Loop-and-allied QG bibliography

[Coin]

http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)

"The Poincaré group generalizes the Galilei group for high-velocity kinematics. The de Sitter group is here assumed to go one step further, generalizing Poincaré as the group governing high-energy kinematics. Algebraically, this is done by supplementing spacetime translations with proper conformal transformations. This change in special relativity implies concomitant changes in general relativity -- yielding a de Sitter general relativity. The source current turns out to include now, in addition to energy-momentum, the proper conformal current, which appears as the origin of the cosmological constant. In consequence, it is no longer a free parameter, and can be determined in terms of other quantities. When applied to the propagation of ultra-high energy photons, de Sitter general relativity gives a good estimate of the time delay observed in extragalactic gamma-ray flares. It can, for this reason, be considered a new approach to quantum gravity."

So I'm skimming this and I'm trying to understand... they say the de sitter group is SO(4,1)/L, where "L" is the lorentz group?

I'm getting kind of curious exactly which models there are that try to deny poincare invariance or replace it with something different. Ones I can think of include:

- This De Sitter Relativity thing ( SO(4,1)/Lorentz )
- LQG ( SO(3,1), which is the "restricted lorentz subgroup"? or just the lorentz subgroup? or does LQG assert poincare invariance as well as SO(3,1)? )
- Lisi's E8 theory ( SO(3,1) )

Anything else? Do doubly-special relativity or MOND imply any specific spacetime symmetry? Is doubly-special relativity related to this de sitter relativity in any way? (The de sitter relativity paper seems to be using some buzzwords similar to doubly-special relativity near the beginning, but maybe I am imagining this...)

I'm kinda confused here, so maybe everything I ask above is nonsense. In particular, are SO(3,1) and SO(1,3) the same thing?! I'm finding a couple of places using the two interchangeably :O

But basically I am curious whether there are any commonalities between all of these models which reject the poincare group (besides of course the detail that they reject the poincare group...)

Actually I am only just now noticing this thread, perhaps I should read this?

 

[marcus]

...
But basically I am curious whether there are any commonalities between all of these models which reject the poincare group (besides of course the detail that they reject the poincare group...)

Actually I am only just now noticing this thread, perhaps I should read this?

Hi Coin, the custom is to start discussions in a separate thread. Otherwise this biblio link thread would get overloaded.
You are new and there's no reason you would know. Anyway I thought you raised a bunch of related issues, not merely having to do with the Pereira Aldrovandi paper, so I started a thread in case people want to discuss

I hope the title accurately reflects the cluster of issues you have in mind
How extensions of relativity apply to quantum gravity (Coin's gambit)
https://www.physicsforums.com/showthread.php?t=202438

If I got the drift wrong (I apologize and) let me know so i can try to correct and get it right.
===================================================

Continuing the biblio-linkage---this important 4-page june 2007 paper used to have a title that made it difficult to recommend to people because you had to explain the title, but now, as a couple of weeks ago, it has been accepted for publication in PRL and has a new, more descriptive title:
http://arxiv.org/abs/0706.3690
Conceptual Explanation for the Algebra in the Noncommutative Approach to the Standard Model
Ali H. Chamseddine, Alain Connes
Phys. Rev. Lett. 99, 191601 (2007)
(Submitted on 25 Jun 2007)

"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states."

This is just the sort of thing we need more of-----conceptual (hopefully intuitive) explanations of what may seem, like Connes' algebraic/geometric derivation of the Standard Model, like an extremely strange and puzzling coincidence.
==============================

Frank Saueressig and Pedro Machado are both at Utrecht, or were until recently. Saueressig has co-authored some Asymptotic Safety papers with Martin Reuter, including a recent pedagogical survey of A.S. Now, with this new one, Saueressig seems to be going out on his own, without Reuter as senior author.
http://arxiv.org/abs/0712.0445
On the renormalization group flow of f(R)-gravity
Pedro F. Machado, Frank Saueressig
55 pages, 7 figures
(Submitted on 4 Dec 2007)

"We use the functional renormalization group equation for quantum gravity to construct a non-perturbative flow equation for modified gravity theories of the form S = \int d^dx \sqrt{g} f(R). Based on this equation we show that certain gravitational interactions monomials can be consistently decoupled from the renormalization group (RG) flow and reproduce recent results on the asymptotic safety conjecture. The non-perturbative RG flow of non-local extensions of the Einstein-Hilbert truncation including \int d^dx \sqrt{g} \ln(R) and \int d^dx \sqrt{g} R^{-n} interactions is investigated in detail. The inclusion of such interactions resolves the infrared singularities plaguing the RG trajectories with positive cosmological constant in previous truncations. In particular, in some R^{-n}-truncations all physical trajectories emanate from a Non-Gaussian (UV) fixed point and are well-defined on all RG scales. The RG flow of the \ln(R)-truncation contains an infrared attractor which drives a positive cosmological constant to zero, thereby providing a dynamical explanation of the tiny value of Lambda observed today."

Pedro Machado is a PhD student of Loll at Utrecht. Just as a refresher, look at Loll's group
http://www.phys.uu.nl/~loll/Web/group/group.html
She gets quality people and they move on to places like Perimeter, Marseille.
Saueressig is a young guy who has published mainly string research (plus 4 or 5 papers with Reuter), he could easily be pursuing a string carreer, but for some reason he has moved over into nonstring QG at least for the moment.

===========================
Rudy Vaas has been the main popularizer of LQG, LQC, and nonstring QG in general. He writes in German for the SciAm-like Bild der Wissenschaft, Ashtekar has English translations of Vaas QG articles at his personal website, and some are on arxiv as well. Vaas is a Philosophy of Science expert who also writes science for wide audience. Now Springer Press has commissioned Vaas to do a book scheduled to come out in 2008 called BEYOND THE BIG BANG.

We will be seeing preprints of chapters contributed by various people, I expect. Today a preprint chapter by Tony Aguirre showed up:
http://arxiv.org/abs/0712.0571
Eternal Inflation, past and future
Authors: Anthony Aguirre
38 pp., 6 color figures. Contribution to R. Vaas (ed.): Beyond the Big Bang. Springer 2008
(Submitted on 4 Dec 2007)

"Cosmological inflation, if it occurred, radically alters the picture of the 'big bang', which would merely point to reheating at the end of inflation. Moreover, this reheating may be only local, so that inflation continues elsewhere and forever, continually spawning big-bang-like regions. This chapter reviews this idea of 'eternal inflation', then focuses on what this may mean for the ultimate beginning of the universe. In particular, I will argue that given eternal inflation, the universe may be free of a cosmological initial singularity, might be eternal (and eternally inflating) to the past, and might obey an interesting sort of cosmological time-symmetry."

I don't know of any evidence that points to the "Eternal Inflation" picture being real, or that makes it a theoretical necessity. It may be just a fantasy that appeals to certain people. But Vaas is evidently including all kinds of idea of before big bang in his book, which should be quite interesting.
===========================

http://arxiv.org/abs/0712.0315
Very-High Energy Gamma Astrophysics
Alessandro De Angelis, Oriana Mansutti, Massimo Persic
Invited Review Talk at the Sixth International Workshop on New Worlds in Astroparticle Physics, September 6-8, 2007, University of the Algarve, Faro, Portugal
(Submitted on 3 Dec 2007)

"High energy photons are a powerful probe for astrophysics and for fundamental physics under extreme conditions. During the recent years, our knowledge or the most violent phenomena in the Universe has impressively progressed thanks to the advent of new detectors for high energy gamma rays. Observation of gamma rays gives an exciting view of the high energy universe thanks to the current (AGILE) and future (GLAST) satellite-based telescopes and to the current and future ground-based telescopes like the Cherenkov telescopes (H.E.S.S. and MAGIC in particular), which discovered in the recent years more than 60 new very-high energy sources. The progress achieved with the latest generation of Cherenkov telescopes is comparable to the one drawn by EGRET satellite-borne observatory with respect to the previous gamma-ray satellite detectors. This article reviews the present status of high-energy gamma astrophysics, with emphasis on the recent results and on the experimental developments."

Stephon Alexander has co-authored with Lee Smolin and is currently postdoc at Ashtekar's institute at Penn State
http://arxiv.org/abs/0712.0370
Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae
Stephon Alexander, Tirthabir Biswas, Alessio Notari, Deepak Vaid
26 pages, 11 figures
(Submitted on 3 Dec 2007)

"It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an 'apparent acceleration', even when any form of dark energy is absent. In this paper, we present a 'Minimal Void'' scenario, i.e. a 'void' with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only be consistent with the supernovae data, but also with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter. We also point out possible other observable signatures."

 

[marcus]

http://arxiv.org/abs/0712.0977
The Plebanski action extended to a unification of gravity and Yang-Mills theory
Lee Smolin
13 pages, one figure
(Submitted on 6 Dec 2007)

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

http://arxiv.org/abs/0712.0817
Loop quantization of spherically symmetric midi-superspaces : the interior problem
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
12 Pages, to appear in Proceedings of the Third Mexican Meeting on Mathematical and Experimental Physics, A. Macias, C. Laemmerzahl, A. Camacho, editors
(Submitted on 5 Dec 2007)

"We continue the study of spherically symmetric vacuum space-times in loop quantum gravity by treating the interior of a black hole. We start from a midi-superspace approach, but a simple gauge fixing leads to a Kantowski--Sachs form for the variables. We show that one can solve the quantum theory exactly in the (periodic) connection representation, including the inner product. The evolution can be solved exactly by de-parameterizing the theory and can be easily interpreted as a semi-classical evolution plus quantum corrections. A relational evolution can also be introduced in a precise manner, suggesting what may happen in situations where it is not possible to de-parameterize. We show that the singularity is replaced by a bounce at which quantum effects are important and that the extent of the region at the bounce where one departs from classical general relativity depends on the initial data."

http://arxiv.org/abs/0712.0921
Going Beyond Bell's Theorem
Daniel M. Greenberger, Michael A. Horne, Anton Zeilinger
Originally published in: 'Bell's Theorem, Quantum Theory, and Conceptions of the Universe', M. Kafatos (Ed.), Kluwer, Dordrecht, 69-72 (1989)
This original suggestion of GHZ-states and of what is today called multi-qubit entanglement is not generally accessible, so here we make it available electronically.
(Submitted on 6 Dec 2007)

"Bell's Theorem proved that one cannot in general reproduce the results of quantum theory with a classical, deterministic local model. However, Einstein originally considered the case where one could define an 'element of reality', namely for the much simpler case where one could predict with certainty a definite outcome for an experiment. For this simple case, Bell's Theorem says nothing. But by using a slightly more complicated model than Bell, one can show that even in this simple case where one can make definite predictions, one still cannot generally introduce deterministic, local models to explain the results."

http://arxiv.org/abs/0712.0683
Loop Quantization of Polarized Gowdy Model on T³: Classical Theory
Kinjal Banerjee, Ghanashyam Date
20 pages
(Submitted on 5 Dec 2007)

"The vacuum Gowdy models provide much studied, non-trivial midi-superspace examples. Various technical issues within Loop Quantum Gravity can be studied in these models as well as one can hope to understand singularities and their resolution in the loop quantization. The first step in this program is to reformulate the model in real connection variables in a manner that is amenable to loop quantization. We begin with the unpolarized model and carry out a consistent reduction to the polarized case. Carrying out complete gauge fixing, the known solutions are recovered."

http://arxiv.org/abs/0712.0687
Loop Quantization of Polarized Gowdy Model on T³: Quantum Theory
Kinjal Banerjee, Ghanashyam Date
24 pages
(Submitted on 5 Dec 2007)

"In this second paper on loop quantization of Gowdy model, we introduce the kinematical Hilbert space on which appropriate holonomies and fluxes are well represented. The quantization of the volume operator and the Gauss constraint is straight forward. Imposition of the Gauss constraint can be done on the kinematical Hilbert space to select subspace of gauge invariant states. We carry out the quantization of the Hamiltonian constraint making specific choices. Alternative choices are briefly discussed. It appears that to get spatial correlations reflected in the Hamiltonian constraint, one may have to adopt the so called 'effective operator viewpoint'."

 

[marcus]

http://arxiv.org/abs/0712.2485
Planckian Birth of the Quantum de Sitter Universe
J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll
10 pages, 3 figures
(Submitted on 17 Dec 2007)

"We show that the quantum universe emerging from a nonperturbative, Lorentzian sum-over-geometries can be described with high accuracy by a four-dimensional de Sitter spacetime. By a scaling analysis involving Newton's constant, we establish that the linear size of the quantum universes under study is in between 17 and 28 Planck lengths. Somewhat surprisingly, the measured quantum fluctuations around the de Sitter universe in this regime are to good approximation still describable semiclassically. The numerical evidence presented comes from a regularization of quantum gravity in terms of causal dynamical triangulations."

http://arxiv.org/abs/0712.2811
A Note on Quantum Field Theories with a Minimal Length Scale
S. Hossenfelder
(Submitted on 17 Dec 2007)

"The aim of this note is to address the low energy limit of quantum field theories with a minimal length scale. The essential feature of these models is that the minimal length acts as a regulator in the asymptotic high energy limit which is incorporated through an infinite series of higher order derivatives. If one investigates a perturbative expansion in inverse powers of the Planck mass, one generically obtains extra poles in the propagator, and instabilities connected with the higher order derivative Lagrangian, that are however artifacts of truncating the series."

 

[marcus]

http://arxiv.org/abs/0712.3242
Unspeakables and the Epistemological path towards Quantum Gravity
Daniel Sudarsky
Invited article for "GRF2007 Special Issue" of the IJMPD . In press
(Submitted on 19 Dec 2007)

"We offer a critical assessment of some generic features of various of the current approaches towards the construction of a Theory of Quantum Gravity. We will argue that there is a need for further conceptual clarifications before such an enterprise can be launched on a truly well grounded setting, and that one of the guiding principles that can be viewed as part of the reasons for successes of the past theoretical developments is the identification of Unspeakables: Concepts that should not only play no role in the formulation of the theories, but ones that the formalism of the theory itself should prevent from ever been spoken about."

http://arxiv.org/abs/0712.2843
Correlation of the highest-energy cosmic rays with the positions of nearby active galactic nuclei
The Pierre Auger Collaboration
33 pages, 8 figures, submitted to Astropart. phys
(Submitted on 17 Dec 2007)

"Data collected by the Pierre Auger Observatory provide evidence for anisotropy in the arrival directions of the cosmic rays with the highest energies, which are correlated with the positions of relatively nearby active galactic nuclei (AGN) [1]. The correlation has maximum significance for cosmic rays with energy greater than ~ 6x10^{19} eV and AGN at a distance less than ~ 75 Mpc. We have confirmed the anisotropy at a confidence level of more than 99% through a test with parameters specified a priori, using an independent data set. The observed correlation is compatible with the hypothesis that cosmic rays with the highest energies originate from extra-galactic sources close enough so that their flux is not significantly attenuated by interaction with the cosmic background radiation (the Greisen-Zatsepin-Kuz'min effect). The angular scale of the correlation observed is a few degrees, which suggests a predominantly light composition unless the magnetic fields are very weak outside the thin disk of our galaxy. Our present data do not identify AGN as the sources of cosmic rays unambiguously, and other candidate sources which are distributed as nearby AGN are not ruled out. We discuss the prospect of unequivocal identification of individual sources of the highest-energy cosmic rays within a few years of continued operation of the Pierre Auger Observatory."

http://arxiv.org/abs/0712.3099
Dark-Energy Dynamics Required to Solve the Cosmic Coincidence
Chas A. Egan, Charles H. Lineweaver
16 pages, 8 figures, Submitted to Phys. Rev. D
(Submitted on 19 Dec 2007)

"Dynamic dark energy (DDE) models are often designed to solve the cosmic coincidence (why, just now, is the dark energy density \rho_{de}, the same order of magnitude as the matter density \rho_m?) by guaranteeing \rho_{de} \sim \rho_m for significant fractions of the age of the universe. This typically entails ad-hoc tracking or oscillatory behaviour in the model. However, such behaviour is neither sufficient nor necessary to solve the coincidence problem. What must be shown is that a significant fraction of observers see \rho_{de} \sim \rho_m. Precisely when, and for how long, must a DDE model have \rho_{de} \sim \rho_{m} in order to solve the coincidence? We explore the coincidence problem in dynamic dark energy models using the temporal distribution of terrestrial-planet-bound observers. We find that any dark energy model fitting current observational constraints on \rho_{de} and the equation of state parameters w_0 and w_a, does have \rho_{de} \sim \rho_m for a large fraction of observers in the universe. This demotivates DDE models specifically designed to solve the coincidence using long or repeated periods of \rho_{de} \sim \rho_m."

http://arxiv.org/abs/0712.3219
Habitability of Super-Earths: Gliese 581c and 581d
W. von Bloh, C. Bounama, M. Kuntz, S. Franck
3 pages, 1 figure; submitted to: Exoplanets: Detection, Formation and Dynamics, IAU Symposium 249, eds. Y.S. Sun and S. Ferraz-Mello (San Francisco: Astr. Soc. Pac.)
(Submitted on 19 Dec 2007)

"The unexpected diversity of exoplanets includes a growing number of super-Earth planets, i.e. exoplanets with masses smaller than 10 Earth masses and a similar chemical and mineralogical composition as Earth. We present a thermal evolution model for super-Earth planets to identify the sources and sinks of atmospheric carbon dioxide. The photosynthesis-sustaining habitable zone (pHZ) is determined by the limits of biological productivity on the planetary surface. We apply our model to calculate the habitability of the two super-Earths in the Gliese 581 system. The super-Earth Gl 581c is clearly outside the pHZ, while Gl 581d is at the outer edge of the pHZ, and therefore could at least harbor some primitive forms of life."

http://arxiv.org/abs/0712.2865
New Frontiers in Cosmology and Galaxy Formation: Challenges for the Future
Richard Ellis (Caltech), Joseph Silk (Oxford)
29 pages. To appear in "Structure Formation in the Universe", ed. Chabrier, G., Cambridge University Press.

"(Abridged) Cosmology faces three distinct challenges in the next decade. (1) The dark sector, both dark matter and dark energy, dominates the Universe. Key questions include determining the nature of both. Improved observational probes are crucial. (2) Galaxy formation was initiated at around the epoch of reionization: we need to understand how and when as well as to develop probes of earlier epochs. (3) Our simple dark matter-driven picture of galaxy assembly is seemingly at odds with several observational results, including the presence of ULIRGs at high z, the `downsizing' signature, chemical signatures of alpha-element ratios and suggestions that merging may not be important in defining the Hubble sequence. Understanding the physical implications is a major challenge for theorists and refiniing the observational uncertainties a major goal for observers."

An ULIRG is an ultra luminous infrared galaxy http://www.daviddarling.info/encyclopedia/U/ULIRG.html
thought to be the site of intense star formation activity

 

[marcus]

http://arxiv.org/abs/0712.3457
A general test of the Copernican Principle
Chris Clarkson, Bruce A. Bassett, Teresa Hui-Ching Lu (UCT & SAAO, Cape Town)
4 pages
(Submitted on 20 Dec 2007)

"Here we present an observational test for the Copernican assumption which can be automatically implemented while we search for dark energy in the coming decade. Our test, which relies on the constant curvature of FLRW models, is entirely independent of any model for dark energy or theory of gravity and thereby represents a model-independent test of the Copernican Principle."

http://arxiv.org/abs/0712.4143
Cosmological Plebanski theory
Karim Noui, Alejandro Perez, Kevin Vandersloot
20 pages
(Submitted on 26 Dec 2007)

"We consider the cosmological symmetry reduction of the Plebanski action as a toy-model to explore, in this simple framework, some issues related to loop quantum gravity and spin-foam models. We make the classical analysis of the model and perform both path integral and canonical quantizations. As for the full theory, the reduced model admits two types of classical solutions: topological and gravitational ones. The quantization mixes these two solutions, which prevents the model to be equivalent to standard quantum cosmology. Furthermore, the topological solution dominates at the classical limit. We also study the effect of an Immirzi parameter in the model."

http://arxiv.org/abs/0712.3545
The Higgs Phenomenon in Quantum Gravity
R. Percacci
This paper was published in 1991 but was not previously available in the archive. Some updates have been added in a postscript
(Submitted on 20 Dec 2007)

"The Higgs phenomenon occurs in theories of gravity in which the connection is an independent dynamical variable. The role of order parameters is played by the soldering form and a fiber metric. The breaking of the original gauge symmetry is linked to the appearance of geometrical structures on spacetime. These facts suggest certain modifications and generalizations of the theory. We propose a Higgs-like model which provides a dynamical explanation for the nondegeneracy of the metric and a framework for the unification of gravity with the other interactions."

The next paper might be of pedagogical value.
http://arxiv.org/abs/0712.3709
Complete calculations of the perihelion precession of Mercury and the deflection of light by the Sun in General Relativity
17 pages, 3 figures
Christian Magnan
(Submitted on 21 Dec 2007)

"Taking up a method devised by Taylor and Wheeler and collecting pieces of their work we offer a self-contained derivation of the formulae giving both the precession of the orbit of a planet around the Sun and the deflection angle of a light pulse passing near the Sun in the framework of General Relativity. The demonstration uses only elementary algebra without resorting to tensor formalism. No prior knowledge in relativity is needed to follow the presentation."

 

[marcus]

http://arxiv.org/abs/0801.0502
Exact solutions for Big Bounce in loop quantum cosmology
Jakub Mielczarek, Tomasz Stachowiak, Marek Szydlowski
12 pages, 12 figures
(Submitted on 3 Jan 2008)

"In this paper we study the cosmological FRW model k=0 with holonomy corrections of Loop Quantum Gravity. The considered universe contains a massless scalar field and cosmological constant Lambda. We find analytical solutions for this model in different configurations and investigate its dynamical behaviour in the whole phase space. We show the explicit influence of Lambda on the qualitative and quantitative character of solutions. For the case of positive Lambda the oscillating solutions without the initial and final singularity appear as a generic case for some quantisation schemes."

My comment is that I don't know the authors from their previous work. Ashtekar has a close collaborator at the jagellonian University, and so also do Ambjorn and Loll. These authors are also Jagellonians. Last year there was the international QG school in Poland.
The senior author in this case has 64 preprints on arxiv going back to 1997
http://arxiv.org/find/grp_physics/1/au:+Szydlowski_M/0/1/0/all/0/1CODATA has the accepted collection of recommended values of the fundamental physical constants. They just brought out the 2006 revised edition on arxiv.
http://arxiv.org/abs/0801.0028
CODATA Recommended Values of the Fundamental Physical Constants: 2006
Peter J. Mohr, Barry N. Taylor, David B. Newell
(Submitted on 29 Dec 2007)

"This paper gives the 2006 self-consistent set of the basic constants and conversion factors of physics and chemistry recommended by the Committee on Data for Science and Technology (CODATA) for international use..."

You can always get the latest CODATA values of the constants online from the NIST.gov/constants website. But it's sometimes more convenient to have it all in a few pages of hardcopy. The whole batch can be printed off in about 10 pages: you can either print off pages 94-105-----or the short one-page list on page 94

Thanks to Garth for pointing out the following paper by George Ellis et al.
http://arxiv.org/abs/0801.0068
Time drift of cosmological redshifts as a test of the Copernican principle
Jean-Philippe Uzan, Chris Clarkson, George F.R. Ellis
4 pages
(Submitted on 29 Dec 2007)

"The time drift of the cosmological redshift in a general spherically symmetric spacetime is derived. It is shown that its observation would offer the possibility to construct a test of the Copernican principle. In particular, it allows to close the reconstruction problem of a Lemaitre-Tolman-Bondi spacetime from background observations."

 

[marcus]

http://arxiv.org/abs/0801.0861
Quantum Graphity: a model of emergent locality
Tomasz Konopka, Fotini Markopoulou, Simone Severini
25 pages
(Submitted on 6 Jan 2008)

"Quantum graphity is a background independent model for emergent locality, spatial geometry and matter. The states of the system correspond to dynamical graphs on N vertices. At high energy, the graph describing the system is highly connected and the physics is invariant under the full symmetric group acting on the vertices. We present evidence that the model also has a low-energy phase in which the graph describing the system breaks permutation symmetry and appears to be ordered, low-dimensional and local. Consideration of the free energy associated with the dominant terms in the dynamics shows that this low-energy state is thermodynamically stable under local perturbations. The model can also give rise to an emergent U(1) gauge theory in the ground state by the string-net condensation mechanism of Levin and Wen. We also reformulate the model in graph-theoretic terms and compare its dynamics to some common graph processes."


http://arxiv.org/abs/0801.0705
Realistic Equations of State for the Primeval Universe
R. Aldrovandi, R.R. Cuzinatto, L. G. Medeiros
29 pages, 5 figures
(Submitted on 4 Jan 2008)

"Early universe equations of state including realistic interactions between constituents are built up. Under certain reasonable assumptions, these equations are able to generate an inflationary regime prior to the nucleosynthesis period. The resulting accelerated expansion is intense enough to solve the flatness and horizon problems. In the cases of curvature parameter kappa equal to 0 or +1, the model is able to avoid the initial singularity and offers a natural explanation for why the universe is in expansion."

http://arxiv.org/abs/0801.1073
\mathcal{O}(\mu^4) corrections from holonomies in Loop Quantum Gravity and its effect on flat FRW models
Jakub Mielczarek, Marek Szydlowski
6 pages, 1 figure
(Submitted on 7 Jan 2008)

"In this paper we calculate \mathcal{O}(\mu^4) corrections from holonomies in Loop Quantum Gravity. We apply these corrections to the flat FRW cosmological model and calculate the modified Friedmann equation. We show that the bounce appeared for in the models with lowest \mathcal{O}(\mu^2) order corrections is shifted to the higher energies \rho_{\text{bounce}} = 3 \rho_{\text{c}}. Also a pole in the Hubble parameter appear for \rho_{\text{pole}} = {3/2} \rho_{\text{c}}. This result suggests that an ordinary bouncing solution appears only when quantum corrections in the lowest order are considered. Higher order corrections can lead to the nonperturbative effects.

The next paper happens to cite a Derek Wise' paper
http://arxiv.org/abs/0801.0905
Cosmological Solutions with Torsion in a Model of de Sitter Gauge Theory of Gravity
Chao-Guang Huang, Hai-Qing Zhang, Han-Ying Guo
16 pages, 2 figures
(Submitted on 7 Jan 2008)

"The torsion is shown to be vitally important in the explanation of the evolution of the universe in a large class of gravitational theories containing quadratic terms of curvature and torsion. The cosmological solutions with homogeneous and isotropic torsion in a model of de Sitter gauge theory of gravity are presented, which may explain the observation data for SN Ia when parameters are suitably chosen and supply a natural transit from decelerating expansion to accelerating expansion without the help of the introduction of other strange fields in the theory."

[22] D.K. Wise, MacDowell-Mansouri, Gravity and Cartan Geometry, arXiv: gr-qc/0611154.

 

[marcus]

http://arxiv.org/abs/0801.1547
Einstein-aether gravity: a status report
Ted Jacobson
18 pages, for the proceedings of the conference "From Quantum to Emergent Gravity: Theory and Phenomenology", June 11-15 2007, SISSA; Trieste, Italy
(Submitted on 10 Jan 2008)

"This paper reviews the theory, phenomenology, and observational constraints on the coupling parameters of Einstein-aether gravity, i.e. General Relativity coupled to a dynamical unit timelike vector field. A discussion of open questions concerning both phenomenology and fundamental issues is included."

http://arxiv.org/abs/0801.1565
Dynamics of dark energy with a coupling to dark matter
Christian G. Boehmer, Gabriela Caldera-Cabral, Ruth Lazkoz, Roy Maartens
11 pages, 4 figures
(Submitted on 10 Jan 2008)

"Dark energy and dark matter are the dominant sources in the evolution of the late universe. They are currently only indirectly detected via their gravitational effects, and there could be a coupling between them without violating observational constraints. We investigate the background dynamics when dark energy is modeled as exponential quintessence, and is coupled to dark matter via simple models of energy exchange. We introduce a new form of dark sector coupling, which leads to a more complicated dynamical phase space and has a better physical motivation than previous mathematically similar couplings."

 

[marcus]

http://arxiv.org/abs/0801.1847
What is the entropy of the universe?
Paul Frampton, Stephen D.H. Hsu, Thomas W. Kephart, David Reeb
4 pages, 2 figures
(Submitted on 11 Jan 2008)

"Standard calculations suggest that the entropy of the universe is dominated by black holes, although they comprise only a tiny fraction of its total energy. We give a physical interpretation of this result. Statistical entropy is the logarithm of the number of microstates consistent with the observed macroscopic properties of a system, hence a measure of uncertainty about its precise state. The largest uncertainty in the present and future state of the universe is due to the (unknown) internal microstates of its black holes. We also discuss the qualitative gap between the entropies of black holes and ordinary matter."

The following is presented as a pedagogical paper
http://arxiv.org/abs/0801.1734
The Volume Inside a Black Hole
Brandon S. DiNunno, Richard A. Matzner
17 pages, 5 figures
(Submitted on 11 Jan 2008)

"The horizon (the surface) of a black hole is a null surface, defined by those hypothetical "outgoing" light rays that just hover under the influence of the strong gravity at the surface. Because the light rays are orthogonal to the spatial 2-dimensional surface at one instant of time, the surface of the black hole is the same for all observers (i.e. the same for all coordinate definitions of "instant of time"). This value is 4*(pi)* (2Gm/c^2)^2 for nonspinning black holes, with G= Newton's constant, c= speed of light, and m= mass of the black hole.
The 3-dimensional spatial volume inside a black hole, in contrast, depends explicitly on the definition of time, and can even be time dependent, or zero. We give examples of the volume found inside a standard, nonspinning spherical black hole, for several different standard time-coordinate definitions. Elucidating these results for the volume provides a new pedagogical resource of facts already known in principle to the relativity community, but rarely worked out."

http://arxiv.org/abs/0801.1811
Information is Not Lost in the Evaporation of 2-dimensional Black Holes
Abhay Ashtekar, Victor Taveras, Madhavan Varadarajan
4 pages, 2 figures
(Submitted on 11 Jan 2008)

"We analyze Hawking evaporation of the Callen-Giddings-Harvey-Strominger (CGHS) black holes from a quantum geometry perspective and show that information is not lost, primarily because the quantum space-time is sufficiently larger than the classical. Using suitable approximations to extract physics from quantum space-times we establish that: i)future null infinity of the quantum space-time is sufficiently long for the the past vacuum to evolve to a pure state in the future; ii) this state has a finite norm in the future Fock space; and iii) all the information comes out at future infinity; there are no remnants."

http://arxiv.org/abs/0801.1928
Supersymmetry in Elementary Particle Physics
Michael E. Peskin
75 pages, 36 figures
(Submitted on 13 Jan 2008)

"These lectures, presented at the 2006 TASI summer school, give a general introduction to supersymmetry, emphasizing its application to models of elementary particle physics at the 100 GeV energy scale. I discuss the following topics: the construction of supersymmetric Lagrangians with scalars, fermions, and gauge bosons, the structure and mass spectrum of the Minimal Supersymmetric Standard Model (MSSM), the measurement of the parameters of the MSSM at high-energy colliders, and the solutions that the MSSM gives to the problems of electroweak symmetry breaking and dark matter."

http://arxiv.org/abs/0801.1852
Detecting the Glint of Starlight on the Oceans of Distant Planets
D.M. Williams, E. Gaidos
41 pages, 7 figures. Icarus in press
(Submitted on 11 Jan 2008)

"We propose that astronomers will be eventually be able to discriminate between extrasolar Earth-like planets with surface oceans and those without using the shape of phase light curves in the visible and near-IR spectrum. We model the visible light curves of planets having Earth-like surfaces, seasons, and optically-thin atmospheres with idealized diffuse-scattering clouds. We show that planets partially covered by water will appear measurably brighter near crescent phase (relative to Lambertian planets) because of the efficient specular reflection (i.e., glint) of starlight incident on their surfaces at a highly oblique angle. Planets on orbits within 30 degrees of edge-on orientation (half of all planets) will show pronounced glint over a sizeable range of orbital longitudes, from quadrature to crescent, all outside the glare of their parent stars. Also, water-covered planets will appear darker than a Lambertian disk near full illumination. Finally, we show that planets with a mixed land/water surface will polarize the reflected signal by as much as 30-70 percent. These results suggest several new ways of directly identifying water on distant planets."

 

[Fra]

I didn't realize that this was somehow a special thread. I'm sorry. I removed my post to shorten this. Thanks for the new thread.

/Fredrik

 

[marcus]

Fra thanks for moving the discussion to
https://www.physicsforums.com/showthread.php?p=1572779#post1572779
By custom this thread is not for discussion. I am grateful for this courtesy, because discussion would interfere with its function as a list of abstracts with links. When questions like this come up, we can start a separate thread.
============================================

http://arxiv.org/abs/0801.2564
Modern space-time and undecidability
Rodolfo Gambini, Jorge Pullin
8 pages, contribution to the volume "Minkowski spacetime: a hundred years later", edited by Vesselin Petkov
(Submitted on 16 Jan 2008)

"The picture of space-time that Minkowski created in 1907 has been followed by two important developments in physics not contained in the original picture: general relativity and quantum mechanics. We will argue that the use of concepts of those theories to construct space-time implies conceptual modifications in quantum mechanics. In particular one can construct a viable picture of quantum mechanics without a reduction process that has outcomes equivalent to a picture with a reduction process. One therefore has two theories that are entirely equivalent experimentally but profoundly different in the description of reality they give. This introduces a fundamental level of undecidability in physics of a kind that has not been present before. We discuss some of the implications.

 

[marcus]

This is a nice invited review paper by a top cosmologist
http://arxiv.org/abs/0801.2968
Mapping the Cosmological Expansion
Eric V. Linder
49 pages, 29 figures; Review invited for Reports on Progress in Physics
(Submitted on 18 Jan 2008)

"The ability to map the cosmological expansion has developed enormously, spurred by the turning point one decade ago of the discovery of cosmic acceleration. The standard model of cosmology has shifted from a matter dominated, standard gravity, decelerating expansion to the present search for the origin of acceleration in the cosmic expansion. We present a wide ranging review of the tools, challenges, and physical interpretations. The tools include direct measures of cosmic scales through Type Ia supernova luminosity distances, and angular distance scales of baryon acoustic oscillation and cosmic microwave background density perturbations, as well as indirect probes such as the effect of cosmic expansion on the growth of matter density fluctuations. Accurate mapping of the expansion requires understanding of systematic uncertainties in both the measurements and the theoretical framework, but the result will give important clues to the nature of the physics behind accelerating expansion and to the fate of the universe."

He put some work into it. It's thorough and well-written. And it has the perspective of an old hand.

 

[marcus]

http://arxiv.org/abs/0801.3287
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
Martin Reuter, Holger Weyer
4 figures
(Submitted on 21 Jan 2008)

"We analyze the conceptual role of background independence in the application of the effective average action to quantum gravity. Insisting on a background independent renormalization group (RG) flow the coarse graining operation must be defined in terms of an unspecified variable metric since no rigid metric of a fixed background spacetime is available. This leads to an extra field dependence in the functional RG equation and a significantly different RG flow in comparison to the standard flow equation with a rigid metric in the mode cutoff. The background independent RG flow can possesses a non-Gaussian fixed point, for instance, even though the corresponding standard one does not. We demonstrate the importance of this universal, essentially kinematical effect by computing the RG flow of Quantum Einstein Gravity in the 'conformally reduced' Einstein--Hilbert approximation which discards all degrees of freedom contained in the metric except the conformal one. Without the extra field dependence the resulting RG flow is that of a simple \phi^4-theory. Including it one obtains a flow with exactly the same qualitative properties as in the full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian fixed point which is necessary for asymptotic safety."

 

[marcus]

http://arxiv.org/abs/0801.4001
Quantum nature of cosmological bounces
Martin Bojowald
26 pages
(Submitted on 25 Jan 2008)

"Several examples are known where quantum gravity effects resolve the classical big bang singularity by a bounce. The most detailed analysis has probably occurred for loop quantum cosmology of isotropic models sourced by a free, massless scalar. Once a bounce has been realized under fairly general conditions, the central questions are how strongly quantum it behaves, what influence quantum effects can have on its appearance, and what quantum space-time beyond the bounce may look like. This, then, has to be taken into account for effective equations which describe the evolution properly and can be used for further phenomenological investigations. Here, we provide the first analysis with interacting matter with new effective equations valid for weak self-interactions or small masses. They differ from the free scalar equations by crucial terms and have an important influence on the bounce and the space-time around it. Especially the role of squeezed states, which have often been overlooked in this context, is highlighted. The presence of a bounce is proven for uncorrelated states, but as squeezing is a dynamical property and may change in time, further work is required for a general conclusion."

http://arxiv.org/abs/0801.4241
A Immirzi-like parameter for 3d quantum gravity
Valentin Bonzom, Etera R. Livine
14 pages
(Submitted on 28 Jan 2008)

"We study an Immirzi-like ambiguity in three-dimensional quantum gravity. It shares some features with the Immirzi parameter of four-dimensional loop quantum gravity: it does not affect the equations of motion, but modifies the Poisson brackets and the constraint algebra at the canonical level. We focus on the length operator and show how to define it through non-commuting fluxes. We compute its spectrum and show the effect of this Immirzi-like ambiguity. Finally, we extend these considerations to 4d gravity and show how the different topological modifications of the action affect the canonical structure of loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0802.0896
Topology change in causal quantum gravity
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
4 pages, proceedings of the workshop JGRG 17 (Nagoya, Japan, December 2007)
(Submitted on 6 Feb 2008)

"The role of topology change in a fundamental theory of quantum gravity is still a matter of debate. However, when regarding string theory as two-dimensional quantum gravity, topological fluctuations are essential. Here we present a third quantization of two-dimensional surfaces based on the method of causal dynamical triangulation (CDT). Formally, our construction is similar to the c = 0 non-critical string field theory developed by Ishibashi, Kawai and others, but physically it is quite distinct. Unlike in non-critical string theory the topology change of spatial slices is well controlled and regulated by Newton's constant."

http://arxiv.org/abs/0802.0864
Area-angle variables for general relativity
Bianca Dittrich, Simone Speziale
7 pages, 1 figure
(Submitted on 6 Feb 2008)

"We introduce a modified Regge calculus for general relativity on a triangulated four dimensional Riemannian manifold where the fundamental variables are areas and a certain class of angles. These variables satisfy constraints which are local in the triangulation. We expect the formulation to have applications to classical discrete gravity and non-perturbative approaches to quantum gravity."

http://arxiv.org/abs/0802.0880
Entanglement Entropy in Loop Quantum Gravity
William Donnelly
4 pages
(Submitted on 6 Feb 2008)

The entanglement entropy between quantum fields inside and outside a black hole horizon is a promising candidate for the microscopic origin of black hole entropy. We show that the entanglement entropy may be defined in loop quantum gravity, and compute its value for spin network states. The entanglement entropy for an arbitrary region of space is expressed as a sum over punctures where the spin network intersects the region's boundary. Our result agrees asymptotically with results previously obtained from the isolated horizon framework, and we give a justification for this agreement. We conclude by proposing a new method for studying corrections to the area law and its implications for quantum corrections to the gravitational action."

http://arxiv.org/abs/0802.0719
A String Field Theory based on Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
29 pages, 4 figures
(Submitted on 5 Feb 2008)

"We formulate the string field theory in zero-dimensional target space corresponding to the two-dimensional quantum gravity theory defined through Causal Dynamical Triangulations. This third quantization of the quantum gravity theory allows us in principle to calculate the transition amplitudes of processes in which the topology of space changes in time, and to include non-trivial topologies of space-time. We formulate the corresponding Dyson-Schwinger equations and illustrate how they can be solved iteratively."

http://arxiv.org/abs/0802.0767
A fundamental length as a candidate for dark energy: a DSR inspired FRW spacetime
N. Khosravi, H. R. Sepangi
8 pages, to appear in PLA
(Submitted on 6 Feb 2008)

"We show that the existence of a fundamental length, introduced in Deformed Special Relativity (DSR) inspired minisuper (phase-) space, causes the behavior of the scale factor of the universe to change from that of a universe filled with dust to an accelerating universe driven by a cosmological constant."

http://arxiv.org/abs/0802.0702
Strategies for Determining the Nature of Dark Matter
Dan Hooper, Edward A. Baltz
25 pages, 5 figures, Review intended for the Annual Review of Nuclear and Particle Science
(Submitted on 5 Feb 2008)

"In this review, we discuss the role of the various experimental programs taking part in the broader effort to identify the particle nature of dark matter. In particular, we focus on electroweak scale dark matter particles and discuss a wide range of search strategies being carried out and developed to detect them. These efforts include direct detection experiments, which attempt to observe the elastic scattering of dark matter particles with nuclei, indirect detection experiments, which search for photons, antimatter and neutrinos produced as a result of dark matter annihilations, and collider searches for new TeV-scale physics. Each of these techniques could potentially provide a different and complementary set of information related to the mass, interactions and distribution of dark matter. Ultimately, it is hoped that these many different tools will be used together to conclusively identify the particle or particles that constitute the dark matter of our universe."

http://arxiv.org/abs/0802.0013
Cosmic Neutrinos
Chris Quigg
27 pages, 16 figures, lecture at 2007 SLAC Summer Institute
(Submitted on 31 Jan 2008)

"I recall the place of neutrinos in the electroweak theory and summarize what we know about neutrino mass and flavor change. I next review the essential characteristics expected for relic neutrinos and survey what we can say about the neutrino contribution to the dark matter of the Universe. Then I discuss the standard-model interactions of ultrahigh-energy neutrinos, paying attention to the consequences of neutrino oscillations, and illustrate a few topics of interest to neutrino observatories. I conclude with short comments on the remote possibility of detecting relic neutrinos through annihilations of ultrahigh-energy neutrinos at the Z resonance."

 

[marcus]

http://arxiv.org/abs/0802.1201
Tensorial Structure of the LQG graviton propagator
Emanuele Alesci
4 pages,; to appear in the proceedings of the II Stueckelberg Workshop, Int.J.Mod.Phys.A
(Submitted on 8 Feb 2008)

"We review the construction of the tensorial structure of the graviton propagator in the context of loop quantum gravity and spinfoam formalism. The main result of this analysis is that applying the same strategy used to compute the diagonal terms, the Barrett-Crane vertex is unable to yield the correct propagator in the long distance limit. The problem is in the intertwiner-independence of the Barrett-Crane vertex. We also review the asymptotic behavior of an alternative vertex that is able to give the correct propagator."

http://arxiv.org/abs/0802.1129
Doubly special relativity in de Sitter spacetime
Salvatore Mignemi
18 pages
(Submitted on 8 Feb 2008)

"We discuss the generalization of Doubly Special Relativity to a curved de Sitter background. The model has three observer-independent scales, the velocity of light c, the radius of curvature of the geometry alpha, and the Planck energy kappa, and can be realized in a noncommutative position space. It is possible to construct a model exhibiting a duality for the interchange of positions and momenta together with the exchange of alpha and kappa."

http://arxiv.org/abs/0802.1221
Immirzi parameter and fermions with non-minimal coupling
Sergei Alexandrov
4 pages
(Submitted on 8 Feb 2008)

"We clarify the role played by the Immirzi parameter in classical gravity coupled to fermions. Considering the general non-minimal coupling, we show that, although the torsion depends explicitly on the Immirzi parameter, in a suitable parametrization the effective action obtained by integrating out the spin-connection is independent of it. Thus the Immirzi parameter is not detectable in classical theory even after coupling of fermions."

http://arxiv.org/abs/0802.1215
TeVeS gets caught on caustics
Carlo R. Contaldi, Toby Wiseman, Benjamin Withers
21 pages,12 figures
(Submitted on 10 Feb 2008)

"TeVeS uses a dynamical vector field with timelike unit norm constraint to specify a preferred local frame. When matter moves slowly in this frame - the so-called quasi-static regime - Modified Newtonian Dynamics (MoND) results. Theories with such vectors (such as Einstein-aether) are prone to the vector dynamics forming singularities which render their classical evolution problematic. Here we analyse the dynamics of the vector in TeVeS in various situations. We find that, quite generically, the vector field develops caustic singularities on time scales of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein's original formulation of TeVeS appears dynamically problematic. We argue that TeVeS might be saved as a relativistic theory with a MoND limit by modifying the vector field kinetic terms to the more general form used by Einstein-aether."

 

[marcus]

http://arxiv.org/abs/0802.1783
On Spectral Triples in Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
84 pages, 8 figures
(Submitted on 13 Feb 2008)

"This paper establishes a link between Noncommutative Geometry and canonical quantum gravity. A semi-finite spectral triple over a space of connections is presented. The triple involves an algebra of holonomy loops and a Dirac type operator which resembles a global functional derivation operator. The interaction between the Dirac operator and the algebra reproduces the Poisson structure of General Relativity. Moreover, the associated Hilbert space corresponds, up to a discrete symmetry group, to the Hilbert space of diffeomorphism invariant states known from Loop Quantum Gravity. Correspondingly, the square of the Dirac operator has, in terms of canonical quantum gravity, the form of a global area-squared operator. Furthermore, the spectral action resembles a partition function of Quantum Gravity. The construction is background independent and is based on an inductive system of triangulations. This paper is the first of two papers on the subject."

http://arxiv.org/abs/0802.1784
On Spectral Triples in Quantum Gravity II
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
43 pages, 1 figure
(Submitted on 13 Feb 2008)

"A semifinite spectral triple for an algebra canonically associated to canonical quantum gravity is constructed. The algebra is generated by based loops in a triangulation and its barycentric subdivisions. The underlying space can be seen as a gauge fixing of the unconstrained state space of Loop Quantum Gravity. This paper is the second of two papers on the subject."


http://arxiv.org/abs/0802.1561
Have we tested Lorentz invariance enough?
David Mattingly
17 pages, Talk given at 'From Quantum to Emergent Gravity: Theory and Phenomenology', SISSA, June 2007
(Submitted on 12 Feb 2008)

"Motivated by ideas from quantum gravity, Lorentz invariance has undergone many stringent tests over the past decade and passed every one. Since there is no conclusive reason from quantum gravity that the symmetry \textit{must} be violated at some point we should ask the questions: a) are the existing tests sufficient that the symmetry is already likely exact at the Planck scale? b) Are further tests simply blind searches for new physics without reasonable expectation of a positive signal? Here we argue that the existing tests are not quite sufficient and describe some theoretically interesting areas of existing parameterizations for Lorentz violation in the infrared that are not yet ruled out but are accessible (or almost accessible) by current experiments. We illustrate this point using a vector field model for Lorentz violation containing operators up to mass dimension six and analyzing how terrestrial experiments, neutrino observatories, and Auger results on ultra-high energy cosmic rays limit this model."

http://arxiv.org/abs/0802.1798
Emergent gravity and Dark Energy
T. Padmanabhan
26 pages
(Submitted on 13 Feb 2008)

"This is an invited contribution to be included in a multi-authored book on 'Dark Energy', to be edited by Pilar Ruiz-Lapuente and published by Cambridge University Press."

 

[marcus]

http://arxiv.org/abs/0802.2527
Asymptotics and Hamiltonians in a First order formalism
Abhay Ashtekar, Jonathan Engle, David Sloan
18 pages
(Submitted on 18 Feb 2008)

"We consider 4-dimensional space-times which are asymptotically flat at spatial infinity and show that, in the first order framework, action principle is well-defined without the need of infinite counter terms. It naturally leads to a covariant phase space in which the Hamiltonians generating asymptotic symmetries provide the total energy-momentum and angular momentum of the space-time. We address the subtle but important problems that arise because of logarithmic translations and super-translations both in the Langrangian and Hamiltonian frameworks. As a forthcoming paper will show, the treatment of higher dimensions is considerably simpler. Our first order framework also suggests a new direction for generalizing the spectral action of non-commutative geometry."


http://arxiv.org/abs/0802.2230
Canonical Lagrangian Dynamics and General Relativity
Andrew Randono
(Submitted on 15 Feb 2008)

"Building towards a more covariant approach to canonical classical and quantum gravity we outline an approach to constrained dynamics that de-emphasizes the role of the Hamiltonian phase space and highlights the role of the Lagrangian phase space. We identify a 'Lagrangian one-form' to replace the standard symplectic one-form, which we use to construct the canonical constraints and an associated constraint algebra. The method is particularly useful for generally covariant systems and systems with a degenerate canonical symplectic form, such as Einstein Cartan gravity, to which we apply the method explicitly. We find that one can demonstrate the closure of the constraints without gauge fixing the Lorentz group or introducing primary constraints on the phase space variables. Finally, using geometric quantization techniques, we briefly discuss implications of the formalism for the quantum theory."

 

[marcus]

http://arxiv.org/abs/0802.3188
Black hole state degeneracy in Loop Quantum Gravity
Ivan Agullo, Jacobo Diaz-Polo, Enrique Fernandez-Borja
22 pages, 7 figures
(Submitted on 21 Feb 2008)

"The combinatorial problem of counting the black hole quantum states within the Isolated Horizon framework in Loop Quantum Gravity is analyzed. A qualitative understanding of the origin of the band structure shown by the degeneracy spectrum, which is responsible for the black hole entropy quantization, is reached. Even when motivated by simple considerations, this picture allows to obtain analytical expressions for the most relevant quantities associated to this effect."

 

[marcus]

http://arxiv.org/abs/0802.3389
Simplicity and closure constraints in spin foam models of gravity
Sergei Alexandrov
16 pages
(Submitted on 23 Feb 2008)

"We revise imposition of various constraints in spin foam models of 4-dimensional general relativity. We argue that the usual simplicity constraint must be supplemented by a constraint on holonomies and together they must be inserted explicitly into the discretized path integral. At the same time, the closure constraint must be relaxed so that the new constraint expresses covariance of intertwiners assigned to tetrahedra by spin foam quantization. As a result, the spin foam boundary states are shown to be realized in terms of projected spin networks of the covariant loop approach to quantum gravity."


http://arxiv.org/abs/0802.3422
Black Holes without Event Horizons
Alex B. Nielsen
Talk at APCTP Winter School, Daejeon, Korea, 2008. 7 pages
(Submitted on 23 Feb 2008)

"We discuss some of the drawbacks of using event horizons to define black holes. The reasons are both practical, physical and theoretical. We argue that locally defined trapping horizons can remedy many of these drawbacks. We examine of the question of whether black hole thermodynamics should be associated with event horizons or trapping horizons. To this end we discuss what role trapping horizons may play in black hole thermodynamics. In addition, we show how trapping horizons may give rise to Hawking radiation and discuss the issue of gravitational entropy."

 

[marcus]

http://arxiv.org/abs/0802.3983
Towards the graviton from spinfoams: the complete perturbative expansion of the 3d toy model
Valentin Bonzom, Etera R. Livine, Matteo Smerlak, Simone Speziale
16 pages, 3 figs
(Submitted on 27 Feb 2008)

"We consider an exact expression for the 6j-symbol for the isosceles tetrahedron, involving integrals over SU(2), and use it to write the two-point function of 3d gravity on a single tetrahedron as a group integral. The perturbative expansion of this expression is then performed with respect to the boundary geometry using a simple saddle-point analysis. We derive the complete expansion in inverse powers of the length scale and evaluate explicitly the quantum corrections up to second order. Finally, we use the same method to provide the complete expansion of the isosceles 6j-symbol with the explicit phases at all orders and the next-to-leading correction to the Ponzano-Regge asymptotics."

http://arxiv.org/abs/0802.4077
Black hole state counting in LQG: A number theoretical approach
Ivan Agullo, J. Fernando Barbero G., Jacobo Diaz-Polo, Enrique Fernandez-Borja, Eduardo J. S. Villaseñor
4 pages
(Submitted on 27 Feb 2008)

"We give a practical method to exactly compute black hole entropy in the framework of Loop Quantum Gravity. Along the way we provide a complete characterization of the relevant sector of the spectrum of the area operator, including degeneracies, and determine the number of solutions to the projection constraint analytically. We use a computer implementation of the proposed algorithm to confirm and extend previous results on the detailed structure of the black hole degeneracy spectrum."

 

[marcus]

http://arxiv.org/abs/0802.4274
Loop Quantum Cosmology: Effective theories and oscillating universes
Martin Bojowald, Reza Tavakol
24 pages, 3 figures, Chapter contributed to: Beyond the Big Bang, edited by R. Vaas (Springer Verlag, 2008)


"Despite its great successes in accounting for the current observations, the so called 'standard' model of cosmology faces a number of fundamental unresolved questions. Paramount among these are those relating to the nature of the origin of the universe and its early evolution. Regarding the question of origin, the main difficulty has been the fact that within the classical general relativistic framework, the 'origin' is almost always a singular event at which the laws of physics break down, thus making it impossible for such an event, or epochs prior to it, to be studied. Recent studies have shown that Loop Quantum Cosmology may provide a non-singular framework where these questions can be addressed. The crucial role here is played by quantum effects, i.e. corrections to the classical equations of motion, which are incorporated in effective equations employed to develop cosmological scenarios.
In this chapter we shall consider the three main types of quantum effects expected to be present within such a framework and discuss some of their consequences for the effective equations. In particular we discuss how such corrections can allow the construction of non-singular emergent scenarios for the origin of the universe, which are past-eternal, oscillating and naturally emerge into an inflationary phase. These scenarios provide a physically plausible picture for the origin and early phases of the universe, which is in principle testable. We pay special attention to the interplay between these different types of correction terms. Given the absence, so far, of a complete derivation of such corrections in general settings, it is important to bear in mind the questions of consistency and robustness of scenarios based on partial inclusion of such effects."

Amazon is now taking advance orders on the new book Beyond the Big Bang, edited by R. Vaas
https://www.amazon.com/dp/3540714227/?tag=pfamazon01-20
The book is 600 pages, several different experts in quantum cosmology have contributed chapters.
The publication date is 1 July 2008.

 

[marcus]

http://arxiv.org/abs/0803.0982
Dark Energy and the Accelerating Universe
Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan)
Invited review for Annual Reviews of Astronomy and Astrophysics; 53 pages, 18 figures
(Submitted on 7 Mar 2008)

"The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years."

 

[marcus]

http://arxiv.org/abs/0803.2546
Primordial Entropy Production and Lambda-driven Inflation from Quantum Einstein Gravity
Alfio Bonanno, Martin Reuter
12 pages, 4 figures, IGCG-07 Pune
(Submitted on 17 Mar 2008)

"We review recent work on renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values. In particular we argue that QEG effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. This phase is a pure quantum effect and requires no classical inflaton field."

This is the paper Martin Reuter gave at an international conference that was held in December 2007 in India.

Reuter is also scheduled to give a talk at the QG2 conference at Nottingham, first week of July.

 

[marcus]

Sets of slides for two important talks at the Zakopane workshop. More PDF files from the workshop are available at Kostecki's website:

* Abhay Ashtekar - An Overview of Loop Quantum Cosmology of FRW Models
http://cift.fuw.edu.pl/users/kostecki/zakopane08/ashtekar.pdf

* Carlo Rovelli - Looppy & Foammy: at long last, falling in love
http://cift.fuw.edu.pl/users/kostecki/zakopane08/rovelli.pdfhttp://arxiv.org/abs/0803.2926
Matrix universality of gauge field and gravitational dynamics
Lee Smolin
21 pages,
(Submitted on 20 Mar 2008)

"A simple cubic matrix model is presented, which has truncations that, it is argued, lead at the classical level to a variety of theories of gauge fields and gravity. These include Yang-Mills theories and background independent theories of connections. The latter includes Chern-Simons theory in d=3, and BF theory and general relativity in d=4. General relativity coupled to Yang-mills theory for any SU(N) may also arise from quantum corrections.
On the basis of these results we conjecture that there are large universality classes of cut-off gauge and gravity theories, connected by transformations that mix up local and spacetime symmetries. If our universe is described by one of these theories then the question of the choice of the laws of physics is to a large extent subsumed in the problem of the choice of initial conditions in cosmology."

I don't mean to suggest that the next one is a big deal, only a reminder that things can be more complicated than at first suspected and require correction. Linder has a solid track record---perhaps I'm biased but I tend to pay extra attention.
http://arxiv.org/abs/0803.2877
Shifting the Universe: Early Dark Energy and Standard Rulers
Eric V. Linder, Georg Robbers
6 pages, 3 figures
(Submitted on 19 Mar 2008)

"The presence of dark energy at high redshift influences both the cosmic sound horizon and the distance to last scattering of the cosmic microwave background. We demonstrate that through the degeneracy in their ratio, early dark energy can lie hidden in the CMB temperature and polarization spectra, leading to an unrecognized shift in the sound horizon. If the sound horizon is then used as a standard ruler, as in baryon acoustic oscillations, then the derived cosmological parameters can be nontrivially biased. Fitting for the absolute ruler scale (just as supernovae must be fit for the absolute candle magnitude) removes the bias but decreases the leverage of the BAO technique by a factor 2."

 

[marcus]

http://arxiv.org/abs/0803.3203
Conserved Quantities for Interacting Four Valent Braids in Quantum Gravity
Jonathan Hackett, Yidun Wan
18 pages, 7 figures
(Submitted on 21 Mar 2008)

"We derive conservation laws from interactions of actively-interacting braid-like excitations of embedded framed spin networks in Quantum Gravity. Additionally we demonstrate that actively-interacting braid-like excitations interact in such a way that the product of interactions involving two actively-interacting braid-like excitations produces a resulting actively-interacting form."

 

[marcus]

http://arxiv.org/abs/0803.3456
Is Quantum Gravity Necessary?
S. Carlip
based on a talk given at Peyresq Physics 11, to appear in Class. Quant. Grav
(Submitted on 24 Mar 2008)

"In view of the enormous difficulties we seem to face in quantizing general relativity, we should perhaps consider the possibility that gravity is a fundamentally classical interaction. Theoretical arguments against such mixed classical-quantum models are strong, but not conclusive, and the question is ultimately one for experiment. I review some work in progress on the possibility of experimental tests, exploiting the nonlinearity of the classical-quantum coupling, that could help settle this question."

http://arxiv.org/abs/0803.2309
Cosmic Microwave Weak lensing data as a test for the dark universe
Erminia Calabrese, Anze Slosar, Alessandro Melchiorri, George F. Smoot, Oliver Zahn
7 Pages, 3 Figures
(Submitted on 17 Mar 2008)

"Combined analyses of WMAP 3-year and ACBAR Cosmic Microwave Anisotropies angular power spectra have presented evidence for gravitational lensing >3 sigma level. This signal could provide a relevant test for cosmology. After evaluating and confirming the statistical significance of the detection in light of the new WMAP 5-year data, we constrain a new parameter A_L that scales the lensing potential such that A_L=0 corresponds to unlensed while A_L=1 is the expected lensed result. We find from WMAP5+ACBAR a 2.5 sigma indication for a lensing contribution larger than expected, with A_L=3.1_{-1.5}^{+1.8} at 95% c.l.. The result is stable under the assumption of different templates for an additional Sunyaev-Zel'dovich foreground component or the inclusion of an extra background of cosmic strings. We find negligible correlation with other cosmological parameters as, for example, the energy density in massive neutrinos. While unknown systematics may be present, dark energy or modified gravity models could be responsible for the over-smoothness of the power spectrum. Near future data, most notably from the Planck satellite mission, will scrutinize this interesting possibility."

http://arxiv.org/abs/0803.3559
Loop Quantum Cosmology of Diagonal Bianchi Type I model: simplified theory
Lukasz Szulc
10 pages, 10 figures
(Submitted on 25 Mar 2008)

"A simplified theory of diagonal Bianchi type I model coupled with a massless scalar field in Loop Quantum Cosmology is constructed. The quantum constraint operator and physical sector are under good analytical control. The problem of finding semi-classical states is reduced to the following one: how to compute an ordinary, continuous, three-dimensional Fourier Transform with an amplitude given analytically? Moreover, the evolution of the three gravitational degrees of freedom is numerically shown to be non-singular."

http://arxiv.org/abs/0803.3659
Phenomenological dynamics of loop quantum cosmology in Kantowski-Sachs spacetime
Dah-Wei Chiou
36 pages, 4 figures, 1 table
(Submitted on 26 Mar 2008)

"The full theory and the semiclassical description of loop quantum cosmology (LQC) have been studied in the Friedmann-Robertson-Walker and Bianchi I models. As an extension to include both anisotropy and intrinsic curvature, this paper investigates the cosmological model of Kantowski-Sachs spacetime with a free massless scalar field at the level of phenomenological dynamics with the LQC discreteness corrections. The LQC corrections are implemented in two different improved quantization schemes. In both schemes, the big bang and big crunch singularities of the classical solution are resolved and replaced by the big bounces when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. Symmetries of scaling are also noted and suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale. The bouncing scenarios are in an analogous fashion of the Bianchi I model, naturally extending the observations obtained earlier."

 

[marcus]

http://arxiv.org/abs/0803.4484
Recollapsing quantum cosmologies and the question of entropy
Martin Bojowald, Reza Tavakol
23 pages, 2 figures
(Submitted on 31 Mar 2008)

"Recollapsing homogeneous and isotropic models present one of the key ingredients for cyclic scenarios. This is considered here within a quantum cosmological framework in presence of a free scalar field with, in turn, a negative cosmological constant and spatial curvature. Effective equations shed light on the quantum dynamics around a recollapsing phase and the evolution of state parameters such as fluctuations and correlations through such a turn around. In the models considered here, the squeezing of an initial state is found to be strictly monotonic in time during the expansion, turn around and contraction phases. The presence of such monotonicity is of potential importance in relation to a long standing intensive debate concerning the (a)symmetry between the expanding and contracting phases in a recollapsing universe. Furthermore, together with recent analogous results concerning a bounce one can extend this monotonicity throughout an entire cycle. This provides a strong motivation for employing the degree of squeezing as an alternative measure of (quantum) entropy. It may also serve as a new concept of emergent time described by a variable without classical analog. The evolution of the squeezing in emergent oscillating scenarios can in principle provide constraints on the viability of such models."

It is an important issue. Penrose has made it central to all his discussion of cosmology, including opposition to the bounce explanation of the big bang. They may have answered Penrose. London is getting to be an important place for studying quantum cosmology. Tavakol is at Queen Mary London, but there is also a group at King's College.

http://arxiv.org/abs/0803.4483
Numerical techniques for solving the quantum constraint equation of generic lattice-refined models in loop quantum cosmology
William Nelson, Mairi Sakellariadou (King's College London)
17 pages, 14 figures
(Submitted on 31 Mar 2008)

"To avoid instabilities in the continuum semi-classical limit of loop quantum cosmology models, refinement of the underlying lattice is necessary. The lattice refinement leads to new dynamical difference equations which, in general, do not have a uniform step-size, implying complications in their analysis and solutions. We propose a numerical method based on Taylor expansions, which can give us the necessary information to calculate the wave-function at any given lattice point. The method we propose can be applied in any lattice-refined model, while in addition the accuracy of the method can be estimated. Moreover, we confirm numerically the stability criterion which was earlier found following a von Neumann analysis. Finally, the `motion' of the wave-function due to the underlying discreteness of the space-time is investigated, for both a constant lattice, as well as lattice refinement models."

http://arxiv.org/abs/0803.4446
Anti-deSitter universe dynamics in LQC
Eloisa Bentivegna, Tomasz Pawlowski
29 pages, 20 figures
(Submitted on 31 Mar 2008)

"A model for a flat isotropic universe with a negative cosmological constant $\Lambda$ and a massless scalar field as sole matter content is studied within the framework of Loop Quantum Cosmology. By application of the methods introduced for the model with Lambda=0, the physical Hilbert space and the set of Dirac observables are constructed. As in that case, the scalar field plays here the role of an emergent time. The properties of the system are found to be similar to those of the k=1 FRW model: for small energy densities, the quantum dynamics reproduces the classical one, whereas, due to modifications at near-Planckian densities, the big bang and big crunch singularities are replaced by a quantum bounce connecting deterministically the large semiclassical epochs. Thus in Loop Quantum Cosmology the evolution is qualitatively cyclic."

http://arxiv.org/abs/0804.0030
A Pointless Model for the Continuum as the Foundation for Quantum Gravity
Louis Crane
Submitted to GRF
(Submitted on 31 Mar 2008)

"In this paper, we outline a new approach to quantum gravity; describing states for a bounded region of spacetime as eigenstates for two classes of physically plausible gedanken experiments. We end up with two complementary descriptions in which the point set continuum disappears.
The first replaces the continuum of events with a handlebody decomposition of loop space. We conjecture that techniques fron algebraic topology will allow us to extend state sum models on spacetime to loop space.
The second picture replaces the continuum with a nondistributive lattice; the classical limit seems more tractible in this picture."

http://arxiv.org/abs/0804.0054
Formalism Locality in Quantum Theory and Quantum Gravity
Lucien Hardy
To appear in "Philosophy of Quantum Information and Entanglement" Eds A. Bokulich and G. Jaeger (CUP)
(Submitted on 1 Apr 2008)

"We expect a theory of Quantum Gravity to be both probabilistic and have indefinite causal structure. Indefinite causal structure poses particular problems for theory formulation since many of the core ideas used in the usual approaches to theory construction depend on having definite causal structure. For example, the notion of a state across space evolving in time requires that we have some definite causal structure so we can define a state on a space-like hypersurface. We will see that many of these problems are mitigated if we are able to formulate the theory in a "formalism local" (or F-local) fashion. A formulation of a physical theory is said to be F-local if, in making predictions for any given arbitrary space-time region, we need only refer to mathematical objects pertaining to that region. This is a desirable property both on the grounds of efficiency and since, if we have indefinite causal structure, it is not clear how to select some other space-time region on which our calculations may depend. The usual ways of formulating physical theories (the time evolving state picture, the histories approach, and the local equations approach) are not F-local.
We set up a framework for probabilistic theories with indefinite causal structure. This, the causaloid framework, is F-local. We describe how Quantum Theory can be formulated in the causaloid framework (in an F-local fashion). This provides yet another formulation of Quantum Theory. This formulation, however, may be particularly relevant to the problem of finding a theory of Quantum Gravity."

 

[marcus]

http://arxiv.org/abs/0804.0037
Particle Identifications from Symmetries of Braided Ribbon Network Invariants
Sundance Bilson-Thompson, Jonathan Hackett, Lou Kauffman, Lee Smolin
9 pages, 7 figures
(Submitted on 1 Apr 2008)

"We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge."

Here is a quote from the Conclusions, section 5 on page 8:

We have presented an embedding of the fermion and weak vector boson states of the standard model in a class of loop quantum gravity models. These are models in which the states are based on embeddings of framed trivalent spin networks, with possibly arbitrary labellings, whose dynamics is given by the standard dual Pachner trivalent moves, plus additional moves consistent with the conservation of the topological invariants (a; b; c).

There are a number of interrelated questions that remain open before the promise of this development can be fully understood...

http://arxiv.org/abs/0804.0328
A locally finite model for gravity
Gerard 't Hooft
26 pages, 9 figures
(Submitted on 2 Apr 2008)

"Matter interacting classically with gravity in 3+1 dimensions usually gives rise to a continuum of degrees of freedom, so that, in any attempt to quantize the theory, ultraviolet divergences are nearly inevitable. Here, we investigate matter of a form that only displays a finite number of degrees of freedom in compact sections of space-time. In finite domains, one has only exact, analytic solutions. This is achieved by limiting ourselves to straight pieces of string, surrounded by locally flat sections of space-time. Globally, however, the model is not finite, because solutions tend to generate infinite fractals. The model is not (yet) quantized, but could serve as an interesting setting for analytical approaches to classical general relativity, as well as a possible stepping stone for quantum models. Details of its properties are explained, but some problems remain unsolved, such as a complete description of the most violent interactions, which can become quite complex."

http://arxiv.org/abs/0804.0252
A Matrix Model for 2D Quantum Gravity defined by Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
13 pages, 1 figure
(Submitted on 1 Apr 2008)

"A novel continuum theory of two-dimensional quantum gravity, based on a version of Causal Dynamical Triangulations which incorporates topology change, has recently been formulated as a genuine string field theory in zero-dimensional target space (arXiv:0802.0719). Here we show that the Dyson-Schwinger equations of this string field theory are reproduced by a cubic matrix model. This matrix model also appears in the so-called Dijkgraaf-Vafa correspondence if the superpotential there is required to be renormalizable. In the spirit of this model, as well as the original large-N expansion by 't Hooft, we need no special double-scaling limit involving a fine tuning of coupling constants to obtain the continuum quantum-gravitational theory. Our result also implies a matrix model representation of the original, strictly causal quantum gravity model."

http://arxiv.org/abs/0804.0279
A Discrete Representation of Einstein's Geometric Theory of Gravitation: The Fundamental Role of Dual Tessellations in Regge Calculus
Jonathan R. McDonald, Warner A. Miller
25 pages, 12 figures, submitted to "Tessellations in the Science: Virtues, Techniques and Applications of Geometric Tilings," ed. R. van de Weijgaert, G. Vegter, J. Ritzerveld and V. Icke
(Submitted on 2 Apr 2008)

"In 1961 Tullio Regge provided us with a beautiful lattice representation of Einstein's geometric theory of gravity. This Regge Calculus (RC) is strikingly different from the more usual finite difference and finite element discretizations of gravity. In RC the fundamental principles of General Relativity are applied directly to a tessellated spacetime geometry. In this manuscript, and in the spirit of this conference, we reexamine the foundations of RC and emphasize the central role that the Voronoi and Delaunay lattices play in this discrete theory. In particular we describe, for the first time, a geometric construction of the scalar curvature invariant at a vertex. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding and more familiar hinge-based expression in RC (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas. What is most striking to us is how naturally spacetime is represented by Voronoi and Delaunay structures and that the laws of gravity appear to be encoded locally on the lattice spacetime with less complexity than in the continuum, yet the continuum is recovered by convergence in mean. Perhaps these prominent features may enable us to transcend the details of any particular discrete model gravitation and yield clues to help us discover how we may begin to quantize this fundamental interaction."

 

[marcus]

http://arxiv.org/abs/0804.0632
Reconstructing AdS/CFT
Laurent Freidel
34 pages
(Submitted on 4 Apr 2008)

"In this note we clarify the dictionary between pure quantum gravity on the bulk in the presence of a cosmological constant and a CFT on the boundary. We show for instance that there is a general correspondence between quantum gravity 'radial states' and a pair of CFT's. Restricting to one CFT is argued to correspond to states possessing an asymptotic infinity. This point of view allows us to address the problem of reconstructing the bulk from the boundary. And in the second part of this paper we present an explicit formula which gives, from the partition function of any 2 dimensional conformal field theory, a wave functional solution to the 3-dimensional Wheeler-DeWitt equation. This establishes at the quantum level a precise dictionary between 2d CFT and pure gravity."

http://arxiv.org/abs/0804.0672
Quantum Cosmology
Claus Kiefer, Barbara Sandhoefer
29 pages, 9 figures, contribution to "Beyond the Big Bang", ed. by R. Vaas (Springer 2008)
(Submitted on 4 Apr 2008)

"We give an introduction into quantum cosmology with emphasis on its conceptual parts. After a general motivation we review the formalism of canonical quantum gravity on which discussions of quantum cosmology are usually based. We then present the minisuperspace Wheeler--DeWitt equation and elaborate on the problem of time, the imposition of boundary conditions, the semiclassical approximation, the origin of irreversibility, and singularity avoidance. Restriction is made to quantum geometrodynamics; loop quantum gravity and string theory are discussed in other contributions to this volume."

http://arxiv.org/abs/0804.0619
A model for delayed emission in a very-high energy gamma-ray flare in Markarian 501
W. Bednarek (1), R. M. Wagner (2) ((1) University of Lodz, Poland (2) MPI für Physik, München, Germany)
3 pages, no figures, submitted to Astronomy & Astrophysics
(Submitted on 4 Apr 2008)

"Recently the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a gamma-ray flare from the blazar Markarian 501 on 2005 July 9. We aim at describing the observed delayed high-energy emission. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. Such a modified SSC model predicts the appearance of the gamma-ray flare first at lower energies and subsequently at higher energies. Moreover, we argue that the time delay between the flare observed at different energies depends on the gamma-ray energy. Based on the reported time delay of approx. 240 s between the flare observed at 190 GeV and 2.7 TeV, we predict it should be on the order of 1 h if it was observed between 10 GeV and 100 GeV. Such delay time scales can be tested in the future by simultaneous observations of flares with GLAST and Cherenkov telescopes."

http://arxiv.org/abs/0804.1098
Hybrid Quantum Gowdy Cosmology: Combining Loop and Fock Quantizations
Mercedes Martin-Benito, Luis J. Garay, Guillermo A. Mena Marugan
4 pages
(Submitted on 7 Apr 2008)

"We quantize the linearly polarized Gowdy T^3 family of cosmologies by combining loop and Fock techniques. The loop quantization of the degrees of freedom that describe the subfamily of homogeneous cosmologies proves sufficient to solve the initial singularity. We obtain the general expression of the solutions to the quantum constraints. These solutions are determined by their value on an initial spatial section, where they arise from nothing. They can be provided with a Hilbert structure that reproduces the conventional Fock quantization of the inhomogeneities."

 

[marcus]

http://arxiv.org/abs/0804.1686
On the origin of the particles in black hole evaporation
Ralf Schützhold, William G. Unruh
4 pages
(Submitted on 10 Apr 2008)

"We present an analytic derivation of Hawking radiation for an arbitrary (spatial) dispersion relation \omega(k) as a model for ultra-high energy deviations from general covariance. It turns out that the Hawking temperature is proportional to the product of the group d\omega/dk and phase \omega/k velocities evaluated at the frequency \omega of the outgoing radiation far away, which suggests that Hawking radiation is basically a low-energy phenomenon. Nevertheless, a group velocity growing too fast at ultra-short distances would generate Hawking radiation at ultra-high energies ('ultra-violet catastrophe') and hence should not be a realistic model for the microscopic structure of quantum gravity."

http://arxiv.org/abs/0804.1771
The cosmic variance of Omega
T. P. Waterhouse, J. P. Zibin
10 pages, 2 figures
(Submitted on 10 Apr 2008)

"How much can we know about our Universe? All of our observations are restricted to a finite volume, and therefore our estimates of presumably global cosmological parameters are necessarily based on incomplete information. Even assuming that the Standard Model of cosmology is correct, this means that some cosmological questions may be unanswerable. For example, is the curvature parameter Omega_K positive, negative, or identically zero? If its magnitude is sufficiently small, then due to cosmic variance no causal observation can ever answer that question. In this article, we first describe the gauge problems associated with defining the cosmic variance of cosmological parameters, then describe a solution involving the use of parameters defined on the surface of last scattering, and finally calculate the statistical variance of ideal measurements of the matter, radiation, and curvature density parameters. We find that Omega_K cannot be measured to better than about 1.5x10^(-5) (1 sigma), and that this limit has already begun to decrease due to the flattening effect of dark energy. Proposed 21 cm hydrogen experiments, for example, make this limit more than just a theoretical curiosity."

 

[nrqed]

The thread has a very interesting beginning where there was an attempt at a introductory level discussion of the concepts of loop quantum gravity. Would it be possible to start a new thread doing that? Where someone could start explaining the ideas of LQG at a level appropriate for, say, beginning graduate students.

That would be awesome.

 

[marcus]

Thanks nrqed, that's a good suggestion! There are people who post here who are graduate students in QG who, if not too busy, could do a great tutorial.
Maybe if I get inspired I'll start one and count on someone else taking over if it catches on.

http://arxiv.org/abs/0804.1797
Chern-Simons Modified Gravity as a Torsion Theory and its Interaction with Fermions
Stephon Alexander, Nicolas Yunes
11 pages, submitted to Phys. Rev. D
(Submitted on 10 Apr 2008)

"We study the tetrad formulation of Chern-Simons (CS) modified gravity, which adds a Pontryagin term to the Einstein-Hilbert action with a spacetime-dependent coupling field. We first verify that CS modified gravity leads to a theory with torsion, where this tensor is given by an antisymmetric product of the Riemann tensor and derivatives of the CS coupling. We then calculate the torsion in the far field of a weakly gravitating source within the parameterized post-Newtonian formalism, and specialize the result to Earth. We find that CS torsion vanishes only if the coupling vanishes, thus generically leading to a modification of gyroscopic precession, irrespective of the coupling choice. Perhaps most interestingly, we couple fermions to CS modified gravity via the standard Dirac action and find that these further correct the torsion tensor. Such a correction leads to two new results: (i) a generic enhancement of CS modified gravity by the Dirac equation and axial fermion currents; (ii) a new two-fermion interactions, mediated by an axial current and the CS correction. We conclude with a discussion of the consequences of these results in particle detectors and realistic astrophysical systems."

 

[marcus]

http://arxiv.org/abs/0804.2541
Phase space quantization and Loop Quantum Cosmology: A Wigner function for the Bohr-compactified real line
Christopher J. Fewster, Hanno Sahlmann
26 pages, 3 figures
(Submitted on 16 Apr 2008)

"We give a definition for the Wigner function for quantum mechanics on the Bohr compactification of the real line and prove a number of simple consequences of this definition. We then discuss how this formalism can be applied to loop quantum cosmology. As an example, we use the Wigner function to give a new quantization of an important building block of the Hamiltonian constraint."
http://arxiv.org/abs/0804.2475
The Void Phenomenon Explained
Jeremy L. Tinker (KICP, UChicago), Charlie Conroy (Princeton)
8 pages, 5 figures, submitted to ApJ
(Submitted on 16 Apr 2008)

"We use high-resolution N-body simulations, combined with a halo occupation model of galaxy bias, to investigate voids in the galaxy distribution. Our goal is to address the 'void phenomenon' of Peebles (2001), which presents the observed dearth of faint galaxies in voids as a challenge to the current cosmology. In our model, galaxy luminosity is determined only as a function of dark matter halo mass. With this simple assumption, we demonstrate that large, empty voids of ~15 Mpc/h in diameter are expected even for galaxies seven magnitudes fainter than L*. The predictions of our model are in excellent agreement with several statistical measures; (i) the luminosity function of galaxies in underdense regions, (ii) nearest neighbor statistics of dwarf galaxies, (iii) the void probability function of faint galaxies. In the transition between filaments and voids in the dark matter, the halo mass function changes abruptly, causing the maximum galaxy luminosity to decrease by ~5 magnitudes over a range of ~1 Mpc/h. Thus the boundary between filaments and voids in the galaxy distribution is nearly as sharp for dwarfs as for ~L* objects. These results support a picture in which galaxy formation is driven predominantly by the mass of the host dark matter halo, and is nearly independent of the larger-scale halo environment. Further, they demonstrate that LCDM, combined with a straightforward bias model, naturally explains the existence of the void phenomenon."

http://arxiv.org/abs/0804.2082
BTZ Black Hole Entropy: A spin foam model description
J.Manuel Garcia-Islas
9 pages, 2 figures
(Submitted on 13 Apr 2008)

"We present a microscopical explanation of the entropy of the BTZ black hole using discrete spin foam models of quantum gravity. The entropy of a black hole is given in geometrical terms which lead us to think that its statistical description must be given in terms of a quantum geometry. In this paper we present it in terms of spin foam geometrical observables at the horizon of the black hole."

 

[marcus]

http://arxiv.org/abs/0804.2720
Black Hole Thermodynamics and Lorentz Symmetry
Ted Jacobson, Aron C. Wall
4 pages, prepared for the 2008 Gravity Research Foundation Essay Competition
(Submitted on 17 Apr 2008)

"Recent developments point to a breakdown in the generalized second law of thermodynamics for theories with Lorentz symmetry violation. It appears possible to construct a perpetual motion machine of the second kind in such theories, using a black hole to catalyze the conversion of heat to work. Here we describe the arguments leading to that conclusion. We suggest the implication that Lorentz symmetry should be viewed as an emergent property of the macroscopic world, required by the second law of black hole thermodynamics."


http://arxiv.org/abs/0804.2778
Effects of the quantisation ambiguities on the Big Bounce dynamics
Orest Hrycyna, Jakub Mielczarek, Marek Szydlowski
26 pages, 10 figs
(Submitted on 17 Apr 2008)

"In this paper we investigate dynamics of the modified loop quantum cosmology models using dynamical systems methods. Modifications considered come from the choice of the different field strength operator \hat{F} and result in different forms of the effective Hamiltonian. Such an ambiguity of the choice of this expression from some class of functions is allowed in the framework of loop quantisation. Our main goal is to show how such modifications can influence the bouncing universe scenario in the loop quantum cosmology. In effective models considered we classify all evolutional paths for all admissible initial conditions. The dynamics is reduced to the form of a dynamical system of the Newtonian type on a 2-dimensional phase plane. These models are equivalent dynamically to the FRW models with the decaying effective cosmological term parametrised by the canonical variable p (or by the scale factor a). We find that for the positive cosmological constant there is a class of oscillating models without the initial and final singularities. The new phenomenon is the appearance of curvature singularities for the finite values of the scale factor, but we find that for the positive cosmological constant these singularities can be avoided. For the positive cosmological constant the evolution begins at the asymptotic state in the past represented by the deSitter contracting (deS_-) spacetime or the static Einstein universe H=0 or H=- infinity state and reaches the deSitter expanding state (deS₊) , the state H=0 or H=+infinity state. In the case of the negative cosmological constant we obtain the past and future asymptotic states as the Einstein static universes.

http://arxiv.org/abs/0804.2811
Spin Foam Perturbation Theory for Three-Dimensional Quantum Gravity
Joao Faria Martins, Aleksandar Mikovic
34 pages, 17 figures
(Submitted on 17 Apr 2008)

"We develop the spin foam perturbation theory for three-dimensional Euclidean Quantum Gravity. We analyse the perturbative expansion of the partition function in the dilute gas limit and argue that a conjecture due to Baez does not hold for arbitrary triangulations. However, the conjecture holds for a special class of triangulations which are based on subdivisions of certain 3-manifold cubulations. In this case we calculate the partition function."

 

[marcus]

http://arxiv.org/abs/0804.3157
Loop Quantization of Vacuum Bianchi I Cosmology
M. Martin-Benito, G. A. Mena Marugan, T. Pawlowski
10 pages
(Submitted on 19 Apr 2008)

"We analyze the loop quantization of the family of vacuum Bianchi I spacetimes, a gravitational system whose classical solutions describe homogeneous anisotropic cosmologies. We rigorously construct the operator that represents the Hamiltonian constraint, showing that the states of zero volume completely decouple from the rest of quantum states. This fact ensures that the classical cosmological singularity is resolved in the quantum theory. In addition, this allows us to adopt an equivalent quantum description in terms of a well defined densitized Hamiltonian constraint. This latter constraint can be regarded in a certain sense as a difference evolution equation in an internal time provided by one of the triad components, which is polymerically quantized. Generically, this evolution equation is a relation between the projection of the quantum states in three different sections of constant internal time. Nevertheless, around the initial singularity the equation involves only the two closest sections with the same orientation of the triad. This has a double effect: on the one hand, physical states are determined just by the data on one section, on the other hand, the evolution defined in this way never crosses the singularity, without the need of any special boundary condition. Finally, we provide these physical states with a Hilbert structure, completing the quantization."

http://arxiv.org/abs/0804.3365
Effective Constraints for Quantum Systems
Martin Bojowald, Barbara Sandhoefer, Aureliano Skirzewski, Artur Tsobanjan
40 pages
(Submitted on 21 Apr 2008)

"An effective formalism for quantum constrained systems is presented which allows manageable derivations of solutions and observables, including a treatment of physical reality conditions without requiring full knowledge of the physical inner product. Instead of a state equation from a constraint operator, an infinite system of constraint functions on the quantum phase space of expectation values and moments of states is used. The examples of linear constraints as well as the free non-relativistic particle in parameterized form illustrate how standard problems of constrained systems can be dealt with in this framework."

 

[marcus]

http://arxiv.org/abs/0804.3726
Quantum Geometry and Quantum Gravity
J. Fernando Barbero G.
To appear in the AIP Conference Proceedings of the XVI International Fall Workshop on Geometry and Physics, Lisbon - Portugal, 5-8 September 2007
(Submitted on 23 Apr 2008)

"The purpose of this contribution is to give an introduction to quantum geometry and loop quantum gravity for a wide audience of both physicists and mathematicians. From a physical point of view the emphasis will be on conceptual issues concerning the relationship of the formalism with other more traditional approaches inspired in the treatment of the fundamental interactions in the standard model. Mathematically I will pay special attention to functional analytic issues, the construction of the relevant Hilbert spaces and the definition and properties of geometric operators: areas and volumes."

http://arxiv.org/abs/0804.3765
Groups of generalized flux transformations in loop quantum gravity
J. M. Velhinho
6 pages
(Submitted on 23 Apr 2008)

"We present a group of transformations in the space of generalized connections that contains the set of transformations generated by the flux variables of loop quantum gravity. This group is labelled by certain SU(2)-valued functions on the bundle of directions in the spatial manifold. A further generalization is obtained by considering functions that depend on germs of analytic curves, rather than just on directions."

 

[marcus]

I can't evaluate this paper adequately (taking Brans-Dicke modified gravity seriously strikes me as highly innovative at this point). However two of the authors, Schuller and Wohlfarth, have good publication track records and have co-authored with well-knowns such as Paul Townsend and Mark Trodden. This is not the first paper of theirs developing this new approach. It's not safe to ignore it.

http://arxiv.org/abs/0804.4067
Brans-Dicke geometry
Raffaele Punzi, Frederic P. Schuller, Mattias N.R. Wohlfarth
8 pages
(Submitted on 25 Apr 2008)

"We reveal the non-metric geometry underlying omega-->0 Brans-Dicke theory by unifying the metric and scalar field into a single geometric structure. Taking this structure seriously as the geometry to which matter universally couples, we show that the theory is fully consistent with solar system tests. This is in striking constrast with the standard metric coupling, which grossly violates post-Newtonian experimental constraints."

Brief mention:

http://arxiv.org/abs/0804.4161
Discrete Quantum Gravity I
P. Kramer (University of Tuebingen) M. Lorente (University of Oviedo)
24 pages
(Submitted on 25 Apr 2008)

"...The crucial step for the Barrett-Crane model in Quantum Gravity is the description of the amplitude for the quantum 4-simplex that is used in the state sum partition function. We obtain the zonal spherical functions for the construction of the SO(4,R) invariant weight and associate them to the triangular faces of the 4-simplices."

http://arxiv.org/abs/0804.4162
Discrete Quantum Gravity II
P. Kramer (University of Tuebingen), M. Lorete (University of Oviedo)
19 pages
(Submitted on 25 Apr 2008)

"In part I of [1] we have developed the tensor and spin representation of SO(4) in order to apply it to the simplicial decomposition of the Barrett-Crane model. We attach to each face of a triangle the spherical function constructed from the Dolginov-Biedenharn function.
In part II we apply the same technique to the Lorentz invariant state sum model. We need three new ingredients: the classification of the edges and the corresponding subspaces that arises in the simplicial decomposition, the irreps of SL(2,C) and its isomorphism to the bivectors appearing in the 4-simplices, the need of a zonal spherical function from the intertwining condition of the tensor product for the simple representations attached to the faces of the simplicial decomposition."

 

[marcus]

Bill Unruh (remember the Unruh effect?) is a prominent figure in Quantum Gravity. A recent paper by Ashtekar et al was devoted to disposing of an objection to Loop Quantum Cosmology that Unruh had raised. Here is a new one from Unruh and Bojan Losic.

http://arxiv.org/abs/0804.4296
Cosmological perturbation theory near de Sitter spacetime
B. Losic, W.G. Unruh
Submitted to PRL
(Submitted on 27 Apr 2008)

"We present an argument that a nonlocal measure of second order metric and matter perturbations dominates that of linear fluctuations in its effect on the gravitational field in spacetimes close to de Sitter spacetime."

There seems to be growing interest in the quantum geometry/gravity of (near) deSitter space. The next paper is by lesser-known authors and already has been accepted for publication in the International Journal of Modern Physics D

http://arxiv.org/abs/0804.4326
Thermodynamics of noncommutative de Sitter spacetime
B. Vakili, N. Khosravi, H. R. Sepangi
11 pages, accepted for publication in IJMPD
(Submitted on 28 Apr 2008)

"We study the effects of noncommutativity of spacetime geometry on the thermodynamical properties of the de Sitter horizon. We show that noncommutativity results in modifications in temperature, entropy and vacuum energy and that these modifications are of order of the Planck scale, suggesting that the size of the noncommutative parameter should be close to that of the Planck. In an alternative way to deal with noncommutativity, we obtain a quantization rule for the entropy. Since noncommutativity in spacetime geometry modifies the Heisenberg algebra and introduces the general uncertainty principle, we also investigate the above problem in this framework."

 

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http://arxiv.org/abs/0804.4784
Generating functions for black hole entropy in Loop Quantum Gravity
J. Fernando Barbero G., Eduardo J. S. Villaseñor
(Submitted on 30 Apr 2008)

"We introduce, in a systematic way, a set of generating functions that solve all the different combinatorial problems that crop up in the study of black hole entropy in Loop Quantum Gravity. Specifically we give generating functions for: The different sources of degeneracy related to the spectrum of the area operator, the solutions to the projection constraint, and the black hole degeneracy spectrum. Our methods are capable of handling the different countings proposed and discussed in the literature. The generating functions presented here provide the appropriate starting point to extend the results already obtained for microscopic black holes to the macroscopic regime --in particular those concerning the area law and the appearance of an effectively equidistant area spectrum."

 

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

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

 

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

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

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

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

 

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

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

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

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

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

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


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

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

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

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

 

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

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

 

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

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

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

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

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

 

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

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

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

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

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

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

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

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

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

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

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

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

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

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

 

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

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

 

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

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

 

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

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

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