Loop-and-allied QG bibliography

[marcus]

Recent contributions from Baez, Smolin, Rovelli

a couple of these are discussed in https://www.physicsforums.com/showthread.php?t=114140

http://math.ucr.edu/home/baez/where_we_stand/
"Fundamental Physics: Where We Stand Today"
John Baez

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

http://www.nyas.org/publications/UpdateUnbound.asp?UpdateID=41
"A Crisis in Fundamental Physics"
Lee Smolin
published in the New York Academy of Sciences magazine.

The Houghton-Mifflin publishing house plans to release a new book by Lee Smolin this year---what the book is about is suggested by:

http://www.lse.ac.uk/collections/CPNSS/events/Conferences/AnnualPublicLecture.htm
Abstract for something called "The Annual Lecture" to be given by Smolin at The London School of Economics CPNSS, in June 2006."...I will begin by proposing an answer to the question of what science is and why it works. Part of the answer is that scientists form a community that is defined by adherence to a set of ethics which encourages honest reporting of observations and results as well as an awareness that future generations will know far more than us. As such science is based on a tragic understanding of how easily we fool ourselves and others and its success is due to the discovery of techniques whose use helps us discover error. In this and other ways I will discuss science is tied to democracy in that both require members of a community to adhere to ethics designed to allow us to achieve as a community far more than would be possible each on our own..."

[my comment: a healthy scientific community requires ethical behavior and some "checks and balances" because even prestigeous elites can succumb to self-delusion and error. it is not simply an egalitarian democracy, nor is it simply an amoral turf-based aristocratic hierarchy---a key ingredient is the empirical ethic]

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

Recent contributions from Rovelli are mentioned here:

https://www.physicsforums.com/showthread.php?t=114024

A 59 slide lecture given at Lyon, Introduction to Loop Quantum Gravity
http://www.cpt.univ-mrs.fr/~rovelli/Lyon2006II.pdf

Course outline for a History of Science course that Rovelli is currently teaching is here
http://www.cpt.univ-mrs.fr/~rovelli/

=============================
We recently had a thread at PF about George Ellis finding misinformation in Susskind's book
https://www.physicsforums.com/showthread.php?t=114068

http://arxiv.org/abs/astro-ph/0603266
On horizons and the cosmic landscape
George F R Ellis

"Susskind claims in his recent book The Cosmic Landscape that evidence for the existence and nature of 'pocket universes' in a multiverse would be available via detailed study of the Cosmic Blackbody Background Radiation. I point out that apart from any other queries one might have about the chain of argument involved, this claim is invalid because it rests on a confusion between the nature of a particle horizon and an event horizon in cosmology."

Ellis is co-author, with Stephen Hawking, of a well-known reference "The Large-Scale Structure of Spacetime". He gives this link to an erroneous passage of Susskind's book.
http://www.twbookmark.com/books/28/0316155799/chapter_excerpt22014.html

 

[marcus]

new Loll and Benedetti, plus others of interest

http://arxiv.org/abs/hep-lat/0603013
Unexpected Spin-Off from Quantum Gravity
D. Benedetti (U. Utrecht), R. Loll (U. Utrecht)
10 pages, 4 figures

"We propose a novel way of investigating the universal properties of spin systems by coupling them to an ensemble of causal dynamically triangulated lattices, instead of studying them on a fixed regular or random lattice. Somewhat surprisingly, graph-counting methods to extract high- or low-temperature series expansions can be adapted to this case. For the two-dimensional Ising model, we present evidence that this ameliorates the singularity structure of thermodynamic functions in the complex plane, and improves the convergence of the power series."

title seems to involve a pun. Benedetti got his Masters from Rome IIRC and joined Loll's group as a PhD student sometime in the past two years---I have the feeling it was about a year ago

http://arxiv.org/abs/gr-qc/0603062
Conformal decomposition in canonical general relativity
Charles H.-T. Wang
6 pages
"A new canonical transformation is found that enables the direct canonical treatment of the conformal factor in general relativity. The resulting formulation significantly simplifies the previously presented conformal geometrodynamics. It provides a further theoretical basis for the conformal approach to loop quantum gravity and offers a generic framework for the conformal analysis of spacetime dynamics."

Charles Wang is physics prof IIRC at Aberdeen, UK. He does LQG his own independent way and is someone to watch out of the corner of one's eye. He gave a paper at last September QG conference on Sardinia. My fear is that if I ignore mavericks that seem disconnected from the rest of the researchers one of them may sneak up and surprise me. So without recommendation, I keep a link on Wang.

===============
Cosmology is a principle feed and motivation for QG. This book by Loeb (Harvard) will be a valuable reference, I suspect. Around 160 pages.
It is being published by Springer in Germany.

http://arxiv.org/abs/astro-ph/0603360
First Light
Abraham Loeb (Harvard)
158 pages, lecture notes for the 36th Swiss (SAAS-Fee) Winter School, April 2006 (to be published by Springer Verlag); http://obswww.unige.ch/saas-fee2006/preannouncement/course_pres/overview_f.html

"The first dwarf galaxies, which constitute the building blocks of the collapsed objects we find today in the Universe, had formed hundreds of millions of years after the big bang. This pedagogical review describes the early growth of their small-amplitude seed fluctuations from the epoch of inflation through dark matter decoupling and matter-radiation equality, to the final collapse and fragmentation of the dark matter on all mass scales above \~10^{-4} solar masses. The condensation of baryons into halos in the mass range of ~10^5-10^{10} solar masses led to the formation of the first stars and the re-ionization of the cold hydrogen gas, left over from the big bang. The production of heavy elements by the first stars started the metal enrichment process that eventually led to the formation of rocky planets and life. A wide variety of instruments currently under design [including large-aperture infrared telescopes on the ground or in space (JWST), and low-frequency arrays for the detection of redshifted 21cm radiation], will establish better understanding of the first sources of light during an epoch in cosmic history that was largely unexplored so far. Numerical simulations of reionization are computationally challenging, as they require radiative transfer across large cosmological volumes as well as sufficently high resolution to identify the sources of the ionizing radiation. The technological challenges for observations and the computational challenges for numerical simulations, will motivate intense work in this field over the coming decade."

 

[arivero]

Duff on three generations?

Just noticed this one:
http://arxiv.org/abs/hep-th/0602160
Hidden symmetries of the Nambu-Goto action

It seems a sort of follow-up of

http://arxiv.org/abs/hep-th/0601134
String triality, black hole entropy and Cayley's hyperdeterminant

 

[marcus]

thanks! good to have your contributions.

 

[arivero]

hmmm and sorry I put it using "new reply" instead of "new thread"

 

[marcus]

hmmm and sorry I put it using "new reply" instead of "new thread"

well actually it might be simpler to keep this thread specialized for non-string QG papers, as a general rule. and there is a sticky for string literature.

but I hope you will keep watch for interesting Loop-and-allied QG papers and contribute links here---I think you have an eye for what research could be important

 

[marcus]

baez "loop braid group" paper

no-brainer, this has to go on the link list

http://arxiv.org/abs/gr-qc/0603085
Exotic Statistics for Loops in 4d BF Theory
John C. Baez, Derek K. Wise, Alissa S. Crans
40 pages, many figures

"After a review of exotic statistics for point particles in 3d BF theory, and especially 3d quantum gravity, we show that loop-like defects in 4d BF theory obey exotic statistics governed by the 'loop braid group'. This group has a set of generators that switch two loops just as one would normally switch point particles, but also a set of generators that switch two loops by passing one through the other. The first set generates a copy of the symmetric group, while the second generates a copy of the braid group. Thanks to recent work of Xiao-Song Lin, we can give a presentation of the whole loop braid group, which turns out to be isomorphic to the 'braid permutation group' of Fenn, Rimanyi and Rourke. In the context 4d BF theory this group naturally acts on the moduli space of flat G-bundles on the complement of a collection of unlinked unknotted circles in R^3. When G is unimodular, this gives a unitary representation of the loop braid group. We also discuss 'quandle field theory', in which the gauge group G is replaced by a quandle."here's a discussion thread, with exerpts

https://www.physicsforums.com/showthread.php?t=115082

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

f-h called attention to this new paper, also about the interesting (4D) case of BF theory:
http://arxiv.org/abs/gr-qc/0603076
Alternative symplectic structures for SO(3,1) and SO(4) four-dimensional BF theories
Merced Montesinos
13 pages
Class. Quantum Grav. 23 (2006) 2267-2278
"The most general action, quadratic in the B fields as well as in the curvature F, having SO(3,1) or SO(4) as the internal gauge group for a four-dimensional BF theory is presented and its symplectic geometry is displayed. It is shown that the space of solutions to the equations of motion for the BF theory can be endowed with symplectic structures alternative to the usual one. The analysis also includes topological terms and cosmological constant. The implications of this fact for gravity are briefly discussed."

========briefly noted========
http://arxiv.org/abs/gr-qc/0603090
Fundamental decoherence from quantum gravity: a pedagogical review
Rodolfo Gambini, Rafael Porto, Jorge Pullin
9 pages, dedicated to Octavio Obregon on his 60th birthday

I have no reason to recommend the next one. Just want to stay aware of other (particle) approaches to the c.c. problem. here is an example.
http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."

No special reason to recommend this either. philosophical/historical paper suggesting that there's something wrong with the ideas of b.i. and diffeo-invariance. If true, a serious critique of LQG and allied research.
http://arxiv.org/abs/gr-qc/0603087
Some remarks on the notions of general covariance and background independence
Domenico Giulini
26 pages, 3 figures. Contribution to "An assessment of current paradigms in the physics of fundamental interactions'', edited by I.O. Stamatescu (Springer Verlag, to appear)

 

[marcus]

John Swain about a way of seeing Higgs

John Swain is a experimental particle physicist/phenomenologist who works part at CERN and part in the physics department at Northeastern.

He interests me in part because he also writes LQG papers. He is a working nutsbolts high energy physicist who has enough style to also think creatively in Quantum Gravity.

So when he has new-sounding ideas about anything, I try to pay attention. This has been sitting on my desktop since last September.
Maybe someone here at PF could be interested. It is a way of seeing Higgs field by mass shifts, not by actually detecting the particle quantum of the field.

I suspect that the higgs field will ultimately be found built into the quantum geometry of some QG----intrinsic to the quantum state of spacetime. I would bet this thought has crossed Swain's mind too.

http://arxiv.org/hep-ph/0509151
Probing the Higgs Field Using Massive Particles as Sources and Detectors
S. Reucroft, Y.N. Srivastava, J. Swain, A. Widom
6 pages, no figures; Version 2 corrects some typographical errors of factors of 2 in equations 14, 17, 18 and 19 (all of the same origin) and mentions a linear collider as an interesting place to test the results of this paper

"In the Standard Model, all massive elementary particles acquire their masses by coupling to a background Higgs field with a non-zero vacuum expectation value. What is often overlooked is that each massive particle is also a source of the Higgs field. A given particle can in principle shift the mass of a neighboring particle. The mass shift effect goes beyond the usual perturbative Feynman diagram calculations which implicitly assume that the mass of each particle is rigidly fixed. Local mass shifts offer a unique handle on Higgs physics since they do not require the production of on-shell Higgs bosons. We provide theoretical estimates showing that the mass shift effect can be large and measurable, especially near pair threshold, at both the Tevatron and the LHC."

The general idea (which has some historical validity I believe) is to look for the field instead of for the particle which represents the field in certain situations.

======================
TIMBUQTU AND F-H thanks for your help with this biblio-links thread! About CD's blog as time goes on it is getting more and more active and useful isn't it, contributions from f-h and also from John Baez among others. It is an odd coincidence that we should post a link to Timb.'s professor's paper at the same time as he assigns homework from it.

 

[Timbuqtu]

http://arxiv.org/abs/gr-qc/0603088
A solution to the cosmological constant problem
Tomislav Prokopec (Utrecht University)
22 pages, 4 figures
"We argue that, when coupled to Einstein's theory of gravity, the Yukawa theory may solve the cosmological constant problem in the following sense: The radiative corrections of fermions generate an effective potential for the scalar field, such that the effective cosmological term Lambda_eff is dynamically driven to zero. Thence, for any initial positive cosmological constant Lambda_0, Lambda_eff = 0 is an attractor of the semiclassical Einstein theory coupled to fermionic and scalar matter fields. When the initial cosmological term is negative, Lambda_eff=Lambda_0 does not change. Next we argue that the dark energy of the Universe may be explained by a GUT scale fermion with a mass, m = 4.3 * 10^15 (Lambda_0/10^13GeV)^(1/2) GeV.
Finally, we comment on how the inflationary paradigm, BEH mechanism and phase transitions in the early Universe get modified in the light of our findings."

That's funny. Last week, one day before this paper was posted, Prokopec gave us a homework exercise to derive this result (the mass calculation). Still have to do it. But apparently there is a squareroot missing in the mass-formula in this abstract, because it must be m = 4.3 * 10^15 ((Lambda_0)^(1/2)/10^13GeV)^(1/2) GeV (according to his own paper).

 

[f-h]

"f-h called attention to this new paper, also about the interesting (4D) case of BF theory"

Actuall I first saw it on CDs blog.

 

[marcus]

http://arxiv.org/abs/gr-qc/0603110
Quantum Cosmology
Martin Bojowald
10 pages, published in Encyclopedia of Mathematical Physics, eds. J.-P. Franccoise, G. L. Naber and Tsou S. T., Oxford: Elsevier, 2006 (ISBN 978-0-1251-2666-3), volume 4, page 153

"Quantum cosmology in general denotes the application of quantum physics to the whole universe and thus gives rise to many realizations and examples, covering problems at different mathematical and conceptual levels. It is related to quantum gravity and more specifically describes the application to cosmological situations rather than the construction and analysis of quantum field equations. As there are several different approaches to quantum gravity, equations for quantum cosmology are not unique. Most investigations have been performed in the context of canonical quantization, where Wheeler--DeWitt like equations are the prime object. Applications are mostly conceptual, ranging from possible resolutions of classical singularities and explanations of the uniqueness of the universe to the origin of seeds for a classical world and its initial conditions."

this is in rather dry handbook style and covers the general subject instead of focusing on Loop cosmology. For a more focused and detailed discussion see Bojowald's Living Reviews article
http://arxiv.org/abs/gr-qc/0601085
or this short survey for non-specialists, also by Bojowald:
http://arxiv.org/abs/astro-ph/0511557

while on the subject, a recent cosmology overview by Ned Wright just came out
http://arxiv.org/abs/astro-ph/0603750
A Century of Cosmology
E. L. Wright (UCLA Astronomy)GLAST mission reference
http://arxiv.org/abs/astro-ph/0603762
The GLAST mission, LAT and GRBs
Nicola Omodei, for the GLAST/LAT GRB Science Group
6 pages, 3 figures. Proceedings for the 16th Annual October Astrophysics Conference in Maryland: Gamma Ray Bursts in the Swift Era

 

[marcus]

Mohammad Ansari is at Perimeter and has been sitting in on the Smolin lectures
Several times Smolin has referred to Ansari's work, and they've talked back and forth, but Smolin has the mike and I can't distinguish what Mohammad says.

Here is some recent work by him

http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."
=====================
http://arxiv.org/abs/gr-qc/0601013
Fermions in Ashtekar-Barbero Connections Formalism for Arbitrary Values of the Immirzi Parameter
Simone Mercuri
15 pp., no figures, title changed, version accepted for publication on Phys. Rev. D"The Ashtekar-Barbero-Immirzi formulation of General Relativity is extended to include spinor matter fields. Our formulation applies to generic values of the Immirzi parameter and reduces to the Ashtekar-Romano-Tate approach when the Immirzi parameter is taken equal to the imaginary unit. The dynamics of the gravity-fermions coupled system is described by the Holst plus Dirac action with a non-minimal coupling term. The non-minimal interaction together with the Holst modification to the Hilbert-Palatini action reconstruct the Nieh-Yan invariant, so that the effective action coming out is the one of Einstein-Cartan theory with a typical Fermi-like interaction term: in spite of the presence of spinor matter fields, the Immirzi parameter plays no role in the classical effective dynamics and results to be only a multiplicative factor in front of a total divergence.
We reduce the total action of the theory to the sum of dynamically independent Ashtekar-Romano-Tate actions for self and anti-self dual connections, with different weights depending on the Immirzi parameter. This allows to calculate the constraints of the complete theory in a simple way, it is only necessary to realize that the Barbero-Immirzi connection is a weighted sum of the self and anti-self dual Ashtekar connections. Finally the obtained constraints for the separated action result to be polynomial in terms of the self and anti-self dual connections, this could have implications in the inclusion of spinor matter in the framework of non-perturbative quantum gravity."From page 5, discussion of equation (22):

"This result shows that the Immirzi parameter, which appears in the action for the gravitational field used as starting point in the construction of LQG, not only appears in non-perturbative quantum effects but also in the classical equations of motion, when fermions are present, leading to (independently from the quantum theory) possible observable effects. It plays the role of coupling constant in front of the four fermions interacting term..."

 

[marcus]

new Ashtekar paper, quantum nature of big bang

http://arxiv.org/abs/gr-qc/0604013
Quantum Nature of the Big Bang: An Analytical and Numerical Investigation I
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
59 pages, 19 figures

"Analytical and numerical methods are developed to analyze the quantum nature of the big bang in the setting of loop quantum cosmology. They enable one to explore the effects of quantum geometry both on the gravitational and matter sectors and significantly extend the known results on the resolution of the big bang singularity. Specifically, the following results are established for the homogeneous isotropic model with a massless scalar field: i) the scalar field is shown to serve as an internal clock, thereby providing a detailed realization of the `emergent time' idea; ii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously; iii) the Hamiltonian constraint is solved numerically to show that the big bang is replaced by a big bounce. Thanks to the non-perturbative, background independent methods, unlike in other approaches the quantum evolution is deterministic across the deep Planck regime. Our constructions also provide a conceptual framework and technical tools which can be used in more general models. In this sense, they provide foundations for analyzing physical issues associated with the Planck regime of loop quantum cosmology as a whole."
===============
not sure about this. any discussion of making QM deterministic is bound to be over my head. but there was discussion of this paper in a separate thread---some folks might be interested in checking it out

http://arxiv.org/abs/quant-ph/0604008
The mathematical basis for deterministic quantum mechanics
Gerard 't Hooft
15 pages, 3 figures

"If there exists a classical, i.e. deterministic theory underlying quantum mechanics, an explanation must be found of the fact that the Hamiltonian, which is defined to be the operator that generates evolution in time, is bounded from below. The mechanism that can produce exactly such a constraint is identified in this paper. It is the fact that not all classical data are registered in the quantum description. Large sets of values of these data are assumed to be indistinguishable, forming equivalence classes. It is argued that this should be attributed to information loss, such as what one might suspect to happen during the formation and annihilation of virtual black holes.
The nature of the equivalence classes is further elucidated, as it follows from the positivity of the Hamiltonian. Our world is assumed to consist of a very large number of subsystems that may be regarded as approximately independent, or weakly interacting with one another. As long as two (or more) sectors of our world are treated as being independent, they all must be demanded to be restricted to positive energy states only. What follows from these considerations is a unique definition of energy in the quantum system in terms of the periodicity of the limit cycles of the deterministic model."

 

[marcus]

new Freidel paper

http://arxiv.org/abs/gr-qc/0604016
Hidden Quantum Gravity in 3d Feynman diagrams
Aristide Baratin, Laurent Freidel
35 pages, 4 figures
"In this work we show that 3d Feynman amplitudes of standard QFT in flat and homogeneous space can be naturally expressed as expectation values of a specific topological spin foam model. The main interest of the paper is to set up a framework which gives a background independent perspective on usual field theories and can also be applied in higher dimensions. We also show that this Feynman graph spin foam model, which encodes the geometry of flat space-time, can be purely expressed in terms of algebraic data associated with the Poincare group. This spin foam model turns out to be the spin foam quantization of a BF theory based on the Poincare group, and as such is related to a quantization of 3d gravity in the limit where the Newton constant G_N goes to 0. We investigate the 4d case in a companion paper where the strategy proposed here leads to similar results."

 

[marcus]

Rovelli, graviton propagator

http://arxiv.org/abs/gr-qc/0604044
Graviton propagator in loop quantum gravity
Eugenio Bianchi, Leonardo Modesto, Carlo Rovelli, Simone Speziale
41 pages, 6 figures
"We compute some components of the graviton propagator in loop quantum gravity, using the spinfoam formalism, up to some second order terms in the expansion parameter."

===========================
Chapter for Oriti's book "Towards Quantum Gravity"

http://arxiv.org/abs/gr-qc/0604045
Unfinished revolution
Carlo Rovelli
8 pages

"Introductive chapter of a book on Quantum Gravity, edited by Daniele Oriti, that will appear with Cambridge University Press."

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

http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
7 pages
"We argue that EPR-type correlations do not entail any form of "non-locality", when viewed in the context of a relational interpretation of quantum mechanics. The abandonment of strict Einstein realism advocated by this interpretation permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."

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

http://arxiv.org/abs/gr-qc/0604050
Tachyon Matter in Loop-Inspired Cosmology
A. A. Sen
5 pages

"An analytical approach for studying the cosmological scenario with a homogeneous tachyon field within the framework of loop quantum gravity is developed. Our study is based on the semi-classical regime where space time can be approximated as a continuous manifold, but matter Hamiltonian gets non-perturbative quantum corrections. A formal correspondence between classical and loop quantum cosmology is also established. The Hamilton-Jacobi method for getting exact solutions is constructed and some exact power-law as well as bouncing solutions are presented."

http://arxiv.org/abs/gr-qc/0604040
Macroscopic observables and Lorentz violation in discrete quantum gravity
Joe Henson
15 pages, 1 figure

"This article concerns the fate of local Lorentz invariance in quantum gravity, particularly for approaches in which a discrete structure replaces continuum spacetime. Some features of standard quantum mechanics, presented in a sum-over-histories formulation, are reviewed, and their consequences for such theories are discussed. It is argued that, if the individual histories of a theory give bad approximations to macroscopic continuum properties in some frames, then it is inevitable that the theory violates Lorentz symmetry.

 

[marcus]

I don't think discussion of anthropics is so important because I expect nonperturbative approaches to Quantum Gravity (the new understanding of spacetime and matter that is emerging, see Oriti's book when it comes out will explain the CC---what its role is and why it is what it is.

And then people will lose interest in the "Anthropic Lack of Principles" as it has been called. But meanwhile, here's something to check out!

http://arxiv.org/abs/astro-ph/0604242
An Observational Test for the Anthropic Origin of the Cosmological Constant
Abraham Loeb (Harvard)
5 pages, submitted to JCAP

"The existence of multiple regions of space beyond the observable Universe (within the so-called "multiverse") where the vacuum energy density takes different values, has been postulated as an explanation for the low non-zero value observed for it in our Universe. It is often argued that our existence pre-selects regions where the cosmological constant is sufficiently small to allow galaxies like the Milky Way to form and intelligent life to emerge. Here we propose a simple empirical test for this anthropic argument within the boundaries of the observable Universe. We make use of the fact that dwarf galaxies formed in our Universe at redshifts as high as z~10 when the mean matter density was larger by a factor of ~10^3 than today. Existing technology enables to check whether planets form in nearby dwarf galaxies and globular clusters by searching for microlensing or transit events of background stars. The oldest of these nearby systems may have formed at z~10. If planets are as common per stellar mass in these descendents as they are in the Milky Way galaxy, then the anthropic argument would be weakened considerably since planets could have formed in our Universe even if the cosmological constant was three orders of magnitude larger than observed. For a flat probability distribution, this would imply that the probability for us to reside in a region where the cosmological constant obtains its observed value is lower than approx. 10^{-3}. A precise version of the anthropic argument could then be ruled-out at a confidence level of ~99.9%, which constitutes a satisfactory measure of a good experimental test."http://arxiv.org/abs/gr-qc/0604053
Violation of Strong Energy Condition in Effective Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages

must keep tabs on cousins at Beijing Normal----getting quite strong in LQG and LQC.
several presentations from Beijing Normal at Loops '05 last October.

 

[marcus]

Joao Magueijo is a hero of mine, ever since I watched an online video seminar talk of him from February 2006 at Perimeter.

I really liked a MOND paper he wrote with Bekenstein posted this year.
Now he has a paper with Rafael Sorkin----familiar to everybody too I guess.
I personally could not get much out of this Magueijo Sorkin paper. But because of who they are, and because AXIS OF EVIL is topical in cosmology and because of WMAP3, I figure there might be someone to whom this is interesting. Even though I don't someone else might.

http://arxiv.org/abs/astro-ph/0604410
Occam's razor meets WMAP
Joao Magueijo, Rafael D. Sorkin

"Using a variety of quantitative implementations of Occam's razor we examine the low quadrupole, the ``axis of evil'' effect and other detections recently made appealing to the excellent WMAP data. We find that some razors fully demolish the much lauded claims for departures from scale-invariance. They all reduce to pathetic levels the evidence for a low quadrupole (or any other low ell cut-off), both in the first and third year WMAP releases. The 'axis of evil' effect is the only anomaly examined here that survives the humiliations of Occam's razor, and even then in the category of 'strong' rather than 'decisive' evidence. Statistical considerations aside, differences between the various renditions of the datasets remain worrying."Hey look again, this shows ago that Magueijo is a lean-mean phenomenologist. Lots of other folks are talking about the axis of evil and believing departure from scale-invariance at large angles and he says NO! the confidence levels for most of that are pitiful. At least he is frank and willing to take contrary stands. It might be a good paper.

Magueijo has QG importance because he co-authors with Lee Smolin and is into QG phenomenology.

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

Couple of Shahn Majid QG-related papers came out today:
OTE=marcus]http://arxiv.org/abs/hep-th/0604130
Algebraic approach to quantum gravity II: noncommutative spacetime
S. Majid
26 pages, 2 figures; book chapter to appear in D. Oriti, ed., Cambridge Univ. Press
"We provide a self-contained introduction to the quantum group approach to noncommutative geometry as the next-to-classical effective geometry that might be expected from any successful quantum gravity theory. We focus particularly on a thorough account of the bicrossproduct model noncommutative spacetimes of the form [t,x_i]=i lambda x_i and the correct formulation of predictions for it including a variable speed of light. We also study global issues in the Poincaré group in the model with the 2D case as illustration. We show that any off-shell momentum can be boosted to infinite negative energy by a finite Lorentz transformaton."

http://arxiv.org/abs/hep-th/0604132
Algebraic approach to quantum gravity III: noncommmutative Riemannian geometry
S. Majid
25 pages, 1 figure; to appear in collection B. Fauser and J. Tolksdorf, eds., Birkhauser

"This is a self-contained introduction to quantum Riemannian geometry based on quantum groups as frame groups, and its proposed role in quantum gravity. Much of the article is about the generalisation of classical Riemannian geometry that arises naturally as the classical limit; a theory with nonsymmetric metric and a skew version of metric compatibilty. Meanwhile, in quantum gravity a key ingredient of our approach is the proposal that the differential structure of spacetime is something that itself must be summed over or 'quantise' as a physical degree of freedom. We illustrate such a scheme for quantum gravity on small finite sets."

 

[marcus]

http://arxiv.org/abs/gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
"Non-minimally coupling a scalar field to gravity introduces an additional curvature term into the action which can change the general behavior in strong curvature regimes, in particular close to classical singularities. While one can conformally transform any non-minimal model to a minimally coupled one, that transformation can itself become singular. It is thus not guaranteed that all qualitative properties are shared by minimal and non-minimal models. This paper addresses the classical singularity issue in isotropic models and extends singularity removal in quantum gravity to non-minimal models."

http://arxiv.org/abs/hep-th/0604181
6J Symbols Duality Relations
L. Freidel, K. Noui, P. Roche
28 pages, 2 figures
"It is known that the Fourier transformation of the square of (6j) symbols has a simple expression in the case of su(2) and U_q(su(2)) when q is a root of unity. The aim of the present work is to unravel the algebraic structure behind these identities. We show that the double crossproduct construction H_1\bowtie H_2 of two Hopf algebras and the bicrossproduct construction
H_2^{*}\bowtie H_1
are the Hopf algebras structures behind these identities by analysing different examples. We study the case where
D= H_1\bowtie H_2
is equal to the group algebra of ISU(2), SL(2,C) and where D is a quantum double of a finite group, of SU(2) and of U_q(su(2)) when q is real."

the year 2006 is turning out to be the wonderyear of Laurent Freidel

http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schrödinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories.

http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vacuum and gauge invariant variables
Laurent Freidel
35 pages
In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian.
-------------------

the next paper is discussed in Bee's thread about it here
https://www.physicsforums.com/showthread.php?t=118641

http://arxiv.org/abs/gr-qc/0508013
Anti-Gravitation
S. Hossenfelder
Phys. Lett. B 636 (2006) 119-125

 

[marcus]

http://arxiv.org/abs/gr-qc/0604112
Background independent quantizations: the scalar field II
W. Kaminski, J. Lewandowski, A. Okolow
51 pages
"We are concerned with the issue of quantization of a scalar field in a diffeomorphism invariant manner. We apply the method used in Loop Quantum Gravity. It relies on the specific choice of scalar field variables referred to as the polymer variables. The quantization, in our formulation, amounts to introducing the `quantum' polymer *-star algebra and looking for positive linear functionals, called states. Assumed in our paper homeomorphism invariance allows to derive the complete class of the states. They are determined by the homeomorphism invariant states defined on the CW-complex *-algebra. The corresponding GNS representations of the polymer *-algebra and their self-adjoint extensions are derived, the equivalence classes are found and invariant subspaces characterized. In the preceding letter (the part I) we outlined those results. Here, we present the technical details."

can't evaluate the next one. list it in part to keep tabs on the Karpacz Winterschool, which several times has chosen a topic related to DSR or quantum gravity

http://arxiv.org/abs/gr-qc/0604120
The Chrono-geometrical Structure of Special and General Relativity: a Re-Visitation of Canonical Geometrodynamics
Luca Lusanna (INFN, Firenze)
33 pages, Lectures given at the 42nd Karpacz Winter School of Theoretical Physics, "Current Mathematical Topics in Gravitation and Cosmology", Ladek, Poland, 6-11 February 2006
A modern re-visitation of the consequences of the lack of an intrinsic notion of instantaneous 3-space in relativistic theories leads to a reformulation of their kinematical basis emphasizing the role of non-inertial frames centered on an arbitrary accelerated observer. In special relativity the exigence of predictability implies the adoption of the 3+1 point of view, which leads to a well posed initial value problem for field equations in a framework where the change of the convention of synchronization of distant clocks is realized by means of a gauge transformation. This point of view is also at the heart of the canonical approach to metric and tetrad gravity in globally hyperbolic asymptotically flat space-times, where the use of Shanmugadhasan canonical transformations allows the separation of the physical degrees of freedom of the gravitational field (the tidal effects) from the arbitrary gauge variables. Since a global vision of the equivalence principle implies that only global non-inertial frames can exist in general relativity, the gauge variables are naturally interpreted as generalized relativistic inertial effects, which have to be fixed to get a deterministic evolution in a given non-inertial frame. As a consequence, in each Einstein's space-time in this class the whole chrono-geometrical structure, including also the clock synchronization convention, is dynamically determined and a new approach to the Hole Argument leads to the conclusion that "gravitational field" and "space-time" are two faces of the same entity. This view allows to get a classical scenario for the unification of the four interactions in a scheme suited to the description of the solar system or our galaxy with a deperametrization to special relativity and the subsequent possibility to take the non-relativistic limit."

several former string theorists are publishing mainly in pure Mathematics now,
I don't know about Michael Douglas but he has posted some papers in the math part of arxiv.
the most recent is today
http://arxiv.org/abs/math.AG/0604597

Algebraic Geometry no less! classy!

 

[marcus]

http://arxiv.org/abs/hep-th/0604212
Quantum Gravity, or The Art of Building Spacetime
J. Ambjorn, J. Jurkiewicz, R. Loll
22 pages, 6 figures. Contribution to the book "Approaches to Quantum Gravity", ed. D. Oriti, Cambridge University Press

"The method of four-dimensional Causal Dynamical Triangulations provides a background-independent definition of the sum over geometries in quantum gravity, in the presence of a positive cosmological constant. We present the evidence accumulated to date that a macroscopic four-dimensional world can emerge from this theory dynamically. Using computer simulations we observe in the Euclidean sector a universe whose scale factor exhibits the same dynamics as that of the simplest mini-superspace models in quantum cosmology, with the distinction that in the case of causal dynamical triangulations the effective action for the scale factor is not put in by hand but obtained by integrating out in the quantum theory the full set of dynamical degrees of freedom except for the scale factor itself."



http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year

"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."

http://arxiv.org/abs/gr-qc/0605006
Discreteness without symmetry breaking: a theorem
Luca Bombelli, Joe Henson, Rafael D. Sorkin
7 pages
"This paper concerns sprinklings into Minkowski space (Poisson processes). It proves that there exists no equivariant measurable map from sprinklings to spacetime directions (even locally). Therefore, if a discrete structure is associated to a sprinkling in an intrinsic manner, then the structure will not pick out a preferred frame, locally or globally. This implies that the discreteness of a sprinkled causal set will not give rise to 'Lorentz breaking'' effects like modified dispersion relations. Another consequence is that there is no way to associate a finite-valency graph to a sprinkling consistently with Lorentz invariance."

 

[marcus]

http://arxiv.org/abs/gr-qc/0605011
Gravity, Geometry and the Quantum
Abhay Ashtekar
16 pages, 2 figures. To appear in the Proceedings of the `Einstein Century' Conference, 15-22 July, Paris, edited by J-M Alimi et al (American Institute of Physics)

"After a brief introduction, basic ideas of the quantum Riemannian geometry underlying loop quantum gravity are summarized. To illustrate physical ramifications of quantum geometry, the framework is then applied to homogeneous isotropic cosmology. Quantum geometry effects are shown to replace the big bang by a big bounce. Thus, quantum physics does not stop at the big-bang singularity. Rather there is a pre-big-bang branch joined to the current post-big-bang branch by a 'quantum bridge'. Furthermore, thanks to the background independence of loop quantum gravity, evolution is deterministic across the bridge."

http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures

"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A_0 are computed and the statistical entropy, as a function of the area, is obtained for A_0 up to 550 l^2_P. The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to \gamma=0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."

=============
a lot depends on the GLAST mission, I am hoping very much that it flies in 2007 as scheduled
this technical article is a sign that the mission is still going ahead
http://arxiv.org/abs/astro-ph/0605050
GLAST Tracker
Hiroyasu Tajima, for GLAST Tracker Team (Stanford Linear Accelerator Center)
5 pages, 5 figues, Invited talk at Vertex 2005, Chuzenji Lake, Nikko, Japan, November 7-November 11, 2005, To be published in Nuclear Instruments and Methods A
SLAC-PUB-11833
"The Large Area Telescope (LAT) on board the Gamma-ray Large-Area Space Telescope (GLAST) is a pair-conversion gamma-ray detector designed to explore the gamma-ray universe in the 20 MeV-300 GeV energy band. The Tracker subsystem of the LAT will perform tracking of electron and positrons to determine the origin of the gamma-ray. The design and performance of the GLAST LAT Tracker are described in this paper."

 

[marcus]

http://arxiv.org/abs/hep-th/0605052
Generic predictions of quantum theories of gravity
Lee Smolin
For inclusion in "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"I discuss generic consequences (sometimes called "soft predictions") of a class of background independent quantum theories of spacetime called causal spin network theories. These are theories whose kinematics and dynamics is based on the evolution of labeled graphs, by local moves, such as in loop quantum gravity and spin foam models. Some generic consequences are well known, including the discreteness of quantum geometry, the elimination of spacetime singularities, the entropy of black hole and cosmological horizons and the fact that positive cosmological constant spacetimes are hot. Within the last few years three possible generic consequences have come to light. These are:

1) Deformed special relativity as the symmetry of the ground state,

2) Elementary particles as coherent excitations of quantum geometry,

3) Locality is disordered.
I discuss some possible experimental consequences of each."

 

[marcus]

Larry Krauss is a prominent cosmophysicist and neutrinos get into everything including ultimately QG and how we will observe the very early universe. Neutrinos-in-cosmology deserves a wide-audience survey and Krauss has the writing style to give a good one.

http://arxiv.org/abs/astro-ph/0605378
What is the Role of Neutrinos in Shaping the Universe?
Lawrence M. Krauss (Case Western Reserve University)
10 pages. Invited review lecture, International Workshop on NO-VE, Venice, 2006

"I review various aspects of the role neutrinos have played in shaping various cosmological observables: the nature of large scale structure, observed fluctuations in the CMB, the nature of matter, and the shape of things to come. (Invited review lecture III International Workshop on NO-VE, Venice, 2006)"

==========

Back in post 471, in March, I mentioned this paper of Mohammad Ansari, which Danny recently brought up in a post:
"Recently, I read a beautiful paper in which it is proven that ANY surface in LQG contains degeneracy, no matter it being a boundary horizon or whatever else. (http://uk.arxiv.org/abs/gr-qc/0603121) This degeneracy is such that the surface degeneracy is A/4. This is a critial discovery in LQG and can follow up Padmanabhan's idea of associating the lack of information in (http://uk.arxiv.org/abs/gr-qc/0405072) into quantum gravity."

post 471 is here
https://www.physicsforums.com/showpost.php?p=951694&postcount=471
https://www.physicsforums.com/showthread.php?p=951694#post951694
http://arxiv.org/abs/gr-qc/0603121
Entanglement entropy in loop quantum geometry
Mohammad H. Ansari
4 pages, 3 figures
"The notion of entanglement entropy of a non-rotating black hole in the context of loop quantum geometry is introduced. By defining the notion of degeneracy of spin network states and determining the Barbero-Immirzi parameter from the highly damping quasinormal modes, we show that the entropy of SO(3) representations coincides with the Bekenstein-Hawking entropy."

that post also had an article about the Immirzi parameter by Simone Mercuri which was accepted for
publication by Physical Review D

Danny's thread about the Ansari paper is here
https://www.physicsforums.com/showthread.php?t=120887
called "Ansari's degeneracy in LQG"
=============================
this appeared yesterday

http://arxiv.org/abs/gr-qc/0605087
Quantization of strings and branes coupled to BF theory
John C. Baez, Alejandro Perez
"BF theory is a topological theory that can be seen as a natural generalization of 3-dimensional gravity to arbitrary dimensions. Here we show that the coupling to point particles that is natural in three dimensions generalizes in a direct way to BF theory in d dimensions coupled to (d-3)-branes. In the resulting model, the connection is flat except along the membrane world-sheet, where it has a conical singularity whose strength is proportional to the membrane tension. As a step towards canonically quantizing these models, we show that a basis of kinematical states is given by 'membrane spin networks', which are spin networks equipped with extra data where their edges end on a brane."

it is a followup on the Baez Wise Crans article mentioned earlier in this thread
Exotic statistics for loops in 4d BF theory
http://arxiv.org/gr-qc/0603085 =====================

Ignacio Navarro and Karel Van Acoleyen recently posted two articles on a variant of MOND
http://arxiv.org/abs/astro-ph/0605322
Dark energy, MOND and sub-millimeter tests of gravity
I. Navarro, K. Van Acoleyen
6 pages, to appear in proceedings of the XLIrst Rencontres de Moriond

"We consider modifications of General Relativity obtained by adding the logarithm of some curvature invariants to the Einstein-Hilbert action. These non-linear actions can explain the late-time acceleration of the universe giving an expansion history that differs from that of a pure cosmological constant. We show that they also modify the Newtonian potential below a fixed acceleration scale given by the late-time Hubble constant times the speed of light. This is exactly what is required in MOND, a phenomenological modification of the Newtonian potential that is capable of explaining galactic rotation curves without the need to introduce dark matter. We show that this kind of modification also predicts short distance deviations of Newton's law at the sub-mm scale and an anomalous shift in the precession of the Moon's orbit around the Earth, both effects of a size that is less than an order of magnitude below current bounds."http://arxiv.org/abs/gr-qc/0605091
Long distance modifications of gravity in four dimensions
I. Navarro, K. Van Acoleyen
4 pages, contribution to the proceedings of the Rencontres de Moriond: Contents and Structures of the Universe, March 18-25, 2006, La Thuile

"We discuss some general characteristics of modifications of the 4D Einstein-Hilbert action that become important for low space-time curvatures. In particular we focus on the chameleon-like behaviour of the massive gravitational degrees of freedom. Generically there is at least one extra scalar that is light on cosmic scales, but for certain models it becomes heavy close to any mass source."

an earlier paper from last year by Navarro and Van Acoleyen
http://arxiv.org/abs/gr-qc/0512109
Modified gravity, Dark Energy and MOND
Ignacio Navarro, Karel Van Acoleyen
24 pages, 2 figures.

"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 ~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 ~ 0.1mm, while this distance would be bigger in space where the local curvature is significantly lower."

==========
more work on black hole state counting and determining the Immirzi parameter
http://arxiv.org/abs/hep-th/0605125
Counting of isolated horizon states
A. Ghosh, P. Mitra
4 pages
"The entropy of an isolated horizon has been obtained by counting states in loop quantum gravity. We revisit the calculation of the horizon states using statistical methods and find the possibility of additional states, leading to an increase in the entropy. Apart from this, an isolated horizon temperature is introduced in this framework."

the paper of Ghosh and Mitra cites this recent one of Corichi et al
http://arxiv.org/abs/gr-qc/0605014
Entropy counting for microscopic black holes in LQG
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 6 figures

"Quantum black holes within the loop quantum gravity (LQG) framework are considered. The number of microscopic states that are consistent with a black hole of a given horizon area A₀ are computed and the statistical entropy, as a function of the area, is obtained for A₀ up to 550 L²_P The results are consistent with an asymptotic linear relation and a logarithmic correction with a coefficient equal to -1/2. The Barbero-Immirzi parameter that yields the asymptotic linear relation compatible with the Bekenstein-Hawking entropy is shown to coincide with a value close to gamma = 0.274, which has been previously obtained analytically. However, a new and unexpected functional form for the entropy is found for small, Planck size, black holes that calls for a physical interpretation."

The recent paper of Ghosh Mitra also cites one of theirs that was published in Physical Review Letters B, in 2005. It gives the same value of the Immirzi parameter, namely about 0.274.

http://arxiv.org/abs/gr-qc/0411035
An improved estimate of black hole entropy in the quantum geometry approach
A. Ghosh, P. Mitra
5 pages, LaTeX
Journal-ref: Phys.Lett. B616 (2005) 114-117

"A proper counting of states for black holes in the quantum geometry approach shows that the dominant configuration for spins are distributions that include spins exceeding one-half at the punctures. This raises the value of the Immirzi parameter and the black hole entropy. However, the coefficient of the logarithmic correction remains -1/2 as before."

=====================
not sure about this: an odd thing, don't remember ever seeing anything like it
http://arxiv.org/abs/gr-qc/0605098
Area and Entropy: A New Perspective
Jarmo Mäkelä
4 pages. This paper received an honorary mention in the annual Awards for Essays in Gravitation contest

"We consider a spacelike two-plane originally at rest with respect to electromagnetic radiation in equilibrium. We find that if the plane is moved with respect to the radiation, the plane shrinks such that the maximum amount of entropy carried by radiation through the plane is, in natural units, exactly one-half of the decrease in the area of the plane. This result suggests that the equivalence between area and entropy may not be limited in black holes, nor even in the spacetime horizons only, but the equivalence between horizon area and entropy may be a special case of some general and simple, still undiscovered fundamental principle of nature."

 

[marcus]

http://arxiv.org/abs/gr-qc/0605113
Avoidance of future singularities in loop quantum cosmology
M. Sami, Parampreet Singh, Shinji Tsujikawa
6 pages, 2 figures

"We consider the fate of future singularities in the effective dynamics of loop quantum cosmology. Non-perturbative quantum geometric effects which lead to \rho^2 modification of the Friedmann equation at high energies result in generic resolution of singularities whenever energy density \rho diverges at future singularities of Friedmann dynamics. Such quantum effects lead to the avoidance of a Big Rip, which is followed by a recollapsing universe stable against perturbations. Resolution of sudden singularity, the case when pressure diverges but energy density approaches a finite value depends on the ratio of the latter to a critical energy density of the order of Planck. If the value of this ratio is greater than unity, the universe escapes the sudden future singularity and becomes oscillatory."

I list the following since is to appear in Oriti's book
http://arxiv.org/abs/hep-th/0605202
String Field Theory
Washington Taylor (MIT, Stanford)
To appear in "Towards Quantum Gravity", ed. Daniele Oriti, Cambridge University Press; 22 pages
"This elementary introduction to string field theory highlights the features and the limitations of this approach to quantum gravity as it is currently understood. String field theory is a formulation of string theory as a field theory in space-time with an infinite number of massive fields. Although existing constructions of string field theory require expanding around a fixed choice of space-time background, the theory is in principle background-independent, in the sense that different backgrounds can be realized as different field configurations in the theory. String field theory is the only string formalism developed so far which, in principle, has the potential to systematically address questions involving multiple asymptotically distinct string backgrounds. Thus, although it is not yet well defined as a quantum theory, string field theory may eventually be helpful for understanding questions related to cosmology in string theory."

the following really are raw lecture notes, very sketchy, with some spelling mistakes and awkward wording, by someone at Ben-Gurion university who wants to cover all the QM you ever heard of, at an undergraduate level, in 200-some pages. It is an exercise in writing down one or two sentences and half dozen equations, and moving on. Like a stand-up comedian or serial exhibitionist. Frankly I thought it was a good idea---it had more EFFRONTERY than the usual textbook, I thought, so I wanted to include it:

http://arxiv.org/abs/quant-ph/0605180
Lecture Notes in Quantum Mechanics
Doron Cohen
239 pages
"These lecture notes cover undergraduate textbook topics (e.g. as in Sakurai), and also additional advanced topics at the same level of presentation. In particular: EPR and Bell; Basic postulates; The probability matrix; Measurement theory; Entanglement; Quantum computation; Wigner-Weyl formalism; The adiabatic picture; Berry phase; Linear response theory; Kubo formula; Modern approach to scattering theory with mesoscopic orientation; Theory of the resolvent and the Green function; Gauge and Galilei Symmetries; Motion in magnetic field; Quantum Hall effect; Quantization of the electromagnetic field; Fock space formalism."

Let me know if you check it out and think I should not have included it.

 

[marcus]

http://arxiv.org/abs/gr-qc/0605123
Towards the graviton from spinfoams: higher order corrections in the 3d toy model
Etera R. Livine, Simone Speziale, Joshua L. Willis
24 pages, many figures
"We consider the recent calculation gr-qc/0508124 of the graviton propagator in the spinfoam formalism. Within the 3d toy model introduced in gr-qc/0512102, we test how the spinfoam formalism can be used to construct the perturbative expansion of graviton amplitudes. We compute explicitly the next to leading and next to next to leading orders, corresponding to one-loop and two-loop corrections. We show that while the first arises entirely from the expansion of the Regge action around the flat background, the latter receives contributions from the microscopic, non Regge-like, quantum geometry. Surprisingly, this new contribution reduces the magnitude of the next to next to leading order. It thus appears that the spinfoam formalism is likely to substantially modify the conventional perturbative expansion at higher orders.
This result supports the interest in this approach. We then address a number of open issues in the rest of the paper. First, we discuss the boundary state ansatz, which is a key ingredient in the whole construction. We propose a way to enhance the ansatz in order to make the edge lengths and dihedral angles conjugate variables in a mathematically well-defined way. Second, we show that the leading order is stable against different choices of the face weights of the spinfoam model; the next to leading order, on the other hand, is changed in a simple way, and we show that the topological face weight minimizes it. Finally, we extend the leading order result to the case of a regular, but not equilateral, tetrahedron."

 

[marcus]

just keeping the link-basket up to date:
===========
http://arxiv.org/abs/gr-qc/0605141
Time Delay Predictions in a Modified Gravity Theory
J. W. Moffat
5 pages
"The time delay effect for planets and spacecraft is obtained from a fully relativistic modified gravity theory including a fifth force skew symmetric field by fitting to the Pioneer 10/11 anomalous acceleration data. A possible detection of the predicted time delay corrections to general relativity for the outer planets and future spacecraft missions is considered. The time delay correction to GR predicted by the modified gravity is consistent with the observational limit of the Doppler tracking measurement reported by the Cassini spacecraft on its way to Saturn, and the correction increases to a value that could be measured for a spacecraft approaching Neptune and Pluto."

?
========
http://arxiv.org/abs/gr-qc/0605138
Quantization of diffeomorphism invariant theories of connections with a non-compact structure group - an example
Andrzej Okolow
76 pages,
"A simple diffeomorphism invariant theory of connections with the non-compact structure group R of real numbers is quantized. The theory is defined on a four-dimensional 'space-time' by an action resembling closely the self-dual Plebanski action for general relativity. The space of quantum states is constructed by means of projective techniques by Kijowski. Except this point the applied quantization procedure is based on Loop Quantum Gravity methods."

Okolow is the "O" in the L.O.S.T theorem

 

[marcus]

http://arxiv.org/abs/gr-qc/0606032
Non-Singular Bouncing Universes in Loop Quantum Cosmology
Parampreet Singh, Kevin Vandersloot, G. V. Vereshchagin
12 pages, 9 figures

"Non-perturbative quantum geometric effects in Loop Quantum Cosmology predict a rho^2 modification to the Friedmann equation at high energies. The quadratic term is negative definite and can lead to generic bounces when the matter energy density becomes equal to a critical value of the order of the Planck density. The non-singular bounce is achieved for arbitrary matter without violation of positive energy conditions. By performing a qualitative analysis we explore the nature of the bounce for inflationary and Cyclic model potentials. For the former we show that inflationary trajectories are attractors of the dynamics after the bounce implying that inflation can be harmoniously embedded in LQC. For the latter difficulties associated with singularities in cyclic models can be overcome. We show that non-singular cyclic models can be constructed with a small variation in the original Cyclic model potential by making it slightly positive in the regime where scalar field is negative."

http://arxiv.org/abs/gr-qc/0604124
On Recovering Continuum Topology from a Causal Set
Seth Major, David Rideout, Sumati Surya
31 pages, 5 figs. Dedicated to our friend and teacher Rafael Sorkin, to celebrate his 60th year.

"An important question that discrete approaches to quantum gravity must address is how continuum features of spacetime can be recovered from the discrete substructure. Here, we examine this question within the causal set approach to quantum gravity, where the substructure replacing the spacetime continuum is a locally finite partial order. A new topology on causal sets using 'thickened antichains' is constructed. This topology is then used to recover the homology of a globally hyperbolic spacetime from a causal set which faithfully embeds into it at sufficiently high sprinkling density. This implies a discrete-continuum correspondence which lends support to the fundamental conjecture or 'Hauptvermutung' of causal set theory."

http://arxiv.org/abs/gr-qc/0606026
The black hole horizon as a dynamical system
Gerard 't Hooft
16 pages. Presented at "Einstein's Legacy in the New Millennium", Puri, India, 15-22 Dec. 2005. An extension of gr-qc/0504120 with chapter added and small sign changes

"Interactions between outgoing Hawking particles and ingoing matter are determined by gravitational forces and Standard Model interactions. In particular the gravitational interactions are responsible for the unitarity of the scattering against the horizon, as dictated by the holographic principle, but the Standard Model interactions also contribute, and understanding their effects is an important first step towards a complete understanding of the horizon's dynamics. The relation between in- and outgoing states is described in terms of an operator algebra. In this contribution, in which earlier results are rederived and elaborated upon, we first describe the algebra induced on the horizon by U(1) vector fields and scalar fields, including the case of an Englert-Brout-Higgs mechanism, and a more careful consideration of the transverse vector field components. We demonstrate that, unlike classical black holes, the quantized black hole has on its horizon an imprint of its (recent) past history, i.e., quantum hair. The relation between in- and outgoing states depends on this imprint. As a first step towards the inclusion of non-Abelian interactions, we then compute the effects of magnetic monopoles both in the in-states and in the out-states. They completely modify, and indeed simplify, our algebra."John Baez has an online video of an interesting talk given 31 May at Perimeter

Higher-Dimensional Algebra: A Langauge for Quantum Spacetime

http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
select "seminar series" from the left margin menu

the slides/lecturenotes for this talk are available at Baez site

http://math.ucr.edu/home/baez/quantum_spacetime/

http://math.ucr.edu/home/baez/quantum_spacetime/qs.pdf

 

[marcus]

Online videos of 3 talks at last week's "Theory Canada" conference

Gravitational collapse in quantum gravity
Viqar Husain

"I will describe a framework for studying the dynamical collapse of a scalar field in a Hamiltonian formulation of quantum gravity. The approach makes use of operators that test states for trapped surface formation as a state is evolves, and allows the extraction of invariant information about the scalar field and geometry."

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

Nonlocal remnants of microscopic discreteness?
Rafael Sorkin

"If 'quantum spacetime' is discrete, then any propagation equation compatible with the Lorentz transformations is necessarily nonlocal. I will present evidence that this sort of nonlocality survives on intermediate length scales much greater than that of the Planck length, yielding for example a nonlocal equation of motion for a scalar field."

================Protected information and emergent locality in background independent quantum gravity
Fotini Markopoulou

"A successful quantum theory of gravity needs to explain spacetime as an effective, approximate description of a more fundamental theory. I describe an approach to quantum gravity in which the pre-spacetime structure is a directed graph of operator algebras. They may also be thought of as a network of quantum information flow. Methods from quantum information theory can be used to extract degrees of freedom protected from the microscopic evolution and which define the causal structure of the emergent spacetime. As a result of the background independence we find that emergent spacetime means emergent locality and hence locality corrections at all scales. This is a new direction in quantum gravity phenomenology currently under investigation."

==========
There are plenty of others to choose from as well. I didnt sample all by any means, and mention only these.
To get the videos, go here
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
and scroll down to "Theory Canada 2" in the menu on the left

The format is split screen so you get to see both the lecturer and the projected slide.
Several of the talks are brief (20-25 minutes). You may find other talks of particular interest to you.
Among the short talks, I was especially impressed by those given by Sorkin and by Husain. Sorkin's abstract does not indicate the full scope of what he presented.

==============
the talk by Robert Leigh is not yet put online, but it may appear later.
The Spectrum of 2+1 Dimensional Yang-Mills Theory (so far this talk is not available on line)
Robert Leigh
"I review our recent work on an analytic solution of Yang-Mills theory in 2+1 dimensions."

It was about http://arxiv.org/abs/hep-th/0604060
and there was a follow-up
http://arxiv.org/abs/hep-th/0604184
of which Laurent Freidel was co-author

======================
now for something completely different

http://arxiv.org/abs/gr-qc/0606061
Gravity: A New Holographic Perspective
T. Padmanabhan
Plenary talk at the International Conference on Einstein's Legacy in the New Millennium, December 15 - 22, 2005, Puri, India; to appear in the Proceedings to be published in IJMPD; 16 pages

"A general paradigm for describing classical (and semiclassical) gravity is presented. This approach brings to the centre-stage a holographic relationship between the bulk and surface terms in a general class of action functionals and provides a deeper insight into several aspects of classical gravity which have no explanation in the conventional approach. After highlighting a series of unresolved issues in the conventional approach to gravity, I show that (i) principle of equivalence, (ii) general covariance and (iii)a reasonable condition on the variation of the action functional, suggest a generic Lagrangian for semiclassical gravity of the form L=Q_a^{bcd}R^a_{bcd} with \nabla_b Q_a^{bcd}=0 . The expansion of Q_a^{bcd} in terms of the derivatives of the metric tensor determines the structure of the theory uniquely. The zeroth order term gives the Einstein-Hilbert action and the first order correction is given by the Gauss-Bonnet action. Any such Lagrangian can be decomposed into a surface and bulk terms which are related holographically. The equations of motion can be obtained purely from a surface term in the gravity sector. Hence the field equations are invariant under the transformation T_{ab} \to T_{ab} + \lambda g_{ab} and gravity does not respond to the changes in the bulk vacuum energy density. The cosmological constant arises as an integration constant in this approach. The implications are discussed."

 

[marcus]

http://arxiv.org/abs/hep-th/0606135
Fixed points of quantum gravity in higher dimensions
Peter Fischer, Daniel F. Litim
Talk presented at Einstein Century Meeting, Paris, 15-22 July 2005
SHEP-06-14, CERN-PH-TH-2006/066, PITHA 06/06
"We study quantum gravity in more than four dimensions by means of an exact functional flow. A non-trivial ultraviolet fixed point is found in the Einstein-Hilbert theory. It is shown that our results for the fixed point and universal scaling exponents are stable. If the fixed point persists in extended truncations, quantum gravity in the metric field is asymptotically safe. We indicate physical consequences of this scenario in phenomenological models with low-scale quantum gravity and large extra dimensions."

cites several papers of Martin Reuter and by Ambjorn, Jurkiewicz, Loll.
suggests ways to make higher dimensional models testable, by showing that einstein metric gravity is assympt. safe, so manageable, in higher dimensions.

http://arxiv.org/abs/gr-qc/0606065
Evidence for an entropy bound from fundamentally discrete gravity
D. Rideout, S. Zohren
25 pages, 9 figures
IMPERIAL/TP/06/SZ/01
"The various entropy bounds that exist in the literature suggest that spacetime is fundamentally discrete, and hint at an underlying relationship between geometry and 'information'. The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We present a measure for the maximal entropy of spherically symmetric spacelike regions within the causal set approach to quantum gravity. In terms of the proposal, a bound for the entropy contained in this region can be derived from a counting of potential 'degrees of freedom' associated to the Cauchy horizon of its future domain of dependence. For different spherically symmetric spacelike regions in Minkowski spacetime of arbitrary dimension, we show that this proposal leads, in the continuum approximation, to Susskind's well-known spherical entropy bound."

Stefan Zohren got his masters at Utrecht, in Loll's group, and then went to London Imperial for PhD. London is strong in Causal Sets. Rideout is experienced researcher in that approach. Did his PhD at Syracuse with Rafael Sorkin in 2002. (Dynamics of Causal Sets gr-qc/0212064)

http://arxiv.org/abs/gr-qc/0606067
How often does the Unruh-DeWitt detector click? Regularisation by a spatial profile
Jorma Louko, Alejandro Satz
28 pages, 1 figure
"We analyse within first-order perturbation theory the instantaneous transition rate of an accelerated Unruh-DeWitt particle detector whose coupling to a massless scalar field on four-dimensional Minkowski space is regularised by a spatial profile. For the Lorentzian profile introduced by Schlicht, the zero size limit is computed explicitly and expressed as manifestly finite integral formula that no longer involves regulators or limits. The same transition rate is obtained for an arbitrary profile of compact support under a modified definition of spatial smearing. Consequences for the asymptotic behaviour of the transition rate are discussed. A number of stationary and nonstationary trajectories are analysed, recovering in particular the Planckian spectrum for uniform acceleration."

Alejandro Satz is an Argentine studying for PhD in Nottingham. He has a blog called REALITY CONDITIONS
Jorma Louko has co-authored with Carlo Rovelli, Don Marolf, Rafael Sorkin, Kirill Krasnov. 35 of his papers are here:
http://arxiv.org/find/gr-qc/1/au:+Louko_J/0/1/0/all/0/1

 

[marcus]

http://arxiv.org/abs/gr-qc/0606074
A semiclassical tetrahedron
Carlo Rovelli, Simone Speziale
10 pages
"We construct a macroscopic semiclassical state state for a quantum tetrahedron. The expectation values of the geometrical operators representing the volume, areas and dihedral angles are peaked around assigned classical values, with vanishing relative uncertainties."

 

[marcus]

http://arxiv.org/abs/gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
"Non-minimal actions with matter represented by a scalar field coupled to gravity are considered in the context of a homogeneous and isotropic universe. The coupling is of the form -\xi/2 \phi^2 R. The possibility of successful inflation is investigated taking into account features of loop cosmology. For that end a conformal transformation is performed. That brings the theory into the standard minimally coupled form (Einstein frame) with some effective field and its potential. Both analytical and numerical estimates show that a negative coupling constant is preferable for successful inflation. Moreover, provided fixed initial conditions, larger|\xi| leads to a greater number of {\em e}-folds. The latter is obtained for a reasonable range of initial conditions and the coupling parameter and indicates a possibility for successful inflation."http://arxiv.org/abs/gr-qc/0606078
On obtaining classical mechanics from quantum mechanics
Ghanashyam Date
22 pages

"Constructing a classical mechanical system associated with a given quantum mechanical one, entails construction of a classical phase space and a corresponding Hamiltonian function from the available quantum structures and a notion of coarser observations. The Hilbert space of any quantum mechanical system naturally has the structure of an infinite dimensional symplectic manifold (`quantum phase space'). There is also a systematic, quotienting procedure which imparts a bundle structure to the quantum phase space and extracts a classical phase space as the base space. This works straight forwardly when the Hilbert space carries weakly continuous representation of the Heisenberg group and recovers the linear classical phase space \mathbb{R}^{\mathrm{2N}}. We report on how the procedure also allows extraction of non-linear classical phase spaces and illustrate it for Hilbert spaces being finite dimensional (spin-j systems), infinite dimensional but separable (particle on a circle) and infinite dimensional but non-separable (Bohr quantization). To construct a corresponding classical dynamics, one needs to choose a suitable section and identify an effective Hamiltonian. The effective dynamics mirrors the quantum dynamics provided the section satisfies conditions of semiclassicality and tangentiality. There is also an alternative and a priori distinct notion of effective Hamiltonian used in the context of a WKB approximation. These two notions are compared for the example of loop quantum cosmology."

 

[marcus]

http://arxiv.org/abs/astro-ph/0606506
Strong gravitational lens probability in TeVeS
Da-Ming Chen, HongSheng Zhao
5 pages, 4 figures, submitted to ApJL

"We calculate the strong lensing probability with the image-separation greater than a given value \Delta\theta and the image flux ratio q_r less than 10 in a TeVeS (tensor-vector-scalar) cosmology, which is a relativistic version of the MOND (modified Newtonian dynamics). The lensing galaxy is modeled by the Hernquist profile. We assume a flat cosmology with \Omega_b=0.04 and the simplest interpolating function \mu(x)
with \mu(x)=x for x<1 and
\mu(x)=1 for x>1. Our calculations show that MOND exhibits a very high lensing efficiency compared with the CDM paradigm. In order to match the well defined sample of the combined radio Cosmic Lens All-Sky Survey (CLASS) and Jodrell Bank/Very Large Array Astrometric Survey (JVAS), the upper limit of the mass of galaxies should be 1.7\times 10^{11} M_\sun. The galaxies produce much more large image-separation lenses in MOND regime than that in CDM regime, while at small image-separation both regimes match the observational data well."

I wish to highlight this because it is Modified Gravity phenomenology----testing the "Dark Matter" hypotheis against the TeVeS relativistic MOND.

I noticed the article initially because of a confusion of names, a different person (Hongbao Zhang) has reported from Beijing about some talks given this week by Carlo Rovelli.
Here is H-B Zhang's post on N.E.W.
http://www.math.columbia.edu/~woit/wordpress/?p=415#comment-12456
Here is H-B Zhang's blog
http://hongbaozhang.blog.edu.cn/user1/3890/
Carlo Rovelli is giving a series of talks about LQG scattering amplitudes at Beijing Normal coinciding with the international String 2006 conference.

In the following paper STUFF if a technical mathematical term
don't knock it might be interesting------comes from UC Riverside
http://arxiv.org/abs/math.QA/0601458
Categorified Algebra and Quantum Mechanics
Jeffrey Morton (University of California, Riverside)
67 pages, 25 figures
"Interest in combinatorial interpretations of mathematical entities stems from the convenience of the concrete models they provide. Finding a bijective proof of a seemingly obscure identity can reveal unsuspected significance to it. Finding a combinatorial model for some mathematical entity is a particular instance of the process called "categorification''. Examples include the interpretation of N as the Burnside rig of the category of finite sets with product and coproduct, and the interpretation of N[x] as the category of combinatorial species. This has interesting applications to quantum mechanics, and in particular the quantum harmonic oscillator, via Joyal's ``species'', a new generalization called ``stuff types'', and operators between these, which can be represented as rudimentary Feynman diagrams for the oscillator. In quantum mechanics, we want to represent states in an algebra over the complex numbers, and also want our Feynman diagrams to carry more structure than these ``stuff operators'' can do, and these turn out to be closely related. We will show how to construct a combinatorial model for the quantum harmonic oscillator in which the group of phases, U(1), plays a special role. We describe a general notion of "M-Stuff Types'' for any monoid M, and see that the case M=U(1) provides an interpretation of time evolution in the combinatorial setting, as well as other quantum mechanical features of the harmonic oscillator."

 

[marcus]

http://arxiv.org/abs/hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics "Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006

"Effective equations are often useful to extract physical information from quantum theories without having to face all technical and conceptual difficulties. One can then describe aspects of the quantum system by equations of classical type, which correct the classical equations by modified coefficients and higher derivative terms. In gravity, for instance, one expects terms with higher powers of curvature. Such higher derivative formulations are discussed here with an emphasis on the role of degrees of freedom and on differences between Lagrangian and Hamiltonian treatments. A general scheme is then provided which allows one to compute effective equations perturbatively in a Hamiltonian formalism. Here, one can expand effective equations around any quantum state and not just a perturbative vacuum. This is particularly useful in situations of quantum gravity or cosmology where perturbations only around vacuum states would be too restrictive. The discussion also demonstrates the number of free parameters expected in effective equations, used to determine the physical situation being approximated, as well as the role of classical symmetries such as Lorentz transformation properties in effective equations. An appendix collects information on effective correction terms expected from loop quantum gravity and string theory."

===========
I didn't catch some of these when they first went on arxiv.

http://arxiv.org/abs/math.DG/0511710
Higher Gauge Theory
John C. Baez, Urs Schreiber
28 pages, 10 figures

"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."

this is a pretty good paper, which clears up doubts about why (from viewpoint of someone to whom spin networks and spinfoam make sense) one should know a bit about twogroups and twoholonomy.
You have to listen when somebody says allows parallel transport along paths and surfaces in a parametrization-independent way-----parallel transport along SURFACES---and then comes up with a necessary condition for that which says that "something" has to equal zero. things that have to be zero for what you want to happen---in this case setting it equal to zero apparently gives something that looks like the BF equation, which I was always wondering where it came from. The paper seems to show that serious old fogeys with gravitas can be interested in twogroups---it is not only just fun, it is required.
Here is an earlier, much longer, related paper:

http://arxiv.org/abs/hep-th/0412325
Higher Gauge Theory: 2-Connections on 2-Bundles
John Baez, Urs Schreiber
73 pages, 4 figures
"Connections and curvings on gerbes are beginning to play a vital role in differential geometry and mathematical physics -- first abelian gerbes, and more recently nonabelian gerbes. These concepts can be elegantly understood using the concept of '2-bundle' recently introduced by Bartels. A 2-bundle is a generalization of a bundle in which the fibers are categories rather than sets. Here we introduce the concept of a '2-connection' on a principal 2-bundle. We describe principal 2-bundles with connection in terms of local data, and show that under certain conditions this reduces to the cocycle data for nonabelian gerbes with connection and curving subject to a certain constraint -- namely, the vanishing of the 'fake curvature', as defined by Breen and Messing. This constraint also turns out to guarantee the existence of '2-holonomies': that is, parallel transport over both curves and surfaces, fitting together to define a 2-functor from the `path 2-groupoid' of the base space to the structure 2-group. We give a general theory of 2-holonomies and show how they are related to ordinary parallel transport on the path space of the base manifold."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, each preserved by the complex structure and parallel transport. In particular, the product of Calabi-Yau manifolds of dimensions 4 and 6 gives such a G-manifold. Moreover, any such G-manifold is naturally a spin manifold, and Dirac spinors on this manifold transform in the representation of G corresponding to one generation of Standard Model fermions and their antiparticles.

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

http://arxiv.org/abs/gr-qc/0606104
Numerical evolutions of a black hole-neutron star system in full General Relativity
Frank Löffler, Luciano Rezzolla, Marcus Ansorg
16 pages, 12 figures

"We present the first simulations in full General Relativity of the head-on collision between a neutron star and a black hole of comparable mass. These simulations are performed through the solution of the Einstein equations combined with an accurate solution of the relativistic hydrodynamics equations via high-resolution shock-capturing techniques. The initial data is obtained by following the York-Lichnerowicz conformal decomposition with the assumption of time symmetry. Unlike other relativistic studies of such systems, no limitation is set for the mass ratio between the black hole and the neutron star, nor on the position of the black hole, whose apparent horizon is entirely contained within the computational domain. The latter extends over 400 M and is covered with six levels of fixed mesh refinement. Concentrating on a prototypical binary system with mass ratio ~6, we find that although a tidal disruption is evident the neutron star is accreted promptly and entirely into the black hole. While the collision is completed before ~300 M, the evolution is carried over up to ~1700 M, thus providing time for the extraction of the gravitational-wave signal produced and allowing for a first estimate of the radiative efficiency of processes of this type."

Basically, the question addressed is: "What kind of ripples does it cause when a black hole eats a neutron star?"

But they were considering the head-on case, and I don't think that makes as much ripples as the spiraling-in case. either way, though, somebody has to compute the gravitational waves that ripple off such events, in case the detectors ever get good enough to pick them up.

================
Recalling that Robert Leigh recently co-authored with Laurent Freidel about 4D Yang-Mills
and there were two papers posted which got a fair amount of attention, in case anyone wants to see a video about this, there is one now:
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/

in the left sidebar menu, scroll down and click where it says THEORY CANADA 2
and you get a menu of 40 presentations at the recent Theory Canada conference,
displayed 12 to a page. Robert Leigh is on page 3, right at the end (he is #36)

I was impressed and thought he gave a good 30 minute talk and even that he and Freidel are making real
progress toward understanding Y-M, and it helps to see the guy talking with his slides instead of only reading the paper.

but if you want the paper too, it is
http://arxiv.org/abs/hep-th/0604184
Towards a solution of pure Yang-Mills theory in 3+1 dimensions
Laurent Freidel, Robert G. Leigh, Djordje Minic
12 pages
"We discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime. The approach is based on the use of local gauge invariant variables in the Schr\"odinger representation and the large N, planar limit. In particular, within this approach we point out unexpected parallels between pure Yang-Mills theory in 2+1 and 3+1 dimensions. The most important parallel shows up in the analysis of the ground state wave-functional especially in view of the numerical similarity of the existing large N lattice simulations of the spectra of 2+1 and 3+1 Yang Mills theories."

and the accompanying Freidel solo paper was
http://arxiv.org/abs/hep-th/0604185
On pure Yang-Mills theory in 3+1 dimensions: Hamiltonian, vacuum and gauge invariant variables
Laurent Freidel
35 pages
"In this work we discuss an analytic approach towards the solution of pure Yang-Mills theory in 3+1 dimensional spacetime which strongly suggests that the recent strategy already applied to pure Yang-Mills theory in 2+1 can be extended to 3+1 dimensions. We show that the local gauge invariant variables introduced by Bars gives a natural generalisation to any dimension of the formalism of Karabali and Nair which recently led to a new understanding of the physics of QCD in dimension 2+1. After discussing the kinematics of these variables, we compute the jacobian between the Yang-Mills and Bars variables and propose a regularization procedure which preserves a generalisation of holomorphic invariance. We discuss the construction of the QCD hamiltonian properly regularized and compute the behavior of the vacuum wave functional both at weak and strong coupling. We argue that this formalism allows the developpement of a strong coupling expansion in the continuum by computing the first local eigenstate of the kinetic part of Yang-Mills hamiltonian."

 

[Mike2]

So the spinfoam goes from one spin network to another in the fashion of a path integral. And each spin network is the eigenfunction of the QG Hamiltonion, blah, blah, blah.

OK. But by what process is the spin network actually measured? What "interaction" causes the superposition of spin networks to "colapse" to a particular eigenstate? I have to wonder if such a measurement process can exist at that level. And if not, then could it be that by definition spacetime must always be assumed to be in a superposition? And what would be the implications of that?

 

[marcus]

So the spinfoam goes from one spin network to another in the fashion of a path integral...

this thread is being used purely for bibliography, Mike, discussion is off topic. Please start a separate thread to ask questions like this--I'm sure folks will be happy to respond. I would not want to reply to your question here in this context (and hope no one else would either) because making a practice of that would clutter the link library and make it harder to use.

 

[marcus]

http://arxiv.org/abs/gr-qc/0606120
A possible experimental test of quantized gravity
P. J. Salzman, S. Carlip
9 pages, 2 figures

"While it is widely believed that gravity should ultimately be treated as a quantum theory, there remains a possibility that general relativity should not be quantized. If this is the case, the coupling of classical gravity to the expectation value of the quantum stress-energy tensor will naturally lead to nonlinearities in the Schrodinger equation. By numerically investigating time evolution in the nonrelativistic 'Schrodinger-Newton' approximation, we show that such nonlinearities may be observable in the next generation of molecular interferometry experiments."
===============
Wolram called attention to this newly posted paper by Floyd Stecker (NASA, Goddard Space Flight Center), an authority on high energy astronomy----cosmic rays, GRB.

http://arxiv.org/abs/astro-ph/0606641
Testing Relativity at High Energies Using Spaceborne Detectors
F. W. Stecker (NASA/GSFC)
13 pages, Text of invited talk at the "From Quantum to Cosmos: Fundamental Physics Studies from Space" meeting

"The Gamma-ray Large Area Space Telescope (GLAST) will measure the spectra of distant extragalactic sources of high energy gamma-rays. GLAST can look for energy dependent propagation effects from such sources as a signal of Lorentz invariance violation (LIV). Such sources should also exhibit high energy spectral cutoffs from pair production interactions with low energy photons. The properties of such cutoffs can also be used to test LIV. ..."
=========
http://arxiv.org/abs/gr-qc/0606121
Uniform discretizations: a quantization procedure for totally constrained systems including gravity
Miguel Campiglia, Cayetano Di Bartolo, Rodolfo Gambini, Jorge Pullin
4 pages

"We present a new method for the quantization of totally constrained systems including general relativity. The method consists in constructing discretized theories that have a well defined and controlled continuum limit. The discrete theories are constraint-free and can be readily quantized. This provides a framework where one can introduce a relational notion of time and that nevertheless approximates in a well defined fashion the theory of interest. The method is equivalent to the group averaging procedure for many systems where the latter makes sense and provides a generalization otherwise. In the continuum limit it can be shown to contain, under certain assumptions, the 'master constraint' of the 'Phoenix project'. It also provides a correspondence principle with the classical theory that does not require to consider the semiclassical limit."
==============
this seems very strange. hovering drops of superfluid helium ?
maybe they could be good for something, if not necessarily to detect gravity waves
I may have to erase this because it is so crazy.

http://arxiv.org/gr-qc/0606118
Proposed observations of gravity waves from the early Universe via "Millikan oil drops"
Raymond. Y. Chiao
10 pages, 2 figures, NASA "Quantum-to-Cosmos" conference proceedings to be published in IJMPD

"Pairs of Planck-mass drops of superfluid helium coated by electrons (i.e., 'Millikan oil drops'), when levitated in a superconducting magnetic trap, can be efficient quantum transducers between electromagnetic (EM) and gravitational (GR) radiation. This leads to the possibility of a Hertz-like experiment, in which EM waves are converted at the source into GR waves, and then back-converted at the receiver from GR waves back into EM waves. Detection of the gravity-wave analog of the cosmic microwave background using these drops can discriminate between various theories of the early Universe."

Maybe (maybe!) it is not so crazy after all. It seems that Raymond Chiao was awarded the Lamb Medal in 2006 or in any case delivered the Lamb Medal Lecture at a January 2006 Conference in Utah. (Remember the Lamb Shift?) He is at the new UC Merced campus jointly in the Schools of Natural Science and of Engineering. His Lamb Medal talk was also about these charged helium droplets (!)
http://arxiv.org/abs/quant-ph/0601193
The Interface between Quantum Mechanics and General Relativity
Raymond Y. Chiao
22 pages, 7 figures; Lamb medal lecture on January 5, 2006 at the Physics of Quantum Electronics Winter Colloquium at Snowbird, Utah; accepted for publication in J. Mod. Optics

"The generation, as well as the detection, of gravitational radiation by means of charged superfluids is considered. One example of such a 'charged superfluid' consists of a pair of Planck-mass-scale, ultracold 'Millikan oil drops,' each with a single electron on its surface, in which the oil of the drop is replaced by superfluid helium. When levitated in a magnetic trap, and subjected to microwave-frequency electromagnetic radiation, a pair of such "Millikan oil drops" separated by a microwave wavelength can become an efficient quantum transducer between quadrupolar electromagnetic and gravitational radiation. This leads to the possibility of a Hertz-like experiment, in which the source of microwave-frequency gravitational radiation consists of one pair of 'Millikan oil drops' driven by microwaves, and the receiver of such radiation consists of another pair of 'Millikan oil drops' in the far field driven by the gravitational radiation generated by the first pair. The second pair then back-converts the gravitational radiation into detectable microwaves. The enormous enhancement of the conversion efficiency for these quantum transducers over that for electrons arises from the fact that there exists macroscopic quantum phase coherence in these charged superfluid systems."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607014
Particles as Wilson lines of gravitational field
L. Freidel, J. Kowalski--Glikman, A. Starodubtsev
19 pages

"Since the work of Mac-Dowell-Mansouri it is well known that gravity can be written as a gauge theory for the de Sitter group. In this paper we consider the coupling of this theory to the simplest gauge invariant observables that is, Wilson lines. The dynamics of these Wilson lines is shown to reproduce exactly the dynamics of relativistic particles coupled to gravity, the gauge charges carried by Wilson lines being the mass and spin of the particles. Insertion of Wilson lines breaks in a controlled manner the diffeomorphism symmetry of the theory and the gauge degree of freedom are transmuted to particles degree of freedom."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607032
The group field theory approach to quantum gravity
Daniele Oriti
17 pages, 2 figures; expanded version of a contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"We give a very concise review of the group field theory formalism for non-perturbative quantum gravity, a higher dimensional generalisation of matrix models. We motivate it as a simplicial and local realisation of the idea of 3rd quantization of the gravitational field, equivalently of a quantum field theory of simplicial geometry, in which also the topology of space is fully dynamical. We highlight the basic structure of the formalism, and discuss briefly various models that are being studied, some recent results and the many open issues that future research should face. Finally, we point out the connections with other approaches to quantum gravity, such as loop quantum gravity, quantum Regge calculus and dynamical triangulations, and causal sets."

http://arxiv.org/abs/gr-qc/0607013
The emergence of background geometry from quantum fluctuations
J. Ambjorn, R. Janik, W. Westra, S. Zohren
12 pages

"We show how the quantization of two-dimensional gravity leads to an (Euclidean) quantum space-time where the average geometry is that of constant negative curvature and where the Hartle-Hawking boundary condition arises naturally."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607039
Quantum Nature of the Big Bang: Improved dynamics
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh
40 pages, 9 figures

"An improved Hamiltonian constraint operator is introduced in loop quantum cosmology. Quantum dynamics of the spatially flat, isotropic model with a massless scalar field is then studied in detail using analytical and numerical methods. The scalar field continues to serve as `emergent time', the big bang is again replaced by a quantum bounce, and quantum evolution remains deterministic across the deep Planck regime. However, while with the Hamiltonian constraint used so far in loop quantum cosmology the quantum bounce can occur even at low matter densities, with the new Hamiltonian constraint it occurs only at a Planck-scale density. Thus, the new quantum dynamics retains the attractive features of current evolutions in loop quantum cosmology but, at the same time, cures their main weakness."

Also posted today:
http://arxiv.org/abs/gr-qc/0607033
Variational Methods in Loop Quantum Cosmology
http://arxiv.org/abs/gr-qc/0607034
On the Green's function and iterative solutions of Loop Quantum Cosmology

More on Loop Quantum Cosmology:
http://arxiv.org/abs/gr-qc/0607044
Instabilities in numerical loop quantum cosmology
Jessica Rosen, Jae-Hun Jung, Gaurav Khanna
6 pages, 8 figures

"In this article we perform von Neumann analysis of the difference equations that arise as a result of loop quantum gravity being applied to models of cosmology and black holes. In particular, we study the numerical stability of Bianchi I LRS (symmetric and non-symmetric constraint) and Schwarzschild interior (symmetric constraint) models, and find that there exist domains over which there are instabilities, generically. We also present explicit evolutions of wave-packets in these models and clearly demonstrate the presence of these instabilities."

Khanna is in the Physics Department at Dartmouth. His papers go back a ways and he has co-authored with Bojowald. His two co-authors are in the Dartmouth Math Department. E.g. J-H Jung has 26 papers in arxiv grp_physics going back to 1998. Good for Dartmouth.

http://arxiv.org/abs/astro-ph/0607227
Why anthropic reasoning cannot predict Lambda
Glenn D. Starkman (Case Western Reserve University and Oxford U.), Roberto Trotta (Oxford University)
4 pages, 1 figure
"We revisit anthropic arguments purporting to explain the measured value of the cosmological constant. We argue that different ways of assigning probabilities to candidate universes lead to totally different anthropic predictions. As an explicit example, we show that weighting different universes by the total number of possible observations leads to an extremely small probability for observing a value of Lambda equal to or greater than what we now measure. We conclude that anthropic reasoning within the framework of probability as frequency is ill-defined and that it cannot be used to explain the value of Lambda, nor, likely, any other physical parameters."

 

[marcus]

cant evaluate, but anybody's new idea of a way to test QG, I want to know about it.

http://arxiv.org/abs/hep-ph/0607145
GRBs Neutrinos as a Tool to Explore Quantum Gravity induced Lorentz Violation
Uri Jacob, Tsvi Piran

"Lorentz Invariance Violation (LIV) arises in various quantum-gravity theories. As the typical energy for quantum gravity is the Planck mass, M_{pl}, LIV will, most likely, be manifested at very high energies that are not accessible on Earth in the foreseeable future. One has to turn to astronomical observations. Time of flight measurement from different astronomical sources set current limits on the energy scale of possible LIV to > 0.01 M_{pl} (for n=1 models) and > 10^{-9} M_{pl} (for n=2). According to current models Gamma-Ray Bursts (GRBs) are accompanied by bursts of high energy (\gsim 100TeV) neutrinos. At this energy range the background level of currently constructed neutrino detectors is so low that a detection of a single neutrino from the direction of a GRB months or even years after the burst would imply an association of the neutrino with the burst and will establish a measurement of a time of flight delay. Such time of flight measurements provide the best way to observe (or set limits) on LIV. Detection of a single GRB neutrino would open a new window on LIV and would improve current limits by many orders of magnitude."

and Tsvi Piran is a recognized specialist in figuring out how to test QG
he was invited to the February 2004 Polish winter symposium on testing QG
and gave a bunch of talks (at the time not so many people had thought to investigate
observational tests of QG) I can't tell if his ideas are good but other people pay attention.
Maybe synching neutrino detection with gammaray observation makes sense---leastwise we have the tools
to do it.

 

[marcus]

this thread is non-string QG bibliography, it gives links to recent non-string QG papers. usually I just keep track of postings on arxiv.org.
this time I am going to add the latest edition of John Baez "This Week's Finds".
the reason is I have a hunch that eventually we are all going to want to understand something about SO(4,1)

I think that particular Lie group is going to be the kitchen stool by which humans reach the high shelf, so it is apt to become familiar, the way the poincaré group and lorentz group are familiar.

Curiously enough, I don't know anything online that is a real kindergarten introduction to SO(4,1). Hey, don't knock kindergarten, it is probably the basis of western civilization or something We all at some point need a basic introduction to things!

So maybe someone can suggest a better ONLINE intuitive intro to DeSitter group?

Until then, the best I know is a few paragraphs in TWF 235, around page 5 out of 8 pages that talks about a lot of other stuff.

http://www.math.ucr.edu/home/baez/week235.html

 

[marcus]

http://arxiv.org/abs/astro-ph/0607380
Solving the Problem of Time in General Relativity and Cosmology with Phantoms and k -- Essence
Thomas Thiemann
38 pages

"We show that if the Lagrangean for a scalar field coupled to General Relativity only contains derivatives, then it is possible to completely deparametrise the theory. This means that
1.Physical observables, i.e. functions which Poisson commute with the spatial diffeomorphism and Hamiltonian constraints of General Relativity, can be easily constructed.
2. The physical time evolution of those observables is generated by a natural physical Hamiltonian which is (constrained to be) positive.

The mechanism by which this works is due to Brown and Kuchar. In order that the physical Hamiltonian is close to the Hamiltonian of the standard model and the one used in cosmology, the required Lagrangean must be that of a Dirac -- Born -- Infeld type. Such matter has been independently introduced previously by cosmologists in the context of k -- essence due to Armendariz-Picon, Mukhanov and Steinhardt in order to solve the cosmological coincidence (dark energy) problem. We arrive at it by totally unrelated physical considerations originating from quantum gravity. Our manifestly gauge invariant approach leads to important modifications of the interpretation and the the analytical appearance of the standard FRW equations of classical cosmology in the late universe. In particular, our concrete model implies that the universe should recollapse at late times on purely classical grounds."http://arxiv.org/abs/gr-qc/0607075
Approximating the physical inner product of Loop Quantum Cosmology
Benjamin Bahr, Thomas Thiemann
35 pages===================

Quantum Theory in Cosmology
The seminar series at Perimeter Institute has a talk given by Martin Bojowald on 28 July
http://streamer.perimeterinstitute.ca:81/mediasite/viewer/FrontEnd/Front.aspx?&shouldResize=False
It is about Quantum Gravity in Cosmology and is particularly interesting because it discusses
the role QG plays in early structure formation in the universe---in the conversion of quantum fluctuations
into corresponding classcial ones.

========at FH's suggestion===========

http://arxiv.org/gr-qc/0607068
Dirac Quantization of Parametrized Field Theory
Madhavan Varadarajan
33 pages

"Parametrized field theory (PFT) is free field theory on flat spacetime in a diffeomorphism invariant disguise. It describes field evolution on arbitrary foliations of the flat spacetime instead of only the usual flat ones, by treating the `embedding variables' which describe the foliation as dynamical variables to be varied in the action in addition to the scalar field. A formal Dirac quantization turns the constraints of PFT into functional Schrodinger equations which describe evolution of quantum states from an arbitrary Cauchy slice to an infinitesimally nearby one.This formal Schrodinger picture- based quantization is unitarily equivalent to the standard Heisenberg picture based Fock quantization of the free scalar field if scalar field evolution along arbitrary foliations is unitarily implemented on the Fock space. Torre and Varadarajan (TV) showed that for generic foliations emanating from a flat initial slice in spacetimes of dimension greater than 2, evolution is not unitarily implemented, thus implying an obstruction to Dirac quantization.
We construct a Dirac quantization of PFT, unitarily equivalent to the standard Fock quantization, using techniques from Loop Quantum Gravity (LQG) which are powerful enough to supercede the no-go implications of the TV results. The key features of our quantization include an LQG type representation for the embedding variables, embedding dependent Fock spaces for the scalar field, an anomaly free representation of (a generalization of) the finite transformations generated by the constraints and group averaging techniques. The difference between 2 and higher dimensions is that in the latter, only finite gauge transformations are defined in the quantum theory, not the infinitesimal ones."

 

[marcus]

Mann and Popescu have co-authored with Freidel. This one sounds like it might connect with Freidel papers on the 3D case. Also with Higher Gauge Theory (HGT) discussed by John Baez----several of whose papers they cite.
http://arxiv.org/abs/gr-qc/0607076
Higher Gauge Theory and Gravity in (2+1) Dimensions
R.B. Mann, Eugeniu M. Popescu
22 pages
"Non-abelian higher gauge theory has recently emerged as a generalization of standard gauge theory to higher dimensional (2-dimensional in the present context) connection forms, and as such, it has been successfully applied to the non-abelian generalizations of the Yang-Mills theory and 2-form electrodynamics. (2+1)-dimensional gravity, on the other hand, has been a fertile testing ground for many concepts related to classical and quantum gravity, and it is therefore only natural to investigate whether we can find an application of higher gauge theory in this latter context. In the present paper we investigate the possibility of applying the formalism of higher gauge theory to gravity in (2+1) dimensions, and we show that a nontrivial model of (2+1)-dimensional gravity coupled to scalar and tensorial matter fields - the \Sigma\Phi EA model - can be formulated both as a standard gauge theory and as a higher gauge theory. Since the model has a very rich structure - it admits as solutions black-hole BTZ-like geometries, particle-like geometries as well as Robertson-Friedman-Walker cosmological-like expanding geometries - this opens a wide perspective for higher gauge theory to be tested and understood in a relevant gravitational context. Additionally, it offers the possibility of studying gravity in (2+1) dimensions coupled to matter in an entirely new framework."

 

[selfAdjoint]

Kristina Giesl & Thomas Thiemann - AQG

Krisitna Giesel and Thomas Thiemann have submitted three papers on a new subject in QG: Algebraic Quantum Gravity. Here are the descriptions.

http://www.arxiv.org/abs/gr-qc/0607099
gr-qc/0607099
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:23:01 GMT (40kb)
Algebraic Quantum Gravity (AQG) I. Conceptual Setup
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-058
"We introduce a new top down approach to canonical quantum gravity, called Algebraic Quantum Gravity (AQG):The quantum kinematics of AQG is determined by an abstract $*-$algebra generated by a countable set of elementary operators labelled by an algebraic graph. The quantum dynamics of AQG is governed by a single Master Constraint operator. While AQG is inspired by Loop Quantum Gravity (LQG), it differs drastically from it because in AQG there is fundamentally no topology or differential structure. A natural Hilbert space representation acquires the structure of an infinite tensor product (ITP) whose separable strong equivalence class Hilbert subspaces (sectors) are left invariant by the quantum dynamics. The missing information about the topology and differential structure of the spacetime manifold as well as about the background metric to be approximated is supplied by coherent states. Given such data, the corresponding coherent state defines a sector in the ITP which can be identified with a usual QFT on the given manifold and background. Thus, AQG contains QFT on all curved spacetimes at once, possibly has something to say about topology change and provides the contact with the familiar low energy physics. In particular, in two companion papers we develop semiclassical perturbation theory for AQG and LQG and thereby show that the theory admits a semiclassical limit whose infinitesimal gauge symmetry agrees with that of General Relativity. In AQG everything is computable with sufficient precision and no UV divergences arise due to the background independence of the undamental combinatorial structure. Hence, in contrast to lattice gauge theory on a background metric, no continuum limit has to be taken, there simply is no lattice regulator that must be sent to zero."

http://www.arxiv.org/abs/gr-qc/0607100
gr-qc/0607100
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:27:43 GMT (43kb)
Algebraic Quantum Gravity (AQG) II. Semiclassical Analysis
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-59
"In the previous article a new combinatorial and thus purely algebraical approach to quantum gravity, called Algebraic Quantum Gravity (AQG), was introduced. In the framework of AQG existing semiclassical tools can be applied to operators that encode the dynamics of AQG such as the Master constraint operator. In this article we will analyse the semiclassical limit of the (extended) algebraic Master constraint operator and show that it reproduces the correct infinitesimal generators of General Relativity. Therefore the question whether General Relativity is included in the semiclassical sector of the theory, which is still an open problem in LQG, can be significantly improved in the framework of AQG. For the calculations we will substitute SU(2) by U(1)^3. That this substitution is justified will be demonstrated in the third article of this series "

http://www.arxiv.org/abs/gr-qc/0607101
gr-qc/0607101
From: Kristina Giesel [view email]
Date: Sun, 23 Jul 2006 21:30:29 GMT (27kb)
Algebraic Quantum Gravity (AQG) III. Semiclassical Perturbation Theory
Authors: K. Giesel, T. Thiemann
Report-no: AEI-2006-60
"In the two previous papers of this series we defined a new combinatorical approach to quantum gravity, Algebraic Quantum Gravity (AQG). We showed that AQG reproduces the correct infinitesimal dynamics in the semiclassical limit, provided one incorrectly substitutes the non -- Abelean group SU(2) by the Abelean group $U(1)^3$ in the calculations. The mere reason why that substitution was performed at all is that in the non -- Abelean case the volume operator, pivotal for the definition of the dynamics, is not diagonisable by analytical methods. This, in contrast to the Abelean case, so far prohibited semiclassical computations. In this paper we show why this unjustified substitution nevertheless reproduces the correct physical result: Namely, we introduce for the first time semiclassical perturbation theory within AQG (and LQG) which allows to compute expectation values of interesting operators such as the master constraint as a power series in $\hbar$ with error control. That is, in particular matrix elements of fractional powers of the volume operator can be computed with extremely high precision for sufficiently large power of $\hbar$ in the $\hbar$ expansion. With this new tool, the non -- Abelean calculation, although technically more involved, is then exactly analogous to the Abelean calculation, thus justifying the Abelean analysis in retrospect. The results of this paper turn AQG into a calculational discipline."

I can see this new area as being a magnet for young trained theoretical physicsts because it is both Algebraic (and algebraic is so cool!) and Perturbative (and hence at least potentially accessible to people with non-LQG training).

 

[marcus]

Good find, selfAdjoint! I was just about to post the Thiemann and Giesel work myself.

Here is another that arrived today

http://arxiv.org/abs/gr-qc/0607097
Semiclassical states for quantum cosmology
Viqar Husain, Oliver Winkler
13 pages
"In a metric variable based Hamiltonian quantization, we give a prescription for constructing semiclassical matter-geometry states for homogeneous and isotropic cosmological models. These "collective" states arise as infinite linear combinations of fundamental excitations in an unconventional "polymer" quantization. They satisfy a number of properties characteristic of semiclassicality, such as peaking on classical phase space configurations. We describe how these states can be used to determine quantum corrections to the classical evolution equations, and to compute the initial state of the universe by a backward time evolution."

keeping an eye on the ekpyrotic alternative:
http://arxiv.org/abs/hep-th/0607164
Cosmic Perturbations Through the Cyclic Ages

Mattingly may have an improved way to constrain the causal set approach to QG
http://arxiv.org/abs/astro-ph/0607485
Low energy bounds on Poincare violation in causal set theory
Nemanja Kaloper, David Mattingly
8 pages
"In the causal set approach to quantum gravity, Poincaré symmetry is modified by swerving in spacetime, induced by the random lattice discretization of the space-time structure. The broken translational symmetry at short distances is argued to lead to a residual diffusion in momentum space, whereby a particle can acquire energy and momentum by drift along its mass shell and a system in equilibrium can spontaneously heat up. We consider bounds on the rate of momentum space diffusion coming from astrophysical molecular clouds, nuclear stability and cosmological neutrino background. We find that the strongest limits come from relic neutrinos, which we estimate to constrain the momentum space diffusion constant by k < 10^{-61} {\rm GeV}^3 for neutrinos with masses m_\nu > 0.01 {\rm eV}, improving the previously quoted bounds by roughly 17 orders of magnitude."

 

[marcus]

On Thursday 3 August, a few days from now, Andy Randono will be giving a talk at Perimeter. Andy is a graduate student at University of Texas. He did his undergraduate work at Tufts. he has been studying the Kodama state---a QG state with connections to DeSitter space. here is the abstract of the talk:

Andy Randono
Generalizing the Kodama State
Thursday August 3, 2006, 1:30 AM
"The Kodama state is unique in being an exact solution to all the constraints of quantum gravity that also has a well defined semi-classical interpretation as a quantum version of a classical spacetime, namely de-Sitter spacetime. In addition, the exact form of the state is known in both the connection and spin network bases, thereby concretely realizing the beautiful connection between spin networks and functionals on the space of connections. However, the state is riddled with difficulties that we will track down to the complexification necessary in its construction. This suggests a generalization to real variables, which, we will argue, can be accomplished. This solves most of the standard problems associated with the state, but in addition opens up a large Hilbert class of states vastly generalizing the de-Sitter/Chern-Simons solution."

Perimeter Streamers website sometimes puts these seminar talks online. I hope they do this one. Andy also has a couple of papers on arxiv:

http://arxiv.org/abs/gr-qc/0504010
A Generalization of the Kodama State for Arbitrary Values of the Immirzi Parameter
Andrew Randono
16 pages
"The Kodama State for Lorentzian gravity presupposes a particular value for the Immirzi-parameter, namely \beta=-i. However, the derivation of black hole entropy in Loop Quantum Gravity suggests that the Immirzi parameter is a fixed value whose magnitude is on the order of unity but larger than one. Since the Kodama state has de-Sitter spacetime as its classical limit, to get the proper radiation temperature, the Kodama state should be extended to incorporate a more physical value for \beta. Thus, we present an extension of the Kodama state for arbitrary values of the Immirzi parameter, \beta, that reduces to the ordinary Chern-Simons state for the particular value \beta=-i. The state for real values of \beta is free of several of the outstanding problems that cast doubts on the original Kodama state as a ground state for quantum general relativity. We show that for real values of \beta, the state is invariant under large gauge transformations, it is CPT invariant (but not CP invariant), and it is expected to be delta-function normalizable with respect to the kinematical inner product. To aid in the construction, we first present a general method for solving the Hamiltonian constraint for imaginary values of \beta that allows one to use the simpler self-dual and anti-self-dual forms of the constraint as an intermediate step."

http://arxiv.org/abs/hep-th/0510001
A Note on Parity Violation and the Immirzi Parameter
Andrew Randono
7 pages
There has been considerable recent interest in the Immirzi parameter as a measure of parity violating effects in the classical theory of gravitation with fermion coupling. Most recently it was shown that the Immirzi parameter together with the non-minimal coupling constant of Dirac spinors provides the measure for parity violating spin-spin interaction terms in the effective field theory. For complex values of the Immirzi parameter, the resulting effective field theory yields complex values for the torsion, and a non-unitary effective field theory that blows up for the special cases \gamma=\pm i where the gravitational kinetic term is the Ashtekar action. We show that by restricting ourselves to real values for the torsion, there is a natural set of choice for the non-minimal coupling constant that yields real and unitary effective field theory that does not blow up for the special cases \gamma=\pm i. We then show that these particular values for the non-minimal coupling coefficients most naturally follow from a non-minimal pseudo-kinetic term in the fermion Lagrangian."

Some pictures of Andy and friends hiking in the Colorado Rockies
http://www.ma.utexas.edu/users/stirling/0508colorado/0508colorado.html
The best one of Andy is scroll halfway down to where the guy snapping the pictures has captioned one:
"I think that Andy looks like a dork with his walking stick."
In fact he does not look dorky. He is dressed and equipped practically for high mountain hiking
and he looks cheerful, which some of the others don't.

It is amusing to think of Randono occupying the same physics department as Jacques (death-to-Loop-Gravity!) Distler. It shows a certain fortitude and independent spirit IMO.
=======================
http://arxiv.org/abs/gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at "Einstein's Legacy in the New Millenium,'' Puri, India, December 2005

"General relativity successfully describes space-times at scales that we can observe and probe today, but it cannot be complete as a consequence of singularity theorems. For a long time there have been indications that quantum gravity will provide a more complete, non-singular extension which, however, was difficult to verify in the absence of a quantum theory of gravity. By now there are several candidates which show essential hints as to what a quantum theory of gravity may look like. In particular, loop quantum gravity is a non-perturbative formulation which is background independent, two properties which are essential close to a classical singularity with strong fields and a degenerate metric. In cosmological and black hole settings one can indeed see explicitly how classical singularities are removed by quantum geometry: there is a well-defined evolution all the way down to, and across, the smallest scales. As for black holes, their horizon dynamics can be studied showing characteristic modifications to the classical behavior. Conceptual and physical issues can also be addressed in this context, providing lessons for quantum gravity in general. Here, we conclude with some comments on the uniqueness issue often linked to quantum gravity in some form or another."

 

[marcus]

http://arxiv.org/abs/gr-qc/0607136
Quantum Gowdy T³ model: A uniqueness result
Alejandro Corichi, Jeronimo Cortez, Guillermo A. Mena Marugan, Jose M. Velhinho
27 pages

"Modulo a homogeneous degree of freedom and a global constraint, the linearly polarised Gowdy T³ cosmologies are equivalent to a free scalar field propagating in a fixed nonstationary background. Recently, a new field parameterisation was proposed for the metric of the Gowdy spacetimes such that the associated scalar field evolves in a flat background in 1+1 dimensions with the spatial topology of S¹, although subject to a time dependent potential. Introducing a suitable Fock quantisation for this scalar field, a quantum theory was constructed for the Gowdy model in which the dynamics is implemented as a unitary transformation. A question that was left open is whether one might adopt a different, nonequivalent Fock representation by selecting a distinct complex structure. The present work proves that the chosen Fock quantisation is in fact unique (up to unitary equivalence) if one demands unitary implementation of the dynamics and invariance under the group of constant S¹translations. These translations are precisely those generated by the global constraint that remains on the Gowdy model. It is also shown that the proof of uniqueness in the choice of complex structure can be applied to more general field dynamics than that corresponding to the Gowdy cosmologies."

 

[marcus]

http://arxiv.org/abs/astro-ph/0608034
The Accelerating Universe and a Limiting Curvature Proposal
Damien A. Easson
21 pages, 7 figures

"We consider the hypothesis of a limiting minimal curvature in gravity as a way to construct a class of theories exhibiting late-time cosmic acceleration. Guided by the minimal curvature conjecture (MCC) we are naturally lead to a set of scalar tensor theories in which the scalar is non-minimally coupled to the matter Lagrangian. The model is compared to the Lambda Cold Dark Matter concordance model and to the observational data using the gold SNeIa sample of Riess et. al. (2004). We present a toy model designed to demonstrate the possibility that such a new, possibly fundamental, principle may be responsible for the recent period of cosmological acceleration."

Damien Easson is a young fellow in UK who has co-authored with Sean Carroll, Mark Trodden, Michael Turner, Robert Brandenburger. this is just a toy model illustrating a preliminary idea. It is an odd idea---though not too different from just admitting a bare CC in the Einstein equation. He proposes a permanent minimal warp as substitute for Dark Energy. IIRC there is a type of relativistic MOND which dispenses with Dark Energy. In any case, since DE is a puzzle, we might keep an eye out for variations of gravity which explain acceleration some other way. even if the idea seems peculiar and preliminary.
==================
http://arxiv.org/abs/gr-qc/0608018
The History and Present Status of Quantum Field Theory in Curved Spacetime
Robert M. Wald
15 pages, no figures; contribution to 7th International Conference on the History of General Relativity

"Quantum field theory in curved spacetime is a theory wherein matter is treated fully in accord with the principles of quantum field theory, but gravity is treated classically in accord with general relativity. It is not expected to be an exact theory of nature, but it should provide a good approximate description when the quantum effects of gravity itself do not play a dominant role. A major impetus to the theory was provided by Hawking's calculation of particle creation by black holes, showing that black holes radiate as perfect black bodies. During the past 30 years, considerable progress has been made in giving a mathematically rigorous formulation of quantum field theory in curved spacetime. Major issues of principle with regard to the formulation of the theory arise from the lack of Poincare symmetry and the absence of a preferred vacuum state or preferred notion of 'particles'. By the mid-1980's, it was understood how all of these difficulties could be overcome for free (i.e., non-self-interacting) quantum fields by formulating the theory via the algebraic approach and focusing attention on the local field observables rather than a notion of 'particles'. However, these ideas, by themselves, were not adequate for the formulation of interacting quantum field theory, even at a perturbative level, since standard renormalization prescriptions in Minkowski spacetime rely heavily on Poincare invariance and the existence of a Poincare invariant vacuum state. However, during the past decade, great progress has been made, mainly due to the importation into the theory of the methods of 'microlocal analysis'. This article will describe the historical development of the subject and describe some of the recent progress."

survey by a senior guy
================
http://arxiv.org/abs/quant-ph/0608035
Quantum measurements without sums
Bob Coecke, Dusko Pavlovic
36 pages and 46 pictures; earlier version circulated since November 2005 with as title 'Quantum Measurements as Coalgebras''. Invited paper to appear in: The Mathematics of Quantum Computation and Technology; Chen, Kauffman and Lomonaco (eds.); Taylor and Francis

"Sums play a prominent role in the formalisms of quantum mechanics, be it for mixing and superposing states, or for composing state spaces. Surprisingly, a conceptual analysis of quantum measurement seems to suggest that quantum mechanics can be done without direct sums, expressed entirely in terms of the tensor product. The corresponding axioms define classical spaces as objects that allow copying and deleting data. Indeed, the information exchange between the quantum and the classical worlds is essentially determined by their distinct capabilities to copy and delete data. The sums turn out to be an implicit implementation of this capabilities. Realizing it through explicit axioms not only dispenses with the unnecessary structural baggage, but also allows a simple and intuitive graphical calculus. In category-theoretic terms, classical data types are dagger-compact Frobenius algebras, and quantum spectra underlying quantum measurements are Eilenberg-Moore coalgebras induced by these Frobenius algebras."

---sample exerpt from page 2 of the article---
A recent research thread, initiated by Abramsky and the first author [2], aims at recasting the quantum mechanical formalism in categorical terms. The upshot of categorical semantics is that it displays concepts in a compositional and typed framework. In the case of quantum mechanics, it uncovers the quantum information-flows [6] which are hidden in the usual formalism. Moreover, while the investigations of quantum structures have so far been predominantly academic, categorical semantics open an alley towards a practical, low-overhead tool for the design and analysis of quantum informatic protocols, versatile enough to capture both quantitative and qualitative aspects of quantum information [2, 7, 10, 13, 31]. In fact, some otherwise complicated quantum informatic protocols become trivial exercises in this framework [8]. On the other hand, compared with the order-theoretic framework for quantum mechanics in terms of Birkhoff-von Neumann’s quantum logic [29], this categorical setting comes with logical derivations, topologically embodied into something as simple as “yanking a rope”. 2[/color] Moreover, in terms of deductive machanism, it turns out to be some kind of “super-logic” as compared to the Birkhoff-von Neumann “non-logic”.
---endquote---

Baez was talking about stretching out a piece of wet spaghetti. curious propositions in quantum theory, seeming paradoxes, become trivial exercises as Coecke says. Baez was trying to get that idea across---basically one of the reasons why one might see categorical semantics infiltrate into physics.

I am guessing that this paper of Coecke et al will be mentioned in the #237 issue of This Weeks Finds.

 

[marcus]

http://arxiv.org/abs/gr-qc/0608074
A Modified Gravity and its Consequences for the Solar System, Astrophysics and Cosmology
J. W. Moffat
Comments: 15 pages. Talk given at the International Workshop "From Quantum to Cosmos: Fundamental Physics in Space", 22-24 May, 2006, Warrenton, Virginia, USA

"A relativistic modified gravity (MOG) theory leads to a self-consistent, stable gravity theory that can describe the solar system, galaxy and clusters of galaxies data and cosmology."

http://arxiv.org/abs/astro-ph/0608344
The rapid formation a large rotating disk galaxy three billion years after the Big Bang
R. Genzel, L. Tacconi, F. Eisenhauer, N. M. Forster Schreiber, A. Cimatti, E. Daddi, N. Bouche, R. Davies, M. D. Lehnert, D. Lutz, N. Nesvadba, A. Verma, R. Abuter, K. Shapiro, A. Sternberg, A. Renzini, X. Kong, N. Arimoto, M. Mignoli
Nature, accepted (Released Aug 17th)

"Over the past two decades observations and theoretical simulations have established a global frame-work of galaxy formation and evolution in the young Universe. Galaxies formed as baryonic gas cooled at the centres of collapsing dark matter halos. Mergers of halos led to the build up of galaxy mass. A major step forward in understanding these issues requires well resolved physical information on individual galaxies at high redshift. Here we report adaptive optics, spectroscopic observations of a representative luminous star forming galaxy when the Universe was only twenty percent of its age. The superior angular resolution of these data reveals the physical and dynamical properties of a high redshift galaxy in unprecedented detail. A large and massive rotating proto-disk is channelling gas towards a growing central stellar bulge hosting an accreting massive black hole."

http://arxiv.org/abs/astro-ph/0608345
Quark Matter in Neutron Stars: An aperçu
Prashanth Jaikumar, Sanjay Reddy, Andrew W. Steiner
15 pages, 1 figure. Invited review for Modern Physics Letters A

"The existence of deconfined quark matter in the superdense interior of neutron stars is a key question that has drawn considerable attention over the past few decades. Quark matter can comprise an arbitrary fraction of the star, from 0 for a pure neutron star to 1 for a pure quark star, depending on the equation of state of matter at high density. From an astrophysical viewpoint, these two extreme cases are generally expected to manifest different observational signatures. An intermediate fraction implies a hybrid star, where the interior consists of mixed or homogeneous phases of quark and nuclear matter, depending on surface and Coulomb energy costs, as well as other finite size and screening effects. In this brief review article, we discuss what we can deduce about quark matter in neutron stars in light of recent exciting developments in neutron star observations. We state the theoretical ideas underlying the equation of state of dense quark matter, including color superconducting quark matter. We also highlight recent advances stemming from re-examination of an old paradigm for the surface structure of quark stars and discuss possible evolutionary scenarios from neutron stars to quark stars, with emphasis on astrophysical observations."

http://arxiv.org/abs/math.CT/0608420
Lectures on n-Categories and Cohomology
John C. Baez, Michael Shulman
68 pages, 2 ps and 2 eps figures

"The goal of these talks was to explain how cohomology and other tools of algebraic topology are seen through the lens of n-category theory. Special topics include nonabelian cohomology, Postnikov towers, the theory of "n-stuff", and n-categories for n = -1 and -2. The talks were very informal, and so are these notes. A lengthy appendix clarifies certain puzzles and ventures into deeper waters such as higher topos theory. For readers who want more details, we include an annotated bibliography."

http://arxiv.org/abs/hep-th/0608120
Holography of Gravitational Action Functionals
A. Mukhopadhyay, T. Padmanabhan
17 pages

"Einstein-Hilbert (EH) action can be separated into a bulk and a surface term, with a specific ("holographic") relationship between the two, so that either can be used to extract information about the other. The surface term can also be interpreted as the entropy of the horizon in a wide class of spacetimes. Since EH action is likely to just the first term in the derivative expansion of an effective theory, it is interesting to ask whether these features continue to hold for more general gravitational actions. We provide a comprehensive analysis of lagrangians of the form L=Q_a^{bcd}R^a_{bcd}, in which Q_a^{bcd} is a tensor with the symmetries of the curvature tensor, made from metric and curvature tensor and satisfies the condition \nabla_cQ^{abcd}=0, and show that they share these features. The Lanczos-Lovelock lagrangians are a subset of these in which Q^{abcd} is a homogeneous function of the curvature tensor. They are all holographic, in a specific sense of the term, and -- in all these cases -- the surface term can be interpreted as the horizon entropy. The thermodynamics route to gravity, in which the field equations are interpreted as TdS=dE+pdV, seems to have greater degree of validity than the field equations of Einstein gravity itself. The results suggest that the holographic feature of EH action could also serve as a new symmetry principle in constraining the semiclassical corrections to Einstein gravity. The implications are discussed."

 

[marcus]

http://arxiv.org/abs/gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
4 pages, 1 figure

"An exactly solvable bounce model in loop quantum cosmology is identified which serves as a perturbative basis for realistic bounce scenarios. Its bouncing solutions are derived analytically, demonstrating why recent numerical simulations robustly led to smooth bounces under the assumption of semiclassicality. Several effects, easily included in a perturbative analysis, can however change this smoothness. The effective theory is not only applicable to such situations where numerical techniques become highly involved but also allows one to discuss conceptual issues. For instance, consequences of the notoriously difficult physical inner product can be implemented at the effective level. This indicates that even physical predictions from full quantum gravity can be obtained from perturbative effective equations."

http://arxiv.org/abs/astro-ph/0608407
A direct empirical proof of the existence of dark matter
Douglas Clowe (1), Marusa Bradac (2), Anthony H. Gonzalez (3), Maxim Markevitch (4), Scott W. Randall (4), Christine Jones (4), Dennis Zaritsky (1) ((1) Steward Observatory, Tucson, (2) KIPAC, Stanford, (3) Department of Astronomy, Gainesville, (4) CfA, Cambridge)
Accepted for publication in ApJL
"We present new weak lensing observations of 1E0657-558 (z=0.296), a unique cluster merger, that enable a direct detection of dark matter, independent of assumptions regarding the nature of the gravitational force law. Due to the collision of two clusters, the dissipationless stellar component and the fluid-like X-ray emitting plasma are spatially segregated. By using both wide-field ground based images and HST/ACS images of the cluster cores, we create gravitational lensing maps which show that the gravitational potential does not trace the plasma distribution, the dominant baryonic mass component, but rather approximately traces the distribution of galaxies. An 8-sigma significance spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law, and thus proves that the majority of the matter in the system is unseen."

http://arxiv.org/abs/astro-ph/0608408
Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56
Marusa Bradac (1,2), Douglas Clowe (3), Anthony H. Gonzalez (4), Phil Marshall (1), William Forman (5), Christine Jones (5), Maxim Markevitch (5), Scott Randall (5), Tim Schrabback (2), Dennis Zaritsky (3) ((1) KIPAC, Stanford, (2) AIfA, Bonn, (3) Steward Observatory, Tucson, (4) Department of Astronomy, Gainesville, (5) CfA, Cambridge)
Accepted for publication in ApJ; Version with full-resolution figures available at this URL:
http://www.slac.stanford.edu/~marusa/Work/bradac_strong_weak_III.pdf

"The galaxy cluster 1E0657-56 (z = 0.296) is remarkably well-suited for addressing outstanding issues in both galaxy evolution and fundamental physics. We present a reconstruction of the mass distribution from both strong and weak gravitational lensing data. Multi-color, high-resolution HST ACS images allow detection of many more arc candidates than were previously known, especially around the subcluster. Using the known redshift of one of the multiply imaged systems, we determine the remaining source redshifts using the predictive power of the strong lens model. Combining this information with shape measurements of "weakly" lensed sources, we derive a high-resolution, absolutely-calibrated mass map, using no assumptions regarding the physical properties of the underlying cluster potential. This map provides the best available quantification of the total mass of the central part of the cluster. We also confirm the result from Clowe et al. (2004,2006a)."

http://arxiv.org/abs/quant-ph/0605070
High Energy Quantum Teleportation Using Neutral Koans
Yu Shi
6 pages, 4 figures. Appearing in Physics Letters B

[sic]

for traveling Zen masters

http://arxiv.org/abs/astro-ph/0608417
Planetesimals To Brown Dwarfs: What is a Planet?
Gibor Basri, Michael E. Brown (Univ. of California, Berkeley and California Inst. of Technology)
23 pages
Annual Reviews of Earth and Planetary Science, 2006, v. 34, pp. 193-216
"The past 15 years have brought about a revolution in our understanding of our Solar System and other planetary systems. During this time, discoveries include the first Kuiper Belt Objects, the first brown dwarfs, and the first extra-solar planets. Although discoveries continue apace, they have called into question our previous perspectives on planets, both here and elsewhere. The result has been a debate about the meaning of the word ''planet'' itself. It became clear that scientists do not have a widely accepted or clear definition of what a planet is, and both scientists and the public are confused (and sometimes annoyed) by its use in various contexts. Because ''planet'' is a very widely used term, it seems worth the attempt to resolve this problem. In this essay, we try to cover all the issues that have come to the fore, and bring clarity (if not resolution) to the debate."

Mike Brown has good sense. I've heard him talk. I'd take his advice on terminology and science+culture issues.
Besides, he found Sedna.

http://arxiv.org/abs/physics/0608156
Threat Mitigation: The Asteroid Tugboat
Russell Schweickart (B612 Foundation), Clark Chapman, Dan Durda, (Southwest Research Institute), Piet Hut (Institute for Advanced Studies)
Submitted to NASA Workshop on Near-Earth Objects, Vail, Colorado, June 2006

"The Asteroid Tugboat (AT) is a fully controlled asteroid deflection concept using a robotic spacecraft powered by a high efficiency, electric propulsion system (ion or plasma) which docks with and attaches to the asteroid, conducts preliminary operations, and then thrusts continuously parallel to the asteroid velocity vector until the desired velocity change is achieved. Based on early warning, provided by ground tracking and orbit prediction, it would be deployed a decade or more prior to a potential impact. On completion of the initial rendezvous with the near-Earth object (NEO) the AT would first reduce the uncertainty in the orbit of the asteroid via Earth tracking of its radio transponder while it is station keeping with the asteroid. If on analysis of tracking data a deflection is required the AT would execute a reconnaissance phase collecting and processing information about the physical characteristics of the asteroid to support subsequent operations. The AT would then dock at the appropriate pole (i.e. on the spin axis), attach to the asteroid surface, and initiate a NEO reorientation maneuver. Following completion of the NEO reorientation the AT would initiate the deflection phase by thrusting continuously parallel to the asteroid velocity vector until the resultant target orbit is achieved. The orbit of the asteroid is continuously monitored throughout the deflection process and the end state is known in real time. If one assumes a nuclear-electric propulsion (NEP) system similar to that formerly under development in the recently canceled Prometheus Program, the AT would be capable of deflecting threatening NEOs up to 800 meters in diameter or more."