Loop-and-allied QG bibliography

[MTd2]

I don`t know if this paper belongs to this thread. But I am posting here anyway because the authors is a frequent dweller here:

http://arxiv.org/abs/1206.2605

Towards a finite quantum supergravity

Leonardo Modesto
(Submitted on 12 Jun 2012)
In this paper we study an N=1 supersymmetric extension of a perturbatively super-renormalizable (nonlocal)theory of gravity in four dimensions. The nonlocal supergravity theory is power-counting super-renormalizable and tree level unitary with the same particle content of the local N=1 supergravity (as simple example, unitarity of the three dimensional N=1 and N=2 supergravity is proved). We believe that extended SO(N) supergravity, for N=4 or N=8, might be free from divergences also at one loop. The extended supergravities would then result finite at any order in the loop expansion.

 

[marcus]

http://arxiv.org/abs/1206.2939
Constraints on the Topology of the Universe: Extension to General Geometries
Pascal M. Vaudrevange, Glenn D. Starkman, Neil J. Cornish, David N. Spergel
(Submitted on 13 Jun 2012)
We present an update to the search for a non-trivial topology of the universe by searching for matching circle pairs in the cosmic microwave background using the WMAP 7 year data release. We extend the exisiting bounds to encompass a wider range of possible topologies by searching for matching circle pairs with opening angles 10 degree < α < 90 degree and separation angles 11 degree < θ < 180 degree. The extended search reveal two small anomalous regions in the CMB sky. Numerous pairs of well-matched circles are found where both circles pass through one or the other of those regions. As this is not the signature of any known manifold, but is a likely consequence of contamination in those sky regions, we repeat the search excluding circle pairs where both pass through either of the two regions. We then find no statistically significant pairs of matched circles, and so no hints of a non-trivial topology. The absence of matched circles increases the lower limit on the length of the shortest closed null geodesic that self-intersects at our location in the universe (equivalently the injectivity radius at our location) to 98.5% of the diameter of the last scattering surface or approximately 26 Gpc. It extends the limit to any manifolds in which the intersecting arcs of said geodesic form an angle greater than 10^o.
11 pages, 11 figures

http://arxiv.org/abs/1206.3067
Comment on "Can we measure structures to a precision better than the Planck length?"[arXiv:1205.3636], by Sabine Hossenfelder
Sergio Doplicher, Gherardo Piacitelli, Luca Tomassini, Stefano Viaggiu
(Submitted on 14 Jun 2012)
First principles do imply a non-zero minimal distance between events in spacetime, but no positive lower bound to the precision of the measurement of a single coordinate.
5 pages

 

[marcus]

http://arxiv.org/abs/1206.3411
Absence of log correction in entropy of large black holes
A. Ghosh, P. Mitra
(Submitted on 15 Jun 2012)
Earlier calculations of black hole entropy in loop quantum gravity have led to a dominant term proportional to the area, but there was a correction involving the logarithm of the area. We find however that calculations yield an entropy proportional to the area eigenvalue with no such correction if the area eigenvalue is taken to be much larger than the classical area.
5 pages

http://arxiv.org/abs/1206.3497
Different Theories of Gravity in Plebanski Formulation
D. L. Bennett, L. V. Laperashvili, H. B. Nielsen, A. Tureanu
(Submitted on 15 Jun 2012)
We present several theories of four-dimensional gravity in the Plebanski formulation, in which the tetrads and the connections are the independent dynamical variables. We consider the relation between different versions of gravitational theories: Einstenian, topological, 'mirror' gravities and gravity with torsion. According to Plebanski's assumption, our world, in which we live, is described by the self-dual left-handed gravity. We propose that if the Mirror World exists in Nature, then the 'mirror gravity' is the right-handed anti-self-dual gravity with broken mirror parity. Considering a special version of the Riemann--Cartan space-time, which has torsion as additional geometric property, we have shown that in the Plebanski formulation the ordinary and topological sectors of gravity, as well as the gravity with torsion, are equivalent. In this context, we have also developed a 'pure connection gravity' -- a diffeomorphism-invariant gauge theory of gravity. We have calculated the partition function and the effective Lagrangian of this four-dimensional gravity and have investigated the limit of this theory at small distances.
13 pages (LQG references on pages 2, 11)

http://arxiv.org/abs/1206.3457
Introduction to Spin Networks and Towards a Generalization of the Decomposition Theorem
Hans-Christian Ruiz
(Submitted on 15 Jun 2012)
The objective of this work is twofold. On one hand, it is intended as a short introduction to spin networks and invariants of 3-manifolds. It covers the main areas needed to have a first understanding of the topics involved in the development of spin networks, which are described in a detailed but not exhaustive manner and in order of their conceptual development such that the reader is able to use this work as a first reading. A motivation due to R. Penrose for considering spin networks as a way of constructing a 3-D Euclidean space is presented, as well as their relation to Ponzano-Regge theory. Furthermore, the basic mathematical framework for the algebraic description of spin networks via quantum groups is described and the notion of a spherical category and its correspondence to the diagrammatic representation given by the Temperley-Lieb recoupling theory are presented. In order to give an example of topological invariants and their relation to TQFT the construction of the Turaev-Viro invariant is depicted and related to the Kauffman-Lins invariant. On the other hand, some results aiming at a decomposition theorem for non-planar spin networks are presented. For this, Moussouris' algorithm and some basic concepts of topological graph theory are explained and used, especially Kuratowski's theorem and the Rotation Scheme theorem.
99 pages

 

[marcus]

http://arxiv.org/abs/1206.3903
How to detect an anti-spacetime
Marios Christodoulou, Aldo Riello, Carlo Rovelli
(Submitted on 18 Jun 2012)
Is it possible, in principle, to measure the sign of the Lapse? We show that fermion dynamics distinguishes spacetimes having the same metric but different tetrads, for instance a Lapse with opposite sign. This sign might be a physical quantity not captured by the metric. We discuss its possible role in quantum gravity.
6 pages, 8 figures. Article awarded with an "Honorable Mention" from the 2012 Gravity Foundation Award.

http://arxiv.org/abs/1206.3807
Scalar Material Reference Systems and Loop Quantum Gravity
Kristina Giesel, Thomas Thiemann
(Submitted on 17 Jun 2012)
In the past, the possibility to employ (scalar) material reference systems in order to describe classical and quantum gravity directly in terms of gauge invariant (Dirac) observables has been emphasised frequently. This idea has been picked up more recently in Loop Quantum Gravity (LQG) with the aim to perform a reduced phase space quantisation of the theory thus possibly avoiding problems with the (Dirac) operator constraint quantisation method for constrained system. In this work, we review the models that have been studied on the classical and/or the quantum level and parametrise the space of theories so far considered. We then describe the quantum theory of a model that, to the best of our knowledge, so far has only been considered classically. This model could arguably called the optimal one in this class of models considered as it displays the simplest possible true Hamiltonian while at the same time reducing all constraints of General Relativity.
28 pages

http://arxiv.org/abs/1206.3689
Do Barbero-Immirzi connections exist in different dimensions and signatures?
L. Fatibene, M. Francaviglia, S.Garruto
(Submitted on 16 Jun 2012)
We shall show that no reductive splitting of the spin group exists in dimension 3 ≤ m ≤ 20 other than in dimension m = 4. In dimension 4 there are reductive splittings in any signature. Euclidean and Lorentzian signatures are reviewed in particular and signature (2, 2) is investigated explicitly in detail. Reductive splittings allow to define a global SU(2)-connection over spacetime which encodes in a weird way the holonomy of the standard spin connection. The standard Barbero-Immirzi (BI) connection used in LQG is then obtained by restriction to a spacelike slice. This mechanism provides a good control on globality and covariance of BI connection showing that in dimension other than 4 one needs to provide some other mechanism to define the analogous of BI connection and control its globality.
8 pages

http://arxiv.org/abs/1206.3805
Relative locality in a quantum spacetime and the pregeometry of κ-Minkowski
Giovanni Amelino-Camelia, Valerio Astuti, Giacomo Rosati
(Submitted on 17 Jun 2012)
We develop a new description of the much-studied κ-Minkowski noncommutative spacetime, centered on representing on a single Hilbert space not only the κ-Minkowski coordinates, but also the associated differential calculus and the κ-Poincaré symmetry generators.
In this "pregeometric" representation the relevant operators act on the kinematical Hilbert space of the covariant formulation of quantum mechanics, which we argue is the natural framework for studying the implications of the step from commuting spacetime coordinates to the κ-Minkowski case, where the spatial coordinates do not commute with the time coordinate. The empowerment provided by this kinematical-Hilbert space representation allows us to give a crisp characterization of the "fuzziness" of κ-Minkowski spacetime, whose most striking aspect is a relativity of spacetime locality. We show that relative locality, which had been previously formulated exclusively in classical-spacetime setups, for a quantum spacetime takes the shape of a dependence of the fuzziness of a spacetime point on the distance at which an observer infers properties of the event that marks the point.
9 pages

http://arxiv.org/abs/1206.4021
The Wheeler-DeWitt Quantization Can Solve the Singularity Problem
F. T. Falciano, Roberto Pereira, N. Pinto-Neto, E. Sergio Santini
(Submitted on 18 Jun 2012)
We study the Wheeler-DeWitt quantum cosmology of a spatially flat Friedmann cosmological model with a massless free scalar field. We compare the consistent histories approach with the de Broglie-Bohm theory when applied to this simple model under two different quantization schemes: the Schrödinger-like quantization, which essentially takes the square-root of the resulting Klein-Gordon equation through the restriction to positive frequencies and their associated Newton-Wigner states, or the induced Klein-Gordon quantization, that allows both positive and negative frequencies together. We show that the consistent histories approach can give a precise answer to the question concerning the existence of a quantum bounce if and only if one takes the single frequency approach and within a single family of histories, namely, a family containing histories concerning properties of the quantum system at only two specific moments of time: the infinity past and the infinity future. In that case, as shown by Craig and Singh [CS], there is no quantum bounce. In any other situation, the question concerning the existence of a quantum bounce has no meaning in the consistent histories approach. On the contrary, we show that if one considers the de Broglie-Bohm theory, there are always states where quantum bounces occur in both quantization schemes. Hence the assertion that the Wheeler-DeWitt quantization does not solve the singularity problem in cosmology is not precise. To address this question, one must specify not only the quantum interpretation adopted but also the quantization scheme chosen.
13 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1206.4203
Counting Line-Colored D-ary Trees
Valentin Bonzom, Razvan Gurau
Random tensor models are generalizations of matrix models which also support a 1/N expansion. The dominant observables are in correspondence with some trees, namely rooted trees with vertices of degree at most D and lines colored by a number i from 1 to D such that no two lines connecting a vertex to its descendants have the same color. In this Letter we study by independent methods a generating function for these observables. We prove that the number of such trees with exactly p_i lines of color i is
\frac{1}{\sum_{i=1}^D p_i +1} \binom{\sum_{i=1}^D p_i+1}{p_1} ... \binom{\sum_{i=1}^D p_i+1}{p_D}
6 pages

 

[marcus]

http://arxiv.org/abs/1206.4553
Decoherent Histories of Spin Networks
David P.B. Schroeren
(Submitted on 20 Jun 2012)
The decoherent histories formalism, developed by Griffiths, Gell-Mann, and Hartle is a general framework in which to formulate a timeless, 'generalised' quantum theory and extract predictions from it. Recent advances in spin foam models allow for loop gravity to be cast in this framework. In this paper, I propose a decoherence functional for loop gravity and interpret existing results as showing that coarse grained histories follow quasiclassical trajectories in the appropriate limit.
13 pages
(newcomer: Cambridge MSci adv. Jeremy Butterfield, now PhD student Potsdam/Marseille adv. Rovelli http://arxiv.org/abs/1007.0664 )

brief mention (of general interest although not directly involving quantum gravity)
http://arxiv.org/abs/1206.4823
Symmetry Doubling: Doubly General Relativity
Henrique Gomes, Tim Koslowski
(Submitted on 21 Jun 2012)
Using a BRST treatment, we show that the equivalence of General Relativity and Shape Dynamics can be extended to a theory that respects the BRST-symmetries of General Relativity as well as the ones of an extended version of Shape Dynamics. This version of Shape Dynamics implements local spatial Weyl transformations as well as a local and abstract analogue of special conformal transformations. Standard effective field theory arguments suggest that the definition of a gravity theory should implement this duality between General Relativity and Shape Dynamics, thus the name "Doubly General Relativity." We briefly discuss several consequences: bulk/bulk- duality in classical gravity, experimental falsification of Doubly General Relativity and possible implications for the renormalization of quantum gravity in the effective field theory framework.
24 pages

http://arxiv.org/abs/1206.4796
On topological restrictions of the spacetime in cosmology
T. Asselmeyer-Maluga, J. Krol
(Submitted on 21 Jun 2012)
In this paper we discuss the restrictions of the spacetime for the standard model of cosmology by using results of the differential topology of 3- and 4-manifolds. The smoothness of the cosmic evolution is the strongest restriction. The Poincare model (dodecaeder model), the Picard horn and the 3-torus are ruled out by the restrictions but a sum of two Poincare spheres is allowed.
11 pages, 3 figures, accepted in Mod. Phys. Lett. A

 

[MTd2]

http://arxiv.org/abs/1206.4949

Fundamental quantum optics experiments conceivable with satellites -- reaching relativistic distances and velocities

David Rideout, Thomas Jennewein, Giovanni Amelino-Camelia, Tommaso F. Demarie, Brendon L. Higgins, Achim Kempf, Adrian Kent, Raymond Laflamme, Xian Ma, Robert B. Mann, Eduardo Martin-Martinez, Nicolas C. Menicucci, John Moffat, Christoph Simon, Rafael Sorkin, Lee Smolin, Daniel R. Terno
(Submitted on 21 Jun 2012)
Physical theories are developed to describe phenomena in particular regimes, and generally are valid only within a limited range of scales. For example, general relativity provides an effective description of the Universe at large length scales, and has been tested from the cosmic scale down to distances as small as 10 meters. In contrast, quantum theory provides an effective description of physics at small length scales. Direct tests of quantum theory have been performed at the smallest probeable scales at the Large Hadron Collider, ${\sim} 10^{-20}$ meters, up to that of hundreds of kilometers. Yet, such tests fall short of the scales required to investigate potentially significant physics that arises at the intersection of quantum and relativistic regimes. We propose to push direct tests of quantum theory to larger and larger length scales, approaching that of the radius of curvature of spacetime, where we begin to probe the interaction between gravity and quantum phenomena. In particular, we review a wide variety of potential tests of fundamental physics that are conceivable with artificial satellites in Earth orbit and elsewhere in the solar system, and attempt to sketch the magnitudes of potentially observable effects. The tests have the potential to determine the applicability of quantum theory at larger length scales, eliminate various alternative physical theories, and place bounds on phenomenological models motivated by ideas about spacetime microstructure from quantum gravity. From a more pragmatic perspective, as quantum communication technologies such as quantum key distribution advance into Space towards large distances, some of the fundamental physical effects discussed here may need to be taken into account to make such schemes

 

[marcus]

http://arxiv.org/abs/1206.5765
A no-singularity scenario in loop quantum gravity
Martin Bojowald, George M. Paily
(Submitted on 25 Jun 2012)
Canonical methods allow the derivation of effective gravitational actions from the behavior of space-time deformations reflecting general covariance. With quantum effects, the deformations and correspondingly the effective actions change, revealing dynamical implications of quantum corrections. A new systematic way of expanding these actions is introduced showing as a first result that inverse-triad corrections of loop quantum gravity simplify the asymptotic dynamics near a spacelike collapse singularity. By generic quantum effects, the singularity is removed.
10 pages

 

[marcus]

http://arxiv.org/abs/1206.6088
Mathematical structure of loop quantum cosmology: Homogeneous models
Martin Bojowald
(Submitted on 26 Jun 2012)
The mathematical structure of homogeneous loop quantum cosmology is analyzed, starting with and taking into account the general classification of homogeneous connections not restricted to be Abelian. As a first consequence, it is seen that the usual approach of quantizing Abelian models using spaces of functions on the Bohr compactification of the real line does not capture all properties of homogeneous connections. A new, more general quantization is introduced which applies to non-Abelian models and, in the Abelian case, can be mapped by an isometric, but not unitary, algebra morphism onto common representations making use of the Bohr compactification. Physically, the Bohr compactification of spaces of Abelian connections leads to a degeneracy of edge lengths and representations of holonomies. Lifting this degeneracy, the new quantization gives rise to several dynamical properties, including lattice refinement seen as a direct consequence of state-dependent regularizations of the Hamiltonian constraint of loop quantum gravity. The representation of basic operators -- holonomies and fluxes -- can be derived from the full theory specialized to lattices. With the new methods of this article, loop quantum cosmology comes closer to the full theory and is in a better position to produce reliable predictions when all quantum effects of the theory are taken into account.
55 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1206.5961

Relativistic kinematics beyond Special Relativity

J. M. Carmona, J. L. Cortes, F. Mercati
(Submitted on 26 Jun 2012)
In the context of departures from Special Relativity written as a momentum power expansion in the inverse of an ultraviolet energy scale M, we derive the constraints that the relativity principle imposes between coefficients of a deformed composition law, dispersion relation, and transformation laws, at first order in the power expansion. In particular, we find that, at that order, the consistency of a modification of the energy-momentum composition law fixes the modification in the dispersion relation. We therefore obtain the most generic modification of Special Relativity that preserves the relativity principle at leading order in 1/M.

 

[marcus]

brief mention, possibly of general interest:
http://arxiv.org/abs/1206.6296
Horava-Lifgarbagez theory as a Fermionic Aether in Ashtekar gravity
Stephon Alexander, Joao Magueijo, Antonino Marciano
(Submitted on 27 Jun 2012)
We show how Horava-Lifgarbagez (HL) theory appears naturally in the Ashtekar formulation of relativity if one postulates the existence of a fermionic field playing the role of aether. The spatial currents associated with this field must be switched off for the equivalence to work. Therefore the field supplies the preferred frame associated with breaking refoliation (time diffeomorphism) invariance, but obviously the symmetry is only spontaneously broken if the field is dynamic. When Dirac fermions couple to the gravitational field via the Ashtekar variables, the low energy limit of HL gravity, recast in the language of Ashtekar variables, naturally emerges (provided the spatial fermion current identically vanishes). HL gravity can therefore be interpreted as a time-like current, or a Fermi aether, that fills space-time, with the Immirzi parameter, a chiral fermionic coupling, and the fermionic charge density fixing the value of the parameter λ determining HL theory. This reinterpretation sheds light on some features of HL theory, namely its good convergence properties.
==excerpts from page 1 and 8==
It is interesting that the discreteness of space-time in Loop Quantum Gravity (LQG) also provides a natural UV regulator [3] and one is led to wonder if the finiteness in HL gravity is connected to the non-perturbative discreteness found in LQG. A way to begin analyzing this possible connection is to see if HL gravity can be reexpressed in terms of the Asthekar canonical variables which naturally lead to the the holonomy representation of LQG.
...
To summarize, Horava’s theory can be seen as a specific case of the covariant first-order gravity theory (Einstein-Cartan-Kibble-Holst). When the covariant theory is rewritten in Ashtekar variables, the imposition of the York-time yields the Horava theory with the Cotton tensor, in the presence of a fermion aether which breaks time-refoliation invariance.
==endquote==

 

[sbrothy]

Sorry for my low-brow "input". I mostly camp here trying to figure as much as I can from you people's exchanges. Often with less result than I would wish for though, heh.

http://arxiv.org/abs/1206.6296
Stephon Alexander, Joao Magueijo, Antonino Marciano
Horava-Lifgarbagez theory as a Fermionic Aether in Ashtekar gravity


Ironic how the big man's efforts always find themselves enshrouded in some kind of aether. This time fermionic even!

http://arxiv.org/abs/1103.4192
Nikodem J. Poplawski
On the mass of the Universe born in a black hole

Since HL-Gravity seems to have the torsion Mr N. Poplawski is missing, should we anytime soon throw something in a black hole and see what we hit (Yes I know...) ? Or maybe someone else "somewhere" else already did that and we got pretty lucky?

 

[MTd2]

This thread is for archival only purpose. If you want to discuss anything, open another thread. Thank you.

 

[marcus]

http://arxiv.org/abs/1206.6736
Consistency of holonomy-corrected scalar, vector and tensor perturbations in Loop Quantum Cosmology
Thomas Cailleteau, Aurelien Barrau, Julien Grain, Francesca Vidotto
(Submitted on 28 Jun 2012)
Loop Quantum Cosmology yields two kinds of quantum corrections to the effective equations of motion for cosmological perturbations. Here we focus on the holonomy kind and we study the problem of the closure of the resulting algebra of constraints. Up to now, tensor, vector and scalar perturbations were studied independently, leading to different algebras of constraints. The structures of the related algebras were imposed by the requirement of anomaly freedom. In this article we show that the algebra can be modified by a very simple quantum correction, holding for all types of perturbations. This demonstrates the consistency of the theory and shows that lessons from the study of scalar perturbations should be taken into account when studying tensor modes. The Mukhanov-Sasaki equations of motion are similarly modified by a simple term.
5 pages

brief mention, not Loop-and-allied QG but probably of general interest:
http://arxiv.org/abs/1206.6559
The quantum geometric limit
Seth Lloyd
(Submitted on 28 Jun 2012)
This letter analyzes the limits that quantum mechanics imposes on the accuracy to which spacetime geometry can be measured. By applying the fundamental physical bounds to measurement accuracy to ensembles of clocks and signals moving in curved spacetime -- e.g., the global positioning system -- I derive a covariant version of the quantum geometric limit: the total number of ticks of clocks and clicks of detectors that can be contained in a four volume of spacetime of radius r and temporal extent t is less than or equal to rt/π x_P t_P, where x_P, t_P are the Planck length and time. The quantum geometric limit bounds the number of events or `ops' that can take place in a four-volume of spacetime: each event is associated with a Planck-scale area. Conversely, I show that if each quantum event is associated with such an area, then Einstein's equations must hold. The quantum geometric limit is consistent with and complementary to the holographic bound which limits the number of bits that can exist within a spatial three-volume.
10 pages

 

[marcus]

brief mention (not directly QG, but of general interest):
http://arxiv.org/abs/1206.7114
Origins of Mass
Frank Wilczek
(Submitted on 29 Jun 2012)
Newtonian mechanics posited mass as a primary quality of matter, incapable of further elucidation. We now see Newtonian mass as an emergent property. Most of the mass of standard matter, by far, arises dynamically, from back-reaction of the color gluon fields of quantum chromodynamics (QCD). The equations for massless particles support extra symmetries - specifically scale, chiral, and gauge symmetries. The consistency of the standard model relies on a high degree of underlying gauge and chiral symmetry, so the observed non-zero masses of many elementary particles (W and Z bosons, quarks, and leptons) requires spontaneous symmetry breaking. Superconductivity is a prototype for spontaneous symmetry breaking and for mass-generation, since photons acquire mass inside superconductors. A conceptually similar but more intricate form of all-pervasive (i.e. cosmic) superconductivity, in the context of the electroweak standard model, gives us a successful, economical account of W and Z boson masses. It also allows a phenomenologically successful, though profligate, accommodation of quark and lepton masses. The new cosmic superconductivity, when implemented in a straightforward, minimal way, suggests the existence of a remarkable new particle, the so-called Higgs particle. The mass of the Higgs particle itself is not explained in the theory, but appears as a free parameter. Earlier results suggested, and recent observations at the Large Hadron Collider (LHC) may indicate, the actual existence of the Higgs particle, with mass m_H ≈ 125 GeV. In addition to consolidating our understanding of the origin of mass, a Higgs particle with m_H ≈ 125 GeV could provide an important clue to the future, as it is consistent with expectations from supersymmetry.
33 pages, 6 figures. Invited review for the Central European Journal of Physics. This is the supplement to my 2011 Solvay Conference talk promised there. It is adapted from an invited talk given at the Atlanta APS meeting, April 2012.

 

[marcus]

http://arxiv.org/abs/1207.0416
Some classes of renormalizable tensor models
Joseph Ben Geloun, Etera R. Livine
(Submitted on 2 Jul 2012)
We identify new families of renormalizable of tensor models from anterior renormalizable tensor models via a mapping capable of reducing or increasing the rank of the theory without having an effect on the renormalizability property. Mainly, the rank 3 tensor model as defined in [arXiv:1201.0176 [hep-th]], the Grosse-Wulkenhaar model in 4D and 2D generate three different classes of renormalizable models.
10 pages, 4 figures
[The application is to Group Field Theory (GFT) which is a close cousin of Loop gravity, hence included here.]

http://arxiv.org/abs/1207.0423
Singularity Avoidance of Charged Black Holes in Loop Quantum Gravity
Mojtaba Taslimi Tehrani, Hoshang Heydari
(Submitted on 2 Jul 2012)
Based on spherically symmetric reduction of loop quantum gravity, quantization of the portion interior to the horizon of a Reissner-Nordström black hole is studied. Classical phase space variables of all regions of such a black hole are calculated for the physical case M²> Q². This calculation suggests a candidate for a classically unbounded function of which all divergent components of the curvature scalar are composed. The corresponding quantum operator is constructed and is shown explicitly to possesses a bounded operator. Comparison of the obtained result with the one for the Swcharzschild case shows that the upper bound of the curvature operator of a charged black hole reduces to that of Schwarzschild at the limit Q → 0. This local avoidance of singularity together with non-singular evolution equation indicates the role quantum geometry can play in treating classical singularity of such black holes.
14 pages. To appear in International Journal of Theoretical Physics

 

[marcus]

http://arxiv.org/abs/1207.0671
Lorentz Symmetry in QFT on Quantum Bianchi I Space-Time
Andrea Dapor, Jerzy Lewandowski, Yaser Tavakoli
(Submitted on 3 Jul 2012)
We develop the quantum theory of a scalar field on LQC Bianchi I geometry. In particular, we focus on single modes of the field: the evolution equation is derived from the quantum scalar constraint, and it is shown that the same equation can be obtained from QFT on an "classical" effective geometry. We investigate the dependence of this effective space-time on the wavevector of the mode (which could in principle generate a deformation in local Lorentz-symmetry), focusing our attention on the dispersion relation. We prove that when we disregard backreaction no Lorentz-violation is present, despite the effective metric being different than the classical Bianchi I one. A preliminary analysis of the correction due to inclusion of backreaction is briefly discussed in the context of Born-Oppenheimer approximation.
13 pages

http://arxiv.org/abs/1207.0637
New ground state for quantum gravity
Joao Magueijo, Laura Bethke
(Submitted on 3 Jul 2012)
In this paper we conjecture the existence of a new "ground" state in quantum gravity, supplying a wave function for the inflationary Universe. We present its explicit perturbative expression in the connection representation, exhibiting the associated inner product. The state is chiral, dependent on the Immirzi parameter, and is the vacuum of a second quantized theory of graviton particles. We identify the physical and unphysical Hilbert sub-spaces. We then contrast this state with the perturbed Kodama state and explain why the latter can never describe gravitons in a de Sitter background. Instead, it describes self-dual excitations, which are composites of the positive frequencies of the right-handed graviton and the negative frequencies of the left-handed graviton. These excitations are shown to be unphysical under the inner product we have identified. Our rejection of the Kodama state has a moral tale to it: the semi-classical limit of quantum gravity can be the wrong path for making contact with reality (which may sometimes be perturbative but nonetheless fully quantum). Our results point towards a non-perturbative extension, and we present some conjectures on the nature of this hypothetical state.

http://arxiv.org/abs/1207.0505
Emergent perspective of Gravity and Dark Energy
T. Padmanabhan
(Submitted on 2 Jul 2012)
There is sufficient amount of internal evidence in the nature of gravitational theories to indicate that gravity is an emergent phenomenon like, e.g, elasticity. Such an emergent nature is most apparent in the structure of gravitational dynamics. It is, however, possible to go beyond the field equations and study the space itself as emergent in a well-defined manner in (and possibly only in) the context of cosmology. In the first part of this review, I describe various pieces of evidence which show that gravitational field equations are emergent. In the second part, I describe a novel way of studying cosmology in which I interpret the expansion of the universe as equivalent to the emergence of space itself. In such an approach, the dynamics evolves towards a state of holographic equipartition, characterized by the equality of number of bulk and surface degrees of freedom in a region bounded by the Hubble radius. This principle correctly reproduces the standard evolution of a Friedmann universe. Further, (a) it demands the existence of an early inflationary phase as well as late time acceleration for its successful implementation and (b) allows us to link the value of late time cosmological constant to the e-folding factor during inflation.
38 pages; 5 figures

brief mention:
http://arxiv.org/abs/1207.0670
Scale hierarchy in Horava-Lifgarbagez gravity: a strong constraint from synchrotron radiation in the Crab nebula
Stefano Liberati, Luca Maccione, Thomas P. Sotiriou
(Submitted on 3 Jul 2012)
4 pages. 2 figures.

 

[marcus]

brief mention, Loop-gravity-related (see references) and of general interest:
http://arxiv.org/abs/1207.1002
Quantum superpositions of the speed of light
Sabine Hossenfelder
(Submitted on 4 Jul 2012)
While it has often been proposed that, fundamentally, Lorentz-invariance is not respected in a quantum theory of gravity, it has been difficult to reconcile deviations from Lorentz-invariance with quantum field theory. The most commonly used mechanisms either break Lorentz-invariance explicitly or deform it at high energies. However, the former option is very tightly constrained by experiment already, the latter generically leads to problems with locality. We show here that there exists a third way to integrate deviations from Lorentz-invariance into quantum field theory that circumvents the problems of the other approaches. The way this is achieved is an extension of the standard model in which photons can have different speeds without singling out a preferred restframe, but only as long as they are in a quantum superposition. Once a measurement has been made, observables are subject to the laws of special relativity, and the process of measurement introduces a preferred frame. The speed of light can take on different values, both superluminal and subluminal (with respect to the usual value of the speed of light), without the need for Lorentz-invariance violating operators and without tachyons. We briefly discuss the relation to deformations of special relativity and phenomenological consequences.
9 pages, 1 figure
[my comment: this paper was the subject of a talk by the author at the MG13 conference 2-6 July.
http://ntsrvg9-5.icra.it/mg13/FMPro...&-max=1200&-recid=35256&-token.0=19&-findall=
How to beat a cosmic speeding ticket
In this talk I will argue it is possible that, within a theory of quantum gravity, special relativity is modified so as to allow for superluminal information exchange. This is possible without inducing higher order operators coupling to a preferred frame, and without causing problems with locality or causality.]

http://arxiv.org/abs/1207.0887
Energy on black hole spacetimes
Alejandro Corichi
(Submitted on 4 Jul 2012)
We consider the issue of defining energy for test particles on a background black hole spacetime. We revisit the different notions of energy as defined by different observers. The existence of a time-like isometry allows for the notion of a total conserved energy to be well defined, and subsequently the notion of a gravitational potential energy is also meaningful. We then consider the situation in which the test particle is adsorbed by the black hole, and analyze the energetics in detail. In particular, we show that the notion of horizon energy es defined by the isolated horizons formalism provides a satisfactory notion of energy compatible with the particle's conserved energy. As another example, we comment a recent proposal to define energy of the black hole as seen by an observer at rest. This account is intended to be pedagogical and is aimed at the level of and as a complement to the standard textbooks on the subject.
7 pages
[my comment: contrasting perspective on definitions in refs. 4 and 5, arXiv:1110.4055 and arXiv:1204.5122]

 

[atyy]

I guess advances in thermodynamics are relevant to QG because of Hawking, Jacobson, Padmanabhan etc.

http://arxiv.org/abs/1207.1026
Stochastic thermodynamics for inhomogeneous media
Matteo Smerlak
(Submitted on 4 Jul 2012)
A unifying framework for the thermodynamics of fluctuating systems with Fokker-Planck dynamics has been developed by Seifert and others using the notion of stochastic entropy. Here we consider the extension of this formalism to the case of inhomogeneous media, where the diffusivity is state-dependent (multiplicative noise) and the usual fluctuation theorems can be violated. We introduce to this effect the concept of "relative stochastic entropy", and use it to generalize (i) the maximum-entropy principle for the Gibbs canonical ensemble, (ii) the second law of thermodynamics and (iii) Seifert's integral fluctuation theorems. Our "relative stochastic thermodynamics" can be used e.g. to describe the stochastic motion of colloidal particles dragged in viscous fluids with space-dependent viscosity and/or temperature.

 

[John86]

Not directly related, but food for thought !

An essay by Pullin, and Gambini
http://fqxi.org/data/essay-contest-files/Pullin_essay.pdf


http://arxiv.org/abs/1207.1635
Time in quantum gravity
Nick Huggett, Tiziana Vistarini, Christian Wuthrich
(Submitted on 3 Jul 2012)
Quantum gravity--the marriage of quantum physics with general relativity--is bound to contain deep and important lessons for the nature of physical time. Some of these lessons shall be canvassed here, particularly as they arise from quantum general relativity and string theory and related approaches. Of particular interest is the question of which of the intuitive aspects of time will turn out to be fundamental, and which 'emergent' in some sense.

http://arxiv.org/abs/1207.1568
The structure of causal sets
Christian Wuthrich
(Submitted on 6 Jul 2012)
More often than not, recently popular structuralist interpretations of physical theories leave the central concept of a structure insufficiently precisified. The incipient causal sets approach to quantum gravity offers a paradigmatic case of a physical theory predestined to be interpreted in structuralist terms. It is shown how employing structuralism lends itself to a natural interpretation of the physical meaning of causal sets theory. Conversely, the conceptually exceptionally clear case of causal sets is used as a foil to illustrate how a mathematically informed rigorous conceptualization of structure serves to identify structures in physical theories. Furthermore, a number of technical issues infesting structuralist interpretations of physical theories such as difficulties with grounding the identity of the places of highly symmetrical physical structures in their relational profile and what may resolve these difficulties can be vividly illustrated with causal sets.

 

[MTd2]

http://arxiv.org/abs/1207.2504

Challenges for Emergent Gravity

S. Carlip
Comments: 18 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
The idea of gravity as an "emergent" phenomenon has gained popularity in recent years. I discuss some of the obstacles that any such model must overcome in order to agree with the observational underpinnings of general relativity.

http://arxiv.org/abs/1207.2509

Gravity's weight on worldline fuzziness

Giovanni Amelino-Camelia, Valerio Astuti, Giacomo Rosati
Comments: 8 pages
Subjects: General Relativity and Quantum Cosmology (gr-qc); High Energy Physics - Theory (hep-th)
We investigate a connection between recent results in 3D quantum gravity, providing an effective noncommutative-spacetime description, and some earlier heuristic descriptions of a quantum-gravity contribution to the fuzziness of the worldlines of particles. We show that 3D-gravity-inspired spacetime noncommutativity reflects some of the features suggested by previous heuristic arguments. Most notably, gravity-induced worldline fuzziness, while irrelevantly small on terrestrial scales, could be observably large for propagation of particles over cosmological distances.

 

[marcus]

http://arxiv.org/abs/1207.2585
Spherically symmetric Einstein-Maxwell theory and loop quantum gravity corrections
Rakesh Tibrewala
(Submitted on 11 Jul 2012)
Effects of inverse triad corrections and (point) holonomy corrections, occurring in loop quantum gravity, are considered on the properties of Reissner-Nordstrom black holes. Version of inverse triad corrections with unmodified constraint algebra reveal the possibility of occurence of three horizons (over a finite range of mass) and also show a mass threshold beyond which the inner horizon disappears. For the version with modified constraint algebra, coordinate transformations are no longer a good symmetry. The covariance property of spacetime is regained by using a quantum notion of mapping from phase space to spacetime. The resulting quantum effects in both versions of these corrections can be associated with renormalization of either mass, charge or wave function. In neither version is the Newton's constant renormalized. (Point) Holonomy corrections are shown to preclude undeformed version of constraint algebra as also a static solution, though time independent solutions exist. Possible reason for difficulty in constructing a covariant metric for these corrections is highlighted. Furthermore, the deformed algebra with holonomy corrections is shown to imply signature change.
36 pages, 9 figures

http://arxiv.org/abs/1207.2323
Modified constraint algebra in loop quantum gravity and spacetime interpretation
Rakesh Tibrewala
(Submitted on 10 Jul 2012)
Classically the constraint algebra of general relativity, which generates gauge transformations, is equivalent to spacetime covariance. In LQG, inverse triad corrections lead to an effective Hamiltonian constraint which can lead to a modified constraint algebra. We show, using example of spherically symmetric spacetimes, that a modified constraint algebra does not correspond to spacetime coordinate transformation. In such a scenario the notion of black hole horizon, which is based on spacetime notions, also needs to be reconsidered. A possible modification to the classical trapping horizon condition leading to consistent results is suggested. In the case where the constraint algebra is not modified a spacetime picture is valid and one finds mass threshold for black holes and small corrections to Hawking temperature.
6 pages. Prepared for ICGC2011, Goa (India) proceedings

 

[marcus]

Relevant to LQG black hole study but also of general interest:
http://arxiv.org/abs/1207.3123
Black Holes: Complementarity or Firewalls?
Ahmed Almheiri, Donald Marolf, Joseph Polchinski, James Sully
(Submitted on 13 Jul 2012)
We argue that the following three statements cannot all be true: (i) Hawking radiation is in a pure state, (ii) the information carried by the radiation is emitted from the region near the horizon, with low energy effective field theory valid beyond some microscopic distance from the horizon, and (iii) the infalling observer encounters nothing unusual at the horizon. Perhaps the most conservative resolution is that the infalling observer burns up at the horizon. Alternatives would seem to require novel dynamics that nevertheless cause notable violations of semiclassical physics at macroscopic distances from the horizon.
19 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1207.4090

Complementarity And Firewalls

Leonard Susskind
(Submitted on 17 Jul 2012)
Almheiri, Marolf, Polchinski, and Sully, recently reported a remarkable and very surprising phenomenon involving old black holes. The authors argue that after a black hole has radiated more than half its initial entropy, the horizon is replaced by a "firewall" at which infalling observers burn up, in apparent violation of one of the postulates of black hole complementarity. In this note I will give a different interpretation of the firewall phenomenon in which the properties of the horizon are conventional, but the dynamics of the singularity are strongly modified. In this formulation the postulates of complementarity are left intact. But the reader is nevertheless warned: black holes could be more dangerous than you thought.

http://arxiv.org/abs/1207.4059

Black Holes as Critical Point of Quantum Phase Transition

Gia Dvali, Cesar Gomez
(Submitted on 17 Jul 2012)
We reformulate the quantum black hole portrait in the language of modern condensed matter physics. We show that black holes can be understood as a graviton Bose-Einstein condensate at the critical point of a quantum phase transition, identical to what has been observed in systems of cold atoms. The Bogoliubov modes that become degenerate and nearly gapless at this point are the holographic quantum degrees of freedom responsible for the black hole entropy and the information storage. They have no (semi)classical counterparts and become inaccessible in this limit. These findings indicate a deep connection between the seemingly remote systems and suggest a new quantum foundation of holography. They also open an intriguing possibility of simulating black hole information processing in table-top labs.

 

[marcus]

http://arxiv.org/abs/1207.4353
Inflation from non-minimally coupled scalar field in loop quantum cosmology
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 18 Jul 2012)
The FRW model with non-minimally coupled massive scalar field has been investigated in LQC framework. Considered form of the potential and coupling allows applications to Higgs driven inflation. The resulting dynamics qualitatively modifies the standard bounce paradigm in LQC in two ways: (i) the bounce point is no longer marked by critical matter energy density, (ii) the Planck scale physics features the "mexican hat" trajectory with two consecutive bounces and rapid expansion and recollapse between them. Furthermore, for physically viable coupling strength and initial data the subsequent inflation exceeds 60 e-foldings.
14 pages, 5 figures

http://arxiv.org/abs/1207.4473
Quantum mechanics in fractional and other anomalous spacetimes
Gianluca Calcagni, Giuseppe Nardelli, Marco Scalisi
(Submitted on 18 Jul 2012)
We formulate quantum mechanics in spacetimes with real-order fractional geometry and more general factorizable measures. In spacetimes where coordinates and momenta span the whole real line, Heisenberg's principle is proven and the wave-functions minimizing the uncertainty are found. In spite of the fact that ordinary time and spatial translations are broken and the dynamics is not unitary, the theory is in one-to-one correspondence with a unitary one, thus allowing us to employ standard tools of analysis. These features are illustrated in the examples of the free particle and the harmonic oscillator. While fractional (and the more general anomalous-spacetime) free models are formally indistinguishable from ordinary ones at the classical level, at the quantum level they differ both in the Hilbert space and for a topological term fixing the classical action in the path integral formulation. Thus, all non-unitarity in fractional quantum dynamics is encoded in a contribution depending only on the initial and final state.
32 pages, 1 figure

 

[John86]

http://arxiv.org/abs/1207.4657
Signature change in loop quantum cosmology
Jakub Mielczarek
(Submitted on 19 Jul 2012)
The Wick rotation is commonly considered only as an useful computational trick. However, as it was suggested by Hartle and Hawking already in early eighties, Wick rotation may gain physical meaning at the Planck epoch. While such possibility is conceptually interesting, leading to no-boundary proposal, mechanism behind the signature change remains mysterious. We show that the signature change anticipated by Hartle and Hawking naturally appear in loop quantum cosmology. Theory of cosmological perturbations with the effects of quantum holonomies is discussed. It was shown by Cailleteau \textit{et al.} (Class. Quant. Grav. {\bf 29} (2012) 095010) that this theory can be uniquely formulated in the anomaly-free manner. The obtained algebra of effective constraints turns out to be modified such that the metric signature is changing from Lorentzian in low curvature regime to Euclidean in high curvature regime. Implications of this phenomenon on propagation of cosmological perturbations are discussed and corrections to inflationary power spectra of scalar and tensor perturbations are derived. Possible relations with other approaches to quantum gravity are outlined. We also propose an intuitive explanation of the observed signature change using analogy with spontaneous symmetry breaking in "wired" metamaterials.

http://arxiv.org/abs/1207.4503
Spontaneous Dimensional Reduction?
S. Carlip
(Submitted on 18 Jul 2012)
Over the past few years, evidence has begun to accumulate suggesting that spacetime may undergo a "spontaneous dimensional reduction" to two dimensions near the Planck scale. I review some of this evidence, and discuss the (still very speculative) proposal that the underlying mechanism may be related to short-distance focusing of light rays by quantum fluctuations

http://arxiv.org/abs/1207.4603
Towards superconformal and quasi-modular representation of exotic smooth R^4 from superstring theory II
Torsten Asselmeyer-Maluga, Jerzy Król
(Submitted on 19 Jul 2012)
This is the second part of the work where quasi-modular forms emerge from small exotic smooth $\mathbb{R}^4$'s grouped in a fixed radial family. SU(2) Seiberg-Witten theory when formulated on exotic $\mathbb{R}^4$ from the radial family, in special foliated topological limit can be described as SU(2) Seiberg-Witten theory on flat standard $\mathbb{R}^4$ with the gravitational corrections derived from coupling to ${\cal N}=2$ supergravity.
Formally, quasi-modular expressions which follow the Connes-Moscovici construction of the universal Godbillon-Vey class of the codimension-1 foliation, are related to topological correlation functions of superstring theory compactified on special Callabi-Yau manifolds. These string correlation functions, in turn, generate Seiberg-Witten prepotential and the couplings of Seiberg-Witten theory to ${\cal N}=2$ supergravity sector. Exotic 4-spaces are conjectured to serve as a link between supersymmetric and non-supersymmetric Yang-Mills theories in dimension 4.

http://arxiv.org/abs/1207.4602
Towards superconformal and quasi-modular representation of exotic smooth R^4 from superstring theory I
Torsten Asselmeyer-Maluga, Jerzy Król
(Submitted on 19 Jul 2012)
We show that superconformal ${\cal N}=4,2$ algebras are well-suited to represent some invariant constructions characterizing exotic $\mathbb{R}^4$ relative to a given radial family. We examine the case of ${\cal N}=4, \hat{c}=4$ (at $r=1$ level) superconformal algebra which is realized on flat $\mathbb{R}^4$ and curved $S^3\times \mathbb{R}$. While the first realization corresponds naturally to standard smooth $\mathbb{R}^4$ the second describes the algebraic end of some small exotic smooth $\mathbb{R}^4$'s from the radial family of DeMichelis-Freedman and represents the linear dilaton background $SU(2)_k\times \mathbb{R}_Q$ of superstring theory.
From the modular properties of the characters of the algebras one derives Witten-Reshetikhin-Turaev and Chern-Simons invariants of homology 3-spheres. These invariants are represented rather by false, quasi-modular, Ramanujan mock-type functions. Given the homology 3-spheres one determines exotic smooth structures of Freedman on $S^3\times \mathbb{R}$. In this way the fake ends are related to the SCA ${\cal N}=4$ characters.
The case of the ends of small exotic $\mathbb{R}^4$'s is more complicated. One estimates the complexity of exotic $\mathbb{R}^4$ by the minimal complexity of some separating from the infinity 3-dimensional submanifold. These separating manifolds can be chosen, in some exotic $\mathbb{R}^4$'s, to be homology 3-spheres. The invariants of such homology 3-spheres are, again, obtained from the characters of SCA, ${\cal N}=4$.

 

[marcus]

http://arxiv.org/abs/1207.4596
The Construction of Spin Foam Vertex Amplitudes
Eugenio Bianchi, Frank Hellmann
(Submitted on 19 Jul 2012)
Spin foam vertex amplitudes are the key ingredient of spin foam models for quantum gravity. They fall into the realm of discretized path integral, and can be seen as generalized lattice gauge theories. They can be seen as an attempt at a 4 dimensional generalization of the Ponzano-Regge model for 3d quantum gravity. We motivate and review the construction of the vertex amplitudes of recent spin foam models, giving two different and complementary perspectives of this construction. The first proceeds by extracting geometric configurations from a topological theory of the BF type, and can be seen to be in the tradition of the work of Barret and Crane and Freidel and Krasnov. The second keeps closer contact to the structure of Loop Quantum Gravity and tries to identify an appropriate set of constraints to define a Lorentz-invariant interaction of its quanta of space. This approach is in the tradition of the work of Smolin, Markopoulous, Engle, Pereira, Rovelli and Livine.
22 Pages. 1 Figure. Invited review for SIGMA Special Issue "Loop Quantum Gravity and Cosmology"

Link to the SIGMA Special Issue TOC (still in progress):
http://www.emis.de/journals/SIGMA/LQGC.html

http://arxiv.org/abs/1207.4689
On the Nature of Black Holes in Loop Quantum Gravity
Christian Röken
(Submitted on 19 Jul 2012)
A genuine notion of black holes can only be obtained in the fundamental framework of quantum gravity resolving the curvature singularities and giving an account of the statistical mechanical, microscopic degrees of freedom able to explain the black hole thermodynamic properties. As for all quantum systems, a quantum realization of black holes requires an operator algebra of the fundamental observables of the theory which is introduced in this study based on aspects of loop quantum gravity. From the eigenvalue spectra of the operators for the black hole area, charge and angular momentum, it is demonstrated that a more general bound on the extensive parameters, other than the relation arising in general relativity, holds, implying that the extremal black hole state can merely be reached asymptotically, while the lowest eigenvalue of the black hole mass spectrum indicates, on the one hand, a Planck scale cutoff ending the final phase of the evaporation process with a massive, ultra-dense, extremely hot remnant and, on the other hand, gives a rough estimate of the numerical value of the Immirzi parameter. This analysis provides an approximative description of the dynamics and the nature of quantum black holes.
15 pages

 

[marcus]

http://arxiv.org/abs/1207.5156
Divergences and Orientation in Spinfoams
Marios Christodoulou, Miklos Långvik, Aldo Riello, Christian Röken, Carlo Rovelli
(Submitted on 21 Jul 2012)
We suggest that large radiative corrections appearing in the spinfoam framework might be tied to the implicit sum over orientations. Specifically, we show that in a suitably simplified context the characteristic "spike" divergence of the Ponzano-Regge model disappears when restricting the theory to just one of the two orientations appearing in the asymptotic limit of the vertex amplitude.
10 pages, 5 figures

not Loop QG related but possibly of some general interest:
http://arxiv.org/abs/1207.5501
SUGRA Grand Unification, LHC and Dark Matter
Pran Nath
(Submitted on 23 Jul 2012)
A brief review is given of recent developments related to the Higgs signal and its implications for supersymmetry in the supergravity grand unification framework. The Higgs data indicates that the allowed parameter space largely lies on focal curves and focal surfaces of the Hyperbolic Branch of radiative breaking of the electroweak symmetry where TeV size scalars naturally arise. The high mass of the Higgs leads to a more precise prediction for the allowed range of the spin independent neutralino -proton cross section which is encouraging for the detection of dark matter in future experiments with greater sensitivity. Also discussed is the status of grand unification and a natural solution to breaking the GUT group at one scale and resolving the doublet-triplet problem. It is shown that the cosmic coincidence can be compatible within a supersymmetric framework in a muticomponent dark matter picture with one component charged under $B-L$ while the other component is the conventional supersymmetric dark matter candidate, the neutralino.
11 pages. Based on Talk at Pascos2012, Merida, Mexico, Jun 3-8, 2012

 

[marcus]

http://arxiv.org/abs/1207.5601
Dynamical eigenfunctions and critical density in loop quantum cosmology
David A. Craig
(Submitted on 24 Jul 2012)
We offer a new, physically transparent argument for the existence of the critical, universal maximum matter density in loop quantum cosmology for the case of a flat Friedmann-Lemaitre-Robertson-Walker cosmology with scalar matter. The argument is based on the existence of a sharp exponential ultraviolet cutoff in momentum space on the eigenfunctions of the quantum cosmological dynamical evolution operator (the gravitational part of the Hamiltonian constraint), attributable to the fundamental discreteness of spatial volume in loop quantum cosmology. The existence of the cutoff is proved directly from recently found exact solutions for the eigenfunctions for this model. As a consequence, the operators corresponding to the momentum of the scalar field and the spatial volume approximately commute. The ultraviolet cutoff then implies that the scalar momentum, though not a bounded operator, is in effect bounded on subspaces of constant volume, leading to the upper bound on the expectation value of the matter density. The maximum matter density is independent of the quantum state essentially because of the linear scaling of the cutoff with volume. These heuristic arguments are supplemented by a new proof in the volume representation of the existence of the maximum matter density. The techniques employed to demonstrate the existence of the cutoff also allow us to extract the large volume limit of the exact eigenfunctions, confirming earlier numerical and analytical work showing the eigenfunctions approach superpositions of the eigenfunctions of the Wheeler-DeWitt quantization of the same model. We argue that generic (not just semiclassical) quantum states approach symmetric superpositions of expanding and contracting universes.
23 pages, 8 figures

http://arxiv.org/abs/1207.5730
Self-adjointness in the Hamiltonians of deparameterized totally constrained theories: a model
Rodolfo Gambini, Jorge Pullin
(Submitted on 24 Jul 2012)
Several proposals to deal with the dynamics of general relativity involve gauge fixings or the introduction matter fields in terms of which the theory is deparameterized. The resulting theories have true Hamiltonians for their evolution that usually involve square roots, and this poses certain challenges for their implementation as self-adjoint quantum operators. We show in the context of a simple model of totally constrained theory that one can introduce related, well defined operators that reproduce semiclassically the same physics as the original ones, at least for states peaked in the regions of phase space where their associated classical quantities are well defined.
5 pages

 

[marcus]

http://arxiv.org/abs/1207.6348
The twistorial structure of loop-gravity transition amplitudes
Simone Speziale, Wolfgang M. Wieland
(Submitted on 26 Jul 2012)
The spin foam formalism provides transition amplitudes for loop quantum gravity. Important aspects of the dynamics are understood, but many open questions are pressing on. In this paper we address some of them using a twistorial description, which brings new light on both classical and quantum aspects of the theory. At the classical level, we clarify the covariant properties of the discrete geometries involved, and the role of the simplicity constraints in leading to SU(2) Ashtekar-Barbero variables. We identify areas and Lorentzian dihedral angles in twistor space, and show that they form a canonical pair. The primary simplicity constraints are solved by simple twistors, parametrized by SU(2) spinors and the dihedral angles. We construct an SU(2) holonomy and prove it to correspond to the Ashtekar-Barbero connection. We argue that the role of secondary constraints is to provide a non trivial embedding of the cotangent bundle of SU(2) in the space of simple twistors. At the quantum level, a Schroedinger representation leads to a spinorial version of simple projected spin networks, where the argument of the wave functions is a spinor instead of a group element. We rewrite the Liouville measure on the cotangent bundle of SL(2,C) as an integral in twistor space. Using these tools, we show that the Engle-Pereira-Rovelli-Livine transition amplitudes can be derived from a path integral in twistor space. We construct a curvature tensor, show that it carries torsion off-shell, and that its Riemann part is of Petrov type D. Finally, we make contact between the semiclassical asymptotic behaviour of the model and our construction, clarifying the relation of the Regge geometries with the original phase space.
39 pages

perhaps of some general interest:
http://arxiv.org/abs/1207.6243
Complementarity, not Firewalls
Daniel Harlow
(Submitted on 26 Jul 2012)
In this note I argue that a strict interpretation of complementarity is possible which evades the need for the "firewalls" recently proposed by Almheiri, Marolf, Polchinski, and Sully to burn up observers falling into black hole horizons. In particular I argue that it is consistent for an infalling observer to fall through an "old" black hole horizon without burning up, without this observer or any other seeing information loss or a violation of low energy effective field theory away from a stretched horizon. The reason that AMPS find the opposite conclusion is because they attempt to use low energy physics to translate between the quantum mechanics of different observers rather than to describe the experiments of only a single observer. The validity of this position is tested by two concrete calculations.
11 pages, 3 figures

http://arxiv.org/abs/1207.6370
A no-go theorem for slowly rotating black holes in Horava-Lifgarbagez gravity
Enrico Barausse, Thomas P. Sotiriou
(Submitted on 26 Jul 2012)
We consider slowly rotating, stationary, axisymmetric black holes in the infrared limit of Horava-Lifgarbagez gravity. We show that such solutions do not exist, provided that they are regular everywhere apart from the central singularity, and we comment on the implications for the viability of the theory.
5 pages, 1 figure