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Intuitive content of Loop Gravity-Rovelli's program

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marcus
#1891
Jan28-13, 07:15 PM
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PF Gold
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http://arxiv.org/abs/1301.6210
Embedding loop quantum cosmology without piecewise linearity
Jonathan Engle
(Submitted on 26 Jan 2013)
An important goal is to understand better the relation between full loop quantum gravity (LQG) and the simplified, reduced theory known as loop quantum cosmology (LQC), directly at the quantum level. Such a firmer understanding would increase confidence in the reduced theory as a tool for formulating predictions of the full theory, as well as permitting lessons from the reduced theory to guide further development in the full theory. The present paper constructs an embedding of the usual state space of LQC into that of standard LQG, that is, LQG based on piecewise analytic paths. The embedding is well-defined even prior to solving the diffeomorphism constraint, at no point is a graph fixed, and at no point is the piecewise linear category used. This motivates for the first time a definition of operators in LQC corresponding to holonomies along non-piecewise-linear paths, without changing the usual kinematics of LQC in any way. The new embedding intertwines all operators corresponding to such holonomies, and all elements in its image satisfy an operator equation which classically implies homogeneity and isotropy. The construction is made possible by a recent result proven by Fleischhack.
18 pages

http://arxiv.org/abs/1301.6173
Scale Anomaly as the Origin of Time
Julian Barbour, Matteo Lostaglio, Flavio Mercati
(Submitted on 25 Jan 2013)
We explore the problem of time in quantum gravity in a point-particle analogue model of scale-invariant gravity. If quantized after reduction to true degrees of freedom, it leads to a time-independent Schrödinger equation. As with the Wheeler--DeWitt equation, time disappears, and a frozen formalism that gives a static wavefunction on the space of possible shapes of the system is obtained. However, if one follows the Dirac procedure and quantizes by imposing constraints, the potential that ensures scale invariance gives rise to a conformal anomaly, and the scale invariance is broken. A behaviour closely analogous to renormalization-group (RG) flow results. The wavefunction acquires a dependence on the scale parameter of the RG flow. We interpret this as time evolution and obtain a novel solution of the problem of time in quantum gravity. We apply the general procedure to the three-body problem, showing how to fix a natural initial value condition, introducing the notion of complexity. We recover a time-dependent Schrödinger equation with a repulsive cosmological force in the `late-time' physics and we analyse the role of the scale invariant Planck constant. We suggest that several mechanisms presented in this model could be exploited in more general contexts.
31 pages, 5 figures

http://arxiv.org/abs/1301.6259
Inconsistencies from a Running Cosmological Constant
Herbert W. Hamber, Reiko Toriumi
(Submitted on 26 Jan 2013)
We examine the general issue of whether a scale dependent cosmological constant can be consistent with general covariance, a problem that arises naturally in the treatment of quantum gravitation where coupling constants generally run as a consequence of renormalization group effects. The issue is approached from several points of view, which include the manifestly covariant functional integral formulation, covariant continuum perturbation theory about two dimensions, the lattice formulation of gravity, and the non-local effective action and effective field equation methods. In all cases we find that the cosmological constant cannot run with scale, unless general covariance is explicitly broken by the regularization procedure. Our results are expected to have some bearing on current quantum gravity calculations, but more generally should apply to phenomenological approaches to the cosmological vacuum energy problem.
34 pages.

http://arxiv.org/abs/1301.6483
Coupling dimers to CDT - conceptual issues
Lisa Glaser
(Submitted on 28 Jan 2013)
Causal dynamical triangulations allows for a non perturbative approach to quantum gravity. In this article a solution for dimers coupled to CDT is presented and some of the conceptual problems that arise are reflected upon.
3 pages. To appear in the Proceedings of the 13th Marcel Grossmann Meeting on General Relativity

brief mention:
http://arxiv.org/abs/1301.6440
The Preon Sector of the SLq(2) (Knot) Model
Robert J. Finkelstein
(Submitted on 28 Jan 2013)
We describe a Lagrangian defining the preon sector of the knot model. The preons are the elements of the fundamental representation of SLq(2). They exactly agree with the preons conjectured by Harari and Shupe. The coupling constants and masses required by this Lagrangian are in principle experimentally measurable...
26 Pages
marcus
#1892
Jan31-13, 11:09 PM
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PF Gold
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P: 23,218
http://arxiv.org/abs/1301.6795
Inhomogenous loop quantum cosmology with matter
Daniel Martín-de Blas, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 28 Jan 2013)
The linearly polarized Gowdy T3 model with a massless scalar field with the same symmetries as the metric is quantized by applying a hybrid approach. The homogeneous geometry degrees of freedom are loop quantized, fact which leads to the resolution of the cosmological singularity, while a Fock quantization is employed for both matter and gravitational inhomogeneities. Owing to the inclusion of the massless scalar field this system allows us to modelize flat Friedmann-Robertson-Walker cosmologies filled with inhomogeneities propagating in one direction. It provides a perfect scenario to study the quantum back-reaction between the inhomogeneities and the polymeric homogeneous and isotropic background.
4 pages, for the proceedings of the Loops 11-Madrid conference.

http://arxiv.org/abs/1301.7466
Report on the session QG4 of the 13th Marcel Grossmann Meeting
Jorge Pullin, Parampreet Singh
(Submitted on 30 Jan 2013)
We summarize the talks presented at the QG4 session (loop quantum gravity: cosmology and black holes) of the 13th Marcel Grossmann Meeting held in Stockholm, Sweden.
5 pages, to appear in the proceedings

http://arxiv.org/abs/1301.7688
Shape Dynamics and Gauge-Gravity Duality
Henrique Gomes, Tim Koslowski
(Submitted on 31 Jan 2013)
The dynamics of gravity can be described by two different systems. The first is the familiar spacetime picture of General Relativity, the other is the conformal picture of Shape Dynamics. We argue that the bulk equivalence of General Relativity and Shape Dynamics is a natural setting to discuss familiar bulk/boundary dualities. We discuss consequences of the Shape Dynamics description of gravity as well as the issue why the bulk equivalence is not explicitly seen in the General Relativity description of gravity.
4 pages, contribution to the 13th Marcel Grossmann Meeting

brief mention:
http://arxiv.org/abs/1301.7750
Quantization maps, algebra representation and non-commutative Fourier transform for Lie groups
Carlos Guedes, Daniele Oriti, Matti Raasakka
(Submitted on 31 Jan 2013)
marcus
#1893
Feb3-13, 07:36 PM
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PF Gold
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http://arxiv.org/abs/1302.0254
The pre-inflationary dynamics of loop quantum cosmology: Confronting quantum gravity with observations
Ivan Agullo, Abhay Ashtekar, William Nelson
(Submitted on 1 Feb 2013)
Using techniques from loop quantum gravity, the standard theory of cosmological perturbations was recently generalized to encompass the Planck era. We now apply this framework to explore pre-inflationary dynamics. The framework enables us to isolate and resolve the true trans-Planckian difficulties, with interesting lessons both for theory and observations. Specifically, for a large class of initial conditions at the bounce, we are led to a self consistent extension of the inflationary paradigm over the 11 orders of magnitude in density and curvature, from the big bounce to the onset of slow roll. In addition, for a narrow window of initial conditions, there are departures from the standard paradigm, with novel effects ---such as a modification of the consistency relation between the ratio of the tensor to scalar power spectrum and the tensor spectral index, as well as a new source for non-Gaussianities--- which could extend the reach of cosmological observations to the deep Planck regime of the early universe.
64 pages, 15 figures

http://arxiv.org/abs/1302.0168
Warm inflation in loop quantum cosmology: a model with a general dissipative coefficient
Xiao-Min Zhang, Jian-Yang Zhu
(Submitted on 1 Feb 2013)
A general form of warm inflation with the dissipative coefficient Γ = Γ0 (φ/φ0)n (T/τ0)m in loop quantum cosmology is studied. In this case, we obtain conditions for the existence of a warm inflationary attractor in the context of loop quantum cosmology by using the method of stability analysis. The two cases when the dissipative coefficient is independent (m=0) and dependent (m≠0) on temperature are analyzed specifically. In the latter case, we use the new power spectrum which should be used when considering temperature dependence in the dissipative coefficient. We find that the thermal effect is enhanced in the case m>0. As in the standard inflation in loop quantum cosmology, we also reach the conclusion that quantum effect leaves a tiny imprint on the cosmic microwave background (CMB) sky.
12 pages, accepted for publication in Phys. Rev. D

http://arxiv.org/abs/1212.5226
Nine-Year Wilkinson Microwave Anisotropy Probe (WMAP) Observations: Cosmological Parameter Results
G. Hinshaw, D. Larson, E. Komatsu, D. N. Spergel, C. L. Bennett, J. Dunkley, M. R. Nolta, M. Halpern, R. S. Hill, N. Odegard, L. Page, K. M. Smith, J. L. Weiland, B. Gold, N. Jarosik, A. Kogut, M. Limon, S. S. Meyer, G. S. Tucker, E. Wollack, E. L. Wright
(Submitted on 20 Dec 2012 (v1), last revised 30 Jan 2013 (this version, v2))
We present cosmological parameter constraints based on the final nine-year WMAP data, in conjunction with additional cosmological data sets. The WMAP data alone, and in combination, continue to be remarkably well fit by a six-parameter LCDM model. When WMAP data are combined with measurements of the high-l CMB anisotropy, the BAO scale, and the Hubble constant, the densities, Ωbh2, Ωch2, and ΩΛ, are each determined to a precision of ~1.5%. The amplitude of the primordial spectrum is measured to within 3%, and there is now evidence for a tilt in the primordial spectrum at the 5σ level, confirming the first detection of tilt based on the five-year WMAP data. At the end of the WMAP mission, the nine-year data decrease the allowable volume of the six-dimensional LCDM parameter space by a factor of 68,000 relative to pre-WMAP measurements. We investigate a number of data combinations and show that their LCDM parameter fits are consistent. New limits on deviations from the six-parameter model are presented, for example: the fractional contribution of tensor modes is limited to r<0.13 (95% CL); the spatial curvature parameter is limited to -0.0027 (+0.0039/-0.0038); the summed mass of neutrinos is Ʃmv< 0.44 eV (95% CL); and the number of relativistic species is found to be 3.84±0.40 when the full data are analyzed. The joint constraint on Neff and the primordial helium abundance agrees with the prediction of standard Big Bang nucleosynthesis. We compare recent PLANCK measurements of the Sunyaev-Zel'dovich effect with our seven-year measurements, and show their mutual agreement. Our analysis of the polarization pattern around temperature extrema is updated. This confirms a fundamental prediction of the standard cosmological model and provides a striking illustration of acoustic oscillations and adiabatic initial conditions in the early universe.
31 pages, 12 figures
For enlightening comment on the latest WMAP estimates see http://resonaances.blogspot.com/2013...os-in-sky.html
marcus
#1894
Feb4-13, 07:35 PM
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PF Gold
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P: 23,218
http://arxiv.org/abs/1302.0724
Death and resurrection of the zeroth principle of thermodynamics
Hal M. Haggard, Carlo Rovelli
(Submitted on 4 Feb 2013)
The zeroth principle of thermodynamics in the form "temperature is uniform at equilibrium" is notoriously violated in relativistic gravity. Temperature uniformity is often derived from the maximization of the total number of microstates of two interacting systems under energy exchanges. Here we discuss a generalized version of this derivation, based on informational notions, which remains valid in the general context. The result is based on the observation that the time taken by any system to move to a distinguishable (nearly orthogonal) quantum state is a universal quantity that depends solely on the temperature. At equilibrium the net information flow between two systems must vanish, and this happens when two systems transit the same number of distinguishable states in the course of their interaction.
5 pages, 2 figures

brief mention:
http://arxiv.org/abs/1302.0451
Macroscopic superpositions and black hole unitarity
Stephen D.H. Hsu
(Submitted on 3 Feb 2013)
We discuss the black hole information problem, including the recent claim that unitarity requires a horizon firewall, emphasizing the role of decoherence and macroscopic superpositions. We consider the formation and evaporation of a large black hole as a quantum amplitude, and note that during intermediate stages (e.g., after the Page time), the amplitude is a superposition of macroscopically distinct (and decohered) spacetimes, with the black hole itself in different positions on different branches. Small but semiclassical observers (who are themselves part of the quantum amplitude) that fall into the hole on one branch will miss it entirely on other branches and instead reach future infinity. This observation can reconcile the subjective experience of an infalling observer with unitarity. We also discuss implications for the nice slice formulation of the information problem, and to complementarity.
3 pages, 1 figure.
MTd2
#1895
Feb6-13, 07:52 PM
PF Gold
P: 1,961
http://arxiv.org/abs/1302.1357

A consistent Horava gravity without extra modes and equivalent to general relativity at the linearized level

J. Bellorin, A. Restuccia, A. Sotomayor
(Submitted on 6 Feb 2013)
We consider a Horava theory that has a consistent structure of constraints and propagates two physical degrees of freedom. The Lagrangian includes the terms of Blas, Pujolas ans Sibiryakov. The theory can be obtained from the general Horava's formulation by setting lambda = 1/3. This value of lambda is protected in the quantum formulation of the theory by the presence of a constraint. The theory has two second-class constraints that are absent for other values of lambda. They remove the extra scalar mode. There is no strong-coupling problem in this theory since there is no extra mode. We perform explicit computations on a model that put together a z=1 term and the IR effective action. We also show that the lowest-order perturbative version of the IR effective theory has a dynamics identical to the one of linearized general relativity. Therefore, this theory is smoothly recovered at the deepest IR without discontinuities in the physical degrees of freedom.
marcus
#1896
Feb6-13, 07:54 PM
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PF Gold
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P: 23,218
http://arxiv.org/abs/1302.1245
Dynamical behaviors of FRW Universe containing a positive/negative potential scalar field in loop quantum cosmology
Xiao Liu, Kui Xiao, Jian-Yang Zhu
(Submitted on 6 Feb 2013)
The dynamical behaviors of FRW Universe containing a posivive/negative potential scalar field in loop quantum cosmology scenario are discussed. The method of the phase-plane analysis is used to investigate the stability of the Universe. It is found that the stability properties in this situation are quite different from the classical cosmology case. For a positive potential scalar field coupled with a barotropic fluid, the cosmological autonomous system has five fixed points and one of them is stable if the adiabatic index γ satisfies 0<γ<2. This leads to the fact that the universe just have one bounce point instead of the singularity which lies in the quantum dominated area and it is caused by the quantum geometry effect. There are four fixed points if one considers a scalar field with a negative potential, but none of them is stable. Therefore, the universe has two kinds of bounce points, one is caused by the quantum geometry effect and the other is caused by the negative potential, the Universe may enter a classical re-collapse after the quantum bounce. This hints that the spatially flat FRW Universe containing a negative potential scalar field is cyclic.
6 pages, 2 figures, accepted for publication in General Relativity and Gravitation

brief mention:
http://arxiv.org/abs/1302.1312
Fixed Functionals in Asymptotically Safe Gravity
Maximilian Demmel, Frank Saueressig, Omar Zanusso
(Submitted on 6 Feb 2013)
We summarize the status of constructing fixed functionals within the f(R)-truncation of Quantum Einstein Gravity in three spacetime dimensions. Focusing on curvatures much larger than the IR-cutoff scale, it is shown that the fixed point equation admits three different scaling regimes: for classical and quantum dominance the equation becomes linear and has power-law solutions, while the balanced case gives rise to a generalized homogeneous equation whose order is reduced by one and whose solutions are non-analytical.
4 pages, to appear in Proceedings of the Thirteenth Marcel Grossman Meeting on General Relativity

http://arxiv.org/abs/1302.1206
Thermality and Heat Content of horizons from infinitesimal coordinate transformations
Bibhas Ranjan Majhi, T. Padmanabhan
(Submitted on 5 Feb 2013)

http://arxiv.org/abs/1302.1498
"The Waters I am Entering No One yet Has Crossed": Alexander Friedman and the Origins of Modern Cosmology
Ari Belenkiy
(Submitted on 6 Feb 2013)
Ninety years ago, in 1922, Alexander Friedman (1888-1925) demonstrated for the first time that the General Relativity equations admit non-static solutions and thus the Universe may expand, contract, collapse, and even be born. The fundamental equations he derived still provide the basis for the current cosmological theories of the Big Bang and the Accelerating Universe. Later, in 1924, he was the first to realize that General Relativity allows the Universe to be infinite. Friedman's ideas initially met strong resistance from Einstein, yet from 1931 he became their staunchest supporter. This essay connects Friedman's cosmological ideas with the 1998-2004 results of the astronomical observations that led to the 2011 Nobel Prize in Physics. It also describes Friedman's little known topological ideas of how to check General Relativity in practice and compares his contributions to those of Georges Lemaitre. Recently discovered corpus of Friedman's writings in the Ehrenfest Archives at Leiden University sheds some new light on the circumstances surrounding his 1922 work and his relations with Paul Ehrenfest.
26 pages, 11 figures. Accepted for publication in the proceedings of the conference "Origins of the Expanding Universe: 1912-1932", M. J. Way & D. Hunter, eds., ASP Conf. Ser., Vol. 471 in press
marcus
#1897
Feb7-13, 07:17 PM
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PF Gold
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http://arxiv.org/abs/1302.1781
Self-Energy in the Lorentzian ERPL-FK Spin Foam Model of Quantum Gravity
Aldo Riello
(Submitted on 7 Feb 2013)
We calculate the most divergent contribution to the self-energy (or "melonic") graph in the context of the Lorentzian EPRL-FK Spin Foam model of Quantum Gravity. We find that such a contribution is logarithmically divergent in the cut-off over the SU(2)-representation spins when one chooses the face amplitude guaranteeing the face-splitting invariance of the foam. We also find that the dependence on the boundary data is different from that of the bare propagator. This fact has its origin in the non-commutativity of the EPRL-FK Y-map with the projector onto SL(2,C)-invariant states. In the course of the paper, we discuss in detail the approximations used during the calculations, its geometrical interpretation as well as the physical consequences of our result.
55 pages, 8 figures

http://arxiv.org/abs/1302.1841
Cosmological Parameters from Pre-Planck CMB Measurements
Erminia Calabrese, Renée A. Hlozek, Nick Battaglia, Elia S. Battistelli, J. Richard Bond, Jens Chluba, Devin Crichton, Sudeep Das, Mark J. Devlin, Joanna Dunkley, Rolando Dünner, Marzieh Farhang, Megan B. Gralla, Amir Hajian, Mark Halpern, Matthew Hasselfield, Adam D. Hincks, Kent D. Irwin, Arthur Kosowsky, Thibaut Louis, Tobias A. Marriage, Kavilan Moodley, Laura Newburgh, Michael D. Niemack, Mike R. Nolta, Lyman A. Page, Neelima Sehgal, Blake D. Sherwin, Jonathan L. Sievers, Cristóbal Sifón, David N. Spergel, Suzanne T. Staggs, Eric R. Switzer, Ed Wollack
(Submitted on 7 Feb 2013)
Recent data from the WMAP, ACT and SPT experiments provide precise measurements of the cosmic microwave background temperature power spectrum over a wide range of angular scales. The combination of these observations is well fit by the standard, spatially flat LCDM cosmological model, constraining six free parameters to within a few percent. The scalar spectral index, ns = 0.9678 ± 0.0088, is less than unity at the 3.6 sigma level, consistent with simple models of inflation. The damping tail of the power spectrum at high resolution, combined with the amplitude of gravitational lensing measured by ACT and SPT, constrains the effective number of relativistic species to be Neff = 3.24 ± 0.39, in agreement with the standard model's three species of light neutrinos.
5 pages, 4 figures
There is a slight inconsistency with the range of Neff given in a similar paper by some of the same people a couple of days ago. See post #1893 about WMAP9 paper http://arxiv.org/abs/1212.5226 . See page 17, and Table 7: Neff = 3.84 ± 0.40 (with all relevant data sets combined).

brief mention:
http://arxiv.org/abs/1302.1617
What if Planck's Universe isn't flat?
Philip Bull, Marc Kamionkowski
(Submitted on 6 Feb 2013)

http://arxiv.org/abs/1302.1860
On cosmic hair and "de Sitter breaking" in linearized quantum gravity
Ian A. Morrison
(Submitted on 7 Feb 2013)
marcus
#1898
Feb11-13, 07:18 PM
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PF Gold
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P: 23,218
http://arxiv.org/abs/1302.2173
Quantum Gravity via Causal Dynamical Triangulations
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 8 Feb 2013)
"Causal Dynamical Triangulations" (CDT) represent a lattice regularization of the sum over spacetime histories, providing us with a non-perturbative formulation of quantum gravity. The ultraviolet fixed points of the lattice theory can be used to define a continuum quantum field theory, potentially making contact with quantum gravity defined via asymptotic safety. We describe the formalism of CDT, its phase diagram, and the quantum geometries emerging from it. We also argue that the formalism should be able to describe a more general class of quantum-gravitational models of Horava-Lifgarbagez type.
31 pages. To appear in "Handbook of Spacetime", Springer Verlag.

http://arxiv.org/abs/1302.2181
Quantum Spacetime, from a Practitioner's Point of View
J. Ambjorn, S. Jordan, J. Jurkiewicz, R. Loll
(Submitted on 9 Feb 2013)
We argue that theories of quantum gravity constructed with the help of (Causal) Dynamical Triangulations have given us the most informative, quantitative models to date of quantum spacetime. Most importantly, these are derived dynamically from nonperturbative and background-independent quantum theories of geometry. In the physically relevant case of four spacetime dimensions, the ansatz of Causal Dynamical Triangulations produces - from a fairly minimal set of quantum field-theoretic inputs - an emergent spacetime which macroscopically looks like a de Sitter universe, and on Planckian scales possesses unexpected quantum properties. Important in deriving these results are a regularized version of the theory, in which the quantum dynamics is well defined, can be studied with the help of numerical Monte Carlo methods and extrapolated to infinite lattice volumes.
7 pages, 5 figures, submission to Multicosmofun '12, Szczecin.

http://arxiv.org/abs/1302.2210
The transfer matrix method in four-dimensional causal dynamical triangulations
J. Ambjorn, J. Gizbert-Studnicki, A.T. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 9 Feb 2013)
The Causal Dynamical Triangulation model of quantum gravity (CDT) is a proposition to evaluate the path integral over space-time geometries using a lattice regularization with a discrete proper time and geometries realized as simplicial manifolds. The model admits a Wick rotation to imaginary time for each space-time configuration. Using computer simulations we determined the phase structure of the model and discovered that it predicts a de Sitter phase with a four-dimensional spherical semi-classical background geometry. The model 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 labelled 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.
6 pages, 6 figures, contribution to the MULTIVERSE conference, Szczecin, Poland, September 2012

brief mention:
http://arxiv.org/abs/1302.2440
Universality of 2d causal dynamical triangulations
J. Ambjorn, A. Ipsen
(Submitted on 11 Feb 2013)
The formalism of Causal Dynamical Triangulations (CDT) attempts to provide a non-perturbative regularization of quantum gravity, viewed as an ordinary quantum field theory. In two dimensions one can solve the lattice theory analytically and the continuum limit is universal, not depending on the details of the lattice regularization.
11 pages

http://arxiv.org/abs/1302.2285
Quantum Gravity: Meaning and Measurement
John Stachel, Kaća Bradonjić
(Submitted on 10 Feb 2013)
A discussion of the meaning of a physical concept cannot be separated from discussion of the conditions for its ideal measurement. We assert that quantization is no more than the invocation of the quantum of action in the explanation of some process or phenomenon, and does not imply an assertion of the fundamental nature of such a process. This leads to an ecumenical approach to the problem of quantization of the gravitational field. There can be many valid approaches,.. We advocate an approach to general relativity based on the unimodular group, which emphasizes the physical significance and measurability of the conformal and projective structures. ...
24 pages; Submitted to Studies in the History and Philosophy of Modern Physics special Quantum Gravity issue
John86
#1899
Feb12-13, 05:37 AM
P: 247
http://arxiv.org/abs/1302.2151
Lanczos-Lovelock models of gravity
T. Padmanabhan, D. Kothawala
(Submitted on 8 Feb 2013)
Lanczos-Lovelock models of gravity represent a natural and elegant generalization of Einstein's theory of gravity to higher dimensions. They are characterized by the fact that the field equations only contain up to second derivatives of the metric even though the action functional can be a quadratic or higher degree polynomial in the curvature tensor. Because these models share several key properties of Einstein's theory they serve as a useful set of candidate models for testing the emergent paradigm for gravity. This review highlights several geometrical and thermodynamical aspects of Lanczos-Lovelock models which have attracted recent attention.

http://arxiv.org/abs/1302.2336
Constraints of NonCommutative Spectral Action from Gravity Probe B
Gaetano Lambiase, Mairi Sakellariadou, Antonio Stabile
(Submitted on 10 Feb 2013)
Noncommutative spectral geometry offers a purely geometric explanation for the standard model of particle physics, including a geometric explanation for the origin of the Higgs field. Within this framework, gravity together with the electroweak and the strong forces are all described as purely gravitational forces on a unified noncommutative spacetime. In this letter, we infer a constraint on the parameter characterising the coupling constants at unification, by linearising the field equations in the limit of weak gravitational fields generated by a rotating gravitational source and by making use of the recent experimental data obtained by Gravity Probe B. We find a lower bound on the Weyl term appearing in the noncommutative spectral action, namely \beta > 1/ (10^6 m), which is much stronger than any limit imposed so far to curvature squared terms.

http://arxiv.org/abs/1302.2383
Surface gravities for non-Killing horizons
Bethan Cropp (SISSA/INFN), Stefano Liberati (SISSA/INFN), Matt Visser (Victoria University of Wellington)
(Submitted on 11 Feb 2013)
There are many logically and computationally distinct characterizations of the surface gravity of a horizon, just as there are many logically rather distinct notions of horizon. Fortunately, in standard general relativity, for stationary horizons, most of these characterizations are degenerate. However, in modified gravity, or in analogue spacetimes, horizons may be non-Killing or even non-null, and hence these degeneracies can be lifted. We present a brief overview of the key issues, specifically focusing on horizons in analogue spacetimes and universal horizons in modified gravity.

http://arxiv.org/abs/1302.2613
Nonviolent information transfer from black holes: a field theory parameterization
Steven B. Giddings
(Submitted on 11 Feb 2013)
A candidate parameterization is introduced, in an effective field theory framework, for the quantum information transfer from a black hole that is necessary to restore unitarity. This in particular allows description of the effects of such information transfer in the black hole atmosphere, for example seen by infalling observers. In the presence of such information transfer, it is shown that infalling observers need not experience untoward violence. Moreover, the presence of general moderate-frequency couplings to field modes with high angular momenta offers a mechanism to enhance information transfer rates, commensurate with the increased energy flux, when a string is introduced to "mine" a black hole. Generic such models for nonviolent information transfer predict extra energy flux from a black hole, beyond that of Hawking.
marcus
#1900
Feb12-13, 07:38 PM
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http://arxiv.org/abs/1302.2810
Four-dimensional Causal Dynamical Triangulations and an effective transfer matrix
Andrzej Görlich
(Submitted on 12 Feb 2013)
Causal Dynamical Triangulations is a background independent approach to quantum gravity. We show that there exists an effective transfer matrix labeled by the scale factor which properly describes the evolution of the quantum universe. In this framework no degrees of freedom are frozen, but, the obtained effective action agrees with the minisuperspace model.
Comments: To appear in the Proceedings of the 13th Marcel Grossmann Meeting on General
John86
#1901
Feb13-13, 04:35 AM
P: 247
http://arxiv.org/abs/1302.2849
Disappearance and emergence of space and time in quantum gravity
Daniele Oriti
(Submitted on 12 Feb 2013)
We discuss the hints for the disappearance of continuum space and time at microscopic scale. These include arguments for a discrete nature of them or for a fundamental non-locality, in a quantum theory of gravity. We discuss how these ideas are realized in specific quantum gravity approaches. Turning then the problem around, we consider the emergence of continuum space and time from the collective behaviour of discrete, pre-geometric atoms of quantum space, and for understanding spacetime as a kind of "condensate", and we present the case for this emergence process being the result of a phase transition, dubbed "geometrogenesis". We discuss some conceptual issues of this scenario and of the idea of emergent spacetime in general. As a concrete example, we outline the GFT framework for quantum gravity, and illustrate a tentative procedure for the emergence of spacetime in this framework. Last, we re-examine the conceptual issues raised by the emergent spacetime scenario in light of this concrete example.

http://arxiv.org/abs/1302.2850
The universal path integral
Seth Lloyd, Olaf Dreyer
(Submitted on 12 Feb 2013)
Path integrals represent a powerful route to quantization: they calculate probabilities by summing over classical configurations of variables such as fields, assigning each configuration a phase equal to the action of that configuration. This paper defines a universal path integral, which sums over all computable structures. This path integral contains as sub-integrals all possible computable path integrals, including those of field theory, the standard model of elementary particles, discrete models of quantum gravity, string theory, etc. The universal path integral possesses a well-defined measure that guarantees its finiteness, together with a method for extracting probabilities for observable quantities. The universal path integral supports a quantum theory of the universe in which the world that we see around us arises out of the interference between all computable structures.

http://arxiv.org/abs/1302.2687
Massive gravity as a limit of bimetric gravity
Prado Martin-Moruno (Victoria University of Wellington), Valentina Baccetti (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 12 Feb 2013)
Massive gravity may be viewed as a suitable limit of bimetric gravity. The limiting procedure can lead to an interesting interplay between the "background" and "foreground" metrics in a cosmological context. The fact that in bimetric theories one always has two sets of metric equations of motion continues to have an effect even in the massive gravity limit. Thus, solutions of bimetric gravity in the limit of vanishing kinetic term are also solutions of massive gravity, but the contrary statement is not necessarily true.

http://arxiv.org/abs/1302.2731
Quantum correlations which imply causation
Joseph Fitzsimons, Jonathan Jones, Vlatko Vedral
(Submitted on 12 Feb 2013)
In ordinary, non-relativistic, quantum physics, time enters only as a parameter and not as an observable: a state of a physical system is specified at a given time and then evolved according to the prescribed dynamics. While the state can, and usually does, extend across all space, it is only defined at one instant of time, in conflict with special relativity where space and time are treated on an equal footing. Here we ask what would happen if we defined the notion of the quantum density matrix for multiple spatial and temporal measurements. We introduce the concept of a pseudo-density matrix which treats space and time indiscriminately. This matrix in general fails to be positive for timelike separated measurements, motivating us to define a measure of causality that discriminates between spacelike and timelike correlations. Important properties of this measure, such as monotonicity under local operations, are proved. Two qubit NMR experiments are presented that illustrate how a temporal pseudo-density matrix approaches a genuinely allowed density matrix as the amount of decoherence is increased between two consecutive measurements.
John86
#1902
Feb14-13, 03:06 AM
P: 247
http://arxiv.org/abs/1302.2928
Modulated Ground State of Gravity Theories with Stabilized Conformal Factor
Alfio Bonanno, Martin Reuter
(Submitted on 12 Feb 2013)
We discuss the stabilization of the conformal factor by higher derivative terms in a conformally reduced $R+R^2$ Euclidean gravity theory. The flat spacetime is unstable towards the condensation of modes with nonzero momentum, and they "condense" in a modulated phase above a critical value of the coupling $\beta$ of the $R^2$ term. By employing a combination of variational, numerical and lattice methods we show that in the semiclassical limit the corresponding functional integral is dominated by a single nonlinear plane wave of frequency $\approx 1/\sqrt{\beta} \lp$. We argue that the ground state of the theory is characterized by a spontaneous breaking of translational invariance at Planckian scales.
marcus
#1903
Feb14-13, 07:22 PM
Astronomy
Sci Advisor
PF Gold
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P: 23,218
http://arxiv.org/abs/1302.3226
Solution to the cosmological constant problem
T. Padmanabhan, Hamsa Padmanabhan
(Submitted on 13 Feb 2013)
The current, accelerated, phase of expansion of our universe can be modeled in terms of a cosmological constant. A key issue in theoretical physics is to explain the extremely small value of the dimensionless parameter Λ LP2 ~ 3.4 x 10-122, where LP is the Planck length. We show that this value can be understood in terms of a new dimensionless parameter N, which counts the number of modes inside a Hubble volume crossing the Hubble radius, from the end of inflation until the beginning of the accelerating phase. Theoretical considerations suggest that N = 4π. On the other hand, N is related to ln(ΛLP2) and two other parameters which will be determined by high energy particle physics: (a) the ratio between the number densities of photons and matter and (b) the energy scale of inflation. For realistic values of (nγ/nm) ~ 4.3 x 1010 and Einf ~ 1015 GeV, our postulate N =4π leads to the observed value of the cosmological constant. This provides a unified picture of cosmic evolution relating the early inflationary phase to the late accelerating phase.
15 pages; 2 figures
MTd2
#1904
Feb14-13, 08:52 PM
PF Gold
P: 1,961
http://arxiv.org/abs/1302.3406

Spontaneous Lorentz Violation in Gauge Theories

A. P. Balachandran, S. Vaidya
(Submitted on 14 Feb 2013)
Frohlich, Morchio and Strocchi long ago proved that Lorentz invariance is spontaneously broken in QED because of infrared effects. We develop a simple model where consequences of this breakdown can be explicitly and easily calculated. For this purpose, the superselected U(1) charge group of QED is extended to a superselected "Sky" group containing direction-dependent gauge transformations at infinity. It is the analog of the Spi group of gravity. As Lorentz transformations do not commute with Sky, they are spontaneously broken. These abelian considerations and model are extended to non-Abelian gauge symmetries. Basic issues regarding the observability of twisted non-Abelian gauge symmetries and of the asymptotic ADM symmetries of quantum gravity are raised.
marcus
#1905
Feb17-13, 07:27 PM
Astronomy
Sci Advisor
PF Gold
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P: 23,218
http://arxiv.org/abs/1302.3833
Loop Quantum Cosmology
Ivan Agullo, Alejandro Corichi
(Submitted on 15 Feb 2013)
This Chapter provides an up to date, pedagogical review of some of the most relevant advances in loop quantum cosmology. We review the quantization of homogeneous cosmological models, their singularity resolution and the formulation of effective equations that incorporate the main quantum corrections to the dynamics. We also summarize the theory of quantized metric perturbations propagating in those quantum backgrounds. Finally, we describe how this framework can be applied to obtain a self-consistent extension of the inflationary scenario to incorporate quantum aspects of gravity, and to explore possible phenomenological consequences.
52 pages, 5 figures. To appear as a Chapter of "The Springer Handbook of Spacetime," edited by A. Ashtekar and V. Petkov. (Springer-Verlag, at Press).
John86
#1906
Feb18-13, 06:18 AM
P: 247
http://arxiv.org/abs/1302.1496
Standard Model Higgs field and energy scale of gravity
F.R. Klinkhamer
(Submitted on 6 Feb 2013 (v1), last revised 14 Feb 2013 (this version, v3))
The effective potential of the Higgs scalar field in the Standard Model may have a second degenerate minimum at an ultrahigh vacuum expectation value. This second minimum then determines, by radiative corrections, the values of the top-quark and Higgs-boson masses at the standard minimum corresponding to the electroweak energy scale. An argument is presented that this ultrahigh vacuum expectation value is proportional to the energy scale of gravity, E_{Planck} \equiv \sqrt{\hbar c^5/G_N}, considered to be characteristic of a spacetime foam. In the context of a simple model, the existence of kink-type wormhole solutions places a lower bound on the ultrahigh vacuum expectation value and this lower bound is of the order of E_{Planck}.

http://arxiv.org/abs/1302.3680
Quantum Gravity on a Quantum Computer?
Achim Kempf
(Submitted on 15 Feb 2013)
EPR-type measurements on spatially separated entangled spin qubits allow one, in principle, to detect curvature. Also the entanglement of the vacuum state is affected by curvature. Here, we ask if the curvature of spacetime can be expressed entirely in terms of the spatial entanglement structure of the vacuum. This would open up the prospect that quantum gravity could be simulated on a quantum computer and that quantum information techniques could be fully employed in the study of quantum gravity.

http://arxiv.org/abs/1302.3648
Causality and non-equilibrium second-order phase transitions in inhomogeneous systems
A. del Campo, T. W. B. Kibble, W. H. Zurek
(Submitted on 14 Feb 2013)
When a second-order phase transition is crossed at fine rate, the evolution of the system stops being adiabatic as a result of the critical slowing down in the neighborhood of the critical point. In systems with a topologically nontrivial vacuum manifold, disparate local choices of the ground state lead to the formation of topological defects. The universality class of the transition imprints a signature on the resulting density of topological defects: It obeys a power law in the quench rate, with an exponent dictated by a combination of the critical exponents of the transition. In inhomogeneous systems the situation is more complicated, as the spontaneous symmetry breaking competes with bias caused by the influence of the nearby regions that already chose the new vacuum. As a result, the choice of the broken symmetry vacuum may be inherited from the neighboring regions that have already entered the new phase. This competition between the inherited and spontaneous symmetry breaking enhances the role of causality, as the defect formation is restricted to a fraction of the system where the front velocity surpasses the relevant sound velocity and phase transition remains effectively homogeneous. As a consequence, the overall number of topological defects can be substantially suppressed. When the fraction of the system is small, the resulting total number of defects is still given by a power law related to the universality class of the transition, but exhibits a more pronounced dependence on the quench rate. This enhanced dependence complicates the analysis but may also facilitate experimental test of defect formation theories.
marcus
#1907
Feb21-13, 07:47 PM
Astronomy
Sci Advisor
PF Gold
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P: 23,218
http://arxiv.org/abs/1302.5265
The loop quantum gravity black hole
Rodolfo Gambini, Jorge Pullin
(Submitted on 21 Feb 2013)
We quantize spherically symmetric vacuum gravity without gauge fixing the diffeomorphism constraint. Through a rescaling, we make the algebra of Hamiltonian constraints Abelian and therefore the constraint algebra is a true Lie algebra. This allows the completion of the Dirac quantization procedure using loop quantum gravity techniques. We can construct explicitly the exact solutions of the physical Hilbert space annihilated by all constraints. New observables living in the bulk appear at the quantum level (analogous to spin in quantum mechanics) that are not present at the classical level and are associated with the discrete nature of the spin network states of loop quantum gravity. The resulting quantum space-times resolve the singularity present in the classical theory inside black holes. The new observables that arise suggest a possible resolution for the "firewall" problem of evaporating black holes.
Comments: 4 pages,
MTd2
#1908
Feb22-13, 08:48 AM
PF Gold
P: 1,961
http://arxiv.org/abs/1302.5273

There exist no 4-dimensional geodesically equivalent metrics with the same stress-energy tensor

Volodymir Kiosak, Vladimir S. Matveev
(Submitted on 21 Feb 2013)
We show that if two 4-dimensional metrics of arbitrary signature on one manifold are geodesically equivalent (i.e., have the same geodesics considered as unparameterized curves) and are solutions of the Einstein field equation with the same stress-energy tensor, then they are affinely equivalent or flat. Under the additional assumption that the metrics are complete or the manifold is closed, the result survives in all dimensions >2.

http://arxiv.org/abs/1302.5162

On CCC-predicted concentric low-variance circles in the CMB sky

V. G. Gurzadyan, R. Penrose
(Submitted on 21 Feb 2013)
A new analysis of the CMB, using WMAP data, supports earlier indications of non-Gaussian features of concentric circles of low temperature variance. Conformal cyclic cosmology (CCC) predicts such features from supermassive black-hole encounters in an aeon preceding our Big Bang. The significance of individual low-variance circles in the true data has been disputed; yet a recent independent analysis has confirmed CCC's expectation that CMB circles have a non-Gaussian temperature distribution. Here we examine concentric sets of low-variance circular rings in the WMAP data, finding a highly non-isotropic distribution. A new "sky-twist" procedure, directly analysing WMAP data, without appeal to simulations, shows that the prevalence of these concentric sets depends on the rings being circular, rather than even slightly elliptical, numbers dropping off dramatically with increasing ellipticity. This is consistent with CCC's expectations; so also is the crucial fact that whereas some of the rings' radii are found to reach around $15^\circ$, none exceed $20^\circ$. The non-isotropic distribution of the concentric sets may be linked to previously known anomalous and non-Gaussian CMB features.


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