Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/0903.5303

Disordered Locality as an Explanation for the Dark Energy
Authors: Chanda Prescod-Weinstein, Lee Smolin
(Submitted on 30 Mar 2009)

Abstract: We discuss a novel explanation of the dark energy as a manifestation of macroscopic non-locality coming from quantum gravity, as proposed by Markopoulou. It has been previously suggested that in a transition from an early quantum geometric phase of the universe to a low temperature phase characterized by an emergent spacetime metric, locality might have been "disordered". This means that there is a mismatch of micro-locality, as determined by the microscopic quantum dynamics and macro-locality as determined by the classical metric that governs the emergent low energy physics. In this paper we discuss the consequences for cosmology by studying a simple extension of the standard cosmological models with disordered locality. We show that the consequences can include a naturally small vacuum energy.

 

[MTd2]

http://arxiv.org/abs/0904.0184

Some surprising implications of background independence in canonical quantum gravity

Authors: Abhay Ashtekar
(Submitted on 1 Apr 2009)

Abstract: There is a precise sense in which the requirement of background independence suffices to uniquely select the kinematics of loop quantum gravity (LQG). Specifically, the fundamental kinematic algebra of LQG admits a unique diffeomorphism invariant state. Although this result has been established rigorously, it comes as a surprise to researchers working with other approaches to quantum gravity. The goal of this article is to explain the underlying reasons in a pedagogical fashion using geometrodynamics, keeping the technicalities at their minimum. This discussion will bring out the surprisingly powerful role played by diffeomorphism invariance (and covariance) in non-perturbative, canonical quantum gravity.

 

[MTd2]

http://arxiv.org/abs/0904.0400

Noncommutative Quantum Mechanics and Quantum Cosmology
Authors: Catarina Bastos, Orfeu Bertolami, Nuno Dias, Joao Nuno Prata
(Submitted on 2 Apr 2009)

Abstract: We present a phase-space noncommutative version of quantum mechanics and apply this extension to Quantum Cosmology. We motivate this type of noncommutative algebra through the gravitational quantum well (GQW) where the noncommutativity between momenta is shown to be relevant. We also discuss some qualitative features of the GQW such as the Berry phase. In the context of quantum cosmology we consider a Kantowski-Sachs cosmological model and obtain the Wheeler-DeWitt (WDW) equation for the noncommutative system through the ADM formalism and a suitable Seiberg-Witten (SW) map. The WDW equation is explicitly dependent on the noncommutative parameters, $\theta$ and $\eta$. We obtain numerical solutions of the noncommutative WDW equation for different values of the noncommutative parameters. We conclude that the noncommutativity in the momenta sector leads to a damped wave function implying that this type of noncommmutativity can be relevant for a selection of possible initial states for the universe.

 

[marcus]

http://arxiv.org/abs/0904.0423
Plebanski Formulation of General Relativity: A Practical Introduction
Kirill Krasnov
13 pages
(Submitted on 2 Apr 2009)
"We give a pedagogical introduction into an old, but unfortunately not very well-known formulation of GR in terms of self-dual two-forms due to Plebanski. Our presentation is rather explicit in that we show how the familiar textbook solutions: Schwarzschild, Volkoff-Oppenheimer, as well as those describing the Newtonian limit, graviton and homogeneous isotropic Universe can be obtained within this formalism. Our description shows how Plebanski formulation gives quite an economical alternative to the usual metric and frame-based schemes for deriving Einstein equations."

 

[MTd2]

http://arxiv.org/abs/0904.0573

The Role of Time Gauge in Quantizing Gravity

Authors: Francesco Cianfrani, Giovanni Montani
(Submitted on 3 Apr 2009)

Abstract: We present the Hamiltonian formulation of General Relativity with the Holst formulation in a generic local Lorentz frame. In particular, we outline that a Gauss constraint is inferred by a proper generalization of Ashtekar-Barbero-Immirzi connections. This feature allow to extend the Loop Quantum Gravity quantization procedure to the case in which no gauge fixing at all is performed of the Lorentz frame.

 

[marcus]

http://arxiv.org/abs/0904.0829
Cosmology of the Lifgarbagez universe
Gianluca Calcagni
21 pages
(Submitted on 6 Apr 2009)
"We study the ultraviolet complete non-relativistic theory recently proposed by Horava. After introducing a Lifgarbagez scalar for a general background, we analyze the cosmology of the model in Lorentzian and Euclidean signature. Vacuum solutions are found and it is argued the existence of non-singular bouncing profiles. We find a general qualitative agreement with both the picture of Causal Dynamical Triangulations and Quantum Einstein Gravity. However, inflation driven by a Lifgarbagez scalar field on a classical background might not produce a scale-invariant spectrum."

Gianluca is in Ashtekar's group at Penn State.

 

[marcus]

http://arxiv.org/abs/0904.1303
Semiclassical analysis of the Loop Quantum Gravity volume operator: Area Coherent States
Cecilia Flori
(Submitted on 8 Apr 2009)
"We continue the semiclassical analysis of the Loop Quantum Gravity (LQG) volume operator that was started in the companion paper [23]. In the first paper we prepared the technical tools, in particular the use of complexifier coherent states that use squares of flux operators as the complexifier. In this paper, the complexifier is chosen for the first time to involve squares of area operators.
Both cases use coherent states that depend on a graph. However, the basic difference between the two choices of complexifier is that in the first case the set of surfaces involved is discrete, while, in the second it is continuous. This raises the important question of whether the second set of states has improved invariance properties with respect to relative orientation of the chosen graph in the set of surfaces on which the complexifier depends. In this paper, we examine this question in detail, including a semiclassical analysis.
The main result is that we obtain the correct semiclassical properties of the volume operator for i) artificial rescaling of the coherent state label; and ii) particular orientations of the 4- and 6-valent graphs that have measure zero in the group SO(3). Since such requirements are not present when analysing dual cell complex states, we conclude that coherent states whose complexifiers are squares of area operators are not an appropriate tool with which to analyse the semiclassical properties of the volume operator. Moreover, if one intends to go further and sample over graphs in order to obtain embedding independence, then the area complexifier coherent states should be ruled out altogether as semiclassical states."

 

[marcus]

http://arxiv.org/abs/0904.1556
The Algebra of Grand Unified Theories
John C. Baez, John Huerta
72 pages, 20 figures
(Submitted on 9 Apr 2009)
"The Standard Model of particle physics may seem complicated and arbitrary, but it has hidden patterns that are revealed by the relationship between three "grand unified theories": theories that unify forces and particles by extending the Standard Model symmetry group U(1) x SU(2) x SU(3) to a larger group. These three theories are Georgi and Glashow's SU(5) theory, Georgi's theory based on the group Spin(10), and the Pati-Salam model based on the group SU(2) x SU(2) x SU(4). In this expository account for mathematicians, we explain only the portion of these theories that involves finite-dimensional group representations. This allows us to reduce the prerequisites to a bare minimum while still giving a taste of the profound puzzles that physicists are struggling to solve."

 

[MTd2]

This paper can be discussed here:
http://golem.ph.utexas.edu/category/2009/03/the_algebra_of_grand_unified_t_1.html

 

[atyy]

Horava-Lifgarbagez Cosmology
Elias Kiritsis (U. of Crete), Georgios Kofinas (U. of Crete)
http://arxiv.org/abs/0904.1334

 

[marcus]

http://arxiv.org/abs/0904.1738
Symmetric space Cartan connections and gravity in three and four dimensions
Derek K. Wise
18 pages; Article prepared for special journal issue dedicated to Elie Cartan
(Submitted on 10 Apr 2009)
"Einstein gravity in both 3 and 4 dimensions, as well as some interesting generalizations, can be written as gauge theories in which the connection is a Cartan connection for geometry modeled on a symmetric space. The relevant theories in 3 dimensions include Einstein gravity in Chern-Simons form, as well as a new formulation of topologically massive gravity, with arbitrary cosmological constant, as a single constrained Chern-Simons action. In 4 dimensions the main model of interest is MacDowell-Mansouri gravity, generalized to include the Immirzi parameter in a natural way. I formulate these theories in Cartan geometric language, emphasizing also the role played by the symmetric structure of the model. I also explain how, from the perspective of these Cartan-geometric formulations, both the topological mass in 3d and the Immirzi parameter in 4d are the result of non-simplicity of the Lorentz Lie algebra so(3,1) and its relatives. Finally, I suggest that the language of Cartan geometry provides a guiding principle for elegantly reformulating any 'gauge theory of geometry'."

http://arxiv.org/abs/0904.1595
Solutions to Horava Gravity
H. Lu, Jianwei Mei, C.N. Pope
8 pages
(Submitted on 10 Apr 2009)
"Recently Horava proposed a non-relativistic renormalisable theory of gravitation, which reduces to Einstein's general relativity at large distances, and that may provide a candidate for a UV completion of Einstein's theory. In this paper, we derive the full set of equations of motion, and then we obtain spherically symmetric solutions and discuss their properties. We also obtain the Friedman-Lemaitre-Robertson-Walker cosmological metric."

http://arxiv.org/abs/0904.1657
Topological Interpretation of Barbero-Immirzi Parameter through a Rescaling of Wavefunctional
Sandipan Sengupta
(Submitted on 10 Apr 2009)
"The topological character of Barbero-Immirzi parameter ($\eta$) can emerge in a quantum description through a rescaling of the wavefunctional. This makes it possible to arrive at the canonical formulation for the action made up of the Hilbert-Palatini term and the Nieh-Yan invariant starting from the Hilbert-Palatini canonical theory. Here we set up a general rescaling procedure for gravity with or without matter. This needs a systematic treatment of the second class constraints (of Hilbert-Palatini theory) which are not solved before quantization. This allows a direct topological interpretation of eta in a quantum framework. This analysis can be carried out without choosing time gauge."

 

[MTd2]

Well, in the spirit of spin networks calculations, I guess this article is relevant to this thread... :

http://arxiv.org/abs/0904.1734

On the volume conjecture for classical spin networks
Authors: Abdelmalek Abdesselam
(Submitted on 10 Apr 2009)

Abstract: We prove an upper bound for the evaluation of all classical SU(2) spin networks conjectured by Garoufalidis and van der Veen. This implies one half of the analogue of the volume conjecture which they proposed for classical spin networks. We are also able to obtain the other half, namely, an exact determination of the spectral radius, for the special class of generalized drum graphs. Our proof uses a version of Feynman diagram calculus which we developed as a tool for the interpretation of the symbolic method of classical invariant theory, in a manner which is rigorous yet true to the spirit of the classical literature.

 

[marcus]

http://arxiv.org/abs/0904.1965
hbar as parameter of Minkowski metric in effective theory
G.E. Volovik
16 pages
(Submitted on 13 Apr 2009)
"With the proper choice of the dimensionality of the metric components, the action for all fields becomes dimensionless. Such quantities as the vacuum speed of light c, the Planck constant hbar, the electric charge e, the particle mass m, the Newton constant G never enter equations written in the covariant form, i.e., via the metric g^{\mu\nu}. The speed of light c and the Planck constant are parameters of a particular two-parametric family of solutions of general relativity equations describing the flat isotropic Minkowski vacuum in effective theory emerging at low energy: g^{\mu\nu}=diag(-\hbar^2, (\hbar c)^2, (\hbar c)^2, (\hbar c)^2). They parametrize the equilibrium quantum vacuum state. The physical quantities which enter the covariant equations are dimensionless quantities and dimensionful quantities of dimension of rest energy M or its power. Dimensionless quantities include the running coupling 'constants' \alpha_i; topological and geometric quantum numbers (angular moméntum quantum number j, weak charge, electric charge q, hypercharge, baryonic and leptonic charges, number of atoms N, etc). Dimensionful parameters include the rest energies of particles M_n (or/and mass matrices); the gravitational coupling K with dimension of M²; cosmological constant with dimension M⁴; etc. In effective theory, the interval s has the dimension of 1/M; it characterizes the dynamics of particles in the quantum vacuum rather than geometry of space-time. We discuss the effective action, and the measured physical quantities resulting from the action, including parameters which enter the Josepson effect, quantum Hall effect, etc."

 

[marcus]

http://arxiv.org/abs/0904.2510
Conformally reduced quantum gravity revisited
P.F. Machado, R. Percacci
(Submitted on 16 Apr 2009)
"Applying functional renormalization group methods, we describe two inequivalent ways of defining the renormalization group of matter-coupled four dimensional gravity, in the approximation where only the conformal factor is dynamical and taking the trace anomaly explicitly into account. We make contact with earlier work and briefly discuss the presence or absence of fixed points, depending on the truncation of the action and other approximations."

Roberto Percacci we know. Recently spent some time as a visitor at Utrecht.
Pedro Machado is one of Renate Loll's PhD students.
There've been several cases of CDT and Asymptotic Safe QG people collaborating recently.

 

[marcus]

http://arxiv.org/abs/0904.2835
Matter Bounce in Horava-Lifgarbagez Cosmology
Robert Brandenberger (McGill University and CERN)
6 pages, 1 figure
(Submitted on 18 Apr 2009)
"Horava-Lifgarbagez gravity, a recent proposal for a UV-complete renormalizable gravity theory, may lead to a bouncing cosmology. In this note we argue that Horava-LIfgarbagez cosmology may yield a concrete realization of the matter bounce scenario, and thus give rise to an alternative to inflation for producing a scale-invariant spectrum of cosmological perturbations. In this scenario, quantum vacuum fluctuations exit the Hubble radius in the pre-bounce phase and the spectrum is transformed into a scale-invariant one on super-Hubble scales before the bounce because the long wavelength modes undergo squeezing of their wave-functions for a longer period of time than shorter wavelength modes. The scale-invariance of the spectrum of curvature fluctuations is preserved during and after the bounce. A distinctive prediction of this scenario is the amplitude and shape of the bispectrum."

 

[MTd2]

http://arxiv.org/abs/0904.3184

Search for Lorentz Invariance Violation effects with PKS 2155-304 flaring period in 2006 by H.E.S.S
Authors: J. Bolmont, R. Buehler, A. Jacholkowska, S. J. Wagner, for the H.E.S.S. Collaboration
(Submitted on 21 Apr 2009)

Abstract: Highly energetic, variable and distant sources such as Active Galactic Nuclei provide a good opportunity to evaluate effects due to the emission and the propagation of high energy photons. In this note, a study of possible energy-dependent time-lags with PKS 2155-304 light curve as measured by H.E.S.S. in July 2006 is presented. These time-lags could either come from the emission processes or also sign a Lorentz Symmetry breaking as predicted in some Quantum Gravity models. A Cross-Correlation function and a Wavelet Transform were used to measure the time-lags. The 95% Confidence Limit on the Quantum Gravity energy scale based on the statistical and systematic error evaluation was found to be 7x10^17 GeV considering a linear correction in the standard photon dispersion relations and assuming that emission-induced time-lags are negligible. For now, this limit is the best ever obtained with a blazar.

 

[marcus]

http://arxiv.org/abs/0904.3412
Horava-Lifgarbagez gravity, absolute time, and objective particles in curved space
H. Nikolic
5 pages
(Submitted on 22 Apr 2009)
"Recently, Horava formulated a renormalizable theory of quantum gravity that reduces to general relativity at large distances but violates Lorentz invariance at small distances. The absolute time involved in this theory allows to define an objective notion of particles associated with quantization of fields in classical gravitational backgrounds. The Unruh effect and other observer-dependent notions of particles in curved space are interpreted as effects caused by interaction between the objective vacuum and the measuring apparatus made up of objective particles."

 

[marcus]

http://arxiv.org/abs/0904.3730
Second-order amplitudes in loop quantum gravity
Davide Mamone, Carlo Rovelli
22 pages, 18 figures
(Submitted on 23 Apr 2009)
"We explore some second-order amplitudes in loop quantum gravity. In particular, we compute some second-order contributions to diagonal components of the graviton propagator in the large distance limit, using the old version of the Barrett-Crane vertex amplitude. We illustrate the geometry associated to these terms. We find some peculiar phenomena in the large distance behavior of these amplitudes, related with the geometry of the generalized triangulations dual to the Feynman graphs of the corresponding group field theory. In particular, we point out a possible further difficulty with the old Barrett-Crane vertex: it appears to lead to flatness instead of Ricci-flatness, at least in some situations. The observation raises the question whether this difficulty remains with the new version of the vertex."

 

[John86]

These two are interesting from the emergent prospective. a few months ago Fra brought under atention Olaf Dreyer

http://arxiv.org/abs/0904.3520
Time is not the problem
Authors: Olaf Dreyer
(Submitted on 22 Apr 2009)
]
Abstract: Attempts to quantize general relativity encounter an odd problem. The Hamiltonian that normally generates time evolution vanishes in the case of general relativity as a result of diffeomorphism invariance. The theory seems to be saying that time does not exist. The most obvious feature of our world, namely that time seems to progress and that the world changes accordingly becomes a problem in this presumably fundamental theory. This is called the problem of time. In this essay we argue that this problem is the result of an unphysical idealization. We are caught in this "problem of time" trap because we took a wrong turn in the early days of relativity by permanently including a split of geometry and
matter into our physical theories. We show that another possibility exists that circumvents the problem of time and also sheds new light on other problems like the cosmological constant problem and the horizon problem in early universe cosmology. http://arxiv.org/abs/0904.3276
Gluon condensate, modified gravity, and the accelerating Universe
Authors: F.R. Klinkhamer
(Submitted on 21 Apr 2009)

Abstract: The dynamics of a gravitating gluon condensate q is studied in the context of a spatially flat Friedmann-Robertson-Walker universe. With a quadratic approximation of the gluon-condensate vacuum energy density \rho_{V}(q) near the equilibrium value q_{0} and a small modified-gravity coupling constant \eta of a nonanalytic \tilde{f}(R,q) term in the action, an "accelerating universe" is obtained which more or less resembles the present Universe. The unknown component 'X' of this model universe (here, due to the combined effects of vacuum energy density and modified gravity) has an effective equation-of-state parameter \bar{w}_{X} which is found to evolve towards the value -1 from above. Klinkhamer is one of the Volovik school they published some papers together.

A few days ago i went to the bookshop and found a very fascinating book by Grigori Volovik about condensed matter physics called "Universe in a helium droplet". I scammed the book and it looked really fascinating. Although a little expensive though !...

 

[John86]

http://arxiv.org/abs/0904.3627
Condensed matter lessons about the origin of time
Authors: Gil Jannes
(Submitted on 23 Apr 2009)

Abstract: It is widely hoped that quantum gravity will shed a profound light on the origin of time in physics. The currently dominant approaches to a candidate quantum theory of gravity have quite naturally evolved from general relativity, on the one hand, and from particle physics, on the other hand. In this essay, I will argue that a third important branch of 20th century `fundamental' physics, namely condensed-matter physics, also offers an interesting perspective on quantum gravity, and thereby on the problem of time. The bottomline might sound disappointing to those who have become used to claims that quantum gravity or a `Theory of Everything' will solve most of the conceptual problems of fundamental physics: To understand the origin of time, experimental input is needed at much higher energies than what is available today. Moreover, it is far from obvious that we will ever discover the true origin of physical time, even if we become able to directly probe physics at the Planck scale. But we might learn plenty of interesting lessons about time and the structure of our universe in the process.

 

[marcus]

http://arxiv.org/abs/0904.3914
Spherically symmetric black holes in minimally modified self-dual gravity
Akihiro Ishibashi, Simone Speziale
42 pages, many colour figures
(Submitted on 24 Apr 2009)

"We discuss spherically symmetric black holes in the modified self-dual theory of gravity recently studied by Krasnov, obtained adding a Weyl-curvature dependent `cosmological term' to the Plebanski lagrangian for general relativity. This type of modified gravity admits two different types of singularities: one is a true singularity for the theory where the fundamental fields of the theory, as well as the (auxiliary) spacetime metric, become singular, and the other one is a milder "non-metric singularity" where the metric description of the spacetime breaks down but the fundamental fields themselves are regular. We first generalise this modified self-dual gravity to include Maxwell's field and then study basic features of spherically symmetric, charged black holes, with particular focus on whether these two types of singularities are hidden or naked. We restrict our attention to minimal forms of the modification, and find that the theory exhibits `screening' effects of the electric charge (or `anti-screening', depending upon the sign of the modification term), in the sense that it leads to the possibility of charging the black hole more (or less) than it would be possible in general relativity without exposing a naked singularity. We also find that for any (even arbitrarily large) value of charge, true singularities of the theory appear to be either achronal (non-timelike) covered by the hypersurface of a harmless non-metric singularity, or simply hidden inside at least one Killing horizon."

 

[marcus]

http://arxiv.org/abs/0904.4036
Star product and interacting fields on kappa-Minkowski space
J. Kowalski-Glikman, A. Walkus
7 pages
(Submitted on 26 Apr 2009)
"In this note we extend the methods developed by Freidel et al. [arXiv:hep-th/0612170] to derive the form of phi⁴ interaction term in the case of scalar field theory on kappa-Minkowski space, defined in terms of star product. We present explicit expressions for the kappa-Minkowski star product. Having obtained the the interaction term we use the resulting deformed conservation rules to investigate if they lead to any threshold anomaly, and we find that in the leading order they do not, as expected."

http://arxiv.org/abs/0904.4454
Large classical universes emerging from quantum cosmology
Nelson Pinto-Neto
8 pages, 1 figure
(Submitted on 28 Apr 2009)
"It is generally believed that one cannot obtain a large Universe from quantum cosmological models without an inflationary phase in the classical expanding era because the typical size of the Universe after leaving the quantum regime should be around the Planck length, and the standard decelerated classical expansion after that is not sufficient to enlarge the Universe in the time available. For instance, in many quantum minisuperspace bouncing models studied in the literature, solutions where the Universe leave the quantum regime in the expanding phase with appropriate size have negligible probability amplitude with respect to solutions leaving this regime around the Planck length. In this paper, I present a general class of moving gaussian solutions of the Wheeler-DeWitt equation where the velocity of the wave in minisuperspace along the scale factor axis, which is the new large parameter introduced in order to circumvent the abovementioned problem, induces a large acceleration around the quantum bounce, forcing the Universe to leave the quantum regime sufficiently big to increase afterwards to the present size, without needing any classical inflationary phase in between, and with reasonable relative probability amplitudes with respect to models leaving the quantum regime around the Planck scale. Furthermore, linear perturbations around this background model are free of any transplanckian problem."http://arxiv.org/abs/0904.4435
Matter in Loop Quantum Gravity without time gauge: a non-minimally coupled scalar field
F. Cianfrani, G. Montani
4 pages
(Submitted on 28 Apr 2009)
"We analyze the phase space of gravity non-minimally coupled to a scalar field in a generic local Lorentz frame. We reduce the set of constraints to a first-class one by fixing a specific hypersurfaces in the phase space. The main issue of our analysis is to extend the features of the vacuum case to the presence of scalar matter by recovering the emergence of an SU(2) gauge structure and the non-dynamical role of boost variables. Within this scheme, the super-momentum and the super-Hamiltonian are those ones associated with a scalar field minimally coupled to the metric in the Einstein frame. Hence, the kinematical Hilbert space is defined as in canonical Loop Quantum Gravity with a scalar field, but the differences in the area spectrum are outlined to be the same as in the time-gauge approach."

http://arxiv.org/abs/0904.4379
Alternative quantization of the Hamiltonian in loop quantum cosmology II: Including the Lorentz term
Jinsong Yang, You Ding, Yongge Ma
8 pages, 2 figures
(Submitted on 28 Apr 2009)
"Since there are quantization ambiguities in constructing the Hamiltonian constraint operator in isotropic loop quantum cosmology, it is crucial to check whether the key features of loop quantum cosmology are robust against the ambiguities. In this paper, we quantize the Lorentz term of the gravitational Hamiltonian constraint in the spatially flat FRW model by two approaches different from that of the Euclidean term. One of the approaches is very similar to the treatment of the Lorentz part of Hamiltonian in loop quantum gravity and hence inherits more features from the full theory. Two symmetric Hamiltonian constraint operators are constructed respectively in the improved scheme. Both of them are shown to have the correct classical limit by the semiclassical analysis. In the loop quantum cosmological model with a massless scalar field, the effective Hamiltonians and Friedmann equations are derived. It turns out that the classical big bang is again replaced by a quantum bounce in both cases. Moreover, there are still great possibilities for the expanding universe to recollapse due to the quantum gravity effect."

Yongge Ma leads the LQG section at Beijing Normal. His institution will be hosting this year's Loops 2009 conference.

 

[MTd2]

Wait a minute, why didn't you post todays new a articles in a new post?

 

[marcus]

Doubling up saves space and memory: get two days preprints in one post.

http://arxiv.org/abs/0904.4841
The quantization of unimodular gravity and the cosmological constant problem
Lee Smolin
22 pages
(Submitted on 30 Apr 2009)
"A quantization of unimodular gravity is described, which results in a quantum effective action which is also unimodular, ie a function of a metric with fixed determinant. A consequence is that contributions to the energy momentum tensor of the form of the metric times a spacetime constant, whether classical or quantum, are not sources of curvature in the equations of motion derived from the quantum effective action. This solves the first cosmological constant problem, which is suppressing the enormous contributions to the cosmological constant coming from quantum corrections. We discuss several forms of uniodular gravity and put two of them, including one proposed by Henneaux and Teitelboim, in constrained Hamiltonian form. The path integral is constructed from the latter. Furthermore, the second cosmological constant problem, which is why the measured value is so small, is also addressed by this theory. We argue that a mechanism first proposed by Ng and van Dam for suppressing the cosmological constant by quantum effects obtains at the semiclassical level."

http://arxiv.org/abs/0904.4779
Questioning Newton's second law: What is the structure of equations of motion?
Claus Lämmerzahl, Patricia Rademaker
13 pages
(Submitted on 30 Apr 2009)
"Interactions are explored through the observation of the dynamics of particles. On the classical level the basic underlying assumption in that scheme is that Newton's second law holds. Relaxing the validity of this axiom by, e.g., allowing for higher order time derivatives in the equations of motion would allow for a more general structure of interactions. We derive the structure of interactions by means of a gauge principle and discuss the physics emerging from equations of motion of higher order. One main result is higher order derivatives induce a zitterbewegung. As a consequence the main motion resulting from the second order equation of motion is rather robust against modifications. The gauge principle leads to a gauge field with the property of a space metric. We confront this general scheme with experimental data."

http://arxiv.org/abs/0904.4865
Line element in quantum gravity: the examples of DSR and noncommutative geometry
Pierre Martinetti
Proceedings of the Second Workshop on Quantum Gravity and Noncommutative Geometry, Universidade Lusofona, Lisbon 22-24 September 2008
(Submitted on 30 Apr 2009)
"We question the notion of line element in some quantum spaces that are expected to play a role in quantum gravity, namely non-commutative deformations of Minkowski spaces. We recall how the implementation of the Leibniz rule forbids to see some of the infinitesimal deformed Poincare transformations as good candidates for Noether symmetries. Then we recall the more fundamental view on the line element proposed in noncommutative geometry, and re-interprete at this light some previous results on Connes' distance formula."

 

[MTd2]

http://arxiv.org/abs/0904.4786
Correlated optical and gamma emissions from GRB 081126
Authors: Alain Klotz (OHP, LATT), B. Gendre (LAM), J.L. Atteia (LATT), Michel Boër (OHP), David M. Coward, Alan C. Imerito
(Submitted on 30 Apr 2009)

Abstract: We present an analysis of time-resolved optical emissions observed from the gamma-ray burst GRB 081126 during the prompt phase. The analysis employed time-resolved photometry using optical data obtained by the TAROT telescope, using BAT data from the Swift spacecraft , and time-resolved spectroscopy at high energies from the GBM instrument onboard the Fermi spacecraft . The optical emission of GRB 081126 is found to be compatible with the second gamma emission pulse shifted by a positive time lag of 8.4 $\pm$ 3.9 s. This is the first well-resolved observation of a time lag between optical and gamma emissions during a gamma-ray burst. Our observations could potentially provide new constraints on the fireball model for gamma-ray burst early emissions. Furthermore, observations of time lags between optical and gamma ray photons provides an exciting opportunity to constrain quantum gravity theories. http://arxiv.org/abs/0904.4878
Can we probe the Lorentz factor of gamma-ray bursts from GeV-TeV spectra integrated over internal shocks?
Authors: Junichi Aoi, Kohta Murase, Keitaro Takahashi, Kunihito Ioka, Shigehiro Nagataki
(Submitted on 30 Apr 2009)

Abstract: We revisit the high-energy spectral cutoff originating from the electron-positron pair creation in the prompt phase of gamma-ray bursts (GRBs) with numerical and analytical calculations. We show that the conventional exponential cutoff should be drastically modified to a steepened power-law in practical observations that integrate emissions from different internal shocks. Since the steepening is tiny for observations, this "smearing" effect can generally reduce the previous estimates of the Lorentz factor of the GRB outflows. We apply our formulation to GRB 080916C, recently detected by the LAT detector on the Fermi satellite, and find that the minimum Lorentz factor can be ~600 (or even smaller values), which is below but consistent with the previous result of ~900. Observing the steepening energy (so-called "pair-break energy") is crucial to diagnose the Lorentz factor and/or the emission site in the future observations.http://arxiv.org/abs/0904.4299
Modifying Gravity in the Infra-Red by imposing an "Ultra-Strong" Equivalence Principle
Authors: Federico Piazza (Perimeter Institute)
(Submitted on 28 Apr 2009)

Abstract: The equivalence principle suggests to consider gravity as an infrared phenomenon, whose effects are visible only outside Einstein's free-falling elevator. By curving spacetime, General Relativity leaves the smallest systems free of classical gravitational effects. However, according to the standard semi-classical treatment, indirect effects of gravity can be experienced inside the elevator through the well-known mechanism of quantum particle production. Here we try a different path than the one historically followed: rather than imposing field quantization on top of a curved manifold, we attempt to upgrade the equivalence principle and extend it to the quantum phenomena. Therefore, we consider, and try to realize in a theoretical framework, a stronger version of the equivalence principle, in which all the effects of gravity are definitely banned from the elevator and confined to the infra-red. For this purpose, we introduce infrared modified commutation relations for the global field operators (Fourier modes) that allow to reabsorb the time-dependent quadratic divergence of the vacuum expectation value of the stress-energy tensor. The proposed modification is effective on length scales comparable to the inverse curvature and, therefore, does no add any dimensional parameter to the theory.

 

[MTd2]

http://arxiv.org/abs/0905.0213

A causal perspective on random geometry
Authors: Stefan Zohren
(Submitted on 2 May 2009)

Abstract: In this thesis we investigate the importance of causality in non-perturbative approaches to quantum gravity. Firstly, causal sets are introduced as a simple kinematical model for causal geometry. It is shown how causal sets could account for the microscopic origin of the Bekenstein entropy bound. Holography and finite entropy emerge naturally from the interplay between causality and discreteness. Going beyond causal set kinematics is problematic however. It is a hard problem to find the right amplitude to attach to each causal set that one needs to define the non-perturbative quantum dynamics of gravity. One approach which is ideally suited to define the non-perturbative gravitational path integral is dynamical triangulation. Without causality this method leads to unappealing features of the quantum geometry though. It is shown how causality is instrumental in regulating this pathological behavior. In two dimensions this approach of causal dynamical triangulations has been analytically solved by transfer matrix methods. In this thesis considerable progress has been made in the development of more powerful techniques for this approach. The formulation through matrix models and a string field theory allow us to study interesting generalizations. Particularly, it has become possible to define the topological expansion. A surprising twist of the new matrix model is that it partially disentangles the large-N and continuum limit. This makes our causal model much closer in spirit to the original idea by 't Hooft than the conventional matrix models of non-critical string theory.

 

[John86]

http://arxiv.org/abs/0905.0113

Consequences of Kaluza-Klein Covariance
Authors: Paul S. Wesson
(Submitted on 1 May 2009)

The group of coordinate transformations for 5D noncompact Kaluza-Klein theory is broader than the 4D group for Einstein's general relativity. Therefore, a 4D quantity can take on different forms depending on the choice for the 5D coordinates. We illustrate this by deriving the physical consequences for several forms of the canonical metric, where the fifth coordinate is altered by a translation, an inversion and a change from spacelike to timelike. These cause, respectively, the 4D cosmological 'constant' to become dependent on the fifth coordinate, the rest mass of a test particle to become measured by its Compton wavelength, and the dynamics to become wave-mechanical with a small mass quantum. These consequendes of 5D covariance -- whether viewed as positive or negative -- help to determine the viability of current attempts to unify gravity with the interactions of particles.


http://arxiv.org/abs/0905.0119

Time as an Illusion
Authors: Paul S. Wesson
(Submitted on 1 May 2009)

We review the idea, due to Einstein, Eddington, Hoyle and Ballard, that time is a subjective label, whose primary purpose is to order events, perhaps in a higher-dimensional universe. In this approach, all moments in time exist simultaneously, but they are ordered to create the illusion of an unfolding experience by some physical mechanism. This, in the language of relativity, may be connected to a hypersurface in a world that extends beyond spacetime. Death in such a scenario may be merely a phase change.


http://arxiv.org/abs/0905.0017

Emergence of spatial structure from causal sets
Authors: David Rideout, Petros Wallden
(Submitted on 30 Apr 2009)

There are numerous indications that a discrete substratum underlies continuum spacetime. Any fundamentally discrete approach to quantum gravity must provide some prescription for how continuum properties emerge from the underlying discreteness. The causal set approach, in which the fundamental relation is based upon causality, finds it easy to reproduce timelike distances, but has a more difficult time with spatial distance, due to the unique combination of Lorentz invariance and discreteness within that approach. We describe a method to deduce spatial distances from a causal set. In addition, we sketch how one might use an important ingredient in deducing spatial distance, the `$n$-link', to deduce whether a given causal set is likely to faithfully embed into a continuum spacetime.

 

[marcus]

http://arxiv.org/abs/0905.0997
Beyond the Standard Model: A Noncommutative Approach
Christoph A. Stephan
To be published in the Proceedings of the XLIVth Rencontres de Moriond: Electroweak Interactions and Unified Theories (La Thuile, Italy, 7-14 March 2009)
(Submitted on 7 May 2009)
"During the last two decades Alain Connes developed Noncommutative Geometry (NCG), which allows to unify two of the basic theories of modern physics: General Relativity (GR) and the Standard Model (SM) of Particle Physics as classical field theories. In the noncommutative framework the Higgs boson, which had previously to be put in by hand, and many of the ad hoc features of the standard model appear in a natural way.
The aim of this presentation is to motivate this unification from basic physical principles and to give a flavour of its derivation. One basic prediction of the noncommutative approach to the SM is that the mass of the Higgs Boson should be of the order of 170 GeV if one assumes the Big Desert. This mass range is with reasonable probability excluded by the Tevatron and therefore it is interesting to investigate models beyond the SM that are compatible with NCG. Going beyond the SM is highly non-trivial within the NCG approach but possible extensions have been found and provide for phenomenologically interesting models. We will present in this article a short introduction into the NCG framework and describe one of these extensions of the SM. This model contains new scalar bosons (and fermions) which constitute a second Higgs-like sector mixing with theordinary Higgs sector and thus considerably modifying the mass eigenvalues."

 

[marcus]

http://arxiv.org/abs/0905.0695
A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder
Adam G. Riess (JHU, STScI), Lucas Macri (Texas A&M), Stefano Casertano (STScI), Megan Sosey (STScI), Hubert Lampeitl (UPort), Henry C. Ferguson (STScI), Alexei V. Filippenko (UCB), Saurabh W. Jha (Rutgers), Weidong Li (UCB), Ryan Chornock (UCB), Devdeep Sarkar (UCI)
60 pages, 15 figures Accepted for Publication, ApJ. This is the second of two papers reporting results from a program to determine the Hubble constant to 5% precision from a refurbished distance ladder based on extensive use of differential measurements
(Submitted on 5 May 2009)
We report observations of 240 Cepheid variables obtained with the Near Infrared Camera (NICMOS) through the F160W filter on the Hubble Space Telescope (HST). The Cepheids are distributed across six recent hosts of Type Ia supernovae (SNe Ia) and the "maser galaxy" NGC 4258, allowing us to directly calibrate the peak luminosities of the SNe Ia from the precise, geometric distance measurements provided by the masers. New features of our measurement include the use of the same instrument for all Cepheid measurements across the distance ladder and homogeneity of the Cepheid periods and metallicities thus necessitating only a differential measurement of Cepheid fluxes and reducing the largest systematic uncertainties in the determination of the fiducial SN Ia luminosity. The NICMOS measurements reduce differential extinction in the host galaxies by a factor of 5 over past optical data. Combined with an expanded of 240 SNe Ia at z<0.1 which define their magnitude-redshift relation, we find H₀=74.2 +/-3.6, a 4.8% uncertainty including both statistical and systematic errors. We show that the factor of 2.2 improvement in the precision of H₀ is a significant aid to the determination of the equation-of-state of dark energy, w = P/(rho c²). Combined with the WMAP 5-year measurement of Omega_M h², we find w= -1.12 +/- 0.12 independent of high-redshift SNe Ia or baryon acoustic oscillations (BAO). This result is also consistent with analyses based on the combination of high-z SNe Ia and BAO. The constraints on w(z) now with high-z SNe Ia and BAO are consistent with a cosmological constant and improved by a factor of 3 from the refinement in H₀ alone. We show future improvements in H₀ are likely and will further contribute to multi-technique studies of dark energy."

 

[MTd2]

http://arxiv.org/abs/0905.1665

Fractal Quantum Space-Time
Authors: Leonardo Modesto
(Submitted on 11 May 2009)

Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.