Loop-and-allied QG bibliography

[marcus]

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.

Fixed broken link:
http://arxiv.org/abs/0809.3190
excellent choices IMHO, thanks Francesca!

 

[marcus]

http://arxiv.org/abs/0809.3718
Asymptotics of LQG fusion coefficients
Emanuele Alesci, Eugenio Bianchi, Elena Magliaro, Claudio Perini
14 pages
(Submitted on 22 Sep 2008)

"The fusion coefficients from SO(3) to SO(4) play a key role in the definition of spin foam models for the dynamics in Loop Quantum Gravity. In this paper we give a simple analytic formula of the EPRL fusion coefficients. We study the large spin asymptotics and show that they map SO(3) semiclassical intertwiners into SU(2)_L\times SU(2)_R semiclassical intertwiners. This non-trivial property opens the possibility for an analysis of the semiclassical behavior of the model."

 

[marcus]

http://arxiv.org/abs/0809.3850
Black holes and black hole thermodynamics without event horizons
Alex B. Nielsen
Invited review article for General Relativity and Gravitation. 43 pages
(Submitted on 23 Sep 2008)

"We investigate whether black holes can be defined without using event horizons. In particular we focus on the thermodynamic properties of event horizons and the alternative, locally defined horizons. We discuss the assumptions and limitations of the proofs of the zeroth, first and second laws of black hole mechanics for both event horizons and trapping horizons. This leads to the possibility that black holes may be more usefully defined in terms of trapping horizons. We also show how Hawking radiation can also be seen to arise from trapping horizons and discuss which horizon area should be associated with the gravitational entropy."

The black hole "event horizon" is poorly defined in some situations because its definition assumes we know the future with certainty, to infinite time. In quantum geometry/gravity there is geometrical uncertainty and limitation on what an observer knows. Several authors have tackled the problem of defining a different kind of horizon which can serve equally well but which at least in principle is defined based on locally observed and measured stuff. Ashtekar has several papers about this. This paper of Nielsen could provide a useful up-to-date review of where this business stands.

http://arxiv.org/abs/0809.4235
Conditional probabilities with Dirac observables and the problem of time in quantum gravity
Rodolfo Gambini, Rafael Porto, Sebastian Torterolo, Jorge Pullin
4 pages
(Submitted on 24 Sep 2008)

"We combine the 'evolving constants' approach to the construction of observables in canonical quantum gravity with the Page--Wootters formulation of quantum mechanics with a relational time for generally covariant systems. This overcomes the objections levied by Kuchar against the latter formalism. The construction is formulated entirely in terms of Dirac observables, avoiding in all cases the physical observation of quantities that do not belong in the physical Hilbert space. We work out explicitly the example of the parameterized particle, including the calculation of the propagator. The resulting theory also predicts a fundamental mechanism of decoherence."

http://arxiv.org/abs/0809.4170
Fine-grained state counting for black holes in loop quantum gravity
A. Ghosh, P. Mitra
5 pages,
(Submitted on 24 Sep 2008)

"The degeneracy of a black hole corresponding to a definite area eigenvalue is counted within the framework of loop quantum gravity. This is shown to involve several microscopic parameters depending on the area. Their fluctuations yield an explanation of the appearance of a fine structure seen in numerical calculations of the number of states."

 

[marcus]

http://arxiv.org/abs/0809.4464
Effective Theory of Braid Excitations of Quantum Geometry in terms of Feynman Diagrams
Yidun Wan
24 pages, 7 figures
(Submitted on 25 Sep 2008)

"We study interactions amongst topologically conserved excitations of quantum theories of gravity, in particular the braid excitations of four-valent spin networks. These have been shown previously to propagate and interact under evolution rules of spin foam models. We show that the dynamics of these braid excitations can be described by an effective theory based on Feynman diagrams. In this language, braids which are actively interacting are analogous to bosons, in that the topological conservation laws permit them to be singly created and destroyed. Exchanges of these excitations give rise to interactions between braids which are charged under the topological conservation rules."

 

[marcus]

http://arxiv.org/abs/0809.4763
Hamiltonian Analysis of non-chiral Plebanski Theory and its Generalizations
Sergei Alexandrov, Kirill Krasnov
10 pages
(Submitted on 27 Sep 2008)

"We consider non-chiral, full Lorentz group-based Plebanski formulation of general relativity in its version that utilizes the Lagrange multiplier field Phi with "internal" indices. The Hamiltonian analysis of this version of the theory turns out to be simpler than in the previously considered in the literature version with Phi carrying spacetime indices. We then extend the Hamiltonian analysis to a more general class of theories whose action contains scalars invariants constructed from Phi. Such theories have recently been considered in the context of unification of gravity with other forces. We show that these more general theories have six additional propagating degrees of freedom as compared to general relativity, something that has not been appreciated in the literature treating them as being not much different from GR."

 

[MTd2]

http://arxiv.org/abs/0809.5093
Observer Dependent Horizon Temperatures: a Coordinate-Free Formulation of Hawking Radiation as Tunneling
10 pages, 1 2-part figure
Sean Stotyn, Kristin Schleich, Don Witt
(Submitted on 29 Sep 2008)

"We reformulate the Hamilton-Jacobi tunneling method for calculating Hawking radiation in static, spherically-symmetric spacetimes by explicitly incorporating a preferred family of frames. These frames correspond to a family of observers tied to a locally static timelike Killing vector of the spacetime. This formulation separates the role of the coordinates from the choice of vacuum and thus provides a coordinate-independent formulation of the tunneling method. In addition, it clarifies the nature of certain constants and their relation to these preferred observers in the calculation of horizon temperatures. We first use this formalism to obtain the expected temperature for a static observer at finite radius in the Schwarzschild spacetime. We then apply this formalism to the Schwarzschild-de Sitter spacetime, where there is no static observer with 4-velocity equal to the static timelike Killing vector. It is shown that a preferred static observer, one whose trajectory is geodesic, measures the lowest temperature from each horizon. Furthermore, this observer measures horizon temperatures corresponding to the well-known Bousso-Hawking normalization."

-----

I don't think this is a breakthrough, nor is the only article with this point of view, but this is an important analysis nonetheless, and something that urges to be highlitghted: people, either from string or lqg, it seems, ignore the fact that temperature, or even more, perhaps the entropy of black hole is observer dependent. The Hawking temperature is a special and very simplified case.

 

[marcus]

http://arxiv.org/abs/0810.0104
The gravitational wave background from super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
8 pages, 3 figures
(Submitted on 1 Oct 2008)

"We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with n_t \approx 2. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where n_t \approx 0."

 

[marcus]

http://arxiv.org/abs/0810.0514
Quantum Space-times: Beyond the Continuum of Minkowski and Einstein
Abhay Ashtekar
31 pages, 6 figures. To appear in the volume Minkowski Space-time: A Hundred Years Later, edited by V. Petkov (Springer-Verlag,Berlin, 2008) commemorating the 100th anniversary of Minkowski's fusion of space and time into a 4 dimensional space-time continuum
(Submitted on 2 Oct 2008)

"In general relativity space-time ends at singularities. The big bang is considered as the Beginning and the big crunch, the End. However these conclusions are arrived at by using general relativity in regimes which lie well beyond its physical domain of validity. Examples where detailed analysis is possible show that these singularities are naturally resolved by quantum geometry effects. Quantum space-times can be vastly larger than what Einstein had us believe. These non-trivial space-time extensions enable us to answer of some long standing questions and resolve of some puzzles in fundamental physics. Thus, a century after Minkowski's revolutionary ideas on the nature of space and time, yet another paradigm shift appears to await us in the wings."

 

[MTd2]

http://arxiv.org/abs/0810.0613
On the condensed matter scheme for emergent gravity and interferometry
Authors: G. Jannes
11 pages; to appear in: F. Columbus (ed.), "Interferometers: Research, Technology and Applications"
(Submitted on 3 Oct 2008)

"An increasingly popular approach to quantum gravity rests on the idea that gravity (and maybe electromagnetism and the other gauge fields) might be an 'emergent phenomenon', in the sense of representing a collective behaviour resulting from a very different microscopic physics. A prominent example of this approach is the condensed matter scheme for quantum gravity, which considers the possibility that gravity emerges as an effective low-energy phenomenon from the quantum vacuum in a way similar to the emergence of collective excitations in condensed matter systems. This condensed matter view of the quantum vacuum clearly hints that, while the term 'ether' has been discredited for about a century, quantum gravity holds many (if not all) of the characteristics that have led people in the past to label various hypothetical substances with the term 'ether'. Since the last burst of enthusiasm for an ether, at the end of the 19th century, was brought to the grave in part by the performance of a series of important experiments in interferometry, the suggestion then naturally arises that maybe interferometry could also play a role in the current discussion on quantum gravity. We will highlight some aspects of this suggestion in the context of the condensed matter scheme for emergent gravity."

 

[marcus]

Westra is one of Loll's Utrecht bunch:
http://arxiv.org/abs/0810.0771
Topology Change and the Emergence of Geometry in Two Dimensional Causal Quantum Gravity
Willem Westra
110 pages, 30 figures, PhD thesis
(Submitted on 4 Oct 2008)

"In this thesis we analyze a very simple model of two dimensional quantum gravity based on causal dynamical triangulations (CDT). We present an exactly solvable model which indicates that it is possible to incorporate spatial topology changes in the nonperturbative path integral. It is shown that if the change in spatial topology is accompanied by a coupling constant it is possible to evaluate the path integral to all orders in the coupling and that the result can be viewed as a hybrid between causal and Euclidian dynamical triangulation. The second model we describe shows how a classical geometry with constant negative curvature emerges naturally from a path integral over noncompact manifolds. No initial singularity is present, hence the quantum geometry is naturally compatible with the Hartle Hawking boundary condition. Furthermore, we demonstrate that under certain conditions the quantum fluctuations are small! To conclude, we treat the problem of spacetime topology change. Although we are not able to completely solve the path integral over all manifolds with arbitrary topology, we do obtain results that indicate that such a path integral might be consistent, provided suitable causality restrictions are imposed."

 

[marcus]

http://arxiv.org/abs/0810.1599
On the computation of black hole entropy in loop quantum gravity
J. Fernando Barbero G., Eduardo J. S. Villaseñor
25 pages
(Submitted on 9 Oct 2008)

"We discuss some issues related to the computation of black hole entropy in loop quantum gravity from the novel point of view provided by the recent number-theoretical methods introduced by the authors and their collaborators. In particular we give exact expressions, in the form of integral transforms, for the black hole entropy in terms of the area. We do this by following several approaches based both on our combinatorial techniques and also on functional equations similar to those employed by Meissner in his pioneering work on this subject. To put our results in perspective we compare them with those of Meissner. We will show how our methods confirm some of his findings, extend the validity of others, and correct some mistakes. At the end of the paper we will discuss the delicate issue of the asymptotics of black hole entropy."

http://arxiv.org/abs/0810.1653
Evaluation of new spin foam vertex amplitudes with boundary states
Igor Khavkine
18 pages, 4 figures
(Submitted on 9 Oct 2008)

"The numerical evaluation algorithms for the new spin foam vertex amplitudes proposed by Engle, Pereira & Rovelli and Freidel & Krasnov, recently developed by the author, are extended to efficiently include a large class of boundary states (factored states). A concrete pragmatic proposal is made for a semi-classical state, encompassing to both the Barrett-Crane and new models. Two computations using this boundary state are described together with a uniform comparison methodology for the three different models: semi-classical wave packet propagation and graviton 2-point function evaluation. The new algorithms are applied to the wave packet propagation problem, indicating that the Magliaro, Rovelli and Perini's hypothesis of good semiclassical behavior of the new models may not hold under more general conditions (with unfrozen j-spins)."

http://arxiv.org/abs/0810.1714
Self-energy and vertex radiative corrections in LQG
Claudio Perini, Carlo Rovelli, Simone Speziale
11 pages, 3 figures
(Submitted on 9 Oct 2008)

"We consider the elementary radiative-correction terms in loop quantum gravity. These are a two-vertex 'elementary bubble' and a five-vertex 'ball'; they correspond to the one-loop self-energy and the one-loop vertex correction of ordinary quantum field theory. We compute their naive degree of (infrared) divergence."

 

[francesca]

...and don't forget today's news in cosmology

http://arxiv.org/abs/0810.0104
The gravitational wave background from super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
Comments: 8 pages, 3 figures

"We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with $n_t \approx 2$. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where $n_t \approx 0$."

 

[marcus]

Hi Francesca,
we got that one already, around 1 October, at post #766.

http://arxiv.org/abs/0810.0104
The gravitational wave background from super-inflation in Loop Quantum Cosmology
E. J. Copeland, D. J. Mulryne, N. J. Nunes, M. Shaeri
8 pages, 3 figures
(Submitted on 1 Oct 2008)

"We investigate the behaviour of tensor fluctuations in Loop Quantum Cosmology, focusing on a class of scaling solutions which admit a near scale-invariant scalar field power spectrum. We obtain the spectral index of the gravitational field perturbations, and find a strong blue tilt in the power spectrum with n_t \approx 2. The amplitude of tensor modes are, therefore, suppressed by many orders of magnitude on large scales compared to those predicted by the standard inflationary scenario where n_t \approx 0."

I agree with you that it looks interesting----may have phenomenological implications. Something to look for that could eventually help test? Maybe we should open a separate thread on this one. BTW thanks for helping spot QG/QC papers! Hope your research is going well.

 

[marcus]

http://arxiv.org/abs/0810.2408
A new continuum limit of matrix models
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
17 pages, 2 figures
(Submitted on 14 Oct 2008)

"We define a new scaling limit of matrix models which can be related to the method of causal dynamical triangulations (CDT) used when investigating two-dimensional quantum gravity. Surprisingly, the new scaling limit of the matrix models is also a matrix model, thus explaining why the recently developed CDT continuum string field theory (arXiv:0802.0719) has a matrix-model representation (arXiv:0804.0252)."

http://arxiv.org/abs/0810.2503
A Causal Alternative for c=0 Strings
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
10 pages, 4 figures, Presented at "The 48th Cracow School of Theoretical Physics: Aspects of Duality", June 13-22, 2008, Zakopane, Poland
(Submitted on 14 Oct 2008)

"We review a recently discovered continuum limit for the one-matrix model which describes 'causal' two-dimensional quantum gravity. The behaviour of the quantum geometry in this limit is different from the quantum geometry of Euclidean two-dimensional quantum gravity defined by taking the 'standard' continuum limit of the one-matrix model. Geodesic distance and time scale with canonical dimensions in this new limit, contrary to the situation in Euclidean two-dimensional quantum gravity. Remarkably, whenever we compare, the known results of (generalized) causal dynamical triangulations are reproduced exactly by the one-matrix model. We complement previous results by giving a geometrical interpretation of the new model in terms of a generalization of the loop equation of Euclidean dynamical triangulations. In addition, we discuss the time evolution of the quantum geometry."

Garrett Lisi's TED talk is online:
http://www.ted.com/index.php/talks/garrett_lisi_on_his_theory_of_everything.html

http://arxiv.org/abs/0810.1978
Background-free propagation in loop quantum gravity
Simone Speziale
Invited contribution to a special issue of Advanced Science Letters edited by Martin Bojowald. 14 pages
(Submitted on 10 Oct 2008)

"I review the definition of n-point functions in loop quantum gravity, discussing what has been done and what are the main open issues. Particular attention is dedicated to gauge aspects and renormalization."

http://arxiv.org/abs/0810.2091
A unitary invariant in Riemannian geometry
Alain Connes
25 pages, 1 figure
(Submitted on 12 Oct 2008)

"We introduce an invariant of Riemannian geometry which measures the relative position of two von Neumann algebras in Hilbert space, and which, when combined with the spectrum of the Dirac operator, gives a complete invariant of Riemannian geometry. We show that the new invariant plays the same role with respect to the spectral invariant as the Cabibbo--Kobayashi--Maskawa mixing matrix in the Standard Model plays with respect to the list of masses of the quarks."

Slide sets for the September Sussex QG school (John Barrett, Renate Loll, Martin Reuter,...)
http://www.ippp.dur.ac.uk/Workshops/08/NPMQFT/Programme/
Slide sets for the Sussex conference (John Barrett, Laurent Freidel, Roberto Percacci, Jan Ambjorn, Jerzy Lewandowski,...)
http://www.ippp.dur.ac.uk/Workshops/08/CLAQG/Programme/
For example, here are slides for Percacci's talk "A particle physicists view of gravity"
http://www.pact.cpes.sussex.ac.uk/~dl79/CLAQG/Percacci.pdf
I would especially recommend looking at the slides for the Percacci talk. I think his view of gravity (with the LHC in mind) parallels and clarifies the perspective in Frank Wilczek's book (and is in a sense easier to understand because in unpopularized style.)

http://arxiv.org/abs/0810.1768
Spacelike distance from discrete causal order
David Rideout, Petros Wallden
31 pages, 16 figures
(Submitted on 9 Oct 2008)

"Any discrete approach to quantum gravity must provide some prescription as to how to deduce continuum properties from the discrete substructure. In the causal set approach it is straightforward to deduce timelike distances, but surprisingly difficult to extract spacelike distances, because of the unique combination of discreteness with local Lorentz invariance in that approach. We propose a number of methods to overcome this difficulty, one of which reproduces the spatial distnce between two points in a finite region of Minkowski space. We demonstrate numerically that this definition can be used to define a 'spatial nearest neighbor' relation on a causal set, and conjecture that this can be exploited to define the length of 'continuous curves' in causal sets which are approximated by curved spacetime. This provides important evidence in support of the 'Hauptvermutung' of causal sets."

brief mention:
http://arxiv.org/abs/0810.2177
The Fate of Substructures in Cold Dark Matter Haloes
R. E. Angulo, C. G. Lacey, C. M. Baugh, C. S Frenk
14 pages, 11 figures. Submitted to MNRAS
(Submitted on 13 Oct 2008)

"We use the Millennium Simulation, a large, high resolution N-body simulation of the evolution of structure in a LambdaCDM cosmology, to study the properties and fate of..." http://arxiv.org/abs/0810.1959
Core-Collapse Astrophysics with a Five-Megaton Neutrino Detector
Matthew D. Kistler, Hasan Yuksel (Ohio State), Shin'ichiro Ando (Caltech), John F. Beacom (Ohio State), Yoichiro Suzuki (Tokyo)
7 pages, 4 figures
(Submitted on 10 Oct 2008)

"The legacy of solar neutrinos suggests that large neutrino detectors should be sited underground. However, to instead go underwater bypasses the need to move mountains, allowing much larger contained water Cherenkov detectors. Reaching a scale of ~5 Megatons, the size of the proposed Deep-TITAND, would permit observations of 'mini-bursts' of neutrinos from supernovae in the nearby universe on a yearly basis. Importantly, these mini-bursts would be detected over backgrounds without the need for optical evidence of the supernova, guaranteeing the beginning of time-domain MeV neutrino astronomy. The ability to identify, to the second, every core collapse would allow a continuous 'death watch' of all stars within ~5 Mpc, making previously-impossible tasks practical. These include the abilities to promptly detect otherwise-invisible prompt black hole formation, provide advance warning for supernova shock-breakout searches, define tight time windows for gravitational-wave searches, and identify "supernova impostors" by the non-detection of neutrinos."

 

[MTd2]

A MECHANICS FOR THE RICCI FLOW

The action functional for Einstein–Hilbert gravity equals the sum of the action functional for Schroedinger quantum mechanics, plus Perelman’s functional, plus the Coulomb functional (see eqns. (35), (36) below).

http://arxiv.org/PS_cache/arxiv/pdf/0810/0810.2356v1.pdf

I didn't post the abstract, because I didn't think it suited this paper.

 

[marcus]

http://arxiv.org/abs/0810.2992
Quantum group connections
Jerzy Lewandowski, Andrzej Okolow
40 pages
(Submitted on 16 Oct 2008)

"The aim of our work is a quantum group generalization of the Ashtekar-Isham C* algebra of the cylindrical functions defined on the space of connections with a compact structure Lie group. The algebra can be constructed by some inductive techniques from the C* algebra of continuous functions on the group and from a family of graphs embedded in the manifold underlying the connections. We generalize the latter construction replacing the algebra of continuous functions by a C* algebra defining a compact quantum group."

brief mention (proof or disproof of the occurrence of inflation would substantially impact quantum cosmology):
http://arxiv.org/abs/0810.2787
Proving Inflation: A Bootstrap Approach
Latham Boyle (CITA), Paul J. Steinhardt (Princeton)
4 pages, 2 figures
(Submitted on 16 Oct 2008)

"We propose a way to test the essential idea underlying the inflationary paradigm: that the universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion."

 

[marcus]

Advanced Science Letters is a new (online) journal. It will be publishing some special issues on specific topics. Martin Bojowald has been asked to serve as guest editor for a special issue on quantum cosmology. This paper of Bianca Dittrich will be part of one of the special issues of ASL---I don't know whether or not it will be Bojowald's..very likely will. The paper was Dittrich's contribution to the September Utrecht workshop ( http://www1.phys.uu.nl/wwwitf/MMQS/ )

http://arxiv.org/abs/0810.3594
Diffeomorphism symmetry in quantum gravity models
Bianca Dittrich
Invited constribution to a special issue of Advanced Science Letters, 16 pages
(Submitted on 20 Oct 2008)

"We review and discuss the role of diffeomorphism symmetry in quantum gravity models. Such models often involve a discretization of the space-time manifold as a regularization method. Generically this leads to a breaking of the symmetries to approximate ones, however there are incidences in which the symmetries are exactly preserved. Both kind of symmetries have to be taken into account in covariant and canonical theories in order to ensure the correct continuum limit. We will sketch how to identify exact and approximate symmetries in the action and how to define a corresponding canonical theory in which such symmetries are reflected as exact and approximate constraints."

The next paper has to do with the possibility of detecting dark matter, an important side issue to quantum cosmology:
http://arxiv.org/abs/0810.3508
First Results from the PAMELA Space Mission
M. Boezio et al.
Parallel talk at ICHEP08, Philadelphia, USA, July 2008. 5 pages, 3 figures
(Submitted on 20 Oct 2008)

"On the 15th of June 2006, the PAMELA satellite-borne experiment was launched from the Baikonur cosmodrome and it has been collecting data since July 2006. The apparatus comprises a time-of-flight system, a silicon-microstrip magnetic spectrometer, a silicon-tungsten electromagnetic calorimeter, an anticoincidence system, a shower tail counter scintillator and a neutron detector. The combination of these devices allows precision studies of the charged cosmic radiation to be conducted over a wide energy range (100 MeV - 100's GeV) with high statistics. The primary scientific goal is the measurement of the antiproton and positron energy spectrum in order to search for exotic sources, such as dark matter particle annihilations. PAMELA is also searching for primordial antinuclei (anti-helium), and testing cosmic-ray propagation models through precise measurements of the antiparticle energy spectrum and precision studies of light nuclei and their isotopes. We review the status of the apparatus and present preliminary results concerning antiparticle measurements and dark-matter indirect searches.



brief mention:
http://arxiv.org/abs/0803.0547 (revised WMAP5 cosmology report issued October as published, some numbers changed)
http://arxiv.org/abs/0810.3172 (LQC big bounce reviewed)
http://arxiv.org/abs/0810.3562 (the first published report from the GLAST gammaray observatory collaboration)
http://arxiv.org/abs/0810.3512 (observing the passage of the meter in 1983)

 

[marcus]

http://arxiv.org/abs/0810.3675
Fixed Points of Quantum Gravity and the Renormalisation Group
Daniel F. Litim
18 pages, 4 figures. Plenary talk. To appear in the proceedings of "From Quantum to Emergent Gravity: Theory and Phenomenology", June 11-15 2007, Trieste
(Submitted on 21 Oct 2008)

"We review the asymptotic safety scenario for quantum gravity and the role and implications of an underlying ultraviolet fixed point. We discuss renormalisation group techniques employed in the fixed point search, analyse the main picture at the example of the Einstein-Hilbert theory, and provide an overview of the key results in four and higher dimensions. We also compare findings with recent lattice simulations and evaluate phenomenological implications for collider experiments."

 

[marcus]

http://arxiv.org/abs/0810.5119
Dilaton Gravity, Poisson Sigma Models and Loop Quantum Gravity
Martin Bojowald, Juan D. Reyes
31 pages
(Submitted on 28 Oct 2008)

"Spherically symmetric gravity in Ashtekar variables coupled to Yang-Mills theory in two dimensions and its relation to dilaton gravity and Poisson sigma models are discussed. After introducing its loop quantization, quantum corrections for inverse triad components are shown to provide a consistent deformation without anomalies. The relation to Poisson sigma models provides a covariant action principle of the quantum corrected theory with effective couplings. Results are also used to provide loop quantizations of spherically symmetric models in arbitrary D space-time dimensions."


http://arxiv.org/abs/0810.5039
Deformed spaces and loop cosmology
Marco Valerio Battisti
5 pages, talk given at NEB-XIII Recent Developments in Gravity, Thessaloniki June 2008
(Submitted on 28 Oct 2008)

"The non-singular bouncing solution of loop quantum cosmology is reproduced by a deformed minisuperspace Heisenberg algebra. This algebra is a realization of the Snyder space, is almost unique and is related to the kappa-Poincaré one. Since the sign of the deformation parameter is not fixed, the Friedmann eqn of braneworlds theory can also be obtained. Moreover, the sign is the only freedom in the picture and these frameworks are the only ones which can be reproduced by our deformed scheme. A generalized uncertainty princple for loop quantum cosmology is also proposed."

 

[marcus]

http://arxiv.org/abs/0810.5356
Lattice refinement in loop quantum cosmology
Mairi Sakellariadou
17 pages, 1 figure, to appear in the Proceedings of "Recent Developments in Gravity-NEB XIII"; Thessaloniki (Greece), June 2008
(Submitted on 29 Oct 2008)

"Lattice refinement in LQC, its meaning and its necessity are discussed. The rôle of lattice refinement for the realisation of a successful inflationary model is explicitly shown. A simple and effective numerical technique to solve the constraint equation for any choice of lattice refinement model is briefly illustrated. Phenomenological and consistency requirements leading to a particular choice of lattice refinement model are presented, while it is subsequently proved that only this choice of lattice refinement leads to a unique factor ordering in the Wheeler-De Witt equation, which is the continuum limit of LQC."

http://arxiv.org/abs/0810.4330
Loop quantum cosmology and tensor perturbations in the early universe
Gianluca Calcagni, Golam Mortuza Hossain
12 pages. Invited contribution to the special issue of Advanced Science Letters on "Quantum gravity, Cosmology and Black Holes"
(Submitted on 23 Oct 2008)

"We study the tensor modes of linear metric perturbations within an effective framework of loop quantum cosmology. After a review of inverse-volume and holonomy corrections in the background equations of motion, we solve the linearized tensor modes equations and extract their spectrum. Ignoring holonomy corrections, the tensor spectrum is blue tilted in the near-Planckian superinflationary regime and may be observationally disfavoured. However, in this case background dynamics is highly nonperturbative, hence the use of standard perturbative techniques may not be very reliable. On the other hand, in the quasi-classical regime the tensor index receives a small negative quantum correction, slightly enhancing the standard red tilt in slow-roll inflation. We discuss possible interpretations of this correction, which depends on the choice of semiclassical state."

http://pirsa.org/08100080/
If the CMB is right, it is inconsistent with standard inflationary Lambda CDM
Glenn Starkman - Case Western Reserve University
video of talk given 30 October 2008 at Perimeter Institute

"The Cosmic Microwave Background Radiation is our most important source of information about the early universe. Many of its features are in good agreement with the predictions of the so-called standard model of cosmology -- the Lambda Cold Dark Matter Inflationary Big Bang. However, the large-angle correlations in the microwave background exhibit several statistically significant anomalies compared to the predictions of the standard model. On the one hand, the lowest multipoles seem to be correlated not just with each other but with the geometry of the solar system. On the other hand, when we look at the part of the sky that we most trust -- the part outside the galactic plane, there is a dramatic lack of large angle correlations. So much so that no choice of angular powerspectrum can explain it if the alms are Gaussian random statistically isotropic variables of zero mean."

Some background to Starkman's talk, briefly noted:

http://arxiv.org/abs/0808.3767
No large-angle correlations on the non-Galactic microwave sky
Craig J. Copi, Dragan Huterer, Dominik J. Schwarz, Glenn D. Starkman
(Submitted on 27 Aug 2008)

http://arxiv.org/abs/astro-ph/0605135
The Uncorrelated Universe: Statistical Anisotropy and the Vanishing Angular Correlation Function in WMAP Years 1-3
Craig J. Copi (CWRU), Dragan Huterer (KICP Chicago), Dominik J. Schwarz (Universitat Bielefeld), Glenn D. Starkman (CWRU and Oxford)
(Submitted on 4 May 2006)

 

[marcus]

http://arxiv.org/abs/0811.1462
The non-commutative geometry of matrix models: the Spinfoam way
Etera R. Livine
13 pages
(Submitted on 10 Nov 2008)

"Group field theories provide a non-perturbative formulation of spin foam models for quantum gravity. Focusing on 2d group field theories, we review their explicit relation to matrix models and show their link to a class of non-commutative field theories invariant under a quantum deformed Poincare symmetry. This provides a simple relation between matrix models and non-commutative geometry. Finally, we review the derivation of effective 2d group field theories with non-trivial propagators from Boulatov's group field theory for 3d quantum gravity. Besides the fact that this gives a simple and direct derivation of non-commutative field theories for the matter dynamics coupled to (3d) quantum gravity, these effective field theories can be expressed as multi-matrix models with non-trivial coupling between matrices of different sizes. It should be interesting to analyze this new class of theories, both from the point of view of matrix models as integrable systems and for the study of non-commutative field theories."

http://arxiv.org/abs/0811.1572
Gauge invariant cosmological perturbation equations with corrections from loop quantum gravity
Martin Bojowald, Golam Mortuza Hossain, Mikhail Kagan, S. Shankaranarayanan
(Submitted on 10 Nov 2008)

"A consistent implementation of quantum gravity is expected to change the familiar notions of space, time and the propagation of matter in drastic ways. This will have consequences on very small scales, but also gives rise to correction terms in evolution equations of modes relevant for observations. In particular, the evolution of inhomogeneities in the very early universe should be affected. In this paper consistent evolution equations for gauge-invariant perturbations in the presence of inverse triad corrections of loop quantum gravity are derived. Some immediate effects are pointed out, for instance concerning conservation of power on large scales and non-adiabaticity. It is also emphasized that several critical corrections can only be seen to arise in a fully consistent treatment where the gauge freedom of canonical gravity is not fixed before implementing quantum corrections. In particular, metric modes must be allowed to be inhomogeneous: it is not consistent to assume only matter inhomogeneities on a quantum corrected homogeneous background geometry. In this way, stringent consistency conditions arise for possible quantization ambiguities which will eventually be further constrained observationally."

http://arxiv.org/abs/0811.1229
Unambiguous Quantum Gravity Phenomenology Respecting Lorentz Symmetry
Yuri Bonder, Daniel Sudarsky
(Submitted on 7 Nov 2008)

"We describe a refined version of a previous proposal for the exploration of quantum gravity phenomenology. Unlike the original scheme, the one presented here is free from sign ambiguities while it shares with the previous one the essential features. It focuses on effects that could be thought as arising from a fundamental granularity of quantum space-time. The sort of schemes we consider are in sharp contrast with the simplest scenarios in that such granularity is assumed to respect Lorentz Invariance but it remains otherwise unspecified. The proposal is fully observer covariant, it involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. We present the effective Hamiltonian which could be used to analyze concrete experimental situations, and we shortly review the degree to which this proposal is in line with the fundamental ideas behind the equivalence principle."

Briefly noted:
http://arxiv.org/abs/0811.1178
Emergent Continuum Spacetime from a Random, Discrete, Partial Order
David Rideout, Petros Wallden
8 pages, 2 figures, talk by P. Wallden at the NEB XIII conference
(Submitted on 7 Nov 2008)

http://arxiv.org/abs/0811.1086
Translational Invariance and the Anisotropy of the Cosmic Microwave Background
Sean M. Carroll, Chien-Yao Tseng, Mark B. Wise
(Submitted on 7 Nov 2008)

http://arxiv.org/abs/0811.1197
TeV Gamma-ray Astronomy: The Story So Far
Trevor C. Weekes
Proceedings of the 4th Heidelberg International Symposium on High Energy Gamma-Ray Astronomy, 2008
(Submitted on 7 Nov 2008)

 

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http://arxiv.org/abs/0811.1396
Fractal properties of quantum spacetime
Dario Benedetti
4 pages, 2 figures
(Submitted on 10 Nov 2008)

"We show that in general a spacetime having a quantum group symmetry has also a scale dependent fractal dimension which deviates from its classical value at short scales, a phenomenon that resembles what observed in some approaches to quantum gravity. In particular we analyze the cases of a quantum sphere and of k-Minkowski, the latter being relevant in the context of quantum gravity."

Benedetti is a Loll PhD now at Perimeter. He may have found an explanation for what the Loll and Reuter continuums have in common. Both continuums exhibit normal 4D dimensionality at large scale but they become fractally at microscopic scale and their dimensionality gradually goes down to around 2D, as the scale goes down to Planck. This is one of the most novel non-classical features of these new QG model continuums and it would be nice to understand it better. I hope Benedetti has a handle on it, as this paper suggests he may. I will start a discussion thread for this paper, in case anyone wants to comment.

http://arxiv.org/abs/0811.1676
Solving the Observer Metric
Charles Hellaby, Alnadhief H. A. Alfedeel
(Submitted on 11 Nov 2008)

"The analysis of modern cosmological data is becoming an increasingly important task as the amount of data multiplies. An important goal is to extract geometric information, i.e. the metric of the cosmos, from observational data. The observer metric is adapted to the reality of observations: information received along the past null cone, and matter flowing along timelike lines. It provides a potentially very good candidate for a developing general numerical data reduction program. As a basis for this, we elucidate the spherically symmetric solution, for which there is to date single presentation that is complete and correct. With future numerical implementation in mind, we give a clear presentation of the mathematical solution in terms of 4 arbitrary functions, the solution algorithm given observational data on the past null cone, and we argue that the evolution from one null cone to the next necessarily involves integrating down each null cone."

http://arxiv.org/abs/0811.1614
Dark Energy, Inflation and Extra Dimensions
Paul J. Steinhardt, Daniel Wesley
26 pages, 1 figure
(Submitted on 11 Nov 2008)

"We consider how accelerated expansion, whether due to inflation or dark energy, imposes strong constraints on fundamental theories obtained by compactification from higher dimensions. For theories that obey the null energy condition (NEC), we find that inflationary cosmology is impossible for a wide range of compactifications; and a dark energy phase consistent with observations is only possible if both Newton's gravitational constant and the dark energy equation-of-state vary with time. If the theory violates the NEC, inflation and dark energy are only possible if the NEC-violating elements are inhomogeneously distributed in the compact dimensions and vary with time in precise synchrony with the matter and energy density in the non-compact dimensions. Although our proofs are derived assuming general relativity applies in both four and higher dimensions and certain forms of metrics, we argue that similar constraints must apply for more general compactifications."

=============
EDIT (partly in response to popular demand) Here is a thread for discussing the Benedetti paper---your comments are cordially invited
https://www.physicsforums.com/showthread.php?t=270975

 

[MTd2]

Hey! I would post that!

Now that you wasted my time here, you must open a thread for this paper, or at least bump some older thread to discuss this paper.

 

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http://arxiv.org/abs/0811.1799
Covariant Effective Action for Loop Quantum Cosmology from Order Reduction
Thomas P. Sotiriou
(Submitted on 11 Nov 2008)

"Loop quantum cosmology (LQC) seems to be predicting modified effective Friedmann equations without extra degrees of freedom. A puzzle arises if one decides to seek for a covariant effective action which would lead to the given Friedmann equation: The Einstein--Hilbert action is the only action that leads to second order field equations and, hence, there exists no covariant action which, under metric variation, leads to modified Friedmann equations without extra degrees of freedom. It is shown that, at least for isotropic models in LQC, this issue is naturally resolved and a covariant effective action can be found if one considers higher order theories of gravity but faithfully follows effective field theory techniques. However, our analysis also raises doubts on whether a covariant description without background structures can be found for anisotropic models."

http://arxiv.org/abs/0811.1916
Towards Loop Quantum Gravity without the time gauge
Francesco Cianfrani, Giovanni Montani
5 pages
(Submitted on 12 Nov 2008)

"The Hamiltonian formulation of the Holst action is reviewed and it is provided a solution of second-class constraints corresponding to a generic local Lorentz frame. Within this scheme the form of rotation constraints can be reduced to a Gauss-like one by a proper generalization of Ashtekar-Barbero-Immirzi connections. This result emphasizes that the Loop Quantum Gravity quantization procedure can be applied when the time-gauge condition does not stand."

 

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http://arxiv.org/abs/0811.1998
Gravity theory that passes the classical tests but does not have black holes
Alexander Torres-Gomez, Kirill Krasnov
5 pages
(Submitted on 12 Nov 2008)

"We revisit a propagating torsion gravity theory obtained by introducing a field coupled to the topological term in the first-order Einstein-Cartan action. The resulting theory has second order field equations, no adjustable coupling constants, and one more propagating degree of freedom as compared to general relativity. Thus, one might suspect that it should be easily ruled out by e.g. the solar system tests. To see whether this is the case, we obtain the spherically-symmetric solution of the theory, and show that it is characterized by the usual mass and an additional parameter. To our great surprise we find that the leading order corrections to the usual Newtonian behaviour are exactly as in general relativity, and, in particular, are independent of the new parameter. Thus, the theory passes the classical gravity tests. The analysis of the global structure of the solution leads to yet another surprising feature of the theory: the spherically-symmetric solution is never a black hole. One either has a naked curvature singularity or a wormhole solution connecting two asymptotic regions."

http://arxiv.org/abs/0811.2196
Space-Time Structure of Loop Quantum Black Hole
Leonardo Modesto
17 pages, 19 figures
(Submitted on 13 Nov 2008)

"In this paper we have improved the semiclassical analysis of loop quantum black hole (LQBH) in the conservative approach of constant polymeric parameter. In particular we have focused our attention on the space-time structure. We have introduced a very simple modification of the spherically symmetric Hamiltonian constraint in its holonomic version. The new quantum constraint reduces to the classical constraint when the polymeric parameter goes to zero.Using this modification we have obtained a large class of semiclassical solutions parametrized by a generic function of the polymeric parameter. We have found that only a particular choice of this function reproduces the black hole solution with the correct asymptotic flat limit. In r=0 the semiclassical metric is regular and the Kretschmann invariant has a maximum peaked in L-Planck. The radial position of the pick does not depend on the black hole mass and the polymeric parameter. The semiclassical solution is very similar to the Reissner-Nordstrom metric. We have constructed the Carter-Penrose diagrams explicitly, giving a causal description of the space-time and its maximal extension. The LQBH metric interpolates between two asymptotically flat regions, the r to infinity region and the r to 0 region. We have studied the thermodynamics of the semiclassical solution. The temperature, entropy and the evaporation process are regular and could be defined independently from the polymeric parameter. We have studied the particular metric when the polymeric parameter goes towards to zero. This metric is regular in r=0 and has only one event horizon in r = 2m. The Kretschmann invariant maximum depends only on L-Planck. The polymeric parameter does not play any role in the black hole singularity resolution. The thermodynamics is the same."

http://arxiv.org/abs/0811.2161
Infinite Degeneracy of States in Quantum Gravity
Jonathan Hackett, Yidun Wan
10 pages, 14 figures
(Submitted on 13 Nov 2008)

"The setting of Braided Ribbon Networks is used to present a general result in spin-networks embedded in manifolds: the existence of an infinite number of species of conserved quantities. Restricted to three-valent networks the number of such conserved quantities in a given network is shown to be invariant barring a single case. The implication of these conserved quantities is discussed in the context of Loop Quantum Gravity."

 

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part of a give-and-take discussion which has been carried out with civility
and deliberateness on the arxiv rather than descending to blog level.

http://arxiv.org/abs/0811.2790
Comment on "Quantum bounce and cosmic recall" [arXiv:0710.4543]
Martin Bojowald
1 page
(Submitted on 17 Nov 2008)

"A recently derived inequality on volume fluctuations of a bouncing cosmology, valid for states which are semiclassical long after the bounce, does not restrict pre-bounce fluctuations sufficiently strongly to conclude that the pre-bounce state was semiclassical except in a very weak sense."

This was a reply to http://arxiv.org/abs/0710.4543
which, in turn, was in response to an earlier paper by Bojowald

http://arxiv.org/abs/0811.2622
Cosmological parameters after WMAP5: forecasts for Planck and future galaxy surveys
L.P.L. Colombo, E. Pierpaoli (USC), J.R. Pritchard (CfA)
16 pages, 15 figures
(Submitted on 17 Nov 2008)

"The Planck satellite is expected to improve the measurement of most cosmological parameters by several factors with respect to current WMAP results...
...Finally, Planck will greatly help future missions like LSST and CIP reach their potentials by providing tight constraints on parameters like n_s and n_run. Considering Planck together with these probes will help in breaking degeneracies between Omega_K and Omega_Lambda or Omega_dm and f_nu, resulting in improvements of several factors in the error associated to these parameters."

A two-page reply to Bojowald's one-page comment, extending the scholarly exchange:
http://arxiv.org/abs/0811.2983
Reply to `Comment on "Quantum Bounce and Cosmic Recall"' [arXiv:0811.2790]
Alejandro Corichi, Parampreet Singh
2 pages
(Submitted on 18 Nov 2008)

"A recent Comment [arXiv:0811.2790] on the Letter 'Quantum Bounce and Cosmic Recall' by the authors is shown to arise from an incorrect understanding of the issues at hand and of our analysis. The conclusions of Bojowald's Comment are shown to add little to our work, to be irrelevant at best, and are further shown to be in contradiction with his own claims in the literature."

http://arxiv.org/abs/0811.2814
Semi-classical limit and minimum decoherence in the Conditional Probability Interpretation of Quantum Mechanics
Vincent Corbin, Neil J. Cornish
8 pages
(Submitted on 17 Nov 2008)

"The Conditional Probability Interpretation of Quantum Mechanics replaces the abstract notion of time used in standard Quantum Mechanics by the time that can be read off from a physical clock. The use of physical clocks leads to apparent non-unitary and decoherence. Here we show that a close approximation to standard Quantum Mechanics can be recovered from conditional Quantum Mechanics for semi-classical clocks, and we use these clocks to compute the minimum decoherence predicted by the Conditional Probability Interpretation."

Carlo Rovelli has written several papers on this way to treat time, going back some 10 years, and has included a treatment in his 2004 book Quantum Gravity. It will be interesting to see if Neil Cornish cites the Loop community papers.

 

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http://arxiv.org/abs/0811.3147
Plebanski gravity without the simplicity constraints
Kirill Krasnov
7 pages
(Submitted on 19 Nov 2008)

"In Plebanski's self-dual formulation general relativity becomes SO(3) BF theory supplemented with the so-called simplicity (or metricity) constraints for the B-field. The main dynamical equation of the theory states that the curvature of the B-compatible SO(3) connection is self-dual, with the notion of self-duality being defined by the B-field. We describe a theory obtained by dropping the metricity constraints, keeping only the requirement that the curvature of the B-compatible connection is self-dual. It turns out that the theory one obtains is to a very large degree fixed by the Bianchi identities. Moreover, it is still a gravity theory, with just two propagating degrees of freedom as in GR."

 

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http://arxiv.org/abs/0811.3240
A Complete, Single-Horizon Quantum Corrected Black Hole Spacetime
A. Peltola, G. Kunstatter
7 pages, 2 figures
(Submitted on 20 Nov 2008)

"We show that a Loop Quantum Gravity motivated semi-classical polymerization of the interior of generic Schwarzschild-like black holes gives rise to a tantalizing candidate for a complete, regular, single horizon black hole spacetime. The exterior has non-zero quantum stress energy but closely approximates the classical spacetime for macroscopic black holes. The interior exhibits a bounce at a microscopic scale and then asymptotes to the non-singular product spacetime of a spacelike R and an expanding 2+1 positive curvature FRW cosmology. The polymer dynamics thus drives the system into an asymptotic interior end-state that is not a small correction to the classical spacetime. The scenario is suggestive of past proposals for universe creation via quantum effects inside a black hole."

 

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http://arxiv.org/abs/0811.3888
Bare Action and Regularized Functional Integral of Asymptotically Safe Quantum Gravity
Elisa Manrique, Martin Reuter
44 pages
(Submitted on 24 Nov 2008)

"Investigations of Quantum Einstein Gravity (QEG) based upon the effective average action employ a flow equation which does not contain any ultraviolet (UV) regulator. Its renormalization group trajectories emanating from a non-Gaussian fixed point define asymptotically safe quantum field theories. A priori these theories are, somewhat unusually, given in terms of their effective rather than bare action. In this paper we construct a functional integral representation of these theories. We fix a regularized measure and show that every trajectory of effective average actions, depending on an IR cutoff only, induces an associated trajectory of bare actions which depend on a UV cutoff. Together with the regularized measure these bare actions give rise to a functional integral which reproduces the prescribed effective action when the UV cutoff is removed. In this way we are able to reconstruct the underlying microscopic ('classical') system and identify its fundamental degrees of freedom and interactions. The bare action of the Einstein-Hilbert truncation is computed and its flow is analyzed as an example. Various conceptual issues related to the completion of the asymptotic safety program are discussed."

Subjects:

 

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http://arxiv.org/abs/0811.4129
Consistent Loop Quantum Cosmology
Martin Bojowald
13 pages
(Submitted on 25 Nov 2008)

"A consistent combination of quantum geometry effects rules out a large class of models of loop quantum cosmology and their critical densities as they have been used in the recent literature. In particular, the critical density at which an isotropic universe filled with a free, massless scalar field would bounce must be well below the Planck density. In the presence of anisotropy, no model of the Schwarzschild black hole interior analyzed so far is consistent."

http://arxiv.org/abs/0811.4160
Hamiltonian General Relativity and the Belinskii, Khalatnikov, Lifgarbagez Conjecture
Abhay Ashtekar, Adam Henderson, David Sloan
10 pages, 2 figures
(Submitted on 25 Nov 2008)

"The Belinkskii, Khalatnokov and Lifgarbagez conjecture says that as one approaches space-like singularities in general relativity, 'time derivatives dominate over spatial derivatives' so that the dynamics at any spatial point is well captured by an ordinary differential equation. By now considerable evidence has accumulated in favor of these ideas. Starting with a Hamiltonian framework, we provide a formulation of this conjecture in terms of variables that are tailored to non-perturbative quantization. Our formulation serves as a first step in the analysis of the fate of generic space-like singularities in loop quantum gravity."

http://arxiv.org/abs/0811.4023
Effective Actions from Loop Quantum Cosmology: Correspondence with Higher Curvature Gravity
Ghanashyam Date, Sandipan Sengupta
(Submitted on 25 Nov 2008)

"Quantum corrections of certain types and relevant in certain regimes can be summarised in terms of an effective action calculable, in principle, from the underlying theory. The demands of symmetries, local form of terms and dimensional considerations limit the form of the effective action to a great extent leaving only the numerical coefficients to distinguish different underlying theories. The effective action can be restricted to particular symmetry sectors to obtain the corresponding, reduced effective action. Alternatively, one can also quantize a classically (symmetry) reduced theory and obtain the corresponding effective action. These two effective actions can be compared. As an example, we compare the effective action(s) known in isotropic loop quantum cosmology with the Lovelock actions, as well as with more general actions, specialized to homogeneous isotropic space-times and find that the \bar{\mu}-scheme is singled out."

http://arxiv.org/abs/0811.4153
Decaying Dark Matter can explain the electron/positron excesses
Enrico Nardi, Francesco Sannino, Alessandro Strumia
20 pages, 7 figures
(Submitted on 25 Nov 2008)

"PAMELA and ATIC recently reported excesses in e+ e- cosmic rays. Since the interpretation in terms of DM annihilations was found to be not easily compatible with constraints from photon observations, we consider the DM decay hypothesis and find that it can explain the e+ e- excesses compatibly with all constraints, and can be tested by dedicated HESS observations of the Galactic Ridge. ATIC data indicate a DM mass of about 2 TeV: this mass naturally implies the observed DM abundance relative to ordinary matter if DM is a quasi-stable composite particle with a baryon-like matter asymmetry. Technicolor naturally yields these type of candidates."

http://arxiv.org/abs/0811.4132
Dark Energy and Modified Gravity
Ruth Durrer, Roy Maartens
(Submitted on 25 Nov 2008)

"Explanations of the late-time cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are mostly based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy, and a failure to solve the so-called coincidence problem. Two broad alternatives to dark energy have emerged as candidate models: these typically address only the coincidence problem and not the vacuum energy problem. The first is based on general relativity and attempts to describe the acceleration as an effect of inhomogeneity in the universe. If this alternative could be shown to work, then it would provide a dramatic resolution of the coincidence problem; however, a convincing demonstration of viability has not yet emerged. The second alternative is based on infrared modifications to general relativity, leading to a weakening of gravity on the largest scales and thus to acceleration. Most examples investigated so far are scalar-tensor or brane-world models, and we focus on the simplest candidates of each type: f(R) models and DGP models respectively. Both of these provide a new angle on the problem, but they also face serious difficulties. However, investigation of these models does lead to valuable insights into the properties of gravity and structure formation, and it also leads to new strategies for testing the validity of General Relativity itself on cosmological scales."

 

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http://arxiv.org/abs/0811.4235
A Causal Set Black Hole
Song He, David Rideout
21 pages, 9 figures
(Submitted on 26 Nov 2008)

"We explicitly compute the causal structure of the Schwarzschild black hole spacetime, by providing an algorithm to decide if any pair of events is causally related. The primary motivation for this study comes from discrete quantum gravity, in particular the causal set approach, in which the fundamental variables can be thought of as the causal ordering of randomly selected events in spacetime. This work opens the way to simulating non-conformally flat spacetimes within the causal set approach, which may allow one to study important questions such as black hole entropy and Hawking radiation on a full four dimensional causal set black hole."

 

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Some words used here are relation and circle and heat if y'want to look them up.
==================================================http://arxiv.org/abs/0812.0177
Loop Quantum Cosmology: An Overview
Abhay Ashtekar
To appear in the Proceedings of the Bad Honef Workshop entitled Quantum Gravity: Challenges and Perspectives, dedicated to the memory of John A. Wheeler
(Submitted on 30 Nov 2008)

"A brief overview of loop quantum cosmology of homogeneous isotropic models is presented with emphasis on the origin of and subtleties associated with the resolution of big bang and big crunch singularities. These results bear out the remarkable intuition that John Wheeler had. Discussion is organized at two levels. The the main text provides a bird's eye view of the subject that should be accessible to non-experts. Appendices address conceptual and technical issues that are often raised by experts in loop quantum gravity and string theory."

http://arxiv.org/abs/0812.0240
On the Flow of Time
George F R Ellis
9 pages, 2 figures. Essay for the Fqxi essay contest on THE NATURE OF TIME
(Submitted on 1 Dec 2008)

"Current theoretical physics suggests the flow of time is an illusion: the entire universe just is, with no special meaning attached to the present time. This paper points out that this view, in essence represented by usual space-time diagrams, is based on time-reversible microphysical laws, which fail to capture essential features of the time-irreversible nature of decoherence and the quantum measurement process, as well as macro-physical behaviour and the development of emergent complex systems, including life, which exist in the real universe. When these are taken into account, the unchanging block universe view of spacetime is best replaced by an evolving block universe which extends as time evolves, with the potential of the future continually becoming the certainty of the past; spacetime itself evolves, as do the entities within it. However this time evolution is not related to any preferred surfaces in spacetime; rather it is associated with the evolution of proper time along families of world lines. The default state of fundamental physics should not be taken to be a time irreversible evolution of physical states: it is an ongoing irreversible development of time itself."

http://arxiv.org/abs/0812.0165
The Uncanny Precision of the Spectral Action
Ali H. Chamseddine, Alain Connes
33 pages, 1 figure
(Submitted on 30 Nov 2008)

"Noncommutative geometry has been slowly emerging as a new paradigm of geometry which starts from quantum mechanics. One of its key features is that the new geometry is spectral in agreement with the physical way of measuring distances. In this paper we present a detailed introduction with an overview on the study of the quantum nature of space-time using the tools of noncommutative geometry. In particular we examine the suitability of using the spectral action as action functional for the theory. To demonstrate how the spectral action encodes the dynamics of gravity we examine the accuracy of the approximation of the spectral action by its asymptotic expansion in the case of the round three sphere. We find that the two terms corresponding to the cosmological constant and the scalar curvature term already give the full result with remarkable accuracy. This is then applied to the physically relevant case of the product of the three sphere by a circle where we show that the spectral action in this case is also given, for any test function, by the sum of two terms up to an astronomically small correction, and in particular all higher order terms vanish. This result is confirmed by evaluating the spectral action using the heat kernel expansion where we check that the higher order terms a4 and a6 both vanish due to remarkable cancelations. We also show that the Higgs potential appears as an exact perturbation when the test function used is a smooth cutoff function."

http://arxiv.org/abs/0812.0272
On the resolution of the big bang singularity in isotropic Loop Quantum Cosmology
Madhavan Varadarajan
26 pages
(Submitted on 1 Dec 2008)

"In contrast to previous work in the field, we construct the Loop Quantum Cosmology (LQC) of the flat isotropic model with a massless scalar field in the absence of higher order curvature corrections to the gravitational part of the Hamiltonian constraint. The matter part of the constraint contains the inverse triad operator which can be quantized with or without the use of a Thiemann- like procedure. With the latter choice, we show that the LQC quantization is identical to that of the standard Wheeler DeWitt theory (WDW) wherein there is no singularity resolution. We argue that the former choice leads to singularity resolution in the sense of a well defined, regular (backward) evolution through and beyond the epoch where the size of the universe vanishes.
Our work along with that of the seminal work of Ashtekar, Pawlowski and Singh (APS) clarifies the role, in singularity resolution, of the three `exotic' structures in this LQC model, namely: curvature corrections, inverse triad definitions and the `polymer' nature of the kinematic representation. We also critically examine certain technical assumptions made by APS in their analysis of WDW semiclassical states and point out some problems stemming from the infrared behaviour of their wave functions."

http://arxiv.org/abs/0811.4496
Topological Interpretation of Barbero-Immirzi Parameter
Ghanashyam Date, Romesh K. Kaul, Sandipan Sengupta
14 pages
(Submitted on 27 Nov 2008)

"We set up a canonical Hamiltonian formulation for a theory of gravity based on a Lagrangian density made up of the Hilbert-Palatini term and, instead of the Holst term, the Nieh-Yan topological density. The resulting set of constraints in the time gauge are shown to lead to a theory in terms of a real SU(2) connection which is exactly the same as that of Barbero and Immirzi with the coefficient of the Nieh-Yan term identified as the inverse of Barbero-Immirzi parameter. This provides a topological interpretation for this parameter. Matter coupling can then be introduced in the usual manner, without changing the universal topological Nieh-Yan term."

http://arxiv.org/abs/0812.0295
Quantum geometrodynamics: whence, whither?
Claus Kiefer
25 pages; invited contribution for the Proceedings of the seminar "Quantum Gravity: Challenges and Perspectives", Bad Honnef, Germany, April 2008
(Submitted on 1 Dec 2008)

"Quantum geometrodynamics is canonical quantum gravity with the three-metric as the configuration variable. Its central equation is the Wheeler--DeWitt equation. Here I give an overview of the status of this approach. The issues discussed include the problem of time, the relation to the covariant theory, the semiclassical approximation as well as applications to black holes and cosmology. I conclude that quantum geometrodynamics is still a viable approach and provides insights into both the conceptual and technical aspects of quantum gravity."

http://arxiv.org/abs/0811.4684
Six Puzzles for LCDM Cosmology
Authors: L. Perivolaropoulos
9 pages, 3 figures. Invited article to the TSPU anniversary volume "The Problems of Modern Cosmology" on the occasion of the 50th birthday of Prof. S. D. Odintsov
(Submitted on 28 Nov 2008)

"The LCDM cosmological model is a well defined, simple and predictive model which is consistent with the majority of current cosmological observations. Despite of these successes there are specific cosmological observations which differ from the predictions of LCDM at a level of 2\sigma or higher. These observations include the following: 1. Large Scale Velocity Flows (LCDM predicts significantly smaller amplitude and scale of flows than what observations indicate), 2. Brightness of Type Ia Supernovae (SnIa) at High Redshift z (LCDM predicts fainter SnIa at High z), 3. Emptiness of Voids (LCDM predicts more dwarf or irregular galaxies in voids than observed), 4. Profiles of Cluster Haloes (LCDM predicts shallow low concentration and density profiles in contrast to observations which indicate denser high concentration cluster haloes) 5. Profiles of Galaxy Haloes (LCDM predicts halo mass profiles with cuspy cores and low outer density while lensing and dynamical observations indicate a central core of constant density and a flattish high dark mass density outer profile), 6. Sizable Population of Disk Galaxies (LCDM predicts a smaller fraction of disk galaxies due to recent mergers expected to disrupt cold rotationally supported disks). Even though the origin of some of the above challenges may be astrophysical or related to dark matter properties, it should be stressed that even on galactic and cluster scales, the effects of dark energy on the equilibrium and stability of astrophysical systems are not negligible and they may play a key role in the resolution of the above puzzles. Here, I briefly review these six challenges of LCDM and discuss the possible dark energy properties required for their resolution."

http://arxiv.org/abs/0811.3806
Slow relaxation of rapidly rotating black holes
Shahar Hod
5 pages
(Submitted on 24 Nov 2008)

"We study analytically the relaxation phase of perturbed, rapidly rotating black holes. In particular, we derive a simple formula for the fundamental quasinormal resonances of near-extremal Kerr black holes. The formula is expressed in terms of the black-hole physical parameters: omega=m Omega - i2 pi T_BH(n+1/2), where T_BH and Omega are the temperature and angular velocity of the black hole, and m is the azimuthal harmonic index of a co-rotating equatorial mode. This formula implies that the relaxation period tau ~ 1/Im(omega) of the black hole becomes extremely long as the extremal limit T -> 0 is approached. The analytically derived formula is shown to agree with direct numerical computations of the black-hole resonances. We use our results to demonstrate analytically the fact that near-extremal Kerr black holes saturate the recently proposed universal relaxation bound."

 

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Autolink for heat if needed
================

http://arxiv.org/abs/0812.0399
Thermodynamics and phases in quantum gravity
Viqar Husain, R. B. Mann
6 pages, 1 figure
(Submitted on 2 Dec 2008)

"We give an approach for studying quantum gravity effects on black hole thermodynamics. This combines a quantum framework for gravitational collapse with quasi-local definitions of energy and surface gravity. Our arguments suggest that (i) the specific heat of a black hole becomes positive after a phase transition near the Planck scale,(ii) its entropy acquires a logarithmic correction, and (iii) the mass loss rate is modified such that Hawking radiation stops near the Planck scale. These results are due essentially to a realization of fundamental discreteness in quantum gravity, and are in this sense potentially theory independent."

 

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http://pirsa.org/08120001/
Quantum Nature of the Big bang in Simple Models.
Abhay Ashtekar
Perimeter video with slides PDF
3 December 2008

"According to general relativity, space-time ends at singularities and classical physics just stops. In particular, the big bang is regarded as The Beginning. However, general relativity is incomplete because it ignores quantum effects. Through simple models, I will illustrate how the quantum nature of space-time geometry resolves the big bang singularity. Quantum physics does not stop there. Indeed, quantum space-times can be vastly larger than what general relativity had us believe, with unforeseen physical effects in the deep Planck regime."

http://pirsa.org/08120016/
Why there is no information loss
Abhay Ashtekar
Perimeter video with slides PDF
4 December 2008

"Using 2-dimensional CGHS black holes, I will argue that information is not lost in the Hawking evaporation because the quantum space-time is significantly larger than the classical one. I will begin with a discussion of the conceptual underpinnings of problem and then introduce a general, non-perturbative framework to describe quantum CGHS black holes. I will show that the Hawking effect emerges from it in the first approximation. Finally, I will introduce a mean field approximation to argue that, when the back reaction is included, future null infinity is `long enough' to capture full information contained in pure states at past null infinity and that the S-matrix is unitary. There are no macroscopic remnants."

http://arxiv.org/abs/0812.0921
Effective equations of motion for constrained quantum systems: A study of the Bianchi I loop quantum cosmology
Dah-Wei Chiou
30 pages
(Submitted on 4 Dec 2008)

"A new mathematical framework is formulated to derive the effective equations of motion for the constrained quantum system which possesses an internal clock. In the realm close to classical behavior, the quantum evolution is approximated by a finite system of coupled but ordinary differential equations adhered to the weakly imposed Hamiltonian constraint. For the simplified version of loop quantum cosmology in the Bianchi I model with a free massless scalar filed, the resulting effective equations of motion affirm the bouncing scenario predicted by the previous studies: The big bang singularity is resolved and replaced by the big bounces, which take place up to three times, once in each diagonal direction, whenever the directional density approaches the critical value in the regime of Planckian density. It is also revealed that back-reaction arises from the quantum corrections and modifies the precise value of the directional density at the bouncing epoch. Additionally, as an example of symmetry reduction, we study isotropy emerging from the anisotropic Bianchi I model in the context of effective equations of motion."

http://arxiv.org/abs/0812.1015
Replacing Anthropy with entropy: Does it work?
Irit Maor, Thomas W. Kephart, Lawrence M. Krauss, Y. Jack Ng, Glenn D. Starkman
6 pages, 1 figure
(Submitted on 4 Dec 2008)

"Probably not, because there are lots of manifestly unanthropic ways of producing entropy. We demonstrate that the Causal Entropic Principle (CEP), as a replacement for the anthropic principle to explain the properties of the observed universe, suffers from many of the same problems of adopting myopic assumptions in order to predict that various fundamental parameters take approximately the observed values. In particular, we demonstrate that four mechanisms -- black hole production, black hole decay, phase transitions, and dark matter annihilations or decays -- will manifestly change the conclusions of the CEP to predict that we should live in a universe quite different than the one in which we find ourselves."

 

[marcus]

http://arxiv.org/abs/0812.1349
The cosmological background of vector modes
Teresa Hui-Ching Lu, Kishore Ananda, Chris Clarkson, Roy Maartens
9 pages, 2 figures
(Submitted on 7 Dec 2008)

"We investigate the spectrum of vector modes today which is generated at second order by density perturbations. The vector mode background that is generated by structure formation is small but in principle it contributes to the integrated Sachs-Wolfe effect, to redshift-space distortions and to weak lensing. We recover, clarify and extend previous results, and explain carefully why no vorticity is generated in the fluid at second order. The amplitude of the induced vector mode in the metric is around 1% that of the first-order scalars on small scales. We also calculate the power spectrum and the energy density of the vector part of the shear at second order."

 

[marcus]

http://arxiv.org/abs/0812.1657
Cosmology and the Noncommutative approach to the Standard Model
William Nelson, Mairi Sakellariadou
4 pages
(Submitted on 9 Dec 2008)

"We study cosmological consequences of the noncommutative approach to the standard model. Neglecting the nonminimal coupling of the Higgs field to the curvature, noncommutative corrections to Einstein's equations are present only for inhomogeneous and anisotropic space-times. Considering the nominimal coupling however, we obtain corrections even for background cosmologies. A link with dilatonic gravity as well as chameleon cosmology are briefly discussed, and potential experimental consequences are mentioned."

http://arxiv.org/abs/0812.1889
Magnetic Bianchi I Universe in Loop Quantum Cosmology
Roy Maartens, Kevin Vandersloot
(Submitted on 10 Dec 2008)

"We examine the dynamical consequences of homogeneous cosmological magnetic fields in the framework of loop quantum cosmology. We show that a big-bounce occurs in a collapsing magnetized Bianchi I universe, thus extending the known cases of singularity-avoidance. Previous work has shown that perfect fluid Bianchi I universes in loop quantum cosmology avoid the singularity via a bounce. The fluid has zero anisotropic stress, and the shear anisotropy in this case is conserved through the bounce. By contrast, the magnetic field has nonzero anisotropic stress, and shear anisotropy is not conserved through the bounce. After the bounce, the universe enters a classical phase. The addition of a dust fluid does not change these results qualitatively."

 

[John86]

Hello yesterday i found this interesting paper by Martin Kober.


http://arxiv.org/abs/0812.0713

The relation of a Unified Quantum Field Theory of Spinors to the structure of General Relativity
Authors: Martin Kober
(Submitted on 3 Dec 2008)

Abstract: Based on a unified quantum field theory of spinors assumed to describe all matter fields and its interactions we construct the space time structure of general relativity according to a general connection within the corresponding spinor space. The tetrad field and the corresponding metric field are composed from a space time dependent basis of spinors within the internal space of the fundamental matter field. Similar to twistor theory the Minkowski signature of the space time metric is related to this spinor nature of elementary matter, if we assume the spinor space to be endowed with a symplectic structure. The equivalence principle and the property of background independence arise from the fact that all elementary fields are composed from the fundamental spinor field. This means that the structure of space time according to general relativity seems to be a consequence of a fundamental theory of matter fields and not a presupposition as in the usual setting of relativistic quantum field theories.


and this one by Thiemann


http://arxiv.org/abs/0812.1537

Semiclassical analysis of the Loop Quantum Gravity volume operator: I. Flux Coherent States
Authors: C. Flori, T. Thiemann
(Submitted on 8 Dec 2008)

Abstract: The volume operator plays a pivotal role for the quantum dynamics of Loop Quantum Gravity (LQG), both in the full theory and in truncated models adapted to cosmological situations coined Loop Quantum Cosmology (LQC). It is therefore crucial to check whether its semiclassical limit coincides with the classical volume operator plus quantum corrections. In the present article we investigate this question by generalizing and employing previously defined coherent states for LQG which derive from a cylindrically consistently defined complexifier operator which is the quantization of a known classical function. These coherent states are not normalizable due to the non separability of the LQG Hilbert space but they define uniquely define cut off states depending on a finite graph. The result of our analysis is that the expectation value of the volume operator with respect to coherent states depending on a graph with only n valent verticies reproduces its classical value at the phase space point at which the coherent state is peaked only if n = 6. In other words, the semiclassical sector of LQG defined by those states is described by graphs with cubic topology! This has some bearing on current spin foam models which are all based on four valent boundary spin networks.


greetings John.

 

[marcus]

greetings, would you like to start a discussion thread for the Martin Kober paper?
I don't know if any members would be interested, but some might be.

http://arxiv.org/abs/0812.2214
Fractal Structure of Loop Quantum Gravity
Leonardo Modesto
5 pages, 5 figures
(Submitted on 11 Dec 2008)

"In this paper we have calculated the spectral dimension of loop quantum gravity (LQG) using simple arguments coming from the area spectrum at different length scales. We have obtained that the spectral dimension of the spatial section runs from 2 to 3, across a 1.5 phase, when the energy of a probe scalar field decrees from high to low energy. We have calculated the spectral dimension of the space-time also using results from spin-foam models, obtaining a 2-dimensional effective manifold at high energy. Our result is consistent with other two approach to non perturbative quantum gravity: causal dynamical triangulation and asymptotic safety quantum gravity."

 

[marcus]

http://arxiv.org/abs/0812.3200
Modified gravity without new degrees of freedom
Laurent Freidel
19 pages
(Submitted on 17 Dec 2008)
"We show that the new type of "non-metric" gravity theories introduced independently by Bengtsson and Krasnov can in fact be reexpressed explicitely as a metrical theory coupled to an auxiliary field. We unravel why such theories possesses only one propagating graviton by looking at the quadratic perturbation around a fixed solution. And we give a general construction principle with a new class of example of such modified gravity theories still possessing only two propagating degrees of freedom."

Part of a scholarly discussion carried out on the arxiv (rather than in less orderly fashion on blogs)
http://arxiv.org/abs/0812.3266
A short answer to critics of our article "Eppur si espande" [arXiv:astro-ph/0612155]
Marek A. Abramowicz, Stanislaw Bajtlik, Jean-Pierre Lasota, Audrey Moudens
5 pages, 2 figures, submitted to Acta Astronomica
(Submitted on 17 Dec 2008)
"Recently [arXiv:astro-ph/0612155] we presented a formal mathematical proof that, contrary to a widespread misconception, cosmological expansion cannot be understood as the motion of galaxies in non-expanding space. We showed that the cosmological redshift must be physically interpreted as the expansion of space. Although our proof was generally accepted, a few authors disagreed. We rebut their criticism in this Note."

 

[marcus]

A Lagrangian approach to the Barrett-Crane spin foam model

http://arxiv.org/abs/0812.3456
A Lagrangian approach to the Barrett-Crane spin foam model
Valentin Bonzom, Etera R. Livine
25 pages, 4 figures
(Submitted on 18 Dec 2008)
"We provide the Barrett-Crane spin foam model for quantum gravity with a discrete action principle, consisting in the usual BF term with discretized simplicity constraints which in the continuum turn topological BF theory into gravity. The setting is the same as usually considered in the literature: space-time is cut into 4-simplices, the connection describes how to glue these 4-simplices together and the action is a sum of terms depending on the holonomies around each triangle. We impose the discretized simplicity constraints on disjoints tetrahedra and we show how the Lagrange multipliers for the simplicity constraints distort the parallel transport and the correlations between neighbouring 4-simplices. We then construct the discretized BF action using a non-commutative product between $\SU(2)$ plane waves. We show how this naturally leads to the Barrett-Crane model. This clears up the geometrical meaning of the model. We discuss the natural generalization of this action principle and the spin foam models it leads to. We show how the recently introduced spinfoam fusion coefficients emerge with a non-trivial measure. In particular, we recover the Engle-Pereira-Rovelli spinfoam model by weakening the discretized simplicity constraints. Finally, we identify the two sectors of Plebanski's theory and we give the analog of the Barrett-Crane model in the non-geometric sector."

http://arxiv.org/abs/0812.3554
Quasi-Local Energy in Loop Quantum Gravity
Jinsong Yang, Yongge Ma
27 pages
(Submitted on 18 Dec 2008)
"Although there is no known meaningful notion of the energy density of the gravitational field in general relativity, a few notions of quasi-local energy of gravity associated to extended but finite domains have been proposed. In this paper, the notions of quasi-local energy are studied in the framework of loop quantum gravity, in order to see whether these notions can be carried out at quantum level. Two basic quasi-local geometric quantities are quantized, which lead to well-defined operators in the kinematical Hilbert space of loop quantum gravity. We then use them as basic building blocks to construct different versions of quasi-local energy operators. The operators corresponding to Brown-York energy, Liu-Yau energy, Hawking energy, and Geroch energy are obtained respectively. The virtue of the Geroch energy operator is beneficial for us to derive a rather general entropy-area relation from loop quantum gravity."

http://arxiv.org/abs/0812.3532
Averaged null energy condition in Loop Quantum Cosmology
Li-Fang Li, Jian-Yang Zhu
5 pages
(Submitted on 18 Dec 2008)
"Wormhole and time machine are very interesting objects in general relativity. However, they need exotic matters which are impossible in classical level to support them. But if we introduce the quantum effects of gravity into the stress-energy tensor, these peculiar objects can be constructed self-consistently. Fortunately, loop quantum cosmology (LQC) has the potential to serve as a bridge connecting the classical theory and quantum gravity. Therefore it provides a simple way for the study of quantum effect in the semiclassical case. As is well known, loop quantum cosmology is very successful to deal with the behavior of early universe. In the early stage, if taken the quantum effect into consideration, inflation is natural because of the violation of every kind of local energy conditions. Similar to the inflationary universe, the violation of the averaged null energy condition is the necessary condition for the traversable wormholes. In this paper, we investigate the averaged null energy condition in LQC in the framework of effective Hamiltonian, and find out that LQC do violate the averaged null energy condition in the massless scalar field coupled model."

http://arxiv.org/abs/0812.3544
Thermodynamics in Loop Quantum Cosmology
Li-Fang Li, Jian-Yang Zhu
4 pages
(Submitted on 18 Dec 2008)
"Loop quantum cosmology (LQC) is very powerful to deal with the behavior of early universe. And the effective loop quantum cosmology gives a successful description of the universe in the semiclassical region. We consider the apparent horizon of the Friedmann-Robertson-Walker universe as a thermodynamical system and investigate the thermodynamics of LQC in the semiclassical region. The effective density and effective pressure in the modified Friedmann equation from LQC not only determine the evolution of the universe in LQC scenario but are actually also found to be the thermodynamic quantities. This result comes from the energy definition in cosmology (the Misner-Sharp gravitational energy) and is consistent with thermodynamic laws. We prove that within the framework of loop quantum cosmology, the elementary equation of equilibrium thermodynamics is still valid."

http://arxiv.org/abs/0812.3552
Tachyon field in loop quantum cosmology: an example of traversable singularity
Li-Fang Li, Jian-Yang Zhu
5 pages, 3 figures
(Submitted on 18 Dec 2008)
"Loop quantum cosmology (LQC) predicts a nonsingular evolution of the universe through a bounce in the high energy region. But LQC has an ambiguity about quantization scheme. Recently, the authors in [Phys. Rev. D 77, 124008 (2008)] proposed one new quantization scheme. Similar to other quantization scheme, this new quantization scheme will replace the big bang singularity with the quantum bounce also. More interestingly, it introduces a quantum singularity, which is traversable. We investigate this novel dynamics quantitatively with tachyon scalar field, which gives us a concrete example. Our result shows that our universe can evolve through the quantum singularity regularly, which is different from the classical big bang singularity."

http://arxiv.org/abs/0812.3438
De Sitter Special Relativity: Effects on Cosmology
R. Aldrovandi, J. G. Pereira
12 pages
(Submitted on 18 Dec 2008)
"The main consequences of de Sitter Special Relativity to the Standard Cosmological Model of Physical Cosmology are examined. The cosmological constant Lambda appears, in this theory, as a manifestation of the proper conformal current, which must be added to the usual energy-momentum density. As that conformal current itself vanishes in absence of sources, Lambda is ultimately dependent on the matter content, and can in principle be calculated. A present-day value very close to that given by the crossed supernova/BBR data is obtained through simple and reasonable approximations. Also a primeval inflation of polynomial type is found, and the usual notion of co-moving observer is slightly modified."

http://arxiv.org/abs/0812.3488
Noncommutative Quantum Cosmology
C. Bastos, O. Bertolami, N. Dias, J. Prata
Proceedings of DICE2008, Castiglioncello, 22nd-26th September 2008, Italy
(Submitted on 18 Dec 2008)
One presents a phase-space noncommutative extension of Quantum Cosmology in the context of a Kantowski-Sachs (KS) minisuperspace model. We obtain the Wheeler-DeWitt (WDW) equation for the noncommutative system through the ADM formalism and a suitable Seiberg-Witten map. The resulting WDW equation explicitly depends on the phase-space noncommutative parameters, theta and eta. Numerical solutions of the noncommutative WDW equation are found and, interestingly, also bounds on the values of the noncommutative parameters. Moreover, we conclude that the noncommutativity in the momenta sector leads to a damped wave function implying that this type of noncommutativity can be relevant for a selection of possible initial states for the universe.

http://arxiv.org/abs/0812.3521
Polemic Notes On IR Perturbative Quantum Gravity
Ilya L. Shapiro
8 pages, Based on plenary talk at the Seventh Alexander Friedmann International Seminar on Gravitation and Cosmology, Joao Pessoa, 2008, to be published in the special issue of Int.J.Mod.Phys. A, Editors: V.B.Bezerra, V.M.Mostepanenko and C.Romero
(Submitted on 18 Dec 2008)
"Quantum gravity is an important and to great extent unsolved problem. There are many different approaches to the quantization of the metric field, both perturbative and non-perturbative. The current situation in the perturbative quantum gravity is characterized by a number of different models, some of them well elaborated but no one perfect nor mathematically neither phenomenologically, mainly because there are no theoretically derived observables which can be experimentally measured. A very interesting one is an effective approach which separates the low-energy quantum effects from the UV sector. In this way one can calculate quantities which are potentially relevant for establishing certain universal features of quantum gravity. In this presentation we give a polemic consideration of the effective approach to the infrared quantum gravity. We question the validity of the recent results in this area and also discuss how one can check the alleged universality of the effective approach."

 

[MTd2]

you forgot this article marcus, this is closely related to http://arxiv.org/abs/0812.3200, Modified gravity without new degrees of freedom, that you posted yesterday. Freidel made that article closely related to Kiril Krasnov, if you check the acknoledegments and introduction.

http://arxiv.org/abs/0812.3603

Motion of a "small body" in non-metric gravity
Authors: Kirill Krasnov
(Submitted on 18 Dec 2008)

"Abstract: We describe "small bodies" in a non-metric gravity theory previously studied by this author. The main dynamical field of the theory is a certain triple of two-forms rather than the metric, with only the spacetime conformal structure, not metric, being canonically defined. The theory is obtained from general relativity (GR) in Plebanski formulation by adding to the action a certain potential. Importantly, the modification does not change the number of propagating degrees of freedom as compared to GR. We find that "small bodies" move along geodesics of a certain metric that is constructed with the help of a new potential function that appears in the matter sector. We then use the "small body" results to formulate a prescription for coupling the theory to general stress-energy tensor. In its final formulation the theory takes an entirely standard form, with matter propagating in a metric background and only the matter-gravity coupling and the gravitational dynamics being modified. This completes the construction of the theory and opens way to an analysis of its physical predictions."

 

[marcus]

http://arxiv.org/abs/0812.4055
The Holst Spin Foam Model via Cubulations
Aristide Baratin, Cecilia Flori, Thomas Thiemann
28 pages, 1 figure
(Submitted on 21 Dec 2008)
"Spin Foam Models (SFM) are an attempt at a covariant or path integral formulation of canonical Loop Quantum Gravity (LQG). Traditionally, SFM rely on
1. the Plebanski formulation of GR as a constrained BF Theory.
2. simplicial triangulations as a UV regulator and
3. a sum over all triangulations via group field techniques (GFT) in order to get rid off triangulation dependence.
Subtle tasks for current SFM are to establish
1. the correct quantum implementation of Plebanski's constraints.
2. the existence of a semiclassical sector implementing additional Regge constraints arising from simplicial triangulations and
3. the physical inner product of LQG via GFT.
We propose a new approach which deals with these issues as follows:
1. The simplicity constraints are correctly implemented by starting directly from the Holst action which is also a proper starting point for canonical LQG.
2. Cubulations are chosen rather than triangulations as a regulator.
3. We give a direct interpretation of our spin foam model as a generating functional of n-point functions on the physical Hilbert space at finite regulator.
This paper focuses on ideas and tasks to be performed before the model can be taken seriously, however, it transpires that
1. this model's amplitudes differ from those of current SFM,
2. tetrad n-point functions reveal a Wick like structure and
3. the restriction to simple representations does not occur automatically but must employ the time gauge just as in the classical theory."

http://arxiv.org/abs/0812.3912
An Accelerating Cosmology Without Dark Energy
G. Steigman, R. C. Santos, J. A. S. Lima
12 pages, 8 figures
(Submitted on 19 Dec 2008)
"The negative pressure accompanying gravitationally-induced particle creation can lead to a cold dark matter (CDM) dominated, accelerating Universe (Lima et al. 1996) without requiring the presence of dark energy or a cosmological constant. In a recent study Lima et al. (2008, LSS) demonstrated that particle creation driven cosmological models are capable of accounting for the SNIa observations of the recent transition from a decelerating to an accelerating Universe. Here we test the evolution of such models at high redshift using the constraint on z_eq, the redshift of the epoch of matter radiation equality, provided by the WMAP constraints on the early Integrated Sachs-Wolfe effect. Since the contribution of baryons and radiation was ignored in the work of LSS, we include them in our study of this class of models. The parameters of these more realistic models with continuous creation of CDM is tested and constrained at widely-separated epochs (z = z_eq and z = 0) in the evolution of the Universe. This comparison reveals a tension between the high redshift CMB constraint on z_eq and that which follows from the low redshift SNIa data, challenging the viability of this class of models."

 

[John86]

Don't forget this one


Imposing causality on a matrix model
Authors: Dario Benedetti, Joe Henson
(Submitted on 22 Dec 2008)


Abstract: We introduce a new matrix model that describes Causal Dynamical Triangulations (CDT) in two dimensions. In order to do so, we introduce a new, simpler definition of 2D CDT and show it to be equivalent to the old one. The model makes use of ideas from dually weighted matrix models, combined with multi-matrix models, and can be studied by the method of character expansion.

 

[MTd2]

SURE INDEED! A GREAT MISS Benedetti is the author of the article fractal space time

You forgot the link:

http://arxiv.org/abs/0812.4261

Nearly all his citations of this article goes for Loll. According to him, this is a leap frog from his earlier article with Renate Loll.

 

[marcus]

thanks John86 and MTd2. The Benedetti Henson paper is a good find.
BTW John Baez has a new paper
http://math.ucr.edu/home/baez/2rep.pdf
Infinite-Dimensional Representations of 2-Groups
John Baez, Aristide Baratin, Laurent Freidel, Derek K. Wise

 

[MTd2]

thanks john86 and mtd2. The benedetti henson paper is a good find.
Btw john baez has a new paper
http://math.ucr.edu/home/baez/2rep.pdf
infinite-dimensional representations of 2-groups
john baez, aristide baratin, laurent freidel, derek k. Wise

wow! Is John Baez back to LQG?

 

[marcus]

wow! Is John Baez back to LQG?

No, this is still pure math. One never knows if and for what pure mathematics might prove useful.
In this case, it isn't clear how 2-groups and higher category theory might eventually apply to QG. It might or it might not.
His three co-authors' research has been mostly in QG. He has taken them on a remarkable excursion into higher category theory.

The original rationale was that Freidel and Livine (also with Baratin) did some very interesting work combining 3D spinfoam and matter and Feynman diagrams of usual field theory in the zero gravity limit. That was in 2006. But try as they might, their results would not extend up to 4D. Baez suggested they venture into n-category theory because it offers a general machinery for jacking up dimensions by one step. Today's paper could provide a missing ingredient for Freidel to jack his 2006 work on 3D QG and matter up to 4D, though it seems like long-odds.

As a rule it is better if we don't burden this bibliography thread with discussion. I will fetch a Baez thread that relates to this paper, so we can talk there if you want, instead of here.

Here is the Baez thread
https://www.physicsforums.com/showthread.php?p=2012718#post2012718
https://www.physicsforums.com/showthread.php?t=123902

 

[marcus]

http://arxiv.org/abs/0812.4512
The Problem Of Gauge Theory
Edward Witten
This article is the written form of a lecture presented at the conference "Geometric Analysis: Past and Future" (Harvard University, August 27-September 1, 2008), in honor of the 60th birthday of S.-T. Yau.
(Submitted on 24 Dec 2008)
"I sketch what it is supposed to mean to quantize gauge theory, and how this can be made more concrete in perturbation theory and also by starting with a finite-dimensional lattice approximation. Based on real experiments and computer simulations, quantum gauge theory in four dimensions is believed to have a mass gap. This is one of the most fundamental facts that makes the Universe the way it is."

http://arxiv.org/abs/0812.2696
Emergent Electroweak Gravity
Bob McElrath
4 pages
(Submitted on 15 Dec 2008)
"We show that any massive cosmological relic particle with small self-interactions is a super-fluid today, due to the broadening of its wave packet, and lack of any elastic scattering. The WIMP dark matter picture is only consistent its mass M >> M_Pl in order to maintain classicality. The dynamics of a super-fluid are given by the excitation spectrum of bound state quasi-particles, rather than the center of mass motion of constituent particles. If this relic is a fermion with a repulsive interaction mediated by a heavy boson, such as neutrinos interacting via the Z⁰, the condensate has the same quantum numbers as the vierbein of General Relativity. Because there exists an enhanced global symmetry SO(3,1)_space x SO(3,1)_spin among the fermion's self-interactions broken only by its kinetic term, the long wavelength fluctuation around this condensate is a Goldstone graviton. A gravitational theory exists in the low energy limit of the Standard Model's Electroweak sector below the weak scale, with a strength that is parametrically similar to G_N."
some helpful interpretive comment:
http://blogs.discovermagazine.com/cosmicvariance/2008/12/27/gravity-emergesfrom-neutrinos/
by John Conway:
http://www.physics.ucdavis.edu/~conway/

 

[marcus]

http://arxiv.org/abs/0812.5018
LQG propagator: III. The new vertex
Emanuele Alesci, Eugenio Bianchi, Carlo Rovelli
9 pages
(Submitted on 30 Dec 2008)
"In the first article of this series, we pointed out a difficulty in the attempt to derive the low-energy behavior of the graviton two-point function, from the loop-quantum-gravity dynamics defined by the Barrett-Crane vertex amplitude. Here we show that this difficulty disappears when using the corrected vertex amplitude recently introduced in the literature. In particular, we show that the asymptotic analysis of the new vertex amplitude recently performed by Barrett, Fairbairn and others, implies that the vertex has precisely the asymptotic structure that, in the second article of this series, was indicated as the key necessary condition for overcoming the difficulty."http://arxiv.org/abs/0812.4703
Singularity Resolution in Loop Quantum Cosmology: A Brief Overview
Abhay Ashtekar
12 pages. Addressed to experts in classical general relativity. Based on a talk at the NEB XIII conference held at Thessaloniki. To appear in Journal of Physics Proceedings Series
(Submitted on 26 Dec 2008)
"A brief summary of the singularity resolution in loop quantum cosmology of homogeneous isotropic models is presented. The article is addressed to relativists who do not specialize in quantum gravity. For further details, and answers to more technical asked questions, the reader is directed to the original papers and to more comprehensive recent reviews.
Dedicated to the memory of Basilis Xanthopoulos, a founder of this NEB series of conferences."

http://arxiv.org/abs/0812.4969
Infinite-Dimensional Representations of 2-Groups
John C. Baez, Aristide Baratin, Laurent Freidel, Derek K. Wise
101 pages
(Submitted on 29 Dec 2008)
"A '2-group' is a category equipped with a multiplication satisfying laws like those of a group. Just as groups have representations on vector spaces, 2-groups have representations on '2-vector spaces', which are categories analogous to vector spaces. Unfortunately, Lie 2-groups typically have few representations on the finite-dimensional 2-vector spaces introduced by Kapranov and Voevodsky. For this reason, Crane, Sheppeard and Yetter introduced certain infinite-dimensional 2-vector spaces called 'measurable categories' (since they are closely related to measurable fields of Hilbert spaces), and used these to study infinite-dimensional representations of certain Lie 2-groups. Here we continue this work. We begin with a detailed study of measurable categories. Then we give a geometrical description of the measurable representations, intertwiners and 2-intertwiners for any skeletal measurable 2-group. We study tensor products and direct sums for representations, and various concepts of subrepresentation. We describe direct sums of intertwiners, and sub-intertwiners--features not seen in ordinary group representation theory. We classify irreducible and indecomposable representations and intertwiners. We also classify 'irretractable' representations--another feature not seen in ordinary group representation theory. Finally, we argue that measurable categories equipped with some extra structure deserve to be considered 'separable 2-Hilbert spaces', and compare this idea to a tentative definition of 2-Hilbert spaces as representation categories of commutative von Neumann algebras."

 

[marcus]

http://arxiv.org/abs/0901.0010
The Evolution of Lambda Black Holes in the Mini-Superspace Approximation of Loop Quantum Gravity
J. Brannlund, S. Kloster, A. DeBenedictis
25 pages, 7 figures
(Submitted on 30 Dec 2008)
"Using the improved quantization technique to the mini-superspace approximation of loop quantum gravity, we study the evolution of black holes supported by a cosmological constant. The addition of a cosmological constant allows for classical solutions with planar, cylindrical, toroidal and higher genus black holes. Here we study the quantum analog of these space-times. In all scenarios studied, the singularity present in the classical counter-part is avoided in the quantized version and is replaced by a bounce, and in the late evolution, a series of less severe bounces. Interestingly, although there are differences during the evolution between the various symmetries and topologies, the evolution on the other side of the bounce aymptotes to space-times of Nariai-type. This is independent of the symmetry and topology of the system, indicating that Nariai-type universes are attractors in the quantum evolution, albeit with different parameters. We study here the quantum evolution of each symmetry in detail."

http://arxiv.org/pdf/0901.0041v1
Gamma-ray Bursts: Light on the distant Universe
Jonathan Grindlay
News and Views article for Nature vol. 455, 177 (Sept. 11, 2008)
(Submitted on 31 Dec 2008)
"Observations of a long-lasting Gamma-ray burst, one that has the brightest optical counterpart yet discovered, challenge theoretical understanding of these bursts but may enhance their usefulness as cosmic probes."