Loop-and-allied QG bibliography

In summary, Rovelli's program for loop gravity involves coupling the standard model to quantized QG loops, allowing for interactions between eigenvalues of length and momentum. This approach allows for non-perturbative calculations without infinity problems and does not require a continuum limit. The main difference in loop gravity is that the excitations of space are represented by polymers, or ball-and-stick models, that can be labeled with numbers to determine the volume and area of any region or surface. This allows for a more intuitive understanding of the geometry of the universe.
  • #631
http://arxiv.org/abs/0708.3288
The probability of inflation in Loop Quantum Cosmology
William Nelson
Proceedings for the International Workshop "From Quantum to Emergent Gravity: Theory and Phenomenology'', SISSA, Trieste (Italy), June 11-15 2007
(Submitted on 24 Aug 2007)

"The probability of there being sufficient inflation to solve the fine-tuning associated with the horizon and flatness problems has recently been shown to be exponentially small, within the context of classical general relativity. Here this result is extended by considering loop quantum gravity effects, that are significant at small scales. In addition to accounting for high-energy departures from classicality, it is shown that, in contrast to the classical case, the loop quantum cosmological probability measure is naturally finite, at least in some well defined region. It is also shown that these loop quantum gravity corrections can overcome the classical suppression of the probability only for extremely unnatural choices of ambiguity parameters, implying that single field, slow-roll inflation is exponentially unlikely."

special mention (PF poster)
http://arxiv.org/abs/0708.3563
Renormalization of vacuum energy in linearized quantum gravity
H. Nikolic
5 pages
(Submitted on 27 Aug 2007)

"In linearized quantum gravity, a shift of the average energy-momentum can be compensated by a shift of the average gravitational field. This allows a renormalization scheme that naturally removes the contribution of quantum vacuum fluctuations to the cosmological constant, solving the old cosmological-constant problem for weak gravitational fields."
 
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Physics news on Phys.org
  • #632
http://arxiv.org/abs/0708.3813
Non-Commutativity of Effective Space-Time Coordinates and the Minimal Length
Florian Girelli, Etera R. Livine
5 pages
(Submitted on 28 Aug 2007)

"Considering that a position measurement can effectively involve a momentum-dependent shift and rescaling of the "true" space-time coordinates, we construct a set of effective space-time coordinates which are naturally non-commutative. They lead to a minimum length and are shown to be related to Snyder's coordinates and the five-dimensional formulation of Deformed Special Relativity. This effective approach then provides a natural physical interpretation for both the extra fifth dimension and the deformed momenta appearing in this context."
 
  • #633
http://arxiv.org/abs/0708.2889
Probing Quantum Gravity using Photons from a Mkn 501 Flare Observed by MAGIC
J. Albert, et al., for the MAGIC Collaboration, John Ellis, N.E. Mavromatos, D.V. Nanopoulos, A.S. Sakharov, E.K.G. Sarkisyan
5 pages, 3 figures, submitted to Phys. Rev. Lett
(Submitted on 21 Aug 2007)

"We use the timing of photons observed by the MAGIC gamma-ray telescope during a flare of the active galaxy Markarian 501 to probe a vacuum refractive index ~ 1-(E/MQGn)n, n = 1,2, that might be induced by quantum gravity. The peaking of the flare is found to maximize for quantum-gravity mass scales MQG1 ~ 0.4x1018 GeV or MQG2 ~ 0.6x1011 GeV, and we establish lower limits MQG1 > 0.26x1018 GeV or MQG2 > 0.39x1011 GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC sensitivity to propagation effects at these levels. Thermal plasma effects in the source are negligible, but we cannot exclude the importance of some other source effect." http://arxiv.org/abs/0708.2934
Observation of Galactic Sources of Very High Energy Gamma-Rays with the MAGIC Telescope
H. Bartko, for the MAGIC collaboration
Mod. Phys. Lett. A, Vol. 22, No. 29 (2007) pp. 2167-2174
(Submitted on 21 Aug 2007 (v1), last revised 25 Aug 2007 (this version, v2))

"The MAGIC telescope with its 17m diameter mirror is today the largest operating single-dish Imaging Air Cherenkov Telescope (IACT). It is located on the Canary Island La Palma, at an altitude of 2200m above sea level, as part of the Roque de los Muchachos European Northern Observatory. The MAGIC telescope detects celestial very high energy gamma-radiation in the energy band between about 50 GeV and 10 TeV. Since the autumn of 2004 MAGIC has been taking data routinely, observing various objects, like supernova remnants (SNRs), gamma-ray binaries, Pulsars, Active Galactic Nuclei (AGN) and Gamma-ray Bursts (GRB). We briefly describe the observational strategy, the procedure implemented for the data analysis, and discuss the results of observations of Galactic Sources."

http://arxiv.org/abs/0708.3571
MAGIC multiwavelength observations: policy, and some recent results
MAGIC Collaboration: Alessandro De Angelis
5 pages, 5 figures; to be published in the Proceedings The Second Multiwavelength Workshop for Next Generation Gamma-Ray Experiments, Adler Planetarium, Chicago, Illinois, August 9-10, 2007
(Submitted on 27 Aug 2007 (v1), last revised 28 Aug 2007 (this version, v2))

"MAGIC, 17 meters of diameter, is the world's largest single dish Imaging Atmospheric Cherenkov Telescope, and reaches in the analysis the lowest energy threshold (60 GeV) among the VHE gamma detectors. Completed in September 2004, MAGIC started full operation with its first cycle of data taking in February 2005. MAGIC observations in the galaxy cover, among others, supernova remnants, the Galactic Center and binary systems. The low threshold makes of MAGIC the IACT looking deepest in the Universe: the record of extragalactic sources detected includes Active Galactic Nuclei (AGN) at z > 0.2. Here we discuss the present performance of MAGIC and the policy for the use of MAGIC data in multiwavelength campaigns. After a review of some recent highlights from MW studies, including the discovery of the most distant source ever detected (the AGN 3C279 at z = 0.54), we present the expected performance of MAGIC after the inauguration of the second telescope, scheduled for September 21st, 2008. Multiwavelength studies are a key for the study of emission mechanisms from galactic and extragalactic sources, and Very-High Energy photon detectors are becoming crucial as the GLAST era approaches."

http://arxiv.org/abs/astro-ph/0702008
Variable VHE gamma-ray emission from Markarian 501
MAGIC Collaboration, (J. Albert, et al)
51 pages, 28 figures. Submitted to ApJ. Changes with respect to V1: a) More detailed abstract, b)Section 2 (technical details of instrument and analysis) significantly shortened, c) Corrected formula 8, d) Corrected figures 20 and 21, e) Discussion on Quantum Gravity limits is removed f) Corrected some typos in text and added some references
(Submitted on 1 Feb 2007 (v1), last revised 8 Jun 2007 (this version, v2))

"The blazar Markarian 501 (Mrk 501) was observed at energies above 0.10 TeV with the MAGIC telescope from May through July 2005. The high sensitivity of the instrument enabled the determination of the flux and spectrum of the source on a night-by-night basis. Throughout our observational campaign, the flux from Mrk 501 was found to vary by an order of magnitude. Intra-night flux variability with flux-doubling times down to 2 minutes was observed during the two most active nights, namely June 30 and July 9. These are the fastest flux variations ever observed in Mrk 501. The ~20-minute long flare of July 9 showed an indication of a 4 +/- 1 min time delay between the peaks of F(<0.25 TeV) and F(>1.2 TeV), which may indicate a progressive acceleration of electrons in the emitting plasma blob. The flux variability was quantified for several energy ranges, and found to increase with the energy of the gamma-ray photons. The spectra hardened significantly with increasing flux, and during the two most active nights, a spectral peak was clearly detected at 0.43 +/- 0.06 TeV and 0.25 +/- 0.07 TeV, respectively for June 30 and July 9. There is no evidence of such spectral feature for the other nights at energies down to 0.10 TeV, thus suggesting that the spectral peak is correlated with the source luminosity. These observed characteristics could be accommodated in a Synchrotron-Self-Compton (SSC) framework in which the increase in gamma-ray flux is produced by a freshly injected (high energy) electron population."


the next two papers form a pair.
the first, by an eminent European astrophysicist Simon White (director of the Garching MPI for Astrophysics) warned of degradation to his field's creative talent, intelligence and credibility resulting from collaboration with hordes of experimental particle physicists in singleminded quest to detect "dark energy"

the second is a REPLY to Simon White's April paper, from an American astrophysicist Edward W. "Rocky" Kolb.
He is director of Particle Astrophysics Center at Fermilab, and led a joint Dark Energy Task Force made up of astrophysicists and particle physicists commissioned to advise the DOE, NSF, and NASA. Here is the task force report:
http://arxiv.org/abs/astro-ph/0609591

http://arxiv.org/abs/0704.2291
Fundamentalist physics: why Dark Energy is bad for Astronomy
Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics. 19 pages including 3 figures
(Submitted on 18 Apr 2007)

"Astronomers carry out observations to explore the diverse processes and objects which populate our Universe. High-energy physicists carry out experiments to approach the Fundamental Theory underlying space, time and matter. Dark Energy is a unique link between them, reflecting deep aspects of the Fundamental Theory, yet apparently accessible only through astronomical observation. Large sections of the two communities have therefore converged in support of astronomical projects to constrain Dark Energy. In this essay I argue that this convergence can be damaging for astronomy. The two communities have different methodologies and different scientific cultures. By uncritically adopting the values of an alien system, astronomers risk undermining the foundations of their own current success and endangering the future vitality of their field. Dark Energy is undeniably an interesting problem to attack through astronomical observation, but it is one of many and not necessarily the one where significant progress is most likely to follow a major investment of resources." http://arxiv.org/abs/0708.1199
A Thousand Invisible Cords Binding Astronomy and High-Energy Physics
Rocky Kolb (Department of Astronomy & Astrophysics, The Enrico Fermi Institute, and The Kavli Institute for Cosmological Physics, The University of Chicago)
Why "Fundamentalist" Physics Is Good for Astronomy (in response to the paper of Simon White, http://arxiv.org/abs/0704.2291
(Submitted on 9 Aug 2007)

"The traditional realm of astronomy is the observation and study of the largest objects in the Universe, while the traditional domain of high-energy physics is the study of the smallest things in nature. But these two sciences concerned with opposite ends of the size spectrum are, in Muir's words, bound fast by a thousand invisible cords that cannot be broken. In this essay I propose that collaborations of astronomers and high-energy physicists on common problems are beneficial for both fields, and that both astronomy and high-energy physics can advance by this close and still growing relationship. Dark matter and dark energy are two of the binding cords I will use to illustrate how collaborations of astronomers and high-energy physicists on large astronomical projects can be good for astronomy, and how discoveries in astronomy can guide high-energy physicists in their quest for understanding nature on the smallest scales. Of course, the fields have some different intellectual and collaborative traditions, neither of which is ideal. The cultures of the different fields cannot be judged to be right or wrong; they either work or they don't. When astronomers and high-energy physicists work together, the binding cords can either encourage or choke creativity. The challenge facing the astronomy and high-energy physics communities is to adopt the best traditions of both fields. It is up to us to choose wisely."
 
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  • #634
http://arxiv.org/abs/0709.0076
Entropy calculation for a toy black hole
Hanno Sahlmann
12 pages, 3 figures
(Submitted on 2 Sep 2007)

"In this note we carry out the counting of states for a black hole in loop quantum gravity, however assuming an equidistant area spectrum. We find that this toy-model is exactly solvable, and we show that its behavior is very similar to that of the correct model. Thus this toy-model can be used as a nice and simplifying `laboratory' for questions about the full theory."


http://arxiv.org/abs/0708.4361
Fundamental Constants
Frank Wilczek
20 pages
(Submitted on 31 Aug 2007)

"The notion of 'fundamental constant' is heavily theory-laden. A natural, fairly precise formulation is possible in the context of the standard model (here defined to include gravity). Some fundamental constants have profound geometric meaning. The ordinary gravitational constant parameterizes the stiffness, or resistance to curvature, of space-time. The cosmological term parameterizes space-time's resistance to expansion -- which may be, and apparently is at present, a negative resistance, i.e. a tendency toward expansion. The three gauge couplings of the strong, electromagnetic, and weak interactions parameterize resistance to curvature in internal spaces. The remaining fundamental couplings, of which there are a few dozen, supply an ungainly accommodation of inertia. The multiplicity and variety of fundamental constants are esthetic and conceptual shortcomings in our present understanding of foundational physics. I discuss some ideas for improving the situation. I then briefly discuss additional constants, primarily cosmological, that enter into our best established present-day world model. Those constants presently appear as macroscopic state parameters, i.e. as empirical 'material constants' of the Universe. I mention a few ideas for how they might become fundamental constants in a future theory. In the course of this essay I've advertised several of my favorite speculations, including a few that might be tested soon."http://arxiv.org/abs/0708.4236
Anticipating a New Golden Age
Frank Wilczek
25 pages, 15 figures
(Submitted on 31 Aug 2007)

"The standard model of fundamental interactions is remarkably successful, but it leaves an unfinished agenda. Several major questions seem ripe for exploration in the near future. I anticipate that the coming decade will be a Golden Age of discovery in fundamental physics."
 
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  • #635
http://arxiv.org/abs/0709.0502
Anthropics and Myopics: Conditional Probabilities and the Cosmological Constant
Irit Maor (1), Lawrence Krauss (1,2), Glenn Starkman (1,2) ((1) CERCA, Case Western Reserve University, (2) Dept of Astronomy, CWRU)
13 pages and 3 figures
(Submitted on 4 Sep 2007)

"The Anthropic Principle is claimed by many to provide a possible explanation for the observed smallness of the cosmological constant. However, correlations between the value of the cosmological constant and the existence of life can be demonstrated only under quite restrictive assumptions. Even allowing for a possible correlation, we demonstrate here that suggesting any such correlation is in fact causative is a much more subtle issue, and in general this latter claim will not be implied by the former."

exerpt: "We thank ... Raphael Bousso for lively discussions."
I can believe those were lively discussions :biggrin:
Apparently reduces anthropic "prediction" of the CC to absurdity. Bousso is a Landscape supporter. He wouldn't have liked that!

http://arxiv.org/abs/0709.0539
Causal sets and conservation laws in tests of Lorentz symmetry
David Mattingly
7 pages
(Submitted on 4 Sep 2007)

"Many of the most important astrophysical tests of Lorentz symmetry also assume that energy-momentum of the observed particles is exactly conserved. In the causal set approach to quantum gravity Lorentz symmetry holds but energy-momentum conservation is violated. We show that incorrectly assuming exact conservation can give rise to a spurious signal of Lorentz symmetry violation for a causal set. However, the size of this spurious signal is much smaller than can be currently detected and hence astrophysical Lorentz symmetry tests as currently performed are safe from Lorentz invariant violations of energy-momentum conservation."

http://arxiv.org/abs/0709.0551
Quantum Gravity Phenomenology without Lorentz Invariance Violation: a detailed proposal
Yuri Bonder, Daniel Sudarsky
23 pages
(Submitted on 5 Sep 2007)

"We propose a scheme for quantum gravity phenomenology which could be thought as arising form a granularity of space-time. Such granularity is considered as unspecified but is assumed to respect Lorentz Invariance. The proposal involves non-trivial couplings of curvature to matter fields and leads to a well defined phenomenology. Finally, we present the effective hamiltonian which could be used to analyze concrete experimental situations, some of which are briefly described."
 
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  • #636
Bojowald's article in the August 2007 issue of NATURE PHYSICS is available to non-subscribers here
http://npg.nature.com/nphys/journal/v3/n8/index.html
this gives the TOC for the August issue.
Scroll halfway down the page to where it says LETTERS and click on the PDF for the Bojo piece. it is called
What happened before the Big Bang?
In this piece Bojowald derives some definite limitations on what can be known about the universe prior to the beginning of expansion. Even though the singularity is no longer there, in the Loop Cosmology model, a Heisenberg-like principle of indeterminacy limits knowledge in some (but not all) respects.

The same August issue of Nature Physics also has a short commentary by Carlo Rovelli which discusses Bojowald's findings.

The next article is not directly related to the publications in Nature Physics. It reports ongoing work which attempts to go beyond the simple "minisuperspace" Loop Cosmology model. Current work is perturbative and goes outside the confines of the earlier models.

http://arxiv.org/abs/0709.0872
Cosmological vector modes and quantum gravity effects
Martin Bojowald, Golam Mortuza Hossain
20 pages
(Submitted on 6 Sep 2007)

"In contrast to scalar and tensor modes, vector modes of linear perturbations around an expanding Friedmann--Robertson--Walker universe decay. This makes them largely irrelevant for late time cosmology, assuming that all modes started out at a similar magnitude at some early stage. By now, however, bouncing models are frequently considered which exhibit a collapsing phase. Before this phase reaches a minimum size and re-expands, vector modes grow. Such modes are thus relevant for the bounce and may even signal the breakdown of perturbation theory if the growth is too strong. Here, a gauge invariant formulation of vector mode perturbations in Hamiltonian cosmology is presented. This lays out a framework for studying possible canonical quantum gravity effects, such as those of loop quantum gravity, at an effective level. As an explicit example, typical quantum corrections, namely those coming from inverse densitized triad components and holonomies, are shown to increase the growth rate of vector perturbations in the contracting phase, but only slightly. Effects at the bounce of the background geometry can, however, be much stronger." brief mention
http://arxiv.org/abs/0709.0942
Matters of Gravity, The Newsletter of the Topical Group in Gravitation of the American Physical Society, Volume 30, Fall 2007
David Garfinkle, Greg Comer
(Submitted on 6 Sep 2007 (v1), last revised 6 Sep 2007 (this version, v2))

sample exerpt, from Jorge Pullin's report on GRG 18 conference:

"The 18th International Conference on General Relativity and Gravitation (GR18) ... in Sydney, Australia, July 8-14 2007.
Over 600 scientists converged on the Sydney Convention and Exhibition Center at spectacular Darling Harbour. There were 15 plenary talks and 55 parallel sessions...

During the conference the Committee of the International Society of General Relativity met. Among other topics, the results for the election of the president of the society were announced, Abhay Ashtekar was elected. ...

The Basilis Xanthopoulos prize was presented jointly to Martin Bojowald (PennState) and Thomas Thiemann (Albert Einstein Institute) for their seminal contributions in loop quantum gravity..."

Here is some information about the prize
http://www.physics.uoc.gr/Xanthopoulos/index.html
The six previous recipients of the Xantho include Gary Horowitz (1993), Carlo Rovelli (1995), and Juan Maldacena (2001).
 
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  • #637
http://arxiv.org/abs/0709.1625
Dark energy from corrections to the Wheeler-DeWitt equation
William Nelson, Mairi Sakellariadou (King's College, University of London, U.K.)
4 pages, 1 figure
(Submitted on 11 Sep 2007)

"We present a method for approximating the effective consequence of generic quantum gravity corrections to the Wheeler-DeWitt equation. We show that in many cases these corrections can produce departures from classical physics at large scales and that this behaviour is equivalent to additional matter components. This opens up the possibility that dark energy (and possible dark matter) could be large scale manifestations of quantum gravity corrections to classical general relativity. We examine the first order corrections to the Wheeler-DeWitt equation arising from loop quantum cosmology in the absence of lattice refinement and show how the ultimate breakdown in large scale physics occurs."

http://arxiv.org/abs/0709.1680
Towards new background independent representations for Loop Quantum Gravity
Madhavan Varadarajan
22 pages
(Submitted on 11 Sep 2007 (v1), last revised 11 Sep 2007 (this version, v2))

"Recently, uniqueness theorems were constructed for the representation used in Loop Quantum Gravity. We explore the existence of alternate representations by weakening the assumptions of the so called LOST uniqueness theorem. The weakened assumptions seem physically reasonable and retain the key requirement of explicit background independence. For simplicity, we restrict attention to the case of gauge group U(1)."

http://arxiv.org/abs/0709.1694
Successful ToO triggers on the extragalactic sources with the MAGIC telescope
D. Mazin, E. Lindfors, for the MAGIC Collaboration
Contribution to the 30th ICRC, Merida Mexico, July 2007 on behalf of the MAGIC Collaboration
(Submitted on 11 Sep 2007)

"The MAGIC collaboration has been performing Target of Opportunity (ToO) observations whenever alerted that known or potential very high energy gamma-ray emitting extragalactic sources were in a high flux state in the optical, X-ray band or/and in the TeV energy range. Here we report on MAGIC observations performed after such triggers, results of the analysis, and a possible optical-TeV correlation seen in the data. Detections as well as spectral and temporal characterestics of Mkn 180, PKS 2155-304, and 1ES 1011+496 are reported.

My note: this is a routine report of MAGIC observing AGN flares which describes some of the relevant details. It was their report on one flare, Makarian 501, that caused such a lot of controversy last month. This paper serves to provide background and perspective on the Makarian 501 findings.

http://arxiv.org/abs/0709.1621
On the Configuration Spaces of Homogeneous Loop Quantum Cosmology and Loop Quantum Gravity
Johannes Brunnemann, Christian Fleischhack
11 pages
(Submitted on 11 Sep 2007)

"The set of homogeneous isotropic connections, as used in loop quantum cosmology,forms a line [tex]l[/tex] in the space of all connections [tex]\cal A[/tex]. This embedding, however, does not continuously extend to an embedding of the configuration space [tex]\bar l[/tex] of homogeneous isotropic loop quantum cosmology into that of loop quantum gravity, [tex]\bar{\cal A}[/tex]. This follows from the fact that the parallel transports for general, non-straight paths in the base manifold do not depend almost periodically on [tex]l[/tex]. Analogous results are given for the anisotropic case.

http://arxiv.org/abs/0709.1258
Fermi-point scenario of emergent gravity
G.E. Volovik
10 pages, 6 figures, draft for proceedings of conference "From Quantum to Emergent Gravity: Theory and Phenomenology", Trieste June 11-15 (2007)
(Submitted on 9 Sep 2007)

"Let us assume, that gravity is emergent low-energy phenomenon arising from the topologically stable defect in momentum space -- the Fermi point. What are the consequences? We discuss the natural values of fermion masses and cosmological constant; flatness of the Universe; bounds on the Lorentz violation; etc "

http://arxiv.org/abs/0709.1144
Cosmic Microwave Background Statistics for a Direction-Dependent Primordial Power Spectrum
Anthony R. Pullen, Marc Kamionkowski
8 pages, submitted to Phys Rev D
(Submitted on 7 Sep 2007)

"Statistical isotropy of primordial perturbations is a common assumption in cosmology, but it is an assumption that should be tested. To this end, we develop cosmic microwave background statistics for a primordial power spectrum that depends on the direction, as well as the magnitude, of the Fourier wavevector. We first consider a simple estimator that searches in a model-independent way for anisotropy in the square of the temperature (and/or polarization) fluctuation. We then construct the minimum-variance estimators for the coefficients of a spherical-harmonic expansion of the direction-dependence of the primordial power spectrum. To illustrate, we apply these statistics to an inflation model with a quadrupole dependence of the primordial power spectrum on direction and find that a power quadrupole as small as 2.3% can be detected with the Planck satellite."
 
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  • #638
http://arxiv.org/abs/0709.2051
The perturbative Regge-calculus regime of Loop Quantum Gravity
Eugenio Bianchi, Leonardo Modesto
40 pages
(Submitted on 13 Sep 2007)

"The relation between Loop Quantum Gravity and Regge calculus has been pointed out many times in the literature. In particular the large spin asymptotics of the Barrett-Crane vertex amplitude is known to be related to the Regge action. In this paper we study a semiclassical regime of Loop Quantum Gravity and show that it admits an effective description in terms of perturbative area-Regge-calculus. The regime of interest is identified by a class of states given by superpositions of four-valent spin networks, peaked on large spins.
As a probe of the dynamics in this regime, we compute explicitly two- and three-area correlation functions at the vertex amplitude level. We find that they match with the ones computed perturbatively in area-Regge-calculus with a single 4-simplex, once a specific perturbative action and measure have been chosen in the Regge-calculus path integral. Correlations of other geometric operators and the existence of this regime for other models for the dynamics are briefly discussed."

see page 33 (section 8) for question of how Regge fits with the new Spinfoam vertex proposed by Rovelli and others this year. The same analysis by Bianchi and Modesto presumably must be performed on the new vertex.

http://arxiv.org/abs/0709.2129
Loop Quantum Dynamics of the Schwarzschild Interior
Christian G. Boehmer, Kevin Vandersloot
15 pages, 13 figures
(Submitted on 13 Sep 2007)

"We examine the Schwarzschild interior of a black hole, incorporating quantum gravitational modifications due to loop quantum gravity. We consider an improved loop quantization using techniques that have proven successful in loop quantum cosmology. The central Schwarzschild singularity is resolved and the implications for the fate of an in-falling test particle in the interior region is discussed. The singularity is replaced by a Nariai type Universe. We discuss the resulting conformal diagram, providing a clear geometrical interpretation of the quantum effects.


brief mention:
http://arxiv.org/abs/0709.1965
Natural extension of the Generalised Uncertainty Principle
C. Bambi, F. R. Urban
4 pages, 1 figure
(Submitted on 13 Sep 2007)

"We discuss a gedanken experiment for the simultaneous measurement of position and momentum of a particle in de Sitter spacetime. We propose a further extension of the so called Generalised Uncertainty Principle (GUP) which suggests the existence of a minimum observable momentum. The new GUP is directly connected to the non--zero cosmological constant, which becomes a necessary ingredient for a more complete picture of the quantum spacetime."
 
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  • #639
http://arxiv.org/abs/0709.2365
Loop quantum gravity corrections to gravitational wave dispersion
Martin Bojowald, Golam Mortuza Hossain
27 pages
(Submitted on 14 Sep 2007)

"Cosmological tensor perturbations equations are derived for Hamiltonian cosmology based on Ashtekar's formulation of general relativity, including typical quantum gravity effects in the Hamiltonian constraint as they are expected from loop quantum gravity. This translates to corrections of the dispersion relation for gravitational waves. The main application here is the preservation of causality which is shown to be realized due to the absence of anomalies in the effective constraint algebra used."

Johannes Tambornino ILQGS talk, 4 September 2007
Relational observables and cosmological perturbation theory

Slides
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.pdf

Audio (4MB)
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.aif

Audio (38MB)
http://relativity.phys.lsu.edu/ilqgs/tambornino090407.wav

Here is the main page for the International LQG Seminar, which lists upcoming and past semester talks
http://relativity.phys.lsu.edu/ilqgs/
 
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  • #640
Today Tuesday 18 September there was an important meeting of the ILQGS
http://relativity.phys.lsu.edu/ilqgs/
The scheduled presentation was by Jonathan Engle at Marseille, talking about the new spinfoam vertices. Several people joined in lively discussion (Abhay at Penn State, Carlo at Marseille, Laurent at Perimeter, Jorge at Louisiana, who was hosting the teleseminar.

Engle's notes PDF
http://relativity.phys.lsu.edu/ilqgs/engle091807.pdf

The better of the two audio options
http://relativity.phys.lsu.edu/ilqgs/engle091807.wav

http://arxiv.org/abs/0709.2433
Toward explaining black hole entropy quantization in loop quantum gravity
Hanno Sahlmann
14 pages, 5 figures
(Submitted on 15 Sep 2007)

"In a remarkable numerical analysis of the spectrum of states for a spherically symmetric black hole in loop quantum gravity, Corichi, Diaz-Polo and Fernandez-Borja found that the entropy of the black hole horizon increases in what resembles discrete steps as a function of area. In the present article we reformulate the combinatorial problem of counting horizon states in terms of paths through a certain space. This formulation sheds some light on the origins of this step-like behavior of the entropy. In particular, using a few extra assumptions we arrive at a formula that reproduces the observed step-length to a few tenths of a percent accuracy. However, in our reformulation the periodicity ultimately arises as a property of some complicated process, the properties of which, in turn, depend on the properties of the area spectrum in loop quantum gravity in a rather opaque way. Thus, in some sense, a deep explanation of the observed periodicity is still lacking."Alain Connes and Matilde Marcolli NCG lectures on YouTube
http://noncommutativegeometry.blogspot.com/2007/09/you-tube-qft.html
I haven't had time to adequately sample these. They are from two years ago. My guess is that they are going to prove quite useful. Matilde gives the first one, racing thru QFT.http://arxiv.org/abs/0709.2784
Putting a cap on causality violations in CDT
J. Ambjorn, R. Loll, W. Westra, S. Zohren
17 pages, 4 figures
(Submitted on 18 Sep 2007)

"The formalism of causal dynamical triangulations (CDT) provides us with a non-perturbatively defined model of quantum gravity, where the sum over histories includes only causal space-time histories. Path integrals of CDT and their continuum limits have been studied in two, three and four dimensions. Here we investigate a generalization of the two-dimensional CDT model, where the causality constraint is partially lifted by introducing weighted branching points, and demonstrate that the system can be solved analytically in the genus-zero sector."
http://arxiv.org/abs/0709.2905
In Search of Quantum de Sitter Space: Generalizing the Kodama State
Andrew Randono
Ph.D. dissertation, University of Texas at Austin. 150 pages
(Submitted on 18 Sep 2007)

"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 the quantum version of a classical spacetime, namely de Sitter or anti-de sitter space. Despite this, the state fails to pass some of the key tests of a physically realistic quantum state. In an attempt to resolve this problem, we track down the root of the problem to a choice for a particular parameter: the Immirzi parameter. The Kodama state takes this parameter to be complex, whereas modern formulations of canonical quantum gravity require that the parameter is real. We generalize the Kodama state to real values of the Immirzi parameter, and find that the generalization opens up a large Hilbert space of states, one of which can be directly interpreted as particular slicing of de Sitter space. We then show that these states resolve, or are expected to resolve many of the problems associated with the original version of the Kodama state. In order to resolve the interpretation of the multitude of states, we develop a new model of covariant classical and quantum gravity where the full Lorentz group is retained as a local symmetry group, and the canonical evolution generated by the constraints has a close relation to a larger group: the de Sitter group. This formalism gives strong evidence that the multitude of generalized Kodama states can be unified into a single quantum state that is quantum de Sitter space."


Thomas Larsson, an occasional poster here at PF Beyond forum, recently put two papers up on arxiv that have been published in past years.
http://arxiv.org/abs/0709.2539
Multi-dimensional Virasoro algebra and quantum gravity
T. A. Larsson
In: Mathematical physics research at the leading edge, ed: Charles V. Benton, pp 91-111, 2004 Nova Science Publishers Inc. ISBN 1-59033-905-3
(Submitted on 17 Sep 2007)

"I review the multi-dimensional generalizations of the Virasoro algebra, i.e. the non-central Lie algebra extensions of the algebra vect(N) of general vector fields in N dimensions, and its Fock representations. Being the Noether symmetry of background independent theories such as N-dimensional general relativity, this algebra is expected to be relevant to the quantization of gravity. To this end, more complicated modules which depend on dynamics in the form of Euler-Lagrange equations are described. These modules can apparently only be interpreted as quantum fields if spacetime has four dimensions and both bosons and fermions are present."

http://arxiv.org/abs/0709.2540
Manifestly covariant canonical quantization of gravity and diffeomorphism anomalies in four dimensions
T. A. Larsson
In: Focus on quantum gravity research, ed: David C. Moore, pp 261-310, 2006 Nova Science Publishers Inc. ISBN 1-59454-660-6
(Submitted on 17 Sep 2007)

"Canonical quantization of gravity requires knowledge about the representation theory of its constraint algebra, which is physically equivalent to the algebra of arbitrary 4-diffeomorphisms. All interesting lowest-energy representations are projective, making the relevant algebra into a four-dimensional generalization of the Virasoro algebra. Such diffeomorphism anomalies are invisible in field theory, because the relevant cocycles are functionals of the observer's trajectory in spacetime. The multi-dimensional Virasoro algebra acts naturally in the phase space of arbitrary histories, with dynamics playing the role of first-class constraints. General relativity is regularized by expanding all fields in Taylor series around the observer's trajectory, and truncating at some fixed order. This regularized but manifestly general-covariant theory is quantized in the history phase space, and dynamics is imposed afterwards, in analogy with BRST quantization. Infinities arise when the regularization is removed; it is presently unclear how these should be dealt with."
 
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  • #641
http://arxiv.org/abs/0709.3120
The flat FRW model in LQC: the self-adjointness
Wojciech Kamiński, Jerzy Lewandowski
13 pages
(Submitted on 19 Sep 2007)

"The flat FRW model coupled to the massless scalar field according to the improved, background scale independent version of Ashtekar, Pawlowski and Singh is considered. The core of the theory is addressed directly: the APS construction of the quantum Hamiltonian is analyzed under the assumption that cosmological constant Lambda <= 0. We prove the essential self-adjointness of the operator whose square-root defines the quantum Hamiltonian operator and therefore, provide the explicit definition. If Lambda<0, then the spectrum is discrete. In the Lambda=0 case, the essential and absolutely continues spectra of the operator are derived. The latter operator is related in the unitary way to the absolutely continuous part of the Quantum Mechanics operator [tex]a(-\frac{\partial^2}{\partial y^2} - \frac{b}{{\rm cosh}^2\hat{y}})[/tex]
(a,b>0 being some constants) plus a trace class operator."



The following describes what seems like an important new way to get a handle on dark energy and the expansion history of the universe

http://arxiv.org/abs/0709.2955
Baryonic Acoustic Oscillations in 21cm Emission: A Probe of Dark Energy out to High Redshifts
Stuart Wyithe, Abraham Loeb, Paul Geil
16 pages, 9 figures. Submitted to MNRAS
(Submitted on 19 Sep 2007)

"Low-frequency observatories are currently being constructed with the goal of detecting redshifted 21cm emission from the epoch of reionization. These observatories will also be able to detect intensity fluctuations in the cumulative 21cm emission after reionization, from hydrogen in unresolved damped Ly-alpha absorbers (such as gas rich galaxies) down to a redshift z~3.5. The inferred power spectrum of 21cm fluctuations at all redshifts will show acoustic oscillations, whose co-moving scale can be used as a standard ruler to infer the evolution of the equation of state for the dark energy. We find that the first generation of low-frequency experiments (such as MWA or LOFAR) will be able to constrain the acoustic scale to within a few percent in a redshift window just prior to the end of the reionization era, provided that foregrounds can be removed over frequency band-passes of >8MHz. This sensitivity to the acoustic scale is comparable to the best current measurements from galaxy redshift surveys, but at much higher redshifts. Future extensions of the first generation experiments (involving an order of magnitude increase in the antennae number of the MWA) could reach sensitivities below one percent in several redshift windows and could be used to study the dark energy in the unexplored redshift regime of 3.5<z<12. Moreover, new experiments with antennae designed to operate at higher frequencies would allow precision measurements (<1%) of the acoustic peak to be made at more moderate redshifts (1.5<z<3.5), where they would be competitive with ambitious spectroscopic galaxy surveys covering more than 1000 square degrees. Together with other data sets, observations of 21cm fluctuations will allow full coverage of the acoustic scale from the present time out to z~12."

http://arxiv.org/abs/0709.2948
The Contribution of the Cosmological Constant to the Relativistic Bending of Light Revisited
Wolfgang Rindler, Mustapha Ishak (The University of Texas at Dallas)
5 pages, 2 figures
(Submitted on 19 Sep 2007)

"We study the effect of the cosmological constant Lambda on the bending of light by a concentrated spherically symmetric mass. Contrarily to previous claims, we show that when the Schwarzschild-de Sitter geometry is taken into account, Lambda does indeed contribute to the bending."
 
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  • #642
http://arxiv.org/abs/0709.3851
Asymptotic Safety
R. Percacci
To appear in "Approaches to Quantum Gravity: Towards a New Understanding of Space, Time and Matter", ed. D. Oriti, Cambridge University Press
(Submitted on 24 Sep 2007)

"Asymptotic safety is a set of conditions, based on the existence of a nontrivial fixed point for the renormalization group flow, which would make a quantum field theory consistent up to arbitrarily high energies. After introducing the basic ideas of this approach, I review the present evidence in favor of an asymptotically safe quantum field theory of gravity".

I suspect this Percacci paper on Asymptotic Safety is important, so I just started a thread on it:
https://www.physicsforums.com/showthread.php?t=187046

http://arxiv.org/abs/0709.4225
The status of Quantum Geometry in the dynamical sector of Loop Quantum Cosmology
Wojciech Kaminski, Jerzy Lewandowski, Lukasz Szulc
12 pages
(Submitted on 26 Sep 2007)

"This letter is motivated by the recent papers by Ditrich and Thiemann and, respectively, by Rovelli discussing the status of Quantum Geometry in the dynamical sector of Loop Quantum Geometry. Since the papers consider model examples, we also study the issue in the case of an example, namely on the Loop Quantum Cosmology model of space-isotropic universe. We derive the Rovelli-Thiemann-Ditrich partial observables corresponding to the quantum geometry operators of LQC in both Hilbert spaces: the kinematical one and, respectively, the physical Hilbert space of solutions to the quantum constraints. We find, that Quantum Geometry can be used to characterize the physical solutions, and the operators of quantum geometry preserve many of their kinematical properties."

http://arxiv.org/abs/0709.4235
Extended matter coupled to BF theory
Winston J. Fairbairn (Phys-ENS), Alejandro Perez (CPT)
27 pages, 7 figures
(Submitted on 26 Sep 2007)

"Recently, a topological field theory of membrane-matter coupled to BF theory in arbitrary spacetime dimensions was proposed [1]. In this paper, we discuss various aspects of the four-dimensional theory. Firstly, we study classical solutions leading to an interpretation of the theory in terms of strings propagating on a flat spacetime. We also show that the general classical solutions of the theory are in one-to-one correspondence with solutions of Einstein's equations in the presence of distributional matter (cosmic strings). Secondly, we quantize the theory and present, in particular, a prescription to regularize the physical inner product of the canonical theory. We show how the resulting transition amplitudes are dual to evaluations of Feynman diagrams coupled to three-dimensional quantum gravity. Finally, we remove the regulator by proving the topological invariance of the transition amplitudes.

http://arxiv.org/abs/0709.4157
A combinatorial and field theoretic path to quantum gravity: the new challenges of group field theory
Daniele Oriti
16 pages, 7 figures; RevTex4; to appear in the proceedings of the Conference on Combinatorics and Physics, MPIM Bonn, March 19-23, 2007
(Submitted on 26 Sep 2007)

"Group field theories are a new type of field theories over group manifolds and a generalization of matrix models, that have recently attracted much interest in quantum gravity research. They represent a development of and a possible link between different approaches such as loop quantum gravity and simplicial quantum gravity. After a brief introduction to the GFT formalism we put forward a long but still far from exhaustive list of open issues that this line of research faces, and that could be represent interesting challenges for mathematicians and mathematical physicists alike."http://arxiv.org/abs/0709.3947
Lorentz transformations in de Sitter relativity
R. Aldrovandi, J. P. Beltran Almeida, C. S. O. Mayor, J. G. Pereira
7 pages
(Submitted on 25 Sep 2007)

"The properties of Lorentz transformations in de Sitter relativity are studied. It is shown that, in addition to leaving invariant the velocity of light, they also leave invariant the length-scale related to the curvature of the de Sitter spacetime. The basic conclusion is that it is possible to have an invariant length parameter without breaking the Lorentz symmetry. This result may have important implications for the study of quantum kinematics, and in particular for quantum gravity."
 
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  • #643
a Baez student's thesis
http://arxiv.org/abs/0710.0032
Extended TQFT's and Quantum Gravity
Jeffrey Morton
112 pages, 17 figures - Ph.D Thesis (University of California, Riverside - 2007)
(Submitted on 29 Sep 2007)

"This paper gives a definition of an extended topological quantum field theory (TQFT) as a weak 2-functor Z: nCob_2 -> 2Vect, by analogy with the description of a TQFT as a functor Z: nCob -> Vect. We also show how to obtain such a theory from any finite group G. This theory is related to a topological gauge theory, the Dijkgraaf-Witten model. To give this definition rigorously, we first define a bicategory of cobordisms between cobordisms. We also give some explicit description of a higher-categorical version of Vect, denoted 2Vect, a bicategory of "2-vector spaces". Along the way, we prove several results showing how to construct 2-vector spaces of "Vect-valued presheaves" on certain kinds of groupoids. In particular, we use the case when these are groupoids whose objects are connections, and whose morphisms are gauge transformations, on the manifolds on which the extended TQFT is to be defined. On cobordisms between these manifolds, we show how a construction of ``pullback and pushforward'' of presheaves gives both the morphisms and 2-morphisms in 2Vect for the extended TQFT, and that these satisfy the axioms for a weak 2-functor. Finally, we discuss the motivation for this research in terms of Quantum Gravity. If the results can be extended from a finite group G to a Lie group, then for some choices of G this theory will recover an existing theory of Euclidean quantum gravity in 3 dimensions. We suggest extensions of these ideas which may be useful to further this connection and apply it in higher dimensions.


http://arxiv.org/abs/0709.4619
Gauge-invariant coherent states for Loop Quantum Gravity I: Abelian gauge groups
Benjamin Bahr, Thomas Thiemann
36 pages
(Submitted on 28 Sep 2007)

"In this paper we investigate the properties of gauge-invariant coherent states for Loop Quantum Gravity, for the gauge group U(1). This is done by projecting the corresponding complexifier coherent states, which have been applied in numerous occasions to investigate the semiclassical limit of the kinematical sector, to the gauge-invariant Hilbert space. This being the first step to construct physical coherent states, we arrive at a set of gauge-invariant states that approximate well the gauge-invariant degrees of freedom of abelian LQG. Furthermore, these states turn out to encode explicit information about the graph topology, and show the same pleasant peakedness properties known from the gauge-variant complexifier coherent states."

http://arxiv.org/abs/0709.4636
Gauge-invariant coherent states for Loop Quantum Gravity II: Non-abelian gauge groups
Benjamin Bahr, Thomas Thiemann
60 pages, 8 figures
(Submitted on 28 Sep 2007)

"This is the second paper concerning gauge-invariant coherent states for Loop Quantum Gravity. Here, we deal with the gauge group SU(2), this being a significant complication compared to the abelian U(1) case encountered in the previous article. We study gauge-invariant coherent states on certain special graphs by analytical and numerical methods. We find that their overlap is Gauss-peaked in gauge-invariant quantities, as long as states are not labeled by degenerate gauge orbits, i.e. points where the gauge-invariant configuration space has singularities. In these cases the overlaps are still concentrated around these points, but the peak profile exhibits a plateau structure. This shows how the semiclassical properties of the states are influenced by the geometry of the gauge-invariant phase space."

http://arxiv.org/abs/0710.0323
A Dual Algorithm for Non-abelian Yang-Mills coupled to Dynamical Fermions
J. Wade Cherrington
17 pages, 11 figures
(Submitted on 1 Oct 2007)

"We extend the dual algorithm recently described for pure, non-abelian Yang-Mills on the lattice to the case of lattice fermions coupled to Yang-Mills, by constructing an ergodic Metropolis algorithm for dynamic fermions that is local, exact, and built from gauge-invariant boson-fermion coupled configurations. For concreteness, we present in detail the case of three dimensions, for the group SU(2) and staggered fermions, however the algorithm readily generalizes with regard to group and dimension. The treatment of the fermion determinant makes use of a polymer expansion; as with previous proposals making use of the polymer expansion in higher than two dimensions, the critical question for practical applications is whether the presence of negative amplitudes can be managed in the continuum limit."
 
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  • #644
http://arxiv.org/abs/0710.0617
Numerical evidence of regularized correlations in spin foam gravity
J. Daniel Christensen, Etera R. Livine, Simone Speziale
5 pages, 3 figures
(Submitted on 2 Oct 2007)

"We report on the numerical analysis of the area correlations in spin foam gravity on a single 4-simplex considered by Rovelli in PRL 97 (2006) 151301. We compare the asymptotics and confirm the inverse squared distance leading behaviour at large scales. This supports the recent advances on testing the semiclassical limit of the theory. Furthermore, we show that the microscopic discreteness of the theory dynamically suppresses and regularizes the correlations at the Planck scale."


http://arxiv.org/abs/0710.0416
Effective Dynamics, Big Bounces and Scaling Symmetry in Bianchi Type I Loop Quantum Cosmology
Dah-Wei Chiou
19 pages, 6 figures, 1 table
(Submitted on 2 Oct 2007)

"The detailed formulation for loop quantum cosmology (LQC) in the Bianchi I model with a scalar massless field has been constructed. In this paper, its effective dynamics is studied in two improved strategies for implementing the LQC discreteness corrections. Both schemes show that the big bang is replaced by the big bounces, which take place up to three times, once in each diagonal direction, when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. These two strategies give different evolutions: In one scheme, the effective dynamics is independent of the choice of the finite sized cell prescribed to make Hamiltonian finite; in the other, the effective dynamics reacts to the macroscopic scales introduced by the boundary conditions. Both schemes reveal interesting symmetries of scaling, which are reminiscent of the relational interpretation of quantum mechanics and also suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale."

http://arxiv.org/abs/0710.0610
de Sitter Relativity and Quantum Physics
Authors: R. Aldrovandi, J. P. Beltran Almeida, C. S. O. Mayor, J. G. Pereira
10 pages. To appear in "Quantum Theory: Reconsideration of Foundations 4", 11-16 June 2007, Vaxjo University, Sweden (AIP Conference Proceedings, ed. by G. Adenier, A. Khrennikov and T. Nieuwenhuizen).
(Submitted on 2 Oct 2007)

"In the presence of a cosmological constant, interpreted as a purely geometric entity, absence of matter is represented by a de Sitter spacetime. As a consequence, ordinary Poincare' special relativity is no longer valid and must be replaced by a de Sitter special relativity. By considering the kinematics of a spinless particle in a de Sitter spacetime, we study the geodesics of this spacetime, the ensuing definitions of canonical momenta, and explore possible implications for quantum mechanics."
 
  • #645
http://arxiv.org/abs/0710.1548
Propagation and interaction of chiral states in quantum gravity
Lee Smolin, Yidun Wan
34 pages, 30 figures
(Submitted on 5 Oct 2007)

"We study the stability, propagation and interactions of braid states in models of quantum gravity in which the states are four-valent spin networks embedded in a topological three manifold and the evolution moves are given by the dual Pachner moves. There are results for both the framed and unframed case. We study simple braids made up of two nodes which share three edges, which are possibly braided and twisted. We find three classes of such braids, those which both interact and propagate, those that only propagate, and the majority that do neither."


http://arxiv.org/abs/0710.1312
On Braid Excitations in Quantum Gravity
Yidun Wan
24 pages, 16 figures, 5 tables
(Submitted on 5 Oct 2007)

"We propose a new notation for the states in some models of quantum gravity, namely 4-valent spin networks embedded in a topological three manifold. With the help of this notation, equivalence moves, namely translations and rotations, can be defined, which relate the projections of diffeomorphic embeddings of a spin network. Certain types of topological structures, viz 3-strand braids as local excitations of embedded spin networks, are defined and classified by means of the equivalence moves. This paper formulates a mathematical approach to the further research of particle-like excitations in quantum gravity."

http://arxiv.org/abs/0710.1675
Is the cosmological "constant" a nonlocal quantum residue of discreteness of the causal set type?
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
13 pages. To appear in the proceedings of the PASCOS-07 Conference, held July, 2007, London
(Submitted on 9 Oct 2007)

"The evidence for an accelerating Hubble expansion appears to have confirmed the heuristic prediction, from causal set theory, of a fluctuating and 'ever-present' cosmological term in the Einstein equations. A more concrete phenomenological model incorporating this prediction has been devised and tested, but it remains incomplete. I will review these developments and also mention a possible consequence for the dimensionality of spacetime."

http://arxiv.org/abs/0710.1562
Dark energy and 3-manifold topology
Torsten Asselmeyer-Maluga, Helge Rose
7 pages
(Submitted on 8 Oct 2007)

"We show that the differential-geometric description of matter by differential structures of spacetime leads to a unifying model of the three types of energy in the cosmos: matter, dark matter and dark energy. Using this model we are able to calculate the ratio of dark energy to the total energy of the cosmos."

Torsten and Helge have both posted here at Beyond forum.
There was quite a bit of discussion of some earlier work of theirs around 2005, if I remember right.
 
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  • #646
http://arxiv.org/abs/0710.2886
kappa-Minkowski space, scalar field, and the issue of Lorentz invariance
Laurent Freidel, Jerzy Kowalski-Glikman
Based on the talk given at the conference "From Quantum to Emergent Gravity", SISSA, June, 2007; to appear in the proceedings
(Submitted on 15 Oct 2007)

"We describe kappa-Minkowski space and its relation to group theory. The group theoretical picture makes it possible to analyze the symmetries of this space. As an application of this analysis we analyze in detail free field theory on kappa-Minkowski space and the Noether charges associated with deformed spacetime symmetries."

http://arxiv.org/abs/0710.2679
A New Class of Group Field Theories for 1st Order Discrete Quantum Gravity
Daniele Oriti, Tamer Tlas
48 pages, 4 figures
(Submitted on 14 Oct 2007)

"Group Field Theories, a generalization of matrix models for 2d gravity, represent a 2nd quantization of both loop quantum gravity and simplicial quantum gravity. In this paper, we construct a new class of Group Field Theory models, for any choice of spacetime dimension and signature, whose Feynman amplitudes are given by path integrals for clearly identified discrete gravity actions, in 1st order variables. In the 3-dimensional case, the corresponding discrete action is that of 1st order Regge calculus for gravity (generalized to include higher order corrections), while in higher dimensions, they correspond to a discrete BF-theory (again, generalized to higher order) with an imposed orientation restriction on hinge volumes, similar to that characterizing discrete gravity. The new models shed also light on the large distance or semi-classical approximation of spin foam models. This new class of group field theories may represent a concrete unifying framework for loop quantum gravity and simplicial quantum gravity approaches."
 
  • #647
http://arxiv.org/abs/0710.3565
On the robustness of key features of loop quantum cosmology
Abhay Ashtekar, Alejandro Corichi, Parampreet Singh
(Submitted on 18 Oct 2007)

"A small simplification based on well motivated approximations is shown to make loop quantum cosmology of the k=0 FRW model (with a massless scalar field) exactly soluble. Analytical methods are then used i) to show that the quantum bounce is generic; ii) to establish that the matter density has an absolute upper bound which, furthermore, equals the critical density that first emerged in numerical simulations and effective equations; iii) to bring out the precise sense in which the Wheeler DeWitt theory approximates loop quantum cosmology and the sense in which this approximation fails; and iv) to show that discreteness underlying LQC is fundamental. Finally, the model is compared to analogous discussions in the literature and it is pointed out that some of their expectations do not survive a more careful examination. An effort has been made to make the underlying structure transparent also to those who are not familiar with details of loop quantum gravity."http://arxiv.org/abs/0710.3540
N=2 supersymmetric spin foams in three dimensions
Etera R. Livine, James P. Ryan
12 pages
(Submitted on 18 Oct 2007)

"We construct the spin foam model for N=2 supergravity in three dimensions. Classically, it is a BF theory with gauge algebra osp(2|2). This algebra has representations which are not completely reducible. This complicates the procedure when building a state sum. Fortunately, one can and should excise these representations. We show that the restricted subset of representations form a subcategory closed under tensor product. The resulting state-sum is once again a topological invariant. Furthermore, within this framework one can identify positively and negatively charged fermions propagating on the spin foam. These results on osp(2|2) representations and intertwiners apply more generally to spin network states for N=2 loop quantum supergravity (in 3+1 dimensions) where it allows to define a notion of BPS states. "


http://arxiv.org/abs/0710.3276
Group field theory as the microscopic description of the quantum spacetime fluid: a new perspective on the continuum in quantum gravity
Daniele Oriti
38 pages, 6 figures; contribution to the proceedings of the conference 'From quantum to emergent gravity: theory and phenomenology', SISSA, Trieste, Italy, June 11-15 (2007)
(Submitted on 17 Oct 2007)

"We introduce the group field theory (GFT) formalism for non-perturbative quantum gravity, and present it as a potential unifying framework for several other quantum gravity approaches, i.e. loop quantum gravity and simplicial quantum gravity ones. We then argue in favor of and present in detail what we believe is a new GFT perspective on the emergence of continuum spacetime from discrete quantum structures, based on the idea of quantum space as a condensed matter system. We put forward a more specific, albeit still very much tentative, proposal for the relevant phase of the GFT corresponding to the continuum: a Bose-Einstein condensate of GFT quanta. Finally, we sketch how the proposal may be realized and its effective dynamics could be extracted in the GFT setting and compared with continuum gravity theories."

http://arxiv.org/abs/0710.3415
Modified gravity and the origin of inertia
J. W. Moffat, V. T. Toth
4 pages, 2 figures
(Submitted on 18 Oct 2007)

"Modified gravity theory is known to violate Birkhoff's theorem. We explore a key consequence of this violation, the effect of distant matter in an Einstein-de Sitter universe on the motion of test particles. We find that when a particle is accelerated, a force is experienced that is proportional to the particle's mass and acceleration and acts in the direction opposite to that of the acceleration. We identify this force with inertia. At very low accelerations, our inertial law deviates slightly from that of Newton, yielding a testable prediction that may be verified with relatively simple experiments."

http://arxiv.org/abs/0710.3438
Can one detect passage of small black hole through the Earth?
I.B. Khriplovich, A.A. Pomeransky, N. Produit, G.Yu. Ruban
6 pages
(Submitted on 18 Oct 2007)

"The energy losses of a small black hole passing through the Earth are examined. In particular, we investigate the excitations in the frequency range accessible to modern acoustic detectors. The main contribution to the effect is given by the coherent sound radiation of the Cherenkov type."


http://arxiv.org/abs/0710.3187
On the Quantum Origin of Structure in the Inflationary Universe
Daniel Baumann
29 pages; pedagogical lecture notes
(Submitted on 16 Oct 2007)

"In this lecture I give a pedagogical introduction to inflationary cosmology with a special focus on the quantum generation of cosmological perturbations."
 
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  • #648
http://arxiv.org/abs/0710.4543
Quantum bounce and cosmic recall
Alejandro Corichi, Parampreet Singh
4 pages
(Submitted on 24 Oct 2007)

"Loop quantum cosmology predicts that, in simple models, the big bang singularity of classical general relativity is replaced by a quantum bounce. Because of the extreme physical conditions near the bounce, a natural question is whether the universe can retain, after the bounce, its memory about the previous epoch. More precisely, does the universe recall various properties of the state after evolving unitarily through the bounce or does it suffer from cosmic amnesia as has been recently suggested? Here we show that this issue can be answered unambiguously by means of an exactly solvable model, derived from a small simplification of loop quantum cosmology, for which there is full analytical control on the quantum theory. We show that if there exists a semi-classical state at late times on one side, peaked around a pair of canonically conjugate variables, then there are very strong bounds on the fluctuations on the other side of the bounce, implying semi-classicality. For a model universe which grows to a size of 1 megaparsec at late times, the change in relative fluctuations of the only non-trivial observable of the model across the bounce is less than 10-57 (becoming smaller for universes which grow larger). The universe maintains (an almost) total recall.

http://arxiv.org/abs/0710.4345
A Primer for Black Hole Quantum Physics
Robert Brout, Serge Massar, Renaud Parentani, Philippe Spindel
An old review article on black hole evaporation and black hole thermodynamics, put on the archive following popular demand, 178 pages, 21 figures
Physics Report 260 (6), 329-454 (1995)
(Submitted on 23 Oct 2007)

"The mechanisms which give rise to Hawking radiation are revealed by analyzing in detail pair production in the presence of horizons. In preparation for the black hole problem, three preparatory problems are dwelt with at length: pair production in an external electric field, thermalization of a uniformly accelerated detector and accelerated mirrors. In the light of these examples, the black hole evaporation problem is then presented.
The leitmotif is the singular behavior of modes on the horizon which gives rise to a steady rate of production. Special emphasis is put on how each produced particle contributes to the mean albeit arising from a particular vacuum fluctuation. It is the mean which drives the semiclassical back reaction. This aspect is analyzed in more detail than heretofore and in particular its drawbacks are emphasized. It is the semiclassical theory which gives rise to Hawking's famous equation for the loss of mass of the black hole due to evaporation [tex]dM/dt \simeq -1/M^2[/tex]. Black hole thermodynamics is derived from the evaporation process whereupon the reservoir character of the black hole is manifest. The relation to the thermodynamics of the eternal black hole through the Hartle--Hawking vacuum and the Killing identity are displayed.
It is through the analysis of the fluctuations of the field configurations which give rise to a particular Hawking photon that the dubious character of the semiclassical theory is manifest. The present frontier of research revolves around this problem and is principally concerned with the fact that one calls upon energy scales that are greater than Planckian and the possibility of a non unitary evolution as well. These last subjects are presented in qualitative fashion only, so that this review stops at the threshold of quantum gravity."

===================
The following are included because I've found previous papers by Dreyer and by Girelli interesting. Did not have time today to evaluate the current work.
http://arxiv.org/abs/0710.4350
Why things fall
Olaf Dreyer
Talk given at the conference "From Quantum to Emergent Gravity: Theory and Phenomenology", June 11-15, 2007, Trieste, Italy. To appear in the proceedings
(Submitted on 23 Oct 2007)

http://arxiv.org/abs/0710.4393
Quantum reference frames and deformed symmetries
Florian Girelli, David Poulin
11 pages
(Submitted on 24 Oct 2007)
 
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  • #649
http://arxiv.org/abs/0710.4919
Harmonic cosmology: How much can we know about a universe before the big bang?
Martin Bojowald
16 pages
(Submitted on 25 Oct 2007)

"Quantum gravity may remove classical space-time singularities and thus reveal what a universe at and before the big bang could be like. In loop quantum cosmology, an exactly solvable model is available which allows one to address precise dynamical coherent states and their evolution in such a setting. It is shown here that quantum fluctuations before the big bang are generically unrelated to those after the big bang. A reliable determination of pre-big bang quantum fluctuations would require exceedingly precise observations."

This paper can be seen as responding to one by Corichi and Singh noted in the previous post:

http://arxiv.org/abs/0710.4543
Quantum bounce and cosmic recall
Alejandro Corichi, Parampreet Singh
4 pages
(Submitted on 24 Oct 2007)

"Loop quantum cosmology predicts that, in simple models, the big bang singularity of classical general relativity is replaced by a quantum bounce. Because of the extreme physical conditions near the bounce, a natural question is whether the universe can retain, after the bounce, its memory about the previous epoch. More precisely, does the universe recall various properties of the state after evolving unitarily through the bounce or does it suffer from cosmic amnesia...?
... The universe maintains (an almost) total recall."
 
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  • #650
http://arxiv.org/abs/0710.5034
Numerical indications on the semiclassical limit of the flipped vertex
Elena Magliaro, Claudio Perini, Carlo Rovelli
4 pages, 8 figures
(Submitted on 26 Oct 2007 (v1), last revised 27 Oct 2007 (this version, v2))

"We introduce a technique for testing the semiclassical limit of a quantum gravity vertex amplitude. The technique is based on the propagation of a semiclassical wave packet. We apply this technique to the newly introduced "flipped" vertex in loop quantum gravity, in order to test the intertwiner dependence of the vertex. Under some drastic simplifications, we find very preliminary, but surprisingly good numerical evidence for the correct classical limit."

http://arxiv.org/abs/0710.5017
Coherent states, constraint classes, and area operators in the new spin-foam models
Jonathan Engle, Roberto Pereira
21 pages
(Submitted on 26 Oct 2007)

"Recently, two new spin-foam models have appeared in the literature, both motivated by a desire to modify the Barrett-Crane model in such a way that the imposition of certain second class constraints, called cross-simplicity constraints, are weakened. We refer to these two models as the FKLS model, and the flipped model. Both of these models are based on a reformulation of the cross-simplicity constraints. This paper has two main parts. First, we clarify the structure of the reformulated cross-simplicity constraints and the nature of their quantum imposition in the new models. In particular we show that in the FKLS model, quantum cross-simplicity implies no restriction on states. The deeper reason for this is that, with the symplectic structure relevant for FKLS, the reformulated cross-simplicity constraints, in a certain relevant sense, are now first class, and this causes the coherent state method of imposing the constraints, key in the FKLS model, to fail to give any restriction on states. Nevertheless, the cross-simplicity can still be seen as implemented via suppression of intertwiner degrees of freedom in the dynamical propagation. In the second part of the paper, we investigate area spectra in the models. The results of these two investigations will highlight how, in the flipped model, the Hilbert space of states, as well as the spectra of area operators exactly match those of loop quantum gravity, whereas in the FKLS (and Barrett-Crane) models, the boundary Hilbert spaces and area spectra are different."

http://arxiv.org/abs/0710.5043
Lorentzian LQG vertex amplitude
Roberto Pereira
9 pages
(Submitted on 26 Oct 2007)

"We generalize a model recently proposed for Euclidean quantum gravity to the case of Lorentzian signature. The main features of the Euclidean model are preserved in the Lorentzian one. In particular, the boundary Hilbert space matches the one of SU(2) loop quantum gravity. As in the Euclidean case, the model can be obtained from the Lorentzian Barrett-Crane model from a flipping of the Poisson structure, or alternatively, by adding a topological term to the action and taking the small Barbero-Immirzi parameter limit."

http://arxiv.org/abs/0710.4953
Different canonical formulations of Einstein's theory of gravity
V.A. Franke
18 pages. Theor.Math.Phys. 148(1), 2006, 995-1010; Teor.Mat.Fiz. 148(1), 2006, 143-160
(Submitted on 25 Oct 2007)

"We describe the four most famous versions of the classical canonical formalism in the Einstein theory of gravity: the Arnovitt-Deser-Misner formalism, the Faddeev-Popov formalism, the tetrad formalism in the usual form, and the tetrad formalism in the form best suited for constructing the loop theory of gravity, which is now being developed. We present the canonical transformations relating these formalisms. The paper is written mainly for pedagogical purposes."

http://arxiv.org/abs/0710.4959
Outlook from SUSY07
John Ellis
Submitted for the SUSY07 proceedings, 9 pages, 12 figures
(Submitted on 25 Oct 2007)

"Make-or-break time is near for the Higgs boson and supersymmetry. The LHC will soon put to the sword many theoretical ideas, and define the future for collider physics."
 
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http://arxiv.org/abs/0710.5734
Dirac Fields in Loop Quantum Gravity and Big Bang Nucleosynthesis
Martin Bojowald, Rupam Das, Robert J. Scherrer
15 pages, 2 figures
(Submitted on 30 Oct 2007)

"Big Bang nucleosynthesis requires a fine balance between equations of state for photons and relativistic fermions. Several corrections to equation of state parameters arise from classical and quantum physics, which are derived here from a canonical perspective. In particular, loop quantum gravity allows one to compute quantum gravity corrections for Maxwell and Dirac fields. Although the classical actions are very different, quantum corrections to the equation of state are remarkably similar. To lowest order, these corrections take the form of an overall expansion-dependent multiplicative factor in the total density. We use these results, along with the predictions of Big Bang nucleosynthesis, to place bounds on these corrections."

http://arxiv.org/abs/0710.5721
The radiation equation of state and loop quantum gravity corrections
Martin Bojowald, Rupam Das
11 pages, 1 figure
Phys. Rev. D 75 (2007) 123521
(Submitted on 30 Oct 2007)

"The equation of state for radiation is derived in a canonical formulation of the electromagnetic field. This allows one to include correction terms expected from canonical quantum gravity and to infer implications to the universe evolution in radiation dominated epochs. Corrections implied by quantum geometry can be interpreted in physically appealing ways, relating to the conformal invariance of the classical equations."

http://arxiv.org/abs/0710.5722
Canonical Gravity with Fermions
Martin Bojowald, Rupam Das
27 pages
(Submitted on 30 Oct 2007)

"Canonical gravity in real Ashtekar-Barbero variables is generalized to allow for fermionic matter. The resulting torsion changes several expressions in Holst's original vacuum analysis, which are explicitly displayed here. This in turn requires adaptations to the known canonical (loop) quantization of gravity coupled to fermions, which is discussed on the basis of the classical analysis."

http://arxiv.org/abs/0710.5608
Measurement of the space-time interval between two events using the retarded and advanced times of each event with respect to a time-like world-line
Giovanni Amelino-Camelia, John Stachel
17 pages
(Submitted on 30 Oct 2007)

"Several recent studies have been devoted to investigating the limitations that ordinary quantum mechanics and/or quantum gravity might impose on the measurability of space-time observables. These analyses are often confined to the simplified context of two-dimensional flat space-time and rely on a simple procedure for the measurement of space-like distances based on the exchange of light signals. We present a generalization of this measurement procedure applicable to all three types of space-time intervals between two events in space-times of any number of dimensions. We also present some preliminary observations on an alternative measurement procedure that can be applied taking into account the gravitational field of the measuring apparatus, and briefly discuss quantum limitations of measurability in this context.

Alejandro Satz has the blog called Reality Conditions, which had the best blog reporting of the Zakopane school and Loops '07
http://arxiv.org/abs/0710.5671
Transition rate of the Unruh-DeWitt detector in curved spacetime
Jorma Louko, Alejandro Satz
(Submitted on 30 Oct 2007)

"We examine the Unruh-DeWitt particle detector coupled to a scalar field in an arbitrary Hadamard state in four-dimensional curved spacetime. Using smooth switching functions to turn on and off the interaction, we obtain a regulator-free integral formula for the total excitation probability, and we show that an instantaneous transition rate can be recovered in a suitable limit. Previous results in Minkowski space are recovered as a special case. As applications, we consider an inertial detector in the Rindler vacuum and a detector at rest in a static Newtonian gravitational field. Gravitational corrections to decay rates in atomic physics laboratory experiments on the surface of the Earth are estimated to be suppressed by 42 orders of magnitude."

Didn't want to omit this (classical, not quantum geometry) paper, on the chance it might give additional understanding of behavior at the classical singularity which could be of use to quantum gravitists.
http://arxiv.org/abs/0710.5692
Describing general cosmological singularities in Iwasawa variables
Thibault Damour, Sophie de Buyl
50 pages, 4 figures
(Submitted on 30 Oct 2007)

"Belinskii, Khalatnikov, and Lifshîtz (BKL) conjectured that the description of the asymptotic behavior of a generic solution of Einstein equations near a spacelike singularity could be drastically simplified by considering that the time derivatives of the metric asymptotically dominate (except at a sequence of instants, in the `chaotic case') over the spatial derivatives. We present a precise formulation of the BKL conjecture (in the chaotic case) that consists of basically three elements: (i) we parametrize the spatial metric [tex]g_{ij}[/tex] by means of Iwasawa variables ([tex]\beta^a, {\cal N}^a{}_i[/tex]); (ii) we define, at each spatial point, a (chaotic) asymptotic evolution system made of ordinary differential equations for the Iwasawa variables; and (iii) we characterize the exact Einstein solutions [tex]\beta, {\cal{N}}[/tex] whose asymptotic behavior is described by a solution [tex]\beta_{[0]}, {\cal N}_{[0]}[/tex] of the previous evolution system by means of a `generalized Fuchsian system' for the differenced variables [tex]\bar \beta = \beta - \beta_{[0]}, \bar {\cal N} = {\cal N} - {\cal N}_{[0]}[/tex], and by requiring that [tex]\bar \beta[/tex] and [tex]\bar {\cal N}[/tex] tend to zero on the singularity. We also show that, in spite of the apparently chaotic infinite succession of `Kasner epochs' near the singularity, there exists a well-defined asymptotic geometrical structure on the singularity : it is described by a partially framed flag. Our treatment encompasses Einstein-matter systems (comprising scalar and p-forms), and also shows how the use of Iwasawa variables can simplify the usual (`asymptotically velocity term dominated') description of non-chaotic systems.

http://arxiv.org/abs/0710.5590
Gauguin's questions in particle physics: Where are we coming from? What are we? Where are we going?
John Ellis
16 pages, 11 figures, plenary talk at the International Europhysics Conference on High-Energy Physics (EPS-HEP2007), Manchester, England, July 2007
(Submitted on 30 Oct 2007)

"Within particle physics itself, Gauguin's questions may be interpreted as: P1 - What is the status of the Standard Model? P2 - What physics may lie beyond the Standard Model? P3 - What is the `Theory of Everything'? Gauguin's questions may also asked within a cosmological context: C1 - What were the early stages of the Big Bang? C2 - What is the material content of the Universe today? C3 - What is the future of the Universe? In this talk I preview many of the topics to be discussed in the plenary sessions of this conference, highlighting how they bear on these fundamental questions."

http://arxiv.org/abs/0710.5844
Lessons from (2+1)-dimensional quantum gravity
B J Schroers
17 pages, two figures, based on talk given at the conference ``From Quantum to Emergent Gravity: Theory and Phenomenology'',June 11-15 2007, Trieste, Italy
(Submitted on 31 Oct 2007)

"Proposals that quantum gravity gives rise to non-commutative spacetime geometry and deformations of Poincare symmetry are examined in the context of (2+1)-dimensional quantum gravity. The results are expressed in five lessons, which summarise how the gravitational constant, Planck's constant and the cosmological constant enter the non-commutative and non-cocommutative structures arising in (2+1)-dimensional quantum gravity. It is emphasised that the much studied bicrossproduct kappa-Poincare algebra does not arise directly in (2+1)-dimensional quantum gravity."
 
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http://arxiv.org/abs/0711.0146
LQG vertex with finite Immirzi parameter
Jonathan Engle, Etera Livine, Roberto Pereira, Carlo Rovelli
(Submitted on 1 Nov 2007)

"We extend the definition of the "flipped" loop-quantum-gravity vertex to the case of a finite Immirzi parameter gamma. We cover the euclidean as well as the lorentzian case. We show that the resulting dynamics is defined on a Hilbert space isomorphic to the one of loop quantum gravity, and that the area operator has the same discrete spectrum as in loop quantum gravity. This includes the correct dependence on gamma, and, remarkably, holds in the lorentzian case as well. The ad hoc flip of the symplectic structure that was required to derive the flipped vertex is not anymore required for finite gamma. These results establish a bridge between canonical loop quantum gravity and the spinfoam formalism in four dimensions."

The above paper is probably important. I put some comment on it as the second post in this discussion thread
https://www.physicsforums.com/showthread.php?t=194651
Additional comment would be welcome there.

http://arxiv.org/abs/0711.0090
On deformations of Ashtekar's constraint algebra
Kirill Krasnov
3 pages
(Submitted on 1 Nov 2007)

"We show that the constraint algebra of Ashtekar's Hamiltonian formulation of general relativity can be non-trivially deformed by allowing the cosmological constant to become an arbitrary function of the (Weyl) curvature. Our result implies that there is not one but infinitely many (parameterized by an arbitrary function) four-dimensional gravity theories propagating two degrees of freedom."

http://arxiv.org/abs/0711.0115
Manifestly Gauge-Invariant General Relativistic Perturbation Theory: I. Foundations
K. Giesel, S. Hofmann, T. Thiemann, O. Winkler
77 pages
(Submitted on 1 Nov 2007)

"Linear cosmological perturbation theory is pivotal to a theoretical understanding of current cosmological experimental data provided e.g. by cosmic microwave anisotropy probes. A key issue in that theory is to extract the gauge invariant degrees of freedom which allow unambiguous comparison between theory and experiment. When one goes beyond first (linear) order, the task of writing the Einstein equations expanded to n'th order in terms of quantities that are gauge invariant up to terms of higher orders becomes highly non-trivial and cumbersome. This fact has prevented progress for instance on the issue of the stability of linear perturbation theory and is a subject of current debate in the literature. In this series of papers we circumvent these difficulties by passing to a manifestly gauge invariant framework. In other words, we only perturb gauge invariant, i.e. measurable quantities, rather than gauge variant ones. Thus, gauge invariance is preserved non perturbatively while we construct the perturbation theory for the equations of motion for the gauge invariant observables to all orders. In this first paper we develop the general framework which is based on a seminal paper due to Brown and Kuchar as well as the realtional formalism due to Rovelli. In the second, companion, paper we apply our general theory to FRW cosmologies and derive the deviations from the standard treatment in linear order. As it turns out, these deviations are negligible in the late universe, thus our theory is in agreement with the standard treatment. However, the real strength of our formalism is that it admits a straightforward and unambiguous, gauge invariant generalisation to higher orders. This will also allow us to settle the stability issue in a future publication."

http://arxiv.org/abs/0711.0117
Manifestly Gauge-Invariant General Relativistic Perturbation Theory: II. FRW Background and First Order
K. Giesel, S. Hofmann, T. Thiemann, O. Winkler
51 pages
(Submitted on 1 Nov 2007)

"In our companion paper we identified a complete set of manifestly gauge-invariant observables for general relativity. This was possible by coupling the system of gravity and matter to pressureless dust which plays the role of a dynamically coupled observer. The evolution of those observables is governed by a physical Hamiltonian and we derived the corresponding equations of motion. Linear perturbation theory of those equations of motion around a general exact solution in terms of manifestly gauge invariant perturbations was then developed. In this paper we specialise our previous results to an FRW background which is also a solution of our modified equations of motion. We then compare the resulting equations with those derived in standard cosmological perturbation theory (SCPT). We exhibit the precise relation between our manifestly gauge-invariant perturbations and the linearly gauge-invariant variables in SCPT. We find that our equations of motion can be cast into SCPT form plus corrections. These corrections are the trace that the dust leaves on the system in terms of a conserved energy momentum current density. It turns out that these corrections decay, in fact, in the late universe they are negligible whatever the value of the conserved current. We conclude that the addition of dust which serves as a test observer medium, while implying modifications of Einstein's equations without dust, leads to acceptable agreement with known results, while having the advantage that one now talks about manifestly gauge-invariant, that is measurable, quantities, which can be used even in perturbation theory at higher orders."

http://arxiv.org/abs/0711.0119
Algebraic Quantum Gravity (AQG) IV. Reduced Phase Space Quantisation of Loop Quantum Gravity
K. Giesel, T. Thiemann
31 pages
(Submitted on 1 Nov 2007)

"We perform a canonical, reduced phase space quantisation of General Relativity by Loop Quantum Gravity (LQG) methods. The explicit construction of the reduced phase space is made possible by the combination of 1. the Brown -- Kuchar mechanism in the presence of pressure free dust fields which allows to deparametrise the theory and 2. Rovelli's relational formalism in the extended version developed by Dittrich to construct the algebra of gauge invariant observables. Since the resulting algebra of observables is very simple, one can quantise it using the methods of LQG. Basically, the kinematical Hilbert space of non reduced LQG now becomes a physical Hilbert space and the kinematical results of LQG such as discreteness of spectra of geometrical operators now have physical meaning. The constraints have disappeared, however, the dynamics of the observables is driven by a physical Hamiltonian which is related to the Hamiltonian of the standard model (without dust) and which we quantise in this paper."

The following is an overview article for a special issue of General Relativity and Gravitation (Springer) which will be devoted to the topic of Dark Energy. Martin Bojowald contributed one of the articles in that issue---indicating a way that the effect associated with dark energy might arise naturally in LQG from an already present quantum correction, without need for assuming a new type of exotic "dark energy" field.
The co-editors of GRG are George Ellis and Hermann Nicolai. The special edition on Dark Energy should be interesting, with many competing ideas presented.

http://arxiv.org/abs/0711.0077
Dark Energy and Dark Gravity
Ruth Durrer, Roy Maartens
21 pages 10 figures Overview article for the dark energy issue of GRG
(Submitted on 1 Nov 2007)

"Observations provide increasingly strong evidence that the universe is accelerating. This revolutionary advance in cosmological observations confronts theoretical cosmology with a tremendous challenge, which it has so far failed to meet. Explanations of cosmic acceleration within the framework of general relativity are plagued by difficulties. General relativistic models are nearly all based on a dark energy field with fine-tuned, unnatural properties. There is a great variety of models, but all share one feature in common -- an inability to account for the gravitational properties of the vacuum energy. Speculative ideas from string theory may hold some promise, but it is fair to say that no convincing model has yet been proposed. An alternative to dark energy is that gravity itself may behave differently from general relativity on the largest scales, in such a way as to produce acceleration. The alternative approach of modified gravity (or dark gravity) provides a new angle on the problem, but also faces serious difficulties, including in all known cases severe fine-tuning and the problem of explaining why the vacuum energy does not gravitate. The lack of an adequate theoretical framework for the late-time acceleration of the universe represents a deep crisis for theory -- but also an exciting challenge for theorists. It seems likely that an entirely new paradigm is required to resolve this crisis."

http://arxiv.org/abs/0711.0150
Space-based research in fundamental physics and quantum technologies
S.G. Turyshev, U.E. Israelsson, M. Shao, N. Yu, A. Kusenko, E.L. Wright, C.W.F. Everitt, M.A. Kasevich, J.A. Lipa, J.C. Mester, R.D. Reasenberg, R.L. Walsworth, N. Ashby, H. Gould, H.-J.Paik
a white paper, 27 pages
(Submitted on 1 Nov 2007)

"Space-based experiments today can uniquely address important questions related to the fundamental laws of Nature. In particular, high-accuracy physics experiments in space can test relativistic gravity and probe the physics beyond the Standard Model; they can perform direct detection of gravitational waves and are naturally suited for precision investigations in cosmology and astroparticle physics. In addition, atomic physics has recently shown substantial progress in the development of optical clocks and atom interferometers. If placed in space, these instruments could turn into powerful high-resolution quantum sensors greatly benefiting fundamental physics.
We discuss the current status of space-based research in fundamental physics, its discovery potential, and its importance for modern science. We offer a set of recommendations to be considered by the upcoming National Academy of Sciences' Decadal Survey in Astronomy and Astrophysics. In our opinion, the Decadal Survey should include space-based research in fundamental physics as one of its focus areas. We recommend establishing an Astronomy and Astrophysics Advisory Committee's interagency ``Fundamental Physics Task Force'' to assess the status of both ground- and space-based efforts in the field, to identify the most important objectives, and to suggest the best ways to organize the work of several federal agencies involved. We also recommend establishing a new NASA-led interagency program in fundamental physics that will consolidate new technologies, prepare key instruments for future space missions, and build a strong scientific and engineering community. Our goal is to expand NASA's science objectives in space by including ``laboratory research in fundamental physics'' as an element in agency's ongoing space research efforts."
 
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http://arxiv.org/abs/0711.0273
The Emergence of Spacetime, or, Quantum Gravity on Your Desktop
R. Loll
21 pages, 11 figures, write-up of plenary talk at GR18, Sydney, July 2007
(Submitted on 2 Nov 2007)

"Is there an approach to quantum gravity which is conceptually simple, relies on very few fundamental physical principles and ingredients, emphasizes geometric (as opposed to algebraic) properties, comes with a definite numerical approximation scheme, and produces robust results, which go beyond showing mere internal consistency of the formalism? The answer is a resounding yes: it is the attempt to construct a nonperturbative theory of quantum gravity, valid on all scales, with the technique of so-called Causal Dynamical Triangulations. Despite its conceptual simplicity, the results obtained up to now are far from trivial. Most remarkable at this stage is perhaps the fully dynamical emergence of a classical background (and solution to the Einstein equations) from a nonperturbative sum over geometries, without putting in any preferred geometric background at the outset. In addition, there is concrete evidence for the presence of a fractal spacetime foam on Planckian distance scales. The availability of a computational framework provides built-in reality checks of the approach, whose importance can hardly be overestimated."

http://arxiv.org/abs/0711.0373
Automorphisms in Loop Quantum Gravity
Benjamin Bahr, Thomas Thiemann
57 pages, 7 figures
(Submitted on 2 Nov 2007)

"We investigate a certain distributional extension of the group of spatial diffeomorphisms in Loop Quantum Gravity. This extension, which is given by the automorphisms Aut(P) of the path groupoid P, was proposed by Velhinho and is inspired by category theory. This group is much larger than the group of piecewise analytic diffeomorphisms. In particular, we will show that graphs with the same combinatorics but different generalized knotting classes can be mapped into each other. We describe the automorphism-invariant Hilbert space and comment on how a combinatorial formulation of LQG might arise."
 
  • #654
http://arxiv.org/abs/0711.0697
Non-metric gravity: A status report
Kirill Krasnov
13 pages, no figures, invited review for Modern Physics Letters A
(Submitted on 5 Nov 2007)

"We review the status of a certain (infinite) class of four-dimensional generally covariant theories propagating two degrees of freedom that are formulated without any direct mention of the metric. General relativity itself (in its Plebanski formulation) belongs to the class, so these theories are examples of modified gravity. We summarize the current understanding of the nature of the modification, of the renormalizability properties of these theories, of their coupling to matter fields, and describe some of their physical properties."

http://arxiv.org/abs/0711.0788
On a Continuum Limit for Loop Quantum Cosmology
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
8 pages. Contribution for the Proceedings of the Third Mexican Meeting on Mathematical and Experimental Physics
(Submitted on 6 Nov 2007)

"The use of non-regular representations of the Heisenberg-Weyl commutation relations has proved to be useful for studying conceptual and technical issues in quantum gravity. Of particular relevance is the study of Loop Quantum Cosmology (LQC), a symmetry reduced theory that is related to Loop Quantum Gravity, and that is based on a non-regular, polymeric representation. Recently, a soluble model was used by Ashtekar, Corichi and Singh to study the relation between Loop Quantum Cosmology and the standard Wheeler-DeWitt theory and in particular the passage to the limit in which the auxiliary parameter (interpreted as "quantum geometry discreetness") is sent to zero in hope to get rid of this `regulator ambiguity' in the LQC dynamics. In this note we outline the first steps toward reformulating this question within the program developed by the authors for studying the continuum limit of polymeric theories, which was successfully applied to simple systems such as a Simple Harmonic Oscillator and the Free Particle."

http://arxiv.org/abs/0711.0770
An Exceptionally Simple Theory of Everything
A. Garrett Lisi
31 pages, 7 figures
(Submitted on 6 Nov 2007)

"All fields of the standard model and gravity are unified as an E8 principal bundle connection. A non-compact real form of the E8 Lie algebra has G2 and F4 subalgebras which break down to strong su(3), electroweak su(2) x u(1), gravitational so(3,1), the frame-Higgs, and three generations of fermions related by triality. The interactions and dynamics of these 1-form and Grassmann valued parts of an E8 superconnection are described by the curvature and action over a four dimensional base manifold."

http://arxiv.org/abs/0711.0743
Gravitational Wave Sirens as a Triple Probe of Dark Energy
Eric V. Linder
7 pages, 5 figures
(Submitted on 5 Nov 2007)

"Gravitational wave standard sirens have been considered as precision distance indicators to high redshift; however, at high redshift standard sirens or standard candles such as supernovae suffer from lensing noise. We investigate lensing noise as a signal instead and show how measurements of the maximum demagnification (minimum convergence) probe cosmology in a highly complementary manner to the distance itself. Revisiting the original form for minimum convergence we quantify the bias arising from the commonly used approximation. Furthermore, after presenting a new lensing probability function we discuss how the width of the lensed standard siren amplitude distribution also probes growth of structure. Thus standard sirens and candles can serve as triple probes of dark energy, measuring both the cosmic expansion history and growth history."

http://arxiv.org/abs/0711.0757
Universal quantum mechanics
Steven B. Giddings
15 pages
(Submitted on 5 Nov 2007)

"If gravity respects quantum mechanics, it is important to identify the essential postulates of a quantum framework capable of incorporating gravitational phenomena. Such a construct likely requires elimination or modification of some of the "standard" postulates of quantum mechanics, in particular those involving time and measurement. This paper proposes a framework that appears sufficiently general to incorporate some expected features of quantum gravity. These include the statement that space and time may only emerge approximately and relationally. One perspective on such a framework is as a sort of generalization of the S-matrix approach to dynamics. Within this framework, more dynamical structure is required to fully specify a theory; this structure is expected to lack some of the elements of local quantum field theory. Some aspects of this structure are discussed, both in the context of scattering of perturbations about a flat background, and in the context of cosmology."
 
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http://arxiv.org/abs/0711.1284
The complete LQG propagator: II. Asymptotic behavior of the vertex
Emanuele Alesci, Carlo Rovelli
16 pages
(Submitted on 8 Nov 2007)

"In a previous article we have show that there are difficulties in obtaining the correct graviton propagator from the loop-quantum-gravity dynamics defined by the Barrett-Crane vertex amplitude. Here we show that a vertex amplitude that depends nontrivially on the intertwiners can yield the correct propagator. We give an explicit example of asymptotic behavior of a vertex amplitude that gives the correct full graviton propagator in the large distance limit."
 
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  • #656
Making a 1978 classic available online
http://arxiv.org/abs/0711.2445
An introduction to quantum gravity
Bryce S. DeWitt, Giampiero Esposito
68 pages. Sections 2 to 17, with kind permission of Springer
(Submitted on 15 Nov 2007)

"After an overview of the physical motivations for studying quantum gravity, we reprint THE FORMAL STRUCTURE OF QUANTUM GRAVITY, i.e. the 1978 Cargese Lectures by Professor B.S. DeWitt, with kind permission of Springer. The reader is therefore introduced, in a pedagogical way, to the functional integral quantization of gravitation and Yang-Mills theory. It is hoped that such a paper will remain useful for all lecturers or Ph.D. students who face the task of introducing (resp. learning) some basic concepts in quantum gravity in a relatively short time. In the second part, we outline selected topics such as the braneworld picture with the same covariant formalism of the first part, and spectral asymptotics of Euclidean quantum gravity with diffeomorphism-invariant boundary conditions. The latter might have implications for singularity avoidance in quantum cosmology."

http://arxiv.org/abs/0711.2455
Physical boundary state for the quantum tetrahedron
Etera R. Livine, Simone Speziale
20 pages, 6 figures
(Submitted on 15 Nov 2007)

"We consider stability under evolution as a criterion to select a physical boundary state for the spinfoam formalism. As an example, we apply it to the simplest spinfoam defined by a single quantum tetrahedron and solve the associated eigenvalue problem at leading order in the large spin limit. We show that this fixes uniquely the free parameters entering the boundary state. Remarkably, the state obtained this way gives a correlation between edges which runs at leading order with the inverse distance between the edges, in agreement with the linearized continuum theory. Finally, we give an argument why this correlator represents the propagation of a pure gauge, consistently with the absence of physical degrees of freedom in 3d general relativity."

http://arxiv.org/abs/0711.2198
Stability, Singularities and Mass Thresholds in Child Universe Production: a concise survey including some recent results and prospects
Stefano Ansoldi, Eduardo I. Guendelman, Idan Shilon
19 pages, including 68 references; to appear in the Proceedings of "BH2, Dynamics and Thermodynamics of Blackholes and Naked Singularities", May 10-12 2007, Milano, Italy; conference website: http://www.mate.polimi.it/bh2/
(Submitted on 14 Nov 2007)

"We present a review of selected topics concerning the creation and evolution of child universes, together with a concise account of some recent progress in the field."

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

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

The next paper draws conclusions which are unfamiliar and seem radical to me. Not qualified to evaluate, but decided it was safer to include it.
http://arxiv.org/abs/0711.1879
Area, ladder symmetry, degeneracy and fluctuations of a horizon
Mohammad H. Ansari
12 pages, 1 figure
(Submitted on 13 Nov 2007)

"Loop quantum gravity admits a kind of area quantization that is characterized by three quantum numbers. We show the complete spectrum of area is the union of equidistant subsets and a universal reformulation with fewer parameters is possible. Associated with any area there is also another number that determines its degeneracy. One application is that a quantum horizon manifests harmonic modes in vacuum fluctuations. It is discussed the physical fluctuations of a space-time horizon should include all the excluded area eigenvalues, where quantum amplification effect occurs. Due to this effect the uniformity of transition matrix elements between near levels could be assumed. Based on these, a modification to the previous method of analyzing the radiance intensities in hep-th/0607081 is presented that makes the result one step further precise. A few of harmonic modes appear to be extremely amplified on top of the Hawking's radiation. They are expected to form a few brightest lines with the wavelength not larger than the black hole size."
 
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  • #657
http://arxiv.org/abs/0711.2875
Two-dimensional topological field theories coupled to four-dimensional BF theory
Merced Montesinos, Alejandro Perez
(Submitted on 19 Nov 2007)

"Four dimensional BF theory admits a natural coupling to extended sources supported on two dimensional surfaces or string world-sheets. Solutions of the theory are in one to one correspondence with solutions of Einstein equations with distributional matter (cosmic strings). We study new (topological field) theories that can be constructed by adding extra degrees of freedom to the two dimensional world-sheet. We show how two dimensional Yang-Mills degrees of freedom can be added on the world-sheet, producing in this way, an interactive (topological) theory of Yang-Mills fields with BF fields in four dimensions. We also show how a world-sheet tetrad can be naturally added. As in the previous case the set of solutions of these theories are contained in the set of solutions of Einstein's equations if one allows distributional matter supported on two dimensional surfaces. These theories are argued to be exactly quantizable. In the context of quantum gravity, one important motivation to study these models is to explore the possibility of constructing a background independent quantum field theory where local degrees of freedom at low energies arise from global topological (world-sheet) degrees of freedom at the fundamental level."http://arxiv.org/abs/0711.3107
The theta parameter in loop quantum gravity: effects on quantum geometry and black hole entropy
Danilo Jimenez Rezende, Alejandro Perez
(Submitted on 20 Nov 2007)

"The precise analog of the theta-quantization ambiguity of Yang-Mills theory exists for the real SU(2) connection formulation of general relativity. As in the former case theta labels representations of large gauge transformations, which are super-selection sectors in loop quantum gravity. We show that unless theta=0, the (kinematical) geometric operators such as area and volume are not well defined on spin network states. More precisely the intersection of their domain with the dense set Cyl in the kinematical Hilbert space H of loop quantum gravity is empty. The absence of a well defined notion of area operator acting on spin network states seems at first in conflict with the expected finite black hole entropy. However, we show that the black hole (isolated) horizon area--which in contrast to kinematical area is a (Dirac) physical observable--is indeed well defined, and quantized so that the black hole entropy is proportional to the area. The effect of theta is negligible in the semiclassical limit where proportionality to area holds."

edit to reply to next:
Thanks. I made the correction.
 
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  • #658
The links should be switched around.
jal
 
  • #659
There is an interesting tie-in between this paper (which I already listed above) and two other new ones from Fulvio Melia, which are listed below:
http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)

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

http://arxiv.org/abs/0711.4181
The Cosmic Horizon
Fulvio Melia
Accepted for publication in MNRAS
(Submitted on 27 Nov 2007)

"The cosmological principle, promoting the view that the universe is homogeneous and isotropic, is embodied within the mathematical structure of the Robertson-Walker (RW) metric. The equations derived from an application of this metric to the Einstein Field Equations describe the expansion of the universe in terms of comoving coordinates, from which physical distances may be derived using a time-dependent expansion factor. These coordinates, however, do not explicitly reveal properties of the cosmic spacetime manifested in Birkhoff's theorem and its corollary. In this paper, we compare two forms of the metric--written in (the traditional) comoving coordinates, and a set of observer-dependent coordinates--first for the well-known de Sitter universe containing only dark energy, and then for a newly derived form of the RW metric, for a universe with dark energy and matter. We show that Rindler's event horizon--evident in the co-moving system--coincides with what one might call the 'curvature horizon' appearing in the observer-dependent frame. The advantage of this dual prescription of the cosmic spacetime is that with the latest WMAP results, we now have a much better determination of the universe's mass-energy content, which permits us to calculate this curvature with unprecedented accuracy. We use it here to demonstrate that our observations have probed the limit beyond which the cosmic curvature prevents any signal from having ever reached us. In the case of de Sitter, where the mass-energy density is a constant, this limit is fixed for all time. For a universe with a changing density, this horizon expands until de Sitter is reached asymptotically, and then it too ceases to change."

http://arxiv.org/abs/0711.4810
Dark Energy in Light of the Cosmic Horizon
Fulvio Melia
Submitted to MNRAS
(Submitted on 29 Nov 2007)

"Based on dramatic observations of the CMB with WMAP and of Type Ia supernovae with the Hubble Space Telescope and ground-based facilities, it is now generally believed that the Universe's expansion is accelerating. Within the context of standard cosmology, the Universe must therefore contain a third 'dark' component of energy, beyond matter and radiation. However, the current data are still deemed insufficient to distinguish between an evolving dark energy component and the simplest model of a time-independent cosmological constant. In this paper, we examine the role played by our cosmic horizon R0 in our interrogation of the data, and reach the rather firm conclusion that the existence of a cosmological constant is untenable. The observations are telling us that R0=c t0, where t0 is the perceived current age of the Universe, yet a cosmological constant would drive R0 towards ct (where t is the cosmic time) only once, and that would have to occur right now. In contrast, scaling solutions simultaneously eliminate several conundrums in the standard model, including the 'coincidence' and 'flatness' problems, and account very well for the fact that R0=c t0. We show here that for such dynamical dark energy models, either R0=ct for all time (thus eliminating the apparent coincidence altogether), or that what we believe to be the current age of the universe is actually the horizon time th=R0/c, which is always shorter than t0. Our best fit to the Type Ia supernova data indicates that t0 would then have to be ~16.9 billion years. Though surprising at first, an older universe such as this would actually eliminate several other long-standing problems in cosmology, including the (too) early appearance of supermassive black holes (at a redshift > 6) and the glaring deficit of dwarf halos in the local group."

Thanks to Wolram, Patty144, and Wallace for alerting us to these new papers by Melia, which I suspect may be important and may also clarify some of the issues raised by Pereira and Aldrovandi.
 
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  • #660
marcus said:
http://arxiv.org/abs/0711.2274
de Sitter Relativity: a New Road to Quantum Gravity
R. Aldrovandi, J. G. Pereira
17 pages
(Submitted on 14 Nov 2007)

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

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

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

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

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

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

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

Actually I am only just now noticing this thread, perhaps I should read this?
 
  • #661
Coin said:
...
But basically I am curious whether there are any commonalities between all of these models which reject the poincare group (besides of course the detail that they reject the poincare group...)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

"It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an 'apparent acceleration', even when any form of dark energy is absent. In this paper, we present a 'Minimal Void'' scenario, i.e. a 'void' with minimal underdensity contrast (of about -0.4) and radius (~ 200-250 Mpc/h) that can, not only be consistent with the supernovae data, but also with the 3-yr WMAP data. We also discuss consistency of our model with various other measurements such as Big Bang Nucleosynthesis, Baryon Acoustic Oscillations and local measurements of the Hubble parameter. We also point out possible other observable signatures."
 
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  • #662
http://arxiv.org/abs/0712.0977
The Plebanski action extended to a unification of gravity and Yang-Mills theory
Lee Smolin
13 pages, one figure
(Submitted on 6 Dec 2007)

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

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

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

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

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

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

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

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

"In this second paper on loop quantization of Gowdy model, we introduce the kinematical Hilbert space on which appropriate holonomies and fluxes are well represented. The quantization of the volume operator and the Gauss constraint is straight forward. Imposition of the Gauss constraint can be done on the kinematical Hilbert space to select subspace of gauge invariant states. We carry out the quantization of the Hamiltonian constraint making specific choices. Alternative choices are briefly discussed. It appears that to get spatial correlations reflected in the Hamiltonian constraint, one may have to adopt the so called 'effective operator viewpoint'."
 
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  • #663
http://arxiv.org/abs/0712.2485
Planckian Birth of the Quantum de Sitter Universe
J. Ambjorn, A. Gorlich, J. Jurkiewicz, R. Loll
10 pages, 3 figures
(Submitted on 17 Dec 2007)

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

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

"The aim of this note is to address the low energy limit of quantum field theories with a minimal length scale. The essential feature of these models is that the minimal length acts as a regulator in the asymptotic high energy limit which is incorporated through an infinite series of higher order derivatives. If one investigates a perturbative expansion in inverse powers of the Planck mass, one generically obtains extra poles in the propagator, and instabilities connected with the higher order derivative Lagrangian, that are however artifacts of truncating the series."
 
  • #664
http://arxiv.org/abs/0712.3242
Unspeakables and the Epistemological path towards Quantum Gravity
Daniel Sudarsky
Invited article for "GRF2007 Special Issue" of the IJMPD . In press
(Submitted on 19 Dec 2007)

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

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

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

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

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

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

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

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

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

An ULIRG is an ultra luminous infrared galaxy http://www.daviddarling.info/encyclopedia/U/ULIRG.html
thought to be the site of intense star formation activity
 
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  • #665
http://arxiv.org/abs/0712.3457
A general test of the Copernican Principle
Chris Clarkson, Bruce A. Bassett, Teresa Hui-Ching Lu (UCT & SAAO, Cape Town)
4 pages
(Submitted on 20 Dec 2007)

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

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

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

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

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

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

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