Loop-and-allied QG bibliography

[John86]

http://arxiv.org/abs/1401.3416
Wormholes and Entanglement
John C. Baez, Jamie Vicary
(Submitted on 15 Jan 2014)
Maldacena and Susskind have proposed a correspondence between wormholes and entanglement, dubbed ER=EPR. We study this in the context of 3d topological quantum field theory, where we show that the formation of a wormhole is the same process as creating a particle-antiparticle pair. A key feature of the ER=EPR proposal is that certain apparently entangled degrees of freedom turn out to be the same. We name this phenomenon "fake entanglement", and show how it arises in our topological quantum field theory model.

http://arxiv.org/abs/1401.3578
Conceptual Problems in Quantum Gravity and Quantum Cosmology
Claus Kiefer
(Submitted on 15 Jan 2014)
The search for a consistent and empirically established quantum theory of gravity is among the biggest open problems of fundamental physics. The obstacles are of formal and of conceptual nature. Here, I address the main conceptual problems, discuss their present status and outline further directions of research. For this purpose, the main current approaches to quantum gravity are briefly reviewed and compared.

http://arxiv.org/abs/1401.3393
Universal coordinates for Schwarzschild black holes
W G Unruh
(Submitted on 15 Jan 2014)
A variety of historical coordinates in which the Schwarzschild metric is regular over the whole of the extended spacetime are compared and the hypersurfaces of constant coordinate are graphically presented. While the Kruscal form (one of the later forms) is probably the simplest, each of the others has some interesting features.

 

[MTd2]

http://arxiv.org/abs/1401.4452

Black holes within Asymptotic Safety

Benjamin Koch, Frank Saueressig
(Submitted on 17 Jan 2014)
Black holes are probably among the most fascinating objects populating our universe. Their characteristic features found within general relativity, encompassing spacetime singularities, event horizons, and black hole thermodynamics, provide a rich testing ground for quantum gravity ideas. We review the status of black holes within a particular proposal for quantum gravity, Weinberg's asymptotic safety program. Starting from a brief survey of the effective average action and scale setting procedures, an improved quantum picture of the black hole is developed. The Schwarzschild black hole and its generalizations including angular momenta, higher-derivative corrections and the implications of extra dimensions are discussed in detail. In addition, the quantum singularity emerging for the inclusion of a cosmological constant is elucidated and linked to the phenomenon of a dynamical dimensional reduction of spacetime.

 

[marcus]

http://arxiv.org/abs/1401.5262
Spacetime thermodynamics without hidden degrees of freedom
Goffredo Chirco, Hal M. Haggard, Aldo Riello, Carlo Rovelli
(Submitted on 21 Jan 2014)
A celebrated result by Jacobson is the derivation of Einstein's equations from Unruh's temperature, the Bekenstein-Hawking entropy and the Clausius relation. This has been repeatedly taken as evidence for an interpretation of Einstein's equations as equations of state for unknown degrees of freedom underlying the metric. We show that a different interpretation of Jacobson result is possible, which does not imply the existence of additional degrees of freedom, and follows only from the quantum properties of gravity. We introduce the notion of quantum gravitational Hadamard states, which give rise to the full local thermodynamics of gravity.
12 pages, 1 figure

http://arxiv.org/abs/1401.5083
Non-Commutative Geometry, Non-Associative Geometry and the Standard Model of Particle Physics
Latham Boyle, Shane Farnsworth
(Submitted on 20 Jan 2014)
Connes has developed a notion of non-commutative geometry (NCG) that generalizes Riemannian geometry, and provides a framework in which the standard model of particle physics, coupled to Einstein gravity, may be concisely and elegantly reformulated. We point out that his formalism may be recast in a way that generalizes immediately from non-commutative to non-associative geometry. In the process, several of the standard axioms and formulae are conceptually reinterpreted. This reformulation also suggests a new constraint on the finite NCG corresponding to the standard model of particle physics. Remarkably, this new condition resolves a long-standing puzzle about the NCG embedding of the standard model, by precisely eliminating from the action the collection of 7 unwanted terms that previously had to be removed by an extra (empirically-motivated) assumption.
5 pages

http://arxiv.org/abs/1401.4275
Strict Deformation Quantisation of the G-connections via Lie Groupoid
Alan Lai
(Submitted on 17 Jan 2014)
Motivated by the compactification process of the space of connections in loop quantum gravity literature. A description of the space of G-connections using the tangent groupoid is given. As the tangent groupoid parameter is away from zero, the G-connections are (strictly) deformation quantised to noncommuting elements using C*-algebraic formalism. The approach provides a means to obtain a semi-classical limit in loop quantum gravity.
9 pages

http://arxiv.org/abs/1401.5352
Effective Casimir Conditions and Group Coherent States
Martin Bojowald, Artur Tsobanjan
(Submitted on 21 Jan 2014)
Properties of group coherent states can be derived "effectively" without knowing full wave functions. The procedure is detailed in this article as an example of general methods for effective constraints. The role of constraints in the present context is played by a Casimir condition that puts states within an irreducible representation of a Lie group (or, equivalently, on a quantization of a co-adjoint orbit of the dual Lie algebra). Simplifications implied by a Casimir condition, compared with general first-class constraints, allows one to show that the correct number of degrees of freedom is obtained after imposing the condition. When combined with conditions to saturate uncertainty relations, moments of group coherent states can be derived. A detailed example in quantum cosmology (cosmic forgetfulness) illustrates the usefulness of the methods.
23 pages

http://arxiv.org/abs/1401.5313
Hot big bang or slow freeze?
C.Wetterich
(Submitted on 21 Jan 2014)
9 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1401.5495
RG flows of Quantum Einstein Gravity on maximally symmetric spaces
Maximilian Demmel, Frank Saueressig, Omar Zanusso
(Submitted on 21 Jan 2014)
We use the Wetterich-equation to study the renormalization group flow of f(R)-gravity in a three-dimensional, conformally reduced setting. Building on the exact heat kernel for maximally symmetric spaces, we obtain a partial differential equation which captures the scale-dependence of f(R) for positive and, for the first time, negative scalar curvature. The effects of different background topologies are studied in detail and it is shown that they affect the gravitational RG flow in a way that is not visible in finite-dimensional truncations. Thus, while featuring local background independence, the functional renormalization group equation is sensitive to the topological properties of the background. The detailed analytical and numerical analysis of the partial differential equation reveals two globally well-defined fixed functionals with at most a finite number of relevant deformations. Their properties are remarkably similar to two of the fixed points identified within the R2-truncation of full Quantum Einstein Gravity. As a byproduct, we obtain a nice illustration of how the functional renormalization group realizes the "integrating out" of fluctuation modes on the three-sphere.
35 pages, 6 figures

http://arxiv.org/abs/1401.5256
Effective dynamics of scalar perturbations in a flat Friedmann-Robertson-Walker spacetime in Loop Quantum Cosmology
Mikel Fernández-Méndez, Guillermo A. Mena Marugán, Javier Olmedo
(Submitted on 21 Jan 2014)
We study the evolution of a homogeneous and isotropic spacetime whose spatial sections have three-torus topology, coupled to a massless scalar field with small scalar perturbations within loop quantum cosmology. We consider a proposal for the effective dynamics based on a previous hybrid quantization completed by us. Consequently, we introduce a convenient gauge fixing and adopt reduced canonical variables adapted to that hybrid quantum description. Besides, we keep backreaction contributions on the background coming from terms quadratic in the perturbations in the action of the system. We carry out a numerical analysis assuming that the inhomogeneities were in a massless vacuum state at distant past (where the initial data are set). At distant future, we observe a statistical amplification of the modes amplitude in the infrared region, as well as a phase synchronization arising from quantum gravity phenomena. A description of the perturbations in terms of the Mukhanov-Sasaki gauge invariants provides the same qualitative results. Finally, we analyze some consequences of the backreaction in our effective description.
15 pages, 9 figures. Accepted for publication in Phys. Rev. D

not QG but possibly of general interest:
http://arxiv.org/abs/1401.5761
Information Preservation and Weather Forecasting for Black Holes
S. W. Hawking
(Submitted on 22 Jan 2014)
It has been suggested [1] that the resolution of the information paradox for evaporating black holes is that the holes are surrounded by firewalls, bolts of outgoing radiation that would destroy any infalling observer. Such firewalls would break the CPT invariance of quantum gravity and seem to be ruled out on other grounds. A different resolution of the paradox is proposed, namely that gravitational collapse produces apparent horizons but no event horizons behind which information is lost. This proposal is supported by ADS-CFT and is the only resolution of the paradox compatible with CPT. The collapse to form a black hole will in general be chaotic and the dual CFT on the boundary of ADS will be turbulent. Thus, like weather forecasting on Earth, information will effectively be lost, although there would be no loss of unitarity.
4 pages, talk given at KITP workshop, Santa Barbara, August 2013

 

[marcus]

http://arxiv.org/abs/1401.5819
Asymptotic analysis of Ponzano-Regge model with non-commutative metric variables
Daniele Oriti, Matti Raasakka
(Submitted on 22 Jan 2014)
We apply the non-commutative Fourier transform for Lie groups to formulate the non-commutative metric representation of the Ponzano-Regge spin foam model for 3d quantum gravity. The non-commutative representation allows to express the amplitudes of the model as a first order phase space path integral, whose properties we consider. In particular, we study the asymptotic behavior of the path integral in the semi-classical limit. First, we compare the stationary phase equations in the classical limit for three different non-commutative structures corresponding to symmetric, Duflo and Freidel-Livine-Majid quantization maps. We find that in order to unambiguously recover discrete geometric constraints for non-commutative metric data through stationary phase method, the deformation structure of the phase space must be accounted for in the variational calculus. When this is understood, our results demonstrate that the non-commutative metric representation facilitates a convenient semi-classical analysis of the Ponzano-Regge model, which yields as the dominant contribution to the amplitude the cosine of the Regge action in agreement with previous studies. We also consider the asymptotics of the SU(2) 6j-symbol using the non-commutative phase space path integral for the Ponzano-Regge model, and explain the connection of our results to the previous asymptotic results in terms of coherent states.
32 pages, 2 figures

 

[marcus]

http://arxiv.org/abs/1401.6441
A new vacuum for Loop Quantum Gravity
Bianca Dittrich, Marc Geiller
(Submitted on 24 Jan 2014)
We construct a new vacuum for loop quantum gravity, which is dual to the Ashtekar-Lewandowski vacuum. Because it is based on BF theory, this new vacuum is physical for (2+1)-dimensional gravity, and much closer to the spirit of spin foam quantization in general. To construct this new vacuum and the associated representation of quantum observables, we introduce a modified holonomy-flux algebra which is cylindrically consistent with respect to the notion of refinement by time evolution suggested in [1]. This supports the proposal for a construction of a physical vacuum made in [1,2], also for (3+1)-dimensional gravity. We expect that the vacuum introduced here will facilitate the extraction of large scale physics and cosmological predictions from loop quantum gravity.
10 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1401.6562
Planck stars
Carlo Rovelli, Francesca Vidotto
(Submitted on 25 Jan 2014)
A star that collapses gravitationally can reach a further stage of its life, where quantum-gravitational pressure counteracts weight. The duration of this stage is very short in the star proper time, yielding a bounce, but extremely long seen from the outside, because of the huge gravitational time dilation. Since the onset of quantum-gravitational effects is governed by energy density --not by size-- the star can be much larger than Planckian in this phase. The object emerging at the end of the Hawking evaporation of a black hole can then be larger than Planckian by a factor (m/m_P)ⁿ, where m is the mass fallen into the hole, m_P is the Planck mass, and n is positive. The existence of these objects alleviates the black-hole information paradox. More interestingly, these objects could have astrophysical and cosmological interest: they produce a detectable signal, of quantum gravitational origin, around the 10⁻¹⁴cm wavelength.
5 pages, 3 figures. Nice paper.

 

[marcus]

http://arxiv.org/abs/1401.6940
Towards measuring the Archimedes force of vacuum
Enrico Calloni, Martina De Laurentis, Rosario De Rosa, Luciano Di Fiore, Giampiero Esposito, Fabio Garufi, Luigi Rosa, Carlo Rovelli, Paolo Ruggi, Francesco Tafuri
(Submitted on 27 Jan 2014)
We discuss the force exerted by the gravitational field on a Casimir cavity in terms of Archimedes' force of vacuum, we identify the force that can be tested against observation and we show that the present technology makes it possible to perform the first experimental tests. We motivate the use of suitable high-Tc superconductors as modulators of Archimedes' force. We analyze the possibility of using gravitational wave interferometers as detectors of the force, transported through an optical spring from the Archimedes vacuum force apparatus to the gravitational interferometers test masses to maintain the two systems well separated. We also analyze the use of balances to actuate and detect the force, we compare different solutions and discuss the most important experimental issues.
20 pages, 6 figures

http://arxiv.org/abs/1401.7358
Propagator with Positive Cosmological Constant in the 3D Euclidian Quantum Gravity Toy Model
William Bunting, Carlo Rovelli
(Submitted on 28 Jan 2014)
We study the propagator on a single tetrahedron in a three dimensional toy model of quantum gravity with positive cosmological constant. The cosmological constant is included in the model via q-deformation of the spatial symmetry algebra, that is, we use the Tuarev-Viro amplitude. The expected repulsive effect of dark energy is recovered in numerical and analytic calculations of the propagator at large scales comparable to the infrared cutoff. However, due to the simplicity of the model we do not obtain the exact Newton limit of the propagator. This is a first step toward the similar calculation in the full 3+1 dimensional theory with larger numbers of simplicies.
7 pages, 2 figures

http://pirsa.org/14010098/
Black hole entropy and the case for self-dual loop quantum gravity
Marc Geiller
By focusing on aspects of black hole thermodynamics, I will present some evidences supporting the unexpected role of the complex self-dual variables in quantum gravity. This will also be the occasion of revisiting some aspects of three-dimensional gravity, and in particular the link between the BTZ black hole and the Turaev-Viro state sum model.
Date: 29/01/2014 - 3:30 pm

 

[marcus]

http://arxiv.org/abs/1401.6062
Quantization of systems with temporally varying discretization I: Evolving Hilbert spaces
Philipp A Hoehn
(Submitted on 23 Jan 2014)
A temporally varying discretization often features in discrete gravitational systems and appears in lattice field theory models subject to a coarse graining or refining dynamics. To better understand such discretization changing dynamics in the quantum theory, an according formalism for constrained variational discrete systems is constructed. While the present manuscript focuses on global evolution moves and, for simplicity, restricts to Euclidean configuration spaces, a companion article discusses local evolution moves. In order to link the covariant and canonical picture, the dynamics of the quantum states is generated by propagators which satisfy the canonical constraints and are constructed using the action and group averaging projectors. This projector formalism offers a systematic method for tracing and regularizing divergences in the resulting state sums. Non-trivial coarse graining evolution moves lead to non-unitary, and thus irreversible, projections of physical Hilbert spaces and Dirac observables such that these concepts become evolution move dependent on temporally varying discretizations. The formalism is illustrated in a toy model mimicking a `creation from nothing'. Subtleties arising when applying such a formalism to quantum gravity models are discussed.
44 pages, 1 appendix, 6 figures

http://arxiv.org/abs/1401.7731
Quantization of systems with temporally varying discretization II: Local evolution moves
Philipp A Hoehn
(Submitted on 30 Jan 2014)
Several quantum gravity approaches and field theory on an evolving lattice involve a discretization changing dynamics generated by evolution moves. Local evolution moves in variational discrete systems (1) are a generalization of the Pachner evolution moves of simplicial gravity models, (2) update only a small subset of the dynamical data, (3) change the number of kinematical and physical degrees of freedom, and (4) generate a dynamical coarse graining or refining of the underlying discretization. To systematically explore such local moves and their implications in the quantum theory, this article suitably expands the quantum formalism for global evolution moves, constructed in a companion paper, by employing that global moves can be decomposed into sequences of local moves. This formalism is spelled out for systems with Euclidean configuration spaces. Various types of local moves, the different kinds of constraints generated by them, the constraint preservation and possible divergences in resulting state sums are discussed. It is shown that non-trivial local coarse graining moves entail a non-unitary projection of (physical) Hilbert spaces and `fine grained' Dirac observables defined on them. Identities for undoing a local evolution move with its (time reversed) inverse are derived. Finally, the implications of these results for a Pachner move generated dynamics in simplicial quantum gravity models are commented on.
36 pages, many figures, 2 appendices (5 pages)

 

[marcus]

http://arxiv.org/abs/1402.1038
A note on entanglement entropy and quantum geometry
Norbert Bodendorfer
(Submitted on 5 Feb 2014)
It has been argued that the entropy which one is computing in the isolated horizon framework of loop quantum gravity is closely related to the entanglement entropy of the gravitational field and that the calculation performed is not restricted to horizons. We recall existing work on this issue and explain how recent work on generalising these computations to arbitrary spacetime dimensions D+1>2 supports this point of view and makes the duality between entanglement entropy and the entropy computed from counting boundary states manifest. In a certain semiclassical regime in 3+1 dimensions, this entropy is given by the Bekenstein-Hawking formula.
14 pages

 

[marcus]

http://arxiv.org/abs/1402.1427
The Black Hole Uncertainty Principle Correspondence
B. J. Carr
(Submitted on 6 Feb 2014)
The Black Hole Uncertainty Principle correspondence proposes a connection between the Uncertainty Principle on microscopic scales and black holes on macroscopic scales. This is manifested in a unified expression for the Compton wavelength and Schwarzschild radius. It is a natural consequence of the Generalized Uncertainty Principle, which suggests corrections to the Uncertainty Principle as the energy increases towards the Planck value. It also entails corrections to the event horizon size as the black hole mass falls to the Planck value, leading to the concept of a Generalized Event Horizon. One implication of this is that there could be sub-Planckian black holes with a size of order their Compton wavelength. Loop quantum gravity suggests the existence of black holes with precisely this feature. The correspondence leads to a heuristic derivation of the black hole temperature and suggests how the Hawking formula is modified in the sub-Planckian regime.
8 pages, 4 figures, to appear in Proceedings of 2013 Karl Schwarzschild Meeting on Gravitational Physics.

http://arxiv.org/abs/1402.1437
Primordial Black Holes and Quantum Effects
B. J. Carr
(Submitted on 6 Feb 2014)
Primordial black holes (PBHs) are of special interest because of the crucial role of quantum effects in their formation and evaporation. This means that they provide a unique probe of the early universe, high-energy physics and quantum gravity. We highlight some recent developments in the subject, including improved limits on the fraction of the Universe going into evaporating PBHs in the mass range 10⁹−10¹⁷ g and the possibility of using PBHs to probe a cosmological bounce.
8 pages, 6 figures, to appear in Proceedings of 2013 Karl Schwarzschild Meeting on Gravitational Physics.

 

[marcus]

http://arxiv.org/abs/1402.2084
Black Hole Entropy in Loop Quantum Gravity, Analytic Continuation, and Dual Holography
Muxin Han
(Submitted on 10 Feb 2014)
A new approach to black hole thermodynamics is proposed in Loop Quantum Gravity (LQG), by defining a new black hole partition function, followed by analytic continuations of Barbero-Immirzi parameter to γ ∈ iℝ and Chern-Simons level to k∈iℝ. The analytic continued partition function has remarkable features: The black hole entropy S=A/4ℓ²_P is reproduced correctly for infinitely many γ=iη, at least for η ∈ ℤ∖{0}. The near-horizon Unruh temperature emerges as the pole of partition function. Interestingly, by analytic continuation the partition function can have a dual statistical interpretation corresponding to a dual quantum theory of γ ∈ iℤ. The dual quantum theory implies a semiclassical area spectrum for γ ∈ iℤ. It also implies that at a given near horizon (quantum) geometry, the number of quantum states inside horizon is bounded by a holographic degeneracy d=e^A/4ℓ_P, which produces the Bekenstein bound from LQG.
On the other hand, the result in http://arxiv.org/abs/1212.4060 receives a justification here.
5 pages

 

[marcus]

http://arxiv.org/abs/1402.2323
Deformed phase space for 3d loop gravity and hyperbolic discrete geometries
Valentin Bonzom, Maité Dupuis, Florian Girelli, Etera R. Livine
(Submitted on 10 Feb 2014)
We revisit the loop gravity space phase for 3D Riemannian gravity by algebraically constructing the phase space T*SU(2)∼ISO(3) as the Heisenberg double of the Lie group SO(3) provided with the trivial cocyle. Tackling the issue of accounting for a non-vanishing cosmological constraint Λ≠0 in the canonical framework of 3D loop quantum gravity, SL(2,ℂ) viewed as the Heisenberg double of SU(2) provided with a non-trivial cocyle is introduced as a phase space. It is a deformation of the flat phase space ISO(3) and reproduces the latter in a suitable limit. The SL(2,ℂ) phase space is then used to build a new, deformed LQG phase space associated to graphs. It can be equipped with a set of Gauss constraints and flatness constraints, which form a first class system and Poisson-generate local 3D rotations and deformed translations. We provide a geometrical interpretation for this lattice phase space with constraints in terms of consistently glued hyperbolic triangles, i.e. hyperbolic discrete geometries, thus validating our construction as accounting for a constant curvature Λ<0. Finally, using ribbon diagrams, we show that our new model is topological.
30 pages 12 figures

http://arxiv.org/abs/1402.2274
Anyonic statistics and large horizon diffeomorphisms for Loop Quantum Gravity Black Holes
Andreas G. A. Pithis, Hans-Christian Ruiz Euler
(Submitted on 10 Feb 2014)
In this article we investigate the role played by large diffeomorphisms of quantum Isolated Horizons for the statistics of LQG Black Holes by means of their relation to the braid group. To this aim the symmetries of Chern-Simons theory are recapitulated with particular regard to the aforementioned type of diffeomorphisms. For the punctured spherical horizon, these are elements of the mapping class group of S², which is almost isomorphic to a corresponding braid group on this particular manifold. The mutual exchange of quantum entities in 2-dimensions is communicated by the braid group, rendering the statistics anyonic. With this we argue that the quantum Isolated Horizon model of LQG based on SU(2)_k-Chern-Simons theory exhibits non-abelian anyonic statistics. In this way a connection to theory behind the fractional quantum Hall effect and that of topological quantum computation is established, where non-abelian anyons play a significant role.
17 pages, largely based on work taken from AP's diploma thesis from 09/2012

http://arxiv.org/abs/1402.2384
Turaev-Viro amplitudes from 2+1 Loop Quantum Gravity
Daniele Pranzetti
(Submitted on 11 Feb 2014)
The Turaev-Viro state sum model provides a covariant spin foam quantization of three-dimensional Riemannian gravity with a positive cosmological constant Λ. We complete the program to canonically quantize the theory in the BF formulation using the formalism of Loop Quantum Gravity. In particular, we show first how quantum group structures arise from the requirement of the constraint algebra to be anomaly free. This allows us to generalize the construction of the physical scalar product, from the Λ = 0 case, in presence of a positive Λ. We prove the equivalence between the covariant and canonical quantizations by recovering the spin foam amplitudes.
23 pages, many figures

 

[marcus]

Non-technical piece on QG for wide audience:
http://arxiv.org/abs/1402.2757
Quantum Gravity for Dummies
Deepak Vaid
(Submitted on 12 Feb 2014)
I have been asked to write brief, gentle introduction to the basic idea behind the field of "quantum gravity" in 1500 words or less. Doing so appears to be almost as great a challenge as coming up with a consistent theory of quantum gravity. However, I will try. Disclaimer: The views expressed in this article are my own and do not represent the consensus of the quantum gravity community.
7 pages; non-technical article; invited contribution for NITK annual magazine "Vitruvian"

 

[marcus]

http://arxiv.org/abs/1402.3155
Quantum Reduced Loop Gravity: Semiclassical limit
Emanuele Alesci, Francesco Cianfrani
(Submitted on 13 Feb 2014)
We discuss the semiclassical limit of Quantum Reduced Loop Gravity, a recently proposed model to address the quantum dynamics of the early Universe. We apply the techniques developed in full Loop Quantum Gravity to define the semiclassical states in the kinematical Hilbert space and evaluating the expectation value of the euclidean scalar constraint we demonstrate that it coincides with the classical expression, i.e. the one of a local Bianchi I dynamics. The result holds as a leading order expansion in the scale factors of the Universe and opens the way to study the subleading corrections to the semiclassical dynamics. We outline how by retaining a suitable finite coordinate length for holonomies our effective Hamiltonian at the leading order coincides with the one expected from LQC. This result is an important step in fixing the correspondence between LQG and LQC.
23 pages

http://arxiv.org/abs/1402.3071
On R+αR² Loop Quantum Cosmology
J. Amorós, J. de Haro, S.D. Odintsov
(Submitted on 13 Feb 2014)
Working in Einstein frame we introduce, in order to avoid singularities, holonomy corrections to the f(R)=R+αR² model. We perform a detailed analytical and numerical study when holonomy corrections are taken into account in both Jordan and Einstein frames obtaining, in Jordan frame, a dynamics which differs qualitatively, at early times, from the one of the original model. More precisely, when holonomy corrections are taken into account the universe is not singular, starting at early times in the contracting phase and bouncing to enter in the expanding one where, as in the original model, it inflates. This dynamics is completely different from the one obtained in the original R+αR² model, where the universe is singular at early times and never bounces. Moreover, we show that these holonomy corrections may lead to better predictions for the inflationary phase as compared with current observations.
22 pages, 5 figures

http://arxiv.org/abs/1402.3009
Non-singular bounce scenarios in loop quantum cosmology and the effective field description
Yi-Fu Cai, Edward Wilson-Ewing
(Submitted on 13 Feb 2014)
A non-singular bouncing cosmology is generically obtained in loop quantum cosmology due to non-perturbative quantum gravity effects. A similar picture can be achieved in standard general relativity in the presence of a scalar field with a non-standard kinetic term such that at high energy densities the field evolves into a ghost condensate and causes a non-singular bounce. During the bouncing phase, the perturbations can be stabilized by introducing a Horndeski operator. Taking the matter content to be a dust field and an ekpyrotic scalar field, we compare the dynamics in loop quantum cosmology and in a non-singular bouncing effective field model with a non-standard kinetic term at both the background and perturbative levels. We find that these two settings share many important properties, including the result that they both generate scale-invariant scalar perturbations. This shows that some quantum gravity effects of the very early universe may be mimicked by effective field models.
12 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1402.3586
LQG for the Bewildered
Sundance Bilson-Thompson, Deepak Vaid
(Submitted on 14 Feb 2014)
We present a pedagogical introduction to the notions underlying the connection formulation of General Relativity - Loop Quantum Gravity (LQG) - with an emphasis on the physical aspects of the framework. We explain, in a concise and clear manner, the steps leading from the Einstein-Hilbert action for gravity to the construction of the quantum states of geometry, known as spin-networks, which provide the basis for the kinematical Hilbert space of quantum general relativity. Along the way we introduce the various associated concepts of tetrads, spin-connection and holonomies which are a pre-requisite for understanding the LQG formalism. Having provided a minimal introduction to the LQG framework, we discuss its applications to the problems of black hole entropy and of quantum cosmology. A list of the most common criticisms of LQG is presented, which are then tackled one by one in order to convince the reader of the physical viability of the theory.
An extensive set of appendices provide accessible introductions to several key notions such as the Peter-Weyl theorem, duality} of differential forms and Regge calculus, among others. The presentation is aimed at graduate students and researchers who have some familiarity with the tools of quantum mechanics and field theory and/or General Relativity, but are intimidated by the seeming technical prowess required to browse through the existing LQG literature. Our hope is to make the formalism appear a little less bewildering to the un-initiated and to help lower the barrier for entry into the field.
84 pages, many figures

http://arxiv.org/abs/1402.3544
Quantum fields in curved space-time, semiclassical gravity, quantum gravity phenomenology, and analogue models: Report on session D4 at GR20
Christopher J Fewster, Stefano Liberati
(Submitted on 14 Feb 2014)
The talks given in parallel session D4 at the 20th International Conference on General Relativity and Gravitation (Warsaw 2013) are summarized.
15 pages. To appear in the GR20 Proceedings issue of General Relativity and Gravitation

 

[kindlin]

Wow, there is a lot of information in this thread. It's too bad I can't understand any of it. Whenever I try and read into any part of the physics beyond GR and QM, I'm flooded with higher level mathematics which keep compounding on themselves until I'm thoroughly confused. I think if if I could figure out what a manifold is, that would be a big step in the right direction, but I'm always just as confused by the explanations as I am about the manifold itself.

 

[MTd2]

I think this might be very interesting:

http://arxiv.org/abs/1402.3055
Black Holes, Firewalls and Chaos from Gravitational Collapse

Pankaj S. Joshi, Ramesh Narayan
(Submitted on 13 Feb 2014)
One of the most spectacular predictions of the general theory of relativity is the black hole, an object that plays a central role in modern physics [1,2,3] and astrophysics [4,5]. Black holes are, however, plagued by fundamental paradoxes that remain unresolved to this day. First, the black hole event horizon is teleological in nature [6], which means that we need to know the entire future space-time of the universe to determine the current location of the horizon. This is essentially impossible. Second, any information carried by infalling matter is lost once the material falls through the event horizon. Even though the black hole may later evaporate by emitting Hawking radiation [7], the lost information does not reappear, which has the rather serious and disturbing consequence that quantum unitarity is violated [8]. Here we propose that the above paradoxes are restricted to a particular idealized model of collapse first studied in the 1930s [9, 10] in which the event horizon, which defines the boundary of the black hole, forms initially, and the singularity in the interior of the black hole forms at a later time. In contrast, gravitational collapse from more reasonable and/or physically more realistic initial conditions often leads to models in which the event horizon and the singularity form simultaneously. We show that this apparently simple modification mitigates the causality and teleological paradoxes and at the same time lends support to two recently proposed solutions to the information paradox, namely, the "firewall" [11] and "classical chaos" [12].

 

[marcus]

http://arxiv.org/abs/1402.4138
Near-Horizon Radiation and Self-Dual Loop Quantum Gravity
Marc Geiller, Karim Noui
(Submitted on 17 Feb 2014)
We compute the near-horizon radiation of quantum black holes in the context of self-dual loop quantum gravity. For this, we first use the unitary spinor basis of SL(2,ℂ) to decompose states of Lorentzian spin foam models into their self-dual and anti self-dual parts, and show that the reduced density matrix obtained by tracing over one chiral component describes a thermal state at Unruh temperature. Then, we show that the analytically-continued dimension of the SU(2) Chern-Simons Hilbert space, which reproduces the Bekenstein-Hawking entropy in the large spin limit in agreement with the large spin effective action, takes the form of a partition function for states thermalized at Unruh temperature, with discrete energy levels given by the near-horizon energy of Frodden-Gosh-Perez, and with a degenerate ground state which is holographic and responsible for the entropy.
6+2 pages

 

[marcus]

http://arxiv.org/abs/1402.5130
Discreteness corrections and higher spatial derivatives in effective canonical quantum gravity
Martin Bojowald, George M. Paily, Juan D. Reyes
(Submitted on 20 Feb 2014)
Canonical quantum theories with discrete space may imply interesting effects. This article presents a general effective description, paying due attention to the role of higher spatial derivatives in a local expansion and differences to higher time derivatives. In a concrete set of models, it is shown that spatial derivatives one order higher than the classical one are strongly restricted in spherically symmetric effective loop quantum gravity. Moreover, radial holonomy corrections cannot be anomaly-free to this order.
46 pages

http://arxiv.org/abs/1402.4854
Quantum Einstein-Cartan theory with the Holst term
Ilya L. Shapiro, Poliane M. Teixeira
(Submitted on 20 Feb 2014)
Holst term represents an interesting addition to the Einstein-Cartan theory of gravity with torsion. When this term is present the contact interactions between vector and axial vector fermion currents gain an extra parity-violating component. We re-derive this interaction using a simple representation for the Holst term. The same representation serves as a useful basis for the calculation of one-loop divergences in the theory with external fermionic currents and cosmological constant. Furthermore, we explore the possibilities of the on-shell version of renormalization group and construct the equations for the running of dimensionless parameters related to currents and for the effective Barbero-Immirzi parameter.
Comments: 19 pages

Probably the current favorite in the Dark Matter sweepstakes is the "keV scale sterile neutrino" for instance:
http://arxiv.org/abs/1402.2301
Detection of An Unidentified Emission Line in the Stacked X-ray spectrum of Galaxy Clusters
Esra Bulbul, Maxim Markevitch, Adam Foster, Randall K. Smith, Michael Loewenstein, Scott W. Randall
(Submitted on 10 Feb 2014)
We detect a weak unidentified emission line at E=(3.55-3.57)+/-0.03 keV in a stacked XMM spectrum of 73 galaxy clusters spanning a redshift range 0.01-0.35. MOS and PN observations independently show the presence of the line at consistent energies. When the full sample is divided into three subsamples (Perseus, Centaurus+Ophiuchus+Coma, and all others), the line is significantly detected in all three independent MOS spectra and the PN "all others" spectrum. It is also detected in the Chandra spectra of Perseus with the flux consistent with XMM (though it is not seen in Virgo). However, it is very weak and located within 50-110eV of several known faint lines, and so is subject to significant modeling uncertainties. On the origin of this line, we argue that there should be no atomic transitions in thermal plasma at this energy. An intriguing possibility is the decay of sterile neutrino, a long-sought dark matter particle candidate. Assuming that all dark matter is in sterile neutrinos with m_s=2E=7.1 keV, our detection in the full sample corresponds to a neutrino decay mixing angle sin²(2θ)=7e-11, below the previous upper limits. …
25 pages, 14 figures, submitted to ApJ

 

[marcus]

http://arxiv.org/abs/1402.5719
Singularity avoidance in classical gravity from four-fermion interaction
Cosimo Bambi, Daniele Malafarina, Antonino Marciano, Leonardo Modesto
(Submitted on 24 Feb 2014)
We derive the dynamics of the gravitational collapse of a homogeneous and spherically symmetric cloud in a classical set-up endowed with a topological sector of gravity and a non-minimal coupling to fermions. The effective theory consists of the Einstein-Hilbert action plus Dirac fermions interacting through a four-fermion vertex. At the classical level, we obtain the same picture that has been recently studied by some of us within a wide range of effective theories inspired by a super-renormalizable and asymptotically free theory of gravity. The classical singularity is replaced by a bounce, beyond which the cloud re-expands indefinitely. We thus show that, even at a classical level, if we allow for a non-minimal coupling of gravity to fermions, black holes may never form for a suitable choice of some parameters of the theory.
5 pages

http://arxiv.org/abs/1402.5880
Fermi-bounce Cosmology and scale invariant power-spectrum
Stephon Alexander, Cosimo Bambi, Antonino Marciano, Leonardo Modesto
(Submitted on 24 Feb 2014)
We develop a novel non-singular bouncing cosmology, due to the non-trivial coupling of general relativity to fermionic fields. The resolution of the singularity arises from the negative energy density provided by fermions. Our theory is ghost-free because the fermionic operator that generates the bounce is equivalent to torsion, which has no kinetic terms. The physical system is minimal in that it consists of standard general relativity plus a topological sector for gravity, a U(1) gauge field reducing to radiation at late times and fermionic matter described by Dirac fields with a non-minimal coupling. We show that a scale invariant power-spectrum generated in the contracting phase can be recovered for a suitable choice of the fermion number density and the bare mass, hence providing a possible alternative to the inflationary scenario.
Comments: 6 pages

 

[marcus]

http://arxiv.org/abs/1402.6613
Numerical simulations of a loop quantum cosmos: robustness of the quantum bounce and the validity of effective dynamics
Peter Diener, Brajesh Gupt, Parampreet Singh
(Submitted on 26 Feb 2014)
A key result of isotropic loop quantum cosmology is the existence of a quantum bounce which occurs when the energy density of the matter field approaches a universal maximum close to the Planck density. Though the bounce has been exhibited in various matter models, due to severe computational challenges some important questions have so far remained unaddressed. These include the demonstration of the bounce for widely spread states, its detailed properties for the states when matter field probes regions close to the Planck volume and the reliability of the continuum effective spacetime description in general. In this manuscript we rigorously answer these questions using the Chimera numerical scheme for the isotropic spatially flat model sourced with a massless scalar field. We show that as expected from an exactly solvable model, the quantum bounce is a generic feature of states even with a very wide spread, and for those which bounce much closer to the Planck volume. We perform a detailed analysis of the departures from the effective description and find some expected, and some surprising results. At a coarse level of description, the effective dynamics can be regarded as a good approximation to the underlying quantum dynamics unless the states correspond to small scalar field momenta, in which case they bounce closer to the Planck volume, or are very widely spread. Quantifying the amount of discrepancy between the quantum and the effective dynamics, we find that the departure between them depends in a subtle and non-monotonic way on the field momentum and different fluctuations. Interestingly, the departures are generically found to be such that the effective dynamics overestimates the spacetime curvature, and underestimates the volume at the bounce.
46 pages, 26 figures

briefly noted as possibly of interest:
http://arxiv.org/abs/1402.6332
Common origin of reactor and sterile neutrino mixing
Alexander Merle, Stefano Morisi, Walter Winter
(Submitted on 25 Feb 2014)
If the hints for light sterile neutrinos from short-baseline anomalies are to be taken seriously, global fits indicate active-sterile mixings of a magnitude comparable to the known reactor mixing. We therefore study the conditions under which the active-sterile and reactor mixings could have the same origin in an underlying flavour model. As a starting point, we use μ−τ symmetry in the active neutrino sector,…

http://arxiv.org/abs/1402.6319
A Globular Cluster Toward M87 with a Radial Velocity < -1000 km/s: The First Hypervelocity Cluster
Nelson Caldwell (CfA), Jay Strader (Michigan St), Aaron J. Romanowsky (San Jose St/Santa Cruz), Jean P. Brodie (Santa Cruz), Ben Moore (Zurich), Jurg Diemand (Zurich), Davide Martizzi (Berkeley)
...

 

[marcus]

http://arxiv.org/abs/1402.6708
A one-dimensional action for simplicial gravity in three dimensions
Wolfgang M. Wieland
(Submitted on 26 Feb 2014)
This article presents a derivation of the Ponzano--Regge model from a one-dimensional spinor action. The construction starts from the first-order Palatini formalism in three dimensions. We then introduce a simplicial decomposition of the three-dimensional manifold and study the discretised action in the spinorial representation of loop gravity. A one-dimensional refinement limit along the edges of the discretisation brings us back to a continuum formulation. The three-dimensional action turns into a line integral over the one-skeleton of the simplicial manifold. All fields are continuous but have support only along the one-dimensional edges. We define the path integral, and remove the redundant integrals over the local gauge orbits through the usual Faddeev--Popov procedure. The resulting state sum model reproduces the Ponzano--Regge amplitudes.
19 pages, two figures

not loop-and-allied QG but possibly of general interest:
http://arxiv.org/abs/1402.6980
Inflationary schism after Planck2013
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 27 Feb 2014)
Classic inflation, the theory described in textbooks, is based on the idea that, beginning from typical initial conditions and assuming a simple inflaton potential with a minimum of fine-tuning, inflation can create exponentially large volumes of space that are generically homogeneous, isotropic and flat, with nearly scale-invariant spectra of density and gravitational wave fluctuations that are adiabatic, Gaussian and have generic predictable properties. In a recent paper, we showed that, in addition to having certain conceptual problems known for decades, classic inflation is for the first time also disfavored by data, specifically the most recent data from WMAP, ACT and Planck2013. Guth, Kaiser and Nomura and Linde have each recently published critiques of our paper, but, as made clear here, we all agree about one thing: the problematic state of classic inflation. Instead, they describe an alternative inflationary paradigm that revises the assumptions and goals of inflation, and perhaps of science generally.
7 pages, 2 tables


http://arxiv.org/abs/1402.6703
The Characterization of the Gamma-Ray Signal from the Central Milky Way: A Compelling Case for Annihilating Dark Matter
Tansu Daylan, Douglas P. Finkbeiner, Dan Hooper, Tim Linden, Stephen K. N. Portillo, Nicholas L. Rodd, Tracy R. Slatyer
(Submitted on 26 Feb 2014)
Past studies have identified a spatially extended excess of ~1-3 GeV gamma rays from the region surrounding the Galactic Center, consistent with the emission expected from annihilating dark matter. We revisit and scrutinize this signal with the intention of further constraining its characteristics and origin. By applying cuts to the Fermi event parameter CTBCORE, we suppress the tails of the point spread function and generate high resolution gamma-ray maps, enabling us to more easily separate the various gamma-ray components. Within these maps, we find the GeV excess to be robust and highly statistically significant, with a spectrum, angular distribution, and overall normalization that is in good agreement with that predicted by simple annihilating dark matter models. For example, the signal is very well fit by a 31-40 GeV dark matter particle annihilating to b quarks with an annihilation cross section of sigma v = (1.4-2.0) x 10^-26 cm^3/s (normalized to a local dark matter density of 0.3 GeV/cm^3). Furthermore, we confirm that the angular distribution of the excess is approximately spherically symmetric and centered around the dynamical center of the Milky Way (within ~0.05 degrees of Sgr A*), showing no sign of elongation along or perpendicular to the Galactic Plane. The signal is observed to extend to at least 10 degrees from the Galactic Center, disfavoring the possibility that this emission originates from millisecond pulsars.
Comments: 26 pages, 28 figures

http://arxiv.org/abs/1402.6795
Multidimensional finite quantum gravity
Leonardo Modesto
(Submitted on 27 Feb 2014)
We advance a class of unitary higher derivative theories of gravity that realize an ultraviolet completion of Einstein general relativity in any dimension. This range of theories is marked by an entire function, which averts extra degrees of freedom (including poltergeists) and improves the high energy behavior of the loop amplitudes. It is proved that only one-loop divergences survive and the theory can be made super-renormalizable regardless of the spacetime dimension. Moreover, using the Pauli-Villars regularization procedure introduced by Diaz-Troost-van Nieuwenhuizen-van Proeyen (DTPN) and applied to Einstein's gravity by Anselmi, we are able to remove the divergences also at one-loop, making the theory completely finite in any dimension as expected by Anselmi and Asorey-Lopez-Shapiro.
8 pages, 2 figures

 

[marcus]

Useful review, could serve as source for Planck star discussion:
http://arxiv.org/abs/1403.1198
Primordial Black Holes: sirens of the early Universe
Anne M. Green
(Submitted on 5 Mar 2014)
Primordial Black Holes (PBHs) are, typically light, black holes which can form in the early Universe. There are a number of formation mechanisms, including the collapse of large density perturbations, cosmic string loops and bubble collisions. The number of PBHs formed is tightly constrained by the consequences of their evaporation and their lensing and dynamical effects. Therefore PBHs are a powerful probe of the physics of the early Universe, in particular models of inflation. They are also a potential cold dark matter candidate.
21 pages. To be published in "Quantum Aspects of Black Holes", ed. X. Calmet (Springer, 2014)

Briefly noted as possibly relevant to discussion thread:
http://arxiv.org/abs/1403.1146
Why I am not a QBist
Louis Marchildon
(Submitted on 5 Mar 2014)
Quantum Bayesianism, or QBism, is a recent development of the epistemic view of quantum states, according to which the state vector represents knowledge about a quantum system, rather than the true state of the system. QBism explicitly adopts the subjective view of probability, wherein probability assignments express an agent's personal degrees of belief about an event. QBists claim that most if not all conceptual problems of quantum mechanics vanish if we simply take a proper epistemic and probabilistic perspective. Although this judgement is largely subjective and logically consistent, I explain why I do not share it...

 

[MTd2]

One thing that I miss from discussions about cosmogony, cosmology in general, star system formation, is the role of turbulence (if any) in structure formation. The author made some mention talking about fluctuations, which made me happy.

There are some quite good papers about it, but I am afraid of posting them here since people could call it crackpotish. Not that I agree with everything, but they have a flavor of oddness on them, like you'd expect from the mind of Alven (though they have nothing to do with electric universe or similar).

 

[marcus]

Chronos keeps spotting good papers about the possible composition of DM by 7 keV sterile neutrinos. Here is another that came out this month
http://arxiv.org/abs/1403.0954
Resonantly-Produced 7 keV Sterile Neutrino Dark Matter Models and the Properties of Milky Way Satellites
Kevork N. Abazajian
(Submitted on 4 Mar 2014)
Sterile neutrinos produced through a resonant Shi-Fuller mechanism are arguably the simplest model for a dark matter interpretation origin of the recent unidentified X-ray line seen toward a number of objects harboring dark matter. Here, I calculate the exact parameters required in this mechanism to produce the signal. The suppression of small scale structure predicted by these models is consistent with Local Group and high-z galaxy count constraints. Very significantly, the parameters necessary in these models to produce the full dark matter density fulfill previously determined requirements to successfully match the Milky Way Galaxy's total satellite abundance, the satellites' radial distribution and their mass density profile, or "too big to fail problem." I also discuss how further precision determinations of the detailed properties of the candidate sterile neutrino dark matter can probe the nature of the quark-hadron transition, which takes place during the dark matter production.
4 pages, 3 figures.
The author (PhD 2001, 60 papers with good average cites) is someone I think I will be keeping track of.

 

[marcus]

http://arxiv.org/abs/1403.2080
The Soccer-ball Problem
S. Hossenfelder
(Submitted on 9 Mar 2014)
The idea that Lorentz-symmetry in momentum space could be modified but still remain observer-independent has received quite some attention in the recent years. Motivated by Loop Quantum Gravity, this modified Lorentz-symmetry is being used as a phenomenological model to test possibly observable effects of quantum gravity. The most pressing problem in these models is the treatment of multi-particle states, known as the "soccer-ball problem." This article briefly reviews the problem and the status of existing solution attempts.
9 pages, prepared for the SIGMA Special Issue on Deformations of Space-Time

 

[MTd2]

http://backreaction.blogspot.com.br/2014/03/what-is-asymptotically-safe-gravity-and.html

Bee's blog:

What is asymptotically safe gravity and what does it save?

 

[marcus]

http://arxiv.org/abs/1403.2815
FRW cosmology of the generalized model of LQG
S. Chattopadhyay, A. Ashurov, M. Khurshudyan, K. Myrzakulov, A. Pasqua, R. Myrzakulov
(Submitted on 12 Mar 2014)
In this paper, we study the main cosmological properties of the classical Friedmann equations in the case of homogeneous and isotropic Friedmann-Robertson-Walker Universe and we also generalized the expression of the Friedmann equation in the case of Loop Quantum Cosmology (LQC). Considering the M₃₅-model, we found the solutions of the equations considered for two particular cases, i.e. Q=0 (i.e., the de Sitter solution) and Q>0. Moreover, we considered and studied two exact cosmological solutions of the M₃₅-model, in particular the power-law and the exponential ones. Futhermore, we also considered a third more complicated case and we derived the solution for an arbitrary function of the time f(t). A scalar field description of the model is presented by constructing its self-interacting potential.
16 pages

http://arxiv.org/abs/1403.2974
Separable Hilbert space for loop quantization
J. Fernando Barbero G., Tomasz Pawlowski, Eduardo J. S. Villasenor
(Submitted on 12 Mar 2014)
We discuss, within the simplified context provided by the polymeric harmonic oscillator, a construction leading to a separable Hilbert space that preserves some of the most important features of the spectrum of the Hamiltonian operator. This construction can be generalized to loop quantum cosmology and is helpful to sidestep some of the issues that appear in that context. In particular those related to superselection and the definition of suitable ensembles for the statistical mechanics of these types of systems.
5 pages

 

[marcus]

http://arxiv.org/abs/1403.3190
A curvature operator for LQG
Emanuele Alesci, Mehdi Assanioussi, Jerzy Lewandowski
(Submitted on 13 Mar 2014)
We introduce a new operator in Loop Quantum Gravity - the 3D curvature operator - related to the 3-dimensional scalar curvature. The construction is based on Regge Calculus. We define it starting from the classical expression of the Regge curvature, then we derive its properties and discuss some explicit checks of the semi-classical limit.
20 pages.