Loop-and-allied QG bibliography

[marcus]

It seems to me that there is a third option.
Assuming that a measurement in a given physical reality can be seen as a higher order self-expression, I don't consider it unreasonable that there would be information loss.

Hi Jennifer, this thread is unfortunately not for discussion---I just collect non-string QG links here. We normally start discussion of papers in a separate discussion thread, not to overcrowd this one.
So I will set up a discussion thread for you, so that you and others can comment on Appleby's paper.

 

[MeJennifer]

Ok, no problem. Thanks for setting up a new topic.

 

[marcus]

You are most welcome! Here is a link to the discussion thread

https://www.physicsforums.com/showthread.php?t=155348

https://www.physicsforums.com/showthread.php?p=1238609#post1238609

 

[marcus]

http://arxiv.org/abs/gr-qc/0702065
Some Implications of the Cosmological Constant to Fundamental Physics
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
15 pages, lecture presented at the "XIIth Brazilian School of Cosmology and Gravitation", Mangaratiba, Rio de Janeiro, September 10-23, 2006

"In the presence of a cosmological constant, ordinary Poincaré special relativity is no longer valid and must be replaced by a de Sitter special relativity, in which Minkowski space is replaced by a de Sitter spacetime. In consequence, the ordinary notions of energy and momentum change, and will satisfy a different kinematic relation. Such a theory is a different kind of a doubly special relativity. Since the only difference between the Poincaré and the de Sitter groups is the replacement of translations by certain linear combinations of translations and proper conformal transformations, the net result of this change is ultimately the breakdown of ordinary translational invariance. From the experimental point of view, therefore, a de Sitter special relativity might be probed by looking for possible violations of translational invariance. If we assume the existence of a connection between the energy scale of an experiment and the local value of the cosmological constant, there would be changes in the kinematics of massive particles which could hopefully be detected in high-energy experiments. Furthermore, due to the presence of a horizon, the usual causal structure of spacetime would be significantly modified at the Planck scale."

http://arxiv.org/abs/gr-qc/0606122
de Sitter special relativity
R. Aldrovandi, J. P. Beltran Almeida, J. G. Pereira
24 pages, to be published in Classical and Quantum Gravity

"A special relativity based on the de Sitter group is introduced, which is the theory that might hold up in the presence of a non-vanishing cosmological constant. Like ordinary special relativity, it retains the quotient character of spacetime, and a notion of homogeneity. As a consequence, the underlying spacetime will be a de Sitter spacetime, whose associated kinematics will differ from that of ordinary special relativity. The corresponding modified notions of energy and momentum are obtained, and the exact relationship between them, which is invariant under a re-scaling of the involved quantities, explicitly exhibited. Since the de Sitter group can be considered a particular deformation of the Poincaré group, this theory turns out to be a specific kind of deformed (or doubly) special relativity. Some experimental consequences, as well as the causal structure of spacetime--modified by the presence of the de Sitter horizon--are briefly discussed."

http://arxiv.org/abs/astro-ph/0702275
Escaping from MOND
Benoit Famaey, Jean-Philippe Bruneton, HongSheng Zhao
4 pages, 1 figure, submitted to MNRAS

"We present a new test of modified Newtonian dynamics (MOND) on galactic scales, based on the escape speed in the solar neighbourhood. This test is independent from other empirical successes of MOND at reproducing the phenomenology of galactic rotation curves. The galactic escape speed in MOND is entirely determined by the baryonic content of the Galaxy and the external field in which it is embedded. We estimate that the external field in which the Milky Way must be embedded to produce the observed local escape speed of 544 km/s is of the order of a_0/100, where a_0 is the dividing acceleration scale below which gravity is boosted in MOND. This is compatible with the external gravitational field actually acting on the Milky Way."

http://arxiv.org/abs/astro-ph/0702298
Missing Pages in Our Photo Album of the Infant Universe
Abraham Loeb (Harvard)
Comments: Popular level review, to appear in "Physica Plus" magazine, Vol. 8; a modified version with original artwork appeared in Scientific American, 295, 46, 2006---see this http URL:
http://cfa-www.harvard.edu/~loeb/sciam.pdf

"Existing data sets include an image of the Universe when it was 0.4 million years old (in the form of the cosmic microwave background), as well as images of individual galaxies when the Universe was older than a billion years. But there is a serious challenge: in between these two epochs was a period when the Universe was dark, stars had not yet formed, and the cosmic microwave background no longer traced the distribution of matter. And this is precisely the most interesting period, when the primordial soup evolved into the rich zoo of objects we now see. In this popular-level overview, I describe how astronomers plan to observe this nearly-invisible yet crucial period."

 

[marcus]

http://arxiv.org/abs/hep-th/0702124
Lorentz violation and perpetual motion
Christopher Eling, Brendan Z. Foster, Ted Jacobson, Aron C. Wall
5 pages, 1 figure

"We show that any Lorentz violating theory with two or more propagation speeds is in conflict with the generalized second law of black hole thermodynamics. We do this by identifying a classical energy-extraction method, analogous to the Penrose process, which would decrease the black hole entropy. Although the usual definitions of black hole entropy are ambiguous in this context, we require only very mild assumptions about its dependence on the mass. This extends the result found by Dubovsky and Sibiryakov, which uses the Hawking effect and applies only if the fields with different propagation speeds interact just through gravity. We also point out instabilities that could interfere with their black hole perpetuum mobile, but argue that these can be neglected if the black hole mass is sufficiently large."

Watch out for anything by Ted Jacobson. This is part of a cluster of recent papers dispelling hopes that GLAST might observe gammaray dispersion (energy dependent speed). There was one by Jerzy K-G and one that Laurent Freidel co-authored I think with K-G, and one by Bee Hossenfelder, and now this by Jacobson et al.

http://arxiv.org/abs/gr-qc/0702094
Black Hole Entropy and the Problem of Universality
Steven Carlip
10 pages; talk at DICE 2006, Piombino, Italy

"A key test of any quantum theory of gravity is its ability to reproduce the known thermodynamic properties of black holes. A statistical mechanical description of the Bekenstein-Hawking entropy once seemed remote, but today we suffer an embarrassment of riches: many different approaches to quantum gravity yield the same entropy, despite counting very different states. This 'universality' suggests that some underlying feature of the classical theory may control the quantum density of states. I discuss the possibility that this feature is an approximate two-dimensional conformal symmetry near the horizon."

http://arxiv.org/abs/gr-qc/0702093
Gauge invariant perturbations around symmetry reduced sectors of general relativity: applications to cosmology
Bianca Dittrich, Johannes Tambornino
39 pages, 1 figure

"We develop a gauge invariant canonical perturbation scheme for perturbations around symmetry reduced sectors in generally covariant theories, such as general relativity. The central objects of investigation are gauge invariant observables which encode the dynamics of the system. We apply this scheme to perturbations around a homogeneous and isotropic sector (cosmology) of general relativity. The background variables of this homogeneous and isotropic sector are treated fully dynamically which allows us to approximate the observables to arbitrary high order in a self--consistent and fully gauge invariant manner. Methods to compute these observables are given. The question of backreaction effects of inhomogeneities onto a homogeneous and isotropic background can be addressed in this framework. We illustrate the latter by considering homogeneous but anisotropic Bianchi--I cosmologies as perturbations around a homogeneous and isotropic sector."

Bianca Dittrich has worked with both Thomas Thiemann and Renate Loll (as the junior author) and has clearly aided TT's program. He keeps on citing a solo paper she did which has turned out valuable. I think her method of operation is to do contributory research that she sees needs to be done---to crack some difficult technical obstacle in somebody else's path. So although I don't immediately see where this paper fits in, i guess it's a significant piece is somebody's jigsaw.

http://arxiv.org/abs/gr-qc/0702082
Cosmic clocks, cosmic variance and cosmic averages
David L. Wiltshire
72 pages, 5 figures

"Cosmic acceleration is explained quantitatively, purely in general relativity, as an apparent effect due to quasilocal gravitational energy differences that arise in the decoupling of bound systems from the global expansion of the universe. "Dark energy" is recognised as a misidentification of those aspects of gravitational energy which by virtue of the equivalence principle cannot be localised, namely gradients in the energy associated with the expansion of space and spatial curvature variations in an inhomogeneous universe, as we observe. Gravitational energy differences between observers in bound systems, such as galaxies, and volume-averaged comoving locations within voids in freely expanding space can be so large that the time dilation between the two significantly affects the parameters of any effective homogeneous isotropic model one fits to the universe. A new approach to cosmological averaging is presented, which implicitly solves the Sandage-de Vaucouleurs paradox. When combined with a nonlinear scheme for cosmological evolution with back-reaction via the Buchert equations, a new observationally viable quantitative model of the universe is obtained. The expansion age is increased, allowing more time for structure formation. The baryon density fraction obtained from primordial nucleosynthesis bounds can be significantly larger, yet consistent with primordial lithium abundance measurements. The angular scale of the first Doppler peak in the CMB anisotropy spectrum fits the new model despite an average negative spatial curvature at late epochs, resolving the anomaly associated with ellipticity in the CMB anisotropies. A number of other testable consequences are discussed, with the potential to profoundly change the whole of theoretical and observational cosmology. [Abridged] "

Special thanks to Kea, who is mentioned in the acknowledgments :-), for calling attention to this new article by David Wiltshire.

http://www.arxiv.org/abs/hep-ex/0702026
Charting the Course for Elementary Particle Physics
Burton Richter
AAAS National Meeting, San Francisco, 2007, Symposium, A New Frontier in Particle Physics, 15 pages, 8 figures
SLAC-PUB-12345

"It was the best of times; it was the worst of times is the way Dickens begins the Tale of Two Cities. The line is appropriate to our time in particle physics. It is the best of times because we are in the midst of a revolution in understanding, the third to occur during my career. It is the worst of times because accelerator facilities are shutting down before new ones are opening, restricting the opportunity for experiments, and because of great uncertainty about future funding. My task today is to give you a view of the most important opportunities for our field under a scenario that is constrained by a tight budget. It is a time when we cannot afford the merely good, but must give first priority to the really important."

 

[marcus]

Today (20 Feb) Martin Bojowald gave a seminar talk for the ILQGS in which he summarized the 3-week workshop on
The Quantum Nature of Spacetime Singularities at Santa Barbara KITP
The lecture notes are here:
http://relativity.phys.lsu.edu/ilqgs/
http://relativity.phys.lsu.edu/ilqgs/bojowald022007.pdf

I have not yet encountered the audio for the talk. The ILQGS webpage has an error, a different talk was given on 13 Feb.

http://arxiv.org/abs/gr-qc/0702107
Black Hole Thermodynamics from Euclidean Horizon Constraints
S. Carlip
4 pages

"To explain black hole thermodynamics in quantum gravity, one must introduce constraints to ensure that a black hole is actually present. I show that for a large class of black holes, the presence of such 'horizon constraints' makes it possible to use conformal field theory techniques to compute the density of states, reproducing the Bekenstein-Hawking entropy in a nearly model-independent manner. I argue that the relevant degrees of freedom may be Goldstone-boson-like excitations arising from the weak breaking of symmetry by the horizon constraints."

The second paper by Steve Carlip on BH entropy this week.

http://arxiv.org/abs/hep-th/0702159
Deformed Special Relativity in a Canonical Framework
Subir Ghosh (Indian Statistical Institute, India), Probir Pal (Uluberia College, India)
17 pages

"In this paper we have studied the nature of kinematical and dynamical laws in $\kappa$-Minkowski spacetime from a new perspective: the canonical phase space approach. We have introduced a new form of $\kappa$-Minkowski phase space algebra from which we recover the $\kappa$-extended finite Lorentz transformations derived in [13]. This is a particular form of a Deformed Special Relativity model that admits a modified energy-momentum dispersion law as well as noncommutative $\kappa$-Minkowski phase space. We show that this system can be completely mapped to a set of phase space variables that obey canonical (and not $\kappa$-Minkowski) phase space algebra and Special Relativity Lorentz transformation (and not $\kappa$-extended Lorentz transformation). We demonstrate the usefulness and simplicity of this approach through a number of applications both in classical and quantum mechanics. We also construct a Lagrangian for the $\kappa$-particle."

 

[marcus]

William Donnelly has started a blog called "Uncommon Information"
and one of the first posts called attention to a 1995 paper by Ted Jacobson and a related 2006 workshop talk he gave at Perimeter
http://arxiv.org/abs/gr-qc/9504004
Thermodynamics of Spacetime: The Einstein Equation of State
Ted Jacobson
8 pages, 1 figure.
Phys.Rev.Lett. 75 (1995) 1260-1263

"The Einstein equation is derived from the proportionality of entropy and horizon area together with the fundamental relation $\delta Q=TdS$ connecting heat, entropy, and temperature. The key idea is to demand that this relation hold for all the local Rindler causal horizons through each spacetime point, with $\delta Q$ and T interpreted as the energy flux and Unruh temperature seen by an accelerated observer just inside the horizon. This requires that gravitational lensing by matter energy distorts the causal structure of spacetime in just such a way that the Einstein equation holds. Viewed in this way, the Einstein equation is an equation of state. This perspective suggests that it may be no more appropriate to canonically quantize the Einstein equation than it would be to quantize the wave equation for sound in air."

Donnelly's blog is here:
http://williamdonnelly.blogspot.com/
and the post about T.J.'s Perimeter workshop talk is here:
http://williamdonnelly.blogspot.com/2006/09/natural-ultraviolet-cutoffs-in_09.html

I couldn't find a PIRSA video of this talk. the date would have been Wednesday 6 September 2006.
Sounds interesting both from Donnelly's summary and from the related 1995 paper----you assume that all causal horizons have the famous S = A/4 relation between horizon area and entropy and from that simple relation you can derive the Einstein equation of GR. The roots of intuition that grew out of must be the most interesting thing about it.
============================

there is a new blog about NonCommutative Geometry
http://noncommutativegeometry.blogspot.com/2007/02/rosetta-stone-of-noncommutative.html
It just started in the past month or so. One of the people posting is Masoud Khalkhali.
Alain Connes submitted a comment to the blog recently.

There is also a new journal called Journal of Non-Commutative Geometry (JNCG)
http://www.ems-ph.org/journals/jncg/jncg.php
I see that Marc Rieffel and Vaughn Jones are on the editorial board. Connes is the journal's top editor.
The first issue of JNCG appeared this month.

NCG and Connes are especially notable now because there is an actual prediction that the higgs boson mass will be about 170 GeV.
http://resonaances.blogspot.com/2007/02/alain-connes-standard-model.html
https://www.physicsforums.com/showthread.php?t=127342
http://www.sciam.com/print_version.cfm?articleID=00039831-4051-14C0-AFE483414B7F4945
Here are PF predictions---Jim Graber set up a poll
https://www.physicsforums.com/showthread.php?t=147265

briefly noted:
http://arxiv.org/abs/hep-th/0702178
Eternal inflation and its implications
Alan H. Guth
21 pages, 5 figures. Talk presented at the "2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC 2006)," Barcelona, Spain, 11-15 July 2006, to be published in J. Phys. A

 

[marcus]

http://math.ucr.edu/home/baez/week246.html
John Baez TWF 246 discusses issues raised in
the Smolin book The Trouble with Physics...and What Comes Next and the Woit book Not Even Wrong

Thanks to Christine Dantas for calling attention to the new JB essay!

http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages

"The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way. Such SU(2)-connection locally agrees with the usual BI connection and it can be defined on pretty general bundles; in particular triviality is not assumed. As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice."

http://arxiv.org/abs/gr-qc/0702125
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
Winston Fairbairn, Etera R. Livine
17 pages, 1 figure

"An effective field theory for matter coupled to three-dimensional quantum gravity was recently derived in the context of spinfoam models in hep-th/0512113. In this paper, we show how this relates to group field theories and generalized matrix models. In the first part, we realize that the effective field theory can be recasted as a matrix model where couplings between matrices of different sizes can occur. In a second part, we provide a family of classical solutions to the three-dimensional group field theory. By studying perturbations around these solutions, we generate the dynamics of the effective field theory. We identify a particular case which leads to the action of hep-th/0512113 for a massive field living in a flat non-commutative space-time. The most general solutions lead to field theories with non-linear redefinitions of the momentum which we propose to interpret as living on curved space-times. We conclude by discussing the possible extension to four-dimensional spinfoam models."

http://arxiv.org/abs/gr-qc/0702132
A generalized Schroedinger equation for loop quantum cosmology
D. C. Salisbury, A. Schmitz
5 pages, to appear in the Proceedings of the Eleventh Marcel Grossmann Meeting

"A temporally discrete Schroedinger time evolution equation is proposed for isotropic quantum cosmology coupled to a massless scalar source. The approach employs dynamically determined intrinsic time and produces the correct semiclassical limit."

http://arxiv.org/abs/astro-ph/0702670
Dynamical Dark Energy or Simply Cosmic Curvature?
Chris Clarkson, Marina Cortes, Bruce A. Bassett
5 pages, 1 figure

"We show that the assumption of a flat universe induces critically large errors in reconstructing the dark energy equation of state at z>~0.9 even if the true cosmic curvature is very small, O(1%) or less. The spuriously reconstructed w(z) shows a range of unusual behaviour, including crossing of the phantom divide and mimicking of standard tracking quintessence models. For 1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching (50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble rate measurements have opposite trends due to the asymmetric influence of the curved geodesics. These results show that including curvature as a free parameter is imperative in any future analyses attempting to pin down the dynamics of dark energy, especially at moderate or high redshifts."briefly noted:

http://arxiv.org/abs/hep-th/0702192
The Universe as a topological defect
Andres Anabalon, Steven Willison, Jorge Zanelli

http://arxiv.org/abs/hep-th/0702115
Predicting the Cosmological Constant from the Causal Entropic Principle
Raphael Bousso, Roni Harnik, Graham D. Kribs, Gilad Perez

 

[marcus]

http://arxiv.org/abs/gr-qc/0702144
Singularities and Quantum Gravity
Martin Bojowald
41 pages, lecture course at the XIIth Brazilian School on Cosmology and Gravitation, September 2006
IGPG-07/2-4, NSF-KITP-07-19

"Although there is general agreement that a removal of classical gravitational singularities is not only a crucial conceptual test of any approach to quantum gravity but also a prerequisite for any fundamental theory, the precise criteria for non-singular behavior are often unclear or controversial. Often, only special types of singularities such as the curvature singularities found in isotropic cosmological models are discussed and it is far from clear what this implies for the very general singularities that arise according to the singularity theorems of general relativity. In these lectures we present an overview of the current status of singularities in classical and quantum gravity, starting with a review and interpretation of the classical singularity theorems. This suggests possible routes for quantum gravity to evade the devastating conclusion of the theorems by different means, including modified dynamics or modified geometrical structures underlying quantum gravity. The latter is most clearly present in canonical quantizations which are discussed in more detail. Finally, the results are used to propose a general scheme of singularity removal, quantum hyperbolicity, to show cases where it is realized and to derive intuitive semiclassical pictures of cosmological bounces."

briefly noted:
http://arxiv.org/abs/hep-th/0702219
String Theory: Progress and Problems
John H. Schwarz
14 pages; Presented at the Yukawa--Tomonaga Centennial Symposium
CALT-68-2627

"...This talk will give an overview of some of the progress and some of the unsolved problems that characterize string theory today..."

 

[marcus]

http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations
Kirill Krasnov
21 pages

"We describe and study a certain class of modified gravity theories. Our starting point is Plebanski formulation of gravity in terms of a triple of 2-forms, a connection A and a 'Lagrange multiplier' field Psi. The generalization we consider stems from presence in the action of an extra term proportional to a scalar function of Psi. As in the usual Plebanski general relativity (GR) case, the equations coming from variations with respect to Psi imply that a certain metric can be introduced. However, unlike in GR, the connection A no longer coincides with the self-dual part of the metric-compatible spin-connection. Field equations of the theory are shown to be relations between derivatives of the metric and components of field Psi, as well as its derivatives, the later being in contrast to the GR case. The equations are of second order in derivatives. An analog of the Bianchi identity is still present in the theory, as well as its contracted version tantamount to energy conservation equation. The arising modifications to the later are possibly of experimental significance."

 

[marcus]

Extending Quant. Cos. to the ANISOTROPIC case

http://arxiv.org/abs/gr-qc/0703010
Effective Dynamics for the Cosmological Bounces in Bianchi Type I Loop Quantum Cosmology
Dah-Wei Chiou
7 pages, 4 figures
IGPG-07/2-5

"The detailed formulation for loop quantum cosmology (LQC) in Bianchi I models was recently constructed. In this paper, the effective dynamics with the LQC discreteness corrections is studied and exactly solved, showing that the big bang is replaced by big bounces, which take place up to three times, once in each diagonal direction, whenever each of the area scale factors approaches its critical value in the Planck regime measured by the reference of the scalar field momentum."

Dah-Wei has already published the prequel to this paper in Physical Review series D:

http://arxiv.org/abs/gr-qc/0609029
Loop Quantum Cosmology in Bianchi Type I Models: Analytical Investigation
Dah-Wei Chiou
53 pages, 2 figures
Phys.Rev. D75 (2007) 024029

The comprehensive formulation for loop quantum cosmology in the spatially flat, isotropic model was recently constructed. In this paper, the methods are extended to the anisotropic Bianchi I cosmology. Both the precursor and the improved strategies are applied and the expected results are established: (i) the scalar field again serves as an internal clock and is treated as emergent time; (ii) the total Hamiltonian constraint is derived by imposing the fundamental discreteness and gives the evolution as a difference equation; and (iii) the physical Hilbert space, Dirac observables and semi-classical states are constructed rigorously. It is also shown that the state in the kinematical Hilbert space associated with the classical singularity is decoupled in the difference evolution equation, indicating that the big bounce may take place when any of the area scales undergoes the vanishing behavior. The investigation affirms the robustness of the framework used in the isotropic model by enlarging its domain of validity and provides foundations to conduct the detailed numerical analysis.

Dah-Wei Chiou got his PhD in theoretical physics from UC-BERKELEY in IIRC 2005. He was still doing string in 2005, but he escaped out of string and made it to Penn State to work in Ashtekar's group on singularity resolution in quantum cosmology. In my humble private opinion DahWei Chiou made a smart move and he has already done very significant research.
Before, one could handle the big bounce of the universe only in case it was not LOP-SIDED. If it was perfectly symmetrical you could get a collapse turning into an expansion. But what about the lop-sided case? What happens when the space and matter are not perfectly isotropic? Will the collapse get screwed-up and not bounce right? This is the important ANisotropic, or "not-isotropic", asymmetrical case. Dah-Wei has addressed this in one particular version called "Bianchi #1".
Carlo Rovelli and Matteo Smerlak have gotten their Relational EPR accepted for publication and they have made the final revision on it.
They posted the final version yesterday 4 March and it seems actually a little different from the first version, which we discussed here at PF. I will put the link again in case anyone wants to check out the revised version of the paper.
http://arxiv.org/abs/quant-ph/0604064
Relational EPR
Matteo Smerlak, Carlo Rovelli
Revised, published version

"We study the EPR-type correlations from the perspective of the relational interpretation of quantum mechanics. We argue that these correlations do not entail any form of 'non-locality', when viewed in the context of this interpretation. The abandonment of strict Einstein realism implied by the relational stance permits to reconcile quantum mechanics, completeness, (operationally defined) separability, and locality."

 

[marcus]

http://arxiv.org/abs/gr-qc/0703044
de Sitter space and the equivalence between f(R) and scalar-tensor gravity
Valerio Faraoni (Bishop's University)
4 pages, to appear in Phys. Rev. D

"It is shown that, when f'' is non-vanishing, metric f(R) gravity is completely equivalent to a scalar-tensor theory (with zero Brans-Dicke parameter) with respect to perturbations of de Sitter space, contrary to previous expectations. Moreover, the stability conditions of de Sitter space with respect to homogeneous and inhomogeneous perturbations coincide in most scalar-tensor theories, as is the case in metric f(R) gravity."

http://arxiv.org/abs/gr-qc/0703050
Vacuum properties of nonsymmetric gravity in de Sitter space
Tomas Janssen, Tomislav Prokopec (ITP & Spinoza Institute, Utrecht University)
32 pages, 2 figures
ITP-UU-07/9, SPIN-07/9

"We consider quantum effects of a massive antisymmetric tensor field on the dynamics of de Sitter space-time. Our starting point is the most general, stable, linearized Lagrangian arising in nonsymmetric gravitational theories (NGTs), where part of the antisymmetric field mass is generated by the cosmological term. We construct a renormalization group (RG) improved effective action by integrating out one loop vacuum fluctuations of the antisymmetric tensor field and show that, in the limit when the RG scale goes to zero, the Hubble parameter -- and thus the effective cosmological constant -- relaxes rapidly to zero. We thus conclude that quantum loop effects in de Sitter space can dramatically change the infrared sector of the on-shell gravity, making the expansion rate insensitive to the original (bare) cosmological constant."

http://arxiv.org/abs/quant-ph/0703060
A Topos Foundation for Theories of Physics: I. Formal Languages for Physics
A. Doering, C.J. Isham
36 pages

"This paper is the first in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper we discuss two different types of language that can be attached to a system, S. The first is a propositional language, PL(S); the second is a higher-order, typed language L(S). Both languages provide deductive systems with an intuitionistic logic. The reason for introducing PL(S) is that, as shown in paper II of the series, it is the easiest way of understanding, and expanding on, the earlier work on topos theory and quantum physics. However, the main thrust of our programme utilises the more powerful language L(S) and its representation in an appropriate topos."

http://arxiv.org/abs/quant-ph/0703062
A Topos Foundation for Theories of Physics: II. Daseinisation and the Liberation of Quantum Theory
A. Doering, C.J. Isham
34 pages

"This paper is the second in a series whose goal is to develop a fundamentally new way of constructing theories of physics. The motivation comes from a desire to address certain deep issues that arise when contemplating quantum theories of space and time. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. In this paper, we study in depth the topos representation of the propositional language, PL(S), for the case of quantum theory. In doing so, we make a direct link with, and clarify, the earlier work on applying topos theory to quantum physics. The key step is a process we term `daseinisation' by which a projection operator is mapped to a sub-object of the spectral presheaf--the topos quantum analogue of a classical state space. In the second part of the paper we change gear with the introduction of the more sophisticated local language L(S). From this point forward, throughout the rest of the series of papers, our attention will be devoted almost entirely to this language. In the present paper, we use L(S) to study `truth objects' in the topos. These are objects in the topos that play the role of states: a necessary development as the spectral presheaf has no global elements, and hence there are no microstates in the sense of classical physics. Truth objects therefore play a crucial role in our formalism."

http://arxiv.org/abs/quant-ph/0703064
A Topos Foundation for Theories of Physics: III. The Representation of Physical Quantities With Arrows
A. Doering, C.J. Isham
38 pages

"This paper is the third in a series whose goal is to develop a fundamentally new way of viewing theories of physics. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. In paper II, we studied the topos representations of the propositional language PL(S) for the case of quantum theory, and in the present paper we do the same thing for the, more extensive, local language L(S). One of the main achievements is to find a topos representation for self-adjoint operators. This involves showing that, for any physical quantity A, there is an arrow $\breve{\delta}^o(A):\Sig\map$symbol, where symbol is the quantity-value object for this theory. The construction of $\breve{\delta}^o(A)$ is an extension of the daseinisation of projection operators that was discussed in paper II. The object symbol is a monoid-object only in the topos, $\tau_\phi$, of the theory, and to enhance the applicability of the formalism, we apply to symbol a topos analogue of the Grothendieck extension of a monoid to a group. The resulting object, symbol, is an abelian group-object in $\tau_\phi$. We also discuss another candidate, PR, for the quantity-value object. In this presheaf, both inner and outer daseinisation are used in a symmetric way. Finally, there is a brief discussion of the role of unitary operators in the quantum topos scheme."
[Comment: I couldn't get many of the symbols in this abstract to translate into LaTex and eventually left a substantial portion untranslated.]

http://arxiv.org/abs/quant-ph/0703066
A Topos Foundation for Theories of Physics: IV. Categories of Systems
A. Doering, C.J. Isham
38 pages

"This paper is the fourth in a series whose goal is to develop a fundamentally new way of building theories of physics. The motivation comes from a desire to address certain deep issues that arise in the quantum theory of gravity. Our basic contention is that constructing a theory of physics is equivalent to finding a representation in a topos of a certain formal language that is attached to the system. Classical physics arises when the topos is the category of sets. Other types of theory employ a different topos. The previous papers in this series are concerned with implementing this programme for a single system. In the present paper, we turn to considering a collection of systems: in particular, we are interested in the relation between the topos representation for a composite system, and the representations for its constituents. We also study this problem for the disjoint sum of two systems. Our approach to these matters is to construct a category of systems and to find a topos representation of the entire category."

http://arxiv.org/abs/gr-qc/0703027
Conserved Quantities in Background Independent Theories
Fotini Markopoulou
11 pages, 3 figures

"We discuss the difficulties that background independent theories based on quantum geometry encounter in deriving general relativity as the low energy limit. We follow a geometrogenesis scenario of a phase transition from a pre-geometric theory to a geometric phase which suggests that a first step towards the low energy limit is searching for the effective collective excitations that will characterize it. Using the correspondence between the pre-geometric background independent theory and a quantum information processor, we are able to use the method of noiseless subsystems to extract such coherent collective excitations. We illustrate this in the case of locally evolving graphs." http://arxiv.org/abs/gr-qc/0703052
Existence of generalized semiclassical Kodama states. I. The Ashtekar--Klein--Gordon model
Eyo Eyo Ita
32 pages

"This is the first in a series of papers aimed at outlining an algorithm to explicitly construct a finite quantum theory of gravity in Ashtekar variables. The algorithm is based upon extending some properties of a special state, the Kodama state for pure gravity, to more general models. In this paper we analyse a simple case, gravity coupled to a Klein-Gordon scalar field in the minisuperspace Ansatz, in order to derive a criterion for a new semiclassical state and its corresponding semiclassical orbits of spacetime. We then illustrate a presciption for nonperturbatively constructing the analog of the Kodama state for a general case, in preparation for subsequent works in this series."

http://arxiv.org/abs/gr-qc/0703056
Existence of generalized quantum Kodama states. II. The minisuperspace Ashtekar--Klein--Gordon model
Eyo Eyo Ita
41 pages

"This is the second in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. In Part I we constructed a generalized semiclassical Kodama state by solving the classical Hamiltonian constraint under the condition of a broken semiclassical-quantum correspondence due to a Klein-Gordon scalar field. In Part II we will demonstrate a method of restoring this correspondence by generalizing the self-duality condition for the Ashtekar electromagnetic field. The end result will be to establish the existence of a generalized quantum Kodama state devoid of quantum corrections in the minisuperspace model. We also derive the equations needed to solve for the full theory of a finite theory of quantum gravity within the context of this new interpretation."

http://arxiv.org/abs/gr-qc/0703057
Existence of generalized Kodama quantum states. III. A new approach to finite, full quantum gravity
Eyo Eyo Ita
18 pages

"This is the third in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper is a first attempt at the quantization of the full theory coupled to matter, in this case to a spatially inhomogeneous Klein-Gordon scalar field. We delineate the conditions required to construct a solution to the quantum Hamiltonian constraint under the Ansatz of an isotropic, but spatially inhomogeneous, Ashtekar connection, and highlight some differences relative to the minisuperspace case."

briefly mentioned:
http://arxiv.org/abs/gr-qc/0703055
Hawking radiation as tunneling from Gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu

http://arxiv.org/abs/gr-qc/0703058
Asymptotic quasinormal modes of scalar field in a gravity's rainbow
Cheng-Zhou Liu, Jian-Yang Zhu

briefly mentioned:
http://arxiv.org/abs/hep-th/0703055
(reminder about Vaas new book "Beyond the big bang" Springer 2007)

 

[marcus]

http://arxiv.org/abs/gr-qc/0703078
Snyder's Model -- de Sitter Special Relativity Duality and de Sitter Gravity
Han-Ying Guo, Chao-Guang Huang, Yu Tian, Hong-Tu Wu, Bin Zhou
28 pages

"Between Snyder's quantized space-time model in de Sitter space of momenta and the dS special relativity on dS-spacetime of radius R with Beltrami coordinates, there is a one-to-one dual correspondence supported by a minimum uncertainty-like argument. Together with Planck length $\ell_P, R\simeq (3/\Lambda)^{1/2}$ should be a fundamental constant. They lead to a dimensionless constant $g{\sim\ell_PR^{-1}}=(G\hbar c^{-3}\Lambda/3)^{1/2}\sim 10^{-61}$. These indicate that physics at these two scales should be dual to each other and there is in between gravity of local dS-invariance characterized by g. A simple model of dS-gravity with a gauge-like action on umbilical manifolds may show these characters. It can pass the observation tests and support the duality."

http://arxiv.org/abs/gr-qc/0703074
Three-geometry and reformulation of the Wheeler-DeWitt equation
Chopin Soo
10 pages
Class. Quantum Grav. 24 (2007) 1547-1555

"A reformulation of the Wheeler-DeWitt equation which highlights the role of gauge-invariant three-geometry elements is presented. It is noted that the classical super-Hamiltonian of four-dimensional gravity as simplified by Ashtekar through the use of gauge potential and densitized triad variables can furthermore be succinctly expressed as a vanishing Poisson bracket involving three-geometry elements. This is discussed in the general setting of the Barbero extension of the theory with arbitrary non-vanishing value of the Immirzi parameter, and when a cosmological constant is also present. A proposed quantum constraint of density weight two which is polynomial in the basic conjugate variables is also demonstrated to correspond to a precise simple ordering of the operators, and may thus help to resolve the factor ordering ambiguity in the extrapolation from classical to quantum gravity. Alternative expression of a density weight one quantum constraint which may be more useful in the spin network context is also discussed, but this constraint is non-polynomial and is not motivated by factor ordering. The article also highlights the fact that while the volume operator has become a preeminient object in the current manifestation of loop quantum gravity, the volume element and the Chern-Simons functional can be of equal significance, and need not be mutually exclusive. Both these fundamental objects appear explicitly in the reformulation of the Wheeler-DeWitt constraint."http://arxiv.org/abs/astro-ph/0703352
Confrontation of MOND with the rotation curves of early-type disc galaxies
R.H. Sanders, E. Noordermeer
9 pages, 2 figures, submitted MNRAS

brief mention:
http://arxiv.org/abs/hep-th/0703115
Transient Observers and Variable Constants, or Repelling the Invasion of the Boltzmann's Brains
S. Carlip
4 pages

"If the universe expands exponentially without end, 'ordinary observers' like ourselves may be vastly outnumbered by 'Boltzmann's brains,' transient observers who briefly flicker into existence as a result of quantum or thermal fluctuations. One might then wonder why we are so atypical. I show that tiny changes in physics--for instance, extremely slow variations of fundamental constants--can drastically change this result, and argue that one should be wary of conclusions that rely on exact knowledge of the laws of physics in the very distant future."http://arxiv.org/abs/hep-th/0703116
Quantization in black hole backgrounds
Steven B. Giddings
28 pages, 4 figures

first sentences read in part:
"1. Introduction
Hawking’s discovery of black hole radiance[1] has produced a paradox that may be as important to finding a quantum description of gravity as the paradox of the classical instability of matter was in the foundation of quantum mechanics. There is no commonly accepted explanation for what is wrong with Hawking’s original argument that black holes destroy information[2]. This is despite widespread belief that black holes respect unitary quantum evolution, which is now shared by originator of the paradox himself[3]."

To fill out the picture somewhat, AFAIK Hawking may have conceded his BET at Dublin in 2004, but he did not thereby establish anything to change experts opinion about quantum geometry, information loss, black holes. Hawking notwithstanding it is quite possible that some information that falls into hole is indeed lost from this universe. So Hawking's error may ultimately turn out to be not what he originally proposed, but the fact that he mistakenly conceded.

I wonder why Giddings didn't cite Steve Hsu? And why Hsu didn't cite a bunch of LQG work relevant to his paper that Ted Jacobson could have told him about. It seems this field of inquiry is fragmented, not to say "atomized". Hsu and Giddings know each other personally, so the omission is especially puzzling. Here is Hsu's recent paper on the subject:

http://arxiv.org/abs/hep-th/0608175
Spacetime topology change and black hole information
Stephen D.H. Hsu
5 pages, 5 figures, to appear in Physics Letters B
Phys.Lett. B644 (2007) 67-71

"Topology change -- the creation of a disconnected baby universe -- due to black hole collapse may resolve the information loss paradox. Evolution from an early time Cauchy surface to a final surface which includes a slice of the disconnected region can be unitary and consistent with conventional quantum mechanics. We discuss the issue of cluster decomposition, showing that any violations thereof are likely to be unobservably small. Topology change is similar to the black hole remnant scenario and only requires assumptions about the behavior of quantum gravity in Planckian regimes. It does not require non-locality or any modification of low-energy physics."

 

[marcus]

http://arxiv.org/abs/astro-ph/0703566
Thermal fluctuations in loop cosmology
Joao Magueijo, Parampreet Singh
10 pages

"Quantum gravitational effects in loop quantum cosmology lead to a resolution of the initial singularity and have the potential to solve the horizon problem and generate a quasi scale-invariant spectrum of density fluctuations. We consider loop modifications to the behavior of the inverse scale factor below a critical scale in closed models and assume a purely thermal origin for the fluctuations. We show that the no-go results for scale invariance in classical thermal models can be evaded even if we just consider modifications to the background (zeroth order) gravitational dynamics. Since a complete and systematic treatment of the perturbed Einstein equations in loop cosmology is still lacking, we simply parameterize their expected modifications. These change quantitatively, but not qualitatively, our conclusions. We thus urge the community to more fully work out this complex aspect of loop cosmology, since the full picture would not only fix the free parameters of the theory, but also provide a model for a non-inflationary, thermal origin for the structures of the Universe."

http://arxiv.org/abs/gr-qc/0703098
Relativity theory does not imply that the future already exists: a counterexample
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
Comments: plainTeX, 12 pages, no figures. To appear in Vesselin Petkov (editor), Relativity and the Dimensionality of the World (Springer 2007, in press). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/

"It is often said that the relativistic fusion of time with space rules out genuine change or 'becoming'. I offer the classical sequential growth models of causal set theory as counterexamples."

http://arxiv.org/abs/gr-qc/0703099
Does Locality Fail at Intermediate Length-Scales
Rafael D. Sorkin (Perimeter Institute and Syracuse University)
24 pages, 2 figures. To appear in Daniele Oriti (ed.), Towards Quantum Gravity (Cambridge University Press, 2007). Most current version is available at http://www.physics.syr.edu/~sorkin/some.papers/

"If quantum gravity implies a fundamental spatiotemporal discreteness, and if its 'laws of motion' are compatible with the Lorentz transformations, then physics cannot remain local. One might expect this nonlocality to be confined to the fundamental discreteness scale, but I will present evidence that it survives at much lower energies, yielding for example a nonlocal equation of motion for a scalar field propagating on an underlying causal set."

http://arxiv.org/abs/gr-qc/0703097
New directions in Background Independent Quantum Gravity
Fotini Markopoulou
26 pages. Contribution to "Approaches to Quantum Gravity - toward a new understanding of space, time, and matter", edited by D. Oriti, to be published by Cambridge University Press

"We discuss the meaning of background independence in quantum theories of gravity where geometry and gravity are emergent and illustrate the possibilities using the framework of quantum causal histories."

brief mention:
http://arxiv.org/abs/physics/0703191
The Expansion of the Universe and the Cosmological Constant Problem
R. F. O'Connell
Phys. Lett. A, in press
http://arxiv.org/find/grp_physics/1/au:OConnell_R_F/0/1/0/all/0/1

http://arxiv.org/abs/quant-ph/0703179
Disproof of Bell's Theorem by Clifford Algebra Valued Local Variables
Joy Christian (Perimeter and Oxford)
4 pages

 

[marcus]

http://arxiv.org/abs/gr-qc/0703114
Note on non-metric gravity
Ingemar Bengtsson
9 pages

We discuss a class of alternative gravity theories that are specific to four dimensions, do not introduce new degrees of freedom, and come with a physical motivation. In particular we sketch their Hamiltonian formulation, and their relation to some earlier constructions."

This refers to two recent papers by Kirill Krasnov
http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
http://arxiv.org/abs/gr-qc/0703002
Non-Metric Gravity I: Field Equations

For anyone not familiar with Bengtsson here are 34 papers in gr-qc, hep-th, and quant-ph going back to 1992
http://arxiv.org/find/grp_physics/1/au:+Bengtsson_I/0/1/0/all/0/1

He picked up fast on Krasnov's idea of a way to make gravity renormalizable,
relating it to the asymptotic safety program.

http://arxiv.org/abs/gr-qc/0703116
Loop quantum gravity and Planck-size black hole entropy
Alejandro Corichi, Jacobo Diaz-Polo, Enrique Fernandez-Borja
21 pages, 5 figures. Contribution to the Proceedings of the NEB XII International Conference

"The Loop Quantum Gravity (LQG) program is briefly reviewed and one of its main applications, namely the counting of black hole entropy within the framework is considered. In particular, recent results for Planck size black holes are reviewed. These results are consistent with an asymptotic linear relation (that fixes uniquely a free parameter of the theory) and a logarithmic correction with a coefficient equal to -1/2. The account is tailored as an introduction to the subject for non-experts."

 

[marcus]

http://arxiv.org/abs/gr-qc/0703144
Dynamical coherent states and physical solutions of quantum cosmological bounces
Martin Bojowald
30 pages, 3 figures
IGPG-07/3-5, NSF-KITP-07-55

"A new model is studied which describes the quantum behavior of transitions through an isotropic quantum cosmological bounce in loop quantum cosmology sourced by a free and massless scalar field. As an exactly solvable model even at the quantum level, it illustrates properties of dynamical coherent states and provides the basis for a systematic perturbation theory of loop quantum gravity. The detailed analysis is remarkably different from what is known for harmonic oscillator coherent states. Results are evaluated with regard to their implications in cosmology, including a demonstration that in general quantum fluctuations before and after the bounce are unrelated. Thus, even within this solvable model the condition of classicality at late times does not imply classicality at early times before the bounce without further assumptions. Nevertheless, the quantum state does evolve deterministically through the bounce."

http://arxiv.org/abs/gr-qc/0703135
Loop quantization of spherically symmetric midi-superspaces
Miguel Campiglia, Rodolfo Gambini, Jorge Pullin
18 pages

"We quantize the exterior of spherically symmetric vacuum space-times using a midi-superspace reduction within the Ashtekar new variables. Through a partial gauge fixing we eliminate the diffeomorphism constraint and are left with a Hamiltonian constraint that is first class. We complete the quantization in the loop representation. We also use the model to discuss the issues that will arise in more general contexts in the 'uniform discretization' approach to the dynamics."

http://arxiv.org/abs/gr-qc/0703137
Decoherence in Quantum Gravity: Issues and Critiques
C. Anastopoulos, B. L. Hu
25 pages, proceedings of DICE06 (Piombino)

"An increasing number of papers have appeared in recent years on decoherence in quantum gravity at the Planck energy. We discuss the meaning of decoherence in quantum gravity starting from the common notion that quantum gravity is a theory for the microscopic structures of spacetime, and invoking some generic features of quantum decoherence from the open systems viewpoint. We dwell on a range of issues bearing on this process including the relation between statistical and quantum, noise from effective field theory, the meaning of stochasticity, the origin of non-unitarity and the nature of nonlocality in this and related contexts. To expound these issues we critique on two representative theories: One claims that decoherence in quantum gravity scale leads to the violation of CPT symmetry at sub-Planckian energy which is used to explain today's particle phenomenology. The other uses this process in place with the Brownian motion model to prove that spacetime foam behaves like a thermal bath."

could be a useful text and reference work:
http://arxiv.org/abs/astro-ph/0703730
Why CMB physics?
Massimo Giovannini
179 pages, 27 figures
CERN-PH-TH/2007-048

"The aim of these lectures is to introduce some basic problems arising in gravitation and modern cosmology. All along the discussion the guiding theme is provided by the phenomenological and theoretical properties of the Cosmic Microwave Background (CMB). These lectures have been prepared for a regular Phd course of the University of Milan-Bicocca."

briefly noted:
http://arxiv.org/abs/astro-ph/0703722
Observable Signatures of a Black Hole Ejected by Gravitational Radiation Recoil in a Galaxy Merger
Abraham Loeb (Harvard)
4 pages, submitted to PRL

"According to recent general-relativistic simulations, the coalescence of two spinning black holes (BHs) could lead to recoil speeds of the BH remnant of up to thousands of km/s as a result of the emission of gravitational radiation. Such speeds would enable the merger product to escape its host galaxy. Here we examine the circumstances resulting from a gas-rich galaxy merger under which the ejected BH would carry an accretion disk with it and be observable. As the initial BH binary emits gravitational radiation and its orbit tightens, a hole is opened around it in the disk which delays the consumption of gas prior to the eventual BH ejection. The punctured disk remains bound to the ejected BH within the region where the gas orbital velocity is larger than the ejection speed. For a ~10^7 solar mass BH the ejected disk has a characteristic size of tens of thousands of Schwarzschild radii and an accretion lifetime of ~10^7 years. During that time, the ejected BH could traverse a considerable distance and appear as an off-center quasar with a feedback trail along the path it left behind. A small fraction of all quasars could be associated with an escaping BH."

briefly noted:

http://arxiv.org/abs/gr-qc/0703150
A gravitational explanation for quantum theory - non-time-orientable manifolds
Mark J Hadley
7 pages Talk given at FFP8 in Madrid 2006

"Spacetime manifolds that are not time orientable play a key role in a gravitational explanation of quantum theory. Such manifolds allow topology change, but also have fascinating additional properties such as net charge from source-free equations and spin half transformation properties. It is shown how the logical structure of propositions and the probabilities of quantum theory arise from such acausal space times."

http://arxiv.org/abs/hep-th/0703265
A new PPN parameter to test Chern-Simons gravity
Stephon Alexander, Nicolas Yunes
4 pages, submitted to PRL

"We study Chern-Simons (CS) gravity in the parameterized post-Newtonian (PPN) framework through weak-field solutions of the modified field equations for a perfect fluid source... This new term encodes the key physical effect of CS gravity in the weak-field limit, leading to a modification of frame dragging and, thus, the Lense-Thirring contribution to gyroscopic precession. We provide a physical interpretation for the new term, as well as an estimate of the size of this effect relative to the general relativistic Lense-Thirring prediction. This correction to frame dragging might be used in experiments,.."

 

[marcus]

http://arxiv.org/abs/0704.0145
Singularity Resolution in Isotropic Loop Quantum Cosmology: Recent Developments
Ghanashyam Date
(Submitted on 2 Apr 2007)

"Since the past Iarge meeting in December 2004, new developments in loop quantum cosmology have taken place, especially with regards to the resolution of the Big Bang singularity in the isotropic models. The singularity resolution issue has been discussed in terms of physical quantities (expectation values of Dirac observables) and there is also an 'improved' quantization of the Hamiltonian constraint. These developments are briefly discussed. This is an expanded version of the review talk given at the 24-th IAGRG meeting in February 2007."

http://arxiv.org/abs/0704.0007
Polymer Quantum Mechanics and its Continuum Limit
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
(Submitted on 31 Mar 2007)

"A rather non-standard quantum representation of the canonical commutation relations of quantum mechanics systems, known as the polymer representation has gained some attention in recent years, due to its possible relation with Planck scale physics. In particular, this approach has been followed in a symmetric sector of loop quantum gravity known as loop quantum cosmology (LQC). Here we explore different aspects of the relation between the ordinary Schroedinger theory and the polymer description. The paper has two parts. In the first one, we derive the polymer quantum mechanics starting from the ordinary Schroedinger theory and show that the polymer description arises as an appropriate limit. In the second part we consider the continuum limit of this theory, namely, the reverse process in which one starts from the discrete theory and tries to recover back the ordinary Schroedinger quantum mechanics. We consider several examples of interest, including the harmonic oscillator, the free particle and a simple cosmological model."

http://arxiv.org/abs/0704.0221
The Return of a Static Universe and the End of Cosmology
Lawrence M. Krauss (1,2), Robert J. Scherrer (2) ((1) Case Western Reserve University, (2) Vanderbilt University)
(Submitted on 2 Apr 2007)

"We demonstrate that as we extrapolate the current LambdaCDM universe forward in time, all evidence of the Hubble expansion will disappear, so that observers in our 'island universe' will be fundamentally incapable of determining the true nature of the universe, including the existence of the highly dominant vacuum energy, the existence of the CMB, and the primordial origin of light elements. With these pillars of the modern Big Bang gone, this epoch will mark the end of cosmology and the return of a static universe. In this sense, the coordinate system appropriate for future observers will perhaps fittingly resemble the static coordinate system in which the de Sitter universe was first presented."

briefly noted:
http://arxiv.org/abs/0704.0058
Intelligent Life in Cosmology
Frank J. Tipler

 

[marcus]

http://arxiv.org/abs/0704.0278
q-Deformed spin foam models of quantum gravity
Igor Khavkine, J. Daniel Christensen
(Submitted on 2 Apr 2007)

"We numerically study Barrett-Crane models of Riemannian quantum gravity. We have extended the existing numerical techniques to handle q-deformed models and arbitrary space-time triangulations. We present and interpret expectation values of a few selected observables for each model, including a spin-spin correlation function which gives insight into the behaviour of the models. We find the surprising result that, as the deformation parameter q goes to 1 through roots of unity, the limit is discontinuous."

Dan Christensen accesses the supercomputer at U. West Ontario (UWO) where he has charge of programs in mathematics, computing, and quantum gravity. Co-authored spinfoam papers with John Baez several years back and with others since. Use of numerical techniques notable here---probably a Beowolf cluster.

http://arxiv.org/abs/0704.0367
Existence of generalized Kodama states. IV. The search for a quantization of 4-dimensional gravity
Eyo Eyo Ita III

"This is the fourth in a series of papers outlining an algorithm to consistently construct a finite quantum theory of gravity in Ashtekar variables. This paper continues essentially from where papers II and III left off, treating the kinematic constraints in greater depth and moving on to a higher level of complexity with regard to the Hamiltonian constraint of the full, unrestricted theory. First we identify some of the traditional obstacles to the consistent quantization of four-dimensional gravity, then provide suggestions for how these obstacles may possibly be surmounted within the context of our new approach. This inevitably entails a critical analysis of the relationship of gauge transformations to diffeomorphisms, which in turn leads to the reduced phase space approach to quantization of the kinematic constraints, as well as the implementation of the semiclassical-quantum correspondence. We also compute some more of the terms needed to implement the full quantum Hamiltonian constraint, focusing in this work on its antisymmetric CDJ components. Some of the relationships among generalized Kodama states for the more general model are clearly elucidated due to the calculation of such terms."
33 pages

E.E.Ita is a PhD student at Cambridge.

http://arxiv.org/abs/0704.0299
Parametrized Post-Newtonian Expansion of Chern-Simons Gravity
Stephon Alexander, Nicolas Yunes

"We investigate the weak-field, post-Newtonian expansion to the solution of the field equations in Chern-Simons gravity with a perfect fluid source. In particular, we study the mapping of this solution to the parameterized post-Newtonian formalism to 1 PN order in the metric. We find that the PPN parameters of Chern-Simons gravity are identical to those of general relativity, with the exception of the inclusion of a new term that is proportional to the Chern-Simons coupling parameter and the curl of the PPN vector potentials. We also find that the new term is naturally enhanced by the non-linearity of spacetime and we provide a physical interpretation for it. By mapping this correction to the gravito-electro-magnetic framework, we study the corrections that this new term introduces to the acceleration of point particles and the frame-dragging effect in gyroscopic precession. We find that the Chern-Simons correction to these classical predictions could be used by current and future experiments to place bounds on intrinsic parameters of Chern-Simons gravity and, thus, string theory."
14 pages

 

[marcus]

http://arxiv.org/abs/0704.1137
Lattice refining loop quantum cosmology, anisotropic models and stability
Martin Bojowald, Daniel Cartin, Gaurav Khanna
24 pages

"A general class of loop quantizations for anisotropic models is introduced and discussed, which enhances loop quantum cosmology by relevant features seen in inhomogeneous situations. The main new effect is an underlying lattice which is being refined during dynamical changes of the volume. In general, this leads to a new feature of dynamical difference equations which may not have constant step-size, posing new mathematical problems. It is discussed how such models can be evaluated and what lattice refinements imply for semiclassical behavior. Two detailed examples illustrate that stability conditions can put strong constraints on suitable refinement models, even in the absence of a fundamental Hamiltonian which defines changes of the underlying lattice. Thus, a large class of consistency tests of loop quantum gravity becomes available. In this context, it will also be seen that quantum corrections due to inverse powers of metric components in a constraint are much larger than they appeared recently in more special treatments of isotropic, free scalar models where they were artificially suppressed."

http://arxiv.org/abs/0704.0992
Compatibility of radial, Lorenz and harmonic gauges
Elena Magliaro, Claudio Perini, Carlo Rovelli
9 pages

"We observe that the radial gauge can be consistently imposed together with the Lorenz gauge in Maxwell theory, and with the harmonic traceless gauge in linearized general relativity. This simple observation has relevance for some recent developments in quantum gravity where the radial gauge is implicitly utilized.

 

[Kea]

Marcus

Perhaps you could enlighten us as to the LQG program predictions for the MiniBooNE results, which come out this week. I haven't seen any anywhere...

 

[marcus]

http://arxiv.org/abs/0704.2397
The Quantum Configuration Space of Loop Quantum Cosmology
Jose Manuel Velhinho
14 pages

"The article gives an account of several aspects of the space known as the Bohr compactification of the line, featuring as the quantum configuration space in loop quantum cosmology, as well as of the corresponding configuration space realization of the so-called polymer representation. Analogies with loop quantum gravity are explored, providing an introduction to (part of) the mathematical structure of loop quantum gravity, in a technically simpler context."

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

"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."

 

[Kea]

Simon D.M. White
Essay commissioned for publication in Reports on Progress in Physics.

Goodness, Marcus! Do such articles usually use such strong language?

 

[ensabah6]

Hey Marcus
not sure if you've ever mentioned this (I can't search for this personally)

arXiv:gr-qc/0611156
Title: On Loop States in Loop Quantum Gravity
Authors: N. D. Hari Dass, Manu Mathur
Comments: 12 pages, 3 figures, the version to be published in Classical and Quantum Gravity
We explicitly construct and characterize all possible independent loop states in 3+1 dimensional loop quantum gravity by regulating it on a 3-d regular lattice in the Hamiltonian formalism. These loop states, characterized by the (dual) angular momentum quantum numbers, describe SU(2) rigid rotators on the links of the lattice. The loop states are constructed using the Schwinger bosons which are harmonic oscillators in the fundamental (spin half) representation of SU(2). Using generalized Wigner Eckart theorem, we compute the matrix elements of the volume operator in the loop basis. Some simple loop eigenstates of the volume operator are explicitly constructed.

 

[marcus]

the preceding post duplicates an entry made last year in #543 of this thread

http://arxiv.org/abs/0704.3214
(2+1)-Dimensional Quantum Gravity as the Continuum Limit of Causal Dynamical Triangulations
D. Benedetti, R. Loll, F. Zamponi
38 pages, 13 figures

"We perform a non-perturbative sum over geometries in a (2+1)-dimensional quantum gravity model given in terms of Causal Dynamical Triangulations. Inspired by the concept of triangulations of product type introduced previously, we impose an additional notion of order on the discrete, causal geometries. This simplifies the combinatorial problem of counting geometries just enough to enable us to calculate the transfer matrix between boundary states labelled by the area of the spatial universe, as well as the corresponding quantum Hamiltonian of the continuum theory. This is the first time in dimension larger than two that a Hamiltonian has been derived from such a model by mainly analytical means, and opens the way for a better understanding of scaling and renormalization issues."

http://arxiv.org/abs/0704.2667
Wormholes as Black Hole Foils
Thibault Damour, Sergey N. Solodukhin
13 pages

"We study to what extent wormholes can mimic the observational features of black holes. It is surprisingly found that many features that could be thought of as 'characteristic' of a black hole (endowed with an event horizon) can be closely mimicked by a globally static wormhole, having no event horizon. This is the case for: the apparently irreversible accretion of matter down a hole, no-hair properties, quasi-normal-mode ringing, and even the dissipative properties of black hole horizons, such as a finite surface resistivity equal to 377 Ohms. The only way to distinguish the two geometries on an observationally reasonable time scale would be through the detection of Hawking's radiation, which is, however, too weak to be of practical relevance for astrophysical black holes. We point out the existence of an interesting spectrum of quantum microstates trapped in the throat of a wormhole which could be relevant for storing the information 'lost' during a gravitational collapse."

http://arxiv.org/abs/0704.1746
Tunnelling from black holes in the Hamilton Jacobi approach
Bhramar Chatterjee, Amit Ghosh, P. Mitra
5 pages

"It has recently been shown that it is possible to understand Hawking radiation as tunnelling across black hole horizons using appropriate Hamilton-Jacobi boundary conditions. The procedure is applied to the non-rotating black hole in different coordinate systems and to the rotating charged black hole. Differences with the earlier literature are pointed out."

 

[ensabah6]

Thanks, how do you search that? When I type it in I get wrong hits.

Anyhow did you also mention this?
http://arxiv.org/abs/gr-qc/0606100

A lattice bosonic model as a quantum theory of gravity
Authors: Zheng-Cheng Gu, Xiao-Gang Wen
(Submitted on 23 Jun 2006)


A local quantum bosonic model on a lattice is constructed whose low energy excitations are gravitons described by linearized Einstein action. Thus the bosonic model is a quantum theory of gravity, at least at the linear level. We find that the compactification and the discretization of metric tenor are crucial in obtaining a quantum theory of gravity.

 

[marcus]

John Baez is at Les Treilles (interesting people at rustic estate in south France, QG discussion this year) to present this talk
http://math.ucr.edu/home/baez/treilles/
the slides are downloadable.

ftp://ftp.alainconnes.org/Inteng.pdf[/URL]
recent [b]Alain Connes interview[/b] with G. Skandalis and C. Goldstein

[url]http://arxiv.org/abs/0704.2630[/url]
[b]Are We Typical?[/b]
James B. Hartle, Mark Srednicki
6 pages

"Bayesian probability theory is used to analyze the oft-made assumption that humans are typical observers in the universe. Some theoretical calculations make the {\it selection fallacy} that we are randomly chosen from a class of objects by some physical process, despite the absence of any evidence for such a process, or any observational evidence favoring our typicality. It is possible to favor theories in which we are typical by appropriately choosing their prior probabilities, but such assumptions should be made explicit to avoid confusion."

[url]http://arxiv.org/abs/0704.3429[/url]
[b]Light Propagation on Quantum Curved Spacetime and Back reaction effects[/b]
Carlos Kozameh, Florencia Parisi

"We study the electromagnetic field equations on an arbitrary quantum curved background in the semiclassical approximation of Loop Quantum Gravity. The effective interaction hamiltonian for the Maxwell and gravitational fields is obtained and the corresponding field equations, which can be expressed as a modified wave equation for the Maxwell potential, are derived. We use these results to analyze electromagnetic wave propagation on a quantum Robertson-Walker space time and show that Lorentz Invariance is not preserved. The formalism developed can be applied to the case where back reaction effects on the metric due to the electromagnetic field are taken into account, leading to non-covariant field equations."

[url]http://arxiv.org/abs/0704.3595[/url]
[b]Generalized Kodama states. V. Evidence of the normalizability and renormalizability of 4D QGRA[/b]
Eyo Eyo Ita III

"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. In this work we transform the pure Kodama state into the metric representation for the Bianchi IX minisuperspace model. Previous such calculations have been carried out to semiclassical order for a particular choice of gauge, revealing the existence of five topologically distinct states in the metric representation. We have performed our calculation to all orders in perturbation theory by maintaining the gauge degrees of freedom explicit, revealing the existence of a sixth state. We propose a resolution to the issue of reality conditions for the Ashtekar variables, and also for the recasting of 4-dimensional general relativity as a renormalizable field theory, stemming from the nonperturbative result obtained from this work. We also address the issue of normalizability of the Kodama state."briefly noted

[url]http://arxiv.org/abs/0704.3306[/url]
[b]Quantum mechanics on Hilbert manifolds: The principle of functional relativity[/b]
Alexey A. Kryukov
45 pages, 9 figures

"Quantum mechanics is formulated as a geometric theory on a Hilbert manifold..."[url]http://arxiv.org/abs/0704.3414[/url]
[b]Phantom field dynamics in loop quantum cosmology[/b]
Daris Samart, Burin Gumjudpai (TPTP Naresuan U.)

"We consider a dynamical system of phantom scalar field under exponential potential in background of loop quantum cosmology... "

 

[marcus]

http://arxiv.org/abs/0705.0006
Multiple-event probability in general-relativistic quantum mechanics: a discrete model
Mauricio Mondragon, Alejandro Perez, Carlo Rovelli
(Submitted on 30 Apr 2007)

"We introduce a simple quantum mechanical model in which time and space are discrete and periodic. These features avoid the complications related to continuous-spectrum operators and infinite-norm states. The model provides a tool for discussing the probabilistic interpretation of generally-covariant quantum systems, without the confusion generated by spurious infinities. We use the model to illustrate the formalism of general-relativistic quantum mechanics, and to test the definition of multiple-event probability introduced in a companion paper. We consider a version of the model with unitary time-evolution and a version without unitary time-evolution."

http://arxiv.org/abs/0705.0235
The canonical versus path integral quantization approach to generalized Kodama states (Part II)
Eyo Eyo Ita III
41 pages

"This is the sixth paper in the series outlining an algorithm to consistently quantize four-dimensional gravity. A major feature of the quantization programme resides in the inherent ability of the path integral to exhaustively pick out a complete basis of quantum states precisely matching the canonically determined wavefunctions for an arbitrary model coupled to gravity. First we develop the notation and tools necessary to expose this hidden feature of the path integral, and then we apply it to the nonperturbative construction of the pure Kodama state (Part I) and the generalized Kodama state (Part II) for an arbitraty matter coupling, in analogy to the no-boundary proposal. We argue that the well-definedness and the explicit convergence of the path integral is intimately connected to its equivalence to the canonical approach, the principle of the SQC, and in particular to the existence of the generalized Kodama states (GKodS). We then indicate how these features, combined, can ultimately lead to a resolution of the problem of time in quantum gravity. In the end, we recast the equations necessary to eliminate the quantum counterterms for a general model into a standard form suitable for model-specific expansion of the GKodS about the pure Kodama state. The explicit solution of these equations is reserved for future work."

http://arxiv.org/abs/0705.0165
Is Modified Gravity Required by Observations? An Empirical Consistency Test of Dark Energy Models
Sheng Wang (Brookhaven; Columbia), Lam Hui (Columbia; ISCAP), Morgan May (Brookhaven), Zoltan Haiman (Columbia)

"We apply the technique of parameter-splitting to existing cosmological data sets, to check for a generic failure of dark energy models. Given a dark energy parameter, such as the energy density Omega_Lambda or equation of state w, we split it into two meta-parameters with one controlling geometrical distances, and the other controlling the growth of structure. Observational data spanning Type Ia Supernovae, the cosmic microwave background (CMB), galaxy clustering, and weak gravitational lensing statistics are fit without requiring the two meta-parameters to be equal. This technique checks for inconsistency between different data sets, as well as for internal inconsistency within anyone data set (e.g., CMB or lensing statistics) that is sensitive to both geometry and growth. We find that the cosmological constant model is consistent with current data. Theories of modified gravity generally predict a relation between growth and geometry that is different from that of general relativity. Parameter-splitting can be viewed as a crude way to parametrize the space of such theories. Our analysis of current data already appears to put sharp limits on these theories: assuming a flat universe, current data constrain the difference Omega_Lambda(geom) - Omega_Lambda(grow) to be -0.0044 +/- 0.0058 (68% C.L.); allowing the equation of state w to vary, the difference w(geom) - w(grow) is constrained to be 0.37 +/- 0.37 (68% C.L.). Interestingly, the region w(grow) > w(geom), which should be generically favored by theories that slow structure formation relative to general relativity, is quite restricted by data already. We find w(grow) < -0.80 at 2 sigma."

briefly noted:
http://arxiv.org/abs/0705.0164
Inflationary Cosmology
Andrei Linde
60 pages, 10 figs, based on a talk given at the 22nd IAP Colloquium, "Inflation+25", Paris, June 2006

 

[marcus]

http://arxiv.org/abs/0705.0674
A new spinfoam vertex for quantum gravity
Etera R. Livine, Simone Speziale
17+8 pages, 6 figures

"We introduce a new spinfoam vertex to be used in models of 4d quantum gravity based on SU(2) and SO(4) BF theory plus constraints. It can be seen as the conventional vertex of SU(2) BF theory, the 15j symbol, in a particular basis constructed using SU(2) coherent states. This basis makes the geometric interpretation of the variables transparent: they are the vectors normal to the triangles within each tetrahedron. We study the condition under which these states can be considered semiclassical, and we show that the semiclassical ones dominate the evaluation of quantum correlations. Finally, we describe how the constraints reducing BF to gravity can be directly written in terms of the new variables, and how the semiclassicality of the states might improve understanding the correct way to implement the constraints."
.

 

[marcus]

http://arxiv.org/abs/0705.0991
From semiconductors to quantum gravity: to centenary of Matvei Bronstein
G.E. Volovik
12 pages, 1 figure, draft for proceedings of the conference devoted to centenary of Marvei Bronstein, St. Petersburg, November 2006

"Investigation of the many-body condensed-matter systems allows us to connect the microscopic physics at the atomic energy scale and the macroscopic physics emerging in the low-energy corner. It gives some hints on the mechanisms of the formation of the physical laws which our Universe obeys. The paper is devoted to the centenary of Matvei Petrovich Bronstein."


briefly noted:
http://arxiv.org/abs/0705.1060
Obtaining the spacetime metric from cosmological observations

http://arxiv.org/abs/0705.1029
No Way Back: Maximizing survival time below the Schwarzschild event horizon

 

[marcus]

http://arxiv.org/abs/0705.1178
A global picture of quantum de Sitter space
Steven B. Giddings, Donald Marolf
17 pages, 1 figure

"Perturbative gravity about a de Sitter background motivates a global picture of quantum dynamics in `eternal de Sitter space,' the theory of states which are asymptotically de Sitter to both future and past. Eternal de Sitter physics is described by a finite dimensional Hilbert space in which each state is precisely invariant under the full de Sitter group. This resolves a previously-noted tension between de Sitter symmetry and finite entropy. Observables, implications for Boltzmann brains, and Poincare recurrences are briefly discussed."

Giddings is known as a string theorist but this is not a string paper. It finds a mistake in a 2002 paper by Leonard Susskind and others called "The Trouble with deSitter Space" http://arxiv.org/hep-th/0212209
Many papers by people in the LQG community are cited---four by Rovelli, several by Ambjorn, by Thiemann, Ashtekar, Gambini and Pullin. Doubtless others, I didnt make a careful count. Since Giddings and Marolf are well-known people at KITP Santa Barbara it suggests a significant direction of interest. At the same time on the nonstring QG side a substantial amoung of research is involved with deSitter space---Baez student Derek Wise was discussing Cartan geometry using the deSitter group instead of Poincaré---DSR (deformed special relativity) deals a lot with deSitter group. What happens when quantum spacetime geometry is "locally deSitter"? This paper will be of interest and will have usefulness outside of string research.

http://arxiv.org/abs/0705.1032
Matter density perturbations and effective gravitational constant in modified gravity models of dark energy
Shinji Tsujikawa
10 pages

"We derive the equation of matter density perturbations on sub-horizon scales for a general Lagrangian density f(R, phi, X) that is a function of a Ricci scalar R, a scalar field phi and a kinetic term X=-(nabla phi)^2/2. This is useful to constrain modified gravity dark energy models from observations of large-scale structure and weak lensing..."

http://arxiv.org/abs/0705.1158
Models of f(R) Cosmic Acceleration that Evade Solar-System Tests
Wayne Hu, Ignacy Sawicki (KICP, U. Chicago)
13 pages, 10 figures. Submitted to Phys. Rev. D

"We study a class of metric-variation f(R) models that accelerates the expansion without a cosmological constant and satisfies both cosmological and solar-system tests in the small-field limit of the parameter space. Solar-system tests alone place only weak bounds on these models, since the additional scalar degree of freedom is locked to the high-curvature general-relativistic prediction across more than 25 orders of magnitude in density, out through the solar corona. This agreement requires that the galactic halo be of sufficient extent to maintain the galaxy at high curvature in the presence of the low-curvature cosmological background. If the galactic halo and local environment in f(R) models do not have substantially deeper potentials than expected in LCDM, then cosmological field amplitudes |f_R| > 10^{-6} will cause the galactic interior to evolve to low curvature during the acceleration epoch. Viability of large-deviation models therefore rests on the structure and evolution of the galactic halo, requiring cosmological simulations of f(R) models, and not directly on solar-system tests. Even small deviations that conservatively satisfy both galactic and solar-system constraints can still be tested by future, percent-level measurements of the linear power spectrum, while they remain undetectable to cosmological-distance measures. Although we illustrate these effects in a specific class of models, the requirements on f(R) are phrased in a nearly model-independent manner."

My impression is that Wayne Hu is very highly regarded in cosmology. It seems significant to me that he is seriously talking about ways to get away from having "Dark Energy" by modifying the law of gravity.

http://arxiv.org/abs/0705.1170
The Collision Between The Milky Way And Andromeda
T.J. Cox, Abraham Loeb (Harvard/CfA)
submitted to MNRAS

"We use a N-body/hydrodynamic simulation to forecast the future encounter between the Milky Way and the Andromeda galaxies, given current observational constraints on their relative distance, relative velocity, and masses. Allowing for a comparable amount of diffuse mass to fill the volume of the Local Group, we find that the two galaxies are likely to collide in a few billion years - within the Sun's lifetime. During the first close encounter of the two galaxies, there is a 12% chance that the Sun will be pulled from its present position and reside in the extended tidal material. After the second close encounter, there is a 30% chance that the Sun will reside in the extended tidal material, and a 2.7% chance that our Sun will be more tightly bound to Andromeda than to the Milky Way. Eventually, after the merger has completed, the Sun is likely to be scattered to the outer halo and reside at much larger radii (>30 kpc). The density profiles of the stars, gas and dark matter in the merger product resemble those of elliptical galaxies. Our Local Group model therefore provides a prototype progenitor of late--forming elliptical galaxies."

 

[marcus]

http://arxiv.org/abs/0705.2222
Loop Quantum Gravity: Four Recent Advances and a Dozen Frequently Asked Questions
Abhay Ashtekar
21 pages, to appear in the Proceedings of the 11th Marcel Grossmann Conference

"As per organizers' request, my talk at the 11th Marcel Grossmann Conference consisted of two parts. In the first, I illustrated recent advances in loop quantum gravity through examples. In the second, I presented an overall assessment of the status of the program by addressing some frequently asked questions. This account is addressed primarily to researchers outside the loop quantum gravity community."http://arxiv.org/abs/0705.2047
Non-Metric Gravity II: Spherically Symmetric Solution, Missing Mass and Redshifts of Quasars
Kirill Krasnov, Yuri Shtanov
37 pages, 2 figures

"We continue the study of the non-metric theory of gravity introduced in hep-th/0611182 and gr-qc/0703002 and obtain its general spherically symmetric vacuum solution. It respects the analog of the Birkhoff theorem, i.e., the vacuum spherically symmetric solution is necessarily static. As in general relativity, the spherically symmetric solution is seen to describe a black hole. The exterior geometry is essentially the same as in the Schwarzschild case, with power-law corrections to the Newtonian potential. The behavior inside the black-hole region is different from the Schwarzschild case in that the usual spacetime singularity gets replaced by a singular of a new type, where all basic fields of the theory remain finite but metric ceases to exist. The theory does not admit arbitrarily small black holes: for small objects, the curvature on the would-be horizon is so strong that non-metric modifications prevent the horizon from being formed. The theory allows for modifications of gravity of very interesting nature. We discuss three physical effects, namely, (i) correction to Newton's law in the neighborhood of the source, (ii) renormalization of effective gravitational and cosmological constants at large distances from the source, and (iii) additional redshift factor between spatial regions of different curvature. The first two effects can be responsible, respectively, for the observed anomaly in the acceleration of the Pioneer spacecraft and for the alleged missing mass in spiral galaxies and other astrophysical objects. The third effect can be used to propose a non-cosmological explanation of high redshifts of quasars."

http://arxiv.org/abs/0705.2197
Black holes, information, and locality
Steven B. Giddings
Essay submitted to the Gravity Research Foundation essay contest; with minor updates

"Thirty years of a deepening information paradox suggest the need to revise our basic physical framework. Multiple indicators point toward reassessment of the principle of locality: lack of a precise definition in quantum gravity, behavior of high-energy scattering, hints from strings and AdS/CFT, conundrums of quantum cosmology, and finally lack of good alternative resolutions of the paradox. A plausible conjecture states that the non-perturbative dynamics of gravity is unitary but nonlocal. String theory may directly address these issues but so far important aspects remain elusive. If this viewpoint is correct, critical questions are to understand the 'correspondence' limit where nonlocal physics reduces to local quantum field theory, and beyond, to unveil principles of an underlying nonlocal theory."

http://arxiv.org/abs/0705.2388
The loop-quantum-gravity vertex-amplitude
Jonathan Engle, Roberto Pereira, Carlo Rovelli
6 pages

"Spinfoam theories are hoped to provide the dynamics of non-perturbative loop quantum gravity. But a number of their features remain elusive. The best studied one -the euclidean Barrett-Crane model- does not have the boundary state space needed for this, and there are recent indications that, consequently, it may fail to yield the correct low-energy n-point functions. These difficulties can be traced to the SO(4) -> SU(2) gauge fixing and the way certain second class constraints are imposed, arguably incorrectly, strongly. We present an alternative model, that can be derived as a bona fide quantization of a Regge discretization of euclidean general relativity, and where the constraints are imposed weakly. Its state space is a natural subspace of the SO(4) spin-network space and matches the SO(3) hamiltonian spin network space. The model provides a long sought SO(4)-covariant vertex amplitude for loop quantum gravity."

http://arxiv.org/abs/0705.2357
The inevitable nonlinearity of quantum gravity falsifies the many-worlds interpretation of quantum mechanics
T. P. Singh
6 pages. Honorable Mention in Gravity Research Foundation Essay Competition 2007. This is also a summary of a talk given at the Meeting `Himalayan Relativity Dialogue', Mirik, India, 18-20 April, 2007. Detailed version of this essay is in preparation

"There are fundamental reasons as to why there should exist a reformulation of quantum mechanics which does not refer to a classical spacetime manifold. It follows as a consequence that quantum mechanics as we know it is a limiting case of a more general nonlinear quantum theory, with the nonlinearity becoming significant at the Planck mass/energy scale. This nonlinearity is responsible for a dynamically induced collapse of the wave-function, during a quantum measurement, and it hence falsifies the many-worlds interpretation of quantum mechanics. We illustrate this conclusion using a mathematical model based on a generalized Doebner-Goldin equation. The non-Hermitian part of the Hamiltonian in this norm-preserving, nonlinear, Schrodinger equation dominates during a quantum measurement, and leads to a breakdown of linear superposition."

briefly noted:
http://arxiv.org/abs/0705.2306
Near-Extreme Black Holes and the Universal Relaxation Bound
Shahar Hod
2 Pages. Submitted to PRD

=========
Still able to edit this, I reply to Francesca here so as not to make an extra post:

Francesca, thanks for the two references! Both the Percacci et al, and the Chamseddine-Connes look interesting and relevant as you say.

 

[Ontoplankton]

So does anyone else think "Carlo Rovelli" is secretly Karl Rove with a wig? I think he's trying to destroy string theory as part of the Republican War on Science. That sneaky bastard.

 

[francesca]

Sorry, I didn't want to replay but I can't...
ST is right, QG is left!
That's an Italian play started by the movie maker Nanni Moretti in a movie of him, "Aprile"... the ham steak is right, the Bologna ham is left...

 

[francesca]

Do you remember the great interest for Reuter's lecture "Asymptotic Safety in Quantum Einstein Gravity"? This is a related paper:

http://arxiv.org/abs/0705.1769"
Title: Ultraviolet properties of f(R)-Gravity
Authors: Alessandro Codello, Roberto Percacci, Christoph Rahmede
Comments: 4 pages

We discuss the existence and properties of a nontrivial fixed point in f(R)-gravity, where f is a polynomial of order up to six. Within this seven-parameter class of theories, the fixed point has three ultraviolet-attractive and four ultraviolet-repulsive directions; this brings further support to the hypothesis that gravity is nonperturbatively renormalizabile.


and BTW:

http://arxiv.org/abs/0705.1786"
Title: Quantum Gravity Boundary Terms from Spectral Action
Authors: Ali H. Chamseddine, Alain Connes
Comments: RevTex 4 pages

We study the boundary terms of the spectral action of the noncommutative space, defined by the spectral triple dictated by the physical spectrum of the standard model, unifying gravity with all other fundamental interactions. We prove that the spectral action predicts uniquely the gravitational boundary term required for consistency of quantum gravity with the correct sign and coefficient. This is a remarkable result given the lack of freedom in the spectral action to tune this term.

 

[marcus]

http://arxiv.org/abs/0705.2440
Quantum Structure of Geometry: Loopy and fuzzy?
Alejandro Corichi, Jose A. Zapata
9 pages

"In any attempt to build a quantum theory of gravity, a central issue is to unravel the structure of space-time at the smallest scale. Of particular relevance is the possible definition of coordinate functions within the theory and the study of their algebraic properties, such as non-commutativity. Here we approach this issue from the perspective of loop quantum gravity and the picture of quantum geometry that the formalism offers. In particular, as we argue here, this emerging picture has two main elements: i) The nature of the quantum geometry at Planck scale is one-dimensional, polymeric with quantized geometrical quantities and; ii) Appropriately defined operators corresponding to coordinates by means of intrinsic, relational, constructions become non-commuting. This particular feature of the operators, that operationally localize points on space, gives rise to an emerging geometry that is also, in a precise sense, fuzzy."

http://arxiv.org/abs/0705.2533
Dark Energy and Gravity
T. Padmanabhan
Invited Review for a special Gen.Rel.Grav. issue on Dark Energy, edited by G.F.R.Ellis, R.Maartens and H.Nicolai; revtex; 22 pages; 2 figures

"I review the problem of dark energy focusing on the cosmological constant as the candidate and discuss its implications for the nature of gravity. Part 1 briefly overviews the currently popular 'concordance cosmology' and summarises the evidence for dark energy. It also provides the observational and theoretical arguments in favour of the cosmological constant as the candidate and emphasises why no other approach really solves the conceptual problems usually attributed to the cosmological constant. Part 2 describes some of the approaches to understand the nature of the cosmological constant and attempts to extract the key ingredients which must be present in any viable solution. I argue that (i)the cosmological constant problem cannot be satisfactorily solved until gravitational action is made invariant under the shift of the matter lagrangian by a constant and (ii) this cannot happen if the metric is the dynamical variable. Hence the cosmological constant problem essentially has to do with our (mis)understanding of the nature of gravity. Part 3 discusses an alternative perspective on gravity in which the action is explicitly invariant under the above transformation. Extremizing this action leads to an equation determining the background geometry which gives Einstein's theory at the lowest order with Lanczos-Lovelock type corrections. (Condensed abstract)."

http://arxiv.org/abs/0705.2525
On the physical meaning of the Unruh effect
Emil T.Akhmedov, Douglas Singleton
7 pages

"We present simple arguments that detectors moving with constant acceleration (even acceleration for a finite time) should detect particles. The effect is seen to be universal. Moreover, detectors undergoing linear acceleration and uniform, circular motion both detect particles for the same physical reason. We show that if one uses a circularly orbiting electron in a constant external magnetic field as the Unruh--DeWitt detector, then the Unruh effect physically coincides with the experimentally verified Sokolov--Ternov effect."

http://arxiv.org/abs/0705.2462
LambdaCDM cosmology: how much suppression of credible evidence, and does the model really lead its competitors, using all evidence?
Richard Lieu
14 pages, 3 figures and 3 tables

"Astronomy can never be a hard core physics discipline, because the Universe offers no control experiment, i.e. with no independent checks it is bound to be highly ambiguous and degenerate. Thus e.g. while superluminal motion can be explained by Special Relativity. data on the former can never on their own be used to establish the latter. This is why traditionally astrophysicists have been content with (and proud of) their ability to use known physical laws and processes established in the laboratory to explain celestial phenomena. Cosmology is not even astrophysics: all the principal assumptions in this field are unverified (or unverifiable) in the laboratory, and researchers are quite comfortable with inventing unknowns to explain the unknown. How then could, after fifty years of failed attempt in finding dark matter, the fields of dark matter and now dark energy have become such lofty priorities in astronomy funding, to the detriment of all other branches of astronomy? I demonstrate in this article that while some of is based upon truth, at least just as much of LambdaCDM cosmology has been propped by a paralyzing amount of propaganda which suppress counter evidence and subdue competing models. The recent WMAP3 paper of Spergel et al (2007) will be used as case in point on selective citation. I also show that when all evidence are taken into account, two of the competing models that abolish dark energy and/or dark matter do not trail behind LambdaCDM by much. Given all of the above, I believe astronomy is no longer heading towards a healthy future, unless funding agencies re-think their master plans by backing away from such high a emphasis on groping in the dark."