Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/gr-qc/0612144
Three Dimensional Loop Quantum Gravity: Particles and the Quantum Double
Karim Noui
36 pages, published in J. Math. Phys. 47, 102501 (2006)

"It is well known that the quantum double structure plays an important role in three dimensional quantum gravity coupled to matter field. In this paper, we show how this algebraic structure emerges in the context of three dimensional Riemannian loop quantum gravity (LQG) coupled to a finite number of massive spinless point particles. In LQG, physical states are usually constructed from the notion of SU(2) cylindrical functions on a Riemann surface Sigma and the Hilbert structure is defined by the Ashtekar-Lewandowski measure. In the case where Sigma is the sphere S^2, we show that the physical Hilbert space is in fact isomorphic to a tensor product of simple unitary representations of the Drinfeld double DSU(2): the masses of the particles label the simple representations, the physical states are tensor products of vectors of simple representations and the physical scalar product is given by intertwining coefficients between simple representations. This result is generalized to the case of any Riemann surface Sigma.

http://arxiv.org/abs/gr-qc/0612145
Three dimensional Loop Quantum Gravity: towards a self-gravitating Quantum Field Theory
Karim Noui
36 pages, published in Class. Quant. Grav. 24 (2007)

In a companion paper, we have emphasized the role of the Drinfeld double DSU(2) in the context of three dimensional Riemannian Loop Quantum Gravity coupled to massive spinless point particles. We make use of this result to propose a model for a self-gravitating quantum field theory (massive spinless non-causal scalar field) in three dimensional Riemannian space. We start by constructing the Fock space of the free self-gravitating field: the vacuum is the unique DSU(2) invariant state, one-particle states correspond to DSU(2) unitary irreducible simple representations and any multi-particles states is obtained as the symmetrized tensor product between simple representations. The associated quantum field is defined by the usual requirement of covariance under DSU(2). Then, we introduce a DSU(2)-invariant self-interacting potential (the obtained model is a Group Field Theory) and compute explicitely the lowest order terms (in the self-interaction coupling constant lambda) of the propagator and of the three-points function. Finally, we compute the lowest order quantum gravity corrections (in the Newton constant G) to the propagator and to the three-points function.

http://arxiv.org/abs/gr-qc/0612147
Spectral Analysis of the Volume Operator in Loop Quantum Gravity
J. Brunnemann, D. Rideout
5 pages, 1 figure. Talk given by D. Rideout at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006
Imperial/TP/2006/DR/02

"We describe preliminary results of a detailed numerical analysis of the volume operator as formulated by Ashtekar and Lewandowski. Due to a simplified explicit expression for its matrix elements, it is possible for the first time to treat generic vertices of valence greater than four. It is found that the vertex geometry characterizes the volume spectrum."

 

[marcus]

http://arxiv.org/abs/hep-th/0612280
Doubly Special Relativity at the age of six
Jerzy Kowalski-Glikman
To appear in the Proceedings of 22nd Max Born Symposium

"The current status of Doubly Special Relativity research program is shortly presented.
I dedicate this paper to my teacher and friend Professor Jerzy Lukierski on occasion of his seventieth birthday."

http://arxiv.org/abs/gr-qc/0612167
Deformed Special Relativity in Position Space
S. Hossenfelder "We investigate how deformations of special relativity in momentum space can be extended to position space in a consistent way, such that the dimensionless contraction between wave-vector and coordinate-vector remains invariant. By using a parametrization in terms of an energy dependent speed of light, and an energy dependent Planck's constant, we are able to formulate simple requirements that completely determine the active transformations in position space. These deviate from the standard transformations for large velocities of the observed object. Some examples are discussed, and it is shown how the relativistic mass gain of a massive particle is affected. We finally study the construction of passive Lorentz-transformations."

http://arxiv.org/abs/gr-qc/0612170
The Ponzano-Regge model and Reidemeister torsion
John W. Barrett, Ileana Naish-Guzman
4 pages. MG11 conference proceedings

"The Ponzano-Regge model of three-dimensional quantum gravity is well-defined when the observables satisfy a certain condition involving the twisted cohomology. In this case, the partition function is defined in terms of the Reidemeister torsion. Some consequences for the special cases of planar graphs and knots are given."

 

[marcus]

Francesca flagged this one by Lucien Hardy of PI Waterloo

http://arxiv.org/abs/quant-ph/0701019
Quantum gravity computers: On the theory of computation with indefinite causal structure

A quantum gravity computer is one for which the particular effects of quantum gravity are relevant. In general relativity, causal structure is non-fixed. In quantum theory non-fixed quantities are subject to quantum uncertainty. It is therefore likely that, in a theory of quantum gravity, we will have indefinite causal structure. This means that there will be no matter of fact as to whether a particular interval is timelike or not. We study the implications of this for the theory of computation. Classical and quantum computations consist in ivolving the state of the computer through a sequence of time steps. This will, most likely, not be possible for a quantum gravity computer because the notion of a time step makes no sense if we have indefinite causal structure. We show that it is possible to set up a model for computation even in the absence of definite causal structure by using a certain framework (the causaloid formalism) that was developed for the purpose of correlating data taken in this type of situation. Corresponding to a physical theory is a causaloid, Lambda (this is a mathematical object containing information about the causal connections between different spacetime regions). A computer is given by the pair {Lambda, S} where S is a set of gates. Working within the causaloid formalism, we explore the question of whether universal quantum gravity computers are possible. We also examine whether a quantum gravity computer might be more powerful than a quantum (or classical) computer. In particular, we ask whether indefinite causal structure can be used as a computational resource.

 

[marcus]

papers by 't Hooft, by Freidel, and by Sudarsky

http://arxiv.org/abs/quant-ph/0701097
The Free-Will Postulate in Quantum Mechanics
Gerard 't Hooft
8 pages, 1 figure

"The so-called 'free will axiom' is an essential ingredient in many discussions concerning hidden variables in quantum mechanics. In this paper we argue that 'free will' can be defined in different ways. The definition usually employed is clearly invalid in strictly deterministic theories. A different, more precise formulation is proposed here, defining a condition that may well be a more suitable one to impose on theoretical constructions and models. Our axiom, to be referred to as the `unconstrained initial state' condition, has consequences similar to 'free will', but does not clash with determinism, and appears to lead to different conclusions concerning causality and locality in quantum mechanics. Models proposed earlier by this author fall in this category. Imposing our 'unconstrained initial state' condition on a deterministic theory underlying Quantum Mechanics, appears to lead to a restricted free-will condition in the quantum system: an observer has the free will to modify the setting of a measuring device, but has no control over the phase of its wave function. The dismissal of the usual 'free will' concept does not have any consequences for our views and interpretations of human activities in daily life, and the way our minds function, but it requires a more careful discussion on what, in practice, free will actually amounts to."


http://arxiv.org/abs/hep-th/0701113
The Relativistic Particle: Dirac observables and Feynman propagator
Laurent Freidel, Florian Girelli, Etera R. Livine
14 pages

"We analyze the algebra of Dirac observables of the relativistic particle in four space-time dimensions. We show that the position observables become non-commutative and the commutation relations lead to a structure very similar to the non-commutative geometry of Deformed Special Relativity (DSR). In this framework, it appears natural to consider the 4d relativistic particle as a five dimensional massless particle. We study its quantization in terms of wave functions on the 5d light cone. We introduce the corresponding five-dimensional action principle and analyze how it reproduces the physics of the 4d relativistic particle. The formalism is naturally subject to divergences and we show that DSR arises as a natural regularization: the 5d light cone is regularized as the de Sitter space. We interpret the fifth coordinate as the particle's proper time while the fifth moment can be understood as the mass. Finally, we show how to formulate the Feynman propagator and the Feynman amplitudes of quantum field theory in this context in terms of Dirac observables. This provides new insights for the construction of observables and scattering amplitudes in DSR."

http://arxiv.org/abs/gr-qc/0701071
A signature of quantum gravity at the source of the seeds of cosmic structure?
Daniel Sudarsky
Prepared for the proceedings the Third International Workshop DICE 2006, "Quantum Mechanics between Decoherence and Determinism"

"This article reviews a recent work by a couple of colleagues and myself about the shortcomings of the standard explanations of the quantum origin of cosmic structure in the inflationary scenario, and a proposal to address them. The point it that in the usual accounts the inhomogeneity and anisotropy of our universe seem to emerge from an exactly homogeneous and isotropic initial state through processes that do not break those symmetries. We argued that some novel aspect of physics must be called upon to able to address the problem in a fully satisfactory way. The proposed approach is inspired on Penrose's ideas regarding an quantum gravity induced, real and dynamical collapse of the wave function."

It has been announced that this year Loops '07 conference will be in Mexico City (Sudarsky's home base) this year. I believe he is one of the conference organizers. I will keep special track of QG at Uni Mex, to keep prospective participants apprised of their hosts research interests.

 

[marcus]

CORRECTION:
LOOPS '07, this year's international conference on background independent (i.e. non-string) quantum gravity, will be at Morelia, about 200 miles west of Mexico City. Morelia is the site of the National University campus and several related institutes. Earlier I was under the mistaken impression that the conference was to be at Mexico City. The old city of Morelia has the reputation of being a picturesque place for sight-seeing, as well as having the University.

here is Francesca's post about the conference:
https://www.physicsforums.com/showthread.php?p=1212084#post1212084

here is the Morelia Math Institute website:
http://www.matmor.unam.mx/

it has this link to some tourist information in whichever language you prefer:
http://www.matmor.unam.mx/Morelia/espanol/default.html
http://www.matmor.unam.mx/Morelia/ingles/default.html
=========
this paper by Jonathan Engle appeared today:
http://arxiv.org/abs/gr-qc/0701132
On the physical interpretation of states in loop quantum cosmology
Jonathan Engle
20 pages

"In this paper we address the physical meaning of states in loop quantum cosmology (LQC). A first step in this is the completion of the program begun in [1], applied to LQC. Specifically, we introduce a family of (what are called) b-embeddings of isotropic loop quantum cosmology (LQC) into full loop quantum gravity. As a side note, we exhibit a large class of operators preserving each of these embeddings, and show their consistency with the LQC quantization. Embedding at the gauge and diffeomorphism invariant level is discussed in the conclusion section."

Engle got his PhD at Penn State in 2006 and is now in Rovelli's group at Marseille (CPT Luminy)
====================

http://arxiv.org/abs/hep-th/0701239
Loop quantum gravity and black hole singularity
Leonardo Modesto
17 pages, 7 figures. Proceedings of the XVII SIGRAV Conference, Turin, September 4-7, 2006

"In this paper we summarize loop quantum gravity (LQG) and we show how ideas developed in LQG can solve the black hole singularity problem when applied to a minisuperspace model."

=====================

The January 2007 KITP workshop on The Quantum Nature of Spacetime Singularities menu of talks online is here:
http://online.kitp.ucsb.edu/online/singular_m07/

Videos of the talks are available for download.
Some of the talks which may be of special interest to people concentrating on background independent (non-string) QG are those of

Ashtekar
http://online.kitp.ucsb.edu/online/singular_m07/ashtekar/

Thiemann (first hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann/

Thiemann (second hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann1/

Bojowald
http://online.kitp.ucsb.edu/online/singular_m07/bojowald/

===================
http://arxiv.org/abs/astro-ph/0701584
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
Edward L. Wright (UCLA)
16 pages, 8 figure

"The luminosity distance vs. redshift law is now measured using supernovae and gamma ray bursts, and the angular size distance is measured at the surface of last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In this paper this data is fit to models for the equation of state with w = -1, w = const, and w(z) = w_0+w_a(1-a). The last model is poorly constrained by the distance data, leading to unphysical solutions where the dark energy dominates at early times unless the large scale structure and acoustic scale constraints are modified to allow for early time dark energy effects. A flat LambdaCDM model is consistent with all the data."

Note discussion section page 14:

"Using all the data together
gives the plot shown in Figure 5. The best fit model is slightly closed with
Omega_tot = 1.011 and M = 0.315. "

===============
http://arxiv.org/abs/gr-qc/0701140
New Energy Definition for Higher Curvature Gravities
S. Deser, Bayram Tekin
4 pages

"We propose a novel but natural definition of conserved quantities for gravity models quadratic and higher in curvature. Based on the spatial asymptotics of curvature rather than of metric, it avoids the GR energy machinery's more egregious problems--such as zero energy 'theorems' and failure in flat backgrounds -- in this fourth-derivative realm. In D>4, the present expression indeed correctly discriminates between second derivative Gauss-Bonnet and generic, fourth derivative, actions."

 

[marcus]

http://arxiv.org/abs/gr-qc/0701142
Quantum gravity and cosmological observations
Martin Bojowald
8 pages, plenary talk at the VIth Latin American Symposium on High Energy Physics (Puerto Vallarta, Mexico, Nov. 2006)

"Quantum gravity places entirely new challenges on the formulation of a consistent theory as well as on an extraction of potentially observable effects. Quantum corrections due to the gravitational field are commonly expected to be tiny because of the smallness of the Planck length. However, a consistent formulation now shows that key features of quantum gravity imply magnification effects on correction terms which are especially important in cosmology with its long stretches of evolution. After a review of the salient features of recent canonical quantizations of gravity and their implications for the quantum structure of space-time a new example for potentially observable effects is given."

 

[marcus]

a possibly interesting challenge to LQC:
http://arxiv.org/abs/gr-qc/0701172
On the Onset of Inflation in Loop Quantum Cosmology
Cristiano Germani (SISSA), William Nelson (KCL London), Mairi Sakellariadou (KCL London)
18 pages, 5 figures

"Using a Liouville measure, similar to the one proposed recently by Gibbons and Turok, we investigate the probability that single-field inflation with a polynomial potential can last long enough to solve the shortcomings of the standard hot big bang model, within the semiclassical regime of loop quantum cosmology. We conclude that, for such a class of inflationary models and for natural values of the loop quantum cosmology parameters, a successful inflationary scenario is highly improbable."

http://arxiv.org/abs/astro-ph/0701848
The modified Newtonian dynamics-MOND-and its implications for new physics
Jacob D. Bekenstein
Pedagogical introduction to MOND and review of modern developments. To appear shortly in Contemporary Physics; 6 figures, 15 pages

"No more salient issue exists in contemporary astrophysics and cosmology than that of the elusive 'dark matter'. For many years already Milgrom's paradigm of modified Newtonian dynamics (MOND) has provided an alternative way to interpret observations without appeal to invisible dark matter. MOND had been successful in elucidating economically the dynamics of disk galaxies of all scales, while doing less well for clusters of galaxies; in its original form it could not address gravitational lensing or cosmology. After reviewing some of the evidence in favor of MOND, I recollect the development of relativistic formulations for it to cope with the last deficiency. I comment on recent work by various groups in confronting TeVeS, a relativistic embodiment of MOND, with observational data on gravitational lensing and cosmology. Throughout I ask what sort of physics can be responsible for the efficacy of MOND, and conclude with an appraisal of what theoretical developments are still needed to reach a full description of the world involving no unobserved matter."

http://arxiv.org/abs/gr-qc/0702002
Effect of the Inverse Volume Modification in Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages

"It is known that in loop quantum cosmology the universe avoids the singularity by a bounce when the matter density approaches the critical density \rho_c (the order of Planck density). In the framework of the effective Hamiltonian incorporating the inverse volume modifications both in the gravitational and matter part, we find that two modifications play different roles for the effective dynamics (the modified Friedmann equation). In the case a_{g*}>a_{bounce}, a_{bounce} denotes the bounce scale and the scale a_{g*} marked the different region below which the modification in the gravitational part become notable, the modification in the gravitational part decrease the matter density at the bounce point such that \rho_{bounce}<\rho _c. For the case a_{m*}>a_{bounce} (the role of a_{m*} is same as a_{g*}), the modification in the matter part helps the matter density to approach the critical density \rho_c quickly. Furthermore, based on the physical reason it is suitable to neglect the inverse volume modification in the gravitational part."

http://arxiv.org/abs/gr-qc/0702003
Tachyon field in Loop Quantum Cosmology: inflation and evolution picture
Hua-Hui Xiong, Jian-Yang Zhu
7 pages, 3 figures

"Loop quantum cosmology (LQC) predicts a nonsingular evolution of the universe through a bounce in the high energy region. We show that this is always true in tachyon matter LQC. Different from the classical FRW cosmology, the superinflation can appear in the tachyon matter LQC; furthermore, the inflation can be extended to the region where classical inflation stops. Using numerical method, we give an evolution picture of the tachyon field with an exponential potential in the context of LQC. It indicates that the quantum dynamical solutions have the attractor behavior as the classical solutions does. And, the whole evolution of the tachyon field is that: at the far past, the tachyon field, being in the contracting cosmology, is accelerated to climb up the potential hill with a negative velocity; and then, the tachyon field at the boundary is bounced into an expanding universe with positive velocity rolling down to the bottom of the potential."

http://arxiv.org/abs/astro-ph/0702010
Universe's Skeleton Sketched
Eric V. Linder
News & Views, Nature 445, 273 (2007)

"The deepest and clearest maps yet of the Universe's skeleton of dark matter structure present a picture broadly in concord with favoured models - although puzzling discrepancies remain."

 

[marcus]

new one from Bee Hossenfelder

http://arxiv.org/abs/hep-th/0702016
Multi-Particle States in Deformed Special Relativity
Sabine Hossenfelder

"We investigate the properties of multi-particle states in Deformed Special Relativity (DSR). Starting from the Lagrangian formalism with an energy dependent metric, the conserved Noether current is derived and is found to be additive in the usual way. This quantity had previously been discarded because it was correctly realized that it does no longer obey the DSR transformations. We identify the reason for this mismatch in the fact that DSR depends only on the extensive quantity of total four-momentum instead of the energy-momentum densities as would be appropriate for a field theory. We argue that the reason for the failure of DSR to reproduce the standard transformation behavior in the well established limits is due to the missing sensitivity to the volume inside which energy is accumulated. We show that the soccer-ball problem is absent if one formulates DSR instead for the field densities. As a consequence, estimates for predicted effects have to be corrected by many orders of magnitude. Further, we derive that the modified quantum field theory implies a locality bound."

=========================
briefly noted:

http://arxiv.org/abs/gr-qc/0702023
Evolutionary Reformulation of Quantum Gravity
Giovanni Montani
5 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin

"We present a critical analysis of the Canonical approach to quantum gravity, which relies on the ambiguity of implementing a space-time slicing on the quantum level. We emphasize that such a splitting procedure is consistent only if a real matter fluid is involved in the dynamics."


http://arxiv.org/abs/gr-qc/0702025
Electro-weak Model within a 5-dimensional Lorentz group theory
Orchidea Maria Lecian, Giovanni Montani
3 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin

"The Electroweak model will be geometrized in a 5-D Riemann-Cartan framework: U(1) weak hyper-charge group will be worked out in a Klauza-Klein scheme, while SU(2) weak isospin group will be identified to suitable bein projections of the contortion field. The possibility of introducing Ashtekar formalism in 5-D Klauza-Klein theories will be investigated."



http://arxiv.org/abs/gr-qc/0702026
Electro-weak Model within the framework of Lorentz gauge theory: Ashtekar variables?
Orchidea Maria Lecian, Giovanni Montani
5 pages, to appear in the Proceedings of the 1st Stueckelberg Workshop

"The Electroweak (EW) model is geometrized in the framework of a 5D gauge theory of the Lorentz group, after the implementation of the Kaluza-Klein (KK) paradigm. The possibility of introducing Ashtekar variables on a 5D KK manifold is considered on the ground of its geometrical structure."

 

[marcus]

http://arxiv.org/abs/gr-qc/0702030
An Introduction to Loop Quantum Gravity Through Cosmology
Abhay Ashtekar
20 pages, 4 figures, Introductory Review

"This introductory review is addressed to beginning researchers. Some of the distinguishing features of loop quantum gravity are illustrated through loop quantum cosmology of FRW models. In particular, these examples illustrate: i) how 'emergent time' can arise; ii) how the technical issue of solving the Hamiltonian constraint and constructing the physical sector of the theory can be handled; iii) how questions central to the Planck scale physics can be answered using such a framework; and, iv) how quantum geometry effects can dramatically change physics near singularities and yet naturally turn themselves off and reproduce classical general relativity when space-time curvature is significantly weaker than the Planck scale."

this paper of Ashtekar's grew out of a talk he gave in Summer 2006 at the First Stueckelberg Workshop at Pescara, italy.
http://www.icra.it/ICRA_Networkshops/INw20_Stueckelberg/Welcome.htmhttp://arxiv.org/abs/astro-ph/0702146
The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
J. R. Brownstein, J. W. Moffat
25 pages, 14 figures, 5 tables

"A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006)."http://arxiv.org/abs/math.QA/0702140
Lectures on Noncommutative Geometry
Masoud Khalkhali
112 pages

"This text is an introduction to a few selected areas of Alain Connes' noncommutative geometry written for the volume of the school/conference 'Noncommutative Geometry 2005' held at IPM Tehran. It is an expanded version of my lectures which was directed at graduate students and novice in the subject."

 

[marcus]

http://arxiv.org/abs/gr-qc/0702044
Disordered locality in loop quantum gravity states
Fotini Markopoulou, Lee Smolin
11 pages, 4 figures

"We show that loop quantum gravity suffers from a potential problem with non-locality, coming from a mismatch between micro-locality, as defined by the combinatorial structures of their microscopic states, and macro-locality, defined by the metric which emerges from the low energy limit. As a result, the low energy limit may suffer from a disordered locality characterized by identifications of far away points. We argue that if such defects in locality are rare enough they will be difficult to detect."

http://arxiv.org/abs/hep-th/0702051
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
M. Reuter, H. Weyer
18 pages, 4 figures. Invited contribution to the Int. J. Mod. Phys. D special issue on dark matter and dark energy

"The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that at large distances there could be strong renormalization effects, including a scale dependence of Newton's constant, which mimic the presence of dark matter at galactic and cosmological scales."

http://arxiv.org/abs/gr-qc/0702036
Phase-space and Black Hole Entropy of Toroidal Horizons in Loop Quantum Gravity
S. Kloster, J. Brannlund, A. DeBenedictis
14 pages, 6 figures

"In the context of loop quantum gravity, we construct the phase-space of the isolated horizon with toroidal topology. Within the loop quantum gravity framework, this horizon is described by a torus with N punctures and the dimension of the corresponding phase-space is calculated including the toroidal cycles as degrees of freedom. From this, the black hole entropy can be calculated by counting the microstates which correspond to a black hole of fixed area. We find that the leading term agrees with the A/4 law and that the sub-leading contribution is modified by the toroidal cycles."http://arxiv.org/abs/hep-th/0702064
A fine tuning free resolution of the cosmological constant problem
Stephon Alexander, Deepak Vaid
5 pages, 2 figures

"In a recent paper we discovered that a fermionic condensate is formed from gravitational interactions due to the covariant coupling of fermions in the presence of a torsion-fermion contact interaction. The condensate gap gives a negative contribution to the bare cosmological constant. In this letter, we show that the cosmological constant problem can be solved without fine tuning of the bare cosmological constant. We demonstrate how a universe with a large initial cosmological constant undergoes inflation, during which time the energy gap grows as the volume of the universe. Eventually the gap becomes large enough to cancel out the bare cosmological term, inflation ends and we end up in a universe with an almost vanishing cosmological term. We provide a detailed numerical analysis of the system of equations governing the self regulating relaxation of the cosmological constant."http://arxiv.org/abs/gr-qc/0702049
Canonical quantization and the spectral action, a nice example
Fabien Besnard (CPL)
22 pages. Revised version submitted to Journal of Geometry and Physics

"We study the canonical quantization of the theory given by Chamseddine-Connes spectral action on a particular finite spectral triple with algebra M_2(C)\oplus C. We define a quantization of the natural distance associated with this noncommutative space and show that the quantum distance operator has a discrete spectrum. We also show that it would be the same for any other geometric quantity. Finally we propose a physical Hilbert space for the quantum theory. This spectral triple had been previously considered by Rovelli as a toy model, but with a different action which was not gauge-invariant. The results are similar in both cases, but the gauge-invariance of the spectral action manifests itself by the presence of a non-trivial degeneracy structure for our distance operator."

http://arxiv.org/abs/astro-ph/0702207
It's Never Too Late For Matter
Lawrence M. Krauss (Case Western Reserve University and Vanderbilt University), Robert J. Scherrer (Vanderbilt University)
submitted to PRL

"We demonstrate that in a vacuum-energy-dominated expansion phase, neither the decay of matter nor matter-antimatter annihilation into relativistic particles can ever cause radiation to once again dominate over matter in the future history of the universe."

 

[MeJennifer]

http://arxiv.org/abs/quant-ph/0611261
Concerning Dice and Divinity
D.M.Appleby
Contribution to proceedings of Foundations of Probability and Physics, Vaxjo, 2006

"Einstein initially objected to the probabilistic aspect of quantum mechanics - the idea that God is playing at dice. Later he changed his ground, and focussed instead on the point that the Copenhagen Interpretation leads to what Einstein saw as the abandonment of physical realism. We argue here that Einstein's initial intuition was perfectly sound, and that it is precisely the fact that quantum mechanics is a fundamentally probabilistic theory which is at the root of all the controversies regarding its interpretation. Probability is an intrinsically logical concept. This means that the quantum state has an essentially logical significance. It is extremely difficult to reconcile that fact with Einstein's belief, that it is the task of physics to give us a vision of the world apprehended sub specie aeternitatis. Quantum mechanics thus presents us with a simple choice: either to follow Einstein in looking for a theory which is not probabilistic at the fundamental level, or else to accept that physics does not in fact put us in the position of God looking down on things from above. There is a widespread fear that the latter alternative must inevitably lead to a greatly impoverished, positivistic view of physical theory. It appears to us, however, that the truth is just the opposite. The Einsteinian vision is much less attractive than it seems at first sight. In particular, it is closely connected with philosophical reductionism."

It seems to me that there is a third option.
Assuming that a measurement in a given physical reality can be seen as a higher order self-expression, I don't consider it unreasonable that there would be information loss.

 

[marcus]

It seems to me that there is a third option.
Assuming that a measurement in a given physical reality can be seen as a higher order self-expression, I don't consider it unreasonable that there would be information loss.

Hi Jennifer, this thread is unfortunately not for discussion---I just collect non-string QG links here. We normally start discussion of papers in a separate discussion thread, not to overcrowd this one.
So I will set up a discussion thread for you, so that you and others can comment on Appleby's paper.

 

[MeJennifer]

Ok, no problem. Thanks for setting up a new topic.

 

[marcus]

You are most welcome! Here is a link to the discussion thread

https://www.physicsforums.com/showthread.php?t=155348

https://www.physicsforums.com/showthread.php?p=1238609#post1238609

 

[marcus]

http://arxiv.org/abs/gr-qc/0702065
Some Implications of the Cosmological Constant to Fundamental Physics
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
15 pages, lecture presented at the "XIIth Brazilian School of Cosmology and Gravitation", Mangaratiba, Rio de Janeiro, September 10-23, 2006

"In the presence of a cosmological constant, ordinary Poincaré special relativity is no longer valid and must be replaced by a de Sitter special relativity, in which Minkowski space is replaced by a de Sitter spacetime. In consequence, the ordinary notions of energy and momentum change, and will satisfy a different kinematic relation. Such a theory is a different kind of a doubly special relativity. Since the only difference between the Poincaré and the de Sitter groups is the replacement of translations by certain linear combinations of translations and proper conformal transformations, the net result of this change is ultimately the breakdown of ordinary translational invariance. From the experimental point of view, therefore, a de Sitter special relativity might be probed by looking for possible violations of translational invariance. If we assume the existence of a connection between the energy scale of an experiment and the local value of the cosmological constant, there would be changes in the kinematics of massive particles which could hopefully be detected in high-energy experiments. Furthermore, due to the presence of a horizon, the usual causal structure of spacetime would be significantly modified at the Planck scale."

http://arxiv.org/abs/gr-qc/0606122
de Sitter special relativity
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
24 pages, to be published in Classical and Quantum Gravity

"A special relativity based on the de Sitter group is introduced, which is the theory that might hold up in the presence of a non-vanishing cosmological constant. Like ordinary special relativity, it retains the quotient character of spacetime, and a notion of homogeneity. As a consequence, the underlying spacetime will be a de Sitter spacetime, whose associated kinematics will differ from that of ordinary special relativity. The corresponding modified notions of energy and momentum are obtained, and the exact relationship between them, which is invariant under a re-scaling of the involved quantities, explicitly exhibited. Since the de Sitter group can be considered a particular deformation of the Poincaré group, this theory turns out to be a specific kind of deformed (or doubly) special relativity. Some experimental consequences, as well as the causal structure of spacetime--modified by the presence of the de Sitter horizon--are briefly discussed."

http://arxiv.org/abs/astro-ph/0702275
Escaping from MOND
Benoit Famaey, Jean-Philippe Bruneton, HongSheng Zhao
4 pages, 1 figure, submitted to MNRAS

"We present a new test of modified Newtonian dynamics (MOND) on galactic scales, based on the escape speed in the solar neighbourhood. This test is independent from other empirical successes of MOND at reproducing the phenomenology of galactic rotation curves. The galactic escape speed in MOND is entirely determined by the baryonic content of the Galaxy and the external field in which it is embedded. We estimate that the external field in which the Milky Way must be embedded to produce the observed local escape speed of 544 km/s is of the order of a_0/100, where a_0 is the dividing acceleration scale below which gravity is boosted in MOND. This is compatible with the external gravitational field actually acting on the Milky Way."

http://arxiv.org/abs/astro-ph/0702298
Missing Pages in Our Photo Album of the Infant Universe
Abraham Loeb (Harvard)
Comments: Popular level review, to appear in "Physica Plus" magazine, Vol. 8; a modified version with original artwork appeared in Scientific American, 295, 46, 2006---see this http URL:
http://cfa-www.harvard.edu/~loeb/sciam.pdf

"Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see. In this popular-level overview, I describe how astronomers plan to observe this nearly-invisible yet crucial period."

 

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http://arxiv.org/abs/hep-th/0702124
Lorentz violation and perpetual motion
Christopher Eling, Brendan Z. Foster, Ted Jacobson, Aron C. Wall
5 pages, 1 figure

"We show that any Lorentz violating theory with two or more propagation speeds is in conflict with the generalized second law of black hole thermodynamics. We do this by identifying a classical energy-extraction method, analogous to the Penrose process, which would decrease the black hole entropy. Although the usual definitions of black hole entropy are ambiguous in this context, we require only very mild assumptions about its dependence on the mass. This extends the result found by Dubovsky and Sibiryakov, which uses the Hawking effect and applies only if the fields with different propagation speeds interact just through gravity. We also point out instabilities that could interfere with their black hole perpetuum mobile, but argue that these can be neglected if the black hole mass is sufficiently large."

Watch out for anything by Ted Jacobson. This is part of a cluster of recent papers dispelling hopes that GLAST might observe gammaray dispersion (energy dependent speed). There was one by Jerzy K-G and one that Laurent Freidel co-authored I think with K-G, and one by Bee Hossenfelder, and now this by Jacobson et al.

http://arxiv.org/abs/gr-qc/0702094
Black Hole Entropy and the Problem of Universality
Steven Carlip
10 pages; talk at DICE 2006, Piombino, Italy

"A key test of any quantum theory of gravity is its ability to reproduce the known thermodynamic properties of black holes. A statistical mechanical description of the Bekenstein-Hawking entropy once seemed remote, but today we suffer an embarrassment of riches: many different approaches to quantum gravity yield the same entropy, despite counting very different states. This 'universality' suggests that some underlying feature of the classical theory may control the quantum density of states. I discuss the possibility that this feature is an approximate two-dimensional conformal symmetry near the horizon."

http://arxiv.org/abs/gr-qc/0702093
Gauge invariant perturbations around symmetry reduced sectors of general relativity: applications to cosmology
Bianca Dittrich, Johannes Tambornino
39 pages, 1 figure

"We develop a gauge invariant canonical perturbation scheme for perturbations around symmetry reduced sectors in generally covariant theories, such as general relativity. The central objects of investigation are gauge invariant observables which encode the dynamics of the system. We apply this scheme to perturbations around a homogeneous and isotropic sector (cosmology) of general relativity. The background variables of this homogeneous and isotropic sector are treated fully dynamically which allows us to approximate the observables to arbitrary high order in a self--consistent and fully gauge invariant manner. Methods to compute these observables are given. The question of backreaction effects of inhomogeneities onto a homogeneous and isotropic background can be addressed in this framework. We illustrate the latter by considering homogeneous but anisotropic Bianchi--I cosmologies as perturbations around a homogeneous and isotropic sector."

Bianca Dittrich has worked with both Thomas Thiemann and Renate Loll (as the junior author) and has clearly aided TT's program. He keeps on citing a solo paper she did which has turned out valuable. I think her method of operation is to do contributory research that she sees needs to be done---to crack some difficult technical obstacle in somebody else's path. So although I don't immediately see where this paper fits in, i guess it's a significant piece is somebody's jigsaw.

http://arxiv.org/abs/gr-qc/0702082
Cosmic clocks, cosmic variance and cosmic averages
David L. Wiltshire
72 pages, 5 figures

"Cosmic acceleration is explained quantitatively, purely in general relativity, as an apparent effect due to quasilocal gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. "Dark energy" is recognised as a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localised, namely gradients in the energy associated with the expansion of space and spatial curvature variations in an inhomogeneous universe, as we observe. Gravitational energy differences between observers in bound systems, such as galaxies, and volume-averaged comoving locations within voids in freely expanding space can be so large that the time dilation between the two significantly affects the parameters of any effective homogeneous isotropic model one fits to the universe. A new approach to cosmological averaging is presented, which implicitly solves the Sandage-de Vaucouleurs paradox. When combined with a nonlinear scheme for cosmological evolution with back-reaction via the Buchert equations, a new observationally viable quantitative model of the universe is obtained. The expansion age is increased, allowing more time for structure formation. The baryon density fraction obtained from primordial nucleosynthesis bounds can be significantly larger, yet consistent with primordial lithium abundance measurements. The angular scale of the first Doppler peak in the CMB anisotropy spectrum fits the new model despite an average negative spatial curvature at late epochs, resolving the anomaly associated with ellipticity in the CMB anisotropies. A number of other testable consequences are discussed, with the potential to profoundly change the whole of theoretical and observational cosmology. [Abridged] "

Special thanks to Kea, who is mentioned in the acknowledgments :-), for calling attention to this new article by David Wiltshire.

http://www.arxiv.org/abs/hep-ex/0702026
Charting the Course for Elementary Particle Physics
Burton Richter
AAAS National Meeting, San Francisco, 2007, Symposium, A New Frontier in Particle Physics, 15 pages, 8 figures
SLAC-PUB-12345

"It was the best of times; it was the worst of times is the way Dickens begins the Tale of Two Cities. The line is appropriate to our time in particle physics. It is the best of times because we are in the midst of a revolution in understanding, the third to occur during my career. It is the worst of times because accelerator facilities are shutting down before new ones are opening, restricting the opportunity for experiments, and because of great uncertainty about future funding. My task today is to give you a view of the most important opportunities for our field under a scenario that is constrained by a tight budget. It is a time when we cannot afford the merely good, but must give first priority to the really important."

 

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Today (20 Feb) Martin Bojowald gave a seminar talk for the ILQGS in which he summarized the 3-week workshop on
The Quantum Nature of Spacetime Singularities at Santa Barbara KITP
The lecture notes are here:
http://relativity.phys.lsu.edu/ilqgs/
http://relativity.phys.lsu.edu/ilqgs/bojowald022007.pdf

I have not yet encountered the audio for the talk. The ILQGS webpage has an error, a different talk was given on 13 Feb.

http://arxiv.org/abs/gr-qc/0702107
Black Hole Thermodynamics from Euclidean Horizon Constraints
S. Carlip
4 pages

"To explain black hole thermodynamics in quantum gravity, one must introduce constraints to ensure that a black hole is actually present. I show that for a large class of black holes, the presence of such 'horizon constraints' makes it possible to use conformal field theory techniques to compute the density of states, reproducing the Bekenstein-Hawking entropy in a nearly model-independent manner. I argue that the relevant degrees of freedom may be Goldstone-boson-like excitations arising from the weak breaking of symmetry by the horizon constraints."

The second paper by Steve Carlip on BH entropy this week.

http://arxiv.org/abs/hep-th/0702159
Deformed Special Relativity in a Canonical Framework
Subir Ghosh (Indian Statistical Institute, India), Probir Pal (Uluberia College, India)
17 pages

"In this paper we have studied the nature of kinematical and dynamical laws in \kappa-Minkowski spacetime from a new perspective: the canonical phase space approach. We have introduced a new form of \kappa-Minkowski phase space algebra from which we recover the \kappa-extended finite Lorentz transformations derived in [13]. This is a particular form of a Deformed Special Relativity model that admits a modified energy-momentum dispersion law as well as noncommutative \kappa-Minkowski phase space. We show that this system can be completely mapped to a set of phase space variables that obey canonical (and not \kappa-Minkowski) phase space algebra and Special Relativity Lorentz transformation (and not \kappa-extended Lorentz transformation). We demonstrate the usefulness and simplicity of this approach through a number of applications both in classical and quantum mechanics. We also construct a Lagrangian for the \kappa-particle."

 

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William Donnelly has started a blog called "Uncommon Information"
and one of the first posts called attention to a 1995 paper by Ted Jacobson and a related 2006 workshop talk he gave at Perimeter
http://arxiv.org/abs/gr-qc/9504004
Thermodynamics of Spacetime: The Einstein Equation of State
Ted Jacobson
8 pages, 1 figure.
Phys.Rev.Lett. 75 (1995) 1260-1263

"The Einstein equation is derived from the proportionality of entropy and horizon area together with the fundamental relation \delta Q=TdS connecting heat, entropy, and temperature. The key idea is to demand that this relation hold for all the local Rindler causal horizons through each spacetime point, with \delta Q and T interpreted as the energy flux and Unruh temperature seen by an accelerated observer just inside the horizon. This requires that gravitational lensing by matter energy distorts the causal structure of spacetime in just such a way that the Einstein equation holds. Viewed in this way, the Einstein equation is an equation of state. This perspective suggests that it may be no more appropriate to canonically quantize the Einstein equation than it would be to quantize the wave equation for sound in air."

Donnelly's blog is here:
http://williamdonnelly.blogspot.com/
and the post about T.J.'s Perimeter workshop talk is here:
http://williamdonnelly.blogspot.com/2006/09/natural-ultraviolet-cutoffs-in_09.html

I couldn't find a PIRSA video of this talk. the date would have been Wednesday 6 September 2006.
Sounds interesting both from Donnelly's summary and from the related 1995 paper----you assume that all causal horizons have the famous S = A/4 relation between horizon area and entropy and from that simple relation you can derive the Einstein equation of GR. The roots of intuition that grew out of must be the most interesting thing about it.
============================

there is a new blog about NonCommutative Geometry
http://noncommutativegeometry.blogspot.com/2007/02/rosetta-stone-of-noncommutative.html
It just started in the past month or so. One of the people posting is Masoud Khalkhali.
Alain Connes submitted a comment to the blog recently.

There is also a new journal called Journal of Non-Commutative Geometry (JNCG)
http://www.ems-ph.org/journals/jncg/jncg.php
I see that Marc Rieffel and Vaughn Jones are on the editorial board. Connes is the journal's top editor.
The first issue of JNCG appeared this month.

NCG and Connes are especially notable now because there is an actual prediction that the higgs boson mass will be about 170 GeV.
http://resonaances.blogspot.com/2007/02/alain-connes-standard-model.html
https://www.physicsforums.com/showthread.php?t=127342
http://www.sciam.com/print_version.cfm?articleID=00039831-4051-14C0-AFE483414B7F4945
Here are PF predictions---Jim Graber set up a poll
https://www.physicsforums.com/showthread.php?t=147265

briefly noted:
http://arxiv.org/abs/hep-th/0702178
Eternal inflation and its implications
Alan H. Guth
21 pages, 5 figures. Talk presented at the "2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC 2006)," Barcelona, Spain, 11-15 July 2006, to be published in J. Phys. A

 

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http://math.ucr.edu/home/baez/week246.html
John Baez TWF 246 discusses issues raised in
the Smolin book The Trouble with Physics...and What Comes Next and the Woit book Not Even Wrong

Thanks to Christine Dantas for calling attention to the new JB essay!

http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages

"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."

http://arxiv.org/abs/gr-qc/0702125
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
Winston Fairbairn, Etera R. Livine
17 pages, 1 figure

"An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models in hep-th/0512113. In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recasted as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of hep-th/0512113 for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which we propose to interpret as living on curved space-times. We conclude by discussing the possible extension to four-dimensional spinfoam models."

http://arxiv.org/abs/gr-qc/0702132
A generalized Schroedinger equation for loop quantum cosmology
D. C. Salisbury, A. Schmitz
5 pages, to appear in the Proceedings of the Eleventh Marcel Grossmann Meeting

"A temporally discrete Schroedinger time evolution equation is proposed for isotropic quantum cosmology coupled to a massless scalar source. The approach employs dynamically determined intrinsic time and produces the correct semiclassical limit."

http://arxiv.org/abs/astro-ph/0702670
Dynamical Dark Energy or Simply Cosmic Curvature?
Chris Clarkson, Marina Cortes, Bruce A. Bassett
5 pages, 1 figure

"We show that the assumption of a flat universe induces critically large errors in reconstructing the dark energy equation of state at z>~0.9 even if the true cosmic curvature is very small, O(1%) or less. The spuriously reconstructed w(z) shows a range of unusual behaviour, including crossing of the phantom divide and mimicking of standard tracking quintessence models. For 1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching (50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble rate measurements have opposite trends due to the asymmetric influence of the curved geodesics. These results show that including curvature as a free parameter is imperative in any future analyses attempting to pin down the dynamics of dark energy, especially at moderate or high redshifts."briefly noted:

http://arxiv.org/abs/hep-th/0702192
The Universe as a topological defect
Andres Anabalon, Steven Willison, Jorge Zanelli

http://arxiv.org/abs/hep-th/0702115
Predicting the Cosmological Constant from the Causal Entropic Principle
Raphael Bousso, Roni Harnik, Graham D. Kribs, Gilad Perez

 

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http://arxiv.org/abs/gr-qc/0702144
Singularities and Quantum Gravity
Martin Bojowald
41 pages, lecture course at the XIIth Brazilian School on Cosmology and Gravitation, September 2006
IGPG-07/2-4, NSF-KITP-07-19

"Although there is general agreement that a removal of classical gravitational singularities is not only a crucial conceptual test of any approach to quantum gravity but also a prerequisite for any fundamental theory, the precise criteria for non-singular behavior are often unclear or controversial. Often, only special types of singularities such as the curvature singularities found in isotropic cosmological models are discussed and it is far from clear what this implies for the very general singularities that arise according to the singularity theorems of general relativity. In these lectures we present an overview of the current status of singularities in classical and quantum gravity, starting with a review and interpretation of the classical singularity theorems. This suggests possible routes for quantum gravity to evade the devastating conclusion of the theorems by different means, including modified dynamics or modified geometrical structures underlying quantum gravity. The latter is most clearly present in canonical quantizations which are discussed in more detail. Finally, the results are used to propose a general scheme of singularity removal, quantum hyperbolicity, to show cases where it is realized and to derive intuitive semiclassical pictures of cosmological bounces."

briefly noted:
http://arxiv.org/abs/hep-th/0702219
String Theory: Progress and Problems
John H. Schwarz
14 pages; Presented at the Yukawa--Tomonaga Centennial Symposium
CALT-68-2627

"...This talk will give an overview of some of the progress and some of the unsolved problems that characterize string theory today..."

 

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http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations
Kirill Krasnov
21 pages

"We describe and study a certain class of modified gravity theories. Our starting point is Plebanski formulation of gravity in terms of a triple of 2-forms, a connection A and a 'Lagrange multiplier' field Psi. The generalization we consider stems from presence in the action of an extra term proportional to a scalar function of Psi. As in the usual Plebanski general relativity (GR) case, the equations coming from variations with respect to Psi imply that a certain metric can be introduced. However, unlike in GR, the connection A no longer coincides with the self-dual part of the metric-compatible spin-connection. Field equations of the theory are shown to be relations between derivatives of the metric and components of field Psi, as well as its derivatives, the later being in contrast to the GR case. The equations are of second order in derivatives. An analog of the Bianchi identity is still present in the theory, as well as its contracted version tantamount to energy conservation equation. The arising modifications to the later are possibly of experimental significance."

 

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Extending Quant. Cos. to the ANISOTROPIC case

http://arxiv.org/abs/gr-qc/0703010
Effective Dynamics for the Cosmological Bounces in Bianchi Type I Loop Quantum Cosmology
Dah-Wei Chiou
7 pages, 4 figures
IGPG-07/2-5

"The detailed formulation for loop quantum cosmology (LQC) in Bianchi I models was recently constructed. In this paper, the effective dynamics with the LQC discreteness corrections is studied and exactly solved, showing that the big bang is replaced by big bounces, which take place up to three times, once in each diagonal direction, whenever each of the area scale factors approaches its critical value in the Planck regime measured by the reference of the scalar field momentum."

Dah-Wei has already published the prequel to this paper in Physical Review series D:

http://arxiv.org/abs/gr-qc/0609029
Loop Quantum Cosmology in Bianchi Type I Models: Analytical Investigation
Dah-Wei Chiou
53 pages, 2 figures
Phys.Rev. D75 (2007) 024029

The comprehensive formulation for loop quantum cosmology in the spatially flat, isotropic model was recently constructed. In this paper, the methods are extended to the anisotropic Bianchi I cosmology. Both the precursor and the improved strategies are applied and the expected results are established: (i) the scalar field again serves as an internal clock and is treated as emergent time; (ii) the total Hamiltonian constraint is derived by imposing the fundamental discreteness and gives the evolution as a difference equation; and (iii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously. It is also shown that the state in the kinematical Hilbert space associated with the classical singularity is decoupled in the difference evolution equation, indicating that the big bounce may take place when any of the area scales undergoes the vanishing behavior. The investigation affirms the robustness of the framework used in the isotropic model by enlarging its domain of validity and provides foundations to conduct the detailed numerical analysis.

Dah-Wei Chiou got his PhD in theoretical physics from UC-BERKELEY in IIRC 2005. He was still doing string in 2005, but he escaped out of string and made it to Penn State to work in Ashtekar's group on singularity resolution in quantum cosmology. In my humble private opinion DahWei Chiou made a smart move and he has already done very significant research.
Before, one could handle the big bounce of the universe only in case it was not LOP-SIDED. If it was perfectly symmetrical you could get a collapse turning into an expansion. But what about the lop-sided case? What happens when the space and matter are not perfectly isotropic? Will the collapse get screwed-up and not bounce right? This is the important ANisotropic, or "not-isotropic", asymmetrical case. Dah-Wei has addressed this in one particular version called "Bianchi #1".
Carlo Rovelli and Matteo Smerlak have gotten their Relational EPR accepted for publication and they have made the final revision on it.
They posted the final version yesterday 4 March and it seems actually a little different from the first version, which we discussed here at PF. I will put the link again in case anyone wants to check out the revised version of the paper.
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
Revised, published version

"We study the EPR-type correlations from the perspective of the relational interpretation of quantum mechanics. We argue that these correlations do not entail any form of 'non-locality', when viewed in the context of this interpretation. The abandonment of strict Einstein realism implied by the relational stance permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."

 

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http://arxiv.org/abs/gr-qc/0703044
de Sitter space and the equivalence between f(R) and scalar-tensor gravity
Valerio Faraoni (Bishop's University)
4 pages, to appear in Phys. Rev. D

"It is shown that, when f'' is non-vanishing, metric f(R) gravity is completely equivalent to a scalar-tensor theory (with zero Brans-Dicke parameter) with respect to perturbations of de Sitter space, contrary to previous expectations. Moreover, the stability conditions of de Sitter space with respect to homogeneous and inhomogeneous perturbations coincide in most scalar-tensor theories, as is the case in metric f(R) gravity."

http://arxiv.org/abs/gr-qc/0703050
Vacuum properties of nonsymmetric gravity in de Sitter space
Tomas Janssen, Tomislav Prokopec (ITP & Spinoza Institute, Utrecht University)
32 pages, 2 figures
ITP-UU-07/9, SPIN-07/9

"We consider quantum effects of a massive antisymmetric tensor field on the dynamics of de Sitter space-time. Our starting point is the most general, stable, linearized Lagrangian arising in nonsymmetric gravitational theories (NGTs), where part of the antisymmetric field mass is generated by the cosmological term. We construct a renormalization group (RG) improved effective action by integrating out one loop vacuum fluctuations of the antisymmetric tensor field and show that, in the limit when the RG scale goes to zero, the Hubble parameter -- and thus the effective cosmological constant -- relaxes rapidly to zero. We thus conclude that quantum loop effects in de Sitter space can dramatically change the infrared sector of the on-shell gravity, making the expansion rate insensitive to the original (bare) cosmological constant."

http://arxiv.org/abs/quant-ph/0703060
A Topos Foundation for Theories of Physics: I. Formal Languages for Physics
A. Doering, C.J. Isham
36 pages

"This paper is the first in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper we discuss two different types of language that can be attached to a system, S. The first is a propositional language, PL(S); the second is a higher-order, typed language L(S). Both languages provide deductive systems with an intuitionistic logic. The reason for introducing PL(S) is that, as shown in paper II of the series, it is the easiest way of understanding, and expanding on, the earlier work on topos theory and quantum physics. However, the main thrust of our programme utilises the more powerful language L(S) and its representation in an appropriate topos."

http://arxiv.org/abs/quant-ph/0703062
A Topos Foundation for Theories of Physics: II. Daseinisation and the Liberation of Quantum Theory
A. Doering, C.J. Isham
34 pages

"This paper is the second in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper, we study in depth the topos representation of the propositional language, PL(S), for the case of quantum theory. In doing so, we make a direct link with, and clarify, the earlier work on applying topos theory to quantum physics. The key step is a process we term `daseinisation' by which a projection operator is mapped to a sub-object of the spectral presheaf--the topos quantum analogue of a classical state space. In the second part of the paper we change gear with the introduction of the more sophisticated local language L(S). From this point forward, throughout the rest of the series of papers, our attention will be devoted almost entirely to this language. In the present paper, we use L(S) to study `truth objects' in the topos. These are objects in the topos that play the role of states: a necessary development as the spectral presheaf has no global elements, and hence there are no microstates in the sense of classical physics. Truth objects therefore play a crucial role in our formalism."

http://arxiv.org/abs/quant-ph/0703064
A Topos Foundation for Theories of Physics: III. The Representation of Physical Quantities With Arrows
A. Doering, C.J. Isham
38 pages

"This paper is the third in a series whose goal is to develop a fundamentally new way of viewing theories of physics. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. In paper II, we studied the topos representations of the propositional language PL(S) for the case of quantum theory, and in the present paper we do the same thing for the, more extensive, local language L(S). One of the main achievements is to find a topos representation for self-adjoint operators. This involves showing that, for any physical quantity A, there is an arrow \breve{\delta}^o(A):\Sig\mapsymbol, where symbol is the quantity-value object for this theory. The construction of \breve{\delta}^o(A) is an extension of the daseinisation of projection operators that was discussed in paper II. The object symbol is a monoid-object only in the topos, \tau_\phi, of the theory, and to enhance the applicability of the formalism, we apply to symbol a topos analogue of the Grothendieck extension of a monoid to a group. The resulting object, symbol, is an abelian group-object in \tau_\phi. We also discuss another candidate, PR, for the quantity-value object. In this presheaf, both inner and outer daseinisation are used in a symmetric way. Finally, there is a brief discussion of the role of unitary operators in the quantum topos scheme."
[Comment: I couldn't get many of the symbols in this abstract to translate into LaTex and eventually left a substantial portion untranslated.]

http://arxiv.org/abs/quant-ph/0703066
A Topos Foundation for Theories of Physics: IV. Categories of Systems
A. Doering, C.J. Isham
38 pages

"This paper is the fourth in a series whose goal is to develop a fundamentally new way of building theories of physics. The motivation comes from a desire to address certain deep issues that arise in the quantum theory of gravity. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. The previous papers in this series are concerned with implementing this programme for a single system. In the present paper, we turn to considering a collection of systems: in particular, we are interested in the relation between the topos representation for a composite system, and the representations for its constituents. We also study this problem for the disjoint sum of two systems. Our approach to these matters is to construct a category of systems and to find a topos representation of the entire category."

http://arxiv.org/abs/gr-qc/0703027
Conserved Quantities in Background Independent Theories
Fotini Markopoulou
11 pages, 3 figures

"We discuss the difficulties that background independent theories based on quantum geometry encounter in deriving general relativity as the low energy limit. We follow a geometrogenesis scenario of a phase transition from a pre-geometric theory to a geometric phase which suggests that a first step towards the low energy limit is searching for the effective collective excitations that will characterize it. Using the correspondence between the pre-geometric background independent theory and a quantum information processor, we are able to use the method of noiseless subsystems to extract such coherent collective excitations. We illustrate this in the case of locally evolving graphs." http://arxiv.org/abs/gr-qc/0703052
Existence of generalized semiclassical Kodama states. I. The Ashtekar--Klein--Gordon model
Eyo Eyo Ita
32 pages

"This is the first in a series of papers aimed at outlining an algorithm to explicitly construct a finite quantum theory of gravity in Ashtekar variables. The algorithm is based upon extending some properties of a special state, the Kodama state for pure gravity, to more general models. In this paper we analyse a simple case, gravity coupled to a Klein-Gordon scalar field in the minisuperspace Ansatz, in order to derive a criterion for a new semiclassical state and its corresponding semiclassical orbits of spacetime. We then illustrate a presciption for nonperturbatively constructing the analog of the Kodama state for a general case, in preparation for subsequent works in this series."

http://arxiv.org/abs/gr-qc/0703056
Existence of generalized quantum Kodama states. II. The minisuperspace Ashtekar--Klein--Gordon model
Eyo Eyo Ita
41 pages

"This is the second in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. In Part I we constructed a generalized semiclassical Kodama state by solving the classical Hamiltonian constraint under the condition of a broken semiclassical-quantum correspondence due to a Klein-Gordon scalar field. In Part II we will demonstrate a method of restoring this correspondence by generalizing the self-duality condition for the Ashtekar electromagnetic field. The end result will be to establish the existence of a generalized quantum Kodama state devoid of quantum corrections in the minisuperspace model. We also derive the equations needed to solve for the full theory of a finite theory of quantum gravity within the context of this new interpretation."

http://arxiv.org/abs/gr-qc/0703057
Existence of generalized Kodama quantum states. III. A new approach to finite, full quantum gravity
Eyo Eyo Ita
18 pages

"This is the third in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper is a first attempt at the quantization of the full theory coupled to matter, in this case to a spatially inhomogeneous Klein-Gordon scalar field. We delineate the conditions required to construct a solution to the quantum Hamiltonian constraint under the Ansatz of an isotropic, but spatially inhomogeneous, Ashtekar connection, and highlight some differences relative to the minisuperspace case."

briefly mentioned:
http://arxiv.org/abs/gr-qc/0703055
Hawking radiation as tunneling from Gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu

http://arxiv.org/abs/gr-qc/0703058
Asymptotic quasinormal modes of scalar field in a gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu

briefly mentioned:
http://arxiv.org/abs/hep-th/0703055
(reminder about Vaas new book "Beyond the big bang" Springer 2007)

 

[marcus]

http://arxiv.org/abs/gr-qc/0703078
Snyder's Model -- de Sitter Special Relativity Duality and de Sitter Gravity
Han-Ying Guo, Chao-Guang Huang, Yu Tian, Hong-Tu Wu, Bin Zhou
28 pages

"Between Snyder's quantized space-time model in de Sitter space of momenta and the dS special relativity on dS-spacetime of radius R with Beltrami coordinates, there is a one-to-one dual correspondence supported by a minimum uncertainty-like argument. Together with Planck length \ell_P, R\simeq (3/\Lambda)^{1/2} should be a fundamental constant. They lead to a dimensionless constant g{\sim\ell_PR^{-1}}=(G\hbar c^{-3}\Lambda/3)^{1/2}\sim 10^{-61}. These indicate that physics at these two scales should be dual to each other and there is in between gravity of local dS-invariance characterized by g. A simple model of dS-gravity with a gauge-like action on umbilical manifolds may show these characters. It can pass the observation tests and support the duality."

http://arxiv.org/abs/gr-qc/0703074
Three-geometry and reformulation of the Wheeler-DeWitt equation
Chopin Soo
10 pages
Class. Quantum Grav. 24 (2007) 1547-1555

"A reformulation of the Wheeler-DeWitt equation which highlights the role of gauge-invariant three-geometry elements is presented. It is noted that the classical super-Hamiltonian of four-dimensional gravity as simplified by Ashtekar through the use of gauge potential and densitized triad variables can furthermore be succinctly expressed as a vanishing Poisson bracket involving three-geometry elements. This is discussed in the general setting of the Barbero extension of the theory with arbitrary non-vanishing value of the Immirzi parameter, and when a cosmological constant is also present. A proposed quantum constraint of density weight two which is polynomial in the basic conjugate variables is also demonstrated to correspond to a precise simple ordering of the operators, and may thus help to resolve the factor ordering ambiguity in the extrapolation from classical to quantum gravity. Alternative expression of a density weight one quantum constraint which may be more useful in the spin network context is also discussed, but this constraint is non-polynomial and is not motivated by factor ordering. The article also highlights the fact that while the volume operator has become a preeminient object in the current manifestation of loop quantum gravity, the volume element and the Chern-Simons functional can be of equal significance, and need not be mutually exclusive. Both these fundamental objects appear explicitly in the reformulation of the Wheeler-DeWitt constraint."http://arxiv.org/abs/astro-ph/0703352
Confrontation of MOND with the rotation curves of early-type disc galaxies
R.H. Sanders, E. Noordermeer
9 pages, 2 figures, submitted MNRAS

brief mention:
http://arxiv.org/abs/hep-th/0703115
Transient Observers and Variable Constants, or Repelling the Invasion of the Boltzmann's Brains
S. Carlip
4 pages

"If the universe expands exponentially without end, 'ordinary observers' like ourselves may be vastly outnumbered by 'Boltzmann's brains,' transient observers who briefly flicker into existence as a result of quantum or thermal fluctuations. One might then wonder why we are so atypical. I show that tiny changes in physics--for instance, extremely slow variations of fundamental constants--can drastically change this result, and argue that one should be wary of conclusions that rely on exact knowledge of the laws of physics in the very distant future."http://arxiv.org/abs/hep-th/0703116
Quantization in black hole backgrounds
Steven B. Giddings
28 pages, 4 figures

first sentences read in part:
"1. Introduction
Hawking’s discovery of black hole radiance[1] has produced a paradox that may be as important to finding a quantum description of gravity as the paradox of the classical instability of matter was in the foundation of quantum mechanics. There is no commonly accepted explanation for what is wrong with Hawking’s original argument that black holes destroy information[2]. This is despite widespread belief that black holes respect unitary quantum evolution, which is now shared by originator of the paradox himself[3]."

To fill out the picture somewhat, AFAIK Hawking may have conceded his BET at Dublin in 2004, but he did not thereby establish anything to change experts opinion about quantum geometry, information loss, black holes. Hawking notwithstanding it is quite possible that some information that falls into hole is indeed lost from this universe. So Hawking's error may ultimately turn out to be not what he originally proposed, but the fact that he mistakenly conceded.

I wonder why Giddings didn't cite Steve Hsu? And why Hsu didn't cite a bunch of LQG work relevant to his paper that Ted Jacobson could have told him about. It seems this field of inquiry is fragmented, not to say "atomized". Hsu and Giddings know each other personally, so the omission is especially puzzling. Here is Hsu's recent paper on the subject:

http://arxiv.org/abs/hep-th/0608175
Spacetime topology change and black hole information
Stephen D.H. Hsu
5 pages, 5 figures, to appear in Physics Letters B
Phys.Lett. B644 (2007) 67-71

"Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in Planckian regimes. It does not require non-locality or any modification of low-energy physics."

 

[marcus]

http://arxiv.org/abs/astro-ph/0703566
Thermal fluctuations in loop cosmology
Joao Magueijo, Parampreet Singh
10 pages

"Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations. We show that the no-go results for scale invariance in classical thermal models can be evaded even if we just consider modifications to the background (zeroth order) gravitational dynamics. Since a complete and systematic treatment of the perturbed Einstein equations in loop cosmology is still lacking, we simply parameterize their expected modifications. These change quantitatively, but not qualitatively, our conclusions. We thus urge the community to more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe."

http://arxiv.org/abs/gr-qc/0703098
Relativity theory does not imply that the future already exists: a counterexample
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
Comments: plainTeX, 12 pages, no figures. To appear in Vesselin Petkov (editor), Relativity and the Dimensionality of the World (Springer 2007, in press). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/

"It is often said that the relativistic fusion of time with space rules out genuine change or 'becoming'. I offer the classical sequential growth models of causal set theory as counterexamples."

http://arxiv.org/abs/gr-qc/0703099
Does Locality Fail at Intermediate Length-Scales
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
24 pages, 2 figures. To appear in Daniele Oriti (ed.), Towards Quantum Gravity (Cambridge University Press, 2007). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/

"If quantum gravity implies a fundamental spatiotemporal discreteness, and if its 'laws of motion' are compatible with the Lorentz transformations, then physics cannot remain local. One might expect this nonlocality to be confined to the fundamental discreteness scale, but I will present evidence that it survives at much lower energies, yielding for example a nonlocal equation of motion for a scalar field propagating on an underlying causal set."

http://arxiv.org/abs/gr-qc/0703097
New directions in Background Independent Quantum Gravity
Fotini Markopoulou
26 pages. Contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"We discuss the meaning of background independence in quantum theories of gravity where geometry and gravity are emergent and illustrate the possibilities using the framework of quantum causal histories."

brief mention:
http://arxiv.org/abs/physics/0703191
The Expansion of the Universe and the Cosmological Constant Problem
R. F. O'Connell
Phys. Lett. A, in press
http://arxiv.org/find/grp_physics/1/au:OConnell_R_F/0/1/0/all/0/1

http://arxiv.org/abs/quant-ph/0703179
Disproof of Bell's Theorem by Clifford Algebra Valued Local Variables
Joy Christian (Perimeter and Oxford)
4 pages

 

[marcus]

http://arxiv.org/abs/gr-qc/0703114
Note on non-metric gravity
Ingemar Bengtsson
9 pages

We discuss a class of alternative gravity theories that are specific to four dimensions, do not introduce new degrees of freedom, and come with a physical motivation. In particular we sketch their Hamiltonian formulation, and their relation to some earlier constructions."

This refers to two recent papers by Kirill Krasnov
http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations

For anyone not familiar with Bengtsson here are 34 papers in gr-qc, hep-th, and quant-ph going back to 1992
http://arxiv.org/find/grp_physics/1/au:+Bengtsson_I/0/1/0/all/0/1

He picked up fast on Krasnov's idea of a way to make gravity renormalizable,
relating it to the asymptotic safety program.

http://arxiv.org/abs/gr-qc/0703116
Loop quantum gravity and Planck-size black hole entropy
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
21 pages, 5 figures. Contribution to the Proceedings of the NEB XII International Conference

"The Loop Quantum Gravity (LQG) program is briefly reviewed and one of its main applications, namely the counting of black hole entropy within the framework is considered. In particular, recent results for Planck size black holes are reviewed. These results are consistent with an asymptotic linear relation (that fixes uniquely a free parameter of the theory) and a logarithmic correction with a coefficient equal to -1/2. The account is tailored as an introduction to the subject for non-experts."

 

[marcus]

http://arxiv.org/abs/gr-qc/0703144
Dynamical coherent states and physical solutions of quantum cosmological bounces
Martin Bojowald
30 pages, 3 figures
IGPG-07/3-5, NSF-KITP-07-55

"A new model is studied which describes the quantum behavior of transitions through an isotropic quantum cosmological bounce in loop quantum cosmology sourced by a free and massless scalar field. As an exactly solvable model even at the quantum level, it illustrates properties of dynamical coherent states and provides the basis for a systematic perturbation theory of loop quantum gravity. The detailed analysis is remarkably different from what is known for harmonic oscillator coherent states. Results are evaluated with regard to their implications in cosmology, including a demonstration that in general quantum fluctuations before and after the bounce are unrelated. Thus, even within this solvable model the condition of classicality at late times does not imply classicality at early times before the bounce without further assumptions. Nevertheless, the quantum state does evolve deterministically through the bounce."

http://arxiv.org/abs/gr-qc/0703135
Loop quantization of spherically symmetric midi-superspaces
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
18 pages

"We quantize the exterior of spherically symmetric vacuum space-times using a midi-superspace reduction within the Ashtekar new variables. Through a partial gauge fixing we eliminate the diffeomorphism constraint and are left with a Hamiltonian constraint that is first class. We complete the quantization in the loop representation. We also use the model to discuss the issues that will arise in more general contexts in the 'uniform discretization' approach to the dynamics."

http://arxiv.org/abs/gr-qc/0703137
Decoherence in Quantum Gravity: Issues and Critiques
C. Anastopoulos, B. L. Hu
25 pages, proceedings of DICE06 (Piombino)

"An increasing number of papers have appeared in recent years on decoherence in quantum gravity at the Planck energy. We discuss the meaning of decoherence in quantum gravity starting from the common notion that quantum gravity is a theory for the microscopic structures of spacetime, and invoking some generic features of quantum decoherence from the open systems viewpoint. We dwell on a range of issues bearing on this process including the relation between statistical and quantum, noise from effective field theory, the meaning of stochasticity, the origin of non-unitarity and the nature of nonlocality in this and related contexts. To expound these issues we critique on two representative theories: One claims that decoherence in quantum gravity scale leads to the violation of CPT symmetry at sub-Planckian energy which is used to explain today's particle phenomenology. The other uses this process in place with the Brownian motion model to prove that spacetime foam behaves like a thermal bath."

could be a useful text and reference work:
http://arxiv.org/abs/astro-ph/0703730
Why CMB physics?
Massimo Giovannini
179 pages, 27 figures
CERN-PH-TH/2007-048

"The aim of these lectures is to introduce some basic problems arising in gravitation and modern cosmology. All along the discussion the guiding theme is provided by the phenomenological and theoretical properties of the Cosmic Microwave Background (CMB). These lectures have been prepared for a regular Phd course of the University of Milan-Bicocca."

briefly noted:
http://arxiv.org/abs/astro-ph/0703722
Observable Signatures of a Black Hole Ejected by Gravitational Radiation Recoil in a Galaxy Merger
Abraham Loeb (Harvard)
4 pages, submitted to PRL

"According to recent general-relativistic simulations, the coalescence of two spinning black holes (BHs) could lead to recoil speeds of the BH remnant of up to thousands of km/s as a result of the emission of gravitational radiation. Such speeds would enable the merger product to escape its host galaxy. Here we examine the circumstances resulting from a gas-rich galaxy merger under which the ejected BH would carry an accretion disk with it and be observable. As the initial BH binary emits gravitational radiation and its orbit tightens, a hole is opened around it in the disk which delays the consumption of gas prior to the eventual BH ejection. The punctured disk remains bound to the ejected BH within the region where the gas orbital velocity is larger than the ejection speed. For a ~10^7 solar mass BH the ejected disk has a characteristic size of tens of thousands of Schwarzschild radii and an accretion lifetime of ~10^7 years. During that time, the ejected BH could traverse a considerable distance and appear as an off-center quasar with a feedback trail along the path it left behind. A small fraction of all quasars could be associated with an escaping BH."

briefly noted:

http://arxiv.org/abs/gr-qc/0703150
A gravitational explanation for quantum theory - non-time-orientable manifolds
Mark J Hadley
7 pages Talk given at FFP8 in Madrid 2006

"Spacetime manifolds that are not time orientable play a key role in a gravitational explanation of quantum theory. Such manifolds allow topology change, but also have fascinating additional properties such as net charge from source-free equations and spin half transformation properties. It is shown how the logical structure of propositions and the probabilities of quantum theory arise from such acausal space times."

http://arxiv.org/abs/hep-th/0703265
A new PPN parameter to test Chern-Simons gravity
Stephon Alexander, Nicolas Yunes
4 pages, submitted to PRL

"We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN) framework through weak-field solutions of the modified field equations for a perfect fluid source... This new term encodes the key physical effect of CS gravity in the weak-field limit, leading to a modification of frame dragging and, thus, the Lense-Thirring contribution to gyroscopic precession. We provide a physical interpretation for the new term, as well as an estimate of the size of this effect relative to the general relativistic Lense-Thirring prediction. This correction to frame dragging might be used in experiments,.."

 

[marcus]

http://arxiv.org/abs/0704.0145
Singularity Resolution in Isotropic Loop Quantum Cosmology: Recent Developments
Ghanashyam Date
(Submitted on 2 Apr 2007)

"Since the past Iarge meeting in December 2004, new developments in loop quantum cosmology have taken place, especially with regards to the resolution of the Big Bang singularity in the isotropic models. The singularity resolution issue has been discussed in terms of physical quantities (expectation values of Dirac observables) and there is also an 'improved' quantization of the Hamiltonian constraint. These developments are briefly discussed. This is an expanded version of the review talk given at the 24-th IAGRG meeting in February 2007."

http://arxiv.org/abs/0704.0007
Polymer Quantum Mechanics and its Continuum Limit
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
(Submitted on 31 Mar 2007)

"A rather non-standard quantum representation of the canonical commutation relations of quantum mechanics systems, known as the polymer representation has gained some attention in recent years, due to its possible relation with Planck scale physics. In particular, this approach has been followed in a symmetric sector of loop quantum gravity known as loop quantum cosmology (LQC). Here we explore different aspects of the relation between the ordinary Schroedinger theory and the polymer description. The paper has two parts. In the first one, we derive the polymer quantum mechanics starting from the ordinary Schroedinger theory and show that the polymer description arises as an appropriate limit. In the second part we consider the continuum limit of this theory, namely, the reverse process in which one starts from the discrete theory and tries to recover back the ordinary Schroedinger quantum mechanics. We consider several examples of interest, including the harmonic oscillator, the free particle and a simple cosmological model."

http://arxiv.org/abs/0704.0221
The Return of a Static Universe and the End of Cosmology
Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
(Submitted on 2 Apr 2007)

"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented."

briefly noted:
http://arxiv.org/abs/0704.0058
Intelligent Life in Cosmology
Frank J. Tipler

 

[marcus]

http://arxiv.org/abs/0704.0278
q-Deformed spin foam models of quantum gravity
Igor Khavkine, J. Daniel Christensen
(Submitted on 2 Apr 2007)

"We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous."

Dan Christensen accesses the supercomputer at U. West Ontario (UWO) where he has charge of programs in mathematics, computing, and quantum gravity. Co-authored spinfoam papers with John Baez several years back and with others since. Use of numerical techniques notable here---probably a Beowolf cluster.

http://arxiv.org/abs/0704.0367
Existence of generalized Kodama states. IV. The search for a quantization of 4-dimensional gravity
Eyo Eyo Ita III

"This is the fourth in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper continues essentially from where papers II and III left off, treating the kinematic constraints in greater depth and moving on to a higher level of complexity with regard to the Hamiltonian constraint of the full, unrestricted theory. First we identify some of the traditional obstacles to the consistent quantization of four-dimensional gravity, then provide suggestions for how these obstacles may possibly be surmounted within the context of our new approach. This inevitably entails a critical analysis of the relationship of gauge transformations to diffeomorphisms, which in turn leads to the reduced phase space approach to quantization of the kinematic constraints, as well as the implementation of the semiclassical-quantum correspondence. We also compute some more of the terms needed to implement the full quantum Hamiltonian constraint, focusing in this work on its antisymmetric CDJ components. Some of the relationships among generalized Kodama states for the more general model are clearly elucidated due to the calculation of such terms."
33 pages

E.E.Ita is a PhD student at Cambridge.

http://arxiv.org/abs/0704.0299
Parametrized Post-Newtonian Expansion of Chern-Simons Gravity
Stephon Alexander, Nicolas Yunes

"We investigate the weak-field, post-Newtonian expansion to the solution of the field equations in Chern-Simons gravity with a perfect fluid source. In particular, we study the mapping of this solution to the parameterized post-Newtonian formalism to 1 PN order in the metric. We find that the PPN parameters of Chern-Simons gravity are identical to those of general relativity, with the exception of the inclusion of a new term that is proportional to the Chern-Simons coupling parameter and the curl of the PPN vector potentials. We also find that the new term is naturally enhanced by the non-linearity of spacetime and we provide a physical interpretation for it. By mapping this correction to the gravito-electro-magnetic framework, we study the corrections that this new term introduces to the acceleration of point particles and the frame-dragging effect in gyroscopic precession. We find that the Chern-Simons correction to these classical predictions could be used by current and future experiments to place bounds on intrinsic parameters of Chern-Simons gravity and, thus, string theory."
14 pages

 

[marcus]

http://arxiv.org/abs/0704.1137
Lattice refining loop quantum cosmology, anisotropic models and stability
Martin Bojowald, Daniel Cartin, Gaurav Khanna
24 pages

"A general class of loop quantizations for anisotropic models is introduced and discussed, which enhances loop quantum cosmology by relevant features seen in inhomogeneous situations. The main new effect is an underlying lattice which is being refined during dynamical changes of the volume. In general, this leads to a new feature of dynamical difference equations which may not have constant step-size, posing new mathematical problems. It is discussed how such models can be evaluated and what lattice refinements imply for semiclassical behavior. Two detailed examples illustrate that stability conditions can put strong constraints on suitable refinement models, even in the absence of a fundamental Hamiltonian which defines changes of the underlying lattice. Thus, a large class of consistency tests of loop quantum gravity becomes available. In this context, it will also be seen that quantum corrections due to inverse powers of metric components in a constraint are much larger than they appeared recently in more special treatments of isotropic, free scalar models where they were artificially suppressed."

http://arxiv.org/abs/0704.0992
Compatibility of radial, Lorenz and harmonic gauges
Elena Magliaro, Claudio Perini, Carlo Rovelli
9 pages

"We observe that the radial gauge can be consistently imposed together with the Lorenz gauge in Maxwell theory, and with the harmonic traceless gauge in linearized general relativity. This simple observation has relevance for some recent developments in quantum gravity where the radial gauge is implicitly utilized.