Loop-and-allied QG bibliography

[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)

 

[marcus]

http://arxiv.org/abs/1111.4997
A Renormalizable 4-Dimensional Tensor Field Theory
Joseph Ben Geloun, Vincent Rivasseau
(Submitted on 21 Nov 2011)
We prove that an integrated version of the Gurau colored tensor model supplemented with the usual Bosonic propagator on U(1)⁴ is renormalizable to all orders in perturbation theory. The model is of the type expected for quantization of space-time in 4D Euclidean gravity and is the first example of a renormalizable model of this kind. Its vertex and propagator are four-stranded like in 4D group field theories, but without gauge averaging on the strands. Surprisingly perhaps, the model is of the φ⁶ rather than of the φ⁴ type, since two different φ⁶-type interactions are log-divergent, i.e. marginal in the renormalization group sense. The renormalization proof relies on a multiscale analysis. It identifies all divergent graphs through a power counting theorem. These divergent graphs have internal and external structure of a particular kind called melonic. Melonic graphs dominate the 1/N expansion of colored tensor models and generalize the planar ribbon graphs of matrix models. A new locality principle is established for this category of graphs which allows to renormalize their divergences through counterterms of the form of the bare Lagrangian interactions. The model also has an unexpected anomalous log-divergent (∫φ²)² term, which can be interpreted as the generation of a scalar matter field out of pure gravity.
41 pages, 9 figures

brief mention:
http://arxiv.org/abs/1111.4627
Topology of quantum vacuum
G.E. Volovik
(Submitted on 20 Nov 2011)
Topology in momentum space is the main characteristics of the ground states of a system at zero temperature, the quantum vacua. The gaplessness of fermions in bulk, on the surface or inside the vortex core is protected by topology. Irrespective of the deformation of the parameters of the microscopic theory, the energy spectrum of these fermions remains strictly gapless. This solves the main hierarchy problem in particle physics. The quantum vacuum of Standard Model is one of the representatives...
...The topological invariants in extended momentum and coordinate space determine the bulk-surface and bulk-vortex correspondence, connecting the topology in bulk with the real space. The momentum space topology gives some lessons for quantum gravity. In effective gravity emerging at low energy, the collective variables are the tetrad field and spin connections, while the metric is the composite object of tetrad field...
40 pages, 19 figures, draft for Chapter in proceedings the Como Summer School on analogue gravity

classical result of interest and possible use to quantum relativists:
http://arxiv.org/abs/1111.4962
A local Hamiltonian for spherically symmetric gravity coupled to a scalar field
Nestor Alvarez, Rodolfo Gambini, Jorge Pullin
(Submitted on 21 Nov 2011)
We present a gauge fixing of gravity coupled to a scalar field in spherical symmetry such that the Hamiltonian is an integral over space of a local density. Such a formulation had proved elusive over the years. As in any gauge fixing, it works for a restricted set of initial data. We argue that the set could be large enough to attempt a quantization the could include the important case of an evaporating black hole.
4 pages

αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔~≈≠≡ ≤≥½∞(⇐⇑⇒⇓⇔∴∃ℝℤℕℂ⋅)
_

 

[marcus]

http://arxiv.org/abs/1111.5558
Palatini approach to bouncing cosmologies and DSR-like effects
Gonzalo J. Olmo
(Submitted on 23 Nov 2011)
It is shown that a quadratic gravitational Lagrangian in the Palatini formulation is able to capture different aspects of quantum gravity phenomenology in a single framework. In particular, in this theory field excitations propagating with different energy-densities perceive different background metrics, a fundamental characteristic of the DSR and Rainbow Gravity approaches. This theory, however, avoids the so-called soccer ball problem. Also, the resulting isotropic and anisotropic cosmologies are free from the big bang singularity. This singularity avoidance occurs non-perturbatively and shares some similitudes with the effective dynamics of loop quantum cosmology.
4 pages. Proceedings of Loops'11, Madrid. To appear in Journal of Physics: Conference Series (JPCS)

 

[marcus]

Though not a quantum gravity paper, this could (if not explained away) conceivably have important consequences for Loop and allied research. MTd2 noted it earlier, in September when the first version appeared. It has now been extensively revised with data from a second experiment. So "version 2" really constitutes a new paper:
http://arxiv.org/abs/1109.4897
Measurement of the neutrino velocity with the OPERA detector in the CNGS beam
The OPERA Collaboraton
(Submitted on 22 Sep 2011 (v1), last revised 17 Nov 2011 (this version, v2))
The OPERA neutrino experiment at the underground Gran Sasso Laboratory has measured the velocity of neutrinos from the CERN CNGS beam over a baseline of about 730 km with much higher accuracy than previous studies conducted with accelerator neutrinos. The measurement is based on high-statistics data taken by OPERA in the years 2009, 2010 and 2011. Dedicated upgrades of the CNGS timing system and of the OPERA detector, as well as a high precision geodesy campaign for the measurement of the neutrino baseline, allowed reaching comparable systematic and statistical accuracies. An early arrival time of CNGS muon neutrinos with respect to the one computed assuming the speed of light in vacuum of (57.8 ± 7.8 (stat.)^+8.3_-5.9 (sys.)) ns was measured. This anomaly corresponds to a relative difference of the muon neutrino velocity with respect to the speed of light (v-c)/c = (2.37 ± 0.32 (stat.)^+.34_-.24 (sys.)) x 10^-5. The above result, obtained by comparing the time distributions of neutrino interactions and of protons hitting the CNGS target in 10.5 microsecond long extractions, was confirmed by a test performed using a beam with a short-bunch time-structure allowing to measure the neutrino time of flight at the single interaction level.
32 pages, 18 figures
According to a CERN press release quoted here:
http://www.science20.com/quantum_diaries_survivor/opera_confirms_neutrinos_travel_faster_light-84763
on 17 November the paper was submitted to JHEP (Journal of High Energy Physics) so it will be undergoing peer-review.

 

[marcus]

http://arxiv.org/abs/1111.5842
Group field theory and simplicial gravity path integrals: A model for Holst-Plebanski gravity
Aristide Baratin, Daniele Oriti
(Submitted on 24 Nov 2011)
In a recent work, a dual formulation of group field theories as non-commutative quantum field theories has been proposed, providing an exact duality between spin foam models and non-commutative simplicial path integrals for constrained BF theories. In light of this new framework, we define a model for 4d gravity which includes the Immirzi parameter gamma. It reproduces the Barrett-Crane amplitudes when gamma goes to infinity, but differs from existing models otherwise; in particular it does not require any rationality condition for gamma. We formulate the amplitudes both as BF simplicial path integrals with explicit non-commutative B variables, and in spin foam form in terms of Wigner 15j-symbols. Finally, we briefly discuss the correlation between neighboring simplices, often argued to be a problematic feature, for example, in the Barrett-Crane model.
26 pages, 1 figure

http://arxiv.org/abs/1111.5685
A Topos Model for Loop Quantum Gravity
Tore Dahlen
(Submitted on 24 Nov 2011)
One of the main motivations behind so-called topos physics, as developed by Chris Isham and Andreas Doering [4-7], is to provide a framework for new theories of quantum gravity. In this article we do not search for such theories, but ask instead how one of the known candidates for a final theory, loop quantum gravity (LQG), fits into the topos-theoretical approach. In the construction to follow, we apply the 'Bohrification' method developed by Heunen, Landsman and Spitters [10, 11] to the C*-algebra version of LQG introduced by Christian Fleischhack [9]. We then bring together LQG results and methods from topos physics in a proof of the non-sobriety of the external state space S of the Bohrified LQG theory, and show that the construction obeys the standard requirements of diffeomorphism and gauge invariance.
21 pages

brief mention:
http://arxiv.org/abs/1111.5643
Particle-dependent deformations of Lorentz symmetry
Giovanni Amelino-Camelia
(Submitted on 23 Nov 2011)
I here investigate what is arguably the most significant residual challenge for the proposal of phenomenologically viable "DSR deformations" of relativistic kinematics, which concerns the description of composite particles, such as atoms. ...
... I show that it is possible to implement a fully consistent DSR-relativistic description of kinematics endowing different types of particles with suitably different deformed-Lorentz-symmetry properties...
...Some of the new elements here introduced in the formulation of relativistic kinematics appear to also provide the starting point for the development of a correspondingly novel mathematical formulation of spacetime-symmetry algebras.
24 pages

http://arxiv.org/abs/1111.6090
Holographic No-Boundary Measure
Thomas Hertog, James Hartle
(Submitted on 25 Nov 2011)
We show that the complex saddle points of the no-boundary wave function with a positive cosmological constant and a positive scalar potential have a representation in which the geometry consists of a regular Euclidean AdS domain wall that makes a smooth transition to a Lorentzian, inflationary universe that is asymptotically deSitter. The transition region between AdS and dS regulates the volume divergences ...
... We conjecture that the resulting dS/CFT duality holds also beyond the leading order approximation.
35 pages, 6 figures

 

[MTd2]

http://arxiv.org/abs/1111.6634

Quantizing Horava-Lifgarbagez Gravity via Causal Dynamical Triangulations

Christian Anderson, Steven Carlip, Joshua H. Cooperman, Petr Horava, Rajesh Kommu, Patrick R. Zulkowski
(Submitted on 28 Nov 2011)
We extend the discrete Regge action of causal dynamical triangulations to include discrete versions of the curvature squared terms appearing in the continuum action of (2+1)-dimensional projectable Horava-Lifgarbagez gravity. Focusing on an ensemble of spacetimes whose spacelike hypersurfaces are 2-spheres, we employ Markov chain Monte Carlo simulations to study the path integral defined by this extended discrete action. We demonstrate the existence of known and novel macroscopic phases of spacetime geometry, and we present preliminary evidence for the consistency of these phases with solutions to the equations of motion of classical Horava-Lifgarbagez gravity. Apparently, the phase diagram contains a phase transition between a time-dependent de Sitter-like phase and a time-independent phase. We speculate that this phase transition may be understood in terms of deconfinement of the global gravitational Hamiltonian integrated over a spatial 2-sphere.

 

[marcus]

http://arxiv.org/abs/1111.7192
Gauge invariance in Loop Quantum Cosmology : Hamilton-Jacobi and Mukhanov-Sasaki equations for scalar perturbations
Thomas Cailleteau, Aurelien Barrau
(Submitted on 30 Nov 2011)
Gauge invariance of scalar perturbations is studied together with the associated equations of motion. Extending methods developed in the framework of hamiltonian General Relativity, the Hamilton-Jacobi equation is investigated into the details in Loop Quantum Cosmology. The gauge-invariant observables are built and their equations of motions are reviewed both in Hamiltonian and Lagrangian approaches. This method is applied to scalar perturbations with either holonomy or inverse-volume corrections.
16 pages

http://arxiv.org/abs/1111.7195
Spontaneously broken Lorentz symmetry for Hamiltonian gravity
Steffen Gielen, Derek K. Wise
(Submitted on 30 Nov 2011)
In Ashtekar's Hamiltonian formulation of general relativity, and in loop quantum gravity, Lorentz covariance is a subtle issue that has been strongly debated. Maintaining manifest Lorentz covariance seems to require introducing either complex-valued fields or second class constraints, and either option presents a significant obstacle to quantization. After reviewing the sources of difficulty, we present a Lorentz covariant, real formulation free of second class constraints. Rather than a foliation of spacetime, we use a gauge field y, interpreted as a field of observers, to break the SO(3,1) symmetry down to a subgroup SO(3)_y. This symmetry breaking plays a role analogous to that in MacDowell-Mansouri gravity, which is based on Cartan geometry, leading us to a picture of gravity as 'Cartan geometrodynamics.' We study both Lorentz gauge transformations and transformations of the observer field to show that the apparent breaking of SO(3,1) to SO(3) is not in conflict with Lorentz covariance.
10 pages

brief mention:
http://arxiv.org/abs/1111.7127
Quantum astrometric observables I: time delay in classical and quantum gravity
Igor Khavkine
(Submitted on 30 Nov 2011)
A class of diffeomorphism invariant, physical observables, so-called astrometric observables, is introduced. A particularly simple example, the time delay, which expresses the difference between two initially synchronized proper time clocks in relative inertial motion, is analyzed in detail. It is found to satisfy some interesting inequalities related to the causal structure of classical Lorentzian spacetimes. Thus it can serve as a probe of causal structure and in particular of violations of causality. A quantum model of this observable as well as the calculation of its variance due to vacuum fluctuations in quantum linearized gravity are sketched. The question of whether the causal inequalities are still satisfied by quantized gravity, which is pertinent to the nature of causality in quantum gravity, is raised, but it is shown that perturbative calculations cannot provide a definite answer. Some potential applications of astrometric observables in quantum gravity are discussed.
20 pages, 7 figures

 

[marcus]

http://arxiv.org/abs/1112.0291
Isolated Horizons and Black Hole Entropy In Loop Quantum Gravity
Jacobo Diaz-Polo, Daniele Pranzetti
(Submitted on 1 Dec 2011)
We review the black hole entropy calculation in the framework of Loop Quantum Gravity based on the quasi-local definition of a black hole encoded in the isolated horizon formalism. We show, by means of the covariant phase space formalism, the appearance in the conserved symplectic structure of a boundary term corresponding to a Chern-Simons theory on the horizon and present its quantization both in the U(1) gauge fixed version and in the fully SU(2) invariant one. We then describe the boundary degrees of freedom counting techniques developed for an infinite value of the Chern-Simons level case and, less rigorously, for the case of a finite value. This allows us to perform a comparison between the U(1) and SU(2) approaches and provide a state of the art analysis of their common features and different implications for the entropy calculations. In particular, we comment on different points of view regarding the nature of the horizon degrees of freedom and the role played by the Barbero-Immirzi parameter. We conclude by presenting some of the most recent results concerning possible observational tests for theory.
79 pages, 7 figures. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

 

[marcus]

http://arxiv.org/abs/1112.0360
Positive cosmological constant in loop quantum cosmology
Tomasz Pawlowski, Abhay Ashtekar
(Submitted on 1 Dec 2011)
The k=0 Friedmann Lemaitre Robertson Walker model with a positive cosmological constant and a massless scalar field is analyzed in detail. If one uses the scalar field as relational time, new features arise already in the Hamiltonian framework of classical general relativity: In a finite interval of relational time, the universe expands out to infinite proper time and zero matter density. In the deparameterized quantum theory, the true Hamiltonian now fails to be essentially self-adjoint both in the Wheeler DeWitt (WDW) approach and in LQC. Irrespective of the choice of the self-adjoint extension, the big bang singularity persists in the WDW theory while it is resolved and replaced by a big bounce in loop quantum cosmology (LQC). Furthermore, the quantum evolution is surprisingly insensitive to the choice of the self-adjoint extension. This may be a special case of an yet to be discovered general property of a certain class of symmetric operators that fail to be essentially self-adjoint.
36 pages, 6 figures

http://arxiv.org/abs/1112.0374
The Coupling of Shape Dynamics to Matter
Henrique Gomes
(Submitted on 2 Dec 2011)
Shape Dynamics (SD) is a theory dynamically equivalent to vacuum General Relativity (GR), which has a different set of symmetries. It trades refoliation invariance, present in GR, for local 3-dimensional conformal invariance. This contribution to the Loops 11 conference addresses one of the more urgent questions regarding the equivalence: is it possible to incorporate normal matter in the new framework? The answer is yes, in certain regimes. We present general criteria for coupling and apply it to a few examples.The outcome presents bounds and conditions on scalar densities (such as the Higgs potential and the cosmological constant) not present in GR.
4 pages. Contribution to Loops '11 conference in Madrid, to appear in Journal of Physics: Conference Series (JPCS)

brief mention:
http://arxiv.org/abs/1112.0434
Test of the Law of Gravitation at small Accelerations
H.Meyer (Bergische Universitaet Wuppertal), E.Lohrmann, S.Schubert (Universitaet Hamburg), W.Bartel, A.Glazov, B.Loehr, C.Niebuhr, E.Wuensch (DESY), L.Joensson (University of Lund), G.Kempf (Hamburgische Schiffbau-Versuchsanstalt)
(Submitted on 2 Dec 2011)
Newton's Law of Gravitation has been tested at small values of the acceleration, down to a=10^-10 m/s², the approximate value of MOND's constant a₀. No deviations were found.
11 pages, 5 figures, to be submitted to EPJ

http://arxiv.org/abs/1112.0438
Dark matter in dwarf galaxies of the Local Group
Ewa L. Lokas
(Submitted on 2 Dec 2011)
We review basic properties of the population of dwarf galaxies in the Local Group focusing on dwarf spheroidal galaxies found in the immediate vicinity of the Milky Way. The evidence for dark matter in these objects is critically assessed. We describe the methods of dynamical modelling of such objects, using a few examples of the best-studied dwarfs and discuss the sources of uncertainties in mass estimates. We conclude with perspectives for dwarf galaxies as targets for dark matter detection experiments.
8 pages, 3 figures, contribution to the proceedings of XXXV International Conference of Theoretical Physics "Matter to the Deepest: Recent Developments in Physics of Fundamental Interactions", held in Ustron, Poland in September 2011

 

[marcus]

http://arxiv.org/abs/1112.0994
Generalized uncertainty principles and localization in discrete space
Martin Bojowald, Achim Kempf
(Submitted on 5 Dec 2011)
Generalized uncertainty principles are able to serve as useful descriptions of some of the phenomenology of quantum gravity effects, providing an intuitive grasp on non-trivial space-time structures such as a fundamental discreteness of space, a universal bandlimit or an irreducible extendedness of elementary particles. In this article, uncertainty relations are derived by a moment expansion of states for quantum systems with a discrete coordinate, and correspondingly a periodic momentum. Corrections to standard uncertainty relations are found, with some similarities but also key differences to what is often assumed in this context. The relations provided can be applied to discrete models of matter or space-time, including loop quantum cosmology.
26 pages

 

[John86]

http://arxiv.org/abs/1112.1466
Inertial frames without the relativity principle
Authors: Valentina Baccetti (Victoria University of Wellington), Kyle Tate (Victoria University of Wellington), Matt Visser (Victoria University of Wellington)
(Submitted on 7 Dec 2011)
Abstract: Ever since the work of von Ignatowsky circa 1910 it has been known that the relativity principle, combined with the mild physical assumption of linearity and the less mild assumption of isotropy, leads almost uniquely to either the Lorentz transformations of special relativity or to Galileo's transformations of classical Newtonian mechanics. Consequently, if one wishes to entertain the possibility of Lorentz symmetry breaking, then it seems likely that one will have to abandon or at the very least grossly modify the relativity principle. We reassess the notion of spacetime transformations between inertial frames in the absence of the relativity principle, arguing that significant nontrivial physics can still be extracted as long as the transformations are at least linear. An interesting technical aspect of the analysis is that the transformations now form a groupoid/pseudo-group - it is this technical point that permits one to evade the von Ignatowsky argument. Even in the absence of a relativity principle we can nevertheless deduce clear and compelling rules for the transformation of space and time, rules for the composition of 3-velocities, and rules for the transformation of energy and momentum. The energy-momentum transformations are in general affine, but may be chosen to be linear. As part of the analysis we identify two particularly elegant and compelling models implementing "minimalist" violations of Lorentz invariance - in one of these minimalist models all Lorentz violations are confined to the neutrino sector, while the second minimalist Lorentz-violating model depends on one free function of absolute velocity, but otherwise preserves as much as possible of standard Lorentz invariant physics. In many ways these models serve as a "gold standard" when studying possible violations of Lorentz invariance.


http://arxiv.org/abs/1112.1469
Optimal Probabilistic Simulation of Quantum Channels from the Future to the Past
Authors: Dina Genkina, Giulio Chiribella, Lucien Hardy
(Submitted on 7 Dec 2011)
Abstract: We introduce the study of quantum protocols that probabilistically simulate quantum channels from a sender in the future to a receiver in the past.
The maximum probability of simulation is determined by causality and depends on the amount and type (classical or quantum) of information that the channel can transmit. We illustrate this dependence in several examples, including ideal classical and quantum channels, measure-and-prepare channels, partial trace channels, and universal cloning channels. For the simulation of partial trace channels, we consider generalized teleportation protocols that take N input copies of a pure state in the future and produce M < N output copies of the same state in the past. In this case, we show that the maximum probability of successful teleportation increases with the number of input copies, a feature that was impossible in classical physics. In the limit of asymptotically large N, the probability converges to the probability of simulation for an ideal classical channel.
Similar results are found for universal cloning channels from N copies to M > N approximate copies, exploiting a time-reversal duality between universal cloning and partial trace.

 

[marcus]

http://arxiv.org/abs/1112.1899
Deformed General Relativity and Effective Actions from Loop Quantum Gravity
Martin Bojowald, George M. Paily
(Submitted on 8 Dec 2011)
Canonical methods can be used to construct effective actions from deformed covariance algebras, as implied by quantum-geometry corrections of loop quantum gravity. To this end, classical constructions are extended systematically to effective constraints of canonical quantum gravity and applied to model systems as well as general metrics, with the following conclusions:
(i) Dispersion relations of matter and gravitational waves are deformed in related ways, ensuring a consistent realization of causality.
(ii) Inverse-triad corrections modify the classical action in a way clearly distinguishable from curvature effects. In particular, these corrections can be significantly larger than often expected for standard quantum-gravity phenomena.
(iii) Finally, holonomy corrections in high-curvature regimes do not signal the evolution from collapse to expansion in a "bounce," but rather the emergence of the universe from Euclidean space at high density. This new version of signature-change cosmology suggests a natural way of posing initial conditions, and a solution to the entropy problem.
44 pages

http://arxiv.org/abs/1112.1781
New insights in quantum geometry
Hanno Sahlmann
(Submitted on 8 Dec 2011)
Quantum geometry, i.e., the quantum theory of intrinsic and extrinsic spatial geometry, is a cornerstone of loop quantum gravity. Recently, there have been many new ideas in this field, and I will review some of them. In particular, after a brief description of the main structures and results of quantum geometry, I review a new description of the quantized geometry in terms of polyhedra, new results on the volume operator, and a way to incorporate a classical background metric into the quantum description. Finally I describe a new type of exponentiated flux operator, and its application to Chern-Simons theory and black holes.
10 pages, 3 figures; Proceedings of Loops'11, Madrid, submitted to Journal of Physics: Conference Series (JPCS)

http://arxiv.org/abs/1112.1825
Non-commutative holonomies in 2+1 LQG and Kauffman's brackets
Karim Noui, Alejandro Perez, Daniele Pranzetti
(Submitted on 8 Dec 2011)
We investigate the canonical quantization of 2+1 gravity with Λ > 0 in the canonical framework of LQG. A natural regularization of the constraints of 2+1 gravity can be defined in terms of the holonomies of A_± = A ± √Λe, where the SU(2) connection A and the triad field e are the conjugated variables of the theory. As a first step towards the quantization of these constraints we study the canonical quantization of the holonomy of the connection A_λ = A + λe acting on spin network links of the kinematical Hilbert space of LQG. We provide an explicit construction of the quantum holonomy operator, exhibiting a close relationship between the action of the quantum holonomy at a crossing and Kauffman's q-deformed crossing identity. The crucial difference is that the result is completely described in terms of standard SU(2) spin network states.
4 pages; Proceedings of Loops'11, Madrid, to appear in Journal of Physics: Conference Series (JPCS)

αβγδεζηθικλμνξοπρσςτυφχψωΓΔΘΛΞΠΣΦΨΩ∏∑∫∂√±←↓→↑↔ ~≈≠≡ ≤≥½∞⇐⇑⇒⇓⇔∴∃⋅ℝℤℕℂ

 

[marcus]

http://arxiv.org/abs/1112.1961
Spin Foams and Canonical Quantization
Sergei Alexandrov, Marc Geiller, Karim Noui
(Submitted on 8 Dec 2011)
This review is devoted to the analysis of the mutual consistency of the spin foam and canonical loop quantizations in three and four spacetime dimensions. In the three-dimensional context, where the two approaches are in good agreement, we show how the canonical quantization \`a la Witten of Riemannian gravity with a positive cosmological constant is related to the Turaev-Viro spin foam model, and how the Ponzano-Regge amplitudes are related to the physical scalar product of Riemannian loop quantum gravity without cosmological constant. In the four-dimensional case, we recall a Lorentz-covariant formulation of loop quantum gravity using projected spin networks, compare it with the new spin foam models, and identify interesting relations and their pitfalls. Finally, we discuss the properties which a spin foam model is expected to possesses in order to be consistent with the canonical quantization, and suggest a new model illustrating these results.
88 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1112.1965
Testing the imposition of the Spin Foam Simplicity Constraints
Marc Geiller, Karim Noui
(Submitted on 8 Dec 2011)
We introduce a three-dimensional Plebanski action for the gauge group SO(4). In this model, the $B$ field satisfies quadratic simplicity constraints similar to that of the four-dimensional Plebanski theory, but with the difference that the $B$ field is now a one-form. We exhibit a natural notion of "simple one-form", and identify a gravitational sector, a topological sector and a degenerate sector in the space of solutions to the simplicity constraints. Classically, in the gravitational sector, the action is shown to be equivalent to that of three-dimensional first order Riemannian gravity. This enables us to perform the complete spin foam quantization of the theory once the simplicity constraints are solved at the classical level, and to compare this result with the various models that have been proposed for the implementation of the constraints after quantization. In particular, we impose the simplicity constraints following the prescriptions of the so-called BC and EPRL models. We observe that the BC prescription cannot lead to the proper vertex amplitude. The EPRL prescription allows to recover the expected result when, in this three-dimensional model, it is supplemented with additional secondary second class constraints.
30 pages. 18 figures

http://arxiv.org/abs/1112.2085
Extension of Loop Quantum Gravity to Metric Theories beyond General Relativity
Yongge Ma
(Submitted on 9 Dec 2011)
The successful background-independent quantization of Loop Quantum Gravity relies on the key observation that classical General Relativity can be cast into the connection-dynamical formalism with the structure group of SU(2). Due to this particular formalism, Loop Quantum Gravity was generally considered as a quantization scheme that applies only to General Relativity. However, we will show that the nonperturbative quantization procedure of Loop Quantum Gravity can be extended to a rather general class of metric theories of gravity, which have received increased attention recently due to motivations coming form cosmology and astrophysics. In particular, we will first introduce how to reformulate the 4-dimensional metric f(R) theories of gravity, as well as Brans-Dicke theory, into connection-dynamical formalism with real SU(2) connections as configuration variables. Through these formalisms, we then outline the nonpertubative canonical quantization of the f(R) theories and Brans-Dicke theory by extending the loop quantization scheme of General Relativity.
10 pages; Proceedings of Loops'11, Madrid, submitted to Journal of Physics: Conference Series

brief mention:
http://arxiv.org/abs/1112.2048
What if ... General Relativity is not the theory?
Orfeu Bertolami
(Submitted on 9 Dec 2011)
The nature of gravity is fundamental to understand the scaffolding of the Universe and its evolution... In this discussion we briefly review the experimental bounds on the foundational principles of general relativity, and present three recent proposals to extend general relativity or, at least, to regard it under different perspectives.
17 pages. Based on talks delivered at the "QSO Astrophysics, Fundamental Physics and Astrometric Cosmology in the Gaia era" Meeting, 6-9 June 2011, Faculty of Sciences of University of Porto, Portugal

 

[marcus]

http://arxiv.org/abs/1112.2511
q-Deformation of Lorentzian spin foam models
Winston J. Fairbairn, Catherine Meusburger
(Submitted on 12 Dec 2011)
We construct and analyse a quantum deformation of the Lorentzian EPRL model. The model is based on the representation theory of the quantum Lorentz group with real deformation parameter. We give a definition of the quantum EPRL intertwiner, study its convergence and braiding properties and construct an amplitude for the four-simplexes. We find that the resulting model is finite.
12 pages, 2 figures, Proceedings of the 3rd Quantum Gravity and Quantum Geometry School (Zakopane, 2011), to appear in PoS

http://arxiv.org/abs/1112.2390
The geometric role of symmetry breaking in gravity
Derek K. Wise
(Submitted on 11 Dec 2011)
In gravity, breaking symmetry from a group G to a group H plays the role of describing geometry in relation to the geometry the homogeneous space G/H. The deep reason for this is Cartan's "method of equivalence," giving, in particular, an exact correspondence between metrics and Cartan connections. I argue that broken symmetry is thus implicit in any gravity theory, for purely geometric reasons. As an application, I explain how this kind of thinking gives a new approach to Hamiltonian gravity in which an observer field spontaneously breaks Lorentz symmetry and gives a Cartan connection on space.
4 pages. Contribution written for proceedings of the conference "Loops 11" (Madrid, May 2011)

Video talk by Sahlmann (LQG black holes)
http://pirsa.org/11110097/
In general relativity, the fields on a black hole horizon are obtained from those in the bulk by pullback and restriction. In quantum gravity, it would be natural to obtain them in the same manner. This is not fully realized in the quantum theory of isolated horizons in loop quantum gravity, in which a Chern-Simons phase space on the horizon is quantized separately from the bulk. I will outline an approach in which the quantum horizon degrees of freedom are simply components of the quantized bulk degrees of freedom. A condition is imposed on the quantum states to encode the existence of a horizon. I will present evidence that solutions to this condition have properties on the horizon that are remarkably similar to those of Chern-Simons theory. Instrumental in formulating the horizon condition are novel flux operators that use the Duflo isomorphism and seem to represent some type of quantum deformed SU(2). I will review their definition and summarize what I know about their properties.

Video talk by Muxin Han (q-deformed spinfoam)
http://pirsa.org/11120047/
We construct the q-deformed spinfoam vertex amplitude using Chern-Simons theory on the boundary 3-sphere of the 4-simplex. The rigorous definition involves the construction of Vassiliev-Kontsevich invariant for trivalent knot graph. Under the semiclassical asymptotics, the q-deformed spinfoam amplitude reproduce Regge gravity with cosmological constant at nondegenerate critical configurations.

Video talk by Philipp Höhn (canonical time evo in simplicial QG)
http://pirsa.org/11120050/

brief mention:
http://arxiv.org/abs/1112.2311
Experimenting with Quantum Fields in Curved Spacetime in the Lab
Isabeau Prémont-Schwarz
(Submitted on 11 Dec 2011)
In this paper we will investigate how one can create emergent curved spacetimes by locally tuning the coupling constants of condensed matter systems. In the continuum limit we thus obtain continuous effective quantum fields living on curved spacetimes. In particular, using stringnet condensates we can obtain effective electromagnetism. We will show for example how we obtain quantum electrodynamics in a black hole (Schwarzschild) spacetime.
5 pages, to be published in the proceedings of LOOPS '11 in Journal of Physics: Conference Series

http://arxiv.org/abs/1112.2470
Antigravity and the big crunch/big bang transition
Itzhak Bars, Shih-Hung Chen, Paul J. Steinhardt, Neil Turok
(Submitted on 12 Dec 2011)
We point out a new phenomenon which seems to be generic in 4d effective theories of scalar fields coupled to Einstein gravity, when applied to cosmology. A lift of such theories to a Weyl-invariant extension allows one to define classical evolution through cosmological singularities unambiguously, and hence construct geodesically complete background spacetimes. An attractor mechanism ensures that, at the level of the effective theory, generic solutions undergo a big crunch/big bang transition by contracting to zero size, passing through a brief antigravity phase, shrinking to zero size again, and re-emerging into an expanding normal gravity phase. The result may be useful for the construction of complete bouncing cosmologies like the cyclic model.
4 pages, 2 figures

just for fun:
http://arxiv.org/abs/1112.2347
(a playful paper by Ingemar Bengtsson and friends)

 

[marcus]

http://arxiv.org/abs/1112.2785
Non-Gaussian features from the inverse volume corrections in loop quantum cosmology
Li-Fang Li, Rong-Gen Cai, Zong-Kuan Guo, Bin Hu
(Submitted on 13 Dec 2011)
In this paper we study the non-Gaussian features of the primordial fluctuations in loop quantum cosmology with the inverse volume corrections. The detailed analysis is performed in the single field slow-roll inflationary models. However, our results reflect the universal characteristics of bispectrum in loop quantum cosmology. The main corrections to the scalar bispectrum come from two aspects: one is the modifications to the standard Bunch-Davies vacuum, the other is the corrections to the background dependent variables, such as slow-roll parameters. Our calculations show that the loop quantum corrections make f_NL of the inflationary models increase 0.1%. Moreover, we find that two new shapes arise, namely F₁ and F₂ . The former gives a unique loop quantum feature which is less correlated with the local, equilateral and single types, while the latter is highly correlated with the local one.
30 pages, 4 figures

brief mention:
http://arxiv.org/abs/1112.2860
Quantum back-reaction in a universe with positive cosmological constant
David Brizuela
(Submitted on 13 Dec 2011)
4 pages. Proceedings of the Loops'11 conference. Submitted to Journal of Physics: Conference Series

 

[marcus]

http://arxiv.org/abs/1112.3270
Ten questions on Group Field Theory (and their tentative answers)
Aristide Baratin, Daniele Oriti
(Submitted on 14 Dec 2011)
We provide a short and non-technical summary of our current knowledge and some possible perspectives on the group field theory formalism for quantum gravity, in the form of a (partial) FAQ (with answers). Some of the questions and answers relate to aspects of the formalism that concern loop quantum gravity. This summary also aims at giving a brief, rough guide to the recent literature on group field theory (and tensor models).
10 pages; contribution to the proceedings of the Loops'11 conference (CSIC, Madrid, Spain, 23-28 May 2011); to be published by Journal of Physics: Conference Series (JPCS)

 

[marcus]

http://arxiv.org/abs/1112.3567
Operator Spin Foams: holonomy formulation and coarse graining
Benjamin Bahr
(Submitted on 15 Dec 2011)
A dual holonomy version of operator spin foam models is presented, which is particularly adapted to the notion of coarse graining. We discuss how this leads to a natural way of comparing models on different discretization scales, and a notion of renormalization group flow on the partially ordered set of 2-complexes.
5 pages, 3 figures, to appear in Journal of Physics: Conference Series. (JPCS)

My comment: this was one of the presenations at the May 2011 Loops conference in Madrid.

http://arxiv.org/abs/1112.3457
Stability of Quantum Isolated Horizon with Energy Spectrum Linear in Area
Abhishek Majhi
(Submitted on 15 Dec 2011)
Considering the microstates of the SU(2) Chern-Simons theory of an uncharged, non-rotating Quantum Isolated Horizon(QIH), it is shown that such a QIH, with energy spectrum linear in area, is locally unstable as a thermodynamic system. The result is derived in two different ways. Firstly, the specific heat of the QIH is shown to be negative definite through a quantum statistical analysis. Then, it is shown that the energy spectrum of the QIH violates the stability criterion obtained from the finiteness condition of the canonical partition function, implying the thermodynamic instability of the QIH. Also, a local temperature is obtained for the QIH which points towards a possible link between thermodynamics of quantum black holes and the removal of classical black hole singularity problem in Loop Quantum Gravity.
9 pages, no figure

My comment: this may be important since it critically goes over the same ground as the July 2011 paper by Ghosh Perez and takes issue on several points.