Recognitions:
Gold Member

## Intuitive content of Loop Gravity--Rovelli's program

http://arxiv.org/abs/1204.2550
Diffusion in quantum gravity
Gianluca Calcagni
(Submitted on 11 Apr 2012)
The change of the effective dimension of spacetime with the probed scale is a universal phenomenon shared by independent models of quantum gravity. Using tools of probability theory and multifractal geometry, we show how dimensional flow is controlled by a multiscale fractional diffusion equation, and physically interpreted as a composite stochastic process. The simplest example is a fractional telegraph process, describing quantum spacetimes with a spectral dimension equal to 2 in the ultraviolet and monotonically rising to 4 towards the infrared.
4 pages, 1 figure

http://arxiv.org/abs/1204.1530
Ayan Chatterjee, Bhramar Chatterjee, Amit Ghosh
(Submitted on 6 Apr 2012)
In completely local settings, we establish that a dynamically evolving black hole horizon can be assigned a Hawking temperature. Moreover, we calculate the Hawking flux and show that the radius of the horizon shrinks.
5 Pages

brief mention:
http://arxiv.org/abs/1204.2520
Quantum Black Holes from Cosmic Rays
Xavier Calmet, Lauretiu Ioan Caramete, Octavian Micu
(Submitted on 11 Apr 2012)
We investigate the possibility for cosmic ray experiments to discover non-thermal small black holes with masses in the TeV range. Such black holes would result due to the impact between ultra high energy cosmic rays or neutrinos with nuclei from the upper atmosphere and decay instantaneously. They could be produced copiously if the Planck scale is in the few TeV region. As their masses are close to the Planck scale, these holes would typically decay into two particles emitted back-to-back. Depending on the angles between the emitted particles with respect to the center of mass direction of motion, it is possible for the simultaneous showers to be measured by the detectors.
4 pages, 1 figure

 Recognitions: Science Advisor http://arxiv.org/abs/1112.0302 Spacetime as a topological insulator: Mechanism for the origin of the fermion generations David B. Kaplan, Sichun Sun (Submitted on 1 Dec 2011 (v1), last revised 13 Apr 2012 (this version, v3)) We suggest a mechanism whereby the three generations of quarks and leptons correspond to surface modes in a five-dimensional theory. These modes arise from a nonlinear fermion dispersion relation in the extra dimension, much in the same manner as fermion surface modes in a topological insulator or lattice implementation of domain wall fermions. We also show that the topological properties can persist in a deconstructed version of the model in four dimensions.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1204.3039 The analogue cosmological constant in Bose-Einstein condensates: a lesson for quantum gravity Stefano Finazzi, Stefano Liberati, Lorenzo Sindoni (Submitted on 13 Apr 2012) For almost a century, the cosmological constant has been a mysterious object, in relation to both its origin and its very small value. By using a Bose-Einstein condensate analogue model for gravitational dynamics, we address here the cosmological constant issue from an analogue gravity standpoint. Starting from the fundamental equations describing a system of condensed bosons, we highlight the presence of a vacuum source term for the analogue gravitational field, playing the role of a cosmological constant. In this simple system it is possible to compute from scratch the value of this constant, to compare it with other characteristic energy scales and hence address the problem of its magnitude within this framework, suggesting a different path for the solution of this longstanding puzzle. We find that, even though this constant term is related with quantum vacuum effects, it is not immediately related to the ground state energy of the condensate. On the gravity side this result suggests that the interpretation and computation of the cosmological term as a form of renormalized vacuum energy might be misleading, its origin being related to the mechanism that instead produces spacetime from its pregeometric progenitor, shedding a different light on the subject and at the same time suggesting a potentially relevant role of analogue models in the understanding of quantum gravity. 24 pages, 1 figure, Proceedings of the II Amazonian Symposium on Physics http://arxiv.org/abs/1204.3505 Conserved quantities in isotropic loop quantum cosmology Daniel Cartin (Submitted on 16 Apr 2012) We develop an action principle for those models arising from isotropic loop quantum cosmology, and show that there is a natural conserved quantity Q for the discrete difference equation arising from the Hamiltonian constraint. This quantity Q relates the semi-classical limit of the wavefunction at large values of the spatial volume, but opposite triad orientations. Moreover, there is a similar quantity for generic difference equations of one parameter arising from a self-adjoint operator. 6 pages, to be published in Europhysics Letters http://arxiv.org/abs/1204.3541 The local potential approximation in quantum gravity Dario Benedetti, Francesco Caravelli (Submitted on 16 Apr 2012) Within the context of the functional renormalization group flow of gravity, we suggest that a generic f(R) ansatz (i.e. not truncated to any specific form, polynomial or not) for the effective action plays a role analogous to the local potential approximation (LPA) in scalar field theory. In the same spirit of the LPA, we derive and study an ordinary differential equation for f(R) to be satisfied by a fixed point of the renormalization group flow. As a first step in trying to assess the existence of global solutions (i.e. true fixed point) for such equation, we investigate here the properties of its solutions by a comparison of various series expansions and numerical integrations. In particular, we study the analyticity conditions required because of the presence of fixed singularities in the equation, and we develop an expansion of the solutions for large R up to order N=29. Studying the convergence of the fixed points of the truncated solutions with respect to N, we find a characteristic pattern for the location of the fixed points in the complex plane, with one point stemming out for its stability. Finally, we establish that if a non-Gaussian fixed point exists within the full f(R) approximation, it corresponds to an R2 theory. 31 pages, 7 figures http://arxiv.org/abs/1204.3531 Interacting Generalised Cosmic Chaplygin gas in Loop quantum cosmology: A singularity free universe Ratul Chowdhury (Jadavpur U.), Prabir Rudra (Bengal Engin. Sci. U., Howrah) (Submitted on 13 Apr 2012) In this work we investigate the background dynamics when dark energy is coupled to dark matter with a suitable interaction in the universe described by Loop quantum cosmology. Dark energy in the form of Generalised Cosmic Chaplygin gas is considered. A suitable interaction between dark energy and dark matter is taken into account in order to at least alleviate (if not solve) the cosmic coincidence problem. The dynamical system of equations is solved numerically and a stable scaling solution is obtained. A significant attempt towards the solution of the cosmic coincidence problem is taken. The statefinder parameters are also calculated to classify the dark energy model. Graphs and phase diagrams are drawn to study the variations of these parameters. It is seen that the background dynamics of Generalised Cosmic Chaplygin gas is completely consistent with the notion of an accelerated expansion in the late universe. From the graphs, generalised cosmic Chaplygin gas is identified as a dark fluid with a lesser negative pressure compared to Modified Chaplygin gas, thus supporting a 'No Big Rip' cosmology. It has also been shown that in this model the universe follows the power law form of expansion around the critical point, which is consistent with the known results. Future singularities that may be formed in this model as an ultimate fate of the universe has been studied in detail. It was found that the model is completely free from any types of future singularities. 10 pages, 10 figures brief mention (of general interest though not directly LoopQG-related) http://arxiv.org/abs/1204.3138 Quantization of area for event and Cauchy horizons of the Kerr-Newman black hole Matt Visser (Victoria University of Wellington) (Submitted on 14 Apr 2012) Based on various string theoretic constructions, there have been repeated suggestions that the areas of black hole event horizons should be quantized in a quite specific manner, involving linear combinations of square roots of natural numbers. It is important to realise the significant physical limitations of such proposals when one attempts to extend them outside their original framework. Specifically, in their most natural and direct physical interpretations, these specific proposals for horizon areas fail for the ordinary Kerr-Newman black holes in (3+1) dimensions, essentially because the fine structure constant is not an integer. A more baroque interpretation involves asserting the fine structure constant is the square root of a rational number; but such a proposal has its own problems. Insofar as one takes (3+1) general relativity (plus the usual quantization of angular momentum and electric charge) as being paramount, the known explicitly calculable spectra of horizon areas for the physically compelling Kerr-Newman spacetimes do not resemble those of currently available string theoretic constructions. 15 pages
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1204.4345 The spacetime in the neighborhood of a general isolated black hole Badri Krishnan (Submitted on 19 Apr 2012) We construct the spacetime in the vicinity of a general isolated, rotating, charged black hole. The black hole is modeled as a weakly isolated horizon, and we use the characteristic initial value formulation of the Einstein equations with the horizon as an inner boundary. The spacetime metric and other geometric fields are expanded in a power series in a radial coordinate away from the horizon by solving the characteristic field equations in the Newman-Penrose formalism. This is the first in a series of papers which investigate the near horizon geometry and its physical applications using the isolated horizon framework. 23 pages, 1 figure http://arxiv.org/abs/1204.4344 Reply to the comment on "Black hole entropy and isolated horizons thermodynamics" Amit Ghosh, Alejandro Perez (Submitted on 19 Apr 2012) The comment http://arxiv.org/abs/1204.2729v1 is completely wrong. The author makes serious mistakes in calculations and judgement... brief mention: http://arxiv.org/abs/1204.4339 Teleparallel Gravity as a Higher Gauge Theory John C. Baez, Derek K. Wise (Submitted on 19 Apr 2012) We show that general relativity can be viewed as a higher gauge theory involving a categorical group, or 2-group, called the teleparallel 2-group. On any semi-Riemannian manifold M, we first construct a principal 2-bundle with the Poincaré 2-group as its structure 2-group. Any flat metric-preserving connection on M gives a flat 2-connection on this 2-bundle, and the key ingredient of this 2-connection is the torsion. Conversely, every flat strict 2-connection on this 2-bundle arises in this way if M is simply connected and has vanishing 2nd deRham cohomology. Extending from the Poincaré 2-group to the teleparallel 2-group, a 2-connection includes an additional piece: a coframe field. Taking advantage of the teleparallel reformulation of general relativity, in which a coframe field, a flat connection and its torsion are the key ingredients, this lets us rewrite general relativity as a theory with a 2-connection for the teleparallel 2-group as its only field. 35 pages
 Recognitions: Gold Member Science Advisor Unfortunately not available online, a talk given today at Princeton Institute for Advanced Studies: Monday, April 23, 2012 High Energy Theory Seminar “Loop Quantum Gravity: Recent Results and Open Problems” Location: Bloomberg Lecture Hall Time: 2:30 PM Speaker(s): Carlo Rovelli, Centre de Physique Théorique de Luminy, Aix-Marseille University, France Description: The loop approach to quantum gravity has developed considerably during the last few years, especially in its covariant ('spinfoam') version. I present the current definition of the theory and the results that have been proven. I discuss what I think is still missing towards of the goal of defining a consistent tentative quantum field theory genuinely background independent and having general relativity as classical limit. http://www.princeton.edu/physics/eve...ent.xml?id=347 http://arxiv.org/abs/1204.5122 Entropy of Non-Extremal Black Holes from Loop Gravity Eugenio Bianchi (Submitted on 23 Apr 2012) We compute the entropy of non-extremal black holes using the quantum dynamics of Loop Gravity. The horizon entropy is finite, scales linearly with the area A, and reproduces the Bekenstein-Hawking expression S = A/4 with the one-fourth coefficient for all values of the Immirzi parameter. The near-horizon geometry of a non-extremal black hole - as seen by a stationary observer - is described by a Rindler horizon. We introduce the notion of a quantum Rindler horizon in the framework of Loop Gravity. The system is described by a quantum surface and the dynamics is generated by the boost Hamiltonion of Lorentzian Spinfoams. We show that the expectation value of the boost Hamiltonian reproduces the local horizon energy of Frodden, Ghosh and Perez. We study the coupling of the geometry of the quantum horizon to a two-level system and show that it thermalizes to the local Unruh temperature. The derived values of the energy and the temperature allow one to compute the thermodynamic entropy of the quantum horizon. The relation with the Spinfoam partition function is discussed. 6 pages, 1 figure brief mention--not Loop-and-allied QG, but possibly of general interest: http://arxiv.org/abs/1204.4926 Relating the quantum mechanics of discrete systems to standard canonical quantum mechanics Gerard 't Hooft (Submitted on 22 Apr 2012) Discrete quantum mechanics is here defined to be a quantum theory of wave functions defined on integers P_i and Q_i, while canonical quantum mechanics is assumed to be based on wave functions on the real numbers, R^n. We study reversible mappings from the position operators q_i and their quantum canonical operators p_i of a canonical theory, onto the discrete, commuting operators Q_i and P_i. In this paper we are particularly interested in harmonic oscillators. In the discrete system, these turn into deterministic models, which is our motivation for this study. We regard the procedure worked out here as a "canonical formalism" for discrete dynamics, and as a stepping stone to handling discrete deterministic systems in a quantum formalism. 20 pages (incl. title page), 2 figures http://arxiv.org/abs/1204.4683 A Long View of Particle Physics Frank Wilczek (Submitted on 20 Apr 2012) 2011 marked the hundredth anniversary both of the famous Solvay conferences, and of the Geiger-Marsden experiment that launched the modern understanding of subatomic structure. I was asked to survey the status and prospects of particle physics for the anniversary Solvay conference, with appropriate perspective. This is my attempt. 8 pages, no figures. Rapporteur talk at the 25th Solvay Conference on Physics, "Theory of the Quantum World", October 2011. To be published in the Proceedings
 I was at the talk at IAS, everyone was pretty confused by what he was doing. One point brought up was that there exist other models, like YM in 5d i think, whose discrete version has the correct classical limit and is uv and ir finite, but does not make sense quantum mechanically. And there was no concrete argument for why lqg would be a better example. Some numerical checks are needed, and he said they are very hard to do but people are working on them.
 This thread is for bibliographical help. You might wish to open a thread to tell your experience.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1204.5394 Discrete gravity models and Loop Quantum Gravity: a short review Maite Dupuis, James P. Ryan, Simone Speziale (Submitted on 24 Apr 2012) We review the relation between loop quantum gravity on a fixed graph and discrete models of gravity. We compare Regge and twisted geometries, and discuss discrete actions based on twisted geometries and on the discretization of the Plebanski action. We discuss the role of discrete geometries in the spin foam formalism, with particular attention to the definition of the simplicity constraints. 32 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology" brief mention: http://arxiv.org/abs/1204.5455 Quantization of Lorentzian 3d Gravity by Partial Gauge Fixing Rodrigo M S Barbosa, Clisthenis P Constantinidis, Zui Oporto, Olivier Piguet (Submitted on 24 Apr 2012) D = 2+1 gravity with a cosmological constant has been shown by Bonzom and Livine to present a Barbero-Immirzi like ambiguity depending on a parameter. We make use of this fact to show that, for positive cosmological constant, the Lorentzian theory can be partially gauge fixed and reduced to an SU(2) Chern-Simons theory... 19 pages
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1204.6349 Gravitation and vacuum entanglement entropy Ted Jacobson (Submitted on 28 Apr 2012) The vacuum of quantum fields contains correlated fluctuations. When restricted to one side of a surface these have a huge entropy of entanglement that scales with the surface area. If UV physics renders this entropy finite, then a thermodynamic argument implies the existence of gravity. That is, the causal structure of spacetime must be dynamical and governed by the Einstein equation with Newton's constant inversely proportional to the entropy density. Conversely, the existence of gravity makes the entanglement entropy finite. This thermodynamic reasoning is powerful despite the lack of a detailed description of the dynamics at the cutoff scale, but it has its limitations. In particular, we should not expect to understand corrections to Einstein gravity in this way. 9 pages; Essay written for the Gravity Research Foundation 2012 Awards for Essays on Gravitation
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1205.0733 Discrete Symmetries in Covariant LQG Carlo Rovelli, Edward Wilson-Ewing (Submitted on 3 May 2012) We study time-reversal and parity ---on the physical manifold and in internal space--- in covariant loop gravity. We consider a minor modification of the Holst action which makes it transform coherently under such transformations. The classical theory is not affected but the quantum theory is slightly different. In particular, the simplicity constraints are slightly modified and this restricts orientation flips in a spinfoam to occur only across degenerate regions, thus reducing the sources of potential divergences. 8 pages http://pirsa.org/12050050 (online talk) Shape Dynamics and General Relativity Speaker(s): Julian Barbour Abstract: Shape Dynamics first arose as a theory of particle interactions formulated without any of Newton's absolute structures. Its fundamental arena is shape space, which is obtained by quotienting Newton's kinematic framework with respect to translations, rotations and dilatations. This leads to a universe defined purely intrinsically in relational terms. It is then postulated that a dynamical history is determined by the specification in shape space of an initial shape and an associated rate of change of shape. There is a very natural way to create a theory that meets such a requirement. It fully implements Mach's principle and shows how time and local inertial frames are determined by the universe as whole. If the same principles are applied to a spatially closed universe in which geometry is dynamical, they lead rather surprisingly to a theory that, modulo some caveats, is dynamically equivalent to general relativity but dual to it in that refoliation invariance is traded for three-dimensional conformal invariance. This shows that there is a hidden three-dimensional conformal symmetry within general relativity. It is in fact what underlies York's crucial method of solution of the initial-value problem in general relativity. It is also remarkable that, as in York's work, shape dynamics inescapably introduces a mathematically distinguished notion of absolute simultaneity, the desirability of which has been found in two currently popular approaches to quantum gravity: causal dynamical triangulations and Horava gravity. I aim to express the key ideas and techniques of shape dynamics as simply as possible. Date: 09/05/2012 - 2:00 pm Series: Colloquium
 Recognitions: Science Advisor http://arxiv.org/abs/1205.0911 The new spin foam models and quantum gravity Alejandro Perez (Submitted on 4 May 2012) In this article we give a systematic definition of the recently introduced spin foam models for four dimensional quantum gravity reviewing the main results on their semiclassical limit on fixed discretizations.
 Recognitions: Science Advisor Seems to be ay very interesting paper - especially as the author points out that the construction of the SF measure due to the presence of second-class constraints is still a matter of debate and by no means fully understood.
 Recognitions: Science Advisor Seems to be ay very interesting paper - especially as the author points out that the construction of the SF measure due to the presence of second-class constraints is still a matter of debate and by no means fully understood.
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1205.1229 Second- and First-Order Phase Transitions in CDT J. Ambjorn, S. Jordan, J. Jurkiewicz, R. Loll (Submitted on 6 May 2012) Causal Dynamical Triangulations (CDT) is a proposal for a theory of quantum gravity, which implements a path-integral quantization of gravity as the continuum limit of a sum over piecewise flat spacetime geometries. We use Monte Carlo simulations to analyse the phase transition lines bordering the physically interesting de Sitter phase of the four-dimensional CDT model. Using a range of numerical criteria, we present strong evidence that the so-called A-C transition is first order, while the B-C transition is second order. The presence of a second-order transition may be related to an ultraviolet fixed point of quantum gravity and thus provide the key to probing physics at and possibly beyond the Planck scale. 24 pages, 9 figures http://arxiv.org/abs/1205.1304 Curved Momentum Space and Relative Locality Jerzy Kowalski-Glikman (Submitted on 7 May 2012) I briefly discuss the construction of a theory of particles with curved momentum space and its consequence, the principle of relative locality. 10 pages; based on the talk given at 29-th Max Born Symposium, to appear in the proceedings not Loop-or-allied QG, but possibly of general interest: http://arxiv.org/abs/1205.1256 Relationalism Edward Anderson (Submitted on 6 May 2012) This article contributes to the debate of the meaning of relationalism and background independence, which has remained of interest in theoretical physics from Newton versus Leibniz through to foundational issues for today's leading candidate theories of quantum gravity. I contrast and compose the substantially different Leibniz--Mach--Barbour (LMB) and Rovelli--Crane (RC) uses of the word 'relational'. Leibniz advocated primary timelessness and Mach that 'time is to be abstracted from change'. I consider 3 distinct viewpoints on Machian time: Barbour's, Rovelli's and my own. I provide four expansions on Barbour's taking configuration space to be primary: to (perhaps a weakened notion of) phase space, categorizing, perspecting and propositioning. Categorizing means considering not only object spaces but also the corresponding morphisms and then functors between such pairs. Perspecting means considering the set of subsystem perspectives; this is an arena in which the LMB and Rovelli approaches make contact. By propositioning, I mean considering the set of propositions about a physical (sub)system. I argue against categorization being more than a formal pre-requisite for quantization in general; however, perspecting is a categorical operation, and propositioning leads one to considering topoi, with Isham and Doering's work represents one possibility for a mathematically sharp implementation of propositioning. Further applications of this article are arguing for Ashtekar variables as being relational in LMB as well as just the usually-ascribed RC sense, relationalism versus supersymmetry, string theory and M-theory. The question of whether scale is relational is also considered, with quantum cosmology in mind. 39 pages, 4 figures.
 http://arxiv.org/abs/1205.1788 Clockwork Quantum Universe Donatello Dolce (Submitted on 7 May 2012) Besides the purely digital or analog interpretation of reality there is a third possibility which incorporates important aspects of both. This is the cyclic formulation of elementary systems, in which elementary particles are represented as classical strings vibrating in compact space-time dimensions with periodic boundary conditions. We will address these cyclic solutions as "de Broglie internal clocks". They constitute the deterministic gears of a consistent semi-classical description of quantum relativistic physics, providing in addition an appealing formulation of the notion of time. http://arxiv.org/abs/1205.1636 Born's prophecy leaves no space for quantum gravity Giovanni Amelino-Camelia (Submitted on 8 May 2012) I stress that spacetime is a redundant abstraction, since describing the physical content of all so-called "space-time measurements" only requires timing (by a physical/material clock) of particle detections (at a physical/material detector). It is interesting then to establish which aspects of our current theories afford us the convenient abstraction of a spacetime. I emphasize the role played by the assumed triviality of the geometry of momentum space, which makes room for an observer-independent notion of locality. This is relevant for some recent studies of the quantum-gravity problem that stumbled upon hints of a nontrivial geometry of momentum space, something which had been strikingly envisaged for quantum gravity already in 1938 by Max Born. If indeed momentum space has nontrivial geometry then the abstraction of a spacetime becomes more evidently redundant and less convenient: one may still abstract a spacetime but only allowing for the possibility of a relativity of spacetime locality. I also provide some examples of how all this could affect our attitude toward the quantum-gravity problem, including some for the program of emergent gravity and emergent spacetime and an indication of triviality of the holographic description of black holes. And in order to give an illustrative example of possible logical path for the "disappearance of spacetime" I rely on formulas inspired by the $\kappa$-Poincar\'e framework.
 http://arxiv.org/abs/1205.1296 A Distinguished Vacuum State for a Quantum Field in a Curved Spacetime: Formalism, Features, and Cosmology Niayesh Afshordi, Siavash Aslanbeigi, Rafael D. Sorkin (Submitted on 7 May 2012) We define a distinguished "ground state" or "vacuum" for a free scalar quantum field in a globally hyperbolic region of an arbitrarily curved spacetime. Our prescription is motivated by the recent construction of a quantum field theory on a background causal set using only knowledge of the retarded Green's function. We generalize that construction to continuum spacetimes and find that it yields a distinguished vacuum or ground state for a non-interacting, massive or massless scalar field. This state is defined for all compact regions and for many noncompact ones. In a static spacetime we find that our vacuum coincides with the usual ground state. We determine it also for a radiation-filled, spatially homogeneous and isotropic cosmos, and show that the super-horizon correlations are approximately the same as those of a thermal state. Finally, we illustrate the inherent non-locality of our prescription with the example of a spacetime which sandwiches a region with curvature in-between flat initial and final regions. http://arxiv.org/abs/1205.1308[ Testing MOND over a large acceleration range in x-ray ellipticals Mordehai Milgrom (Submitted on 7 May 2012) The gravitational fields of two isolated ellipticals, NGC 720 and NGC 1521, have been recently measured, assuming hydrostatic balance of the hot gas enshrouding them. These galaxies are worthy of special interest: They afford, for the first time to my knowledge, testing MOND in ellipticals with force and quality that, arguably, approach those of rotation-curve tests in disc galaxies: The fields have been probed to very large galactic radii, revealing a large range of mass discrepancies. In the context of MOND, it is noteworthy that the measured accelerations span a wide range, from more than 10a0 to about a0/10, unprecedented in individual ellipticals. I compare the predictions of MOND, based on only the baryonic mass, for reasonable stellar M/L values, with the deduced dynamical mass runs of these galaxies. I find that MOND predicts correctly the runs of the mass discrepancies: from no discrepancy in the inner parts, to approximately a-factor-of-ten discrepancy in the outermost regions probed. For NGC 1521, this is achieved with the same M/L value as best fitted the data in the Newtonian analysis with dark matter, and for NGC 720, with a somewhat larger value than preferred by the Newtonian fit. http://arxiv.org/abs/1205.1317 A novel MOND effect in isolated high acceleration systems Mordehai Milgrom (Submitted on 7 May 2012) I discuss a novel MOND effect that entails a small correction to the dynamics of isolated mass systems even when they are deep in the Newtonian regime. [These are systems whose extent R<< Rm, where Rm=sqrt(GM/a0) is the MOND radius of the system, of total mass M.] Interestingly, even if the MOND equations approach Newtonian dynamics arbitrarily fast at high accelerations, this correction decreases only as a power of R/Rm. The effect appears in formulations of MOND as modified gravity governed by generalizations of the Poisson equation. The MOND correction to the potential is a quadrupole field \phi_{a} \approx GP_{ij}r^ir^j, where r is the radius from the center of mass. In QUMOND, P_{ij}=-q Q_{ij}/Rm^5, where Q_{ij} is the quadrupole moment of the system, and q>0 is a numerical factor that depends on the interpolating function. For example, the correction to the Newtonian force between two masses, m and M, a distance L apart (L<
 Recognitions: Gold Member Science Advisor http://arxiv.org/abs/1205.2019 The Spin Foam Approach to Quantum Gravity Alejandro Perez (Submitted on 9 May 2012) This article reviews the present status of the spin foam approach to the quantization of gravity. Special attention is payed to the pedagogical presentation of the recently introduced new models for four dimensional quantum gravity. The models are motivated by a suitable implementation of the path integral quantization of the Plebanski formulation of gravity on a simplicial regularization. The article also includes a self-contained treatment of the 2+1 gravity. The simple nature of the latter provides the basis and a perspective for the analysis of both conceptual and technical issues that remain open in four dimensions. 121 pages. To appear in Living Reviews in Relativity http://arxiv.org/abs/1205.1917 Hybrid quantization of an inflationary universe Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo (Submitted on 9 May 2012) We quantize to completion an inflationary universe with small inhomogeneities in the framework of loop quantum cosmology. The homogeneous setting consists of a massive scalar field propagating in a closed, homogeneous scenario. We provide a complete quantum description of the system employing loop quantization techniques. After introducing small inhomogeneities as scalar perturbations, we identify the true physical degrees of freedom by means of a partial gauge fixing, removing all the local degrees of freedom except the matter perturbations. We finally combine a Fock description for the inhomogeneities with the polymeric quantization of the homogeneous background, providing the quantum Hamiltonian constraint of the composed system. Its solutions are then completely characterized, owing to the suitable choice of quantum constraint, and the physical Hilbert space is constructed. Finally, we consider the analog description for an alternate gauge and, moreover, in terms of gauge-invariant quantities. In the deparametrized model, all these descriptions are unitarily equivalent at the quantum level. 16 pages [final paragraph of conclusion section: "In conclusion, we have been able to provide a full quantum description of an inflationary universe with small in- homogeneities propagating on it, in the context of LQC. The model is now ready to produce physical predictions, which will be the aim of future work."] http://arxiv.org/abs/1205.1974 On solutions of loop quantum cosmology and their thermodynamics properties H. Mohseni Sadjadi (Submitted on 9 May 2012) Loop quantum cosmology is considered in inflationary era. A slow rolling scalar field solution with power law potential is presented in the neighborhood of transition time, i.e. when the universe enters inflationary phase from super-inflation era. The second and the generalized second laws of thermodynamics and their validities and violations are discussed and elucidated through some examples. 11 pages, 3 figures brief mention--not loop-related QG but possibly of general interest: http://arxiv.org/abs/1205.1992 Relativistic Quantum Mechanics and Quantum Field Theory H. Nikolic (Submitted on 9 May 2012) A general formulation of classical relativistic particle mechanics is presented, with an emphasis on the fact that superluminal velocities and nonlocal interactions are compatible with relativity. Then a manifestly relativistic-covariant formulation of relativistic quantum mechanics (QM) of fixed number of particles (with or without spin) is presented, based on many-time wave functions and the spacetime probabilistic interpretation. These results are used to formulate the Bohmian interpretation of relativistic QM in a manifestly relativistic-covariant form. The results are also generalized to quantum field theory (QFT), where quantum states are represented by wave functions depending on an infinite number of spacetime coordinates. The corresponding Bohmian interpretation of QFT describes an infinite number of particle trajectories. Even though the particle trajectories are continuous, the appearance of creation and destruction of a finite number of particles results from quantum theory of measurements describing entanglement with particle detectors. 43 pages, Chapter 8. of the book "Applied Bohmian Mechanics: From Nanoscale Systems to Cosmology", edited by X. Oriols and J. Mompart (Pan Stanford Publishing, 2012)

 Tags loop quantum gravity