Loop-and-allied QG bibliography

[MTd2]

http://arxiv.org/abs/1305.4582

Causal Dynamical Triangulations without Preferred Foliation

S. Jordan, R. Loll
(Submitted on 20 May 2013)
We introduce a generalized version of the Causal Dynamical Triangulations (CDT) formulation of quantum gravity, in which the regularized, triangulated path integral histories retain their causal properties, but do not have a preferred proper-time foliation. An extensive numerical study of the associated nonperturbative path integral in 2+1 dimensions shows that it can nevertheless reproduce the emergence of an extended de Sitter universe on large scales, a key feature of CDT quantum gravity. This suggests that the preferred foliation normally used in CDT is not a crucial (albeit convenient) part of its background structure.

 

[marcus]

http://arxiv.org/abs/1305.4790
Quantum-gravity inspired collapse with far observable bounce
Cosimo Bambi, Daniele Malafarina, Leonardo Modesto
(Submitted on 21 May 2013)
We consider general relativistic homogeneous gravitational collapses for dust and radiation. We show that replacing the density profile with an effective density justified by some quantum gravity framework leads to the avoidance of the final singularity. The effective density acts on the collapsing cloud by introducing an isotropic pressure, which is negligible at the beginning of the collapse and becomes negative and dominant in the strong field regime. For sufficiently small sources, the horizon never forms. For larger objects, the horizon forms, it disappears when the matter density approaches a critical value and gravity becomes very weak (asymptotic freedom regime), it forms again after the bounce as a consequence of the decrease in the matter density, and it eventually disappears when the density becomes too low and the matter is radiated away. In both cases, the bounce stage regulated by quantum gravity effects turns out to be visible to distant observers. The possibility of detecting radiation coming from the high density region of a collapsing astrophysical object in which classically there would be the creation of a singularity could open a new window to experimentally test theories of quantum gravity.
10 pages, 6 figures

http://arxiv.org/abs/1305.4702
Exploring Torus Universes in Causal Dynamical Triangulations
T.G. Budd, R. Loll
(Submitted on 21 May 2013)
Motivated by the search for new observables in nonperturbative quantum gravity, we consider Causal Dynamical Triangulations (CDT) in 2+1 dimensions with the spatial topology of a torus. This system is of particular interest, because one can study not only the global scale factor, but also global shape variables in the presence of arbitrary quantum fluctuations of the geometry. Our initial investigation focusses on the dynamics of the scale factor and uncovers a qualitatively new behaviour, which leads us to investigate a novel type of boundary conditions for the path integral. Comparing large-scale features of the emergent quantum geometry in numerical simulations with a classical minisuperspace formulation, we find partial agreement. By measuring the correlation matrix of volume fluctuations we succeed in reconstructing the effective action for the scale factor directly from the simulation data. Apart from setting the stage for the analysis of shape dynamics on the torus, the new set-up highlights the role of nontrivial boundaries and topology.
30 pages, 12 figures

brief mention:
http://arxiv.org/abs/1305.4685
Machian Classical and Semiclassical Emergent Time
Edward Anderson
(Submitted on 21 May 2013)
Classical and semiclassical schemes are presented that are timeless at the primary level and recover time from Mach's "time is to be abstracted from change" principle at the emergent secondary level. The semiclassical scheme is a Machian variant of the Semiclassical Approach to the Problem of Time in Quantum Gravity. The classical scheme is Barbour's, cast here explicitly as the classical precursor of the Semiclassical Approach...
22 pages and 1 figure.

 

[marcus]

http://arxiv.org/abs/1305.5203
A computable framework for Loop Quantum Gravity
Viqar Husain, Tomasz Pawlowski
(Submitted on 22 May 2013)
We present a non-perturbative quantization of general relativity coupled to dust and other matter fields. The dust provides a natural time variable, leading to a physical Hamiltonian with spatial diffeomorphism symmetry. The methods of loop quantum gravity applied to this model lead to a physical Hilbert space and Hamiltonian. This provides a framework for physical calculations in the theory.
3 pages. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

http://pirsa.org/13050000/
A la recherche du temps perdu...in quantum gravity
Speaker(s): Fay Dowker
Abstract: Causal set quantum gravity is based on the marriage between the concept of causality as an organising principle more basic even than space or time and fundamental atomicity. Causal sets suggest novel possibilities for "dynamical laws" in which spacetime grows by the accumulation of new spacetime atoms, potentially realising within physics C.D. Broad's concept of a growing block universe in which the past is real and the future is not.
To do justice to relativity and general covariance, the atoms must accumulate in a partial order, exactly the order that the atoms have physically amongst themselves. That this is possible is demonstrated by the Rideout-Sorkin Classical Stochastic Growth models. This proof of concept -- of the compatibility of relativity and "becoming'' -- is, however, classical and is challenged by the global character of the physical world within a path integral framework for quantum theory. Out of the struggle to reconcile the global and local natures of the physical world may arise a quantal dynamics for causal sets.
Date: 22/05/2013 - 2:00 pm

http://arxiv.org/abs/1305.4986
Mechanical laws of the Rindler horizon
Eugenio Bianchi, Alejandro Satz
(Submitted on 21 May 2013)
Gravitational perturbations of flat Minkowski space make the Rindler horizon dynamical: the horizon satisfies mechanical laws analogous to the ones followed by black holes. We describe the gravitational perturbation of Minkowski space using perturbative field-theoretical methods. The change in the area of the Rindler horizon is described in terms of the deflection of light rays by the gravitational field. The difference between the area of the perturbed and the unperturbed horizon is related to the energy of matter crossing the horizon. We derive consistency conditions for the validity of our approximations, and compare our results to similar ones present in the literature. Finally, we discuss how this setting can be used in perturbative quantum gravity to extend the classical mechanical laws to thermodynamic laws, with the entanglement of field modes across the Rindler horizon providing a notion of thermodynamic entropy.
10 pages, 2 figures

http://arxiv.org/abs/1305.5191
A small cosmological constant due to non-perturbative quantum effects
Jan Holland, Stefan Hollands
(Submitted on 22 May 2013)
We propose that the expectation value of the stress energy tensor of the Standard Model should be given by ⟨T_μν⟩ = ρ_vac η_μν, with a vacuum energy ρvac that differs from the usual “dimensional analysis” result by an exponentially small factor associated with non-perturbative effects. We substantiate our proposal by a rigorous analysis of a toy model, namely the 2-dimensional Gross-Neveu model. In particular, we address, within this model, the key question of the renormalization ambiguities affecting the calculation. The stress energy operator is constructed concretely via the operator-product-expansion. The non-perturbative factor in the vacuum energy is seen as a consequence of the facts that a) the OPE-coefficients have an analytic dependence on g, b) the vacuum correlations have a non-analytic (=non-perturbative) dependence on g, which we propose to be a generic feature of QFT. Extrapolating our result from the Gross-Neveu model to the Standard Model, one would expect to find ρ_vac ∼ Λ⁴e^−O(1)/g2 , where Λ is an energy scale such as Λ = MH, and g is a gauge coupling such as g²/4π = α_EW. The exponentially small factor due to non-perturbative effects could explain the “unnatural” smallness of this quantity.
11 pages.

 

[MTd2]

http://arxiv.org/abs/1305.3326

A Discrete and Coherent Basis of Intertwiners

Laurent Freidel, Jeff Hnybida
(Submitted on 15 May 2013)
We construct a new discrete basis of 4-valent SU(2) intertwiners. This basis possesses both the advantage of being discrete, while at the same time representing accurately the classical degrees of freedom; hence it is coherent. The closed spin network amplitude obtained from these intertwiners depends on twenty spins and can be evaluated by a generalization of the Racah formula for an arbitrary graph. The asymptotic limit of these amplitudes is found. We give, for the first time, the asymptotics of 15j symbols in the real basis. Remarkably it gives a generalization of the Regge action to twisted geometries.

 

[John86]

http://arxiv.org/abs/1305.5062
Dual redshift on Planck-scale-curved momentum spaces
Giovanni Amelino-Camelia, Leonardo Barcaroli, Giulia Gubitosi, Niccoló Loret
(Submitted on 22 May 2013)
Several approaches to the investigation of the quantum-gravity problem have provided "theoretical evidence" of a role for the Planck scale in characterizing the geometry of momentum space. One of the main obstructions for a full exploitation of this scenario is the understanding of the role of the Planck-scale-curved geometry of momentum space in the correlations between emission and detection times, the "travel times" for a particle to go from a given emitter to a given detector. These travel times appear to receive Planck-scale corrections for which no standard interpretation is applicable, and the associated implications for spacetime locality gave rise to the notion of "relative locality" which is still in the early stages of investigation. We here show that these Planck-scale corrections to travel times can be described as "dual redshift" (or "lateshift"): they are manifestations of momentum-space curvature of the same type already known for ordinary redshift produced by spacetime curvature. In turn we can identify the novel notion of "relative momentum-space locality" as a known but under-appreciated feature associated to ordinary redshift produced by spacetime curvature, and this can be described in complete analogy with the relative spacetime locality that became of interest in the recent quantum-gravity literature. We also briefly comment on how these findings may be relevant for an approach to the quantum-gravity problem proposed by Max Born in 1938 and centered on Born duality.

 

[atyy]

Lee Smolin on the future of physics
Outside the box
May 28th 2013, 13:14 by Economist.com
http://www.economist.com/blogs/babbage/2013/05/lee-smolin-future-physics

 

[marcus]

http://arxiv.org/abs/1305.6315
Why gravity codes the renormalization of conformal field theories
Henrique Gomes, Sean Gryb, Tim Koslowski, Flavio Mercati, Lee Smolin
(Submitted on 27 May 2013)
We give a new demonstration that General Relativity in d+1 dimensions with negative or positive cosmological constant codes the renormalization group behaviour of conformal field theories (CFT) in d dimensions. This utilizes Shape Dynamics, which is a conformally invariant theory known to be equivalent to General Relativity. A key result of Shape Dynamics is that the evolution of observables under local conformal transformations and spatial diffeomorphisms is shown to be equivalent to many fingered time, i.e., d+1-dimensional spacetime diffeomorphisms. This relationship explains why the renormalization group flow of a CFT is governed by a geometry with d+1-dimensional spacetime diffeomorphism invariance.
25 pages

brief mention:
http://arxiv.org/abs/1305.6565
Path Integrals and Reality
Adrian Kent
(Submitted on 28 May 2013)
We define the idea of real path quantum theory, a realist generalisation of quantum theory in which it is postulated that the configuration space path actually followed by a closed quantum system is probabilistically chosen. This is done a postulate defining probabilities for paths, which we propose are determined by an expression involving path amplitudes and a distance function that quantifies path separation. We suggest a possible form for a path probability postulate and explore possible choices of distance function, including choices suitable for Lorentz or generally covariant versions of real path quantum theory. We set out toy models of quantum interferometry and show that in these models the probability postulate and specific distance functions do indeed give a physically sensible path ontology. These functions can be chosen so as to predict quantum interference for interference of microscopic quantum systems and the failure of interference for macroscopic quantum systems. More generally, they predict interference when the beams are close, and its failure when they are far apart, as determined by the distance function. If taken seriously in its present relatively unconstrained form, real path quantum theory thus motivates experimental tests of quantum interference in all unexplored regimes defined by potentially physically interesting parameters, including the mass of the beam object, the beam separation distance, the beam separation time, and many others. We discuss open questions raised by these ideas.
20 pages

symptom of shift towards bounce models:
http://arxiv.org/abs/1305.6346
Bouncing cosmologies in massive gravity on de Sitter
David Langlois, Atsushi Naruko

 

[marcus]

http://arxiv.org/abs/1305.6714
Black hole entropy from KMS-states of quantum isolated horizons
Daniele Pranzetti
(Submitted on 29 May 2013)
By reintroducing Lorentz invariance via a complex connection formulation in canonical loop quantum gravity, we define a geometrical notion of temperature for quantum isolated horizons. Upon imposition of the reality conditions in the form of the linear simplicity constraints for an imaginary Barbero-Immirzi parameter, the exact formula for the temperature can be derived by demanding that the horizon state satisfying the boundary conditions be a KMS-state. In this way, our analysis reveals the connection between the passage to the Ashtekar self-dual variables and the thermality of the horizon. The horizon equilibrium state can then be used to compute both the von Neumann and the Boltzmann entropies. By means of a natural cut-off introduced by the topological theory on the boundary, we show that the two provide the same finite answer which allows us to recover the Bekenstein-Hawking formula in the semi-classical limit. The connection with Connes-Rovelli thermal time proposal for a general relativistic statistical mechanics is worked out.
10 pages, 1 figure

http://arxiv.org/abs/1305.6680
CDT and the Search for a Theory of Quantum Gravity
J. Ambjorn, A. Goerlich, J. Jurkiewicz, R. Loll
(Submitted on 29 May 2013)
Causal Dynamical Triangulations provide a non-perturbative regularization of a theory of quantum gravity. We describe how this approach connects with the asymptotic safety program and Ho\vrava-Lifgarbagez gravity theory, and present the most recent results from computer simulations.
23 pages, 5 figures. Based on plenary talk at MG13. To appear in the proceeding

brief mention:
http://arxiv.org/abs/1305.6859
Dark Matter
P. J. E. Peebles
(Submitted on 29 May 2013)
The evidence for the dark matter of the hot big bang cosmology is about as good as it gets in natural science. The exploration of its nature is now led by direct and indirect detection experiments, to be complemented by advances in the full range of cosmological tests, including judicious consideration of the rich phenomenology of galaxies. The results may confirm ideas about DM already under discussion. If we are lucky we also will be surprised once again.
3 pages. Essay for the Dark Matter Sackler Colloquium, October 2012, Irvine CA

http://arxiv.org/abs/1305.6838
The Physics of the Far Future
Ruxandra Bondarescu, Andrew P. Lundgren, Mihai Bondarescu
(Submitted on 29 May 2013)
3 pages. Proceeding for the 13th Marcel Grossmann Meeting in Stockholm, Sweden

 

[atyy]

http://arxiv.org/abs/1305.6741
Finite Quantum Gravity
Leonardo Modesto
(Submitted on 29 May 2013)
We hereby present a class of multidimensional higher derivative theories of gravity that realizes an ultraviolet completion of Einstein general relativity. This class is marked by a "non-polynomal" entire function (form factor), which averts extra degrees of freedom (including ghosts) and improves the high energy behavior of the loop amplitudes. By power counting arguments, it is proved that the theory is super-renormalizable in any dimension, i.e. only one-loop divergences survive. Furthermore, in odd dimensions there are no counter terms for pure gravity and the theory turns out to be "finite." Finally, considering the infinite tower of massive states coming from dimensional reduction, quantum gravity is finite in even dimension as well.

 

[marcus]

brief mention:
http://arxiv.org/abs/1305.7190
Vector Fields in Holographic Cosmology
James B. Hartle, S. W. Hawking, Thomas Hertog
(Submitted on 30 May 2013)
We extend the holographic formulation of the semiclassical no-boundary wave function (NBWF) to models with Maxwell vector fields. It is shown that the familiar saddle points of the NBWF have a representation in which a regular, Euclidean asymptotic AdS geometry smoothly joins onto a Lorentzian asymptotically de Sitter universe through a complex transition region. The tree level probabilities of Lorentzian histories are fully specified by the action of the AdS region of the saddle points. The scalar and vector matter profiles in this region are complex from an AdS viewpoint, with universal asymptotic phases. The dual description of the semiclassical NBWF thus involves complex deformations of Euclidean CFTs.
17 pages, 3 figs

 

[John86]

http://arxiv.org/abs/1305.7289
Twisting loops and global momentum non-conservation in Relative Locality
Andrzej Banburski
(Submitted on 31 May 2013)
Recent work in Relative Locality has shown that the theory allows for a solution of an on-shell causal loop. We show that the theory contains a different type of a loop in which locally momenta are conserved, but there is no global momentum conservation. Thus a freely propagating particle can decay into two particles, which later recombine to give a particle with momentum and mass different than the original one.

http://arxiv.org/abs/1305.7479
Generalized fuzzy torus and its modular properties
Paul Schreivogl, Harold Steinacker
(Submitted on 31 May 2013)
We consider a generalization of the basic fuzzy torus to a fuzzy torus with non-trivial modular parameter, based on a finite matrix algebra. We discuss the modular properties of this fuzzy torus, and compute the matrix Laplacian for a scalar field. In the semi-classical limit, the generalized fuzzy torus can be used to approximate a generic commutative torus represented by two generic vectors in the complex plane, with generic modular parameter \tau. The effective classical geometry and the spectrum of the Laplacian are correctly reproduced in the limit. The spectrum of a matrix Dirac operator is also computed.

 

[marcus]

brief mention:
http://arxiv.org/abs/1306.0527
Quantum Field Theory on Curved Backgrounds - A Primer
Marco Benini, Claudio Dappiaggi, Thomas-Paul Hack
(Submitted on 3 Jun 2013)
Goal of this review is to introduce the algebraic approach to quantum field theory on curved backgrounds. Based on a set of axioms, first written down by Haag and Kastler, this method consists of a two-step procedure. In the first one, a suitable algebra of observables is assigned to a physical system, which is meant to encode all algebraic relations among observables, such as commutation relations, while, in the second step, one must select an algebraic state in order to recover the standard Hilbert space interpretation of a quantum system. As quantum field theories possesses infinitely many degrees of freedom, many unitarily inequivalent Hilbert space representations exist and the power of such approach is the ability to treat them all in a coherent manner. We will discuss in detail the algebraic approach for free fields in order to give to the reader all necessary information to deal with the recent literature, which focuses on the applications to specific problems, mostly in cosmology.
36 pages, invited review to appear on IJMPA

 

[MTd2]

http://arxiv.org/abs/1306.0861

Matrix Elements of Lorentzian Hamiltonian Constraint in LQG

Emanuele Alesci, Klaus Liegener, Antonia Zipfel
(Submitted on 4 Jun 2013)
The Hamiltonian constraint is the key element of the canonical formulation of LQG coding its dynamics. In Ashtekar-Barbero variables it naturally splits into the so called Euclidean and Lorentzian parts. However, due to the high complexity of this operator, only the matrix elements of the Euclidean part have been considered so far. Here we evaluate the action of the full constraint, including the Lorentzian part. The computation requires an heavy use of SU(2) recoupling theory and several tricky identities among n-j symbols are used to find the final result: these identities, together with the graphical calculus used to derive them, also simplify the Euclidean constraint and are of general interest in LQG computations.

 

[John86]

http://arxiv.org/abs/1306.0820
Quantum cosmology from the de Broglie-Bohm perspective
N. Pinto-Neto, J.C. Fabris
(Submitted on 4 Jun 2013)
We review the main results that have been obtained in quantum cosmology from the perspective of the de Broglie-Bohm quantum theory. As it is a dynamical theory of assumed objectively real trajectories in the configuration space of the physical system under investigation, this quantum theory is not essentially probabilistic and dispenses the collapse postulate, turning it suitable to be applied to cosmology. In the framework of minisuperspace models, we show how quantum cosmological effects in the de-Broglie-Bohm's approach can avoid the initial singularity, and isotropize the Universe. We then extend minisuperspace in order to include linear cosmological perturbations. We present the main equations which govern the dynamics of quantum cosmological perturbations evolving in non-singular quantum cosmological backgrounds, and calculate some of their observational consequences. These results are not known how to be obtained in other approaches to quantum theory. In the general case of full superspace, we enumerate the possible structures of quantum space and time that emerge from the de Broglie-Bohm picture. Finally, we compare some of the results coming from the de Broglie-Bohm theory with other approaches, and discuss the physical reasons for some discrepancies that occur.

http://arxiv.org/abs/1306.0515
Passing through the Firewall
Erik Verlinde, Herman Verlinde
(Submitted on 3 Jun 2013 (v1), last revised 4 Jun 2013 (this version, v2))
We propose that black hole information is encoded in non-local correlations between microscopic interior and exterior degrees of freedom. We give a simple qubit representation of this proposal, and show herein that for every black hole state, the apparent firewall can be removed via a universal, state independent unitary transformation. A central element in our discussion is the distinction between virtual qubits, which are in a specified vacuum state, and real qubits, that carry the free quantum information of the black hole. We outline how our proposal may be realized in AdS/CFT

http://arxiv.org/abs/1306.0516
Black Hole Information as Topological Qubits
Erik Verlinde, Herman Verlinde
(Submitted on 3 Jun 2013 (v1), last revised 4 Jun 2013 (this version, v2))
The principle of balanced holography, introduced in [1], posits that black hole information is stored in non-local correlations between the interior and exterior. Based on this concept, we propose that black hole information decomposes into elementary units in the form of topological qubits, and is protected from local sources of decoherence. The topological protection mechanism ensures that the horizon of an evaporating black hole stays young and smooth.

http://arxiv.org/abs/1306.0353
Wheeler-De Witt equation and AdS/CFT correspondence
Francesco Cianfrani, Jerzy Kowalski-Glikman
(Submitted on 3 Jun 2013)
The radial Wheeler-De Witt equation on the asymptotically AdS spacetime proposed in [9] has as its semiclassical solution the wave function that asymptotically satisfies the conformal Ward identity, exemplifying the AdS/CFT correspondence. In this paper we show that this results holds also in the case of a complete quantum solution of the radial Wheeler-De Witt equation. It turns out that if the wavefunction is expanded in the parameter $\rho$ with $\rho\rightarrow0$ defines the asymptotic boundary of the spacetime, the quantum loop corrections to the semiclassical wave are of subleading order.

 

[marcus]

http://arxiv.org/abs/1306.1546
Structural aspects of asymptotically safe black holes
Benjamin Koch, Frank Saueressig
(Submitted on 6 Jun 2013)
We study the quantum modifications of classical, spherically symmetric Schwarzschild (Anti-) de Sitter black holes within Quantum Einstein Gravity. The quantum effects are incorporated through the running coupling constants G_k and Λ_k, computed within the exact renormalization group approach, and a common scale-setting procedure. We find that, in contrast to common intuition, it is actually the cosmological constant that determines the short-distance structure of the RG-improved black hole: in the asymptotic UV the structure of the quantum solutions is universal and given by the classical Schwarzschild-de Sitter solution, entailing a self-similarity between the classical and quantum regime. As a consequence asymptotically safe black holes evaporate completely and no Planck-size remnants are formed. Moreover, the thermodynamic entropy of the critical Nariai-black hole is shown to agree with the microstate count based on the effective average action, suggesting that the entropy originates from quantum fluctuations around the mean-field geometry.
25 pages 10 figures

 

[MTd2]

http://arxiv.org/abs/1306.1668
Terminating black holes in quantum gravity
Cosimo Bambi, Daniele Malafarina, Leonardo Modesto
(Submitted on 7 Jun 2013)
We study the homogeneous gravitational collapse of a spherical cloud composed of radiation or dust in a super-renormalizable and asymptotically free theory of gravity. The central singularity appearing in classical general relativity is resolved in both cases. The singularity is replaced by a bounce, after which the cloud re-expands indefinitely. In this model, strictly speaking, a black hole never forms and the high density state governed by quantum-gravitational physics is visible to faraway observers. Our result is quite general, and it holds for gravity theories with form factors suggested by string field theory and non-commutative geometries.

 

[MTd2]

http://arxiv.org/abs/1306.1489

Affine group formulation of the Standard Model coupled to gravity

Ching-Yi Chou, Eyo Ita, Chopin Soo
(Submitted on 6 Jun 2013 (v1), last revised 8 Jun 2013 (this version, v2))
This work demonstrates that a complete description of the interaction of matter and all forces, gravitational and non-gravitational, can in fact be realized within a quantum affine algebraic framework. Using the affine group formalism, we construct elements of a physical Hilbert space for full, Lorentzian quantum gravity coupled to the Standard Model in four spacetime dimensions. Affine algebraic quantization of gravitation and matter on equal footing implies a fundamental uncertainty relation which is predicated upon a non-vanishing cosmological constant.

 

[marcus]

http://arxiv.org/abs/1306.2987
Coarse graining of spin net models: dynamics of intertwiners
Bianca Dittrich, Mercedes Martín-Benito, Erik Schnetter
(Submitted on 12 Jun 2013)
Spin foams are models of quantum gravity and therefore quantum space time. A key open issue is to determine the possible continuum phases of these models. Progress on this issue has been prohibited by the complexity of the full four--dimensional models. We consider here simplified analogue models, so called spin nets, that retain the main dynamical ingredient of spin foams, the simplicity constraints. For a certain class of these spin net models we determine the phase diagram and therefore the continuum phases via a coarse graining procedure based on tensor network renormalization. This procedure will also reveal an unexpected fixed point, which turns out to define a new triangulation invariant vertex model.
33 pages, 17 figures

brief mention:
http://arxiv.org/abs/1306.3021
The Trace-Free Einstein Equations and inflation
George F R Ellis
(Submitted on 13 Jun 2013)
The trace-free version of the Einstein Gravitational equations, essentially equivalent to unimodular gravity, can solve the troubling issue of the huge discrepancy between quantum field theory estimates of the vacuum energy density and the astronomically observed value of the cosmological constant. However it has been suggested that this proposal cannot work because it prevents the inflaton potential energy from driving inflation. It is shown here that that concern is unjustified: inflation proceeds as usual if we adopt the trace free gravitational equations.
10 pages

http://arxiv.org/abs/1306.2964
The Sterile Neutrino Field and Late Time Acceleration
Stephon H.S. Alexander
(Submitted on 12 Jun 2013)
We show that when the neutrino minimal standard model (νMSM) with a light sterile neutrino has a long range interaction between the neutrino vacuum charge density and a dark U(1) gauge potential, late time acceleration is dynamically realized. Moreover, the relation between the scale of dark energy, the weak and Planck masses derived by Arkani-Hamed et. al that addresses the coincidence problem arises naturally in this model. Assuming that the cosmological constant is zero by some as yet [un]known mechanism, the only fine tuning in our model is the mass of the sterile neutrino. We show that perturbations of the vector fields are oscillatory and hence, there are no instabilities.
4 pages, 1 figure

A strange paper, which some people may be curious to know about:
http://arxiv.org/abs/1306.2993
How all of quantum mechanics can be derived from a single law of physics
Holger F. Hofmann
(Submitted on 12 Jun 2013)
...
Author's comment: I sincerely believe that this paper presents a comprehensive answer to all interpretational problems of quantum mechanics. It contains new and surprising insights from recent experiments that should be considered with great care and thoroughness. Please take a look and see for yourself
18 pages

 

[MTd2]

http://arxiv.org/abs/1306.3021v1

The Trace-Free Einstein Equations and inflation

George F R Ellis
(Submitted on 13 Jun 2013)
The trace-free version of the Einstein Gravitational equations, essentially equivalent to unimodular gravity, can solve the troubling issue of the huge discrepancy between quantum field theory estimates of the vacuum energy density and the astronomically observed value of the cosmological constant. However it has been suggested that this proposal cannot work because it prevents the inflaton potential energy from driving inflation. It is shown here that that concern is unjustified: inflation proceeds as usual if we adopt the trace free gravitational equations.

 

[marcus]

http://arxiv.org/abs/1306.3241
Testing the role of the Barbero-Immirzi parameter and the choice of connection in Loop Quantum Gravity
Jibril Ben Achour, Marc Geiller, Karim Noui, Chao Yu
(Submitted on 13 Jun 2013)
We study the role of the Barbero-Immirzi parameter γ and the choice of connection in the construction of (a symmetry-reduced version of) loop quantum gravity. We start with the four-dimensional Lorentzian Holst action that we reduce to three dimensions in a way that preserves the presence of γ. In the time gauge, the phase space of the resulting three-dimensional theory mimics exactly that of the four-dimensional one. Its quantization can be performed, and on the kinematical Hilbert space spanned by SU(2) spin network states the spectra of geometric operators are discrete and γ-dependent. However, because of the three-dimensional nature of the theory, its SU(2) Ashtekar-Barbero Hamiltonian constraint can be traded for the flatness constraint of an sl(2,C) connection, and we show that this latter has to satisfy a linear simplicity-like condition analogous to the one used in the construction of spin foam models. The physically relevant solution to this constraint singles out the non-compact subgroup SU(1,1), which in turn leads to the disappearance of the Barbero-Immirzi parameter and to a continuous length spectrum, in agreement with what is expected from Lorentzian three-dimensional gravity.
36 pages

http://arxiv.org/abs/1306.3246
Spectra of geometric operators in three-dimensional LQG: From discrete to continuous
Jibril Ben Achour, Marc Geiller, Karim Noui, Chao Yu
(Submitted on 13 Jun 2013)
We study and compare the spectra of geometric operators (length and area) in the quantum kinematics of two formulations of three-dimensional Lorentzian loop quantum gravity. In the SU(2) Ashtekar-Barbero framework, the spectra are discrete and depend on the Barbero-Immirzi parameter γ exactly like in the four-dimensional case. However, we show that when working with the self-dual variables and imposing the reality conditions the spectra become continuous and γ-independent.
13 pages. 2 figures

http://arxiv.org/abs/1306.3458
Dynamics of apparent horizons in quantum gravitational collapse
Yaser Tavakoli, Joao Marto, Andrea Dapor
(Submitted on 14 Jun 2013)
We study the gravitational collapse of a massless scalar field within the effective scenario of loop quantum gravity. Classical singularity is avoided and replaced by a quantum bounce in this model. It is shown that, quantum gravity effects predict a threshold scale below which no horizon can form as the collapse evolves towards the bounce.
4 pages. Contribution to the Spanish Relativity Meeting in Portugal 2012 (ERE2012), Guimaraes, Portugal

 

[marcus]

http://arxiv.org/abs/1306.4035
Graviton scattering amplitudes and Pure Connection Formulation of GR
Gianluca Delfino
(Submitted on 17 Jun 2013)
We show how the recently introduced "Pure Connection Formulation" of gravity provides a natural framework for approaching the problem of computing graviton scattering amplitudes. In particular, we show that the interaction vertices are greatly simplified in this formalism as compared to the Einstein-Hilbert perturbation theory. This, in turns, leads to very simple Feynman rules that we employ for the direct computations. Furthermore, this framework naturally extends to wider class of gravitational theories, which encompasses General Relativity as a special case. We compute all the possible tree-level graviton-graviton scattering amplitudes for a general theory from this class. In the GR case the results are in complete accordance with the known expressions in the literature. Moreover, for the general theory distinct from GR, we find new tree-level parity-violating amplitudes. The presence of this new amplitudes is a direct consequence of the fact that the general theory does not exhibit explicit parity invariance.
122 pages, PhD Thesis

 

[marcus]

http://arxiv.org/abs/1306.5206
The boundary is mixed
Eugenio Bianchi, Hal M. Haggard, Carlo Rovelli
(Submitted on 21 Jun 2013)
We show that Oeckl's boundary formalism incorporates quantum statistical mechanics naturally, and we formulate general-covariant quantum statistical mechanics in this language. We illustrate the formalism by showing how it accounts for the Unruh effect. We observe that the distinction between pure and mixed states weakens in the general covariant context, and surmise that local gravitational processes are indivisibly statistical with no possible quantal versus probabilistic distinction.
8 pages, 2 figures

http://arxiv.org/abs/1306.4995
Dust time in quantum cosmology
Viqar Husain, Tomasz Pawlowski
(Submitted on 20 Jun 2013)
We give a formulation of quantum cosmology with a pressureless dust and arbitrary additional matter fields. The dust provides a natural time gauge corresponding to a cosmic time, yielding a physical time independent Hamiltonian. The approach simplifies the analysis of both Wheeler-deWitt and loop quantum cosmology models, broadening the applicability of the latter.
3 pages. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

brief mention:
http://arxiv.org/abs/1306.5063
Entropy from near-horizon geometries of Killing horizons
Olaf Dreyer, Amit Ghosh, Avirup Ghosh
(Submitted on 21 Jun 2013)
We derive black hole entropy based on the near-horizon symmetries of black hole space-times. To derive these symmetries we make use of an (R,T)-plane close to a Killing horizon. We identify a set of vector fields that preserves this plane and forms a Witt algebra. The corresponding algebra of Hamiltonians is shown to have a non-trivial central extension. Using the Cardy formula and the central charge we obtain the Bekenstein-Hawking entropy.
8 pages.

 

[John86]

http://www.fqxi.org/community/forum/topic/1816
Essay Abstract
Relative information at the foundation of physics by Carlo Rovelli
I observe that Shannon's notion of relative information between two physical systems can effectively function as a foundation for statistical mechanics and quantum mechanics, without referring to any subjectivism or idealism. It can also represent the key missing element in the foundation of the naturalistic picture of the world, providing the conceptual tool for dealing with its apparent limitations. I comment on the relation between these ideas and Democritus.

http://www.fqxi.org/community/forum/topic/1831
Information-Based Physics and the Influence Network by Kevin H Knuth
This essay considers a simple model of observers that are influenced by the world around them. Consistent quantification of information about such influences results in a great deal of familiar physics. The end result is a new perspective on relativistic quantum mechanics, which includes both a way of conceiving of spacetime as well as particle “properties” that may be amenable to a unification of quantum mechanics and gravity. Rather than thinking about the universe as a computer, perhaps it is more accurate to think about it as a network of influences where the laws of physics derive from both consistent descriptions and optimal information-based inferences made by embedded observers.

 

[John86]

http://arxiv.org/abs/1306.5332
Super-accelerating bouncing cosmology in asymptotically-free non-local gravity
Gianluca Calcagni, Leonardo Modesto, Piero Nicolini
(Submitted on 22 Jun 2013)
Recently, evidence has been collected that a class of gravitational theories with certain non-local operators is renormalizable. We consider one such model which, at the linear perturbative level, reproduces the effective non-local action for the light modes of bosonic closed string field theory. Using the property of asymptotic freedom in the ultraviolet and fixing the classical behaviour of the scale factor at late times, an algorithm is proposed to find general homogeneous cosmological solutions valid both at early and late times. Imposing a power-law classical limit, these solutions (including anisotropic ones) display a bounce instead of a big-bang singularity, and super-accelerate near the bounce even in the absence of an inflaton or phantom field.

http://arxiv.org/abs/1306.5697
Dynamical Black Holes: Approach to the Final State
Abhay Ashtekar, Miguel Campiglia, Samir Shah
(Submitted on 24 Jun 2013)
Since black holes can be formed through widely varying processes, the horizon structure is highly complicated in the dynamical phase. Nonetheless, as numerical simulations show, the final state appears to be universal, well described by the Kerr geometry. How are all these large and widely varying deviations from the Kerr horizon washed out? To investigate this issue, we introduce a well-suited notion of horizon multipole moments and equations governing their dynamics, thereby providing a coordinate and slicing independent framework to investigate the approach to equilibrium. In particular, our flux formulas for multipoles can be used as analytical checks on numerical simulations and, in turn, the simulations could be used to fathom possible universalities in the way black holes approach their final equilibrium.

http://arxiv.org/abs/1306.5470
Bimetric Gravity, Variable Speed of Light Cosmology and Planck2013
J. W. Moffat
(Submitted on 23 Jun 2013)
A bimetric gravity model with a variable speed of light is shown to be in agreement with the results reported from the Planck satellite in 2013. The predicted scalar mode spectral index is $n_s\approx 0.96$ and its running is $\alpha_s\approx 8\times 10^{-4}$ when the fundamental length scale $\ell_0$ in the model is fixed to be $\ell_0\approx 10^5\ell_P$, where $\ell_P$ is the Planck length $\ell_P=1.62\times 10^{-33}\,{\rm cm}$, giving the observed CMB fluctuations: $\delta_H\approx 10^{-5}$. The enlarged lightcone ensures that horizon and flatness problems are solved. The model is free from many of the fine-tuning problems of the inflationary models and the fluctuations that form the seeds of structure formation do not lead to a chaotic inhomogeneous universe and the need for a multiverse.

http://arxiv.org/abs/1306.5471
Quantum Structures: A View of the Quantum World
Jasmina Jeknic-Dugic, Momir Arsenijevic, Miroljub Dugic
(Submitted on 23 Jun 2013)
We oer a systematic account of decomposition of quantum systems into parts. Dierent decompositions (structures) are mutually linked via the proper linear canonical transformations. Dierent kinds of structures, as well as their relations, are considered. Emphasis is placed on mutually global and irreducible structures. Is there a privileged structure of the closed system? Is there a preferred (bipartite) structure of an open system? Are there any practical advantages of certain alternative structures of the system? Is there a simple dynamical relation for a pair of structures? The [necessarily partial] answers are rather intriguing. Relativity of quantum correlations (that include entanglement as well as the "one-way" and "two-way" discord) is carefully presented. Emphasis is placed on the "parallel occurrence of decoherence" in the quantum Brownian motion. The environment-selected preferred structure of an open composite system is presented for a pair of harmonic oscillators (or the ?eld modes). A limitation of the Nakajima-Zwanzig projection method appears as a consequence of quantum correlations relativity. Hence, describing dynamics of an alternate open-system is a delicate task. Certain interpretational issues, which include "quantum reference frames" issue, are discussed. Some recent experiments are also discussed. To this end, the option that "there are no particles" on the most fundamental physical level naturally follows. Certain open questions and prospects for further research are highlighted.

 

[marcus]

http://arxiv.org/abs/1306.6126
The generator of spatial diffeomorphisms in the Koslowski- Sahlmann representation
Madhavan Varadarajan
(Submitted on 26 Jun 2013)
A generalization of the representation underlying the discrete spatial geometry of Loop Quantum Gravity, to accommodate states labelled by smooth spatial geometries, was discovered by Koslowski and further studied by Sahlmann. We show how to construct the diffeomorphism constraint operator in this Koslowski- Sahlmann (KS) representation from suitable connection and triad dependent operators. We show that the KS representation supports the action of hitherto unnoticed "background exponential" operators which, in contrast to the holonomy-flux operators, change the smooth spatial geometry label of the states. These operators are shown to be quantizations of certain connection dependent functions and play a key role in the construction of the diffeomorphism constraint operator.
8 pages

http://arxiv.org/abs/1306.6142
Consistent probabilities in loop quantum cosmology
David A. Craig, Parampreet Singh
(Submitted on 26 Jun 2013)
A fundamental issue for any quantum cosmological theory is to specify how probabilities can be assigned to various quantum events or sequences of events such as the occurrence of singularities or bounces. In previous work, we have demonstrated how this issue can be successfully addressed within the consistent histories approach to quantum theory for Wheeler-DeWitt-quantized cosmological models. In this work, we generalize that analysis to the exactly solvable loop quantization of a spatially flat, homogeneous and isotropic cosmology sourced with a massless, minimally coupled scalar field known as sLQC. We provide an explicit, rigorous and complete decoherent histories formulation for this model and compute the probabilities for the occurrence of a quantum bounce vs. a singularity. Using the scalar field as an emergent internal time, we show for generic states that the probability for a singularity to occur in this model is zero, and that of a bounce is unity, complementing earlier studies of the expectation values of the volume and matter density in this theory. We also show from the consistent histories point of view that all states in this model, whether quantum or classical, achieve arbitrarily large volume in the limit of infinite 'past' or 'future' scalar 'time', in the sense that the wave function evaluated at any arbitrary fixed value of the volume vanishes in that limit. Finally, we briefly discuss certain misconceptions concerning the utility of the consistent histories approach in these models.
22 pages, 3 figures

 

[John86]

http://arxiv.org/abs/1306.5805
Is the Pusey-Barrett-Rudolph Theorem Compatible with Quantum Nonseparability?
Maximilian Schlosshauer, Arthur Fine
(Submitted on 24 Jun 2013)
The Pusey-Barrett-Rudolph (PBR) no-go theorem targets a class of "epistemic" hidden-variables models in which hidden variables associated with distinct quantum states overlap. We show that the PBR strategy leads to a no-go result that would also rule out nonepistemic ("ontic") models. Moreover, it would rule out a vast class of deterministic hidden-variables theories, even those known to be consistent. The strength of this result calls into question a central assumption of the PBR strategy about how hidden variables of composite systems relate to hidden variables of components.

http://www.fqxi.org/community/forum/topic/1842
Essay Abstract
It from Qubit by Giacomo Mauro D'Ariano
In this essay I will embark on the venture of changing the realist reader’s mind about the informational viewpoint for physics: “It from Bit”. I will try to convince him of the amazing theoretical power of such paradigm. Contrary to the common belief, the whole history of physics is indeed a winding road making the notion of “physical object”–the “It”–fade away. Such primary concept, on which the structure of contemporary theoretical physics is still grounded, is no longer logically tenable. The thesis I advocate here is that the “It” is emergent from pure information, an information of special kind: quantum. The paradigm then becomes: “It from Qubit”. Quantum fields, particles, space-time and relativity simply emerge from countably infinitely many quantum systems in interaction. Don’t think that, however, I can cheat by suitably programming a “simulation” of what we see. On the contrary: the quantum software is constrained by very strict rules of topological nature, which minimize the algorithmic complexity. These are locality, unitariety, homogeneity, and isotropy of the processing, with minimal quantum dimension. What is amazing is that from just such simple rules, and without using relativity, we obtain the Dirac field dynamics as emergent.

 

[member 11137]

http://arxiv.org/abs/1306.3021v1

The Trace-Free Einstein Equations and inflation

George F R Ellis
(Submitted on 13 Jun 2013)
The trace-free version of the Einstein Gravitational equations, essentially equivalent to unimodular gravity, can solve the troubling issue of the huge discrepancy between quantum field theory estimates of the vacuum energy density and the astronomically observed value of the cosmological constant. However it has been suggested that this proposal cannot work because it prevents the inflaton potential energy from driving inflation. It is shown here that that concern is unjustified: inflation proceeds as usual if we adopt the trace free gravitational equations.

I have read both, the rules for that forum and the article. I find that it is an interesting and very clear exposé of a controversial and difficult problem (The inflation of our universe and the equation of state for the vacuum remain two puzzling item, so far I know).

Staying exclusively at the mathematical and physical level of the discussion, I get some confusion because of the fact that the conversation introduces (see p. 6) three different mass densities: the effective, the gravitational and the inertial one. Furthermore, equation (21) page 6 indicates an obvious difference between the gravitational and the inertial mass density. Isn't it in contradiction with some fundamental principle stating the equivalence between inerty and gravitation? What did I certainly miss? Thanks for explaining better, if possible.

 

[marcus]

Blackforest, our custom here is to reserve this thread for bibliography and start a separate discussion thread for any paper we want to discuss. I have quoted your post, with your question, in order to start a discussion thread.
https://www.physicsforums.com/showthread.php?p=4428693#post4428693
The idea is not to overload this thread and interfere with bibliography function.

 

[MTd2]

http://arxiv.org/abs/1307.0096

Elementary Particles as Gates for Universal Quantum Computation
Deepak Vaid
(Submitted on 29 Jun 2013)
It is shown that there exists a mapping between the fermions of the Standard Model (SM) represented as braids in the Bilson-Thompson model, and a set of gates which can perform Universal Quantum Computation (UQC). This leads us to conjecture that the "Computational Universe Hypothesis" (CUH) can be given a concrete implementation in a new physical framework where elementary particles and the gauge bosons (which intermediate interactions between fermions) are interpreted as the components of a quantum computational network, with the particles serving as quantum computational gates and the gauge fields as the information carrying entities.

 

[MTd2]

http://arxiv.org/abs/1101.5185

Einstein Manifolds As Yang-Mills Instantons

John J. Oh, Hyun Seok Yang
(Submitted on 27 Jan 2011 (v1), last revised 2 Jul 2013 (this version, v4))
It is well-known that Einstein gravity can be formulated as a gauge theory of Lorentz group where spin connections play a role of gauge fields and Riemann curvature tensors correspond to their field strengths. One can then pose an interesting question: What is the Einstein equations from the gauge theory point of view? Or equivalently, what is the gauge theory object corresponding to Einstein manifolds? We show that the Einstein equations in four dimensions are precisely self-duality equations in Yang-Mills gauge theory and so Einstein manifolds correspond to Yang-Mills instantons in SO(4) = SU(2)_L x SU(2)_R gauge theory. Specifically, we prove that any Einstein manifold with or without a cosmological constant always arises as the sum of SU(2)_L instantons and SU(2)_R anti-instantons. This result explains why an Einstein manifold must be stable because two kinds of instantons belong to different gauge groups, instantons in SU(2)_L and anti-instantons in SU(2)_R, and so they cannot decay into a vacuum. We further illuminate the stability of Einstein manifolds by showing that they carry nontrivial topological invariants.

 

[John86]

http://arxiv.org/abs/1307.0256
Quantum astrometric observables II: time delay in linearized quantum gravity
Béatrice Bonga, Igor Khavkine
(Submitted on 1 Jul 2013)
A clock synchronization thought experiment is modeled by a diffeomorphism invariant "time delay" observable. In a sense, this observable probes the causal structure of the ambient Lorentzian spacetime. Thus, upon quantization, it is sensitive to the long expected smearing of the light cone by vacuum fluctuations in quantum gravity. After perturbative linearization, its mean and variance are computed in the Minkowski Fock vacuum of linearized gravity. The na\"ive divergence of the variance is meaningfully regularized by a length scale $\mu$, the physical detector resolution. This is the first time vacuum fluctuations have been fully taken into account in a similar calculation. Despite some drawbacks this calculation provides a useful template for the study of a large class of similar observables in quantum gravity. Due to their large volume, intermediate calculations were performed using computer algebra software. The resulting variance scales like $(s \ell_p/\mu)^2$, where $\ell_p$ is the Planck length and $s$ is the distance scale separating the ("lab" and "probe") clocks. Additionally, the variance depends on the relative velocity of the lab and the probe, diverging for low velocities. This puzzling behavior may be due to an oversimplified detector resolution model or a neglected second order term in the time delay.

http://arxiv.org/abs/1307.0745
Rainbow gravity and scale-invariant fluctuations
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 2 Jul 2013)
We re-examine a recently proposed scenario where the deformed dispersion relations associated with a flow of the spectral dimension to a UV value of 2 leads to a scale-invariant spectrum of cosmological fluctuations, without the need for inflation. In that scenario Einstein gravity was assumed. The theory displays a wavelength-dependent speed of light but by transforming to a suitable "rainbow frame" this feature can be removed, at the expense of modifying gravity. We find that the ensuing rainbow gravity theory is such that gravity switches off at high energy (or at least leads to a universal conformal coupling). This explains why the fluctuations are scale-invariant on all scales: there is no horizon scale as such. For dispersion relations that do not lead to exact scale invariance we find instead esoteric inflation in the rainbow frame. We argue that these results shed light on the behaviour of gravity under the phenomenon of dimensional reduction

http://arxiv.org/abs/1307.0722
Linear lattice gauge theory
C.Wetterich
(Submitted on 2 Jul 2013)
Linear lattice gauge theory is based on link variables that are arbitrary complex or real $N\times N$ matrices. This contrasts with the usual (non-linear) formulation with unitary or orthogonal matrices. The additional degrees of freedom correspond to massive particles. We discuss a limit in parameter space where linear lattice gauge theory becomes equivalent to the standard formulation. We argue that the continuum limit of linear lattice gauge theory may be a useful setting for an analytic description of confinement. The running gauge coupling corresponds to the flow of the minimum of a "link potential". This minimum occurs for nonzero values $l_0$ in the perturbative regime, while $l_0$ vanishes in the confinement regime.

http://arxiv.org/abs/1307.0246
Noisy soccer balls
Giovanni Amelino-Camelia, Laurent Freidel, Jerzy Kowalski-Glikman, Lee Smolin
(Submitted on 30 Jun 2013)
In her Comment arXiv:1202.4066 [hep-th] Hossenfelder proposes a generalization of the results we reported in Phys. Rev. D84 (2011) 087702 and argues that thermal fluctuations introduce incurable pathologies for the description of macroscopic bodies in the relative-locality framework. We here show that Hossenfelder's analysis, while raising a very interesting point, is incomplete and leads to incorrect conclusions. Her estimate for the fluctuations did not take into account some contributions from the geometry of momentum space which must be included at the relevant order of approximation. Using the full expression here derived one finds that thermal fluctuations are not in general large for macroscopic bodies in the relative-locality framework. We find that such corrections can be unexpectedly large only for some choices of momentum-space geometry, and we comment on the possibility of developing a phenomenology suitable for possibly ruling out such geometries of momentum space.

 

[MTd2]

Relative Locality in $κ$-Poincaré

http://arxiv.org/abs/1106.5710

Giulia Gubitosi, Flavio Mercati
(Submitted on 28 Jun 2011 (v1), last revised 3 Jul 2013 (this version, v2))
We show that the $\kappa$-Poincar\'e Hopf algebra can be interpreted in the framework of curved momentum space leading to the relativity of locality \cite{AFKS}. We study the geometric properties of the momentum space described by $\kappa$-Poincar\'e, and derive the consequences for particles propagation and energy-momentum conservation laws in interaction vertices, obtaining for the first time a coherent and fully workable model of the deformed relativistic kinematics implied by $\kappa$-Poincar\'e. We describe the action of boost transformations on multi-particles systems, showing that in order to keep covariant the composed momenta it is necessary to introduce a dependence of the rapidity parameter on the particles momenta themselves. Finally, we show that this particular form of the boost transformations keeps the validity of the relativity principle, demonstrating the invariance of the equations of motion under boost transformations.

 

[marcus]

http://arxiv.org/abs/1307.1420
Approximation methods in Loop Quantum Cosmology: From Gowdy cosmologies to inhomogeneous models in Friedmann-Robertson-Walker geometries
Mercedes Martín-Benito, Daniel Martín-de Blas, Guillermo A. Mena Marugán
(Submitted on 4 Jul 2013)
We develop approximation methods in the hybrid quantization of the Gowdy model with linear polarization and a massless scalar field, for the case of three-torus spatial topology. The loop quantization of the homogeneous gravitational sector of the Gowdy model (according to the improved dynamics prescription) and the presence of inhomogeneities lead to a very complicated Hamiltonian constraint. Therefore, the extraction of physical results calls for the introduction of well justified approximations. We first show how to approximate the homogeneous part of the Hamiltonian constraint, corresponding to Bianchi I geometries, as if it described a Friedmann-Robertson-Walker (FRW) model corrected with anisotropies. This approximation is valid in the high-energy sector of the FRW geometry (concerning its contribution to the constraint) and for anisotropy profiles that are sufficiently smooth. In addition, for certain families of states associated to regimes of physical interest, with negligible effects of the anisotropies and small inhomogeneities, one can approximate the Hamiltonian constraint of the inhomogeneous system by that of an FRW geometry with a relatively simple matter content, and then obtain its solutions.
20 pages, 3 figures

not QG, however possibly of general interest:
http://arxiv.org/abs/1307.1331
A new perspective on CP and T violation
Abhay Ashtekar
(Submitted on 4 Jul 2013)
It is shown that the results of CP and T violation experiments can be interpreted using a very general framework that does not require a Hilbert space of states or linear operators to represent the symmetries or dynamics. Analysis within this general framework brings out the aspects of quantum mechanics that are essential for this interpretation. More importantly, should quantum mechanics be eventually replaced by a new paradigm, this framework could still be used to establish violation of CP and T invariance from the already known experimental results.
This work arose as a 'formal response' to a talk "Three Merry Roads to T-Violation" by Dr. Bryan Roberts [1]. Both talks were given at a "Workshop on Cosmology and Time" held at Penn State in April 2013, which brought together physicists and philosophers of science.
10 pages; 'Formal Response' to the talk "Three Merry Roads to T-Violation" by Bryan Roberts at the Workshop on Cosmology and Time, held at the Institute for Gravitation and the Cosmos, Penn State, in April 2013

 

[marcus]

http://arxiv.org/abs/1307.1679
Holonomy spin foam models: Asymptotic geometry of the partition function
Frank Hellmann, Wojciech Kaminski
(Submitted on 5 Jul 2013)
We study the asymptotic geometry of the spin foam partition function for a large class of models, including the models of Barrett and Crane, Engle, Pereira, Rovelli and Livine, and, Freidel and Krasnov.
The asymptotics is taken with respect to the boundary spins only, no assumption of large spins is made in the interior. We give a sufficient criterion for the existence of the partition function. We find that geometric boundary data is suppressed unless its interior continuation satisfies certain accidental curvature constraints. This means in particular that most Regge manifolds are suppressed in the asymptotic regime. We discuss this explicitly for the case of the configurations arising in the 3-3 Pachner move. We identify the origin of these accidental curvature constraints as an incorrect twisting of the face amplitude upon introduction of the Immirzi parameter and propose a way to resolve this problem, albeit at the price of losing the connection to the SU(2) boundary Hilbert space.
The key methodological innovation that enables these results is the introduction of the notion of wave front sets, and the adaptation of tools for their study from micro local analysis to the case of spin foam partition functions.
63 pages, 5 figures
Note: Frank Hellmann is giving a plenary talk at Loops 2013 later this month, quite possibly the topic will be holonomy spin foams, but i haven't yet seen the title of his talk listed.

 

[marcus]

http://arxiv.org/abs/1307.1848
High Energy Physics - Theory
Local Conformal Symmetry in Physics and Cosmology
Itzhak Bars, Paul Steinhardt, Neil Turok
(Submitted on 7 Jul 2013)
We review some of the arguments for why scale symmetry may be a fundamental principle in nature and, if so, why it is likely to be manifest as a local conformal symmetry including gravity. We show how to lift a generic non-scale invariant theory in Einstein frame into a Weyl-invariant theory and present the general form for Lagrangians consistent with Weyl symmetry. Various applications to cosmology are then discussed: the construction of classically geodesically complete cosmologies, the determination of initial conditions after the big bang, inflation, the metastability of the Higgs and cyclic cosmology. As examples, we focus on the standard model and Higgs cosmology, exploring the notion that the Higgs alone could be sufficient to explain the stages of cosmic evolution after (and perhaps before) the big bang and the large-scale features of the universe.
28 pages, 1 figure

brief mention:
http://arxiv.org/abs/1307.2060
Pseudo Weyl invariance is still anomalous
Enrique Alvarez, Mario Herrero-Valea
(Submitted on 8 Jul 2013)
A Weyl invariant extension of Einstein gravity is studied. It simply consists in the group averaging of Einstein's action under Weyl transformations. Contradicting cherished beliefs, a conformal anomaly is found in the trace of the equations of motion ...It is however possible to keep Weyl invariance as a bona fide symmetry at the price of losing full diffeomorphism invariance. This is what happens in unimodular gravity, a closely related theory.
19 pages

 

[marcus]

http://arxiv.org/abs/1307.2227
The two faces of Hawking radiation
Matteo Smerlak
(Submitted on 5 Jul 2013)
What happens when Alice falls into a black hole? In spite of recent challenges by Almheiri et al. -- the ""firewall" hypothesis -- the consensus on this question tends to remain "nothing special". Here I argue that something rather special can happen near the horizon, already at the semiclassical level: besides the standard Hawking outgoing modes, Alice can record a quasi-thermal spectrum of ingoing modes, whose temperature and intensity diverges as Alice's Killing energy E goes to zero. I suggest that this effect can be thought of in terms a "horizon-infinity duality", which relates the perception of near-horizon and asymptotic geodesic observers -- the two faces of Hawking radiation.
7 pages Honorable Mention in the Gravity Research Foundation 2013 Essay Competition

 

[marcus]

http://arxiv.org/abs/1307.2719
Deformations of Polyhedra and Polygons by the Unitary Group
Etera R. Livine
(Submitted on 10 Jul 2013)
We introduce the set of framed (convex) polyhedra with N faces as the symplectic quotient C^2N //SU(2). A framed polyhedron is then parametrized by N spinors living in C² satisfying suitable closure constraints and defines a usual convex polyhedron plus extra U(1) phases attached to each face. We show that there is a natural action of the unitary group U(N) on this phase space, which changes the shape of faces and allows to map any (framed) polyhedron onto any other with the same total (boundary) area. This identifies the space of framed polyhedra to the Grassmannian space U(N )/ (SU(2)×U(N −2)). We show how to write averages of geometrical observables (polynomials in the faces’ area and the angles between them) over the ensemble of polyhedra (distributed uniformly with respect to the Haar measure on U(N)) as polynomial integrals over the unitary group and we provide a few methods to compute these integrals systematically. We also use the Itzykson-Zuber formula from matrix models as the generating function for these averages and correlations.
In the quantum case, a canonical quantization of the framed polyhedron phase space leads to the Hilbert space of SU(2) intertwiners (or, in other words, SU(2)-invariant states in tensor products of irreducible representations). The total boundary area as well as the individual face areas are quantized as half-integers (spins), and the Hilbert spaces for fixed total area form irreducible representations of U(N). We define semi-classical coherent intertwiner states peaked on classical framed polyhedra and transforming consistently under U(N) transformations. And we show how the U(N) character formula for unitary transformations is to be considered as an extension of the Itzykson-Zuber to the quantum level and generates the traces of all polynomial observables over the Hilbert space of intertwiners.
We finally apply the same formalism to two dimensions and show that classical (convex) polygons can be described in a similar fashion trading the unitary group for the orthogonal group. We conclude with a discussion of the possible (deformation) dynamics that one can define on the space of polygons or polyhedra. This work is a priori useful in the context of discrete geometry but it should hopefully also be relevant to (loop) quantum gravity in 2+1 and 3+1 dimensions when the quantum geometry is defined in terms of gluing of (quantized) polygons and polyhedra.
33 pages

 

[marcus]

http://arxiv.org/abs/1307.3228
Maximal acceleration in covariant loop gravity and singularity resolution
Carlo Rovelli, Francesca Vidotto
(Submitted on 11 Jul 2013)
A simple argument indicates that covariant loop gravity (spinfoam theory) predicts a maximal acceleration, and hence forbids the development of curvature singularities. This supports the results obtained for cosmology and black holes using canonical methods.
4 pages, 1 figure

 

[MTd2]

http://arxiv.org/abs/1102.1592

From Dimensional Reduction of 4d Spin Foam Model to Adding Non-Gravitational Fields to 3d Spin Foam Model
Somayeh Fani, Kamran Kaviani
(Submitted on 8 Feb 2011 (v1), last revised 14 Jul 2013 (this version, v5))
A Kaluza-Klein like approach for a 4d spin foam model is considered. By applying this approach to a model based on group field theory in 4d (TOCY model), and using the Peter-Weyl expansion of the gravitational field, reconstruction of new non gravitational fields and interactions in the action are found. The perturbative expansion of the partition function produces graphs colored with su(2) algebraic data, from which one can reconstruct a 3d simplicial complex representing space-time and its geometry; (like in the Ponzano-Regge formulation of pure 3d quantum gravity), as well as the Feynman graph for typical matter fields. Thus a mechanism for generation of matter and construction of new dimensions are found from pure gravity.

 

[marcus]

http://arxiv.org/abs/1307.4747
Asymptotic of Lorentzian Polyhedra Propagator
Jacek Puchta
(Submitted on 17 Jul 2013)
A certain operator T=\int_{SL(2,C)}dgY^{\dagger}gY can be found in various Lorentzian EPRL calculations. The properties of this operator has been studied here in large j limit. The leading order of T is proportional to the identity operator.
Knowing the operator T one can renormalize spin-foam's edge self-energy by computing the amplitude of sum of a series of edges with increasing number of vertices and bubbles. This amplitude is calculated and is shown to be convergent.
Moreover some technical tools useful in Lorentzian Spin-Foam calculation have been developed.
28 pages, 1 figure

http://arxiv.org/abs/1307.4687
Comment on "Time and a Physical Hamiltonian for Quantum Gravity"
Jedrzej Świeżewski
(Submitted on 17 Jul 2013)
The aim of this comment is to present a simple argument showing that in the irrotational dust model the Hamilotnian constraint is indeed free of the square root if the time-gauge is chosen. This feature requires no additional assumptions, namely no choices of signs.
2 pages (comment on Husain Pawlowski http://arxiv.org/abs/1108.1145 )

brief mention:
http://arxiv.org/abs/1307.4706
Gauge/Gravity Duality and the Black Hole Interior
Donald Marolf, Joseph Polchinski
(Submitted on 17 Jul 2013)
We present a further argument that typical black holes with field theory duals have firewalls at the horizon...
... We also address the ER=EPR conjecture of Maldacena and Susskind, arguing that the correlations in generic highly entangled states cannot be geometrized as a smooth wormhole.
5 pages
[my comment: Ashtekar pointed out at GR20 that LQG BH eliminates singularity and necessity of "firewall"]

http://gr20-amaldi10.edu.pl/userfiles/book_07_07_2013.pdf
Quantum space-times and unitarity of black hole evaporation
Ashtekar A
There is growing evidence that, because of the singularity resolution, quantum space-times can be vastly larger than what classical general relativity would lead us to believe. We review arguments that, thanks to this enlargement, unitarity is restored in the evaporation of black holes. In contrast to ADS/CFT, these arguments deal with the evaporation process directly in the physical space-time.
page 216 of the book of abstracts of the July 2013 GR20 conference.

There is also a paper by Don Marolf presented at the same joint session

Ads/cft, unitary black hole evaporation, and firewalls
Marolf D
We review arguments that black hole evaporation is unitary in AdS/CFT. As a result, the physics expe- rienced by infalling observers at the horizon of at least sufficiently old black holes described by AdS/CFT must be dramatically different from that described by familiar field theory in a smooth spacetime.
page 217 of the GR20 abstracts.

 

[MTd2]

http://www.nature.com/news/cosmologist-claims-universe-may-not-be-expanding-1.13379

http://arxiv.org/abs/1303.6878

A Universe without expansion
C. Wetterich
(Submitted on 27 Mar 2013 (v1), last revised 5 Jul 2013 (this version, v2))
We discuss a cosmological model where the universe shrinks rather than expands during the radiation and matter dominated periods. Instead, the Planck mass and all particle masses grow exponentially. Together with a preceding inflationary phase and a late dark energy dominated epoch this model is compatible with all observations. The curvature is almost constant during all epochs. Cosmology has no big bang singularity. There exist other, equivalent choices of field variables for which the universe shows the usual expansion or is static during the radiation or matter dominated epochs. For those ``field coordinates`` the big bang is singular. Thus the big bang singularity turns out to be related to a singular choice of field coordinates.

 

[marcus]

http://arxiv.org/abs/1307.5029
Black hole entropy from loop quantum gravity in higher dimensions
Norbert Bodendorfer
(Submitted on 18 Jul 2013)
We propose a derivation for computing black hole entropy for spherical non-rotating isolated horizons from loop quantum gravity in four and higher dimensions. The state counting problem effectively reduces to the well studied 3+1-dimensional one based on an SU(2)-Chern-Simons theory, differing only in the precise form of the area spectrum and the restriction to integer spins.
5 pages

http://arxiv.org/abs/1307.5026
Melonic phase transition in group field theory
Aristide Baratin, Sylvain Carrozza, Daniele Oriti, James P. Ryan, Matteo Smerlak
(Submitted on 18 Jul 2013)
Group field theories have recently been shown to admit a 1/N expansion dominated by so-called `melonic graphs', dual to triangulated spheres. In this note, we deepen the analysis of this melonic sector. We obtain a combinatorial formula for the melonic amplitudes in terms of a graph polynomial related to a higher dimensional generalization of the Kirchhoff tree-matrix theorem. Simple bounds on these amplitudes show the existence of a phase transition driven by melonic interaction processes. We restrict our study to the Boulatov-Ooguri models, which describe topological BF theories and are the basis for the construction of four dimensional models of quantum gravity.
8 pages, 4 figures

 

[marcus]

http://arxiv.org/abs/1307.5238
Anomaly-free perturbations with inverse-volume and holonomy corrections in Loop Quantum Cosmology
Thomas Cailleteau, Linda Linsefors, Aurelien Barrau
(Submitted on 19 Jul 2013)
This article addresses the issue of the closure of the algebra of constraints for generic (cosmological) perturbations when taking into account simultaneously the two main corrections of effective loop quantum cosmology, namely the holonomy and the inverse-volume terms. Previous works on either the holonomy or the inverse volume case are reviewed and generalized. In the inverse-volume case, we point out new possibilities. An anomaly-free solution including both corrections is found for perturbations, and the corresponding equations of motion are derived.
19 pages.

http://arxiv.org/abs/1307.5281
Double Scaling in Tensor Models with a Quartic Interaction
Stephane Dartois, Razvan Gurau, Vincent Rivasseau
(Submitted on 19 Jul 2013)
In this paper we identify and analyze in detail the subleading contributions in the 1/N expansion of random tensors, in the simple case of a quartically interacting model. The leading order for this 1/N expansion is made of graphs, called melons, which are dual to particular triangulations of the D-dimensional sphere, closely related to the "stacked" triangulations. For D<6 the subleading behavior is governed by a larger family of graphs, hereafter called cherry trees, which are also dual to the D-dimensional sphere. They can be resummed explicitly through a double scaling limit. In sharp contrast with random matrix models, this double scaling limit is stable. Apart from its unexpected upper critical dimension 6, it displays a singularity at fixed distance from the origin and is clearly the first step in a richer set of yet to be discovered multi-scaling limits.
40 pages.

brief mention:
http://arxiv.org/abs/1307.5303
Invariant Connections in Loop Quantum Gravity
Maximilian Hanusch
(Submitted on 19 Jul 2013)
38 pages

 

[marcus]

http://arxiv.org/abs/1307.5461
Quantum hyperbolic geometry in loop quantum gravity with cosmological constant
Maite Dupuis, Florian Girelli
(Submitted on 20 Jul 2013)
Loop Quantum Gravity (LQG) is an attempt to describe the quantum gravity regime. Introducing a non-zero cosmological constant Λ in this context has been a withstanding problem. Other approaches, such as Chern-Simons gravity, suggest that quantum groups can be used to introduce Λ in the game. Not much is known when defining LQG with a quantum group. Tensor operators can be used to construct observables in any type of discrete quantum gauge theory with a classical/quantum gauge group. We illustrate this by constructing explicitly geometric observables for LQG defined with a quantum group and show for the first time that they encode a quantized hyperbolic geometry. This is a novel argument pointing out the usefulness of quantum groups as encoding a non-zero cosmological constant. We conclude by discussing how tensor operators provide the right formalism to unlock the LQG formulation with a non-zero cosmological constant.
6 pages, 1 figure

http://arxiv.org/abs/1307.5469
De Sitter Universe from Causal Dynamical Triangulations without Preferred Foliation
S. Jordan, R. Loll
(Submitted on 20 Jul 2013)
We present a detailed analysis of a recently introduced version of Causal Dynamical Triangulations (CDT) that does not rely on a distinguished time slicing. Focussing on the case of 2+1 spacetime dimensions, we analyze its geometric and causal properties, present details of the numerical set-up and explain how to extract "volume profiles". Extensive Monte Carlo measurements of the system show the emergence of a de Sitter universe on large scales from the underlying quantum ensemble, similar to what was observed previously in standard CDT quantum gravity. This provides evidence that the distinguished time slicing of the latter is not an essential part of its kinematical set-up.
44 pages, 29 figures

http://arxiv.org/abs/1307.5527
Loop Quantum Cosmology and the Fine Structure Constant
David Sloan
(Submitted on 21 Jul 2013)
The cosmological implications of introducing a variation to the fine structure 'constant', α are examined within the context of Loop Quantum Cosmology. The evolution of α is described using the model introduced by Bekenstein, Sandvik, Barrow and Magueijo, in which a ghost scalar field produces the variation. The dynamics of the system are examined in flat and closed cosmological settings. Matter consisting of the scalar field and radiation are examined with a thermodynamically motivated coupling between the two, which can lead to a series of bounces induced by both the negative density effects of the ghost field and the loop effects.
10 pages, 5 figures

http://arxiv.org/abs/1307.5352
Cosine problem in EPRL/FK spinfoam model
Marko Vojinovic
(Submitted on 19 Jul 2013)
We calculate the classical limit effective action of the EPRL/FK spinfoam model of quantum gravity coupled to matter fields. By employing the standard QFT background field method adapted to the spinfoam setting, we find that the model has many different classical effective actions. Most notably, these include the ordinary Einstein-Hilbert action coupled to matter, but also an action which describes antigravity. All those multiple classical limits appear as a consequence of of the fact that the EPRL/FK vertex amplitude has cosine-like large spin asymptotics. We discuss some possible ways to eliminate the unwanted classical limits.
20 pages

possibly of general interest:
http://arxiv.org/abs/1307.5737
Structures in the Planck map of the CMB
Daniel An, Krzysztof A. Meissner, Pawel Nurowski
(Submitted on 22 Jul 2013)
We present the results of the quest for ring-type structures on the maps observed by the Planck satellite.

 

[marcus]

http://arxiv.org/abs/1307.6167
The Universe as a Process of Unique Events
Marina Cortês, Lee Smolin
(Submitted on 23 Jul 2013)
We describe a new class of models of quantum space-time based on energetic causal sets and show that under natural conditions space-time emerges from them. These are causal sets whose causal links are labelled by energy and momentum and conservation laws are applied at events. The models are motivated by principles we propose govern microscopic physics which posit a fundamental irreversibility of time. One consequence is that each event in the history of the universe has a distinct causal relationship to the rest; this requires a novel form of dynamics which an be applied to uniquely distinctive events. We hence introduce a new kind of deterministic dynamics for a causal set in which new events are generated from pairs of progenitor events by a rule which is based on extremizing the distinctions between causal past sets of events. This dynamics is asymmetric in time, but we find evidence from numerical simulations of a 1+1 dimensional model, that an effective dynamics emerges which restores approximate time reversal symmetry. Finally we also present a natural twistorial representation of energetic causal sets.
26 pages, 5 figures

http://arxiv.org/abs/1307.5885
Linking covariant and canonical LQG II: Spin foam projector
Thomas Thiemann, Antonia Zipfel
(Submitted on 22 Jul 2013)
In a seminal paper, Kaminski, Kisielowski an Lewandowski for the first time extended the definition of spin foam models to arbitrary boundary graphs. This is a prerequisite in order to make contact to the canonical formulation of Loop Quantum Gravity (LQG) and allows to investigate the question whether any of the presently considered spin foam models yield a rigging map for any of the presently defined Hamiltonian constraint operators. The KKL extension cannot be described in terms of Group Field Theory (GFT) since arbitrary foams are involved while GFT is tied to simplicial complexes. Therefore one has to define the sum over spin foams with given boundary spin networks in an independent fashion using natural axioms, most importantly a gluing property for 2-complexes. These axioms are motivated by the requirement that spin foam amplitudes should define a rigging map (physical inner product) induced by the Hamiltonian constraint. This is achieved by constructing a spin foam operator based on abstract 2-complexes that acts on the kinematical Hilbert space of Loop Quantum Gravity. In the analysis of the resulting object we are able to identify an elementary spin foam transfer matrix that allows to generate any finite foam as a finite power of the transfer matrix. It transpires that the sum over spin foams, as written, does not define a projector on the physical Hilbert space. This statement is independent of the concrete spin foam model and Hamiltonian constraint. However, the transfer matrix potentially contains the necessary ingredient in order to construct a proper rigging map in terms of a modified transfer matrix.
62 pages, 14 figures

http://arxiv.org/abs/1307.5979
The Large-Volume Limit of a Quantum Tetrahedron is a Quantum Harmonic Oscillator
John Schliemann
(Submitted on 23 Jul 2013)
It is shown that the volume operator of a quantum tetrahedron is, in the sector of large eigenvalues, accurately described by a quantum harmonic oscillator. This result relies on the fact that (i) the volume operator couples only neighboring states of its standard basis, and (ii) its matrix elements show a unique maximum as a function of internal angular momentum quantum numbers. These quantum numbers, considered as a continuous variable, are the coordinate of the oscillator describing its quadratic potential, while the corresponding derivative defines a momentum operator. We also analyze the scaling properties of the oscillator parameters as a function of the size of the tetrahedron, and the role of different angular momentum coupling schemes.
10 pages, 3 figures; comments welcome

not directly QG-related, but possibly of interest:
http://arxiv.org/abs/1307.6122
Black-hole entropy and minimal diffusion
Michele Arzano, Gianluca Calcagni
(Submitted on 23 Jul 2013)
The density of states reproducing the Bekenstein-Hawking entropy-area scaling can be modeled via a nonlocal field theory. We define a diffusion process based on the kinematics of this theory and find a spectral dimension whose flow exhibits surprising properties. While it asymptotes the infrared value of four from above, in the ultraviolet the spectral dimension diverges at a finite (Planckian) value of the diffusion length, signalling a breakdown of the notion of diffusion on a continuum spacetime below that scale. The correlation length remains finite throughout the flow. We comment on the implications of this minimal diffusion scale for the entropy bound in a holographic and field-theoretic context.
4 pages, 1 figure

http://arxiv.org/abs/1307.6169
The world is discrete
Olaf Dreyer
(Submitted on 23 Jul 2013)
We argue that the scale-free spectrum that is observed in the cosmic microwave background is the result of a phase transition in the early universe. The observed tilt of the spectrum, which has been measured to be 0.04, is shown to be equal to the anomalous scaling dimension of the correlation function. The phase transition replaces inflation as the mechanism that produces this spectrum. The tilt further indicates that there is a fundamental small length scale in nature that we have not yet observed in any other way.
12 pages, 1 figure

 

[John86]

Maybe Freidel is trying to close the gap between strings and loops ?

http://arxiv.org/abs/1307.7080
Born Reciprocity in String Theory and the Nature of Spacetime
Laurent Freidel, Robert G. Leigh, Djordje Minic
(Submitted on 26 Jul 2013)
After many years, the deep nature of spacetime in string theory remains an enigma. In this letter we incorporate the concept of Born reciprocity in order to provide a new point of view on string theory in which spacetime is a derived dynamical concept. This viewpoint may be thought of as a dynamical chiral phase space formulation of string theory, in which Born reciprocity is implemented as a choice of a Lagrangian submanifold of the phase space, and amounts to a generalization of T-duality. In this approach the fundamental symmetry of string theory contains phase space diffeomorphism invariance and the underlying string geometry should be understood in terms of dynamical bi-Lagrangian manifolds and an apparently new geometric structure, somewhat reminiscent of para-quaternionic geometry, which we call Born geometry.

 

[John86]

http://arxiv.org/abs/1307.7376
Multiversality
Frank Wilczek
(Submitted on 28 Jul 2013)
Valid ideas that physical reality is vastly larger than human perception of it, and that the perceived part may not be representative of the whole, exist on many levels and have a long history. After a brief general inventory of those ideas and their implications, I consider the cosmological "multiverse" much discussed in recent scientific literature. I review its theoretical and (broadly) empirical motivations, and its disruptive implications for the traditional program of fundamental physics. I discuss the inflationary axion cosmology, which provides an example where firmly rooted, plausible ideas from microphysics lead to a well-characterized "mini-multiverse" scenario, with testable phenomenological consequences.

 

[marcus]

not directly Loop-or-allied gravity but of general interest:
http://arxiv.org/abs/1307.8106
Cyclic Cosmology, Conformal Symmetry and the Metastability of the Higgs
Itzhak Bars, Paul J. Steinhardt, Neil Turok
(Submitted on 30 Jul 2013)
Recent measurements at the LHC suggest that the current Higgs vacuum could be metastable with a modest barrier (height 10^10-12GeV⁴) separating it from a ground state with negative vacuum density of order the Planck scale. We note that metastability is problematic for big bang to end one cycle, bounce, and begin the next. In this paper, motivated by the approximate scaling symmetry of the standard model of particle physics and the primordial large-scale structure of the universe, we use our recent formulation of the Weyl-invariant version of the standard model coupled to gravity to track the evolution of the Higgs in a regularly bouncing cosmology. We find a band of solutions in which the Higgs field escapes from the metastable phase during each big crunch, passes through the bang into an expanding phase, and returns to the metastable vacuum, cycle after cycle after cycle. We show that, due to the effect of the Higgs, the infinitely cycling universe is geodesically complete, in contrast to inflation.
16 pages, 4 figures

http://arxiv.org/abs/1307.7988
Pathways to relativistic curved momentum spaces: de Sitter case study
Giovanni Amelino-Camelia, Giulia Gubitosi, Giovanni Palmisano
(Submitted on 30 Jul 2013)
Several arguments suggest that the Planck scale could be the characteristic scale of curvature of momentum space. As other recent studies we assume that the metric of momentum space determines the condition of on-shellness while the momentum-space affine connection governs the form of the law of composition of momenta. We show that the possible choices of laws of composition of momenta are more numerous than the possible choices of affine connection on a momentum space. This motivates us to propose a new prescription for associating an affine connection to momentum composition, which we compare to the one most used in the recent literature. We find that the two prescriptions lead to the same picture of the so-called κ-momentum space, with de Sitter metric and κ-Poincaré connection. We also examine in greater detail than ever before the DSR-relativistic properties of κ-momentum space, particularly in relation to its noncommutative law of composition of momenta. We then show that in the case of "proper de Sitter momentum space", with the de Sitter metric and its Levi-Civita connection, the two prescriptions are inequivalent. Our novel prescription leads to a picture of proper de Sitter momentum space which is DSR-relativistic and is characterized by a commutative law of composition of momenta, a possibility for which no explicit curved-momentum-space picture had been previously found. We argue that our construction provides a natural test case for the study of momentum spaces with commutative, and yet deformed, laws of composition of momenta. Moreover, it can serve as laboratory for the exploration of the properties of DSR-relativistic theories which are not connected to group-manifold momentum spaces and Hopf algebras.
36 pages, 1 figure

Footnote: one of the authors gives a 20 minute talk on this here (start at minute 49:00 of the recording):
http://pirsa.org/13070052/
Phenomenology - 1
Jonathan Granot, Julien Bolmont, Giulia Gubitosi, Giovanni Palmisano, Linqing Chen
21:00, 49:00, 75:00, 95:00
===========
For the complete list of parallel session talks, with minute-marks, go here:
https://www.physicsforums.com/showthread.php?p=4458326#post4458326

 

[atyy]

http://arxiv.org/abs/1308.0040
Spinning geometry = Twisted geometry
Laurent Freidel, Jonathan Ziprick
(Submitted on 31 Jul 2013)
It is well known that the $\SU(2)$-gauge invariant phase space of loop gravity can be represented in terms of twisted geometries. These are piecewise-linear-flat geometries obtained by gluing together polyhedra, but the resulting geometries are not continuous across the faces. Here we show that this phase space can also be represented by continuous, piecewise-flat three-geometries called spinning geometries. These are composed of metric-flat three-cells glued together consistently. The geometry of each cell and the manner in which they are glued is compatible with the choice of fluxes and holonomies.
We first remark that the fluxes provide each edge with an angular momentum. By studying the piecewise-flat geometries which minimize edge lengths, we show that these angular momenta can be literally interpreted as the spin of the edges: the geometries of all edges are necessarily helices. We also show that the compatibility of the gluing maps with the holonomy data results in the same conclusion. This shows that a spinning geometry represents a way to glue together the three-cells of a twisted geometry to form a continuous geometry which represents a point in the loop gravity phase space.

http://arxiv.org/abs/1308.0300
Snyder Momentum Space in Relative Locality
Andrzej Banburski, Laurent Freidel
(Submitted on 1 Aug 2013)
The standard approaches of phenomenology of Quantum Gravity have usually explicitly violated Lorentz invariance, either in the dispersion relation or in the addition rule for momenta. We investigate whether it is possible in 3+1 dimensions to have a non local deformation that preserves fully Lorentz invariance, as it is the case in 2+1D Quantum Gravity. We answer positively to this question and show for the first time how to construct a homogeneously curved momentum space preserving the full action of the Lorentz group in dimension 4 and higher, despite relaxing locality. We study the property of this relative locality deformation and show that this space leads to a noncommutativity related to Snyder spacetime.

http://arxiv.org/abs/1212.5233
Causal loop in the theory of Relative Locality
Lin-Qing Chen
(Submitted on 20 Dec 2012 (v1), last revised 3 Feb 2013 (this version, v2))
We find that Relative Locality, a recently proposed Planck-scale deformation of Special Relativity, suffers from the existence of causal loops. A simple and general construction of such on-shell loop processes is studied. We then show that even in one of the weakest deformations of the Poincar\'e group in Relative Locality, causality can be violated.

http://arxiv.org/abs/1308.0318
Orientability of loop processes in Relative Locality
Lin-Qing Chen
(Submitted on 1 Aug 2013)
We introduce a way to classify loop processes in relative locality in the case of Kappa-Poincare momentum space. We show that orientability is connected to a few essential properties in loop processes. Non-orientable loops have "effective curvature", which explicitly breaks translation symmetry, and can lead to breaking of causality and global momentum conservation. Orientable loops are "flat". Causality and global momentum conservation are all preserved in this kind of loops. We argue that the non-trivial classical loops in relative locality might be understood as dual effects from general relativity.

 

[marcus]

http://arxiv.org/abs/1308.0687
Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 3 Aug 2013)
The dynamics of a homogeneous, anisotropic universe are investigated within the context of spinfoam cosmology. Transition amplitudes are calculated for a graph consisting of a single node and three links - the `Daisy graph' - probing the behaviour a classical Bianchi I spacetime. It is shown further how the use of such single node graphs gives rise to a simplification of states such that all orders in the spin expansion can be calculated, indicating that it is the vertex expansion that contains information about quantum dynamics.
28 pages, 1 figure

http://arxiv.org/abs/1308.1012
A look at area Regge calculus
Yasha Neiman
(Submitted on 5 Aug 2013)
Area Regge calculus is a candidate theory of simplicial gravity, based on the Regge action with triangle areas as the dynamical variables. It is characterized by metric discontinuities and vanishing deficit angles. Area Regge calculus arises in the large-spin limit of the Barrett-Crane spinfoam model, but not in the newer EPRL/FK model. We address the viability of area Regge calculus as a discretization of General Relativity. We argue that when all triangles are spacelike and all tetrahedra have the same signature, non-trivial solutions of the area calculus are associated with a nonzero Ricci scalar. Our argument rests on a seemingly natural regularization of the metric discontinuities. It rules out the Euclidean area calculus, as well as the Lorentzian sector with all tetrahedra spacelike - the two setups usually considered in spinfoam models. On the other hand, we argue that the area calculus has attractive properties from the point of view of finite-region observables in quantum gravity.
15 pages, 2 figures

general interest (brief mention):
http://arxiv.org/abs/1308.1007
The Fate of the Quantum
Gerard 't Hooft
(Submitted on 5 Aug 2013)
Although the suspicion that quantum mechanics is emergent has been lingering for a long time, only now we begin to understand how a bridge between classical and quantum mechanics might be squared with Bell's inequalities and other conceptual obstacles...
14 pages, 1 figure, talk given at the "Time and Matter" conference, Venice March 2013.