Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/0805.0136
Is loop quantization in cosmology unique?
Alejandro Corichi, Parampreet Singh
(Submitted on 1 May 2008)

"We re-examine the process of loop quantization for flat isotropic models in cosmology. In particular, we contrast different inequivalent 'loop quantizations' of these simple models through their respective successes and limitations and asses whether they can lead to any viable physical description. We propose three simple requirements which any such admissible quantum model should satisfy: i) independence from any auxiliary structure, such as a fiducial interval/cell introduced to define the phase space when integrating over non-compact manifolds; ii) existence of a well defined classical limit and iii) provide a sensible 'Planck scale' where quantum gravitational effects become manifest. We show that even when it may seem that one can have several possible loop quantizations, these physical requirements considerably narrow down the consistent choices. Apart for the so called improved dynamics of LQC, none of the other available inequivalent loop quantizations pass above tests, showing the limitations of lattice refinement models to approximate the homogeneous sector and loop modified quantum geometrodynamics. We conclude that amongst a large class of loop quantizations in isotropic cosmology, there is a unique consistent choice."

 

[marcus]

http://arxiv.org/abs/0805.0453
Conserved Quantities and the Algebra of Braid Excitations in Quantum Gravity
Song He, Yidun Wan
25 pages, 2 figures
(Submitted on 5 May 2008)

"We derive conservation laws from interactions of braid-like excitations of embedded framed spin networks in Quantum Gravity. We also demonstrate that the set of stable braid-like excitations form a noncommutative algebra under braid interaction, in which the set of actively-interacting braids is a subalgebra."

http://arxiv.org/abs/0805.0543
"So what will you do if string theory is wrong?"
Moataz H. Emam
Americal Journal of Physics, July 2008
(Submitted on 5 May 2008)

"I briefly discuss the accomplishments of string theory that would survive a complete falsification of the theory as a model of nature and argue the possibility that such a survival may necessarily mean that string theory would become its own discipline, independently of both physics and mathematics."

 

[marcus]

http://arxiv.org/abs/0805.1265
C, P, and T of Braid Excitations in Quantum Gravity
Song He, Yidun Wan
28 pages, 5 figures
(Submitted on 9 May 2008)

"We study the discrete transformations of four-valent braid excitations of framed spin networks embedded in a topological three-manifold. We show that four-valent braids allow seven and only seven discrete transformations. These transformations can be uniquely mapped to C, P, T, and their products. Each CPT multiplet of actively-interacting braids is found to be uniquely characterized by a non-negative integer. Finally, braid interactions turn out to be invariant under C, P, and T."

http://arxiv.org/abs/0805.1187
Black holes in loop quantum gravity: the complete space-time
Rodolfo Gambini, Jorge Pullin
4 pages, 2 figures
(Submitted on 8 May 2008)

"We consider the quantization of the complete extension of the Schwarzschild space-time using spherically symmetric loop quantum gravity. We find an exact solution corresponding to the semi-classical theory. The singularity is eliminated but the space-time still contains a horizon. Although the solution is known partially numerically and therefore a proper global analysis is not possible, a global structure akin to a singularity-free Reissner--Nordstrom space-time including a Cauchy horizon is suggested."

http://arxiv.org/abs/0805.1178
Loop and braneworlds cosmologies from a deformed Heisenberg algebra
Marco Valerio Battisti
9 pages, submitted to PRD
(Submitted on 8 May 2008)

"The implications of a deformed Heisenberg algebra on the Friedmann-Robertson-Walker cosmological models are investigated. In particular, we consider generalized commutation relations which leave undeformed the translation group and preserve the rotational invariance. The resulting algebra is related to the kappa-Poincaré one and no sign in the deformation term is selected at all. The analysis of the models is performed at classical level by studying the modifications induced on the symplectic geometry by the deformed algebra. We show that this framework leads to a modified Friedmann equation which coincide with that one found in loop quantum cosmology as well as in the Randall-Sundrum braneworlds scenario. In fact, the complementary sign of the loop and brane term, in the effective cosmological dynamics, naturally emerges from the free sign of the deformed algebra. This way, a common phenomenological description for both these theories is obtained and a relation with the low energy quantum gravity framework established."


http://arxiv.org/abs/0805.1192
How quantum is the big bang?
Martin Bojowald
4 pages
(Submitted on 8 May 2008)

"When quantum gravity is used to discuss the big bang singularity, the most important, though rarely addressed, question is what role genuine quantum degrees of freedom play. Here, complete effective equations are derived for isotropic models with an interacting scalar to all orders in the expansions involved. The resulting coupling terms show that quantum fluctuations do not affect the bounce much. Quantum correlations, however, do have an important role and could even eliminate the bounce. How quantum gravity regularizes the big bang depends crucially on properties of the quantum state."

http://arxiv.org/abs/0805.1219
GLAST and Lorentz violation
Raphael Lamon
11 pages, 7 figures and 2 tables
(Submitted on 8 May 2008)

"We study possible Lorentz violations by means of gamma-ray bursts (GRB) with special focus on the Large Array Telescope (LAT) of GLAST. We simulate bursts with gtobssim and introduce a Lorentz violating term in the arrival times of the photons. We further perturb these arrival times and energies with a Gaussian distribution corresponding to the time resp. energy resolution of GLAST. We then vary the photon flux in gtobssim in order to derive a relation between the photon number and the standard deviation of the Lorentz violating term. We conclude with the fact that our maximum likelihood method as first developed in [1] is able to make a statement whether Nature breaks the Lorentz symmetry if the number of bursts with known redshifts is of the order of 100."

 

[marcus]

http://arxiv.org/abs/0805.2124
Noncommutative gravity, a 'no strings attached' quantum-classical duality, and the cosmological constant puzzle
Tejinder P. Singh
7 pages. Second Prize in Gravity Research Foundation Essay Competition, 2008...To appear in Gen. Rel. Grav.
(Submitted on 14 May 2008)

"There ought to exist a reformulation of quantum mechanics which does not refer to an external classical spacetime manifold. Such a reformulation can be achieved using the language of noncommutative differential geometry. A consequence which follows is that the 'weakly quantum, strongly gravitational' dynamics of a relativistic particle whose mass is much greater than Planck mass is dual to the 'strongly quantum, weakly gravitational' dynamics of another particle whose mass is much less than Planck mass. The masses of the two particles are inversely related to each other, and the product of their masses is equal to the square of Planck mass. This duality explains the observed value of the cosmological constant, and also why this value is nonzero but extremely small in Planck units."

 

[marcus]

http://arxiv.org/abs/0805.2373
Exotic Statistics for Ordinary Particles in Quantum Gravity
John Swain
Awarded an honourable mention in the 2008 Gravity Research Foundation Essay Competition
(Submitted on 15 May 2008)

"Objects exhibiting statistics other than the familiar Bose and Fermi ones are natural in theories with topologically nontrivial objects including geons, strings, and black holes. It is argued here from several viewpoints that the statistics of ordinary particles with which we are already familiar are likely to be modified due to quantum gravity effects. In particular, such modifications are argued to be present in loop quantum gravity and in any theory which represents spacetime in a fundamentally piecewise-linear fashion. The appearance of unusual statistics may be a generic feature (such as the deformed position-momentum uncertainty relations and the appearance of a fundamental length scale) which are to be expected in any theory of quantum gravity, and which could be testable."

http://arxiv.org/abs/0805.2183
Gravity and its Mysteries: Some Thoughts and Speculations
A. Zee
18 pages, conference talk
(Submitted on 14 May 2008)

"I gave a rambling talk about gravity and its many mysteries at Chen-Ning Yang's 85th Birthday Celebration held in November 2007. I don't have any answers."

Some people will know Tony Zee from his textbook Quantum Field Theory in a Nutshell
or from his popular book Fearful Symmetry
http://www.kitp.ucsb.edu/~zee/

 

[marcus]

http://arxiv.org/abs/0805.2503
Fundamentals and recent developments in non-perturbative canonical Quantum Gravity
F. Cianfrani, O.M. Lecian, G. Montani
94 pages
(Submitted on 16 May 2008)

"The aim of this review is to provide a detailed account of the physical content emerging from this story of the canonical approach to Quantum Gravity. All the crucial steps in our presentation have a pedagogical character, providing the reader with the necessary tools to become involved in the field. Such a pedagogical aspect is then balanced and completed by subtle discussions on specific topics which we regard as relevant for the physical insight they outline on the treated questions. Our analysis is not aimed at convincing the reader about a pre-constituted point of view, bu instead our principal goal is to review the picture of Canonical Quantum Gravity on the basis of the concrete facts at the ground of its clear successes, but also of its striking shortcomings."

[Giovanni Montani is a senior guy at Rome. 87 papers on arxiv. This pedagogical review of LQG looks like it may prove useful.]

http://arxiv.org/abs/0805.2536
The entropic boundary law in BF theory
Etera R. Livine, Daniel R. Terno
17 pages
(Submitted on 16 May 2008)

"We compute the entropy of a closed bounded region of space for pure 3d Riemannian gravity formulated as a topological BF theory for the gauge group SU(2) and show its holographic behavior. More precisely, we consider a fixed graph embedded in space and study the flat connection spin network state without and with particle-like topological defects. We regularize and compute exactly the entanglement for a bipartite splitting of the graph and show it scales at leading order with the number of vertices on the boundary (or equivalently with the number of loops crossing the boundary). More generally these results apply to BF theory with any compact gauge group in any space-time dimension."

http://arxiv.org/abs/0805.2411
Measuring the Scalar Curvature with Clocks and Photons: Voronoi-Delaunay Lattices in Regge Calculus
Jonathan R. McDonald, Warner A. Miller
7 pages, 2 figures, submitted to Classical and Quantum Gravity
(Submitted on 15 May 2008)

"The Riemann scalar curvature plays a central role in Einstein's geometric theory of gravity. We describe a new geometric construction of this scalar curvature invariant at an event (vertex) in a discrete spacetime geometry. This allows one to constructively measure the scalar curvature using only clocks and photons. Given recent interest in discrete pre-geometric models of quantum gravity, we believe is it ever so important to reconstruct the curvature scalar with respect to a finite number of communicating observers. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the original simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding hinge-based expression in Regge calculus (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas."

http://arxiv.org/abs/0805.2584
Is Physics Asking for a New Kinematics?
R. Aldrovandi, J. G. Pereira
8 pages. Honorable Mention in the Gravity Research Foundation essay contest, 2008
(Submitted on 16 May 2008)

"It is discussed whether some of the consistency problems of present-day physics could be solved by replacing special relativity, whose underlying kinematics is ruled by the Poincare' group, by de Sitter relativity, with underlying kinematics ruled by the de Sitter group. In contrast to ordinary special relativity, which seems to fail at the Planck scale, this new relativity is "universal" in the sense that it holds at all energy scales."

http://arxiv.org/abs/0805.2555
Quantum black-hole information missing in the semiclassical treatment
H. Nikolic
7 pages
(Submitted on 16 May 2008)

"In the semiclassical treatment of gravity, an external observer can measure only the mean (not the exact) mass of the black hole (BH). By contrast, in fully quantum gravity the exact (not only mean) BH mass is measurable by the external observer. This additional information (missing in the semiclassical treatment) available to the external observer significantly helps to understand how information leaks out during the BH evaporation."

[Harvey Nikolic is a PF Beyond regular---Demystifier. Might like to discuss/explain the paper.]

http://arxiv.org/abs/0805.2429
Galactic Neutrino Communication
John G. Learned, Sandip Pakvasa, A. Zee
6 pages, 2 figures
(Submitted on 16 May 2008)

"We examine the possibility to employ neutrinos to communicate within the galaxy. We discuss various issues associated with transmission and reception, and suggest that the resonant neutrino energy near 6.3 PeV may be most appropriate. In one scheme we propose to make Z^o particles in an overtaking e^+ - e^- collider such that the resulting decay neutrinos are near the W^- resonance on electrons in the laboratory. Information is encoded via time structure of the beam. In another scheme we propose to use a 30 PeV pion accelerator to create neutrino or anti-neutrino beams. The latter encodes information via the particle/anti-particle content of the beam, as well as timing. Moreover, the latter beam requires far less power, and can be accomplished with presently foreseeable technology. Such signals from an advanced civilization, should they exist, will be eminently detectable in neutrino detectors now under construction."

 

[marcus]

http://arxiv.org/abs/0805.2668
Halos of Modified Gravity
Kirill Krasnov, Yuri Shtanov
Honorable Mention in the 2008 Essay Competition of the Gravity Research Foundation; 9 pages, 2 figures
(Submitted on 17 May 2008)

"We describe how a certain simple modification of general relativity, in which the local cosmological constant is allowed to depend on the space-time curvature, predicts the existence of halos of modified gravity surrounding spherically-symmetric objects. We show that the gravitational mass of an object weighed together with its halo can be much larger than its gravitational mass as seen from inside the halo. This effect could provide an alternative explanation of the dark-matter phenomenon in galaxies. In this case, the local cosmological constant in the solar system must be some six orders of magnitude larger than its cosmic value obtained in the supernovae type Ia experiments. This is well within the current experimental bounds, but may be directly observable in the future high-precision experiments." Interesting collaboration. I think of Krasnov as one of the central people in nonstring QG who has co-authored with Ashtekar, Baez, Freidel, and Rovelli. On the other hand, I think of Shtanov's background as string---braneworld cosmology, at least until recently. Now they are both working on what is a classical (nonquantum) gravity modification. It's an intriguing modification that Krasnov has written already maybe 6 papers about. Another one of these longshot gambles that you don't necessarily hear about.

 

[marcus]

I can't evaluate or vouch for this paper. Randono was at UTex Austin and is now postdoc at Penn State in Ashtekar's group. The conclusion here is unexpected and unlike anything else I have seen. It opens a possible phenomenology for QG at intermediate scale---well before Planck scale. this sounds incredible. But he has been talking with Ashtekar and Golam Hossain about this and we know from past papers that he is a capable and original researcher. So I have to include the link.

http://arxiv.org/abs/0805.2955
A Mesoscopic Quantum Gravity Effect
Andrew Randono
10 pages, 2 figures
(Submitted on 19 May 2008)

 

[marcus]

Yesterday there was that mesoscopic QG effects Randono.
Today another Randono
http://arxiv.org/abs/0805.3169
A New Perspective on Covariant Canonical Gravity
Andrew Randono
25 pages
(Submitted on 20 May 2008)

"We present a new approach to the covariant canonical formulation of Einstein-Cartan gravity that preserves the full Lorentz group as the local gauge group. The method exploits lessons learned from gravity in 2+1 dimensions regarding the relation between gravity and a general gauge theory. The dynamical variables are simply the frame field and the spin-connection pulled-back to the hypersurface, thereby eliminating the need for simplicity constraints on the momenta. A consequence of this is a degenerate (pre)symplectic form, which appears to be a necessary feature of the Einstein-Cartan formulation. A new feature unique to this approach arises when the constraint algebra is computed: the algebra is a deformation of the de Sitter, anti-de Sitter, or Poincaré algebra (depending on the value of the cosmological constant) with the deformation parameter being the conformal Weyl tensor." http://arxiv.org/abs/0805.3175
Conserved Topological Defects in Non-Embedded Graphs in Quantum Gravity
Fotini Markopoulou, Isabeau Prémont-Schwarz
42 pages, 34 figures
(Submitted on 20 May 2008)

"We follow up on previous work which found that commonly used graph evolution moves lead to conserved quantities that can be expressed in terms of the braiding of the graph in its embedding space. We study non-embedded graphs under three distinct sets of dynamical rules and find non-trivial conserved quantities that can be expressed in terms of topological defects in the dual geometry. For graphs dual to 2-dimensional simplicial complexes we identify all the conserved quantities of the evolution. We also indicate expected results for graphs dual to 3-dimensional simplicial complexes."

 

[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.

:rofl:

 

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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."

 

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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."

 

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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."

 

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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."

 

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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."

 

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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).

 

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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."

 

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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.

 

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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."

 

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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

 

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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."

 

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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."

 

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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."

 

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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."