Loop-and-allied QG bibliography

[marcus]

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 SL_q(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 SL_q(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]

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]

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, Ω_bh², Ω_ch², and Ω_Λ, 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 Ʃm_v< 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 N_eff 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/2013/01/how-many-neutrinos-in-sky.html

 

[marcus]

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]

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]

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]

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, n_s = 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 N_eff = 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 N_eff 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: N_eff = 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]

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]

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]

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]

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]

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]

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 L_P² ~ 3.4 x 10^-122, where L_P 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(ΛL_P²) 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 (n_γ/n_m) ~ 4.3 x 10¹⁰ and E_inf ~ 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]

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]

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]

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]

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]

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.

 

[marcus]

http://arxiv.org/abs/1302.5695
Quantum matter in quantum space-time
Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, Casey Tomlin
(Submitted on 22 Feb 2013)
Quantum matter in quantum space-time is discussed using general properties of energy-conservation laws. As a rather radical conclusion, it is found that standard methods of differential geometry and quantum field theory on curved space-time are inapplicable in canonical quantum gravity, even at the level of effective equations.
16 pages

http://arxiv.org/abs/1302.5564
Spin-cube Models of Quantum Gravity
A. Mikovic
(Submitted on 22 Feb 2013)
We study the state-sum models of quantum gravity based on a representation 2-category of the Poincare 2-group. We call them spin-cube models, since they are categorical generalizations of spin-foam models. A spin-cube state sum can be considered as a path integral for a constrained 2-BF theory, and depending on how the constraints are imposed, a spin-cube state sum can be reduced to a path integral for the area-Regge model with the edge-length constraints, or to a path integral for the Regge model. We also show that the effective actions for these spin-cube models have the correct classical limit.
16 pages

brief mention (Shapo is always interesting):
http://arxiv.org/abs/1302.5619
Spontaneously Broken Conformal Symmetry: Dealing with the Trace Anomaly
Roberta Armillis, Alexander Monin, Mikhail Shaposhnikov
(Submitted on 22 Feb 2013)
The majority of renormalizable field theories possessing the scale invariance at the classical level exhibits the trace anomaly once quantum corrections are taken into account. This leads to the breaking of scale and conformal invariance. At the same time any realistic theory must contain gravity and is thus non-renormalizable. We show that discarding the renormalizability it is possible to construct viable models allowing to preserve the scale invariance at the quantum level. We present explicit one-loop computations for two toy models to demonstrate the main idea of the approach. Constructing the renormalized energy momentum tensor we show that it is traceless, meaning that the conformal invariance is also preserved.
20 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1302.6264
The Solution to the Problem of Time in Shape Dynamics
Julian Barbour, Tim Koslowski, Flavio Mercati
(Submitted on 25 Feb 2013)
The absence of unique time evolution in Einstein's spacetime description of gravity leads to the hitherto unresolved 'problem of time' in quantum gravity. Shape Dynamics is an objectively equivalent representation of gravity that trades spacetime refoliation invariance for three-dimensional conformal invariance. Its logical completion presented here gives a dimensionless description of gravitational dynamics. We show that in this framework the classical problem of time is completely solved. Since a comparable definitive solution is impossible within the spacetime description, we believe Shape Dynamics provides a key ingredient for the creation of quantum gravity.
14 pages

http://arxiv.org/abs/1302.6566
Singularity resolution from polymer quantum matter
Andreas Kreienbuehl, Tomasz Pawlowski
(Submitted on 26 Feb 2013)
We study the polymeric nature of quantum matter fields using the example of a Friedmann-Lemaitre-Robertson-Walker universe sourced by a minimally coupled massless scalar field. The model is treated in the symmetry reduced regime via deparametrization techniques, with the scale factor playing the role of time. Subsequently the remaining dynamic degrees of freedom are polymer quantized. The analysis of the resulting dynamic shows that the big bang singularity is resolved, although with the form of the resolution differing significantly from that of the models with matter clocks: dynamically, the singularity is made passable rather than avoided. Furthermore, the results of the genuine quantum analysis expose crucial limitations to the so-called effective dynamics in loop quantum cosmology when applied outside of the simplest isotropic settings.
12 pages and 4 figures

 

[marcus]

http://arxiv.org/abs/1302.7142
Holonomy Operator and Quantization Ambiguities on Spinor Space
Etera R. Livine
(Submitted on 28 Feb 2013)
We construct the holonomy-flux operator algebra in the recently developed spinor formulation of loop gravity. We show that, when restricting to SU(2)-gauge invariant operators, the familiar grasping and Wilson loop operators are written as composite operators built from the gauge-invariant 'generalized ladder operators' recently introduced in the U(N) approach to intertwiners and spin networks. We comment on quantization ambiguities that appear in the definition of the holonomy operator and use these ambiguities as a toy model to test a class of quantization ambiguities which is present in the standard regularization and definition of the Hamiltonian constraint operator in loop quantum gravity.
14 pages

http://arxiv.org/abs/1302.7135
Fields and Laplacians on Quantum Geometries
Johannes Thürigen
(Submitted on 28 Feb 2013)
In fundamentally discrete approaches to quantum gravity such as loop quantum gravity, spin-foam models, group field theories or Regge calculus observables are functions on discrete geometries. We present a bra-ket formalism of function spaces and discrete calculus on abstract simplicial complexes equipped with geometry and apply it to the mentioned theories of quantum gravity. In particular we focus on the quantum geometric Laplacian and discuss as an example the expectation value of the heat kernel trace from which the spectral dimension follows.
3 pages, submitted to the Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, July 1-7, 2012

http://arxiv.org/abs/1302.7037
Loop Quantization of Shape Dynamics
Tim Koslowski
(Submitted on 28 Feb 2013)
Loop Quantum Gravity (LQG) is a promising approach to quantum gravity, in particular because it is based on a rigorous quantization of the kinematics of gravity. A difficult and still open problem in the LQG program is the construction of the physical Hilbert space for pure quantum gravity. This is due to the complicated nature of the Hamilton constraints. The Shape Dynamics description of General Relativity (GR) replaces the Hamilton constraints with spatial Weyl constraints, so the problem of finding the physical Hilbert space reduces to the problem of quantizing the Weyl constraints. Unfortunately, it turns out that a loop quantization of Weyl constraints is far from trivial despite their intuitive physical interpretation. A tentative quantization proposal and interpretation proposal is given in this contribution.
3 pages, talk given at the 13th Marcel Grossmann Meeting, Stockholm 1-7 July 2012

I don't normally list online seminar talks but these might be of particular interest to people following QG research:
http://pirsa.org/13020132/
Quantum Gravity as Random Geometry
Vincent Rivasseau
Abstract: Matrix models, random maps and Liouville field theory are prime tools which connect random geometry and quantum gravity in two dimensions. The tensor track is a new program to extend this connection to higher dimensions through the corresponding notions of tensor models, colored triangulations and tensor group field theories.
27/02/2013

http://pirsa.org/13020146/
The universe as a process of unique events
Lee Smolin
26/02/2013
(Covers material from a new book "Time Reborn" listed on Amazon to appear April 2013)

also of interest, possibly related:
http://arxiv.org/abs/1302.7291
General Relativity and Quantum Cosmology
The arrow of time and the nature of spacetime
George F R Ellis
(Submitted on 28 Feb 2013)
This paper extends the work of a previous paper [arXiv:1108.5261] on top-down causation and quantum physics, to consider the origin of the arrow of time. It proposes that a `past condition' cascades down from cosmological to micro scales, being realized in many microstructures and setting the arrow of time at the quantum level by top-down causation. This physics arrow of time then propagates up, through underlying emergence of higher level structures, to geology, astronomy, engineering, and biology. The appropriate space-time picture to view all this is an emergent block universe (`EBU'), that recognizes the way the present is different from both the past and the future. This essential difference is the ultimate reason the arrow of time has to be the way it is.
56 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1303.0195
Living in Curved Momentum Space
J. Kowalski-Glikman
(Submitted on 1 Mar 2013)
In this paper we review some aspects of relativistic particles' mechanics in the case of a non-trivial geometry of momentum space. We start with showing how the curved momentum space arises in the theory of gravity in 2+1 dimensions coupled to particles, when (topological) degrees of freedom of gravity are solved for. We argue that there might exist a similar topological phase of quantum gravity in 3+1 dimensions. Then we characterize the main properties of the theory of interacting particles with curved momentum space and the symmetries of the action. We discuss the spacetime picture and the emergence of the principle of relative locality, according to which locality of events is not absolute but becomes observer dependent, in the controllable, relativistic way. We conclude with the detailed review of the most studied kappa-Poincare framework, which corresponds to the de Sitter momentum space.
23 pages

http://arxiv.org/abs/1303.0196
Inhomogeneous Universe in Loop Quantum Gravity
Francesco Cianfrani
(Submitted on 1 Mar 2013)
It is discussed a truncation of the kinematical Hilbert space of Loop Quantum Gravity, which describes the dynamical system associated with an inhomogeneous cosmological model.
3 pages, contribution to the Proceedings of the 13th Marcel Grossman Meeting (Stockholm, Sweden, July 1-7 2012)

http://arxiv.org/abs/1303.0060
Differences and similarities between Shape Dynamics and General Relativity
Henrique Gomes, Tim Koslowski
(Submitted on 1 Mar 2013)
The purpose of this contribution is to elucidate some of the properties of Shape Dynamics (SD) and is largely based on a recent longer article. We shall point out some of the key differences between SD and related theoretical constructions, illustrate the central mechanism of symmetry trading in electromagnetism and finally point out some new quantization strategies inspired by SD. We refrain from describing mathematical detail and from citing literature. For both we refer to the longer article.
3 pages, talk given at the 13th Marcel Grossmann Meeting in Stockholm, July 2012

http://arxiv.org/abs/1303.0174
MG13 Proceedings: A lattice Universe as a toy-model for inhomogeneous cosmology
Jean-Philippe Bruneton
(Submitted on 1 Mar 2013)
We briefly report on a previously found new, approximate, solution to Einstein field equations, describing a cubic lattice of spherical masses. This model mimics in a satisfactory way a Universe which can be strongly inhomogeneous at small scales, but quite homogeneous at large ones. As a consequence of field equations, the lattice Universe is found to expand or contract in the same way as the solution of a Friedmann Universe filled with dust having the same average density. The study of observables indicates however the possible existence of a fitting problem, i.e. the fact that the Friedmann model obtained from past-lightcone observables does not match with the one obtained by smoothing the matter content of the Universe.
3 pages. Prepared for MG13 conference

 

[John86]

http://arxiv.org/abs/1303.0762
A new perspective on early cosmology
Emanuele Alesci
(Submitted on 4 Mar 2013)
We present a new perspective on early cosmology based on Loop Quantum Gravity. We use projected spinnetworks, coherent states and spinfoam techniques, to implement a quantum reduction of the full Kinematical Hilbert space of LQG, suitable to describe inhomogeneous cosmological models. Some preliminary results on the solutions of the Scalar constraint of the reduced theory are also presented.

http://arxiv.org/abs/1303.0433
A Dynamics for Discrete Quantum Gravity
Stan Gudder
(Submitted on 2 Mar 2013)
This paper is based on the causal set approach to discrete quantum gravity. We first describe a classical sequential growth process (CSGP) in which the universe grows one element at a time in discrete steps. At each step the process has the form of a causal set (causet) and the "completed" universe is given by a path through a discretely growing chain of causets. We then quantize the CSGP by forming a Hilbert space $H$ on the set of paths. The quantum dynamics is governed by a sequence of positive operators $\rho_n$ on $H$ that satisfy normalization and consistency conditions. The pair $(H,\brac{\rho_n})$ is called a quantum sequential growth process (QSGP). We next discuss a concrete realization of a QSGP in terms of a natural quantum action. This gives an amplitude process related to the sum over histories" approach to quantum mechanics. Finally, we briefly discuss a discrete form of Einstein's field equation and speculate how this may be employed to compare the present framework with classical general relativity theory.

 

[marcus]

http://arxiv.org/abs/1303.0752
Inclusion of matter in inhomogeneous loop quantum cosmology
Daniel Martín-de Blas, Mercedes Martín-Benito, Guillermo A. Mena Marugán
(Submitted on 4 Mar 2013)
We study the hybrid quantization of the linearly polarized Gowdy $T^3$ model with a massless scalar field with the same symmetries as the metric. For simplicity, we quantize its restriction to the model with local rotational symmetry. Using this hybrid approach, the homogeneous degrees of freedom of the geometry are quantized \`a la loop, leading to the resolution of the cosmological singularity. A Fock quantization is employed both for the matter and the 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, providing a perfect scenario to study the quantum back-reaction of the inhomogeneities on the polymeric homogeneous and isotropic background.
4 pages

 

[Leucippus]

I told you I would make several false starts. Eventually there should be a non-technical description of loop gravity in only one to ten pages. Let's keep this thread going until we have one, or find one in the literature.

Hi, sorry for the interruption, I quoted the above from post #17 of this thread.

I'm just wondering if a non-technical discussion of loop gravity in only one to ten pages has ever been achieved?

I would love to have a nice concise and refined synopsis of the main ideas of LQG along with perhaps a well-organized outline of what types of topics are most required.

My specific interests in LQG is to try to understand how and why it is 'background independent", and also any information that might be known on how it might relate to the entropy associated with the horizon area of a black hole. Or actually any information on how a spin network itself relates to the concept of entropy.

Thanks.

 

[marcus]

Hi Leucippus, this thread started out with discussion (in 2003) but evolved into a Loop and allied QG bibliography. We can start separate discussion threads. You have some good questions so I gathered excerpts (including from this latest) and started a new thread. I hope the new one will prove satisfactory.
https://www.physicsforums.com/showthread.php?p=4298277&posted=1#post4298277

 

[marcus]

http://arxiv.org/abs/1303.1687
Quantum states of the bouncing universe
Jean Pierre Gazeau, Jakub Mielczarek, Wlodzimierz Piechocki
(Submitted on 7 Mar 2013)
In this paper we study quantum dynamics of the bouncing cosmological model. We focus on the model of the flat Friedman-Robertson-Walker universe with a free scalar field. The bouncing behavior, which replaces classical singularity, appears due to the modification of general relativity along the methods of loop quantum cosmology. We show that there exist a unitary transformation that enables to describe the system as a free particle with Hamiltonian equal to canonical momentum. We examine properties of the various quantum states of the Universe: boxcar state, standard coherent state, and soliton-like state, as well as Schrödinger's cat states constructed from these states. Characteristics of the states such as quantum moments and Wigner functions are investigated. We show that each of these states have, for some range of parameters, a proper semiclassical limit fulfilling the correspondence principle. Decoherence of the superposition of two universes is described and possible interpretations in terms of triad orientation and Belinsky-Khalatnikov-Lifgarbagez conjecture are given. Some interesting features regarding the area of the negative part of the Wigner function have emerged.
18 pages, 19 figures

brief mention:
http://arxiv.org/abs/1303.1535
The Structure of the Gravitational Action and its relation with Horizon Thermodynamics and Emergent Gravity Paradigm
Krishnamohan Parattu, Bibhas Ranjan Majhi, T. Padmanabhan
(Submitted on 6 Mar 2013)

http://arxiv.org/abs/1303.1782
Non-Associative Geometry and the Spectral Action Principle
Shane Farnsworth, Latham Boyle
(Submitted on 7 Mar 2013)
Chamseddine and Connes have shown how the action for Einstein gravity, coupled to the SU(3) × SU(2) × U(1) standard model of particle physics, may be elegantly recast as the "spectral action" on a certain "non-commutative geometry." In this paper, we show how this formalism may be extended to "non-associative geometries," and explain the motivations for doing so. As a guiding illustration, we present the simplest non-associative geometry (based on the octonions) and evaluate its spectral action: it describes Einstein gravity coupled to a G₂ gauge theory, with 8 Dirac fermions (which transform as a singlet and a septuplet under G₂). We use this example to illustrate how non-associative geometries may be naturally linked to ordinary (associative) geometries by a certain twisting procedure. This is just the simplest example: in a forthcoming paper we show how to construct realistic models that include Higgs fields, spontaneous symmetry breaking and fermion masses.

 

[John86]

Don't miss these

http://arxiv.org/abs/1303.1537
On the theory of composition in physics
Lucien Hardy
(Submitted on 6 Mar 2013)
We develop a theory for describing composite objects in physics. These can be static objects, such as tables, or things that happen in spacetime (such as a region of spacetime with fields on it regarded as being composed of smaller such regions joined together). We propose certain fundamental axioms which, it seems, should be satisfied in any theory of composition. A key axiom is the order independence axiom which says we can describe the composition of a composite object in any order. Then we provide a notation for describing composite objects that naturally leads to these axioms being satisfied. In any given physical context we are interested in the value of certain properties for the objects (such as whether the object is possible, what probability it has, how wide it is, and so on). We associate a generalized state with an object. This can be used to calculate the value of those properties we are interested in for for this object. We then propose a certain principle, the composition principle, which says that we can determine the generalized state of a composite object from the generalized states for the components by means of a calculation having the same structure as the description of the generalized state. The composition principle provides a link between description and prediction.

http://arxiv.org/abs/1303.1538
Reconstructing quantum theory
Lucien Hardy
(Submitted on 6 Mar 2013)
We discuss how to reconstruct quantum theory from operational postulates. In particular, the following postulates are consistent only with for classical probability theory and quantum theory. Logical Sharpness: There is a one-to-one map between pure states and maximal effects such that we get unit probability. This maximal effect does not give probability equal to one for any other pure state. Information Locality: A maximal measurement is effected on a composite system if we perform maximal measurements on each of the components. Tomographic Locality: The state of a composite system can be determined from the statistics collected by making measurements on the components. Permutability: There exists a reversible transformation on any system effecting any given permutation of any given maximal set of distinguishable states for that system. Sturdiness: Filters are non-flattening. To single out quantum theory we need only add any requirement that is inconsistent with classical probability theory and consistent with quantum theory.

http://arxiv.org/abs/1303.1632
Higgs potential and confinement in Yang-Mills theory on exotic R^4
Torsten Asselmeyer-Maluga, Jerzy Król
(Submitted on 7 Mar 2013)
We show that pure SU(2) Yang-Mills theory formulated on certain exotic R^4 from the radial family shows confinement. The condensation of magnetic monopoles and the qualitative form of the Higgs potential are derived from the exotic R^4, e. A relation between the Higgs potential and inflation is discussed. Then we obtain a formula for the Higgs mass and discuss a particular smoothness structure so that the Higgs mass agrees with the experimental value. The singularity in the effective dual U(1) potential has its cause by the exotic 4-geometry and agrees with the singularity in the maximal abelian gauge scenario. We will describe the Yang-Mills theory on e in some limit as the abelian-projected effective gauge theory on the standard R^4. Similar results can be derived for SU(3) Yang-Mills theory on an exotic R^4 provided dual diagonal effective gauge bosons propagate in the exotic 4-geometry.

http://arxiv.org/abs/1303.1803
Classifying gauge anomalies through SPT orders and classifying anomalies through topological orders
Xiao-Gang Wen
(Submitted on 7 Mar 2013)
In this paper, we systematically study gauge anomalies in bosonic and fermionic weak-coupling gauge theories with gauge group G (which can be continuous or discrete). We argue that, in d space-time dimensions, the gauge anomalies are described by the elements in Free[H^{d+1}(G,R/Z)]\oplus H_\pi^{d+1}(BG,R/Z). The well known Adler-Bell-Jackiw anomalies are classified by the free part of the group cohomology class H^{d+1}(G,R/Z) of the gauge group G (denoted as Free[H^{d+1}(G,\R/\Z)]). We refer other kinds of gauge anomalies beyond Adler-Bell-Jackiw anomalies as nonABJ gauge anomalies, which include Witten SU(2) global gauge anomaly. We introduce a notion of \pi-cohomology group, H_\pi^{d+1}(BG,R/Z), for the classifying space BG, which is an Abelian group and include Tor[H^{d+1}(G,R/Z)] and topological cohomology group H^{d+1}(BG,\R/\Z) as subgroups. We argue that H_\pi^{d+1}(BG,R/Z) classifies the bosonic nonABJ gauge anomalies, and partially classifies fermionic nonABJ anomalies. We also show a very close relation between gauge anomalies and symmetry-protected trivial (SPT) orders [also known as symmetry-protected topological (SPT) orders] in one-higher dimensions. Such a connection will allow us to use many well known results and well developed methods for gauge anomalies to study SPT states. In particular, the \pi-cohomology theory may give a more general description of SPT states than the group cohomology theory.

http://arxiv.org/abs/1212.4863
Boundary Degeneracy of Topological Order
Juven Wang, Xiao-Gang Wen
(Submitted on 19 Dec 2012 (v1), last revised 23 Jan 2013 (this version, v2))
We introduce the notion of boundary degeneracy of topologically ordered states on a compact orientable spatial manifold with boundaries, and emphasize that it provides richer information than the bulk degeneracy. Beyond the bulk-edge correspondence, we find the ground state degeneracy of fully gapped edge states depends on boundary gapping conditions. We develop a quantitative description of different types of boundary gapping conditions by viewing them as different ways of non-fractionalized particle condensation on the boundary. Via Chern-Simons theory, this allows us to derive the ground state degeneracy formula in terms of boundary gapping conditions, which reveals more than the fusion algebra of fractionalized quasiparticles. We apply our results to Toric code and Levin-Wen string-net models. By measuring the boundary degeneracy on a cylinder, we predict Z_k gauge theory and U(1)_k x U(1)_k non-chiral fractional quantum hall state at even integer k can be experimentally distinguished. Our work refines definitions of symmetry protected topological order and intrinsic topological order.

 

[marcus]

http://arxiv.org/abs/1303.2773
BTZ Black Hole Entropy in Loop Quantum Gravity and in Spin Foam Models
J.Manuel Garcia-Islas
(Submitted on 12 Mar 2013)
We present a comparison of the calculation of BTZ black hole entropy in loop quantum gravity and in spin foam models. We see that both give the same answer.
6 pages, 3 figures

brief mention:
http://arxiv.org/abs/1303.2719
Another Survey of Foundational Attitudes Towards Quantum Mechanics
Christoph Sommer
(Submitted on 11 Mar 2013)
Although it has been almost 100 years since the beginnings of quantum mechanics, the discussions about its interpretation still do not cease. Therefore, a survey of opinions regarding this matter is of particular interest. This poll was conducted following an idea and using the methodology of Schlosshauer et al. (arXiv:1301.1069 [quant-ph]), but among a slightly different group. It is supposed to give another snapshot of attitudes towards the interpretation of quantum mechanics and keep discourse about this topic alive.
10 pages, 18 figures, 1 table

 

[tom.stoer]

Intuitive content of Loop Gravity--Rovelli's program

There is one problem with these results, namely that some people may agree that they believe in interpretation x, but that a careful analysis may also show, that they do NOT agree what this interpretation x MEANS. So without such an analysis one always misses the fact that people agreeing on Copenhagen do unfortunately not agree on the same Copenhagen :-)

 

[John86]

http://arxiv.org/abs/1303.3576
Cosmology from Group Field Theory
Steffen Gielen, Daniele Oriti, Lorenzo Sindoni
(Submitted on 14 Mar 2013)
We identify a class of condensate states in the group field theory (GFT) approach to quantum gravity that can be interpreted as macroscopic homogeneous spatial geometries. We then extract the dynamics of such condensate states directly from the fundamental quantum GFT dynamics, following the procedure used in ordinary quantum fluids. The effective dynamics is a non-linear and non-local extension of quantum cosmology. We also show that any GFT model with a kinetic term of Laplacian type gives rise, in a semi-classical (WKB) approximation and in the isotropic case, to a modified Friedmann equation. This is the first concrete, general procedure for extracting an effective cosmological dynamics directly from a fundamental theory of quantum geometry.

http://arxiv.org/abs/1303.3497
The DeWitt Equation in Quantum Field Theory
Parikgarbage Dutta, Krzysztof A. Meissner, Hermann Nicolai
(Submitted on 14 Mar 2013)
We take a new look at the DeWitt equation, a defining equation for the effective action functional in quantum field theory. We present a formal solution to this equation, and discuss the equation in various contexts, and in particular for models where it can be made completely well defined, such as the Wess-Zumino model in two dimensions.

 

[marcus]

http://arxiv.org/abs/1303.4636
Spin foams
Jonathan Engle
(Submitted on 19 Mar 2013)
The spin foam framework provides a way to define the dynamics of canonical loop quantum gravity in a spacetime covariant way, by using a path integral over histories of quantum states which can be interpreted as 'quantum space-times'. This chapter provides a basic introduction to spin foams aimed principally at beginning graduate students and, where possible, at broader audiences.
32 pages, 14 figures, 2 tables, to appear as a chapter of "The Springer Handbook of Spacetime," edited by A. Ashtekar and V. Petkov (Springer-Verlag 2013)

http://arxiv.org/abs/1303.4294
Constraint analysis for variational discrete systems
Bianca Dittrich, Philipp A Hoehn
(Submitted on 18 Mar 2013)
A canonical formalism and constraint analysis for discrete systems subject to a variational action principle are devised. The formalism is equivalent to the covariant formulation, encompasses global and local discrete time evolution moves and naturally incorporates both constant and evolving phase spaces, the latter of which is necessary for a time varying discretization. The different roles of constraints in the discrete and the conditions under which they are first or second class and/or symmetry generators are clarified. The (non-) preservation of constraints and the symplectic structure is discussed; on evolving phase spaces the number of constraints at a fixed time step depends on the initial and final time step of evolution. Moreover, the definition of observables and a reduced phase space is provided; again, on evolving phase spaces the notion of an observable as a propagating degree of freedom requires specification of an initial and final step and crucially depends on this choice, in contrast to the continuum. However, upon restriction to translation invariant systems, one regains the usual time step independence of canonical concepts. These results are applicable, e.g., to discrete mechanics, lattice field theory, quantum gravity models and numerical analysis.
48 pages, many figures

brief mention:
http://arxiv.org/abs/1303.3935
Quantum mechanics from invariance laws
Florin Moldoveanu
(Submitted on 16 Mar 2013)

 

[marcus]

http://arxiv.org/abs/1303.4752
Imaginary action, spinfoam asymptotics and the 'transplanckian' regime of loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
(Submitted on 19 Mar 2013)
It was recently noted that the on-shell Einstein-Hilbert action with York-Gibbons-Hawking boundary term has an imaginary part, proportional to the area of the codimension-2 surfaces on which the boundary normal becomes null. We extend this result to first-order formulations of gravity, by generalizing a previously proposed boundary term to closed boundaries. As a side effect, we settle the issue of the Holst modification vs. the Nieh-Yan density by demanding a well-defined variational principle. We then set out to find the imaginary action in the large-spin 4-simplex limit of the Lorentzian EPRL/FK spinfoam. It turns out that the spinfoam's effective action indeed has the correct imaginary part, but only if the Barbero-Immirzi parameter is set to +/- i after the quantum calculation. An interpretation and a connection to other recent results is discussed. In particular, we propose that the large-spin limit of loop quantum gravity can be viewed as a high-energy 'transplanckian' regime.
22 pages, 5 figures

http://arxiv.org/abs/1303.4989
Loop Quantum Gravity and the The Planck Regime of Cosmology
Abhay Ashtekar
(Submitted on 20 Mar 2013)
The very early universe provides the best arena we currently have to test quantum gravity theories. The success of the inflationary paradigm in accounting for the observed inhomogeneities in the cosmic microwave background already illustrates this point to a certain extent because the paradigm is based on quantum field theory on the curved cosmological space-times. However, this analysis excludes the Planck era because the background space-time satisfies Einstein's equations all the way back to the big bang singularity. Using techniques from loop quantum gravity, the paradigm has now been extended to a self-consistent theory from the Planck regime to the onset of inflation, covering some 11 orders of magnitude in curvature. 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 involving the scalar and tensor power spectra and a new source for non-Gaussianities. Thus, the genesis of the large scale structure of the universe can be traced back to quantum gravity fluctuations in the Planck regime. This report provides a bird's eye view of these developments for the general relativity community.
23 pages, 4 figures. Plenary talk at the Conference: Relativity and Gravitation: 100 Years after Einstein in Prague. To appear in the Proceedings to be published by Edition Open Access. Summarizes results that appeared in journal articles [2-13]

http://arxiv.org/abs/1303.4829
Thermodynamics of Quantum Isolated Horizon in Canonical Ensemble
Abhishek Majhi
(Submitted on 20 Mar 2013)
The derivation of the microcanonical entropy of a Quantum Isolated Horizon(QIH), directly from the associated quantum Chern-Simons(CS) theory, in terms of the two parameters characterizing the QIH, namely the level of the associated CS theory (k) and the total number of punctures of the QIH (N), is reviewed. With a view to extend the thermodynamic analysis to the Canonical Ensemble a model energy spectrum for the QIH is proposed from a complete quantum view point using the available elements of Loop Quantum Gravity (LQG). The study of thermodynamics of QIH in the Canonical Ensemble reveals that, in the LQG framework, for a quantum spacetime admitting a thermodynamically stable QIH as its internal boundary, must have a specific bound on the Barbero-Immirzi(BI) parameter if the entropy of the QIH obeys the Bekenstein-Hawking Area Law. To mention, the complete analysis, especially the used energy spectrum of the QIH, has been performed from a complete quantum viewpoint.
23 pages

http://arxiv.org/abs/1303.4832
A Model Hamiltonian for Quantum Isolated Horizon
Abhishek Majhi
(Submitted on 20 Mar 2013)
With a view to clarify some apparent inconsistencies in the classical and quantum theories of Isolated Horizon and motivated by the structure of the area operator in loop quantum gravity, a model Hamiltonian operator for the quantum Isolated Horizon is proposed. Known results of Isolated Horizon thermodynamics are used as inputs to fix the model. The proposal of the model is based on the facts that the Hamiltonian operator and the area operator associated with the quantum Isolated Horizon should have simultaneous eigenstates and in the correspondence limit one must obtain that the area of a classical Isolated Horizon is constant.
9 pages

 

[marcus]

http://arxiv.org/abs/1303.5612
A Gravitational Entropy Proposal
Timothy Clifton, George F R Ellis, Reza Tavakol
(Submitted on 22 Mar 2013)
We propose a thermodynamically motivated measure of gravitational entropy based on the Bel-Robinson tensor, which has a natural interpretation as the effective super-energy-momentum tensor of free gravitational fields. The specific form of this measure differs depending on whether the gravitational field is Coulomb-like or wave-like, and reduces to the Bekenstein-Hawking value when integrated over the interior of a Schwarzschild black hole. For scalar perturbations of a Robertson-Walker geometry we find that the entropy goes like the Hubble weighted anisotropy of the gravitational field, and therefore increases as structure formation occurs. This is in keeping with our expectations for the behaviour of gravitational entropy in cosmology, and provides a thermodynamically motivated arrow of time for cosmological solutions of Einstein's field equations. It is also in keeping with Penrose's Weyl curvature hypothesis.
17 pages

 

[marcus]

http://arxiv.org/abs/1303.6157
Loop quantum dynamics of the gravitational collapse
Yaser Tavakoli, Joao Marto, Andrea Dapor
(Submitted on 25 Mar 2013)
We consider a quantum description for a spherically symmetric gravitational collapse of a massless scalar field. The effective scenario from loop quantum gravity is applied to a homogeneous interior spacetime. Classical singularity that arises at the final stage of our collapsing system, is resolved and replaced by a quantum bounce. Our main purpose is to investigate the evolution of trapped surfaces during the collapse in semiclassical regime. We show that, in this regime, there exists a threshold scale bellow which no horizon can form as collapse evolves towards the bounce. By employing the matching conditions at the boundary shell, quantum effects are carried out to the exterior region, leading to an improved Vaidya geometry. In addition, the effective mass loss emerging in this model predicts an outward energy flux from the interior quantum geometry regime.
11 pages, 5 figures