Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/pdf/0902.0590v1
Lorentz symmetry breaking as a quantum field theory regulator
Matt Visser (Victoria University of Wellington)
(Submitted on 3 Feb 2009)
"Perturbative expansions of relativistic quantum field theories typically contain ultraviolet divergences requiring regularization and renormalization. Many different regularization techniques have been developed over the years, but most regularizations require severe mutilation of the logical foundations of the theory. In contrast, breaking Lorentz invariance, while it is certainly a radical step, at least does not damage the logical foundations of the theory. We shall explore the features of a Lorentz symmetry breaking regulator in a simple polynomial scalar field theory, and discuss its implications. We shall quantify just "how much" Lorentz symmetry breaking is required to fully regulate the theory and render it finite. This scalar field theory provides a simple way of understanding many of the key features of Horava's recent article [arXiv:0901.3775 [hep-th]] on 3+1 dimensional quantum gravity."

(thx to CarlB)

http://arxiv.org/abs/0902.0697
Big Bounce and inhomogeneities
David Brizuela, Guillermo A. Mena Marugan, Tomasz Pawlowski
4 pages, 2 figures
(Submitted on 4 Feb 2009)
"The dynamics of an inhomogeneous universe is studied with the methods of Loop Quantum Cosmology as an example of the quantization of vacuum cosmological spacetimes containing gravitational waves (Gowdy spacetimes). The analysis performed at the effective level shows that: (i) The initial Big Bang singularity is replaced (as in the case of homogeneous cosmological models) by a Big Bounce, joining deterministically two large universes, (ii) the universe size at the bounce is at least of the same order of magnitude as that of the background homogeneous universe, (iii) for each gravitational wave mode, the difference in amplitude at very early and very late times has a vanishing statistical average when the bounce dynamics is strongly dominated by the inhomogeneities, whereas this average is positive when the dynamics is in a near-vacuum regime, so that statistically the inhomogeneities are amplified."

 

[marcus]

http://arxiv.org/abs/0902.0928
Chaplygin inflation in loop quantum cosmology
Xin Zhang, Jingfei Zhang, Jinglei Cui, Li Zhang
6 pages, accepted for publication in Mod. Phys. Lett. A
(Submitted on 5 Feb 2009)
"In this paper we discuss the inflationary universe in the context of a Chaplygin gas equation of state within the framework of the effective theory of loop quantum cosmology. Under the slow-roll approximation, we calculate the primordial perturbations for this model. We give the general expressions of the scalar spectral index, its running, and the tensor-to-scalar ratio, etc. For the chaotic inflation with a quadratic potential, using the WMAP 5-year results, we determine the parameters of the Chaplygin inflation model in loop quantum cosmology. The results are consistent with the WMAP observations."


http://arxiv.org/abs/0902.0957
The Barbero-Immirzi field in canonical formalism of pure gravity
Gianluca Calcagni, Simone Mercuri
9 pages
(Submitted on 5 Feb 2009)
"The Barbero-Immirzi (BI) parameter is promoted to a field and a canonical analysis is performed when it is coupled with a Nieh-Yan topological invariant. It is shown that, in the effective theory, the BI field is a canonical pseudo-scalar minimally coupled with gravity. This framework is argued to be more natural than the one of the usual Holst action. Potential consequences in relation with inflation and the quantum theory are briefly discussed."

 

[MTd2]

http://arxiv.org/abs/0902.1170
Asymptotic analysis of the EPRL four-simplex amplitude
Authors: John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Henrique Gomes, Frank Hellmann
(Submitted on 6 Feb 2009)

Abstract: An asymptotic formula for a certain 4d Euclidean spin foam 4-simplex amplitude is given for the limit of large spins. The analysis covers the model with Immirzi parameter less than one defined separately by Engle, Livine, Pereira and Rovelli (EPRL) and Freidel and Krasnov (FK). We are also able to analyse the EPRL model with Immirzi parameter greater than one. The asymptotic formula has one term which is proportional to the cosine of the Regge action for gravity, and it is shown that this term is present whenever the boundary data determines a non-degenerate Euclidean geometry for the 4-simplex. A scheme for resolving the phase ambiguity of the boundary data in these cases is also presented.

 

[MTd2]

http://arxiv.org/abs/0902.2001v1
Deformed General Relativity and Torsion
Authors: Gary W. Gibbons, Steffen Gielen
(Submitted on 11 Feb 2009)

Abstract: We argue that the natural framework for embedding the ideas of deformed, or doubly, special relativity (DSR) into a curved spacetime is a generalisation of Einstein-Cartan theory, considered by Stelle and West. Instead of interpreting the noncommuting "spacetime coordinates" of the Snyder algebra as endowing spacetime with a fundamentally noncommutative structure, we are led to consider a connection with torsion in this framework. This may lead to the usual ambiguities in minimal coupling. We note that observable violations of charge conservation induced by torsion should happen on a time scale of 10^3 s, which seems to rule out these modifications as a serious theory. Our considerations show, however, that the noncommutativity of translations in the Snyder algebra need not correspond to noncommutative spacetime in the usual sense.

LoL Gary Gibbons on DSL bandwagon!

 

[marcus]

http://arxiv.org/abs/0902.1551
Solving the Problem of Time in Mini-superspace: Measurement of Dirac Observables
Donald Marolf
12 pages
(Submitted on 9 Feb 2009)
"One solution to the so-called problem of time is to construct certain Dirac observables, sometimes called evolving constants of motion. There has been some discussion in the literature about the interpretation of such observables, and in particular whether single Dirac observables can be measured. Here we clarify the situation by describing a class of interactions that can be said to implement measurements of such observables. Along the way, we describe a useful notion of perturbation theory for the rigging map eta of group averaging (sometimes loosely called the physical state 'projector'), which maps states from the auxiliary Hilbert space to the physical Hilbert space."

http://arxiv.org/abs/0902.1913
Alternative quantization of the Hamiltonian in isotropic loop quantum cosmology
Jinsong Yang, You Ding, Yongge Ma
6 pages, 2 figures
(Submitted on 11 Feb 2009)
"Since there are quantization ambiguities in constructing the Hamiltonian constraint operator in isotropic loop quantum cosmology, it is crucial to check whether the key features of loop quantum cosmology, such as the quantum bounce and effective scenario, are robust against the ambiguities. In this paper, we consider a typical quantization ambiguity arising from the quantization of the field strength of the gravitational connection. An alternative Hamiltonian constraint operator is constructed, which is shown to have the correct classical limit by the semiclassical analysis. The effective Hamiltonian incorporating higher order quantum corrections is also obtained. In the spatially flat FRW model with a massless scalar field, the classical big bang is again replaced by a quantum bounce. Moreover, there are still great possibilities for the expanding universe to recollapse due to the quantum gravity effect. Thus, these key features are robust against this quantization ambiguity."

http://arxiv.org/abs/0902.1313
What the small angle CMB really tells us about the curvature of the Universe
Timothy Clifton, Pedro G. Ferreira, Joe Zuntz
4 pages, 4 figures
(Submitted on 8 Feb 2009)
"It is well known that observations of the cosmic microwave background (CMB) are highly sensitive to the spatial curvature of the Universe, k. Here we find that what is in fact being tightly constrained by small angle fluctuations is spatial curvature near the surface of last scattering, and that if we allow k to be a function of position, rather than taking a constant value everywhere, then considerable spatial curvature is permissible within our own locale. This result is of interest for the giant void models that attempt to explain the supernovae observations without Dark Energy. We find such voids to be compatible with the observed small angle CMB, but they must be either very deep (and unnaturally empty) or exist in a positively curved Universe."

 

[MTd2]

http://arxiv.org/abs/0902.2301v1

A note on discrete Holonomy through directed edges, with no lengths
Authors: Stuart Armstrong, Jussi Westergren
(Submitted on 13 Feb 2009)

Abstract: This note demonstrates how both the concept of distance and the concept of holonomy can be constructed from a suitable network with directed edges (and no lengths). The number of different edge types depends on the signature of the metric and the dimension of the holonomy group. If the holonomy group is of dimension one and the metric is positive-definite, a single type of directed edges is needed.

 

[MTd2]

http://arxiv.org/abs/0902.2247The Science Case for STEP
Authors: James Overduin, Francis Everitt, John Mester, Paul Worden

STEP (the Satellite Test of the Equivalence Principle) will advance experimental limits on violations of Einstein's equivalence principle (EP) from their present sensitivity of 2 parts in 10^13 to 1 part in 10^18 through multiple comparison of the motions of four pairs of test masses of different compositions in an earth-orbiting drag-free satellite. Dimensional arguments suggest that violations, if they exist, should be found in this range, and they are also suggested by leading attempts at unified theories of fundamental interactions (e.g. string theory) and cosmological theories involving dynamical dark energy. Discovery of a violation would constitute the discovery of a new force of nature and provide a critical signpost toward unification. A null result would be just as profound, because it would close off any possibility of a natural-strength coupling between standard-model fields and the new light degrees of freedom that such theories generically predict (e.g., dilatons, moduli, quintessence). STEP should thus be seen as the intermediate-scale component of an integrated strategy for fundamental physics experiments that already includes particle acclerators (at the smallest scales) and supernova probes (at the largest). The former may find indirect evidence for new fields via their missing-energy signatures, and the latter may produce direct evidence through changes in cosmological equation of state--but only a gravitational experiment like STEP can go further and reveal how or whether such a field couples to the rest of the standard model. It is at once complementary to the other two kinds of tests, and a uniquely powerful probe of fundamental physics in its own right.

 

[MTd2]

http://arxiv.org/abs/0902.2764v1

Peccei--Quinn mechanism in gravity and the nature of the Barbero--Immirzi parameter
Authors: Simone Mercuri
(Submitted on 16 Feb 2009)

Abstract: A general argument provides the motivation to consider the Barbero--Immirzi parameter as a field. The specific form of the geometrical effective action allows to relate the value of the Barbero--Immirzi parameter to other quantum ambiguities through the analog of the Peccei--Quinn mechanism.

 

[marcus]

http://pirsa.org/09020023/
Graviton propagator from EPRL spinfoam model
Claudio Perini - CPT
"We derive geometric correlation functions in the new spinfoam model with coherent states techniques, making connection with quantum Regge calculus and perturbative quantum gravity. In particular we recover the expected scaling with distance for all components of the propagator. We expect the same technique to be well-suited for other spinfoam models."
Date: 11/02/2009 - 4:00 pm

http://arxiv.org/abs/0902.2490
Tensor power spectrum with holonomy corrections in LQC
Jakub Mielczarek
13 pages, 13 figures
(Submitted on 14 Feb 2009)
"In this paper we consider tensor perturbations produced at a bounce phase in presence of the holonomy corrections. Here bounce phase and holonomy corrections originate from Loop Quantum Cosmology. We re-derive formulas for the of the corrections for the model with a scalar field content. Background dynamics with a free scalar field and multi-fluid potential are considered. Both analytical approximations as well as numerical investigations were performed. We have found analytical solutions on super-horizontal and sub-horizontal regimes and derived corresponding power spectra. Also occupation number n_{\bf k} and parameter \Omega_{\text{gw}} were derived in sub-horizontal limit, leading to its extremely low present value. Final results are numerical power spectra of the gravitational waves produced in the presence of quantum holonomy corrections. In the super-horizontal limit the obtained spectrum behaves like \mathcal{P}_T \propto k^3(C_1+\log^2(k)) while on sub-horizontal scales it exhibits oscillations around \mathcal{P}_T \propto k^2. These results can be directly applied as initial conditions for the inflationary modes. We mention possible resulting observational features of the CMB in particular BB spectrum of polarization."

http://arxiv.org/abs/0902.2764
Peccei--Quinn mechanism in gravity and the nature of the Barbero--Immirzi parameter
Simone Mercuri
5 pages
(Submitted on 16 Feb 2009)
"A general argument provides the motivation to consider the Barbero--Immirzi parameter as a field. The specific form of the geometrical effective action allows to relate the value of the Barbero--Immirzi parameter to other quantum ambiguities through the analog of the Peccei--Quinn mechanism."

 

[MTd2]

http://arxiv.org/abs/0902.3447
Connection between Fermionic Strings and Quantum Gravity States -- A loop space approach
Authors: Luiz C.L. Botelho
(Submitted on 19 Feb 2009)

Abstract: We present physical arguments based on loop space representations for Dirac/Klein gordon determinants that some suitable Fermionic String Ising models at the critical point and defined on the space-time base manifold are formal quantum states of the gravitational field when quantized in the Ashtekar-Sen connection canonical formalism .These results complements our previous Loop Space studies on the subject (Random surface representation for Einstein quantum gravity, Phys.Rev.D52,6941 (1995))

Hmm, I knew that Ashok Sen proposed at least 5 years ago that LQG was on of the limits of M Theory. But many people didnt give credit to that.

 

[marcus]

http://arxiv.org/abs/0902.3416
4d Lorentzian Holst action with topological terms
Danilo Jimenez Rezende, Alejandro Perez
(Submitted on 19 Feb 2009)
"We study the Hamiltonian formulation of the general first order action of general relativity compatible with local Lorentz invariance and background independence. The most general simplectic structure (compatible with diffeomorphism invariance and local Lorentz transformations) is obtained by adding to the Holst action the Pontryagin, Euler and Nieh-Yan invariants with independent coupling constants. We perform a detailed canonical analysis of this general formulation (in the time gauge) exploring the structure of the phase space in terms of connection variables. We explain the relationship of these topological terms, and the effect of large SU(2) gauge transformations in quantum theories of gravity defined in terms of the Ashtekar-Barbero connection."

 

[marcus]

http://arxiv.org/abs/0902.3903
Group field theory and simplicial quantum gravity
Daniele Oriti
11 pages
(Submitted on 23 Feb 2009)
"We present a new Group Field Theory for 4d quantum gravity. It incorporates the constraints that give gravity from BF theory, and has quantum amplitudes with the explicit form of simplicial path integrals for 1st order gravity. The geometric interpretation of the variables and of the contributions to the quantum amplitudes is manifest. This allows a direct link with other simplicial gravity approaches, like quantum Regge calculus, in the form of the amplitudes of the model, and dynamical triangulations, which we show to correspond to a simple restriction of the same."

http://arxiv.org/abs/0902.3657
Spectral Dimension of Spacetime in Quantum Gravity at a Lifgarbagez Point
Petr Horava
11 pages, 1 figure
(Submitted on 23 Feb 2009)
"We extend the definition of "spectral dimension" (usually defined for fractal and lattice geometries) to theories on smooth spacetimes with anisotropic scaling. We show that in quantum gravity dominated by a Lifgarbagez point with dynamical critical exponent z in D+1 spacetime dimensions, the spectral dimension of spacetime is equal to d_s=1+D/z. In the case of gravity in 3+1 dimensions presented in arXiv:0901.3775, which is dominated by z=3 in the UV and flows to z=1 in the IR, the spectral dimension of spacetime flows from d_s=4 at large scales, to d_s=2 at short distances. Remarkably, this is the qualitative behavior of d_s found numerically by Ambjorn, Jurkiewicz and Loll in their causal dynamical triangulations approach to quantum gravity."

Brief mention:
http://arxiv.org/abs/0902.3817
Noncommutative D=4 gravity coupled to fermions
Paolo Aschieri, Leonardo Castellani
19 pages
(Submitted on 23 Feb 2009)

http://arxiv.org/abs/0902.3823
Noncommutative supergravity in D=3 and D=4
Paolo Aschieri, Leonardo Castellani
22 pages
(Submitted on 23 Feb 2009)

 

[marcus]

http://arxiv.org/abs/0902.4191
Holonomy Loops, Spectral Triples & Quantum Gravity
Johannes Aastrup, Jesper M. Grimstrup, Ryszard Nest
24 pages, 7 figures, based on talk given by J.M.G. at the QG2 conference, Nottingham, juli 2008; at the QSTNG conference in Rome in sept/oct 2008; at the AONCG conference, Canberra, dec. 2008
(Submitted on 24 Feb 2009)
"We review the motivation, construction and physical interpretation of a semi-finite spectral triple obtained through a rearrangement of central elements of loop quantum gravity. The triple is based on a countable set of oriented graphs and the algebra consists of generalized holonomy loops in this set. The Dirac type operator resembles a global functional derivation operator and the interaction between the algebra of holonomy loops and the Dirac type operator reproduces the structure of a quantized Poisson bracket of general relativity. Finally we give a heuristic argument as to how a natural candidate for a quantized Hamiltonian might emerge from this spectral triple construction."

 

[MTd2]

http://arxiv.org/abs/0902.4630

Taming perturbative divergences in asymptotically safe gravity
Authors: Dario Benedetti, Pedro F. Machado, Frank Saueressig

Abstract: We use functional renormalization group methods to study gravity minimally coupled to a free scalar field. This setup provides the prototype of a gravitational theory which is perturbatively non-renormalizable at one-loop level, but may possesses a non-trivial renormalization group fixed point controlling its UV behavior. We show that such a fixed point indeed exists within the truncations considered, lending strong support to the conjectured asymptotic safety of the theory. In particular, we demonstrate that the counterterms responsible for its perturbative non-renormalizability have no qualitative effect on this feature.

 

[marcus]

http://arxiv.org/abs/0902.4731
Alternatives to Cosmological Inflation
Robert H. Brandenberger (McGill and IHEP)
14 pages, 5 figures, to be published in the proceedings of CosPA08
(Submitted on 27 Feb 2009)
"The inflationary paradigm, although very successful phenomenologically, suffers from several conceptual problems which motivate the search for alternative scenarios of early universe cosmology. Here, two possible alternatives will be reviewed. - "string gas cosmology" and the "matter bounce". Their successes and problems will be pointed out."

 

[MTd2]

http://arxiv.org/abs/0903.0267

From lattice BF gauge theory to area-angle Regge calculus
Authors: Valentin Bonzom

Abstract: We consider Riemannian 4d BF lattice gauge theory, on a triangulation of spacetime. Introducing the simplicity constraints which turn BF theory into simplicial gravity, some geometric quantities of Regge calculus, areas, and 3d and 4d dihedral angles, are identified. The parallel transport conditions are taken care of to ensure a consistent gluing of simplices. We show that these gluing relations, together with the simplicity constraints, contain the constraints of area-angle Regge calculus in a simple way, via the group structure of the underlying BF gauge theory. This provides a precise road from constrained BF theory to area-angle Regge calculus. Doing so, a framework combining variables of lattice BF theory and Regge calculus is built. The action takes a form {\it \`a la Regge} and includes the contribution of the Immirzi parameter. In the absence of simplicity constraints, the standard spin foam model for BF theory is recovered. Insertions of local observables are investigated, leading to Casimir insertions for areas and 6j-symbols for 3d angles. The present formulation is argued to be suitable for deriving spin foam models from discrete path integrals.

 

[marcus]

http://arxiv.org/abs/0903.0986
Cosmological solutions of emergent noncommutative gravity
Daniela Klammer, Harold Steinacker
4 pages, 2 figures
(Submitted on 5 Mar 2009)
"We find 2-parameter families of (approximate) cosmological solutions of Friedmann-Robertson-Walker type in matrix models of Yang-Mills type. Gravity arises through an intrinsically noncommutative mechanism on these branes, distinct from general relativity. No fine-tuning of a cosmological constant is required, in fact the solutions are stabilized through vacuum fluctuations. They generically have a big bounce, and an early inflation-like phase with graceful exit. The mechanism is purely geometrical, no ad-hoc scalar fields are introduced. The case k=-1 leads to a Milne-like universe after inflation, which appears to be in remarkably good agreement with observation."

http://arxiv.org/abs/0903.1015
Matrix Models, Emergent Gravity, and Gauge Theory
Harold Steinacker
8 pages. Based on invited talks given at the Conferences "Quantum Spacetime and Noncommutative Geometry", Rome, 2008 and at "Workshop on quantum gravity and nocommutative geometry", Lisbon, 2008 and at "Emergent Gravity", Boston, 2008 and at DICE2008, Italy, 2008 and at "QG2 2008 Quantum Geometry and Quantum Gravity", Nottingham, 2008
(Submitted on 5 Mar 2009)
"Matrix models of Yang-Mills type induce an effective gravity theory on 4-dimensional branes, which are considered as models for dynamical space-time. We review recent progress in the understanding of this emergent gravity. The metric is not fundamental but arises effectively in the semi-classical limit, along with nonabelian gauge fields. This leads to a mechanism for protecting certain geometries from corrections due to the vacuum energy."

http://arxiv.org/abs/0903.0902
Observing the Evolution of the Universe
James Aguirre, Alexandre Amblard, Amjad Ashoorioon, Carlo Baccigalupi, Amedeo Balbi, James Bartlett, Nicola Bartolo, Dominic Benford, Mark Birkinshaw, Jamie Bock, Dick Bond, Julian Borrill, Franois Bouchet, Michael Bridges, Emory Bunn, Erminia Calabrese, Christopher Cantalupo, Ana Caramete, Carmelita Carbone, Suchetana Chatterjee, Sarah Church, David Chuss, Carlo Contaldi, Asantha Cooray, Sudeep Das, Francesco De Bernardis, Paolo De Bernardis, Gianfranco De Zotti, Jacques Delabrouille, F.-Xavier Dsert, Mark Devlin, Clive Dickinson, Simon Dicker, Matt Dobbs, Scott Dodelson, Olivier Dore, Jessie Dotson, Joanna Dunkley, Maria Cristina Falvella, Dale Fixsen, Pablo Fosalba, Joseph Fowler, Evalyn Gates, Walter Gear, Sunil Golwala, Krzysztof Gorski, Alessandro Gruppuso, Josh Gundersen, Mark Halpern, et al. (126 additional authors not shown)
Science White Paper submitted to the US Astro2010 Decadal Survey. Full list of 177 author available at this http URL
(Submitted on 5 Mar 2009)
"How did the universe evolve? The fine angular scale (l>1000) temperature and polarization anisotropies in the CMB are a Rosetta stone for understanding the evolution of the universe. Through detailed measurements one may address everything from the physics of the birth of the universe to the history of star formation and the process by which galaxies formed. One may in addition track the evolution of the dark energy and discover the net neutrino mass.
We are at the dawn of a new era in which hundreds of square degrees of sky can be mapped with arcminute resolution and sensitivities measured in microKelvin. Acquiring these data requires the use of special purpose telescopes such as the Atacama Cosmology Telescope (ACT), located in Chile, and the South Pole Telescope (SPT). These new telescopes are outfitted with a new generation of custom mm-wave kilo-pixel arrays. Additional instruments are in the planning stages."

Brief mention:
http://arxiv.org/abs/0903.0878
Emergent Quantum Gravity: Macro/Micro Structures of Spacetime
B. L. Hu
15 pages. Essay for invited talks at the Symposium on "Foundations of Physics", University of Maryland, April 24-27, 2008, Workshop on "Emergent Gravity", MIT, August 25-29, 2008. In Proceedings of DICE 08 Conference, J. Phys. Conf. Ser. (2009)
(Submitted on 4 Mar 2009)

 

[marcus]

http://arxiv.org/abs/0903.1471
Dynamics and entanglement in spherically symmetric quantum gravity
Viqar Husain, Daniel R. Terno
12 pages, 1 figure
(Submitted on 9 Mar 2009)
"The gravity-scalar field system in spherical symmetry provides a natural setting for exploring gravitational collapse and its aftermath in quantum gravity. In a canonical approach, we give constructions of the Hamiltonian operator, and of semiclassical states peaked on constraint free data. Such states provide explicit examples of physical states. We also show that matter-gravity entanglement is an inherent feature of physical states, whether or not there is a black hole."

http://arxiv.org/abs/0903.1520
Natural inflation mechanism in asymptotic noncommutative geometry
William Nelson, Mairi Sakellariadou
3 pages
(Submitted on 9 Mar 2009)
"The possibility of having an inflationary epoch within a noncommutative geometry approach to unifying gravity and the standard model is demonstrated. This inflationary phase occurs without the need to introduce 'ad hoc' additional fields or potentials, rather it is a consequence of a nonminimal coupling between the geometry and the Higgs field."

http://arxiv.org/abs/0903.1105
The Event Horizon of Sagittarius A*
Avery E. Broderick (1), Abraham Loeb (2), Ramesh Narayan (2) ((1) CITA,(2) Harvard CfA)
11 pages, 6 figures, submitted to ApJ
(Submitted on 5 Mar 2009)
"Black hole event horizons, causally separating the external universe from compact regions of spacetime, are one of the most exotic predictions of General Relativity (GR). Until recently, their compact size has prevented efforts to study them directly. Here we show that recent millimeter and infrared observations of Sagittarius A* (Sgr A*), the supermassive black hole at the center of the Milky Way, all but requires the existence of a horizon. Specifically, we show that these observations limit the luminosity of any putative visible compact emitting region to below 0.4% of Sgr A*'s accretion luminosity. Equivalently, this requires the efficiency of converting the gravitational binding energy liberated during accretion into radiation and kinetic outflows to be greater than 99.6%, considerably larger than those implicated in Sgr A*, and therefore inconsistent with the existence of such a visible region. Finally, since we are able to frame this argument entirely in terms of observable quantities, our results apply to all geometric theories of gravity that admit stationary solutions, including the commonly discussed f(R) class of theories."

http://arxiv.org/abs/0903.1376
Particle Topology, Braids, and Braided Belts
Sundance Bilson-Thompson, Jonathan Hackett, Louis H. Kauffman
21 pages, 16 figures
(Submitted on 7 Mar 2009)
"Recent work suggests that topological features of certain quantum gravity theories can be interpreted as particles, matching the known fermions and bosons of the first generation in the Standard Model. This is achieved by identifying topological structures with elements of the framed Artin braid group on three strands, and demonstrating a correspondence between the invariants used to characterise these braids (a braid is a set of non-intersecting curves, that connect one set of N points with another set of N points), and quantities like electric charge, colour charge, and so on. In this paper we show how to manipulate a modified form of framed braids to yield an invariant standard form for sets of isomorphic braids, characterised by a vector of real numbers. This will serve as a basis for more complete discussions of quantum numbers in future work."

 

[MTd2]

http://arxiv.org/abs/0903.1667v1

Computing Black Hole entropy in Loop Quantum Gravity from a Conformal Field Theory perspective

Authors: Ivan Agullo, Enrique F. Borja, Jacobo Diaz-Polo
(Submitted on 10 Mar 2009)

Abstract: Motivated by the analogy proposed by Witten between Chern-Simons and Conformal Field Theories, we explore an alternative way of computing the entropy of a black hole starting from the isolated horizon framework in Loop Quantum Gravity. The consistency of the result opens a window for the interplay between Conformal Field Theory and the description of black holes in Loop Quantum Gravity.

 

[marcus]

http://arxiv.org/abs/0903.1859
Free will, undecidability, and the problem of time in quantum gravity
Rodolfo Gambini, Jorge Pullin
10 pages. This essay received the Second Community Prize in the Foundational Questions Institute (fqxi.org) essay contest
(Submitted on 10 Mar 2009)
"In quantum gravity there is no notion of absolute time. Like all other quantities in the theory, the notion of time has to be introduced 'relationally', by studying the behavior of some physical quantities in terms of others chosen as a 'clock'. We have recently introduced a consistent way of defining time relationally in general relativity. When quantum mechanics is formulated in terms of this new notion of time the resolution of the measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same experimental results of ordinary quantum mechanics, but every time an event is produced or a measurement happens two alternatives are possible: a) the state collapses; b) the system evolves without changing the state. One therefore has two possible behaviors of the quantum mechanical system and physical observations cannot decide between them, not just as a matter of experimental limitations but as an issue of principle. This first-ever example of fundamental undecidability in physics suggests that nature may behave sometimes as described by one alternative and sometimes as described by another. This in particular may give new vistas on the issue of free will."

http://arxiv.org/abs/0903.2065
The Trans-Planckian Problem for Inflationary Cosmology Revisited
Robert Brandenberger, Xinmin Zhang
8 pages, 2 figures
(Submitted on 12 Mar 2009)
"We consider an inflationary universe model in which the phase of accelerated expansion was preceded by a non-singular bounce and a period of contraction which involves a phase of deceleration. We follow fluctuations which exit the Hubble radius in the radiation-dominated contracting phase as quantum vacuum fluctuations, re-enter the Hubble radius in the deflationary period and re-cross during the phase of inflationary expansion. Evolving the fluctuations using the general relativistic linear perturbation equations, we find that they exit the Hubble radius during inflation not with a scale-invariant spectrum, but with a highly red spectrum with index n_s = -3. We also show that the back-reaction of fluctuations limits the time interval of deflation. Our toy model demonstrates the importance for inflationary cosmology both of the trans-Planckian problem for cosmological perturbations and of back-reaction effects . Firstly, without understanding both Planck-scale physics and the phase which preceded inflation, it is a non-trivial assumption to take the perturbations to be in their local vacuum state when they exit the Hubble radius at late times. Secondly, the back-reaction effects of fluctuations can influence the background in an important way."

 

[marcus]

http://arxiv.org/abs/0903.2270
A possible topological interpretation of the Barbero--Immirzi parameter
Simone Mercuri
6 pages, based on a talk given at the III Stueckelberg workshop on Relativistic Field Theories, July 8-18, 2008, Pescara (Italy)
(Submitted on 12 Mar 2009)
A possible topological interpretation of the Barbero--Immirzi parameter is proposed. Classically, by generalizing the Holst action to torsional spacetimes, we construct a precise analogy between the Barbero--Immirzi parameter and the $\theta$-angle of Yang--Mills gauge theories, where the role of the Pontryagin class is played by a well known topological term containing the Holst modification, the Nieh--Yan density. Quantum mechanically, the same analogy requires the study of the large gauge sector of the theory. In particular it is argued that a specific component of the large sector of the gauge group can be correlated with the Nieh--Yan density, where the Barbero--Immirzi parameter plays the role of the free angular parameter of the large gauge transformations.

http://arxiv.org/abs/0903.2438
Quantum cosmic censor: gravitation makes reality undecidable
Rodolfo Gambini, Jorge Pullin
3 Pages, This essay received an honorable mention from the Gravity Research Foundation essay competition in 2008
(Submitted on 13 Mar 2009)
"When one takes into account gravitation, the measurement of space and time cannot be carried out with infinite accuracy. When quantum mechanics is reformulated taking into account this lack of accuracy, the resolution of the measurement problem can be implemented via decoherence without the usual pitfalls. The resulting theory has the same physical predictions of quantum mechanics with a reduction postulate, but is radically different, with the quantum states evolving unitarily in terms of the underlying variables. Gravitation therefore makes this worrisome situation, potentially leading to two completely different views of reality, irrelevant from an empirical point of view. It may however be highly relevant from a philosophical point of view."

Brief mention:
http://arxiv.org/abs/0801.4415
Huge entropy production inside black holes
Colin S. Wallace, Andrew J. S. Hamilton, Gavin Polhemus (JILA)
Paper has been split into two. This paper now confines itself to presenting the general relativistic model. A companion paper, arXiv:0903.2290, discusses the quantum gravity implications.
(Submitted on 29 Jan 2008)
"... may exceed the Bekenstein-Hawking entropy by a large factor. If the black hole subsequently evaporates, radiating only the Bekenstein-Hawking entropy, then the black hole appears to destroy entropy, violating the second law of thermodynamics. A companion paper discusses the implications of this startling result..."
http://arxiv.org/abs/0903.2290
Entropy creation inside black holes points to observer complementarity
Gavin Polhemus, Andrew J. S. Hamilton, Colin S. Wallace
(Submitted on 12 Mar 2009)
Conclusions here seem bizarre, it may be interesting to see if the authors can get the paper published.

 

[marcus]

http://arxiv.org/abs/0903.2971
The role of Background Independence for Asymptotic Safety in Quantum Einstein Gravity
Martin Reuter, Holger Weyer
2 figures. Talk given by M.R. at the WE-Heraeus-Seminar "Quantum Gravity: Challenges and Perspectives", Bad Honnef, April 14-16, 2008; to appear in General Relativity and Gravitation
(Submitted on 17 Mar 2009)
"We discuss various basic conceptual issues related to coarse graining flows in quantum gravity. In particular the requirement of background independence is shown to lead to renormalization group (RG) flows which are significantly different from their analogs on a rigid background spacetime. The importance of these findings for the asymptotic safety approach to Quantum Einstein Gravity (QEG) is demonstrated in a simplified setting where only the conformal factor is quantized. We identify background independence as a (the ?) key prerequisite for the existence of a non-Gaussian RG fixed point and the renormalizability of QEG."

 

[MTd2]

Alright, alright. I regret posting this. You beat me by 10 seconds...

 

[MTd2]

http://arxiv.org/abs/0903.3397

Loop quantum cosmology of Bianchi I models
Authors: Abhay Ashtekar, Edward Wilson-Ewing
(Submitted on 19 Mar 2009)

Abstract: The "improved dynamics" of loop quantum cosmology is extended to include anisotropies of the Bianchi I model. As in the isotropic case, a massless scalar field serves as a relational time parameter. However, the extension is non-trivial because one has to face several conceptual subtleties as well as technical difficulties. These include: a better understanding of the relation between loop quantum gravity (LQG) and loop quantum cosmology (LQC); handling novel features associated with the non-local field strength operator in presence of anisotropies; and finding dynamical variables that make the action of the Hamiltonian constraint manageable. Our analysis provides a conceptually complete description that overcomes limitations of earlier works. We again find that the big bang singularity is resolved by quantum geometry effects but, because of the presence of Weyl curvature, Planck scale physics is now much richer than in the isotropic case. Since the Bianchi I models play a key role in the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture on the nature of generic space-like singularities in general relativity, the quantum dynamics of Bianchi I cosmologies is likely to provide considerable intuition about the fate of generic space-like singularities in quantum gravity. Finally, we show that the quantum dynamics of Bianchi I cosmologies projects down \emph{exactly} to that of the Friedmann model. This opens a new avenue to relate more complicated models to simpler ones, thereby providing a new tool to relate the quantum dynamics of LQG to that of LQC.

 

[marcus]

http://arxiv.org/abs/0903.3475
4d Deformed Special Relativity from Group Field Theories
Florian Girelli, Etera R. Livine, Daniele Oriti
23 pages
(Submitted on 20 Mar 2009)
"We derive a scalar field theory of the deformed special relativity type, living on non-commutative kappa-Minkowski spacetime and with a kappa-deformed Poincare symmetry, from the SO(4,1) group field theory defining the transition amplitudes for topological BF-theory in 4 space-time dimensions. This is done at a non-perturbative level of the spin foam formalism working directly with the group field theory (GFT). We show that matter fields emerge from the fundamental model as perturbations around a specific phase of the GFT, corresponding to a solution of the fundamental equations of motion, and that the non-commutative field theory governs their effective dynamics."

http://arxiv.org/abs/0903.3489
The Nature of Time
Julian Barbour
9 pages. This article won the first prize awarded on 7th March 2009 by the jury in the essay competition of the Foundational Questions Institute (fqxi.org) on The Nature of Time
(Submitted on 20 Mar 2009)
"A review of some basic facts of classical dynamics shows that time, or precisely duration, is redundant as a fundamental concept. Duration and the behaviour of clocks emerge from a timeless law that governs change."

 

[marcus]

http://arxiv.org/abs/0903.3832
"Forget time"
Carlo Rovelli
'First Community Prize' of the FQXi 'The Nature of Time' Essay Contest
(Submitted on 23 Mar 2009)
"Following a line of research that I have developed for several years, I argue that the best strategy for understanding quantum gravity is to build a picture of the physical world where the notion of time plays no role. I summarize here this point of view, explaining why I think that in a fundamental description of nature we must "forget time", and how this can be done in the classical and in the quantum theory. The idea is to develop a formalism that treats dependent and independent variables on the same footing. In short, I propose to interpret mechanics as a theory of relations between variables, rather than the theory of the evolution of variables in time."

http://arxiv.org/abs/0903.3970
Emergent non-commutative matter fields from Group Field Theory models of quantum spacetime
Daniele Oriti
13 pages; contribution to the proceedings of the D.I.C.E. 2008 workshop
(Submitted on 23 Mar 2009)
"We offer a perspective on some recent results obtained in the context of the group field theory approach to quantum gravity, on top of reviewing them briefly. These concern a natural mechanism for the emergence of non-commutative field theories for matter directly from the GFT action, in both 3 and 4 dimensions and in both Riemannian and Lorentzian signatures. As such they represent an important step, we argue, in bridging the gap between a quantum, discrete picture of a pre-geometric spacetime and the effective continuum geometric physics of gravity and matter, using ideas and tools from field theory and condensed matter analog gravity models, applied directly at the GFT level."

 

[marcus]

http://arxiv.org/abs/0903.4407
Interaction of the Barbero--Immirzi Field with Matter and Pseudo-Scalar Perturbations
Simone Mercuri, Victor Taveras
13 pages
(Submitted on 25 Mar 2009)
"In Loop Quantum Gravity the classical point of departure is the Einstein-Hilbert action modified by the addition of the so-called Holst term. Classically, this term does not affect the equations of motion, but it induces a well-known quantization ambiguity in the quantum theory, parametrized by the Barbero--Immirzi parameter. Recently it has been suggested to promote the Barbero--Immirzi parameter to a field. The resulting theory obtainable starting from the usual Holst action is General Relativity coupled to a pseudo-scalar field. However, this theory turns out to have an unconventional kinetic term for the BI field and a rather unnatural coupling with fermions.
The main goal of this work is twofold: Firstly, to propose a further generalization of the Holst action, which yields a theory of gravity and matter with a more natural coupling to the Barbero--Immirzi field; secondly, to study the possible implications for cosmology correlated to the existence of this new pseudo-scalar field."

http://arxiv.org/abs/0903.4329
Graviton propagator as a tool to test spinfoam models
Emanuele Alesci
Proceedings of the 3rd Stueckelberg Workshop on Relativistic Field Theories, July 2008, Pescara - Italy
(Submitted on 25 Mar 2009)
"I briefly review the advancements in the construction of the graviton propagator in the context of LQG and Spinfoam Models. In particular the problems of the Barrett-Crane vertex in giving the correct long-distance limit and the introduction of the new corrected models. This kind of calculation applied to an alternative vertex with given asymptotic can give the correct propagator and is then able to help selecting spinfoam models. In particular the study of the asymptotic properties of the new models shows the predicted behavior able to overcome the BC difficulties and to give the correct propagator."

http://arxiv.org/abs/0903.4342
Matter in Toy Dynamical Geometries
Tomasz Konopka
8 pages, 1 figure, paper presented at DICE 2008
(Submitted on 25 Mar 2009)
"One of the objectives of theories describing quantum dynamical geometry is to compute expectation values of geometrical observables. The results of such computations can be affected by whether or not matter is taken into account. It is thus important to understand to what extent and to what effect matter can affect dynamical geometries. Using a simple model, it is shown that matter can effectively mold a geometry into an isotropic configuration. Implications for "atomistic" models of quantum geometry are briefly discussed."

 

[marcus]

http://arxiv.org/abs/0903.4876
Is the notion of time really fundamental?
Florian Girelli, Stefano Liberati, Lorenzo Sindoni
9 pages, 2 figures. Fourth prize essay for the competition of the Foundational Questions Institute (fqxi.org) on the Nature of Time
(Submitted on 27 Mar 2009)
"From the Physics point of view, time is now best described through General Relativity, as part of space-time which is a dynamical object encoding gravity. Time possesses also some intrinsic irreversibility due to thermodynamics, quantum mechanical effects... This irreversibility can look puzzling since time-like loops (and hence time machines) can appear in General Relativity (for example in the Goedel universe, a solution of Einstein's equations). We take this apparent discrepancy as a warning bell pointing to us that time as we understand it, might not be fundamental and that whatever theory, lying beyond General Relativity, may not include time as we know it as a fundamental structure. We propose therefore, following the philosophy of analog models of gravity, that time and gravity might not be fundamental per se, but only emergent features. We illustrate our proposal using a toy-model where we show how the Lorentzian signature and Nordstroem gravity (a diffeomorphisms invariant scalar gravity theory) can emerge from a timeless non-dynamical space."
Subje

 

[marcus]

Brief mention:

http://arxiv.org/abs/0903.5251
Classical basis for kappa-Poincare algebra and doubly special relativity theories
A. Borowiec (Wroclaw U., JINR, Dubna), A. Pachol (Wroclaw U.)
12 pages
(Submitted on 30 Mar 2009)

http://arxiv.org/abs/0903.5141
Is cosmic acceleration slowing down?
Arman Shafieloo, Varun Sahni, Alexei A. Starobinsky
5 pages, 3 figures
(Submitted on 30 Mar 2009)
"An investigation of dark energy (DE) using the Constitution SnIa sample (which includes recent CfA data at low redshifts) reveals a slight inconsistency (more than 1 sigma) with the standard spatially flat LCDM model, ... In geometrical terms, this suggests that cosmic acceleration may have already peaked and that we are currently witnessing its slowing down. Interestingly, such DE behaviour also provides a better fit to baryon acoustic oscillation (BAO) data. ... The effect we observe could correspond to DE decaying into dark matter (or something else). A toy model which mimics this process agrees well with the combined SnIa+BAO+CMB data..."

 

[MTd2]

Marcus, you missed this one!

http://arxiv.org/abs/0903.5080

Holography, Gauge-Gravity Connection and Black Hole Entropy
Authors: Parthasarathi Majumdar
(Submitted on 29 Mar 2009)

Abstract: The issues of holography and possible links with gauge theories in spacetime physics is discussed, in an approach quite distinct from the more restricted AdS-CFT correspondence. A particular notion of holography in the context of black hole thermodynamics is derived (rather than conjectured) from rather elementary considerations, which also leads to a criterion of thermal stability of radiant black holes, without resorting to specific classical metrics. For black holes that obey this criterion, the canonical entropy is expressed in terms of the microcanonical entropy of an Isolated Horizon which is essentially a local generalization of the very global event horizon and is a null inner boundary of spacetime, with marginal outer trapping. It is argued why degrees of freedom on this horizon must be described by a topological gauge theory. Quantizing this boundary theory leads to the microcanonical entropy of the horizon expressed in terms of an infinite series asymptotic in the cross-sectional area, with the leading `area-law' term followed by finite, unambiguously calculable corrections arising from quantum spacetime fluctuations.

****

Check section 4, equations 22 and 23, and the comment below it

The remarkable aspect of (23) is that, perhaps for the first time since Bekenstein’s pioneering work, one has an ab initio computation of SIH [from LQG] and obtained an infinite series, asymptotic in AIH, of quantum spacetime fluctuation corrections to the Bekenstein-Hawking area law; each term of this series is finite and unambiguously calculable. The leading correction to the area law is logarithmic and has what appears to be a robust coefficient. With due modesty, one may say that these corrections are the only known physical signatures of loop quantum gravity as applied to the computation of microcanonical black hole entropy.