Loop-and-allied QG bibliography

[Fra]

I didn't realize that this was somehow a special thread. I'm sorry. I removed my post to shorten this. Thanks for the new thread.

/Fredrik

 

[marcus]

Fra thanks for moving the discussion to
https://www.physicsforums.com/showthread.php?p=1572779#post1572779
By custom this thread is not for discussion. I am grateful for this courtesy, because discussion would interfere with its function as a list of abstracts with links. When questions like this come up, we can start a separate thread.
============================================

http://arxiv.org/abs/0801.2564
Modern space-time and undecidability
Rodolfo Gambini, Jorge Pullin
8 pages, contribution to the volume "Minkowski spacetime: a hundred years later", edited by Vesselin Petkov
(Submitted on 16 Jan 2008)

"The picture of space-time that Minkowski created in 1907 has been followed by two important developments in physics not contained in the original picture: general relativity and quantum mechanics. We will argue that the use of concepts of those theories to construct space-time implies conceptual modifications in quantum mechanics. In particular one can construct a viable picture of quantum mechanics without a reduction process that has outcomes equivalent to a picture with a reduction process. One therefore has two theories that are entirely equivalent experimentally but profoundly different in the description of reality they give. This introduces a fundamental level of undecidability in physics of a kind that has not been present before. We discuss some of the implications.

 

[marcus]

This is a nice invited review paper by a top cosmologist
http://arxiv.org/abs/0801.2968
Mapping the Cosmological Expansion
Eric V. Linder
49 pages, 29 figures; Review invited for Reports on Progress in Physics
(Submitted on 18 Jan 2008)

"The ability to map the cosmological expansion has developed enormously, spurred by the turning point one decade ago of the discovery of cosmic acceleration. The standard model of cosmology has shifted from a matter dominated, standard gravity, decelerating expansion to the present search for the origin of acceleration in the cosmic expansion. We present a wide ranging review of the tools, challenges, and physical interpretations. The tools include direct measures of cosmic scales through Type Ia supernova luminosity distances, and angular distance scales of baryon acoustic oscillation and cosmic microwave background density perturbations, as well as indirect probes such as the effect of cosmic expansion on the growth of matter density fluctuations. Accurate mapping of the expansion requires understanding of systematic uncertainties in both the measurements and the theoretical framework, but the result will give important clues to the nature of the physics behind accelerating expansion and to the fate of the universe."

He put some work into it. It's thorough and well-written. And it has the perspective of an old hand.

 

[marcus]

http://arxiv.org/abs/0801.3287
Background Independence and Asymptotic Safety in Conformally Reduced Gravity
Martin Reuter, Holger Weyer
4 figures
(Submitted on 21 Jan 2008)

"We analyze the conceptual role of background independence in the application of the effective average action to quantum gravity. Insisting on a background independent renormalization group (RG) flow the coarse graining operation must be defined in terms of an unspecified variable metric since no rigid metric of a fixed background spacetime is available. This leads to an extra field dependence in the functional RG equation and a significantly different RG flow in comparison to the standard flow equation with a rigid metric in the mode cutoff. The background independent RG flow can possesses a non-Gaussian fixed point, for instance, even though the corresponding standard one does not. We demonstrate the importance of this universal, essentially kinematical effect by computing the RG flow of Quantum Einstein Gravity in the 'conformally reduced' Einstein--Hilbert approximation which discards all degrees of freedom contained in the metric except the conformal one. Without the extra field dependence the resulting RG flow is that of a simple \phi^4-theory. Including it one obtains a flow with exactly the same qualitative properties as in the full Einstein--Hilbert truncation. In particular it possesses the non-Gaussian fixed point which is necessary for asymptotic safety."

 

[marcus]

http://arxiv.org/abs/0801.4001
Quantum nature of cosmological bounces
Martin Bojowald
26 pages
(Submitted on 25 Jan 2008)

"Several examples are known where quantum gravity effects resolve the classical big bang singularity by a bounce. The most detailed analysis has probably occurred for loop quantum cosmology of isotropic models sourced by a free, massless scalar. Once a bounce has been realized under fairly general conditions, the central questions are how strongly quantum it behaves, what influence quantum effects can have on its appearance, and what quantum space-time beyond the bounce may look like. This, then, has to be taken into account for effective equations which describe the evolution properly and can be used for further phenomenological investigations. Here, we provide the first analysis with interacting matter with new effective equations valid for weak self-interactions or small masses. They differ from the free scalar equations by crucial terms and have an important influence on the bounce and the space-time around it. Especially the role of squeezed states, which have often been overlooked in this context, is highlighted. The presence of a bounce is proven for uncorrelated states, but as squeezing is a dynamical property and may change in time, further work is required for a general conclusion."

http://arxiv.org/abs/0801.4241
A Immirzi-like parameter for 3d quantum gravity
Valentin Bonzom, Etera R. Livine
14 pages
(Submitted on 28 Jan 2008)

"We study an Immirzi-like ambiguity in three-dimensional quantum gravity. It shares some features with the Immirzi parameter of four-dimensional loop quantum gravity: it does not affect the equations of motion, but modifies the Poisson brackets and the constraint algebra at the canonical level. We focus on the length operator and show how to define it through non-commuting fluxes. We compute its spectrum and show the effect of this Immirzi-like ambiguity. Finally, we extend these considerations to 4d gravity and show how the different topological modifications of the action affect the canonical structure of loop quantum gravity."

 

[marcus]

http://arxiv.org/abs/0802.0896
Topology change in causal quantum gravity
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
4 pages, proceedings of the workshop JGRG 17 (Nagoya, Japan, December 2007)
(Submitted on 6 Feb 2008)

"The role of topology change in a fundamental theory of quantum gravity is still a matter of debate. However, when regarding string theory as two-dimensional quantum gravity, topological fluctuations are essential. Here we present a third quantization of two-dimensional surfaces based on the method of causal dynamical triangulation (CDT). Formally, our construction is similar to the c = 0 non-critical string field theory developed by Ishibashi, Kawai and others, but physically it is quite distinct. Unlike in non-critical string theory the topology change of spatial slices is well controlled and regulated by Newton's constant."

http://arxiv.org/abs/0802.0864
Area-angle variables for general relativity
Bianca Dittrich, Simone Speziale
7 pages, 1 figure
(Submitted on 6 Feb 2008)

"We introduce a modified Regge calculus for general relativity on a triangulated four dimensional Riemannian manifold where the fundamental variables are areas and a certain class of angles. These variables satisfy constraints which are local in the triangulation. We expect the formulation to have applications to classical discrete gravity and non-perturbative approaches to quantum gravity."

http://arxiv.org/abs/0802.0880
Entanglement Entropy in Loop Quantum Gravity
William Donnelly
4 pages
(Submitted on 6 Feb 2008)

The entanglement entropy between quantum fields inside and outside a black hole horizon is a promising candidate for the microscopic origin of black hole entropy. We show that the entanglement entropy may be defined in loop quantum gravity, and compute its value for spin network states. The entanglement entropy for an arbitrary region of space is expressed as a sum over punctures where the spin network intersects the region's boundary. Our result agrees asymptotically with results previously obtained from the isolated horizon framework, and we give a justification for this agreement. We conclude by proposing a new method for studying corrections to the area law and its implications for quantum corrections to the gravitational action."

http://arxiv.org/abs/0802.0719
A String Field Theory based on Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
29 pages, 4 figures
(Submitted on 5 Feb 2008)

"We formulate the string field theory in zero-dimensional target space corresponding to the two-dimensional quantum gravity theory defined through Causal Dynamical Triangulations. This third quantization of the quantum gravity theory allows us in principle to calculate the transition amplitudes of processes in which the topology of space changes in time, and to include non-trivial topologies of space-time. We formulate the corresponding Dyson-Schwinger equations and illustrate how they can be solved iteratively."

http://arxiv.org/abs/0802.0767
A fundamental length as a candidate for dark energy: a DSR inspired FRW spacetime
N. Khosravi, H. R. Sepangi
8 pages, to appear in PLA
(Submitted on 6 Feb 2008)

"We show that the existence of a fundamental length, introduced in Deformed Special Relativity (DSR) inspired minisuper (phase-) space, causes the behavior of the scale factor of the universe to change from that of a universe filled with dust to an accelerating universe driven by a cosmological constant."

http://arxiv.org/abs/0802.0702
Strategies for Determining the Nature of Dark Matter
Dan Hooper, Edward A. Baltz
25 pages, 5 figures, Review intended for the Annual Review of Nuclear and Particle Science
(Submitted on 5 Feb 2008)

"In this review, we discuss the role of the various experimental programs taking part in the broader effort to identify the particle nature of dark matter. In particular, we focus on electroweak scale dark matter particles and discuss a wide range of search strategies being carried out and developed to detect them. These efforts include direct detection experiments, which attempt to observe the elastic scattering of dark matter particles with nuclei, indirect detection experiments, which search for photons, antimatter and neutrinos produced as a result of dark matter annihilations, and collider searches for new TeV-scale physics. Each of these techniques could potentially provide a different and complementary set of information related to the mass, interactions and distribution of dark matter. Ultimately, it is hoped that these many different tools will be used together to conclusively identify the particle or particles that constitute the dark matter of our universe."

http://arxiv.org/abs/0802.0013
Cosmic Neutrinos
Chris Quigg
27 pages, 16 figures, lecture at 2007 SLAC Summer Institute
(Submitted on 31 Jan 2008)

"I recall the place of neutrinos in the electroweak theory and summarize what we know about neutrino mass and flavor change. I next review the essential characteristics expected for relic neutrinos and survey what we can say about the neutrino contribution to the dark matter of the Universe. Then I discuss the standard-model interactions of ultrahigh-energy neutrinos, paying attention to the consequences of neutrino oscillations, and illustrate a few topics of interest to neutrino observatories. I conclude with short comments on the remote possibility of detecting relic neutrinos through annihilations of ultrahigh-energy neutrinos at the Z resonance."

 

[marcus]

http://arxiv.org/abs/0802.1201
Tensorial Structure of the LQG graviton propagator
Emanuele Alesci
4 pages,; to appear in the proceedings of the II Stueckelberg Workshop, Int.J.Mod.Phys.A
(Submitted on 8 Feb 2008)

"We review the construction of the tensorial structure of the graviton propagator in the context of loop quantum gravity and spinfoam formalism. The main result of this analysis is that applying the same strategy used to compute the diagonal terms, the Barrett-Crane vertex is unable to yield the correct propagator in the long distance limit. The problem is in the intertwiner-independence of the Barrett-Crane vertex. We also review the asymptotic behavior of an alternative vertex that is able to give the correct propagator."

http://arxiv.org/abs/0802.1129
Doubly special relativity in de Sitter spacetime
Salvatore Mignemi
18 pages
(Submitted on 8 Feb 2008)

"We discuss the generalization of Doubly Special Relativity to a curved de Sitter background. The model has three observer-independent scales, the velocity of light c, the radius of curvature of the geometry alpha, and the Planck energy kappa, and can be realized in a noncommutative position space. It is possible to construct a model exhibiting a duality for the interchange of positions and momenta together with the exchange of alpha and kappa."

http://arxiv.org/abs/0802.1221
Immirzi parameter and fermions with non-minimal coupling
Sergei Alexandrov
4 pages
(Submitted on 8 Feb 2008)

"We clarify the role played by the Immirzi parameter in classical gravity coupled to fermions. Considering the general non-minimal coupling, we show that, although the torsion depends explicitly on the Immirzi parameter, in a suitable parametrization the effective action obtained by integrating out the spin-connection is independent of it. Thus the Immirzi parameter is not detectable in classical theory even after coupling of fermions."

http://arxiv.org/abs/0802.1215
TeVeS gets caught on caustics
Carlo R. Contaldi, Toby Wiseman, Benjamin Withers
21 pages,12 figures
(Submitted on 10 Feb 2008)

"TeVeS uses a dynamical vector field with timelike unit norm constraint to specify a preferred local frame. When matter moves slowly in this frame - the so-called quasi-static regime - Modified Newtonian Dynamics (MoND) results. Theories with such vectors (such as Einstein-aether) are prone to the vector dynamics forming singularities which render their classical evolution problematic. Here we analyse the dynamics of the vector in TeVeS in various situations. We find that, quite generically, the vector field develops caustic singularities on time scales of order the gravitational in-fall time. Having shown singularity formation is generic with or without matter, Bekenstein's original formulation of TeVeS appears dynamically problematic. We argue that TeVeS might be saved as a relativistic theory with a MoND limit by modifying the vector field kinetic terms to the more general form used by Einstein-aether."

 

[marcus]

http://arxiv.org/abs/0802.1783
On Spectral Triples in Quantum Gravity I
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
84 pages, 8 figures
(Submitted on 13 Feb 2008)

"This paper establishes a link between Noncommutative Geometry and canonical quantum gravity. A semi-finite spectral triple over a space of connections is presented. The triple involves an algebra of holonomy loops and a Dirac type operator which resembles a global functional derivation operator. The interaction between the Dirac operator and the algebra reproduces the Poisson structure of General Relativity. Moreover, the associated Hilbert space corresponds, up to a discrete symmetry group, to the Hilbert space of diffeomorphism invariant states known from Loop Quantum Gravity. Correspondingly, the square of the Dirac operator has, in terms of canonical quantum gravity, the form of a global area-squared operator. Furthermore, the spectral action resembles a partition function of Quantum Gravity. The construction is background independent and is based on an inductive system of triangulations. This paper is the first of two papers on the subject."

http://arxiv.org/abs/0802.1784
On Spectral Triples in Quantum Gravity II
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
43 pages, 1 figure
(Submitted on 13 Feb 2008)

"A semifinite spectral triple for an algebra canonically associated to canonical quantum gravity is constructed. The algebra is generated by based loops in a triangulation and its barycentric subdivisions. The underlying space can be seen as a gauge fixing of the unconstrained state space of Loop Quantum Gravity. This paper is the second of two papers on the subject."


http://arxiv.org/abs/0802.1561
Have we tested Lorentz invariance enough?
David Mattingly
17 pages, Talk given at 'From Quantum to Emergent Gravity: Theory and Phenomenology', SISSA, June 2007
(Submitted on 12 Feb 2008)

"Motivated by ideas from quantum gravity, Lorentz invariance has undergone many stringent tests over the past decade and passed every one. Since there is no conclusive reason from quantum gravity that the symmetry \textit{must} be violated at some point we should ask the questions: a) are the existing tests sufficient that the symmetry is already likely exact at the Planck scale? b) Are further tests simply blind searches for new physics without reasonable expectation of a positive signal? Here we argue that the existing tests are not quite sufficient and describe some theoretically interesting areas of existing parameterizations for Lorentz violation in the infrared that are not yet ruled out but are accessible (or almost accessible) by current experiments. We illustrate this point using a vector field model for Lorentz violation containing operators up to mass dimension six and analyzing how terrestrial experiments, neutrino observatories, and Auger results on ultra-high energy cosmic rays limit this model."

http://arxiv.org/abs/0802.1798
Emergent gravity and Dark Energy
T. Padmanabhan
26 pages
(Submitted on 13 Feb 2008)

"This is an invited contribution to be included in a multi-authored book on 'Dark Energy', to be edited by Pilar Ruiz-Lapuente and published by Cambridge University Press."

 

[marcus]

http://arxiv.org/abs/0802.2527
Asymptotics and Hamiltonians in a First order formalism
Abhay Ashtekar, Jonathan Engle, David Sloan
18 pages
(Submitted on 18 Feb 2008)

"We consider 4-dimensional space-times which are asymptotically flat at spatial infinity and show that, in the first order framework, action principle is well-defined without the need of infinite counter terms. It naturally leads to a covariant phase space in which the Hamiltonians generating asymptotic symmetries provide the total energy-momentum and angular momentum of the space-time. We address the subtle but important problems that arise because of logarithmic translations and super-translations both in the Langrangian and Hamiltonian frameworks. As a forthcoming paper will show, the treatment of higher dimensions is considerably simpler. Our first order framework also suggests a new direction for generalizing the spectral action of non-commutative geometry."


http://arxiv.org/abs/0802.2230
Canonical Lagrangian Dynamics and General Relativity
Andrew Randono
(Submitted on 15 Feb 2008)

"Building towards a more covariant approach to canonical classical and quantum gravity we outline an approach to constrained dynamics that de-emphasizes the role of the Hamiltonian phase space and highlights the role of the Lagrangian phase space. We identify a 'Lagrangian one-form' to replace the standard symplectic one-form, which we use to construct the canonical constraints and an associated constraint algebra. The method is particularly useful for generally covariant systems and systems with a degenerate canonical symplectic form, such as Einstein Cartan gravity, to which we apply the method explicitly. We find that one can demonstrate the closure of the constraints without gauge fixing the Lorentz group or introducing primary constraints on the phase space variables. Finally, using geometric quantization techniques, we briefly discuss implications of the formalism for the quantum theory."

 

[marcus]

http://arxiv.org/abs/0802.3188
Black hole state degeneracy in Loop Quantum Gravity
Ivan Agullo, Jacobo Diaz-Polo, Enrique Fernandez-Borja
22 pages, 7 figures
(Submitted on 21 Feb 2008)

"The combinatorial problem of counting the black hole quantum states within the Isolated Horizon framework in Loop Quantum Gravity is analyzed. A qualitative understanding of the origin of the band structure shown by the degeneracy spectrum, which is responsible for the black hole entropy quantization, is reached. Even when motivated by simple considerations, this picture allows to obtain analytical expressions for the most relevant quantities associated to this effect."

 

[marcus]

http://arxiv.org/abs/0802.3389
Simplicity and closure constraints in spin foam models of gravity
Sergei Alexandrov
16 pages
(Submitted on 23 Feb 2008)

"We revise imposition of various constraints in spin foam models of 4-dimensional general relativity. We argue that the usual simplicity constraint must be supplemented by a constraint on holonomies and together they must be inserted explicitly into the discretized path integral. At the same time, the closure constraint must be relaxed so that the new constraint expresses covariance of intertwiners assigned to tetrahedra by spin foam quantization. As a result, the spin foam boundary states are shown to be realized in terms of projected spin networks of the covariant loop approach to quantum gravity."


http://arxiv.org/abs/0802.3422
Black Holes without Event Horizons
Alex B. Nielsen
Talk at APCTP Winter School, Daejeon, Korea, 2008. 7 pages
(Submitted on 23 Feb 2008)

"We discuss some of the drawbacks of using event horizons to define black holes. The reasons are both practical, physical and theoretical. We argue that locally defined trapping horizons can remedy many of these drawbacks. We examine of the question of whether black hole thermodynamics should be associated with event horizons or trapping horizons. To this end we discuss what role trapping horizons may play in black hole thermodynamics. In addition, we show how trapping horizons may give rise to Hawking radiation and discuss the issue of gravitational entropy."

 

[marcus]

http://arxiv.org/abs/0802.3983
Towards the graviton from spinfoams: the complete perturbative expansion of the 3d toy model
Valentin Bonzom, Etera R. Livine, Matteo Smerlak, Simone Speziale
16 pages, 3 figs
(Submitted on 27 Feb 2008)

"We consider an exact expression for the 6j-symbol for the isosceles tetrahedron, involving integrals over SU(2), and use it to write the two-point function of 3d gravity on a single tetrahedron as a group integral. The perturbative expansion of this expression is then performed with respect to the boundary geometry using a simple saddle-point analysis. We derive the complete expansion in inverse powers of the length scale and evaluate explicitly the quantum corrections up to second order. Finally, we use the same method to provide the complete expansion of the isosceles 6j-symbol with the explicit phases at all orders and the next-to-leading correction to the Ponzano-Regge asymptotics."

http://arxiv.org/abs/0802.4077
Black hole state counting in LQG: A number theoretical approach
Ivan Agullo, J. Fernando Barbero G., Jacobo Diaz-Polo, Enrique Fernandez-Borja, Eduardo J. S. Villaseñor
4 pages
(Submitted on 27 Feb 2008)

"We give a practical method to exactly compute black hole entropy in the framework of Loop Quantum Gravity. Along the way we provide a complete characterization of the relevant sector of the spectrum of the area operator, including degeneracies, and determine the number of solutions to the projection constraint analytically. We use a computer implementation of the proposed algorithm to confirm and extend previous results on the detailed structure of the black hole degeneracy spectrum."

 

[marcus]

http://arxiv.org/abs/0802.4274
Loop Quantum Cosmology: Effective theories and oscillating universes
Martin Bojowald, Reza Tavakol
24 pages, 3 figures, Chapter contributed to: Beyond the Big Bang, edited by R. Vaas (Springer Verlag, 2008)


"Despite its great successes in accounting for the current observations, the so called 'standard' model of cosmology faces a number of fundamental unresolved questions. Paramount among these are those relating to the nature of the origin of the universe and its early evolution. Regarding the question of origin, the main difficulty has been the fact that within the classical general relativistic framework, the 'origin' is almost always a singular event at which the laws of physics break down, thus making it impossible for such an event, or epochs prior to it, to be studied. Recent studies have shown that Loop Quantum Cosmology may provide a non-singular framework where these questions can be addressed. The crucial role here is played by quantum effects, i.e. corrections to the classical equations of motion, which are incorporated in effective equations employed to develop cosmological scenarios.
In this chapter we shall consider the three main types of quantum effects expected to be present within such a framework and discuss some of their consequences for the effective equations. In particular we discuss how such corrections can allow the construction of non-singular emergent scenarios for the origin of the universe, which are past-eternal, oscillating and naturally emerge into an inflationary phase. These scenarios provide a physically plausible picture for the origin and early phases of the universe, which is in principle testable. We pay special attention to the interplay between these different types of correction terms. Given the absence, so far, of a complete derivation of such corrections in general settings, it is important to bear in mind the questions of consistency and robustness of scenarios based on partial inclusion of such effects."

Amazon is now taking advance orders on the new book Beyond the Big Bang, edited by R. Vaas
https://www.amazon.com/dp/3540714227/?tag=pfamazon01-20
The book is 600 pages, several different experts in quantum cosmology have contributed chapters.
The publication date is 1 July 2008.

 

[marcus]

http://arxiv.org/abs/0803.0982
Dark Energy and the Accelerating Universe
Joshua Frieman (Chicago/Fermilab), Michael Turner (Chicago), Dragan Huterer (Michigan)
Invited review for Annual Reviews of Astronomy and Astrophysics; 53 pages, 18 figures
(Submitted on 7 Mar 2008)

"The discovery ten years ago that the expansion of the Universe is accelerating put in place the last major building block of the present cosmological model, in which the Universe is composed of 4% baryons, 20% dark matter, and 76% dark energy. At the same time, it posed one of the most profound mysteries in all of science, with deep connections to both astrophysics and particle physics. Cosmic acceleration could arise from the repulsive gravity of dark energy -- for example, the quantum energy of the vacuum -- or it may signal that General Relativity breaks down on cosmological scales and must be replaced. We review the present observational evidence for cosmic acceleration and what it has revealed about dark energy, discuss the various theoretical ideas that have been proposed to explain acceleration, and describe the key observational probes that will shed light on this enigma in the coming years."

 

[marcus]

http://arxiv.org/abs/0803.2546
Primordial Entropy Production and Lambda-driven Inflation from Quantum Einstein Gravity
Alfio Bonanno, Martin Reuter
12 pages, 4 figures, IGCG-07 Pune
(Submitted on 17 Mar 2008)

"We review recent work on renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values. In particular we argue that QEG effects can account for the entire entropy of the present Universe in the massless sector and give rise to a phase of inflationary expansion. This phase is a pure quantum effect and requires no classical inflaton field."

This is the paper Martin Reuter gave at an international conference that was held in December 2007 in India.

Reuter is also scheduled to give a talk at the QG2 conference at Nottingham, first week of July.

 

[marcus]

Sets of slides for two important talks at the Zakopane workshop. More PDF files from the workshop are available at Kostecki's website:

* Abhay Ashtekar - An Overview of Loop Quantum Cosmology of FRW Models
http://cift.fuw.edu.pl/users/kostecki/zakopane08/ashtekar.pdf

* Carlo Rovelli - Looppy & Foammy: at long last, falling in love
http://cift.fuw.edu.pl/users/kostecki/zakopane08/rovelli.pdfhttp://arxiv.org/abs/0803.2926
Matrix universality of gauge field and gravitational dynamics
Lee Smolin
21 pages,
(Submitted on 20 Mar 2008)

"A simple cubic matrix model is presented, which has truncations that, it is argued, lead at the classical level to a variety of theories of gauge fields and gravity. These include Yang-Mills theories and background independent theories of connections. The latter includes Chern-Simons theory in d=3, and BF theory and general relativity in d=4. General relativity coupled to Yang-mills theory for any SU(N) may also arise from quantum corrections.
On the basis of these results we conjecture that there are large universality classes of cut-off gauge and gravity theories, connected by transformations that mix up local and spacetime symmetries. If our universe is described by one of these theories then the question of the choice of the laws of physics is to a large extent subsumed in the problem of the choice of initial conditions in cosmology."

I don't mean to suggest that the next one is a big deal, only a reminder that things can be more complicated than at first suspected and require correction. Linder has a solid track record---perhaps I'm biased but I tend to pay extra attention.
http://arxiv.org/abs/0803.2877
Shifting the Universe: Early Dark Energy and Standard Rulers
Eric V. Linder, Georg Robbers
6 pages, 3 figures
(Submitted on 19 Mar 2008)

"The presence of dark energy at high redshift influences both the cosmic sound horizon and the distance to last scattering of the cosmic microwave background. We demonstrate that through the degeneracy in their ratio, early dark energy can lie hidden in the CMB temperature and polarization spectra, leading to an unrecognized shift in the sound horizon. If the sound horizon is then used as a standard ruler, as in baryon acoustic oscillations, then the derived cosmological parameters can be nontrivially biased. Fitting for the absolute ruler scale (just as supernovae must be fit for the absolute candle magnitude) removes the bias but decreases the leverage of the BAO technique by a factor 2."

 

[marcus]

http://arxiv.org/abs/0803.3203
Conserved Quantities for Interacting Four Valent Braids in Quantum Gravity
Jonathan Hackett, Yidun Wan
18 pages, 7 figures
(Submitted on 21 Mar 2008)

"We derive conservation laws from interactions of actively-interacting braid-like excitations of embedded framed spin networks in Quantum Gravity. Additionally we demonstrate that actively-interacting braid-like excitations interact in such a way that the product of interactions involving two actively-interacting braid-like excitations produces a resulting actively-interacting form."

 

[marcus]

http://arxiv.org/abs/0803.3456
Is Quantum Gravity Necessary?
S. Carlip
based on a talk given at Peyresq Physics 11, to appear in Class. Quant. Grav
(Submitted on 24 Mar 2008)

"In view of the enormous difficulties we seem to face in quantizing general relativity, we should perhaps consider the possibility that gravity is a fundamentally classical interaction. Theoretical arguments against such mixed classical-quantum models are strong, but not conclusive, and the question is ultimately one for experiment. I review some work in progress on the possibility of experimental tests, exploiting the nonlinearity of the classical-quantum coupling, that could help settle this question."

http://arxiv.org/abs/0803.2309
Cosmic Microwave Weak lensing data as a test for the dark universe
Erminia Calabrese, Anze Slosar, Alessandro Melchiorri, George F. Smoot, Oliver Zahn
7 Pages, 3 Figures
(Submitted on 17 Mar 2008)

"Combined analyses of WMAP 3-year and ACBAR Cosmic Microwave Anisotropies angular power spectra have presented evidence for gravitational lensing >3 sigma level. This signal could provide a relevant test for cosmology. After evaluating and confirming the statistical significance of the detection in light of the new WMAP 5-year data, we constrain a new parameter A_L that scales the lensing potential such that A_L=0 corresponds to unlensed while A_L=1 is the expected lensed result. We find from WMAP5+ACBAR a 2.5 sigma indication for a lensing contribution larger than expected, with A_L=3.1_{-1.5}^{+1.8} at 95% c.l.. The result is stable under the assumption of different templates for an additional Sunyaev-Zel'dovich foreground component or the inclusion of an extra background of cosmic strings. We find negligible correlation with other cosmological parameters as, for example, the energy density in massive neutrinos. While unknown systematics may be present, dark energy or modified gravity models could be responsible for the over-smoothness of the power spectrum. Near future data, most notably from the Planck satellite mission, will scrutinize this interesting possibility."

http://arxiv.org/abs/0803.3559
Loop Quantum Cosmology of Diagonal Bianchi Type I model: simplified theory
Lukasz Szulc
10 pages, 10 figures
(Submitted on 25 Mar 2008)

"A simplified theory of diagonal Bianchi type I model coupled with a massless scalar field in Loop Quantum Cosmology is constructed. The quantum constraint operator and physical sector are under good analytical control. The problem of finding semi-classical states is reduced to the following one: how to compute an ordinary, continuous, three-dimensional Fourier Transform with an amplitude given analytically? Moreover, the evolution of the three gravitational degrees of freedom is numerically shown to be non-singular."

http://arxiv.org/abs/0803.3659
Phenomenological dynamics of loop quantum cosmology in Kantowski-Sachs spacetime
Dah-Wei Chiou
36 pages, 4 figures, 1 table
(Submitted on 26 Mar 2008)

"The full theory and the semiclassical description of loop quantum cosmology (LQC) have been studied in the Friedmann-Robertson-Walker and Bianchi I models. As an extension to include both anisotropy and intrinsic curvature, this paper investigates the cosmological model of Kantowski-Sachs spacetime with a free massless scalar field at the level of phenomenological dynamics with the LQC discreteness corrections. The LQC corrections are implemented in two different improved quantization schemes. In both schemes, the big bang and big crunch singularities of the classical solution are resolved and replaced by the big bounces when the area or volume scale factor approaches the critical values in the Planck regime measured by the reference of the scalar field momentum. Symmetries of scaling are also noted and suggest that the fundamental spatial scale (area gap) may give rise to a temporal scale. The bouncing scenarios are in an analogous fashion of the Bianchi I model, naturally extending the observations obtained earlier."

 

[marcus]

http://arxiv.org/abs/0803.4484
Recollapsing quantum cosmologies and the question of entropy
Martin Bojowald, Reza Tavakol
23 pages, 2 figures
(Submitted on 31 Mar 2008)

"Recollapsing homogeneous and isotropic models present one of the key ingredients for cyclic scenarios. This is considered here within a quantum cosmological framework in presence of a free scalar field with, in turn, a negative cosmological constant and spatial curvature. Effective equations shed light on the quantum dynamics around a recollapsing phase and the evolution of state parameters such as fluctuations and correlations through such a turn around. In the models considered here, the squeezing of an initial state is found to be strictly monotonic in time during the expansion, turn around and contraction phases. The presence of such monotonicity is of potential importance in relation to a long standing intensive debate concerning the (a)symmetry between the expanding and contracting phases in a recollapsing universe. Furthermore, together with recent analogous results concerning a bounce one can extend this monotonicity throughout an entire cycle. This provides a strong motivation for employing the degree of squeezing as an alternative measure of (quantum) entropy. It may also serve as a new concept of emergent time described by a variable without classical analog. The evolution of the squeezing in emergent oscillating scenarios can in principle provide constraints on the viability of such models."

It is an important issue. Penrose has made it central to all his discussion of cosmology, including opposition to the bounce explanation of the big bang. They may have answered Penrose. London is getting to be an important place for studying quantum cosmology. Tavakol is at Queen Mary London, but there is also a group at King's College.

http://arxiv.org/abs/0803.4483
Numerical techniques for solving the quantum constraint equation of generic lattice-refined models in loop quantum cosmology
William Nelson, Mairi Sakellariadou (King's College London)
17 pages, 14 figures
(Submitted on 31 Mar 2008)

"To avoid instabilities in the continuum semi-classical limit of loop quantum cosmology models, refinement of the underlying lattice is necessary. The lattice refinement leads to new dynamical difference equations which, in general, do not have a uniform step-size, implying complications in their analysis and solutions. We propose a numerical method based on Taylor expansions, which can give us the necessary information to calculate the wave-function at any given lattice point. The method we propose can be applied in any lattice-refined model, while in addition the accuracy of the method can be estimated. Moreover, we confirm numerically the stability criterion which was earlier found following a von Neumann analysis. Finally, the `motion' of the wave-function due to the underlying discreteness of the space-time is investigated, for both a constant lattice, as well as lattice refinement models."

http://arxiv.org/abs/0803.4446
Anti-deSitter universe dynamics in LQC
Eloisa Bentivegna, Tomasz Pawlowski
29 pages, 20 figures
(Submitted on 31 Mar 2008)

"A model for a flat isotropic universe with a negative cosmological constant $\Lambda$ and a massless scalar field as sole matter content is studied within the framework of Loop Quantum Cosmology. By application of the methods introduced for the model with Lambda=0, the physical Hilbert space and the set of Dirac observables are constructed. As in that case, the scalar field plays here the role of an emergent time. The properties of the system are found to be similar to those of the k=1 FRW model: for small energy densities, the quantum dynamics reproduces the classical one, whereas, due to modifications at near-Planckian densities, the big bang and big crunch singularities are replaced by a quantum bounce connecting deterministically the large semiclassical epochs. Thus in Loop Quantum Cosmology the evolution is qualitatively cyclic."

http://arxiv.org/abs/0804.0030
A Pointless Model for the Continuum as the Foundation for Quantum Gravity
Louis Crane
Submitted to GRF
(Submitted on 31 Mar 2008)

"In this paper, we outline a new approach to quantum gravity; describing states for a bounded region of spacetime as eigenstates for two classes of physically plausible gedanken experiments. We end up with two complementary descriptions in which the point set continuum disappears.
The first replaces the continuum of events with a handlebody decomposition of loop space. We conjecture that techniques fron algebraic topology will allow us to extend state sum models on spacetime to loop space.
The second picture replaces the continuum with a nondistributive lattice; the classical limit seems more tractible in this picture."

http://arxiv.org/abs/0804.0054
Formalism Locality in Quantum Theory and Quantum Gravity
Lucien Hardy
To appear in "Philosophy of Quantum Information and Entanglement" Eds A. Bokulich and G. Jaeger (CUP)
(Submitted on 1 Apr 2008)

"We expect a theory of Quantum Gravity to be both probabilistic and have indefinite causal structure. Indefinite causal structure poses particular problems for theory formulation since many of the core ideas used in the usual approaches to theory construction depend on having definite causal structure. For example, the notion of a state across space evolving in time requires that we have some definite causal structure so we can define a state on a space-like hypersurface. We will see that many of these problems are mitigated if we are able to formulate the theory in a "formalism local" (or F-local) fashion. A formulation of a physical theory is said to be F-local if, in making predictions for any given arbitrary space-time region, we need only refer to mathematical objects pertaining to that region. This is a desirable property both on the grounds of efficiency and since, if we have indefinite causal structure, it is not clear how to select some other space-time region on which our calculations may depend. The usual ways of formulating physical theories (the time evolving state picture, the histories approach, and the local equations approach) are not F-local.
We set up a framework for probabilistic theories with indefinite causal structure. This, the causaloid framework, is F-local. We describe how Quantum Theory can be formulated in the causaloid framework (in an F-local fashion). This provides yet another formulation of Quantum Theory. This formulation, however, may be particularly relevant to the problem of finding a theory of Quantum Gravity."

 

[marcus]

http://arxiv.org/abs/0804.0037
Particle Identifications from Symmetries of Braided Ribbon Network Invariants
Sundance Bilson-Thompson, Jonathan Hackett, Lou Kauffman, Lee Smolin
9 pages, 7 figures
(Submitted on 1 Apr 2008)

"We develop the idea that the particles of the standard model may arise from excitations of quantum geometry. A previously proposed topological model of preons is developed so that it incorporates an unbounded number of generations. A condition is also found on quantum gravity dynamics necessary for the interactions of the standard model to emerge."

Here is a quote from the Conclusions, section 5 on page 8:

We have presented an embedding of the fermion and weak vector boson states of the standard model in a class of loop quantum gravity models. These are models in which the states are based on embeddings of framed trivalent spin networks, with possibly arbitrary labellings, whose dynamics is given by the standard dual Pachner trivalent moves, plus additional moves consistent with the conservation of the topological invariants (a; b; c).

There are a number of interrelated questions that remain open before the promise of this development can be fully understood...

http://arxiv.org/abs/0804.0328
A locally finite model for gravity
Gerard 't Hooft
26 pages, 9 figures
(Submitted on 2 Apr 2008)

"Matter interacting classically with gravity in 3+1 dimensions usually gives rise to a continuum of degrees of freedom, so that, in any attempt to quantize the theory, ultraviolet divergences are nearly inevitable. Here, we investigate matter of a form that only displays a finite number of degrees of freedom in compact sections of space-time. In finite domains, one has only exact, analytic solutions. This is achieved by limiting ourselves to straight pieces of string, surrounded by locally flat sections of space-time. Globally, however, the model is not finite, because solutions tend to generate infinite fractals. The model is not (yet) quantized, but could serve as an interesting setting for analytical approaches to classical general relativity, as well as a possible stepping stone for quantum models. Details of its properties are explained, but some problems remain unsolved, such as a complete description of the most violent interactions, which can become quite complex."

http://arxiv.org/abs/0804.0252
A Matrix Model for 2D Quantum Gravity defined by Causal Dynamical Triangulations
J. Ambjorn, R. Loll, Y. Watabiki, W. Westra, S. Zohren
13 pages, 1 figure
(Submitted on 1 Apr 2008)

"A novel continuum theory of two-dimensional quantum gravity, based on a version of Causal Dynamical Triangulations which incorporates topology change, has recently been formulated as a genuine string field theory in zero-dimensional target space (arXiv:0802.0719). Here we show that the Dyson-Schwinger equations of this string field theory are reproduced by a cubic matrix model. This matrix model also appears in the so-called Dijkgraaf-Vafa correspondence if the superpotential there is required to be renormalizable. In the spirit of this model, as well as the original large-N expansion by 't Hooft, we need no special double-scaling limit involving a fine tuning of coupling constants to obtain the continuum quantum-gravitational theory. Our result also implies a matrix model representation of the original, strictly causal quantum gravity model."

http://arxiv.org/abs/0804.0279
A Discrete Representation of Einstein's Geometric Theory of Gravitation: The Fundamental Role of Dual Tessellations in Regge Calculus
Jonathan R. McDonald, Warner A. Miller
25 pages, 12 figures, submitted to "Tessellations in the Science: Virtues, Techniques and Applications of Geometric Tilings," ed. R. van de Weijgaert, G. Vegter, J. Ritzerveld and V. Icke
(Submitted on 2 Apr 2008)

"In 1961 Tullio Regge provided us with a beautiful lattice representation of Einstein's geometric theory of gravity. This Regge Calculus (RC) is strikingly different from the more usual finite difference and finite element discretizations of gravity. In RC the fundamental principles of General Relativity are applied directly to a tessellated spacetime geometry. In this manuscript, and in the spirit of this conference, we reexamine the foundations of RC and emphasize the central role that the Voronoi and Delaunay lattices play in this discrete theory. In particular we describe, for the first time, a geometric construction of the scalar curvature invariant at a vertex. This derivation makes use of a new fundamental lattice cell built from elements inherited from both the simplicial (Delaunay) spacetime and its circumcentric dual (Voronoi) lattice. The orthogonality properties between these two lattices yield an expression for the vertex-based scalar curvature which is strikingly similar to the corresponding and more familiar hinge-based expression in RC (deficit angle per unit Voronoi dual area). In particular, we show that the scalar curvature is simply a vertex-based weighted average of deficits per weighted average of dual areas. What is most striking to us is how naturally spacetime is represented by Voronoi and Delaunay structures and that the laws of gravity appear to be encoded locally on the lattice spacetime with less complexity than in the continuum, yet the continuum is recovered by convergence in mean. Perhaps these prominent features may enable us to transcend the details of any particular discrete model gravitation and yield clues to help us discover how we may begin to quantize this fundamental interaction."

 

[marcus]

http://arxiv.org/abs/0804.0632
Reconstructing AdS/CFT
Laurent Freidel
34 pages
(Submitted on 4 Apr 2008)

"In this note we clarify the dictionary between pure quantum gravity on the bulk in the presence of a cosmological constant and a CFT on the boundary. We show for instance that there is a general correspondence between quantum gravity 'radial states' and a pair of CFT's. Restricting to one CFT is argued to correspond to states possessing an asymptotic infinity. This point of view allows us to address the problem of reconstructing the bulk from the boundary. And in the second part of this paper we present an explicit formula which gives, from the partition function of any 2 dimensional conformal field theory, a wave functional solution to the 3-dimensional Wheeler-DeWitt equation. This establishes at the quantum level a precise dictionary between 2d CFT and pure gravity."

http://arxiv.org/abs/0804.0672
Quantum Cosmology
Claus Kiefer, Barbara Sandhoefer
29 pages, 9 figures, contribution to "Beyond the Big Bang", ed. by R. Vaas (Springer 2008)
(Submitted on 4 Apr 2008)

"We give an introduction into quantum cosmology with emphasis on its conceptual parts. After a general motivation we review the formalism of canonical quantum gravity on which discussions of quantum cosmology are usually based. We then present the minisuperspace Wheeler--DeWitt equation and elaborate on the problem of time, the imposition of boundary conditions, the semiclassical approximation, the origin of irreversibility, and singularity avoidance. Restriction is made to quantum geometrodynamics; loop quantum gravity and string theory are discussed in other contributions to this volume."

http://arxiv.org/abs/0804.0619
A model for delayed emission in a very-high energy gamma-ray flare in Markarian 501
W. Bednarek (1), R. M. Wagner (2) ((1) University of Lodz, Poland (2) MPI für Physik, München, Germany)
3 pages, no figures, submitted to Astronomy & Astrophysics
(Submitted on 4 Apr 2008)

"Recently the MAGIC collaboration reported evidence for a delay in the arrival times of photons of different energies during a gamma-ray flare from the blazar Markarian 501 on 2005 July 9. We aim at describing the observed delayed high-energy emission. We apply a homogeneous synchrotron self-Compton (SSC) model under the assumption that the blob containing relativistic electrons was observed in its acceleration phase. Such a modified SSC model predicts the appearance of the gamma-ray flare first at lower energies and subsequently at higher energies. Moreover, we argue that the time delay between the flare observed at different energies depends on the gamma-ray energy. Based on the reported time delay of approx. 240 s between the flare observed at 190 GeV and 2.7 TeV, we predict it should be on the order of 1 h if it was observed between 10 GeV and 100 GeV. Such delay time scales can be tested in the future by simultaneous observations of flares with GLAST and Cherenkov telescopes."

http://arxiv.org/abs/0804.1098
Hybrid Quantum Gowdy Cosmology: Combining Loop and Fock Quantizations
Mercedes Martin-Benito, Luis J. Garay, Guillermo A. Mena Marugan
4 pages
(Submitted on 7 Apr 2008)

"We quantize the linearly polarized Gowdy T^3 family of cosmologies by combining loop and Fock techniques. The loop quantization of the degrees of freedom that describe the subfamily of homogeneous cosmologies proves sufficient to solve the initial singularity. We obtain the general expression of the solutions to the quantum constraints. These solutions are determined by their value on an initial spatial section, where they arise from nothing. They can be provided with a Hilbert structure that reproduces the conventional Fock quantization of the inhomogeneities."

 

[marcus]

http://arxiv.org/abs/0804.1686
On the origin of the particles in black hole evaporation
Ralf Schützhold, William G. Unruh
4 pages
(Submitted on 10 Apr 2008)

"We present an analytic derivation of Hawking radiation for an arbitrary (spatial) dispersion relation \omega(k) as a model for ultra-high energy deviations from general covariance. It turns out that the Hawking temperature is proportional to the product of the group d\omega/dk and phase \omega/k velocities evaluated at the frequency \omega of the outgoing radiation far away, which suggests that Hawking radiation is basically a low-energy phenomenon. Nevertheless, a group velocity growing too fast at ultra-short distances would generate Hawking radiation at ultra-high energies ('ultra-violet catastrophe') and hence should not be a realistic model for the microscopic structure of quantum gravity."

http://arxiv.org/abs/0804.1771
The cosmic variance of Omega
T. P. Waterhouse, J. P. Zibin
10 pages, 2 figures
(Submitted on 10 Apr 2008)

"How much can we know about our Universe? All of our observations are restricted to a finite volume, and therefore our estimates of presumably global cosmological parameters are necessarily based on incomplete information. Even assuming that the Standard Model of cosmology is correct, this means that some cosmological questions may be unanswerable. For example, is the curvature parameter Omega_K positive, negative, or identically zero? If its magnitude is sufficiently small, then due to cosmic variance no causal observation can ever answer that question. In this article, we first describe the gauge problems associated with defining the cosmic variance of cosmological parameters, then describe a solution involving the use of parameters defined on the surface of last scattering, and finally calculate the statistical variance of ideal measurements of the matter, radiation, and curvature density parameters. We find that Omega_K cannot be measured to better than about 1.5x10^(-5) (1 sigma), and that this limit has already begun to decrease due to the flattening effect of dark energy. Proposed 21 cm hydrogen experiments, for example, make this limit more than just a theoretical curiosity."

 

[nrqed]

The thread has a very interesting beginning where there was an attempt at a introductory level discussion of the concepts of loop quantum gravity. Would it be possible to start a new thread doing that? Where someone could start explaining the ideas of LQG at a level appropriate for, say, beginning graduate students.

That would be awesome.

 

[marcus]

Thanks nrqed, that's a good suggestion! There are people who post here who are graduate students in QG who, if not too busy, could do a great tutorial.
Maybe if I get inspired I'll start one and count on someone else taking over if it catches on.

http://arxiv.org/abs/0804.1797
Chern-Simons Modified Gravity as a Torsion Theory and its Interaction with Fermions
Stephon Alexander, Nicolas Yunes
11 pages, submitted to Phys. Rev. D
(Submitted on 10 Apr 2008)

"We study the tetrad formulation of Chern-Simons (CS) modified gravity, which adds a Pontryagin term to the Einstein-Hilbert action with a spacetime-dependent coupling field. We first verify that CS modified gravity leads to a theory with torsion, where this tensor is given by an antisymmetric product of the Riemann tensor and derivatives of the CS coupling. We then calculate the torsion in the far field of a weakly gravitating source within the parameterized post-Newtonian formalism, and specialize the result to Earth. We find that CS torsion vanishes only if the coupling vanishes, thus generically leading to a modification of gyroscopic precession, irrespective of the coupling choice. Perhaps most interestingly, we couple fermions to CS modified gravity via the standard Dirac action and find that these further correct the torsion tensor. Such a correction leads to two new results: (i) a generic enhancement of CS modified gravity by the Dirac equation and axial fermion currents; (ii) a new two-fermion interactions, mediated by an axial current and the CS correction. We conclude with a discussion of the consequences of these results in particle detectors and realistic astrophysical systems."

 

[marcus]

http://arxiv.org/abs/0804.2541
Phase space quantization and Loop Quantum Cosmology: A Wigner function for the Bohr-compactified real line
Christopher J. Fewster, Hanno Sahlmann
26 pages, 3 figures
(Submitted on 16 Apr 2008)

"We give a definition for the Wigner function for quantum mechanics on the Bohr compactification of the real line and prove a number of simple consequences of this definition. We then discuss how this formalism can be applied to loop quantum cosmology. As an example, we use the Wigner function to give a new quantization of an important building block of the Hamiltonian constraint."
http://arxiv.org/abs/0804.2475
The Void Phenomenon Explained
Jeremy L. Tinker (KICP, UChicago), Charlie Conroy (Princeton)
8 pages, 5 figures, submitted to ApJ
(Submitted on 16 Apr 2008)

"We use high-resolution N-body simulations, combined with a halo occupation model of galaxy bias, to investigate voids in the galaxy distribution. Our goal is to address the 'void phenomenon' of Peebles (2001), which presents the observed dearth of faint galaxies in voids as a challenge to the current cosmology. In our model, galaxy luminosity is determined only as a function of dark matter halo mass. With this simple assumption, we demonstrate that large, empty voids of ~15 Mpc/h in diameter are expected even for galaxies seven magnitudes fainter than L*. The predictions of our model are in excellent agreement with several statistical measures; (i) the luminosity function of galaxies in underdense regions, (ii) nearest neighbor statistics of dwarf galaxies, (iii) the void probability function of faint galaxies. In the transition between filaments and voids in the dark matter, the halo mass function changes abruptly, causing the maximum galaxy luminosity to decrease by ~5 magnitudes over a range of ~1 Mpc/h. Thus the boundary between filaments and voids in the galaxy distribution is nearly as sharp for dwarfs as for ~L* objects. These results support a picture in which galaxy formation is driven predominantly by the mass of the host dark matter halo, and is nearly independent of the larger-scale halo environment. Further, they demonstrate that LCDM, combined with a straightforward bias model, naturally explains the existence of the void phenomenon."

http://arxiv.org/abs/0804.2082
BTZ Black Hole Entropy: A spin foam model description
J.Manuel Garcia-Islas
9 pages, 2 figures
(Submitted on 13 Apr 2008)

"We present a microscopical explanation of the entropy of the BTZ black hole using discrete spin foam models of quantum gravity. The entropy of a black hole is given in geometrical terms which lead us to think that its statistical description must be given in terms of a quantum geometry. In this paper we present it in terms of spin foam geometrical observables at the horizon of the black hole."

 

[marcus]

http://arxiv.org/abs/0804.2720
Black Hole Thermodynamics and Lorentz Symmetry
Ted Jacobson, Aron C. Wall
4 pages, prepared for the 2008 Gravity Research Foundation Essay Competition
(Submitted on 17 Apr 2008)

"Recent developments point to a breakdown in the generalized second law of thermodynamics for theories with Lorentz symmetry violation. It appears possible to construct a perpetual motion machine of the second kind in such theories, using a black hole to catalyze the conversion of heat to work. Here we describe the arguments leading to that conclusion. We suggest the implication that Lorentz symmetry should be viewed as an emergent property of the macroscopic world, required by the second law of black hole thermodynamics."


http://arxiv.org/abs/0804.2778
Effects of the quantisation ambiguities on the Big Bounce dynamics
Orest Hrycyna, Jakub Mielczarek, Marek Szydlowski
26 pages, 10 figs
(Submitted on 17 Apr 2008)

"In this paper we investigate dynamics of the modified loop quantum cosmology models using dynamical systems methods. Modifications considered come from the choice of the different field strength operator \hat{F} and result in different forms of the effective Hamiltonian. Such an ambiguity of the choice of this expression from some class of functions is allowed in the framework of loop quantisation. Our main goal is to show how such modifications can influence the bouncing universe scenario in the loop quantum cosmology. In effective models considered we classify all evolutional paths for all admissible initial conditions. The dynamics is reduced to the form of a dynamical system of the Newtonian type on a 2-dimensional phase plane. These models are equivalent dynamically to the FRW models with the decaying effective cosmological term parametrised by the canonical variable p (or by the scale factor a). We find that for the positive cosmological constant there is a class of oscillating models without the initial and final singularities. The new phenomenon is the appearance of curvature singularities for the finite values of the scale factor, but we find that for the positive cosmological constant these singularities can be avoided. For the positive cosmological constant the evolution begins at the asymptotic state in the past represented by the deSitter contracting (deS_-) spacetime or the static Einstein universe H=0 or H=- infinity state and reaches the deSitter expanding state (deS₊) , the state H=0 or H=+infinity state. In the case of the negative cosmological constant we obtain the past and future asymptotic states as the Einstein static universes.

http://arxiv.org/abs/0804.2811
Spin Foam Perturbation Theory for Three-Dimensional Quantum Gravity
Joao Faria Martins, Aleksandar Mikovic
34 pages, 17 figures
(Submitted on 17 Apr 2008)

"We develop the spin foam perturbation theory for three-dimensional Euclidean Quantum Gravity. We analyse the perturbative expansion of the partition function in the dilute gas limit and argue that a conjecture due to Baez does not hold for arbitrary triangulations. However, the conjecture holds for a special class of triangulations which are based on subdivisions of certain 3-manifold cubulations. In this case we calculate the partition function."

 

[marcus]

http://arxiv.org/abs/0804.3157
Loop Quantization of Vacuum Bianchi I Cosmology
M. Martin-Benito, G. A. Mena Marugan, T. Pawlowski
10 pages
(Submitted on 19 Apr 2008)

"We analyze the loop quantization of the family of vacuum Bianchi I spacetimes, a gravitational system whose classical solutions describe homogeneous anisotropic cosmologies. We rigorously construct the operator that represents the Hamiltonian constraint, showing that the states of zero volume completely decouple from the rest of quantum states. This fact ensures that the classical cosmological singularity is resolved in the quantum theory. In addition, this allows us to adopt an equivalent quantum description in terms of a well defined densitized Hamiltonian constraint. This latter constraint can be regarded in a certain sense as a difference evolution equation in an internal time provided by one of the triad components, which is polymerically quantized. Generically, this evolution equation is a relation between the projection of the quantum states in three different sections of constant internal time. Nevertheless, around the initial singularity the equation involves only the two closest sections with the same orientation of the triad. This has a double effect: on the one hand, physical states are determined just by the data on one section, on the other hand, the evolution defined in this way never crosses the singularity, without the need of any special boundary condition. Finally, we provide these physical states with a Hilbert structure, completing the quantization."

http://arxiv.org/abs/0804.3365
Effective Constraints for Quantum Systems
Martin Bojowald, Barbara Sandhoefer, Aureliano Skirzewski, Artur Tsobanjan
40 pages
(Submitted on 21 Apr 2008)

"An effective formalism for quantum constrained systems is presented which allows manageable derivations of solutions and observables, including a treatment of physical reality conditions without requiring full knowledge of the physical inner product. Instead of a state equation from a constraint operator, an infinite system of constraint functions on the quantum phase space of expectation values and moments of states is used. The examples of linear constraints as well as the free non-relativistic particle in parameterized form illustrate how standard problems of constrained systems can be dealt with in this framework."

 

[marcus]

http://arxiv.org/abs/0804.3726
Quantum Geometry and Quantum Gravity
J. Fernando Barbero G.
To appear in the AIP Conference Proceedings of the XVI International Fall Workshop on Geometry and Physics, Lisbon - Portugal, 5-8 September 2007
(Submitted on 23 Apr 2008)

"The purpose of this contribution is to give an introduction to quantum geometry and loop quantum gravity for a wide audience of both physicists and mathematicians. From a physical point of view the emphasis will be on conceptual issues concerning the relationship of the formalism with other more traditional approaches inspired in the treatment of the fundamental interactions in the standard model. Mathematically I will pay special attention to functional analytic issues, the construction of the relevant Hilbert spaces and the definition and properties of geometric operators: areas and volumes."

http://arxiv.org/abs/0804.3765
Groups of generalized flux transformations in loop quantum gravity
J. M. Velhinho
6 pages
(Submitted on 23 Apr 2008)

"We present a group of transformations in the space of generalized connections that contains the set of transformations generated by the flux variables of loop quantum gravity. This group is labelled by certain SU(2)-valued functions on the bundle of directions in the spatial manifold. A further generalization is obtained by considering functions that depend on germs of analytic curves, rather than just on directions."

 

[marcus]

I can't evaluate this paper adequately (taking Brans-Dicke modified gravity seriously strikes me as highly innovative at this point). However two of the authors, Schuller and Wohlfarth, have good publication track records and have co-authored with well-knowns such as Paul Townsend and Mark Trodden. This is not the first paper of theirs developing this new approach. It's not safe to ignore it.

http://arxiv.org/abs/0804.4067
Brans-Dicke geometry
Raffaele Punzi, Frederic P. Schuller, Mattias N.R. Wohlfarth
8 pages
(Submitted on 25 Apr 2008)

"We reveal the non-metric geometry underlying omega-->0 Brans-Dicke theory by unifying the metric and scalar field into a single geometric structure. Taking this structure seriously as the geometry to which matter universally couples, we show that the theory is fully consistent with solar system tests. This is in striking constrast with the standard metric coupling, which grossly violates post-Newtonian experimental constraints."

Brief mention:

http://arxiv.org/abs/0804.4161
Discrete Quantum Gravity I
P. Kramer (University of Tuebingen) M. Lorente (University of Oviedo)
24 pages
(Submitted on 25 Apr 2008)

"...The crucial step for the Barrett-Crane model in Quantum Gravity is the description of the amplitude for the quantum 4-simplex that is used in the state sum partition function. We obtain the zonal spherical functions for the construction of the SO(4,R) invariant weight and associate them to the triangular faces of the 4-simplices."

http://arxiv.org/abs/0804.4162
Discrete Quantum Gravity II
P. Kramer (University of Tuebingen), M. Lorete (University of Oviedo)
19 pages
(Submitted on 25 Apr 2008)

"In part I of [1] we have developed the tensor and spin representation of SO(4) in order to apply it to the simplicial decomposition of the Barrett-Crane model. We attach to each face of a triangle the spherical function constructed from the Dolginov-Biedenharn function.
In part II we apply the same technique to the Lorentz invariant state sum model. We need three new ingredients: the classification of the edges and the corresponding subspaces that arises in the simplicial decomposition, the irreps of SL(2,C) and its isomorphism to the bivectors appearing in the 4-simplices, the need of a zonal spherical function from the intertwining condition of the tensor product for the simple representations attached to the faces of the simplicial decomposition."

 

[marcus]

Bill Unruh (remember the Unruh effect?) is a prominent figure in Quantum Gravity. A recent paper by Ashtekar et al was devoted to disposing of an objection to Loop Quantum Cosmology that Unruh had raised. Here is a new one from Unruh and Bojan Losic.

http://arxiv.org/abs/0804.4296
Cosmological perturbation theory near de Sitter spacetime
B. Losic, W.G. Unruh
Submitted to PRL
(Submitted on 27 Apr 2008)

"We present an argument that a nonlocal measure of second order metric and matter perturbations dominates that of linear fluctuations in its effect on the gravitational field in spacetimes close to de Sitter spacetime."

There seems to be growing interest in the quantum geometry/gravity of (near) deSitter space. The next paper is by lesser-known authors and already has been accepted for publication in the International Journal of Modern Physics D

http://arxiv.org/abs/0804.4326
Thermodynamics of noncommutative de Sitter spacetime
B. Vakili, N. Khosravi, H. R. Sepangi
11 pages, accepted for publication in IJMPD
(Submitted on 28 Apr 2008)

"We study the effects of noncommutativity of spacetime geometry on the thermodynamical properties of the de Sitter horizon. We show that noncommutativity results in modifications in temperature, entropy and vacuum energy and that these modifications are of order of the Planck scale, suggesting that the size of the noncommutative parameter should be close to that of the Planck. In an alternative way to deal with noncommutativity, we obtain a quantization rule for the entropy. Since noncommutativity in spacetime geometry modifies the Heisenberg algebra and introduces the general uncertainty principle, we also investigate the above problem in this framework."