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."

 

[marcus]

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:

 

[marcus]

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."