Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/gr-qc/0610060
A perturbative approach to Dirac observables and their space-time algebra
Bianca Dittrich, Johannes Tambornino
23 pages

"We introduce a general approximation scheme in order to calculate gauge invariant observables in the canonical formulation of general relativity. Using this scheme we will show how the observables and the dynamics of field theories on a fixed background or equivalently the observables of the linearized theory can be understood as an approximation to the observables in full general relativity. Gauge invariant corrections can be calculated up to an arbitrary high order and we will explicitly calculate the first non--trivial correction. Furthermore we will make a first investigation into the Poisson algebra between observables corresponding to fields at different space--time points and consider the locality properties of the observables."

 

[marcus]

http://arxiv.org/abs/gr-qc/0610072
Hamiltonian and physical Hilbert space in polymer quantum mechanics
Alejandro Corichi, Tatjana Vukasinac, Jose A. Zapata
17 pages, 2 figures

"In this paper, a version of polymer quantum mechanics, which is inspired by loop quantum gravity, is considered and shown to be equivalent, in a precise sense, to the standard, experimentally tested, Schroedinger quantum mechanics. The kinematical cornerstone of our framework is the so called polymer representation of the Heisenberg-Weyl (H-W) algebra, which is the starting point of the construction. The dynamics is constructed as a continuum limit of effective theories characterized by a scale, and requires a renormalization of the inner product. The result is a physical Hilbert space in which the continuum Hamiltonian can be represented and that is unitarily equivalent to the Schroedinger representation of quantum mechanics. As a concrete implementation of our formalism, the simple harmonic oscillator is fully developed."

http://arxiv.org/abs/gr-qc/0610074
Gravitational collapse in loop quantum gravity
Leonardo Modesto
16 pages, 2 figures

"In this paper we study the gravitational collapse in loop quantum gravity. We consider the space-time region inside the Schwarzschild black hole event horizon and we divide this region in two parts, the first one where the matter (dust matter) is localized and the other (outside) where the metric is Kantowski-Sachs type. We calculate the state solving Hamiltonian constraint and we obtain a set of three difference equations that give a regular and natural evolution beyond the classical singularity point in "r=0" localized."

http://arxiv.org/abs/hep-th/0610164
Modified gravity as an alternative for Lambda-CDM cosmology
Shin'ichi Nojiri, Sergei D. Odintsov
9 pages, prepared for Special Issue IRGAC 2006, Barcelona

"The reconstruction scheme is developed for modified f(R) gravity with realistic matter (dark matter, baryons, radiation). Two versions of such theory are constructed: the first one describes the sequence of radiation and matter domination, decceleration-acceleration transition and acceleration era and the second one is reconstructed from exact Lambda-CDM cosmology. The asymptotic behaviour of first model at late times coincides with the theory containing positive and negative powers of curvature while second model approaches to General Relativity without singularity at zero curvature."

 

[marcus]

http://arxiv.org/abs/hep-th/0610193
Lessons from the LQG String
Robert C. Helling (IU Bremen)

"We give a non-technical description of the differences of quantisation of the bosonic string between the usual Fock-space approach and the treatment inspired by methods of loop quantum gravity termed the LCQ string. We point out the role of covariant states with continuous representations of the Weyl operators versus invariant states leading to discontinuous polymer representations. In the example of the harmonic oscillator we compare the optical absorption spectrum for the two quantisations and find that the question of distinguishability depends on the order in which limits are taken: For a fixed UV cut-off restricting the Hilbert space to a finite dimensional subspace the spectra can be made arbitrarily similar by an appropriate choice of state. However, if the states are chosen first, they differ at high frequencies."http://arxiv.org/abs/hep-th/0610194
Quantum Gravity as a Broken Symmetry Phase of a BF Theory
A. Mikovic
6 pages, talk given at the LOR2006 Symposium, Budapest, 22-24 June 2006

"We explain how General Relativity with a cosmological constant arises as a broken symmetry phase of a BF theory. In particular we show how to treat de Sitter and anti-de Sitter cases simultaneously. This is then used to formulate a quantization of General Relativity through a spin foam perturbation theory. We then briefly discuss how to calculate the effective action in this quantization procedure."

This paper fails to cite the prior work of Freidel Starodubtsev, to which it is rather similar.
http://arxiv.org/abs/hep-th/0501191
Quantum gravity in terms of topological observables
Laurent Freidel, Artem Starodubtsev
19 pages

"We recast the action principle of four dimensional General Relativity so that it becomes amenable for perturbation theory which doesn't break general covariance. The coupling constant becomes dimensionless (G_{Newton} \Lambda) and extremely small 10^{-120}. We give an expression for the generating functional of perturbation theory. We show that the partition function of quantum General Relativity can be expressed as an expectation value of a certain topologically invariant observable. This sets up a framework in which quantum gravity can be studied perturbatively using the techniques of topological quantum field theory."

 

[marcus]

http://arxiv.org/abs/hep-th/0610241
Gravity and the standard model with neutrino mixing
Ali H. Chamseddine, Alain Connes, Matilde Marcolli
71 pages, 7 figures

"We present an effective unified theory based on noncommutative geometry for the standard model with neutrino mixing, minimally coupled to gravity. The unification is based on the symplectic unitary group in Hilbert space and on the spectral action. It yields all the detailed structure of the standard model with several predictions at unification scale. Besides the familiar predictions for the gauge couplings as for GUT theories, it predicts the Higgs scattering parameter and the sum of the squares of Yukawa couplings. From these relations one can extract predictions at low energy, giving in particular a Higgs mass around 170 GeV and a top mass compatible with present experimental value. The geometric picture that emerges is that space-time is the product of an ordinary spin manifold (for which the theory would deliver Einstein gravity) by a finite noncommutative geometry F. The discrete space F is of KO-dimension 6 modulo 8 and of metric dimension 0, and accounts for all the intricacies of the standard model with its spontaneous symmetry breaking Higgs sector. "http://arxiv.org/abs/hep-th/0610231
Does string theory predict an open universe?
R. Buniy, S. Hsu, A. Zee
3 pages, 1 figure

"It has been claimed that the string landscape predicts an open universe, with negative curvature. The prediction is a consequence of a large number of metastable string vacua, and the properties of the Coleman--De Luccia instanton which describes vacuum tunneling. We examine the robustness of this claim, which is of particular importance since it seems to be string theory's sole claim to falsifiability. We find that, due to subleading tunneling processes, the prediction is sensitive to unknown properties of the landscape. Under plausible assumptions, universes like ours are as likely to be closed as open."

http://arxiv.org/abs/gr-qc/0610101
The emergence of AdS(2) from quantum fluctuations
J. Ambjorn, R. Janik, W. Westra, S. Zohren
4 pages. Talk given by W. Westra at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006

"We have shown how the quantization of two-dimensional quantum gravity with an action which contains only a positive cosmological constant and boundary cosmological constants leads to the emergence of a spacetime which can be described as a constant negative curvature spacetime with superimposed quantum fluctuations."

===============
http://arxiv.org/abs/hep-th/0610236
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. I. BF Yang-Mills representation
Florian Conrady (Penn State U.)
24 pages, 2 figures
IGPG-06/10-4

"In this series of three papers, we generalize the derivation of photons and monopoles by Polyakov and Banks, Myerson and Kogut, to obtain gluon-monpole representations of SU(2) lattice gauge theory. The papers take three different representations as their starting points: the representation as a BF Yang-Mills theory, the spin foam representation and the plaquette representation. The subsequent derivations are based on semiclassical expansions.
In this first article, we cast d-dimensional SU(2) lattice gauge theory in the form of a lattice BF Yang-Mills theory. In several steps, the expectation value of a Wilson loop is transformed into a path integral over a gluon field and monopole-like degrees of freedom. The action contains the tree-level Coulomb interaction and a nonlinear coupling between gluons, monopoles and current.
At the end, we compare the results from all three papers."

http://arxiv.org/abs/hep-th/0610237
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. II Spin foam representation

http://arxiv.org/abs/hep-th/0610238
Analytic derivation of gluons and monopoles from SU(2) lattice Yang-Mills theory. III Plaquette representation

 

[marcus]

http://arxiv.org/abs/gr-qc/0610140
Multiple-event probability in general-relativistic quantum mechanics
Frank Hellmann, Mauricio Mondragon, Alejandro Perez, Carlo Rovelli

"We discuss the definition of quantum probability in the context of 'timeless' general--relativistic quantum mechanics. In particular, we study the probability of sequences of events, or multi-event probability. In conventional quantum mechanics this can be obtained by means of the 'wave function collapse' algorithm. We first point out certain difficulties of some natural definitions of multi-event probability, including the conditional probability widely considered in the literature. We then observe that multi-event probability can be reduced to single-event probability, by taking into account the quantum nature of the measuring apparatus. In fact, by exploiting the von-Neumann freedom of moving the quantum classical boundary, one can always trade a sequence of non-commuting quantum measurements at different times, with an ensemble of simultaneous commuting measurements on the joint system+apparatus system. This observation permits a formulation of quantum theory based only on single-event probability, where the results of the 'wave function collapse' algorithm can nevertheless be recovered. The discussion bears also on the nature of the quantum collapse."

 

[marcus]

the title was too intriguing to pass up. Malcolm Fairbairn is at Perimeter Institute and Stockholm.

http://arxiv.org/abs/astro-ph/0610844
Shining light through the Sun
Malcolm Fairbairn, Timur Rashba, Sergey Troitsky
4 pages

"It is shown that the Sun can become partially transparent to high energy photons in the presence of a pseudo-scalar. In particular, if the axion interpretation of the PVLAS result were true then up to 2% of GeV energy gamma rays might pass through the Sun, while an even stronger effect is expected for some axion parameters. We discuss the possibilities of observing this effect. Present data are limited to the observation of the solar occultation of 3C279 by EGRET in 1991; 98% C.L. detection of a non-zero flux of gamma rays passing through the Sun is not yet conclusive. Future experiments, e.g. GLAST, are expected to have better sensitivity."

http://arxiv.org/abs/gr-qc/0611005
Problems and hopes in nonsymmetric gravity
Tomas Janssen, Tomislav Prokopec (Utrecht University)
9 pages, 1 figure. Based on two talks by the authors at the 2nd International Conference on Quantum Theories and Renormalization Group in Gravity and Cosmology (IRGAC) 2006, Barcelona
SPIN-06-37, ITP-UU-06-47

"We consider the linearized nonsymmetric theory of gravitation (NGT) within the background of an expanding universe and near a Schwarzschild mass. We show that the theory always develops instabilities unless the linearized nonsymmetric lagrangian reduces to a particular simple form. This form contains a gauge invariant kinetic term, a mass term for the antisymmetric metric-field and a coupling with the Ricci curvature scalar. This form cannot be obtained within NGT. Based on the linearized lagrangian we know to be stable, we consider the generation and evolution of quantum fluctuations of the antisymmetric gravitational field (B-field) from inflation up to the present day. We find that a B-field with a mass m ~ 0.03(H_I/10^(13)GeV)^4 eV is an excellent dark matter candidate."

http://arxiv.org/abs/gr-qc/0611004
Universal Bound on Dynamical Relaxation Times and Black-Hole Quasinormal Ringing
Shahar Hod
4 pages

"From information theory and thermodynamic considerations a universal bound on the relaxation time $\tau$ of a perturbed system is inferred, $$\tau \geq \hbar/\pi T$$, where T is the system's temperature. We prove that black holes comply with the bound; in fact they actually saturate it. Thus, when judged by their relaxation properties, black holes are the most extreme objects in nature, having the maximum relaxation rate which is allowed by quantum theory."

http://arxiv.org/abs/hep-th/0611017
Phenomenological Quantum Gravity
S. Hossenfelder
To appear in Proceedings of SUSY06, the 14th International Conference on Supersymmetry and the Unification of Fundamental Interactions, UC Irvine, California, 12-17 June 2006

"Planck scale physics represents a future challenge, located between particle physics and general relativity. The Planck scale marks a threshold beyond which the old description of spacetime breaks down and conceptually new phenomena must appear. In the last years, increased efforts have been made to examine the phenomenology of quantum gravity, even if the full theory is still unknown."

 

[marcus]

http://arxiv.org/abs/hep-th/0611042
Hidden Quantum Gravity in 4d Feynman diagrams: Emergence of spin foams
Aristide Baratin, Laurent Freidel
28 pages, 7 figures

"We show how Feynman amplitudes of standard QFT on flat and homogeneous space can naturally be recast as the evaluation of observables for a specific spin foam model, which provides dynamics for the background geometry. We identify the symmetries of this Feynman graph spin foam model and give the gauge-fixing prescriptions. We also show that the gauge-fixed partition function is invariant under Pachner moves of the triangulation, and thus defines an invariant of four-dimensional manifolds. Finally, we investigate the algebraic structure of the model, and discuss its relation with a quantization of 4d gravity in the limit where the Newton constant goes to zero."

http://arxiv.org/abs/gr-qc/0611024
Phenomenology of Quantum Gravity and Finsler Geometry
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
10 pages

A common feature of all Quantum Gravity (QG) phenomenology approaches is to consider a modification of the mass shell condition of the relativistic particle to take into account quantum gravitational effects. The framework for such approaches is therefore usually set up in the cotangent bundle (phase space). However it was recently proposed that this phenomenology could be associated with an energy dependent geometry that has been coined ``rainbow metric". We show here that the latter actually corresponds to a Finsler Geometry, the natural generalization of Riemannian Geometry. We provide in this way a new and rigorous framework to study the geometrical structure possibly arising in the semiclassical regime of QG. We further investigate the symmetries in this new context and discuss their role in alternative scenarios like Lorentz violation in emergent spacetimes or Deformed Special Relativity-like models."

 

[marcus]

JB taught us to look out for whatever Bob Coecke is up to. He often has quite strange ideas. Here is something new by him:

http://arxiv.org/abs/physics/0611064
Early Greek Thought and Perspectives for the Interpretation of Quantum Mechanics: Preliminaries to an Ontological Approach
K. Verelst, B. Coecke
18 pages
History of Physics

" It will be shown in this article that an ontological approach for some problems related to the interpretation of Quantum Mechanics could emerge from a re-evaluation of the main paradox of early Greek thought: the paradox of Being and non-Being, and the solutions presented to it by Plato and Aristotle. Plato's and Aristotle's systems are argued here to do on the ontological level essentially the same: to introduce stability in the world by introducing the notion of a separable, stable object, for which a principle of contradiction is valid: an object cannot be and not-be at the same place at the same time. After leaving Aristotelian metaphysics, early modern science had to cope with these problems: it did so by introducing 'space' as the seat of stability, and 'time' as the theater of motion. But the ontological structure present in this solution remained the same. Therefore the fundamental notion 'separable system', related to the notions observation and measurement, themselves related to the modern concepts of space and time, appears to be intrinsically problematic, because it is inextricably connected to classical logic on the ontological level. We see therefore the problems dealt with by quantum logic not as merely formal, and the problem of 'non-locality' as related to it, indicating the need to re-think the notions 'system', 'entity', as well as the implications of the operation 'measurement', which is seen here as an application of classical logic (including its ontological consequences) on the material world. "http://arxiv.org/abs/gr-qc/0611043
Black hole interior from loop quantum gravity
Leonardo Modesto
11 pages, 7 figures

"In this paper we calculate modifications to the Schwarzschild solution by using a semiclassical analysis of loop quantum black hole. We obtain a metric inside the event horizon that coincides with the Schwarzschild solution near the horizon but that is substantially different at the Planck scale. In particular we obtain a bounce of the two-sphere for a minimum value of the radius and that it is possible to have another event horizon close to the r=0 point."

Francesca was telling us about the DICE 2006 conference which took place in September at Piombino on the Tuscan coast. Here is one of the papers presented:

http://arxiv.org/abs/quant-ph/0611076
Classicality in Quantum Mechanics
Olaf Dreyer
Converence talk presented at DICE 2006 in Piombino. To be published in the Journal of Physics: Conference Series

"In this article we propose a solution to the measurement problem in quantum mechanics. We point out that the measurement problem can be traced to an a priori notion of classicality in the formulation of quantum mechanics. If this notion of classicality is dropped and instead classicality is defined in purely quantum mechanical terms the measurement problem can be avoided. We give such a definition of classicality. It identifies classicality as a property of large quantum system. We show how the probabilistic nature of quantum mechanics is a result of this notion of classicality. We also comment on what the implications of this view are for the search of a quantum theory of gravity."http://arxiv.org/abs/gr-qc/0611025
Current tests of alternative gravity theories: the Modified Newtonian Dynamics case
Jorge Paramos, Orfeu Bertolami
12 pages. Talk presented by one of us (O.B.) at the Fundamental Physics in Space section of the Comittee on Space Research, 36th. COSPAR Scientific Assembly, Beijing, China, 16-23 July, 2006

"We address the possibility of taking advantage of high accuracy gravitational space experiments in the Solar System and complementary cosmological tests to distinguish between the usual general relativistic theory from the alternative modified Newtonian dynamics paradigm."

Interest persists in modified gravity. Here is another sample:

http://arxiv.org/abs/hep-th/0611071
Modified gravity and its reconstruction from the universe expansion history
Shin'ichi Nojiri, Sergei D. Odintsov
24 pages, prepared for the proceedings of ERE 2006

"We develop the reconstruction program for the number of modified gravities: scalar-tensor theory, f(R), F(G) and string-inspired, scalar-Gauss-Bonnet gravity. The known (classical) universe expansion history is used for the explicit and successful reconstruction of some versions (of special form or with specific potentials) from all above modified gravities. It is demonstrated that cosmological sequence of matter dominance, decceleration-acceleration transition and acceleration era may always emerge as cosmological solutions of such theory. Moreover, the late-time dark energy FRW universe may have the approximate or exact LambdaCDM form consistent with three years WMAP data. The principal possibility to extend this reconstruction scheme to include the radiation dominated era and inflation is briefly mentioned. Finally, it is indicated how even modified gravity which does not describe the matter-dominated epoch may have such a solution before acceleration era at the price of the introduction of compensating dark energy."

 

[marcus]

http://arxiv.org/abs/gr-qc/0611073
Generalizing the Kodama State I: Construction
Andrew Randono
First part in two part series, 20 pages

The Kodama State is unique in being an exact solution to all the ordinary constraints of canonical quantum gravity that also has a well defined semi-classical interpretation as a quantum version of a classical spacetime, namely (anti)de Sitter space. However, the state is riddled with difficulties which can be tracked down to the complexification of the phase space necessary in its construction. This suggests a generalization of the state to real values of the Immirzi parameter. In this first part of a two paper series we show that one can generalize the state to real variables and the result is surprising in that it appears to open up an infinite class of physical states. We show that these states closely parallel the ordinary momentum eigenstates of non-relativistic quantum mechanics with the Levi-Civita curvature playing the role of the momentum. With this identification, the states inherit many of the familiar properties of the momentum eigenstates including delta-function normalizability. In the companion paper we will discuss the physical interpretation, CPT properties, and an interesting connection between the inner product and the Macdowell-Mansouri formulation of general relativity. "

http://arxiv.org/abs/gr-qc/0611074
Generalizing the Kodama State II: Properties and Physical Interpretation
Andrew Randono
Second paper in two part series. 18 pages

In this second part of a two paper series we discuss the properties and physical interpretation of the generalized Kodama states. We first show that the states are the three dimensional boundary degrees of freedom of two familiar 4-dimensional topological invariants: the second Chern class and the Euler class. Using this, we show that the states have the familiar interpretation as WKB states, in this case corresponding not only to de Sitter space, but also to first order perturbations therein. In an appropriate spatial topology, the de Sitter solution has pure Chern-Simons functional form, and is the unique state in the class that is identically diffeomorphism and SU(2) gauge invariant. The q-deformed loop transform of this state yields evidence of a cosmological horizon when the deformation parameter is a root of unity. We then discuss the behavior of the states under discrete symmetries, showing that the states violate P and T due to the presence of the Immirzi parameter, but they are CPT invariant. We conclude with an interesting connection between the physical inner product and the Macdowell Mansouri formulation of gravity."

http://arxiv.org/abs/gr-qc/0611075
Quantum Gravity and Matter: Counting Graphs on Causal Dynamical Triangulations
D. Benedetti, R. Loll
40 pages, 15 figures, 13 tables

"An outstanding challenge for models of non-perturbative quantum gravity is the consistent formulation and quantitative evaluation of physical phenomena in a regime where geometry and matter are strongly coupled. After developing appropriate technical tools, one is interested in measuring and classifying how the quantum fluctuations of geometry alter the behaviour of matter, compared with that on a fixed background geometry.
In the simplified context of two dimensions, we show how a method invented to analyze the critical behaviour of spin systems on flat lattices can be adapted to the fluctuating ensemble of curved spacetimes underlying the Causal Dynamical Triangulations (CDT) approach to quantum gravity. We develop a systematic counting of embedded graphs to evaluate the thermodynamic functions of the gravity-matter models in a high- and low-temperature expansion. For the case of the Ising model, we compute the series expansions for the magnetic susceptibility on CDT lattices and their duals up to orders 6 and 12, and analyze them by ratio method, Dlog Padé and differential approximants. Apart from providing evidence for a simplification of the model's analytic structure due to the dynamical nature of the geometry, the technique introduced can shed further light on criteria à la Harris and Luck for the influence of random geometry on the critical properties of matter systems.

http://arxiv.org/abs/gr-qc/0611080
A new proposal for group field theory I: the 3d case
James Ryan

"In this series of papers, we propose a new rendition of 3d and 4d state sum models based upon the group field theory (GFT) approach to non-perturbative quantum gravity. We will see that the group field theories investigated in the literature to date are, when judged from the position of quantum field theory, an unusual manifestation of quantum dynamics. They are one in which the Hadamard function for the field theory propagates a-causally the physical degrees of freedom of quantum gravity. This is fine if we wish to define a scalar product on the physical state space, but it is not what we generally think of as originating directly from a field theory. We propose a model in 3d more in line with standard quantum field theory, and therefore the field theory precipitates causal dynamics. Thereafter, we couple the model to point matter, and extract from the GFT the effective non-commutative field theory describing the matter dynamics on a quantum gravity background. We identify the symmetries of our new model and clarify their meaning in the GFT setting. We are aided in this process by identifying the category theory foundations of this GFT which, moreover, propel us towards a categorified version for the 4d case. "

 

[marcus]

Krasnov is at Nottingham, where fellow-poster fh has joined the QG group

http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov
5 pages, no figures

"We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological 'constant' and may provide a mechanism for naturally small 'dark energy'. "

http://arxiv.org/abs/astro-ph/0611504
Using globular clusters to test gravity in the weak acceleration regime: NGC 7099
Riccardo Scarpa, Gianni Marconi, Roberto Gilmozzi, Giovanni Carraro
Accepted for publication in A&A Letters. Four pages in total

"A test of Newton's law of gravity in the low acceleration regime using globular clusters is presented. New results for the core collapsed globular cluster NGC 7099 are given. The run of the gravitational potential as a function of distance is probed studying the velocity dispersion profile of the cluster, as derived from a set of 125 radial velocities with accuracy better than 1 km/s. The velocity dispersion profile is traced up to ~18 pc from the cluster center. The dispersion is found to be maximal at the center, then decrease until 10+-2 pc from the center, well inside the cluster tidal radius of 42 pc. After that the dispersion remains constant with average value 2.2+-0.3 km/s. Assuming for NGC 7099 a total V mag of M(V)=-7.43 mags and mass-to-light ratio M/L=1, the acceleration at 10 pc from the center is 1.1e-8 cm/s/s. Thus, the flattening of the velocity dispersion profile occurs for a value of the internal acceleration of gravity fully consistent with a_0=1.2e-8 cm/s/s observed in galaxies. This new result for NGC 7099 brings to 4 the clusters with velocity dispersion profile probing acceleration below a_0. All four have been found to have a flat dispersion profile at large radii where the acceleration is below a_0, mimicking qualitatively and quantitatively elliptical galaxies. Whether this indicates a failure of Newtonian dynamics in the low acceleration limit or some more conventional dynamical effect (e.g., tidal heating) is still unclear. However, the similarities emerging between very different globular clusters, as well as between globular clusters and elliptical galaxies seem to favor the first of these two possibilities. "These two Stanford people have a funny title---couldn't resist.

http://arxiv.org/abs/hep-th/0611183
O'KKLT
Renata Kallosh, Andrei Linde
12 pages, 4 figures
SU-ITP-2006-31

"We propose to combine the quantum corrected O'Raifeartaigh model, which has a dS minimum near the origin of the moduli space, with the KKLT model with an AdS minimum. The combined effective N=1 supergravity model, which we call O'KKLT, has a dS minimum with all moduli stabilized. Gravitino in the O'KKLT model tends to be light in the regime of validity of our approximations. We show how one can construct models with a light gravitino and a high barrier protecting vacuum stability during the cosmological evolution. "

 

[selfAdjoint]

These two Stanford people have a funny title---couldn't resist.

http://arxiv.org/abs/hep-th/0611183
O'KKLT

Years and years ago I bought O'Raifeartaigh's Group Structure of Gauge Theories at Christmas time, and in the flyleaf I wrote:

O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages
O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages,

Thou tellst us not just what you know
But challenge us with quiz also,

O'Raifeartaigh, O'Raifeartaigh
How helpful are thy pages

(Christmas 1988)

 

[marcus]

great guy
http://www.stp.dias.ie/Lochlainn/lochlainn.html
this affectionate one-page obit mentions that book in its concluding paragraph, chosing it to sum up the man

"In all his work, Lochlainn O'Raifeartaigh's love of physics, his interest in understanding fundamental problems clearly, and his ability to present results with precision and clarity shone through. He was an inspiring teacher for physics students in Dublin as well as abroad. His lectures on the Group Structure of Gauge Theories, published by Cambridge University Press, are very popular among students as well as researchers..."

I may have to take a look at that book.

your inscription is delightful

odious as it is to emend another's verse, it's hard to resist

in the interest of the English second person, and parallelism, I suggestThou tellst us not just what you know
But challengest with quiz also,

I particularly like the scansion which makes a stress fall on "not" in the middle of the first line of the couplet.

 

[marcus]

http://arxiv.org/abs/hep-th/0611197
Quantum Graphity
Tomasz Konopka, Fotini Markopoulou, Lee Smolin
14 pages, 3 figures

"We introduce a new model of background independent physics in which the degrees of freedom live on a complete graph and the physics is invariant under the permutations of all the points. We argue that the model has a low energy phase in which physics on a low dimensional lattice emerges and the permutation symmetry is broken to the translation group of that lattice. In the high temperature, or disordered, phase the permutation symmetry is respected and the average distance between degrees of freedom is small. This may serve as a tractable model for the emergence of classical geometry in background independent models of spacetime. We use this model to argue for a cosmological scenario in which the universe underwent a transition from the high to the low temperature phase, thus avoiding the horizon problem."

The next thing is something NASA called a press conference about last week. The technical article may give some additional details of interest.
http://arxiv.org/abs/astro-ph/0611572
New Hubble Space Telescope Discoveries of Type Ia Supernovae at z > 1: Narrowing Constraints on the Early Behavior of Dark Energy
Adam G. Riess (JHU, STScI), Louis-Gregory Strolger (UWK), Stefano Casertano (STScI), Henry C. Ferguson (STScI), Bahram Mobasher (STScI), Ben Gold (JHU), Peter J. Challis (CfA), Alexei V. Filippenko (UCB), Saurabh Jha (UCB), Weidong Li (UCB), John Tonry (IfA), Ryan Foley (UCB), Robert P. Kirshner (CfA), Mark Dickinson (NOAO), Emily MacDonald (NOAO), Daniel Eisenstein (UofA), Mario Livio (STScI), Josh Younger (CfA), Chun Xu (STScI), Tomas Dahlen (STScI), Daniel Stern (JPL)
82 pages, 17 figures, 6 tables. Data also available at: this http URL Accepted, Astrophysical Journal vol. 656 for Feb 10, 2007 "We have discovered 21 new Type Ia supernovae (SNe Ia) with the Hubble Space Telescope (HST) and have used them to trace the history of cosmic expansion over the last 10 billion years. These objects, which include 13 spectroscopically confirmed SNe Ia at z > 1, were discovered during 14 epochs of reimaging of the GOODS fields North and South over two years with the Advanced Camera for Surveys on HST. Together with a recalibration of our previous HST-discovered SNe Ia, the full sample of 23 SNe Ia at z > 1 provides the highest-redshift sample known. Combined with previous SN Ia datasets, we measured H(z) at discrete, uncorrelated epochs, reducing the uncertainty of H(z>1) from 50% to under 20%, strengthening the evidence for a cosmic jerk--the transition from deceleration in the past to acceleration in the present. The unique leverage of the HST high-redshift SNe Ia provides the first meaningful constraint on the dark energy equation-of-state parameter at z >1.
The result remains consistent with a cosmological constant (w(z)=-1), and rules out rapidly evolving dark energy (dw/dz >>1). The defining property of dark energy, its negative pressure, appears to be present at z>1, in the epoch preceding acceleration, with ~98% confidence in our primary fit. Moreover, the z>1 sample-averaged spectral energy distribution is consistent with that of the typical SN Ia over the last 10 Gyr, indicating that any spectral evolution of the properties of SNe Ia with redshift is still below our detection threshold. "The next is listed in case some wish to follow the development of "anthropic predictions".

http://arxiv.org/abs/astro-ph/0611573
Anthropic predictions for vacuum energy and neutrino masses in the light of WMAP-3
Levon Pogosian, Alexander Vilenkin
11 pages, 6 figures

"Anthropic probability distributions for the cosmological constant and for the sum of neutrino masses are updated using the WMAP-3 data release. The new distribution for Lambda is in a better agreement with observation than the earlier one. The typicality of the observed value, defined as the combined probability of all values less likely than the observed, is no less than 22%. We discuss the dependence of our results on the simplifying assumptions used in deriving the distribution for Lambda and show that the agreement of the anthropic prediction with the data is rather robust. The distribution for the sum of the neutrino masses is peaked at 1 eV, suggesting degenerate masses, but a hierarchical mass pattern is still marginally allowed at a 2 sigma level."

Briefly noted:
http://arxiv.org/abs/math.AG/0611524
Langlands duality and G2 spectral curves
Nigel Hitchin

 

[Kea]

Briefly noted:
http://arxiv.org/abs/math.AG/0611524
Langlands duality and G2 spectral curves
Nigel Hitchin

Thanks for this reference, Marcus. It looks interesting.

 

[selfAdjoint]

odious as it is to emend another's verse, it's hard to resist

in the interest of the English second person, and parallelism, I suggest


Thou tellst us not just what you know
But challengest with quiz also,

I particularly like the scansion which makes a stress fall on "not" in the middle of the first line of the couplet.

You are absolutely right. My only excuse is that, as I dimly recall, I was a little squiffed at the time. BTW I had got the book out to check the verse and started to read it again. By comparison with books on Lie Groups and Algebras that I have studied since then, it is a marvel of clarity and kindnness toward the student. I most heartily recommend it!

 

[marcus]

http://arxiv.org/abs/gr-qc/0611112
Effective constraints of loop quantum gravity
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Aureliano Skirzewski
44 pages, 6 figures
IGPG-06/11-4, AEI-2006-086

"Within a perturbative cosmological regime of loop quantum gravity corrections to effective constraints are computed. This takes into account all inhomogeneous degrees of freedom relevant for scalar metric modes around flat space and results in explicit expressions for modified coefficients and of higher order terms. It also illustrates the role of different scales determining the relative magnitude of corrections. Our results demonstrate that loop quantum gravity has the correct classical limit, at least in its sector of cosmological perturbations around flat space, in the sense of perturbative effective theory."

http://arxiv.org/abs/astro-ph/0611685
Formation and Evolution of Structure in Loop Cosmology
Martin Bojowald, Hector Hernandez, Mikhail Kagan, Parampreet Singh, Aureliano Skirzewski
4 pages
IGPG-06/11-3, AEI-2006-085

"Inhomogeneous cosmological perturbation equations are derived in loop quantum gravity, taking into account corrections in particular in gravitational parts. This provides a framework for calculating the evolution of modes in structure formation scenarios related to inflationary or bouncing models. Applications here are corrections to the Newton potential and to the evolution of large scale modes which imply non-conservation of curvature perturbations possibly noticeable in a running spectral index. These effects are sensitive to quantization procedures and test the characteristic behavior of correction terms derived from quantum gravity."

The next was spotted by selfAdjoint, who started a discussion thread here:
https://www.physicsforums.com/showthread.php?t=144841
http://arxiv.org/hep-ph/0611279
Beyond the Standard Model
Dmitri I. Kazakov
11 pages, 11 figures, Plenary talk at XXXIII ICHEP, Moscow 2006

"Review of recent developments in attempts to go beyond the Standard Model is given. We concentrate on three main unresolved problems: mechanism of electroweak symmetry breaking, expected new physics at the TeV scale (mainly SUSY) and the origin of the Dark matter."

 

[marcus]

http://arxiv.org/abs/gr-qc/0611141
The Canonical Approach to Quantum Gravity: General Ideas and Geometrodynamics
Domenico Giulini, Claus Kiefer
21 pages, 6 figures. Contribution to E. Seiler and I.-O. Stamatescu (editors): `Approaches To Fundamental Physics -- An Assessment Of Current Theoretical Ideas' (Springer Verlag, to appear) "We give an introduction to the canonical formalism of Einstein's theory of general relativity. This then serves as the starting point for one approach to quantum gravity called quantum geometrodynamics. The main features and applications of this approach are briefly summarized."

http://arxiv.org/abs/gr-qc/0611135
Quantum causal histories in the light of quantum information
Etera R. Livine, Daniel R. Terno
9 pages, 8 figures

"We use techniques of quantum information theory to analyze the quantum causal histories approach to quantum gravity. We show that while it is consistent to introduce closed timelike curves (CTCs), they cannot generically carry independent degrees of freedom. Moreover, if the effective dynamics of the chronology-respecting part of the system is linear, it should be completely decoupled from the CTCs. In the absence of a CTC not all causal structures admit the introduction of quantum mechanics. It is possible for those and only for those causal structures that can be represented as quantum computational networks. The dynamics of the subsystems should not be unitary or even completely positive. However, we show that other commonly made assumptions ensure the complete positivity of the reduced dynamics."

http://arxiv.org/abs/hep-th/0611294
Scale-dependent metric and causal structures in Quantum Einstein Gravity
Martin Reuter, Jan-Markus Schwindt
52 pages "Within the asymptotic safety scenario for gravity various conceptual issues related to the scale dependence of the metric are analyzed. The running effective field equations implied by the effective average action of Quantum Einstein Gravity (QEG) and the resulting families of resolution dependent metrics are discussed. The status of scale dependent vs. scale independent diffeomorphisms is clarified, and the difference between isometries implemented by scale dependent and independent Killing vectors is explained. A concept of scale dependent causality is proposed and illustrated by various simple examples. The possibility of assigning an 'intrinsic length' to objects in a QEG spacetime is also discussed."

http://arxiv.org/abs/quant-ph/0611261
Concerning Dice and Divinity
D.M.Appleby
Contribution to proceedings of Foundations of Probability and Physics, Vaxjo, 2006

"Einstein initially objected to the probabilistic aspect of quantum mechanics - the idea that God is playing at dice. Later he changed his ground, and focussed instead on the point that the Copenhagen Interpretation leads to what Einstein saw as the abandonment of physical realism. We argue here that Einstein's initial intuition was perfectly sound, and that it is precisely the fact that quantum mechanics is a fundamentally probabilistic theory which is at the root of all the controversies regarding its interpretation. Probability is an intrinsically logical concept. This means that the quantum state has an essentially logical significance. It is extremely difficult to reconcile that fact with Einstein's belief, that it is the task of physics to give us a vision of the world apprehended sub specie aeternitatis. Quantum mechanics thus presents us with a simple choice: either to follow Einstein in looking for a theory which is not probabilistic at the fundamental level, or else to accept that physics does not in fact put us in the position of God looking down on things from above. There is a widespread fear that the latter alternative must inevitably lead to a greatly impoverished, positivistic view of physical theory. It appears to us, however, that the truth is just the opposite. The Einsteinian vision is much less attractive than it seems at first sight. In particular, it is closely connected with philosophical reductionism."

http://arxiv.org/abs/gr-qc/0611148
Fundamental spatiotemporal decoherence: a key to solving the conceptual problems of black holes, cosmology and quantum mechanics
Rodolfo Gambini, Rafael Porto, Jorge Pullin
6 pages, Honorable Mention GRF 2006, to appear in IJMPD "Unitarity is a pillar of quantum theory. Nevertheless, it is also a source of several of its conceptual problems. We note that in a world where measurements are relational, as is the case in gravitation, quantum mechanics exhibits a fundamental level of loss of coherence. This can be the key to solving, among others, the puzzles posed by the black hole information paradox, the formation of inhomogeneities in cosmology and the measurement problem in quantum mechanics."

 

[marcus]

http://arxiv.org/abs/math.CT/0611930
A Double Bicategory of Cobordisms With Corners
Jeffrey Morton
40 pages, 7 figures

"Interest in cobordism categories arises in areas from topology to theoretical physics, and in particular in Topological Quantum Field Theories (TQFT's). These categories have manifolds as objects, and cobordisms between them as morphisms, have - that is, manifolds of one dimension higher whose boundary decomposes into the source and target. Since the boundary of a boundary is empty, this formulation cannot account for cobordisms between manifolds with boundary. This is needed to describe open-closed TQFT's, and more generally, 'extended TQFT's'. We describe a framework for describing these, in the form of what we call a 'Verity Double Bicategory', after Dominic Verity, who introduced them. This is similar to a double category, but with properties holding only up to certain 2-morphisms. We show how a broad class of examples is given by a construction involving spans in suitable settings, and how this gives cobordisms between cobordisms when we start with the category of manifolds."

http://arxiv.org/abs/gr-qc/0611154
MacDowell-Mansouri gravity and Cartan geometry
Derek K. Wise
34 pages, 5 figures

"The geometric content of the MacDowell-Mansouri formulation of general relativity is best understood in terms of Cartan geometry. In particular, Cartan geometry gives clear geometric meaning to the MacDowell-Mansouri trick of combining the Levi-Civita connection and coframe field, or soldering form, into a single physical field. The Cartan perspective allows us to view physical spacetime as tangentially approximated by an arbitrary homogeneous 'model spacetime', including not only the flat Minkowski model, as is implicitly used in standard general relativity, but also de Sitter, anti de Sitter, or other models. A 'Cartan connection' gives a prescription for parallel transport from one 'tangent model spacetime' to another, along any path, giving a natural interpretation of the MacDowell-Mansouri connection as 'rolling' the model spacetime along physical spacetime. I explain Cartan geometry, and 'Cartan gauge theory', in which the gauge field is replaced by a Cartan connection. In particular, I discuss MacDowell-Mansouri gravity, as well as its recent reformulation in terms of BF theory, in the context of Cartan geometry."

http://arxiv.org/abs/gr-qc/0611156
On Loop States in Loop Quantum Gravity
N. D. Hari Dass, Manu Mathur
12 pages, 3 figures

"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]

noted in mid-November (post 535)

http://arxiv.org/abs/hep-th/0611182
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov
5 pages, no figures

"We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological 'constant' and may provide a mechanism for naturally small 'dark energy'. "

there is now a video to go along with it. Krasnov giving a slide presentation at P.I. and lots of questions (from Laurent Freidel and Lee Smolin among others) and discussion.
Krasnov has a version of gravity that departs from usual at large scale and something like a cosmological constant comes out of it.
If you want to jump to the "Dark Energy" slide it is #5 on page 5 of the slide menu---about halfway through. You can start the video there if you want.
The discussion of "interpretation" begins at slide #7 of page 8 of slide menu. Starting there you get a lot of questions by people in audience and Krasnov responding.

the video is #06110041 (dated 30 November) in the P.I. collection
http://www.perimeterinstitute.ca/en/Scientific/Seminars/PIRSA/
one way to get it is just type that PIRSA number in the box
another way, since it is recent, is to click on "catch up" so you get a list
of all the recent video seminars

 

[marcus]

revised version just posted:

http://arxiv.org/abs/gr-qc/0601043
Can Gravitons Be Detected?
Tony Rothman, Stephen Boughn
This version as appeared in Foundations of Physics
Journal-ref: Foundations of Physics, vol. 36, No. 12, 1801-1825 (2006)

"Freeman Dyson has questioned whether any conceivable experiment in the real universe can detect a single graviton. If not, is it meaningful to talk about gravitons as physical entities? We attempt to answer Dyson's question and find it is possible concoct an idealized thought experiment capable of detecting one graviton; however, when anything remotely resembling realistic physics is taken into account, detection becomes impossible, indicating that Dyson's conjecture is very likely true. We also point out several mistakes in the literature dealing with graviton detection and production."

a 1981 paper of Bohm, re-published to make it more accessible:
http://arxiv.org/abs/quant-ph/0612002
Algebraic Quantum Mechanics and Pregeometry
D.J. Bohm P.G. Davies B.J. Hiley
This paper was originally written in 1981 and published as a supplement to my Ph.D. thesis. (Davies, P., (1981) The Weyl Algebra and an Algebraic Mechanics. Ph.D thesis, Birkbeck College, University of London.) It is believed to be one of the "lost papers" of David Bohm as it was is not listed among his completed works and is set forth here for historical completeness

"We discuss the relation between the q-number approach to quantum mechanics suggested by Dirac and the notion of "pregeometry" introduced by Wheeler. By associating the q-numbers with the elements of an algebra and regarding the primitive idempotents as "generalized points" we suggest an approach that may make it possible to dispense with an a priori given space manifold. In this approach the algebra itself would carry the symmetries of translation, rotation, etc. Our suggestion is illustrated in a preliminary way by using a particular generalized Clifford Algebra proposed originally by Weyl, which approaches the ordinary Heisenberg algebra in a suitable limit. We thus obtain a certain insight into how quantum mechanics may be regarded as a purely algebraic theory, provided that we further introduce a new set of "neighbourhood operators", which remove an important kind of arbitrariness that has thus far been present in the attempt to treat quantum mechanics solely in terms of a Heisenberg algebra."

http://arxiv.org/abs/math-ph/0612012
An informal introduction to the ideas and concepts of noncommutative geometry
Thierry Masson
53 pages, Lecture given at the 6th Peyresq meeting "Integrable systems and quantum field theory" "This informal introduction is an extended version of a three hours lecture given at the 6th Peyresq meeting ``Integrable systems and quantum field theory''. In this lecture, we make an overview of some of the mathematical results which motivated the development of what is called noncommutative geometry. The first of these results is the theorem by Gelfand and Neumark about commutative $C^\ast$-algebras; then come some aspects of the $K$-theories, first for topological spaces, then for $C^\ast$-algebras and finally the purely algebraic version. Cyclic homology is introduced, keeping in mind its relation to differential structures. The last result is the construction of the Chern character, which shows how these developments are related to each other."

 

[marcus]

PIRSA numbers for some recent seminar talk videos

Some of these entries do not yet have the title and abstract. Note that there are some here from the Young Researchers Conference that took place at Perimeter this week---Willem Westra (PhD student who has co-authored with Loll), Andy Randono (PhD student with research generalizing the Kodama state).

The videos are split screen, showing a larger-format version of the slide or blackboard. They can be watched at the PI site simply by typing in the PIRSA number.
http://www.perimeterinstitute.ca/en/Scientific/Seminars/PIRSA/

or even easier, check the QG box here and click "do search"
http://www.perimeterinstitute.ca/in...=113&Itemid=167&p=presentations&with_msl=true
then you get a menu---and can just select what you want to watch from the menu

The ones that I starred (***) are talks that I watched and can recommend to anyone interested in the topic. (Benedetti, Randono, Krasnov...)***PIRSA#: 06110041
Renormalizable Non-Metric Quantum Gravity?
Kirill Krasnov - University of Nottingham
We argue that four-dimensional quantum gravity may be essentially renormalizable provided one relaxes the assumption of metricity of the theory. We work with Plebanski formulation of general relativity in which the metric (tetrad), the connection as well as the curvature are all independent variables and the usual relations among these quantities are only on-shell. One of the Euler-Lagrange equations of this theory guarantees its metricity. We show that quantum corrections generate a counterterm that destroys this metricity property, and that there are no other counterterms, at least at the one-loop level. There is a new coupling constant that controls the non-metric character of the theory. Its beta-function can be computed and is negative, which shows that the non-metricity becomes important in the infra red. The new IR-relevant term in the action is akin to a curvature dependent cosmological "constant" and may provide a mechanism for naturally small "dark energy".
30/11/2006

PIRSA#: 06120017
Kappa Deformed Field Theory
Sebastian Nowak (Wroclaw, probably Kowalski-Glikman's group)
The description of noncommutative space will be given. I will show the relation between field theory on kappa-Minkowski space and the one in Minkowski. This construction leads to deformed energy momentum conservation law for energies close to the Planck scale.
04/12/2006

PIRSA#: 06120018
A Cosmological Sector in Loop Quantum Gravity
Tim Koslowski (Würzburg)
A classical Hamiltonian system can be reduced to a subsystem of "relevant observables" using the pull-back under a Poisson embedding of the "relevant phase space" into the "full phase space". Since a quantum theory can be thought of a noncommutative phase space, one encounters the problem of the embedding of noncommutative spaces, when one tries to extend the reduction via a pull-back to a quantum theory. This problem can be solved for a class of physically interesting quantum systems and embeddings using an analogy to finding the base space of an embedded fibre bundle via the projection in the full fibre bundle. The resulting construction is then applied to Loop Quantum Gravity to extract a cosmological sector. This sector turns out to be similar but not equivalent to Loop Quantum Cosmology.
04/12/2006

PIRSA#: 06120019
Emergence of a Background From Background Independent Quantum Gravity
Willem Westra (Utrecht, Loll's group)
04/12/2006

*?*PIRSA#: 06120030
Dario Benedetti (Utrecht, Loll's group)
A Simplicial Path to the Quantum Hamiltonian of Gravity
06/12/2006
[my comment: this talk presents results in work by Loll and Benedetti which has not yet been posted on arxiv or published anywhere. It looks like Loll may have shifted emphasis from doing Monte Carlo simulations back to proving stuff analytically. This research is about the 2+1 dimensional case. Apparently they got an unexpected result. You can skip to the last two slides and start the talk there, near the end. Something problematical apparently came up, that they are trying to explain. I just saw this and can't comment. The talk starts slow, with review of the whole development of CDT, so it helps to look at the slide menu and decide where to jump in and start the talk. If you know some CDT already you don't have to start at the very beginning.]

***PIRSA#: 06120032
Andy Randono (Texas-Austin)
Generalizing the Kodama State
06/12/2006

==================
The menu gives a longer listing of PIRSA numbers for QG seminars (incl. Smolin, Bojowald,... other well-known speakers)
That's where you check the QG box here http://www.perimeterinstitute.ca/in...=113&Itemid=167&p=presentations&with_msl=true
and click "do search". You don't have to type in anything. It's pretty easy. Here is a sample.

***PIRSA#:05090005
Shahn Majid
Noncommutative geometry and the origin of time
14/09/2005

***PIRSA#: 05110009
Sundance Bilson-Thompson
Topological Preon Models: a Braid New World
01/11/2005

 

[marcus]

http://arxiv.org/abs/gr-qc/0612071
Plebanski Theory and Covariant Canonical Formulation
Sergei Alexandrov, Eric Buffenoir, Philippe Roche
18 pages

"We establish an equivalence between the Hamiltonian formulation of the Plebanski action for general relativity and the covariant canonical formulation of the Hilbert-Palatini action. This is done by comparing the symplectic structures of the two theories through the computation of Dirac brackets. We also construct a shifted connection with simplified Dirac brackets, playing an important role in the covariant loop quantization program, in the Plebanski framework. Implications for spin foam models are also discussed."

http://arxiv.org/abs/gr-qc/0612070
Loop quantum cosmology and the k = - 1 RW model
Kevin Vandersloot
14 pages, 3 figures

"The loop quantization of the negatively curved k=-1 RW model poses several technical challenges. We show that the issues can be overcome and a successful quantization is possible that extends the results of the k=0,+1 models in a natural fashion. We discuss the resulting dynamics and show that for a universe consisting of a massless scalar field, a bounce is predicted in the backward evolution in accordance with the results of the k=0,+1 models. We also show that the model predicts a vacuum repulsion in the high curvature regime that would lead to a bounce even for matter with vanishing energy density. We finally comment on the inverse volume modifications of loop quantum cosmology and show that, as in the k=0 model, the modifications depend sensitively on the introduction of a length scale which a priori is independent of the curvature scale or a matter energy scale."

http://arxiv.org/abs/gr-qc/0612084
Evaporating loop quantum black hole
Leonardo Modesto
13 pages, 9 figures

"In this paper we obtain the black hole metric from a semiclassical analysis of loop quantum black hole. Our solution and the Schwarzschild one tend to match well at large distances from Planck region. In r=0 the semiclassical metric is regular and singularity free in contrast to the classical one. By using the new metric we calculate the Hawking temperature and the entropy. For the entropy we obtain the logarithmic correction to the classical area law. Finally we study the mass evaporation process and we show the mass and temperature tend to zero at infinitive time."


http://arxiv.org/abs/gr-qc/0612074
Counting entropy in causal set quantum gravity
D. Rideout, S. Zohren
5 pages, 1 figure. Talk given by S. Zohren at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006

"The finiteness of black hole entropy suggest that spacetime is fundamentally discrete, and hints at an underlying relationship between geometry and "information". The foundation of this relationship is yet to be uncovered, but should manifest itself in a theory of quantum gravity. We review recent attempts to define a microscopic measure for black hole entropy and for the maximum entropy of spherically symmetric spacelike regions, within the causal set approach to quantum gravity."

 

[marcus]

http://arxiv.org/abs/gr-qc/0612093
Can we see gravitational collapse in (quantum) gravity perturbation theory?
J. Kowalski-Glikman, A. Starodubtsev
23 pages

"In this paper, by making use of the perturbative expansion around topological field theory we are trying to understand why the standard perturbation theory for General Relativity, which starts with linearized gravity does not see gravitational collapse. We start with investigating classical equations of motion. For zero Immirzi parameter the ambiguity of the standard perturbative expansion is reproduced. This ambiguity is related to the appearance of the linearized diffeomorphism symmetry, which becomes unlinked from the original diffeomorphism symmetry. Introducing Immirzi parameter makes it possible to restore the link between these two symmetries and thus removes the ambiguity, but at the cost of making classical perturbation theory rather intractable. Then we argue that the two main sources of complexity of perturbation theory, infinite number of degrees of freedom and non-trivial curvature of the phase space of General Relativity could be disentangled when studying quantum amplitudes. As an illustration we consider zero order approximation in quantum perturbation theory. We identify relevant observables, and sketch their quantization. We find some indications that this zero order approximation might be described by Doubly Special Relativity."

 

[marcus]

http://arxiv.org/abs/gr-qc/0612101
Closed FRW model in Loop Quantum Cosmology
Lukasz Szulc, Wojciech Kaminski, Jerzy Lewandowski
19 pages

"The basic idea of the LQC applies to every spatially homogeneous cosmological model, however only the spatially flat (so called k=0) case has been understood in detail in the literature thus far. In the closed (so called: k=1) case certain technical difficulties have been the obstacle that stopped the development. In this work the difficulties are overcome, and a new LQC model of the spatially closed, homogeneous, isotropic universe is constructed. The topology of the spacelike section of the universe is assumed to be that of SU(2) or SO(3). Surprisingly, according to the results achieved in this work, the two cases can be distinguished from each other just by the local properties of the quantum geometry of the universe. The quantum hamiltonian operator of the gravitational field takes the form of a difference operator, where the elementary step is the quantum of the 3-volume derived in the flat case by Ashtekar, Pawlowski and Singh. The mathematical properties of the operator are studied: it is essentially self-adjoint, bounded from above by 0, the 0 itself is not an eigenvalue, the eigenvectors form a basis. An estimate on the dimension of the spectral projection on any finite interval is provided."

http://arxiv.org/abs/gr-qc/0612104
Loop quantum cosmology of k=1 FRW models
Abhay Ashtekar, Tomasz Pawlowski, Parampreet Singh, Kevin Vandersloot
43 pages, 11 figures
IGPG-06/12-1

"The closed, k=1, FRW cosmology coupled to a massless scalar field is investigated in the framework of loop quantum cosmology using analytical and numerical methods. As in the k=0 case, the scalar field can be again used as emergent time to construct the physical Hilbert space and introduce Dirac observables. The resulting framework is then used to address a major challenge of quantum cosmology: resolving the big-bang singularity while retaining agreement with general relativity at large scales. It is shown that the framework fulfills this task. In particular, for states which are semi-classical at some late time, the big-bang is replaced by a quantum bounce and a recollapse occurs at the value of the scale factor predicted by classical general relativity. Thus, the `difficulties' pointed out by Green and Unruh in the k=1 case do not arise in a more systematic treatment. As in k=0 models, quantum dynamics is deterministic across the deep Planck regime. However, because it also retains the classical recollapse, in contrast to the k=0 case one is now led to a cyclic model. Finally, we clarify some issues raised by Laguna's recent work addressed to computational physicists."


http://arxiv.org/abs/gr-qc/0612111
Some comments on the universal constant in DSR
Florian Girelli, Etera R. Livine
12 pages, Proceedings of DICE2006 (Piombino, Italy)

"Deformed Special Relativity is usually presented as a deformation of Special Relativity accommodating a new universal constant, the Planck mass, while respecting the relativity principle. In order to avoid some fundamental problems (e.g. soccer ball problem), we argue that we should switch point of view and consider instead the Newton constant G as the universal constant."

http://arxiv.org/abs/hep-th/0612170
From noncommutative kappa-Minkowski to Minkowski space-time
Laurent Freidel, Jerzy Kowalski-Glikman, Sebastian Nowak
6 pages

"We show that free kappa-Minkowski space field theory is equivalent to a relativistically invariant, non local, free field theory on Minkowski space-time. The field theory we obtain has in spectrum a relativistic mode of arbitrary mass m and a Planck mass tachyon. We show that while the energy momentum for the relativistic mode is essentially the standard one, it diverges for the tachyon, so that there are no asymptotic tachyonic states in the theory. It also follows that the dispersion relation is not modified, so that, in particular, in this theory the speed of light is energy-independent."

http://arxiv.org/abs/hep-th/0612185
The status of cosmological natural selection
Lee Smolin
25 pages

"The problem of making predictions from theories that have landscapes of possible low energy parameters is reviewed. Conditions for such a theory to yield falsifiable predictions for doable experiments are given. It is shown that the hypothesis of cosmological natural selection satisfies these conditions, thus showing that it is possible to continue to do physics on a landscape without invoking the anthropic principle. In particular, this is true whether or not the ensemble of universes generated by black holes bouncing is a sub-ensemble of a larger ensemble that might be generated by a random process such as eternal inflation.
A recent criticism of cosmological natural selection made by Vilenkin in hep-th/0610051 is discussed. It is shown to rely on assumptions about both the infrared and ultraviolet behavior of quantum gravity that are very unlikely to be true."

 

[marcus]

http://arxiv.org/abs/gr-qc/0612144
Three Dimensional Loop Quantum Gravity: Particles and the Quantum Double
Karim Noui
36 pages, published in J. Math. Phys. 47, 102501 (2006)

"It is well known that the quantum double structure plays an important role in three dimensional quantum gravity coupled to matter field. In this paper, we show how this algebraic structure emerges in the context of three dimensional Riemannian loop quantum gravity (LQG) coupled to a finite number of massive spinless point particles. In LQG, physical states are usually constructed from the notion of SU(2) cylindrical functions on a Riemann surface Sigma and the Hilbert structure is defined by the Ashtekar-Lewandowski measure. In the case where Sigma is the sphere S^2, we show that the physical Hilbert space is in fact isomorphic to a tensor product of simple unitary representations of the Drinfeld double DSU(2): the masses of the particles label the simple representations, the physical states are tensor products of vectors of simple representations and the physical scalar product is given by intertwining coefficients between simple representations. This result is generalized to the case of any Riemann surface Sigma.

http://arxiv.org/abs/gr-qc/0612145
Three dimensional Loop Quantum Gravity: towards a self-gravitating Quantum Field Theory
Karim Noui
36 pages, published in Class. Quant. Grav. 24 (2007)

In a companion paper, we have emphasized the role of the Drinfeld double DSU(2) in the context of three dimensional Riemannian Loop Quantum Gravity coupled to massive spinless point particles. We make use of this result to propose a model for a self-gravitating quantum field theory (massive spinless non-causal scalar field) in three dimensional Riemannian space. We start by constructing the Fock space of the free self-gravitating field: the vacuum is the unique DSU(2) invariant state, one-particle states correspond to DSU(2) unitary irreducible simple representations and any multi-particles states is obtained as the symmetrized tensor product between simple representations. The associated quantum field is defined by the usual requirement of covariance under DSU(2). Then, we introduce a DSU(2)-invariant self-interacting potential (the obtained model is a Group Field Theory) and compute explicitely the lowest order terms (in the self-interaction coupling constant lambda) of the propagator and of the three-points function. Finally, we compute the lowest order quantum gravity corrections (in the Newton constant G) to the propagator and to the three-points function.

http://arxiv.org/abs/gr-qc/0612147
Spectral Analysis of the Volume Operator in Loop Quantum Gravity
J. Brunnemann, D. Rideout
5 pages, 1 figure. Talk given by D. Rideout at the Eleventh Marcel Grossmann Meeting on General Relativity at the Freie U. Berlin, July 23 - 29, 2006
Imperial/TP/2006/DR/02

"We describe preliminary results of a detailed numerical analysis of the volume operator as formulated by Ashtekar and Lewandowski. Due to a simplified explicit expression for its matrix elements, it is possible for the first time to treat generic vertices of valence greater than four. It is found that the vertex geometry characterizes the volume spectrum."

 

[marcus]

http://arxiv.org/abs/hep-th/0612280
Doubly Special Relativity at the age of six
Jerzy Kowalski-Glikman
To appear in the Proceedings of 22nd Max Born Symposium

"The current status of Doubly Special Relativity research program is shortly presented.
I dedicate this paper to my teacher and friend Professor Jerzy Lukierski on occasion of his seventieth birthday."

http://arxiv.org/abs/gr-qc/0612167
Deformed Special Relativity in Position Space
S. Hossenfelder "We investigate how deformations of special relativity in momentum space can be extended to position space in a consistent way, such that the dimensionless contraction between wave-vector and coordinate-vector remains invariant. By using a parametrization in terms of an energy dependent speed of light, and an energy dependent Planck's constant, we are able to formulate simple requirements that completely determine the active transformations in position space. These deviate from the standard transformations for large velocities of the observed object. Some examples are discussed, and it is shown how the relativistic mass gain of a massive particle is affected. We finally study the construction of passive Lorentz-transformations."

http://arxiv.org/abs/gr-qc/0612170
The Ponzano-Regge model and Reidemeister torsion
John W. Barrett, Ileana Naish-Guzman
4 pages. MG11 conference proceedings

"The Ponzano-Regge model of three-dimensional quantum gravity is well-defined when the observables satisfy a certain condition involving the twisted cohomology. In this case, the partition function is defined in terms of the Reidemeister torsion. Some consequences for the special cases of planar graphs and knots are given."

 

[marcus]

Francesca flagged this one by Lucien Hardy of PI Waterloo

http://arxiv.org/abs/quant-ph/0701019
Quantum gravity computers: On the theory of computation with indefinite causal structure

A quantum gravity computer is one for which the particular effects of quantum gravity are relevant. In general relativity, causal structure is non-fixed. In quantum theory non-fixed quantities are subject to quantum uncertainty. It is therefore likely that, in a theory of quantum gravity, we will have indefinite causal structure. This means that there will be no matter of fact as to whether a particular interval is timelike or not. We study the implications of this for the theory of computation. Classical and quantum computations consist in ivolving the state of the computer through a sequence of time steps. This will, most likely, not be possible for a quantum gravity computer because the notion of a time step makes no sense if we have indefinite causal structure. We show that it is possible to set up a model for computation even in the absence of definite causal structure by using a certain framework (the causaloid formalism) that was developed for the purpose of correlating data taken in this type of situation. Corresponding to a physical theory is a causaloid, Lambda (this is a mathematical object containing information about the causal connections between different spacetime regions). A computer is given by the pair {Lambda, S} where S is a set of gates. Working within the causaloid formalism, we explore the question of whether universal quantum gravity computers are possible. We also examine whether a quantum gravity computer might be more powerful than a quantum (or classical) computer. In particular, we ask whether indefinite causal structure can be used as a computational resource.

 

[marcus]

papers by 't Hooft, by Freidel, and by Sudarsky

http://arxiv.org/abs/quant-ph/0701097
The Free-Will Postulate in Quantum Mechanics
Gerard 't Hooft
8 pages, 1 figure

"The so-called 'free will axiom' is an essential ingredient in many discussions concerning hidden variables in quantum mechanics. In this paper we argue that 'free will' can be defined in different ways. The definition usually employed is clearly invalid in strictly deterministic theories. A different, more precise formulation is proposed here, defining a condition that may well be a more suitable one to impose on theoretical constructions and models. Our axiom, to be referred to as the `unconstrained initial state' condition, has consequences similar to 'free will', but does not clash with determinism, and appears to lead to different conclusions concerning causality and locality in quantum mechanics. Models proposed earlier by this author fall in this category. Imposing our 'unconstrained initial state' condition on a deterministic theory underlying Quantum Mechanics, appears to lead to a restricted free-will condition in the quantum system: an observer has the free will to modify the setting of a measuring device, but has no control over the phase of its wave function. The dismissal of the usual 'free will' concept does not have any consequences for our views and interpretations of human activities in daily life, and the way our minds function, but it requires a more careful discussion on what, in practice, free will actually amounts to."


http://arxiv.org/abs/hep-th/0701113
The Relativistic Particle: Dirac observables and Feynman propagator
Laurent Freidel, Florian Girelli, Etera R. Livine
14 pages

"We analyze the algebra of Dirac observables of the relativistic particle in four space-time dimensions. We show that the position observables become non-commutative and the commutation relations lead to a structure very similar to the non-commutative geometry of Deformed Special Relativity (DSR). In this framework, it appears natural to consider the 4d relativistic particle as a five dimensional massless particle. We study its quantization in terms of wave functions on the 5d light cone. We introduce the corresponding five-dimensional action principle and analyze how it reproduces the physics of the 4d relativistic particle. The formalism is naturally subject to divergences and we show that DSR arises as a natural regularization: the 5d light cone is regularized as the de Sitter space. We interpret the fifth coordinate as the particle's proper time while the fifth moment can be understood as the mass. Finally, we show how to formulate the Feynman propagator and the Feynman amplitudes of quantum field theory in this context in terms of Dirac observables. This provides new insights for the construction of observables and scattering amplitudes in DSR."

http://arxiv.org/abs/gr-qc/0701071
A signature of quantum gravity at the source of the seeds of cosmic structure?
Daniel Sudarsky
Prepared for the proceedings the Third International Workshop DICE 2006, "Quantum Mechanics between Decoherence and Determinism"

"This article reviews a recent work by a couple of colleagues and myself about the shortcomings of the standard explanations of the quantum origin of cosmic structure in the inflationary scenario, and a proposal to address them. The point it that in the usual accounts the inhomogeneity and anisotropy of our universe seem to emerge from an exactly homogeneous and isotropic initial state through processes that do not break those symmetries. We argued that some novel aspect of physics must be called upon to able to address the problem in a fully satisfactory way. The proposed approach is inspired on Penrose's ideas regarding an quantum gravity induced, real and dynamical collapse of the wave function."

It has been announced that this year Loops '07 conference will be in Mexico City (Sudarsky's home base) this year. I believe he is one of the conference organizers. I will keep special track of QG at Uni Mex, to keep prospective participants apprised of their hosts research interests.

 

[marcus]

CORRECTION:
LOOPS '07, this year's international conference on background independent (i.e. non-string) quantum gravity, will be at Morelia, about 200 miles west of Mexico City. Morelia is the site of the National University campus and several related institutes. Earlier I was under the mistaken impression that the conference was to be at Mexico City. The old city of Morelia has the reputation of being a picturesque place for sight-seeing, as well as having the University.

here is Francesca's post about the conference:
https://www.physicsforums.com/showthread.php?p=1212084#post1212084

here is the Morelia Math Institute website:
http://www.matmor.unam.mx/

it has this link to some tourist information in whichever language you prefer:
http://www.matmor.unam.mx/Morelia/espanol/default.html
http://www.matmor.unam.mx/Morelia/ingles/default.html
=========
this paper by Jonathan Engle appeared today:
http://arxiv.org/abs/gr-qc/0701132
On the physical interpretation of states in loop quantum cosmology
Jonathan Engle
20 pages

"In this paper we address the physical meaning of states in loop quantum cosmology (LQC). A first step in this is the completion of the program begun in [1], applied to LQC. Specifically, we introduce a family of (what are called) b-embeddings of isotropic loop quantum cosmology (LQC) into full loop quantum gravity. As a side note, we exhibit a large class of operators preserving each of these embeddings, and show their consistency with the LQC quantization. Embedding at the gauge and diffeomorphism invariant level is discussed in the conclusion section."

Engle got his PhD at Penn State in 2006 and is now in Rovelli's group at Marseille (CPT Luminy)
====================

http://arxiv.org/abs/hep-th/0701239
Loop quantum gravity and black hole singularity
Leonardo Modesto
17 pages, 7 figures. Proceedings of the XVII SIGRAV Conference, Turin, September 4-7, 2006

"In this paper we summarize loop quantum gravity (LQG) and we show how ideas developed in LQG can solve the black hole singularity problem when applied to a minisuperspace model."

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

The January 2007 KITP workshop on The Quantum Nature of Spacetime Singularities menu of talks online is here:
http://online.kitp.ucsb.edu/online/singular_m07/

Videos of the talks are available for download.
Some of the talks which may be of special interest to people concentrating on background independent (non-string) QG are those of

Ashtekar
http://online.kitp.ucsb.edu/online/singular_m07/ashtekar/

Thiemann (first hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann/

Thiemann (second hour)
http://online.kitp.ucsb.edu/online/singular_m07/thiemann1/

Bojowald
http://online.kitp.ucsb.edu/online/singular_m07/bojowald/

===================
http://arxiv.org/abs/astro-ph/0701584
Constraints on Dark Energy from Supernovae, Gamma Ray Bursts, Acoustic Oscillations, Nucleosynthesis and Large Scale Structure and the Hubble constant
Edward L. Wright (UCLA)
16 pages, 8 figure

"The luminosity distance vs. redshift law is now measured using supernovae and gamma ray bursts, and the angular size distance is measured at the surface of last scattering by the CMB and at z = 0.35 by baryon acoustic oscillations. In this paper this data is fit to models for the equation of state with w = -1, w = const, and w(z) = w_0+w_a(1-a). The last model is poorly constrained by the distance data, leading to unphysical solutions where the dark energy dominates at early times unless the large scale structure and acoustic scale constraints are modified to allow for early time dark energy effects. A flat LambdaCDM model is consistent with all the data."

Note discussion section page 14:

"Using all the data together
gives the plot shown in Figure 5. The best fit model is slightly closed with
Omega_tot = 1.011 and M = 0.315. "

===============
http://arxiv.org/abs/gr-qc/0701140
New Energy Definition for Higher Curvature Gravities
S. Deser, Bayram Tekin
4 pages

"We propose a novel but natural definition of conserved quantities for gravity models quadratic and higher in curvature. Based on the spatial asymptotics of curvature rather than of metric, it avoids the GR energy machinery's more egregious problems--such as zero energy 'theorems' and failure in flat backgrounds -- in this fourth-derivative realm. In D>4, the present expression indeed correctly discriminates between second derivative Gauss-Bonnet and generic, fourth derivative, actions."

 

[marcus]

http://arxiv.org/abs/gr-qc/0701142
Quantum gravity and cosmological observations
Martin Bojowald
8 pages, plenary talk at the VIth Latin American Symposium on High Energy Physics (Puerto Vallarta, Mexico, Nov. 2006)

"Quantum gravity places entirely new challenges on the formulation of a consistent theory as well as on an extraction of potentially observable effects. Quantum corrections due to the gravitational field are commonly expected to be tiny because of the smallness of the Planck length. However, a consistent formulation now shows that key features of quantum gravity imply magnification effects on correction terms which are especially important in cosmology with its long stretches of evolution. After a review of the salient features of recent canonical quantizations of gravity and their implications for the quantum structure of space-time a new example for potentially observable effects is given."

 

[marcus]

a possibly interesting challenge to LQC:
http://arxiv.org/abs/gr-qc/0701172
On the Onset of Inflation in Loop Quantum Cosmology
Cristiano Germani (SISSA), William Nelson (KCL London), Mairi Sakellariadou (KCL London)
18 pages, 5 figures

"Using a Liouville measure, similar to the one proposed recently by Gibbons and Turok, we investigate the probability that single-field inflation with a polynomial potential can last long enough to solve the shortcomings of the standard hot big bang model, within the semiclassical regime of loop quantum cosmology. We conclude that, for such a class of inflationary models and for natural values of the loop quantum cosmology parameters, a successful inflationary scenario is highly improbable."

http://arxiv.org/abs/astro-ph/0701848
The modified Newtonian dynamics-MOND-and its implications for new physics
Jacob D. Bekenstein
Pedagogical introduction to MOND and review of modern developments. To appear shortly in Contemporary Physics; 6 figures, 15 pages

"No more salient issue exists in contemporary astrophysics and cosmology than that of the elusive 'dark matter'. For many years already Milgrom's paradigm of modified Newtonian dynamics (MOND) has provided an alternative way to interpret observations without appeal to invisible dark matter. MOND had been successful in elucidating economically the dynamics of disk galaxies of all scales, while doing less well for clusters of galaxies; in its original form it could not address gravitational lensing or cosmology. After reviewing some of the evidence in favor of MOND, I recollect the development of relativistic formulations for it to cope with the last deficiency. I comment on recent work by various groups in confronting TeVeS, a relativistic embodiment of MOND, with observational data on gravitational lensing and cosmology. Throughout I ask what sort of physics can be responsible for the efficacy of MOND, and conclude with an appraisal of what theoretical developments are still needed to reach a full description of the world involving no unobserved matter."

http://arxiv.org/abs/gr-qc/0702002
Effect of the Inverse Volume Modification in Loop Quantum Cosmology
Hua-Hui Xiong, Jian-Yang Zhu
7 pages

"It is known that in loop quantum cosmology the universe avoids the singularity by a bounce when the matter density approaches the critical density $\rho_c$ (the order of Planck density). In the framework of the effective Hamiltonian incorporating the inverse volume modifications both in the gravitational and matter part, we find that two modifications play different roles for the effective dynamics (the modified Friedmann equation). In the case $a_{g*}>a_{bounce}$, $a_{bounce}$ denotes the bounce scale and the scale $a_{g*}$ marked the different region below which the modification in the gravitational part become notable, the modification in the gravitational part decrease the matter density at the bounce point such that $\rho_{bounce}<\rho _c$. For the case $a_{m*}>a_{bounce}$ (the role of $a_{m*}$ is same as $a_{g*}$), the modification in the matter part helps the matter density to approach the critical density $\rho_c$ quickly. Furthermore, based on the physical reason it is suitable to neglect the inverse volume modification in the gravitational part."

http://arxiv.org/abs/gr-qc/0702003
Tachyon field in Loop Quantum Cosmology: inflation and evolution picture
Hua-Hui Xiong, Jian-Yang Zhu
7 pages, 3 figures

"Loop quantum cosmology (LQC) predicts a nonsingular evolution of the universe through a bounce in the high energy region. We show that this is always true in tachyon matter LQC. Different from the classical FRW cosmology, the superinflation can appear in the tachyon matter LQC; furthermore, the inflation can be extended to the region where classical inflation stops. Using numerical method, we give an evolution picture of the tachyon field with an exponential potential in the context of LQC. It indicates that the quantum dynamical solutions have the attractor behavior as the classical solutions does. And, the whole evolution of the tachyon field is that: at the far past, the tachyon field, being in the contracting cosmology, is accelerated to climb up the potential hill with a negative velocity; and then, the tachyon field at the boundary is bounced into an expanding universe with positive velocity rolling down to the bottom of the potential."

http://arxiv.org/abs/astro-ph/0702010
Universe's Skeleton Sketched
Eric V. Linder
News & Views, Nature 445, 273 (2007)

"The deepest and clearest maps yet of the Universe's skeleton of dark matter structure present a picture broadly in concord with favoured models - although puzzling discrepancies remain."

 

[marcus]

new one from Bee Hossenfelder

http://arxiv.org/abs/hep-th/0702016
Multi-Particle States in Deformed Special Relativity
Sabine Hossenfelder

"We investigate the properties of multi-particle states in Deformed Special Relativity (DSR). Starting from the Lagrangian formalism with an energy dependent metric, the conserved Noether current is derived and is found to be additive in the usual way. This quantity had previously been discarded because it was correctly realized that it does no longer obey the DSR transformations. We identify the reason for this mismatch in the fact that DSR depends only on the extensive quantity of total four-momentum instead of the energy-momentum densities as would be appropriate for a field theory. We argue that the reason for the failure of DSR to reproduce the standard transformation behavior in the well established limits is due to the missing sensitivity to the volume inside which energy is accumulated. We show that the soccer-ball problem is absent if one formulates DSR instead for the field densities. As a consequence, estimates for predicted effects have to be corrected by many orders of magnitude. Further, we derive that the modified quantum field theory implies a locality bound."

=========================
briefly noted:

http://arxiv.org/abs/gr-qc/0702023
Evolutionary Reformulation of Quantum Gravity
Giovanni Montani
5 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin

"We present a critical analysis of the Canonical approach to quantum gravity, which relies on the ambiguity of implementing a space-time slicing on the quantum level. We emphasize that such a splitting procedure is consistent only if a real matter fluid is involved in the dynamics."


http://arxiv.org/abs/gr-qc/0702025
Electro-weak Model within a 5-dimensional Lorentz group theory
Orchidea Maria Lecian, Giovanni Montani
3 pages, proceedings of the XI Marcel Grossmann meeting on Relativistic Astrophysics, July 23-29, 2006, Berlin

"The Electroweak model will be geometrized in a 5-D Riemann-Cartan framework: U(1) weak hyper-charge group will be worked out in a Klauza-Klein scheme, while SU(2) weak isospin group will be identified to suitable bein projections of the contortion field. The possibility of introducing Ashtekar formalism in 5-D Klauza-Klein theories will be investigated."



http://arxiv.org/abs/gr-qc/0702026
Electro-weak Model within the framework of Lorentz gauge theory: Ashtekar variables?
Orchidea Maria Lecian, Giovanni Montani
5 pages, to appear in the Proceedings of the 1st Stueckelberg Workshop

"The Electroweak (EW) model is geometrized in the framework of a 5D gauge theory of the Lorentz group, after the implementation of the Kaluza-Klein (KK) paradigm. The possibility of introducing Ashtekar variables on a 5D KK manifold is considered on the ground of its geometrical structure."

 

[marcus]

http://arxiv.org/abs/gr-qc/0702030
An Introduction to Loop Quantum Gravity Through Cosmology
Abhay Ashtekar
20 pages, 4 figures, Introductory Review

"This introductory review is addressed to beginning researchers. Some of the distinguishing features of loop quantum gravity are illustrated through loop quantum cosmology of FRW models. In particular, these examples illustrate: i) how 'emergent time' can arise; ii) how the technical issue of solving the Hamiltonian constraint and constructing the physical sector of the theory can be handled; iii) how questions central to the Planck scale physics can be answered using such a framework; and, iv) how quantum geometry effects can dramatically change physics near singularities and yet naturally turn themselves off and reproduce classical general relativity when space-time curvature is significantly weaker than the Planck scale."

this paper of Ashtekar's grew out of a talk he gave in Summer 2006 at the First Stueckelberg Workshop at Pescara, italy.
http://www.icra.it/ICRA_Networkshops/INw20_Stueckelberg/Welcome.htmhttp://arxiv.org/abs/astro-ph/0702146
The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter
J. R. Brownstein, J. W. Moffat
25 pages, 14 figures, 5 tables

"A detailed analysis of the November 15, 2006 data release (Clowe et al., 2006) X-ray surface density Sigma-map and the strong and weak gravitational lensing convergence kappa-map for the Bullet Cluster 1E0657-558 is performed and the results are compared with the predictions of a modified gravity (MOG) and dark matter. Our surface density Sigma-model is computed using a King beta-model density, and a mass profile of the main cluster and an isothermal temperature profile are determined by the MOG. We find that the main cluster thermal profile is nearly isothermal. The MOG prediction of the isothermal temperature of the main cluster is T = 15.5 +- 3.9 keV, in good agreement with the experimental value T = 14.8{+2.0}{-1.7} keV. Excellent fits to the two-dimensional convergence kappa-map data are obtained without non-baryonic dark matter, accounting for the 8-sigma spatial offset between the Sigma-map and the kappa-map reported in Clowe et al. (2006). The MOG prediction for the kappa-map results in two baryonic components distributed across the Bullet Cluster 1E0657-558 with averaged mass-fraction of 83% intracluster medium (ICM) gas and 17% galaxies. Conversely, the Newtonian dark matter kappa-model has on average 76% dark matter (neglecting the indeterminant contribution due to the galaxies) and 24% ICM gas for a baryon to dark matter mass-fraction of 0.32, a statistically significant result when compared to the predicted Lambda-CDM cosmological baryon mass-fraction of 0.176{+0.019}{-0.012} (Spergel et al., 2006)."http://arxiv.org/abs/math.QA/0702140
Lectures on Noncommutative Geometry
Masoud Khalkhali
112 pages

"This text is an introduction to a few selected areas of Alain Connes' noncommutative geometry written for the volume of the school/conference 'Noncommutative Geometry 2005' held at IPM Tehran. It is an expanded version of my lectures which was directed at graduate students and novice in the subject."

 

[marcus]

http://arxiv.org/abs/gr-qc/0702044
Disordered locality in loop quantum gravity states
Fotini Markopoulou, Lee Smolin
11 pages, 4 figures

"We show that loop quantum gravity suffers from a potential problem with non-locality, coming from a mismatch between micro-locality, as defined by the combinatorial structures of their microscopic states, and macro-locality, defined by the metric which emerges from the low energy limit. As a result, the low energy limit may suffer from a disordered locality characterized by identifications of far away points. We argue that if such defects in locality are rare enough they will be difficult to detect."

http://arxiv.org/abs/hep-th/0702051
On the Possibility of Quantum Gravity Effects at Astrophysical Scales
M. Reuter, H. Weyer
18 pages, 4 figures. Invited contribution to the Int. J. Mod. Phys. D special issue on dark matter and dark energy

"The nonperturbative renormalization group flow of Quantum Einstein Gravity (QEG) is reviewed. It is argued that at large distances there could be strong renormalization effects, including a scale dependence of Newton's constant, which mimic the presence of dark matter at galactic and cosmological scales."

http://arxiv.org/abs/gr-qc/0702036
Phase-space and Black Hole Entropy of Toroidal Horizons in Loop Quantum Gravity
S. Kloster, J. Brannlund, A. DeBenedictis
14 pages, 6 figures

"In the context of loop quantum gravity, we construct the phase-space of the isolated horizon with toroidal topology. Within the loop quantum gravity framework, this horizon is described by a torus with N punctures and the dimension of the corresponding phase-space is calculated including the toroidal cycles as degrees of freedom. From this, the black hole entropy can be calculated by counting the microstates which correspond to a black hole of fixed area. We find that the leading term agrees with the A/4 law and that the sub-leading contribution is modified by the toroidal cycles."http://arxiv.org/abs/hep-th/0702064
A fine tuning free resolution of the cosmological constant problem
Stephon Alexander, Deepak Vaid
5 pages, 2 figures

"In a recent paper we discovered that a fermionic condensate is formed from gravitational interactions due to the covariant coupling of fermions in the presence of a torsion-fermion contact interaction. The condensate gap gives a negative contribution to the bare cosmological constant. In this letter, we show that the cosmological constant problem can be solved without fine tuning of the bare cosmological constant. We demonstrate how a universe with a large initial cosmological constant undergoes inflation, during which time the energy gap grows as the volume of the universe. Eventually the gap becomes large enough to cancel out the bare cosmological term, inflation ends and we end up in a universe with an almost vanishing cosmological term. We provide a detailed numerical analysis of the system of equations governing the self regulating relaxation of the cosmological constant."http://arxiv.org/abs/gr-qc/0702049
Canonical quantization and the spectral action, a nice example
Fabien Besnard (CPL)
22 pages. Revised version submitted to Journal of Geometry and Physics

"We study the canonical quantization of the theory given by Chamseddine-Connes spectral action on a particular finite spectral triple with algebra $M_2(C)\oplus C$. We define a quantization of the natural distance associated with this noncommutative space and show that the quantum distance operator has a discrete spectrum. We also show that it would be the same for any other geometric quantity. Finally we propose a physical Hilbert space for the quantum theory. This spectral triple had been previously considered by Rovelli as a toy model, but with a different action which was not gauge-invariant. The results are similar in both cases, but the gauge-invariance of the spectral action manifests itself by the presence of a non-trivial degeneracy structure for our distance operator."

http://arxiv.org/abs/astro-ph/0702207
It's Never Too Late For Matter
Lawrence M. Krauss (Case Western Reserve University and Vanderbilt University), Robert J. Scherrer (Vanderbilt University)
submitted to PRL

"We demonstrate that in a vacuum-energy-dominated expansion phase, neither the decay of matter nor matter-antimatter annihilation into relativistic particles can ever cause radiation to once again dominate over matter in the future history of the universe."

 

[MeJennifer]

http://arxiv.org/abs/quant-ph/0611261
Concerning Dice and Divinity
D.M.Appleby
Contribution to proceedings of Foundations of Probability and Physics, Vaxjo, 2006

"Einstein initially objected to the probabilistic aspect of quantum mechanics - the idea that God is playing at dice. Later he changed his ground, and focussed instead on the point that the Copenhagen Interpretation leads to what Einstein saw as the abandonment of physical realism. We argue here that Einstein's initial intuition was perfectly sound, and that it is precisely the fact that quantum mechanics is a fundamentally probabilistic theory which is at the root of all the controversies regarding its interpretation. Probability is an intrinsically logical concept. This means that the quantum state has an essentially logical significance. It is extremely difficult to reconcile that fact with Einstein's belief, that it is the task of physics to give us a vision of the world apprehended sub specie aeternitatis. Quantum mechanics thus presents us with a simple choice: either to follow Einstein in looking for a theory which is not probabilistic at the fundamental level, or else to accept that physics does not in fact put us in the position of God looking down on things from above. There is a widespread fear that the latter alternative must inevitably lead to a greatly impoverished, positivistic view of physical theory. It appears to us, however, that the truth is just the opposite. The Einsteinian vision is much less attractive than it seems at first sight. In particular, it is closely connected with philosophical reductionism."

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