Loop-and-allied QG bibliography

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  • #401
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Ashtekar and Bojowald Black Hole paper (long awaited)

http://www.arxiv.org/abs/gr-qc/0509075
Quantum geometry and the Schwarzschild singularity
Abhay Ashtekar, Martin Bojowald
31 pages, 1 figure
IGPG-05-09/01, AEI-2005-132
"In homogeneous cosmologies, quantum geometry effects lead to a resolution of the classical singularity without having to invoke special boundary conditions at the singularity or introduce ad-hoc elements such as unphysical matter. The same effects are shown to lead to a resolution of the Schwarzschild singularity. The resulting quantum extension of space-time is likely to have significant implications to the black hole evaporation process. Similarities and differences with the situation in quantum geometrodynamics are pointed out."
 
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http://www.arxiv.org/abs/gr-qc/0509080
Positivity in Lorentzian Barrett-Crane Models of Quantum Gravity
J. Wade Cherrington, J. Daniel Christensen
13 pages
"The Barrett-Crane models of Lorentzian quantum gravity are a family of spin foam models based on the Lorentz group. We show that for various choices of edge and face amplitudes, including the Perez-Rovelli normalization, the amplitude for every triangulated closed 4-manifold is a non-negative real number. Roughly speaking, this means that if one sums over triangulations, there is no interference between the different triangulations. We prove non-negativity by transforming the model into a "dual variables'' formulation in which the amplitude for a given triangulation is expressed as an integral over three copies of hyperbolic space for each tetrahedron. Then we prove that, expressed in this way, the integrand is non-negative. In addition to implying that the amplitude is non-negative, the non-negativity of the integrand is highly significant from the point of view of numerical computations, as it allows statistical methods such as the Metropolis algorithm to be used for efficient computation of expectation values of observables."

http://www.arxiv.org/abs/gr-qc/0509078
Loop quantum black hole
Leonardo Modesto
11 pages
"In this paper we consider the Kantowski-Sachs space-time in Ashtekar variables and the quantization of this space-time starting from the complete loop quantum gravity theory. The Kanthowski-Sachs space-time coincides with the Schwarzschild black hole solution inside the horizon. By studying this model we can obtain information about the black hole singularity and about the dynamics across the point r=0. We studied this space-time in ADM variables in two previous papers where we showed that the classical black hole singularity disappears in quantum theory. In this work we study the same model in Ashtekar variables and we obtain a regular space-time inside the horizon region and that the dynamics can be extend further the classical singularity."

---quote---
An important consequence of the quantization is that, unlike the classical evolution, the quantum evolution doesn’t stop at the classical singularity and the “other side” of the singularity corresponds to a new domain where the triad reverses its orientation. This work is useful if we want understand what is the mechanism to resolve the problem of the “information loss” in the process of black hole formation [15].
---end quote---
 
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  • #403
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just a little paper on DSR (doubly special relativity) by a new researcher at the perimeter institute, Jonathan Hackett. havent heard of him before. undergraduate physics major University of Waterloo in 2003, maybe a graduate student there now

http://arxiv.org/gr-qc/0509103 [Broken]
Asymptotic Flatness in Rainbow Gravity
Jonathan Hackett
13 pages
"A construction of conformal infinity in null and spatial directions is constructed for the Rainbow-flat space-time corresponding to doubly special relativity. From this construction a definition of asymptotic DSRness is put forward which is compatible with the correspondence principle of Rainbow gravity. Furthermore a result equating asymptotically flat space-times with asymptotically DSR spacetimes is presented."

curious thing is that rainbow-gravity has a RUNNING METRIC which is something we also saw with some Martin Reuter's work.

in the scheme Hackett uses (due to Smolin and Maguiejo IIRC) the metric runs with energy which is how Reuter's QEG does too. so the metric depends on the scale that you are looking at the geometry---what microscope you use to look at the geometry.

so what does this paper signify? maybe just that Lee Smolin has gotten another potential grad student who is interested in the rainbowgravity scheme (energy or scale dependent metric) and in DSR.

i expect it has to do with the upcoming AUGER and GLAST observational tests

footnote: the "rainbow" could be a reference to the fact that in this approach you dont has just one metric on the manifold, you can have a whole SPECTRUM of metrics. corresponding (as often the case with spectra) to different energies or scales. strange idea, odd that at least superficially it resembles something that came up with Reuter QEG
=======================

Ashtekar presentation, lecture notes
http://www.phys.psu.edu/events/index.html [Broken]
select "this semester" from the menu. Abhay talk is at top of list:


THE OTHER SIDE OF THE BIG BANG An Analytical and Numerical Study
IGPG Seminar by Abhay Ashtekar from Penn State
Monday at 3:00 PM in 318 Osmond (8/29/2005)

click on "presentation" to download

http://www.phys.psu.edu/events/index.html?event_id=1257&event_type_ids=0&span=2005-08-20.2005-12-25 [Broken]
 
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  • #404
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new papers from Perez and from Livine/Oriti

http://arxiv.org/gr-qc/0509118 [Broken]
On the regularization ambiguities in loop quantum gravity
Alejandro Perez
21 pages

http://arxiv.org/hep-th/0509192 [Broken]
Coherent States for 3d Deformed Special Relativity: semi-classical points in a quantum flat spacetime
E.R. Livine, D. Oriti
20 pages
"We analyse the quantum geometry of 3-dimensional deformed special relativity (DSR) and the notion of spacetime points in such a context, identified with coherent states that minimize the uncertainty relations among spacetime coordinates operators. We construct this system of coherent states in both the Riemannian and Lorentzian case, and study their properties and their geometric interpretation."
 
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  • #405
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Thomas Thiemann---Master Constraint paper

http://www.arxiv.org/abs/gr-qc/0510011

Quantum Spin Dynamics VIII. The Master Constraint
Thomas Thiemann
19 pages
AEI-2005-152

"Recently the Master Constraint Programme (MCP) for Loop Quantum Gravity (LQG) was launched which replaces the infinite number of Hamiltonian constraints by a single Master constraint. The MCP is designed to overcome the complications associated with the non -- Lie -- algebra structure of the Dirac algebra of Hamiltonian constraints and was successfully tested in various field theory models. For the case of 3+1 gravity itself, so far only a positive quadratic form for the Master Constraint Operator was derived. In this paper we close this gap and prove that the quadratic form is closable and thus stems from a unique self-adjoint Master Constraint Operator. The proof rests on a simple feature of the general pattern according to which Hamiltonian constraints in LQG are constructed and thus extends to arbitrary matter coupling and holds for any metric signature. With this result the existence of a physical Hilbert space for LQG is established by standard spectral analysis."


As Thomald Thiemannegger said: "I'll be back."
 
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Huzzah! How ya like THEM apples, Lubos!
 
  • #407
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:biggrin:

more tonight.
http://www.arxiv.org/abs/gr-qc/0510014
Master Constraint Operator in Loop Quantum Gravity
Muxin Han, Yongge Ma
11 pages, no figures

"We introduce a Master Constraint Operator ˆMˆ densely defined in the diffeomorphism invariant Hilbert space in loop quantum gravity. The corresponding quadratic form coincides with the one proposed by Thiemann in the master constraint programme. It is shown that ˆMˆ is positive and symmetric, and hence has its Friedrichs self-adjoint extension. So the master constraint programme for loop quantum gravity can be carried out in principle by employing ˆMˆ."

taking root at Beijing Normal
Yongge Ma is presenting a paper at Loops 05 in a week or so

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

something from Thanu Padmanabhan (Snark Hunting fan)
http://www.arxiv.org/abs/gr-qc/0510015
A new perspective on Gravity and the dynamics of Spacetime
T. Padmanabhan

Based on the Essay selected for Honorable Mention in the Gravity Research Foundation Essay Contest, 2005; to appear in the special issue of IJMPD

"The Einstein-Hilbert action has a bulk term and a surface term (which arises from integrating a four divergence). I show that one can obtain Einstein's equations from the surface term alone. This leads to: (i) a novel, completely self contained, perspective on gravity and (ii) a concrete mathematical framework in which the description of spacetime dynamics by Einstein's equations is similar to the description of a continuum solid in the thermodynamic limit."

Fabien Besnard heard T.P. give this talk in Paris at the Einstein Century conference this summer. Fabien blogged about it. thought it was one of the more interesting things from the conference. I can't evaluate. But Padmanabhan has done interesting stuff in the past.
 
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http://www.arxiv.org/abs/gr-qc/0510022
Correspondence between Loop-inspired and Braneworld Cosmology
Edmund J. Copeland, James E. Lidsey, Shuntaro Mizuno

"Braneworld scenarios are motivated by string/M-theory and can be characterized by the way in which they modify the conventional Friedmann equations of Einstein gravity. An alternative approach to quantum gravity, however, is the loop quantum cosmology program. In the semi-classical limit, the cosmic dynamics in this scenario can also be described by a set of modified Friedmann equations. We demonstrate that a dynamical correspondence can be established between these two paradigms at the level of the effective field equations. This allows qualitatively similar features between the two approaches to be compared and contrasted..."

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

Jorge Pullin's newsletter
http://www.arxiv.org/abs/gr-qc/0510021
Matters of Gravity
================

John Baez talk at Loops 05
http://math.ucr.edu/home/baez/loops05/

the 18 page lecture notes:
http://math.ucr.edu/home/baez/loops05/loops05.pdf
 
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Here's another from tonight Two of the authors are the same as the ones of "Master Constraint M" above. Intended as a pedagogical introduction for beginners.

Fundamental Structure of Loop Quantum Gravity
http://www.arxiv.org/abs/gr-qc/0509064


Muxin Han, Weiming Huang, Yongge Ma

From the abstract:
In this review, the fundamental structure of loop quantum gravity is presented pedagogically. Our main aim is to help non-experts to understand the motivations, basic structures, as well as general results. We will focus on the theoretical framework itself, rather than its applications, and do our best to write it in modern and precise langauge while keeping the presentation accessible for beginners. After reviewing the classical connection dynamical formalism of general relativity, as a foundation, the construction of kinematical Ashtekar-Isham-Lewandowski representation is introduced in the content of quantum kinematics. In the content of quantum dynamics, we mainly introduce the construction of a Hamiltonian constraint operator and the master constraint project. It should be noted that this strategy of quantizing gravity can also be extended to obtain other background independent quantum gauge theories. There is no divergence within this background independent and diffeomorphism invariant quantization programme of matter coupled to gravity
 
  • #410
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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 begining 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 [Broken]

In the acknowledgements 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:[itex] c(E) \simeq c_0 (1 - E/M)[/itex], 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.physics.research/browse_frm/thread/6d8038e9fd92cf2/efe70e642e41021f?hl=en&lr=&ie=UTF-8&rnum=1&prev=/groups?hl=en&lr=&ie=ISO-8859-1&q=Lattice+Gauge+Theory&btnG=Google+Search&meta=group%3Dsci.physics.research#efe70e642e41021f[/QUOTE]
 
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  • #411
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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 [Broken]

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
 
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  • #412
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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
 
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  • #413
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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."
 
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  • #414
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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
 
  • #415
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Marcus said:
we have a thread about the Baez resu
And I see you started one on the Jones polynomial too.
 
  • #416
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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 [Broken]

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 doesnt 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
 
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  • #417
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See you got the recent papers marcus! great stuff.
 
  • #418
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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
 
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  • #419
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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?
 
  • #420
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selfAdjoint said:
...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 [Broken]
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."
 
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  • #421
marcus
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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."
 
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  • #422
marcus
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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."
 
  • #423
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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."
 
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  • #424
marcus
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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/ [Broken]

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

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.
 
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  • #425
marcus
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marcus said:
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 realised 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
 
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