Loop-and-allied QG bibliography

[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

 

[marcus]

http://arxiv.org/abs/1201.4151
Group field cosmology: a cosmological field theory of quantum geometry
Gianluca Calcagni, Steffen Gielen, Daniele Oriti
(Submitted on 19 Jan 2012)
Following the idea of a field quantization of gravity as realized in group field theory, we construct a minisuperspace model where the wavefunction of canonical quantum cosmology (either Wheeler-DeWitt or loop quantum cosmology) is promoted to a field, the coordinates are minisuperspace variables, the kinetic operator is the Hamiltonian constraint operator, and the action features a nonlinear and possibly nonlocal interaction term. We discuss free-field classical solutions, the quantum propagator, and a mean-field approximation linearizing the equation of motion and augmenting the Hamiltonian constraint by an effective term mixing gravitational and matter variables. Depending on the choice of interaction, this can reproduce, for example, a cosmological constant, a scalar-field potential, or a curvature contribution.
14 pages, 2 figures

http://arxiv.org/abs/1201.3969
Breaking the uniqueness of the Shape Dynamics Hamiltonian
Henrique Gomes
(Submitted on 19 Jan 2012)
In Gomes 2011, a linear method of solving a particular set of Lichnerowicz-type equations through the implicit function theorem was sketched in order to implicitly construct Shape Dynamics' global Hamiltonian and eliminate second class constraints. This method was completely laid out, and in Gomes 2011 it was used for extending Shape Dynamics (SD) to the non-vacuum case, showing how other fields are coupled to the theory. In the latter paper it was noticed that, unlike the vacuum case, the use of such methods yielded puzzling bounds on the density of some types of fields. Here we show that the original SD cannot be extended beyond such bounds, but that a slight modification of the original can withstand any type of coupling. When the bound is broken, the theory does not come equipped with a single Hamiltonian as in vacuum SD, but with a finite set of weakly commuting Hamiltonians, which we describe.
14 pages

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 (v1), last revised 24 Nov 2011 (this version, v2))
In this work, we study the evolution of Primordial Black Holes within the context of Loop Quantum Gravity. First we calculate the scale factor and energy density of the universe for different cosmic era and then taking these as inputs we study evolution of primordial black holes. From our estimation it is found that accretion of radiation does not affect evolution of primordial black holes in loop quantum gravity even though a larger number of primordial black holes may form in early universe in comparison with Einstein's or scalar-tensor theories.
8 pages, 1 figure

brief mention:
http://arxiv.org/abs/1201.4147
Are OPERA neutrinos faster than light because of non-inertial reference frames?
Claudio Germana
(Submitted on 19 Jan 2012)
Recent results from the OPERA experiment reported a neutrino beam traveling faster than light. The experiment measured the neutrino time of flight (TOF) over a baseline from the CERN to the Gran Sasso site. The neutrino beam arrives 60 ns earlier than a light ray would do. Because the result has an enormous impact on science, it might be worth double-checking the time definitions with respect to the non-inertial system in which the neutrino travel time was measured. Potential problems in the OPERA data analysis connected with the definition of the reference frame and time synchronization are emphasized. We aim to investigate the synchronization of non-inertial clocks on Earth by relating this time to the proper time of an inertial observer at Solar System Barycenter(SSB). The Tempo2 software was used to time-stamp events observed on the geoid with respect to the SSB inertial observer time. Neutrino results from OPERA might carry the fingerprint of non-inertial effects. The CERN-Gran Sasso clock synchronization is accomplished by applying corrections that depend on special and general relativistic time dilation effects at the clocks, depending on the position of the clocks in the solar system gravitational well. As a consequence, TOF distributions are centered on values shorter by tens of ns than expected, integrating over a period from April to December, longer if otherwise. It is worth remarking that the OPERA runs have always been carried out from April/May to November. If the analysis by Tempo2 holds for the OPERA experiment, the excellent measurement by the OPERA collaboration will turn into a proof of the General Relativity theory in a weak field approximation. The analysis presented here is falsifiable because it predicts that performing the experiment from January to March/April, the neutrino beam will be detected to arrive 50 ns later than light.
5 pages, 4 figures, accepted for publication in Astronomy and Astrophysics Letters

 

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http://arxiv.org/abs/1201.4247
On the relations between gravity and BF theories
Laurent Freidel, Simone Speziale
(Submitted on 20 Jan 2012)
We review, in the light of recent developments, the existing relations between gravity and topological BF theories at the classical level. We include the Plebanski action in both self-dual and non-chiral formulations, their generalizations, and the MacDowell-Mansouri action.
16 pages. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

http://arxiv.org/abs/1201.4229
Revisiting the quantum scalar field in spherically symmetric quantum gravity
Enrique F. Borja, Iñaki Garay, Eckhard Strobel
(Submitted on 20 Jan 2012)
We extend previous results in spherically symmetric gravitational systems coupled with a massless scalar field within the loop quantum gravity framework. As starting point, we take the Schwarzschild spacetime. The results presented here rely on the uniform discretization method. We are able to minimize the associated discrete master constraint using a variational method. The trial state for the vacuum consists of a direct product of a Fock vacuum for the matter part and a Gaussian centered around the classical Schwarzschild solution. This paper follows the line of research presented by Gambini, Pullin and Rastgoo and a comparison between their result and the one given in this work is made.
16 pages

http://arxiv.org/abs/1201.4226
Gravitational four-fermion interaction on the Planck scale
I.B. Khriplovich
(Submitted on 20 Jan 2012)
The four-fermion gravitational interaction is induced by torsion, and gets essential on the Planck scale. On this scale, the axial-axial contribution dominates strongly in the discussed interaction. The energy-momentum tensor, generated by this contribution, is analyzed, as well as stability of the problem with respect to compression. The trace of this energy-momentum tensor can be negative.
4 pages

 

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http://arxiv.org/abs/1201.4598
Introduction to Loop Quantum Gravity
Abhay Ashtekar
(Submitted on 22 Jan 2012)
This article is based on the opening lecture at the third quantum geometry and quantum gravity school sponsored by the European Science Foundation and held at Zakopane, Poland in March 2011. The goal of the lecture was to present a broad perspective on loop quantum gravity for young researchers. The first part is addressed to beginning students and the second to young researchers who are already working in quantum gravity.
30 pages, 2 figures.

http://arxiv.org/abs/1201.4853
Effective Dynamics in Bianchi Type II Loop Quantum Cosmology
Alejandro Corichi, Edison Montoya
(Submitted on 23 Jan 2012)
We numerically investigate the solutions to the effective equations of the Bianchi II model within the "improved" Loop Quantum Cosmology (LQC) dynamics. The matter source is a massless scalar field. We perform a systematic study of the space of solutions, and focus on the behavior of several geometrical observables. We show that the big-bang singularity is replaced by a bounce and the point-like singularities do not saturate the energy density bound. There are up to five directional bounces in the scale factors, one global bounce in the expansion, the shear presents up to four local maxima and can be zero at the bounce. This allows for solutions with density larger than the maximal density for the isotropic and Bianchi I cases. The asymptotic behavior is shown to behave like that of a Bianchi I model, and the effective solutions connect anisotropic solutions even when the shear is zero at the bounce. All known facts of Bianchi I are reproduced. In the "vacuum limit", solutions are such that almost all the dynamics is due to the anisotropies. Since Bianchi II plays an important role in the Bianchi IX model and the the Belinskii, Khalatnikov, Lifgarbagez (BKL) conjecture, our results can provide an intuitive understanding of the behavior in the vicinity of general space-like singularities.
23 pages, 26 figures

http://arxiv.org/abs/1201.4543
Aspects of Quantum Gravity in Cosmology
Massimiliano Rinaldi
(Submitted on 22 Jan 2012)
We review some aspects of quantum gravity in the context of cosmology. In particular, we focus on models with a phenomenology accessible to current and near-future observations, as the early Universe might be our only chance to peep through the quantum gravity realm.
15 pages, 1 figure. Invited review for Modern Physics Letter A

 

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http://arxiv.org/abs/1201.4874
Divergences in spinfoam quantum gravity
Matteo Smerlak
(Submitted on 23 Jan 2012)
In this thesis we study the flat model, the main buidling block for the spinfoam approach to quantum gravity, with an emphasis on its divergences. Besides a personal introduction to the problem of quantum gravity, the manuscript consists in two part. In the first one, we establish an exact powercounting formula for the bubble divergences of the flat model, using tools from discrete gauge theory and twisted cohomology. In the second one, we address the issue of spinfoam continuum limit, both from the lattice field theory and the group field theory perspectives. In particular, we put forward a new proof of the Borel summability of the Boulatov-Freidel-Louapre model, with an improved control over the large-spin scaling behaviour. We conclude with an outlook of the renormalization program in spinfoam quantum gravity.
113 pages. PhD thesis, introduction and conclusion in French, main text in English.

http://arxiv.org/abs/1201.4996
Gauge symmetries in spinfoam gravity: the case for "cellular quantization"
Valentin Bonzom, Matteo Smerlak
(Submitted on 24 Jan 2012)
The spinfoam approach to quantum gravity rests on a "quantization" of BF theory using 2-complexes and group representations. We explain why, in dimension three and higher, this "spinfoam quantization" must be amended to be made consistent with the gauge symmetries of discrete BF theory. We discuss a suitable generalization, called "cellular quantization", which (1) is finite, (2) produces a topological invariant, (3) matches with the properties of the continuum BF theory, (4) corresponds to its loop quantization. These results significantly clarify the foundations - and limitations - of the spinfoam formalism, and open the path to understanding, in a discrete setting, the symmetry-breaking which reduces BF theory to gravity.
6 pages

 

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http://arxiv.org/abs/1201.5423
Dirac fields and Barbero-Immirzi parameter in Cosmology
G. de Berredo-Peixoto, L. Freidel, I.L. Shapiro, C.A. de Souza
(Submitted on 26 Jan 2012)
We consider cosmological solution for Einstein gravity with massive fermions with a four-fermion coupling, which emerges from the Holst action and is related to the Barbero-Immirzi (BI) parameter. This gravitational action is an important object of investigation in a non-perturbative formalism of quantum gravity. We study the equation of motion for for the Dirac field within the standard Friedman-Robertson-Walker (FRW) metric. Finally, we show the theory with BI parameter and minimally coupling Dirac field, in the zero mass limit, is equivalent to an additional term which looks like a perfect fluid with the equation of state p = wρ, with w = 1 which is independent of the BI parameter. The existence of mass imposes a variable w, which creates either an inflationary phase with w=-1, or assumes an ultra hard equation of states w = 1 for very early universe. Both phases relax to a pressureless fluid w = 0 for late universe (corresponding to the limit m→∞).
16 pages

http://arxiv.org/abs/1201.5470
New tools for Loop Quantum Gravity with applications to a simple model
Enrique F. Borja, Jacobo Díaz-Polo, Laurent Freidel, Iñaki Garay, Etera R. Livine
(Submitted on 26 Jan 2012)
Loop Quantum Gravity is now a well established approach to quantum gravity. One of the main challenges still faced by the theory is constructing a consistent dynamics which would lead back to the standard dynamics of the gravitational field at large scales. Here we will review the recent U(N) framework for Loop Quantum Gravity and the new spinor representation (that provides a classical setting for the U(N) framework). Then, we will apply these techniques to a simple model in order to propose a dynamics for a symmetry reduced sector of the theory. Furthermore, we will explore certain analogies of this model with Loop Quantum Cosmology.
4 pages, to appear in Proceedings of Spanish Relativity Meeting 2011 (ERE 2011) held in Madrid, Spain

 

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http://arxiv.org/abs/1201.6102
Entropy of quantum black holes
Romesh K. Kaul
(Submitted on 30 Jan 2012)
In the Loop Quantum Gravity, black holes (or even more general Isolated Horizons) are described by a SU(2) Chern-Simons theory. There is an equivalent formulation of the horizon degrees of freedom in terms of a U(1) gauge theory which is just a gauged fixed version of the SU(2) theory. These developments will be surveyed here. Quantum theory based on either formulation can be used to count the horizon micro-states associated with quantum geometry fluctuations and from this the micro-canonical entropy can be obtained. We shall review the computation in SU(2) formulation. Leading term in the entropy is proportional to horizon area with a coefficient depending on the Barbero-Immirzi parameter which is fixed by matching this result with the Bekenstein-Hawking formula. Remarkably there are corrections beyond the area term, the leading one is logarithm of the horizon area with a definite coefficient -3/2, a result which is more than a decade old now. How the same results are obtained in the equivalent U(1) framework will also be indicated. Over years, this entropy formula has also been arrived at from a variety of other perspectives. In particular, entropy of BTZ black holes in three dimensional gravity exhibits the same logarithmic correction. Even in the String Theory, many black hole models are known to possesses such properties. This may suggests a possible universal nature of this logarithmic correction.
42 pages, 1 figure. Invited article for the special issue on Loop Quantum Gravity and Cosmology of SIGMA

brief mention:
http://arxiv.org/abs/1201.6164
Cosmological inflation
K. Enqvist (Univ. Helsinki and Helsinki Inst. Phys.)
(Submitted on 30 Jan 2012)
The very basics of cosmological inflation are discussed. We derive the equations of motion for the inflaton field, introduce the slow-roll parameters, and present the computation of the inflationary perturbations and their connection to the temperature fluctuations of the cosmic microwave background.
9 pages

http://arxiv.org/abs/1201.6094
Gerbert of Aurillac: astronomy and geometry in tenth century Europe
Costantino Sigismondi
5 pages

 

[atyy]

http://arxiv.org/abs/1201.6212
Quantum fermions and quantum field theory from classical statistics
C. Wetterich
(Submitted on 30 Jan 2012)
An Ising-type classical statistical ensemble can describe the quantum physics of fermions if one chooses a particular law for the time evolution of the probability distribution. It accounts for the time evolution of a quantum field theory for Dirac particles in an external electromagnetic field. This yields in the non-relativistic one-particle limit the Schrödinger equation for a quantum particle in a potential. Interference or tunneling arise from classical probabilities.

 

[marcus]

http://arxiv.org/abs/1201.6505
Geometry and symmetries in lattice spinor gravity
C.Wetterich
(Submitted on 31 Jan 2012)
Lattice spinor gravity is a proposal for regularized quantum gravity based on fermionic degrees of freedom. In our lattice model the local Lorentz symmetry is generalized to complex transformation parameters. The difference between space and time is not put in a priori, and the euclidean and Minkowski quantum field theory are unified in one functional integral. The metric and its signature arise as a result of the dynamics, corresponding to a given ground state or cosmological solution. Geometrical objects as the vierbein, spin connection or the metric are expectation values of collective fields built from an even number of fermions. The quantum effective action for the metric is invariant under general coordinate transformations in the continuum limit. The action of our model is found to be also invariant under gauge transformations. We observe a "geometrical entanglement" of gauge- and Lorentz-transformations due to geometrical objects transforming non-trivially under both types of symmetry transformations.
38 pages

My comment: (He refers to Rovelli's http://arxiv.org/abs/1102.3660 the Zakopane Lectures on Loop Gravity, and also to papers on CDT and Regge QG, as part of clarifying why his approach is different.)

 

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http://arxiv.org/abs/1202.0008
Super-renormalizable Higher-Derivative Quantum Gravity
Leonardo Modesto
(Submitted on 31 Jan 2012)
In this paper we study perturbatively an extension of the Stelle higher derivative gravity involving an infinite number of derivative terms. We know that the usual quadratic action is renormalizable but is not unitary because of the presence of a ghost in the theory (pole with negative residue in the propagator). The new theory is instead ghost-free since an entire function (or form factor) is introduced in the model without involving new poles in the propagator. The local high derivative theory is recovered expanding the entire functions to the lowest order in the mass scale of the theory. Any truncation of the entire function gives rise to unitarity violation. The theory is divergent at one loop and finite from two loops upwards: the theory is then super-renormalizable. Using the modified graviton propagator, we demonstrate the regularity of the gravitational potential in r=0.
5 pages, 2 figures, text overlap with arXiv:1107.2403

 

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http://arxiv.org/abs/1202.0412
Emission spectra of self-dual black holes
Sabine Hossenfelder, Leonardo Modesto, Isabeau Prémont-Schwarz
(Submitted on 2 Feb 2012)
We calculate the particle spectra of evaporating self-dual black holes that are potential dark matter candidates. We first estimate the relevant mass and temperature range and find that the masses are below the Planck mass, and the temperature of the black holes is small compared to their mass. In this limit, we then derive the number-density of the primary emission particles, and, by studying the wave-equation of a scalar field in the background metric of the black hole, show that we can use the low energy approximation for the greybody factors. We finally arrive at the expression for the spectrum of secondary particle emission from a dark matter halo constituted of self-dual black holes.
15 pages, 6 figures

This excerpt from page 1 helps clarify what the paper is about:

One approach to quantum gravity, Loop Quantum Gravity (LQG) [1–4], has given rise to models that allow to describe the very early universe. Simplified frameworks of LQG using a minisuperspace approximation has been shown to resolve the initial singularity problem [5, 6]. In the present work we will study the properties of black holes in such a minisuperspace model. The metric of black holes in this model was previously derived in [7], where it was shown in particular that the singularity is removed by a self-duality of the metric that replaces the black hole’s usually singular inside by another asymptotically flat region. The thermodynamical properties of these self-dual black holes have been examined in [8], and in [9] the dynamical aspects of the collapse and evaporation were studied...​

http://arxiv.org/abs/1202.0526
Coherent states for quantum gravity: towards collective variables
Daniele Oriti, Roberto Pereira, Lorenzo Sindoni
(Submitted on 2 Feb 2012)
We investigate the construction of coherent states for quantum theories of connections based on graphs embedded in a spatial manifold, as in loop quantum gravity. We discuss the many subtleties of the construction, mainly related to the diffeomorphism invariance of the theory. Aiming at approximating a continuum geometry in terms of discrete, graph-based data, we focus on coherent states for collective observables characterizing both the intrinsic and extrinsic geometry of the hypersurface, and we argue that one needs to revise accordingly the more local definitions of coherent states considered in the literature so far. In order to clarify the concepts introduced, we work through a concrete example that we hope will be useful to applying coherent state techniques to cosmology.
25 pages, 1 figure

Increasing tendency for LQG papers to simply say "quantum gravity" in the title or not even say it in the title and have "quantum gravity" as a keyword tag for the search engine. So unless it is in the abstract you may have to look at the first page and the leading references cited at the end to see it is Loop.

 

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The FGZ paper seems to be turning out to be important. (Freidel Geiller Ziprick).
http://arxiv.org/abs/1110.4833.
Jonathan Z. just gave an excellent PIRSA video presentation, which can aid us in understanding the paper:
http://pirsa.org/12020096/
The title of the video talk is Continuous Formulation of the Loop Quantum Gravity Phase Space
He also gave a more introductory parallel session talk at Loops 2011 Madrid last May:
http://161.111.24.32/loops11/Archives/Parallel-Sessions/Jonathan-Ziprick_Geometry-of-Loop-Gravity.pdf
The talk was titled Geometry of Loop Gravity
The slides PDF is clear and helpful. It augments what one gets from the February 2012 video.

The first idea in this slide set, from which the rest takes off, is
LOOP CLASSICAL GRAVITY and the diagram of this idea is

GR —discretization→ LCG —quantization→ LQGZiprick is a PhD student at Waterloo/PI, Freidel we know, Marc Geiller is at Paris-Diderot.

This same FGZ paper will be presented by Geiller on 28 February at the ILQGS
http://relativity.phys.lsu.edu/ilqgs/schedulesp12.html
http://relativity.phys.lsu.edu/ilqgs/
Audio and slides PDF are normally available online.

This PIRSA presentation by Ziprick is interesting for several reasons, one is that his presentation went smoothly and was over in 25 minutes but was then followed by 40 minutes of discussion! The people in the audience doing the discussing included Laurent Freidel, Lee Smolin, and Bianca Dittrich.
So the total video is 65 minutes.
But if all you want is the straight presentation you just need to watch the first 25 minutes.
It gets extra interesting around 20 minutes where he is summarizing and drawing conclusions.

 

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brief mention, not primarily QG but potentially of interest:
http://arxiv.org/abs/1202.1101
On classicalization in nonlinear sigma models
Roberto Percacci, Leslaw Rachwal
(Submitted on 6 Feb 2012)
We consider the phenomenon of classicalization in nonlinear sigma models with both positive and negative target space curvature and with any number of derivatives. We find that the theories with only two derivatives exhibit a weak form of classicalization, and that the quantitative results depend on the sign of the curvature. Nonlinear sigma models with higher derivatives show a strong form of the phenomenon which is independent of the sign of curvature. We argue that weak classicalization may actually be equivalent to asymptotic safety, whereas strong classicalization seems to be a genuinely different phenomenon. We also discuss possible ambiguities in the definition of the classical limit.
14 pages

 

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I think this paper will influence how we view the AsymSafe QG approach. It would be reallyi nice to have an explanation for inflation. Easson has been a co-author of Nobelist Smoot.
http://arxiv.org/abs/1202.1285
Higgs Boson in RG running Inflationary Cosmology
Yi-Fu Cai, Damien A. Easson
(Submitted on 6 Feb 2012)
An intriguing hypothesis is that gravity may be non-perturbatively renormalizable via the notion of asymptotic safety. We show that the Higgs sector of the SM minimally coupled to asymptotically safe gravity can generate the observed near scale-invariant spectrum of the Cosmic Microwave Background through the curvaton mechanism. The resulting primordial power spectrum places an upper bound on the Higgs mass, which for canonical values of the curvaton parameters, is compatible with the recently released Large Hadron Collider data.
5 pages
I put this out for discussion in a separate related thread in case anyone wants to ask or comment about it https://www.physicsforums.com/showthread.php?p=3751800#post3751800

If curious about "curvaton" see:
http://en.wikipedia.org/wiki/Curvaton
"The curvaton is a hypothetical elementary particle which mediates a scalar field in early universe cosmology. It can generate fluctuations during inflation, but does not itself drive inflation, instead it generates curvature perturbations at late times after the inflaton field has decayed and the decay products have redshifted away, when the curvaton is the dominant component of the energy density.
The model was proposed by David Wands and David H. Lyth in 2001."

 

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brief mention (though not QG may still be of use in QG):

http://arxiv.org/abs/1202.1818
Measure and Probability in Cosmology
Joshua S. Schiffrin, Robert M. Wald
(Submitted on 8 Feb 2012)
General relativity has a Hamiltonian formulation, which formally provides a canonical (Liouville) measure on the space of solutions. In ordinary statistical physics, the Liouville measure is used to compute probabilities of macrostates, and it would seem natural to use the similar measure arising in general relativity to compute probabilities in cosmology, such as the probability that the universe underwent an era of inflation. Indeed, a number of authors have used the restriction of this measure to the space of homogeneous and isotropic universes with scalar field matter (minisuperspace)---namely, the Gibbons-Hawking-Stewart measure---to make arguments about the likelihood of inflation. We argue here that there are at least four major difficulties with using the measure of general relativity to make probability arguments in cosmology: (1) Equilibration does not occur on cosmological length scales. (2) Even in the minisuperspace case, the measure of phase space is infinite and the computation of probabilities depends very strongly on how the infinity is regulated. (3) The inhomogeneous degrees of freedom must be taken into account (we illustrate how) even if one is interested only in universes that are very nearly homogeneous. The measure depends upon how the infinite number of degrees of freedom are truncated, and how one defines "nearly homogeneous." (4) In a universe where the second law of thermodynamics holds, one cannot make use of our knowledge of the present state of the universe to "retrodict" the likelihood of past conditions.
43 pages, 2 figures

http://arxiv.org/abs/1202.1793
The presence of Primordial Gravitational Waves in the Cosmic Microwave Background
Wytler Cordeiro dos Santos
(Submitted on 8 Feb 2012)
The General Relativity affirms that any field is a source of gravitational field, thus one should affirm that the energy of Cosmic Microwave Background (CMB) generated primordial gravitational waves. The present article shows that a gravitational wave with dimensionless amplitude ~ 10^-5 and large wave length ~ 10 megaparsecs shifts temperature of CMB radiation about of a part in 10⁵
7 pages, 1 figure

 

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http://arxiv.org/abs/1202.1846
Effective constrained polymeric theories and their continuum limit
Alejandro Corichi, Tatjana Vukasinac
(Submitted on 8 Feb 2012)
The classical limit of polymer quantum theories yields a one parameter family of 'effective' theories labeled by λ. Here we consider such families for constrained theories and pose the problem of taking the 'continuum limit', λ→0. We put forward criteria for such question to be well posed, and propose a concrete strategy based in the definition of appropriately constructed Dirac observables. We analyze two models in detail, namely a constrained oscillator and a cosmological model arising from loop quantum cosmology. For both these models we show that the program can indeed be completed, provided one makes a particular choice of λ-dependent internal time with respect to which the dynamics is described and compared. We show that the limiting theories exist and discuss the corresponding limit. These results might shed some light in the problem of defining a renormalization group approach, and its associated continuum limit, for quantum constrained systems.
19 pages, 5 figures

http://arxiv.org/abs/1202.1824
The Hidden Quantum Groups Symmetry of Super-renormalizable Gravity
Stephon Alexander, Antonino Marciano, Leonardo Modesto
(Submitted on 8 Feb 2012)
In this paper we consider the relation between the super-renormalizable theories of quantum gravity (SRQG) studied in [arXiv:1110.5249v2, arXiv:1202.0008] and an underlying non-commutativity of spacetime. For one particular super-renormalizable theory we show that at linear level (quadratic in the Lagrangian) the propagator of the theory is the same we obtain starting from a theory of gravity endowed with θ-Poincaré quantum groups of symmetry. Such a theory is over the so called θ-Minkowski non-commuative spacetime. We shed new light on this link and show that among the theories considered in [arXiv:1110.5249v2, arXiv:1202.0008], there exist only one non-local and Lorentz invariant super-renormalizable theory of quantum gravity that can be described in terms of a quantum group symmetry structure. We also emphasize contact with pre-existent works in the literature and discuss preservation of the equivalence principle in our framework.
10 pages

 

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http://arxiv.org/abs/1202.2274
Quantum Einstein Gravity
Martin Reuter, Frank Saueressig
(Submitted on 10 Feb 2012)
We give a pedagogical introduction to the basic ideas and concepts of the Asymptotic Safety program in Quantum Einstein Gravity. Using the continuum approach based upon the effective average action, we summarize the state of the art of the field with a particular focus on the evidence supporting the existence of the non-trivial renormalization group fixed point at the heart of the construction. As an application, the multifractal structure of the emerging space-times is discussed in detail. In particular, we compare the continuum prediction for their spectral dimension with Monte Carlo data from the Causal Dynamical Triangulation approach.
87 pages, 13 figures, review article prepared for the New Journal of Physics focus issue on Quantum Einstein Gravity

http://arxiv.org/abs/1202.2301
Canonical Partition function of Loop Black Holes
Kinjalk Lochan, Cenalo Vaz
(Submitted on 10 Feb 2012)
We compute the canonical partition for quantum black holes in the approach of Loop Quantum Gravity (LQG). We argue that any quantum theory of gravity in which the horizon area is built of non-interacting constituents cannot yield qualitative corrections to the Bekenstein-Hawking (B-H) area law, but corrections to the area law can arise as a consequence additional constraints inducing interactions between the constituents. In LQG this is implemented by requiring spherical horizons. The canonical approach for LQG favours a logarithmic correction to the B-H law with a coefficient of -1/2, independently of the area spectrum. Our initial calculation of the partition function uses certain approximations that, we show, do not qualitatively affect the expression for the black hole entropy. We later discuss the quantitative corrections to these results when the simplifying approximations are relaxed and the full LQG spectrum is dealt with. We show how these corrections can be recovered to all orders in perturbation theory. However, the convergence properties of the perturbative series remains unknown.
16 pages, 1 figure

brief mention (a sweet bit of history, possibly of general interest)
http://arxiv.org/abs/1202.2347
A Sommerfeld Explanation
Jeremy Bernstein
(Submitted on 10 Feb 2012)
Sommerfeld shows that the Wien displacement formula implies the existence of Planck's constant.

 

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http://arxiv.org/abs/1202.2710
Dynamical dimensional reduction in toy models of 4D causal quantum gravity
Georgios Giasemidis, John F. Wheater, Stefan Zohren
(Submitted on 13 Feb 2012)
In recent years several approaches to quantum gravity have found evidence for a scale dependent spectral dimension of space-time varying from four at large scales to two at small scales of order of the Planck length. The first evidence came from numerical results of four-dimensional causal dynamical triangulations (CDT) [Ambj{\o}rn et al., Phys. Rev. Lett. 95 (2005) 171]. Since then little progress has been made in analytically understanding the numerical results coming from the CDT approach and showing that they remain valid when taking the continuum limit. In this letter we propose a new toy model of "radially reduced" four-dimensional CDT in which we can take the continuum limit analytically and obtain a scale dependent spectral dimension varying from four to two with scale. Furthermore, the functional behaviour of the spectral dimension is exactly of the form which was conjectured on the basis of the numerical results.
5 pages, 2 figures

brief mention:
http://arxiv.org/abs/1202.2539
Quantum Time Crystals
Frank Wilczek
(Submitted on 12 Feb 2012)
Difficulties around the idea of spontaneous breaking of time translation symmetry in a closed quantum mechanical system are identified, and then overcome in a simple model. The possibility of ordering in imaginary time is also discussed.
6 pages,

 

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brief mention (may be of general interest though not directly QG-related):
http://arxiv.org/abs/1202.3324
On the inertia of heat
Matteo Smerlak
(Submitted on 15 Feb 2012)
Does heat have inertia? This question is at the core of a long-standing controversy on Eckart's dissipative relativistic hydrodynamics. Here I show that the troublesome inertial term in Eckart's heat flux arises only if one insists on defining thermal diffusivity as a spacetime constant. I argue that this is the most natural definition, and that all confusion disappears if one considers instead the space-dependent comoving diffusivity, in line with the fact that, in the presence of gravity, space is an inhomogeneous medium.
3 pages

 

[marcus]

Great talk by Frank Saueressig on Asym Safe QG. Perimeter video here:
http://pirsa.org/12020088/
Part of the talk explains the basics--a clear introduction. Part goes into advanced topics.
He's an excellent presenter IMO. He's worked with Martin Reuter for around 10 years and is now at Mainz as junior faculty, so about as authoritative as anyone concerning AS.

http://arxiv.org/abs/1202.3637
Random tensor models in the large N limit: Uncoloring the colored tensor models
Valentin Bonzom, Razvan Gurau, Vincent Rivasseau
(Submitted on 16 Feb 2012)
Tensor models generalize random matrix models in yielding a theory of dynamical triangulations in arbitrary dimensions. Colored tensor models have been shown to admit a 1/N expansion and a continuum limit accessible analytically. In this paper we prove that these results extend to the most general tensor model for a single generic, i.e. non-symmetric, complex tensor. Colors appear in this setting as a canonical book-keeping device and not as a fundamental feature. In the large N limit, we exhibit a set of Virasoro constraints satisfied by the free energy and an infinite family of multicritical behaviors with entropy exponents γ_m=1-1/m.
15 pages

extremely brief mention (a reminder about Ingemar Bengtsson):
http://arxiv.org/abs/1202.3559
http://arxiv.org/abs/1202.3561

 

[marcus]

http://arxiv.org/abs/1202.4066
Comment on arXiv:1104.2019, "Relative locality and the soccer ball problem," by Amelino-Camelia et al
Sabine Hossenfelder
(Submitted on 18 Feb 2012)
It is explained why the argument in arXiv:1104.2019 does not answer the question how to describe multi-particle states in models with a deformed Lorentz-symmetry in momentum space.
3 pages

brief mention:
http://arxiv.org/abs/1202.4435
New multicritical matrix models and multicritical 2d CDT
Jan Ambjorn, Lisa Glaser, Andrzej Gorlich, Yuki Sato
(Submitted on 20 Feb 2012)
We define multicritical CDT models of 2d quantum gravity and show that they are a special case of multicritical generalized CDT models obtained from the new scaling limit, the so-called "classical" scaling limit, of matrix models. The multicritical behavior agrees with the multicritical behavior of the so-called branched polymers.
16 pages, 4 figures

http://arxiv.org/abs/1202.4322
An Analytical Analysis of CDT Coupled to Dimer-like Matter
Max R. Atkin, Stefan Zohren
(Submitted on 20 Feb 2012)
We consider a model of restricted dimers coupled to two-dimensional causal dynamical triangulations (CDT), where the dimer configurations are restricted in the sense that they do not include dimers in regions of high curvature. It is shown how the model can be solved analytically using bijections with decorated trees. At a negative critical value for the dimer fugacity the model undergoes a phase transition at which the critical exponent associated to the geometry changes. This represents the first account of an analytical study of a matter model with two-dimensional interactions coupled to CDT.

http://arxiv.org/abs/1202.4186
http://arxiv.org/abs/1202.4187
Edward Anderson

 

[marcus]

http://arxiv.org/abs/1202.5039
Degenerate Plebanski Sector and its Spin Foam Quantization
Sergei Alexandrov
(Submitted on 22 Feb 2012)
We show that the degenerate sector of Spin(4) Plebanski formulation of four-dimensional gravity is exactly solvable and describes covariantly embedded SU(2) BF theory. This fact provides its spin foam quantization and allows to test various approaches of imposing the simplicity constraints. Our analysis suggests a unique method of imposing the constraints which leads to a consistent and well defined spin foam model.
34 pages