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Intuitive content of Loop Gravity-Rovelli's program

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atyy
#1711
May6-12, 08:21 PM
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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.
tom.stoer
#1712
May7-12, 01:31 AM
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P: 5,464
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.
tom.stoer
#1713
May7-12, 01:31 AM
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P: 5,464
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.
marcus
#1714
May7-12, 07:23 PM
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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.
MTd2
#1715
May9-12, 12:26 AM
PF Gold
P: 1,963
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.
John86
#1716
May9-12, 08:50 AM
P: 247
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<<Rm) is Fa=2q(L/Rm)^3(mM)^2(M+m)^{-3}a0 (attractive). At present I don't see where this effect can be tested. For example, it's predicted strength is rather much below present testing capabilities in the solar system, for which the added acceleration is of order 10^{-12}a0. (Abridged)

http://arxiv.org/abs/1205.1439
Are quantum states real?
Lucien Hardy
(Submitted on 7 May 2012 (v1), last revised 8 May 2012 (this version, v2))
In this paper we give a new argument for the reality of the wavefunction. We consider theories in which reality is described by some underlying variables. Each value of these variables can take represents an ontic state (a particular state of reality). The preparation of a quantum state corresponds to a distribution over the ontic states. Using three basic assumptions, we will show that the distributions over ontic states corresponding to distinct pure states are non-overlapping. This means that we can deduce the quantum state from a knowledge of the ontic state. Hence we can claim that the quantum state is a real thing (it is written into the underlying variables that describe reality). The key assumption we use in this proof is ontic indifference - that quantum transformations that do not effect a given pure quantum state can be implemented in such a way that they do not effect the ontic states in the support of that state. This argument in this paper is different from the recent proof of Pusey, Barrett, and Rudolph. It uses a different key assumption and it pertains to a single copy of the system in question.

http://arxiv.org/abs/1205.1584
Everything is Entangled
Roman V. Buniy, Stephen D.H. Hsu
(Submitted on 8 May 2012)
We show that big bang cosmology implies a high degree of entanglement of particles in the universe. In fact, a typical particle is entangled with many particles far outside our horizon. However, the entanglement is spread nearly uniformly so that two randomly chosen particles are unlikely to be directly entangled with each other -- the reduced density matrix describing any pair is likely to be separable.

http://arxiv.org/abs/1205.1619
Quantum Theory as emergent from an undulatory translocal Sub-Quantum Level
Manfred Requardt
(Submitted on 8 May 2012)
We argue that quantum theory is a low-energy effective theory which emerges from some sub-quantum level theory which is of an undulatory and translocal character. We show the close connection of quantum theory with both gravity and the holographic principle which are different phenomena of one and the same theory on this primordial level. An important role in our analysis is played by the concept of a generalized renormalization group connecting this primordial level and e.g. quantum theory plus a continuous space-time. We show that characteristic phenomena like the seemingly instantaneous state reduction, the EPR-paradox or the problem of polydimensions can be understood in our undulatory translocal theory in a realistic way. Most importantly, we give a realistic interpretation of the phasefunction as a collective action variable in the spirit of Bohm and explain the emergence of a macroscopic notion of time.
marcus
#1717
May9-12, 07:28 PM
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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)
marcus
#1718
May10-12, 06:36 PM
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http://pirsa.org/12050050
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
[My comment: a remarkable and convincing talk partly because of the clear personal perspective it gives on shape dynamics and its history. Best introduction to SD so far IMO.]

brief mention:
http://arxiv.org/abs/1205.2158
Massive gravity from bimetric gravity
Valentina Baccetti (Victoria University of Wellington), Prado Martin-Moruno (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 10 May 2012)
We discuss the subtle relationship between so-called massive gravity (that is, gravity incorporating a non-zero graviton mass) and bimetric gravity, focussing particularly on the manner in which massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure is more delicate than currently appreciated, and in particular, in a cosmological context can lead to an interesting interplay between the "background" and "foreground" metrics...
25 pages
Blackforest
#1719
May13-12, 01:47 AM
P: 450
Quote Quote by marcus View Post

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
I like equation (19). Thanks for the reference.
marcus
#1720
May14-12, 12:21 PM
Astronomy
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PF Gold
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http://pirsa.org/12050061
Conformal Gravity and Black Hole Complementarity
Speaker(s): Gerard t'Hooft
Date: 11/05/2012 - 4:40 pm
Collection: Conformal Nature of the Universe
[Fascinating talk, with possibly ground-breaking conceptual novelty. I would recommend anyone interested in quantum gravity watch at least the first 25-30 minutes. The audio needs a boost though, and requires close attention at some points.]

http://pirsa.org/12050072/
Two-dimensional Conformal Symmetry of Short-distance Spacetime
Speaker(s): Steve Carlip
Abstract: Evidence from several approaches to quantum gravity hints at the possibility that spacetime undergoes a "spontaneous dimensional reduction" at very short distances. If this is the case, the small scale universe might be described by a theory with two-dimensional conformal symmetry. I will summarize the evidence for dimensional reduction and indicate a tentative path towards using this conformal invariance to explore quantum gravity.
Date: 11/05/2012 - 9:00 am

http://arxiv.org/abs/1205.2953
Expressing entropy globally in terms of (4D) field-correlations
Rafael D. Sorkin
(Submitted on 14 May 2012)
We express the entropy of a scalar field φ directly in terms of its spacetime correlation function W(x,y)=<φ(x)φ(y)>, assuming that the higher correlators are of "Gaussian" form. The resulting formula associates an entropy S(R) to any spacetime region R; and when R is globally hyperbolic with Cauchy surface Σ, S(R) can be interpreted as the entropy of the reduced density-matrix belonging to Σ. One acquires in particular a new expression for the entropy of entanglement across an event-horizon. Thanks to its spacetime character, this expression makes sense in a causal set as well as in a continuum spacetime.
14 pages, 2 figures. To appear in proceedings of ICGC2011, held Goa, Journal of Physics Conference Series.

brief mention:
http://arxiv.org/abs/1205.2720
Why there is something rather than nothing: The finite, infinite and eternal
Peter Lynds
marcus
#1721
May15-12, 07:24 PM
Astronomy
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PF Gold
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http://arxiv.org/abs/1205.3370
Lattice loop quantum cosmology: scalar perturbations
Edward Wilson-Ewing
(Submitted on 15 May 2012)
We study the scalar modes of linear perturbations in loop quantum cosmology. This is done on a lattice where each cell is taken to be homogeneous and isotropic and can be quantized via standard homogeneous loop quantum cosmology techniques. The appropriate interactions between nearby cells are included in the Hamiltonian in order to obtain the correct physics. It is shown that the quantum theory is anomaly-free: the scalar and diffeomorphism constraint operators weakly commute with the Hamiltonian. Finally, the effective theory encoding the leading order quantum gravity corrections is derived and is shown to give the same holonomy-corrected effective equations that have been obtained in previous studies.
32 pages

http://arxiv.org/abs/1205.3487
A New Term in the Microcanonical Entropy of Quantum Isolated Horizon
Abhishek Majhi
(Submitted on 15 May 2012)
The quantum geometric framework for Isolated Horizon has led to the Bekenstein-Hawking area law and the quantum logarithmic correction for the black hole entropy. The point to be noted here is that all the results have been derived in a model independent way and completely from within the quantum geometric framework where the quantum degrees of freedom are described by the states of the SU(2) Chern Simons theory on the Isolated Horizon. Here we show that a completely new term independent of the area of the Isolated Horizon appears in the microcanonical entropy. It has a coeffcient which is a function of the Barbero Immirzi parameter.
4 pages

http://arxiv.org/abs/1205.3291
The Unbearable Beingness of Light, Dressing and Undressing Photons in Black Hole Spacetimes
Timothy J. Hollowood, Graham M. Shore
(Submitted on 15 May 2012)
Gravitational tidal forces acting on the virtual e+ e- cloud surrounding a photon endow spacetime with a non-trivial refractive index. This has remarkable properties unique to gravitational theories including superluminal low-frequency propagation, in apparent violation of causality, and amplification of the renormalized photon field, in apparent violation of unitarity. Using the geometry of null congruences and the Penrose limit, we illustrate these phenomena and their resolution by tracing the history of a photon as it falls into the near-singularity region of a black hole.
8 pages, Essay awarded third prize in the Gravity Research Foundation essay competition 2012
marcus
#1722
May16-12, 07:20 PM
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http://arxiv.org/abs/1205.3636
Can we measure structures to a precision better than the Planck length?
Sabine Hossenfelder
(Submitted on 16 May 2012)
It was recently claimed that the Planck length is not a limit to the precision by which we can measure distances, but that instead it is merely the Planck volume that limits the precision by which we can measure volumes. Here, we investigate this claim and show that the argument does not support the conclusion.
9 pages, 1 figure

http://arxiv.org/abs/1205.3583
Running boundary actions, Asymptotic Safety, and black hole thermodynamics
D. Becker, M. Reuter
(Submitted on 16 May 2012)
Previous explorations of the Asymptotic Safety scenario in Quantum Einstein Gravity (QEG) by means of the effective average action and its associated functional renormalization group (RG) equation assumed spacetime manifolds which have no boundaries. Here we take a first step towards a generalization for non-trivial boundaries, restricting ourselves to action functionals which are at most of second order in the derivatives acting on the metric. We analyze two examples of truncated actions with running boundary terms: full fledged QEG within the single-metric Einstein-Hilbert (EH) truncation, augmented by a scale dependent Gibbons-Hawking (GH) surface term, and a bi-metric truncation for gravity coupled to scalar matter fields. The latter contains 17 running couplings, related to both bulk and boundary terms, whose beta-functions are computed in the induced gravity approximation. We find that the bulk and the boundary Newton constant, pertaining to the EH and GH term, respectively, show opposite RG running; proposing a scale dependent variant of the ADM mass we argue that the running of both couplings is consistent with gravitational anti-screening. We describe a simple device for counting the number of field modes integrated out between the infrared cutoff scale and the ultraviolet. This method makes it manifest that, in an asymptotically safe theory, there are effectively no field modes integrated out while the RG trajectory stays in the scaling regime of the underlying fixed point. As an application, we investigate how the semiclassical theory of Black Hole Thermodynamics gets modified by quantum gravity effects and compare the new picture to older work on `RG-improved black holes' which incorporated the running of the bulk Newton constant only. We find, for instance, that the black hole's entropy vanishes and its specific heat capacity turns positive at Planckian scales.
Comments: 65 pages, 2 figures

not loop quantum gravity but could alter foundations and hence prove of general interest:
http://arxiv.org/abs/1205.3707
Precedence and freedom in quantum physics
Lee Smolin
(Submitted on 16 May 2012)
A new interpretation of quantum mechanics is proposed according to which precedence, freedom and novelty play central roles. This is based on a modification of the postulates for quantum theory given by Masanes and Muller. We argue that quantum mechanics is uniquely characterized as the probabilistic theory in which individual systems have maximal freedom in their responses to experiment, given reasonable axioms for the behavior of probabilities in a physical theory. Thus, to the extent that quantum systems are free, in the sense of Conway and Kochen, there is a sense in which they are maximally free.
We also propose that laws of quantum evolution arise from a principle of precedence, according to which the outcome of a measurement on a quantum system is selected randomly from the ensemble of outcomes of previous instances of the same measurement on the same quantum system. This implies that dynamical laws for quantum systems can evolve as the universe evolves, because new precedents are generated by the formation of new entangled states.
12 pages
arivero
#1723
May17-12, 09:27 AM
PF Gold
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Quote Quote by marcus View Post
http://arxiv.org/abs/1205.3636
Can we measure structures to a precision better than the Planck length?
Sabine Hossenfelder
(Submitted on 16 May 2012)
It was recently claimed that the Planck length is not a limit to the precision by which we can measure distances, but that instead it is merely the Planck volume that limits the precision by which we can measure volumes. Here, we investigate this claim and show that the argument does not support the conclusion.
9 pages, 1 figure
Quantum theory of weak gravitational fields by Matvei Bronstein is translated and republished free of charge (EDIT:hmm, no) here http://www.springerlink.com/content/...427080/?MUD=MP
marcus
#1724
May17-12, 07:14 PM
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PF Gold
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Thanks for the Bronstein reference!

http://arxiv.org/abs/1205.3791
The transfer matrix in four-dimensional CDT
Jan Ambjorn, Jakub Gizbert-Studnicki, Andrzej Görlich, Jerzy Jurkiewicz
(Submitted on 16 May 2012)
The Causal Dynamical Triangulation model of quantum gravity (CDT) has a transfer matrix, relating spatial geometries at adjacent (discrete lattice) times. The transfer matrix uniquely determines the theory. We show that the measurements of the scale factor of the (CDT) universe are well described by an effective transfer matrix where the matrix elements are labeled only by the scale factor. Using computer simulations we determine the effective transfer matrix elements and show how they relate to an effective minisuperspace action at all scales.
32 pages, 19 figures

http://arxiv.org/abs/1205.3974
Statistical analysis of entropy correction from topological defects in Loop Black Holes
Kinjalk Lochan, Cenalo Vaz
(Submitted on 17 May 2012)
In this paper we discuss the entropy of quantum black holes in the LQG formalism when the number of punctures on the horizon is treated as a quantum hair, that is we compute the black hole entropy in the grand canonical (area) ensemble. The entropy is a function of both the average area and the average number of punctures and bears little resemblance to the Bekenstein-Hawking entropy. In the thermodynamic limit, both the "temperature" and the chemical potential can be shown to be functions only of the average area per puncture. At a fixed temperature, the average number of punctures becomes proportional to the average area and we recover the Bekenstein-Hawking area-entropy law to leading order provided that the Barbero-Immirzi parameter, γ, is appropriately fixed. This also relates the chemical potential to γ. We obtain a sub-leading correction, which differs in signature from that obtained in the microcanonical and canonical ensembles in its sign but agrees with earlier results in the grand canonical ensemble.
12 pages

brief mention--not loop-and-allied QG but possibly of general interest:

http://arxiv.org/abs/1205.3807
Accelerated Expansion from Negative Λ
James B. Hartle, S. W. Hawking, Thomas Hertog
(Submitted on 16 May 2012)
Wave functions specifying a quantum state of the universe must satisfy the constraints of general relativity, in particular the Wheeler-DeWitt equation (WDWE). We show for a wide class of models with non-zero cosmological constant that solutions of the WDWE exhibit a universal semiclassical asymptotic structure for large spatial volumes. A consequence of this asymptotic structure is that a wave function in a gravitational theory with a negative cosmological constant can predict an ensemble of asymptotically classical histories which expand with a positive effective cosmological constant. This raises the possibility that even fundamental theories with a negative cosmological constant can be consistent with our low-energy observations of a classical, accelerating universe. We illustrate this general framework with the specific example of the no-boundary wave function in its holographic form. The implications of these results for model building in string cosmology are discussed.
28 pages
atyy
#1725
May20-12, 07:54 PM
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http://arxiv.org/abs/1205.4218
Limit cycles and quantum gravity
Daniel Litim, Alejandro Satz
(Submitted on 18 May 2012)
We study renormalization group equations of quantum gravity in four dimensions. We find an ultraviolet fixed point in accordance with the asymptotic safety conjecture, and infrared fixed points corresponding to general relativity with positive, vanishing or negative cosmological constant. In a minisuperspace approximation, we additionally find a renormalization group limit cycle shielding the ultraviolet from the infrared fixed points. We discuss implications of this pattern for asymptotically safe gravity in the continuum and on the lattice.
John86
#1726
May21-12, 10:39 AM
P: 247
http://arxiv.org/abs/1205.4107
Duality between a deterministic cellular automaton and a bosonic quantum field theory in 1+1 dimensions
Gerard 't Hooft
(Submitted on 18 May 2012)
Methods developed in a previous paper are employed to define an exact correspondence between the states of a deterministic cellular automaton in 1+1 dimensions and those of a bosonic quantum field theory. The result may be used to argue that quantum field theories may be much closer related to deterministic automata than what is usually thought possible.
marcus
#1727
May23-12, 07:16 PM
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PF Gold
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http://arxiv.org/abs/1205.5161
Can effects of quantum gravity be observed in the cosmic microwave background?
Claus Kiefer, Manuel Kraemer
(Submitted on 23 May 2012)
We investigate the question whether small quantum-gravitational effects can be observed in the anisotropy spectrum of the cosmic microwave background radiation. An observation of such an effect is needed in order to discriminate between different approaches to quantum gravity. Using canonical quantum gravity with the Wheeler-DeWitt equation, we find a suppression of power at large scales. Current observations only lead to an upper bound on the energy scale of inflation, but the framework is general enough to study other situations in which such effects might indeed be seen.
5 pages, 1 figure, essay awarded first prize in the Gravity Research Foundation essay competition 2012
[my comment: in their conclusions K and K call attention to http://arxiv.org/abs/1101.5391 by Bojowald Calcagni Tsujikawa which derives a detectable prediction about CMB that can discriminate between theories]

brief mention:
http://arxiv.org/abs/1205.5251
Effective temperature, Hawking radiation and quasinormal modes
Christian Corda
(Submitted on 17 May 2012)
Parikh and Wilczek have shown that Hawking radiation's spectrum cannot be strictly thermal. Such a non-strictly thermal character implies that the spectrum is also not strictly continuous... In particular, the formula of the horizon's area quantization and the number of quanta of area are modified becoming functions of the quantum "overtone" number n. Consequently, Bekenstein-Hawking entropy, its sub-leading corrections and the number of microstates, i.e. quantities which are fundamental to realize unitary quantum gravity theory, are also modified. They become functions of the quantum overtone number too...
11 pages, founded on the research paper JHEP 1108, 101 (2011), http://arxiv.org/abs/1107.5334. This essay received an honorable mention in the 2012 Essay Competition of the Gravity Research Foundation.

http://pirsa.org/12050002/
New Probes of Initial State of Quantum Fluctuations During Inflation
Speaker(s): Eiichiro Komatsu
Abstract: ...When we calculate statistical properties of primordial fluctuations generated during inflation, we usually assume that the initial state of quantum fluctuations is in a preferred vacuum state called Bunch-Davies vacuum. While there is some motivation for choosing such a state, this is an assumption, and thus needs to be tested by observations. In this talk I will present new probes of initial state of quantum fluctuations during inflation: the 3-point function of the cosmic microwave background anisotropy, the 2-point function of galaxies, and a spectral distortion of the thermal spectrum of the cosmic microwave background.
Date: 22/05/2012 - 11:00 am

http://arxiv.org/abs/1205.5046
Diffusion in multi-fractional spacetimes
Gianluca Calcagni
(Submitted on 22 May 2012)
We study diffusion processes in anomalous spacetimes regarded as models of quantum geometry... The case of multi-scale (in particular, multi-fractal) spacetimes is then considered through a number of examples and the most general spectral-dimension profile of multi-fractional spaces is constructed.
43 pages, 5 figures
MTd2
#1728
May24-12, 07:47 PM
PF Gold
P: 1,963
http://arxiv.org/abs/1205.5431

Asymptotic Safety, Fractals, and Cosmology

Martin Reuter, Frank Saueressig
(Submitted on 24 May 2012)
These lecture notes introduce the basic ideas of the Asymptotic Safety approach to Quantum Einstein Gravity (QEG). In particular they provide the background for recent work on the possibly multifractal structure of the QEG space-times. Implications of Asymptotic Safety for the cosmology of the early Universe are also discussed.


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