Loop-and-allied QG bibliography

[marcus]

Sergei Alexandrov is one of the Utrecht people
http://www.arxiv.org/abs/gr-qc/0510050
Reality conditions for Ashtekar gravity from Lorentz-covariant formulation
Sergei Alexandrov
14 pages
ITP-UU-05/45, SPIN-05/31
"We show the equivalence of the Lorentz-covariant canonical formulation considered for the Immirzi parameter beta = i to the selfdual Ashtekar gravity. We also propose to deal with the reality conditions in terms of Dirac brackets derived from the covariant formulation and defined on an extended phase space which involves, besides the selfdual variables, also their anti-selfdual counterparts."

If I remember, he was getting his PhD in String at Paris around 2002 or 2003, when he was also sometimes co-authoring in loop/foam with Etera Livine. Then he went to Utrecht for postdoc. All this time he is refusing to go along with anybody and take conventional approaches. He liked to do LQG and spinfoam but only in his own way. He suggests that when Astekar at the beginning put the Immirzi equal to square root -1 this was right and that later Ashtekar and the others took the wrong turn by making Immirzi real. To me Sergei seems determined to eventually prove that his way is right and that everybody else took the wrong turn of the road. I admire what I interpret as a polite independence of thought and civilized willingness to take risks.

here's a picture of Sergei
http://www1.phys.uu.nl/spinoza/members/Sergei.htm

In the acknowledgments he appreciates talks with two other Utrecht people Loll and Hanno Sahlmann.

===============

We all know that in a sense Lee Smolin is chained to a rock and the tide is coming in----the tide called GLAST.

From the core version of LQG, as Smolin presents it, there is a prediction that the speed of light should depend slightly on the energy of the photon. Very energetic photons should go just enough faster that after a GRB blast has traveled for a billion years the harder more energetic one should arrive just a little early.

We have Smolin's "Falsifiable Predictions..." paper assuring us. Now John Ellis and Nick Mavromatos would like to constrain or narrow down the amount of that speed-variation. The more they narrow it the more they pinch LQG.

Or think of it as a tide of data, for photons of higher and higher energy, that still inexorably confirms that they are traveling at indistinguishable speed---all seem so far to be traveling same speed----and the energy level at which one has checked this is rising. Anyway that is the cartoon version.

So let's look at the latest in this story, just posted today:

http://www.arxiv.org/abs/astro-ph/0510172
Robust Limits on Lorentz Violation from Gamma-Ray Bursts
John Ellis (CERN), Nick E. Mavromatos (King's Coll., London), Dimitri V. Nanopoulos (Texas A-M & HARC, Woodlands & Athens Academy), Alexander S. Sakharov (CERN & ETHZ), Edward K.G. Sarkisyan (CERN & Univ. Manchester)
18 pages, 4 figures

"We constrain the possibility of a non-trivial refractive index in free space corresponding to an energy-dependent velocity of light:c(E) \simeq c_0 (1 - E/M), where M is a mass scale that might represent effect of quantum-gravitational space-time foam, using the arrival times of sharp features observed in the intensities of radiation with different energies from a large sample of gamma-ray bursters (GRBs) with known redshifts. We use wavelet techniques to identify genuine features, which we confirm in simulations with artificial added noise. Using the weighted averages of the time-lags calculated using correlated features in all the GRB light curves, we find a systematic tendency for more energetic photons to arrive earlier. However, there is a very strong correlation between the parameters characterizing an intrinsic time-lag at the source and a distance-dependent propagation effect. Moreover, the significance of the earlier arrival times is less evident for a subsample of more robust spectral structures. Allowing for intrinsic stochastic time-lags in these features, we establish a statistically robust lower limit: M > 0.9x10^{16} GeV on the scale of violation of Lorentz invariance."
==============
I am very surprised to see this:
"...Using the weighted averages of the time-lags calculated using correlated features in all the GRB light curves, we find a systematic tendency for more energetic photons to arrive earlier. However..."

From Ellis and Mavromatos I only remember seeing observations ruling out any variation in the speed of light with energy----and this is consistent in spirit with their "However..."
It is very surprising to see that this time they allow for a slight possible tendency for the more energetic ones to arrive earlier.

Maybe the tide is destined to rise only so far up the rock where Smolin is chained, and he will survive this test. It only takes just a very tiny variation, so that the more energetic ones (these super hard gammarays) arrive (after all have been traveling a billion years) just a very tiny bit before the others.

============

John Baez student named Derek Wise has posted a 61 page paper:
http://www.arxiv.org/abs/gr-qc/0510033
Lattice p-Form Electromagnetism and Chain Field Theory
"... 'chain field theory' -- a theory analogous to topological quantum field theory, but with chain complexes replacing manifolds..."

Thanks to Spin_Network for pointing out this preprint and providing some background:
In this post
https://www.physicsforums.com/showthread.php?p=782595#post782595
SN recalled this from Baez on SPR
http://groups.google.com/group/sci....group%3Dsci.physics.research#efe70e642e41021f[/QUOTE]

 

[marcus]

Parampreet Singh seminar talk today at Penn State

today 21 October Parampreet Singh gave a seminar talk on quantum gravitational collapse.

He said that the LIGHTCURVE of a collapsing star could show a distinctive signature that was predicted by the quantum theory (LQG) but not by the classical theory of gravitational collapse.

this seems quite iffy, but still a step in the right direction----looking for observable effects, so as to test LQG

http://www.phys.psu.edu/events/index.html?event_id=1302&event_type_ids=0&span=2005-08-20.2005-12-25

Quantum Gravity Effects and the Fate on Gravitational Collapse

the slides (or else they are his lecture notes) are available for download as well as audio.
PAGE 27:
"this quantum gravitational signature can be in principle observed by an external observer as a slight dimming and subsequent brightening of the collapsing star.

An observer can estimate the loop quantum parameter j by observing the flux profile of the burst based on this mechanism and measuring the variation in luminosity of the collapsing cloud."

P. Singh is a co-author with Bojowald of
http://www.arxiv.org/abs/gr-qc/0503041
A black hole mass threshold from non-singular quantum gravitational collapse
Physical Review Letters 95 (2005) 091302

 

[marcus]

http://arxiv.org/abs/hep-th/0511021
A Minimal Length from the Cutoff Modes in Asymptotically Safe Quantum Gravity
Martin Reuter, Jan-Markus Schwindt
26 pages, 1 figure
MZ-TH/05-23
"Within asymptotically safe Quantum Einstein Gravity (QEG), the quantum 4-sphere is discussed as a specific example of a fractal spacetime manifold. The relation between the infrared cutoff built into the effective average action and the corresponding coarse graining scale is investigated. Analyzing the properties of the pertinent cutoff modes, the possibility that QEG generates a minimal length scale dynamically is explored. While there exists no minimal proper length, the QEG sphere appears to be "fuzzy" in the sense that there is a minimal angular separation below which two points cannot be resolved by the cutoff modes."

http://arxiv.org/abs/gr-qc/0511007
Phenomenological implications of an alternative Hamiltonian constraint for quantum cosmology
Mikhail Kagan
10 pages, 7 figures

"In this paper we review a model based on loop quantum cosmology that arises from a symmetry reduction of the self dual Plebanski action. In this formulation the symmetry reduction leads to a very simple Hamiltonian constraint that can be quantized explicitly in the framework of loop quantum cosmology. We investigate the phenomenological implications of this model in the semi-classical regime and compare those with the known results of the standard Loop Quantum Cosmology."

new Weinberg!

http://arxiv.org/abs/hep-th/0511037
Living in the Multiverse
Steven Weinberg
13 pages
UTTG-12-05
"This is the written version of the opening talk at the symposium "Expectations of a Final Theory," at Trinity College, Cambridge, on September 2, 2005. It is to be published in Universe or Multiverse?, ed. B. Carr (Cambridge University Press)."

IIRC Lee Smolin has an essay in that same book. His is called "Scientific Alternatives to the Anthropic Principle" and we've discussed it some at PF

 

[marcus]

http://arxiv.org/abs/gr-qc/0511031
Deformed Special Relativity as an effective theory of measurements on quantum gravitational backgrounds
R. Aloisio, A. Galante, A. Grillo, S. Liberati, E. Luzio, F. Mendez
11 pages
"In this article we elaborate on a recently proposed interpretation of DSR as an effective measurement theory in the presence of non-negligible (albeit small) quantum gravitational fluctuations. We provide several heuristic arguments to explain how such a new theory can emerge and discuss the possible observational consequences of this framework."

 

[marcus]

http://arxiv.org/abs/quant-ph/0511096
A Polynomial Quantum Algorithm for Approximating the Jones Polynomial
Dorit Aharonov, Vaughan Jones, Zeph Landau
26 pages

"The Jones polynmial, discovered in 1984, is an important knot invariant in topology, which is intimately connected to Topological Quantum Field Theory (TQFT). The works of Freedman, Kitaev, Larsen and Wang provide an efficient simulation of TQFT by a quantum computer, and vice versa. These results implicitly imply the existence of an efficient quantum algorithm that provides a certain additive approximation of the Jones polynomial at the fifth root of unity, and moreover, that this problem is BQP-complete. Unfortunately, this important algorithm was never explicitly formulated. Moreover, the results of Freedman et. al are heavily based on deep knowledge of TQFT, which makes the algorithm essentially inaccessible for computer scientists.
We provide an explicit and simple polynomial algorithm to approximate the Jones polynomial of an n strands braid with m crossings at the primitive k'th root of unity, for any k, where the running time of the algorithm is polynomial in m,n and k. Our algorithm does not use TQFT at all. By the results of Freedman et. al, our algorithm solves a BQP complete problem.
The algorithm we provide exhibits a structure which we hope is generalizable to other quantum algorithmic problems. A candidate of particular interest is the approximation of the partition function of the Potts model."

http://arxiv.org/abs/hep-th/0511086
Calabi-Yau Manifolds and the Standard Model
John C. Baez
4 pages
"For any subgroup G of O(n), define a "G-manifold" to be an n-dimensional Riemannian manifold whose holonomy group is contained in G. Then a G-manifold where G is the Standard Model gauge group is precisely a Calabi-Yau manifold of 10 real dimensions whose tangent spaces split into orthogonal 4- and 6-dimensional subspaces..."

quote:"It would be nice to find a use for these results."

either of these two results, of Vaughn Jones and of John Baez, could turn out to be mathematically fertile, and might have consequences for QG

we have a thread about the Baez result
https://www.physicsforums.com/showthread.php?t=99073

 

[selfAdjoint]

we have a thread about the Baez resu

And I see you started one on the Jones polynomial too.

 

[marcus]

NEW BOJOWALD

http://arxiv.org/abs/math-ph/0511043
Effective Equations of Motion for Quantum Systems
Martin Bojowald, Aureliano Skirzewski
29 pages
AEI-2005-169, NI05063

"In many situations, one can approximate the behavior of a quantum system, i.e. a wave function subject to a partial differential equation, by effective classical equations which are ordinary differential equations. A general method and geometrical picture is developed and shown to agree with effective action results, commonly derived through path integration, for perturbations around a harmonic oscillator ground state. The same methods are used to describe dynamical coherent states, which in turn provide means to compute quantum corrections to the symplectic structure of an effective system."

======================================
Interestingly enough although the main Quantum Gravity conference of 2005 was Loops '05 there was another QG conference in September with the abbreviated title of QG '05
that many of us including myself didn't hear about at the time and quite a few Quantum Gravity people attended!

http://www.phy.olemiss.edu/GR/qg05/info.html

There were 101 participants---compared to some 155 at Loops '05 AEI-Potsdam----and it was at a beautiful spot on the island of Sardinia.

Here is a paper that was presented at QG '05---it sounds related to the work of Laurent Freidel and Artem Starodubtsev that we discussed earlier this year.

http://arxiv.org/abs/gr-qc/0511077
Quantum Gravity as a Deformed Topological Quantum Field Theory
A. Mikovic
7 pages, talk presented at the QG05 conference, 12-16 September 2005, Cala Gonone, Italy
"It is known that the Einstein-Hilbert action with a positive cosmological constant can be represented as a perturbation of the SO(4,1) BF theory by a symmetry-breaking term quadratic in the B field. Introducing fermionic matter generates additional terms in the action which are polynomial in the tetrads and the spin connection. We describe how to construct the generating functional in the spin foam formalism for a generic BF theory when the sources for the B and the gauge field are present. This functional can be used to obtain a path integral for General Relativity with matter as a perturbative series whose the lowest order term is a path integral for a topological gravity coupled to matter."

Yeah, Mikovic reference [9] is
[9] L. Freidel and A. Starodubtsev, hep-th/0501191

The odd thing about the naming of the conferences is that John Baez was just saying here at PF that he thought Loops '05 SHOULD HAVE BEEN CALLED QG '05 and that if Renate Loll wanted to host a conference next year at Utrecht she might not want to call it "Loops '06" because that is too specific and CDT is distinct from LQG. But it seems as if some other organization had preempted the tag, so that they could not have used the tag QG '05 anyway regardless of what JB said was better. It doesn't matter, it is just names and abbreviations, who cares about acronyms? But sometimes little things like a grain of sand mess something up.

Well I still hope they have a 2006 quantum gravity conference-----maybe it could be at perimeter and Smolin could call it Loops '06. He interprets LQG very inclusively and actively promotes a many-approaches philosophy.

here is a curious paper, I keep the link because might conceivably turn out useful
http://arxiv.org/abs/hep-th/0511114
Lectures on Fuzzy and Fuzzy SUSY Physics
A. P. Balachandran, S. Kurkcuoglu, S. Vaidya

"fuzzy suzy" what were they thinking? peach-fuzz? hope it was something nice
===============
just stashing this one to check out later:
http://arxiv.org/abs/gr-qc/0511080

 

[Spin_Network]

See you got the recent papers marcus! great stuff.

 

[marcus]

ANOTHER NEW BOJOWALD

http://arxiv.org/abs/gr-qc/0511058
Perturbative Degrees of Freedom in Loop Quantum Gravity: Anisotropies
Martin Bojowald, Hector H. Hernandez, Hugo A Morales-Tecotl
32 pages
AEI-2005-170, NI05064

"The relation between an isotropic and an anisotropic model in loop quantum cosmology is discussed in detail, comparing the strict symmetry reduction with a perturbative implementation of symmetry. While the latter cannot be done in a canonical manner, it allows to consider the dynamics including the role of small non-symmetric degrees of freedom for the symmetric evolution. This serves as a model for the general situation of perturbative degrees of freedom in a background independent quantization such as loop quantum gravity, and for the more complicated addition of perturbative inhomogeneities. While being crucial for cosmological phenomenology, it is shown that perturbative non-symmetric degrees of freedom do not allow definitive conclusions for the singularity issue and in such a situation could even lead to wrong claims."

=================
(viqar husain and oliver winkler gave talks at Loops '05 about what replaces the cosmological singularity, their talks came right after Ashtekar IIRC, not sure what husain is driving at here but want to watch his moves)

http://arxiv.org/abs/hep-th/0511131
Background independent duals of the harmonic oscillator
Viqar Husain
5 pages

"We show that a class of topological field theories are quantum duals of the harmonic oscillator. This is demonstrated by establishing a correspondence between the creation and annihilation operators and non-local gauge invariant observables of the topological field theory. The example is used to discuss some issues concerning background independence and the relation of vacuum energy to the problem of time in quantum gravity."
===========
I can't judge how interesting this next paper is, but list it as a way to keep track of development in LQG research in China

http://arxiv.org/abs/gr-qc/0511084
"Effective Gauge Group of Pure Loop Quantum Gravity is SO(3)"
Chung-Hsien Chou, Yi Ling, Chopin Soo, Hoi-Lai Yu

 

[selfAdjoint]

I haven't read the paper but since Bojowald is the author of the idea that LQG cosmology removes the initial singularity (at the big bang), and now he says that pertubative consideration of anisometries may have untold results on that, it is an important evolution in his thinking. No?

 

[marcus]

...anisometries may have untold results on that, it is an important evolution in his thinking. No?

definitely. we should check it out-----a real-life gravitational collapse seems apt to have significant asymmetries. the issue is whether LQG can handle that or does it break down (IMO nature does not have singularities, it is only theories that break down and give infinities and other physically meaningless results when they are pushed too far)
so this is a way of testing Loop gravity it seems to me. Would you like to start a thread on this, or should I?

BTW I see Bojo cites this:
[27] http://arxiv.org/gr-qc/0506128
and says on page 23

"It is interesting to compare this situation with recent results in string theory [26] where inhomogeneities on a background are seen to prevent the occurence of a bounce instead of a singularity (which would otherwise be possible in the corresponding homogeneous model). This looks similar to our perturbative quantization of anisotropies on an isotropic background, even though the models and techniques are certainly very different. In contrast, non-perturbative background independent models studied so far are non-singular, including inhomogeneous ones which classically have local physical degrees of freedom [27]. There is a further disadvantage of using perturbative or semiclassical degrees of freedom in order to discuss the singularity issue, as for instance suggested in [28]. Such a perturbative treatment is unlikely to remain valid close to a singularity where potentially all degrees of freedom can be excited strongly. Perturbations certainly allow one to include all degrees of freedom which is important for phenomenology, but properties of the singularity can be extremely blurred as we have seen here..."

he seems to be saying that the trouble is not with LQG, but with using it PERTURBATIVELY to deal with a singularity. maybe perturbative approaces don't mix with singularities very well.

the paper he cited dealt with inhomogeneous case, but NON peturbatively======================
new GAMBINI-PULLIN
http://arxiv.org/abs/gr-qc/0511096
Consistent discretization and canonical classical and quantum Regge calculus
Rodolfo Gambini, Jorge Pullin
4 Pages, 4 figures
"We apply the 'consistent discretization' technique to the Regge action for (Euclidean and Lorentzian) general relativity in arbitrary number of dimensions. The result is a well defined canonical theory that is free of constraints and where the dynamics is implemented as a canonical transformation. This provides a framework for the discussion of topology change in canonical quantum gravity. In the Lorentzian case, the framework appears to be naturally free of the 'spikes' that plague traditional formulations. It also provides a well defined recipe for determining the measure of the path integral."
=================
REUTER-RELATED
http://arxiv.org/abs/hep-th/0511177
Further Evidence for a Gravitational Fixed Point
R. Percacci
4 pages
"A theory of gravity with a generic action functional and minimally coupled to N matter fields has a nontrivial fixed point in the leading large N approximation. At this fixed point, the cosmological constant and Newton's constant are nonzero and UV relevant; the curvature squared terms are asymptotically free with marginal behaviour; all higher order terms are irrelevant and can be set to zero by a suitable choice of cutoff function."

 

[marcus]

A third new Bojowald

we've been getting a new Bojowald paper every few days. this is the the third that have been posted lately, since 14 november actually----so three in just the past week.

http://arxiv.org/abs/gr-qc/0511108
Spherically Symmetric Quantum Geometry: Hamiltonian Constraint
Martin Bojowald, Rafal Swiderski
33 pages
AEI-2005-171, NI05065

"Variables adapted to the quantum dynamics of spherically symmetric models are introduced, which further simplify the spherically symmetric volume operator and allow an explicit computation of all matrix elements of the Euclidean and Lorentzian Hamiltonian constraints. The construction fits completely into the general scheme available in loop quantum gravity for the quantization of the full theory as well as symmetric models. This then presents a further consistency check of the whole scheme in inhomogeneous situations, lending further credence to the physical results obtained so far mainly in homogeneous models. New applications in particular of the spherically symmetric model in the context of black hole physics are discussed."

Ooops, make that FOUR Bojo papers appearing in the past 7 days, here is another, this time in the Astronomy-Astrophysics department:

http://arxiv.org/abs/astro-ph/0511557
Universe scenarios from loop quantum cosmology
Martin Bojowald
16 pages, 8 figures, plenary talk at "Pomeranian Workshop in Fundamental Cosmology", Pobierowo, Sep 2005
AEI-2005-168
"Loop quantum cosmology is an application of recent developments for a non-perturbative and background independent quantization of gravity to a cosmological setting. Characteristic properties of the quantization such as discreteness of spatial geometry entail physical consequences for the structure of classical singularities as well as the evolution of the very early universe. While the singularity issue in general requires one to use difference equations for a wave function of the universe, phenomenological scenarios for the evolution are based on effective equations implementing the main quantum modifications. These equations show generic bounces as well as inflation in diverse models, which have been combined to more complicated scenarios."

 

[marcus]

new Martin Reuter, papers by Garrett, by Torsten and Helge

http://arxiv.org/abs/hep-th/0511260
Asymptotic Safety in Quantum Einstein Gravity: nonperturbative renormalizability and fractal spacetime structure
O. Lauscher, M. Reuter
29 pages, latex, 1 figure, invited paper at the Blaubeuren Workshop 2005 on Mathematical and Physical Aspects of Quantum Gravity
MZ-TH/05-26
"The asymptotic safety scenario of Quantum Einstein Gravity, the quantum field theory of the spacetime metric, is reviewed and it is argued that the theory is likely to be nonperturbatively renormalizable. It is also shown that asymptotic safety implies that spacetime is a fractal in general, with a fractal dimension of 2 on sub-Planckian length scales."

=====================
http://arxiv.org/abs/gr-qc/0511120
Clifford bundle formulation of BF gravity generalized to the standard model
A. Garrett Lisi
24 pages
"The structure and dynamics of the standard model and gravity are described by a Clifford valued connection and its curvature."
congratulations.

===============

http://arxiv.org/abs/gr-qc/0511089
Differential Structures - the Geometrization of Quantum Mechanics
Torsten Asselmeyer-Maluga, Helge Rosé
13 pages, 2 figures
"The usual quantization of a classical space-time field does not touch the non-geometrical character of quantum mechanics. We believe that the deep problems of unification of general relativity and quantum mechanics are rooted in this poor understanding of the geometrical character of quantum mechanics. In Einstein's theory gravitation is expressed by geometry of space-time, and the solutions of the field equation are invariant w.r.t. a certain equivalence class of reference frames. This class can be characterized by the differential structure of space-time. We will show that matter is the transition between reference frames that belong to different differential structures, that the set of transitions of the differential structure is given by a Temperley-Lieb algebra which is extensible to a C*-algebra comprising the field operator algebra of quantum mechanics and that the state space of quantum mechanics is the linear space of the differential structures. Furthermore we are able to explain the appearance of the complex numbers in quantum theory. The strong relation to Loop Quantum Gravity is discussed in conclusion."

 

[marcus]

31 dimensionless physical constants

http://arxiv.org/abs/astro-ph/0511774

Dimensionless constants, cosmology and other dark matters
Max Tegmark (MIT), Anthony Aguirre (UCSC), Martin Rees (Cambridge), Frank Wilczek (MIT)
29 pages, 12 figs
"We identify 31 dimensionless physical constants required by particle physics and cosmology, and emphasize that both microphysical constraints and selection effects might help elucidate their origin. Axion cosmology provides an instructive example, in which these two kinds of arguments must both be taken into account, and work well together. If a Peccei-Quinn phase transition occurred before or during inflation, then the axion dark matter density will vary from place to place with a probability distribution. By calculating the net dark matter halo formation rate as a function of all four relevant cosmological parameters and assessing other constraints, we find that this probability distribution, computed at stable solar systems, is arguably peaked near the observed dark matter density. If cosmologically relevant WIMP dark matter is discovered, then one naturally expects comparable densities of WIMPs and axions, making it important to follow up with precision measurements to determine whether WIMPs account for all of the dark matter or merely part of it."

============
http://arxiv.org/abs/astro-ph/0511780
A Quantitative Occam's Razor
Rafael D. Sorkin (Syracuse University)
16 pages
International Journal of Theoretical Physics, 22:1091-1104 (1983)

"This paper derives an objective Bayesian "prior" based on considerations of entropy/information. By this means, it produces a quantitative measure of goodness of fit (the "H-statistic") that balances higher likelihood against the number of fitting parameters employed. The method is intended for phenomenological applications where the underlying theory is uncertain or unknown.
For example, it can help decide whether the large angle anomalies in the CMB data should be taken seriously.
I am therefore posting it now, even though it was published before the arxiv existed."

================
http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez, Urs Schreiber
10 encapsulated Postscript figures
Differential Geometry; Category Theory

"Just as gauge theory describes the parallel transport of point particles using connections on bundles, higher gauge theory describes the parallel transport of 1-dimensional objects (e.g. strings) using 2-connections on 2-bundles. A 2-bundle is a categorified version of a bundle: that is, one where the fiber is not a manifold but a category with a suitable smooth structure. Where gauge theory uses Lie groups and Lie algebras, higher gauge theory uses their categorified analogues: Lie 2-groups and Lie 2-algebras. We describe a theory of 2-connections on principal 2-bundles and explain how this is related to Breen and Messing's theory of connections on nonabelian gerbes. The distinctive feature of our theory is that a 2-connection allows parallel transport along paths and surfaces in a parametrization-independent way. In terms of Breen and Messing's framework, this requires that the "fake curvature" must vanish. In this paper we summarize the main results of our theory without proofs."

 

[marcus]

Abstracts page for the September QG '05 conference

this fall there were TWO major international quantum gravity conferences Loops '05, which was in October at AEI-Golm outside Berlin, and QG '05, which was held in September on the island of Sardinia

the Loops '05 program is here
http://loops05.aei.mpg.de/index_files/Programme.html
and the recorded talks (usually with slides as well) are online
here is the homepage
http://loops05.aei.mpg.de/

there were 156 registered participants of which 11 were from US institutions, by my count.
there is no separate page with all the abstracts assembled together,
but by clicking on the speaker's name in the program you can get the title and abstract of the talk.
This conference has been discussed in several PF threads, including one that John Baez started.
=======================
If only for completeness, we should also compare the other conference QG '05.
http://www.phy.olemiss.edu/GR/qg05/

here is a page listing the conference talks with abstracts:
http://www.phy.olemiss.edu/GR/qg05/abstracts.html

here is the list of participants---it says there were 101:
http://www.phy.olemiss.edu/cgi-bin/qg05/pr_participants.cgi

At this conference, by my count, 72 people gave talks, of whom 7 were from institutions in the USA. A ten percent showing---roughly comparable to what occurred at the other large Quantum Gravity conference: Loops '05.

Here are some samples of the abstracts, to give a taste:

Daniel Terno (dterno@perimeterinstitute.ca)
Thursday, September 15th, 18:10, Parallel session VI: Black holes
Quantum black holes: entropy and entanglement on the horizon
Abstract: Considering a horizon as a surface beyond which no information is accessible we conclude that the spin network states that are associated with it should be globally SU(2) invariant. We derive the Bekenstein-Hawking entropy and the logarithmic correction with the prefactor 3/2, which is independent from the size of the elementary spin that is used in the calculation. The logarithmic correction turns to be equal to the quantum mutual information (total amount of classical and quantum correlations) between parts of the spin network that describes the horizon. We analyze the relation between the microscopic and the macroscopic surface area, when the elementary patches of the surface are coarse-grained. Joint work with Etera Livine.Charles Wang (c.wang@abdn.ac.uk)
Monday, September 12th, 18:10, Parallel session II: Quantum gravity
Towards conformal loop quantum gravity
Abstract: In a recent publication [C. H.-T. Wang, Phys. Rev. D 71, 124026 (2005)], the author has presented a new canonical formulation of GR by extending the ADM phase space to that consisting of York's mean extrinsic curvature time, conformal three-metric and their momenta. In addition to the Hamiltonian and diffeomorphism constraints, the resulting theory contains a new first class constraint, called the conformal constraint. The extended algebra of constraints has as subalgebra the Lie algebra for the conformorphisms of the spatial hypersurface. The structure of the new constraints suggests that conformal metric may be used to formulate the unitary functional evolution of quantum gravity with respect to the York time. This talk will outline a further enlarged phase space of GR by incorporating spin gauge as well as conformal symmetries. Remarkably, a new set of gauge variables for canonical GR is found that is shown to be free from a parameter of the Barbero- Immirzi type due to the inherent conformal invariance of the formalism. A discussion is then given of the prospect of constructing a theory of conformal loop quantum gravity to address both the conceptual problem of time and technical problem of functional calculus in quantum gravity.

Ruth Williams (rmw7@damtp.cam.ac.uk)
Monday, September 12th, 12:00, Plenary session
Discrete quantum gravity
Abstract: Discrete approaches to quantum gravity, including Regge calculus, dynamical triangulations and spin foam models, will be reviewed briefly. A fuller account will be given of recent progress in quantum Regge calculus.

James Ryan (jpr25@cam.ac.uk)
Tuesday, September 13th, 18:10, Parallel session III: Quantum gravity
A group field theory for 3d quantum gravity coupled to a scalar field
Abstract: We present a new group field theory model, which incorporates both 3-dimensional gravity and matter coupled to gravity. We show that the Feynman diagram amplitudes of this model are given by Riemannian quantum gravity spin foam amplitudes coupled to a scalar matter field. We briefly discuss the features of this model and its possible generalisations.

Matej Pavsic (matej.pavsic@ijs.si)
Thursday, September 15th, 17:45, Parallel session V: Gauge theories and quantisation
Spin gauge theory of gravity in Clifford space
Abstract: A theory in which a 16-dimensional curved Clifford space (C-space) provides a realization of Kaluza-Klein theory is investigated. No extra dimensions of spacetime are needed: "extra dimensions" are in C-space. We explore the spin gauge theory in C-space and show that the the generalized spin connection contains the usual 4-dimensional gravity and Yang-Mills fields of the U(1)xSU(2)xSU(3) gauge group. The representation space for the latter group is provided by 16-component generalized spinors composed of four usual 4-component spinors, defined geometrically as the members of four independent left minimal ideals of Clifford algebra. [my comment: note possible contact with Garrett Lisi work ]

Daniele Oriti (d.oriti@damtp.cam.ac.uk)
Monday, September 12th, 17:20, Parallel session II: Quantum gravity
The group field theory approach to quantum gravity
Abstract: We review the basic ideas of the group field theory approach to non-perturbative quantum gravity, a generalisation of matrix models for 2d gravity, that provides a third quantization of gravity in higher spacetime dimensions. We also discuss several recent developments, including the coupling of matter fields to quantum gravity, the implementation of causality, and the definition of different transition amplitudes for these theories.

Aleksandar Mikovic (amikovic@ulusofona.pt)
Monday, September 12th, 16:55, Parallel session II: Quantum gravity
Quantum gravity as a topological quantum field theory
Abstract: In the discretized approaches to Quantum Gravity, like spin foam models, one needs to perform a sum over the spacetime triangulations, or to define a continious limit, in order to impose the diffeomorphism invariance. If the QG theory was a topological theory, then a single triangulation would suffice. We describe an approach to define quantum gravity theory as a topological quantum field theory by using a BF theory.

Fotini Markopoulou (fmarkopoulou@perimeterinstitute.ca)
Friday, September 16th, 9:15, Plenary session
The low energy problem of background-independent quantum gravity
Abstract: We review the main issue facing background-independent approaches to quantum gravity, the low-energy problem. This is the task of extracting general relativity (and possibly also quantum field theory) from a microscopic Planckian theory. We find that, perhaps not surprisingly, the central issue is dynamics. We then approach this problem from a quantum information theoretic perspective. In any such application, the focus has to be on dynamics. We propose ways to do so.

there were several other interesting titles and abstracts that could have been included in this sample but were dropped because the list was getting too long.

 

[marcus]

this fall there were TWO major international quantum gravity conferences Loops '05, which was in October at AEI-Golm outside Berlin, and QG '05, which was held in September on the island of Sardinia
...
...
...
Daniele Oriti (d.oriti@damtp.cam.ac.uk)
Monday, September 12th, 17:20, Parallel session II: Quantum gravity
The group field theory approach to quantum gravity
Abstract: We review the basic ideas of the group field theory approach to non-perturbative quantum gravity, a generalisation of matrix models for 2d gravity, that provides a third quantization of gravity in higher spacetime dimensions. We also discuss several recent developments, including the coupling of matter fields to quantum gravity, the implementation of causality, and the definition of different transition amplitudes for these theories.

...

One sees from the Sardinia conference that Daniele Oriti was giving the GFT overview---essentially substituting for Laurent Freidel. Today he and Etera Livine posted another GFT paper:

http://arxiv.org/abs/gr-qc/0512002
Coupling of spacetime atoms and spin foam renormalisation from group field theory
Etera R. Livine, Daniele Oriti
18 pages
"We study the issue of coupling among 4-simplices in the context of spin foam models obtained from a group field theory formalism. We construct a generalisation of the Barrett-Crane model in which an additional coupling between the normals to tetrahedra, as defined in different 4-simplices that share them, is present. This is realized through an extension of the usual field over the group manifold to a five argument one. We define a specific model in which this coupling is parametrised by an additional real parameter that allows to tune the degree of locality of the resulting model, interpolating between the usual Barrett-Crane model and a flat BF-type one. Moreover, we define a further extension of the group field theory formalism in which the coupling parameter enters as a new variable of the field, and the action presents derivative terms that lead to modified classical equations of motion. Finally, we discuss the issue of renormalisation of spin foam models, and how the new coupled model can be of help regarding this."

==============================

Dan Christensen has been a co-author with John Baez, computing with spinfoams.
he is at UWO (western ontario) where they have a supercomputer center and does both theoretical and computational physics----they developed a fast algorithm for 10j symbols---they can do stuff with spinfoams that is sort of like what Loll does with dynamical triangulations---that is, run them. He also does spinfoam theory. Josh Willis, an Ashtekar Penn State PhD, has gone to postdoc at UWO with Christensen. Dan Cherrington, who gave a paper at Loops '05 is another UWO postdoc.
http://arxiv.org/abs/gr-qc/0512004
Finiteness of Lorentzian 10j symbols and partition functions
J. Daniel Christensen
8 pages
"We give a short and simple proof that the Lorentzian 10j symbol, which forms a key part of the Barrett-Crane model of Lorentzian quantum gravity, is finite. The argument is very general, and applies to other integrals. For example, we show that the Lorentzian and Riemannian causal 10j symbols are finite, despite their singularities. Moreover, we show that integrals that arise in Cherrington's work are finite. Cherrington has shown that this implies that the Lorentzian partition function for a single triangulation is finite, even for degenerate triangulations. Finally, we also show how to use these methods to prove finiteness of integrals based on other graphs and other homogeneous domains."

============================
Here is Charles Wang's paper he referred to in his talk at Sardinia QG '05, and a follow-up by the same author:
http://arxiv.org/abs/gr-qc/0501024
Conformal geometrodynamics: True degrees of freedom in a truly canonical structure
8 pages
Phys.Rev. D71 (2005) 124026

"The standard geometrodynamics is transformed into a theory of conformal geometrodynamics by extending the ADM phase space for canonical general relativity to that consisting of York's mean exterior curvature time, conformal three-metric and their momenta. Accordingly, an additional constraint is introduced, called the conformal constraint. In terms of the new canonical variables, a diffeomorphism constraint is derived from the original momentum constraint. The Hamiltonian constraint then takes a new form. It turns out to be the sum of an expression that previously appeared in the literature and extra terms quadratic in the conformal constraint. The complete set of the conformal, diffeomorphism and Hamiltonian constraints are shown to be of first class through the explicit construction of their Poisson brackets. The extended algebra of constraints has as subalgebras the Dirac algebra for the deformations and Lie algebra for the conformorphism transformations of the spatial hypersurface. This is followed by a discussion of potential implications of the presented theory on the Dirac constraint quantization of general relativity. An argument is made to support the use of the York time in formulating the unitary functional evolution of quantum gravity. Finally, the prospect of future work is briefly outlined."

http://arxiv.org/abs/gr-qc/0507044
Unambiguous spin-gauge formulation of canonical general relativity with conformorphism invariance
4 pages
Phys.Rev. D72 (2005) 087501

"We present a parameter-free gauge formulation of general relativity in terms of a new set of real spin connection variables. The theory is constructed by extending the phase space of the recently formulated conformal geometrodynamics for canonical gravity to accommodate a spin gauge description. This leads to a further enlarged set of first class gravitational constraints consisting of a reduced Hamiltonian constraint and the canonical generators for spin gauge and conformorphism transformations. Owing to the incorporated conformal symmetry, the new theory is shown to be free from an ambiguity of the Barbero-Immirzi type."

here is Charles Wang homepage---he has a remarkable set of research interests and accomplishments---check this out:
http://www.lancs.ac.uk/depts/physics/staff/chtw.htm
He is now at Aberdeen---the page was from 2004 when he was at Lancaster

 

[marcus]

============================
Here is Charles Wang's paper he referred to in his talk at Sardinia QG '05, and a follow-up by the same author:
http://arxiv.org/abs/gr-qc/0501024
Conformal geometrodynamics: True degrees of freedom in a truly canonical structure
8 pages
Phys.Rev. D71 (2005) 124026

"The standard geometrodynamics is transformed into a theory of conformal geometrodynamics by extending the ADM phase space for canonical general relativity to that consisting of York's mean exterior curvature time, conformal three-metric and their momenta. Accordingly, an additional constraint is introduced, called the conformal constraint. In terms of the new canonical variables, a diffeomorphism constraint is derived from the original momentum constraint. The Hamiltonian constraint then takes a new form. It turns out to be the sum of an expression that previously appeared in the literature and extra terms quadratic in the conformal constraint. The complete set of the conformal, diffeomorphism and Hamiltonian constraints are shown to be of first class through the explicit construction of their Poisson brackets. The extended algebra of constraints has as subalgebras the Dirac algebra for the deformations and Lie algebra for the conformorphism transformations of the spatial hypersurface. This is followed by a discussion of potential implications of the presented theory on the Dirac constraint quantization of general relativity. An argument is made to support the use of the York time in formulating the unitary functional evolution of quantum gravity. Finally, the prospect of future work is briefly outlined."

http://arxiv.org/abs/gr-qc/0507044
Unambiguous spin-gauge formulation of canonical general relativity with conformorphism invariance
4 pages
Phys.Rev. D72 (2005) 087501

"We present a parameter-free gauge formulation of general relativity in terms of a new set of real spin connection variables. The theory is constructed by extending the phase space of the recently formulated conformal geometrodynamics for canonical gravity to accommodate a spin gauge description. This leads to a further enlarged set of first class gravitational constraints consisting of a reduced Hamiltonian constraint and the canonical generators for spin gauge and conformorphism transformations. Owing to the incorporated conformal symmetry, the new theory is shown to be free from an ambiguity of the Barbero-Immirzi type."

here is Charles Wang homepage---he has a remarkable set of research interests and accomplishments---check this out:
http://www.lancs.ac.uk/depts/physics/staff/chtw.htm
He is now at Aberdeen---the page was from 2004 when he was at Lancaster

Today Charles H-T Wang posted another paper:
http://arxiv.org/abs/gr-qc/0512023
Towards conformal loop quantum gravity
Charles H.-T. Wang
6 pages, 1 figure, Talk given at Constrained Dynamics and Quantum Gravity 05, Cala Gonone, Sardinia, Italy, 12-16 September 2005
A discussion is given of recent developments in canonical gravity that assimilates the conformal analysis of gravitational degrees of freedom. The work is motivated by the problem of time in quantum gravity and is carried out at the metric and the triad levels. At the metric level, it is shown that by extending the Arnowitt-Deser-Misner (ADM) phase space of general relativity (GR), a conformal form of geometrodynamics can be constructed. In addition to the Hamiltonian and diffeomorphism constraints, an extra first class constraint is introduced to generate conformal transformations. This phase space consists of York's mean extrinsic curvature time, conformal three-metric and their momenta. At the triad level, the phase space of GR is further enlarged by incorporating spin-gauge as well as conformal symmetries. This leads to a canonical formulation of GR using a new set of real spin connection variables. The resulting gravitational constraints are first class, consisting of the Hamiltonian constraint and the canonical generators for spin-gauge and conformorphism transformations. The formulation has a remarkable feature of being parameter-free. Indeed, it is shown that a conformal parameter of the Barbero-Immirzi type can be absorbed by the conformal symmetry of the extended phase space. This gives rise to an alternative approach to loop quantum gravity that addresses both the conceptual problem of time and the technical problem of functional calculus in quantum gravity."

this guy is a dark horse. I would appreciate help evaluating this work if anyone has any ideas.

 

[marcus]

http://arxiv.org/abs/hep-th/0512033
Thermal gravity, black holes and cosmological entropy
Stephen D. H. Hsu, Brian M. Murray
5 pages, 2 figures

"Taking seriously the interpretation of black hole entropy as the logarithm of the number of microstates, we argue that thermal gravitons may undergo a phase transition to a kind of black hole condensate. The phase transition proceeds via nucleation of black holes at a rate governed by a saddlepoint configuration whose free energy is of order the inverse temperature in Planck units. Whether the universe remains in a low entropy state as opposed to the high entropy black hole condensate depends sensitively on its thermal history. Our results may clarify an old observation of Penrose regarding the very low entropy state of the universe."

Steve Hsu's blog is Information Processing. It is a real good blog.
He also collaborated with Zee on a fun paper called "A Message in the Sky"

New Witten paper
http://arxiv.org/abs/hep-th/0512039

New Arivero paper
http://arxiv.org/abs/hep-ph/0512065

 

[marcus]

Oriti: intro to the Group Field Theory approach to QG

Oriti presented this at the QG '05 conference

http://arxiv.org/abs/gr-qc/0512048
Quantum gravity as a group field theory: a sketch
Daniele Oriti
8 pages, 9 figures; to appear in the Proceedings of the Fourth Meeting on Constrained Dynamics and Quantum Gravity, Cala Gonone, Italy, September 12-16, 2005
DAMTP-2005-123

"We give a very brief introduction to the group field theory approach to quantum gravity, a generalisation of matrix models for 2-dimensional quantum gravity to higher dimension, that has emerged recently from research in spin foam models."

 

[marcus]

another new paper by Oriti
http://arxiv.org/abs/gr-qc/0512069
Generalised group field theories and quantum gravity transition amplitudes
Daniele Oriti
6 pages, 2 figures
DAMTP-2005-127
"We construct a generalised formalism for group field theories, in which the domain of the field is extended to include additional proper time variables, as well as their conjugate mass variables. This formalism allows for different types of quantum gravity transition amplitudes in perturbative expansion, and we show how both causal spin foam models and the usual a-causal ones can be derived from it, within a sum over triangulations of all topologies. We also highlight the relation of the so-derived causal transition amplitudes with simplicial gravity actions."

Oriti is the editor of a book Cambridge University Press has scheduled to bring out in 2006, and here is one of the chapters (contributed by Gambini and Pullin)
according to Oriti, the title of the new book is:
Towards quantum gravity: different approaches to a new understanding of space and time Cambridge University Press (2006); but Gambini and Pullin mention a trivially different title.

http://arxiv.org/abs/gr-qc/0512065
Consistent discretizations as a road to quantum gravity
Rodolfo Gambini, Jorge Pullin
Comments: 17 Pages, Draft chapter contributed to the book "Approaches to quantum gravity", being prepared by Daniele Oriti for Cambridge University Press
LSU-REL-121105
"We present a brief description of the ``consistent discretization'' approach to classical and quantum general relativity. We exhibit a classical simple example to illustrate the approach and summarize current classical and quantum applications. We also discuss the implications for the construction of a well defined quantum theory and in particular how to construct a quantum continuum limit."

a new paper by Freidel and Livine appeared to day:
http://arxiv.org/abs/hep-th/0512113
Effective 3d Quantum Gravity and Non-Commutative Quantum Field Theory
Laurent Freidel, Etera R. Livine
9 pages, Proceeding of the conference "Quantum Theory and Symmetries 4" 2005 (Varna, Bulgaria)
"We show that the effective dynamics of matter fields coupled to 3d quantum gravity is described after integration over the gravitational degrees of freedom by a braided non-commutative quantum field theory symmetric under a kappa-deformation of the Poincaré group."

a new paper by Jerzy Kowalski-Glikman and others
http://arxiv.org/abs/hep-th/0512107
The Free Particle in Deformed Special Relativity
F. Girelli, T. Konopka, J. Kowalski-Glikman, E.R. Livine
15 pages
"The phase space of a classical particle in DSR contains de Sitter space as the space of momenta. We start from the standard relativistic particle in five dimensions with an extra constraint and reduce it to four dimensional DSR by imposing appropriate gauge fixing. We analyze some physical properties of the resulting theories like the equations of motion, the form of Lorentz transformations and the issue of velocity. We also address the problem of the origin and interpretation of different bases in DSR."

 

[marcus]

http://arxiv.org/abs/gr-qc/0512072
Quantum information in loop quantum gravity
Daniel R. Terno
4 pages. Proceedings of QG'05, Cala Gonone, 2005

"A coarse-graining of spin networks is expressed in terms of partial tracing, thus allowing to use tools of quantum information theory. This is illustrated by the analysis of a simple black hole model, where the logarithmic correction of the Hawking-Bekenstein entropy is shown to be equal to the total amount of correlations on the horizon. Finally other applications of entanglement to quantum gravity are briefly discussed."

 

[marcus]

encyclopedic resource for quantum information and computing

http://arxiv.org/abs/quant-ph/0512125
Quantum information and computation
Jeffrey Bub
103 pages, no figures. Forthcoming as a chapter in Handbook of Philosophy of Physics, edited by John Earman and Jeremy Butterfield (Elsevier/NH)

"This article deals with theoretical developments in the subject of quantum information and quantum computation, and includes an overview of classical information and some relevant quantum mechanics. The discussion covers topics in quantum communication, quantum cryptography, and quantum computation, and concludes by considering whether a perspective in terms of quantum information sheds new light on the conceptual problems of quantum mechanics."

given that Daniel Terno is collaborating on LQG with Eteral Livine at Perimeter (where there are a lot of Q. information and computing people as well as QG) we may eventually need reference material in this area. Scott Aaronson, known for his blog among other things, is another QI at Waterloo. At first sight this source seems comprehensive and not too hard. Anyone have comments?

of possible interest
http://arxiv.org/abs/hep-th/0512197
Eric D'Hoker, D.H. Phong
http://arxiv.org/abs/hep-th/0512200
Observables in effective gravity
Steven B. Giddings, Donald Marolf, James B. Hartle
http://arxiv.org/abs/hep-th/0512201
Holography and entropy bounds in the plane wave matrix model
Raphael Bousso, Aleksey L. Mintshttp://arxiv.org/abs/hep-th/0512210
2D Ising Model with non-local links - a study of non-locality
Yidun Wan
4 pages, 6 figures
"Markopoulou and Smolin have argued that the low energy limit of LQG may suffer from a conflict between locality, as defined by the connectivity of spin networks, and an averaged notion of locality that emerges at low energy from a superposition of spin network states. This raises the issue of how much non-locality, relative to the coarse grained metric, can be tolerated in the spin network graphs that contribute to the ground state. To address this question we have been studying statistical mechanical systems on lattices decorated randomly with non-local links. These turn out to be related to a class of recently studied systems called small world networks. We show, in the case of the 2D Ising model, that one major effect of non-local links is to raise the Curie temperature. We report also on measurements of the spin-spin correlation functions in this model and show, for the first time, the impact of not only the amount of non-local links but also of their configuration on correlation functions."

Yidun Wan's "Perimeter name" is Eaton Wan. He gave a talk at Loops '05. Smolin in his talk cited Eaton's results
====================

http://arxiv.org/abs/gr-qc/0512103
Quantum Gravity as a quantum field theory of simplicial geometry
Daniele Oriti
23 pages, 13 figures; to be published in 'Mathematical and Physical Aspects of Quantum Gravity', B. Fauser, J. Tolksdorf and E. Zeidler eds, Birkhaeuser, Basel (2006)
"This is an introduction to the group field theory approach to quantum gravity, with emphasis on motivations and basic formalism, more than on recent results; we elaborate on the various ingredients, both conceptual and formal, of the approach, giving some examples, and we discuss some perspectives of future developments."

=======================

http://arxiv.org/abs/gr-qc/0512102
Towards the graviton from spinfoams: the 3d toy model
Simone Speziale
7 pages, 2 figures
"Recently, a proposal has appeared for the extraction of the 2-point function of linearised quantum gravity, within the spinfoam formalism. This relies on the use of a boundary state, which introduces a semi-classical flat geometry on the boundary. In this paper, we investigate this proposal considering a toy model in the (Riemannian) 3d case, where the semi-classical limit is better understood. We show that in this limit the propagation kernel of the model is the one for the harmonic oscillator. This is at the origin of the expected 1/L behaviour of the 2-point function. Furthermore, we numerically study the short scales regime, where deviations from this behaviour occur."

 

[marcus]

I don't know anything of either author. Navarro is at Cambridge. I will flag this and watch for future papers.

http://arxiv.org/abs/gr-qc/0512109
Modified gravity, Dark Energy and MOND
Ignacio Navarro, Karel Van Acoleyen
24 pages, 2 figures
DAMTP-2005-129, DCPT/05/154, IPPP/05/77

"We propose a class of actions for the spacetime metric that introduce corrections to the Einstein-Hilbert Lagrangian depending on the logarithm of some curvature scalars. We show that for some choices of these invariants the models are ghost free and modify Newtonian gravity below a characteristic acceleration scale given by a_0 = c\mu, where c is the speed of light and \mu is a parameter of the model that also determines the late-time Hubble constant: H_0 \sim \mu. In these models, besides the massless spin two graviton, there is a scalar excitation of the spacetime metric whose mass depends on the background curvature. This dependence is such that this scalar, although almost massless in vacuum, becomes massive and effectively decouples when one gets close to any source and we recover an acceptable weak field limit at short distances. There is also a (classical) 'running' of Newton's constant with the distance to the sources and gravity is easily enhanced at large distances by a large ratio. We comment on the possibility of building a model with a MOND-like Newtonian limit that could explain the rotation curves of galaxies without introducing Dark Matter using this kind of actions. We also explore briefly the characteristic gravitational phenomenology that these models imply: besides a long distance modification of gravity they also predict deviations from Newton's law at short distances. This short distance scale depends on the local background curvature of spacetime, and we find that for experiments on the Earth surface it is of order \sim 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower."

They cite REUTER work (renormalizable QG) as their reference [33] in this passage on page 21

"...there is a second effect in these theories: the Planck mass that controls the coupling strength of the massless graviton also undergoes a rescaling or 'running' with the distance to the sources (or the background curvature). This phenomenon, although a purely classical one in our theory, is reminiscent of the quantum renormalisation group running of couplings. So one might wonder if actions of the type (15) could be an effective classical description of strong renormalisation effects in the infrared that might appear in GR (see e.g. [33] and references therein), as happens in QCD. In fact, corrections depending on the logarithm of the renormalisation scale are ubiquitous in quantum field theory,.."

=======================
this Utrecht master's thesis was flagged by John Baez in TWF #224
I like the way it is written----by a person who gets a kick out of writing clearly and finding the simple way to understand something complex. Baez says he's looking forward to this guy's PhD thesis

http://arxiv.org/abs/math.QA/0512103
Categorical Aspects of Topological Quantum Field Theories
Bruce H. Bartlett
M.Sc Thesis, Utrecht University, 2005. 111 pages, numerous pictures. Supervisors : Dr. S. Vandoren, Prof. I. Moerdijk

"This thesis provides an introduction to the various category theory ideas employed in topological quantum field theory. These theories are viewed as symmetric monoidal functors from topological cobordism categories into the category of vector spaces. In two dimensions, they are classified by Frobenius algebras. In three dimensions, and under certain conditions, they are classified by modular categories. These are special kinds of categories in which topological notions such as braidings and twists play a prominent role. There is a powerful graphical calculus available for working in such categories, which may be regarded as a generalization of the Feynman diagrams method familiar in physics. This method is introduced and the necessary algebraic structure is graphically motivated step by step.
A large subclass of two-dimensional topological field theories can be obtained from a lattice gauge theory construction using triangulations. In these theories, the gauge group is finite. This construction is reviewed, from both the original algebraic perspective as well as using the graphical calculus developed in the earlier chapters.
This finite gauge group toy model can be defined in all dimensions, and has a claim to being the simplest non-trivial quantum field theory. We take the opportunity to show explicitly the calculation of the modular category arising from this model in three dimensions, and compare this algebraic data with the corresponding data in two dimensions, computed both geometrically and from triangulations. We use this as an example to introduce the idea of a quantum field theory as producing a tower of algebraic structures, each dimension related to the previous by the process of categorification."

======================
In the current conversation at Woit blog concerning Cosmological Natural Selection (CNS) Smolin cited this paper as a marginal aside in response to someone's question:
http://arxiv.org/gr-qc/0510052
Geometry from quantum particles
David W. Kribs, Fotini Markopoulou
17 pages
"We investigate the possibility that a background independent quantum theory of gravity is not a theory of quantum geometry. We provide a way for global spacetime symmetries to emerge from a background independent theory without geometry. In this, we use a quantum information theoretic formulation of quantum gravity and the method of noiseless subsystems in quantum error correction. This is also a method that can extract particles from a quantum geometric theory such as a spin foam model."

the CNS discussion is transcribed here:
https://www.physicsforums.com/showthread.php?p=862787#post862787

 

[marcus]

Freidel and Majid

http://arxiv.org/abs/hep-th/0601004

Noncommutative Harmonic Analysis, Sampling Theory and the Duflo Map in 2+1 Quantum Gravity
L. Freidel, S. Majid
"54 pages, 2 figs
We show that the $\star$-product for $U(su_2)$ arising in \cite{EL} in an effective theory for the Ponzano-Regge quantum gravity model is compatible with the noncommutative bicovariant differential calculus previously proposed for 2+1 Euclidean quantum gravity using quantum group methods in \cite{BatMa}. We show that the effective action for this model essentially agrees with the noncommutative scalar field theory coming out of the noncommutative differential geometry. We show that the required Fourier transform essentially agrees with the previous quantum group Fourier transform. In combining these methods we develop practical tools for noncommutative harmonic analysis for the model including radial quantum delta-functions and Gaussians, the Duflo map and elements of `noncommutative sampling theory' applicable to the bounded $SU_2,SO_3$ momentum groups. This allows us to understand the bandwidth limitation in 2+1 quantum gravity arising from the bounded momentum. We also argue that the the anomalous extra `time' dimension seen in the noncommutative differential geometry should be viewed as the renormalisation group flow visible in the coarse graining in going from $SU_2$ to $SO_3$. Our methods also provide a generalised twist operator for the $\star$-product."

http://arxiv.org/abs/hep-th/0601001
The String Landscape, Black Holes and Gravity as the Weakest Force
Nima Arkani-Hamed, Lubos Motl, Alberto Nicolis, Cumrun Vafa
20 pages, 5 figures

http://arxiv.org/abs/math-ph/0601005
Construction of Generalized Connections
Christian Fleischhack
12 pages
"We present a construction method for mappings between generalized connections, comprising, e.g., the action of gauge transformations, diffeomorphisms and Weyl transformations. Moreover, criteria for continuity and measure preservation are stated."

 

[marcus]

back in early December (post #429) I flagged this

...a new paper by Jerzy Kowalski-Glikman and others
http://arxiv.org/abs/hep-th/0512107
The Free Particle in Deformed Special Relativity
F. Girelli, T. Konopka, J. Kowalski-Glikman, E.R. Livine
15 pages
"The phase space of a classical particle in DSR contains de Sitter space as the space of momenta. We start from the standard relativistic particle in five dimensions with an extra constraint and reduce it to four dimensional DSR by imposing appropriate gauge fixing. We analyze some physical properties of the resulting theories like the equations of motion, the form of Lorentz transformations and the issue of velocity. We also address the problem of the origin and interpretation of different bases in DSR."

now I see that this gives a helpful perspective on the work of Freidel by people who are not Freidel. It is an outside perspective that can begin to sum up how they see his line of research going (and theirs in relation to it.)

---quote from conclusions---
In this paper, we have studied a classical particle in five space-time dimensions subject to two constraints defining two energy scales m and kappa. We have shown that, after gauge fixing, the 5d model can give rise to various DSR models in 4d. The reduction from 5d to 4d selects a set of phase space coordinates (x, p) via the requirement that they should commute with both the kappa-shell constraint H_5d and the gauge fixing function C.
...
...
In three space-time dimensions, the link between DSR and gravity has been clarified in [7]. Indeed, in 3d quantum gravity, particles are identified as conical singularities and their momentum is defined through non-local measurements as (a function of) the holonomy around the particle. This explicit characterization allows to rigorously derive DSR from 3d quantum gravity and unambiguously compute the Feynman diagrams for the resulting non-commutative quantum field theory [7].

There is also a proposal attempting to move the similarity between DSR and GR to the level of an explicit relationship in four dimensions [27]. In that proposal, the choice of coordinates p_mu (and x_mu) correspond to the definition of the measured momenta (and positions) in terms of the tetrad field e^I_mu. The issue then becomes: what are we exactly measuring physically when we talk about the energy-momentum p_mu? The answer to this question will determine the “correct” choice of physical coordinates to use in DSR. Regardless, we expect the physical predictions of DSR to be independent of any gauge fixing choice and propose that the “correctness” of a particular choice of coordinates should be measured by how convenient these coordinates are to express the measurements of a particular observer. For instance, one could try to properly define length measurements using clocks and time-of-flight experiments to define the metric operationally.

At the end of the day, we cannot make concrete predictions using DSR as long as we do not find gauge invariant quantities (commuting with the two constraints of the 5d action) and their physical interpretation, or equivalently an explicit link between the choices of gauge fixing and measurement. This avenue of research seems to be a natural one from the 5d perspective. It is also our view that the 5d perspective should be a used when looking at twoparticle systems and studying their properties. Other related topics to be investigated are free spinning particles.

Finally, an important unresolved issue regards the physical interpretation of the fifth dimension. Written as a 5d theory, DSR appears as a large extra dimension theory. We have proposed to see the coordinates in the fifth dimension as some effective degree of freedom coming from quantum gravity. The reformulation of GR as a SO(4, 1) BF gauge field theory proposed in [21] may prove to be a guide in this direction. It is also very tempting to interpret P_4 as the energy scale in a renormalisation scheme, as some kind of dynamical cut-off. X_4 would then be the generator of scale transformations. Such a speculation is supported by the fact that X_4 is (more or less) the 4d dilatation operator in the Snyder basis, but this is truly little evidence. One could look at the renormalisation equation of a scalar field and try to interpret them as equations of motion in the DSR framework. The potential link between DSR and quantum gravity and the fact that the renormalisation flow of general relativity can be associated to a fifth dimension (with an AdS metric) [29] also points toward such an interpretation.
---endquote---

what is emerging is some interconnected treatment of spacetime dynamics, matter, and DSR----in this Kolwalski-Glikman paper they are dealing with the flat DSR limit, but in the context of Freidel papers on spacetime dynamics. especially his seemingly successful treatment of the 3D case. this paper is evidently part of a combined initiative by several people to proceed to the 4D case.

BTW we should watch for possible observational tests of QG. GLAST has been discussed in this context and is scheduled for orbit next year. Also Auger (OH-ZHAY)
which is now beginning to report
http://arxiv.org/abs/astro-ph/0601035
The First Scientific Results from the Pierre Auger Observatory
T. Yamamoto (for The Pierre Auger Observatory Collaboration)
4 pages, 1 figure, Proceedings of the PANIC 2005 conference

"The southern site of the Pierre Auger Observatory is under the construction near Malargue in Argentina and now more than 60% of the detectors are completed. The observatory has been collecting data for over 1 year and the cumulative exposure is already similar to that of the largest forerunner experiments. The hybrid technique provides model-independent energy measurements from the Fluorescence Detector to calibrate the Surface Detector. Based on this technique, the first estimation of the energy spectrum above 3 EeV has been presented and is discussed in this paper."

Smolin has a paper "Falsifiable..." describing how Auger and GLAST may be able to distinguish between certain approaches to QG, and test some assumptions.

 

[marcus]

Carlip is an important figure. In case of interest:
http://arxiv.org/gr-qc/0601041
Horizons, Constraints, and Black Hole Entropy
S. Carlip
16 pages, talk given at the "Peyresq Physics 10 Meeting on Micro and Macro structures of spacetime"

"Black hole entropy appears to be "universal''--many independent calculations, involving models with very different microscopic degrees of freedom, all yield the same density of states. I discuss the proposal that this universality comes from the behavior of the underlying symmetries of the classical theory. To impose the condition that a black hole be present, we must partially break the classical symmetries of general relativity, and the resulting Goldstone boson-like degrees of freedom may account for the Bekenstein-Hawking entropy. In particular, I demonstrate that the imposition of a "stretched horizon'' constraint modifies the algebra of symmetries at the horizon, allowing the use of standard conformal field theory techniques to determine the asymptotic density of states. The results reproduce the Bekenstein-Hawking entropy without any need for detailed assumptions about the microscopic theory."

Mattingly takes off from Carlip's result
http://arxiv.org/gr-qc/0601044
On horizon constraints and Hawking radiation
David Mattingly
"Questions about black holes in quantum gravity generally presuppose the presence of a horizon. Recently Carlip has shown that enforcing an initial data surface to be a horizon leads to the correct form for the Bekenstein-Hawking entropy of the black hole. Requiring a horizon also constitutes fixed background geometry, which generically leads to non-conservation of the matter stress tensor at the horizon. In this work, I show that the generated matter energy flux for a Schwarzschild black hole is in agreement with the first law of black hole thermodynamics, 8 pi G Delta Q = kappa Delta A."

In case anyone is wondering whether gravitons can be detected (Freeman Dyson said not)
http://arxiv.org/gr-qc/0601043
Can Gravitons Be Detected?
Tony Rothman, Stephen Boughn
21 pages, no figures. To be submitted to AJP
"Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson's question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson's conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production."

----------------------as an afterthought-------------------
Two others looked as if they might be interesting as well:

http://arxiv.org/astro-ph/0601219
Constraining Lorentz violations with Gamma Ray Bursts
Maria Rodriguez Martinez, Tsvi Piran
16 pages, 4 figures
"Gamma ray bursts are excellent candidates to constrain physical models which break Lorentz symmetry. We consider deformed dispersion relations which break the boost invariance and lead to an energy-dependent speed of light. In these models, simultaneously emitted photons from cosmological sources reach Earth with a spectral time delay that depends on the symmetry breaking scale. We estimate the possible bounds which can be obtained by comparing the spectral time delays with the time resolution of available telescopes. We discuss the best strategy to reach the strongest bounds. We compute the probability of detecting bursts that improve the current bounds. The results are encouraging. Depending on the model, it is possible to build a detector that within several years will improve the present limits of 0.015 m_pl."

http://arxiv.org/astro-ph/0601247
Alternative proposal to modified Newton dynamics (MOND)
Juan M. Romero, Adolfo Zamora
4 pages. Accepted for publication in PRD
"From a study of conserved quantities of the so-called Modified Newtonian Dynamics (MOND) we propose an alternative to this theory. We show that this proposal is consistent with the Tully-Fisher law, has conserved quantities whose Newtonian limit are the energy and angular momentum, and can be useful to explain cosmic acceleration. The dynamics obtained suggests that, when acceleration is very small, time depends on acceleration. This result is analogous to that of special relativity where time depends on velocity."

note that this paper has been accepted for publication in Physical Review Series D.

 

[marcus]

a string theorist assesses Loop and Spinfoam Gravity (for nonspecialists)

http://arxiv.org/abs/hep-th/0601129
Loop and spin foam quantum gravity: a brief guide for beginners
Hermann Nicolai, Kasper Peeters
18 pages, 11 figures; Contributed article to "An assessment of current paradigms in theoretical physics"
Report-no: AEI-2006-004
"We review aspects of loop quantum gravity and spin foam models at an introductory level, with special attention to questions frequently asked by non-specialists."

Nicolai earlier co-authored an "outsider's view" of LQG but did not discuss recent work (e.g. in the past 5 years) and omitted spinfoam.
So the view had some problems---Lee Smolin replied to Nicolai politely and Peter Woit published the letter.

I don't know how this one is going to play out. Basically it is great of Nicolai, as a string theorist, to take an interest in alternatives like Loop and Spinfoam. It is potentially really constructive.
====================
the next is by two authors who are not familiar to me:

http://arxiv.org/abs/hep-th/0601127
Intersecting Connes Noncommutative Geometry with Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup
19 pages, 4 figures
NORDITA-2006-1
An intersection of Noncommutative Geometry and Loop Quantum Gravity is proposed. Alain Connes' Noncommutative Geometry provides a framework in which the Standard Model of particle physics coupled to general relativity is formulated as a unified, gravitational theory. However, to this day no quantization procedure compatible with this framework is known. In this paper we consider the noncommutative algebra of holonomy loops on a functional space of certain spin-connections. The construction of a spectral triple is outlined and ideas on interpretation and classical limit are presented.

 

[marcus]

http://arxiv.org/abs/gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at "Living Reviews":
http://relativity.livingreviews.org/Articles/lrr-2005-11/

AEI-2005-185, IGPG-06/1-6
Journal-ref: Living Rev. Relativity 8 (2005) 11

"Quantum gravity is expected to be necessary in order to understand situations where classical general relativity breaks down. In particular in cosmology one has to deal with initial singularities, i.e. the fact that the backward evolution of a classical space-time inevitably comes to an end after a finite amount of proper time. This presents a breakdown of the classical picture and requires an extended theory for a meaningful description. Since small length scales and high curvatures are involved, quantum effects must play a role. Not only the singularity itself but also the surrounding space-time is then modified. One particular realization is loop quantum cosmology, an application of loop quantum gravity to homogeneous systems, which removes classical singularities. Its implications can be studied at different levels. Main effects are introduced into effective classical equations which allow to avoid interpretational problems of quantum theory. They give rise to new kinds of early universe phenomenology with applications to inflation and cyclic models. To resolve classical singularities and to understand the structure of geometry around them, the quantum description is necessary. Classical evolution is then replaced by a difference equation for a wave function which allows to extend space-time beyond classical singularities. One main question is how these homogeneous scenarios are related to full loop quantum gravity, which can be dealt with at the level of distributional symmetric states. Finally, the new structure of space-time arising in loop quantum gravity and its application to cosmology sheds new light on more general issues such as time."

To get the movies, go to the Living Reviews version
http://relativity.livingreviews.org/Articles/lrr-2005-11/
and scroll down the sidebar menu all the way to the bottom where it says "figures"

========================

Also in today's arxiv postings:
http://arxiv.org/abs/gr-qc/0601082
Quantum Hamiltonian for gravitational collapse

Viqar Husain, Oliver Winkler
17 pages

"Using a Hamiltonian formulation of the spherically symmetric gravity-scalar field theory adapted to flat spatial slicing, we give a construction of the reduced Hamiltonian operator. This Hamiltonian, together with the null expansion operators presented in an earlier work, form a framework for studying gravitational collapse in quantum gravity. We describe a setting for its numerical implementation, and discuss some conceptual issues associated with quantum dynamics in a partial gauge fixing."

============================

Lee Smolin thinks that if MOND is real then it may have an explanation in quantum gravity. We should keep an eye on MOND research, just in case.
Here is an overview for beginners. Good place to start if you want to learn something about MOND.

http://arxiv.org/abs/astro-ph/0601478
Modified Newtonian Dynamics, an Introductory Review
Riccardo Scarpa
"By the time, in 1937, the Swiss astronomer Zwicky measured the velocity dispersion of the Coma cluster of galaxies, astronomers somehow got acquainted with the idea that the universe is filled by some kind of dark matter. After almost a century of investigations, we have learned two things about dark matter, (i) it has to be non-baryonic -- that is, made of something new that interact with normal matter only by gravitation-- and, (ii) that its effects are observed in stellar systems when and only when their internal acceleration of gravity falls below a fix value a0=1.2x10-8 cm s-2. This systematic, more than anything else, tells us we might be facing a failure of the law of gravity in the weak field limit rather then the effects of dark matter. Thus, in an attempt to avoid the need for dark matter, the Modified Newtonian Dynamics. MOND posits a breakdown of Newton's law of gravity (or inertia) below a0, after which the dependence with distance became linear. Despite many attempts, MOND resisted stubbornly to be falsified as an alternative to dark matter and succeeds in explaining the properties of an impressively large number of objects without invoking the presence of non-baryonic dark matter. In this paper, I will review the basics of MOND and its ability to explain observations without the need of dark matter."

=====================
Of possible interest to the category-minded:
http://arxiv.org/abs/math.QA/0601458
Categorified Algebra and Quantum Mechanics
Jeffrey Morton (University of California, Riverside)
67 pages, 25 figures

Jeffrey Morton has studied Quantum Gravity with John Baez. Here is what he says in the acknowledgments section

"This work grew out of the regular Quantum Gravity seminar taught by John Baez at UCR, notes for which are available online as [2]. I would like to acknowledge his work on this subject (some published as [1]), excellent teaching, and helpful advice and discussions in preparing this paper. Other students in the seminar, especially Toby Bartels, Miguel Carrion-Alvarez, Alissa Crans, and Derek Wise also provided many useful discussions."

We know Miguel and Derek as Baez QG students. Miguel recently finished his thesis and Derek gave a paper at Loops '05. So this comes out of that group, although it is not specifically about gravity.

 

[marcus]

Perez on Spin Foam, a chapter for Oriti's book

http://arxiv.org/abs/gr-qc/0601095
The spin-foam-representation of loop quantum gravity

Alejandro Perez

Draft chapter contributed to the book "Towards quantum gravity", being prepared by Daniele Oriti for Cambridge University Press. 19 pages

"The problem of background independent quantum gravity is the problem of defining a quantum field theory of matter and gravity in the absence of an underlying background geometry. Loop quantum gravity (LQG) is a promising proposal for addressing this difficult task. Despite the steady progress of the field, dynamics remains to a large extend an open issue in LQG. Here we present the main ideas behind a series of proposals for addressing the issue of dynamics. We refer to these constructions as the spin foam representation of LQG. This set of ideas can be viewed as a systematic attempt at the construction of the path integral representation of LQG.
The spin foam representation is mathematically precise in 2+1 dimensions, so we will start this chapter by showing how it arises in the canonical quantization of this simple theory. This toy model will be used to precisely describe the true geometric meaning of the histories that are summed over in the path integral of generally covariant theories.
In four dimensions similar structures appear. We call these constructions spin foam models as their definition is incomplete in the sense that at least one of the following issues remains unclear: 1) the connection to a canonical formulation, and 2) regularization independence (renormalizability). In the second part of this chapter we will describe the definition of these models emphasizing the importance of these open issues. We also discuss the non standard picture of quantum spacetime that follows from background independence.

 

[marcus]

several MOND articles recently, in post #437 we saw this

http://arxiv.org/abs/astro-ph/0601478
Modified Newtonian Dynamics, an Introductory Review
Riccardo Scarpa
"...This systematic, more than anything else, tells us we might be facing a failure of the law of gravity in the weak field limit rather then the effects of dark matter... In this paper, I will review the basics of MOND and its ability to explain observations without the need of dark matter."

now this has appeared
http://arxiv.org/abs/hep-th/0601213
Introduction to Modified Gravity and Gravitational Alternative for Dark Energy
S. Nojiri, S.D. Odintsov
21 pages, lectures for 42nd Karpacz Winter School on Theoretical Physics

"We review various modified gravities considered as gravitational alternative for dark energy. Specifically, we consider the versions of f(R), f(G) or f(R,G) gravity, model with non-linear gravitational coupling or string-inspired model with Gauss-Bonnet-dilaton coupling in the late universe where they lead to cosmic speed-up. It is shown that some of such theories may pass the Solar System tests. On the same time, it is demonstrated that they have quite rich cosmological structure: they may naturally describe the effective (cosmological constant, quintessence or phantom) late-time era with a possible transition from decceleration to acceleration thanks to gravitational terms which increase with scalar curvature decrease. The possibility to explain the coincidence problem as the manifestation of the universe expansion in such models is mentioned. The late (phantom or quintessence) universe filled with dark fluid with inhomogeneous equation of state (where inhomogeneous terms are originated from the modified gravity) is also described."

this paper was prepared for this year's Polish Winterschool. It happens every year in February. Two years ago we got a bunch of interesting papers from the 2004 Winterschool---it was about QG phenomenology, DSR, possible observable effects. Carlo Rovelli was one of the organizers and Lee Smolin and Jerzy K-G gave weeklong lecture courses. Smolin and others discussed MOND.

Now it seems that the 2006 Winterschool is again touching on some of the same topics! The school is held at a ski resort on the Poland Czech border, in south Poland. It has been about all kinds of theoretical physics, not just QG or DSR or MOND. but now for two out of the past three years it will be about these things. We should watch for more papers coming out on arxiv from this year's school.
http://www.ift.uni.wroc.pl/karp42/#Prog
Bojowald is lecturing about Loop Cosmology at the Winterschool this year

mentioned briefly:
http://arxiv.org/abs/physics/0601218
A Theory of Quantum Gravity may not be possible because Quantum Mechanics violates the Equivalence Principle
Mario Rabinowitz
"Easy to follow original proof of the incompatibility of General Relativity and Quantum Mechanics"

Also a recent mathematics PhD thesis at Göttingen:
http://arxiv.org/abs/math.MG/0601744
Coarse geometry and asymptotic dimension
Bernd Grave
Dissertation
Subj-class: Metric Geometry

"We consider asymptotic dimension of coarse spaces. We analyse coarse structures induced by metrisable compactifications. We calculate asymptotic dimension of coarse cell complexes. We calculate the asymptotic dimension of certain negatively curved spaces, e.g. for complete, simply connected manifolds with bounded, strictly negative sectional curvature."

Bernd's thesis advisor was Tom Schick. This is high abstract math, with no obvious connection to QG or other physics. My personal opinion is that it might be interesting to develop a connection. Renate Loll and Hanno Sahlmann gave seminar talks at Göttingen around November-December last year. The physics department there seems to have an interest in QG.

http://arxiv.org/abs/gr-qc/0601121
The causal set approach to quantum gravity
Joe Henson
22 pages, 4 figures.
"Extended version of a review to be published in "Approaches to Quantum Gravity - Towards a new understanding of space and time" (ed. D. Oriti), Cambridge University Press, 2006... Dedicated to Rafael Sorkin on the occasion of his 60th birthday"

Renate Loll impresses me as a team player. Acting for the good of the QG field as a whole. She has taken Joe Henson on as a postdoc but he seems not to be doing Loll-type CDT research. He seems to be going great guns on Causal Sets---with several collaborations with Fay Dowker in the works and at least one with Rafael Sorkin.

I suppose this is what Lee Smolin was asking for (independence for worthy postdocs, don't tie support to one particular research program) and it seems an idealistic attempt to treat QG as a single field where the principle investigators share the job of hosting the postdocs. Instead of dividing up into separate competing factions---jealously guarded bailiwicks of funding

Well, I don't know how it will work in practice. I am a little disappointed, I thought Joe Henson going to Utrecht as a Loll postdoc would mean he crosses over into CDT research. Is there a common ground?

Christine flagged the Joe Henson paper on her blog yesterday and got some discussion:
http://christinedantas.blogspot.com/2006/01/causal-set-approach-to-quantum-gravity.html

 

[selfAdjoint]

Just a note on Thiemann. I'm reading his latest master constraint paper, gr-qc/0510011, where he brings it all home. Unlike the previous papers in the series, this one is based on triangulations, not networks. Influence of CDT?

Later Edit. Reading farther I find he does make some use of spin networks. See my post # 442. However his Master Constraint is defined, as advertised, in terms of triangulations.

 

[marcus]

Just a note on Thiemann. I'm reading his latest master constraint paper, gr-qc/0510011, where he brings it all home. ...

I am imagining the TOC of Oriti's book ("towards a new understanding of space and time") with all these guys lined up to evaluate and compare.

Perez---Spinfoams
Henson---Causal Sets
Thiemann---Masterconstraint
Gambini---"Gambinistics"
Loll---Dynamic Triangulation
Bojowald---Loop Cosmology
Freidel---Finessing matter feynman diagrams from foam spacetime
...
...

so far we only actually know that Perez and Henson have contributed chapters, the others are guesses with varying degrees of seriousness.
But Thiemann certainly should be there!

EDIT: based on next post by selfAdjoint I deleted a non-essential mention of triangulations

 

[selfAdjoint]

Marcus I have to stand corrected. He does use spin networks, or rather diffeomorphic equivalence classes of them, in defining his new inner product. When I finally get my head around it, I'll start a thread describing it; it has some very important consequences, and as you know, was cited along with CDT at the summer meetings as an important step forward in quantum gravity.

 

[marcus]

Oriti doing spacetime and matter in 3D (similar to Freidel)

http://arxiv.org/abs/gr-qc/0602010
Group field theory formulation of 3d quantum gravity coupled to matter fields

Daniele Oriti, James Ryan
28 pages, 21 figures

"We present a new group field theory describing 3d Riemannian quantum gravity coupled to matter fields for any choice of spin and mass. The perturbative expansion of the partition function produces fat graphs colored with SU(2) algebraic data, from which one can reconstruct at once a 3-dimensional simplicial complex representing spacetime and its geometry, like in the Ponzano-Regge formulation of pure 3d quantum gravity, and the Feynman graphs for the matter fields. The model then assigns quantum amplitudes to these fat graphs given by spin foam models for gravity coupled to interacting massive spinning point particles, whose properties we discuss."

 

[marcus]

Christine Dantas blog is turning out to be a real valuable resource.
http://christinedantas.blogspot.com/

Her sidebar has some good references. Not just the Smolin Lectures on Intro to LQG, but also links to a READING LIST to go along with the Smolin Lectures.

For instance Smolin is often recommending Dirac's thin book "Lectures on Quantum Mechanics" but that requires a trip to the library or bookstore. So Christine gives an online substitute:

http://www.tech.port.ac.uk/staffweb/seahras/documents/reviews/quantization.pdf

This is by Sanjeev Seahra
"The Classical and Quantum Mechanics of Systems with Constraints"

Christine does onboard satellite computer code for Brazil government. She running what is, it seems, the world's only QG blog. She also has substantial other demands on her time. ...
[EDIT: correction, selfAdjoint points out another QG blog I didnt know about]

 

[selfAdjoint]

...what is, it seems, the world's only QG blog.

Not quite, there is also http://lqg.blogspot.com/, but I think Dantas is better.

 

[marcus]

James Hartle, Lev Okun

Recent postings by James Hartle and by Lev Okun---both papers are somewhat on the philosophical side, and have a bit of historical perspective. Both Hartle and Okun should perhaps be revered as elder statesmen. Hartle was born in 1939 and Okun in 1929.

http://arxiv.org/abs/gr-qc/0602013
Generalizing Quantum Mechanics for Quantum Spacetime
James B. Hartle (University of California, Santa Barbara)
31 pages, 4 figures, latex, contribution to the 23rd Solvay Conference, The Quantum Structure of Space and Time

"Familiar textbook quantum mechanics assumes a fixed background spacetime to define states on spacelike surfaces and their unitary evolution between them. Quantum theory has changed as our conceptions of space and time have evolved. But quantum mechanics needs to be generalized further for quantum gravity where spacetime geometry is fluctuating and without definite value. This paper reviews a fully four-dimensional, sum-over-histories, generalized quantum mechanics of cosmological spacetime geometry. This generalization is constructed within the framework of generalized quantum theory. This is a minimal set of principles for quantum theory abstracted from the modern quantum mechanics of closed systems, most generally the universe. In this generalization, states of fields on spacelike surfaces and their unitary evolution are emergent properties appropriate when spacetime geometry behaves approximately classically. The principles of generalized quantum theory allow for the further generalization that would be necessary were spacetime not fundamental. Emergent spacetime phenomena are discussed in general and illustrated with the example of the classical spacetime geometries with large spacelike surfaces that emerge from the 'no-boundary' wave function of the universe. These must be Lorentzian with one, and only one, time direction. The essay concludes by raising the question of whether quantum mechanics itself is emergent."

====================

a key reference, in the Hartle paper, is
http://arxiv.org/abs/hep-th/0512200
Observables in effective gravity
Steven B. Giddings, Donald Marolf, James B. Hartle
43 pages
We address the construction and interpretation of diffeomorphism-invariant observables in a low-energy effective theory of quantum gravity. The observables we consider are constructed as integrals over the space of coordinates, in analogy to the construction of gauge-invariant observables in Yang-Mills theory via traces. As such, they are explicitly non-local. Nevertheless we describe how, in suitable quantum states and in a suitable limit, the familiar physics of local quantum field theory can be recovered from appropriate such observables, which we term `pseudo-local.' We consider measurement of pseudo-local observables, and describe how such measurements are limited by both quantum effects and gravitational interactions. These limitations support suggestions that theories of quantum gravity associated with finite regions of spacetime contain far fewer degrees of freedom than do local field theories."

this paper has half a dozen citations to work by Carlo Rovelli

13, 14, 15, 20, 42, 46

also about the same number of citations to papers by Abhay Ashtekar

I would say that a central theme of these two Hartle papers is BACKGROUND INDEPENDENCE the need for quantum observables to be defined in a diffeomorphism invariant way.

Hartle presents this in a PALATABLE way. To me he comes across as a reformer but with a tactful restrained manner. He is actually saying stuff not very different from Lee Smolin in The Case for Background Independence but he says it in a soothing way that does not step on anyone's toes.

All through Hartle section 7 he is talking generalities about something where Renate Loll has tried specifics-----but instead of CITING Loll and Ambjorn work, he puts a footnote where he says "Regge" and cites a paper of Ruth Williams (previous generation triangulation gravity).

I guess to say "Loll" at the 23rd Solvay conference (select old boys chosen by David Gross) would have sounded a jarring note.

Here is Hartle's page 14 footnote with the Ruth Williams citation:

"9 Perhaps, most naturally by discrete approximations to geometry such as the Regge calculus (see, e.g. [43, 44]) "

What he is essentially describing there, in section 7, is an approach that Loll has worked out. But seems unaware of this.

================

I think the Hartle paper could be an important contribution for DIPLOMATIC reasons.
It articulates a reform position but nicely, avoiding backlash. It is admirably intelligent and well-reasoned. Its faults (not explicitly pointing out developments in the non-string QG community) can be considered to be its merits.

=======================

http://arxiv.org/abs/hep-ph/0602037
The Concept of Mass in the Einstein Year
L.B. Okun
19 pages, Presented at the 12th Lomonosov conference on Elementary Particle Physics, Moscow State University, August 25-31

"Various facets of the concept of mass are discussed. The masses of elementary particles and the search for higgs. The masses of hadrons. The pedagogical virus of relativistic mass."

(another by Lev Okun is http://arxiv.org/abs/hep-ph/0602036)
===============
while I can still edit, I will tack on notice of a new paper by Thanu Padmanabhan
http://arxiv.org/abs/astro-ph/0602117
this is just a pedagogical cosmology paper, but he has written interesting articles on QG, so I am inclined to flag it.

 

[Sauron]

http://arxiv.org/abs/physics/0601218
A Theory of Quantum Gravity may not be possible because Quantum Mechanics violates the Equivalence Principle
Mario Rabinowitz
"Easy to follow original proof of the incompatibility of General Relativity and Quantum Mechanics"

I hve just readed and it is very basic and very false. It is obvious that the ordinary shcröedinger equation in a Newtonian potential can´t implement the weak equivalence principle. Nobody clames that.

The question which arise from these paper is ¿how difficoult is to publish in arxiv? ¿isn´t there any peer to peer revisión?

 

[ZapperZ]

I hve just readed and it is very basic and very false. It is obvious that the ordinary shcröedinger equation in a Newtonian potential can´t implement the weak equivalence principle. Nobody clames that.

The question which arise from these paper is ¿how difficoult is to publish in arxiv? ¿isn´t there any peer to peer revisión?

This is why I always caution people who cite papers of arXiv. There is no peer review. There is only a very rudimentary review of submitted papers to make sure it is not pure quackery. But other than that, papers like this can get through especially if a person has posted a paper before the current endorsement system (i.e. you get grandfathered into the new system).

If you check for this author - Mario Rabinowitz - you'll see that he has had a series of equally dubious papers submitted. The alarm bells should ring when you realize that these papers don't appear anywhere else, and especially in peer-reviewed journals.

Always wait (unless it is a proceeding paper or a text of a speech) for an arXiv submission to appear in a peer-reviewed journal. That is your best bet. Unfortunately, arXiv has become a major "citation source" in String theory. I don't know if that's good, or a poor reflection on the field of study itself.

Zz.

 

[marcus]

I hve just readed and it is very basic and very false...

Glad you agree :-)
I thought it was preposterous.
Title had amusement value for me since my area of interest is QG and he was saying that QG is apriori impossible(!)
Next time I put in a joke citation, if there is a next time, I will attach a SMILEY to make clear that the citation is not to be taken seriously.

 

[marcus]

Risto Raitio of Espoo, Finland

finns have great names for people and places sometimes and this denizen of Espoo has a blog called "Small Window"
http://fysix.blogspot.com/

and Risto has reported about a Zhao/Famaey MOND paper
http://fysix.blogspot.com/2006/02/refining-mond-this-is-not-Newtons.html

Here is Z/F paper,
http://arxiv.org/abs/astro-ph/0512425
a Chinese Scotch collaboration (sound potent?)When you survey Quantum Gravity I think you have to keep MOND in your periferal vision, because if it turns out right it will be a test of QG. Because QG will have to EXPLAIN why this particular modification of Newton law happens. Finally a QG will have to predict mondy effects--or darkmattery effects---and then astronomers will measure to see if the QG got it right out to as many decimal places as you can.

I flagged some MOND papers recently in posts #432 and 435----noticed but did not flag the Zhao/Famaey paper. And Risto supplies something more: a "SOFT" discussion, as Risto calls it:

http://www.interactions.org/cms/?pid=1023887

It is a journalistic introduction to the paper. I like their attitude here. they aren't partisans, they just want the best possible MOND so that it can be tested and pitted against Dark Matter models. Let observation decide.
The paper was published this month in Astrophysical Journal Letters. It seems like these days more and more MOND research is passing review and getting published.

 

[marcus]

't Hooft, PCW Davies

these are noted because PCW Davies is a major league cosmologist and because anything by Gerard 't Hooft is likely to be of interest to someone---he has ideas---it is good to keep track of what he is thinking about these days, even if it is not a big breakthrough

http://arxiv.org/abs/gr-qc/0602076
Invariance under complex transformations, and its relevance to the cosmological constant problem
Gerard 't Hooft, Stefan Nobbenhuis
ITP-UU-06/06, SPIN-06/04

"In this paper we study a new symmetry argument that results in a vacuum state with strictly vanishing vacuum energy. This argument exploits the well-known feature that de Sitter and Anti- de Sitter space are related by analytic continuation. When we drop boundary and hermiticity conditions on quantum fields, we get as many negative as positive energy states, which are related by transformations to complex space. The paper does not directly solve the cosmological constant problem, but explores a new direction that appears worthwhile."

this paper makes several references to earlier work by 't Hooft's co-author Nobbenhuis,
http://arxiv.org/gr-qc/0411093
Categorizing Different Approaches to the Cosmological Constant Problem
Stefan Nobbenhuis
Accepted for publication
ITP-UU-04/40, SPIN-04/23

"We have found that proposals addressing the old cosmological constant problem come in various categories. The aim of this paper is to identify as many different, credible mechanisms as possible and to provide them with a code for future reference. We find that they all can be classified into five different schemes of which we indicate the advantages and drawbacks.
Besides, we add a new approach based on a symmetry principle mapping real to imaginary spacetime."
==========
the next is a talk that Davies gave at a Stanford conference to string theorists. Davies has co-authored with Lineweaver. He is a major cosmologist. I don't necessarily recommend the paper but I want to be able to keep tabs on Davies views of current issues like multiverse/anthropics.

http://arxiv.org/abs/astro-ph/0602420
The problem of what exists
P.C.W. Davies
18 pages, one figure

"Popular multiverse models such as the one based on the string theory landscape require an underlying set of unexplained laws containing many specific features and highly restrictive prerequisites. I explore the consequences of relaxing some of these prerequisites with a view to discovering whether any of them might be justified anthropically. Examples considered include integer space dimensionality, the immutable, Platonic nature of the laws of physics and the no-go theorem for strong emergence. The problem of why some physical laws exist, but others which are seemingly possible do not, takes on a new complexion following this analysis, although it remains an unsolved problem in the absence of an additional criterion."

 

[marcus]

Magueijo video on MOND versus Dark Matter

this is a pretty good talk on MOND
Joao Magueijo gave it today at Perimeter and it is already
available as streamer.

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/
where you click on "seminar series" in the sidebar menu on the left

The talk is based on a recent paper Magueijo did with Bekenstein
http://arxiv.org/abs/astro-ph/0602266
MOND habitats within the solar system
Jacob Bekenstein, Joao Magueijo

"MOdified Newtonian Dynamics (MOND) is an interesting alternative to dark matter in extragalactic systems. We here examine the possibility that mild or even strong MOND behavior may become evident well inside the solar system, in particular near saddle points of the total gravitational potential. Whereas in Newtonian theory tidal stresses are finite at saddle points, they are expected to diverge in MOND, and to remain distinctly large inside a sizeable oblate ellipsoid around the saddle point. We work out the MOND effects using the nonrelativistic limit of the TeVeS theory, both in the perturbative nearly Newtonian regime and in the deep MOND regime. While strong MOND behavior would be a spectacular 'backyard'' vindication of the theory, pinpointing the MOND-bubbles in the setting of the realistic solar system may be difficult. Space missions, such as the LISA Pathfinder, equipped with sensitive accelerometers, may be able to explore the larger perturbative region."

in the talk, one or more members of the audience seemed eager to interrupt with comments and questions, there seemed a fair amount of restrained excitement at times

one important thing involves the spaceprobe LISA which, if I understand correctly, will explore the Earth-Sun Lagrange L1 point, and the gravitational field between the Earth and L1.

Magueijo explained how LISA can discount radiation pressure----it has balls floating inside the spacecraft ----the spacecraft shields the balls from radiation pressure.

Magueijo explained the strategy of going to SADDLE POINTS where the acceleration due to gravity is small, and how (according to him) one could test MOND within the confines of the solar system.

He seemed to have a balanced view---conventional dark matter has strong points---mond has strong points----one should try to test the theories, may the best survive, maybe mond will be disproved by these tests (as with LISA) that he described. he did not seem to have his ego tied up in either competing theory DM or MOND.

 

[Sauron]

I am not sure if it has been posted here or not.

Anyway like it is a "must be linked" website i post the url:

http://relativity.livingreviews.org/Articles/

In particular the articles by Astekhar in isolated horizonts (very readable althought it just trate too many aspects and sometimes doesn´t go as deep as i would like, and the one by bojowald in llop quantum cosmology which i have just discovered and can say too much more.

 

[marcus]

I am not sure if it has been posted here or not.

Anyway like it is a "must be linked" website i post the url:

http://relativity.livingreviews.org/Articles/

...

Excellent choice Sauron! In the past we have linked a few selected Living Reviews articles---including the recent one by Bojowald on LQC. But we have never posted a link to the table of contents of the entire collection. It is good to have. Thanks.

 

[marcus]

new Bojowald----black holes this time

http://arxiv.org/abs/gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)
IGPG-06/2-2, AEI-2006-009

"Black Holes have always played a central role in investigations of quantum gravity. This includes both conceptual issues such as the role of classical singularities and information loss, and technical ones to probe the consistency of candidate theories. Lacking a full theory of quantum gravity, such studies had long been restricted to black hole models which include some aspects of quantization. However, it is then not always clear whether the results are consequences of quantum gravity per se or of the particular steps one had undertaken to bring the system into a treatable form. Over a little more than the last decade loop quantum gravity has emerged as a widely studied candidate for quantum gravity, where it is now possible to introduce black hole models within a quantum theory of gravity. This makes it possible to use only quantum effects which are known to arise also in the full theory, but still work in a rather simple and physically interesting context of black holes. Recent developments have now led to the first physical results about non-rotating quantum black holes obtained in this way. Restricting to the interior inside the Schwarzschild horizon, the resulting quantum model is free of the classical singularity, which is a consequence of discrete quantum geometry taking over for the continuous classical space-time picture. This fact results in a change of paradigm concerning the information loss problem. The horizon itself can also be studied in the quantum theory by imposing horizon conditions at the level of states. Thereby one can illustrate the nature of horizon degrees of freedom and horizon fluctuations. All these developments allow us to study the quantum dynamics explicitly and in detail which provides a rich ground to test the consistency of the full theory."

 

[marcus]

http://arxiv.org/abs/gr-qc/0602119
Matters of Gravity, The Newsletter of The Topical Group in Gravitation of the American Physical Society, Volume 27, Spring 2006
Jorge Pullin (Editor)
30 pages

"GGR News:
GGR program at the APS April meeting in Dallas
We hear that..., 100 years ago, by Jorge Pullin
Research Briefs:
What's new in LIGO, by David Shoemaker
LISA Pathfinder, by Paul McNamara
Recent progress in binary black hole simulations, by Thomas Baumgarte
Conference reports:
Workshop on Emergence of Spacetime, by Olaf Dreyer
Quantum gravity subprogram at the Isaac Newton Institute, by Jorma Louko
Global problems in Math Relativity at the Newton Institute, by Jim Isenberg
Loops '05, by Thomas Thiemann
Numrel 2005, by Scott Hawley and Richard Matzner
Apples With Apples Workshop in Argentina, by Sascha Husa"
====================

http://arxiv.org/abs/gr-qc/0602120
Categorical Geometry and the Mathematical Foundations of Quantum General Relativity
Louis Crane
Contribution to the Oxford University Press volume on Quantum Gravity

"We explore the possibility of replacing point set topology by higher category theory and topos theory as the foundation for quantum general relativity. We discuss the BC model and problems of its interpretation, and connect with the construction of causal sites."

 

[marcus]

http://arxiv.org/abs/hep-th/0603022
Quantum Gravity and the Standard Model
Sundance O. Bilson-Thompson, Fotini Markopoulou, Lee Smolin
12 pages, 21 figures

"We show that a class of background independent models of quantum spacetime have local excitations that can be mapped to the first generation fermions of the standard model of particle physics. These states propagate coherently as they can be shown to be noiseless subsystems of the microscopic quantum dynamics. These are identified in terms of certain patterns of braiding of graphs, thus giving a quantum gravitational foundation for the topological preon model proposed by one of us.
These results apply to a large class of theories in which the Hilbert space has a basis of states given by ribbon graphs embedded in a three-dimensional manifold up to diffeomorphisms, and the dynamics is given by local moves on the graphs, such as arise in the representation theory of quantum groups. For such models, matter appears to be already included in the microscopic kinematics and dynamics."

==================
Sundance did a seminar talk at Perimeter on this back in November 2005. The video is online in the PI streaming media center. It is impressive to see him responding to questions and working stuff out at the blackboard. At that time Sundance was collaborating with Smolin and Markopoulou on this and they were putting it together----merging Sundance preon model of matter with Spin-network spacetime.

I will get the link to that video, or a link to the link

Here is a link to a PF thread about the Preons of Sundance Bilson-Thompson that we had back in November 2005
https://www.physicsforums.com/showthread.php?t=100183

that will have a link to the seminar talk

also try this:
http://streamer.perimeterinstitute....fa7-485f-8d5d-3b62fb7d3e4c&shouldResize=False

and flip to page 5 of the seminar series. the date is 16 November and that is currently on page 5------they list latest first

the title is "Braid New World"
==================

Etera Livine must always be kept track of and he posted today as well

http://arxiv.org/abs/gr-qc/0603008
Reconstructing Quantum Geometry from Quantum Information: Area Renormalisation, Coarse-Graining and Entanglement on Spin Networks
Etera R. Livine, Daniel R. Terno
27 pages, 12 figures

"After a brief review of spin networks and their interpretation as wave functions for the (space) geometry, we discuss the renormalisation of the area operator in loop quantum gravity. In such a background independent framework, we propose to probe the structure of a surface through the analysis of the coarse-graining and renormalisation flow(s) of its area. We further introduce a procedure to coarse-grain spin network states and we quantitatively study the decrease in the number of degrees of freedom during this process. Finally, we use these coarse-graining tools to define the correlation and entanglement between parts of a spin network and discuss their potential interpretation as a natural measure of distance in such a state of quantum geometry."

============
briefly noted
Sabine H.
http://arxiv.org/hep-th/0603032
Thanu P.
http://arxiv.org/astro-ph/0603114

 

[marcus]

DSR chapter of Oriti

http://arxiv.org/abs/gr-qc/0603022

Doubly Special Relativity: facts and prospects
Jerzy Kowalski-Glikman
Submitted to the volume "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", D. Oriti ed

"In this short review of Doubly Special Relativity I describe first the relations between DSR and (quantum) gravity. Then I show how, in the case of a field theory with curved momentum space, the Hopf algebra of symmetries naturally emerges. I conclude with some remarks concerning DSR phenomenology and description of open problems."

 

[marcus]

CDT paper from Paris Einstein conference last summer

http://arxiv.org/abs/hep-th/0603079
Nonperturbative sum over topologies in 2D Lorentzian quantum gravity
R. Loll (U. Utrecht), W. Westra (U. Utrecht), S. Zohren (U. Utrecht, RWTH Aachen)
10 pages, 4 figures. Talk given by S. Zohren at the Albert Einstein Century International Conference (Paris, July 18-22 2005)

"The recent progress in the Causal Dynamical Triangulations (CDT) approach to quantum gravity indicates that gravitation is nonperturbatively renormalizable. We review some of the latest results in 1+1 and 3+1 dimensions with special emphasis on the 1+1 model. In particular we discuss a nonperturbative implementation of the sum over topologies in the gravitational path integral in 1+1 dimensions. The dynamics of this model shows that the presence of infinitesimal wormholes leads to a decrease in the effective cosmological constant. Similar ideas have been considered in the past by Coleman and others in the formal setting of 4D Euclidean path integrals. A remarkable property of the model is that in the continuum limit we obtain a finite space-time density of microscopic wormholes without assuming fundamental discreteness. This shows that one can in principle make sense out of a gravitational path integral including a sum over topologies, provided one imposes suitable kinematical restrictions on the state-space that preserve large scale causality."

this is listed for completeness although I think there is nothing new here that was not already in
http://arxiv.org/abs/hep-th/0507012
and its referencesbriefly noted:
http://arxiv.org/abs/gr-qc/0603029
Counting of Black Hole Microstates
A. Ghosh, P. Mitra