Parallel Session Abstracts.pdf

**Plenary Speaker Abstracts:**

**Ivan Agullo**, DAMTP Cambridge

*A Quantum Gravity Extension of the Inflationary Scenario*

Since the standard inflationary paradigm is based on quantum field theory on classical space-times, it excludes the Planck era. Using techniques from loop quantum gravity, the theory is extended to overcome this limitations. The new framework sharpens conceptual issues by distinguishing between the true and apparent trans-Planckian difficulties and provides sufficient conditions under which the true difficulties can be overcome within a quantum gravity theory, with interesting lessons for both theory and observations.

**Abhay Ashtekar**, Pennsylvania State University

*Promising Paths*

In LQG we work in the spirit of Antonio Machado: "Traveler, there is no path; Paths are made by walking." I will present a bird's eye view of some of the paths that have emerged since Loops 11 and offer a few suggestions.

**Aurelien Barrau**, Universite Joseph Fourier

*Some possible ways to observe consequences of loop quantum gravity*

In this talk, I'll briefly review some possible observational consequences of loop quantum gravity. I will first address the issue of the closure of the algebra of constraints in holonomy-corrected effective loop quantum cosmology for tensor, vector, and scalar modes. I will underline some unexpected features like a possible change of signature. The associated primordial power spectrum and the basics of the related CMB analysis will be presented. The "asymptotic silence" hypothesis will be mentioned as a promising alternative. Then, I'll address the issue of the probability for inflation and the prediction of its duration from a new perspective. Finally, I'll present some prospect about the evaporation of black holes in LQG.

**Eugenio Bianchi**, Perimeter Institute

*Entanglement, Bekenstein-Hawking Entropy, and Spinfoams*

I review recent developments on vacuum entanglement perturbations in perturbative quantum gravity and spinfoams, and discuss their relevance for understanding the nature of black hole entropy.

**Steve Carlip**, University of California, Davis

*Spontaneous Dimensional Reduction?*

Several lines of evidence hint that quantum gravity at distances a bit larger than the Planck scale may become effectively two-dimensional. I will summarize the evidence for this "spontaneous dimensional reduction," and suggest a further argument based on the effect of vacuum fluctuations on light cones. If this description proves to be correct, it suggests an interesting relationship between small-scale quantum spacetime and the behavior of cosmologies near a spacelike singularity.

**Fay Dowker**, Imperial College, London

*Causal Sets and the Quantum of Action *

The struggle between local and global concepts in physics comes to a head in causal set quantum gravity. Local physics -- and general relativity in particular -- must be recovered in a continuum approximation if the theory is to be successful but causal sets are inherently non-local entities. I will describe a family of causal set actions labelled by dimension, each of which is nonlocal and yet, when applied to certain causal sets, approximates the Einstein Hilbert action and is therefore effectively local. I will explain why this is a source of hope that causal set theory has a continuum approximation.

**Henrique de Andrade Gomes**, University of California, Davis

*Shape Dynamics: a status report*

I will give an introduction to the theory of Shape Dynamics, and then comment on recent advances, obstacles and future projects.

**Dafne Guetta**, ORT-Braude College & INAF-OAR

*Quantum Gravity Phenomenology with Neutrinos and high energy photons.*

I review the main properties of the Gamma Ray Bursts (GRBs) as possible sources of high energy (E>TeV) neutrinos and confirmed sources of high energy (E>GeV) photons.

I discuss the possibility to use the data of neutrino telescopes, such as IceCube and the GeV-photon telescopes, such as Fermi’s LAT, for precision tests of Einstein's Special Relativity as applied to neutrinos and photons. My focus is on possible departures from Special Relativity that can be motivated by models of quantum space-time. I observe that neutrinos which one would not associate to a GRB, when assuming a classical spacetime picture, may well be GRB neutrinos if the possibility that Lorentz invariance is broken at very high energies is taken into account. I outline how future analyses of neutrino data should be done in order to systematically test the Lorentz Invariance Violation possibility. In addition I consider the possibility that Lorentz Invariance Violation might be responsible for the spectral lags that characterize the GeV signal observed for the remarkable GRB130427A.

A comparison of these features for GRBs at different redshifts provides some encouragement for a redshift dependence of the effects of the type expected for a quantum-spacetime interpretation, but other aspects of the analysis appear to invite the interpretation as intrinsic properties of GRBs.

**Razvan Gurau**, Universite Paris-Sud

*he non perturbative 1/N expansion of Tensor Models*

I will present the recently obtained non perturbative 1/N expansion of tensor models. The correlation functions are shown to be analytic in the coupling constant in some domain of the complex plane and to support appropriate scaling bounds at large N. Surprisingly, the non perturbative setting turns out to be a powerful computational tool allowing the explicit evaluation order by order (with bounded rest terms) of the correlations.

**Muxin Han**, Centre de Physique Theorique

*Spinfoam Formulation of Loop Quanum Gravity*

Recently there are a lot of progresses in developing the spinfoam formulation of loop quantum gravity. In this talk I give an overview of the subject. I introduce the formalism and the motivation of the theory, and I discuss the application of spinfoam formulation in black hole and cosmology. I also discuss the inclusion of the quantum matter fields and cosmological constant in the formalism. The inclusion of cosmological constant motivates a Chern-Simons formulation of LQG. Finally I discuss the semiclassical low-energy approximation of the spinfoam formulation, where Einstein gravity appears as the leading contribution.

**Frank Hellmann**, Max Planck Institute for Gravitational Physics

*Asymptotic dynamics: Spin foam partition functions in the asymptotic regime*

Spin foam models are models for space time built from discrete chunks of quantized geometry. In the asymptotic regime the classical geometry is regained.

In the last year we have seen rapid developments in our understanding of this geometry at the level of the entire partition function. In particular it was found that the geometries that contribute to the partition function in the asymptotic regime satisfy accidental curvature constraints.

I will discuss the classic results and role of asymptotics, the recent results and their impact on the interpretation of these models.

**Viqar Husain**, University of New Brunswick

*Scenes from polymer quantization*

A regime of "polymer quantum field theory on curved spacetime" should emerge in a low energy approximation of quantum gravity based on LQG ideas. This era should be characterized by a polymer scale, and give modifications to the usual semiclassical approximation. I will describe work on gravitational collapse, cosmology, and statistical mechanics in this setting. Results include models of horizon evaporation, inflation and graceful exit without an inflaton potential, and an indication of dimensional reduction from 4 to 2.5 dimensions.

**Kirill Krasnov**, University of Notthingham *Diffeomorphism Invariant Gauge Theories*

I will describe a very large class of gauge theories that do not use any external structure such as e.g. a spacetime metric in their construction. When the gauge group is taken to be SL(2) these theories describe interacting gravitons, with GR being just a particular member of a whole family of gravity theories. Taking larger gauge groups one obtains gravity coupled to various matter systems. In particular, I will show how gravity together with Yang-Mills gauge fields arise from one and the same diffeomorphism invariant gauge theory Lagrangian. Finally, I will describe what is known about these theories quantum mechanically.

**Etera Livine**, Ens de Lyon

*Spinor and Twistor networks in Loop Gravity*

I will review the reformulation of the loop gravity phase space in terms of spinor networks and twistor networks, and present how these techniques can be used to write spinfoam amplitudes as discretized path integrals and to study the dynamics that they define (recursion, Hamiltonian constraints as differential equations).

**Renate Loll,** Radboud University Nijmegen

*Causal Dynamical Triangulations without Preferred Foliation*

We introduce a generalized version of the Causal Dynamical Triangulations (CDT) formulation of quantum gravity, in which the regularized, triangulated path integral histories maintain 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 (although convenient) part of its background structure.

**Alejandro Perez**, Centre de Physique Theorique

*Black holes in loop quantum gravity: new insights and perspectives from semiclassical consistency*

I will argue that the recently introduced quasilocal framework for black hole mechanics (based on the form of the near horizon geometry of stationary black holes (BHs)) together with an additional assumption on the degeneracy of the area spectrum in quantum gravity (holography for non geometric degrees of freedom) leads to agreement between the statistical mechanical treatment of quantum black holes and standard semiclassical results in BH thermodynamics. More precisely, up to small quantum corrections, quantum black holes satisfy the following properties: Entropy is Bekenstein-Hawking entropy, and fluctuations of the horizon area are small. Moreover, under the above assumption, an explicit correspondence between the statistical mechanical treatment of the fundamental LQG degrees of freedom and the semiclassical Euclidean path integral formulation can be explicitly established.

**Vincent Rivasseau**, *Universite Paris-Sud XI Orsay*

The Tensor Track

I will present the tensorial renormalization group approach to quantum gravity and the tentative scenario it suggests for the emergence of space-time.

**Carlo Rovelli**, Le Centre de Physique Théorique

*What have we learned so far about quantum gravity?*

I try to make the point about what we know and what we do not yet know about the possibility of writing a quantum theory of gravity.

**Frank Saueressig**, Radboud University Nijmegen

*Black holes in Asymptotically Safe Gravity*

In this talk, I will briefly review the main ingredients of the gravitational asymptotic safety program before focusing on the phenomenological consequences originating from the scale-dependent couplings characteristic for the theory. In particular, I will discuss recent unexpected developments in unveiling the structure of microscopic black holes within Asymptotic Safety: 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. The relation of these results to previous criticism that Asymptotic Safety does not reproduce the state-count of a conformal field theory will be addressed.

**David Skinner**, DAMTP & Institute for Advanced Study

*Twistor Strings for N=8 Supergravity*

The perturbative S-matrix of General Relativity has a rich and fascinating geometric structure that is completely obscured by its traditional description in terms of Feynman diagrams. I'll explain a new way of looking at four dimensional supergravity: as a string theory in twistor space. All tree-level amplitudes in the theory can be described by algebraic curves in Penrose's nonlinear graviton

**Madhavan Varadarajan**, Raman Research Institute

*Towards a Consistent Quantum Dynamics for Euclidean LQG: a weak coupling limit*

Spacetime covariance in canonical quantum gravity is tied to the existence of an anomaly free representation of its constraint algebra. I will argue that establishing the existence of such a representation in the LQG context requires the consideration of higher than unit density weight Hamiltonian constraints. Smolin's weak coupling limit of Euclidean gravity, while simpler than full blown gravity , still exhibits an open constraint algebra isomorphic to that of gravity and offers an ideal testing ground for the investigation of the quantum constraint algebra of such higher density constraints. I will report on recent progress on this issue in the context of an LQG type quantization of this system. Certain features of the constructions such as the encoding of the action of the quantum constraint in terms of operator valued diffeomorphisms may play a key role in the definition of a consistent quantum dynamics for LQG.