Loop-and-allied QG bibliography

[marcus]

http://arxiv.org/abs/1303.6772
Renormalization of an SU(2) Tensorial Group Field Theory in Three Dimensions
Sylvain Carrozza, Daniele Oriti, Vincent Rivasseau
(Submitted on 27 Mar 2013)
We address in this paper the issue of renormalizability for SU(2) Tensorial Group Field Theories (TGFT) with geometric Boulatov-type conditions in three dimensions. We prove that tensorial interactions up to degree 6 are just renormalizable without any anomaly. Our new models define the renormalizable TGFT version of the Boulatov model and provide therefore a new approach to quantum gravity in three dimensions. Among the many new technical results established in this paper are a general classification of just renormalizable models with gauge invariance condition, and in particular concerning properties of melonic graphs, the second order expansion of melonic two point subgraphs needed for wave-function renormalization.
47 pages, 8 figures, 2 tables

brief mention, not QG but possibly of interest:
http://arxiv.org/abs/1303.6912
News on Right Handed Neutrinos
Marco Drewes
(Submitted on 27 Mar 2013)
Neutrinos are the only particles in the Standard Model of particle physics that have only been observed with left handed chirality to date. If right handed neutrinos exist, they could be responsible for several phenomena that have no explanation within the Standard Model, including neutrino oscillations, the baryon asymmetry of the universe, dark matter and dark radiation. After a pedagogical introduction, we review recent progress in the phenomenology of right handed neutrinos. We in particular discuss the mass ranges suggested by hints for neutrino oscillation anomalies and dark radiation (eV), sterile neutrino dark matter scenarios (keV) and experimentally testable theories of baryogenesis (GeV to TeV). We summarize constraints from theoretical considerations, laboratory experiments, astrophysics and cosmology for each of these.
Comments: 44 pages, 14 figures

 

[marcus]

http://arxiv.org/abs/1303.7139
Symmetry and Evolution in Quantum Gravity
Sean Gryb, Karim Thebault
(Submitted on 28 Mar 2013)
We propose an operator constraint equation for the wavefunction of the Universe that admits genuine evolution. While the corresponding classical theory is equivalent to the canonical decomposition of General Relativity, the quantum theory makes predictions that are distinct from Wheeler-DeWitt cosmology. Furthermore, the local symmetry principle - and corresponding observables - of the theory have a direct interpretation in terms of a conventional gauge theory, where the gauge symmetry group is that of spatial conformal diffeomorphisms (that preserve the spatial volume of the Universe). The global evolution is in terms of an arbitrary parameter that serves only as an unobservable label for successive states of the Universe. Our proposal follows unambiguously from a suggestion of York whereby the independently specifiable initial data in the action principle of General Relativity is given by a conformal geometry and the spatial average of the York time on the spacelike hypersurfaces that bound the variation. Remarkably, such a variational principle uniquely selects the form of the constraints of the theory so that we can establish a precise notion of both symmetry and evolution in quantum gravity.
36 preprint pages. 1 table

http://arxiv.org/abs/1303.7216
Relative Locality in Curved Space-time
Jerzy Kowalski-Glikman, Giacomo Rosati
(Submitted on 28 Mar 2013)
In this paper we construct the action describing dynamics of the particle moving in curved spacetime, with a non-trivial momentum space geometry. Curved momentum space is the core feature of theories where relative locality effects are presents. So far aspects of nonlinearities in momentum space have been studied only for flat or constantly expanding (De Sitter) spacetimes, relying on the their maximally symmetric nature. The extension of curved momentum space frameworks to arbitrary spacetime geometries could be relevant for the opportunities to test Planck-scale curvature/deformation of particles momentum space. As a first example of this construction we describe the particle with kappa-Poincar\'e momentum space on a circular orbit in Schwarzschild spacetime, where the contributes of momentum space curvature turn out to be negligible. The analysis of this problem relies crucially on the solution of the soccer ball problem.

 

[marcus]

brief mention
(relevant to QG, see conclusions):
http://arxiv.org/abs/1303.7371
Spheres are rare
Vincent Rivasseau
(Submitted on 29 Mar 2013)
We prove that triangulations of homology spheres in any dimension grow much slower than general triangulations. Our bound states in particular that the number of triangulations of homology spheres in 3 dimensions grows at most like the power 1/3 of the number of general triangulations.
14 pages, 1 figure

 

[marcus]

http://arxiv.org/abs/1304.0209
Connection Dynamics for Higher Dimensional Scalar-Tensor Theories of Gravity
Yu Han, Yongge Ma, Xiangdong Zhang
(Submitted on 31 Mar 2013)
The scalar-tensor theories of gravity in spacetime dimensions D+1>2 are studied. By doing Hamiltonian analysis, we obtain the geometrical dynamics of the theories from their Lagrangian. The Hamiltonian formalism indicates that the theories are naturally divided into two sectors by the coupling parameter ω. The Hamiltonian structure in both sectors are similar to the corresponding structure of 4-dimensional cases. It turns out that there is a symplectic reduction from the canonical structure of so(D+1) Yang-Mills theories coupled to the scalar field to the canonical structure of the geometrical scalar-tensor theories. Therefore the non-perturbative loop quantum gravity techniques can also be applied to the scalar-tensor theories in D+1 dimensions based on their connection-dynamical formalism.
13 pages

 

[marcus]

http://arxiv.org/abs/1304.2208
Loop-deformed Poincaré algebra
Jakub Mielczarek
(Submitted on 8 Apr 2013)
In this essay we present evidence suggesting that loop quantum gravity leads to deformation of the local Poincar\'e algebra within the limit of high energies. This deformation is a consequence of quantum modification of effective off-shell hypersurface deformation algebra. Surprisingly, the form of deformation suggests that the signature of space-time changes from Lorentzian to Euclidean at large curvatures. We construct particular realization of the loop-deformed Poincar\'e algebra and find that it can be related to curved momentum space, which indicates the relationship with recently introduced notion of relative locality. The presented findings open a new way of testing loop quantum gravity effects.
7 pages, 1 figure. Essay written for the Gravity Research Foundation 2013 Awards for Essays on Gravitation.

 

[marcus]

http://arxiv.org/abs/1304.2679
New Variables for Classical and Quantum Gravity in all Dimensions V. Isolated Horizon Boundary Degrees of Freedom
Norbert Bodendorfer, Thomas Thiemann, Andreas Thurn
(Submitted on 9 Apr 2013)
In this paper, we generalise the treatment of isolated horizons in loop quantum gravity, resulting in a Chern-Simons theory on the boundary in the four-dimensional case, to non-distorted isolated horizons in 2(n+1)-dimensional spacetimes. The key idea is to generalise the four-dimensional isolated horizon boundary condition by using the Euler topological density of a spatial slice of the black hole horizon as a measure of distortion. The resulting symplectic structure on the horizon coincides with the one of higher-dimensional SO(2(n+1))-Chern-Simons theory in terms of a Peldan-type hybrid connection and resembles closely the usual treatment in 3+1 dimensions. We comment briefly on a possible quantisation of the horizon theory. Here, some subtleties arise since higher-dimensional non-Abelian Chern-Simons theory has local degrees of freedom. However, when replacing the natural generalisation to higher dimensions of the usual boundary condition by an equally natural stronger one, it is conceivable that the problems originating from the local degrees of freedom are avoided, thus possibly resulting in a finite entropy.
49 pages

http://arxiv.org/abs/1304.2666
The 1/N Expansion of Tensor Models Beyond Perturbation Theory
Razvan Gurau
(Submitted on 9 Apr 2013)
We analyze in full mathematical rigor the most general quartically perturbed invariant probability measure for a random tensor. Using a version of the Loop Vertex Expansion (which we call the mixed expansion) we show that the cumulants write as explicit series in 1/N plus bounded rest terms. The mixed expansion recasts the problem of determining the subleading corrections in 1/N into a simple combinatorial problem of counting trees decorated by a finite number of loop edges.
As an aside, we use the mixed expansion to show that the (divergent) perturbative expansion of the tensor models is Borel summable and to prove that the cumulants respect an uniform scaling bound. In particular the quartically perturbed measures fall, in the N to infinity limit, in the universality class of Gaussian tensor models.
45 pages

brief mention:
http://arxiv.org/abs/1304.2709
Spectral dimension and diffusion in multi-scale spacetimes

Gianluca Calcagni, Giuseppe Nardelli
(Submitted on 9 Apr 2013)
Starting from a classical-mechanics stochastic model encoded in a Langevin equation, we derive the natural diffusion equation associated with three classes of multi-scale spacetimes (with weighted, ordinary, and 'q-' Poincaré symmetries). As a consistency check, the same result is obtained by inspecting the propagation of a quantum-mechanical particle in a disordered environment. The solution of the diffusion equation displays a time-dependent diffusion coefficient and represents a probabilistic process, classified according to the statistics of the noise in the Langevin equation. We thus illustrate, also with pictorial aids, how spacetime geometries can be more completely catalogued not only through their Hausdorff and spectral dimension, but also by a stochastic process. The spectral dimension of multi-fractional spacetimes is then computed and compared with what found in previous studies, where a diffusion equation with some open issues was assumed rather than derived. These issues are here discussed and solved, pointing in favour of the model with q-Poincaré symmetries.
39 pages, 3 multiple figures, 1 table

 

[marcus]

http://arxiv.org/abs/1304.3025
The Wald entropy formula and loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
(Submitted on 10 Apr 2013)
We outline how the Wald entropy formula naturally arises in loop quantum gravity based on recently introduced dimension-independent connection variables. The key observation is that in a loop quantization of a generalized gravity theory, the analog of the area operator turns out to measure, morally speaking, the Wald entropy rather than the area. We discuss the explicit example of (higher-dimensional) Lovelock gravity and comment on recent work on finding the correct numerical prefactor of the entropy by comparing it to a semiclassical effective action.
16 pages

brief mention:
http://arxiv.org/abs/1304.2858
New perspectives on Hawking radiation
Matteo Smerlak, Suprit Singh
(Submitted on 10 Apr 2013)
We develop an adiabatic formalism to study the Hawking phenomenon from the perspective of Unruh-DeWitt detectors moving along non-stationary, non-asymptotic trajectories. When applied to geodesic trajectories, this formalism yields surprising results: (i) though they have zero acceleration, the temperature measured by detectors on circular orbits is higher than that measured by static detectors at the same distance from the hole, and diverges on the photon sphere, (ii) in the near-horizon region, both outgoing and incoming modes excite infalling detectors, and, for highly bound trajectories ($E\ll1$), the latter actually dominate the former, (iii) in this region, the relationship between the temperature of Hawking radiation and the relative velocity between the detector and the hole is not of Doppler type. We confirm the apparent perception of high-temperature ingoing Hawking radiation by infalling observers with $E\ll1$ by a flux computation. We close by a discussion of the role played by spacetime curvature on the near-horizon Hawking radiation.
14 pages, 7 figures

http://arxiv.org/abs/1304.2785
Inflationary paradigm in trouble after Planck2013
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 9 Apr 2013)
The recent Planck satellite combined with earlier results eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported in the analysis of the collaboration. More important, though, is that all the simplest inflaton models are disfavored by the data while the surviving models -- namely, those with plateau-like potentials -- are problematic. We discuss how the restriction to plateau-like models leads to three independent problems: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new difficulty which we call the inflationary "unlikeliness problem." Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results. In sum, we find that recent experimental data disfavors all the best-motivated inflationary scenarios and introduces new, serious difficulties that cut to the core of the inflationary paradigm. Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive.

possible interest:
http://arxiv.org/abs/1304.2914
Quantum discord cannot be shared
Alexander Streltsov, Wojciech H. Zurek
(Submitted on 10 Apr 2013)
5 pages, 3 figures

http://arxiv.org/abs/1304.3069
Graviweak Unification, Invisible Universe and Dark Energy
C.R. Das, L.V. Laperashvili, A. Tureanu
(Submitted on 10 Apr 2013)
22 pages

 

[John86]

http://arxiv.org/abs/1303.4659
Complementarity of quantum discord and classically accessible information
Michael Zwolak, Wojciech H. Zurek
(Submitted on 19 Mar 2013 (v1), last revised 25 Mar 2013 (this version, v2))
The sum of the Holevo quantity (that bounds the capacity of quantum channels to transmit classical information about an observable) and the quantum discord (a measure of the quantumness of correlations of that observable) yields an observable-independent total given by the quantum mutual information. This split naturally delineates information about quantum systems accessible to observers -- information that is redundantly transmitted by the environment -- while showing that it is maximized for the quasi-classical pointer observable. Other observables are accessible only via correlations with the pointer observable. Further, we prove an anti-symmetry property relating accessible information and discord. It shows that information becomes objective -- accessible to many observers -- only as quantum information is relegated to correlations with the global environment, and, therefore, locally inaccessible. The resulting complementarity explains why, in a quantum Universe, we perceive objective classical reality while flagrantly quantum superpositions are out of reach.

 

[John86]

http://arxiv.org/abs/1304.3122
Planck 2013 results support the simplest cyclic models
Jean-Luc Lehners, Paul J. Steinhardt
(Submitted on 10 Apr 2013)
We show that results from the Planck satellite reported in 2013 are consistent with the simplest cyclic models for natural parameter ranges i.e., order unity dimensionless coefficients, assuming the standard entropic mechanism for generating curvature perturbations. With improved precision, forthcoming results from Planck and other experiments should be able to test the parameter ranges by confirming or refuting the core predictions - i.e., no observable primordial B-mode polarization and detectable local non-gaussianity. A new prediction, given the Planck 2013 constraints on the bispectrum, is a sharp constraint on the local trispectrum parameter g_{NL}; namely, the simplest models predict it is negative, with g_{NL} < -1700.

 

[marcus]

http://arxiv.org/abs/1304.4152
Fully packed loops on random surfaces and the 1/N expansion of tensor models
Valentin Bonzom, Frédéric Combes
(Submitted on 15 Apr 2013)
Starting with the observation that some fully packed loop models on random surfaces can be mapped to random edge-colored graphs, we show that the expansion in the number of loops is organized like the 1/N expansion of rank-three tensor models. In particular, configurations which maximize the number of loops are precisely the melonic graphs of tensor models and a scaling limit which projects onto the melonic sector is found. This also shows that some three-dimensional topologies can be obtained from discrete surfaces decorated with loops. We generalize this approach to higher-rank tensor models, for random tensors of size N^d-1 x τ N^β with beta between 0 and 1. They generate loops with fugacity τ N^β on triangulations in dimension d-1 and we show that the 1/N expansion is beta-dependent.
13 pages

 

[marcus]

http://arxiv.org/abs/1304.4777
Consistent closure of RG flow equations in quantum gravity
Alessandro Codello, Giulio D'Odorico, Carlo Pagani
(Submitted on 17 Apr 2013)
We construct a consistent closure for the beta functions of the cosmological and Newton's constants by evaluating the influence of the fluctuating metric and ghost fields anomalous dimensions on their flow. In this generalized framework we confirm the presence of a UV attractive non-Gaussian fixed-point. Our closure method is general and can be applied systematically to more general truncations of the gravitational effective average action.
5 pages, 2 figures

 

[John86]

http://arxiv.org/abs/1304.3025
The Wald entropy formula and loop quantum gravity
Norbert Bodendorfer, Yasha Neiman
(Submitted on 10 Apr 2013)
We outline how the Wald entropy formula naturally arises in loop quantum gravity based on recently introduced dimension-independent connection variables. The key observation is that in a loop quantization of a generalized gravity theory, the analog of the area operator turns out to measure, morally speaking, the Wald entropy rather than the area. We discuss the explicit example of (higher-dimensional) Lovelock gravity and comment on recent work on finding the correct numerical prefactor of the entropy by comparing it to a semiclassical effective action.

http://arxiv.org/abs/1304.2785
Inflationary paradigm in trouble after Planck2013
Anna Ijjas, Paul J. Steinhardt, Abraham Loeb
(Submitted on 9 Apr 2013)
The recent Planck satellite combined with earlier results eliminate a wide spectrum of more complex inflationary models and favor models with a single scalar field, as reported in the analysis of the collaboration. More important, though, is that all the simplest inflaton models are disfavored by the data while the surviving models -- namely, those with plateau-like potentials -- are problematic. We discuss how the restriction to plateau-like models leads to three independent problems: it exacerbates both the initial conditions problem and the multiverse-unpredictability problem and it creates a new difficulty which we call the inflationary "unlikeliness problem." Finally, we comment on problems reconciling inflation with a standard model Higgs, as suggested by recent LHC results. In sum, we find that recent experimental data disfavors all the best-motivated inflationary scenarios and introduces new, serious difficulties that cut to the core of the inflationary paradigm. Forthcoming searches for B-modes, non-Gaussianity and new particles should be decisive.

http://arxiv.org/abs/1304.4358
Reexamination of inflation in noncommutative space-time after Planck results
Nan Li, Xin Zhang
(Submitted on 16 Apr 2013)
An inflationary model in the framework of noncommutative space-time may generate a nontrivial running of the scalar spectral index, but usually induces a large tensor-to-scalar ratio simultaneously. With the latest observational data from the Planck mission, we reexamine the inflationary scenarios in a noncommutative space-time. We find that either the running of the spectral index is tiny compared with the recent observational result, or the tensor-to-scalar ratio is too large to allow a sufficient number of $e$-folds. As examples, we show that the chaotic and power-law inflation models with the noncommutative effects are not favored by the current Planck data.

 

[marcus]

BTW the first two items in post #1937 were already listed a few posts back, in post #1932.

http://arxiv.org/abs/1304.5430
Extensions of Lorentzian spacetime geometry: from Finsler to Cartan and vice versa
Manuel Hohmann
(Submitted on 19 Apr 2013)
We briefly review two recently developed extensions of the Lorentzian geometry of spacetime and prove that they are in fact closely related. The first is the concept of observer space, which generalizes the space of Lorentzian observers, i.e., future unit timelike vectors, using Cartan geometry. The second is the concept of Finsler spacetimes, which generalizes the Lorentzian metric of general relativity to an observer-dependent Finsler metric. We show that every Finsler spacetime possesses a well-defined observer space that can naturally be equipped with a Cartan geometry. Conversely, we derive conditions under which a Cartan geometry on observer space gives rise to a Finsler spacetime. We further show that these two constructions complement each other. We finally apply our constructions to two gravity theories, MacDowell-Mansouri gravity on observer space and Finsler gravity, and translate their actions from one geometry to the other.
32 pages,

 

[marcus]

http://arxiv.org/abs/1304.5626
Path Integral Representation of Lorentzian Spinfoam Model, Asymptotics, and Simplicial Geometries
Muxin Han, Thomas Krajewski
(Submitted on 20 Apr 2013)
A path integral representation of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model is proposed as a starting point of semiclassical analysis. The relation between the spinfoam model and classical simplicial geometry is studied via the large spin asymptotic expansion of the spinfoam amplitude with all spins uniformaly large. More precisely in the large spin regime, there is an equivalence between the spinfoam critical configuration (with certain nondegeneracy assumption) and a classical Lorentzian simplicial geometry. Such an equivalence relation allows us to classify the spinfoam critical configurations by their geometrical interpretations, via two types of solution-generating maps. The equivalence between spinfoam critical configuration and simplical geometry also allows us to define the notion of globally oriented and time-oriented spinfoam critical configuration. It is shown that only at the globally oriented and time-oriented spinfoam critical configuration, the leading order contribution of spinfoam large spin asymptotics gives precisely an exponential of Lorentzian Regge action of General Relativity. At all other (unphysical) critical configurations, spinfoam large spin asymptotics modifies the Regge action at the leading order approximation.
36 pages

http://arxiv.org/abs/1304.5627
On Spinfoam Model in Large Spin Regime
Muxin Han
We study the semiclassical behavior of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model, by taking into account of the sum over spins in the large spin regime. We also employ the method of stationary phase analysis with parameters and the so called, almost-analytic machinery, in order to find the asymptotic behaviour of the contributions from all possible large spin configurations in the spinfoam model. The spins contributing the sum is written as J_f = λj_f where λ is a large parameter, which resulting in an asymptotic expansion via stationary phase approximation. The analysis shows that at least for the simplicial Lorentzian geometries (as spinfoam critical configurations), they contribute the leading order approximation of spinfoam amplitude only when their deficit angles satisfy γΘ ̊_f ≪ λ^−1/2 mod 4πℤ, when one treats γ ∼ o(1). We also discuss the consequences of such a result.
21 pages.

http://arxiv.org/abs/1304.5628
Semiclassical Analysis of Spinfoam Model with a Small Barbero-Immirzi Parameter
Muxin Han
(Submitted on 20 Apr 2013)
We study the semiclassical behavior of Lorentzian Engle-Pereira-Rovelli-Livine (EPRL) spinfoam model, by taking into account of the sum over spins in the large spin regime. The large spin parameter λ and small Barbero-Immirzi parameter γ are treated as two independent parameters for the asymptotic expansion of spinfoam state-sum. Interestingly, there are two different spin regimes: 1 ≪ γ⁻¹ ≪ λ ≪ γ⁻² and λ ≥ γ⁻². The model in two spin regimes has dramatically different number of effective degrees of freedom. In 1 ≪ γ⁻¹ ≪ λ ≪ γ⁻², the model produces in the leading order a functional integration of Regge action, which gives the discrete Einstein equation for the leading contribution. There is no restriction of Lorentzian deficit angle in this regime. In the other regime λ ≥ γ⁻², only small deficit angle is allowed |Θ_f | ≪ γ⁻¹λ^1/2 mod 4πℤ. When spins go even larger, only zero deficit angle mod 4πℤ is allowed asymptotically. In the transition of the two regimes, only the configurations with small deficit angle can contribute, which means one need a large triangulation in order to have oscillatory behavior of the spinfoam amplitude.
21 pages, 2 figures

http://arxiv.org/abs/1304.5983
Dirac's discrete hypersurface deformation algebras
Valentin Bonzom, Bianca Dittrich
(Submitted on 22 Apr 2013)
The diffeomorphism symmetry of general relativity leads in the canonical formulation to constraints, which encode the dynamics of the theory. These constraints satisfy a complicated algebra, known as Dirac's hypersurface deformation algebra. This algebra has been a long standing challenge for quantization. One reason is that discretizations, on which many quantum gravity approaches rely, generically break diffeomorphism symmetry. In this work we find a representation for the Dirac constraint algebra of hypersurface deformations in a formulation of discrete 3D gravity and for the flat as well as homogeneously curved sector of discrete 4D gravity. In these cases diffeomorphism symmetry can be preserved. Furthermore we present different versions of the hypersurface deformation algebra for the boundary of a simplex in arbitrary dimensions.
30 pages

brief mention (potential technical relevance)
http://arxiv.org/abs/1304.5913
How to Resum Feynman Graphs
Vincent Rivasseau, Zhituo Wang
(Submitted on 22 Apr 2013)
In this paper we reformulate in a simpler way the combinatoric core of constructive quantum field theory We define universal rational combinatoric weights for pairs made of a graph and one of its spanning trees. These weights are nothing but the percentage of Hepp's sectors in which the tree is leading the ultraviolet analysis. We explain how they allow to reshuffle the divergent series formulated in terms of Feynman graphs into convergent series indexed by the trees that these graphs contain. The Feynman graphs to be used are not the ordinary ones but those of the intermediate field representation, and the result of the reshuffling is called the Loop Vertex Expansion.
18 pages, 6 figures; improves and replaces arXiv:1006.4617

 

[marcus]

http://arxiv.org/abs/1304.6688
Towards Anisotropic Spinfoam Cosmology
Julian Rennert, David Sloan
(Submitted on 24 Apr 2013)
We examine spinfoam cosmology by use of a simple graph adapted to homogeneous cosmological models. We calculate dynamics in the isotropic limit, and provide the framework for the aniostropic case. The dynamical behaviour is calculating transition amplitudes between holomorphic coherent states on a single node graph. The resultant dynamics is peaked on solutions which have no support on the zero volume state, indicating that big bang type singularities are avoided within such models.
19 pages, 4 figures

http://arxiv.org/abs/1304.6632
Bubble divergences and gauge symmetries in spin foams
Valentin Bonzom, Bianca Dittrich
(Submitted on 24 Apr 2013)
Spin foams are candidate state-sum models for transition amplitudes in quantum gravity. An active research subject is to identify the possible divergences of spin foam models, or alternatively to show that models are finite. We will discuss in detail the (non--occurrence of) divergences in the Barrett-Crane model, formulated as an integral of delta function weights only. We will furthermore present a simple method to estimate the divergence degree of the so-called bubbles for general spin foam models. Divergences in spin foams are expected to be related to the existence of gauge symmetries (diffeomorphisms). Thus we have to conclude that such gauge symmetries are not (fully) present in the model we consider. But we will identify a class of gauge symmetries which occur at special solutions of the equations imposed by the delta function weights. This situation is surprisingly similar to the case of broken diffeomorphism symmetries in discrete gravity, which are present around flat solutions. We introduce a method to derive (Ward-identity-like) equations for the vertex amplitude of the model in the case of broken gauge symmetries.
28 pages

brief mention (not usual subject matter, included in case might be of interest):
http://arxiv.org/abs/1304.6217
Dark Radiation after Planck
Najla Said, Eleonora Di Valentino, Martina Gerbino
(Submitted on 23 Apr 2013)
We present new constraints on the relativistic neutrino effective number N_eff and on the Cosmic Microwave Background power spectrum lensing amplitude A_L from the recent Planck 2013 data release. Including observations of the CMB large angular scale polarization from the WMAP satellite, we obtain the bounds N_eff = 3.71 +/- 0.40 and A_L = 1.25 +/- 0.13 at 68% c.l.. The Planck dataset alone is therefore suggesting the presence of a dark radiation component at 91.1% c.l. and hinting for a higher power spectrum lensing amplitude at 94.3% c.l.. We discuss the agreement of these results with the previous constraints obtained from the Atacama Cosmology Telescope (ACT) and the South Pole Telescope (SPT). Considering the constraints on the cosmological parameters, we found a very good agreement with the previous WMAP+SPT analysis but a tension with the WMAP+ACT results, with the only exception of the lensing amplitude.
5 pages, 6 figures, 2 tables

http://arxiv.org/abs/1304.6699
Bouncing Anisotropic Universes with Varying Constants
John D. Barrow, David Sloan
(Submitted on 24 Apr 2013)
We examine the evolution of a closed, homogeneous and anisotropic cosmology subject to a variation of the fine structure 'constant', α, within the context of the theory introduced by Bekenstein, Sandvik, Barrow and Magueijo, which generalises Maxwell's equations and general relativity...
...
9 pages, 5 figures

http://arxiv.org/abs/1304.6339
Continuum interpretation of the dynamical-triangulation formulation of quantum Einstein gravity
Jan Smit
(Submitted on 23 Apr 2013)
In the time-space symmetric version of dynamical triangulation, a non-perturbative version of quantum Einstein gravity, numerical simulations without matter have shown two phases, with spacetimes that are either crumpled or elongated like branched polymers, with strong evidence of a first-order transition between them. These properties have generally been considered unphysical. Using previously unpublished numerical results, we give an interpretation in terms of continuum spacetimes that have constant positive an negative curvature, respectively in the 'elongated' and 'crumpled' phase...
...
40 pages, 22 figures

 

[marcus]

http://arxiv.org/abs/1304.6946
Pure connection gravity at one loop: Instanton background
Kai Groh, Kirill Krasnov, Christian F. Steinwachs
(Submitted on 25 Apr 2013)
In the "pure connection" formulation General Relativity becomes a particular diffeomorphism invariant SL(2) gauge theory. Using this formalism, we compute the divergent contributions to the gravitational one-loop effective action. Calculations of the on-shell effective action simplify greatly if one specialises to an instanton background where only the anti-self-dual part of the Weyl curvature is non-vanishing. One of the most striking features of the connection formulation is that the (linearised) Euclidean action is non-negative, unlike in the metric case. As in the metric GR, we find the logarithmically divergent contribution to consist of the volume and Euler character terms, but the arising numerical constants are different. However, surprisingly, the difference between the two results turns out to be always an integer. This suggests that there exists a relation between the connection and metric based quantum theories.
21 pages

http://arxiv.org/abs/1304.6934
Towards a double-scaling limit for tensor models: probing sub-dominant orders
Wojciech Kaminski, Daniele Oriti, James P. Ryan
(Submitted on 25 Apr 2013)
The definition of a double-scaling limit represents an important goal in the development of tensor models. We take the first steps towards this goal by extracting and analysing the next-to-leading order contributions, in the 1/N expansion, for the IID tensor models. We show that the radius of convergence of the NLO series coincides with that of the leading order melonic sector. Meanwhile, the value of the susceptibility exponent at NLO is 3/2, signaling a departure from the leading order behaviour. Both pieces of information provide clues for a non-trivial double-scaling limit, for which we put forward some precise conjecture.
24 pages, 21 figures

 

[marcus]

http://arxiv.org/abs/1304.7247
Probing the quantum nature of spacetime by diffusion
Gianluca Calcagni, Astrid Eichhorn, Frank Saueressig
(Submitted on 26 Apr 2013)
Many approaches to quantum gravity have resorted to diffusion processes to characterize the spectral properties of the resulting quantum spacetimes. We critically discuss these quantum-improved diffusion equations and point out that a crucial property, namely positivity of their solutions, is not preserved automatically. We then construct a novel set of diffusion equations with positive semi-definite probability densities, applicable to Asymptotically Safe gravity, Horava-Lifgarbagez gravity and Loop Quantum Gravity. These recover all previous results on the spectral dimension and shed further light on the structure of the quantum spacetimes by assessing the underlying stochastic processes. Pointing out that manifestly different diffusion processes lead to the same spectral dimension, we propose the probability distribution of the diffusion process as a refined probe of quantum spacetime.
14 pages, 5 figures

 

[marcus]

http://arxiv.org/abs/1304.7686
A quantum gravitational inflationary scenario in Bianchi-I spacetime
Brajesh Gupt, Parampreet Singh
(Submitted on 29 Apr 2013)
We investigate the φ² inflationary model in the Bianchi-I spacetime using effective spacetime description of loop quantum cosmology to understand the issues of the resolution of initial singularity, isotropization, effect of anisotropies on amount of inflation, and the phase space attractors in the presence of non-perturbative quantum gravitational modifications. A comparative analysis with the classical theory by including more general initial conditions than the ones previously considered in the latter is also performed. We show that, in general, the classical singularity is replaced by a bounce of the mean scale factor in loop quantum cosmology. Due to the underlying quantum geometric effects, the energy density of the inflaton and the anisotropic shear remain bounded throughout the non-singular evolution. Starting from arbitrary anisotropic initial conditions, a loop quantum universe isotropizes either before or soon after the onset of slow-roll inflation. We find a double attractor behavior in the phase space dynamics of loop quantum cosmology, similar to the one in classical theory, but with some additional subtle features. Quantum modifications to the dynamical equations are such that, unlike the classical theory, the amount of inflation does not monotonically depend on the initial anisotropy in loop quantum cosmology. Our results suggest that a viable non-singular inflationary model can be constructed from highly anisotropic initial conditions in the Planck regime.
34 pages, 19 figures

brief mention:
http://arxiv.org/abs/1304.7550
Does a Quantum Particle Know its Own Energy?
Rafael D. Sorkin
(Submitted on 29 Apr 2013)
If a wave function does not describe microscopic reality then what does? Reformulating quantum mechanics in path-integral terms leads to a notion of "precluded event" and thence to the proposal that quantal reality differs from classical reality in the same way as a set of worldlines differs from a single worldline. One can then ask, for example, which sets of electron trajectories correspond to a Hydrogen atom in its ground state and how they differ from those of an excited state. We address the analogous questions for simple model that replaces the electron by a particle hopping (in discrete time) on a circular lattice.
15 pages, 2 figures. To appear in Journal of Physics: Conf. Ser.

 

[John86]

http://arxiv.org/abs/1304.7271
A macroscopic challenge for quantum spacetime
Giovanni Amelino-Camelia
(Submitted on 26 Apr 2013)
Over the last decade a growing number of quantum-gravity researchers has been looking for opportunities for the first ever experimental evidence of a Planck-length quantum property of spacetime. These studies are usually based on the analysis of some candidate indirect implications of spacetime quantization, such as a possible curvature of momentum space. Some recent proposals have raised hope that we might also gain direct experimental access to quantum properties of spacetime, by finding evidence of limitations to the measurability of the center-of-mass coordinates of some macroscopic bodies. However I here observe that the arguments that originally lead to speculating about spacetime quantization do not apply to the localization of the center of mass of a macroscopic body. And I also analyze some popular formalizations of the notion of quantum spacetime, finding that when the quantization of spacetime is Planckian for the constituent particles then for the composite macroscopic body the quantization of spacetime is much weaker than Planckian. These results show that finding evidence of spacetime quantization with studies of macroscopic bodies is extremely unlikely. And they also raise some conceptual challenges for theories of mechanics in quantum spacetime, in which for example free protons and free atoms should feel the effects of spacetime quantization differently.

http://arxiv.org/abs/1304.7630
Predictive description of Planck-scale-induced spacetime fuzziness
Giovanni Amelino-Camelia, Valerio Astuti, Giacomo Rosati
(Submitted on 29 Apr 2013)
Several approaches to the quantum-gravity problem predict that spacetime should be "fuzzy", but have been so far unable to provide a crisp physical characterization of this notion. An intuitive picture of spacetime fuzziness has been proposed on the basis of semi-heuristic arguments, and in particular involves an irreducible Planck-scale contribution to the uncertainty of the energy of a particle. These arguments also inspired a rather active phenomenological programme looking for blurring of images of distant astrophysical sources that would result from such energy uncertainties. We here report the first ever physical characterization of spacetime fuzziness derived constructively within a quantum picture of spacetime, the one provided by spacetime noncommutativity. Our results confirm earlier heuristic arguments suggesting that spacetime fuzziness, while irrelevantly small on terrestrial scales, could be observably large for propagation of particles over cosmological distances. However, we find no Planck-scale-induced lower bound on the uncertainty of the energy of particles, and we observe that this changes how we should picture a quantum spacetime and also imposes a reanalysis of the associated phenomenology.

http://arxiv.org/abs/1304.7583
Inner Fluctuations in Noncommutative Geometry without the first order condition
Ali H. Chamseddine, Alain Connes, Walter D. van Suijlekom
(Submitted on 29 Apr 2013)
We extend inner fluctuations to spectral triples that do not fulfill the first-order condition. This involves the addition of a quadratic term to the usual linear terms. We find a semi-group of inner fluctuations, which only depends on the involutive algebra A and which extends the unitary group of A. This has a key application in noncommutative spectral models beyond the Standard Model, of which we consider here a toy model.

http://arxiv.org/abs/1304.7743
Dilaton Quantum Gravity
Tobias Henz, Jan Martin Pawlowski, Andreas Rodigast, Christof Wetterich
(Submitted on 29 Apr 2013)
We propose a simple fixed point scenario in the renormalization flow of a scalar dilaton coupled to gravity. This would render gravity non-perturbatively renormalizable and thus constitute a viable theory of quantum gravity. On the fixed point dilatation symmetry is exact and the quantum effective action takes a very simple form. Realistic gravity with a nonzero Planck mass is obtained through a nonzero expectation value for the scalar field, constituting a spontaneous scale symmetry breaking. Furthermore, relevant couplings for the flow away from the fixed point can be associated with a 'dilatation anomaly' that is responsible for dynamical dark energy. For the proposed fixed point and flow away from it the cosmological 'constant' vanishes for asymptotic time.

 

[marcus]

http://arxiv.org/abs/1304.8050
Beyond the Spectral Standard Model: Emergence of Pati-Salam Unification
Ali H. Chamseddine, Alain Connes, Walter D. van Suijlekom
(Submitted on 30 Apr 2013)
The assumption that space-time is a noncommutative space formed as a product of a continuous four dimensional manifold times a finite space predicts, almost uniquely, the Standard Model with all its fermions, gauge fields, Higgs field and their representations. A strong restriction on the noncommutative space results from the first order condition which came from the requirement that the Dirac operator is a differential operator of order one. Without this restriction, invariance under inner automorphisms requires the inner fluctuations of the Dirac operator to contain a quadratic piece expressed in terms of the linear part. We apply the classification of product noncommutative spaces without the first order condition and show that this leads immediately to a Pati-Salam SU(2)_R x SU(2)_L x SU(4) type model which unifies leptons and quarks in four colors. Besides the gauge fields, there are 16 fermions in the (2,2,4) representation, fundamental Higgs fields in the (2,2,1), (2,1,4) and (1,1,1+15) representations. Interestingly we encounter a new phenomena where the Higgs fields in the high energy sector are composite and depend quadratically on the fundamental Higgs fields. The Pati-Salam symmetries are broken spontaneously at high energies to those of the Standard Model.
37 pages, 6 figures

 

[marcus]

http://arxiv.org/abs/1305.0310
A Birkhoff theorem for Shape Dynamics
Henrique Gomes
(Submitted on 1 May 2013)
Here we use the equations of motion of Shape Dynamics in its asymptotically flat version to derive a Birkhoff theorem. There are three significant differences with respect to the usual Birkhoff theorem in GR. The first regards the posing of the problem: in Shape Dynamics we must establish from the start the boundary conditions of our phase space variables. Thus unlike the GR Birkhoff theorem, which yields a static 4-metric from vacuum and spherical symmetry irrespectively of the boundary conditions, we have to postulate asymptotically flat boundary conditions from the start. The second difference regards the construction of the solution: we heavily use the Shape Dynamics spatial Weyl gauge freedom to simplify the problem. The remaining difference is that the solution obtained is uniquely the isotropic wormhole solution, in which no singularity is present, as opposed to maximally extended Schwarzschild.
6 pages

http://arxiv.org/abs/1305.0394
Relational Evolution of Observables for Hamiltonian-Constrained Systems
Andrea Dapor, Wojciech Kamiński, Jerzy Lewandowski, Jedrzej Świeżewski
(Submitted on 2 May 2013)
Evolution of systems whose Hamiltonians are generators of gauge transformations is a notion that requires more structure than the canonical theory provides. We identify and study this additional structure in the framework of relational observables ("partial observables"). We formulate necessary and sufficient conditions for the resulting evolution in the physical phase space to be a symplectomorphism. We give examples which satisfy those conditions and examples which do not. We point out that several classic positions in the literature on relational observables contain incomplete approach to the issue of evolution and false statements. Our work provides useful clarification and opens the door to studying correctly formulated definitions.
7 pages

brief:
http://arxiv.org/abs/1305.0443
Free Fermions on causal sets
Johan Noldus
(Submitted on 2 May 2013)
We construct a Dirac theory on causal sets; a key element in the construction being that the causet must be regarded as emergent in an appropriate sense too. We further notice that mixed norm spaces appear in the construction allowing for negative norm particles and "ghosts".
7 pages.

http://arxiv.org/abs/1305.0383
An Algebraic Topological Construct of Classical Loop Gravity and the prospect of Higher Dimensions
Madhavan Venkatesh
8 pages

 

[marcus]

http://arxiv.org/abs/1305.0653
Wavelet Quantum Cosmology
Herve Bergeron, Andrea Dapor, Jean Pierre Gazeau, Przemyslaw Malkiewicz
(Submitted on 3 May 2013)
We examine the possibility of dealing with gravitational singularities on a quantum level through the use of coherent state or wavelet quantization instead of canonical quantization. We consider the Robertson-Walker metric coupled to a perfect fluid. It is the simplest model of a gravitational collapse and the results obtained here may serve as a useful starting point for more complex investigations in the future. The coherent states, or equivalently the continuous wavelet basis elements, are built from the unitary irreducible representation of the affine group of the real line with positive dilation. A natural regularization of the singularity is an interesting issue of our approach.
24 pages, 3 figures

http://arxiv.org/abs/1305.0659
Doubling of the Algebra and Neutrino Mixing within Noncommutative Spectral Geometry
Maria Vittoria Gargiulo, Mairi Sakellariadou, Giuseppe Vitiello
(Submitted on 3 May 2013)
We study physical implications of the doubling of the algebra, an essential element in the construction of the noncommutative spectral geometry model, proposed by Connes and his collaborators as offering a geometric explanation for the standard model of strong and electroweak interactions. Linking the algebra doubling to the deformed Hopf algebra, we build Bogogliubov transformations and show the emergence of neutrino mixing.
7 pages

 

[marcus]

http://arxiv.org/abs/1305.1487
Shape Dynamics and Effective Field Theory
Tim Koslowski
(Submitted on 7 May 2013)
Shape Dynamics is a gauge theory based on spatial diffeomorphism- and Weyl-invariance which is locally indistinguishable form classical General Relativity. If taken seriously, it suggests that the spacetime--geometry picture that underlies General Relativity can be replaced by a picture based on spatial conformal geometry. This classically well understood trading of gauge symmetries opens new conceptual avenues in many approaches to quantum gravity. I focus on the general implications for quantum gravity and effective field theory and consider the application of the Shape Dynamics picture in the exact renormalization group approaches to gravity, loop- and polymer- quantization approaches to gravity and low energy effective field theories. I also discuss the interpretation of known results through in the Shape Dynamics picture, in particular holographic renormalization and the problem of time in canonical quantum gravity.
56 pages, 1 figure

 

[MTd2]

I don't know why this was missed:

http://arxiv.org/abs/1305.0822

On the Origin of Gravitational Lorentz Covariance

Justin Khoury, Godfrey E. J. Miller, Andrew J. Tolley
(Submitted on 3 May 2013)
We provide evidence that general relativity is the unique spatially covariant effective field theory of the transverse, traceless graviton degrees of freedom. The Lorentz covariance of general relativity, having not been assumed in our analysis, is thus plausibly interpreted as an accidental or emergent symmetry of the gravitational sector.

 

[marcus]

http://arxiv.org/abs/1305.1971
Goldstone Bosons as Fractional Cosmic Neutrinos
Steven Weinberg
(Submitted on 8 May 2013)
It is suggested that Goldstone bosons may be masquerading as fractional cosmic neutrinos, contributing about 0.39 to what is reported as the effective number of neutrino types in the era before recombination. The broken symmetry associated with these Goldstone bosons is further speculated to be the conservation of the particles of dark matter.
9 pages

 

[marcus]

http://pirsa.org/13050003/
The effective field theory of general relativity, running couplings and Asymptotic Safety
Speaker(s): John Donoghue
Abstract: Effective field theory techniques allow reliable quantum calculations in general relativity at low energy. After a review of these techniques, I will discuss the attempts to define the gravitational corrections to running gauge couplings and to the couplings of gravity itself. I will also describe an attempt to understand the relation between the effective field theory and Asymptotic Safety in the region where they overlap.
Date: 09/05/2013 - 2:30 pm

 

[atyy]

http://arxiv.org/abs/1305.2207
The imaginary part of the gravitational action at asymptotic boundaries and horizons
Yasha Neiman
(Submitted on 9 May 2013)
We study the imaginary part of the Lorentzian gravitational action for bounded regions, as described in arXiv:1301.7041. By comparing to a Euclidean calculation, we explain the agreement between the formula for this imaginary part and the formula for black hole entropy. We also clarify the topological structure of the imaginary part in Lovelock gravity. We then evaluate the action's imaginary part for some special regions. These include cylindrical slabs spanning the exterior of a stationary black hole spacetime, 'maximal diamonds' in various symmetric spacetimes, as well as local near-horizon regions. In the first setup, the black hole's entropy and conserved charges contribute to the action's imaginary and real parts, respectively. In the other two setups, the imaginary part coincides with the relevant entropy.

 

[marcus]

Additional info on the YN paper: 34 pages, 10 figures.

http://arxiv.org/abs/1305.2344
Bouncing Loop Quantum Cosmology from F(T) gravity
Jaume Amorós, Jaume de Haro, Sergei D. Odintsov
(Submitted on 10 May 2013)
The big bang singularity could be understood as a breakdown of Einstein's General Relativity at very high energies. Adopting this viewpoint, other theories, that implement Einstein Cosmology at high energies, might solve the problem of the primeval singularity. One of them is Loop Quantum Cosmology (LQC) with a small cosmological constant that models a universe moving along an ellipse, which prevents singularities like the big bang or the big rip, in the phase space (H,ρ), where H is the Hubble parameter and ρ the energy density of the universe. Using LQC when one considers a model of universe filled by radiation and matter where, due to the cosmological constant, there are a de Sitter and an anti de Sitter solution. This means that one obtains a bouncing non-singular universe which is in the contracting phase at early times. After leaving this phase, i.e., after bouncing, it passes trough a radiation and matter dominated phase and finally at late times it expands in an accelerated way (current cosmic acceleration). This model does not suffer from the horizon and flatness problems as in big bang cosmology, where a period of inflation that increases the size of our universe in more than 60 e-folds is needed in order to solve both problems. The model has two mechanisms to avoid these problems: The evolution of the universe through a contracting phase and a period of super-inflation (H' > 0).
16 pages.

 

[marcus]

http://arxiv.org/abs/1305.2597
A semiclassical Hamiltonian for plane waves in loop quantum gravity
Donald E. Neville
(Submitted on 12 May 2013)
This is the first of two papers which study the semiclassical limit of a loop quantum gravity (LQG) canonical quantization of unidirectional plane gravity waves. Initially I formulate an exact LQG in which each triad, E^x_X for example, grasps only one of the x holonomies present at the vertex. Field strengths are non-local, constructed from holonomies which connect two neighboring vertices of the spin network. The equations are simplified using a semiclassical approximation, meaning eigenvalues of the volume operator are assumed to be large enough that the [volume, holonomy] commutators may be replaced by their quantum field theory limits. Additionally, SU(2) holonomies are expanded in sines and cosines, sines are assumed small, and terms quadratic in sines are dropped. In the semiclassical limit many non-local features disappear. However, differences replace derivatives with respect to z, the propagation direction; and semiclassical triads grasp both holonomies present at each vertex. Gauge-fixing constraints, as well as the constraint that the wave is traveling only in one direction, are formulated in a language appropriate to LQG. A subsequent paper constructs a sinusoidal solution using a Hilbert space of coherent states tailored to the symmetry of the plane wave case.
61 pages.

http://arxiv.org/abs/1305.2599
Sine wave solution for plane waves in loop quantum gravity
Donald E. Neville
(Submitted on 12 May 2013)
This paper constructs an approximate sinusoidal wave packet solution to the equations of loop quantum gravity (LQG). The equations are solved in a semiclassical, small sine approximation. Eigenvalues of the volume operator are assumed to be large enough that the [ volume, holonomy ] commutators may be replaced by their quantum field theory limits; SU(2) holonomies are expanded in sines and cosines, sines are assumed small, and terms up to quadratic in sines are kept. The wave is unidirectional and linearly polarized. The states are coherent states tailored to the symmetry of the plane wave case. Fixing the spatial diffeomorphisms is equivalent to fixing the spatial interval between vertices of the loop quantum gravity lattice. In the classical limit, this spacing can be chosen such that the eigenvalues of the triad operators are large, as required by a semiclassical treatment. Exact continuity of variables at boundaries is not reasonable in LQG, a fundamentally discrete theory. I propose equating averages taken over vertices on opposite sides of the boundary.
39 pages.

http://arxiv.org/abs/1305.2588
Causal set d'Alembertians for various dimensions
Fay Dowker, Lisa Glaser
(Submitted on 12 May 2013)
We propose, for dimension d, a discrete Lorentz invariant operator on scalar fields that approximates the Minkowski spacetime scalar d'Alembertian. For each dimension, this gives rise to a scalar curvature estimator for causal sets, and thence to a proposal for a causal set action.
14 pages, 1 figure

http://arxiv.org/abs/1305.2900
A Dark Sector Extension of the Almost-Commutative Standard Model
Christoph A. Stephan
(Submitted on 13 May 2013)
We consider an extension of the Standard Model within the frame work of Noncommutative Geometry. The model is based on an older model [St09] which extends the Standard Model by new fermions, a new U(1)-gauge group and, crucially, a new scalar field which couples to the Higgs field. This new scalar field allows to lower the mass of the Higgs mass from ~170 GeV, as predicted by the Spectral Action for the Standard Model, to a value of 120-130 GeV. The short-coming of the previous model lay in its inability to meet all the constraints on the gauge couplings implied by the Spectral Action. These shortcomings are cured in the present model which also features a "dark sector" containing fermions and scalar particles.
24 pages.

brief mention:
http://arxiv.org/abs/1305.2614
Consistent NEC-violation: towards creating a universe in the laboratory
V. A. Rubakov
(Submitted on 12 May 2013)
Null Energy Condition (NEC) can be violated in a consistent way in models with unconventional kinetic terms, notably, in Galileon theories and their generalizations. We make use of one of these, the scale-invariant kinetic braiding model, to discuss whether a universe can in principle be created by man-made processes. We find that even though the simplest models of this sort can have both healthy Minkowski vacuum and consistent NEC-violating phase, there is an obstruction for creating a universe in a straightforward fashion. To get around this obstruction, we design a more complicated model, and present a scenario for the creation of a universe in the laboratory.
17 pages.

 

[marcus]

John86 pointed out this taped monologue by Lee Smolin:
http://www.edge.org/conversation/think-about-nature
It gives what I think is an important perspective on several areas of professional research.

http://arxiv.org/abs/1305.2932
A first look at transition amplitudes in (2+1)-dimensional causal dynamical triangulations
Joshua H. Cooperman, Jonah Miller
(Submitted on 13 May 2013)
We study a lattice regularization of the gravitational path integral--causal dynamical triangulations--for (2+1)-dimensional Einstein gravity with positive cosmological constant in the presence of past and future spacelike boundaries of fixed intrinsic geometries. For spatial topology of a 2-sphere, we determine the form of the Einstein-Hilbert action supplemented by the Gibbons-Hawking-York boundary terms within the Regge calculus of causal triangulations. Employing this action we numerically simulate a variety of transition amplitudes from the past boundary to the future boundary. To the extent that we have so far investigated them, these transition amplitudes appear consistent with the gravitational effective action previously found to characterize the ground state of quantum spacetime geometry within the de Sitter-like phase. Certain of these transition amplitudes convincingly demonstrate that the so-called stalks present in this phase are numerical artifacts of the lattice regularization, seemingly indicate that the quantization technique of causal dynamical triangulations differs from that of the no-boundary proposal of Hartle and Hawking, and possibly represent the first numerical simulations of portions of temporally unbounded quantum spacetime geometry within the causal dynamical triangulations approach.
24 pages, 12 figures, 1 table

http://arxiv.org/abs/1305.3153
Dimensional reduction in the sky
Giovanni Amelino-Camelia, Michele Arzano, Giulia Gubitosi, Joao Magueijo
(Submitted on 14 May 2013)
We explore the cosmological implications of a mechanism found in several approaches to quantum-gravity, whereby the spectral dimension of spacetime runs from the standard value of 4 in the infrared (IR) to a smaller value in the ultraviolet (UV). Specifically, we invoke the picture where the phenomenon is associated with modified dispersion relations. With minimal assumptions, we find that UV behaviour leading to 2 spectral dimensions results in an exactly scale-invariant spectrum of vacuum scalar and tensor fluctuations, regardless of the equation of state. The fluctuation production mechanism is analogous to the one known for varying speed of sound/light models and, unlike in inflation, the spectrum is already scale-invariant before leaving the horizon, remaining so after freeze-in. In the light of Planck's recent results we also discuss scenarios that break exact scale-invariance, such as the possibility that the spectral dimension runs down to a value slightly higher than 2, or runs down to 2 but with an extremely slow transient. We further show that the tensor to scalar ratio is fixed by the UV ratio between the speed of gravity and the speed of light. Not only does our model not require inflation, but at its most minimal it seems incompatible with it. In contrast, we find that running spectral dimensions can improve the outlook of the cyclic/ekpyrotic scenario, solving the main problems present in its simplest and most appealing realisations.
9 pages, 7 figures.

brief mention (not loop-and-allied QG, but possibly of interest):
http://arxiv.org/abs/1305.3044
Planck constraints on single-field inflation
Shinji Tsujikawa, Junko Ohashi, Sachiko Kuroyanagi, Antonio De Felice
(Submitted on 14 May 2013)
We place observational constraints on slow-variation single-field inflationary models by carrying out the cosmological Monte Carlo simulation with the recent data of Planck combined with the WMAP large-angle polarization, baryon acoustic oscillations, and ACT/SPT temperature data. Our analysis covers a wide variety of models with second-order equations of motion-- including potential-driven slow-roll inflation, non-minimally coupled models, running kinetic couplings, Brans-Dicke theories, potential-driven Galileon inflation, field-derivative couplings to the Einstein tensor, and k-inflation. ...
27 pages, 8 figures

http://arxiv.org/abs/1305.3258
Holographic Special Relativity
Derek K. Wise
(Submitted on 14 May 2013)
We reinterpret special relativity, or more precisely its de Sitter deformation, in terms of 3d conformal geometry, as opposed to (3+1)d spacetime geometry. An inertial observer, usually described by a geodesic in spacetime, becomes instead a choice of ways to reverse the conformal compactification of a Euclidean vector space up to scale. The observer's "current time," usually given by a point along the geodesic, corresponds to the choice of scale in the decompactification. We also show how arbitrary conformal 3-geometries give rise to "observer space geometries," as defined in recent work, from which spacetime can be reconstructed under certain integrability conditions. We conjecture a relationship between this kind of "holographic relativity" and the "shape dynamics" proposal of Barbour and collaborators, in which conformal space takes the place of spacetime in general relativity. We also briefly survey related pictures of observer space, including the AdS analog and a representation related to twistor theory.
17 pages, 5 illustrations

 

[marcus]

brief mention:
http://arxiv.org/abs/1305.3497
Varying electric charge in multi-scale spacetimes
Gianluca Calcagni, Joao Magueijo, David Rodríguez Fernández
...
28 pages.

http://arxiv.org/abs/1305.3509
Interpretation of quantal manifolds
Johan Noldus
(Submitted on 11 May 2013)
In quantum gravity, one looks for alternative structures to ... ordinary real manifolds. Here, we propose an alternative universal construction containing the latter as an equilibrium state under the action of the universal diffeomorphism group. Our theory contains many other previous proposals ... as special cases. However, the crucial point we make is that those have to be appreciated in the universal context developed here.
16 pages.

 

[MTd2]

http://arxiv.org/abs/1305.3313

Is Quantum Gravity a Super-Quantum Theory?

Lay Nam Chang, Zachary Lewis, Djordje Minic, Tatsu Takeuchi
(Submitted on 14 May 2013)
We argue that quantum gravity should be a super-quantum theory, that is, a theory whose non-local correlations are stronger than those of canonical quantum theory. As a super-quantum theory, quantum gravity should display distinct experimentally observable super-correlations of entangled stringy states.

 

[atyy]

http://arxiv.org/abs/1305.1045
A lattice non-perturbative definition of an SO(10) chiral gauge theory and its induced standard model
Xiao-Gang Wen
(Submitted on 5 May 2013)
The standard model is a chiral gauge theory where the gauge fields couple to the right-hand and the left-hand fermions differently. The standard model is defined perturbatively and describes all elementary particles (except gravitons) very well. However, for a long time, we do not know if we can have a non-perturbative definition of standard model as a Hamiltonian quantum mechanical theory. In this paper, we propose a way to give a modified standard model (with 48 two-component Weyl fermions) a non-perturbative definition by embeding the modified standard model into a SO(10) chiral gauge theory and then putting the SO(10) chiral gauge theory on a 3D spatial lattice with a continuous time. Such a non-perturbatively defined standard model is a Hamiltonian quantum theory with a finite-dimensional Hilbert space for a finite space volum. Using the defining connection between gauge anomalies and the symmetry-protected topological orders, we show that any chiral gauge theory can be non-perturbatively defined by putting it on a lattice in the same dimension, as long as the chiral gauge theory is free of all anomalies.

 

[marcus]

http://arxiv.org/abs/1305.4512
Loop quantum cosmology for nonminimally coupled scalar field
Michal Artymowski, Andrea Dapor, Tomasz Pawlowski
(Submitted on 20 May 2013)
We perform a LQC-quantization of the FRW cosmological model with nonminimally coupled scalar field. Making use of a canonical transformation, we recast the theory in the minimally coupled form (Einstein frame), for which standard LQC techniques can be applied to find the physical Hilbert space and the dynamics. We then focus on the semiclassical sector, obtaining a classical effective Hamiltonian, which can be used to study the dynamics. We show that the classical singularity is replaced by a "mexican hat"-shaped bounce, joining the contracting and expanding branches. The model accommodates Higgs-driven inflation, with more than enough e-folding for any physically meaningful initial condition.
3 pages, 2 figures. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012
[my comment: this was based on a revised version of http://arxiv.org/abs/1207.4353 which went on arxiv 20 May 2013 at the same time and gives a more complete presentation of the same material]

http://arxiv.org/abs/1305.4513
Quantum field theory on LQC Bianchi spacetimes
Andrea Dapor, Jerzy Lewandowski, Yaser Tavakoli
(Submitted on 20 May 2013)
Quantum theory of a scalar field is developed on the LQC Bianchi I space-time. By comparing the quantum field theory for a single mode on classical and quantum background geometries we find that an effective Bianchi I space-time emerges. We show that by disregarding the back-reaction no Lorentz-violation is present, despite the effective metric being different than the classical Bianchi I one.
3 pages. To appear in Proceedings of the 13th Marcel Grossmann Meeting (MG13), Stockholm, Sweden, 1-7 July 2012

http://arxiv.org/abs/1305.4516
Modified Friedmann equation and survey of solutions in effective Bianchi-I loop quantum cosmology
Linda Linsefors, Aurelien Barrau
(Submitted on 20 May 2013)
In this article, we study the equations driving the dynamics of a Bianchi-I universe described by holonomy corrected effective loop quantum cosmology. We derive the LQC-modified generalized Friedmann equation, which is used as a guide to find different types of solutions. It turns out that, in this framework, most solutions never reach the classical behavior.
8 pages, 9 figures.

http://arxiv.org/abs/1305.4526
Variables suitable for constructing quantum states for the Teleparallel Equivalent of General Relativity I
Andrzej Okolow
(Submitted on 20 May 2013)
In this paper we present the first part of an analysis aimed at introducing variables which are suitable for constructing a space of quantum states for the Teleparallel Equivalent of General Relativity via projective techniques - the space of quantum states is meant to be applied in a canonical quantization of the theory. We show that natural configuration variables on the phase space of the theory can be used to construct a space of quantum states which however possesses an undesired property. We introduce then a family of new variables such that some elements of the family can be applied to build a space of quantum states free of that property.
Comments: 33 pages, 5 figures.

brief mention:
http://arxiv.org/abs/1305.4524
The Black Hole Particle Accelerator as a Machine to make Baby Universes
A. J. S. Hamilton
(Submitted on 20 May 2013)
7 pages, 2 figures. Entered in the 2013 Essay Competition of the Gravity Research Foundation

 

[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.