Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/0908.2874
Solar system tests of Hořava-Lifgarbagez gravity
Tiberiu Harko, Zoltan Kovács, Francisco S. N. Lobo
9 pages, 4 figures
(Submitted on 20 Aug 2009)
"Recently, a renormalizable gravity theory with higher spatial derivatives in four dimensions was proposed by Horava. The theory reduces to Einstein gravity with a non-vanishing cosmological constant in IR, but it has improved UV behaviors. The spherically symmetric black hole solutions for an arbitrary cosmological constant, which represent the generalization of the standard Schwarzschild-(A)dS solution, has also been obtained for the Horava-Lifgarbagez theory. The exact asymptotically flat Schwarzschild type solution of the gravitational field equations in Horava gravity contains a quadratic increasing term, as well as the square root of a fourth order polynomial in the radial coordinate, and it depends on one arbitrary integration constant. The IR modified Horava gravity seems to be consistent with the current observational data, but in order to test its viability more observational constraints are necessary. In the present paper we consider the possibility of observationally testing Horava gravity at the scale of the Solar System, by considering the classical tests of general relativity (perihelion precession of the planet Mercury, deflection of light by the Sun and the radar echo delay) for the spherically symmetric black hole solution of Horava-Lifgarbagez gravity. All these gravitational effects can be fully explained in the framework of the vacuum solution of the gravity. Moreover, the study of the classical general relativistic tests also constrain the free parameter of the solution."

 

[atyy]

http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.103.081301
Nicolai, Vanquishing infinity: "Accordingly, we should view the coexistence of several possibly finite candidate theories only as a first step towards the future construction of an underlying theory of quantum gravity, where classical space and time are only emergent concepts, and which would also be viable nonperturbatively."

 

[MTd2]

http://arxiv.org/abs/0908.3388

A note on the Plebanski action with cosmological constant and an Immirzi parameter

Lee Smolin, Simone Speziale
(Submitted on 24 Aug 2009)
We study the field equations of the Plebanski action for general relativity when both the cosmological constant and an Immirzi parameter are present. We show that the Lagrange multiplier, which usually gets identified with the Weyl curvature, now acquires a trace part. Some consequences of this for a class of modified gravity theories recently proposed in the literature are briefly discussed.

 

[atyy]

http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.103.091301
Nastase, Pushing the envelope of general relativity: "A recent theory of gravity has stimulated intense debate and many explorations of its implications. A Viewpoint on: Solutions to Hořava Gravity, H. Lü, Jianwei Mei, and C. N. Pope, Phys. Rev. Lett. 103, 091301 (2009)"

 

[MTd2]

http://arxiv.org/abs/0908.3643

On the spectral dimension of causal triangulations

Bergfinnur Durhuus, Thordur Jonsson, John F. Wheater
(Submitted on 25 Aug 2009)
We introduce an ensemble of infinite causal triangulations, called the uniform infinite causal triangulation, and show that it is equivalent to an ensemble of infinite trees, the uniform infinite planar tree. It is proved that in both cases the Hausdorff dimension almost surely equals 2. The infinite causal triangulations are shown to be almost surely recurrent or, equivalently, their spectral dimension is almost surely less than or equal to 2. We also establish that for certain reduced versions of the infinite causal triangulations the spectral dimension equals 2 both for the ensemble average and almost surely. The triangulation ensemble we consider is equivalent to the causal dynamical triangulation model of two-dimensional quantum gravity and therefore our results apply to that model.

 

[John86]

Don't forget these two ones

http://arxiv.org/abs/0908.3408
Entangled quantum states in a local deterministic theory
Authors: Gerard 't Hooft
(Submitted on 24 Aug 2009)

Abstract: Investigating a class of models that is familiar in studies of cellular automata, we find that quantum operators can be employed to describe their long distance behavior. These operators span a Hilbert space that appears to turn such a model into a genuine quantum field theory, obeying the usual conditions of locality in terms of its quantum commutators. Entangled states can be constructed exactly as in quantum theories.
This raises the question whether such models allow Bell's inequalities to be violated. Being a local, deterministic theory, one would argue that this is impossible, but since at large distance scales the model does not seem to differ from real quantum field theories, there is reason to wonder why it should not allow entangled states. The standard arguments concerning Bell's inequalities are re-examined in this light.

http://arxiv.org/abs/0908.3398
On the Electromagnetic Properties of Matter in Collapse Models
Authors: Angelo Bassi, Detlef Duerr
(Submitted on 24 Aug 2009)

Abstract: We discuss the electromagnetic properties of both a charged free particle, and a charged particle bounded by an harmonic potential, within collapse models. By choosing a particularly simple, yet physically relevant, collapse model, and under only the dipole approximation, we are able to solve the equation of motion exactly. In this way, both the finite time and large time behavior can be analyzed accurately. We discovered new features, which did not appear in previous works on the same subject. Since, so far, the spontaneous photon emission process places the strongest upper bounds on the collapse parameters, our results call for a further analysis of this process for those atomic systems which can be employed in experimental tests of collapse models.

 

[marcus]

http://arxiv.org/abs/0908.3683
Early Universe models from Noncommutative Geometry
Matilde Marcolli (Caltech), Elena Pierpaoli (USC)
49 pages, 26 figures
(Submitted on 25 Aug 2009)
"We investigate cosmological predictions on the early universe based on the noncommutative geometry models of gravity coupled to matter. Using the renormalization group analysis for the Standard Model with right handed neutrinos and Majorana mass terms, which is the particle physics content of the most recent noncommutative geometry models, we analyze the behavior of the coefficients of the gravitational and cosmological terms in the Lagrangian derived from the asymptotic expansion of the spectral action functional of noncommutative geometry. We find emergent Hoyle-Narlikar and conformal gravity at the see-saw scales and a running effective gravitational constant, which affects the propagation of gravitational waves and the evaporation law of primordial black holes and provides Linde models of negative gravity in the early universe. The same renormalization group analysis also governs the running of the effective cosmological constant of the model. The model also provides a Higgs based slow-roll inflationary mechanism, for which one can explicitly compute the slow-roll parameters. The particle physics content allows for dark matter models based on sterile neutrinos with Majorana mass terms."

 

[John86]

http://arxiv.org/abs/0908.4069
Decoherence, measurement and interpretation of quantum mechanics
Authors: Juan Sebastián Ardenghi, Mario Castagnino, Sebastian Fortin, Olimpia Lombardi
(Submitted on 27 Aug 2009)
Abstract: According to our modal-Hamiltonian interpretation (MHI) of quantum mechanics, the Hamiltonian of the closed system defines the set of its definite-valued observables. This definition seems to be incompatible with the pointer basis selected by the environment-induced decoherence (EID) of the open system. In this paper we argue that decoherence can be understood from a closed system perspective which (i) shows that the incompatibility between MHI and EID is only apparent, and (ii) solves certain conceptual challenges that the EID program still has to face.

 

[John86]

http://arxiv.org/abs/0908.2809
Emergent Gravity from Quantized Spacetime
Authors: Hyun Seok Yang, M. Sivakumar
(Submitted on 20 Aug 2009)
Abstract: We examine the picture of emergent gravity arising from a mass deformed matrix model. Due to the mass deformation, a vacuum geometry turns out to be a constant curvature spacetime such as d-dimensional sphere and (anti-)de Sitter spaces. We show that the mass deformed matrix model giving rise to the constant curvature spacetime can be derived from the d-dimensional Snyder algebra. The emergent gravity beautifully confirms all the rationale inferred from the algebraic point of view that the d-dimensional Snyder algebra is equivalent to the Lorentz algebra in (d+1)-dimensional {\it flat} spacetime. For example, a vacuum geometry of the mass deformed matrix model is completely described by a G-invariant metric of coset manifolds G/H defined by the Snyder algebra. We also discuss a nonlinear deformation of the Snyder algebra.

http://arxiv.org/abs/0908.3683
Early Universe models from Noncommutative Geometry
Authors: Matilde Marcolli (Caltech), Elena Pierpaoli (USC)
(Submitted on 25 Aug 2009)
Abstract: We investigate cosmological predictions on the early universe based on the noncommutative geometry models of gravity coupled to matter. Using the renormalization group analysis for the Standard Model with right handed neutrinos and Majorana mass terms, which is the particle physics content of the most recent noncommutative geometry models, we analyze the behavior of the coefficients of the gravitational and cosmological terms in the Lagrangian derived from the asymptotic expansion of the spectral action functional of noncommutative geometry. We find emergent Hoyle-Narlikar and conformal gravity at the see-saw scales and a running effective gravitational constant, which affects the propagation of gravitational waves and the evaporation law of primordial black holes and provides Linde models of negative gravity in the early universe. The same renormalization group analysis also governs the running of the effective cosmological constant of the model. The model also provides a Higgs based slow-roll inflationary mechanism, for which one can explicitly compute the slow-roll parameters. The particle physics content allows for dark matter models based on sterile neutrinos with Majorana mass terms.

http://arxiv.org/abs/0908.4029
Turning big bang into big bounce: Quantum dynamics
Authors: Przemyslaw Malkiewicz, Wlodzimierz Piechocki
(Submitted on 27 Aug 2009)
Abstract: We analyze the big bounce (BB) transition of the quantum FRW model in the setting of the non-standard loop quantum cosmology (LQC). Elementary observables are used to quantize compound observables. The spectrum of the energy density operator is bounded and continuous. The spectrum of the volume operator is bounded from below and discrete. Both standard and non-standard LQC methods have a free parameter that should be fixed in some way to specify the BB transition.

 

[marcus]

http://arxiv.org/abs/0908.4224
Stochastic quantization and the role of time in quantum gravity
J. Ambjorn, R. Loll, W. Westra, S. Zohren
15 pages
(Submitted on 28 Aug 2009)
We show that the noncritical string field theory developed from two-dimensional quantum gravity in the framework of causal dynamical triangulations can be viewed as arising through a stochastic quantization. This requires that the proper time appearing in the string field theory be identified with the stochastic time of the stochastic formulation. The framework of stochastic quantization gives rise to a natural nonperturbative quantum Hamiltonian, which incorporates a sum over all spacetime topologies. We point out that the external character of stochastic time is a feature that pertains more generally to the proper time or distance appearing in nonperturbative correlation functions in quantum gravity."

 

[MTd2]

http://arxiv.org/abs/0908.4196

Towards a Noncommutative Astrophysics

We consider astrophysical objects such as main-sequence stars, white-dwarfs and neutron stars in a noncommutative context. Noncommutativity is implemented via a deformed dispersion relation $$E^{2}=p^{2}c^{2}(1+\lambda E)^{2}+m^{2}c^{4}$$ from which we obtain noncommutative corrections to the pressure, particle number and energy densities for radiation and for a degenerate fermion gas. The main implications of noncommutativity for the considered astrophysical objects are examined and discussed.

 

[marcus]

http://arxiv.org/abs/0908.4329
The Observational Implications of Loop Quantum Cosmology
Jakub Mielczarek
11 pages, 7 figures
(Submitted on 29 Aug 2009)
"In this paper we consider realistic model of inflation embedded in the framework of loop quantum cosmology. Phase of inflation is preceded here by the phase of a quantum bounce. We show how parameters of inflation depend on the initial conditions established in the contracting, pre-bounce phase. Our investigations indicate that phase of the bounce easily sets proper initial conditions for the inflation. Subsequently we study observational effects that might arise due to the quantum gravitational modifications. We perform preliminary observational constraints for the Barbero-Immirzi parameter $$\gamma$$, critical density $$\rho_{\text{c}}$$ and parameter $$\lambda$$. In the next step we study effects on power spectrum of perturbations. We calculate spectrum of perturbations from the bounce and from the joined bounce+inflation phase. Based on these studies we indicate possible way to relate quantum cosmological models with the astronomical observations. Using the Sachs-Wolfe approximation we calculate spectrum of the super-horizontal CMB anisotropies. We show that quantum cosmological effects can, in the natural way, explain suppression of the low CMB multipoles. We show that fine-tuning is not required here and model is consistent with observations. We also analyse other possible probes of the quantum cosmologies and discuss perspectives of their implementation."

 

[MTd2]

http://arxiv.org/abs/0908.4476
Sub-leading asymptotic behaviour of area correlations in the Barrett-Crane model
J. Daniel Christensen, Igor Khavkine, Etera R. Livine, Simone Speziale

The Barrett-Crane spin foam model for quantum gravity provides an excellent setting for testing analytical and numerical tools used to probe spinfoam models. Here, we complete the report on the numerical analysis of the single 4-simplex area correlations begun in Phys. Lett. B670 (2009) 403-406, discussing the next-to-leading order corrections ("one-loop" corrections) with particular attention to their measure dependence, and the difference between the Gaussian and Bessel ansatze for the boundary state.

 

[marcus]

http://arxiv.org/abs/0909.0169
Kerr Black Holes as Particle Accelerators to Arbitrarily High Energy
Máximo Bañados, Joseph Silk, Stephen M. West
4 pages, 3 figures, accepted for publication in Phys.Rev.Lett
(Submitted on 1 Sep 2009)
"We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high centre of mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics."

 

[marcus]

http://arxiv.org/abs/0909.0334
Observational constraints on a power spectrum from super-inflation in Loop Quantum Cosmology
Masahiro Shimano, Tomohiro Harada
17 pages, 3 figures
(Submitted on 2 Sep 2009)
"In loop quantum cosmology there may be a super-inflation phase in the very early universe, in which a single scalar field with a negative power-law potential $$V= -M^4(\phi/M)^\beta$$ plays important roles. Since the effective horizon $$\sqrt{SD}/H$$ controls the behavior of quantum fluctuation instead of the usual Hubble horizon, we assume the following inflation scenario; the super-inflation starts when the quantum state of the scalar field emerges into the classical regime,and ends when the effective horizon becomes the Hubble horizon, and the effective horizon scale never gets shorter than the Planck length. From consistency with the WMAP 5-year data, we place a constraint on the parameters of the potential (beta and M) and the energy density at the end of the super-inflation, depending on the volume correction parameter n."

http://arxiv.org/abs/0909.0459
New variables for 1+1 dimensional gravity
Rodolfo Gambini, Jorge Pullin, Saeed Rastgoo
8 pages
(Submitted on 2 Sep 2009)
"We show that the canonical formulation of a generic action for 1+1-dimensional models of gravity coupled to matter admits a description in terms of Ashtekar-type variables. This includes the CGHS model and spherically symmetric reductions of 3+1 gravity as particular cases. This opens the possibility of discussing models of black hole evaporation using loop representation techniques and verifying which paradigm emerges for the possible elimination of the black hole singularity and the issue of information loss."

 

[MTd2]

http://arxiv.org/abs/0909.0456

Interacting Dark Energy in Hořava-Lifgarbagez Cosmology

M R Setare
(Submitted on 2 Sep 2009)
Motivated by the recent work of Saridakis \cite{sari}, we generalize this work to the non-interacting case. Using an additional canonical scalar field, we formulate Ho\v{r}ava-Lifgarbagez cosmology with an effective interacting dark energy sector.

It turns out that the scalar "bug" in Horava gravity might become a very powerful predictive tool. More than General Relativity. Check the conclusion.

 

[marcus]

http://arxiv.org/abs/0909.0551
Division algebras and supersymmetry
John C. Baez, John Huerta
15 pages
(Submitted on 2 Sep 2009)
"Supersymmetry is deeply related to division algebras. Nonabelian Yang--Mills fields minimally coupled to massless spinors are supersymmetric if and only if the dimension of spacetime is 3, 4, 6 or 10. The same is true for the Green--Schwarz superstring. In both cases, supersymmetry relies on the vanishing of a certain trilinear expression involving a spinor field. The reason for this, in turn, is the existence of normed division algebras in dimensions 1, 2, 4 and 8: the real numbers, complex numbers, quaternions and octonions. Here we provide a self-contained account of how this works."

 

[marcus]

http://arxiv.org/abs/0909.0939
Spin-Foams for All Loop Quantum Gravity
Wojciech Kamiński, Marcin Kisielowski, Jerzy Lewandowski
23 pages, 8 figures
(Submitted on 4 Sep 2009)
"The simplicial framework of spin-foam models is generalized to match the diffeomorphism invariant framework of loop quantum gravity. The simplicial spin-foams are generalized to arbitrary linear 2-cell spin-foams. The resulting framework admits all the spin-network states of loop quantum gravity, not only those defined by triangulations (or cubulations). The notion of embedded spin-foam we introduce allows to consider knotting or linking spin-foam histories. The main tools are successfully generalized: the spin-foam vertex structure, the vertex amplitude, the Barrett-Crane as well as Engle-Pereira-Rovelli-Livine intertwiners. The correspondence between all the SU(2) intertwiners and the SU(2) x SU(2) EPRL intertwiners is proved to be 1-1 in the case of the Barbero-Immirzi parameter $$|\gamma|\ge 1$$."

 

[John86]

this paper sounds interesting. Zurek looking in on condensed matter

http://arxiv.org/abs/0909.0761
Soliton creation during a Bose-Einstein condensation
Authors: Bogdan Damski, Wojciech H. Zurek
(Submitted on 3 Sep 2009)
Abstract: We study the process of soliton creation during a non-equilibrium second order phase transition. We investigate a simple model (stochastic Gross-Pitaevskii equation) that simulates many aspects of the normal gas -- Bose-Einstein condensate transition. We show that the quench leads to creation of solitons, whose density follows a Kibble-Zurek-like scaling law involving critical exponents.

 

[MTd2]

http://arxiv.org/abs/0909.0944
The Feynman propagator on a causal set

Steven Johnston
(Submitted on 4 Sep 2009)
The Feynman propagator for a free bosonic scalar field on the discrete spacetime of a causal set is presented. The formalism includes scalar field operators and a vacuum state which are first steps towards scalar quantum field theory on a causal set. This work can be viewed as a novel regularisation of quantum field theory based on a Lorentz invariant discretisation of spacet

 

[John86]

http://arxiv.org/abs/0909.1044
Osmotic pressure of matter and vacuum energy
Authors: G.E. Volovik
(Submitted on 5 Sep 2009)
Abstract: The walls of the box which contains matter represent a membrane that allows the relativistic quantum vacuum to pass but not matter. That is why the pressure of matter in the box may be considered as the analog of the osmotic pressure. However, we demonstrate that the osmotic pressure of matter is modified due to interaction of matter with vacuum. This interaction induces the nonzero negative vacuum pressure inside the box, as a result the measured osmotic pressure becomes smaller than the matter pressure. As distinct from the Casimir effect, this induced vacuum pressure is the bulk effect and does not depend on the size of the box. This effect dominates in the thermodynamic limit of the infinite volume of the box. Analog of this effect has been observed in the dilute solution of 3He in liquid 4He, where the superfluid 4He plays the role of the non-relativistic quantum vacuum, and 3He atoms play the role of matter.

http://arxiv.org/abs/0909.0749
Average observational quantities in the timescape cosmology
Authors: David L. Wiltshire
(Submitted on 4 Sep 2009)
Abstract: We examine the properties of a recently proposed observationally viable alternative to homogeneous cosmology with smooth dark energy, the timescape cosmology. In the timescape model cosmic acceleration is realized as an apparent effect related to the calibration of clocks and rods of observers in bound systems relative to volume-average observers in an inhomogeneous geometry in ordinary general relativity. The model is based on an exact solution to a Buchert average of the Einstein equations with backreaction. The present paper examines a number of observational tests which will enable the timescape model to be distinguished from homogeneous cosmologies with a cosmological constant or other smooth dark energy, in current and future generations of dark energy experiments. Predictions are presented for: comoving distance measures; H(z); the equivalent of the dark energy equation of state, w(z); the Om(z) measure of Sahni, Shafieloo and Starobinsky; the Alcock-Paczynski test; the baryon acoustic oscillation measure, D_v; the inhomogeneity test of Clarkson, Bassett and Lu; and the time drift of cosmological redshifts. Where possible, the predictions are compared to recent independent studies of similar measures in homogeneous cosmologies with dark energy. Three separate tests with indications of results in possible tension with the Lambda CDM model are found to be consistent with the expectations of the timescape cosmology.

 

[John86]

http://arxiv.org/abs/0909.0160
Lorentz violation and black-hole thermodynamics: Compton scattering process
Authors: E. Kant, F.R. Klinkhamer, M. Schreck
(Submitted on 1 Sep 2009 (v1), last revised 7 Sep 2009 (this version, v2))
Abstract: A Lorentz-noninvariant modification of quantum electrodynamics is considered, which has photons described by the nonbirefringent sector of modified Maxwell theory and electrons described by the standard Dirac theory. These photons and electrons are taken to propagate and interact in a Schwarzschild spacetime background. For appropriate Lorentz-violating parameters, the photons have an effective horizon lying outside the Schwarzschild horizon. A particular type of Compton scattering event, taking place between these two horizons (in the photonic ergoregion) and ultimately decreasing the mass of the black hole, is found to have a nonzero probability. These events perhaps allow for a violation of the generalized second law of thermodynamics in the Lorentz-noninvariant theory considered.

 

[marcus]

http://arxiv.org/abs/0909.1882
A Summary of the asymptotic analysis for the EPRL amplitude
John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Henrique Gomes, Frank Hellmann
8 pages, Proceedings for Planck Scale 2009, talk given by Henrique Gomes
(Submitted on 10 Sep 2009)
"We review the basic steps in building the asymptotic analysis of the Euclidean sector of new spin foam models using coherent states, for Immirzi parameter less than one. We focus on conceptual issues and by so doing omit peripheral proofs and the original discussion on spin structures."

http://arxiv.org/abs/0909.2027
Asymptotic analysis of the Ponzano-Regge model for handlebodies
R. Dowdall, Henrique Gomes, Frank Hellmann
27 pages, multiple figures
(Submitted on 10 Sep 2009)
"Using the coherent state techniques developed for the analysis of the EPRL model we give the asymptotic formula for the Ponzano-Regge model amplitude for non-tardis triangulations of handlebodies in the limit of large boundary spins. The formula produces a sum over all possible immersions of the boundary triangulation and its value is given by the cosine of the Regge action evaluated on these. Furthermore the asymptotic scaling registers the existence of flexible immersions. We verify numerically that this formula approximates the 6j-symbol for large spins."

http://arxiv.org/abs/0909.1861
Space does not exist, so time can
Fotini Markopoulou
Third prize of the FQXi 'The Nature of Time' Essay Contest
(Submitted on 10 Sep 2009)
"It is often said that in general relativity time does not exist. This is because the Einstein equations generate motion in time that is a symmetry of the theory, not true time evolution. In quantum gravity, the timelessness of general relativity clashes with time in quantum theory and leads to the 'problem of time' which, in its various forms, is the main obstacle to a successful quantum theory of gravity. I argue that the problem of time is a paradox, stemming from an unstated faulty premise. Our faulty assumption is that space is real. I propose that what does not fundamentally exist is not time but space, geometry and gravity. The quantum theory of gravity will be spaceless, not timeless. If we are willing to throw out space, we can keep time and the trade is worth it."

http://arxiv.org/abs/0909.1899
Time in quantum physics: From an external parameter to an intrinsic observable
Romeo Brunetti, Klaus Fredenhagen, Marc Hoge
13 pages, submitted for the proceedings of the "Festschrift for Peter Mittelstaedt" to be published on Foundations of Physics
(Submitted on 10 Sep 2009)
"In the Schroedinger equation, time plays a special role as an external parameter. We show that in an enlarged system where the time variable denotes an additional degree of freedom, solutions of the Schroedinger equation give rise to weights on the enlarged algebra of observables. States in the associated GNS representation correspond to states on the original algebra composed with a completely positive unit preserving map. Application of this map to the functions of the time operator on the large system delivers the positive operator valued maps which were previously proposed by two of us as time observables. As an example we discuss the application of this formalism to the Wheeler-DeWitt theory of a scalar field on a Robertson-Walker spacetime."

 

[MTd2]

http://arxiv.org/abs/0909.2075

Generalized uncertainty principle, quantum gravity and Hořava-Lifgarbagez gravity

Yun Soo Myung
(Submitted on 11 Sep 2009)
We investigate a close connection between generalized uncertainty principle (GUP) and deformed Ho\v{r}ava-Lifgarbagez (HL) gravity. The GUP commutation relations correspond to the UV-quantum theory, while the canonical commutation relations represent the IR-quantum theory. Inspired by this UV/IR quantum mechanics, we obtain the GUP-corrected graviton propagator by introducing UV-momentum $p_i=p_{0i}(1+\beta p_{0}^2)$ and compare this with tensor propagators in the HL gravity. Two are the same up to $p_0^4$-order.

http://arxiv.org/abs/0909.2219

An analysis of the phase space of Horava-Lifgarbagez cosmologies

Sante Carloni, Emilio Elizalde, Pedro J. Silva
(Submitted on 11 Sep 2009)
Using the dynamical system approach, properties of cosmological models based on the Horava-Lifgarbagez gravity are systematically studied. In particular, the cosmological phase space of the Horava-Lifgarbagez model is characterized. The analysis allows to compare some key physical consequences of the imposition (or not) of detailed balance. A result of the investigation is that in the detailed balance case one of the attractors in the theory corresponds to an oscillatory behavior. Such oscillations can be associated to a bouncing universe, as previously described by Brandenberger, and will prevent a possible evolution towards a de Sitter universe. Other results obtained show that the cosmological models generated by Horava-Lifgarbagez gravity without the detailed balance assumption have indeed the potential to describe the transition between the Friedmann and the dark energy eras. The whole analysis leads to the plausible conclusion that a cosmology compatible with the present observations of the universe can be achieved only if the detailed balance condition is broken.

http://arxiv.org/abs/0909.2094

Comment on "No-go theorem for bimetric gravity with positive and negative mass"

Sabine Hossenfelder
(Submitted on 11 Sep 2009)
Authors Hohmann and Wohlfarth have put forward a no-go theorem for bimetric gravity with positive and negative mass in arXiv:0908.3384v1 [gr-qc]. This comment shows that their no-go theorem does not apply to arXiv:0807.2838v1 [gr-qc].

 

[MTd2]

http://arxiv.org/abs/0909.2829

Further Improvements in the Understanding of Isotropic Loop Quantum Cosmology

Mercedes Martin-Benito, Guillermo A. Mena Marugan, Javier Olmedo
(Submitted on 15 Sep 2009)
The flat, homogeneous, and isotropic universe with a massless scalar field is a paradigmatic model in Loop Quantum Cosmology. In spite of the prominent role that the model has played in the development of this branch of physics, there still remain some aspects of its quantization which deserve a more detailed discussion. These aspects include the kinematical resolution of the cosmological singularity, the precise relation between the solutions of the densitized and non-densitized versions of the quantum Hamiltonian constraint, the possibility of identifying superselection sectors which are as simple as possible, and a clear comprehension of the Wheeler-DeWitt (WDW) limit associated with the theory in those sectors. We propose an alternative operator to represent the Hamiltonian constraint which is specially suitable to deal with these issues in a satisfactory way. In particular, with our constraint operator, the singularity decouples in the kinematical Hilbert space and can be removed already at this level. Thanks to this fact, we can densitize the quantum Hamiltonian constraint in a rigorous manner. Besides, together with the physical observables, this constraint superselects simple sectors for the universe volume, with a support contained in a single semiaxis of the real line and for which the basic functions that encode the information about the geometry possesses optimal physical properties. Namely, they provide a no-boundary description around the cosmological singularity and admit a well-defined WDW limit in terms of standing waves. Both properties explain the presence of a generic quantum bounce replacing the singularity at a fundamental level, in contrast with previous studies where the bounce was proved in concrete regimes and focusing on states with a marked semiclassical behavior.

http://arxiv.org/abs/0909.2821

Stability of the Einstein static universe in Hořava-Lifgarbagez gravity

Puxun Wu, Hongwei Yu
(Submitted on 15 Sep 2009)
We study the stability of Einstein static universe in the Ho\v{r}ava-Lifgarbagez (HL) gravity with the detailed-balance condition, where the Friedmann equation gets corrected by a $1/{a^4}$ term. We find that, if the cosmological constant $\Lambda$ is negative, there exists a stable Einstein static state. The universe can stay at this stable state eternally and thus the big bang singularity can be avoided. However, in this case, it is difficult for the universe to break this stable state and then enter an inflationary era. For a positive $\Lambda$, the system has both an unstable state and a stable one. The former corresponds to an exponentially expanding phase. The universe can stay at this stable state past-eternally. Once the equation of state $w$ reaches infinity: $w\to\infty$ or $w\to-\infty$, this stable critical point coincides with the unstable one. Thus the stable state is broken and then the universe enters an inflationary era. Therefore, the big bang singularity can be avoided and a subsequent inflation can occur.

 

[MTd2]

http://arxiv.org/abs/0909.3265

Four-derivative interactions in asymptotically safe gravity

Dario Benedetti, Pedro F. Machado, Frank Saueressig
(Submitted on 17 Sep 2009)
We summarize recent progress in understanding the role of higher-derivative terms in the asymptotic safety scenario of gravity. Extending previous computations based on the functional renormalization group approach by including a Weyl-squared term in the ansatz for the effective action, further evidence for the existence of a non-Gaussian fixed point is found. The fixed point also persists upon including a minimally coupled free scalar field, providing an explicit example of perturbative counterterms being non-hazardous for asymptotic safety.

 

[marcus]

http://arxiv.org/abs/0909.3426
Quantum gravity without space-time singularities or horizons
Gerard 't Hooft
10 pages, 3 figures. Presented at the Erice Summerschool of Subnuclear Physics 2009
(Submitted on 18 Sep 2009)
"In an attempt to re-establish space-time as an essential frame for formulating quantum gravity - rather than an 'emergent' one -, we find that exact invariance under scale transformations is an essential new ingredient for such a theory. Use is made of the principle of 'black hole complementarity', the notion that observers entering a black hole describe its dynamics in a way that appears to be fundamentally different from the description by an outside observer. These differences can be boiled down to conformal transformations. If we add these to our set of symmetry transformations, black holes, space-time singularities, and horizons disappear, while causality and locality may survive as important principles for quantum gravity."

http://arxiv.org/abs/0909.3456
Antigravitation
Sabine Hossenfelder
This is the summary of a talk given at the 17th International Conference on Supersymmetry and the Unification of Fundamental Interactions in Boston, June 2009
(Submitted on 18 Sep 2009)
"We discuss why there are no negative gravitational sources in General Relativity and show that it is possible to extend the classical theory such that repulsive gravitational interaction occurs."

 

[MTd2]

This article is necessary to understand the conclusion of the above article from t'Hooft:

http://arxiv.org/abs/0908.3408

Entangled quantum states in a local deterministic theory

Gerard 't Hooft
(Submitted on 24 Aug 2009)
Investigating a class of models that is familiar in studies of cellular automata, we find that quantum operators can be employed to describe their long distance behavior. These operators span a Hilbert space that appears to turn such a model into a genuine quantum field theory, obeying the usual conditions of locality in terms of its quantum commutators. Entangled states can be constructed exactly as in quantum theories.
This raises the question whether such models allow Bell's inequalities to be violated. Being a local, deterministic theory, one would argue that this is impossible, but since at large distance scales the model does not seem to differ from real quantum field theories, there is reason to wonder why it should not allow entangled states. The standard arguments concerning Bell's inequalities are re-examined in this light.

On the topic of the convergence of several approaches to quantum gravity on Planck scale, due dimensional reduction, this is also very interesting:

http://arxiv.org/abs/0909.3329

Spontaneous Dimensional Reduction in Short-Distance Quantum Gravity?

Steven Carlip
(Submitted on 17 Sep 2009)
Several lines of evidence suggest that quantum gravity at very short distances may behave effectively as a two-dimensional theory. I summarize these hints, and offer an additional argument based on the strong-coupling limit of the Wheeler-DeWitt equation. The resulting scenario suggests a novel approach to quantum gravity at the Planck scale.

 

[marcus]

http://arxiv.org/abs/0909.3636
Testing Effective Quantum Gravity with Gravitational Waves from Extreme-Mass-Ratio Inspirals
Nicolas Yunes, C. F. Sopuerta
10 pages, 3 figures, invited paper for the Amaldi 8 Proceedings submitted to Class. Quant. Gravity
(Submitted on 20 Sep 2009)
"Testing deviation of GR is one of the main goals of the proposed Laser Interferometer Space Antenna, a space-based gravitational-wave observatory. For the first time, we consistently compute the generation of gravitational waves from extreme-mass ratio inspirals (stellar compact objects into supermassive black holes) in a well-motivated alternative theory of gravity, that to date remains weakly constrained by double binary pulsar observations. The theory we concentrate on is Chern-Simons (CS) modified gravity, a 4-D, effective theory that is motivated both from string theory and loop-quantum gravity, and which enhances the Einstein-Hilbert action through the addition of a dynamical scalar field and the parity-violating Pontryagin density. We show that although point particles continue to follow geodesics in the modified theory, the background about which they inspiral is a modification to the Kerr metric, which imprints a CS correction on the gravitational waves emitted. CS modified gravitational waves are sufficiently different from the General Relativistic expectation that they lead to significant dephasing after 3 weeks of evolution, but such dephasing will probably not prevent detection of these signals, but instead lead to a systematic error in the determination of parameters. We end with a study of radiation-reaction in the modified theory and show that, to leading-order, energy-momentum emission is not CS modified, except possibly for the subdominant effect of scalar-field emission. The inclusion of radiation-reaction will allow for tests of CS modified gravity with space-borne detectors that might be two orders of magnitude larger than current binary pulsar bounds."

http://arxiv.org/abs/0909.3767
Does time exist in quantum gravity?
Claus Kiefer
10 pages, second prize of the FQXi "The Nature of Time" essay contest
(Submitted on 21 Sep 2009)
"Time is absolute in standard quantum theory and dynamical in general relativity. The combination of both theories into a theory of quantum gravity leads therefore to a 'problem of time'. In my essay I shall investigate those consequences for the concept of time that may be drawn without a detailed knowledge of quantum gravity. The only assumptions are the experimentally supported universality of the linear structure of quantum theory and the recovery of general relativity in the classical limit. Among the consequences are the fundamental timelessness of quantum gravity, the approximate nature of a semiclassical time, and the correlation of entropy with the size of the Universe."

http://arxiv.org/abs/0909.3834
Analogue Models for Emergent Gravity
Stefano Liberati, Florian Girelli, Lorenzo Sindoni
47 pages. To appear in the Proceedings of the XVIII SIGRAV Conference, Cosenza, September 22-25, 2008
(Submitted on 21 Sep 2009)
"Gravity stands out among the fundamental interactions because of its apparent incompatibility with having a quantum description. Moreover, thermodynamic aspects of gravitation theory appears as puzzling features of some classical solutions such as black holes. These and other aspects of gravitational theories have recently lead to the proposal that gravity might not be a fundamental interaction but rather an emergent phenomenon, a sort of hydrodynamic limit of some more fundamental theory. In order to further explore this possibility we shall here discuss two systems where such emergence of a gravitational dynamics is observed. We shall consider first the case of a non-relativistic Bose-Einstein condensate and then a more abstract model based on scalar fields living on a Riemannian manifold. This will allow us to put in evidence the general issues related to emergent gravity scenarios with a particular attention to the role and nature of Lorentz and diffeomorphism invariance."

 

[atyy]

So one coefficient is unconstrained?

http://physics.aps.org/viewpoint-for/10.1103/PhysRevD.80.016002
Catching relativity violations with atoms
Quentin G. Bailey
Chung et al. combined their interferometer analysis with the lunar laser ranging results to tabulate the best constraints yet on gravity coefficients in the standard-model extension. The results show that five of nine possible coefficients are constrained to a value of zero to less than parts per billion, while another three of these nine coefficients are constrained at the level of parts per million.

 

[marcus]

http://arxiv.org/abs/0909.3983
A Larger Estimate of the Entropy of the Universe
Chas A. Egan, Charles H. Lineweaver
(Submitted on 22 Sep 2009)
"Using recent measurements of the supermassive black hole mass function we find that supermassive black holes are the largest contributor to the entropy of the observable Universe, contributing at least an order of magnitude more entropy than previously estimated. The total entropy of the observable Universe is correspondingly higher, and is $$S_{obs} = 3.1^{+3.0}_{-1.7}\times 10^{104} k$$. We calculate the entropy of the current cosmic event horizon to be $$S_{CEH} = 2.6 \pm 0.3 \times 10^{122} k$$, dwarfing the entropy of its interior, $$S_{CEH int} = 1.2^{+1.1}_{-0.7}\times 10^{103} k$$. We make the first tentative estimate of the entropy of dark matter within the observable Universe, $$S_{dm} = 10^{88\pm1} k$$. We highlight several caveats pertaining to these estimates and make recommendations for future work."

http://arxiv.org/abs/0909.4221
Loop Quantum Cosmology and Spin Foams
Abhay Ashtekar, Miguel Campiglia, Adam Henderson
11 pages
(Submitted on 23 Sep 2009)
"Loop quantum cosmology (LQC) is used to provide concrete evidence in support of the general paradigm underlying spin foam models (SFMs). Specifically, it is shown that: i) the physical inner product in the timeless framework equals the transition amplitude in the deparameterized theory; ii) this quantity admits a convergent vertex expansion a la SFMs in which the M-th term refers just to M volume transitions, without any reference to the time at which the transition takes place; iii) the exact physical inner product is obtained by summing over just the discrete geometries; no 'continuum limit' is involved; and, iv) the vertex expansion can be interpreted as a perturbative expansion in the spirit of group field theory. This sum over histories reformulation of LQC also addresses certain other issues which are briefly summarized."

http://arxiv.org/abs/0909.4211
Bianchi I model in terms of non-standard LQC: Classical dynamics
Piotr Dzierzak, Wlodzimierz Piechocki
12 pages
(Submitted on 23 Sep 2009)
"The cosmological singularities of the Bianchi I universe are analyzed in the setting of loop geometry underlying the loop quantum cosmology. We solve the Hamiltonian constraint of the theory and find the Lie algebra of elementary observables. Physical compound observables are defined in terms of elementary ones. Modification of classical theory by holonomy around a loop removes the singularities. However, our model has a free parameter that cannot be determined within our method. Testing the model by the data of observational cosmology may be possible after quantization of our modified classical theory."

 

[MTd2]

http://arxiv.org/abs/0909.4157

Revisiting the semiclassical gravity scenario for gravitational collapse

C. Barcelo, S. Liberati, S. Sonego, M. Visser
(Submitted on 23 Sep 2009)
The existence of extremely dark and compact astronomical bodies is by now a well established observational fact. On the other hand, classical General Relativity predicts the existence of black holes which fit very well with the observations, but do lead to important conceptual problems. In this contribution we ask ourselves the straightforward question: Are the dark and compact objects that we have observational evidence for black holes in the sense of General Relativity? By revising the semiclassical scenario of stellar collapse we find out that as the result of a collapse some alternative objects could be formed which might supplant black holes.

http://arxiv.org/abs/0909.4194

Non-equilibrium Thermodynamics of Spacetime: the Role of Gravitational Dissipation

G. Chirco, S. Liberati
(Submitted on 23 Sep 2009)
In arXiv:gr-qc/9504004 it was shown that the Einstein equation can be derived as a local constitutive equation for an equilibrium spacetime thermodynamics. More recently, in the attempt to extend the same approach to the case of $f(R)$ theories of gravity, it was found that a non-equilibrium setting is indeed required in order to fully describe both this theory as well as classical GR (arXiv:gr-qc/0602001). Here, elaborating on this point, we show that the dissipative character leading to a non-equilibrium spacetime thermodynamics is actually related -- both in GR as well as in $f(R)$ gravity -- to non-local heat fluxes associated with the purely gravitational/internal degrees of freedom of the theory. In particular, in the case of GR we show that the internal entropy production term is identical to the so called tidal heating term of Hartle-Hawking. Similarly, for the case of $f(R)$ gravity, we show that dissipative effects can be associated with the generalization of this term plus a scalar contribution whose presence is clearly justified within the scalar-tensor representation of the theory. Finally, we show that the allowed gravitational degrees of freedom can be fixed by the kinematics of the local spacetime causal structure, through the specific Equivalence Principle formulation. In this sense, the thermodynamical description seems to go beyond Einstein's theory as an intrinsic property of gravitation.

http://arxiv.org/abs/0909.4211

Bianchi I model in terms of non-standard LQC: Classical dynamics

Piotr Dzierzak, Wlodzimierz Piechocki
(Submitted on 23 Sep 2009)
The cosmological singularities of the Bianchi I universe are analyzed in the setting of loop geometry underlying the loop quantum cosmology. We solve the Hamiltonian constraint of the theory and find the Lie algebra of elementary observables. Physical compound observables are defined in terms of elementary ones. Modification of classical theory by holonomy around a loop removes the singularities. However, our model has a free parameter that cannot be determined within our method. Testing the model by the data of observational cosmology may be possible after quantization of our modified classical theory.

http://arxiv.org/abs/0909.4221

Loop Quantum Cosmology and Spin Foams

Abhay Ashtekar, Miguel Campiglia, Adam Henderson
(Submitted on 23 Sep 2009)
Loop quantum cosmology (LQC) is used to provide concrete evidence in support of the general paradigm underlying spin foam models (SFMs). Specifically, it is shown that: i) the physical inner product in the timeless framework equals the transition amplitude in the deparameterized theory; ii) this quantity admits a %convergent vertex expansion a la SFMs in which the $M$-th term refers just to $M$ volume transitions, without any reference to the time at which the transition takes place; iii) the exact physical inner product is obtained by summing over just the discrete geometries; no `continuum limit' is involved; and, iv) the vertex expansion can be interpreted as a perturbative expansion in the spirit of group field theory. This sum over histories reformulation of LQC also addresses certain other issues which are briefly summarized.

http://arxiv.org/abs/0909.4238

2+1 Quantum Gravity with Barbero-Immirzi like parameter on Toric Spatial Foliation

Rudranil Basu, Samir K Paul
(Submitted on 23 Sep 2009)
We consider gravity in 2+1 space-time dimensions, with negative cosmological constant and a `Barbero-Immirzi' (B-I) like parameter, when the space-time topology is of the form $ T^2 \times \mathbbm{R}$. The phase space structure, both in covariant and canonical framework is analyzed. Full quantization of the theory in the 'constrain first' approach reveals a finite dimensional physical Hilbert space. An explicit construction of wave functions is presented. The dimension of the Hilbert space is found to depend on the `Barbero-Immirzi' like parameter in an interesting fashion.

http://arxiv.org/abs/0909.3841

Anisotropic Conformal Infinity

Petr Horava, Charles M. Melby-Thompson
(Submitted on 21 Sep 2009)
We generalize Penrose's notion of conformal infinity of spacetime, to situations with anisotropic scaling. This is relevant not only for Lifgarbagez-type anisotropic gravity models, but also in standard general relativity and string theory, for spacetimes exhibiting a natural asymptotic anisotropy. Examples include the Lifgarbagez and Schrodinger spaces (proposed as AdS/CFT duals of nonrelativistic field theories), warped AdS_3, and the near-horizon extreme Kerr geometry. The anisotropic conformal boundary appears crucial for resolving puzzles of holographic renormalization in such spacetimes.

 

[MTd2]

http://arxiv.org/abs/0909.4472

Gravitation as a Plastic Distortion of the Lorentz Vaccum

Virginia V. Fernandez, Waldyr A. Rodrigues Jr
(Submitted on 24 Sep 2009)
In this paper we present a theory of the gravitational field where this field (a kind of square root of g) is represented by a (1,1)-extensor field h describing a plastic distortion of the Lorentz vacuum (a real substance that lives in a Minkowski spacetime) due to the presence of matter. The field h distorts the Minkowski metric extensor in an appropriate way (see below) generating what may be interpreted as an effective Lorentzian metric extensor g and also it permits the introduction of different kinds of parallelism rules on the world manifold, which may be interpreted as distortions of the parallelism structure of Minkowski spacetime and which may have non null curvature and/or torsion and/or nonmetricity tensors. We thus have different possible effective geometries which may be associated to the gravitational field and thus its description by a Lorentzian geometry is only a possibility, not an imposition from Nature. Moreover, we developed with enough details the theory of multiform functions and multiform functionals that permitted us to successfully write a Lagrangian for h and to obtain its equations of motion, that results equivalent to Einstein field equations of General Relativity (for all those solutions where the manifold M is diffeomorphic to R^4. However, in our theory, differently from the case of General Relativity, trustful energy-momentum and angular momentum conservation laws exist. We express also the results of our theory in terms of the gravitational potential 1-form fields (living in Minkowski spacetime) in order to have results which may be easily expressed with the theory of differential forms. The Hamiltonian formalism for our theory (formulated in terms of the potentials) is also discussed. The paper contains also several important Appendices that complete the material in the main text.

 

[marcus]

http://arxiv.org/abs/0909.4771
Indications of de Sitter Spacetime from Classical Sequential Growth Dynamics of Causal Sets
16 pages, 10 figures
Maqbool Ahmed, David Rideout
(Submitted on 25 Sep 2009)
"A large class of the dynamical laws for causal sets described by a classical process of sequential growth yield a cyclic universe, whose cycles of expansion and contraction are punctuated by single 'origin elements' of the causal set. We present evidence that the effective dynamics of the immediate future of one of these origin elements, within the context of the sequential growth dynamics, yields an initial period of de Sitter like exponential expansion, and argue that the resulting picture has many attractive features as a model of the early universe, with the potential to solve some of the standard model puzzles without any fine tuning."

 

[atyy]

http://arxiv.org/abs/0909.4621
On the Newtonian limit of emergent NC gravity and long-distance corrections
Harold Steinacker
(Submitted on 25 Sep 2009)
We show how Newtonian gravity emerges on 4-dimensional non-commutative spacetime branes in Yang-Mills matrix models. Large matter clusters such as galaxies are embedded in large-scale harmonic deformations of the space-time brane, which screen gravity for long distances. On shorter scales, the local matter distribution reproduces Newtonian gravity via local deformations of the brane and its metric. The harmonic ``gravity bag'' acts as a halo with effective positive energy density. This leads in particular to a significant enhancement of the orbital velocities around galaxies at large distances compared with the Newtonian case, before dropping to zero as the geometry merges with a Milne-like cosmology. Besides these ``harmonic'' solutions, there is another class of solutions which is more similar to Einstein gravity. Thus the IKKT model provides an accessible candidate for a quantum theory of gravity.