Loop-and-allied QG bibliography

[marcus]

It is always possible, even if string does not give a good model of nature, that LQG can be used to give a background independent formulation of a string theory. To comprehend string within the background independent context of LQG would be interesting, if only as an academic exercise. Thiemann took some steps in that direction back in 2004 which at the time many people considered outrageous Now some young LQG postdocs are again investigating this possibility.
http://arxiv.org/abs/0908.0953
Canonical Analysis of the Algebraic String
Winston J. Fairbairn, Karim Noui, Francesco Sardelli
27 pages
(Submitted on 6 Aug 2009)
"We investigate the canonical aspects of the algebraic first order formulation of strings introduced two decades ago by Balachandran and collaborators. We slightly enlarge the Lagrangian framework and show the existence of a self-dual formulation and of an Immirzi-type parameter reminiscent of four-dimensional first order gravity. We perform a full Hamiltonian analysis of the self-dual case: we extract the first class constraints and construct the Dirac bracket associated to the second class constraints. The first class constraints contain the diffeomorphisms algebra on the world sheet, as expected; and the coordinates are shown to be non-commutative with respect to the Dirac bracket. Then, the Hamilton equations in a particular (but very natural) gauge are shown to reproduce the wave equation for the string coordinates. In the general, non-self-dual case, we also explicit the first class constraints of the system and show that, unlike the self-dual formulation, the theory admits an extra propagating degree of freedom than the two degrees of freedom of conventional string theory. This prevents the general algebraic string from being strictly equivalent to the Nambu-Goto string."
==quote from the introduction==
Few years ago, Thiemann reconsidered the Nambu-Goto string and proposed a quantisation of it using the techniques of Loop Quantum Gravity (LQG) [14]. He showed that the LQG techniques, based on background independent quantisation, provides in particular a quantisation of the bosonic string in any dimensions, i.e. there is no need of critical dimensions for the quantum theory to be consistent. This result has sparked off some discussions [15] and certainly deserves to be understood deeper. We think that the algebraic formulation of the bosonic string is a better starting point to test the LQG techniques
than the Nambu-Goto string for it admits a lot of similarities with Ashtekar gravity [16]. It is a first order formulation and possesses an Immirzi-type parameter. In fact, the main motivation of this article is to open an arena for a background independent quantisation of the bosonic string and to compare it to the standard Fock quantisation. In that sense, we want to continue the work initiated by Thiemann from a quite different starting point in order to confirm or not his predictions and even go further."
==endquote==

 

[John86]

a little off topic maybe. Yesterday i found this very interesting article from Stuart Kauffman. One of the topics in this paper handlles about reductionism an the problems with it in physics.

It brought memories from Robert Laughlin book á different universe' which also makes a strong point against redoctionism.

http://www.edge.org/3rd_culture/kauffman09/kauffman09_index.html

 

[marcus]

http://arxiv.org/abs/0908.1764
On the Running of the Cosmological Constant in Quantum General Relativity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
5 pages
(Submitted on 12 Aug 2009)
"We present arguments that show what the running of the cosmological constant means when quantum general relativity is formulated following the prescription developed by Feynman."

Better get on Martin Reuter's train, guys, or you'll get left at the station. :-D

 

[MTd2]

http://arxiv.org/abs/0908.1964

Effective Field Theory, Past and Future

Steven Weinberg
(Submitted on 13 Aug 2009)
This is a written version of the opening talk at the 6th International Workshop on Chiral Dynamics, at the University of Bern, Switzerland, July 6, 2009, to be published in the proceedings of the Workshop. In it, I reminisce about the early development of effective field theories of the strong interactions, comment briefly on some other applications of effective field theories, and then take up the idea that the Standard Model and General Relativity are the leading terms in an effective field theory. Finally, I cite recent calculations that suggest that the effective field theory of gravitation and matter is asymptotically safe.

 

[marcus]

http://arxiv.org/abs/0908.2469
A Prehistory of n-Categorical Physics
John C. Baez, Aaron Lauda
129 pages, 8 figures
(Submitted on 18 Aug 2009)
"This paper traces the growing role of categories and n-categories in physics, starting with groups and their role in relativity, and leading up to more sophisticated concepts which manifest themselves in Feynman diagrams, spin networks, string theory, loop quantum gravity, and topological quantum field theory. Our chronology ends around 2000, with just a taste of later developments such as open-closed topological string theory, the categorification of quantum groups, Khovanov homology, and Lurie's work on the classification of topological quantum field theories."

 

[Finbar]

a little off topic maybe. Yesterday i found this very interesting article from Stuart Kauffman. One of the topics in this paper handlles about reductionism an the problems with it in physics.

It brought memories from Robert Laughlin book á different universe' which also makes a strong point against redoctionism.

http://www.edge.org/3rd_culture/kauffman09/kauffman09_index.html

Thanks for the link. I haven't read it yet but looks interesting. Actually I was thinking yesterday that one of the main problems I wish to address is the mind/body problem or the closly related problem of free will.

 

[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].