Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/0905.0113

Consequences of Kaluza-Klein Covariance
Authors: Paul S. Wesson
(Submitted on 1 May 2009)

The group of coordinate transformations for 5D noncompact Kaluza-Klein theory is broader than the 4D group for Einstein's general relativity. Therefore, a 4D quantity can take on different forms depending on the choice for the 5D coordinates. We illustrate this by deriving the physical consequences for several forms of the canonical metric, where the fifth coordinate is altered by a translation, an inversion and a change from spacelike to timelike. These cause, respectively, the 4D cosmological 'constant' to become dependent on the fifth coordinate, the rest mass of a test particle to become measured by its Compton wavelength, and the dynamics to become wave-mechanical with a small mass quantum. These consequendes of 5D covariance -- whether viewed as positive or negative -- help to determine the viability of current attempts to unify gravity with the interactions of particles.


http://arxiv.org/abs/0905.0119

Time as an Illusion
Authors: Paul S. Wesson
(Submitted on 1 May 2009)

We review the idea, due to Einstein, Eddington, Hoyle and Ballard, that time is a subjective label, whose primary purpose is to order events, perhaps in a higher-dimensional universe. In this approach, all moments in time exist simultaneously, but they are ordered to create the illusion of an unfolding experience by some physical mechanism. This, in the language of relativity, may be connected to a hypersurface in a world that extends beyond spacetime. Death in such a scenario may be merely a phase change.


http://arxiv.org/abs/0905.0017

Emergence of spatial structure from causal sets
Authors: David Rideout, Petros Wallden
(Submitted on 30 Apr 2009)

There are numerous indications that a discrete substratum underlies continuum spacetime. Any fundamentally discrete approach to quantum gravity must provide some prescription for how continuum properties emerge from the underlying discreteness. The causal set approach, in which the fundamental relation is based upon causality, finds it easy to reproduce timelike distances, but has a more difficult time with spatial distance, due to the unique combination of Lorentz invariance and discreteness within that approach. We describe a method to deduce spatial distances from a causal set. In addition, we sketch how one might use an important ingredient in deducing spatial distance, the `$n$-link', to deduce whether a given causal set is likely to faithfully embed into a continuum spacetime.

 

[marcus]

http://arxiv.org/abs/0905.0997
Beyond the Standard Model: A Noncommutative Approach
Christoph A. Stephan
To be published in the Proceedings of the XLIVth Rencontres de Moriond: Electroweak Interactions and Unified Theories (La Thuile, Italy, 7-14 March 2009)
(Submitted on 7 May 2009)
"During the last two decades Alain Connes developed Noncommutative Geometry (NCG), which allows to unify two of the basic theories of modern physics: General Relativity (GR) and the Standard Model (SM) of Particle Physics as classical field theories. In the noncommutative framework the Higgs boson, which had previously to be put in by hand, and many of the ad hoc features of the standard model appear in a natural way.
The aim of this presentation is to motivate this unification from basic physical principles and to give a flavour of its derivation. One basic prediction of the noncommutative approach to the SM is that the mass of the Higgs Boson should be of the order of 170 GeV if one assumes the Big Desert. This mass range is with reasonable probability excluded by the Tevatron and therefore it is interesting to investigate models beyond the SM that are compatible with NCG. Going beyond the SM is highly non-trivial within the NCG approach but possible extensions have been found and provide for phenomenologically interesting models. We will present in this article a short introduction into the NCG framework and describe one of these extensions of the SM. This model contains new scalar bosons (and fermions) which constitute a second Higgs-like sector mixing with theordinary Higgs sector and thus considerably modifying the mass eigenvalues."

 

[marcus]

http://arxiv.org/abs/0905.0695
A Redetermination of the Hubble Constant with the Hubble Space Telescope from a Differential Distance Ladder
Adam G. Riess (JHU, STScI), Lucas Macri (Texas A&M), Stefano Casertano (STScI), Megan Sosey (STScI), Hubert Lampeitl (UPort), Henry C. Ferguson (STScI), Alexei V. Filippenko (UCB), Saurabh W. Jha (Rutgers), Weidong Li (UCB), Ryan Chornock (UCB), Devdeep Sarkar (UCI)
60 pages, 15 figures Accepted for Publication, ApJ. This is the second of two papers reporting results from a program to determine the Hubble constant to 5% precision from a refurbished distance ladder based on extensive use of differential measurements
(Submitted on 5 May 2009)
We report observations of 240 Cepheid variables obtained with the Near Infrared Camera (NICMOS) through the F160W filter on the Hubble Space Telescope (HST). The Cepheids are distributed across six recent hosts of Type Ia supernovae (SNe Ia) and the "maser galaxy" NGC 4258, allowing us to directly calibrate the peak luminosities of the SNe Ia from the precise, geometric distance measurements provided by the masers. New features of our measurement include the use of the same instrument for all Cepheid measurements across the distance ladder and homogeneity of the Cepheid periods and metallicities thus necessitating only a differential measurement of Cepheid fluxes and reducing the largest systematic uncertainties in the determination of the fiducial SN Ia luminosity. The NICMOS measurements reduce differential extinction in the host galaxies by a factor of 5 over past optical data. Combined with an expanded of 240 SNe Ia at z<0.1 which define their magnitude-redshift relation, we find H₀=74.2 +/-3.6, a 4.8% uncertainty including both statistical and systematic errors. We show that the factor of 2.2 improvement in the precision of H₀ is a significant aid to the determination of the equation-of-state of dark energy, w = P/(rho c²). Combined with the WMAP 5-year measurement of Omega_M h², we find w= -1.12 +/- 0.12 independent of high-redshift SNe Ia or baryon acoustic oscillations (BAO). This result is also consistent with analyses based on the combination of high-z SNe Ia and BAO. The constraints on w(z) now with high-z SNe Ia and BAO are consistent with a cosmological constant and improved by a factor of 3 from the refinement in H₀ alone. We show future improvements in H₀ are likely and will further contribute to multi-technique studies of dark energy."

 

[MTd2]

http://arxiv.org/abs/0905.1665

Fractal Quantum Space-Time
Authors: Leonardo Modesto
(Submitted on 11 May 2009)

Abstract: In this paper we calculated the spectral dimension of loop quantum gravity (LQG) using the scaling property of the area operator spectrum on spin-network states and using the scaling property of the volume and length operators on Gaussian states. We obtained that the spectral dimension of the spatial section runs from 1.5 to 3, and under particular assumptions from 2 to 3 across a 1.5 phase when the energy of a probe scalar field decreases from high to low energy in a fictitious time T. We calculated also the spectral dimension of space-time using the scaling of the area spectrum operator calculated on spin-foam models. The main result is that the effective dimension is 2 at the Planck scale and 4 at low energy. This result is consistent with two other approaches to non perturbative quantum gravity: "causal dynamical triangulation" and "asymptotically safe quantum gravity". We studied the scaling properties of all the possible curvature invariants and we have shown that the singularity problem seems to be solved in the covariant formulation of quantum gravity in terms of spin-foam models. For a particular form of the scaling (or for a particular area operator spectrum) all the curvature invariants are regular also in the Trans-Planckian regime.

 

[marcus]

http://arxiv.org/abs/0905.1501
Spin foam models for quantum gravity from lattice path integrals
Valentin Bonzom
19 pages, 1 figure
(Submitted on 10 May 2009)
"Spin foam models for quantum gravity are derived from lattice path integrals. The setting involves variables from both lattice BF theory and Regge calculus. The action consists in a Regge action, which depends on areas, dihedral angles and includes the Immirzi parameter. In addition, a measure is inserted to ensure a consistent gluing of simplices, so that the amplitude is dominated by configurations which satisfy the parallel transport relations. We explicitly compute the path integral as a sum over spin foams for a generic measure. The Freidel-Krasnov and Engle-Pereira-Rovelli models correspond to a special choice of gluing. In this case, the equations of motion describe genuine geometries, where the constraints of area-angle Regge calculus are satisfied. Furthermore, the Immirzi parameter drops out of the on-shell action, and stationarity with respect to area variations requires spacetime geometry to be flat."

http://arxiv.org/abs/0905.1670v1
(Broken) Gauge Symmetries and Constraints in Regge Calculus
Benjamin Bahr, Bianca Dittrich
32 pages, 15 figures
(Submitted on 11 May 2009)
"We will examine the issue of diffeomorphism symmetry in simplicial models of (quantum) gravity, in particular for Regge calculus. We find that for a solution with curvature there do not exist exact gauge symmetries on the discrete level. Furthermore we derive a canonical formulation that exactly matches the dynamics and hence symmetries of the covariant picture. In this canonical formulation broken symmetries lead to the replacements of constraints by so--called pseudo constraints. These considerations should be taken into account in attempts to connect spin foam models, based on the Regge action, with canonical loop quantum gravity, which aims at implementing proper constraints. We will argue that the long standing problem of finding a consistent constraint algebra for discretized gravity theories is equivalent to the problem of finding an action with exact diffeomorphism symmetries. Finally we will analyze different limits in which the pseudo constraints might turn into proper constraints. This could be helpful to infer alternative discretization schemes in which the symmetries are not broken."

 

[MTd2]

http://arxiv.org/abs/0905.2170
Fractal Dimension in 3d Spin-Foams

Francesco Caravelli, Leonardo Modesto
(Submitted on 13 May 2009)
In this paper we perform the calculation of the spectral dimension of the space-time in 3d quantum gravity using the dynamics of the Ponzano-Regge vertex (PR) and its quantum group generalization (Turaev-Viro model (TV)). We realize this considering a very simple decomposition of the 3d space-time and introducing a boundary state which selects a classical geometry on the boundary. We obtain that the spectral dimension of the space-time runs from 2 to 3, across a 1.5 phase, when the energy of a probe scalar field decreases from high to low energy. For the TV model the spectral dimension at hight energy increase with the value of the cosmological constant. At low energy the presence of the cosmological constant does not change the spectral dimension.

http://arxiv.org/abs/0905.2031
L.Fatibene, M.Francaviglia
Global Barbero-Immirzi Connections

The Barbero-Immirzi (BI) connection, as usually introduced out of a spin connection, is a global object though it does not transform properly as a genuine connection with respect to generic spin transformations, unless quite specific and suitable gauges are imposed. We shall here investigate whether and under which global conditions a (properly transforming and hence global) SU(2)-connection can be canonically defined in a gauge covariant way in such a way that SU(2)-connection locally agrees with the usual BI connection and can be defined on pretty general bundles (in particular triviality is not assumed). As a by-product we shall also introduce a global covariant SU(2)-connection over the whole spacetime (while for technical reasons the BI connection in the standard formulation is just introduced on a space slice) which restricts to the usual BI connection on a space slice.

 

[MTd2]

I don't know if it fits on this thread, but since it comes from Ambjorn and Loll, here it goes:
http://arxiv.org/abs/0905.2108

Summing over all Topologies in CDT String Field TheoryJ. Ambjorn, R. Loll, W. Westra, S. Zohren
(Submitted on 13 May 2009 (v1), last revised 13 May 2009 (this version, v2))
By explicitly allowing for topology to change as a function of time, two-dimensional quantum gravity defined through causal dynamical triangulations gives rise to a new continuum string field theory. Within a matrix-model formulation we show that -- rather remarkably -- the associated sum over all genera can be performed in closed form, leading to a nonperturbative definition of CDT string field theory. We also obtain explicit formulas for the n-loop correlation functions. Our construction exhibits interesting parallels with previous, purely Euclidean treatments.

 

[marcus]

Here is an important Rovelli paper which was too early to get on the arxiv.
http://ccdb4fs.kek.jp/cgi-bin/img/allpdf?200034016
The preprint is dated November 1989, and the arxiv collection of preprints only got started later like around 1991.
This paper was published in Physical Review D around 1991. Coin pointed me to this.

http://arxiv.org/abs/0905.2421
On the quantum fate of singularities in a dark-energy dominated universe
Mariam Bouhmadi-Lopez, Claus Kiefer, Barbara Sandhoefer, Paulo Vargas Moniz
20 pages, 6 figures
(Submitted on 14 May 2009)
"Classical models for dark energy can exhibit a variety of singularities, many of which occur for scale factors much bigger than the Planck length. We address here the issue whether some of these singularities, the big freeze and the big demarrage, can be avoided in quantum cosmology. We use the framework of quantum geometrodynamics. We restrict our attention to a class of models whose matter content can be described by a generalized Chaplygin gas and be represented by a scalar field with an appropriate potential. Employing the DeWitt criterium that the wave function be zero at the classical singularity, we show that a class of solutions to the Wheeler-DeWitt equation fulfilling this condition can be found. These solutions thus avoid the classical singularity. We discuss the reasons for the remaining ambiguity in fixing the solution."


http://arxiv.org/abs/0905.2446
Einstein-aether as a quantum effective field theory
Benjamin Withers
21 pages
(Submitted on 14 May 2009)
"The possibility that Lorentz symmetry is violated in gravitational processes is relatively unconstrained by experiment, in stark contrast with the level of accuracy to which Lorentz symmetry has been confirmed in the matter sector. One model of Lorentz violation in the gravitational sector is Einstein-aether theory, in which Lorentz symmetry is broken by giving a vacuum expectation value to a dynamical vector field. In this paper we analyse the effective theory for quantised gravitational and aether perturbations. We show that this theory possesses a controlled effective expansion within dimensional regularisation, that is, for any process there are a finite number of Feynman diagrams which will contribute to a given order of accuracy. We find that there is no log-running of the two-derivative phenomenological parameters, justifying the use of experimental constraints for these parameters obtained over many orders of magnitude in energy scale. Given the stringent experimental bounds on two-derivative Lorentz-violating operators, we estimate the size of matter Lorentz-violation which arises due to loop effects. This amounts to an estimation of the natural size of coefficients for Lorentz-violating dimension-six matter operators, which in turn can be used to obtain a new bound on the two-derivative parameters of this theory."

http://arxiv.org/abs/0905.2798
Quantum gravity without Lorentz invariance
Thomas P Sotiriou (Cambridge University), Matt Visser (Victoria University of Wellington), Silke Weinfurtner (UBC Vancouver)
36 pages
(Submitted on 18 May 2009)
"There has been a significant surge of interest in Horava's model for 3+1 dimensional quantum gravity, this model being based on anisotropic scaling at a z=3 Lifgarbagez point. Horava's model, and its variants, show dramatically improved ultra-violet behaviour at the cost of exhibiting violation of Lorentz invariance at ultra-high momenta. Following up on our earlier note, [arXiv:0904.4464 [hep-th]], we discuss in more detail our variant of Horava's model. In contrast to Horava's original model, we abandon "detailed balance" and restore parity invariance. We retain, however, Horava's "projectability condition" and explore its implications. Under these conditions, we explicitly exhibit the most general model, and extract the full classical equations of motion in ADM form. We analyze both spin-2 and spin-0 graviton propagators around flat Minkowski space. We furthermore analyze the classical evolution of FLRW cosmologies in this model, demonstrating that the higher-derivative spatial curvature terms can be used to mimic radiation fluid and stiff matter. We conclude with some observations concerning future prospects."

 

[marcus]

http://arxiv.org/abs/0905.2983
Single particle in quantum gravity and BGS entropy of a spin network
Carlo Rovelli, Francesca Vidotto
8 pages
(Submitted on 18 May 2009)
"Passerini and Severini have recently shown that the Braunstein-Ghosh-Severini (BGS) entropy S(Gamma) = -Tr[rho(Gamma) log rho(Gamma)] of a certain density matrix rho(Gamma) naturally associated to a graph Gamma, is maximized, among all graphs with a fixed number of links and nodes, by regular graphs. We ask if this result can play a role in quantum gravity, and be related to the apparent regularity of the physical geometry of space. We show that in Loop Quantum Gravity the matrix rho(Gamma) is precisely the Hamiltonian operator (suitably normalized) of a non-relativistic quantum particle interacting with the quantum gravitational field, if we restrict elementary area and volume eigenvalues to a fixed value. This operator provides a spectral characterization of the physical geometry, and can be interpreted as a state describing the spectral information about the geometry available when geometry is measured by its physical interaction with matter. It is then tempting to interpret its BGS entropy S(Gamma) as a genuine physical entropy: we discuss the appeal and the difficulties of this interpretation."

 

[MTd2]

http://arxiv.org/abs/0905.3170

Self-dual Black Holes in LQG: Theory and Phenomenology

Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 20 May 2009)
In this paper we have recalled the semiclassical metric obtained from a classical analysis of the loop quantum black hole (LQBH). We show that the regular Reissner-Nordstrom-like metric is self-dual in the sense of T-duality: the form of the metric obtained in Loop quantum Gravity (LQG) is invariant under the exchange "r <-> a0/r" where "a0" is proportional to the minimum area in LQG and "r" is the standard Schwarzschild radial coordinate at asymptotic infinity. Of particular interest, the symmetry imposes that if an observer at "r" close to infinity sees a black hole of mass "m" an observer in the other asymptotic infinity beyond the horizon (at "r" close to "0") sees a dual mass "mp/m" ("mp" is the Planck mass). We then show that small LQBH are stable and could be a component of dark matter. Ultra-light LQBHs created shortly after the Big Bang would now have a mass of approximately "10^(-5) mp" and emit radiation with a typical energy of about 10^(13) - 10^(14) eV but they would also emit cosmic rays of much higher energies, albeit few of them. If these small LQBHs form a majority of the dark matter of the Milky Way's Halo, the production rate of ultra-high-energy-cosmic-rays (UHECR) by these ultra light black holes would be compatible with the observed rate of the Auger detector.

 

[marcus]

http://arxiv.org/abs/0905.3168
Black hole entropy and SU(2) Chern-Simons theory
Jonathan Engle, Karim Noui, Alejandro Perez
4 pages, 1 figure
(Submitted on 19 May 2009)
"We show that the isolated horizon boundary condition can be treated in a manifestly SU(2) invariant manner. The symplectic structure of gravity with the isolated horizon boundary condition has an SU(2) Chern-Simons symplectic structure contribution at the horizon with level $$k=a_H/ (4\pi \beta \ell^2_p)$$. Upon quantization, state counting is expressed in terms of the dimension of Chern-Simons Hilbert spaces on a sphere with marked points (defects). In the large black hole limit quantum horizon degrees of freedom can be modeled by a single intertwiner. The coupling constant of the defects with the Chern Simons theory on the horizon is precisely given by the ratio of the area contribution of the defect to the macroscopic area $$a_H$$, namely $$\lambda= 16\pi^2 \beta \ell^2_p (j(j+1))^{1/2}/a_H$$."

 

[MTd2]

http://arxiv.org/abs/0905.3465

Flux-area operator and black hole entropy

J. Fernando Barbero G., Jerzy Lewandowski, Eduardo J. S. Villaseñor
(Submitted on 21 May 2009)
We show that, for space-times with inner boundaries, there exists a natural area operator different from the standard one used in loop quantum gravity. This new flux-area operator has equidistant eigenvalues. We discuss the consequences of substituting the standard area operator in the Ashtekar-Baez-Corichi-Krasnov definition of black hole entropy by the new one. Our choice simplifies the definition of the entropy and allows us to consider only those areas that coincide with the one defined by the value of the level of the Chern-Simons theory describing the horizon degrees of freedom. We give a prescription to count the number of relevant horizon states by using spin components and obtain exact expressions for the black hole entropy. Finally we derive its asymptotic behavior, discuss several issues related to the compatibility of our results with the Bekenstein-Hawking area law and the relation with Schwarzschild quasi-normal modes.

 

[marcus]

http://arxiv.org/abs/0905.3492
Semiclassical Quantum Gravity: Obtaining Manifolds from Graphs
Luca Bombelli, Alejandro Corichi, Oliver Winkler
16 pages, 5 figures
(Submitted on 21 May 2009)
"We address the 'inverse problem' for discrete geometry, which consists in determining whether, given a discrete structure of a type that does not in general imply geometrical information or even a topology, one can associate with it a unique manifold in an appropriate sense, and constructing the manifold when it exists. This problem arises in a variety of approaches to quantum gravity that assume a discrete structure at the fundamental level; the present work is motivated by the semiclassical sector of loop quantum gravity, so we will take the discrete structure to be a graph and the manifold to be a spatial slice in spacetime. We identify a class of graphs, those whose vertices have a fixed valence, for which such a construction can be specified. We define a procedure designed to produce a cell complex from a graph and show that, for graphs with which it can be carried out to completion, the resulting cell complex is in fact a PL-manifold. Graphs of our class for which the procedure cannot be completed either do not arise as edge graphs of manifold cell decompositions, or can be seen as cell decompositions of manifolds with structure at small scales (in terms of the cell spacing). We also comment briefly on how one can extend our procedure to more general graphs."

 

[MTd2]

http://arxiv.org/abs/0905.3627

Holomorphic Factorization for a Quantum Tetrahedron

Laurent Freidel, Kirill Krasnov, Etera R. Livine
(Submitted on 22 May 2009)

Abstract: We provide a holomorphic description of the Hilbert space H(j_1,..,j_n) of SU(2)-invariant tensors (intertwiners) and establish a holomorphically factorized formula for the decomposition of identity in H(j_1,..,j_n). Interestingly, the integration kernel that appears in the decomposition formula turns out to be the n-point function of bulk/boundary dualities of string theory. Our results provide a new interpretation for this quantity as being, in the limit of large conformal dimensions, the exponential of the Kahler potential of the symplectic manifold whose quantization gives H(j_1,..,j_n). For the case n=4, the symplectic manifold in question has the interpretation of the space of "shapes" of a geometric tetrahedron with fixed face areas, and our results provide a description for the quantum tetrahedron in terms of holomorphic coherent states. We describe how the holomorphic intertwiners are related to the usual real ones by computing their overlap. The semi-classical analysis of these overlap coefficients in the case of large spins allows us to obtain an explicit relation between the real and holomorphic description of the space of shapes of the tetrahedron. Our results are of direct relevance for the subjects of loop quantum gravity and spin foams, but also add an interesting new twist to the story of the bulk/boundary correspondence.

 

[MTd2]

http://arxiv.org/abs/0905.3772

Group field theory renormalization - the 3d case: power counting of divergences
Authors: Laurent Freidel, Razvan Gurau, Daniele Oriti
(Submitted on 22 May 2009)

Abstract: We take the first steps in a systematic study of Group Field Theory renormalization, focusing on the Boulatov model for 3D quantum gravity. We define an algorithm for constructing the 2D triangulations that characterize the boundary of the 3D bubbles, where divergences are located, of an arbitrary 3D GFT Feynman diagram. We then identify a special class of graphs for which a complete contraction procedure is possible, and prove, for these, a complete power counting. These results represent important progress towards understanding the origin of the continuum and manifold-like appearance of quantum spacetime at low energies, and of its topology, in a GFT framework.

 

[MTd2]

http://arxiv.org/abs/0905.4082

LQG propagator from the new spin foams
Authors: Eugenio Bianchi, Elena Magliaro, Claudio Perini
(Submitted on 25 May 2009)

Abstract: We compute metric correlations in loop quantum gravity with the dynamics defined by the new spin foam models. The analysis is done at the lowest order in a vertex expansion and at the leading order in a large spin expansion. The result is compared to the graviton propagator of perturbative quantum gravity.

http://arxiv.org/abs/0905.4220

Bare vs. Effective Fixed Point Action in Asymptotic Safety: The Reconstruction Problem
Authors: Elisa Manrique, Martin Reuter
(Submitted on 26 May 2009)

Abstract: We propose a method for the (re)-construction of a regularized functional integral, well defined in the ultraviolet limit, from a solution of the functional renormalization group equation of the effective average action. The functional integral is required to reproduce this solution. The method is of particular interest for asymptotically safe theories. The bare action for the Einstein-Hilbert truncation of Quantum Einstein Gravity (QEG) is computed and its flow is analyzed. As a second example conformally reduced gravity is explored. Various conceptual issues related to the reconstruction problem are discussed.

 

[marcus]

http://arxiv.org/abs/0905.4188
Pushing Further the Asymptotics of the 6j-symbol
Maite Dupuis, Etera R. Livine
16 pages
(Submitted on 26 May 2009)
"In the context of spinfoam models for quantum gravity, we investigate the asymptotical behavior of the 6j-symbol at next-to-leading order. We compute it analytically and check our results against numerical calculations. The 6j-symbol is the building block of the Ponzano-Regge amplitudes for 3d quantum gravity, and the present analysis is directly relevant to deriving the quantum corrections to gravitational correlations in the spinfoam formalism."

 

[John86]

http://arxiv.org/abs/0903.5303
Disordered Locality as an Explanation for the Dark Energy
Authors: Chanda Prescod-Weinstein, Lee Smolin
(Submitted on 30 Mar 2009)

Abstract: We discuss a novel explanation of the dark energy as a manifestation of macroscopic non-locality coming from quantum gravity, as proposed by Markopoulou. It has been previously suggested that in a transition from an early quantum geometric phase of the universe to a low temperature phase characterized by an emergent spacetime metric, locality might have been "disordered". This means that there is a mismatch of micro-locality, as determined by the microscopic quantum dynamics and macro-locality as determined by the classical metric that governs the emergent low energy physics. In this paper we discuss the consequences for cosmology by studying a simple extension of the standard cosmological models with disordered locality. We show that the consequences can include a naturally small vacuum energy.

http://arxiv.org/abs/0905.3551
Stellar Black Holes and the Origin of Cosmic Acceleration
Authors: Chanda Prescod-Weinstein (Perimeter Institute/U-Waterloo), Niayesh Afshordi (Perimeter Institute), Michael L. Balogh (U-Waterloo)
(Submitted on 22 May 2009)

Abstract: The discovery of cosmic acceleration has presented a unique challenge for cosmologists. As observational cosmology forges ahead, theorists have struggled to make sense of a standard model that requires extreme fine tuning. This challenge is known as the cosmological constant problem. The theory of gravitational aether is an alternative to general relativity that does not suffer from this fine-tuning problem, as it decouples the quantum field theory vacuum from geometry, while remaining consistent with other tests of gravity. In this paper, we study static black hole solutions in this theory and show that it manifests a UV-IR coupling: Aether couples the spacetime metric close to the black hole horizon, to metric at infinity. We then show that using the Trans-Planckian ansatz (as a quantum gravity effect) close to the black hole horizon, leads to an accelerating cosmological solution, far from the horizon. Interestingly, this acceleration matches current observations for stellar mass black holes. Based on our current understanding of the black hole accretion history in the Universe, we then make a prediction for how the effective dark energy density should evolve with redshift, which can be tested with future dark energy probes.

 

[John86]

http://arxiv.org/abs/0905.3828
Quantum Theory of Ur Objects and General Relativity
Authors: Martin Kober
(Submitted on 24 May 2009)

Abstract: The quantum theory of ur objects postulates that all existing physical objects and their properties are constructed from fundamental objects called ur objects being described by an element of a two dimensional complex Hilbert space. This approach is based on the assumption that quantum theory represents a theory being constitutive for human knowledge. Physical objects are characterized by the information one can gain from them being contained in the quantum state they are described by. Since every Hilbert space can be represented as a tensor product of two dimensional Hilbert spaces, one is led to the ur objects. According to this approach relativistic quantum fields and thus the existence of a Minkowski space-time are the consequence of an iteration of a quantization of binary alternatives. In the original formulation there was only obtained a description of quantum fields on a flat Minkowski space-time. In this work there is made the attempt to incorporate general relativity. Thus the existence of a (3+1)-dimensional space-time in the sense of general relativity is assumed to be a consequence of quantum theory interpreted in an abstract sense.

http://arxiv.org/abs/0905.3828
Cosmological Perturbations in Hořava-Lifgarbagez Gravity
Authors: Xian Gao, Yi Wang, R. Brandenberger, A. Riotto
(Submitted on 23 May 2009)

Abstract: We study cosmological perturbations in Ho\v{r}ava-Lifgarbagez Gravity. We consider scalar metric fluctuations about a homogeneous and isotropic space-time. Starting from the most general metric, we work out the complete second order action for the perturbations. We then make use of the residual gauge invariance and of the constraint equations to reduce the number of dynamical degrees of freedom. After introducing the Sasaki-Mukhanov variable, the combination of spatial metric fluctuation and matter inhomogeneity for which the action in General Relativity has canonical form, we find that this variable has the standard time derivative term in the second order action, and that the extra degree of freedom is non-dynamical. The limit $\lambda \to 1$ is well-behaved, unlike what is obtained when performing incomplete analyses of cosmological fluctuations. Thus, there is no strong coupling problem for Ho\v{r}ava-Lifgarbagez gravity when considering cosmological solutions. We also compute the spectrum of cosmological perturbations. If the potential in the action is taken to be of "detailed balance" form, we find a cancelation of the highest derivative terms in the action for the curvature fluctuations. As a consequence, the initial spectrum of perturbations will not be scale-invariant in a general spacetime background. As an application, we consider fluctuations in an inflationary background and draw connections with the "trans-Planckian problem" for cosmological perturbations. In the special case in which the potential term in the action is of detailed balance form and in which $\lambda = 1$, the equation of motion for cosmological perturbations in the far UV takes the same form as in GR. However, in general the equation of motion is characterized by a modified dispersion relation.

 

[John86]

http://arxiv.org/abs/0905.4222

Undecidability and the problem of outcomes in quantum measurements
Authors: Rodolfo Gambini, Luis Pedro Garcia Pintos, Jorge Pullin
(Submitted on 26 May 2009)

Abstract: We argue that it is fundamentally impossible to recover information about quantum superpositions when a system has interacted with a sufficiently large number of degrees of freedom of the environment. This is due to the fact that gravity imposes fundamental limitations on how accurate measurements can be. This leads to the notion of undecidability: there is no way to tell, due to fundamental limitations, if a quantum system evolved unitarily or suffered wavefunction collapse. This in turn provides a solution to the problem of outcomes in quantum measurement by providing a sharp criterion for defining when an event has taken place. We analyze in detail in examples two situations in which in principle one could recover information about quantum coherence: a) "revivals" of coherence in the interaction of a system with the environment and b) the measurement of global observables of the system plus apparatus plus environment. We show in the examples that the fundamental limitations due to gravity and quantum mechanics in measurement prevent both revivals from occurring and the measurement of global observables. It can therefore be argued that the emerging picture provides a complete resolution to the measurement problem in quantum mechanics.

 

[John86]

http://arxiv.org/abs/0905.4402
The Montevideo interpretation of quantum mechanics: frequently asked questions
Authors: Rodolfo Gambini, Jorge Pullin
(Submitted on 27 May 2009)

Abstract: In a series of recent papers we have introduced a new interpretation of quantum mechanics, which for brevity we will call the Montevideo interpretation. In it, the quantum to classical transition is achieved via a phenomenon called "undecidability" which stems from environmental decoherence supplemented with a fundamental mechanism of loss of coherence due to gravity. Due to the fact that the interpretation grew from several results that are dispersed in the literature, we put together this straightforward-to-read article addressing some of the main points that may confuse readers.

 

[John86]

http://arxiv.org/abs/0905.4117
Thermodynamics and classification of cosmological models in the Horava-Lifgarbagez theory of gravity
Authors: Anzhong Wang, Yumei Wu
(Submitted on 26 May 2009)

Abstract: We study thermodynamics of cosmological models in the Horava-Lifgarbagez theory of gravity, and systematically investigate the evolution of the universe filled with a perfect fluid that has the equation of state $p=w\rho$, where $p$ and $\rho$ denote, respectively, the pressure and energy density of the fluid, and $w$ is an arbitrary real constant. Depending on specific values of the free parameters involved in the models, we classify all of them into various cases. In each case the main properties of the evolution are studied in detail, including the periods of deceleration and/or acceleration, and the existence of big bang, big crunch, and big rip singularities. We pay particular attention on models that may give rise to a bouncing universe

http://arxiv.org/abs/0905.4530
Signature-change events in emergent spacetimes with anisotropic scaling
Authors: Silke Weinfurtner (University of British Columbia), Angela White (Newcastle University), Matt Visser (Victoria University of Wellington)
(Submitted on 28 May 2009)

Abstract: We investigate the behaviour of quantum fields coupled to a spacetime geometry exhibiting finite regions of Euclidean (Riemannian) signature. Although from a gravity perspective this situation might seem somewhat far fetched, we will demonstrate its direct physical relevance for an explicitly realizable condensed matter system whose linearized perturbations experience an effective emergent spacetime geometry with externally controllable signature. This effective geometry is intrinsically quantum in origin, and its signature is determined by the details of the microscopic structure. At the level of the effective field theory arising from our condensed matter system we encounter explicit anisotropic scaling in time and space. Here Lorentz symmetry is an emergent symmetry in the infrared. This anisotropic scaling of time and space cures some of the technical problems that arise when working within a canonical quantisation scheme obeying strict Lorentz invariance at all scales, and so is helpful in permitting signature change events to take place.

http://arxiv.org/abs/0905.4639
Particle decay in de Sitter spacetime via quantum tunneling
Authors: G.E. Volovik
(Submitted on 28 May 2009)

Abstract: The quantum tunneling process of decay of the composite particle in the de Sitter vacuum looks as thermal radiation with the effective temperature twice larger than the Hawking temperature associated with the cosmological horizon

 

[francesca]

http://arxiv.org/abs/0905.4529"
The Internal Spin Angular Momentum of an Asymptotically Flat Spacetime
Andrew Randono, David Sloan

In this paper we investigate the manner in which the internal spin angular momentum of a spinor field is encoded in the gravitational field at asymptotic infinity. The inclusion of internal spin requires us to re-analyze our notion of asymptotic flatness. In particular, the Poincarè symmetry at asymptotic infinity must replaced by a spin-enlarged Poincarè symmetry. Likewise, the generators of the asymptotic symmetry group must be supplemented to account for the internal spin. In the Hamiltonian framework of first order Einstein-Cartan gravity, the extra generator comes from the boundary term of the Gauss constraint in the asymptotically flat context. With the additional term, we establish the relations among the Noether charges of a Dirac field, the Komar integral, and the asymptotic ADM-like geometric integral. We show that by imposing mild restraints on the generating functionals of gauge transformations at asymptotic infinity, the phase space is rendered explicitly finite. We construct the energy-momentum and the new total (spin+orbital) angular momentum boundary integrals that satisfy the appropriate algebra to be the generators of the spin-enlarged Poincarè symmetry. This demonstrates that the internal spin is encoded in the tetrad at asymptotic infinity. In addition, ee find that a new conserved and (spin-enlarged) Poincarè invariant charge emerges that is associated with the global structure of a gauge transformation.

 

[MTd2]

http://arxiv.org/abs/0905.4916

Black holes in full quantum gravity

Kirill Krasnov, Carlo Rovelli
(Submitted on 29 May 2009)
Quantum black holes have been studied extensively in quantum gravity and string theory, using various semiclassical or background dependent approaches. We explore the possibility of studying black holes in the full non-perturbative quantum theory, without recurring to semiclassical considerations, and in the context of loop quantum gravity. We propose a definition of a quantum black hole as the collection of the quantum degrees of freedom that do not influence observables at infinity. From this definition, it follows that for an observer at infinity a black hole is described by an SU(2) intertwining operator. The dimension of the Hilbert space of such intertwiners grows exponentially with the horizon area. These considerations shed some light on the physical nature of the microstates contributing to the black hole entropy. In particular, it can be seen that the microstates being counted for the entropy have the interpretation of describing different horizon shapes. The space of black hole microstates described here is related to the one arrived at recently by Engle, Noui and Perez, and sometime ago by Smolin, but obtained here directly within the full quantum theory.

http://arxiv.org/abs/0905.4949

A geometric perspective on singularity resolution and uniqueness in loop quantum cosmology

Alejandro Corichi, Parampreet Singh
(Submitted on 29 May 2009)
We re-examine the issue of singularity resolution in homogeneous loop quantum cosmology from the perspective of geometrical entities such as expansion rate and the shear scalar. These quantities are very reliable measures of the properties of spacetime and can be defined not only at the classical and effective level, but also at an operator level in the quantum theory. From the spectrum of the corresponding operators and their behavior in the effective loop quantum spacetime, we show that one can severely restrict the ambiguities in regularization of the quantum constraint and rule out unphysical choices. We analyze this in the flat isotropic model and the Bianchi-I spacetimes. In the former case we show that the expansion rate operator has a bounded spectrum only for the so called improved quantization, a result which synergizes with uniqueness of this quantization as proved earlier. For the Bianchi-I spacetime, we show that out of the available choices, the expansion rate and shear operator are bounded for only one regularization of the quantum constraint. It turns out only this choice has a well defined quantum gravity scale.

 

[MTd2]

http://arxiv.org/abs/0906.0292

Lattice Refining Loop Quantum Cosmology from an Isotropic Embedding of Anisotropic Cosmology

William Nelson, Mairi Sakellariadou
(Submitted on 1 Jun 2009)
We demonstrate that it is possible to produce different isotropic embeddings of anisotropic Loop Quantum Cosmology, resulting to "lattice refinement" in the isotropic system. To introduce the general approach, we first use a simple model with only two anisotropic directions. We then employ the specific case of a Bianchi I model, to show how the method extends to three-dimensional systems. To concisely calculate the step-size of the resulting isotropic state, we define the "symmetric dual" of states and operators, for the two- and three-dimensional systems, respectively. Moreover, we prove how the "symmetric dual" of operators may be used to improve the approximation of the curvature operator, for systems with non-constant holonomy shifts.

 

[John86]

http://arxiv.org/abs/0906.0569
LTB spacetimes in terms of Dirac observables
Authors: K. Giesel, J. Tambornino, T. Thiemann
(Submitted on 2 Jun 2009)

Abstract: The construction of Dirac observables, that is gauge invariant objects, in General Relativity is technically more complicated than in other gauge theories such as the standard model due to its more complicated gauge group which is closely related to the group of spacetime diffeomorphisms. However, the explicit and usually cumbersome expression of Dirac observables in terms of gauge non invariant quantities is irrelevant if their Poisson algebra is sufficiently simple. Precisely that can be achieved by employing the relational formalism and a specific type of matter proposed originally by Brown and Kucha{\v r}, namely pressureless dust fields. Moreover one is able to derive a compact expression for a physical Hamiltonian that drives their physical time evolution. The resulting gauge invariant Hamiltonian system is obtained by Higgs -- ing the dust scalar fields and has an infinite number of conserved charges which force the Goldstone bosons to decouple from the evolution. In previous publications we have shown that explicitly for cosmological perturbations. In this article we analyse the spherically symmetric sector of the theory and it turns out that the solutions are in one--to--one correspondence with the class of Lemaitre--Tolman--Bondi metrics. Therefore the theory is capable of properly describing the whole class of gravitational experiments that rely on the assumption of spherical symmetry.

 

[marcus]

http://arxiv.org/abs/0906.0681
Lorentz Violation: Motivation and new constraints
Stefano Liberati (SISSA, Trieste and INFN, Trieste), Luca Maccione (DESY, Hamburg)
Prepared for Annual Review of Nuclear and Particle Science
(Submitted on 3 Jun 2009)
"We review the main theoretical motivations and observational constraints on Planck scale suppressed violations of Lorentz invariance. After introducing the problems related to the phenomenological study of quantum gravitational effects, we discuss the main theoretical frameworks within which possible departures from Lorentz invariance can be described. In particular, we focus on the framework of Effective Field Theory, describing several possible ways of including Lorentz violation therein and discussing their theoretical viability. We review the main low energy effects that are expected in this framework. We discuss the current observational constraints on such a framework, focusing on those achievable through high-energy astrophysics observations. In this context we present a summary of the most recent and strongest constraints on QED with Lorentz violating non-renormalizable operators. Finally, we discuss the present status of the field and its future perspectives."

 

[John86]

http://arxiv.org/abs/0906.0666
Regulating the infrared by mode matching: A massless scalar in expanding spaces with constant deceleration
Authors: T. M. Janssen, T. Prokopec
(Submitted on 3 Jun 2009)

Abstract: In this paper we consider a massless scalar field, with a possible coupling $\xi$ to the Ricci scalar in a $D$ dimensional FLRW spacetime with a constant deceleration parameter $q=\epsilon-1$, $\epsilon=-{\dot{H}}/{H^2}$. Correlation functions for the Bunch-Davies vacuum of such a theory have long been known to be infrared divergent for a wide range of values of $\epsilon$. We resolve these divergences by explicitly matching the spacetime under consideration to a spacetime without infrared divergencies. Such a procedure ensures that all correlation functions with respect to the vacuum in the spacetime of interest are infrared finite. In this newly defined vacuum we construct the coincidence limit of the propagator and as an example calculate the expectation value of the stress energy tensor. We find that this approach gives both in the ultraviolet and in the infrared satisfactory results. Moreover, we find that, unless the effective mass due to the coupling to the Ricci scalar $\xi R$ is negative, quantum contributions to the energy density always dilute away faster, or just as fast, as the background energy density. Therefore, quantum backreaction is insignificant at the one loop order, unless $\xi R$ is negative. Finally we compare this approach with known results where the infrared is regulated by placing the Universe in a finite box. In an accelerating universe, the results are qualitatively the same, provided one identifies the size of the Universe with the physical Hubble radius at the time of the matching. In a decelerating universe however, the two schemes give different late time behavior for the quantum stress energy tensor. This happens because in this case the length scale at which one regulates the infrared becomes sub-Hubble at late times.

 

[MTd2]

http://arxiv.org/abs/0906.0926

Bootstrapping gravity: a consistent approach to energy-momentum self-coupling
Authors: Luke M. Butcher, Michael Hobson, Anthony Lasenby
(Submitted on 4 Jun 2009)

Abstract: It is generally believed that coupling the graviton (a classical Fierz-Pauli massless spin-2 field) to its own energy-momentum tensor successfully recreates the dynamics of the Einstein field equations order by order; however the validity of this idea has recently been brought into serious doubt [1]. To remedy this confusion, we present a graviton action for which energy-momentum self-coupling is indeed consistent with the Einstein field equations. The Hilbert energy-momentum tensor for this graviton is calculated explicitly and shown to supply the correct second-order term in the field equations. A formalism for perturbative expansions of metric-based gravitational theories is then developed, and these techniques employed to demonstrate that our graviton action is a starting point for a straightforward energy-momentum self-coupling procedure that, order by order, generates the Einstein-Hilbert action (up to a classically irrelevant surface term). The perturbative formalism is extended to include matter and a cosmological constant, and interactions between perturbations of a free matter field and the gravitational field are studied in a vacuum background. Finally, the effect of a non-vacuum background is examined, and the graviton is found to develop a non-vanishing "mass-term" in the action.

 

[marcus]

http://arxiv.org/abs/0906.1047
de Sitter equilibrium as a fundamental framework for cosmology
Andreas Albrecht
(Submitted on 5 Jun 2009)
14 pages, 2 figures. To appear in the proceedings of the DICE2008 conference, Thomas Elze ed. (IOP 2009)
"Cosmology might turn out to be the study of fluctuations around a 'de Sitter equilibrium' state. In this article I review the basic ideas and the attractive features of this framework, and respond to a number common questions raised about the de Sitter equilibrium picture. I show that this framework does not suffer from the 'Boltzmann Brain' problem, and relate this cosmological picture to recent work on the 'clock ambiguity' ".

http://arxiv.org/abs/0906.1385
Do Spinors Frame-Drag?
Andrew Randono
(Submitted on 7 Jun 2009)
"We investigate the effect of the intrinsic spin of a fundamental spinor field on the surrounding spacetime geometry. We show that despite the lack of a rotating stress-energy source (and despite claims to the contrary) the intrinsic spin of a spin-half fermion gives rise to a frame-dragging effect analogous to that of orbital angular momentum, even in Einstein-Hilbert gravity where torsion is constrained to be zero. This resolves a paradox regarding the counter-force needed to restore Newton's third law in the well known spin-orbit interaction. In addition, the frame-dragging effect gives rise to a long-range gravitationally mediated spin-spin dipole interaction coupling the internal spins of two sources. We argue that despite the weakness of the interaction, the spin-spin interaction will dominate over the ordinary inverse square Newtonian interaction in any process of sufficiently high-energy for quantum field theoretical effects to be non-negligible."

 

[marcus]

http://arxiv.org/abs/0906.1774
Quantum scalar field in quantum gravity: the vacuum in the spherically symmetric case
Rodolfo Gambini, Jorge Pullin, Saeed Rastgoo
15 pages, one figure
(Submitted on 9 Jun 2009)
"We study gravity coupled to a scalar field in spherical symmetry using loop quantum gravity techniques. Since this model has local degrees of freedom, one has to face 'the problem of dynamics', that is, diffeomorphism and Hamiltonian constraints that do not form a Lie algebra. We tackle the problem using the 'uniform discretization' technique. We study the expectation value of the master constraint and argue that among the states that minimize the master constraint is one that incorporates the usual Fock vacuum for the matter content of the theory."

The next thing is just a nice easy survey about how dark matter is being imaged and studied
http://arxiv.org/abs/0906.1597
Digging into dark matter with weak gravitational lensing
Richard Massey
8 pages. To appear in SnowPAC2009 conference proceedings
(Submitted on 8 Jun 2009)
"Ordinary baryonic particles (such as protons and neutrons) account for only one-sixth of the total matter in the Universe. The remainder is a mysterious "dark matter" component, which does not interact via the electromagnetic force and thus neither emits nor reflects light. However, evidence is mounting for its gravitational influence. The past few years have seen particular progress in observations of weak gravitational lensing, the slight deflection of light from distant galaxies due to the curvature of space around foreground mass. Recent surveys from the Hubble Space Telescope have provided direct proof for dark matter, and the first measurements of its properties. We review recent results, then prospects and challenges for future gravitational lensing surveys."

 

[MTd2]

http://arxiv.org/abs/0906.2016

Planck-scale effects for Chandrasekhar model and TOV equations

Giovanni Amelino-Camelia, Niccolo' Loret, Gianluca Mandanici, Flavio Mercati
(Submitted on 11 Jun 2009)
In a recent paper by Camacho a class of Planck-scale effects that is of interest from a quantum-gravity perspective was studied within the framework of the Chandrasekhar model of white dwarfs, also hoping to use observations of white dwarfs to constrain (or search for) the Planck-scale effects. We generalize Camacho's results by considering a broader class of Planck-scale effects, and taking into account general-relativistic corrections to the Chandrasekhar model. The Planck-scale effects do turn out to be remarkably amplified to macroscopic scales, but only in a regime of ultra-high densities where the Chandrasekhar model no longer provides a meaningful physical description of stars. While our results appear to rule out the possibility of observations of white dwarfs that would provide meaningful experimental insight on the relevant Planck-scale effects, we argue that the analysis does provide some elements of intuition that could be valuable in quantum-gravity research, particularly in as much as they can contribute to a shift of focus from ultra-small length scales to ultra-high densities.

 

[marcus]

http://arxiv.org/abs/0906.2503
Effective dynamics of the closed loop quantum cosmology
Jakub Mielczarek, Orest Hrycyna, Marek Szydlowski
18 pages, 11 figures
(Submitted on 13 Jun 2009)
"In this paper we study dynamics of the closed FRW model with holonomy corrections coming from loop quantum cosmology. We consider models with a scalar field and cosmological constant. In case of the models with cosmological constant and free scalar field, dynamics reduce to 2D system and analysis of solutions simplify. If only free scalar field is included then universe undergoes non-singular oscillations. For the model with cosmological constant, different behaviours are obtained depending on the value of Lambda. If the value of Lambda is sufficiently small, bouncing solutions with asymptotic de Sitter stages are obtained. However if the value of Lambda exceeds critical value Lambda_c =
$$\frac{\sqrt{3}m^2_{\text{Pl}}}{2\pi\gamma^3} \simeq 21 m^2_{\text{Pl}}$$
then solutions become oscillatory. Subsequently we study models with a massive scalar field. We find that this model possesses generic inflationary attractors. In particular field, initially situated in the bottom of the potential, is driven up during the phase of quantum bounce. This subsequently leads to the phase of inflation. Finally we find that, comparing with the flat case, effects of curvature do not change qualitatively dynamics close to the phase of bounce. Possible effects of inverse volume corrections are also briefly discussed."

 

[John86]

I hope Fra wil like this paper i'ts related to Zurek's Quantum Dawrwinism ideas.

http://arxiv.org/abs/0906.2700
Anthropomorphic Quantum Darwinism as an explanation for Classicality
Authors: Thomas Durt
(Submitted on 15 Jun 2009)
Abstract: According to the so-called ``Quantum Darwinist'' approach, the emergence of ``classical islands'' from a quantum background is assumed to obey a (selection) principle of maximal information. We illustrate this idea by considering the coupling of two particles that interact through a position-dependent potential. This approach sheds a new light on the emergence of classical logics and of our classical preconceptions about the world. The distinction between internal and external world, the Cartesian prejudice according to which the whole can be reduced to the sum of its parts and the appearance of preferred representation bases such as the position is seen here as the result of a very long evolution and would correspond to the most useful way of extracting stable and useful information from the quantum correlations.

http://arxiv.org/abs/0906.1772
SystemsEffective Constraints for Relativistic Quantum
Authors: Martin Bojowald, Artur Tsobanjan
(Submitted on 9 Jun 2009)
Abstract: Determining the physical Hilbert space is often considered the most difficult but crucial part of completing the quantization of a constrained system. In such a situation it can be more economical to use effective constraint methods, which are extended here to relativistic systems as they arise for instance in quantum cosmology. By side-stepping explicit constructions of states, such tools allow one to arrive much more feasibly at results for physical observables at least in semiclassical regimes. Several questions discussed recently regarding effective equations and state properties in quantum cosmology, including the spreading of states and quantum back-reaction, are addressed by the examples studied here.

 

[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!