Loop-and-allied QG bibliography

[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."

 

[Kea]

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

"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."

Fighting words indeed. Check out the table on page 6!

 

[marcus]

http://arxiv.org/abs/0705.2656
Inflationary universe in loop quantum cosmology
Xin Zhang, Yi Ling
25 pages, 5 figures

"Loop quantum cosmology provides a nice solution of avoiding the big bang singularity through a big bounce mechanism in the high energy region. In loop quantum cosmology an inflationary universe is emergent after the big bounce, no matter what matter component is filled in the universe. A super-inflation phase without phantom matter will appear in a certain way in the initial stage after the bounce; then the universe will undergo a normal inflation stage. We discuss the condition of inflation in detail in this framework. Also, for slow-roll inflation, we expect the imprint from the effects of the loop quantum cosmology should be left in the primordial perturbation power spectrum. However, we show that this imprint is too weak to be observed." http://arxiv.org/abs/0705.2629
Dual Computations of Non-abelian Yang-Mills on the Lattice
J. Wade Cherrington, Dan Christensen, Igor Khavkine
18 pages, 7 figures

"In the past several decades there have been a number of proposals for computing with dual forms of non-abelian Yang-Mills theories on the lattice. Motivated by the gauge-invariant, geometric picture offered by dual models and successful applications of duality in the U(1) case, we revisit the question of whether it is practical to perform numerical computation using non-abelian dual models. Specifically, we consider three-dimensional SU(2) pure Yang-Mills as an accessible yet non-trivial case in which the gauge group is non-abelian. Using methods developed recently in the context of spin foam quantum gravity, we describe a Metropolis algorithm for sampling the dual ensemble and efficiently computing the dual amplitude. We relate our algorithms to prior work in non-abelian dual computations of Hari Dass and his collaborators, addressing several problems that have (to the best our knowledge) been left open. We report results of spin expectation value computations over a range of lattice sizes and couplings that are in agreement with our conventional lattice computations. We conclude with an outlook on further development of dual methods and their application to problems of current interest."

brief mention:
http://arxiv.org/abs/0705.2643
Symmetries, Singularities and the De-Emergence of Space
Thibault Damour, Hermann Nicolai
10 pages

though in a very different formal context, this paper reminded me of the recent work of Kirill Krasnov and Yuri Shtanov----where the metric disappears near the singularity, but spacetime evolution continues because the theory of gravity is non-metric. you could say that in Krasnov-Shtanov non-metric gravity the conventional idea of space also "de-emerges" near a classical singularity---while the classical singularity is removed. For the Krasnov-Shtanov paper see post #591 or http://arxiv.org/abs/0705.2047

EDITED IN to reply to francesca's next post:[/color]
Dear francesca, thanks for helping by supplying the papers of Corichi et al and Padmanabhan. However look back at post #595
https://www.physicsforums.com/showpost.php?p=1334313&postcount=595
from four days ago. they are the first two papers mentioned in that post, IIRC. :-)

 

[francesca]

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

Abstract: 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
Authors: T. Padmanabhan

Abstract: 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).

 

[marcus]

http://arxiv.org/abs/0705.3024
Symmetries, Horizons, and Black Hole Entropy
S. Carlip
6 pages; first prize essay, 2007 Gravity Research Foundation essay contest

"Black holes behave as thermodynamic systems, and a central task of any quantum theory of gravity is to explain these thermal properties. A statistical mechanical description of black hole entropy once seemed remote, but today we suffer an embarrassment of riches: despite counting very different states, many inequivalent approaches to quantum gravity obtain identical results. Such 'universality' may reflect an underlying two-dimensional conformal symmetry near the horizon, which can be powerful enough to control the thermal characteristics independent of other details of the theory. This picture suggests an elegant description of the relevant degrees of freedom as Goldstone-boson-like excitations arising from symmetry breaking by the conformal anomaly."

 

[ensabah6]

Hey Marcus, I do not know how to search this thread but did you ever post this paper?

http://arxiv.org/abs/gr-qc/0612134
From Quantum Hydrodynamics to Quantum Gravity
G.E. Volovik
(Submitted on 21 Dec 2006 (v1), last revised 17 Jan 2007 (this version, v5))
We discuss some lessons from quantum hydrodynamics to quantum gravity.
Comments: 20 pages, 1 figure, rapporteur article for Proceedings of MG11, session `Analog Models of and for General Relativity', references added
Subjects: General Relativity and Quantum Cosmology (gr-qc); Soft Condensed Matter (cond-mat.soft); High Energy Physics - Phenomenology (hep-ph)
Cite as: arXiv:gr-qc/0612134v5

 

[marcus]

http://arxiv.org/abs/0705.3892
Spin foam model from canonical quantization
Sergei Alexandrov
24 pages
(Submitted on 26 May 2007)

"We suggest a modification of the Barrett-Crane spin foam model of 4-dimensional Lorentzian general relativity motivated by the canonical quantization. The starting point is Lorentz covariant loop quantum gravity. Its kinematical Hilbert space is found as a space of the so-called projected spin networks. These spin networks are identified with the boundary states of a spin foam model and provide a generalization of the unique Barrette-Crane intertwiner. We propose a way to modify the Barrett-Crane quantization procedure to arrive at this generalization: the B field (bi-vectors) should be promoted not to generators of the gauge algebra, but to their certain projection. The modification is also justified by the canonical analysis of Plebanski formulation. Finally, we compare our construction with other proposals to modify the Barret-Crane model."

 

[francesca]

cosmology

http://arxiv.org/abs/0705.4398"
The Dark Side of a Patchwork Universe
Authors: Martin Bojowald

Abstract: While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often http://arxiv.org/abs/associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.

I always like Bojowald jobs, and this one seems very readable at the first sight...
Loop Quantum Cosmology could be the main issue of the month
(see below how many papers this week )
and the conference on phenomenology is coming, we have the program now:
http://www.sissa.it/app/QGconference/program.htm"

http://arxiv.org/abs/0705.4449"
Title: Relic gravitons as the observable for Loop Quantum Cosmology
Authors: Jakub Mielczarek, Marek Szydlowski

http://arxiv.org/abs/0705.3741"
Title: Black hole state counting in loop quantum gravity
Authors: P. Mitra

http://arxiv.org/abs/0705.3375"
Title: Unsuccessful cosmology with Modified Gravity Models
Authors: Antonio De Felice, Mark Hindmarsh

http://arxiv.org/abs/0705.4002"
Title: Dynamics of Quintom and Hessence Energies in Loop Quantum Cosmology
Authors: Hao Wei, Shuang Nan Zhang

 

[marcus]

thanks for spotting these, francesca, especially the Bojowald.
Here are some more that appeared in the past week:
http://arxiv.org/abs/0706.0471http://arxiv.org/abs/0706.0469
Properties of the Volume Operator in Loop Quantum Gravity I: Results
Johannes Brunnemann, David Rideout
37 pages, 7 figures
(Submitted on 4 Jun 2007)

"We analyze the spectral properties of the volume operator of Ashtekar and Lewandowski in Loop Quantum Gravity, which is the quantum analogue of the classical volume expression for regions in three dimensional Riemannian space. Our analysis considers for the first time generic graph vertices of valence greater than four. Here we find that the geometry of the underlying vertex characterizes the spectral properties of the volume operator, in particular the presence of a `volume gap' (a smallest non-zero eigenvalue in the spectrum) is found to depend on the vertex embedding. We compute the set of all non-spatially diffeomorphic non-coplanar vertex embeddings for vertices of valence 5--7, and argue that these sets can be used to label spatial diffeomorphism invariant states. We observe how gauge invariance connects vertex geometry and representation properties of the underlying gauge group in a natural way. Analytical results on the spectrum on 4-valent vertices are included, for which the presence of a volume gap is proved. This paper presents our main results; details are provided by a companion paper arXiv:0706.0382v1."

http://arxiv.org/abs/0706.0382
Properties of the Volume Operator in Loop Quantum Gravity II: Detailed Presentation
Authors: Johannes Brunneman, David Rideout
95 pages, 65 figures
(Submitted on 4 Jun 2007)

"The properties of the Volume operator in Loop Quantum Gravity, as constructed by Ashtekar and Lewandowski, are analyzed for the first time at generic vertices of valence greater than four. The present analysis benefits from the general simplified formula for matrix elements of the Volume operator derived in gr-qc/0405060, making it feasible to implement it on a computer as a matrix which is then diagonalized numerically. The resulting eigenvalues serve as a database to investigate the spectral properties of the volume operator. Analytical results on the spectrum at 4-valent vertices are included. This is a companion paper to arXiv:0706.0469, providing details of the analysis presented there."

See also companion paper arXiv:0706.0469

http://arxiv.org/abs/0706.0283
Cosmography in testing loop quantum gravity
Marek Szydlowski, Wlodzimierz Godlowski, Tomasz Stachowiak
19 pages, 1 figure
(Submitted on 2 Jun 2007)

"It was recently suggested by Martin Bojowald that quantum gravity effects give rise to new, potentially observable effects. We check whether this is the case for astronomical tests by trying to constrain the density parameters of the Friedmann equation with a $(-)(1+z)^6$ type of contribution. We describe different interpretations of such an additional term: geometric effects of Loop Quantum Cosmology, effects of braneworld cosmological models, non-standard cosmological models in metric-affine gravity, and models with spinning fluid. Kinematical (or geometrical) tests based on null geodesics are insufficient to separate individual matter components when they behave like perfect fluid and scale in the same way. Still, it is possible to measure their overall effect. We use recent measurements of the coordinate distances from Fanaroff-Riley type IIb (FRIIb) radio galaxy (RG) data, supernovae type Ia (SNIa) data, baryon oscillation peak and cosmic microwave background radiation (CMBR) observations to obtain stronger bounds for the contribution of the considered type. We demonstrate that, while rho^2 corrections are very small, they can be tested by astronomical observations -- at least in principle. Bayesian criteria of model selection (Bayesian factor, AIC, and BIC) are used to check if additional parameters are detectable in the present epoch. As it turns out, the LambdaCDM model is favoured over the bouncing model driven by loop quantum effects. Or, in other words, the bounds obtained from cosmography are very weak, and from the point of view of the present data this model is indistinguishable from the LambdaCDM one."

http://arxiv.org/abs/0706.0174
Entropy signature of the running cosmological constant
Authors: Alfio Bonanno, Martin Reuter
57 pages, 7 figures
(Submitted on 1 Jun 2007 (v1), last revised 3 Jun 2007 (this version, v2))

"Renormalization group (RG) improved cosmologies based upon a RG trajectory of Quantum Einstein Gravity (QEG) with realistic parameter values are investigated using a system of cosmological evolution equations which allows for an unrestricted energy exchange between the vacuum and the matter sector. It is demonstrated that the scale dependence of the gravitational parameters, the cosmological constant in particular, leads to an entropy production in the matter system. The picture emerges that the Universe started out from a state of vanishing entropy, and that the radiation entropy observed today is essentially due to the coarse graining (RG flow) in the quantum gravity sector which is related to the expansion of the Universe. Furthermore, the RG improved field equations are shown to possesses solutions with an epoch of power law inflation immediately after the initial singularity. The inflation is driven by the cosmological constant and ends automatically once the RG running has reduced the vacuum energy to the level of the matter energy density."

http://arxiv.org/abs/0706.0179
Lattice Refining Loop Quantum Cosmology and Inflation
William Nelson, Mairi Sakellariadou (King's College, London)
12 pages
(Submitted on 1 Jun 2007)

"We study the importance of lattice refinement in achieving a successful inflationary era. We solve, in the continuum limit, the second order difference equation governing the quantum evolution in loop quantun cosmology, assuming both a fixed and a dynamically varying lattice in a suitable refinement model. We thus impose a constraint on the potential of a scalar field, so that the continuum approximation is not broken. Considering that such a scalar field could play the role of the inflaton, we obtain a second constraint on the inflationary potential so that there is consistency with the CMB data on large angular scales. For a $m^2\phi^2/2$ inflationary model, we combine the two constraints on the inflaton potential to impose an upper limit on $m$, which is severely fine-tuned in the case of a fixed lattice. We thus conclude that lattice refinement is necessary to achieve a natural inflationary model."http://arxiv.org/abs/0706.0142
Quantum gravity phenomenology via Lorentz violations
Stephano Liberati
21 pages, 1 figure
(Submitted on 1 Jun 2007)

"The search for a quantum theory of gravity has been one of the main aims of theoretical physics for many years by now. However the efforts in this direction have been often hampered by the lack of experimental/observational tests able to select among, or at least constrain, the numerous quantum gravity models proposed so far. This situation has changed in the last decade thanks to the realization that some QG inspired violations of Lorentz symmetry could be constrained using current experiments and observations. This study it is not only allowing us to test at higher and higher energies a fundamental symmetry of spacetime but it is also providing us with hints and perspectives about the fundamental nature of gravity."

 

[marcus]

http://arxiv.org/abs/0706.1057
Effective equations for isotropic quantum cosmology including matter
Martin Bojowald, Hector Hernandez, Aureliano Skirzewski
42 pages
(Submitted on 7 Jun 2007)

"Effective equations often provide powerful tools to develop a systematic understanding of detailed properties of a quantum system. This is especially helpful in quantum cosmology where several conceptual and technical difficulties associated with the full quantum equations can be avoided in this way. Here, effective equations for Wheeler-DeWitt and loop quantizations of spatially flat, isotropic cosmological models sourced by a massive or interacting scalar are derived and studied. The resulting systems are remarkably different from that given for a free, massless scalar. This has implications for the coherence of evolving states and the realization of a bounce in loop quantum cosmology. http://arxiv.org/abs/0706.0985
Bulk Entropy in Loop Quantum Gravity
Etera R. Livine, Daniel R. Terno
13 pages
(Submitted on 7 Jun 2007)

"In the framework of loop quantum gravity (LQG), having quantum black holes in mind, we generalize the previous boundary state counting (gr-qc/0508085) to a full bulk state counting. After a suitable gauge fixing we are able to compute the bulk entropy of a bounded region (the "black hole") with fixed boundary. This allows us to study the relationship between the entropy and the boundary area in details and we identify a holographic regime of LQG where the leading order of the entropy scales with the area. We show that in this regime we can fine tune the factor between entropy and area without changing the Immirzi parameter."

 

[marcus]

http://arxiv.org/abs/0706.1534
Coupling gauge theory to spinfoam 3d quantum gravity
Simone Speziale
18 pages
(Submitted on 11 Jun 2007)

"We construct a spinfoam model for Yang-Mills theory coupled to quantum gravity in three dimensional riemannian spacetime. We define the partition function of the coupled system as a power series in g_0^2 G that can be evaluated order by order using grasping rules and the recoupling theory. With respect to previous attempts in the literature, this model assigns the dynamical variables of gravity and Yang-Mills theory to the same simplices of the spinfoam, and it thus provides transition amplitudes for the spin network states of the canonical theory. For SU(2) Yang-Mills theory we show explicitly that the partition function has a semiclassical limit given by the Regge discretization of the classical Yang-Mills action."

http://arxiv.org/abs/0706.1279
The wedding ring of MOND and non-exotic dark matter
B. Famaey, G.W. Angus, G. Gentile, H.S. Zhao
2 pages, research note submitted to A&A
(Submitted on 9 Jun 2007)

"The lensing mass reconstruction of the rich galaxy cluster Cl0024+17 has been argued to have revealed a ringlike dark matter structure that is offset from both the gas and the galaxies in the cluster. This has been claimed to be hard to explain in the framework of modified Newtonian dynamics (MOND). We aim to check that claim, taking into account the old-known mass discrepancy of galaxy clusters in MOND, which can be resolved by, e.g., adding a component of 2 eV neutrinos. For that purpose, we derive an upper limit to the density of matter in the ring, and compare it to the Tremaine-Gunn limit on the density of neutrinos. We conclude that the maximum density of matter in the ring is two sigmas below the Tremaine-Gunn limit, and that the ringlike structure in Cl0024+17 does not pose a new challenge to MOND."

http://arxiv.org/abs/0706.1654
Gravity as an emergent phenomenon: A conceptual description
T. Padmanabhan
11 pages; Summary of several plenary talks in the conferences including XXIII Texas Symposium in Relativistic Astrophysics (Melbourne, Dec, 06); IAGRG Meeting (Delhi, Feb, 07); International workshop on theoretical high energy physics (Roorkee, Mar, 2007); to appear in the proceedings of IWTHEP
(Submitted on 12 Jun 2007)

"I describe several broad features of a programme to understand gravity as an emergent, long wavelength, phenomenon (like elasticity) and discuss one concrete framework for realizing this paradigm in the backdrop of several recent results."

 

[marcus]

http://arxiv.org/abs/0706.1899
Spacetime Lagrangian Formulation of Barbero-Immirzi Gravity
L. Fatibene, M. Francaviglia, C. Rovelli
12 pages
(Submitted on 13 Jun 2007)

"We shall here discuss a new spacetime gauge-covariant Lagrangian formulation of General Relativity by means of the Barbero-Immirzi SU(2)-connection on spacetime. To the best of our knowledge the Lagrangian based on SU(2) spacetime fields seems to appear here for the first time."

the above is a continuation of work posted earlier this year by the same authors:

http://arxiv.org/abs/gr-qc/0702134
On a Covariant Formulation of the Barbero-Immirzi Connection
L. Fatibene, M. Francaviglia, C. Rovelli
13 pages
(Submitted on 26 Feb 2007)

"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/0706.1979
Note on black hole radiation spectrum in Loop Quantum Gravity
Jacobo Diaz-Polo, Enrique Fernandez-Borja
4 pages, 2 figures
(Submitted on 14 Jun 2007)

"Recent detailed analysis within the Loop Quantum Gravity calculation of black hole entropy show a stair-like structure in the behavior of entropy as a function of horizon area. The non-trivial distribution of the degeneracy of the black hole horizon area eigenstates is at the origin of this behavior. This degeneracy distribution is analyzed and a phenomenological model is put forward to study the possible implications of this distribution in the black hole radiation spectrum."

 

[marcus]

Accidentally duplicated a post, so I'll edit to avoid repeating content. An important aspect of the papers being posted on arxiv is their rate of publication in peer-review journals. Prime examples of good places to publish Loop cosmology and gravity research are Physical Review D and Physical Review Letters. As a sample publication history: in 2006 Martin Bojowald posted 12 papers on arxiv of which 4 were published in Physical Review D.

For completeness, at the end of the list I'll tally the publication results.

7. arXiv:astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
Phys.Rev.Lett. 98 (2007) 031301

8. arXiv:gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
Phys.Rev. D75 (2007) 064022

9. arXiv:gr-qc/0609057
Hamiltonian cosmological perturbation theory with loop quantum gravity corrections
Martin Bojowald, Hector H. Hernández, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
24 pages, 1 figure
Phys.Rev. D74 (2006) 123512

10. arXiv:gr-qc/0609034
Loop quantum cosmology and inhomogeneities
Martin Bojowald
25 pages, 1 figure
Gen.Rel.Grav. 38 (2006) 1771-1795

11. arXiv:gr-qc/0608100
Large scale effective theory for cosmological bounces
Martin Bojowald
5 pages, 1 figure
Phys. Rev. D 75 (2007) 081301(R)

12. arXiv:gr-qc/0607130
Quantum Geometry and its Implications for Black Holes
Martin Bojowald
16 pages, Plenary talk at 'Einstein's Legacy in the New Millenium,' Puri, India, December 2005
Int.J.Mod.Phys. D15 (2006) 1545-1559

13. arXiv:hep-th/0606232
Quantum Gravity and Higher Curvature Actions
Martin Bojowald, Aureliano Skirzewski
28 pages, based on a lecture course at the 42nd Karpacz Winter School of Theoretical Physics ``Current Mathematical Topics in Gravitation and Cosmology,'' Ladek, Poland, February 6-11, 2006
Int.J.Geom.Meth.Mod.Phys. 4 (2007) 25-52

14. arXiv:gr-qc/0606082
Loop cosmological implications of a non-minimally coupled scalar field
Martin Bojowald, Mikhail Kagan
10 pages, 4 figures
Phys.Rev. D74 (2006) 044033

15. arXiv:gr-qc/0604105
Singularities in Isotropic Non-Minimal Scalar Field Models
M. Bojowald, M. Kagan
12 pages
Class.Quant.Grav. 23 (2006) 4983-4990

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

17. arXiv:gr-qc/0602100
Quantum Riemannian Geometry and Black Holes
Martin Bojowald
45 pages, 4 figures, chapter of "Trends in Quantum Gravity Research" (Nova Science)

18. arXiv:gr-qc/0601085
Loop Quantum Cosmology
Martin Bojowald
104 pages, 10 figures; online version, containing 6 movies, available at this http URL
Living Rev.Rel. 8 (2005) 11

Of the dozen 2006 papers, those published in Physical Review D (PRD) were 4
in Physical Review Letters (PRL) 1
in Classical and Quantum Gravity (CQG) 1
in General Relativity and Gravitation (GRG) 1
in International Journal of Modern Physics 1
in International Journal of Geometrical Methods in Modern Physics 1
in Living Reviews in Relativity 1
chapters in books 2

 

[marcus]

http://arxiv.org/abs/0706.2342
Gravitational Wilson Loop and Large Scale Curvature
Herbert W. Hamber, Ruth M. Williams
22 pages, 6 figures
(Submitted on 15 Jun 2007)

"In a quantum theory of gravity the gravitational Wilson loop, defined as a suitable quantum average of a parallel transport operator around a large near-planar loop, provides important information about the large-scale curvature properties of the geometry. Here we shows that such properties can be systematically computed in the strong coupling limit of lattice regularized quantum gravity, by performing a local average over rotations, using an assumed near-uniform measure in group space. We then relate the resulting quantum averages to an expected semi-classical form valid for macroscopic observers, which leads to an identification of the gravitational correlation length appearing in the Wilson loop with an observed large-scale curvature. Our results suggest that strongly coupled gravity leads to a positively curved (De Sitter-like) quantum ground state, implying a positive effective cosmological constant at large distances.[/color]"

Bojowald also recently showed that a positive effective cosmological constant may not need to be put in "by hand" but could be *derived* from quantum gravity. Is there some connection with Hamber and Williams result? Francesca reported this Bojowald paper a few posts back in this thread, and Jal in a separate thread, but I will repeat it since it seems especially important:

http://arxiv.org/abs/0705.4398
The Dark Side of a Patchwork Universe
Martin Bojowald
24 pages, 2 figures, Contribution to the special issue on Dark Energy by Gen. Rel. Grav
(Submitted on 30 May 2007)

While observational cosmology has recently progressed fast, it revealed a serious dilemma called dark energy: an unknown source of exotic energy with negative pressure driving a current accelerating phase of the universe. All attempts so far to find a convincing theoretical explanation have failed, so that one of the last hopes is the yet to be developed quantum theory of gravity. In this article, loop quantum gravity is considered as a candidate, with an emphasis on properties which might play a role for the dark energy problem. Its basic feature is the discrete structure of space, often associated with quantum theories of gravity on general grounds. This gives rise to well-defined matter Hamiltonian operators and thus sheds light on conceptual questions related to the cosmological constant problem. It also implies typical quantum geometry effects which, from a more phenomenological point of view, may result in dark energy[/color]. In particular the latter scenario allows several non-trivial tests which can be made more precise by detailed observations in combination with a quantitative study of numerical quantum gravity. If the speculative possibility of a loop quantum gravitational origin of dark energy[/color] turns out to be realized, a program as outlined here will help to hammer out our ideas for a quantum theory of gravity, and at the same time allow predictions for the distant future of our universe.


Meanwhile Michael Turner, apparently unaware of Bojowald's result, or the Hamber Williams paper, surveys the problem.

http://arxiv.org/abs/0706.2186
Cosmic Acceleration, Dark Energy and Fundamental Physics
Michael S. Turner, Dragan Huterer (KICP, University of Chicago)
10 pages, 8 figures, invited review for Journal of the Physical Society of Japan, in press
(Submitted on 14 Jun 2007)

"A web of interlocking observations has established that the expansion of the Universe is speeding up and not slowing, revealing the presence of some form of repulsive gravity. Within the context of general relativity the cause of cosmic acceleration is a highly elastic (p\sim -rho), very smooth form of energy called 'dark energy' accounting for about 75% of the Universe. The 'simplest' explanation for dark energy is the zero-point energy density associated with the quantum vacuum; however, all estimates for its value are many orders-of-magnitude too large. Other ideas for dark energy include a very light scalar field or a tangled network of topological defects. An alternate explanation invokes gravitational physics beyond general relativity. Observations and experiments underway and more precise cosmological measurements and laboratory experiments planned for the next decade will test whether or not dark energy is the quantum energy of the vacuum or something more exotic, and whether or not general relativity can self consistently explain cosmic acceleration. Dark energy is the most conspicuous example of physics beyond the standard model and perhaps the most profound mystery in all of science."

brief mention:
Masters Thesis in LQG by Muxin Han, a student of Jorge Pullin at LSU
http://arxiv.org/abs/0706.2623
Quantum Dyanmics of Loop Quantum Gravity
95 pages

http://arxiv.org/abs/0706.2445
GRB Cosmology
Volker Bromm, Abraham Loeb
24 pages, 9 figures, review to appear in "Gamma-ray Bursts" (CUP)
(Submitted on 18 Jun 2007)

"Current observations are about to open up a direct window into the final frontier of cosmology: the first billion years in cosmic history when the first stars and galaxies formed. Even before the launch of the James Webb Space Telescope, it might be possible to utilize Gamma-ray Bursts (GRBs) as unique probes of cosmic star formation and the state of the intergalactic medium (IGM) up to redshifts of several tens, when the first (Population III) stars had formed. The Swift mission, or future satellites such as EXIST, might be the first observatories to detect individual Population III stars, provided that massive metal-free stars were able to trigger GRBs. Spectroscopic follow-up observations of the GRB afterglow emission would allow to probe the ionization state and metal enrichment of the IGM as a function of redshift."

 

[marcus]

http://arxiv.org/abs/0706.3658
Field theory on kappa--Minkowski space revisited: Noether charges and breaking of Lorentz symmetry
L. Freidel, J. Kowalski-Glikman, S. Nowak
22 pages, 1 figure
(Submitted on 25 Jun 2007)

"This paper is devoted to detailed investigations of free scalar field theory on $\kappa$-Minkowski space. After reviewing necessary mathematical tools we discuss in depth the Lagrangian and solutions of field equations. We analyze the spacetime symmetries of the model and construct the conserved charges associated with translational and Lorentz symmetry. We show that the version of the theory usually studied breaks Lorentz invariance in a subtle way: There is an additional trans-Planckian mode present, and an associated conserved charge (the number of such modes) is not a Lorentz scalar."

http://arxiv.org/abs/0706.3688
Why the Standard Model
Ali H. Chamseddine, Alain Connes
13 pages
(Submitted on 25 Jun 2007)

"The Standard Model is based on the gauge invariance principle with gauge group U(1)xSU(2)xSU(3) and suitable representations for fermions and bosons, which are begging for a conceptual understanding. We propose a purely gravitational explanation: space-time has a fine structure given as a product of a four dimensional continuum by a finite noncommutative geometry F. The raison d'etre for F is to correct the K-theoretic dimension from four to ten (modulo eight). We classify the irreducible finite noncommutative geometries of K-theoretic dimension six and show that the dimension (per generation) is a square of an integer k. Under an additional hypothesis of quaternion linearity, the geometry which reproduces the Standard Model is singled out (and one gets k=4)with the correct quantum numbers for all fields. The spectral action applied to the product MxF delivers the full Standard Model,with neutrino mixing, coupled to gravity, and makes predictions(the number of generations is still an input)."

http://arxiv.org/abs/0706.3690
A Dress for SM the Beggar
Ali H. Chamseddine
4 pages
(Submitted on 25 Jun 2007)

"The purpose of this letter is to remove the arbitrariness of the ad hoc choice of the algebra and its representation in the noncommutative approach to the Standard Model, which was begging for a conceptual explanation. We assume as before that space-time is the product of a four-dimensional manifold by a finite noncommmutative space F. The spectral action is the pure gravitational action for the product space. To remove the above arbitrariness, we classify the irreducibe geometries F consistent with imposing reality and chiral conditions on spinors, to avoid the fermion doubling problem, which amounts to have total dimension 10 (in the K-theoretic sense). It gives, almost uniquely, the Standard Model with all its details, predicting the number of fermions per generation to be 16, their representations and the Higgs breaking mechanism, with very little input. The geometrical model is valid at the unification scale, and has relations connecting the gauge couplings to each other and to the Higgs coupling. This gives a prediction of the Higgs mass of around 170 GeV and a mass relation connecting the sum of the square of the masses of the fermions to the W mass square, which enables us to predict the top quark mass compatible with the measured experimental value. We thus manage to have the advantages of both SO(10) and Kaluza-Klein unification, without paying the price of plethora of Higgs fields or the infinite tower of states." http://arxiv.org/abs/0706.3586
Graceful exit via polymerization of pre-big bang cosmology
Giuseppe De Risi, Roy Maartens, Parampreet Singh
(Submitted on 25 Jun 2007)

"We consider a phenomenological modification of the Pre Big Bang scenario using ideas from the resolution of curvature singularities in Loop Quantum Cosmology. We show that non-perturbative Loop modifications to the dynamics, arising from the underlying polymer representation, can resolve the graceful exit problem. The curvature and the dilaton energy stay finite at all times, in both the string and Einstein frames. In the string frame, the dilaton tends to a constant value at late times after the bounce."

Subjects: High Energy Physics - Theory (hep-th)
Report number: IGPG-07/6-9http://arxiv.org/abs/0706.3239
Black hole entropy, curved space and monsters
Stephen D.H. Hsu, David Reeb
4 pages
(Submitted on 21 Jun 2007)

"We investigate the microscopic origin of black hole entropy, in particular the gap between the maximum entropy of ordinary matter and that of black holes. Using curved space, we construct configurations with entropy greater than their area in Planck units. These configurations have pathological properties and we refer to them as monsters. When monsters are excluded we recover the entropy bound on ordinary matter S < A^3/4. This bound implies that essentially all of the microstates of a semiclassical black hole are associated with the growth of a slightly smaller black hole which absorbs some additional energy. Our results suggest that the area entropy of black holes is the logarithm of the number of distinct ways in which one can form the black hole from ordinary matter and smaller black holes, but only after the exclusion of monster states."

Steve Hsu is a proven researcher with a good publication record. Although this paper sounds very strange, I'll gamble on it being of interest.