcus]

http://arxiv.org/abs/1310.7786
Group field theory as the 2nd quantization of Loop Quantum Gravity
Daniele Oriti
(Submitted on 29 Oct 2013)
We construct a 2nd quantized reformulation of canonical Loop Quantum Gravity at both kinematical and dynamical level, in terms of a Fock space of spin networks, and show in full generality that it leads directly to the Group Field Theory formalism. In particular, we show the correspondence between canonical LQG dynamics and GFT dynamics leading to a specific GFT model from any definition of quantum canonical dynamics of spin networks. We exemplify the correspondence of dynamics in the specific example of 3d quantum gravity. The correspondence between canonical LQG and covariant spin foam models is obtained via the GFT definition of the latter.
23 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1310.8654
Why are the effective equations of loop quantum cosmology so accurate?
Carlo Rovelli, Edward Wilson-Ewing
(Submitted on 31 Oct 2013)
We point out that the Heisenberg uncertainty relations vanish for non-compact spaces in loop quantum cosmology, thus explaining the surprising accuracy of the effective equations in describing the dynamics of sharply peaked wave packets. This underlines the fact that minisuperspace models ---where it is global variables that are quantized--- do not capture the local quantum fluctuations of the geometry.
5 pages

http://arxiv.org/abs/1310.8552
Two-Dimensional Quantum Geometry
J. Ambjorn, T. Budd
(Submitted on 31 Oct 2013)
In these lectures we review our present understanding of the fractal structure of two-dimensional Euclidean quantum gravity coupled to matter.
26 pages. Lectures presented at "The 53rd Cracow School of Theoretical Physics: Conformal Symmetry and Perspectives in Quantum and Mathematical Gravity", June 28 - July 7, 2013, Zakopane, Poland

 

[MTd2]

http://arxiv.org/abs/1311.0054

Relative information at the foundation of physics

Carlo Rovelli
(Submitted on 31 Oct 2013)
Shannon's notion of relative information between two physical systems can function as foundation for statistical mechanics and quantum mechanics, without referring to subjectivism or idealism. It can also represent a key missing element in the foundation of the naturalistic picture of the world, providing the conceptual tool for dealing with its apparent limitations. I comment on the relation between these ideas and Democritus.

http://arxiv.org/abs/1311.0186

Twistor relative locality

Lee Smolin
(Submitted on 1 Nov 2013)
We present a version of relative locality based on the geometry of twistor space. This can also be thought of as a new kind of deformation of twistor theory based on the construction of a bundle of twistor spaces over momentum space. Locality in space-time is emergent and is deformed in a precise way when a connection on that bundle is non-flat. This gives a precise and controlled meaning to Penrose's hypothesis that quantum gravity effects will deform twistor space in such a way as to maintain causality and relativistic invariance while weakening the notion that interactions take place at points in spacetime.

 

[John86]

http://arxiv.org/abs/1311.0813
Quantropy
John C. Baez, Blake S. Pollard
(Submitted on 4 Nov 2013)
There is a well-known analogy between statistical and quantum mechanics. In statistical mechanics, Boltzmann realized that the probability for a system in thermal equilibrium to occupy a given state is proportional to exp(-E/kT) where E is the energy of that state. In quantum mechanics, Feynman realized that the amplitude for a system to undergo a given history is proportional to exp(-S/i hbar) where S is the action of that history. In statistical mechanics we can recover Boltzmann's formula by maximizing entropy subject to a constraint on the expected energy. This raises the question: what is the quantum mechanical analogue of entropy? We give a formula for this quantity, and for lack of a better name we call it "quantropy". We recover Feynman's formula from assuming that histories have complex amplitudes, that these amplitudes sum to one, and that the amplitudes give a stationary point of quantropy subject to a constraint on the expected action. Alternatively, we can assume the amplitudes sum to one and that they give a stationary point of a quantity we call "free action", which is analogous to free energy in statistical mechanics. We compute the quantropy, expected action and free action for a free particle, and draw some conclusions from the results.

http://arxiv.org/abs/1311.0595
On the paradox of Hawking radiation in a maximally extended Schwarzschild solution
George F R Ellis
(Submitted on 4 Nov 2013)
This paper considers the effect of Hawking radiation on an eternal black hole - that is. a maximally extended Schwarzschild solution. Symmetry considerations that hold independent of the details of the emission mechanism show there is an inconsistency in the claim that such a black hole evaporates away in a finite time. In essence: because the external domain is static, there is an infinite time available for the process to take place, so whenever the evaporation process is claimed to come to completion, it should have happened earlier. The problem is identified to lie in the claim that the locus of emission of Hawking radiation lies just outside the globally defined event horizon. Rather, the emission domain must be mainly located inside the event horizon, so most of the Hawking radiation ends up at this singularity rather than at infinity and the black hole never evaporates away. This result supports a previous claim [arXiv:1310.4771] that astrophysical black holes do not evaporate.

 

[marcus]

http://arxiv.org/abs/1311.0881
Asymptotically Safe Starobinsky Inflation
Edmund J. Copeland, Christoph Rahmede, Ippocratis D. Saltas
We revisit Starobinsky inflation in a quantum gravitational context, by means of the exact Renormalisation Group (RG). We calculate the non-perturbative beta functions for Newton's `constant' G and the dimensionless R^2 coupling, and show that an attractive asymptotically free UV fixed point exists for the latter, while an asymptotically safe one exists for the former, and we provide the corresponding beta functions. The smallness of the R^2 coupling, required for agreement with inflationary observables, is naturally ensured by the presence of the asymptotically free UV fixed point. We discuss the corresponding RG dynamics, showing both how inflationary and classical observations define the renormalisation conditions for the couplings, and also how the UV regime is connected with lower energies along the RG flow.
9 pages, 1 figure

http://arxiv.org/abs/1311.1121
Black holes and running couplings: A comparison of two complementary approaches
Benjamin Koch, Carlos Contreras, Paola Rioseco, Frank Saueressig
(Submitted on 5 Nov 2013)
Black holes appear as vacuum solutions of classical general relativity which depend on Newton's constant and possibly the cosmological constant. At the level of a quantum field theory, these coupling constants typically acquire a scale-dependence. This proceedings briefly summarizes two complementary ways to incorporate this effect: the renormalization group improvement of the classical black hole solution based on the running couplings obtained within the gravitational Asymptotic Safety program and the exact solution of the improved equations of motion including an arbitrary scale dependence of the gravitational couplings. Remarkably the picture of the "quantum" black holes obtained from these very different improvement strategies is surprisingly similar.
7 pages, 2 figures, prepared for the Karl Schwarzschild meeting 2013

brief mention, possibly of general interest:
http://arxiv.org/abs/1311.1109
The First Billion Years project: dark matter haloes going from contraction to expansion and back again
Andrew J. Davis, Sadegh Khochfar, Claudio Dalla Vecchia
(Submitted on 5 Nov 2013)
We study the effect of baryons on the inner dark matter profile in a large statistical sample of the first galaxies using the First Billion Years simulation between z=16−6. Using two simulations of the same volume and cosmological initial conditions, one with and one without baryons, we are able to directly compare haloes with their baryon-free counterparts, allowing a detailed study of the modifications to the dark matter density profile due to the presence of baryons. For each of the ≈5000 haloes in our sample,...
18 pages, 23 figures; submitted to MNRAS.

http://arxiv.org/abs/1311.1104
The largest structure of the Universe, defined by Gamma-Ray Bursts
I. Horvath, J. Hakkila, Z. Bagoly
(Submitted on 5 Nov 2013)
Research over the past three decades has revolutionized the field of cosmology while supporting the standard cosmological model. However, the cosmological principle of Universal homogeneity and isotropy has always been in question, since structures as large as the survey size have always been found as the survey size has increased. Until now, the largest known structure in our Universe is the Sloan Great Wall (SGW), which is more than 400 Mpc long and located approximately one billion light-years away. Here we report the discovery of a structure at least six times larger than the Sloan Great Wall that is suggested by the distribution of gamma-ray bursts (GRBs). ...
...This huge structure lies ten times farther away than the Sloan Great Wall, at a distance of approximately ten billion light-years. The size of the structure defined by these GRBs is about 2000-3000 Mpc, or more than six times the size of the largest known object (SGW) in the Universe.
7th Huntsville Gamma-Ray Burst Symposium, GRB 2013: paper 33 in eConf Proceedings C1304143

 

[John86]

http://arxiv.org/abs/1311.1095
Universal decoherence due to gravitational time dilation
Igor Pikovski, Magdalena Zych, Fabio Costa, Caslav Brukner
(Submitted on 5 Nov 2013)
Phenomena inherent to quantum theory on curved space-time, such as Hawking radiation, are typically assumed to be only relevant at extreme physical conditions: at high energies and in strong gravitational fields. Here we consider low-energy quantum mechanics in the presence of weak gravitational time dilation and show that the latter leads to universal decoherence of quantum superpositions. Time dilation induces a universal coupling between internal degrees-of-freedom and the centre-of-mass of a composite particle and we show that the resulting entanglement causes the particle's position to decohere. We derive the decoherence timescale and show that the weak time dilation on Earth is already sufficient to decohere micro-scale objects. No coupling to an external environment is necessary, thus even completely isolated composite systems will decohere on curved space-time. In contrast to gravitational collapse models, no modification of quantum theory is assumed. General relativity therefore can account for the emergence of classicality and the effect can in principle be tested in future matter wave experiments with large molecules or with trapped microspheres.

 

[marcus]

http://arxiv.org/abs/1311.1461
The Tensor Track, III
Vincent Rivasseau
(Submitted on 6 Nov 2013)
We provide an informal up-to-date review of the tensor track approach to quantum gravity. In a long introduction we describe in simple terms the motivations for this approach. Then the many recent advances are summarized, with emphasis on some points (Gromov-Hausdorff limit, Loop vertex expansion, Osterwalder-Schrader positivity...) which, while important for the tensor track program, are not detailed in the usual quantum gravity literature. We list open questions in the conclusion and provide a rather extended bibliography.
53 pages, 6 figures

http://arxiv.org/abs/1311.1297
Inhomogeneities, loop quantum gravity corrections, constraint algebra and general covariance
Rakesh Tibrewala
(Submitted on 6 Nov 2013)
Loop quantum gravity corrections, in the presence of inhomogeneities, can lead to a deformed constraint algebra. Such a deformation implies that the effective theory is no longer generally covariant. As a consequence, the geometrical concepts used in the classical theory lose their meaning. In the present paper we propose a method, based on canonical transformation on the phase space of the effective theory, to systematically recover the classical constraint algebra in the presence of the inverse triad corrections as well as in the presence of holonomy corrections. We show, by way of explicit example, that this also leads to the recovery of general covariance of the theory in the presence of inverse triad connections, implying that one can once again use the geometrical concepts to analyze the solutions in the presence of these quantum gravity corrections.
19 pages.

http://arxiv.org/abs/1311.1344
Inflationary power spectra with quantum holonomy corrections
Jakub Mielczarek
(Submitted on 6 Nov 2013)
In this paper we study slow-roll inflation with holonomy corrections from loop quantum cosmology. Both tensor and scalar power spectra of primordial perturbations are computed up to the first order in slow-roll parameters and V/ρ_c, where V is a potential of the scalar field and ρ_c is a critical energy density (expected to be of the order of the Planck energy density). Possible normalizations of modes at short scales are discussed. In case the normalization is performed with use of the Wronskian condition applied to adiabatic vacuum, the tensor and scalar spectral indices are not quantum corrected in the leading order. However, by choosing an alternative method of normalization one can obtain quantum corrections in the leading order. Furthermore, we show that the holonomy-corrected equation of motion for tensor modes can be derived from an effective background metric. This allows us to prove that the Wronskian normalization condition for the tensor modes preserves the classical form.
21 pages

http://arxiv.org/abs/1311.1238
Homogeneous cosmologies as group field theory condensates
Steffen Gielen, Daniele Oriti, Lorenzo Sindoni
(Submitted on 5 Nov 2013)
We give a general procedure, in the group field theory (GFT) formalism for quantum gravity, for constructing states that describe macroscopic, spatially homogeneous universes. These states are close to coherent (condensate) states used in the description of Bose-Einstein condensates. The condition on such states to be (approximate) solutions to the quantum equations of motion of GFT is used to extract an effective dynamics for homogeneous cosmologies directly from the underlying quantum theory. The resulting description in general gives nonlinear and nonlocal equations for the 'condensate wavefunction' which are analogous to the Gross-Pitaevskii equation in Bose-Einstein condensates. We show the general form of the effective equations for current quantum gravity models, as well as some concrete examples. We identify conditions under which the dynamics becomes linear, admitting an interpretation as a quantum-cosmological Wheeler-DeWitt equation, and give its semiclassical (WKB) approximation in the case of a kinetic term that includes a Laplace-Beltrami operator. For isotropic states, this approximation reproduces the classical Friedmann equation in vacuum with positive spatial curvature. We show how the formalism can be consistently extended from Riemannian signature to Lorentzian signature models, and discuss the addition of matter fields, obtaining the correct coupling of a massless scalar in the Friedmann equation from the most natural extension of the GFT action. We also outline the procedure for extending our condensate states to include cosmological perturbations. Our results form the basis of a general programme for extracting effective cosmological dynamics directly from a microscopic non-perturbative theory of quantum gravity.
48 pages