Loop-and-allied QG bibliography

[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

 

[marcus]

http://arxiv.org/abs/1311.1798
Topological lattice field theories from intertwiner dynamics
Bianca Dittrich, Wojciech Kaminski
(Submitted on 7 Nov 2013)
We introduce a class of 2D lattice models that describe the dynamics of intertwiners, or, in a condensed matter interpretation, the fusion and splitting of anyons. We identify different families and instances of triangulation invariant, that is, topological, models inside this class. These models give examples for symmetry protected topologically ordered 1D quantum phases with quantum group symmetries. Furthermore the models provide realizations for anyon condensation into a new effective vacuum. We explain the relevance of our findings for the problem of identifying the continuum limit of spin foam and spin net models.
35+9 pages

possibly of general interest:
http://arxiv.org/abs/1311.1608
On the Reality of Unruh Temperature
Manfred Requardt
(Submitted on 7 Nov 2013)
In contrast to recent criticism we undertake to show that the notion of Unruh temperature describes a real thermal property of the vacuum if viewed from an accelerated reference frame. We embed our investigation in a more general analysis of general relativistic temperature (Tolman-Ehrenfest effect) with the entropy-maximum principle being our guiding principle. We show that the Unruh effect neatly fits into this more general framework. Our criterion of reality is, first, the possibility to transfer a quantum of acceleration radiation to the inertial laboratory where it can be studied in principle under ordinary thrmodynamical conditions. Second, we emphasize as another criterion the importance of the coincidence of the accelerated and inertial observer description as far as the final objective result is concerned.
21 pages

 

[marcus]

http://arxiv.org/abs/1311.2530
Spectral dimension in causal set quantum gravity
Astrid Eichhorn, Sebastian Mizera
(Submitted on 11 Nov 2013)
We evaluate the spectral dimension in causal set quantum gravity by simulating random walks on causal sets. In contrast to other approaches to quantum gravity, we find an increasing spectral dimension at small scales. This observation can be connected to the non-locality of causal set theory that is deeply rooted in its fundamentally Lorentzian nature. Based on its large-scale behaviour, we conjecture that the spectral dimension can serve as a tool to distinguish causal sets that approximate manifolds from those that do not. As a new tool to probe quantum spacetime in different quantum gravity approaches, we introduce a novel dimensional estimator, the causal spectral dimension, based on the meeting probability of two random walkers, which respect the causal structure of the quantum spacetime. We discuss a causal-set example, where the spectral dimension and the causal spectral dimension differ, due to the existence of a preferred foliation.
16 pages, 11 figures

http://arxiv.org/abs/1311.2148
On the Axioms of Causal Set Theory
Benjamin F. Dribus
(Submitted on 9 Nov 2013)
This paper offers suggested improvements to the causal sets program in discrete gravity, which treats spacetime geometry as an emergent manifestation of causal structure at the fundamental scale. This viewpoint, which I refer to as the causal metric hypothesis, is summarized by Rafael Sorkin's phrase, "order plus number equals geometry." Proposed improvements include recognition of a generally nontransitive causal relation more fundamental than the causal order, an improved local picture of causal structure, development and use of relation space methods, and a new background-independent version of the histories approach to quantum theory. Besides causal set theory, à la Bombelli, Lee, Meyer, and Sorkin, this effort draws on Isham's topos-theoretic framework for physics, Sorkin's quantum measure theory, Finkelstein's causal nets, and Grothendieck's structural principles. This approach circumvents undesirable structural features in causal set theory, such as the permeability of maximal antichains, studied by Major, Rideout, and Surya, and the configuration space pathology arising from the asymptotic enumeration of Kleitman and Rothschild. The paper culminates in the theory of co-relative histories and kinematic schemes, combining the causal metric hypothesis, the histories approach to quantum theory, and Grothendieck's relative viewpoint. This leads to the derivation of causal Schrödinger-type equations as dynamical laws for discrete quantum spacetime.
147 pages, 32 figures

 

[marcus]

http://arxiv.org/abs/1311.2898
Matter matters in asymptotically safe quantum gravity
Pietro Donà, Astrid Eichhorn, Roberto Percacci
(Submitted on 12 Nov 2013)
We investigate the compatibility of minimally coupled scalar, fermion and gauge fields with asymptotically safe quantum gravity, using nonperturbative functional Renormalization Group methods. We study d=4,5 and 6 dimensions and within certain approximations find that for a given number of gauge fields there is a maximal number of scalar and fermion degrees of freedom compatible with an interacting fixed point at positive Newton coupling. The bounds impose severe constraints on grand unification with fundamental Higgs scalars. Supersymmetry and universal extra dimensions are also generally disfavored. The standard model and its extensions accommodating right-handed neutrinos, the axion and dark-matter models with a single scalar are compatible with a fixed point.
22 pages, 18 figures, 4 tables

general interest:
http://arxiv.org/abs/1311.2619
The New Quantum Logic
Robert B. Griffiths
(Submitted on 11 Nov 2013)
It is shown how all the major conceptual difficulties of standard (textbook) quantum mechanics, including the two measurement problems and the (supposed) nonlocality that conflicts with special relativity, are resolved in the consistent or decoherent histories interpretation of quantum mechanics by using a modified form of quantum logic to discuss quantum properties (subspaces of the quantum Hilbert space), and treating quantum time development as a stochastic process. The histories approach in turn gives rise to some conceptual difficulties, in particular the correct choice of a framework (probabilistic sample space) or family of histories, and these are discussed. The central issue is that the principle of unicity, the idea that there is a unique single true description of the world, is incompatible with our current understanding of quantum mechanics.
23 pages
[Robert Griffiths was the originator of the "consistent histories" way of understanding quantum mechanics. At his Carnegie Mellon webpage he terms it Consistent Quantum Theory. The approach has been taken up and developed in work by Hartle, Gell-Mann among others.]

 

[John86]

http://lanl.arxiv.org/abs/1311.3135
Planck-scale dimensional reduction without a preferred frame
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 13 Nov 2013)
Several approaches to quantum gravity suggest that the standard description of spacetime as probed at low-energy, with four dimensions, is replaced in the Planckian regime by a spacetime with a spectral dimension of two. The implications for relativistic symmetries can be momentous, and indeed the most tangible picture for "running" of the spectral dimension, found within Horava-Lifschitz gravity, requires the breakdown of relativity of inertial frames. In this Letter we incorporate running spectral dimensions in a scenario that does not require the emergence of a preferred frame. We consider the best studied mechanism for deforming relativistic symmetries whilst preserving the relativity of inertial frames, based on a momentum space with curvature at the Planck scale. We show explicitly how running of the spectral dimension can be derived from these models.

 

[marcus]

http://arxiv.org/abs/1311.3279
Null twisted geometries
Simone Speziale, Mingyi Zhang
(Submitted on 13 Nov 2013)
We define and investigate a quantisation of null hypersurfaces in the context of loop quantum gravity on a fixed graph. The main tool we use is the parametrisation of the theory in terms of twistors, which has already proved useful in discussing the interpretation of spin networks as the quantization of twisted geometries. The classical formalism can be extended in a natural way to null hypersurfaces, with the Euclidean polyhedra replaced by null polyhedra with space-like faces, and SU(2) by the little group ISO(2). The main difference is that the simplicity constraints present in the formalims are all first class, and the symplectic reduction selects only the helicity subgroup of the little group. As a consequence, information on the shapes of the polyhedra is lost, and the result is a much simpler, abelian geometric picture. It can be described by an Euclidean singular structure on the 2-dimensional space-like surface defined by a foliation of space-time by null hypersurfaces. This geometric structure is naturally decomposed into a conformal metric and scale factors, forming locally conjugate pairs. Proper action-angle variables on the gauge-invariant phase space are described by the eigenvectors of the Laplacian of the dual graph. We also identify the variables of the phase space amenable to characterize the extrinsic geometry of the foliation. Finally, we quantise the phase space and its algebra using Dirac's algorithm, obtaining a notion of spin networks for null hypersurfaces. Such spin networks are labelled by SO(2) quantum numbers, and are embedded non-trivially in the unitary, infinite-dimensional irreducible representations of the Lorentz group.
22 pages, 3 figures

 

[marcus]

http://arxiv.org/abs/1311.3340
Spectral dimension of quantum geometries
Gianluca Calcagni, Daniele Oriti, Johannes Thürigen
(Submitted on 13 Nov 2013)
The spectral dimension is an indicator of geometry and topology of spacetime and a tool to compare the description of quantum geometry in various approaches to quantum gravity. This is possible because it can be defined not only on smooth geometries but also on discrete (e.g., simplicial) ones. In this paper, we consider the spectral dimension of quantum states of spatial geometry defined on combinatorial complexes endowed with additional algebraic data: the kinematical quantum states of loop quantum gravity (LQG). Preliminarily, the effects of topology and discreteness of classical discrete geometries are studied in a systematic manner. We look for states reproducing the spectral dimension of a classical space in the appropriate regime. We also test the hypothesis that in LQG, as in other approaches, there is a scale dependence of the spectral dimension, which runs from the topological dimension at large scales to a smaller one at short distances. While our results do not give any strong support to this hypothesis, we can however pinpoint when the topological dimension is reproduced by LQG quantum states. Overall, by exploring the interplay of combinatorial, topological and geometrical effects, and by considering various kinds of quantum states such as coherent states and their superpositions, we find that the spectral dimension of discrete quantum geometries is more sensitive to the underlying combinatorial structures than to the details of the additional data associated with them.
38 pages, 18 figures

 

[John86]

http://arxiv.org/abs/1311.3912
A covariant causal set approach to discrete quantum gravity
Stan Gudder
(Submitted on 15 Nov 2013)
A covariant causal set (c-causet) is a causal set that is invariant under labeling. Such causets are well-behaved and have a rigid geometry that is determined by a sequence of positive integers called the shell sequence. We first consider the microscopic picture. In this picture, the vertices of a c-causet have integer labels that are unique up to a label isomorphism. This labeling enables us to define a natural metric d(a,b) between time-like separated vertices a and b. The time metric d(a,b) results in a natural definition of a geodesic from a to b. It turns out that there can be n≥1 such geodesics. Letting a be the origin (the big bang), we define the curvature K(b) of b to be n−1. Assuming that particles tend to move along geodesics, K(b) gives the tendency that vertex b is occupied. In this way, the mass distribution is determined by the geometry of the c-causet. We next consider the macroscopic picture which describes the growth process of c-causets. We propose that this process is governed by a quantum dynamics given by complex amplitudes. At present, these amplitudes are unknown. But if they can be found, they will determine the (approximate) geometry of the c-causet describing our particular universe. As an illustration, we present a simple example of an amplitude process that may have physical relevance. We also give a discrete analogue of Einstein's field equations.

http://arxiv.org/abs/1311.3787
Cosmic structure, averaging and dark energy
David L. Wiltshire
(Submitted on 15 Nov 2013)
These lecture notes review the theoretical problems associated with coarse-graining the observed inhomogeneous structure of the universe at late epochs, of describing average cosmic evolution in the presence of growing inhomogeneity, and of relating average quantities to physical observables. In particular, a detailed discussion of the timescape scenario is presented. In this scenario, dark energy is realized as a misidentification of gravitational energy gradients which result from gradients in the kinetic energy of expansion of space, in the presence of density and spatial curvature gradients that grow large with the growth of structure. The phenomenology and observational tests of the timescape model are discussed in detail, with updated constraints from Planck satellite data. In addition, recent results on the variation of the Hubble expansion on < 100/h Mpc scales are discussed. The spherically averaged Hubble law is significantly more uniform in the rest frame of the Local Group of galaxies than in the conventional rest frame assumed for the Cosmic Microwave Background. This unexpected result supports a fundamental revision of the notion of the cosmic rest frame, consistent with the expectations of the timescape scenario.

 

[marcus]

http://arxiv.org/abs/1311.4481
Cosmological signature change in Cartan Gravity with dynamical symmetry breaking
Joao Magueijo, Matias Rodriguez-Vazquez, Hans Westman, T.G. Zlosnik
(Submitted on 18 Nov 2013)
We investigate the possibility for classical metric signature change in a straightforward generalization of the first order formulation of gravity, dubbed "Cartan gravity". The mathematical structure of this theory mimics the electroweak theory in that the basic ingredients are an SO(1,4) Yang-Mills gauge field A^ab_μ and a symmetry breaking Higgs field V^a, with no metric or affine structure of spacetime presupposed. However, these structures can be recovered, with the predictions of General Relativity exactly reproduced, whenever the Higgs field breaking the symmetry to SO(1,3) is forced to have a constant (positive) norm V^aV_a. This restriction is usually imposed "by hand", but in analogy with the electroweak theory we promote the gravitational Higgs field Va to a genuine dynamical field, subject to non-trivial equations of motion. Even though we limit ourselves to actions polynomial in these variables, we discover a rich phenomenology. Most notably we derive classical cosmological solutions exhibiting a smooth transition between Euclidean and Lorentzian signature in the four-metric. These solutions are non-singular and arise whenever the SO(1,4) norm of the Higgs field changes sign i.e. the signature of the metric of spacetime is determined dynamically by the gravitational Higgs field. It is possible to find a plethora of such solutions and in some of them this dramatic behaviour is confined to the early universe, with the theory asymptotically tending to Einstein gravity at late times. Curiously the theory can also naturally embody a well-known dark energy model: Peebles-Ratra quintessence.
21 pages, 5 figures

 

[MTd2]

Power Point Ppaer!

It cites Garrett Lisi!

http://arxiv.org/abs/1311.4413

Gravi-Weak Unification and Multiple Point Principle

C.D. Froggatt, C.R. Das, L.V. Laperashvili, H.B. Nielsen, A. Tureanu
(Submitted on 18 Nov 2013)
We construct a model unifying gravity with weak SU(2) gauge and "Higgs" scalar fields. We assume the existence of a visible and an invisible (hidden) sector of the Universe. We used the extension of Plebanski's 4-dimensional gravitational theory, in which the fundamental fields are two-forms containing tetrads, spin connections and additional auxiliary fields. Considering a Spin(4,4) invariant extended Plebanski action, we recover the actions in both (visible and invisible) sectors of the Universe. After symmetry breaking of the graviweak (GW) unification, its physical constants (Newton's constants, cosmological constants, YM-couplings, etc.), are determined by a parameter guni of the GW unification. It is discussed that if this "Higgs" field coming in the GW unification could be the Higgs of the Standard Model, then the idea that its vacuum value could be, according to the Multiple Point Principle, a second minimum of the Higgs field effective potential, turns out not to be viable. Then other scalar "Higgs" field, giving the inflation and axion fields, has a Planck scale expectation value, and could have a better chance of being the scalar field unified with gravity.

 

[MTd2]

http://arxiv.org/abs/1311.4712

Euclidean Dynamical Triangulation revisited: is the phase transition really first order?

Tobias Rindlisbacher, Philippe de Forcrand
(Submitted on 19 Nov 2013)
The transition between the two phases of 4D Euclidean Dynamical Triangulation [1] was long believed to be of second order until in 1996 first order behavior was found for sufficiently large systems [3,4]. However, one may wonder if this finding was affected by the numerical methods used: to control volume fluctuations, in both studies [3,4] an artificial harmonic potential was added to the action; in [4] measurements were taken after a fixed number of accepted instead of attempted moves which introduces an additional error. Finally the simulations suffer from strong critical slowing down which may have been underestimated.
In the present work, we address the above weaknesses: we allow the volume to fluctuate freely within a fixed interval; we take measurements after a fixed number of attempted moves; and we overcome critical slowing down by using an optimized parallel tempering algorithm [6]. With these improved methods, on systems of size up to 64k 4-simplices, we confirm that the phase transition is first order.

 

[marcus]

http://arxiv.org/abs/1311.4979
Cosmology: theory
Mikhail Shaposhnikov
(Submitted on 20 Nov 2013)
The discovery of 126 GeV Higgs boson and observations of no signs of new physics at the LHC implies that the Standard Model of elementary particles is a self-consistent weakly-coupled effective field theory all the way up to the Planck scale without the addition of any new particles. I will discuss possible consequences of these findings for cosmology.
11 pages, 5 figures. Plenary talk delivered at the European Physical Society Conference on High Energy Physics, 18-24 July, 2013, Stockholm, Sweden

 

[marcus]

http://arxiv.org/abs/1311.5325
Note on the super inflation in loop quantum cosmology
Kui Xiao, Xiao-Kai He, Jian-Yang Zhu
(Submitted on 21 Nov 2013)
Phenomenological effect of the super-inflation in loop quantum cosmology (LQC) is discussed. We investigate the case that the Universe is filled with the interacting field between massive scalar field and radiation. Considering the damping coefficient Γ as a constant, the changes of the scale factor during super-inflation with four different initial conditions are discussed, and we find that the changes of the scale factor depend on the initial values of energy density of the scalar field and radiation at the bounce point. But no matter which initial condition is chosen, the radiation always dominated at the late time. Moreover, we investigate whether the super-inflation can provide enough e-folding number. For the super-inflation starts from the quantum bounce point, the initial value of Hubble parameter H(t_i)∼0, then it is possible to solve the flatness problem and horizon problem. As an example, following the method of Amoros to calculate particle horizon on the condition that the radiation dominated at bounce point, we find that the Universe has had enough time to be homogeneous and isotopic.
9 pages, 4 figures; published Physics Letters B (2013)

 

[marcus]

http://inspirehep.net/record/1266031?ln=en
http://arxiv.org/abs/arXiv:1311.6117
The Koslowski-Sahlmann representation: Gauge and diffeomorphism invariance
Miguel Campiglia, Madhavan Varadarajan
(Submitted on 24 Nov 2013)
The discrete spatial geometry underlying Loop Quantum Gravity (LQG) is degenerate almost everywhere. This is at apparent odds with the non-degeneracy of asymptotically flat metrics near spatial infinity. Koslowski generalised the LQG representation so as to describe states labelled by smooth non-degenerate triad fields. His representation was further studied by Sahlmann with a view to imposing gauge and spatial diffeomorphism invariance through group averaging methods. Motivated by the desire to model asymptotically flat quantum geometry by states with triad labels which are non- degenerate at infinity but not necessarily so in the interior, we initiate a generalisation of Sahlmann's considerations to triads of varying degeneracy. In doing so, we include delicate phase contributions to the averaging procedure which are crucial for the correct implementation of the gauge and diffeomorphism constraints, and whose existence can be traced to the background exponential functions recently constructed by one of us. Our treatment emphasizes the role of symmetries of quantum states in the averaging procedure. Semianalyticity, influential in the proofs of the beautiful uniqueness results for LQG, plays a key role in our considerations. As a by product, we re-derive the group averaging map for standard LQG, highlighting the role of state symmetries and explicitly exhibiting the essential uniqueness of its specification.
45 pages.

[comment:see also
TK http://arxiv.org/0709.3465
HS http://arxiv.org/1006.0388
MV http://arxiv.org/1306.6126
MC and MV in progress, refs 24 and 25 on pages 35 and 36.]

 

[marcus]

http://arxiv.org/abs/1311.6841
Observables in Loop Quantum Gravity with a cosmological constant
Maïté Dupuis, Florian Girelli
(Submitted on 26 Nov 2013)
An open issue in loop quantum gravity (LQG) is the introduction of a non-vanishing cosmological constant Λ. In 3d, Chern-Simons theory provides some guiding lines: Λ appears in the quantum deformation of the gauge group. The Turaev-Viro model, which is an example of spin foam model is also defined in terms of a quantum group. By extension, it is believed that in 4d, a quantum group structure could encode the presence of Λ≠0. In this article, we introduce by hand the quantum group U_q(su(2)) into the LQG framework, that is we deal with U_q(su(2))-spin networks. We explore some of the consequences, focusing in particular on the structure of the observables. Our fundamental tools are tensor operators for U_q(su(2)). We review their properties and give an explicit realization of the spinorial and vectorial ones. We construct the generalization of the U(n) formalism in this deformed case, which is given by the quantum group U_q(u(n)). We are then able to build geometrical observables, such as the length, area or angle operators ... We show that these operators characterize a quantum discrete hyperbolic geometry in the 3d LQG case. Our results confirm that the use of quantum group in LQG can be a tool to introduce a non-zero cosmological constant into the theory.
29 pages, 2 figures

http://arxiv.org/abs/1311.6942
A note on the spinor construction of Spin Foam amplitudes
Giorgio Immirzi
(Submitted on 27 Nov 2013)
I discuss the use of spinors in the construction of spin-foam models, in particular the form of the closure and simplicity constraints for triangles that are space-like,
i.e. with (area)² = 1/2 S^IJ S_IJ > 0,
regardless of whether they belong the tetrahedra with a space-like or time-like normal, emphasizing the role of the light-like 4-vector u_tσ^I u ̄_t. In the quantization of the model, with the representations of SL(2,C) acting on spaces of functions of light-like vectors, one may use the canonical basis of SU(2) representations, or the pseudobasis limited to the discrete representations of SU(1,1); in alternative it is proposed to use instead a basis of eigenstates of (L₃,K₃), which might give matrix elements and vertex functions with the same classical limit. A detailed example of a small triangulation is presented, which among other things indicates, on the basis of a classical calculation, that it would be impractical to limit oneself to tetrahedra with time-like normals.
20 pages, 1 figure.