Loop-and-allied QG bibliography

[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.

 

[marcus]

http://arxiv.org/abs/1112.3994
Entropy of generic quantum isolated horizons
Jonathan Engle, Christopher Beetle
(Submitted on 16 Dec 2011)
We review our recent proposal of a method to extend the quantization of spherically symmetric isolated horizons, a seminal result of loop quantum gravity, to a phase space containing horizons of arbitrary geometry. Although the details of the quantization remain formally unchanged, the physical interpretation of the results can be quite different. We highlight several such differences, with particular emphasis on the physical interpretation of black hole entropy in loop quantum gravity.
4 pages, contribution to Loops '11 conference proceedings

http://arxiv.org/abs/1112.4412
Isolated horizons in classical and quantum gravity
Jonathan Engle, Tomas Liko
(Submitted on 19 Dec 2011)
Isolated horizons are a quasi-local framework, developed over the last 15 years by many authors, for modeling black holes 'in equilibrium' that involves assumptions only about geometric structures intrinsic to the horizon. We review the motivations for the framework, and the derivation of the key results, both classical and quantum.
41 pages, 3 figures, contribution to the book "Black Holes: New Horizons", edited by S. Hayward, to be published by World Scientific

http://arxiv.org/abs/1112.4366
Quantum Geometry Phenomenology: Angle and Semiclassical States
Seth A. Major
(Submitted on 19 Dec 2011)
The phenomenology for the deep spatial geometry of loop quantum gravity is discussed. In the context of a simple model of an atom of space, it is shown how purely combinatorial structures can affect observations. The angle operator is used to develop a model of angular corrections to local, continuum flat-space 3-geometries. The physical effects involve neither breaking of local Lorentz invariance nor Planck scale suppression, but rather reply on only the combinatorics of SU(2) recouping theory. Bhabha scattering is discussed as an example of how the effects might be observationally accessible.
5 pages, slightly extended version of the contribution to the Loops'11 conference proceedings

brief mention:
http://arxiv.org/abs/1112.4051
Pauli-Fierz mass term in modified Plebanski gravity
David Beke, Giovanni Palmisano, Simone Speziale
(Submitted on 17 Dec 2011)
We study SO(4) BF theory plus a general quadratic potential, which describes a bi-metric theory of gravity. We identify the profile of the potential leading to a Pauli-Fierz mass term for the massive graviton, thereby avoiding the linearized ghost. We include the Immirzi parameter in our analysis, and find that the mass of the second graviton depends on it. We discuss the non-perturbative removal of the ghost mode in the light of recent literature. Finally, we discuss alternative reality conditions for the case of SO(3,1) BF theory, relevant for Lorentzian signature, and give a new solution to the compatibility equation.
24 pages

 

[marcus]

http://arxiv.org/abs/1112.5097
Spontaneous symmetry breaking and gravity
Kirill Krasnov
(Submitted on 21 Dec 2011)
Gravity is usually considered to be irrelevant as far as the physics of elementary particles is concerned and, in particular, in the context of the spontaneous symmetry breaking (SSB) mechanism. We describe a version of the SSB mechanism in which gravity plays a direct role. We work in the context of diffeomorphism invariant gauge theories, which exist for any non-abelian gauge group G, and which have second order in derivatives field equations. We show that any (non-trivial) vacuum solution of such a theory gives rise to an embedding of the group SU(2) into G, and thus breaks G down to SU(2) times its centralizer in G. The components of the connection charged under SU(2) can then be seen to describe gravitons, with the SU(2) itself playing the role of the chiral half of the Lorentz group. Components charged under the centralizer describe the usual Yang-Mills gauge bosons. The remaining components describe massive particles. This breaking of symmetry explains (in the context of models considered) how gravity and Yang-Mills can come from a single underlying theory while being so different in the physics they describe. Further, varying the vacuum solution, and thus the embedding of SU(2) into G, one can break the Yang-Mills gauge group as desired, with massless gauge bosons of one vacuum acquiring mass in another. There is no Higgs field in our version of the SSB mechanism, the only variable is a connection field. Instead of the symmetry breaking by a dedicated Higgs field pointing in some direction in the field space, our theories break the symmetry by choosing how the group of "internal" gauge rotations of gravity (the chiral half of the Lorentz group) sits inside the full gauge group.
35 pages

http://arxiv.org/abs/1112.4856
Off-diagonal heat-kernel expansion and its application to fields with differential constraints
Kai Groh, Frank Saueressig, Omar Zanusso
(Submitted on 20 Dec 2011)
The off-diagonal heat-kernel expansion of a Laplace operator including a general gauge-connection is computed on a compact manifold without boundary up to third order in the curvatures. These results are used to study the early-time expansion of the traced heat-kernel on the space of transverse vector fields satisfying the differential constraint D^μ v_μ = 0. It is shown that the resulting Seeley-deWitt coefficients generically develop singularities, which vanish if the metric is flat or satisfies the Einstein condition. The implications of our findings for the evaluation of the gravitational functional renormalization group equation are briefly discussed.
32 pages

brief mention:
http://arxiv.org/abs/1112.4918
Local tuning of Coupling Constants allows for Quantum Fields in Curved Spacetime in the Lab
Isabeau Prémont-Schwarz
(Submitted on 21 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 Stingnet condensates we can obtain effective electromagnetism. We will show for example how we obtain quantum electromagnetism (U(1)-Yang-Mills) in a black hole (Schwarzschild) spacetime.
11 pages

http://arxiv.org/abs/1112.4882
Exotic Smoothness and Quantum Gravity II: exotic R^4, singularities and cosmology
T. Asselmeyer-Maluga, J. Krol
(Submitted on 20 Dec 2011)
Since the first work on exotic smoothness in physics, it was folklore to assume a direct influence of exotic smoothness to quantum gravity. In the second paper, we calculate the "smoothness structure" part of the path integral in quantum gravity for the exotic R⁴ as non-compact manifold. We discuss the influence of the "sum over geometries" to the "sum over smoothness structure". There are two types of exotic R⁴: large (no smooth embedded 3-sphere) and small (smooth embedded 3-sphere). A large exotic R⁴ can be produced by using topologically slice but smoothly non-slice knots whereas a small exotic R⁴ is constructed by a 5-dimensional h-cobordism between compact 4-manifolds. The results are applied to the calculation of expectation values, i.e. we discuss the two observables, volume and Wilson loop. Then the appearance of naked singularities is analyzed. By using Mostow rigidity, we obtain a justification of area and volume quantization again. Finally exotic smoothness of the R⁴ produces in all cases (small or large) a cosmological constant.
23 pages, 5 figures,

http://arxiv.org/abs/1112.4885
Exotic R⁴ and quantum field theory
T. Asselmeyer-Maluga, R. Mader
(Submitted on 20 Dec 2011)
Recent work on exotic smooth R⁴'s, i.e. topological R⁴ with exotic differential structure, shows the connection of 4-exotics with the codimension-1 foliations of S³, SU(2) WZW models and twisted K-theory K_H(S³), H in H³(S³,Z). These results made it possible to explicate some physical effects of exotic 4-smoothness. Here we present a relation between exotic smooth R⁴ and operator algebras. The correspondence uses the leaf space of the codimension-1 foliation of S³ inducing a von Neumann algebra W(S³) as description. This algebra is a type III₁ factor lying at the heart of any observable algebra of QFT. By using the relation to factor II, we showed that the algebra W(S³) can be interpreted as Drinfeld-Turaev deformation quantization of the space of flat SL(2,C) connections (or holonomies). Thus, we obtain a natural relation to quantum field theory. Finally we discuss the appearance of concrete action functionals for fermions or gauge fields and its connection to quantum-field-theoretical models like the Tree QFT of Rivasseau.
15 pages, 2 figures, Based on the talk presented at Quantum Theory and Symmetries 7, Prague, August 7-13, 2011

 

[MTd2]

http://arxiv.org/abs/1112.5104

Quantum Gravity and Renormalization: The Tensor Track

Vincent Rivasseau
(Submitted on 21 Dec 2011)
We propose a new program to quantize and renormalize gravity based on recent progress on the analysis of large random tensors. We compare it briefly with other existing approaches.

 

[marcus]

So far as I know there will only be one more posting on arxiv this year: at 5 PM (pacific) or 8 PM (eastern) on 28 December. Could be wrong, so I'll check tomorrow. It's been a big year. Over 200 papers in Lqg, Lqc, spinfoam explicitly and many more in related fields.

 

[marcus]

http://arxiv.org/abs/1112.6391
Curvature invariants, geodesics and the strength of singularities in Bianchi-I loop quantum cosmology
Parampreet Singh
(Submitted on 29 Dec 2011)
We investigate the effects of the underlying quantum geometry in loop quantum cosmology on spacetime curvature invariants and the extendibility of geodesics in the Bianchi-I model for matter with a vanishing anisotropic stress. Using the effective Hamiltonian approach, we find that even though quantum geometric effects bound the energy density and expansion and shear scalars, divergences of curvature invariants are potentially possible under special conditions. However, as in the isotropic models in LQC, these do not necessarily imply a physical singularity. Analysis of geodesics and strength of such singular events, point towards a general resolution of all known types of strong singularities. We illustrate these results for the case of a perfect fluid with an arbitrary finite equation of state w > -1, and show that curvature invariants turn out to be bounded, leading to the absence of strong singularities. Unlike classical theory, geodesic evolution does not break down. We also discuss possible generalizations of sudden singularities which may arise at a non-vanishing volume, causing a divergence in curvature invariants. Such finite volume singularities are shown to be weak and harmless.
24 pages

http://arxiv.org/abs/1112.6215
Horizon entropy and higher curvature equations of state
Raf Guedens, Ted Jacobson, Sudipta Sarkar
(Submitted on 29 Dec 2011)
The Clausius relation between entropy change and heat flux has previously been used to derive Einstein's field equations as an equation of state. In that derivation the entropy is proportional to the area of a local causal horizon, and the heat is the energy flux across the horizon, defined relative to an approximate boost Killing vector. We examine here whether a similar derivation can be given for extensions beyond Einstein gravity to include higher derivative and higher curvature terms. We review previous proposals which, in our opinion, are problematic or incomplete. Refining one of these, we assume that the horizon entropy depends on an approximate local Killing vector in a way that mimics the diffeomorphism Noether charge that yields the entropy of a stationary black hole. We show how this can be made to work if various restrictions are imposed on the nature of the horizon slices and the approximate Killing vector. Also, an integrability condition on the assumed horizon entropy density must hold. This can yield field equations of a Lagrangian constructed algebraically from the metric and Riemann tensor, but appears unlikely to allow for derivatives of curvature in the Lagrangian.

http://arxiv.org/abs/1112.5929
BF gravity with Immirzi parameter and matter fields
Merced Montesinos, Mercedes Velázquez
(Submitted on 27 Dec 2011)
We perform the coupling of the scalar, Maxwell, and Yang-Mills as well as the cosmological constant to BF gravity with Immirzi parameter. The proposed action principles employ auxiliary fields in order to keep a polynomial dependence on the two-forms. By handling the equations of motion for the B field and for the auxiliary fields, the latter can be expressed in terms of the physical fields and by substituting these expressions into the original action principles we recover the first-order (Holst) and second-order actions for gravity coupled to the physical matter fields. We consider these results a relevant step towards the understanding of the coupling of matter fields to gravity in the theoretical framework of BF theory.
9 pages

 

[marcus]

note: EPRL is the current standard spinfoam formulation of Loop Quantum Gravity.

http://arxiv.org/abs/1201.0525
String Field Theory from Quantum Gravity
Louis Crane
(Submitted on 2 Jan 2012)
Recent work on neutrino oscillations suggests that the three generations of fermions in the standard model are related by representations of the finite group A(4), the group of symmetries of the tetrahedron. Motivated by this, we explore models which extend the EPRL model for quantum gravity by coupling it to a bosonic quantum field of representations of A(4). This coupling is possible because the representation category of A(4) is a module category over the representation categories used to construct the EPRL model. The vertex operators which interchange vacua in the resulting quantum field theory reproduce the bosons and fermions of the standard model, up to issues of symmetry breaking which we do not resolve. We are led to the hypothesis that physical particles in nature represent vacuum changing operators on a sea of invisible excitations which are only observable in the A(4) representation labels which govern the horizontal symmetry revealed in neutrino oscillations. The quantum field theory of the A(4) representations is just the dual model on the extended lattice of the Lie group E₆, as explained by the quantum Mckay correspondence of Frenkel Jing and Wang. The coupled model can be thought of as string field theory, but propagating on a discretized quantum spacetime rather than a classical manifold.
15 pages

Start of introduction:"In the last few years, a new development [1] [2] [3] [4] has largely resolved the problems of the old BC model [5] for quantum gravity. It is now a natural task to study extensions of the EPRL model which would include realistic matter fields. It would be extremely desirable to find an algebraic extension of the EPRL model which was essentially unique or at least had a small number of possibilities and which gave us the standard model..."

An excerpt from Crane's conclusions: "It was quite a surprise to have the string appear in this theory, which started from a completely different program. Rather than ending up with an almost infinite landscape as in Kaluza-Klein theories, we get an essentially unique theory, which relates fairly directly to the standard model.
Embedding the string field in a discrete model for spacetime removes the difficulties that beset string field theories in a continuum.
There is no longer the integration over worldsheet metrics which leads to bad behavior on moduli space in the Polyakov string; rather the dual models couple to the quantum geometry of the EPRL model itself."

To save time chasing down references, [10] refers to http://arxiv.org/abs/hep-ph/0211393

 

[marcus]

brief mention:
http://arxiv.org/abs/1201.1877
Schrödinger-Feynman quantization and composition of observables in general boundary quantum field theory
Robert Oeckl (UNAM)
(Submitted on 9 Jan 2012)
47 pages

http://arxiv.org/abs/1201.1590
Pseudo-topological transitions in 2D gravity models coupled to massless scalar fields
J. Ambjorn, A. T. Goerlich, J. Jurkiewicz, H.-G. Zhang
(Submitted on 7 Jan 2012)
19 pages, many figures

http://arxiv.org/abs/1201.1596
On "Law without Law"
David Ritz Finkelstein
(Submitted on 8 Jan 2012)
6 pages

http://arxiv.org/abs/1201.1599
Nature as quantum computer
David Ritz Finkelstein
(Submitted on 8 Jan 2012)
16 pages

possibly a useful pedagogical article of current interest:
http://arxiv.org/abs/1201.1637
Neutrinos and the stars
Georg Raffelt
(Submitted on 8 Jan 2012)
83 pages

 

[marcus]

http://arxiv.org/abs/1201.2120
Spinors and Twistors in Loop Gravity and Spin Foams
Maite Dupuis, Simone Speziale, Johannes Tambornino
(Submitted on 10 Jan 2012)
Spinorial tools have recently come back to fashion in loop gravity and spin foams. They provide an elegant tool relating the standard holonomy-flux algebra to the twisted geometry picture of the classical phase space on a fixed graph, and to twistors. In these lectures we provide a brief and technical introduction to the formalism and some of its applications.
16 pages; to appear in the Proceedings of the 3rd Quantum Gravity and Quantum Geometry School, February 28 - March 13, 2011 Zakopane, Poland

 

[marcus]

http://arxiv.org/abs/1201.2187
A spin-foam vertex amplitude with the correct semiclassical limit
Jonathan Engle
(Submitted on 10 Jan 2012)
Spin-foam models are hoped to provide a dynamics for loop quantum gravity. All 4-d spin-foam models of gravity start from the Plebanski formulation, in which gravity is recovered from a topological field theory, BF theory, by the imposition of constraints, which, however, select not only the gravitational sector, but also unphysical sectors. We show that this is the root cause for terms beyond the required Feynman-prescribed exponential of i times the action in the semiclassical limit of the EPRL spin-foam vertex. By quantizing a condition isolating the gravitational sector, we modify the EPRL vertex, yielding what we call the proper EPRL vertex amplitude. This provides at last a vertex amplitude for loop quantum gravity with the correct semiclassical limit.
4 pages

http://arxiv.org/abs/1201.2329
A critical analysis of the cosmological implementation of Loop Quantum Gravity
Francesco Cianfrani, Giovanni Montani
(Submitted on 11 Jan 2012)
This papers offers a critical discussion on the procedure by which Loop Quantum Cosmology (LQC) is constructed from the full Loop Quantum Gravity (LQG) theory. Revising recent issues in preserving SU(2) symmetry when quantizing the isotropic Universe, we trace a new perspective in approaching the cosmological problem within quantum geometry. The cosmological sector of LQG is reviewed and a critical point of view on LQC is presented. It is outlined how a polymer-like scale for quantum cosmology can be predicted from a proper fundamental graph underlying the homogeneous and isotropic continuous picture. However, such a minimum scale does not coincide with the choice made in LQC. Finally, the perspectives towards a consistent cosmological LQG model based on such a graph structure are discussed.
11 pages, accepted for publication in Modern Physics Letters A


brief mention:
http://arxiv.org/abs/1201.2340
What is the Shape of a Black Hole?
G. W. Gibbons
(Submitted on 11 Jan 2012)
11 pages

 

[marcus]

http://arxiv.org/abs/1201.2588
The picture of the Bianchi I model via gauge fixing in Loop Quantum Gravity
Francesco Cianfrani, Andrea Marchini, Giovanni Montani
(Submitted on 12 Jan 2012)
The implications of the SU(2) gauge fixing associated with the choice of invariant triads in Loop Quantum Cosmology are discussed for a Bianchi I model. In particular, via the analysis of Dirac brackets, it is outlined how the holonomy-flux algebra coincides with the one of Loop Quantum Gravity if paths are parallel to fiducial vectors only. This way the quantization procedure for the Bianchi I model is performed by applying the techniques developed in Loop Quantum Gravity but restricting the admissible paths. Furthermore, the local character retained by the reduced variables provides a relic diffeomorphisms constraint, whose imposition implies homogeneity on a quantum level. The resulting picture for the fundamental spatial manifold is that of a cubical knot with attached SU(2) irreducible representations. The discretization of geometric operators is outlined and a new perspective for the super-Hamiltonian regularization in Loop Quantum Cosmology is proposed.
6 pages

brief mention:
http://arxiv.org/abs/1201.2632
Unification of the state with the dynamical law
Lee Smolin
(Submitted on 12 Jan 2012)
We address the question of why particular laws were selected for the universe, by proposing a mechanism for laws to evolve. Normally in physical theories, timeless laws act on time-evolving states. We propose that this is an approximation, good on time scales shorter than cosmological scales, beyond which laws and states are merged into a single entity that evolves in time. Furthermore the approximate distinction between laws and states, when it does emerge, is dependent on the initial conditions. These ideas are illustrated in a simple matrix model.
13 pages,

http://arxiv.org/abs/1201.2475
A Challenge to Entropic Gravity
Jonathan J. Roveto, Gerardo Munoz
(Submitted on 12 Jan 2012)
In a recent publication in this journal, Erik Verlinde attempts to show that gravity should be viewed not as a fundamental force, but rather as an emergent thermodynamic phenomenon arising from an unspecified microscopic theory via equipartition and holography. This paper presents a challenge to his reformulation of gravity. A detailed examination of Verlinde's derivation leads to a number of questions that severely weaken the claim that such a theory correctly reproduces Newton's laws or Einstein gravity. In particular, we find that neither Newtonian gravity nor the Einstein equations are uniquely determined using Verlinde's postulates.
8 pages

http://arxiv.org/abs/1201.2489
A discrete, unitary, causal theory of quantum gravity
Aron C. Wall
(Submitted on 12 Jan 2012)
A discrete model of Lorentzian quantum gravity is proposed. The theory is fully background free, containing no reference to absolute space, time, or simultaneity. The states at one slice of time are networks in which each vertex is labelled with two arrows, which point along an adjacent edge, or to the vertex itself. The dynamics is specified by a set of unitary replacement rules, which causally propagate the local degrees of freedom. The inner product between any two states is given by a sum over histories. Assuming it converges, this inner product is proven to be hermitian and fully gauge-degenerate under spacetime diffeomorphisms. At least for states with a finite past, the inner product is also positive. This allows a Hilbert space of physical states to be constructed.
35 pages, 9 figures

 

[marcus]

brief mention:
http://arxiv.org/abs/1201.2871
Spinor gravity and diffeomorphism invariance on the lattice
C.Wetterich
(Submitted on 13 Jan 2012)
The key ingredient for lattice regularized quantum gravity is diffeomorphism symmetry. We formulate a lattice functional integral for quantum gravity in terms of fermions. This allows for a diffeomorphism invariant functional measure and avoids problems of boundedness of the action. We discuss the concept of lattice diffeomorphism invariance. This is realized if the action does not depend on the positioning of abstract lattice points on a continuous manifold. Our formulation of lattice spinor gravity also realizes local Lorentz symmetry. Furthermore, the Lorentz transformations are generalized such that the functional integral describes simultaneously euclidean and Minkowski signature. The difference between space and time arises as a dynamical effect due to the expectation value of a collective metric field. The quantum effective action for the metric is diffeomorphism invariant. Realistic gravity can be obtained if this effective action admits a derivative expansion for long wavelengths.
Comments: 13 pages, proceedings 6th Aegean Summer School, Naxos 2011

 

[marcus]

http://arxiv.org/abs/1201.3206
Disordered locality and Lorentz dispersion relations: an explicit model of quantum foam
Francesco Caravelli, Fotini Markopoulou
(Submitted on 16 Jan 2012)
Using the framework of Quantum Graphity, we construct an explicit model of a quantum foam, a quantum spacetime with spatial wormholes. The states depend on two parameters: the minimal size of the wormholes and their density with respect to this length. Macroscopic Lorentz invariance requires that the quantum superposition of spacetimes is suppressed by the length of these wormholes. We parametrize this suppression by the distribution of wormhole lengths in the quantum foam. We discuss the general case and then analyze two specific natural wormhole distributions. Corrections to the Lorentz dispersion relations are calculated using techniques developed in previous work.
22 pg, 3 fig

http://arxiv.org/abs/1201.2864
Exploring the Phase Diagram of Lattice Quantum Gravity
Daniel Coumbe, Jack Laiho
(Submitted on 13 Jan 2012)
We present evidence that a nonperturbative model of quantum gravity defined via Euclidean dynamical triangulations contains a region in parameter space with an extended 4-dimensional geometry when a non-trivial measure term is included in the gravitational path integral. Within our extended region we find a large scale spectral dimension of D_s (σ → ∞) = 4.04±0.26 and a Hausdorff dimension that is consistent with D_H = 4 from finite size scaling. We find that the short distance spectral dimension is D_s (σ → 0) ≈ 3/2, which may resolve the tension between asymptotic safety and holographic entropy scaling.
7 pages; presented at the XXIX International Symposium on Lattice Field Theory, July 10-16 2011, Squaw Valley, Lake Tahoe, California, USA

http://arxiv.org/abs/1201.3164
General Relativity without paradigm of space-time covariance: sensible quantum gravity and resolution of the "problem of time"
Chopin Soo, Hoi-Lai Yu
(Submitted on 16 Jan 2012)
Covariance of space and time in General Relativity (GR) entails a number of technical and conceptual difficulties. These can be resolved by a paradigm shift from full 4-dimensional general coordinate invariance to invariance only with respect to spatial diffeomorphisms. A theory of gravity with this paradigm shift, from quantum to classical regimes, is presented; GR is contained as a special case. Appositely formulated as a master constraint, the Hamiltonian constraint now determines only dynamics; and is relieved of its dual role of generating symmetry transformations, and the consequent baggage of multi-fingered evolution with arbitrary lapse functions is absent. The Dirac algebra, with 4-dimensional diffeomorphism symmetry on-shell, is replaced by the master constraint algebra which possesses only spatial diffeomorphism gauge symmetry, both on- and off-shell. Decomposition of the spatial metric into unimodular and determinant, q, factors results in clean separation of the canonical variables. The classical content of GR can be captured with a Hamiltonian constraint linear in the trace of the momentum; with the quantum theory described by a Schrodinger equation first-order in intrinsic time ln q accompanied with positive semi-definite probability density. The semi-classical Hamilton-Jacobi equation is also first order in intrinsic time, with the implication of completeness; and gauge-invariant observables can be constructed from integration constants of its complete integral solution. Classical space-time, with direct correlation of its proper times and intrinsic time intervals, emerges from constructive interference; and the physical content of GR can be regained from a theory with a true Hamiltonian generating intrinsic time translations, but with only spatial diffeomorphism symmetry. The framework also prompts natural extensions towards a well-behaved quantum theory of gravity.
9 pages

 

[marcus]

http://arxiv.org/abs/1201.3613
On the exact evaluation of spin networks
Laurent Freidel, Jeff Hnybida
(Submitted on 17 Jan 2012)
We introduce a fully coherent spin network amplitude whose expansion generates all SU(2) spin networks associated with a given graph. We then give an explicit evaluation of this amplitude for an arbitrary graph. We show how this coherent amplitude can be obtained from the specialization of a generating functional obtained by the contraction of parametrized intertwiners a la Schwinger. We finally give the explicit evaluation of this generating functional for arbitrary graphs.

http://arxiv.org/abs/1201.3398
The Computing Spacetime
Fotini Markopoulou
(Submitted on 17 Jan 2012)
The idea that the Universe is a program in a giant quantum computer is both fascinating and suffers from various problems. Nonetheless, it can provide a unified picture of physics and this can be very useful for the problem of Quantum Gravity where such a unification is necessary. In previous work we proposed Quantum Graphity, a simple way to model a dynamical spacetime as a quantum computation. In this paper, we give an easily readable introduction to the idea of the universe as a quantum computation, the problem of quantum gravity, and the graphity models.

 

[marcus]

http://arxiv.org/abs/1201.3840
How to construct diffeomorphism symmetry on the lattice
Bianca Dittrich
(Submitted on 18 Jan 2012)
Diffeomorphism symmetry, the fundamental invariance of general relativity, is generically broken under discretization. After discussing the meaning and implications of diffeomorphism symmetry in the discrete, in particular for the continuum limit, we introduce a perturbative framework to construct discretizations with an exact notion of diffeomorphism symmetry. We will see that for such a perturbative framework consistency conditions need to be satisfied which enforce the preservation of the gauge symmetry to the perturbative order under discussion. These consistency conditions will allow structural investigations of diffeomorphism invariant discretizations.
21 pages, to appear in the Proceedings of the 3rd Quantum Gravity and Quantum Geometry School 2011, Zakopane, Poland

http://arxiv.org/abs/1201.3787
On the origin of inflation
Torsten Asselmeyer-Maluga, Jerzy Król
(Submitted on 18 Jan 2012)
In this paper we discuss a space-time having the topology of S³xR but with different smoothness structure. This space-time is not a global hyperbolic space-time. Especially we obtain a time line with a topology change of the space from the 3-sphere to a homology 3-sphere and back but without a topology-change of the space-time. Among the infinite possible smoothness structures of this space-time, we choose a homology 3-sphere with hyperbolic geometry admitting a homogenous metric. Then the topology change can be described by a time-dependent curvature parameter k changing from k=+1 to k=-1 and back. The solution of the Friedman equation for dust matter (p=0) after inserting this function shows an exponential growing which is typical for inflation. In contrast to other inflation models, this process stops after a finite time.
4 pages, 2 figures