Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1110.2768
Rapidly-rotating non-Kerr black holes: an open window on the quantum gravity region
Authors: Cosimo Bambi, Francesco Caravelli, Leonardo Modesto
(Submitted on 12 Oct 2011)
Abstract: Recently, two of us have argued that non-Kerr black holes in gravity theories different from General Relativity may have a topologically non-trivial event horizon. More precisely, the spatial topology of the horizon of non-rotating and slow-rotating objects would be a 2-sphere, like in Kerr space-time, while it would change above a critical value of the spin parameter. When the topology of the horizon changes, the black hole central singularity shows up. The accretion process from a thin disk can potentially overspin these black holes and induce the topology transition, violating the Weak Cosmic Censorship Conjecture. If the astrophysical black hole candidates are not the black holes predicted by General Relativity, we might have the quite unique opportunity to see their central region, where classical physics breaks down and quantum gravity effects must appear. Even if the quantum gravity region turned out to be extremely small, at the level of the Planck scale, the size of its apparent image would be finite and potentially observable with future facilities

http://arxiv.org/abs/1110.2199
False loss of coherence
Authors: William G. Unruh
(Submitted on 10 Oct 2011)
Abstract: The loss of coherence of a quantum system coupled to a heat bath as expressed by the reduced density matrix is shown to lead to the miss-characterization of some systems as being incoherent when they are not. The spin boson problem and the harmonic oscillator with massive scalar field heat baths are given as examples of reduced incoherent density matrices which nevertheless still represent perfectly coherent systems.

http://arxiv.org/abs/1110.2217
Temperature of a Decoherent Oscillator with Strong Coupling
Authors: William G. Unruh
(Submitted on 10 Oct 2011)
Abstract: The temperature of an oscillator coupled to the vacuum state of a heat bath via ohmic coupling is non-zero, as measured by the reduced density matrix of the oscillator. This paper shows that the actual temperature, as measured by a thermometer is still zero (or in the thermal state of the bath, the temperature of the bath). The decoherence temperature is due to "false-decoherence", with the heat bath state being dragged along with the oscillator.

 

[marcus]

http://arxiv.org/abs/1110.2905
General covariant transition amplitudes in quantum cosmology
Francesca Vidotto
(Submitted on 13 Oct 2011)
The path-integral approach to cosmology consists in the computation of transition amplitudes between states of the quantum geometry of the universe. In the past, the concrete computation of these transitions amplitudes has been performed in a perturbative regime, breaking the full general covariance of the theory. Here I present how it is possible to define a general covariant path integral in quantum cosmology, by relying on the most recent results of the canonical and covariant formulations of Loop Quantum Gravity. I present two strategies that have been implemented. The first starts from the full Spinfoam theory, i.e. the path-integral framework for Loop Quantum Gravity, and defines a cosmological system. This is not obtained from symmetry reduced variables that are successively quantized, but directly considering the approximations that are characteristic of the full theory. The Spinfoam Cosmology obtained in this way includes quantum fluctuations beyond standard perturbation theory. The second strategy exploits the Hamiltonian constraint of Loop Quantum Cosmology, that is exponentiated in the formal expression of the usual path integral. The result is a general covariant path integral, that reproduces the form of the amplitude in the full Spinfoam theory. Therefore, this procedure connects the canonical and the covariant formalisms.
5 pages. Review for Scientifica Acta

http://arxiv.org/abs/1110.3020
Learning about quantum gravity with a couple of nodes
Enrique F. Borja, Iñaki Garay, Francesca Vidotto
(Submitted on 13 Oct 2011)
Loop Quantum Gravity provides a natural truncation of the infinite degrees of freedom of gravity, obtained by studying the theory on a given finite graph. We review this procedure and we present the construction of the canonical theory on a simple graph, formed by only two nodes. We review the U(N) framework, which provides a powerful tool for the canonical study of this model, and a formulation of the system based on spinors. We consider also the covariant theory, which permits to derive the model from a more complex formulation, paying special attention to the cosmological interpretation of the theory.
43 pages, 8 figures. Prepared for SIGMA's special issue on Loop Quantum Gravity and Cosmology

 

[marcus]

http://arxiv.org/abs/1110.3272
A new Hamiltonian for the Topological BF phase with spinor networks
Valentin Bonzom, Etera R. Livine
(Submitted on 14 Oct 2011)
We describe fundamental equations which define the topological ground states in the lattice realization of the SU(2) BF phase. We introduce a new scalar Hamiltonian, based on recent works in quantum gravity and topological models, which is different from the plaquette operator. Its gauge-theoretical content at the classical level is formulated in terms of spinors. The quantization is performed with Schwinger's bosonic operators on the links of the lattice. In the spin network basis, the quantum Hamiltonian yields a difference equation based on the spin 1/2. In the simplest case, it is identified as a recursion on Wigner 6j-symbols. We also study it in different coherent states representations, and compare with other equations which capture some aspects of this topological phase.
40 pages

http://arxiv.org/abs/1110.3278
Curved geometry and Graphs
Francesco Caravelli
(Submitted on 14 Oct 2011)
Quantum Graphity is an approach to quantum gravity based on a background independent formulation of condensed matter systems on graphs. We summarize recent results obtained on the notion of emergent geometry from the point of view of a particle hopping on the graph. We discuss the role of connectivity in emergent Lorentzian perturbations in a curved background and the BoseHubbard (BH) model defined on graphs with particular symmetries.
Comments: welcome. 4pp, 1 fig. Proceedings of Loops'11 Conference, Madrid

 

[marcus]

http://arxiv.org/abs/1110.3416
Revisiting canonical gravity with fermions
Ghanashyam Date
(Submitted on 15 Oct 2011)
Fermions constitute an important component of matter and their quantization in presence of dynamical gravity is essential for any theory of quantum gravity. We revisit the classical formulation adapted for a background free quantization. The analysis is carried out with the Hilbert-Palatini form for gravity together with the Nieh-Yan topological term which keeps the nature of Barbero-Immirzi parameter independent of inclusion of arbitrary matter with arbitrary couplings. With dynamical gravity, a priori, there are two distinct notions of `parity' - orientation reversing diffeomorphisms and improper Lorentz rotations. The invariance properties of the action and the canonical framework are different with respect to these and gravitational origin of parity violation seems ambiguous.
29 pages

 

[marcus]

http://arxiv.org/abs/1110.4055
A local first law for isolated horizons
Ernesto Frodden, Amit Ghosh, Alejandro Perez
(Submitted on 18 Oct 2011)
We show that isolated horizons satisfy a unique form of first law (in analogy to the first law of stationary black holes) if a local physical input is introduced. Our considerations single out a unique notion of isolated horizon (local) energy E_{IH}=A/(8\pi \ell) and (local) surface gravity \kappa_{IH}=1/\ell where A is the isolated horizon area and \ell is a proper length characterizing the distance to the horizon of a certain family of local observers (suitable for thermodynamics). Even though this result might be of limited relevance for classical considerations, it provides a clear-cut framework for the thermodynamical and statistical mechanical study of quantum isolated horizon models in loop quantum gravity.
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http://arxiv.org/abs/1110.3874
Matter in Loop Quantum Gravity
Ghanashyam Date, Golam Mortuza Hossain
(Submitted on 18 Oct 2011)
Loop quantum Gravity, a non-perturbative and manifestly background free, quantum theory of gravity implies that at the kinematical level the spatial geometry is discrete in a specific sense. The spirit of background independence also requires a non-standard quantum representation of matter. While loop quantization of standard model fields has been proposed, detail study of its implications is not yet available. This review aims to survey the various efforts and results.
39 pages, invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1110.3837
Coupling Shape Dynamics to Matter Gives Spacetime
Henrique Gomes, Tim Koslowski
(Submitted on 17 Oct 2011)
Shape Dynamics is a metric theory of pure gravity, equivalent to General Relativity, but formulated as a gauge theory of spatial diffeomporphisms and local spatial conformal transformations. In this paper we extend the construction of Shape Dynamics from pure gravity to gravity-matter systems and find that there is no obstruction for the coupling of gravity to standard matter. We use the matter gravity system to construct a clock and rod model for Shape Dynamics which allows us to recover a spacetime interpretation of Shape Dynamics trajectories.
10 pages

http://arxiv.org/abs/1110.3947
Canonical formalism for simplicial gravity
Philipp A. Hoehn
(Submitted on 18 Oct 2011)
We summarise a recently introduced general canonical formulation of discrete systems which is fully equivalent to the covariant formalism. This framework can handle varying phase space dimensions and is applied to simplicial gravity in particular.
4 pages, 5 figures, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.4051
Perfect discretization of path integrals
Sebastian Steinhaus
(Submitted on 18 Oct 2011)
In order to obtain a well-defined path integral one often employs discretizations. In the case of General Relativity these generically break diffeomorphism symmetry, which has severe consequences since these symmetries determine the dynamics of the corresponding system.
In this article we consider the path integral of reparametrization invariant systems as a toy example and present an improvement procedure for the discretized propagator. Fixed points and convergence of the procedure are discussed. Furthermore we show that a reparametrization invariant path integral implies discretization independence and acts as a projector onto physical states.
4 pages, 1 figure, based on a talk given at Loops '11, Madrid, to appear in Journal of Physics: Conference Series (JPCS)

brief mention:
The Frodden Ghosh Perez paper above cites a paper by Jacobson et al which was not noticed here at the time it appeared (back in 2008). It now seems clear that the Jacobson et al paper on horizon surface area is intriguing and should be retroactively added to bibliography.
http://arxiv.org/abs/0806.1677
Horizon surface gravity as 2d geodesic expansion
Ted Jacobson, Renaud Parentani
(Submitted on 10 Jun 2008 (v1), last revised 13 Aug 2008 (this version, v2))
The surface gravity of any Killing horizon, in any spacetime dimension, can be interpreted as a local, two-dimensional expansion rate seen by freely falling observers when they cross the horizon. Any two-dimensional congruence of geodesics invariant under the Killing flow can be used to define this expansion, provided that the observers have unit Killing energy.
8 pages,

 

[marcus]

http://arxiv.org/abs/1110.4578
U(N) and holomorphic methods for LQG and Spin Foams
Enrique F. Borja, Jacobo Diaz-Polo, Iñaki Garay
(Submitted on 20 Oct 2011)
The U(N) framework and the spinor representation for loop quantum gravity are two new points of view that can help us deal with the most fundamental problems of the theory. Here, we review the detailed construction of the U(N) framework explaining how one can endow the Hilbert space of N-leg intertwiners with a Fock structure. We then give a description of the classical phase space corresponding to this system in terms of the spinors, and we will study its quantization using holomorphic techniques. We take special care in constructing the usual holonomy operators of LQG in terms of spinors, and in the description of the Hilbert space of LQG with the different polarization given by these spinors.
16 pages. Proceedings for the 3rd Quantum Geometry and Quantum Gravity School in Zakopane (2011)

brief mention:
http://arxiv.org/abs/1110.4492
Information dynamics and new geometric foundations of quantum theory
Ryszard Paweł Kostecki
(Submitted on 20 Oct 2011)
We discuss new approach to mathematical foundations of quantum theory, which is completely independent of Hilbert spaces and measure spaces. New kinematics is defined by non-linear geometry of spaces of integrals on abstract non-commutative algebras. New dynamics is defined by constrained maximisation of quantum relative entropy. We recover Hilbert space based approach (including unitary evolution and the von Neumann-Lüders rule) and measure theoretic approach to probability theory (including Bayes' rule) as special cases of our approach.
Submitted to Proceedings of the Foundations of Probability and Physics 6 conference, Linneuniversitetet, Vaxjo, June 13-16, 2011

 

[marcus]

http://arxiv.org/abs/1110.4833
Continuous formulation of the Loop Quantum Gravity phase space
Laurent Freidel, Marc Geiller, Jonathan Ziprick
(Submitted on 21 Oct 2011)
In this paper, we study the discrete classical phase space of loop gravity, which is expressed in terms of the holonomy-flux variables, and show how it is related to the continuous phase space of general relativity. In particular, we prove an isomorphism between the loop gravity discrete phase space and the symplectic reduction of the continuous phase space with respect to a flatness constraint. This gives for the first time a precise relationship between the continuum and holonomy-flux variables. Our construction shows that the fluxes depend on the three-geometry, but also explicitly on the connection, explaining their non commutativity. It also clearly shows that the flux variables do not label a unique geometry, but rather a class of gauge-equivalent geometries. This allows us to resolve the tension between the loop gravity geometrical interpretation in terms of singular geometry, and the spin foam interpretation in terms of piecewise flat geometry, since we establish that both geometries belong to the same equivalence class. This finally gives us a clear understanding of the relationship between the piecewise flat spin foam geometries and Regge geometries, which are only piecewise-linear flat: While Regge geometry corresponds to metrics whose curvature is concentrated around straight edges, the loop gravity geometry correspond to metrics whose curvature is concentrated around not necessarily straight edges.
27 pages

http://arxiv.org/abs/1110.4694
Poincaré 2-group and quantum gravity
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 21 Oct 2011)
We show that General Relativity can be formulated as a constrained topological theory for flat 2-connections associated to the Poincaré 2-group. Matter can be consistently coupled to gravity in this formulation. We also show that the edge lengths of the spacetime manifold triangulation arise as the basic variables in the path-integral quantization, while the state-sum amplitude is an evaluation of a colored 3-complex, in agreement with the category theory results.
12 pages

 

[John86]

http://arxiv.org/abs/1110.5224
Fractal space-times under the microscope: A Renormalization Group view on Monte Carlo data
Martin Reuter, Frank Saueressig
(Submitted on 24 Oct 2011)
The emergence of fractal features in the microscopic structure of space-time is a common theme in many approaches to quantum gravity. In this work we carry out a detailed renormalization group study of the spectral dimension $d_s$ and walk dimension $d_w$ associated with the effective space-times of asymptotically safe Quantum Einstein Gravity (QEG). We discover three scaling regimes where these generalized dimensions are approximately constant for an extended range of length scales: a classical regime where $d_s = d, d_w = 2$, a semi-classical regime where $d_s = 2d/(2+d), d_w = 2+d$, and the UV-fixed point regime where $d_s = d/2, d_w = 4$. On the length scales covered by three-dimensional Monte Carlo simulations, the resulting spectral dimension is shown to be in very good agreement with the data. This comparison also provides a natural explanation for the apparent puzzle between the short distance behavior of the spectral dimension reported from Causal Dynamical Triangulations (CDT), Euclidean Dynamical Triangulations (EDT), and Asymptotic Safety.

http://arxiv.org/abs/1110.4649
Roaming moduli space using dynamical triangulations
J. Ambjorn, J. Barkley, T. Budd
(Submitted on 20 Oct 2011)
In critical as well as in non-critical string theory the partition function reduces to an integral over moduli space after integration over matter fields. For non-critical string theory this moduli integrand is known for genus one surfaces. The formalism of dynamical triangulations provides us with a regularization of non-critical string theory. We show how to assign in a simple and geometrical way a moduli parameter to each triangulation. After integrating over possible matter fields we can thus construct the moduli integrand. We show numerically for $c=0$ and $c=-2$ non-critical strings that the moduli integrand converges to the known continuum expression when the number of triangles goes to infinity.

http://arxiv.org/abs/0906.2016
UV and IR quantum-spacetime effects for the Chandrasekhar model
Giovanni Amelino-Camelia, Niccolo' Loret, Gianluca Mandanici, Flavio Mercati
(Submitted on 11 Jun 2009 (v1), last revised 24 Oct 2011 (this version, v2))
We modify the Chandrasekhar model of white dwarfs by introducing novel momentum-space features that characterize the analysis of some quantum-spacetime scenarios. We find that the rather standard ultraviolet effects of spacetime quantization can only be significant in a regime where the Chandrasekhar model anyway lacks any contact with observations. But a new class of quantum-spacetime effects inspired by the mechanism of "ultraviolet/infrared mixing" could be relevant for white dwarfs whose mass is roughly half the mass of the Sun, some of which are described in the literature as "strange white dwarfs". We also offer a preliminary argument suggesting that Planck-scale (ultraviolet) effects could be significant in cases where ultra-high densities are present, even when the relevant star is still gigantic in Planck-length units.

http://arxiv.org/abs/1110.5081
On the fate of Lorentz symmetry in relative-locality momentum spaces
Giovanni Amelino-Camelia
(Submitted on 23 Oct 2011)
The most studied doubly-special-relativity scenarios, theories with both the speed-of-light scale and a length/inverse-momentum scale as non-trivial relativistic invariants, have concerned the possibility of enforcing relativistically some nonlinear laws on momentum space. For the "relative-locality framework" recently proposed in arXiv:1101.0931 a central role is played by nonlinear laws on momentum space, with the guiding principle that they should provide a characterization of the geometry of momentum space. Building on previous doubly-special-relativity results I here identify a criterion for establishing whether or not a given geometry of the relative-locality momentum space is "DSR compatible", i.e. compatible with an observer-independent formulation of theories on that momentum space. I find that given some chosen parametrization of momentum-space geometry the criterion takes the form of an elementary algorithm. I show that relative-locality momentum spaces that fail my criterion definitely "break" Lorentz invariance, i.e. theories on such momentum spaces necessarily are observer-dependent "ether" theories. By working out a few examples I provide evidence that when the criterion is instead satisfied one does manage to produce a relativistic formulation. The examples I use to illustrate the applicability of my criterion also have some intrinsic interest, including two particularly noteworthy cases of $\kappa$-Poincar\'e-inspired momentum spaces.

 

[MTd2]

http://arxiv.org/abs/1110.5088

Simplicial Gravity and Strings

John Swain
(Submitted on 23 Oct 2011)
String theory, as a theory containing quantum gravity, is usually thought to require more dimensions of spacetime than the usual 3+1. Here I argue on physical grounds that needing extra dimensions for strings may well be an artefact of forcing a fixed flat background space. I also show that discrete simplicial approaches to gravity in 3+1 dimensions have natural string-like degrees of freedom which are inextricably tied to the dynamical space in which they evolve. In other words, if simplicial approaches to 3+1 dimensional quantum gravity do indeed give consistent theories, they may essentially contain consistent background-independent string theories.

 

[marcus]

http://arxiv.org/abs/1110.5631
Effective relational dynamics
Philipp A. Hoehn
(Submitted on 25 Oct 2011)
We provide a synopsis of an effective approach to the problem of time in the semiclassical regime. The essential features of this new approach to evaluating relational quantum dynamics in constrained systems are illustrated by means of a simple toy model.
4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.5480
Coherent State Functional Integrals in Quantum Cosmology
Li Qin, Yongge Ma
(Submitted on 25 Oct 2011)
Coherent state functional integrals for the minisuperspace models of quantum cosmology are studied. By the well-established canonical theories, the transition amplitudes in the path-integral representations of Wheeler-DeWitt quantum cosmology and loop quantum cosmology can be formulated through group averaging. The effective action and Hamiltonian with higher-order quantum corrections are thus obtained in both models. It turns out that for a non-symmetric Hamiltonian constraint operator, the Moyal (star)-product emerges naturally in the effective Hamiltonian. This reveals the intrinsic relation among coherent state functional integral, effective theory and star-product. Moreover, both the resulted effective theories imply a possible quantum cosmological effect in large scale limit.
15 pages

http://arxiv.org/abs/1110.5606
The microscopic dynamics of quantum space as a group field theory
Daniele Oriti
(Submitted on 25 Oct 2011)
We provide a rather extended introduction to the group field theory approach to quantum gravity, and the main ideas behind it. We present in some detail the GFT quantization of 3d Riemannian gravity, and discuss briefly the current status of the 4-dimensional extensions of this construction. We also briefly report on recent results obtained in this approach and related open issues, concerning both the mathematical definition of GFT models, and possible avenues towards extracting interesting physics from them.
60 pages. Extensively revised version of the contribution to "Foundations of Space and Time: Reflections on Quantum Gravity", edited by G. Ellis, J. Murugan, A. Weltman, published by Cambridge University Press

 

[marcus]

http://arxiv.org/abs/1110.5885
Coherent states in quantum gravity: a construction based on the flux representation of LQG
Daniele Oriti, Roberto Pereira, Lorenzo Sindoni
(Submitted on 26 Oct 2011)
As part of a wider study of coherent states in (loop) quantum gravity, we introduce a modification to the standard construction, based on the recently introduced (non-commutative) flux representation. The resulting quantum states have some welcome features, in particular concerning peakedness properties, when compared to other coherent states in the literature.
24 pages, 2 figures

http://arxiv.org/abs/1110.5899
Einstein-Regge equations in spinfoams
Claudio Perini
(Submitted on 26 Oct 2011)
We consider spinfoam quantum gravity on a spacetime decomposition with many 4-simplices, in the double scaling limit in which the Immirzi parameter gamma is sent to zero (flipped limit) and the physical area in Planck units gamma times the spin quantum number j) is kept constant. We show that the quantum amplitude takes the form of a Regge-like path integral and enforces Einstein equations in the semiclassical regime. In addition to quantum corrections which vanish when the Planck constant goes to zero, we find new corrections due to the discreteness of geometric spectra which is controlled by the Immirzi parameter.
4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.5694
Realizability of the Lorentzian (n,1)-Simplex
Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 26 Oct 2011)
In a previous article [arXiv:1108.4965] we have developed a Lorentzian version of the Quantum Regge Calculus in which the significant differences between simplices in Lorentzian signature and Euclidean signature are crucial. In this article we extend a central result used in the previous article, regarding the realizability of Lorentzian triangles, to arbitrary dimension. This technical step will be crucial for developing the Lorentzian model in the case of most physical interest: 3+1 dimensions.
We first state (and derive in an appendix) the realizability conditions on the edge-lengths of a Lorentzian n-simplex in total dimension n=d+1, where d is the number of space-like dimensions. We then show that in any dimension there is a certain type of simplex which has all of its time-like edge lengths completely unconstrained by any sort of triangle inequality. This result is the d+1 dimensional analogue of the 1+1 dimensional case of the Lorentzian triangle.
15 pages, 2 figures

brief mention:
http://arxiv.org/abs/1110.5550
Quantum gravity effects on compact star cores
Peng Wang, Haitang Yang, Xiuming Zhang
(Submitted on 25 Oct 2011)
Using the Tolman-Oppenheimer-Volkoff equation and the equation of state of zero temperature ultra-relativistic Fermi gas based on GUP, the quantum gravitational effects on the cores of compact stars are discussed. Our results show that...[I omit some details here.]... All these results are different from those obtained from classical gravity.
9 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1110.6017
Dynamics for a simple graph using the U(N) framework for loop quantum gravity
Enrique F. Borja, Jacobo Diaz-Polo, Laurent Freidel, Iñaki Garay, Etera R. Livine
(Submitted on 27 Oct 2011)
The implementation of the dynamics in loop quantum gravity (LQG) is still an open problem. Here, we discuss a tentative dynamics for the simplest class of graphs in LQG: Two vertices linked with an arbitrary number of edges. We find an interesting global U(N) symmetry in this model that selects the homogeneous/isotropic sector. Then, we propose a quantum Hamiltonian operator for this reduced sector. Finally, we introduce the spinor representation for LQG in order to propose a classical effective dynamics for this model.
Comments: 4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.6114
Effective action for EPRL/FK spin foam models
Aleksandar Mikovic, Marko Vojinovic
(Submitted on 27 Oct 2011)
We show that a natural modification of the EPRL/FK vertex amplitude gives a finite spin foam model whose effective action gives the Einstein-Hilbert action in the limit of large spins and arbitrarily fine spacetime triangulations. The first-order quantum corrections can be easily computed and we show how to calculate the higher-order corrections.
4 pages, proceedings of Loops 11 conference, Madrid; to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1110.6150
Regularized Hamiltonians and Spinfoams
Emanuele Alesci
(Submitted on 27 Oct 2011)
We review a recent proposal for the regularization of the scalar constraint of General Relativity in the context of LQG. The resulting constraint presents strengths and weaknesses compared to Thiemann's prescription. The main improvement is that it can generate the 1-4 Pachner moves and its matrix elements contain 15j Wigner symbols, it is therefore compatible with the spinfoam formalism: the drawback is that Thiemann anomaly free proof is spoiled because the nodes that the constraint creates have volume.
4 pages, based on a talk given at Loops '11 in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

brief mention, for historical interest:
http://arxiv.org/abs/1110.5941
Introduction to Bronstein's "Quantum theory of weak gravitational fields"
S. Deser, A. Starobinsky
(Submitted on 26 Oct 2011)
A scientific introduction and short biography to accompany the translation of Matvei P. Bronstein, "Quantum theory of weak gravitational fields", Phys. Zeitschr. der Sowjetunion 9, 140 157 (1936), to appear as a "Golden Oldie" in JGRG.
Introduction to translation of Bronstein's original 1935 paper as a "Golden oldie", JGRG, Jan 2012

 

[marcus]

http://arxiv.org/abs/1110.6350
Evolution of Primordial Black Holes in Loop Quantum Gravity
Debabrata Dwivedee, Bibekananda Nayak, Mubasher Jamil, Lambodar Prasad Singh
(Submitted on 28 Oct 2011)
As a sequel to our paper on the effects of vacuum energy on the evolution of primordial black holes (PBH) in Einstein gravity (Nayak), we study the evolution of PBH within the context of Loop Quantum Gravity. First we calculate the scale factor a(t) and energy density rho(t) of the universe for different cosmic eras and then taking these as inputs we study PBHs evolution. From our estimation it is found that accretion of radiation could not affect PBH evolution in loop quantum gravity. We also found that, in loop quantum gravity, a large number of PBHs may form in early universe compared with Einstein theory or scalar-tensor theories.
11 pages, 1 figure

http://arxiv.org/abs/1110.6389
A short introduction to asymptotic safety
R. Percacci
(Submitted on 28 Oct 2011)
I discuss the notion of asymptotic safety and possible applications to quantum field theories of gravity and matter.
18 pages. In the proceedings of the conference "Time and matter", Budva, Montenegro, October 2010.

brief mention:
http://arxiv.org/abs/1110.6244
Evidence for a Phase Transition in 2D Causal Set Quantum Gravity
Sumati Surya
(Submitted on 28 Oct 2011)
We present evidence for a phase transition in a theory of 2D causal set quantum gravity which contains a dimensionless non-locality parameter epsilon in the interval (0,1]. The transition is between a continuum phase and a crystalline phase, characterised by a set of covariant observables...
14 pages, 13 figures

 

[marcus]

http://arxiv.org/abs/1110.6866
Path integral measure and triangulation independence in discrete gravity
Bianca Dittrich, Sebastian Steinhaus
(Submitted on 31 Oct 2011)
A path integral measure for gravity should also preserve the fundamental symmetry of general relativity, which is diffeomorphism symmetry. In previous work, we argued that a successful implementation of this symmetry into discrete quantum gravity models would imply discretization independence. We therefore consider the requirement of triangulation independence for the measure in (linearized) Regge calculus, which is a discrete model for quantum gravity, appearing in the semi--classical limit of spin foam models. To this end we develop a technique to evaluate the linearized Regge action associated to Pachner moves in 3D and 4D and show that it has a simple, factorized structure. We succeed in finding a local measure for 3D (linearized) Regge calculus that leads to triangulation independence. This measure factor coincides with the asymptotics of the Ponzano Regge Model, a 3D spin foam model for gravity. We furthermore discuss to which extent one can find a triangulation independent measure for 4D Regge calculus and how such a measure would be related to a quantum model for 4D flat space. To this end, we also determine the dependence of classical Regge calculus on the choice of triangulation in 3D and 4D.
36 pages, 7 figures

http://arxiv.org/abs/1110.6566
Quantum Schwarzschild space-time
Paweł Duch, Ryszard Paweł Kostecki
(Submitted on 29 Oct 2011)
Using new approach to construction of space-times emerging from quantum information theory, we identify the space of quantum states that generates the Schwarzschild space-time. No quantisation procedure is used.

brief mention:
http://arxiv.org/abs/1110.6875
A Validation of Causal Dynamical Triangulations
Rajesh Kommu
(Submitted on 31 Oct 2011)
The Causal Dynamical Triangulation (CDT) approach to quantum gravity is a lattice approximation to the gravitational path integral. Developed by Ambjorn, Jurkiewicz and Loll, it has yielded some important results, notably the emergence of classical spacetime and short scale dimensional reduction. However, virtually all the results reported so far have been based on a single computer code. In this paper we present the first completely independent verification of the CDT algorithm, and report the successful reproduction of the emergence of classical spacetime and smooth reduction in the spectral dimension of the 2+1 and 3+1 dimensional spacetimes.
14 pages, 11 figures

http://arxiv.org/abs/1110.6768
On 4-dimensional Lorentz-structures, Dark energy and Exotic smoothness
T. Asselmeyer-Maluga, R. Mader, J. Krol
(Submitted on 31 Oct 2011)
Usually, the topology of a 4-manifolds M is restricted to admit a global hyperbolic structure Ʃxℝ. The result was obtained by using two conditions: existence of a Lorentz structure and causality (no time-like closed curves). In this paper we study the influence of the smoothness structure to show its independence of the two conditions. Then we obtain the possibility for a topology-change of the 3-manifold Ʃ keeping fixed its homology. We will study the example S³xℝ with an exotic differential structure more carefully to show some implications for cosmology. Especially we obtain an interpretation of the transition in topology as dark energy.
7 pages

 

[John86]

http://arxiv.org/abs/1110.6758
RG flow of Weyl-invariant dilaton gravity
R. Percacci
(Submitted on 31 Oct 2011)
Any theory can be made Weyl invariant by introducing a dilaton. It is shown how to construct renormalization group equations for gravity that maintain this property. Explicit calculations are given only in the simplest approximation, namely for the one loop beta functions of a dilaton conformally coupled to a dynamical metric, but the results have wider validity. This formalism could be used to define the meaning of a theory with a position-dependent cutoff: it is equivalent to a theory with a constant cutoff, but a conformally related metric.

 

[marcus]

brief mention:
http://arxiv.org/abs/1111.0214
Quantum vacuum radiation in optical glass
Stefano Liberati, Angus Prain, Matt Visser
(Submitted on 1 Nov 2011)
A recent experimental claim of the detection of analogue Hawking radiation in an optical system [PRL 105 (2010) 203901] has led to some controversy [PRL 107 (2011) 149401, 149402]. While this experiment strongly suggests some form of particle creation from the quantum vacuum (and hence it is per se very interesting), it is also true that it seems difficult to completely explain all features of the observations by adopting the perspective of a Hawking-like mechanism for the radiation. For instance, the observed photons are emitted parallel to the optical horizon, and the relevant optical horizon is itself defined in an unusual manner by combining group and phase velocities. This raises the question: Is this really Hawking radiation, or some other form of quantum vacuum radiation? Naive estimates of the amount of quantum vacuum radiation generated due to the rapidly changing refractive index --- sometimes called the dynamical Casimir effect --- are not encouraging. ...
9 pages, 3 figures

 

[marcus]

brief mention:
http://arxiv.org/abs/1111.0519
Universality for Random Tensors
Razvan Gurau
(Submitted on 2 Nov 2011)
We prove two universality results for random tensors. We first prove universality for tensors of arbitrary rank assuming that the tensor entries are independent identically distributed random variables, generalizing the universality of random matrices. We then prove a universality property for tensors of rank three or higher under the weaker assumption that the joint distribution of the tensor entries is a trace invariant.
15 pages

 

[John86]

http://arxiv.org/abs/1111.0457
The emergent nature of time and the complex numbers in quantum cosmology
Authors: G. W. Gibbons
(Submitted on 2 Nov 2011)
Abstract: The nature of time in quantum mechanics is closely related to the use of a complex, rather than say real, Hilbert space. This becomes particularly clear when considering quantum field theory in time dependent backgrounds, such as in cosmology, when the notion of positive frequency ceases to be well defined. In spacetimes lacking time orientation, i.e without the possibility of defining an arrow of time, one is forced to abandon complex quantum mechanics. One also has to face this problem in quantum cosmology. I use this to argue that this suggests that, at a fundamental level, quantum mechanics may be really real with not one, but a multitude of complex structures. I relate these ideas to other suggestions that in quantum gravity time evolution may not be unitary, possibly implemented by a super-scattering matrix, and the status of CPT.

 

[MTd2]

http://arxiv.org/abs/1111.0884

Perturbative running of the Immirzi parameter

Dario Benedetti, Simone Speziale
(Submitted on 3 Nov 2011)
We report on the renormalization of the Immirzi parameter found through perturbative one-loop calculations.

http://arxiv.org/abs/1111.0880

Exotic smooth R^4 and quantum matter

Jerzy Król
(Submitted on 3 Nov 2011)
We follow the point of view that superstring theory, as the theory of quantum gravity in the number of spacetime dimensions bigger than 4, serves as mathematics for both, 4 dimensional QG and exotic smoothness on open 4-manifolds. Extra-dimensions, supersymmetry or some other string techniques, belong to the mathematical toolkit suitable for the above purposes. Physics in dimension 4 is reached via exotic 4-geometries on $\mathbb{R}^4$. In the paper we discuss the techniques of exact superstring backgrounds, CFT and $SU(2)_k$ WZW models, as suitable for the description of effects assigned to the magnetic field and its gravitational backreactions on exotic Euklidean $\mathbb{R}^4$ which is the underlying smoothness for the 4-dimensional

 

[marcus]

http://arxiv.org/abs/1111.0967
Towards computational insights into the large-scale structure of spin foams
Bianca Dittrich, Frank C Eckert
(Submitted on 3 Nov 2011)
Understanding the large-scale physics is crucial for the spin foam approach to quantum gravity. We tackle this challenge from a statistical physics perspective using simplified, yet feature-rich models. In particular, this allows us to explicitly answer whether broken symmetries will be restored by renormalization: We observe a weak phase transition in both Migdal-Kadanoff and tensor network renormalization. In this work we give a concise presentation of the concepts, results and promises of this new direction of research.
10 pages, 9 figures, to be published in proceedings of the Loops'11 Madrid international conference on quantum gravity

http://arxiv.org/abs/1111.1125
Holomorphic Simplicity Constraints for 4d Riemannian Spinfoam Models
Maité Dupuis, Etera R. Livine
(Submitted on 4 Nov 2011)
Starting from the reformulation of the classical phase space of Loop Quantum Gravity in terms of spinor variables and spinor networks, we build coherent spin network states and show how to use them to write the spinfoam path integral for topological BF theory in terms of Gaussian integrals in the spinors. Finally, we use this framework to revisit the simplicity constraints reducing topological BF theory to 4d Riemannian gravity. These holomorphic simplicity constraints lead us to a new spinfoam model for quantum gravity whose amplitudes are defined as the evaluation of the coherent spin networks.
4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1111.1000
Running Immirzi Parameter and Asymptotic Safety
Jan-Eric Daum, Martin Reuter
(Submitted on 3 Nov 2011)
We explore the renormalization group (RG) properties of quantum gravity, using the vielbein and the spin connection as the fundamental field variables. We require the effective action to be invariant under the semidirect product of spacetime diffeomorphisms and local frame rotations. Starting from the corresponding functional integral we review the construction of an appropriate theory space and an exact funtional RG equation operating on it. We then solve this equation on a truncated space defined by a three parameter family of Holst-type actions which involve a running Immirzi parameter. We find evidence for the existence of an asymptotically safe fundamental theory. It is probably inequivalent to metric quantum gravity constructed in the same way.
To appear in the proceedings of CORFU 2010

brief mention:
http://arxiv.org/abs/1111.1155
From Analogue Models to Gravitating Vacuum
G.E. Volovik

 

[John86]

http://arxiv.org/abs/1111.0973
Two Aspects of Black hole entropy in Lanczos-Lovelock models of gravity
Sanved Kolekar, Dawood Kothawala, T. Padmanabhan
(Submitted on 3 Nov 2011)
We consider two specific approaches to evaluate the black hole entropy which are known to produce correct results in the case of Einstein's theory and generalize them to Lanczos-Lovelock models. In the first approach (which could be called extrinsic) we use a procedure motivated by earlier work by Pretorius, Vollick and Israel, and by Oppenheim, and evaluate the entropy of a configuration of densely packed gravitating shells on the verge of forming a black hole in Lanczos-Lovelock theories of gravity. We find that this matter entropy is not equal to (it is less than) Wald entropy, except in the case of Einstein theory, where they are equal. The matter entropy is proportional to the Wald entropy if we consider a specific m-th order Lanczos-Lovelock model, with the proportionality constant depending on the spacetime dimensions D and the order m of the Lanczos-Lovelock theory as (D-2m)/(D-2). Since the proportionality constant depends on m, the proportionality between matter entropy and Wald entropy breaks down when we consider a sum of Lanczos-Lovelock actions involving different m. In the second approach (which could be called intrinsic) we generalize a procedure, previously introduced by Padmanabhan in the context of GR, to study off-shell entropy of a class of metrics with horizon using a path integral method. We consider the Euclidean action of Lanczos-Lovelock models for a class of metrics off-shell and interpret it as a partition function. We show that in the case of spherically symmetric metrics, one can interpret the Euclidean action as the free energy and read off both the entropy and energy of a black hole spacetime. Surprisingly enough, this leads to exactly the Wald entropy and the energy of the spacetime in Lanczos-Lovelock models obtained by other methods. We comment on possible implications of the result.

http://arxiv.org/abs/1111.1178
Probes of Lorentz Violation
John Ellis, Nick E. Mavromatos
(Submitted on 4 Nov 2011)
Lorentz invariance is such an important principle of fundamental physics that it should constantly be subjected to experimental scrutiny as well as theoretical questioning. Distant astrophysical sources of energetic photons with rapid time variations, such as active galactic nuclei (AGNs) and gamma-ray bursters (GRBs), provide ideal experimental opportunities for testing Lorentz invariance. The Cherenkov Telescope Array (CTA) is an excellent experimental tool for making such tests with sensitivities exceeding those possible using other detectors.

 

[marcus]

brief mention:
http://arxiv.org/abs/1111.1472
The Problem of Time and Quantum Cosmology in the Relational Particle Mechanics Arena
Edward Anderson
(Submitted on 7 Nov 2011)

http://arxiv.org/abs/1111.1252
Topspin Networks in Loop Quantum Gravity
Christopher L Duston
(Submitted on 4 Nov 2011)
We discuss the extension of loop quantum gravity to topspin networks, a proposal which allows topological information to be encoded in spin networks. We will show that this requires minimal changes to the phase space, C*-algebra and Hilbert space of cylindrical functions. We will also discuss the area and Hamiltonian operators, and show these are now dependent on the topology. This extends the idea of "background independence" in loop quantum gravity to include topology as well as geometry...

Duston is a PhD student with Matilde Marcolli as advisor. I can't evaluate, but seems like a novel idea.
http://myweb.fsu.edu/~cduston/cv.pdf

 

[marcus]

http://arxiv.org/abs/1111.1766
Emergence of Loop Quantum Cosmology from Loop Quantum Gravity: Lowest Order in h
Chun-Yen Lin
(Submitted on 7 Nov 2011)
To derive loop quantum cosmology from loop quantum gravity, I apply the model given in [lin1] to a system with coupled gravitational and matter fields. The matter sector consists of a scalar field phi serving as a cosmological clock, and other fields {psi} providing physical spatial coordinates and frames. The physical Hilbert space of the model is constructed from the kinematical Hilbert space of loop quantum gravity, and the local observables in the physical Hilbert space are constructed using the matter coordinates and frames. A specific coherent physical state is then chosen, whose expectation values of the local observables give rise to homogeneous, isotropic and spatially flat gravitational and $phi$ fields at a late clock time. The equations governing these fields may be derived using the symmetry of the physical Hilbert space. When the matter back reactions from {psi} are negligible, the result gives a specific loop quantum cosmological model in the O(hbar⁰) approximation, with calculable higher order corrections.
31 pages 1 figure

http://arxiv.org/abs/1111.1879
Discretisations, constraints and diffeomorphisms in quantum gravity
Benjamin Bahr, Rodolfo Gambini, Jorge Pullin
(Submitted on 8 Nov 2011)
In this review we discuss the interplay between discretization, constraint implementation, and diffeomorphism symmetry in Loop Quantum Gravity and Spin Foam models. To this end we review the Consistent Discretizations approach, which is an application of the master constraint program to construct the physical Hilbert space of the canonical theory, as well as the Perfect Actions approach, which aims at finding a path integral measure with the correct symmetry behavior under diffeomorphisms.
32 pages. Contribution for a special issue of SIGMA on Loop Quantum Gravity and Cosmology

http://arxiv.org/abs/1111.1975
The entropy of large black holes in loop quantum gravity: A combinatorics/analysis approach
Xiangyu Cao, Aurelien Barrau
(Submitted on 8 Nov 2011)
The issue of a possible damping of the entropy periodicity for large black holes in Loop Quantum Gravity is highly debated. Using a combinatorics/analysis approach, we give strong arguments in favor of this damping, at least for prescriptions where the projection constraint is not fully implemented. This means that black holes in loop gravity exhibit an asymptotic Bekenstein-Hawking behavior, provided that a consistent choice of the Immirzi constant is made.
10 pages

http://arxiv.org/abs/1111.1743
Higher Derivative Gravity from the Universal Renormalization Group Machine
F. Saueressig, K. Groh, S. Rechenberger, O. Zanusso
(Submitted on 7 Nov 2011)
We study the renormalization group flow of higher derivative gravity, utilizing the functional renormalization group equation for the average action. Employing a recently proposed algorithm, termed the universal renormalization group machine, for solving the flow equation, all the universal features of the one-loop beta-functions are recovered. While the universal part of the beta-functions admits two fixed points, we explicitly show that the existence of one of them depends on the choice of regularization scheme, indicating that it is most probably unphysical.
7 pages

 

[marcus]

http://arxiv.org/abs/1111.2215
Loop Quantum Brans-Dicke Theory
Xiangdong Zhang, Yongge Ma
(Submitted on 9 Nov 2011)
The loop quantization of Brans-Dicke theory (with coupling parameter omega≠-3/2) is studied. In the geometry-dynamical formalism, the canonical structure and constraint algebra of this theory are similar to those of general relativity coupled with a scalar field. The connection dynamical formalism of the Brans-Dicke theory with real su(2)-connections as configuration variables is obtained by canonical transformations. The quantum kinematical Hilbert space of Brans-Dicke theory is constituted as of that loop quantum gravity coupled with a polymer-like scalar field. The Hamiltonian constraint is promoted as a well defined operator to represent quantum dynamics. This formalism enable us to extend the scheme of non-perturbative loop quantum gravity to the Brans-Dicke theory.
4 pages, to appear in Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1111.2107
Emergence of General Relativity from Loop Quantum Gravity: A Summary
Chun-Yen Lin
(Submitted on 9 Nov 2011)
A model is proposed to demonstrate that classical general relativity can emerge from loop quantum gravity, in a relational description of gravitational field in terms of the coordinates given by matter. Local Dirac observables and coherent states are defined to explore physical content of the model. Expectation values of commutators between the observables for the coherent states recover the four-dimensional diffeomorphism algebra and the large-scale dynamics of the gravitational field relative to the matter coordinates. Both results conform with general relativity up to calculable corrections near singularities.
16 pages with 1 figure

http://arxiv.org/abs/1111.2192
A view on the problems of Quantum Gravity
T. P. Shestakova
(Submitted on 9 Nov 2011)
The existing approaches to quantization of gravity aim at giving quantum description of 3-geometry following to the ideas of the Wheeler--DeWitt geometrodynamics. In this description the role of gauge gravitational degrees of freedom is missed. A probable alternative is to consider gravitational dynamics in extended phase space, taking into account the distinctions between General Relativity and other field theories. The formulation in extended phase space leads to some consequences at classical and quantum levels. At the classical level, it ensures that Hamiltonian dynamics is fully equivalent to Lagrangian dynamics, and the algebra of Poisson brackets is invariant under reparametrizations in a wide enough class including reparametrizations of gauge variables, meantime in the canonical Dirac approach the constraints' algebra is not invariant that creates problems with quantization. At the quantum level, the approach come to the description in which the observer can see various but complementary quantum gravitational phenomena in different reference frames that answers the spirit of General Relativity and Quantum Theory. Though until now the approach was applied to General Relativity in its original formulations, its implementation in different trends, including Quantum Loop Gravity or some other representations of gravitational variables, would also be of interest.
6 pages, talk presented at the International Conference on Quantum Gravity "Loops 11", Madrid, May 2011

 

[marcus]

http://arxiv.org/abs/1111.2341
Quantum Gravity signals in neutrino oscillations
Martin Sprenger, Piero Nicolini, Marcus Bleicher
(Submitted on 9 Nov 2011)
We investigate the effect of a Quantum Gravity-induced minimal length on neutrino oscillations. The minimal length is implemented in a phenomenological framework, allowing us to make predictions independently of any fundamental approach. We obtain clear minimal length signatures and discuss their observability in current and future experiments. We present an overview over other scenarios in which the minimal length leaves its signature and show new results concerning minimal length thermodynamics.
6 pages, 3 figures, presented at First Caribbean Symposium on Nuclear and Astroparticle Physics - STARS2011, La Habana, Cuba, 2011

 

[marcus]

http://arxiv.org/abs/1111.2672
The Space of Connections as the Arena for (Quantum) Gravity
Steffen Gielen
(Submitted on 11 Nov 2011)
We review some properties of the space of connections as the natural arena for canonical (quantum) gravity, and compare to the case of the superspace of 3-metrics. We detail how a 1-parameter family of metrics on the space of connections arises from the canonical analysis for general relativity which has a natural interpretation in terms of invariant tensors on the algebra of the gauge group. We also review the description of canonical GR as a geodesic principle on the space of connections, and comment on the existence of a time variable which could be used in the interpretation of the quantum theory.
12 pages, SIGMA 7 (2011), 104

http://arxiv.org/abs/1111.2671
Equivalent and Alternative Forms for BF Gravity with Immirzi Parameter
Merced Montesinos, Mercedes Velázquez
(Submitted on 11 Nov 2011)
A detailed analysis of the BF formulation for general relativity given by Capovilla, Montesinos, Prieto, and Rojas is performed. The action principle of this formulation is written in an equivalent form by doing a transformation of the fields of which the action depends functionally on. The transformed action principle involves two BF terms and the two Lorentz invariants that appear in the original action principle generically. As an application of this formalism, the action principle used by Engle, Pereira, and Rovelli in their spin foam model for gravity is recovered and the coupling of the cosmological constant in such a formulation is obtained.
13 pages, SIGMA 7 (2011), 103

brief mention:
http://arxiv.org/abs/1111.2732
On Poisson geometries related to noncommutative emergent gravity
Nikolaj Kuntner, Harold Steinacker
(Submitted on 11 Nov 2011)

 

[marcus]

http://arxiv.org/abs/1111.2865
A proposed proper EPRL vertex amplitude
Jonathan Engle
(Submitted on 11 Nov 2011)
As established in a prior work of the author, the linear simplicity constraints used in the construction of the so-called `new' spin-foam models mix three of the five sectors of Plebanski theory, only one of which is gravity in the usual sense, and this is the reason for certain `unwanted' terms in the asymptotics of the EPRL vertex amplitude as calculated by Barrett et al.
In the present paper, an explicit classical discrete condition is derived that isolates the desired gravitational sector, which we call (II+), following other authors. This condition is quantized and used to modify the vertex amplitude, yielding what we call the `proper EPRL vertex amplitude.' This vertex still depends only on standard SU(2) spin-network data on the boundary, is SU(2) gauge invariant, and is linear in the boundary state, as required. In addition, the asymptotics now consist in the single desired term of the form e^iS_Regge, and all degenerate configurations are exponentially suppressed.
25 pages

http://arxiv.org/abs/1111.2867
Classical Setting and Effective Dynamics for Spinfoam Cosmology
Etera R. Livine, Mercedes Martín-Benito
(Submitted on 11 Nov 2011)
We explore how to extract effective dynamics from loop quantum gravity and spinfoams truncated to a finite fixed graph, with the hope of modeling symmetry-reduced gravitational systems. We particularize our study to the 2-vertex graph with N links. We describe the canonical data using the recent formulation of the phase space in terms of spinors, and implement a symmetry-reduction to the homogeneous and isotropic sector. From the canonical point of view, we construct a consistent Hamiltonian for the model and discuss its relation with Friedmann-Robertson-Walker cosmologies. Then, we analyze the dynamics from the spinfoam approach. We compute exactly the transition amplitude between initial and final coherent spin networks states with support on the 2-vertex graph, for the choice of the simplest two-complex (with a single space-time vertex). The transition amplitude verifies an exact differential equation that agrees with the Hamiltonian constructed previously. Thus, in our simple setting we clarify the link between the canonical and the covariant formalisms.
38 pages

http://arxiv.org/abs/1111.3179
Adiabatic and non-adiabatic perturbations for loop quantum cosmology
Yu Li, Jian-Yang Zhu
(Submitted on 14 Nov 2011)
We generalize the perturbations theory of loop quantum cosmology to a hydrodynamical form and define an effective curvature perturbation on an uniform density hypersurfaces $\zeta_e$. As in the classical cosmology, $\zeta_e$ should be gauge-invariant and conservation on the large scales. The evolutions of both the adiabatic and the non-adiabatic perturbations for a multi-fluids model are investigated in the framework of the effective hydrodynamical theory of loop quantum cosmology with the inverse triad correction. We find that, different from the classical cosmology, the evolution of the large-scales non-adiabatic entropy perturbation can be driven by an adiabatic curvature perturbation and this adiabatic source for the non-adiabatic perturbation is a quantum effect. As an application of the related formalism, we study a decay model and give out the numerical results.
10 pages, 3 figures

brief mention:
http://arxiv.org/abs/1111.2875
On the running of the gravitational constant
Mohamed M. Anber, John F. Donoghue
(Submitted on 11 Nov 2011)
We show that there is no useful and universal definition of a running gravitational constant, G(E), in the perturbative regime below the Planck scale. By consideration of the loop corrections to several physical processes, we show that the quantum corrections vary greatly, in both magnitude and sign, and do not exhibit the required properties of a running coupling constant. We comment on the potential challenges of these results for the Asymptotic Safety program.
12 pages

 

[marcus]

http://arxiv.org/abs/1111.3535
Anomaly-free scalar perturbations with holonomy corrections in loop quantum cosmology
Thomas Cailleteau, Jakub Mielczarek, Aurelien Barrau, Julien Grain
(Submitted on 15 Nov 2011)
Holonomy corrections to scalar perturbations are investigated in the loop quantum cosmology framework. Due to the effective approach, modifications of the algebra of constraints generically lead to anomalies. In order to remove those anomalies, counter-terms are introduced. We find a way to explicitly fulfill the conditions for anomaly freedom and we give explicit expressions for the counter-terms. Surprisingly, the "new quantization scheme" naturally arises in this procedure. The gauge invariant variables are found and equations of motion for the anomaly-free scalar perturbations are derived. Finally, some cosmological consequences are discussed qualitatively.
19 pages, 1 figure

http://arxiv.org/abs/1111.3440
Bianchi type-I string cosmological model in the presence of a magnetic field: classical versus loop quantum cosmology approaches
Victor Rikhvitsky, Bijan Saha, Mihai Visinescu
(Submitted on 15 Nov 2011)
A Bianchi type-I cosmological model in the presence of a magnetic flux along a cosmological string is considered. The first objective of this study is to investigate Einstein equations using a tractable assumption usually accepted in the literature. Quantum effects of the present cosmological model are examined in the framework of loop quantum cosmology. Finally we draw a parallel between the classical and quantum approaches.
14 pages, 8 figures

 

[marcus]

http://arxiv.org/abs/1111.3695
Is de Sitter space a fermion?
Andrew Randono
(Submitted on 16 Nov 2011)
Following up on a recent model yielding fermionic geometries, I turn to more familiar territory to address the question of statistics in purely geometric theories. Working in the gauge formulation of gravity, where geometry is characterized by a symmetry broken Cartan connection, I give strong evidence to suggest that de Sitter space itself, and a class of de Sitter-like geometries, can be consistently quantized fermionically. Surprisingly, the underlying mathematics is the same as that of the Skyrme model for strongly interacting baryons. This promotes the question "Is geometry bosonic or fermionic?" [ http://arxiv.org/abs/1105.4184 ] beyond the realm of the rhetorical and places it on uncomfortably familiar ground.
15 pages, 4 figures

 

[marcus]

http://arxiv.org/abs/1111.4369
SU(2) gauge symmetry in gravity phase space
Francesco Cianfrani
(Submitted on 18 Nov 2011)
The Hamiltonian formulation of the Holst action in vacuum and in the presence of matter fields is analyzed in a generic local Lorentz frame. It is elucidated how the SU(2) gauge symmetry is inferred by reducing the set of constraints to a first-class one. The consequences of the proposed approach for Loop Quantum Gravity and Spin Foam models are discussed.
4 pages, proceedings of Loops 11 conference, Madrid; to appear in Journal of Physics: Conference Series (JPCS)