Loop-and-allied QG bibliography

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http://arxiv.org/abs/gr-qc/0703144
Dynamical coherent states and physical solutions of quantum cosmological bounces
Martin Bojowald
30 pages, 3 figures
IGPG-07/3-5, NSF-KITP-07-55

"A new model is studied which describes the quantum behavior of transitions through an isotropic quantum cosmological bounce in loop quantum cosmology sourced by a free and massless scalar field. As an exactly solvable model even at the quantum level, it illustrates properties of dynamical coherent states and provides the basis for a systematic perturbation theory of loop quantum gravity. The detailed analysis is remarkably different from what is known for harmonic oscillator coherent states. Results are evaluated with regard to their implications in cosmology, including a demonstration that in general quantum fluctuations before and after the bounce are unrelated. Thus, even within this solvable model the condition of classicality at late times does not imply classicality at early times before the bounce without further assumptions. Nevertheless, the quantum state does evolve deterministically through the bounce."

http://arxiv.org/abs/gr-qc/0703135
Loop quantization of spherically symmetric midi-superspaces
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
18 pages

"We quantize the exterior of spherically symmetric vacuum space-times using a midi-superspace reduction within the Ashtekar new variables. Through a partial gauge fixing we eliminate the diffeomorphism constraint and are left with a Hamiltonian constraint that is first class. We complete the quantization in the loop representation. We also use the model to discuss the issues that will arise in more general contexts in the 'uniform discretization' approach to the dynamics."

http://arxiv.org/abs/gr-qc/0703137
Decoherence in Quantum Gravity: Issues and Critiques
C. Anastopoulos, B. L. Hu
25 pages, proceedings of DICE06 (Piombino)

"An increasing number of papers have appeared in recent years on decoherence in quantum gravity at the Planck energy. We discuss the meaning of decoherence in quantum gravity starting from the common notion that quantum gravity is a theory for the microscopic structures of spacetime, and invoking some generic features of quantum decoherence from the open systems viewpoint. We dwell on a range of issues bearing on this process including the relation between statistical and quantum, noise from effective field theory, the meaning of stochasticity, the origin of non-unitarity and the nature of nonlocality in this and related contexts. To expound these issues we critique on two representative theories: One claims that decoherence in quantum gravity scale leads to the violation of CPT symmetry at sub-Planckian energy which is used to explain today's particle phenomenology. The other uses this process in place with the Brownian motion model to prove that spacetime foam behaves like a thermal bath."

could be a useful text and reference work:
http://arxiv.org/abs/astro-ph/0703730
Why CMB physics?
Massimo Giovannini
179 pages, 27 figures
CERN-PH-TH/2007-048

"The aim of these lectures is to introduce some basic problems arising in gravitation and modern cosmology. All along the discussion the guiding theme is provided by the phenomenological and theoretical properties of the Cosmic Microwave Background (CMB). These lectures have been prepared for a regular Phd course of the University of Milan-Bicocca."

briefly noted:
http://arxiv.org/abs/astro-ph/0703722
Observable Signatures of a Black Hole Ejected by Gravitational Radiation Recoil in a Galaxy Merger
Abraham Loeb (Harvard)
4 pages, submitted to PRL

"According to recent general-relativistic simulations, the coalescence of two spinning black holes (BHs) could lead to recoil speeds of the BH remnant of up to thousands of km/s as a result of the emission of gravitational radiation. Such speeds would enable the merger product to escape its host galaxy. Here we examine the circumstances resulting from a gas-rich galaxy merger under which the ejected BH would carry an accretion disk with it and be observable. As the initial BH binary emits gravitational radiation and its orbit tightens, a hole is opened around it in the disk which delays the consumption of gas prior to the eventual BH ejection. The punctured disk remains bound to the ejected BH within the region where the gas orbital velocity is larger than the ejection speed. For a ~10^7 solar mass BH the ejected disk has a characteristic size of tens of thousands of Schwarzschild radii and an accretion lifetime of ~10^7 years. During that time, the ejected BH could traverse a considerable distance and appear as an off-center quasar with a feedback trail along the path it left behind. A small fraction of all quasars could be associated with an escaping BH."

briefly noted:

http://arxiv.org/abs/gr-qc/0703150
A gravitational explanation for quantum theory - non-time-orientable manifolds
Mark J Hadley
7 pages Talk given at FFP8 in Madrid 2006

"Spacetime manifolds that are not time orientable play a key role in a gravitational explanation of quantum theory. Such manifolds allow topology change, but also have fascinating additional properties such as net charge from source-free equations and spin half transformation properties. It is shown how the logical structure of propositions and the probabilities of quantum theory arise from such acausal space times."

http://arxiv.org/abs/hep-th/0703265
A new PPN parameter to test Chern-Simons gravity
Stephon Alexander, Nicolas Yunes
4 pages, submitted to PRL

"We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN) framework through weak-field solutions of the modified field equations for a perfect fluid source... This new term encodes the key physical effect of CS gravity in the weak-field limit, leading to a modification of frame dragging and, thus, the Lense-Thirring contribution to gyroscopic precession. We provide a physical interpretation for the new term, as well as an estimate of the size of this effect relative to the general relativistic Lense-Thirring prediction. This correction to frame dragging might be used in experiments,.."
 
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  • #577
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http://arxiv.org/abs/0704.0145
Singularity Resolution in Isotropic Loop Quantum Cosmology: Recent Developments
Ghanashyam Date
(Submitted on 2 Apr 2007)

"Since the past Iarge meeting in December 2004, new developments in loop quantum cosmology have taken place, especially with regards to the resolution of the Big Bang singularity in the isotropic models. The singularity resolution issue has been discussed in terms of physical quantities (expectation values of Dirac observables) and there is also an 'improved' quantization of the Hamiltonian constraint. These developments are briefly discussed. This is an expanded version of the review talk given at the 24-th IAGRG meeting in February 2007."

http://arxiv.org/abs/0704.0007
Polymer Quantum Mechanics and its Continuum Limit
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
(Submitted on 31 Mar 2007)

"A rather non-standard quantum representation of the canonical commutation relations of quantum mechanics systems, known as the polymer representation has gained some attention in recent years, due to its possible relation with Planck scale physics. In particular, this approach has been followed in a symmetric sector of loop quantum gravity known as loop quantum cosmology (LQC). Here we explore different aspects of the relation between the ordinary Schroedinger theory and the polymer description. The paper has two parts. In the first one, we derive the polymer quantum mechanics starting from the ordinary Schroedinger theory and show that the polymer description arises as an appropriate limit. In the second part we consider the continuum limit of this theory, namely, the reverse process in which one starts from the discrete theory and tries to recover back the ordinary Schroedinger quantum mechanics. We consider several examples of interest, including the harmonic oscillator, the free particle and a simple cosmological model."

http://arxiv.org/abs/0704.0221
The Return of a Static Universe and the End of Cosmology
Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
(Submitted on 2 Apr 2007)

"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented."

briefly noted:
http://arxiv.org/abs/0704.0058
Intelligent Life in Cosmology
Frank J. Tipler
 
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  • #578
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http://arxiv.org/abs/0704.0278
q-Deformed spin foam models of quantum gravity
Igor Khavkine, J. Daniel Christensen
(Submitted on 2 Apr 2007)

"We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous."

Dan Christensen accesses the supercomputer at U. West Ontario (UWO) where he has charge of programs in mathematics, computing, and quantum gravity. Co-authored spinfoam papers with John Baez several years back and with others since. Use of numerical techniques notable here---probably a Beowolf cluster.

http://arxiv.org/abs/0704.0367
Existence of generalized Kodama states. IV. The search for a quantization of 4-dimensional gravity
Eyo Eyo Ita III

"This is the fourth in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper continues essentially from where papers II and III left off, treating the kinematic constraints in greater depth and moving on to a higher level of complexity with regard to the Hamiltonian constraint of the full, unrestricted theory. First we identify some of the traditional obstacles to the consistent quantization of four-dimensional gravity, then provide suggestions for how these obstacles may possibly be surmounted within the context of our new approach. This inevitably entails a critical analysis of the relationship of gauge transformations to diffeomorphisms, which in turn leads to the reduced phase space approach to quantization of the kinematic constraints, as well as the implementation of the semiclassical-quantum correspondence. We also compute some more of the terms needed to implement the full quantum Hamiltonian constraint, focusing in this work on its antisymmetric CDJ components. Some of the relationships among generalized Kodama states for the more general model are clearly elucidated due to the calculation of such terms."
33 pages

E.E.Ita is a PhD student at Cambridge.

http://arxiv.org/abs/0704.0299
Parametrized Post-Newtonian Expansion of Chern-Simons Gravity
Stephon Alexander, Nicolas Yunes

"We investigate the weak-field, post-Newtonian expansion to the solution of the field equations in Chern-Simons gravity with a perfect fluid source. In particular, we study the mapping of this solution to the parameterized post-Newtonian formalism to 1 PN order in the metric. We find that the PPN parameters of Chern-Simons gravity are identical to those of general relativity, with the exception of the inclusion of a new term that is proportional to the Chern-Simons coupling parameter and the curl of the PPN vector potentials. We also find that the new term is naturally enhanced by the non-linearity of spacetime and we provide a physical interpretation for it. By mapping this correction to the gravito-electro-magnetic framework, we study the corrections that this new term introduces to the acceleration of point particles and the frame-dragging effect in gyroscopic precession. We find that the Chern-Simons correction to these classical predictions could be used by current and future experiments to place bounds on intrinsic parameters of Chern-Simons gravity and, thus, string theory."
14 pages
 
  • #579
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http://arxiv.org/abs/0704.1137
Lattice refining loop quantum cosmology, anisotropic models and stability
Martin Bojowald, Daniel Cartin, Gaurav Khanna
24 pages

"A general class of loop quantizations for anisotropic models is introduced and discussed, which enhances loop quantum cosmology by relevant features seen in inhomogeneous situations. The main new effect is an underlying lattice which is being refined during dynamical changes of the volume. In general, this leads to a new feature of dynamical difference equations which may not have constant step-size, posing new mathematical problems. It is discussed how such models can be evaluated and what lattice refinements imply for semiclassical behavior. Two detailed examples illustrate that stability conditions can put strong constraints on suitable refinement models, even in the absence of a fundamental Hamiltonian which defines changes of the underlying lattice. Thus, a large class of consistency tests of loop quantum gravity becomes available. In this context, it will also be seen that quantum corrections due to inverse powers of metric components in a constraint are much larger than they appeared recently in more special treatments of isotropic, free scalar models where they were artificially suppressed."




http://arxiv.org/abs/0704.0992
Compatibility of radial, Lorenz and harmonic gauges
Elena Magliaro, Claudio Perini, Carlo Rovelli
9 pages

"We observe that the radial gauge can be consistently imposed together with the Lorenz gauge in Maxwell theory, and with the harmonic traceless gauge in linearized general relativity. This simple observation has relevance for some recent developments in quantum gravity where the radial gauge is implicitly utilized.
 
  • #580
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Marcus

Perhaps you could enlighten us as to the LQG program predictions for the MiniBooNE results, which come out this week. I haven't seen any anywhere...
 
  • #581
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http://arxiv.org/abs/0704.2397
The Quantum Configuration Space of Loop Quantum Cosmology
Jose Manuel Velhinho
14 pages

"The article gives an account of several aspects of the space known as the Bohr compactification of the line, featuring as the quantum configuration space in loop quantum cosmology, as well as of the corresponding configuration space realization of the so-called polymer representation. Analogies with loop quantum gravity are explored, providing an introduction to (part of) the mathematical structure of loop quantum gravity, in a technically simpler context."

http://arxiv.org/abs/0704.2291
Fundamentalist physics: why Dark Energy is bad for Astronomy
Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics. 19 pages including 3 figures

"Astronomers carry out observations to explore the diverse processes and objects which populate our Universe. High-energy physicists carry out experiments to approach the Fundamental Theory underlying space, time and matter. Dark Energy is a unique link between them, reflecting deep aspects of the Fundamental Theory, yet apparently accessible only through astronomical observation. Large sections of the two communities have therefore converged in support of astronomical projects to constrain Dark Energy. In this essay I argue that this convergence can be damaging for astronomy. The two communities have different methodologies and different scientific cultures. By uncritically adopting the values of an alien system, astronomers risk undermining the foundations of their own current success and endangering the future vitality of their field. Dark Energy is undeniably an interesting problem to attack through astronomical observation, but it is one of many and not necessarily the one where significant progress is most likely to follow a major investment of resources."
 
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  • #582
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Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics.

Goodness, Marcus! Do such articles usually use such strong language?

:smile:
 
  • #583
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Hey Marcus
not sure if you've ever mentioned this (I can't search for this personally)

arXiv:gr-qc/0611156
Title: On Loop States in Loop Quantum Gravity
Authors: N. D. Hari Dass, Manu Mathur
Comments: 12 pages, 3 figures, the version to be published in Classical and Quantum Gravity
We explicitly construct and characterize all possible independent loop states in 3+1 dimensional loop quantum gravity by regulating it on a 3-d regular lattice in the Hamiltonian formalism. These loop states, characterized by the (dual) angular momentum quantum numbers, describe SU(2) rigid rotators on the links of the lattice. The loop states are constructed using the Schwinger bosons which are harmonic oscillators in the fundamental (spin half) representation of SU(2). Using generalized Wigner Eckart theorem, we compute the matrix elements of the volume operator in the loop basis. Some simple loop eigenstates of the volume operator are explicitly constructed.
 
  • #584
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the preceding post duplicates an entry made last year in #543 of this thread

http://arxiv.org/abs/0704.3214
(2+1)-Dimensional Quantum Gravity as the Continuum Limit of Causal Dynamical Triangulations
D. Benedetti, R. Loll, F. Zamponi
38 pages, 13 figures

"We perform a non-perturbative sum over geometries in a (2+1)-dimensional quantum gravity model given in terms of Causal Dynamical Triangulations. Inspired by the concept of triangulations of product type introduced previously, we impose an additional notion of order on the discrete, causal geometries. This simplifies the combinatorial problem of counting geometries just enough to enable us to calculate the transfer matrix between boundary states labelled by the area of the spatial universe, as well as the corresponding quantum Hamiltonian of the continuum theory. This is the first time in dimension larger than two that a Hamiltonian has been derived from such a model by mainly analytical means, and opens the way for a better understanding of scaling and renormalization issues."

http://arxiv.org/abs/0704.2667
Wormholes as Black Hole Foils
Thibault Damour, Sergey N. Solodukhin
13 pages

"We study to what extent wormholes can mimic the observational features of black holes. It is surprisingly found that many features that could be thought of as 'characteristic' of a black hole (endowed with an event horizon) can be closely mimicked by a globally static wormhole, having no event horizon. This is the case for: the apparently irreversible accretion of matter down a hole, no-hair properties, quasi-normal-mode ringing, and even the dissipative properties of black hole horizons, such as a finite surface resistivity equal to 377 Ohms. The only way to distinguish the two geometries on an observationally reasonable time scale would be through the detection of Hawking's radiation, which is, however, too weak to be of practical relevance for astrophysical black holes. We point out the existence of an interesting spectrum of quantum microstates trapped in the throat of a wormhole which could be relevant for storing the information 'lost' during a gravitational collapse."

http://arxiv.org/abs/0704.1746
Tunnelling from black holes in the Hamilton Jacobi approach
Bhramar Chatterjee, Amit Ghosh, P. Mitra
5 pages

"It has recently been shown that it is possible to understand Hawking radiation as tunnelling across black hole horizons using appropriate Hamilton-Jacobi boundary conditions. The procedure is applied to the non-rotating black hole in different coordinate systems and to the rotating charged black hole. Differences with the earlier literature are pointed out."
 
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  • #585
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Thanks, how do you search that? When I type it in I get wrong hits.

Anyhow did you also mention this?
http://arxiv.org/abs/gr-qc/0606100

A lattice bosonic model as a quantum theory of gravity
Authors: Zheng-Cheng Gu, Xiao-Gang Wen
(Submitted on 23 Jun 2006)


A local quantum bosonic model on a lattice is constructed whose low energy excitations are gravitons described by linearized Einstein action. Thus the bosonic model is a quantum theory of gravity, at least at the linear level. We find that the compactification and the discretization of metric tenor are crucial in obtaining a quantum theory of gravity.
 
  • #586
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John Baez is at Les Treilles (interesting people at rustic estate in south France, QG discussion this year) to present this talk
http://math.ucr.edu/home/baez/treilles/
the slides are downloadable.

ftp://ftp.alainconnes.org/Inteng.pdf[/URL]
recent [b]Alain Connes interview[/b] with G. Skandalis and C. Goldstein

[url]http://arxiv.org/abs/0704.2630[/url]
[b]Are We Typical?[/b]
James B. Hartle, Mark Srednicki
6 pages

"Bayesian probability theory is used to analyze the oft-made assumption that humans are typical observers in the universe. Some theoretical calculations make the {\it selection fallacy} that we are randomly chosen from a class of objects by some physical process, despite the absence of any evidence for such a process, or any observational evidence favoring our typicality. It is possible to favor theories in which we are typical by appropriately choosing their prior probabilities, but such assumptions should be made explicit to avoid confusion."

[url]http://arxiv.org/abs/0704.3429[/url]
[b]Light Propagation on Quantum Curved Spacetime and Back reaction effects[/b]
Carlos Kozameh, Florencia Parisi

"We study the electromagnetic field equations on an arbitrary quantum curved background in the semiclassical approximation of Loop Quantum Gravity. The effective interaction hamiltonian for the Maxwell and gravitational fields is obtained and the corresponding field equations, which can be expressed as a modified wave equation for the Maxwell potential, are derived. We use these results to analyze electromagnetic wave propagation on a quantum Robertson-Walker space time and show that Lorentz Invariance is not preserved. The formalism developed can be applied to the case where back reaction effects on the metric due to the electromagnetic field are taken into account, leading to non-covariant field equations."

[url]http://arxiv.org/abs/0704.3595[/url]
[b]Generalized Kodama states. V. Evidence of the normalizability and renormalizability of 4D QGRA[/b]
Eyo Eyo Ita III

"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. In this work we transform the pure Kodama state into the metric representation for the Bianchi IX minisuperspace model. Previous such calculations have been carried out to semiclassical order for a particular choice of gauge, revealing the existence of five topologically distinct states in the metric representation. We have performed our calculation to all orders in perturbation theory by maintaining the gauge degrees of freedom explicit, revealing the existence of a sixth state. We propose a resolution to the issue of reality conditions for the Ashtekar variables, and also for the recasting of 4-dimensional general relativity as a renormalizable field theory, stemming from the nonperturbative result obtained from this work. We also address the issue of normalizability of the Kodama state."


briefly noted

[url]http://arxiv.org/abs/0704.3306[/url]
[b]Quantum mechanics on Hilbert manifolds: The principle of functional relativity[/b]
Alexey A. Kryukov
45 pages, 9 figures

"Quantum mechanics is formulated as a geometric theory on a Hilbert manifold..."


[url]http://arxiv.org/abs/0704.3414[/url]
[b]Phantom field dynamics in loop quantum cosmology[/b]
Daris Samart, Burin Gumjudpai (TPTP Naresuan U.)

"We consider a dynamical system of phantom scalar field under exponential potential in background of loop quantum cosmology... "
 
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  • #587
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http://arxiv.org/abs/0705.0006
Multiple-event probability in general-relativistic quantum mechanics: a discrete model
Mauricio Mondragon, Alejandro Perez, Carlo Rovelli
(Submitted on 30 Apr 2007)

"We introduce a simple quantum mechanical model in which time and space are discrete and periodic. These features avoid the complications related to continuous-spectrum operators and infinite-norm states. The model provides a tool for discussing the probabilistic interpretation of generally-covariant quantum systems, without the confusion generated by spurious infinities. We use the model to illustrate the formalism of general-relativistic quantum mechanics, and to test the definition of multiple-event probability introduced in a companion paper. We consider a version of the model with unitary time-evolution and a version without unitary time-evolution."

http://arxiv.org/abs/0705.0235
The canonical versus path integral quantization approach to generalized Kodama states (Part II)
Eyo Eyo Ita III
41 pages

"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. A major feature of the quantization programme resides in the inherent ability of the path integral to exhaustively pick out a complete basis of quantum states precisely matching the canonically determined wavefunctions for an arbitrary model coupled to gravity. First we develop the notation and tools necessary to expose this hidden feature of the path integral, and then we apply it to the nonperturbative construction of the pure Kodama state (Part I) and the generalized Kodama state (Part II) for an arbitraty matter coupling, in analogy to the no-boundary proposal. We argue that the well-definedness and the explicit convergence of the path integral is intimately connected to its equivalence to the canonical approach, the principle of the SQC, and in particular to the existence of the generalized Kodama states (GKodS). We then indicate how these features, combined, can ultimately lead to a resolution of the problem of time in quantum gravity. In the end, we recast the equations necessary to eliminate the quantum counterterms for a general model into a standard form suitable for model-specific expansion of the GKodS about the pure Kodama state. The explicit solution of these equations is reserved for future work."

http://arxiv.org/abs/0705.0165
Is Modified Gravity Required by Observations? An Empirical Consistency Test of Dark Energy Models
Sheng Wang (Brookhaven; Columbia), Lam Hui (Columbia; ISCAP), Morgan May (Brookhaven), Zoltan Haiman (Columbia)

"We apply the technique of parameter-splitting to existing cosmological data sets, to check for a generic failure of dark energy models. Given a dark energy parameter, such as the energy density Omega_Lambda or equation of state w, we split it into two meta-parameters with one controlling geometrical distances, and the other controlling the growth of structure. Observational data spanning Type Ia Supernovae, the cosmic microwave background (CMB), galaxy clustering, and weak gravitational lensing statistics are fit without requiring the two meta-parameters to be equal. This technique checks for inconsistency between different data sets, as well as for internal inconsistency within any one data set (e.g., CMB or lensing statistics) that is sensitive to both geometry and growth. We find that the cosmological constant model is consistent with current data. Theories of modified gravity generally predict a relation between growth and geometry that is different from that of general relativity. Parameter-splitting can be viewed as a crude way to parametrize the space of such theories. Our analysis of current data already appears to put sharp limits on these theories: assuming a flat universe, current data constrain the difference Omega_Lambda(geom) - Omega_Lambda(grow) to be -0.0044 +/- 0.0058 (68% C.L.); allowing the equation of state w to vary, the difference w(geom) - w(grow) is constrained to be 0.37 +/- 0.37 (68% C.L.). Interestingly, the region w(grow) > w(geom), which should be generically favored by theories that slow structure formation relative to general relativity, is quite restricted by data already. We find w(grow) < -0.80 at 2 sigma."

briefly noted:
http://arxiv.org/abs/0705.0164
Inflationary Cosmology
Andrei Linde
60 pages, 10 figs, based on a talk given at the 22nd IAP Colloquium, "Inflation+25", Paris, June 2006
 
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  • #588
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http://arxiv.org/abs/0705.0674
A new spinfoam vertex for quantum gravity
Etera R. Livine, Simone Speziale
17+8 pages, 6 figures

"We introduce a new spinfoam vertex to be used in models of 4d quantum gravity based on SU(2) and SO(4) BF theory plus constraints. It can be seen as the conventional vertex of SU(2) BF theory, the 15j symbol, in a particular basis constructed using SU(2) coherent states. This basis makes the geometric interpretation of the variables transparent: they are the vectors normal to the triangles within each tetrahedron. We study the condition under which these states can be considered semiclassical, and we show that the semiclassical ones dominate the evaluation of quantum correlations. Finally, we describe how the constraints reducing BF to gravity can be directly written in terms of the new variables, and how the semiclassicality of the states might improve understanding the correct way to implement the constraints."



.
 
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  • #589
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http://arxiv.org/abs/0705.0991
From semiconductors to quantum gravity: to centenary of Matvei Bronstein
G.E. Volovik
12 pages, 1 figure, draft for proceedings of the conference devoted to centenary of Marvei Bronstein, St. Petersburg, November 2006

"Investigation of the many-body condensed-matter systems allows us to connect the microscopic physics at the atomic energy scale and the macroscopic physics emerging in the low-energy corner. It gives some hints on the mechanisms of the formation of the physical laws which our Universe obeys. The paper is devoted to the centenary of Matvei Petrovich Bronstein."


briefly noted:
http://arxiv.org/abs/0705.1060
Obtaining the spacetime metric from cosmological observations

http://arxiv.org/abs/0705.1029
No Way Back: Maximizing survival time below the Schwarzschild event horizon
 
  • #590
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http://arxiv.org/abs/0705.1178
A global picture of quantum de Sitter space
Steven B. Giddings, Donald Marolf
17 pages, 1 figure

"Perturbative gravity about a de Sitter background motivates a global picture of quantum dynamics in `eternal de Sitter space,' the theory of states which are asymptotically de Sitter to both future and past. Eternal de Sitter physics is described by a finite dimensional Hilbert space in which each state is precisely invariant under the full de Sitter group. This resolves a previously-noted tension between de Sitter symmetry and finite entropy. Observables, implications for Boltzmann brains, and Poincare recurrences are briefly discussed."

Giddings is known as a string theorist but this is not a string paper. It finds a mistake in a 2002 paper by Leonard Susskind and others called "The Trouble with deSitter Space" http://arxiv.org/hep-th/0212209 [Broken]
Many papers by people in the LQG community are cited---four by Rovelli, several by Ambjorn, by Thiemann, Ashtekar, Gambini and Pullin. Doubtless others, I didnt make a careful count. Since Giddings and Marolf are well-known people at KITP Santa Barbara it suggests a significant direction of interest. At the same time on the nonstring QG side a substantial amoung of research is involved with deSitter space---Baez student Derek Wise was discussing Cartan geometry using the deSitter group instead of Poincaré---DSR (deformed special relativity) deals a lot with deSitter group. What happens when quantum spacetime geometry is "locally deSitter"? This paper will be of interest and will have usefulness outside of string research.

http://arxiv.org/abs/0705.1032
Matter density perturbations and effective gravitational constant in modified gravity models of dark energy
Shinji Tsujikawa
10 pages

"We derive the equation of matter density perturbations on sub-horizon scales for a general Lagrangian density f(R, phi, X) that is a function of a Ricci scalar R, a scalar field phi and a kinetic term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing..."

http://arxiv.org/abs/0705.1158
Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
Wayne Hu, Ignacy Sawicki (KICP, U. Chicago)
13 pages, 10 figures. Submitted to Phys. Rev. D

"We study a class of metric-variation f(R) models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in f(R) models do not have substantially deeper potentials than expected in LCDM, then cosmological field amplitudes |f_R| > 10^{-6} will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of f(R) models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on f(R) are phrased in a nearly model-independent manner."

My impression is that Wayne Hu is very highly regarded in cosmology. It seems significant to me that he is seriously talking about ways to get away from having "Dark Energy" by modifying the law of gravity.

http://arxiv.org/abs/0705.1170
The Collision Between The Milky Way And Andromeda
T.J. Cox, Abraham Loeb (Harvard/CfA)
submitted to MNRAS

"We use a N-body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the first close encounter of the two galaxies, there is a 12% chance that the Sun will be pulled from its present position and reside in the extended tidal material. After the second close encounter, there is a 30% chance that the Sun will reside in the extended tidal material, and a 2.7% chance that our Sun will be more tightly bound to Andromeda than to the Milky Way. Eventually, after the merger has completed, the Sun is likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies."
 
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  • #591
marcus
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http://arxiv.org/abs/0705.2222
Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
Abhay Ashtekar
21 pages, to appear in the Proceedings of the 11th Marcel Grossmann Conference

"As per organizers' request, my talk at the 11th Marcel Grossmann Conference consisted of two parts. In the first, I illustrated recent advances in loop quantum gravity through examples. In the second, I presented an overall assessment of the status of the program by addressing some frequently asked questions. This account is addressed primarily to researchers outside the loop quantum gravity community."


http://arxiv.org/abs/0705.2047
Non-Metric Gravity II: Spherically Symmetric Solution, Missing Mass and Redshifts of Quasars
Kirill Krasnov, Yuri Shtanov
37 pages, 2 figures

"We continue the study of the non-metric theory of gravity introduced in hep-th/0611182 and gr-qc/0703002 and obtain its general spherically symmetric vacuum solution. It respects the analog of the Birkhoff theorem, i.e., the vacuum spherically symmetric solution is necessarily static. As in general relativity, the spherically symmetric solution is seen to describe a black hole. The exterior geometry is essentially the same as in the Schwarzschild case, with power-law corrections to the Newtonian potential. The behavior inside the black-hole region is different from the Schwarzschild case in that the usual spacetime singularity gets replaced by a singular of a new type, where all basic fields of the theory remain finite but metric ceases to exist. The theory does not admit arbitrarily small black holes: for small objects, the curvature on the would-be horizon is so strong that non-metric modifications prevent the horizon from being formed. The theory allows for modifications of gravity of very interesting nature. We discuss three physical effects, namely, (i) correction to Newton's law in the neighborhood of the source, (ii) renormalization of effective gravitational and cosmological constants at large distances from the source, and (iii) additional redshift factor between spatial regions of different curvature. The first two effects can be responsible, respectively, for the observed anomaly in the acceleration of the Pioneer spacecraft and for the alleged missing mass in spiral galaxies and other astrophysical objects. The third effect can be used to propose a non-cosmological explanation of high redshifts of quasars."

http://arxiv.org/abs/0705.2197
Black holes, information, and locality
Steven B. Giddings
Essay submitted to the Gravity Research Foundation essay contest; with minor updates

"Thirty years of a deepening information paradox suggest the need to revise our basic physical framework. Multiple indicators point toward reassessment of the principle of locality: lack of a precise definition in quantum gravity, behavior of high-energy scattering, hints from strings and AdS/CFT, conundrums of quantum cosmology, and finally lack of good alternative resolutions of the paradox. A plausible conjecture states that the non-perturbative dynamics of gravity is unitary but nonlocal. String theory may directly address these issues but so far important aspects remain elusive. If this viewpoint is correct, critical questions are to understand the 'correspondence' limit where nonlocal physics reduces to local quantum field theory, and beyond, to unveil principles of an underlying nonlocal theory."

http://arxiv.org/abs/0705.2388
The loop-quantum-gravity vertex-amplitude
Jonathan Engle, Roberto Pereira, Carlo Rovelli
6 pages

"Spinfoam theories are hoped to provide the dynamics of non-perturbative loop quantum gravity. But a number of their features remain elusive. The best studied one -the euclidean Barrett-Crane model- does not have the boundary state space needed for this, and there are recent indications that, consequently, it may fail to yield the correct low-energy n-point functions. These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way certain second class constraints are imposed, arguably incorrectly, strongly. We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly. Its state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network space. The model provides a long sought SO(4)-covariant vertex amplitude for loop quantum gravity."

http://arxiv.org/abs/0705.2357
The inevitable nonlinearity of quantum gravity falsifies the many-worlds interpretation of quantum mechanics
T. P. Singh
6 pages. Honorable Mention in Gravity Research Foundation Essay Competition 2007. This is also a summary of a talk given at the Meeting `Himalayan Relativity Dialogue', Mirik, India, 18-20 April, 2007. Detailed version of this essay is in preparation

"There are fundamental reasons as to why there should exist a reformulation of quantum mechanics which does not refer to a classical spacetime manifold. It follows as a consequence that quantum mechanics as we know it is a limiting case of a more general nonlinear quantum theory, with the nonlinearity becoming significant at the Planck mass/energy scale. This nonlinearity is responsible for a dynamically induced collapse of the wave-function, during a quantum measurement, and it hence falsifies the many-worlds interpretation of quantum mechanics. We illustrate this conclusion using a mathematical model based on a generalized Doebner-Goldin equation. The non-Hermitian part of the Hamiltonian in this norm-preserving, nonlinear, Schrodinger equation dominates during a quantum measurement, and leads to a breakdown of linear superposition."

briefly noted:
http://arxiv.org/abs/0705.2306
Near-Extreme Black Holes and the Universal Relaxation Bound
Shahar Hod
2 Pages. Submitted to PRD




=========
Still able to edit this, I reply to Francesca here so as not to make an extra post:

Francesca, thanks for the two references! Both the Percacci et al, and the Chamseddine-Connes look interesting and relevant as you say.
 
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  • #592
150
0
So does anyone else think "Carlo Rovelli" is secretly Karl Rove with a wig? I think he's trying to destroy string theory as part of the Republican War on Science. That sneaky bastard.
 
  • #593
147
45
Sorry, I didn't want to replay but I can't...
ST is right, QG is left!
That's an Italian play started by the movie maker Nanni Moretti in a movie of him, "Aprile"... the ham steak is right, the Bologna ham is left...
 
  • #594
147
45
Do you remember the great interest for Reuter's lecture "Asymptotic Safety in Quantum Einstein Gravity"? This is a related paper:

http://arxiv.org/abs/0705.1769" [Broken]
Title: Ultraviolet properties of f(R)-Gravity
Authors: Alessandro Codello, Roberto Percacci, Christoph Rahmede
Comments: 4 pages

We discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile.


and BTW:

http://arxiv.org/abs/0705.1786" [Broken]
Title: Quantum Gravity Boundary Terms from Spectral Action
Authors: Ali H. Chamseddine, Alain Connes
Comments: RevTex 4 pages

We study the boundary terms of the spectral action of the noncommutative space, defined by the spectral triple dictated by the physical spectrum of the standard model, unifying gravity with all other fundamental interactions. We prove that the spectral action predicts uniquely the gravitational boundary term required for consistency of quantum gravity with the correct sign and coefficient. This is a remarkable result given the lack of freedom in the spectral action to tune this term.
 
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  • #595
marcus
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http://arxiv.org/abs/0705.2440
Quantum Structure of Geometry: Loopy and fuzzy?
Alejandro Corichi, Jose A. Zapata
9 pages

"In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy."

http://arxiv.org/abs/0705.2533
Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures

"I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular 'concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract)."

http://arxiv.org/abs/0705.2525
On the physical meaning of the Unruh effect
Emil T.Akhmedov, Douglas Singleton
7 pages

"We present simple arguments that detectors moving with constant acceleration (even acceleration for a finite time) should detect particles. The effect is seen to be universal. Moreover, detectors undergoing linear acceleration and uniform, circular motion both detect particles for the same physical reason. We show that if one uses a circularly orbiting electron in a constant external magnetic field as the Unruh--DeWitt detector, then the Unruh effect physically coincides with the experimentally verified Sokolov--Ternov effect."

http://arxiv.org/abs/0705.2462
LambdaCDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?
Richard Lieu
14 pages, 3 figures and 3 tables

"Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter and now dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of LambdaCDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind LambdaCDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark."
 
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  • #596
Kea
859
0
http://arxiv.org/abs/0705.2462
LambdaCDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?
Richard Lieu

"Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark."

Fighting words indeed. Check out the table on page 6!

:smile:
 
  • #597
marcus
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http://arxiv.org/abs/0705.2656
Inflationary universe in loop quantum cosmology
Xin Zhang, Yi Ling
25 pages, 5 figures

"Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed."


http://arxiv.org/abs/0705.2629
Dual Computations of Non-abelian Yang-Mills on the Lattice
J. Wade Cherrington, Dan Christensen, Igor Khavkine
18 pages, 7 figures

"In the past several decades there have been a number of proposals for computing with dual forms of non-abelian Yang-Mills theories on the lattice. Motivated by the gauge-invariant, geometric picture offered by dual models and successful applications of duality in the U(1) case, we revisit the question of whether it is practical to perform numerical computation using non-abelian dual models. Specifically, we consider three-dimensional SU(2) pure Yang-Mills as an accessible yet non-trivial case in which the gauge group is non-abelian. Using methods developed recently in the context of spin foam quantum gravity, we describe a Metropolis algorithm for sampling the dual ensemble and efficiently computing the dual amplitude. We relate our algorithms to prior work in non-abelian dual computations of Hari Dass and his collaborators, addressing several problems that have (to the best our knowledge) been left open. We report results of spin expectation value computations over a range of lattice sizes and couplings that are in agreement with our conventional lattice computations. We conclude with an outlook on further development of dual methods and their application to problems of current interest."

brief mention:
http://arxiv.org/abs/0705.2643
Symmetries, Singularities and the De-Emergence of Space
Thibault Damour, Hermann Nicolai
10 pages

though in a very different formal context, this paper reminded me of the recent work of Kirill Krasnov and Yuri Shtanov----where the metric disappears near the singularity, but spacetime evolution continues because the theory of gravity is non-metric. you could say that in Krasnov-Shtanov non-metric gravity the conventional idea of space also "de-emerges" near a classical singularity---while the classical singularity is removed. For the Krasnov-Shtanov paper see post #591 or http://arxiv.org/abs/0705.2047

EDITED IN to reply to francesca's next post:
Dear francesca, thanks for helping by supplying the papers of Corichi et al and Padmanabhan. However look back at post #595
https://www.physicsforums.com/showpost.php?p=1334313&postcount=595
from four days ago. they are the first two papers mentioned in that post, IIRC. :-)
 
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  • #598
147
45
http://arxiv.org/abs/0705.2440" [Broken]
Quantum Structure of Geometry: Loopy and fuzzy?
Authors: Alejandro Corichi, Jose A. Zapata

Abstract: In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy.



http://arxiv.org/abs/0705.2533" [Broken]
Dark Energy and Gravity
Authors: T. Padmanabhan

Abstract: I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular `concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract).
 
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  • #599
marcus
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http://arxiv.org/abs/0705.3024
Symmetries, Horizons, and Black Hole Entropy
S. Carlip
6 pages; first prize essay, 2007 Gravity Research Foundation essay contest

"Black holes behave as thermodynamic systems, and a central task of any quantum theory of gravity is to explain these thermal properties. A statistical mechanical description of black hole entropy once seemed remote, but today we suffer an embarrassment of riches: despite counting very different states, many inequivalent approaches to quantum gravity obtain identical results. Such 'universality' may reflect an underlying two-dimensional conformal symmetry near the horizon, which can be powerful enough to control the thermal characteristics independent of other details of the theory. This picture suggests an elegant description of the relevant degrees of freedom as Goldstone-boson-like excitations arising from symmetry breaking by the conformal anomaly."
 
  • #600
695
0
Hey Marcus, I do not know how to search this thread but did you ever post this paper?

http://arxiv.org/abs/gr-qc/0612134
From Quantum Hydrodynamics to Quantum Gravity
G.E. Volovik
(Submitted on 21 Dec 2006 (v1), last revised 17 Jan 2007 (this version, v5))
We discuss some lessons from quantum hydrodynamics to quantum gravity.
Comments: 20 pages, 1 figure, rapporteur article for Proceedings of MG11, session `Analog Models of and for General Relativity', references added
Subjects: General Relativity and Quantum Cosmology (gr-qc); Soft Condensed Matter (cond-mat.soft); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:gr-qc/0612134v5
 

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