Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/0805.3511
The covariant entropy bound and loop quantum cosmology
Abhay Ashtekar, Edward Wilson-Ewing
15 pages, 3 figures
(Submitted on 22 May 2008)

"We examine Bousso's covariant entropy bound conjecture in the context of radiation filled, spatially flat, Friedmann-Robertson-Walker models. The bound is violated near the big bang. However, the hope has been that quantum gravity effects would intervene and protect it. Loop quantum cosmology provides a near ideal setting for investigating this issue. For, on the one hand, quantum geometry effects resolve the singularity and, on the other hand, the wave function is sharply peaked at a quantum corrected but smooth geometry which can supply the structure needed to test the bound. We find that the bound is respected. We suggest that the bound need not be an essential ingredient for a quantum gravity theory but may emerge from it under suitable circumstances."

 

[marcus]

I believe this is an important paper supporting the existence of a positive cosmological constant (aka "dark energy") and tending to dispell alternative explanations of accelerated expansion
http://arxiv.org/abs/0805.3695
An Imprint of Super-Structures on the Microwave Background due to the Integrated Sachs-Wolfe Effect
Benjamin R. Granett, Mark C. Neyrinck, István Szapudi (IfA, Hawaii)
5 pages, 1 figure
(Submitted on 25 May 2008)

"We measure hot and cold spots on the microwave background associated with supercluster and supervoid structures identified in the Sloan Digital Sky Survey Luminous Red Galaxy catalog. The mean temperature deviation is 9.6 +/- 2.2 microK. We interpret this as a detection of the late-time Integrated Sachs-Wolfe (ISW) effect, in which cosmic acceleration from dark energy causes gravitational potentials to decay, heating or cooling photons passing through density crests or troughs. In a flat universe, the linear ISW effect is a direct signal of dark energy. The statistical significance of our detection is over 4 sigma, making it the clearest detection to date using a single galaxy dataset. Moreover, our method produces a compelling visual image of the effect."

http://arxiv.org/abs/0805.3750
Direct Detection of Gravity Waves from Neutron Stars
Redouane Al Fakir, William G. Unruh
19 pages, 2 figures
(Submitted on 24 May 2008)

"In light of the discovery of the first-ever double pulsar system, PSR J0737-3039, we re-examine an earlier proposal to directly detect gravity waves from neutron stars, which was predicated on a hypothetical system almost identical to the later discovered double pulsar. We re-derive the effect in more detail, and confirm the initial estimate--sometimes doubted in the literature--that it includes a 1/b dependence, where b is the impact parameter of a pulsar with respect to its foreground, gravity-wave emitting, neutron star companion. A coherent modulation in pulsar time-of-arrival measurements of 10 nano-sec/sec is possible. A one-year intermittent experiment on an instrument comparable to the SKA could thus detect the exceedingly faint gravity waves from individual neutron stars."

 

[marcus]

This is our PF Francesca. Great going! Homogeneity needs to be relaxed in LQC, and they have found a way to do it in a stepwise gradual fashion. Rovelli has delivered an online seminar talk about this at ILQGS.

http://arxiv.org/abs/0805.4585
Stepping out of Homogeneity in Loop Quantum Cosmology
Carlo Rovelli, Francesca Vidotto
16 pages
(Submitted on 29 May 2008)

"We explore the extension of quantum cosmology outside the homogeneous approximation, using the formalism of loop quantum gravity. We introduce a model where some of the inhomogeneous degrees of freedom are present, providing a tool for describing general fluctuations of quantum geometry near the initial singularity. We show that the dynamical structure of the model reduces to that of loop quantum cosmology in the Born-Oppenheimer approximation. This result corroborates the assumptions that ground loop cosmology, sheds light on the physical and mathematical relation between loop cosmology and full loop quantum gravity, and on the nature of the cosmological approximation. Finally, we show that the non-graph-changing Hamiltonian constraint considered in the context of algebraic quantum gravity provides a viable effective dynamics within this approximation."

Song He is familiar to us as the co-author of braid-matter research with Yidun Wan. While he was at Perimeter Institute working on braid-matter, Song He took over Sabine Hossenfelder's office (she was back in Germany). I am happy about this paper. It seems very clever. Extend Bousso covariant entropy bound, apply it to the standard cosmological model, take account of black holes, and bingo! you get a nice upper bound on the cosmological constant.

http://arxiv.org/abs/0805.4614
A covariant entropy bound conjecture on the dynamical horizon
Song He, Hongbao Zhang
JHEP style, 9 pages, 1 figure, honorable mention award received from Gravity Research Foundation for 2008 Essay Competition
(Submitted on 29 May 2008 (v1), last revised 29 May 2008 (this version, v2))

"As a compelling pattern for the holographic principle, our covariant entropy bound conjecture is proposed for more general dynamical horizons. Then we apply our conjecture to LambdaCDM cosmological models, where we find it imposes a novel upper bound 10^^-90 on the cosmological constant for our own universe by taking into account the dominant entropy contribution from super-massive black holes, which thus provides an alternative macroscopic perspective to understand the longstanding cosmological constant problem. As an intriguing implication of this conjecture, we also discuss the possible profound relation between the present cosmological constant, the origin of mass, and the anthropic principle."

http://arxiv.org/abs/0805.4545
Not so non-renormalizable gravity
Dirk Kreimer
7 pages
(Submitted on 29 May 2008)

"We review recent progress with the understanding of quantum fields, including ideas how gravity might turn out to be a renormalizable theory after all."

http://arxiv.org/abs/0805.2909
Investigating the Ultraviolet Properties of Gravity with a Wilsonian Renormalization Group Equation
Alessandro Codello, Roberto Percacci, Christoph Rahmede
(Submitted on 19 May 2008)

"We review and extend in several directions recent results on the asymptotic safety approach to quantum gravity. The central issue in this approach is the search of a Fixed Point having suitable properties, and the tool that is used is a type of Wilsonian renormalization group equation. We begin by discussing various cutoff schemes, i.e. ways of implementing the Wilsonian cutoff procedure. We compare the beta functions of the gravitational couplings obtained with different schemes, studying first the contribution of matter fields and then the so-called Einstein-Hilbert truncation, where only the cosmological constant and Newton's constant are retained. In this context we make connection with old results, in particular we reproduce the results of the epsilon expansion and the perturbative one loop divergences. We then apply the Renormalization Group to higher derivative gravity. In the case of a general action quadratic in curvature we recover, within certain approximations, the known asymptotic freedom of the four-derivative terms, while Newton's constant and the cosmological constant have a nontrivial fixed point. In the case of actions that are polynomials in the scalar curvature of degree up to eight we find that the theory has a fixed point with three UV-attractive directions, so that the requirement of having a continuum limit constrains the couplings to lie in a three-dimensional subspace, whose equation is explicitly given. We emphasize throughout the difference between scheme-dependent and scheme-independent results, and provide several examples of the fact that only dimensionless couplings can have 'universal' behavior.

 

[marcus]

http://arxiv.org/abs/0805.4696
Regularization and finiteness of the Lorentzian LQG vertices
Jonathan Engle, Roberto Pereira
13 pages
(Submitted on 30 May 2008)

"We give an explicit form for the Lorentzian vertices recently introduced for possibly defining the dynamics of loop quantum gravity. As a result of so doing, a natural regularization of the vertices is found and suggested. The regularized vertices are then proven to be finite."

 

[marcus]

http://arxiv.org/abs/0806.0339
Quantum Gravity Phenomenology
Giovanni Amelino-Camelia
82 pages
(Submitted on 2 Jun 2008)

"I review the present status of the development of Quantum Gravity Phenomenology. Among the accomplishments of this young research area I stress in particular the significance of studies which established that some appropriate data analyses provide sensitivity to effects introduced genuinely at the Planck scale. The objective of testing/falsifying theories that provide comprehensive solutions to the quantum-gravity problem appears to be still rather far, but we might soon be in a position to investigate some 'falsifiable quantum-gravity theories of not everything' ".

 

[marcus]

The Self-Organized de Sitter Universe

http://arxiv.org/abs/0806.0397
The Self-Organized de Sitter Universe
J. Ambjorn, J. Jurkiewicz, R. Loll
7 pp, 1 figure, entry to 2008 GRF essay competition, honourable mention
(Submitted on 2 Jun 2008)

"We propose a theory of quantum gravity which formulates the quantum theory as a nonperturbative path integral, where each spacetime history appears with a weight given by the exponentiated Einstein-Hilbert action of the corresponding causal geometry. The path integral is diffeomorphism-invariant (only geometries appear) and background-independent. The theory can be investigated by computer simulations, which show that a de Sitter universe emerges on large scales. This emergence is of an entropic, self-organizing nature, with the weight of the Einstein-Hilbert action playing a minor role. Also the quantum fluctuations around this de Sitter universe can be studied quantitatively and remain small until one gets close to the Planck scale. The structures found to describe Planck-scale gravity are reminiscent of certain aspects of condensed-matter systems."

http://arxiv.org/abs/0806.0595
Unique factor ordering in the continuum limit of LQC
William Nelson, Mairi Sakellariadou (KCL London)
12 pages
(Submitted on 3 Jun 2008)

"We show that the factor ordering ambiguities associated with the loop quantisation of the gravitational part of the cosmological Hamiltonian constraint, disappear at the level of Wheeler-DeWitt equation only for a particular choice of lattice refinement model, which coincides with constraints imposed from phenomenological and consistency arguments."

 

[marcus]

http://arxiv.org/abs/0806.2593
Lemaitre-Tolman-Bondi collapse from the perspective of loop quantum gravity
Martin Bojowald, Tomohiro Harada, Rakesh Tibrewala
56 pages, 42 figures
(Submitted on 16 Jun 2008)

"Lemaitre-Tolman-Bondi models as specific spherically symmetric solutions of general relativity simplify in their reduced form some of the mathematical ingredients of black hole or cosmological applications. The conditions imposed in addition to spherical symmetry turn out to take a simple form at the kinematical level of loop quantum gravity, which allows a discussion of their implications at the quantum level. Moreover, the spherically symmetric setting of inhomogeneity illustrates several non-trivial properties of lattice refinements of discrete quantum gravity. Nevertheless, the situation at the dynamical level is quite non-trivial and thus provides insights to the anomaly problem. At an effective level, consistent versions of the dynamics are presented which implement the conditions together with the dynamical constraints of gravity in an anomaly-free manner. These are then used for analytical as well as numerical investigations of the fate of classical singularities, including non-spacelike ones, as they generically develop in these models. None of the corrections used here resolve those singularities by regular effective geometries. However, there are numerical indications that the collapse ends in a tamer shell-crossing singularity prior to the formation of central singularities for mass functions giving a regular conserved mass density. Moreover, we find quantum gravitational obstructions to the existence of exactly homogeneous solutions within this class of models. This indicates that homogeneous models must be seen in a wider context of inhomogeneous solutions and their reduction in order to provide reliable dynamical conclusions."

 

[marcus]

http://arxiv.org/abs/0806.2783
Effective Action for Loop Quantum Cosmology à la Palatini
Gonzalo J. Olmo, Parampreet Singh
4 pages, 2 figures
(Submitted on 17 Jun 2008)

"The resolution of the big bang singularity in loop quantum cosmology can be described by quantum gravitational modifications to the Friedman dynamics without introducing any new degrees of freedom. A challenging question is to construct a covariant effective action able to reproduce that new physics emergent at the Planck scale. By enlarging our scope to the realm of non-metric theories, we show that apparent tensions with conventional approaches can be overcome. We provide an algorithm to construct an effective action for loop quantum cosmology in the Palatini framework and obtain an action which involves an infinite series in the scalar curvature of the independent connection. This effective action successfully captures non-local quantum geometric effects and the non-singular bounce at the Planck scale and leads to general relativity at low curvatures."

http://arxiv.org/abs/0806.2821
Fermions in Loop Quantum Cosmology and the Role of Parity
Martin Bojowald, Rupam Das
17 pages
(Submitted on 17 Jun 2008)

"Fermions play a special role in homogeneous models of quantum cosmology because the exclusion principle prevents them from forming sizable matter contributions. They can thus describe the matter ingredients only truly microscopically and it is not possible to avoid strong quantum regimes by positing a large matter content. Moreover, possible parity violating effects are important especially in loop quantum cosmology whose basic object is a difference equation for the wave function of the universe defined on a discrete space of triads. The two orientations of a triad are interchanged by a parity transformation, which leaves the difference equation invariant for ordinary matter. Here, we revisit and extend loop quantum cosmology by introducing fermions and the gravitational torsion they imply, which renders the parity issue non-trivial. A treatable locally rotationally symmetric Bianchi model is introduced which clearly shows the role of parity. General wave functions cannot be parity-even or odd, and parity violating effects in matter influence the microscopic big bang transition which replaces the classical singularity in loop quantum cosmology."

 

[francesca]

http://arxiv.org/abs/0806.3082"
Anomalous CMB polarization and gravitational chirality
Carlo R. Contaldi, Joao Magueijo, Lee Smolin
5 pages (Submitted on 18 Jun 2008)

We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation.

 

[marcus]

Francesca, thanks for catching that one by Smolin, Magueijo, Contaldi
good luck on your presentation at QG2!
The next listing is by a relatively new author on the scene, whose previous LQG paper was published by CGG in 2007.

http://arxiv.org/abs/0806.3356
Spontaneous symmetry breaking in Loop Quantum Gravity
Gabor Helesfai
24 pages
(Submitted on 20 Jun 2008)

"In this paper we investigate the question how spontaneous symmetry breaking works in the framework of Loop Quantum Gravity and we compare it to the results obtained in the case of the Proca field. We obtained that the Hamiltonian of the two systems are very similar, the only difference is an extra scalar field in the case of spontaneous symmetry breaking. This field can be identified as the field that carries the mass of the vector field. In the quantum regime this becomes a well defined operator, which turns out to be a self adjoint operator with continuous spectrum. To calculate the spectrum we used a new representation in the case of the scalar fields, which in addition enabled us to rewrite the constraint equations to a finite system of linear partial differential equations. This made it possible to solve part of the constraints explicitly."

 

[ccdantas]

http://arxiv.org/abs/0806.3082"
Anomalous CMB polarization and gravitational chirality
Carlo R. Contaldi, Joao Magueijo, Lee Smolin
5 pages (Submitted on 18 Jun 2008)

We consider the possibility that gravity breaks parity, with left and right handed gravitons coupling to matter with a different Newton's constant and show that this would affect their zero-point vacuum fluctuations during inflation. Should there be a cosmic background of gravity waves, the effect would translate into anomalous CMB polarization. Non-vanishing TB (and EB) polarization components emerge, revealing interesting experimental targets. Indeed if reasonable chirality is present a TB measurement would provide the easiest way to detect a gravitational wave background. We speculate on the theoretical implications of such an observation.

Hmm. I think "Uncle Al" will like this one.

 

[George Jones]

Hmm. I think "Uncle Al" will like this one.

 

[marcus]

http://arxiv.org/abs/0806.3506
Shaken, but not stirred - Potts model coupled to quantum gravity
J. Ambjorn, K.N. Anagnostopoulos, R. Loll, I. Pushkina
19 pages, 9 figures
(Submitted on 21 Jun 2008)

"We investigate the critical behaviour of both matter and geometry of the three-state Potts model coupled to two-dimensional Lorentzian quantum gravity in the framework of causal dynamical triangulations. Contrary to what general arguments of the effects of disorder suggest, we find strong numerical evidence that the critical exponents of the matter are not changed under the influence of quantum fluctuations in the geometry, compared to their values on fixed, regular lattices. This lends further support to previous findings that quantum gravity models based on causal dynamical triangulations are in many ways better behaved than their Euclidean counterparts."

http://arxiv.org/abs/0806.3713
Cosmic Rays and the Search for a Lorentz Invariance Violation
Wolfgang Bietenholz
76 pages, 15 figures
(Submitted on 23 Jun 2008)

"This is an introductory review about the on-going search for a signal of Lorentz Invariance Violation (LIV) in cosmic rays. We first summarise basic aspects of cosmic rays, focusing on rays of ultra high energy (UHECRs). We discuss the Greisen-Zatsepin-Kuz'min (GZK) energy cutoff for cosmic protons, which is predicted due to photopion production in the Cosmic Microwave Background (CMB). This is a process of modest energy in the proton rest frame. It can be investigated to a high precision in the laboratory, if Lorentz transformations apply even at factors \gamma \sim O(10^{11}). For heavier nuclei the energy attenuation is even faster due to photo-disintegration, again if this process is Lorentz invariant. Hence the viability of Lorentz symmetry up to tremendous gamma-factors - far beyond accelerator tests - is a central issue. Next we comment on conceptual aspects of Lorentz Invariance and the possibility of its spontaneous breaking. This could lead to slightly particle dependent 'Maximal Attainable Velocities'. We discuss their effect in decays, Cerenkov radiation, the GZK cutoff and neutrino oscillation in cosmic rays. We also review the search for LIV in cosmic gamma-rays. For multi TeV gamma-rays we possibly encounter another puzzle related to the transparency of the CMB, similar to the GZK cutoff. The photons emitted in a Gamma Ray Burst occur at lower energies, but their very long path provides access to information not far from the Planck scale. No LIV has been observed so far. However, even extremely tiny LIV effects could change the predictions for cosmic ray physics drastically. An Appendix is devoted to the recent hypothesis by the Pierre Auger Collaboration, which identifies nearby Active Galactic Nuclei - or objects next to them - as probable UHECR sources."

 

[marcus]

http://arxiv.org/abs/0806.3929
Anomaly freedom in perturbative loop quantum gravity
Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, S. Shankaranarayanan
54 pages
(Submitted on 24 Jun 2008)

"A fully consistent linear perturbation theory for cosmology is derived in the presence of quantum corrections as they are suggested by properties of inverse volume operators in loop quantum gravity. The underlying constraints present a consistent deformation of the classical system, which shows that the discreteness in loop quantum gravity can be implemented in effective equations without spoiling space-time covariance. Nevertheless, non-trivial quantum corrections do arise in the constraint algebra. Since correction terms must appear in tightly controlled forms to avoid anomalies, detailed insights for the correct implementation of constraint operators can be gained. The procedures of this article thus provide a clear link between fundamental quantum gravity and phenomenology."

http://arxiv.org/abs/0806.3776
The quasiclassical realms of this quantum universe
James B. Hartle (University of California, Santa Barbara)
24 pages
(Submitted on 23 Jun 2008)

"The most striking observable feature of our indeterministic quantum universe is the wide range of time, place, and scale on which the deterministic laws of classical physics hold to an excellent approximation. This essay describes how this domain of classical predictability of every day experience emerges from a quantum theory of the universe's state and dynamics."

 

[marcus]

http://arxiv.org/abs/0806.4239
On the emergence of time and gravity
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
10 pages
(Submitted on 26 Jun 2008)

"In recent years, a growing momentum has been gained by the emergent gravity framework. Within the latter, the very concepts of geometry and gravitational interaction are not seen as elementary aspects of Nature but rather as collective phenomena associated to the dynamics of more fundamental objects. In this paper we want to further explore this possibility by proposing a model of emergent time and scalar gravity. Assuming that the dynamics of the fundamental objects can give rise in first place to a Riemannian manifold and a set of scalar fields we show how time (in the sense of hyperbolic equations) can emerge as a property of perturbations dynamics around some specific class of solutions of the field equations. Moreover, we show that these perturbations can give rise to a full theory of gravity via a suitable redefinition of the fields that identifies the relevant degrees of freedom. In particular, we find that our model gives rise to Nordström gravity. Since this theory is invariant under general coordinate transformations, this also shows how diffeomorphism invariance can emerge from much simpler systems."

since tests of QG may come out gammaray astronomy, we should occasionally check up on that field. here is a recent survey perspective on it
http://arxiv.org/abs/0806.4363
From MAGIC to CTA: the INAF participation to Cherenkov Telescopes experiments for Very High Energy Astrophysics
L. Angelo Antonelli (INAF MAGIC Collaboration)
5 pages, 2 figures, to appear in Mem. SAIt, Proceedings of the "LII congresso della Societa' Astronomica Italiana", Teramo, May 4-8 2008
(Submitted on 26 Jun 2008)

"The next decade can be considered the 'golden age' of the Gamma Ray Astronomy with the two satellites for Gamma Ray Astronomy (AGILE and GLAST) in orbit. Therefore, thanks to many other X-ray experiments already in orbit (e.g. Swift, Chandra, NewtonXMM, etc.) it will be possible to image the Universe for the first time all over the electromagnetic spectrum almost contemporarily. The new generations of ground-based very high gamma-ray instruments are ready to extend the observed band also to the very high frequencies. Scientists from the Italian National Institute for Astrophysics (INAF) are involved in many, both space- and ground- based gamma ray experiments, and recently such an involvement has been largely improved in the field of the Imaging Atmospheric Cherenkov Telescopes (IACT). INAF is now member of the MAGIC collaboration and is participating to the realization of the second MAGIC telescope. MAGIC, as well other IACT experiments, is not operated as an observatory so a proper guest observer program does not exist. A consortium of European scientists (including INAF scientists) is thus now thinking to the design of a new research infrastructure: the Cherenkov Telescope Array (CTA). CTA is conceived to provide 10 times the sensitivity of current instruments, combined with increased flexibility and increased coverage from some 10 GeV to some 100 TeV. CTA will be operated as an observatory to serve a wider community of astronomer and astroparticle physicists."

 

[marcus]

http://arxiv.org/abs/0806.4382
Superconducting loop quantum gravity and the cosmological constant
Stephon H.S. Alexander, Gianluca Calcagni
5 pages
(Submitted on 26 Jun 2008)

"We argue that the cosmological constant is exponentially suppressed in a candidate ground state of loop quantum gravity as a nonperturbative effect of a holographic Fermi-liquid theory living on a two-dimensional spacetime. Ashtekar connection components, corresponding to degenerate gravitational configurations breaking large gauge invariance and CP symmetry, behave as composite fermions that condense as in Bardeen--Cooper--Schrieffer theory of superconductivity. Cooper pairs admit a description as wormholes on a de Sitter boundary."

 

[marcus]

http://arxiv.org/abs/0806.4640
Path integral representation of spin foam models of 4d gravity
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
29 pages, 6 figures
(Submitted on 28 Jun 2008)

"We give a unified description of all recent spin foam models introduced by Engle, Livine, Pereira and Rovelli (ELPR) and by Freidel and Krasnov (FK). We show that the FK models are, for all values of the Immirzi parameter, equivalent to path integrals of a discrete theory and we provide an explicit formula for the associated actions. We discuss the relation between the FK and ELPR models and also study the corresponding boundary states. For general Immirzi parameter, these are given by Alexandrov's and Livine's SO(4) projected states. For 0 <= gamma < 1, the states can be restricted to SU(2) spin networks."

http://arxiv.org/abs/0806.4710
The length operator in Loop Quantum Gravity
Eugenio Bianchi
33 pages
(Submitted on 28 Jun 2008)

"The dual picture of quantum geometry provided by a spin network state is discussed. From this perspective, we introduce a new operator in Loop Quantum Gravity - the length operator. We describe its quantum geometrical meaning and derive some of its properties. In particular we show that the operator has a discrete spectrum and is diagonalized by appropriate superpositions of spin network states. A series of eigenstates and eigenvalues is presented and an explicit check of its semiclassical properties is discussed."

 

[marcus]

http://arxiv.org/abs/0807.0160
Loop Quantum Cosmology corrections to inflationary models
Michal Artymowski, Zygmunt Lalak, Lukasz Szulc
16 pages, 1 figure
(Submitted on 1 Jul 2008)

"In the recent years the quantization methods of Loop Quantum Gravity have been successfully applied to the homogeneous and isotropic Friedmann-Robertson-Walker space-times. The resulting theory, called Loop Quantum Cosmology (LQC), resolves the Big Bang singularity by replacing it with the Big Bounce. We argue that LQC generates also certain corrections to field theoretical inflationary scenarios. These corrections imply that in the LQC the effective sonic horizon becomes infinite at some point after the bounce and that the scale of the inflationary potential implied by the COBE normalisation increases. The evolution of scalar fields immediately after the Bounce becomes modified in an interesting way. We point out that one can use COBE normalisation to establish an upper bound on the quantum of length of LQG."

Zygmunt Lalak is a new author in LQC. He has some 71 publications going back to 1986, in various other research areas including inflation scenarios, braneworld models, string-related topics. But this seems to be his first venture into Loop Quantum Cosmology. Lukasz Szulc has co-authored several papers with Jerzy Lewandowski (a longtime LQG and LQC expert and collaborator of Ashtekar's)
The conclusion that Cosmic Microwave Background data might be used to bound the smallest eigenvalue of the LQG length operator surprised me initially, but the bound one finds in the paper is not very tight. Still it is a beginning and may suggest ways to get improved results (assuming both LQC and some particular inflation parameters).

 

[marcus]

http://arxiv.org/abs/0807.0225
Quantum gravity as a Fermi liquid
Stephon H.S. Alexander, Gianluca Calcagni
43 pages, 1 figure
(Submitted on 1 Jul 2008)

"We present a reformulation of loop quantum gravity with a cosmological constant and no matter as a Fermi-liquid theory. When the topological sector is deformed and large gauge symmetry is broken, we show that the Chern-Simons state reduces to Jacobson's degenerate sector describing 1+1 dimensional propagating fermions with nonlocal interactions. The Hamiltonian admits a dual description which we realize in the simple BCS model of superconductivity. On one hand, Cooper pairs are interpreted as wormhole correlations at the de Sitter horizon; their number yields the de Sitter entropy. On the other hand, BCS is mapped into a deformed conformal field theory reproducing the structure of quantum spin networks. When area measurements are performed, Cooper-pair insertions are activated on those edges of the spin network intersecting the given area, thus providing a description of quantum measurements in terms of excitations of a Fermi sea to superconducting levels. The cosmological constant problem is naturally addressed as a nonperturbative mass-gap effect of the true Fermi-liquid vacuum."

 

[marcus]

http://arxiv.org/abs/0807.0712
Gravitational waves from the Big Bounce
Jakub Mielczarek
19 pages, 9 figures
(Submitted on 4 Jul 2008)

"In this paper we investigate gravitational waves production during the Big Bounce phase inspired by the Loop Quantum Cosmology. We consider the influence of the holonomy corrections to the equation for tensor modes. We show that they act like additional effective graviton mass, suppressing gravitational waves creation. However, this effects can be treated perturbatively. We investigate the simplified model without these corrections and find its exact analytical solution. For this model we calculate a spectrum of the gravitational waves from the Big Bounce phase. The obtained spectrum decreases to zero for the low energy modes. Based on this observation we indicate that this effect can lead to the low CMB multipoles suppression and gives a potential way to test Loop Quantum Cosmology models. We also consider a scenario with a post-bounce inflationary phase. The obtained power spectrum gives qualitative explanation of the CMB spectra, including low multipoles suppression. This result is a challenge to construct a consistent bounce+inflation model in the Loop Quantum Cosmology."

Jakub Mielczarek has co-authored seven papers with Marek Szydlowski
http://www.slac.stanford.edu/spires/find/hep/www?rawcmd=FIND+A+SZYDLOWSKI&FORMAT=www&SEQUENCE=citecount%28d%29
this is J.M.'s first solo paper. Someone it might be good to watch.

http://arxiv.org/abs/0807.0665
Phenomenological loop quantum geometry of the Schwarzschild black hole
Dah-Wei Chiou
26 pages, 6 figures, 1 table
(Submitted on 4 Jul 2008)

"The interior of a Schwarzschild black hole is investigated at the level of phenomenological dynamics with the discreteness corrections of loop quantum geometry implemented in two different improved quantization schemes. In one scheme, the classical black hole singularity is resolved by the quantum bounce, which bridges the black hole interior with a white hole interior..."

Briefly noted as part of an ongoing effort to understand the black hole interior in terms of LQG. D-W Chiou is a postdoc in Ashtekar's group at Penn State who has already several papers on the LQG black hole. I don't claim ability to judge but I think at this point the results are interesting but not conclusive.

 

[marcus]

http://arxiv.org/abs/0807.1481
Is gravitational entropy quantized?
Dawood Kothawala, T. Padmanabhan, Sudipta Sarkar
4 pages
(Submitted on 9 Jul 2008)

"In Einstein's gravity, the entropy of horizons is proportional to their area. Several arguments given in the literature suggest that, in this context, both area and entropy should be quantized with an equally spaced spectrum for large quantum numbers. But in more general theories (like, for e.g, in the black hole solutions of Gauss-Bonnet or Lanczos-Lovelock gravity) the horizon entropy is not proportional to area and the question arises as to which of the two (if at all) will have this property. We give a general argument that in all Lanczos-Lovelock theories of gravity, it is the entropy that has equally spaced spectrum. In the case of Gauss-Bonnet gravity, we use the asymptotic form of quasi normal mode frequencies to explicitly demonstrate this result. Hence, the concept of a quantum of area in Einstein Hilbert (EH) gravity needs to be replaced by a concept of quantum of entropy in a more general context."

 

[marcus]

http://arxiv.org/abs/0807.1854
DSR as an explanation of cosmological structure
Joao Magueijo
(Submitted on 11 Jul 2008)

"Deformed special relativity (DSR) is one of the possible realizations of a varying speed of light (VSL). It deforms the usual quadratic dispersion relations so that the speed of light becomes energy dependent, with preferred frames avoided by postulating a non-linear representation of the Lorentz group. The theory may be used to induce a varying speed of sound capable of generating (near) scale-invariant density fluctuations, as discussed in a recent Letter. We identify the non-linear representation of the Lorentz group that leads to scale-invariance, finding a universal result. We also examine the higher order field theory that could be set up to represent it."


http://arxiv.org/abs/0807.1726
Axion Cosmology and the Energy Scale of Inflation
Mark P Hertzberg (MIT), Max Tegmark (MIT), Frank Wilczek (MIT)
5 pages, 4 figures

 

[marcus]

special Quantum Gravity issue of ASL, edited by Bojowald---Padma's invited article

http://arxiv.org/abs/0807.2356
Dark Energy and its Implications for Gravity
Thanu Padmanabhan
Invited article to appear in Advanced Science Letters Special Issue on Quantum Gravity, Cosmology and Black holes (editor: M. Bojowald)
(Submitted on 15 Jul 2008)

"The cosmological constant is the most economical candidate for dark energy. No other approach really alleviates the difficulties faced by the cosmological constant because, in all other attempts to model the dark energy, one still has to explain why the bulk cosmological constant (treated as a low-energy parameter in the action principle) is zero. I argue that until the theory is made invariant under the shifting of the Lagrangian by a constant, one cannot obtain a satisfactory solution to the cosmological constant problem. This is impossible in any generally covariant theory with the conventional low-energy matter action, if the metric is varied in the action to obtain the field equations. I review an alternative perspective in which gravity arises as an emergent, long wavelength phenomenon and can be described in terms of an effective theory using an action associated with null vectors in the spacetime. This action is explicitly invariant under the shift of the energy momentum tensor T_{ab}\to T_{ab}+\Lambda g_{ab} and any bulk cosmological constant can be gauged away. Such an approach seems to be necessary for addressing the cosmological constant problem and can easily explain why its bulk value is zero. I describe some possibilities for obtaining its observed value from quantum gravitational fluctuations."

 

[marcus]

http://arxiv.org/abs/0807.2806
Phase space descriptions for simplicial 4d geometries
Bianca Dittrich, James P. Ryan
(Submitted on 17 Jul 2008)

"Starting from the canonical phase space for discretised (4d) BF--theory, we implement a canonical version of the simplicity constraints and construct phase spaces for simplicial geometries. Our construction allows us to study the connection between different versions of Regge calculus and approaches using connection variables, such as loop quantum gravity. We find that on a fixed triangulation the (gauge invariant) phase space associated to loop quantum gravity is genuinely larger than the one for length and even area Regge calculus. Rather, it corresponds to the phase space of area--angle Regge calculus, as defined by Dittrich and Speziale in [arXiv:0802.0864] (prior to the imposition of gluing constraints, that ensure the metricity of the triangulation). We argue that this is due to the fact that the simplicity constraints are not fully implemented in canonical loop quantum gravity. Finally, we show that for a subclass of triangulations one can construct first class Hamiltonian and Diffeomorphism constraints leading to flat 4d space--times."

http://arxiv.org/abs/0807.2808
Emergent diffeomorphism invariance in a discrete loop quantum gravity model
Rodolfo Gambini, Jorge Pullin
12 pages
(Submitted on 17 Jul 2008)
"Several approaches to the dynamics of loop quantum gravity involve discretizing the equations of motion. The resulting discrete theories are known to be problematic since the first class algebra of constraints of the continuum theory becomes second class upon discretization. If one treats the second class constraints properly, the resulting theories have very different dynamics and number of degrees of freedom than those of the continuum theory. It is therefore questionable how these theories could be considered a starting point for quantization and the definition of a continuum theory through a continuum limit. We show explicitly in a model that the uniform discretizations approach to the quantization of constrained systems overcomes these difficulties. We consider here a simple diffeomorphism invariant one dimensional model and complete the quantization using uniform discretizations. The model can be viewed as a spherically symmetric reduction of the well known Husain--Kuchar model of diffeomorphism invariant theory. We show that the correct quantum continuum limit can be satisfactorily constructed for this model. This opens the possibility of treating 1+1 dimensional dynamical situations of great interest in quantum gravity taking into account the full dynamics of the theory and preserving the space-time covariance at a quantum level."

http://arxiv.org/abs/0807.2652
The Barbero-Immirzi Parameter as a Scalar Field: K-Inflation from Loop Quantum Gravity?
Victor Taveras, Nicolas Yunes
11 pages, submitted to Phys. Rev. D
(Submitted on 16 Jul 2008)

"We consider a loop-quantum gravity inspired modification of general relativity, where the Holst action is generalized by making the Barbero-Immirzi (BI) parameter a scalar field, whose value could be dynamically determined. The modified theory leads to a non-zero torsion tensor that corrects the field equations through quadratic first-derivatives of the BI field. Such a correction is equivalent to general relativity in the presence of a scalar field with non-trivial kinetic energy. This stress-energy of this field is automatically covariantly conserved by its own dynamical equations of motion, thus satisfying the strong equivalence principle. Every general relativistic solution remains a solution to the modified theory for any constant value of the BI field. For arbitrary time-varying BI fields, a study of cosmological solutions reduces the scalar field stress-energy to that of a pressureless perfect fluid in a comoving reference frame, forcing the scale factor dynamics to be equivalent to those of a stiff equation of state. Upon ultraviolet completion, this model could provide a natural mechanism for k-inflation, where the role of the inflaton is played by the BI field and inflation is driven by its non-trivial kinetic energy instead of a potential."

http://arxiv.org/abs/0807.2838
A Bi-Metric Theory with Exchange Symmetry
S. Hossenfelder
8 pages
(Submitted on 17 Jul 2008)

"We propose an extension of General Relativity with two different metrics. To each metric we define a Levi-Cevita connection and a curvature tensor. We then consider two types of fields, each of which moves according to one of the metrics and its connection. To obtain the field equations for the second metric we impose an exchange symmetry on the action. As a consequence of this ansatz, additional source terms for Einstein's field equations are generated. We discuss the properties of these additional fields, and consider the examples of the Schwarzschild solution, and the Friedmann-Robertson-Walker metric."

 

[marcus]

Taveras paper here is likely to be quite useful. It is just the sort of thing one would want to have available
http://arxiv.org/abs/0807.3325
Corrections to the Friedmann Equations from LQG for a Universe with a Free Scalar Field
Victor Taveras
9 pages
(Submitted on 21 Jul 2008)

"In loop quantum cosmology the quantum dynamics is well understood. We approximate the full quantum dynamics in the infinite dimensional Hilbert space by projecting it on a finite dimensional submanifold thereof, spanned by suitably chosen semiclassical states. This submanifold is isomorphic with the classical phase space and the projected dynamical flow provides effective equations incorporating the leading quantum corrections to the classical equations of motion. Numerical work has been done using quantum states which are semiclassical at late times. These states follow the classical trajectory until the density is on the order of 1% of the Planck density then deviate strongly from the classical trajectory. The effective equations we obtain reproduce this behavior to surprising accuracy."

Taveras is in Ashtekar's group at Penn State. The next paper develops the theme of adding matter to simplicial quantum gravity
which the Ambjorn Loll triangulations people are also currently concerned with.

http://arxiv.org/abs/0807.3041
A Kirchoff-like conservation law in Regge calculus
Adrian P. Gentle, Arkady Kheyfets, Jonathan R. McDonald, Warner A. Miller
13 pages, 4 figures, submitted to Class. Quantum Grav
(Submitted on 18 Jul 2008)

"Simplicial lattices provide an elegant framework for discrete spacetimes. The inherent orthogonality between a simplicial lattice and its circumcentric dual yields an austere representation of spacetime which provides a conceptually simple form of Einstein's geometric theory of gravitation. A sufficient understanding of simplicial spacetimes has been demonstrated in the literature for spacetimes devoid of all non-gravitational sources. However, this understanding has not been adequately extended to non-vacuum spacetime models. Consequently, a deep understanding of the diffeomorphic structure of the discrete theory is lacking. Conservation laws and symmetry properties are attractive starting points for coupling matter with the lattice. We present a simplicial form of the contracted Bianchi identities which is based on the E. Cartan moment of rotation operator. These identities manifest themselves in the conceptually-simple form of a Kirchoff-like conservation law. This conservation law enables one to extend Regge Calculus to non-vacuum spacetimes, and provides a deeper understanding of the simplicial diffeomorphism group."

http://arxiv.org/abs/0807.3042
Stability of the Schwarzschild Interior in Loop Quantum Gravity
Christian G. Boehmer, Kevin Vandersloot
4 pages, 4 figures
(Submitted on 18 Jul 2008)

"In recent work, the Schwarzschild interior of a black hole was investigated, incorporating quantum gravitational modifications due to loop quantum gravity. The central Schwarzschild singularity was shown to be replaced by a Nariai type universe. In this brief report we show that this interior solution is stable with respect to small perturbations, in contrast to the classical Nariai universe."

http://arxiv.org/abs/0807.3188
On gravitational defects, particles and strings
Winston J. Fairbairn
30 pages
(Submitted on 20 Jul 2008)

"We study the inclusion of point and string matter in the deSitter gauge theory, or MacDowell-Mansouri formulation of four dimensional gravity. We proceed by locally breaking the gauge symmetries of general relativity along worldlines and worldsheets embedded in the spacetime manifold. Restoring full gauge invariance introduces new dynamical fields which describe the dynamics of spinning matter coupled to gravity. We discuss the physical interpretation of the obtained formalism by studying the flat limit and the spinless case on arbitrary backgrounds. It turns out that the worldline action describes a massive spinning particle, while the worldsheet action contains the Nambu-Goto string augmented with spinning contributions. Finally, we study the gravity/matter variational problem and conclude by discussing potential applications of the formalism to the inclusion of the Nambu-Goto string in spinfoam models of four dimensional quantum gravity."

Winston Fairbairn was one of Rovelli's bunch, at Marseille. The first I heard of him was when he co-authored a really interesting paper with Rovelli, as he was a PhD student, back around 2004. Now he is at John Barrett's at Nottingham. This looks exciting, maybe one can get some stringy goodies in the context of 4D quantum gravity without all those extra dimensions.

Additional sample exerpt to help assess the paper's importance:
"Our common quantum relativistic understanding of matter in terms of finite dimensional, irreducible representations of the Poincare algebra is a very rough approximation of reality. This description is tied to the isometries of the flat, Minkowski solution to general relativity and yields a good approximation only in very weak gravitational fields, like for instance, in our particle accelerators where the successes of quantum field theory have been crowned.
In a fundamental theory of Nature, one cannot expect this approximation to be valid since in the early, Planckian universe, spacetime is undoubtedly not flat. Accordingly, the search of the fundamental structure of matter is tied to non-trivial, and certainly quantum configurations of the gravitational field. In turn, a complete theory of quantum gravity will have to incorporate a precise description of the degrees of freedom of matter.

As a first step, it seems therefore natural to look for an understanding of matter which does not rely on a particular fixed background geometry at the classical level. This will automatically render the formulation compatible with non-perturbative attempts to the quantisation of gravity which cannot, consistently, rely on a fixed, background metric structure.

A very old and appealing idea consists in considering the Einstein equations as defining the notion of matter. In other words, to consider matter as particular, possibly singular, configurations of the gravitational field. In this framework, we are reversing the standard picture where matter is defined on flat spacetime and then tentatively extended to other solutions of general relativity. Here, we are starting from the gravitational perspective, without selecting a preferred solution, and deriving matter from the geometry of spacetime. Obviously, this formulation should reproduce the standard properties of matter in the flat limit, but will also select a preferred formulation from the gravitational perspective.
For example, such a reversed approach has recently led to conceptually and technically strong results regarding the coupling of matter to three dimensional quantum gravity [2], [3].

The concrete implementation of this procedure relies on a the gauge symmetries
of gravity,..."

http://arxiv.org/abs/0807.3281
Definition of a time variable with Entropy of a perfect fluid in Canonical Quantum Gravity
Giovanni Montani (1, 2 and 3), Simone Zonetti (1) ((1) Dipartimento di Fisica, Universita degli Studi di Roma "La Sapienza", (2) ENEA C.R. Frascati (Dipartimento F.P.N.), (3) ICRANet C.C. Pescara.)
14 pages, no figures. Submitted to Phys. Rev. D
(Submitted on 21 Jul 2008)

"The Brown-Kuchar mechanism is applied in the case of General Relativity coupled with the Schutz' model for a perfect fluid. Using the canonical formalism and manipulating the set of modified constraints one is able to recover the definition of a time evolution operator, i.e. a physical Hamiltonian, expressed as a functional of gravitational variables and the entropy. Entropy then reveals to be, in the comoving frame, the time variable for the system, and a simple evolution operator is obtained."

http://arxiv.org/abs/0807.3161
A comparative review of recent researches in geometry
Felix C. Klein
26 pages
(Submitted on 20 Jul 2008)

"Felix Klein's so-called Erlangen Program was published in 1872 as professoral dissertation. It proposed a new solution to the problem how to classify and characterize geometries on the basis of projective geometry and group theory. The given translation was made in 1892 by Dr. M. W. Haskell and transcribed by N. C. Rughoonauth. We replaced bibliographical data in text and footnotes with pointers to a complete bibliography section."

 

[marcus]

http://arxiv.org/abs/0807.3561
Spin-Foam Models and the Physical Scalar Product
Emanuele Alesci, Karim Noui, Francesco Sardelli
24 pages
(Submitted on 22 Jul 2008)

"This paper aims at clarifying the link between Loop Quantum Gravity and Spin-Foam models in four dimensions. Starting from the canonical framework, we construct an operator P acting on the space of cylindrical functions Cyl(Gamma), where Gamma is the 4-simplex graph, such that its matrix elements are, up to some normalization factors, the vertex amplitude of Spin-Foam models. The Spin-Foam models we are considering are the topological model, the Barrett-Crane model and the Engle-Pereira-Rovelli model. The operator P is usually called the "projector" into physical states and its matrix elements gives the physical scalar product. Therefore, we relate the physical scalar product of Loop Quantum Gravity to vertex amplitudes of some Spin-Foam models. We discuss the possibility to extend the action of P to any cylindrical functions on the space manifold."

 

[marcus]

New paper by Ambjorn, Goerlich, Jurkiewicz, and Loll

http://arxiv.org/abs/0807.4481
The Nonperturbative Quantum de Sitter Universe
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
37 pages, many figures
(Submitted on 28 Jul 2008)

"The dynamical generation of a four-dimensional classical universe from nothing but fundamental quantum excitations at the Planck scale is a long-standing challenge to theoretical physicists. A candidate theory of quantum gravity which achieves this goal without invoking exotic ingredients or excessive fine-tuning is based on the nonperturbative and background-independent technique of Causal Dynamical Triangulations. We demonstrate in detail how in this approach a macroscopic de Sitter universe, accompanied by small quantum fluctuations, emerges from the full gravitational path integral, and how the effective action determining its dynamics can be reconstructed uniquely from Monte Carlo data. We also provide evidence that it may be possible to penetrate to the sub-Planckian regime, where the Planck length is large compared to the lattice spacing of the underlying regularization of geometry." http://arxiv.org/abs/0807.4192
Black Hole Entropy and the Problem of Universality
Steven Carlip
14 pages, Springer macros; to appear in 'Quantum Mechanics of Fundamental Systems: the Quest for Beauty and Simplicity' (Claudio Bunster Festschrift)
(Submitted on 25 Jul 2008)

"To derive black hole thermodynamics in any quantum theory of gravity, one must introduce constraints that ensure that a black hole is actually present. For a large class of black holes, the imposition of such 'horizon constraints' allows the use of conformal field theory methods to compute the density of states, reproducing the correct Bekenstein-Hawking entropy in a nearly model-independent manner. This approach may explain the 'universality' of black hole entropy, the fact that many inequivalent descriptions of quantum states all seem to give the same thermodynamic predictions. It also suggests an elegant picture of the relevant degrees of freedom, as Goldstone-boson-like excitations arising from symmetry breaking by a conformal anomaly induced by the horizon constraints.

Claudio Bunster, who is being celebrated by this collection of papers, used to be Claudio Teitelboim---a safer name to have in Chile during the Pinochet years. The name Bunster which he has now adopted was associated with opposition to military dictatorship. To find Bunster's research writings, search under his previous name.

http://arxiv.org/abs/0807.4152
The Constraint Algebra of Topologically Massive AdS Gravity
Steven Carlip
17 pages
(Submitted on 25 Jul 2008)

"Three-dimensional topologically massive AdS gravity has a complicated constraint algebra, making it difficult to count nonperturbative degrees of freedom. I show that a new choice of variables greatly simplifies this algebra, and confirm that the theory contains a single propagating mode for all values of the mass parameter and the cosmological constant. As an added benefit, I rederive the central charges and conformal weights of the boundary conformal field theory from an explicit analysis of the asymptotic algebra of constraints."

http://arxiv.org/abs/0807.4520
Black Hole Thermodynamics and Statistical Mechanics
Steven Carlip
35 pages, Springer macros; for the Proceedings of the 4th Aegean Summer School on Black Holes
(Submitted on 28 Jul 2008)

"We have known for more than thirty years that black holes behave as thermodynamic systems, radiating as black bodies with characteristic temperatures and entropies. This behavior is not only interesting in its own right; it could also, through a statistical mechanical description, cast light on some of the deep problems of quantizing gravity. In these lectures, I review what we currently know about black hole thermodynamics and statistical mechanics, suggest a rather speculative 'universal' characterization of the underlying states, and describe some key open questions."

http://arxiv.org/abs/0807.4468
The cosmological BCS mechanism and the Big Bang Singularity
Stephon Alexander, Tirthabir Biswas
11 pages, 1 figure
(Submitted on 28 Jul 2008)

"We provide a novel mechanism that resolves the Big Bang Singularity present in FRW space-times without the need for ghost fields. Building on the fact that a four-fermion interaction arises in General Relativity when fermions are covariantly coupled, we show that at early times the decrease in scale factor enhances the correlation between pairs of fermions. This enhancement leads to a BCS-like condensation of the fermions and opens a gap dynamically driving the Hubble parameter to zero and results in a non-singular bounce. We derive the four fermion interaction and the effective potential for the gap, demonstrating a consistency condition necessary for the non-singular bounce."

 

[MTd2]

http://arxiv.org/abs/0807.4574v1
Proof of the entropy bound on dynamical horizons
Authors: Sijie Gao, Xiaoning Wu
(Submitted on 29 Jul 2008)

"The entropy bound conjecture concerning black hole dynamical horizons is proved. The conjecture states, if a dynamical horizon, $D_H$, is bounded by two surfaces with areas of $A_B$ and $\abp$ ($\abp>A_B$), then the entropy, $S_D$, that crosses $D_H$ must satisfy $S_D\leq {1/4}(\abp-A_B)$. We show that this conjecture is implied by the generalized Bousso bound. Consequently, the generalized second law holds for dynamical horizons. Finally, we show that the lightlike bousso bound and its spacelike counterpart can be unified as one bound."

This is a simple but very insightful paper. By unifying both components, you can just now work with just the lightsheet of causal diamonds.

Now, this is something from my guts. If we consider that bound true, we can start thinking about a universe as a duality between the union of the domain of RP3 projections of matter and fields, because now we can the spacelike component of the causal diamond, and one holographic screen, which is the union of the images of all RP3.

 

[marcus]

http://arxiv.org/abs/0807.4748
Diffeomorphism invariance in spherically symmetric loop quantum gravity
Rodolfo Gambini, Jorge Pullin
5 pages, invited paper for a special issue of Advanced Science Letters
(Submitted on 29 Jul 2008)

"We study the issue of the recovery of diffeomorphism invariance in the recently introduced loop quantum gravity treatment of the exterior Schwarzschild space-time. Although the loop quantization agrees with the quantization in terms of metric variables in identifying the physical Hilbert space, we show that diffeomorphism invariance in space-time is recovered with certain limitations due to the use of holonomic variables in the loop treatment of the model. This resembles behaviors that are expected in the full theory."

http://arxiv.org/abs/0807.4874
Canonical Relativity and the Dimensionality of the World
Martin Bojowald
17 pages, 2 figures; Chapter in: Relativity and the Dimensionality of the World, Ed. V. Petkov (Springer, 2007), pp. 137-152
(Submitted on 30 Jul 2008)

"Different aspects of relativity, mainly in a canonical formulation, relevant for the question 'Is spacetime nothing more than a mathematical space (which describes the evolution in time of the ordinary three-dimensional world) or is it a mathematical model of a real four-dimensional world with time entirely given as the fourth dimension?' are presented. The availability as well as clarity of the arguments depend on which framework is being used, for which currently special relativity, general relativity and some schemes of quantum gravity are available. Canonical gravity provides means to analyze the field equations as well as observable quantities, the latter even in coordinate independent form. This allows a unique perspective on the question of dimensionality since the space-time manifold does not play a prominent role. After re-introducing a Minkowski background into the formalism, one can see how distinguished coordinates of special relativity arise, where also the nature of time is different from that in the general perspective. Just as it is of advantage to extend special to general relativity, general relativity itself has to be extended to some theory of quantum gravity. This suggests that a final answer has to await a thorough formulation and understanding of a fundamental theory of space-time. Nevertheless, we argue that current insights into quantum gravity do not change the picture of the role of time obtained from general relativity."

http://arxiv.org/abs/0807.4910
Gravitational dynamics in Bose Einstein condensates
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
15 pages
(Submitted on 30 Jul 2008)

"Analogue models for gravity intend to provide a framework where matter and gravity, as well as their intertwined dynamics, emerge from degrees of freedom that have a priori nothing to do with what we call gravity or matter. Bose Einstein condensates (BEC) are a natural example of analogue model since one can identify matter propagating on a (pseudo-Riemannian) metric with collective excitations above the condensate of atoms. However, until now, a description of the 'analogue gravitational dynamics' for such model was missing. We show here that in a BEC system with massive quasi-particles, the gravitational dynamics can be encoded in a modified (semi-classical) Poisson equation. In particular, gravity is of extreme short range (characterized by the healing length) and the cosmological constant appears from the non-condensed fraction of atoms in the quasi-particle vacuum. While some of these features make the analogue gravitational dynamics of our BEC system quite different from standard Newtonian gravity, we nonetheless show that it can be used to draw some interesting lessons about 'emergent gravity' scenarios."

 

[marcus]

http://arxiv.org/abs/0808.0025
A Note on B-observables in Ponzano-Regge 3d Quantum Gravity
Etera R. Livine, James P. Ryan
17 pages
(Submitted on 31 Jul 2008)

"We study the insertion and value of metric observables in the (discrete) path integral formulation of the Ponzano-Regge spinfoam model for 3d quantum gravity. In particular, we discuss the length spectrum and the relation between insertion of such B-observables and gauge fixing in the path integral."

 

[marcus]

http://arxiv.org/abs/0808.0190
Singularities in loop quantum cosmology
Thomas Cailleteau, Antonio Cardoso, Kevin Vandersloot, David Wands
5 pages, 3 figures
(Submitted on 1 Aug 2008)

"We show that simple scalar field models can give rise to curvature singularities in Loop Quantum Cosmology (LQC). We find singular solutions for spatially flat Friedmann-Robertson-Walker cosmologies with a canonical scalar field and a negative exponential potential, or with a phantom scalar field and a positive potential. While LQC avoids big bang or big rip type singularities, we find sudden singularities where the Hubble rate is bounded, but the Ricci curvature scalar diverges. We conclude that the effective equations of LQC are not in themselves sufficient to avoid the occurrence of singularities."

http://arxiv.org/abs/0808.0246
Categorified Symplectic Geometry and the Classical String
John C. Baez, Alexander E. Hoffnung, Christopher L. Rogers
28 pages
(Submitted on 2 Aug 2008)

"A Lie 2-algebra is a 'categorified' version of a Lie algebra: that is, a category equipped with structures analogous those of a Lie algebra, for which the usual laws hold up to isomorphism. In the classical mechanics of point particles, the phase space is often a symplectic manifold, and the Poisson bracket of functions on this space gives a Lie algebra of observables. Multisymplectic geometry describes an n-dimensional field theory using a phase space that is an 'n-plectic manifold': a finite-dimensional manifold equipped with a closed nondegenerate (n+1)-form. Here we consider the case n = 2. For any 2-plectic manifold, we construct a Lie 2-algebra of observables. We then explain how this Lie 2-algebra can be used to describe the dynamics of a classical bosonic string. Just as the presence of an electromagnetic field affects the symplectic structure for a charged point particle, the presence of a B field affects the 2-plectic structure for the string."

 

[marcus]

On 25 July Lee Smolin posted a revision of his December 2007 paper, containing new information worth noting. Thanks to MTd2 for alerting us to this.

http://arxiv.org/abs/0712.0977
The Plebanski action extended to a unification of gravity and Yang-Mills theory
Lee Smolin
12 pages, one figure
(Submitted on 6 Dec 2007 (v1), last revised 25 Jul 2008 (this version, v2))

"We study a unification of gravity with Yang-Mills fields based on a simple extension of the Plebanski action to a Lie group G which contains the local lorentz group. The Coleman-Mandula theorem is avoided because the theory has no global spacetime symmetry. This may be applied to Lisi's proposal of an E8 unified theory, giving a fully E8 invariant action. The extended form of the Plebanski action suggests a new class of spin foam models."

 

[marcus]

http://arxiv.org/abs/0808.0701
Effective theory for the cosmological generation of structure
Martin Bojowald, Aureliano Skirzewski
8 pages, 1 figure
(Submitted on 5 Aug 2008)

"The current understanding of structure formation in the early universe is mainly built on a magnification of quantum fluctuations in an initial vacuum state during an early phase of accelerated universe expansion. One usually describes this process by solving equations for a quantum state of matter on a given expanding background space-time, followed by decoherence arguments for the emergence of classical inhomogeneities from the quantum fluctuations. Here, we formulate the coupling of quantum matter fields to a dynamical gravitational background in an effective framework which allows the inclusion of back-reaction effects. It is shown how quantum fluctuations couple to classical inhomogeneities and can thus manage to generate cosmic structure in an evolving background. Several specific effects follow from a qualitative analysis of the back-reaction, including a likely reduction of the overall amplitude of power in the cosmic microwave background, the occurrence of small non-Gaussianities, and a possible suppression of power for odd modes on large scales without parity violation."

 

[marcus]

http://arxiv.org/abs/0808.0949
Critical behaviour in quantum gravitational collapse
Viqar Husain
8 pages, 3 figures
(Submitted on 7 Aug 2008)

"We study the gravitational collapse of an inhomogeneous scalar field with quantum gravity corrections associated with singularity avoidance. Numerical simulations indicate that there is critical behaviour at the onset of black hole formation as in the classical theory, but with the difference that black holes form with a mass gap."

http://arxiv.org/abs/0808.0990
Effective Scenario of Loop Quantum Cosmology
You Ding, Yongge Ma, Jinsong Yang
4 pages, 2 figures
(Submitted on 7 Aug 2008)

"Semiclassical states in isotropic loop quantum cosmology are employed to show that the improved dynamics has the correct classical limit. The effective Hamiltonian for the quantum cosmological model with a massless scalar field is thus obtained, which incorporates also the next to leading order quantum corrections. The possibility that the higher order correction terms may lead to significant departure from the leading order effective scenario is revealed. If the semiclassicality of the model is maintained in the large scale limit, even k=0 Friedmann expanding universe will undergo a collapse in the future due to the quantum gravity effect. Thus the quantum bounce and collapse may contribute a cyclic universe in the new scenario."

http://arxiv.org/abs/0808.1081
The kinematic origin of the cosmological redshift
Emory F. Bunn, David W. Hogg
14 pages. Submitted to Am. J. Phys
(Submitted on 7 Aug 2008)

"A common belief among cosmologists is that the cosmological redshift cannot be properly viewed as a Doppler shift (that is, as evidence for a recession velocity), but must instead be viewed in terms of the stretching of space. We argue that the most natural interpretation of the redshift is in fact as a Doppler shift, or rather as the accumulation of many infinitesimal Doppler shifts. The stretching-of-space interpretation obscures a central idea of relativity, namely that of coordinate freedom, specifically the idea that it is always valid to choose a coordinate system that is locally Minkowski. We show that, in any spacetime, an observed frequency shift can be interpreted either as a kinematic (Doppler) shift or a gravitational shift by imagining a family of observers along the photon's path. In the context of the expanding Universe, the kinematic interpretation corresponds to a family of comoving observers and hence seems to be the more natural one."

 

[marcus]

http://arxiv.org/abs/0808.1107
Semiclassical regime of Regge calculus and spin foams
Eugenio Bianchi, Alejandro Satz
30 pages
(Submitted on 7 Aug 2008)

"Recent attempts to recover the graviton propagator from spin foam models involve the use of a boundary quantum state peaked on a classical geometry. The question arises whether beyond the case of a single simplex this suffices for peaking the interior geometry in a semiclassical configuration. In this paper we explore this issue in the context of quantum Regge calculus with a general triangulation. Via a stationary phase approximation, we show that the boundary state succeeds in peaking the interior in the appropriate configuration, and that boundary correlations can be computed order by order in an asymptotic expansion. Further, we show that if we replace at each simplex the exponential of the Regge action by its cosine -- as expected from the semiclassical limit of spin foam models -- then the contribution from the sign-reversed terms is suppressed in the semiclassical regime and the results match those of conventional Regge calculus."

http://arxiv.org/abs/0808.1223
Constraints and gauge transformations: Dirac's theorem is not always valid
Julian Barbour, Brendan Z. Foster
14 pages
(Submitted on 8 Aug 2008)

"A standard tenet of canonical quantum gravity is that evolution generated by a Hamiltonian constraint is just a gauge transformation on the phase space and therefore does not change the physical state. The basis for this belief is a theorem of Dirac that identifies primary first-class constraints as generators of physically irrelevant motions. We point out that certain assumptions on which Dirac based his argument do not hold for reparametrization invariant systems, and show that the primary Hamiltonian constraint of these systems does generate physical motion. We show explicitly how the argument fails for systems described by Jacobi's principle, which has a structure closely resembling that of general relativity. We defer discussion of general relativity and the implications for quantum gravity to a later paper."

 

[marcus]

Hongbao Zhang is at Beijing Normal, same place as Yongge Ma. We should watch the quantum gravity/cosmology group there. I have already noted several of their papers.
In this note Hongbao improves on Steven Weinberg's treatment of a very interesting problem---how does the expansion of the universe slow down massive particles? E.g. neutrinos originally produced "hot" in the early universe will have slowed down due to expansion.

Note on the thermal history of decoupled massive particles
Hongbao Zhang
JHEP style, 4 pages, to appear in CQG
(Submitted on 11 Aug 2008)

"This note provides an alternative approach to the momentum decay and thermal evolution of decoupled massive particles. Although the ingredients in our results have been addressed in Weinberg's Cosmology, the strategies employed here are simpler, and the results obtained here are more general."

==sample excerpt==

"As is well known, for the freely traveling massless particle like photon in an expanding
FLRW universe, the frequency or energy will vary inversely proportional to the scale
factor, which implies that the number density of massless particles still keeps its thermal
spectrum form with a redshifted effective temperature although these particles went
out of the thermal equilibrium into the free expansion as time passed. This is the
physical foundation for the cosmic microwave radiation background currently observed
by us. Now a natural question arises, namely, does the above fact also apply to the
massive particle? Not only does this question possesses a theoretical interest by itself,
but also acquires a practical implication in cosmology since neutrinos and antineutrinos
are believed to be massive. However, to my best knowledge, this issue has not been
addressed in literatures except in Weinberg’s cosmology book published recently[1].

The purpose of this note is to provide an alternative approach to this issue. The
strategies employed here are simpler, but the results obtained here are more general.
Notations and conventions follow Ref.[2]."

In case Yongge Ma's name is not familiar, a recent paper of his was noted in post #743, two posts back.

I'm told that next year's Loop or QG conference will be in Beijing. I should be watching the QG research output of people like Yongge Ma and Hongbao Zhang. Here are three researchers at Hunan Normal, whose paper has already been accepted for publication in Physical Review D. and their names are new to me. Hongwei Yu is the senior researcher.

http://arxiv.org/abs/0808.1382
Dynamics of interacting phantom scalar field dark energy in Loop Quantum Cosmology
Xiangyun Fu, Hongwei Yu, Puxun Wu
18 pages, 7 figures, to be published in PRD
(Submitted on 10 Aug 2008)

"We study the dynamics of a phantom scalar field dark energy interacting with dark matter in loop quantum cosmology (LQC). Two kinds of coupling of the form \alpha{\rho_m}{\dot\phi} (case I) and 3\beta H (\rho_\phi +\rho_m) (case II) between the phantom energy and dark matter are examined with the potential for the phantom field taken to be exponential. For both kinds of interactions, we find that the future singularity appearing in the standard FRW cosmology can be avoided by loop quantum gravity effects. In case II, if the phantom field is initially rolling down the potential, the loop quantum effect has no influence on the cosmic late time evolution and the universe will accelerate forever with a constant energy ratio between the dark energy and dark matter."

 

[marcus]

http://arxiv.org/abs/0808.1971
Intertwiner dynamics in the flipped vertex
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
12 pages, 7 figures
(Submitted on 14 Aug 2008)

"We continue the semiclassical analysis, started in a previous paper, of the intertwiner sector of the flipped vertex spinfoam model. We use independently both a semi-analytical and a purely numerical approach, finding the correct behavior of wave packet propagation and physical expectation values. In the end, we show preliminary results about correlation functions."

brief mention
http://arxiv.org/abs/0808.1913
Transiting Exoplanets with JWST
S. Seager (MIT), D. Deming (NASA/GSFC), J. A. Valenti (STScI)
22 pages, 11 figures. In press in "Astrophysics in the Next Decade: JWST and Concurrent Facilities, Astrophysics & Space Science Library, Thronson, H. A., Tielens, A., Stiavelli, M., eds., Springer: Dordrecht (2008)."
(Submitted on 13 Aug 2008)

 

[francesca]

http://arxiv.org/abs/0808.2069"
Action and Hamiltonians in higher dimensional general relativity: First order framework
Abhay Ashtekar, David Sloan
12 pages, no figures
(Submitted on 18 Aug 08)
We consider d>4 dimensional space-times which are asymptotically flat at spatial infinity and show that, in the first order framework, the action principle is well-defined without the need of infinite counter terms. It naturally leads to a covariant phase space in which the Hamiltonians generating asymptotic symmetries provide the total energy-momentum and angular momentum of the isolated system. This work runs parallel to our previous analysis in four dimensions. The higher dimensional analysis is in fact simpler because of absence of logarithmic and super translation ambiguities.

 

[marcus]

http://arxiv.org/abs/0808.2259
Time delay of light signals in an energy-dependent spacetime metric
A. F. Grillo, E. Luzio, F. Mendez
5 pages. Physical Review D77, 104033 (2008)
(Submitted on 16 Aug 2008)

"In this note we review the problem of time delay of photons propagating in a spacetime with a metric that explicitly depends on the energy of the particles (Gravity-Rainbow approach). We show that corrections due to this approach -- which is closely related to DSR proposal -- produce for small redshifts (z<<1) smaller time delays than in the generic Lorentz Invariance Violating case."

 

[marcus]

http://arxiv.org/abs/0808.2842
Unitarity and Holography in Gravitational Physics
Donald Marolf
14 pages
(Submitted on 21 Aug 2008)

http://arxiv.org/abs/0808.2845
Holographic Thought Experiments
Donald Marolf
24 pages
(Submitted on 21 Aug 2008)

 

[marcus]

http://arxiv.org/abs/0808.3124
Planck Scale Cosmology in Resummed Quantum Gravity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
7 pages
(Submitted on 22 Aug 2008)

"We show that, by using resummation techniques based on the extension of the methods of Yennie, Frautschi and Suura to Feynman's formulation of Einstein's theory, we get quantum field theoretic predictions for the UV fixed-point values of the dimensionless gravitational and cosmological constants. Connections to the phenomenological asymptotic safety analysis of Planck scale cosmology by Bonanno and Reuter are discussed."

Asymptotic safety papers cited by Reuter et al., Percacci et al.
Expectation mentioned of possible observational test. Running (Newton and cosmological) G and Lambda constants. UV limit of Lambda derived.
Conclusions mention a followup paper. Bennie Ward has over 300 papers at Spires going back to around 1973. Seems to be new arrival in asymptotic safety-related QG.

 

[marcus]

http://arxiv.org/abs/0808.3505
A note on DSR
Carlo Rovelli
This is a note circulated privately for a while. 5 pages, 2 figures
(Submitted on 26 Aug 2008)

"I study the physical meaning of Doubly Special Relativity (DSR) and argue that DSR can be physically relevant in a certain large distance limit. I present a direct physical interpretation of the 5-dimensional Girelli-Livine DSR formalism, by deriving it explicitly from a concrete physical effect: the gravitational time contraction and its effect on the dynamics of massive point particles. I give a physical interpretation to the corresponding 5d spacetime. I also present some speculations on the interpretation of the corresponding SO(4,1) transformations."

http://arxiv.org/abs/0808.3482
Dynamics of interacting dark energy model in Einstein and Loop Quantum Cosmology
Songbai Chen, Bin Wang, Jiliang Jing
15 pages, 4 figures
(Submitted on 26 Aug 2008)

"We investigate the background dynamics when dark energy is coupled to dark matter in the universe described by Einstein cosmology and Loop Quantum Cosmology. We introduce a new general form of dark sector coupling, which presents us a more complicated dynamical phase space. Differences in the phase space in obtaining the accelerated scaling attractor in Einstein cosmology and Loop Quantum Cosmology are disclosed."

 

[marcus]

http://arxiv.org/abs/0808.3765
Could deformed special relativity naturally arise from the semiclassical limit of quantum gravity?
Lee Smolin
12 pages
(Submitted on 27 Aug 2008)

"A argument is described for how deformed or doubly special relativity may arise in the semiclassical limit of a quantum theory of gravity. We consider a generic quantum theory of gravity coupled to matter, from which we use only the assumption that a Hamiltonian constraint is imposed. We study circumstances in which Lambda, G and hbar all may be separately neglected, but there may arise terms in the ratio of particle energies to the Planck mass which are small but measurable. Such cases include probes of an energy dependent speed of light such as are possible in experiments such as MAGIC and GLAST. We show that in such cases the leading order effect of quantum gravity will, if certain scaling relations are satisfied, be to deform the metric in the effective Hamiltonian of the matter quantum field theory by terms linear in energies. As the Hamiltonian constraint has been imposed there can be no preferred time coordinate or frame of reference, hence this is a modification rather than a breaking of special relativity."

http://arxiv.org/abs/0808.3725
New Positron Spectral Features from Supersymmetric Dark Matter - a Way to Explain the PAMELA Data?
Lars Bergstrom, Torsten Bringmann, Joakim Edsjo
6 pages, 4 figures
(Submitted on 27 Aug 2008)

"The space-borne antimatter experiment PAMELA has recently reported a surprising rise in the positron to electron ratio at high energies. It has also recently been found that electromagnetic radiative corrections in some cases may boost the gamma-ray yield from supersymmetric dark matter annihilations in the galactic halo by up to three or four orders of magnitude, providing distinct spectral signatures for indirect dark matter searches to look for. Here, we investigate whether the same type of corrections can also lead to sizeable enhancements in the positron yield. We find that this is indeed the case, albeit for a smaller region of parameter space than for gamma rays; selecting models with a small mass difference between the neutralino and sleptons, like in the stau coannihilation region in mSUGRA, the effect becomes more pronounced. The resulting, rather hard positron spectrum with a relatively sharp cutoff may potentially fit the rising positron ratio measured by the PAMELA satellite. To do so, however, as seems also to be the case for most other dark matter models, very large 'boost factors' have to be invoked that are not expected in current models of halo structure. If the predicted cutoff would also be confirmed by later PAMELA data or upcoming experiments, one could either assume non-thermal production in the early universe or non-standard halo formation to explain such a spectral feature as an effect of dark matter annihilation. At the end of the paper, we briefly comment on the impact of radiative corrections on other annihilation channels, in particular antiprotons and neutrinos."

 

[marcus]

http://arxiv.org/abs/0808.4056
Black hole entropy for the general area spectrum
Tomo Tanaka, Takashi Tamaki
5 pages
(Submitted on 29 Aug 2008)

"We consider the possibility that the horizon area is expressed by the general area spectrum in loop quantum gravity and calculate the black hole entropy by counting the degrees of freedom in spin-network states related to its area. Although the general area spectrum has a complex expression, we succeeded in obtaining the result that the black hole entropy is proportional to its area as in previous works where the simplified area formula has been used. This gives new values for the Barbero-Immirzi parameter (\gamma =0.5802... or 0.7847...) which are larger than that of previous works."

both of the authors are new to me.

 

[marcus]

http://arxiv.org/abs/0809.0304
Towards a spin foam model description of black hole entropy
J.Manuel Garcia-Islas
5 pages, 1 figure
(Submitted on 1 Sep 2008)

"We propose a way to describe the origin of black hole entropy in the spin foam models of quantum gravity. This stimulates a new way to study the relation of spin foam models and loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0809.0469
Dark spinors with torsion in cosmology
Christian G. Boehmer, James Burnett
7 pages, 3 figures
(Submitted on 2 Sep 2008)

"We solve one of the open problems in Einstein-Cartan theory, namely we find a natural matter source whose spin angular momentum tensor is compatible with the cosmological principle. We analyze the resulting evolution equations and find that an epoch of accelerated expansion is an attractor. The torsion field quickly decays in that period. Our results are interpreted in the context of the standard model of cosmology."

Christian Boehmer has already co-authored in loop cosmology. Although this particular paper of his is formally classical, I think it likely to contribute to research in quantum cosmology, possible causes of inflation, and the dark energy effect.

http://arxiv.org/abs/0804.0616
Dark spinor inflation -- theory primer and dynamics
Christian G. Boehmer
13 pages Phys. Rev. D 77 (2008) 123535
(Submitted on 3 Apr 2008)

"Inflation driven by a single dark spinor field is discussed. We define the notion of a dark spinor field and derive the cosmological field equations for such a matter source. The conditions for inflation are determined and an exactly solvable model is presented. We find the power spectrum of the quantum fluctuation of this field and compare the results with scalar field inflation."

 

[marcus]

http://arxiv.org/abs/0809.2022
Cosmology without inflation
Patrick Peter, Nelson Pinto-Neto
8 pages, Phys. Rev. D 78, 063506 (2008)
(Submitted on 11 Sep 2008)

"We propose a new cosmological paradigm in which our observed expanding phase is originated from an initially large contracting Universe that subsequently experienced a bounce. This category of models, being geodesically complete, is non-singular and horizon-free, and can be made to prevent any relevant scale to ever have been smaller than the Planck length. In this scenario, one can find new ways to solve the standard cosmological puzzles. One can also obtain scale invariant spectra for both scalar and tensor perturbations: this will be the case, for instance, if the contracting Universe is dust-dominated at the time at which large wavelength perturbations get larger than the curvature scale. We present a particular example based on a dust fluid classically contracting model, where a bounce occurs due to quantum effects, in which these features are explicit."

 

[marcus]

Freidel-Conrady on semiclassical limit

http://arxiv.org/abs/0809.2280
On the semiclassical limit of 4d spin foam models
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
32 pages, 5 figures
(Submitted on 15 Sep 2008)

"We study the semiclassical properties of the Riemannian spin foam models with Immirzi parameter that are constructed via coherent states. We show that in the semiclassical limit the quantum spin foam amplitudes of an arbitrary triangulation are exponentially suppressed, if the face spins do not correspond to a discrete geometry. When they do arise from a geometry, the amplitudes reduce to the exponential of i times the Regge action. Remarkably, the dependence on the Immirzi parameter disappears in this limit."

http://arxiv.org/abs/0809.2590
Physical time and other conceptual issues of QG on the example of LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
18 pages, 1 figure
(Submitted on 15 Sep 2008)

"Several conceptual aspects of quantum gravity are studied on the example of the homogeneous isotropic LQC model. In particular: (i) The relativistic time of the co-moving observers is showed to be a quantum operator and a quantum spacetime metric tensor operator is derived. (ii) Solutions of the quantum scalar constraint for two different choices of the lapse function are compared and contrasted. (iii) The mechanism of the singularity avoidance is analyzed via detailed studies of an energy density operator. (iv) The relation between the kinematical and the physical quantum geometry is discussed on the level of relation between observables."


http://arxiv.org/abs/0809.2469
Multi-fluid potential in the loop cosmology
Jakub Mielczarek
4 pages, 5 figures
(Submitted on 15 Sep 2008)

"The scalar field can behaves like a fluid with equation of state p_{\phi}=w\rho_{\phi}, where w \in [-1,1]. In this letter we derive a class of the scalar field potentials for which w= const. Scalar field with such a potential can mimic ordinary matter, radiation, cosmic strings etc. We perform our calculations in the framework of the loop cosmology with a holonomy corrections. We solve the model analytically for the whole parameter space. Subsequently, we perform similar consideration for the model with a phantom field (w<-1). We show that scalar field is monotonic function in both cases. This indicate that it can be treated as a well defined internal time for these models."

 

[francesca]

Quaternion-Loop Quantum Gravity

http://arxiv.org/abs/0809.2916"
Quaternion-Loop Quantum Gravity
M. D. Maia, S. S e Almeida Silva, F. S. Carvalho
Comments: 6 pages, latex, no figures

We present a simple realization of loop quantum gravity in terms of quaternion operators acting on the spinor space of the triad holonomy group. We find that the Riemannian curvature of the 3-dimensional hypersurfaces in space-time is induced by the SU(2) gauge potential, but it is constructed differently from the usual gauge curvature. Therefore, quaternion-loop quantum gravity is not a gauge theory of gravitation of the triad holonomy group, but is provides a natural justification for quaternion quantum gravity.

 

[francesca]

Evaluation of new spin foam vertex amplitudes

http://arxiv.org/abs/00809.3190"
Evaluation of new spin foam vertex amplitudes
Igor Khavkine
Comments: 19 pages, 4 figures, amsrefs

A new numerical evaluation algorithm is proposed for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov. The algorithm is applied to compute the high spin behavior of the new vertex amplitudes. Their asymptotics exhibit non-oscillatory, power-law decay, similar to that of the Barrett-Crane model, but with different exponents.