Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/0906.3547

In Search of Fundamental Discreteness in 2+1 Dimensional Quantum Gravity
Authors: T.G. Budd, R. Loll
(Submitted on 18 Jun 2009)

Abstract: Inspired by previous work in 2+1 dimensional quantum gravity, which found evidence for a discretization of time in the quantum theory, we reexamine the issue for the case of pure Lorentzian gravity with vanishing cosmological constant and spatially compact universes of genus larger than 1. Taking as our starting point the Chern-Simons formulation with Poincare gauge group, we identify a set of length variables corresponding to space- and timelike distances along geodesics in three-dimensional Minkowski space. These are Dirac observables, that is, functions on the reduced phase space, whose quantization is essentially unique. For both space- and timelike distance operators, the spectrum is continuous and not bounded away from zero.

http://arxiv.org/abs/0906.3731

Prospects for constraining quantum gravity dispersion with near term observations

Giovanni Amelino-Camelia, Lee Smolin
(Submitted on 19 Jun 2009 (v1), last revised 20 Jun 2009 (this version, v2))
We discuss the prospects for bounding and perhaps even measuring quantum gravity effects on the dispersion of light using the highest energy photons produced in gamma ray bursts measured by the Fermi telescope. These prospects are brigher than might have been expected as in the first 10 months of operation Fermi has reported so far eight events with photons over 100 MeV seen by its Large Area Telescope (LAT). We review features of these events which may bear on Planck scale phenomenology and we discuss the possible implications for the alternative scenarios for in-vacua dispersion coming from breaking or deforming of Poincare invariance. Among these are semi-conservative bounds, which rely on some relatively weak assumptions about the sources, on subluminal and superluminal in-vacuo dispersion. We also propose that it may be possible to look for the arrival of still higher energy photons and neutrinos from GRB's with energies in the range 10^14 - 10^17 eV. In some cases the quantum gravity dispersion effect would predict these arrivals to be delayed or advanced by days to months from the GRB, giving a clean separation of astrophysical source and spacetime propagation effects.

I really think Marcus will enjoy this Smolin's new article!

 

[John86]

http://arxiv.org/abs/0906.3947
Quantum gravity as sum over spacetimes
Authors: Jan Ambjorn, Jerzy Jurkiewicz, Renate Loll
(Submitted on 22 Jun 2009)
Abstract: A major unsolved problem in theoretical physics is to reconcile the classical theory of general relativity with quantum mechanics. These lectures will deal with an attempt to describe quantum gravity as a path integral over geometries known as "Causal Dynamical Triangulations" (CDT).

http://arxiv.org/abs/0906.3751
Physical evolution in Loop Quantum Cosmology: The example of vacuum Bianchi I
Authors: Mercedes Martin-Benito, Guillermo A. Mena Marugan, Tomasz Pawlowski
(Submitted on 22 Jun 2009)
Abstract: We use the vacuum Bianchi I model as an example to investigate the concept of physical evolution in Loop Quantum Cosmology (LQC) in the absence of the massless scalar field which has been used so far in the literature as an internal time. In order to retrieve the system dynamics when no such a suitable clock field is present, we explore different constructions of families of unitarily related partial observables. These observables are parameterized, respectively, by: (i) one of the components of the densitized triad, and (ii) its conjugate momentum; each of them playing the role of an evolution parameter. Exploiting the properties of the considered example, we investigate in detail the domains of applicability of each construction. In both cases the observables possesses a neat physical interpretation only in an approximate sense. However, whereas in case (i) such interpretation is reasonably accurate only for a portion of the evolution of the universe, in case (ii) it remains so during all the evolution (at least in the physically interesting cases). The constructed families of observables are next used to describe the evolution of the Bianchi I universe. The performed analysis confirms the robustness of the bounces, also in absence of matter fields, as well as the preservation of the semiclassicality through them. The concept of evolution studied here and the presented construction of observables are applicable to a wide class of models in LQC, including quantizations of the Bianchi I model obtained with other prescriptions for the improved dynamics.

 

[marcus]

http://arxiv.org/abs/0906.4529
The combinatorics of the SU(2) black hole entropy in loop quantum gravity
Ivan Agullo, J. Fernando Barbero G., Enrique F. Borja, Jacobo Diaz-Polo, Eduardo J. S. Villaseñor
5 pages
(Submitted on 24 Jun 2009)
"We use the combinatorial and number-theoretical methods developed in previous work by the authors to study black hole entropy in the new proposal put forward by Engle, Noui and Perez. Specifically we give the generating functions relevant for the computation of the entropy and use them to derive its asymptotic behavior including the value of the Immirzi parameter and the coefficient of the logarithmic correction."

 

[John86]

http://arxiv.org/abs/0906.4282
Logarithmic nonlinearity in theories of quantum gravity: Origin of time and observational consequences
Authors: Konstantin G. Zloshchastiev
(Submitted on 23 Jun 2009)

Abstract: Within the framework of a generic theory of quantum gravity we introduce the logarithmic correction to the quantum wave equation. We demonstrate by virtue of the Tomita-Takesaki construction the emergence of the evolution time - namely, from the group of automorphisms of the von Neumann algebra governed by this non-linear correction. It turns out that such time parametrization is essentially energy-dependent and becomes universal only asymptotically - when the energies become very small comparing to the effective quantum gravity scale. We show how the logarithmic non-linearity deforms the vacuum wave dispersion relations and explains certain features of the astrophysical data coming from recent observations of high-energy cosmic rays. In general, the estimates imply that ceteris paribus the particles with higher energy propagate slower than those with lower one, therefore, for a high-energy particle the mean free path, lifetime in a high-energy state and, therefore, travel distance from the source can be significantly larger than one would expect from the conventional theory.

http://arxiv.org/abs/0906.4465
Non-classical time evolutions in presence of decoherence and coarse-grained measurements
Authors: Johannes Kofler, Nikola Buric, Caslav Brukner
(Submitted on 24 Jun 2009)

Abstract: In classical physics, such as Newtonian mechanics or Maxwell's electrodynamics, the properties of systems exist independent of and are not influenced by measurement (macroscopic realism) and are evolved continuously through space and time. Quantum theory violates both features. While decoherence can restore macroscopic realism, the question remains open whether it also allows a continuous spatiotemporal description. We show that no such description exists in general for dephasing decoherence, not even under coarse-grained measurements of macroscopically large objects and arbitrarily strong environment. This shines new light on the question how the classical world arises out of the quantum realm.

http://arxiv.org/abs/0906.4516
Notes on the Qubit Phase Space and Discrete Symplectic Structures
Authors: Etera R. Livine
(Submitted on 24 Jun 2009)

Abstract: We start from Wootter's construction of discrete phase spaces and Wigner functions for qubits and more generally for finite dimensional Hilbert spaces. We look at this framework from a non-commutative space perspective and we focus on the Moyal product and the differential calculus on the discrete phase spaces. In particular, the qubit phase space provides the simplest example of a four-point non-commutative phase space. We give an explicit expression of the Moyal bracket as a differential operator. We then compare the quantum dynamics encoded by the Moyal bracket to the classical dynamics: we show that the classical Poisson bracket does not satisfy the Jacobi identity thus leaving the Moyal bracket as the only consistent symplectic structure. We finally generalizes our analysis to Hilbert spaces of prime dimensions d and their associated d*d phase spaces.

http://arxiv.org/abs/0906.4544
Decoherence without decoherence
Authors: Steven Weinstein
(Submitted on 24 Jun 2009)

Abstract: It has been claimed that decoherence of open quantum systems explains the tendency of macroscopic systems to exhibit quasiclassical behavior. We show that quasiclassicality is in fact an unremarkable property, characterizing generic subsystems of environments even in the absence of dynamical decoherence. It is suggested that decoherence is best regarded as explaining the persistence of true classicality, rather than the emergence, rather than the emergence of quasiclassicality.

http://arxiv.org/abs/0906.4428
Noncommutative Black Holes from Gravitationally Collapsing Shells
Authors: John J. Oh, Chanyong Park
(Submitted on 24 Jun 2009)

Abstract: We study the formation of the (noncommutative) Schwarzschild black hole from collapsing polytropic and Chaplygin gas shells. We show that the collapsing shell forms either a black hole or a naked singular shell with the help of the pressure. A slight modification of collapsing shell that consists of the smeared gravitational sources can form a noncommutative Schwarzschild black hole while this noncommutative correction of matters cannot ultimately resolve the naked singularity, which might suggest that we need the full modification of shell collapse formalism from the theory beyond Einstein's gravity.

http://arxiv.org/abs/0905.4480
The Black Hole and Cosmological Solutions in IR modified Horava Gravity
Authors: Mu-in Park
(Submitted on 27 May 2009 (v1), last revised 23 Jun 2009 (this version, v4))

Abstract: Recently Horava proposed a renormalizable gravity theory in four dimensions which reduces to Einstein gravity with a non-vanishing cosmological constant in IR but with improved UV behaviors. Here, I study an IR modification which breaks "softly" the detailed balance condition in Horava model and allows the asymptotically flat limit as well. I obtain the black hole and cosmological solutions for "arbitrary" cosmological constant that represent the analogs of the standard Schwartzschild-(A)dS solutions which can be asymptotically (A)dS as well as flat and I discuss some thermodynamical properties. I also obtain solutions for FRW metric with an arbitrary cosmological constant. I study its implication to the dark energy and find that it seems to be consistent with current observational data.

http://arxiv.org/abs/0906.4275
A Test of Horava Gravity: The Dark Energy
Authors: Mu-in Park
(Submitted on 23 Jun 2009)

Abstract: Recently Horava proposed a renormalizable gravity theory with higher spatial derivatives in four dimensions which reduces to Einstein gravity with a non-vanishing cosmological constant in IR but with improved UV behaviors. Here, I consider a non-trivial test of the new gravity theory in FRW universe by considering an IR modification which breaks "softly" the detailed balance condition in the original Horava model. I separate the dark energy parts from the usual Einstein gravity parts in the Friedman equations and obtain the formula of the equations of state parameter. The IR modified Horava gravity seems to be consistent with the latest observational data.

 

[marcus]

http://arxiv.org/abs/0906.4767
Non-marginal LTB-like models with inverse triad corrections from loop quantum gravity
Martin Bojowald, Juan D. Reyes, Rakesh Tibrewala
35 pages, 1 figure
(Submitted on 25 Jun 2009)
"Marginal LTB models with corrections from loop quantum gravity have recently been studied with an emphasis on potential singularity resolution. This paper corroborates and extends the analysis in two regards: (i) the whole class of LTB models, including non-marginal ones, is considered, and (ii) an alternative procedure to derive anomaly-free models is presented which first implements anomaly-freedom in spherical symmetry and then the LTB conditions rather than the other way around. While the two methods give slightly different equations of motion, not altogether surprisingly given the ubiquitous sprawl of quantization ambiguities, final conclusions remain unchanged: Compared to quantizations of homogeneous models, bounces seem to appear less easily in inhomogeneous situations, and even the existence of homogeneous solutions as special cases in inhomogeneous models may be precluded by quantum effects. However, compared to marginal models, bouncing solutions seem more likely with non-marginal models."

 

[MTd2]

http://arxiv.org/abs/0906.4978

Seeking the Loop Quantum Gravity Barbero-Immirzi Parameter and Field in 4D, $\cal N$ = 1 Supergravity

S. James Gates Jr., Sergei V. Ketov, Nicolas Yunes
(Submitted on 26 Jun 2009 (v1), last revised 27 Jun 2009 (this version, v2))
Abstract: We embed the Loop Quantum Gravity Barbero-Immirzi parameter and field within an action describing 4D, $\cal N$ = 1 supergravity and thus within a Low Energy Effective Action of Superstring/M-Theory. We use the fully gauge-covariant description of supergravity in (curved) superspace. The gravitational constant is replaced with the vacuum expectation value of a scalar field, which in local supersymmetry is promoted to a complex, covariantly chiral scalar superfield. The imaginary part of this superfield couples to a supersymmetric Holst term. The Holst term also serves as a starting point in the Loop Quantum Gravity action. This suggest the possibility of a relation between Loop Quantum Gravity and supersymmetric string theory, where the Barbero-Immirzi parameter and field of the former play the role of the supersymmetric axion in the latter. Adding matter fermions in Loop Quantum Gravity may require the extension of the Holst action through the Nieh-Yan topological invariant, while in pure, matter-free supergravity their supersymmetric extensions are the same. We show that, when the Barbero-Immirzi parameter is promoted to a field in the context of 4D supergravity, it is equivalent to adding a dynamical complex chiral (dilaton-axion) superfield with a non-trivial kinetic term (or K\"ahler potential), coupled to supergravity.http://arxiv.org/abs/0906.4918
Ashtekar's variables without spin

Thomas Schucker
(Submitted on 26 Jun 2009)
Ashtekar's variables are shown to arise naturally from a 3+1 split of general relativity in the Einstein-Cartan formulation. Thereby spinors are exorcised.
Comments: 5 pages, unpublished paper from 1988

 

[marcus]

http://arxiv.org/abs/0906.5477
Scaling behaviour of three-dimensional group field theory
Jacques Magnen (CPHT), Karim Noui (LMPT), Vincent Rivasseau (LPT), Matteo Smerlak (CPT)
(Submitted on 30 Jun 2009)
"Group field theory is a generalization of matrix models, with triangulated pseudomanifolds as Feynman diagrams and state sum invariants as Feynman amplitudes. In this paper, we consider Boulatov's three-dimensional model and its Freidel-Louapre positive regularization (hereafter the BFL model) with a 'ultraviolet' cutoff, and study rigorously their scaling behavior in the large cutoff limit. We prove an optimal bound on large order Feynman amplitudes, which shows that the BFL model is perturbatively more divergent than the former. We then upgrade this result to the constructive level, using, in a self-contained way, the modern tools of constructive field theory: we construct the Borel sum of the BFL perturbative series via a convergent 'cactus' expansion, and establish the 'ultraviolet' scaling of its Borel radius. Our method shows how the 'sum over triangulations' in quantum gravity can be tamed rigorously, and paves the way for the renormalization program in group field theory."

 

[marcus]

http://arxiv.org/abs/0907.0416
The Formulation of Quantum Field Theory in Curved Spacetime
Robert M. Wald
14 pages, no figures; to appear in proceedings of Beyond Einstein conference
(Submitted on 2 Jul 2009)
"The usual formulations of quantum field theory in Minkowski spacetime make crucial use of Poincare symmetry, positivity of total energy, and the existence of a unique, Poincare invariant vacuum state. These and other key features of quantum field theory do not generalize straightforwardly to curved spacetime. We discuss the conceptual obstacles to formulating quantum field theory in curved spacetime and how they can be overcome."

Even though this is not QG and not general relativistic (the geometric background is static, not dynamic as in GR) I think it may be appropriate considering Wald's stature to include this in our biblio as a possibly useful reference.

 

[marcus]

http://arxiv.org/abs/0907.0640
Loop quantum cosmology with higher order holonomy corrections
Dah-Wei Chiou, Li-Fang Li
32 pages, 2 figures
(Submitted on 3 Jul 2009)
"With a well-motivated extension of higher order holonomy corrections, the quantum theory of loop quantum cosmology (LQC) for the k=0 Friedmann-Robertson-Walker model (with a free massless scalar) is rigorously formulated. The analytical investigation reveals that, regardless of the order of holonomy corrections and for any arbitrary states, the matter density remains finite, bounded from above by an upper bound, which equals the critical density obtained at the level of heuristic effective dynamics. Particularly, with all orders of corrections included, the dynamical evolution is shown to follow the bouncing scenario in which two Wheeler-DeWitt (WDW) solutions (expanding and contracting) are bridged together through the quantum bounce. These observations provide further evidence that the quantum bounce is essentially a consequence of the intrinsic discreteness of LQC and LQC is fundamentally different from the WDW theory. Meanwhile, the possibility is also explored that the higher order holonomy corrections can be interpreted as a result of admitting generic SU(2) representations for the Hamiltonian constraint operators."

http://arxiv.org/abs/0907.0822
Black Hole Entropy in Loop Quantum Gravity and Number Theory
J.Manuel Garcia-Islas
6 pages
(Submitted on 5 Jul 2009)
"We show that counting different configurations that give rise to black hole entropy in loop quantum gravity is related to partitions in number theory."

http://arxiv.org/abs/0907.0826
On the black hole singularity issue in loop quantum gravity
A. DeBenedictis
8 pages, 3 figures; Proceedings of Theory Canada IV; Can. J. Phys. 87, 255-262 (2009)
(Submitted on 5 Jul 2009)
"This paper presents a brief overview on the issue of singularity resolution in loop quantum gravity presented at the Theory Canada IV conference at the Centre de Recherches Mathématiques at the Université de Montréal (June 4-7, 2008). The intended audience is theoretical physicists who are non-specialist in the field and therefore much of the technical detail is omitted here. Instead, a brief review of loop quantum gravity is presented, followed by a survey of previous and current work on results concerning the resolution of the classical black hole singularity within loop quantum gravity."
(Slightly expanded version of ...invited talk...)

http://arxiv.org/abs/0907.0714
Fermi Observations of high-energy gamma-ray emissions from GRB 080916C
Hiroyasu Tajima, for the Fermi LAT, Fermi GBM collaborations
4 pages, 4 figures, proceedings for 31st International Cosmic-Ray Conference
(Submitted on 3 Jul 2009)
"Observations of the long-duration Gamma-Ray Burst GRB 080916C by the Fermi Gamma-ray Burst Monitor and Large Area Telescope show that it has a single spectral form from 8 keV to 13.2 GeV. The E>100 MeV emission was ~5 s later than the E<1 MeV emission and lasted much longer even after photons with E<100 MeV became undetectable. The redshift from GROND of z~4.35 means that this GRB has the largest reported apparent isotropic gamma-ray energy release, E_iso ~ 8.8 x10⁵⁴ ergs. It also sets a stringent lower limit on the GRB outflow Lorentz factor, Gamma_min~890, and limits the quantum gravity mass scale, M_QG > 1.3 x 10¹⁸ GeV/c²."

 

[MTd2]

http://arxiv.org/abs/0907.1203

Emergence of helicity +/- 2 modes (gravitons) from qbit models

Zheng-Cheng Gu, Xiao-Gang Wen
(Submitted on 7 Jul 2009)
It was shown that photons (i.e. helicity $\pm 1$ gapless excitations) can emerge from a qbit model (i.e. a quantum spin model) on a 3D lattice. In this paper, we study the possibility of the emergence of helicity $\pm 2$ gapless excitations (i.e. the gravitons) from two quantum spin models. In the first quantum spin model (called the L-type model), the helicity $\pm 2$ gapless excitations are shown to appear as the only type of low energy excitations. Within a perturbative calculation, the dispersion of the gapless helicity $\pm 2$ is found to be $\eps_{\v{k}} \propto |\v{k}|^3$. The appearance of the gapless helicity $\pm2$ modes suggests that the ground state of the quantum spin model is a new state of matter. In the second model (called the N-type model) the collective modes are strongly interacting and there is no reliable approach to understand its low energy dynamics. Using a spin-wave/quantum-freeze approach (which is shown to reproduce the correct emergent U(1) gauge theory in a quantum rotor model), we argue that the second model may contain helicity $\pm 2$ gapless excitations as the only type of low energy excitations with a linear dispersion $\om \propto k$. We believe that the gaplessness of the helicity $\pm 2$ excitations in both models is topologically robust: any translation invariant perturbations cannot generate a gap for those helicity $\pm 2$ excitations. Our results shed light on the quest to find a local bosonic model whose ground state supports helicity $\pm 2$ gapless excitations with a linear $\eps_{\v{k}} \propto |\v{k}|$ dispersion. Such a linearly dispersing helicity $\pm 2$ excitations will correspond to emergent gravitons.

Wow! Finally some good news from Xiao Gang Wen after a long time!

 

[marcus]

This could prove useful in developing observational tests for Quantum Geometry/Gravity:

http://arxiv.org/abs/0907.1169
Stable indications of relic gravitational waves in Wilkinson Microwave Anisotropy Probe data and forecasts for Planck mission
W. Zhao, D. Baskaran, L. P. Grishchuk
21 pages, including 12 figures, 2 tables, 2 appendices
(Submitted on 7 Jul 2009)
"The relic gravitational waves (gw) are the cleanest probe of the violent times in the very early history of the Universe. They are expected to leave signatures in the observed cosmic microwave background anisotropies. We significantly improved our previous analysis [1] of the 5-year WMAP TT and TE data at lower multipoles \ell. This more general analysis returned essentially the same maximum likelihood (ML) result (unfortunately, surrounded by large remaining uncertainties): the relic gw are present and they are responsible for approximately 20% of the temperature quadrupole. We identify and discuss the reasons by which the contribution of gw can be overlooked in a data analysis. One of the reasons is a misleading reliance on data from very high multipoles \ell, another - a too narrow understanding of the problem as the search for B-modes of polarization, rather than the detection of relic gw with the help of all correlation functions. Our analysis of WMAP5 data has led to the identification of a whole family of models characterized by relatively high values of the likelihood function. Using the Fisher matrix formalism we formulated forecasts for Planck mission in the context of this family of models. We explore in details various `optimistic', `pessimistic' and `dream case' scenarios. We show that in some circumstances the B-mode detection may be very inconclusive, at the level of signal-to-noise ratio S/N =1.75, whereas a smarter data analysis can reveal the same gw signal at S/N= 6.48. The final result is encouraging. Even under unfavourable conditions in terms of instrumental noises and foregrounds, the relic gw, if they are characterized by the ML parameters that we found from WMAP5 data, will be detected by Planck at the level S/N = 3.65."

 

[marcus]

http://arxiv.org/abs/0907.1530
The Immirzi parameter from an external scalar field
Francesco Cianfrani, Giovanni Montani
5 pages
(Submitted on 9 Jul 2009)
We promote the Immirzi parameter to be a minimally coupled scalar field and we analyzed the Hamiltonian constraints in the framework of Loop Quantum Gravity without the time gauge. Proper SU(2) connections can be defined and a term containing derivatives of the field $\beta$ enters into their definition. Furthermore, boost degrees of freedom are non-dynamical, while the super-momentum constraints coincide with the scalar field case. Hence, the kinematical Hilbert space can be defined as for gravity in presence of a minimally coupled scalar field. Then, we analyzed the dynamical implications of this scenario and we outlined how a the dynamical relaxation to a non-vanishing vacuum expectation value is predicted."

 

[marcus]

http://arxiv.org/abs/0907.1828
Asymptotically free scalar curvature-ghost coupling in Quantum Einstein Gravity
Astrid Eichhorn, Holger Gies, Michael M. Scherer (Jena U.)
8 pages, 3 figures
(Submitted on 10 Jul 2009)
"We consider the asymptotic-safety scenario for quantum gravity which constructs a non-perturbatively renormalisable quantum gravity theory with the help of the functional renormalisation group. We verify the existence of a non-Gaussian fixed point and include a running curvature-ghost coupling as a first step towards the flow of the ghost sector of the theory. We find that the scalar curvature-ghost coupling is asymptotically free and RG relevant in the ultraviolet. Most importantly, the property of asymptotic safety discovered so far within the Einstein-Hilbert truncation and beyond remains stable under the inclusion of the ghost flow."

http://arxiv.org/abs/0907.1748
Linear perturbations of cosmological models in the Horava-Lifgarbagez theory of gravity without detailed balance
Anzhong Wang, Roy Maartens
10 pages
(Submitted on 10 Jul 2009)
"Recently, Horava proposed a very attractive theory of quantum gravity that is non-relativistic and power-counting UV-renormalizable. In this theory, two conditions, the detailed balance and projectability, were usually assumed. Abandoning the detailed balance but still keeping the projectability condition, Sotiriou, Visser and Weinfurtner (SVW) formulated the most general Horava theory, which does not violate the parity and has the Newton and cosmological constants as independent parameters. In this paper, we study linear scalar perturbations of the FRW model in the SVW setup by taking matter fields into account. We obtain explicitly the perturbation equations for the Hamiltonian, super-momentum constraints and the dynamical equations. We also study the conservation laws of the matter field and find that one of these equations can be integrated explicitly. Applying our formulas to the case where the background is Minkowski, we study the spin-0 scalar mode of graviton, and find that it becomes stabilized in the limit xi = 0, for which general relativity is recovered in the IR limit. It remains stable even when the mode is coupled with a dust fluid."

 

[MTd2]

http://arxiv.org/abs/0907.1658

Signatures of a Graviton Mass in the Cosmic Microwave Background
Authors: Sergei Dubovsky, Raphael Flauger, Alexei Starobinsky, Igor Tkachev
(Submitted on 9 Jul 2009)

Abstract: There exist consistent low energy effective field theories describing gravity in the Higgs phase that allow the coexistence of massive gravitons and the conventional 1/r potential of gravity. In an effort to constrain the value of the graviton mass in these theories, we study the tensor contribution to the CMB temperature anisotropy and polarization spectra in the presence of a non-vanishing graviton mass. We find that the observation of a B-mode signal consistent with the spectrum predicted by inflationary models would provide the strongest limit yet on the mass of an elementary particle -- a graviton -- at a level of m\lesssim 10^(-30) eV\approx(10 Mpc)^(-1). We also find that a graviton mass in the range between (10 Mpc)^(-1) and (10 kpc)^(-1) leads to interesting modifications of the polarization spectrum. The characteristic signature of a graviton mass in this range would be a plateau in the B-mode spectrum up to angular multipoles of l\sim 100. For even larger values of the graviton mass the tensor contribution to the CMB spectra becomes strongly suppressed.

 

[marcus]

http://arxiv.org/abs/0907.2440
Lorentzian spin foam amplitudes: graphical calculus and asymptotics
John W. Barrett, Richard J. Dowdall, Winston J. Fairbairn, Frank Hellmann, Roberto Pereira
30 pages
(Submitted on 14 Jul 2009)
"The amplitude for the 4-simplex in a spin foam model for quantum gravity is defined using a graphical calculus for the unitary representations of the Lorentz group. The asymptotics of this amplitude are studied in the limit when the representation parameters are large, for various cases of boundary data. It is shown that for boundary data corresponding to a Lorentzian simplex, the asymptotic formula has two terms, with phase plus or minus the Lorentzian signature Regge action for the 4-simplex geometry, multiplied by an Immirzi parameter. Other cases of boundary data are also considered, including a surprising contribution from Euclidean signature metrics."

Fairbairn, Hellmann, and Pereira were all at Marseille with Rovelli one time or another as grad students (or maybe postdocs, I think grad students). It looks like Marseille is feeding PhDs to Nottingham. I've noticed it tends to be important when Barrett posts a paper. Congratulations everybody! We have a new Barrett et al! Also a new Reuter

http://arxiv.org/abs/0907.2617
Bimetric Truncations for Quantum Einstein Gravity and Asymptotic Safety
Elisa Manrique, Martin Reuter
48 pages, 5 figures
(Submitted on 15 Jul 2009)
"In the average action approach to the quantization of gravity the fundamental requirement of "background independence" is met by actually introducing a background metric but leaving it completely arbitrary. The associated Wilsonian renormalization group defines a coarse graining flow on a theory space of functionals which, besides the dynamical metric, depend explicitly on the background metric. All solutions to the truncated flow equations known to date have a trivial background field dependence only, namely via the classical gauge fixing term. In this paper we analyze a number of conceptual issues related to the bimetric character of the gravitational average action and explore a first nontrivial bimetric truncation in the simplified setting of conformally reduced gravity. Possible implications for the Asymptotic Safety program and the cosmological constant problem are discussed in detail."

http://arxiv.org/abs/0907.2562
Chern-Simons Modified General Relativity
Stephon Alexander, Nicolas Yunes
104 pages, 2 figures, 186 references, Invited Review accepted for publication in Phys. Repts
(Submitted on 15 Jul 2009)
"Chern-Simons modified gravity is an effective extension of general relativity that captures leading-order, gravitational parity violation. Such an effective theory is motivated by anomaly cancelation in particle physics and string theory. In this review, we begin by providing a pedagogical derivation of the three distinct ways such an extension arises: (1) in particle physics, (2) from string theory and (3) geometrically. We then review many exact and approximate, vacuum solutions of the modified theory, and discuss possible matter couplings. Following this, we review the myriad astrophysical, solar system, gravitational wave and cosmological probes that bound Chern-Simons modified gravity, including discussions of cosmic baryon asymmetry and inflation. The review closes with a discussion of possible future directions in which to test and study gravitational parity violation."

http://arxiv.org/abs/0907.2582
Colored Group Field Theory
Razvan Gurau
(Submitted on 15 Jul 2009)
"Group field theories are higher dimensional generalizations of matrix models. Their Feynman graphs are fat and in addition to vertices, edges and faces, they also contain higher dimensional cells, called bubbles. In this paper, we propose a new, fermionic Group Field Theory, posessing a color symmetry, and take the first steps in a systematic study of the topological properties of its graphs. Unlike its bosonic counterpart, the bubbles of the Feynman graphs of this theory are well defined and readily identified. We prove that this graphs are combinatorial cellular complexes. We define and study the cellular homology of this graphs. Furthermore we define a homotopy transformation appropriate to this graphs. Finally, the amplitude of the Feynman graphs is shown to be related to the fundamental group of the cellular complex."

http://arxiv.org/abs/0907.2652
Gravitational Wilson Loop in Discrete Quantum Gravity
Herbert W. Hamber, Ruth M. Williams
39 pages, 10 figures
(Submitted on 15 Jul 2009)
"Results for the gravitational Wilson loop, in particular the area law for large loops in the strong coupling region, and the argument for an effective positive cosmological constant discussed in a previous paper, are extended to other proposed theories of discrete quantum gravity in the strong coupling limit. We argue that the area law is a generic feature of almost all non-perturbative lattice formulations, for sufficiently strong gravitational coupling. The effects on gravitational Wilson loops of the inclusion of various types of light matter coupled to lattice quantum gravity are discussed as well. One finds that significant modifications to the area law can only arise from extremely light matter particles. The paper ends with some general comments on possible physically observable consequences."

http://arxiv.org/abs/0907.2492
Towards a Post Reductionist Science: The Open Universe
Stuart Kauffman
(Submitted on 15 Jul 2009)
"In this paper I discuss the reality that deductive inference is not the only way we explain in science. I discuss the role of the opportunity for an adaptation in the biosphere and claim that such an opportunity is a 'blind final cause', not an efficient cause, yet shapes evolution. I also argue that Darwinian exaptations are not describable by sufficient natural law. Based on an argument of Sir Karl Popper, I claim that no law, or function, f, maps a decoherence process in a Special Relativity setting from a specific space-time slice into its future. If true this suggests there can be no theory of everything entailing all that happens. I then discuss whether we can view laws as 'enabling constraints' and what they enable. Finally, in place of the weak Anthropic principle in a multiverse, I suggest that we might consider Darwin all the way down. It is not impossible that a single universe has an abiotic natural selection process for laws as enabling constraints and that the single universe that 'wins' is ours. One possible criterion of winning might be 'most rapid growth of the Adjacent Possible of the universe'."

 

[MTd2]

http://arxiv.org/abs/0907.2617

Bimetric Truncations for Quantum Einstein Gravity and Asymptotic Safety

Elisa Manrique, Martin Reuter
(Submitted on 15 Jul 2009)
In the average action approach to the quantization of gravity the fundamental requirement of "background independence" is met by actually introducing a background metric but leaving it completely arbitrary. The associated Wilsonian renormalization group defines a coarse graining flow on a theory space of functionals which, besides the dynamical metric, depend explicitly on the background metric. All solutions to the truncated flow equations known to date have a trivial background field dependence only, namely via the classical gauge fixing term. In this paper we analyze a number of conceptual issues related to the bimetric character of the gravitational average action and explore a first nontrivial bimetric truncation in the simplified setting of conformally reduced gravity. Possible implications for the Asymptotic Safety program and the cosmological constant problem are discussed in detail.

http://arxiv.org/abs/0907.2652
Gravitational Wilson Loop in Discrete Quantum Gravity

Herbert W. Hamber, Ruth M. Williams
(Submitted on 15 Jul 2009)
Results for the gravitational Wilson loop, in particular the area law for large loops in the strong coupling region, and the argument for an effective positive cosmological constant discussed in a previous paper, are extended to other proposed theories of discrete quantum gravity in the strong coupling limit. We argue that the area law is a generic feature of almost all non-perturbative lattice formulations, for sufficiently strong gravitational coupling. The effects on gravitational Wilson loops of the inclusion of various types of light matter coupled to lattice quantum gravity are discussed as well. One finds that significant modifications to the area law can only arise from extremely light matter particles. The paper ends with some general comments on possible physically observable consequences.

 

[MTd2]

Emergent gravity: the BEC paradigm

http://arxiv.org/abs/0907.2839

Gil Jannes
(Submitted on 16 Jul 2009)
We study selected aspects of quantum gravity phenomenology inspired by the gravitational analogy in Bose--Einstein condensates (BECs). We first review the basic ideas and formalism of analogue gravity in BECs, with particular emphasis on the possibility of simulating black holes. The non-relativistic, 'superluminal' modifications of the dispersion relation in a BEC beyond the hydrodynamic limit make it a particularly interesting model for many scenarios of quantum gravity phenomenology which consider a possible violation of local Lorentz invariance at high energies. In particular, these modifications allow the study of kinematical corrections that such quantum gravity scenarios could impose on general relativity.
A simple (1+1)-dimensional acoustic black hole configuration in a BEC is presented, and its dynamical stability and quasinormal mode spectrum are studied. Then, an analysis is performed of the Hawking radiation for a collapsing geometry in which a black hole is created. It is seen that the superluminality of the dispersion relation leads to a frequency-dependence of the horizon, which can cause strong qualitative and quantitative modifications with respect to the standard (Lorentz-invariant) Hawking spectrum. We end with some considerations related to the possibility of constructing a serious toy model for Planck-scale gravity understood as an emergent phenomenon, based on the condensed matter analogy. In particular, we discuss the problem of diffeomorphism invariance in such a seemingly background-dependent approach and indicate some possible ideas for how to recover the Einstein equations in the adequate limit.

http://arxiv.org/abs/0907.2752
Probing Lorentz Invariance at EeV Energy

Reetanjali Moharana (IITB), Nayantara Gupta (IITB)
(Submitted on 16 Jul 2009)
Pierre Auger experiment has detected at least a couple of ray events above energy 60 EeV from the direction of the radio-galaxy Centaurus A. Assuming those events are from Centaurus A, we have calculated the number of neutral cosmic ray events from this source for small values of the degree of violation in Lorentz invariance. Our results show that a comparison of our calculated numbers of events with the observed number of events at EeV energy from the direction of the source can probe extremely low value of the degree of this violation.

 

[MTd2]

http://arxiv.org/abs/0907.3093

Surplus Solid Angle: Toward Astrophysical Test of Horava-Lifgarbagez Gravity

Sung-Soo Kim (ULB), Taekyung Kim, Yoonbai Kim (Sungkyunkwan U.)
(Submitted on 17 Jul 2009)
We consider electrostatic field of a point charge coupled to Horava-Lifgarbagez gravity and find an exact solution describing the space with a surplus (or deficit) solid angle. Although, theoretically in general relativity, a surplus angle is hardly to be obtained in the presence of ordinary matter with positive energy distribution, it seems natural in Horava-Lifgarbagez gravity. [/b]We present sudden disappearance and reappearance of a star image as an astrophysical effect of a surplus angle. We also consider matter configurations of all possible power law behaviors coupled to Horava-Lifgarbagez gravity and obtain a series of exact solutions.

WOW! That's a BOLD prediction!

 

[marcus]

http://arxiv.org/abs/0907.3193
kappa-Rindler space
J. Kowalski-Glikman
10 pages
(Submitted on 18 Jul 2009)
"In this paper we construct, and investigate some thermal properties of, the non-commutative counterpart of Rindler space, which we call kappa-Rindler space. This space is obtained by changing variables in the defining commutators of kappa-Minkowski space. We then rederive the commutator structure of kappa-Rindler space with the help of an appropriate star product, obtained from the kappa-Minkowski one. Using this star product, following the idea of Padmanabhan, we find the leading order, 1/kappa correction to the Hawking thermal spectrum."

 

[marcus]

http://arxiv.org/abs/0907.3797
Hybrid Quantum Cosmology: Combining Loop and Fock Quantizations
Guillermo A. Mena Marugan, Mercedes Martin-Benito
24 pages, published in International Journal of Modern Physics A, Special Issue: Proceedings of the Second Workshop on Quantum Gravity and Noncommutative Geometry at Lisbon, Portugal
(Submitted on 22 Jul 2009)
"As a necessary step towards the extraction of realistic results from Loop Quantum Cosmology, we analyze the physical consequences of including inhomogeneities. We consider in detail the quantization of a gravitational model in vacuo which possesses local degrees of freedom, namely, the linearly polarized Gowdy cosmologies with the spatial topology of a three-torus. We carry out a hybrid quantization which combines loop and Fock techniques. We discuss the main aspects and results of this hybrid quantization, which include the resolution of the cosmological singularity, the polymeric quantization of the internal time, a rigorous definition of the quantum constraints and the construction of their solutions, the Hilbert structure of the physical states, and the recovery of a conventional Fock quantization for the inhomogeneities."

 

[marcus]

http://arxiv.org/abs/0907.4057
Unstable Anisotropic Loop Quantum Cosmology
William Nelson, Mairi Sakellariadou
13 pages, 2 figures
(Submitted on 23 Jul 2009)

"We study stability conditions of the full Hamiltonian constraint equation describing the quantum dynamics of the diagonal Bianchi I model in the context of LQC. Our analysis has shown robust evidence of an instability in the explicit implementation of the difference equation, implying important consequences for the correspondence between the full LQG theory and LQC. As a result, one may question the choice of the quantisation approach, the model of lattice refinement, and/or the role of the ambiguity parameters; all these should in principle be dictated by the full LQG theory."

http://arxiv.org/abs/0907.4064
Gravity as BF theory plus potential
Kirill Krasnov
7 pages, published in Proceedings of the Second Workshop on Quantum Gravity and Noncommutative Geometry (Lisbon, Portugal)
(Submitted on 23 Jul 2009)

"Spin foam models of quantum gravity are based on Plebanski's formulation of general relativity as a constrained BF theory. We give an alternative formulation of gravity as BF theory plus a certain potential term for the B-field. When the potential is taken to be infinitely steep one recovers general relativity. For a generic potential the theory still describes gravity in that it propagates just two graviton polarizations. The arising class of theories is of the type amenable to spin foam quantization methods, and, we argue, may allow one to come to terms with renormalization in the spin foam context."

 

[MTd2]

http://arxiv.org/abs/0907.4103

Recovering General Relativity from massive gravity

E. Babichev, C. Deffayet, R. Ziour
(Submitted on 23 Jul 2009)
We obtain static, spherically symmetric, and asymptotically flat numerical solutions of massive gravity with a source. Those solutions show, for the first time explicitly, a recovery of the Schwarzschild solution of General Relativity via the so-called Vainshtein mechanism.

*****

This is very important for Horava gravity, in order to get rid of the massive gravitons and recover GR. This is stated in the introduction of the article.

 

[marcus]

http://arxiv.org/abs/0907.4238
How Far Are We from the Quantum Theory of Gravity?
R. P. Woodard (University of Florida)
106 page review article solicited by Reports on Progress in Physics
(Submitted on 24 Jul 2009)
"I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology."

http://arxiv.org/abs/0907.4322
Quantum constraints, Dirac observables and evolution: group averaging versus Schroedinger picture in LQC
Wojciech Kaminski, Jerzy Lewandowski, Tomasz Pawlowski
32 pages
(Submitted on 24 Jul 2009)
"A general quantum constraint of the form C= - \partial_T^2 \otimes B - I\otimes H (realized in particular in Loop Quantum Cosmology models) is studied. Group Averaging is applied to define the Hilbert space of solutions and the relational Dirac observables. Two cases are considered. In the first case, the spectrum of the operator (1/2)\pi^2 B - H is assumed to be discrete. The quantum theory defined by the constraint takes the form of a Schroedinger-like quantum mechanics with a generalized Hamiltonian \sqrt{B^{-1} H}. In the second case, the spectrum is absolutely continuous and some peculiar asymptotic properties of the eigenfunctions are assumed. The resulting Hilbert space and the dynamics are characterized by a continuous family of the Schroedinger-like quantum theories. However, the relational observables mix different members of the family. Our assumptions are motivated by new Loop Quantum Cosmology models of quantum FRW spacetime. The two cases considered in the paper correspond to the negative and, respectively, positive cosmological constant. Our results should be also applicable in many other general relativistic contexts."

http://arxiv.org/abs/0907.4303
Detecting relics of a thermal gravitational wave background in the early Universe
W. Zhao, D. Baskaran, P. Coles
6 pages, 3 figures
(Submitted on 24 Jul 2009)
"A thermal gravitational wave background can be produced in the early Universe if a radiation dominated epoch precedes the usual inflationary stage. This background provides a unique way to study the initial state of the Universe. We discuss the imprint of this thermal spectra of gravitons on the cosmic microwave background (CMB) power spectra, and its possible detection by CMB observations. Assuming the inflationary stage is a pure de Sitter expansion we find that, if the number of e-folds of inflation is smaller than 65, the signal of this thermal spectrum can be detected by the observations of Planck and PolarBear experiments, or the planned EPIC experiments. This bound can be even looser if inflation-like stage is the sub-exponential."

http://arxiv.org/abs/0907.4323
Improved and Perfect Actions in Discrete Gravity
Benjamin Bahr, Bianca Dittrich
28 pages, 2 figures
(Submitted on 24 Jul 2009)
"We consider the notion of improved and perfect actions within Regge calculus. These actions are constructed in such a way that they - although being defined on a triangulation - reproduce the continuum dynamics exactly, and therefore capture the gauge symmetries of General Relativity. We construct the perfect action in three dimensions with cosmological constant, and in four dimensions for one simplex. We conclude with a discussion about Regge Calculus with curved simplices, which arises naturally in this context."

http://arxiv.org/abs/0907.4325
Regge calculus from a new angle
Benjamin Bahr, Bianca Dittrich
8 pages
(Submitted on 24 Jul 2009)
"In Regge calculus space time is usually approximated by a triangulation with flat simplices. We present a formulation using simplices with constant sectional curvature adjusted to the presence of a cosmological constant. As we will show such a formulation allows to replace the length variables by 3d or 4d dihedral angles as basic variables. Moreover we will introduce a first order formulation, which in contrast to using flat simplices, does not require any constraints. These considerations could be useful for the construction of quantum gravity models with a cosmological constant."

 

[MTd2]

http://arxiv.org/abs/0907.4325
Regge calculus from a new angle

About this article. The authors thank Freddy Cachazo in the aknoledgments. Interesting.

 

[marcus]

http://arxiv.org/abs/0907.4388
Loop Quantum Gravity a la Aharonov-Bohm
Eugenio Bianchi
19 pages, 1 figure
(Submitted on 24 Jul 2009)
"The state space of Loop Quantum Gravity admits a decomposition into orthogonal subspaces associated to diffeomorphism equivalence classes of graphs. In this paper I discuss the possibility of obtaining this state space from the quantization of a topological field theory with many degrees of freedom. The starting point is a theory of locally-flat connections on a manifold which is non simply-connected because of the presence of a network of defects. The quantization procedure relies on standard field theoretical methods. The functional integral defining the scalar product is shown to reduce to a finite dimensional integral over moduli space. A non-trivial measure given by the Faddeev-Popov determinant is derived. The resulting state space is surprisingly close to the one of ordinary Loop Quantum Gravity. Spin networks arise again and provide the tool for describing gauge- and diffeomorphism-invariant functionals of the connection. The role played by defects and loops in this approach is analogous to the one played by solenoids and Wilson loops in the Aharonov-Bohm effect."

 

[marcus]

http://arxiv.org/abs/0907.5160
A Brief Introduction to Loop Quantum Cosmology
Guillermo A. Mena Marugan
15 pages, published in AIP Conference Proceedings, Volume 1130, Geometry and Physics: XVII International Fall Workshop on Geometry and Physics
(Submitted on 29 Jul 2009)
"In recent years, Loop Quantum Gravity has emerged as a solid candidate for a nonperturbative quantum theory of General Relativity. It is a background independent theory based on a description of the gravitational field in terms of holonomies and fluxes. In order to discuss its physical implications, a lot of attention has been paid to the application of the quantization techniques of Loop Quantum Gravity to symmetry reduced models with cosmological solutions, a line of research that has been called Loop Quantum Cosmology. We summarize its fundamentals and the main differences with respect to the more conventional quantization approaches employed in cosmology until now. In addition, we comment on the most important results that have been obtained in Loop Quantum Cosmology by analyzing simple homogeneous and isotropic models. These results include the resolution of the classical big-bang singularity, which is replaced by a quantum bounce."


Brief mention
http://arxiv.org/abs/0907.4887
Possible solution of the cosmological constant problem
F.R. Klinkhamer, G.E. Volovik
9 pages
(Submitted on 28 Jul 2009)

 

[John86]

Here i found this interesting paper from Chapline, a physicist from the Condensed matter camp.

http://arxiv.org/abs/0907.4397

Dark Energy Stars and AdS/CFT
Authors: George Chapline
(Submitted on 25 Jul 2009)

Abstract: The theory of dark energy stars illustrates how the behavior of matter near to certain kinds of quantum critical phase transitions can be given a geometrical interpretation by regarding the criticality tuning parameter as an extra dimension. In the case of a superfluid with vanishing speed of sound, the implied geometry resembles 5-dimensional anti-de-Sitter. In a dark energy star this geometry applies both inside and outside the horizon radius, so the AdS-CFT correspondence is consistent with the idea that the surface of a compact astrophysical object represents a quantum critical phase transition of space-time. The superfluid transition in a chiron gas, which was originally proposed as a theory of high temperature superconductivity, may provide an exact theory of this transition.

 

[marcus]

http://arxiv.org/abs/0907.5602
Coarse graining dynamical triangulations: a new scheme
Joe Henson
19 pages (14 main body), 3 figures. Accepted for publication in Class. Quant. Grav.
(Submitted on 31 Jul 2009)
"A new procedure for coarse-graining dynamical triangulations is presented. The procedure provides a meaning for the relevant value of observables when 'probing at large scales', e.g. the average scalar curvature. The scheme may also be useful as a starting point for a new type of renormalisation procedure, suitable for dynamically triangulated quantum gravity.
Random Delaunay triangulations have previously been used to produce discretisations of continuous Euclidean manifolds, and the coarse-graining scheme is an extension of this idea, using random simplicial complexes produced from a dynamical triangulation. In order for a coarse-graining process to be useful, it should preserve the properties of the original dynamical triangulation that are relevant when probing at large scales. Some general discussion of this point is given, along with some arguments in favour of the proposed scheme."http://arxiv.org/abs/0907.5591
Non-singular Ekpyrotic/Cyclic model in Loop Quantum Cosmology
Thomas Cailleteau, Parampreet Singh, Kevin Vandersloot
22 pages, 8 figures
(Submitted on 31 Jul 2009)
"We study the role of non-perturbative quantum gravity effects in the Ekpyrotic/Cyclic model using the effective framework of loop quantum cosmology in the presence of anisotropies. We show that quantum geometric modifications to the dynamical equations near the Planck scale as understood in the quantization of Bianchi-I spacetime in loop quantum cosmology lead to the resolution of classical singularity and result in a non-singular transition of the universe from the contracting to the expanding branch. In the Planck regime, the universe undergoes multiple small bounces and the anisotropic shear remains bounded throughout the evolution. A novel feature, which is absent for isotropic models, is a natural turn around of the moduli field from the negative region of the potential leading to a cyclic phenomena as envisioned in the original paradigm. Our work suggests that incorporation of quantum gravitational effects in the Ekpyrotic/Cyclic model may lead to a viable scenario without any violation of the null energy condition."http://arxiv.org/abs/0907.5510
On Semi-Classical States of Quantum Gravity and Noncommutative Geometry
Johannes Aastrup, Jesper M. Grimstrup, Mario Paschke, Ryszard Nest
31 pages, 10 figures
(Submitted on 31 Jul 2009)
"We construct normalizable, semi-classical states for the previously proposed model of quantum gravity which is formulated as a spectral triple over holonomy loops. The semi-classical limit of the spectral triple gives the Dirac Hamiltonian in 3+1 dimensions. Also, time-independent lapse and shift fields emerge from the semi-classical states. Our analysis shows that the model might contain fermionic matter degrees of freedom.
The semi-classical analysis presented in this paper does away with most of the ambiguities found in the initial semi-finite spectral triple construction. The cubic lattices play the role of a coordinate system and a divergent sequence of free parameters found in the Dirac type operator is identified as a certain inverse infinitesimal volume element."

 

[John86]

This is an interesting paper which is giving a quite good overview of the emerging properties of spacetime gravity and so on, from a condensed matter physics stand of view.

http://arxiv.org/abs/0908.0355
Quantum Gravity: Motivations and Alternatives
Authors: Reiner Hedrich
(Submitted on 3 Aug 2009)

Abstract: The mutual conceptual incompatibility between GR and QM/QFT is generally seen as the most essential motivation for the development of a theory of Quantum Gravity (QG). It leads to the insight that, if gravity is a fundamental interaction and QM is universally valid, the gravitational field will have to be quantized, not at least because of the inconsistency of semi-classical theories of gravity. If this means to quantize GR, its identification of the gravitational field with the spacetime metric has to be taken into account. And the resulting quantum theory has to be background-independent. This can not be achieved by means of quantum field theoretical procedures. More sophisticated strategies have to be applied. One of the basic requirements for such a quantization strategy is that the resulting quantum theory has GR as a classical limit. - However, should gravity not be a fundamental, but an residual, emergent interaction, it could very well be an intrinsically classical phenomenon. Should QM be nonetheless universally valid, we had to assume a quantum substrate from which gravity would result as an emergent classical phenomenon. And there would be no conflict with the arguments against semi-classical theories, because there would be no gravity at all on the substrate level. The gravitational field would not have any quantum properties, and a quantization of GR would not lead to any fundamental theory. The objective of a theory of 'QG' would instead be the identification of the quantum substrate from which gravity results. - The paper tries to give an overview over the main options for theory construction in the field of QG. Because of the still unclear status of gravity and spacetime, it pleads for the necessity of a plurality of conceptually different approaches to QG.

http://arxiv.org/abs/0907.5542
Holography for Cosmology
Authors: Paul McFadden, Kostas Skenderis
(Submitted on 31 Jul 2009)

Abstract: We propose a holographic description of four-dimensional single-scalar inflationary universes, and show how cosmological observables, such as the primordial power spectrum, are encoded in the correlation functions of a three-dimensional QFT. The holographic description correctly reproduces standard inflationary predictions in the limit where a perturbative quantization of fluctuations is justified. In the opposite limit, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative large N QFT. Initiating a holographic phenomenological approach, we show that models containing only two parameters, N and a dimensionful coupling constant, are capable of satisfying the current observational constraints.

http://arxiv.org/abs/0907.4986
Zipping and Unzipping of Cosmic String Loops in Collision
Authors: Hassan Firouzjahi, Johanna Karouby, Shahram Khosravi, Robert Brandenberger
(Submitted on 28 Jul 2009)

Abstract: In this paper the collision of two cosmic string loops is studied. After collision junctions are formed and the loops are entangled. We show that after their formation the junctions start to unzip and the loops disentangle. This analysis provides a theoretical understanding of the unzipping effect observed in numerical simulations of a network of cosmic strings with more than one type of cosmic strings. The unzipping phenomena have important effects in the evolution of cosmic string networks when junctions are formed upon collision, such as in a network of cosmic superstrings.

http://arxiv.org/abs/0908.0434
ymmetry Reduction and Exact Solutions in Twisted Noncommutative Gravity
Authors: Alexander Schenkel
(Submitted on 4 Aug 2009)

Abstract: We review the noncommutative gravity of Wess et al. and discuss its physical applications. We define noncommutative symmetry reduction and construct deformed symmetric solutions of the noncommutative Einstein equations. We apply our framework to find explicit deformed cosmological and black hole solutions and discuss their phenomenology.
This article is based on a joint work with Thorsten Ohl.

 

[MTd2]

http://arxiv.org/abs/0908.0745

Geometry vs. Matter: The emergence of scalar matter from modified 3D spinfoam model

Peng Xu, Yongge Ma
(Submitted on 5 Aug 2009)
We provide an alternative model to study the couplings of 3D gravity with massless Klein-Gordon field in spinfoam formalism. After the discretization of the scalar field, the model is worked out precisely in a purely combinational way. It is shown that the quantum physics of the scalar matter are totally encoded into the modified dynamics of SU(2) spin-network states which describe the quantum geometries of space. While the physics of the scalar matter coupled with gravity that manifested in the low energy region can be viewed as emerged from this microscopical construction. This give rise to a radical observation on the issues of the unifications of geometry and matter.

 

[marcus]

It is always possible, even if string does not give a good model of nature, that LQG can be used to give a background independent formulation of a string theory. To comprehend string within the background independent context of LQG would be interesting, if only as an academic exercise. Thiemann took some steps in that direction back in 2004 which at the time many people considered outrageous Now some young LQG postdocs are again investigating this possibility.
http://arxiv.org/abs/0908.0953
Canonical Analysis of the Algebraic String
Winston J. Fairbairn, Karim Noui, Francesco Sardelli
27 pages
(Submitted on 6 Aug 2009)
"We investigate the canonical aspects of the algebraic first order formulation of strings introduced two decades ago by Balachandran and collaborators. We slightly enlarge the Lagrangian framework and show the existence of a self-dual formulation and of an Immirzi-type parameter reminiscent of four-dimensional first order gravity. We perform a full Hamiltonian analysis of the self-dual case: we extract the first class constraints and construct the Dirac bracket associated to the second class constraints. The first class constraints contain the diffeomorphisms algebra on the world sheet, as expected; and the coordinates are shown to be non-commutative with respect to the Dirac bracket. Then, the Hamilton equations in a particular (but very natural) gauge are shown to reproduce the wave equation for the string coordinates. In the general, non-self-dual case, we also explicit the first class constraints of the system and show that, unlike the self-dual formulation, the theory admits an extra propagating degree of freedom than the two degrees of freedom of conventional string theory. This prevents the general algebraic string from being strictly equivalent to the Nambu-Goto string."
==quote from the introduction==
Few years ago, Thiemann reconsidered the Nambu-Goto string and proposed a quantisation of it using the techniques of Loop Quantum Gravity (LQG) [14]. He showed that the LQG techniques, based on background independent quantisation, provides in particular a quantisation of the bosonic string in any dimensions, i.e. there is no need of critical dimensions for the quantum theory to be consistent. This result has sparked off some discussions [15] and certainly deserves to be understood deeper. We think that the algebraic formulation of the bosonic string is a better starting point to test the LQG techniques
than the Nambu-Goto string for it admits a lot of similarities with Ashtekar gravity [16]. It is a first order formulation and possesses an Immirzi-type parameter. In fact, the main motivation of this article is to open an arena for a background independent quantisation of the bosonic string and to compare it to the standard Fock quantisation. In that sense, we want to continue the work initiated by Thiemann from a quite different starting point in order to confirm or not his predictions and even go further."
==endquote==

 

[John86]

a little off topic maybe. Yesterday i found this very interesting article from Stuart Kauffman. One of the topics in this paper handlles about reductionism an the problems with it in physics.

It brought memories from Robert Laughlin book á different universe' which also makes a strong point against redoctionism.

http://www.edge.org/3rd_culture/kauffman09/kauffman09_index.html

 

[marcus]

http://arxiv.org/abs/0908.1764
On the Running of the Cosmological Constant in Quantum General Relativity
B.F.L. Ward (Department of Physics, Baylor University, Waco, TX, USA)
5 pages
(Submitted on 12 Aug 2009)
"We present arguments that show what the running of the cosmological constant means when quantum general relativity is formulated following the prescription developed by Feynman."

Better get on Martin Reuter's train, guys, or you'll get left at the station. :-D

 

[MTd2]

http://arxiv.org/abs/0908.1964

Effective Field Theory, Past and Future

Steven Weinberg
(Submitted on 13 Aug 2009)
This is a written version of the opening talk at the 6th International Workshop on Chiral Dynamics, at the University of Bern, Switzerland, July 6, 2009, to be published in the proceedings of the Workshop. In it, I reminisce about the early development of effective field theories of the strong interactions, comment briefly on some other applications of effective field theories, and then take up the idea that the Standard Model and General Relativity are the leading terms in an effective field theory. Finally, I cite recent calculations that suggest that the effective field theory of gravitation and matter is asymptotically safe.

 

[marcus]

http://arxiv.org/abs/0908.2469
A Prehistory of n-Categorical Physics
John C. Baez, Aaron Lauda
129 pages, 8 figures
(Submitted on 18 Aug 2009)
"This paper traces the growing role of categories and n-categories in physics, starting with groups and their role in relativity, and leading up to more sophisticated concepts which manifest themselves in Feynman diagrams, spin networks, string theory, loop quantum gravity, and topological quantum field theory. Our chronology ends around 2000, with just a taste of later developments such as open-closed topological string theory, the categorification of quantum groups, Khovanov homology, and Lurie's work on the classification of topological quantum field theories."

 

[Finbar]

a little off topic maybe. Yesterday i found this very interesting article from Stuart Kauffman. One of the topics in this paper handlles about reductionism an the problems with it in physics.

It brought memories from Robert Laughlin book á different universe' which also makes a strong point against redoctionism.

http://www.edge.org/3rd_culture/kauffman09/kauffman09_index.html

Thanks for the link. I haven't read it yet but looks interesting. Actually I was thinking yesterday that one of the main problems I wish to address is the mind/body problem or the closly related problem of free will.

 

[marcus]

http://arxiv.org/abs/0908.2874
Solar system tests of Hořava-Lifgarbagez gravity
Tiberiu Harko, Zoltan Kovács, Francisco S. N. Lobo
9 pages, 4 figures
(Submitted on 20 Aug 2009)
"Recently, a renormalizable gravity theory with higher spatial derivatives in four dimensions was proposed by Horava. The theory reduces to Einstein gravity with a non-vanishing cosmological constant in IR, but it has improved UV behaviors. The spherically symmetric black hole solutions for an arbitrary cosmological constant, which represent the generalization of the standard Schwarzschild-(A)dS solution, has also been obtained for the Horava-Lifgarbagez theory. The exact asymptotically flat Schwarzschild type solution of the gravitational field equations in Horava gravity contains a quadratic increasing term, as well as the square root of a fourth order polynomial in the radial coordinate, and it depends on one arbitrary integration constant. The IR modified Horava gravity seems to be consistent with the current observational data, but in order to test its viability more observational constraints are necessary. In the present paper we consider the possibility of observationally testing Horava gravity at the scale of the Solar System, by considering the classical tests of general relativity (perihelion precession of the planet Mercury, deflection of light by the Sun and the radar echo delay) for the spherically symmetric black hole solution of Horava-Lifgarbagez gravity. All these gravitational effects can be fully explained in the framework of the vacuum solution of the gravity. Moreover, the study of the classical general relativistic tests also constrain the free parameter of the solution."

 

[atyy]

http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.103.081301
Nicolai, Vanquishing infinity: "Accordingly, we should view the coexistence of several possibly finite candidate theories only as a first step towards the future construction of an underlying theory of quantum gravity, where classical space and time are only emergent concepts, and which would also be viable nonperturbatively."

 

[MTd2]

http://arxiv.org/abs/0908.3388

A note on the Plebanski action with cosmological constant and an Immirzi parameter

Lee Smolin, Simone Speziale
(Submitted on 24 Aug 2009)
We study the field equations of the Plebanski action for general relativity when both the cosmological constant and an Immirzi parameter are present. We show that the Lagrange multiplier, which usually gets identified with the Weyl curvature, now acquires a trace part. Some consequences of this for a class of modified gravity theories recently proposed in the literature are briefly discussed.

 

[atyy]

http://physics.aps.org/viewpoint-for/10.1103/PhysRevLett.103.091301
Nastase, Pushing the envelope of general relativity: "A recent theory of gravity has stimulated intense debate and many explorations of its implications. A Viewpoint on: Solutions to Hořava Gravity, H. Lü, Jianwei Mei, and C. N. Pope, Phys. Rev. Lett. 103, 091301 (2009)"

 

[MTd2]

http://arxiv.org/abs/0908.3643

On the spectral dimension of causal triangulations

Bergfinnur Durhuus, Thordur Jonsson, John F. Wheater
(Submitted on 25 Aug 2009)
We introduce an ensemble of infinite causal triangulations, called the uniform infinite causal triangulation, and show that it is equivalent to an ensemble of infinite trees, the uniform infinite planar tree. It is proved that in both cases the Hausdorff dimension almost surely equals 2. The infinite causal triangulations are shown to be almost surely recurrent or, equivalently, their spectral dimension is almost surely less than or equal to 2. We also establish that for certain reduced versions of the infinite causal triangulations the spectral dimension equals 2 both for the ensemble average and almost surely. The triangulation ensemble we consider is equivalent to the causal dynamical triangulation model of two-dimensional quantum gravity and therefore our results apply to that model.

 

[John86]

Don't forget these two ones

http://arxiv.org/abs/0908.3408
Entangled quantum states in a local deterministic theory
Authors: Gerard 't Hooft
(Submitted on 24 Aug 2009)

Abstract: Investigating a class of models that is familiar in studies of cellular automata, we find that quantum operators can be employed to describe their long distance behavior. These operators span a Hilbert space that appears to turn such a model into a genuine quantum field theory, obeying the usual conditions of locality in terms of its quantum commutators. Entangled states can be constructed exactly as in quantum theories.
This raises the question whether such models allow Bell's inequalities to be violated. Being a local, deterministic theory, one would argue that this is impossible, but since at large distance scales the model does not seem to differ from real quantum field theories, there is reason to wonder why it should not allow entangled states. The standard arguments concerning Bell's inequalities are re-examined in this light.

http://arxiv.org/abs/0908.3398
On the Electromagnetic Properties of Matter in Collapse Models
Authors: Angelo Bassi, Detlef Duerr
(Submitted on 24 Aug 2009)

Abstract: We discuss the electromagnetic properties of both a charged free particle, and a charged particle bounded by an harmonic potential, within collapse models. By choosing a particularly simple, yet physically relevant, collapse model, and under only the dipole approximation, we are able to solve the equation of motion exactly. In this way, both the finite time and large time behavior can be analyzed accurately. We discovered new features, which did not appear in previous works on the same subject. Since, so far, the spontaneous photon emission process places the strongest upper bounds on the collapse parameters, our results call for a further analysis of this process for those atomic systems which can be employed in experimental tests of collapse models.

 

[marcus]

http://arxiv.org/abs/0908.3683
Early Universe models from Noncommutative Geometry
Matilde Marcolli (Caltech), Elena Pierpaoli (USC)
49 pages, 26 figures
(Submitted on 25 Aug 2009)
"We investigate cosmological predictions on the early universe based on the noncommutative geometry models of gravity coupled to matter. Using the renormalization group analysis for the Standard Model with right handed neutrinos and Majorana mass terms, which is the particle physics content of the most recent noncommutative geometry models, we analyze the behavior of the coefficients of the gravitational and cosmological terms in the Lagrangian derived from the asymptotic expansion of the spectral action functional of noncommutative geometry. We find emergent Hoyle-Narlikar and conformal gravity at the see-saw scales and a running effective gravitational constant, which affects the propagation of gravitational waves and the evaporation law of primordial black holes and provides Linde models of negative gravity in the early universe. The same renormalization group analysis also governs the running of the effective cosmological constant of the model. The model also provides a Higgs based slow-roll inflationary mechanism, for which one can explicitly compute the slow-roll parameters. The particle physics content allows for dark matter models based on sterile neutrinos with Majorana mass terms."

 

[John86]

http://arxiv.org/abs/0908.4069
Decoherence, measurement and interpretation of quantum mechanics
Authors: Juan Sebastián Ardenghi, Mario Castagnino, Sebastian Fortin, Olimpia Lombardi
(Submitted on 27 Aug 2009)
Abstract: According to our modal-Hamiltonian interpretation (MHI) of quantum mechanics, the Hamiltonian of the closed system defines the set of its definite-valued observables. This definition seems to be incompatible with the pointer basis selected by the environment-induced decoherence (EID) of the open system. In this paper we argue that decoherence can be understood from a closed system perspective which (i) shows that the incompatibility between MHI and EID is only apparent, and (ii) solves certain conceptual challenges that the EID program still has to face.

 

[John86]

http://arxiv.org/abs/0908.2809
Emergent Gravity from Quantized Spacetime
Authors: Hyun Seok Yang, M. Sivakumar
(Submitted on 20 Aug 2009)
Abstract: We examine the picture of emergent gravity arising from a mass deformed matrix model. Due to the mass deformation, a vacuum geometry turns out to be a constant curvature spacetime such as d-dimensional sphere and (anti-)de Sitter spaces. We show that the mass deformed matrix model giving rise to the constant curvature spacetime can be derived from the d-dimensional Snyder algebra. The emergent gravity beautifully confirms all the rationale inferred from the algebraic point of view that the d-dimensional Snyder algebra is equivalent to the Lorentz algebra in (d+1)-dimensional {\it flat} spacetime. For example, a vacuum geometry of the mass deformed matrix model is completely described by a G-invariant metric of coset manifolds G/H defined by the Snyder algebra. We also discuss a nonlinear deformation of the Snyder algebra.

http://arxiv.org/abs/0908.3683
Early Universe models from Noncommutative Geometry
Authors: Matilde Marcolli (Caltech), Elena Pierpaoli (USC)
(Submitted on 25 Aug 2009)
Abstract: We investigate cosmological predictions on the early universe based on the noncommutative geometry models of gravity coupled to matter. Using the renormalization group analysis for the Standard Model with right handed neutrinos and Majorana mass terms, which is the particle physics content of the most recent noncommutative geometry models, we analyze the behavior of the coefficients of the gravitational and cosmological terms in the Lagrangian derived from the asymptotic expansion of the spectral action functional of noncommutative geometry. We find emergent Hoyle-Narlikar and conformal gravity at the see-saw scales and a running effective gravitational constant, which affects the propagation of gravitational waves and the evaporation law of primordial black holes and provides Linde models of negative gravity in the early universe. The same renormalization group analysis also governs the running of the effective cosmological constant of the model. The model also provides a Higgs based slow-roll inflationary mechanism, for which one can explicitly compute the slow-roll parameters. The particle physics content allows for dark matter models based on sterile neutrinos with Majorana mass terms.

http://arxiv.org/abs/0908.4029
Turning big bang into big bounce: Quantum dynamics
Authors: Przemyslaw Malkiewicz, Wlodzimierz Piechocki
(Submitted on 27 Aug 2009)
Abstract: We analyze the big bounce (BB) transition of the quantum FRW model in the setting of the non-standard loop quantum cosmology (LQC). Elementary observables are used to quantize compound observables. The spectrum of the energy density operator is bounded and continuous. The spectrum of the volume operator is bounded from below and discrete. Both standard and non-standard LQC methods have a free parameter that should be fixed in some way to specify the BB transition.

 

[marcus]

http://arxiv.org/abs/0908.4224
Stochastic quantization and the role of time in quantum gravity
J. Ambjorn, R. Loll, W. Westra, S. Zohren
15 pages
(Submitted on 28 Aug 2009)
We show that the noncritical string field theory developed from two-dimensional quantum gravity in the framework of causal dynamical triangulations can be viewed as arising through a stochastic quantization. This requires that the proper time appearing in the string field theory be identified with the stochastic time of the stochastic formulation. The framework of stochastic quantization gives rise to a natural nonperturbative quantum Hamiltonian, which incorporates a sum over all spacetime topologies. We point out that the external character of stochastic time is a feature that pertains more generally to the proper time or distance appearing in nonperturbative correlation functions in quantum gravity."

 

[MTd2]

http://arxiv.org/abs/0908.4196

Towards a Noncommutative Astrophysics

We consider astrophysical objects such as main-sequence stars, white-dwarfs and neutron stars in a noncommutative context. Noncommutativity is implemented via a deformed dispersion relation $E^{2}=p^{2}c^{2}(1+\lambda E)^{2}+m^{2}c^{4}$ from which we obtain noncommutative corrections to the pressure, particle number and energy densities for radiation and for a degenerate fermion gas. The main implications of noncommutativity for the considered astrophysical objects are examined and discussed.

 

[marcus]

http://arxiv.org/abs/0908.4329
The Observational Implications of Loop Quantum Cosmology
Jakub Mielczarek
11 pages, 7 figures
(Submitted on 29 Aug 2009)
"In this paper we consider realistic model of inflation embedded in the framework of loop quantum cosmology. Phase of inflation is preceded here by the phase of a quantum bounce. We show how parameters of inflation depend on the initial conditions established in the contracting, pre-bounce phase. Our investigations indicate that phase of the bounce easily sets proper initial conditions for the inflation. Subsequently we study observational effects that might arise due to the quantum gravitational modifications. We perform preliminary observational constraints for the Barbero-Immirzi parameter \gamma, critical density \rho_{\text{c}} and parameter \lambda. In the next step we study effects on power spectrum of perturbations. We calculate spectrum of perturbations from the bounce and from the joined bounce+inflation phase. Based on these studies we indicate possible way to relate quantum cosmological models with the astronomical observations. Using the Sachs-Wolfe approximation we calculate spectrum of the super-horizontal CMB anisotropies. We show that quantum cosmological effects can, in the natural way, explain suppression of the low CMB multipoles. We show that fine-tuning is not required here and model is consistent with observations. We also analyse other possible probes of the quantum cosmologies and discuss perspectives of their implementation."

 

[MTd2]

http://arxiv.org/abs/0908.4476
Sub-leading asymptotic behaviour of area correlations in the Barrett-Crane model
J. Daniel Christensen, Igor Khavkine, Etera R. Livine, Simone Speziale

The Barrett-Crane spin foam model for quantum gravity provides an excellent setting for testing analytical and numerical tools used to probe spinfoam models. Here, we complete the report on the numerical analysis of the single 4-simplex area correlations begun in Phys. Lett. B670 (2009) 403-406, discussing the next-to-leading order corrections ("one-loop" corrections) with particular attention to their measure dependence, and the difference between the Gaussian and Bessel ansatze for the boundary state.

 

[marcus]

http://arxiv.org/abs/0909.0169
Kerr Black Holes as Particle Accelerators to Arbitrarily High Energy
Máximo Bañados, Joseph Silk, Stephen M. West
4 pages, 3 figures, accepted for publication in Phys.Rev.Lett
(Submitted on 1 Sep 2009)
"We show that intermediate mass black holes conjectured to be the early precursors of supermassive black holes and surrounded by relic cold dark matter density spikes can act as particle accelerators with collisions, in principle, at arbitrarily high centre of mass energies in the case of Kerr black holes. While the ejecta from such interactions will be highly redshifted, we may anticipate the possibility of a unique probe of Planck-scale physics."