Loop-and-allied QG bibliography

[marcus]

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."

 

[marcus]

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."

 

[marcus]

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."

 

[marcus]

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."

 

[marcus]

http://arxiv.org/abs/0901.0640
Horizons and the cosmological constant
Krzysztof A. Meissner
6 pages
(Submitted on 6 Jan 2009)
"A new solution of the Einstein equations for the point mass immersed in the de Sitter Universe is presented. The properties of the metric are very different from both the Schwarzschild black hole and the de Sitter Universe: it is everywhere smooth, light can propagate outward through the horizon, there is an antitrapped surface enclosing the point mass and there is necessarily an initial singularity. The solution for any positive cosmological constant is qualitatively different from the Schwarzschild solution and is not its continuous deformation."

http://arxiv.org/abs/0901.0577
Noncommutative Geometry as the Key to Unlock the Secrets of Space-Time
Ali H. Chamseddine
Contribution to a special volume in honor of Alain Connes in occasion of his 60th birthday. To be published in the Proceedings series of the Clay Mathematics Institute. Editors E. Blanchard et al
(Submitted on 6 Jan 2009)
"I give a summary of the progress made on using the elegant construction of Alain Connes noncommutaive geometry to explore the nature of space-time at very high energies. In particular I show that by making very few natural and weak assumptions about the structure of the noncommutative space, one can deduce the structure of all fundamental interactions at low energies."

http://arxiv.org/abs/0901.0662
Gravity from spontaneous Lorentz violation
Alan Kostelecky, Robertus Potting
51 pages
(Submitted on 6 Jan 2009)
"We investigate a class of theories involving a symmetric two-tensor field in Minkowski spacetime with a potential triggering spontaneous violation of Lorentz symmetry. The resulting massless Nambu-Goldstone modes are shown to obey the linearized Einstein equations in a fixed gauge. Imposing self-consistent coupling to the energy-momentum tensor constrains the potential for the Lorentz violation. The nonlinear theory generated from the self-consistent bootstrap is an alternative theory of gravity, containing kinetic and potential terms along with a matter coupling. At energies small compared to the Planck scale, the theory contains general relativity, with the Riemann-spacetime metric constructed as a combination of the two-tensor field and the Minkowski metric. At high energies, the structure of the theory is qualitatively different from general relativity. Observable effects can arise in suitable gravitational experiments."

http://arxiv.org/abs/0901.0632
Dark Matter
Jaan Einasto
UNESCO EOLSS ENCYCLOPEDIA (accepted) 25 pages, 17 figures
(Submitted on 6 Jan 2009)
"A review of the development of the concept of dark matter is given. I begin the review with the description of the discovery of the mass paradox in our Galaxy and in clusters of galaxies. In mid 1970s the amount of observational data was sufficient to suggest the presence of a massive and invisible population around galaxies and in clusters of galaxies. The nature of the dark population was not clear at that time, but the hypotheses of stellar as well as of gaseous nature of the new population had serious difficulties. These difficulties disappeared when non-baryonic nature of dark matter was suggested in early 1980s. In addition to the presence of Dark Matter, recent observations suggest the presence of Dark Energy, which together with Dark Matter and ordinary baryonic matter makes the total matter/energy density of the Universe equal to the critical cosmological density. There are various hypothesis as for the nature of the dark matter particles, and generally some form of weakly interactive massive particles (WIMPs) are strongly favored. Both Dark Matter and Dark Energy are the greatest challenges for modern physics since their nature is unknown."

 

[marcus]

http://arxiv.org/abs/0901.0933
Quantum field theory on a cosmological, quantum space-time
Abhay Ashtekar, Wojciech Kaminski, Jerzy Lewandowski
19 pages
(Submitted on 7 Jan 2009)
"In loop quantum cosmology, Friedmann-LeMaitre-Robertson-Walker (FLRW) space-times arise as well-defined approximations to specific quantum geometries. We initiate the development of a quantum theory of test scalar fields on these quantum geometries. Emphasis is on the new conceptual ingredients required in the transition from classical space-time backgrounds to quantum space-times. These include a 'relational time' a la Leibnitz, the emergence of the Hamiltonian operator of the test field from the quantum constraint equation, and ramifications of the quantum fluctuations of the background geometry on the resulting dynamics. The familiar quantum field theory on classical FLRW models arises as a well-defined reduction of this more fundamental theory."

http://arxiv.org/abs/0901.0964
Quantum Gravity on the Lattice
Herbert W. Hamber
63 pages, 12 figures
(Submitted on 8 Jan 2009)
"I review the lattice approach to quantum gravity, and how it relates to the non-trivial ultraviolet fixed point scenario of the continuum theory. After a brief introduction covering the general problem of ultraviolet divergences in gravity and other non-renormalizable theories, I cover the general methods and goals of the lattice approach. An underlying theme is an attempt at establishing connections between the continuum renormalization group results, which are mainly based on diagrammatic perturbation theory, and the recent lattice results, which should apply to the strong gravity regime and are inherently non-perturbative. A second theme in this review is the ever-present natural correspondence between infrared methods of strongly coupled non-abelian gauge theories on the one hand, and the low energy approach to quantum gravity based on the renormalization group and universality of critical behavior on the other. Towards the end of the review I discuss possible observational consequences of path integral quantum gravity, as derived from the non-trivial ultraviolet fixed point scenario. I argue that the theoretical framework naturally leads to considering a weakly scale-dependent Newton's costant, with a scaling violation parameter related to the observed scaled cosmological constant (and not, as naively expected, to the Planck length)."

http://arxiv.org/abs/0901.0916
CMB Lensing Constraints on Neutrinos and Dark Energy
Roland de Putter, Oliver Zahn, Eric V. Linder
18 pages, 14 figures
(Submitted on 7 Jan 2009)
"Signatures of lensing of the cosmic microwave background radiation by gravitational potentials along the line of sight carry with them information on the matter distribution, neutrino masses, and dark energy properties. We examine the constraints that Planck, PolarBear, and CMBpol future data, including from the B-mode polarization or the lensing potential, will be able to place on these quantities. We simultaneously fit for neutrino mass and dark energy equation of state including time variation and early dark energy density, and compare the use of polarization power spectra with an optimal quadratic estimator of the lensing. Results are given as a function of systematics level from residual foreground contamination. A realistic CMBpol experiment can effectively constrain the sum of neutrino masses to within 0.05 eV and the fraction of early dark energy to 0.002. We also present a surprisingly simple prescription for calculating dark energy equation of state constraints in combination with supernova distances from JDEM."

Some related papers appearing at the same time:
http://arxiv.org/abs/0901.0917
Lensed Cosmic Microwave Background Constraints on Post-General Relativity Parameters
Paolo Serra, Asantha Cooray, Scott F. Daniel, Robert Caldwell, Alessandro Melchiorri

http://arxiv.org/abs/0901.0918
Extending the Gravitational Growth Framework
Eric V. Linder

 

[marcus]

http://arxiv.org/abs/0901.1301
Transcending Big Bang in Loop Quantum Cosmology: Recent Advances
Parampreet Singh
Invited plenary talk at the Sixth International Conference on Gravitation and Cosmology, IUCAA (Pune). 13 pages, 3 figures
(Submitted on 9 Jan 2009)
"We discuss the way non-perturbative quantization of cosmological spacetimes in loop quantum cosmology provides insights on the physics of Planck scale and the resolution of big bang singularity. In recent years, rigorous examination of mathematical and physical aspects of the quantum theory has led to a consistent quantization which is consistent and physically viable and some early ideas have been ruled out. The latter include so called 'physical effects' originating from modifications to inverse scale factors in the flat models. The singularity resolution is understood to originate from the non-local nature of curvature in the quantum theory and the underlying polymer representation. Using an exactly solvable model various insights have been gained. The model predicts a generic occurrence of bounce for states in the physical Hilbert space and a supremum for the spectrum of the energy density operator. It also provides answers to the growth of fluctuations, showing that semi-classicality is preserved to an amazing degree across the bounce."

http://arxiv.org/abs/0901.1302
Black holes and entropy in loop quantum gravity: An overview
Alejandro Corichi
17 pages, 5 figures. Extended version of the contribution for the proceedings of the 6th IGCG Conference and a contribution for Advanced Science Letters
(Submitted on 9 Jan 2009)
"Black holes in equilibrium and the counting of their entropy within Loop Quantum Gravity are reviewed. In particular, we focus on the conceptual setting of the formalism, briefly summarizing the main results of the classical formalism and its quantization. We then focus on recent results for small, Planck scale, black holes, where new structures have been shown to arise, in particular an effective quantization of the entropy. We discuss recent results that employ in a very effective manner results from number theory, providing a complete solution to the counting of black hole entropy. We end with some comments on other approaches that are motivated by loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0901.1757
How loopy is the quantum bounce? A heuristic analysis of higher order holonomy corrections in LQC
Dah-Wei Chiou, Li-Fang Li
5 pages, 1 figure
(Submitted on 13 Jan 2009)
"A well motivated extension of higher order holonomy corrections in loop quantum cosmology (LQC) for the k=0 Friedmann-Robertson-Walker (FRW) model is investigated at the level of heuristic effective dynamics, which reveals that the quantum bounce is generic, regardless of the order of corrections, and the matter density remains finite, bounded by an upper bound in the regime of the Planckian density, even if all orders of corrections are included. This observation provides further evidence that the quantum bounce is essentially a consequence of the loopy nature (i.e. intrinsic discreteness) of LQC and LQC is fundamentally different from the Wheeler-DeWitt (WDW) theory; it also encourages one to construct the quantum theory of LQC with the higher order holonomy corrections, which might be understood as related to the higher j representations in the Hamiltonian operator of loop quantum gravity (LQG)."

brief mention:
http://arxiv.org/abs/0901.1870
Disentangling Forms of Lorentz Violation With Complementary Clock Comparison Experiments
Brett Altschul

 

[marcus]

http://arxiv.org/abs/0901.2414
The weak anthropic principle and the landscape of string theory
George F R Ellis, Lee Smolin
11 pages
(Submitted on 16 Jan 2009)
We note that there is an exception to the general arguments that no falsifiable predictions can be made, on the basis of of presently available data, by applying the weak anthropic principle (WAP) to the landscape of string theory. If there are infinitely more vacua in the landscape for one sign of a parameter than the other, within an anthropically allowed range, then under very weak assumptions about the probability measure one gets a firm prediction favoring that sign of that parameter. It is interesting to note that while the understanding of the string landscape is evolving, present evidence on the nature of the landscape allows such an argument to be made, leading to the conclusion that the WAP favors a negative value for the cosmological constant, Lambda, in contradiction to the result of astronomical observations. The viability of applying the WAP to string theory then requires that either there are found an infinite discretum of anthropically allowed vacua for Lambda >0, or the recently found infinite discretum of solutions for Lambda <0 be reduced to a finite value.

http://arxiv.org/abs/0901.2750
Are loop quantum cosmos never singular?
Parampreet Singh
17 pages, 5 figures
(Submitted on 19 Jan 2009)
"A unified treatment of all known types of singularities for flat, isotropic and homogeneous spacetimes in the framework of loop quantum cosmology (LQC) is presented. These include bangs, crunches and all future singularities. Using effective spacetime description we perform a model independent general analysis of the properties of curvature, behavior of geodesics and strength of singularities. For illustration purposes a phenomenological model based analysis is also performed. We show that all values of the scale factor at which a strong singularity may occur are excluded from the effective loop quantum spacetime. Further, if the evolution leads to either a vanishing or divergent scale factor then the loop quantum universe is asymptotically deSitter in that regime. We also show that there exist a class of sudden extremal events, which includes a recently discussed possibility, for which the curvature or its derivatives will always diverge. Such events however turn out to be harmless weak curvature singularities beyond which geodesics can be extended. Our results point towards a generic resolution of physical singularities in LQC."

 

[marcus]

http://arxiv.org/abs/0901.3156
Conservative solutions to the black hole information problem
Sabine Hossenfelder, Lee Smolin
25 pages, 7 figures
(Submitted on 20 Jan 2009)
"We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence."

http://arxiv.org/abs/0901.2984
Asymptotic safety in higher-derivative gravity
Dario Benedetti, Pedro F. Machado, Frank Saueressig
4 pages
(Submitted on 20 Jan 2009)
"We study the non-perturbative renormalization group flow of higher-derivative gravity employing functional renormalization group techniques. The beta-functions give rise to a non-trivial ultraviolet fixed point with three UV-attractive and one UV-repulsive eigendirections, substantiating previous evidence that gravity is asymptotically safe. We argue that asymptotic safety also resolves the unitarity problem typically haunting higher-derivative gravity theories."

http://arxiv.org/abs/0901.2963
Running of Newton's Constant and Quantum Gravitational Effects
David Reeb
11 pages, 5 figures, latex; contribution to the proceedings of the 46th course of the International School of Subnuclear Physics held in Erice, Sicily/Italy, August 29 - September 7, 2008
(Submitted on 20 Jan 2009)
"Newton's gravitational constant is shown to be a running coupling constant, much like the familiar running gauge couplings of the Standard Model. This implies that, in models with appropriate particle content, the true Planck scale, i.e. the scale at which quantum gravity effects become important, can have a value different from 10^19 GeV, which would be expected from naive dimensional analysis. Then, two scenarios involving this running effect are presented. The first one is a model which employs huge particle content to realize quantum gravity at the TeV scale in 4 dimensions, thereby solving the hierarchy problem of the Standard Model. Secondly, effects of the running of Newton's constant in grand unified theories are examined and shown to introduce new significant uncertainties in their predictions, but possibly also to provide better gauge coupling unification results in some cases."

http://arxiv.org/abs/0901.2932
Exploring Quantum Gravity with Very-High-Energy Gamma-Ray Instruments - Prospects and Limitations
Robert Wagner (MPI für Physik)
11 pages, 4 figures, proceedings of "Science with the New Generation of High Energy Gamma-Ray Experiments", Euganean Spa Area, Padova: October 8-10, 2008
(Submitted on 20 Jan 2009)
"Some models for quantum gravity (QG) violate Lorentz invariance and predict an energy dependence of the speed of light, leading to a dispersion of high-energy gamma-ray signals that travel over cosmological distances. Limits on the dispersion from short-duration substructures observed in gamma-rays emitted by gamma-ray bursts (GRBs) at cosmological distances have provided interesting bounds on Lorentz invariance violation (LIV). Recent observations of unprecedentedly fast flares in the very-high energy gamma-ray emission of the active galactic nuclei (AGNs) Mkn 501 in 2005 and PKS 2155-304 in 2006 resulted in the most constraining limits on LIV from light-travel observations, approaching the Planck mass scale, at which QG effects are assumed to become important. I review the current status of LIV searches using GRBs and AGN flare events, and discuss limitations of light-travel time analyses and prospects for future instruments in the gamma-ray domain."

 

[John86]

http://arxiv.org/abs/0901.3156
Conservative solutions to the black hole information problem
Authors: Sabine Hossenfelder, Lee Smolin
(Submitted on 20 Jan 2009)
Abstract: We review the different options for resolution of the black hole loss of information problem. We classify them first into radical options, which require a quantum theory of gravity which has large deviations from semi-classical physics on macroscopic scales, such as non-locality or endowing horizons with special properties not seen in the semi-classical approximation, and conservative options, which do not need such help. Among the conservative options, we argue that restoring unitary evolution relies on elimination of singularities. We argue that this should hold also in the AdS/CFT correspondence.

 

[MTd2]

http://arxiv.org/abs/0901.3775v1

Quantum Gravity at a Lifgarbagez Point
Authors: Petr Horava
(Submitted on 26 Jan 2009)

Abstract: We present a candidate quantum field theory of gravity with dynamical critical exponent equal to z=3 in the UV. (As in condensed matter systems, z measures the degree of anisotropy between space and time.) This theory, which at short distances describes interacting nonrelativistic gravitons, is power-counting renormalizable in 3+1 dimensions. When restricted to satisfy the condition of detailed balance, this theory is intimately related to topologically massive gravity in three dimensions, and the geometry of the Cotton tensor. At long distances, this theory flows naturally to the relativistic value z=1, and could therefore serve as a possible candidate for a UV completion of Einstein's general relativity or an infrared modification thereof. The effective speed of light, the Newton constant and the cosmological constant all emerge from relevant deformations of the deeply nonrelativistic z=3 theory at short distances.

http://arxiv.org/abs/0901.3879v1

Emergent Einstein Universe under Deconstruction
Authors: Nahomi Kan (Yamaguchi Junior College), Kiyoshi Shiraishi (Yamaguchi University)
(Submitted on 25 Jan 2009)

Abstract: We study self-consistent static solutions for an Einstein universe in a graph-based induced gravity. The one-loop quantum action is computed at finite temperature. In particular, we demonstrate specific results for the models based on cycle graphs.

 

[marcus]

http://arxiv.org/abs/0901.4009
Quantum Histories and Quantum Gravity
Joe Henson
35 pages, 2 figures
(Submitted on 26 Jan 2009)
"This paper reviews the histories approach to quantum mechanics. This discussion is then applied to theories of quantum gravity. It is argued that some of the quantum histories must approximate (in a suitable sense) to classical histories, if the correct classical regime is to be recovered. This observation has significance for the formulation of new theories (such as quantum gravity theories) as it puts a constraint on the kinematics, if the quantum/classical correspondence principle is to be preserved. Consequences for quantum gravity, particularly for Lorentz symmetry and the idea of 'emergent geometry', are discussed."

 

[marcus]

http://arxiv.org/abs/0902.0145
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Accepted by Phys. Rev. Lett., 7 pages, 2 figures
(Submitted on 2 Feb 2009)
"The primordial spectrum of cosmological tensor perturbations is considered as a possible probe of quantum gravity effects. Together with string theory, loop quantum gravity is one of the most promising frameworks to study quantum effects in the early universe. We show that the associated holonomy correction should modify the potential seen by gravitational waves during the inflationary amplification. The resulting power spectrum should exhibit a characteristic tilt. This opens a new window for cosmological tests of quantum gravity."

http://arxiv.org/abs/0902.0351
Quantum geometry from phase space reduction
Florian Conrady, Laurent Freidel (Perimeter Inst. Theor. Phys.)
31 pages, 1 figure
(Submitted on 2 Feb 2009)
"In this work we give an explicit isomorphism between the usual spin network basis and the direct quantization of the reduced phase space of tetrahedra. The main outcome is a formula that describes the space of SU(2) invariant states by an integral over coherent states satisfying the closure constraint exactly, or equivalently, as an integral over the space of classical tetrahedra. This provides an explicit realization of theorems by Guillemin--Sternberg and Hall that describe the commutation of quantization and reduction. In the final part of the paper, we use our result to express the FK spin foam model as an integral over classical tetrahedra and the asymptotics of the vertex amplitude is determined."