Quantum Gravity in Paris 2017

20-23 March 2017




Titles and abstracts:  

John Barrett
Title: Quantum gravity using finite spectral triples
Abstract:
It is proposed to use non-commutative finite spectral triples to model space-time. Then quantum gravity becomes a matrix model for the Dirac operator. One also hopes to recover the usual classical space-time in a suitable limit of non-commutative spectral triples. Some initial results for the proposal will be discussed.
Slides: pdf

Timothy Budd
Title: Escaping universality in two-dimensional quantum gravity
Abstract:
Many models of random (Euclidean) geometry in two dimensions are known to belong to the universality class of "pure" 2d quantum gravity. To escape this universality class, one usualy couples gravity to matter fields with long-range interactions. In this talk I will discuss an alternative involving the geometry of random planar graphs (planar maps) that support vertices of exceptionally high degree. For a suitable range of parameters these geometries are expected to possess self-similar scaling limits that are genuinely different from the "Brownian geometry" of pure 2d quantum gravity. Based mainly on joint work with Nicolas Curien and Cyril Marzouk.
Slides: pdf

Ali Chamseddine
Title: Gravity with Mimetic Matter
Abstract:
We modify the longitudinal mode in General Relativity and show that the extra degree of freedom can mimic cold dark matter. Introducing a potential for the mimetic non-dynamical scalar field we can mimic nearly any gravitational properties of the normal matter. In particular, the mimetic matter can provide us with inflaton, quintessence and even can lead to a bouncing nonsingular universe. By incorporating the idea of limiting curvature, the singularities in contracting Friedmann and Kasner universes as well as black holes are avoided.
Slides: pdf

Stephane Dartois
Title: Topological recursion in six dimensional tensor models
Abstract:
In this presentation I will discuss the topological recursion for a simple, six dimensional, tensor model. I will explain how the recursion formula familiar to matrix models generalizes in the case of this simple tensor model. Then I will present how the observables of this six dimensional tensor model can be extracted from the topological recursion symmetric forms.
Slides: pdf

Frank Ferrari
Title: Black Holes, Matrix Models and Large D
Abstract:
Holography predicts that matrix quantum mechanical models should describe black holes. This in turn implies that these models should have many remarkable properties; in particular, unitarity should be lost at large N and the systems should be, in some sense, maximally chaotic. The aim of my talk will be the review these important ideas and show how they can be explicitly derived in an interesting class of matrix models, using a new large D limit which is inspired from the large N limit of a special class of rank three tensor models. Since the parameter D is related holographically to the number of bulk space-time dimensions, it is natural to speculate that the new large D limit corresponds to a non-trivial infinite dimensional version of holography, possibly along the lines of the large d limit of general relativity recently studied by Emparan and collaborators.
Slides: pdf

Marc Geiller
Title: Quantum gravity phases and topological quantum field theories with defects
Abstract:
In this talk, I will present and explain the structure of vacua underlying the quantum representation and the Hilbert space of loop quantum gravity and related discrete non-perturbative approaches to quantum gravity. This will highlight an unsuspected richness of this class of theories, and make manifest their interpretation as topological quantum field theories with defects. These are theories which posses no local degrees of freedom in the bulk, but can however carry quasi-particle-like excitations on defects of arbitrary co-dimension. Interestingly, such theories have recently attracted lots of attention in condensed matter, where it is believed that they provide a classification of the so-called topologically-ordered phases of matter. On the quantum gravity side, I will explain the relevance of this construction for the study of the continuum limit and for the extraction of physical predictions.
Slides: pdf

Jerzy Jurkiewicz
Title: Diffeomorphism invariant coordinate system on a CDT dynamical geometry with a toroidal topology
Abstract:
We consider a discretized system with a dynamical geometry defined by the CDT ideas. Our earlier studies were devoted to a numerical study of systems periodic in time with a global time foliation and a fixed $S_3$ spatial topology at each time slice. The model in this case has a reach phase structure with four phases, one of which, the de Sitter phase, is particularly interesting from a physical point of view. We extend our study to the case where the spatial topology is that of a $T_3$ torus, We show that in this case we observe the same phase structure. We concentrate on a point in the parameter space corresponding to the de Sitter phase. We try to address a non-trivial question: how in a background independent system with a randomly fluctuating geometry we may observe the appearance of a semi-classical coordinate system suitable to describe the IR limit of a theory. The toroidal topology in spatial directions makes it possible to define such a system of coordinates and and to determine the effective semi-classical action for volume fluctuations in spatial directions.
Slides: pdf

Mercedes Martin-Benito
Title: Fermions in Loop Quantum Cosmology
Abstract:
In this talk we will study the quantization of primordial fermion perturbations in Loop Quantum Cosmology (LQC). More precisely, we consider a Dirac field coupled to a spatially flat homogeneous and isotropic cosmology, sourced by a homogeneous scalar inflaton, and treat the Dirac field as a perturbation, truncating the action of the system at quadratic perturbative order. We motivate a convenient description of this symplectic system and adopt the hybrid LQC approach to quantize it. From the full Hamiltonian constraint, and considering a Born-Oppenheimer ansatz for physical states, we will derive a Schrödinger equation for the quantum evolution of the fermion perturbations. We will carefully analyze this quantum evolution, paying special attention to questions like evolution of the Fock vacuum, unitarity of the dynamics, and back reaction of the fermion perturbations onto the geometry.
Slides: pdf

Kyriakos Papadodimas
Title: The black hole information paradox and the fate of the infalling observer
Abstract:
General relativity predicts that the horizon of a large black hole is smooth. On the other hand, quantum mechanics, and the requirement that no information is lost during black hole evaporation, suggests that the horizon may be dramatically modified at the quantum level, even when the local curvature is small. I will discuss recent developments related to this fundamental conflict between general relativity and quantum mechanics. I will present a proposal, motivated by the AdS/CFT correspondence, which seems to be able to resolve the paradox and which may lead to a new framework for a quantitatively precise description of the black hole interior.
Slides: pdf

Roberto Percacci
Title: On the phase structure of quantum gravity
Abstract:
The low energy structure of gravity suggests that a kind of Higgs phenomenon is at work at the Planck scale, with the metric acting as order parameter. Thus, the main question for a quantum theory of geometry is to explain why there is a non-degenerate metric. I will review some old and new suggestions for a dynamical mechanism generating it, emphasizing recent models of non-commutative geometry.
Slides: pdf

Mairi Sakellariadou
Title: Group Field Theory Condensate Cosmology
Abstract:
After a short introduction in Group Field Theory -- a nonperturbative approach to quantum gravity -- I will discuss aspects of the GFT based quantum cosmology. In particular, I will present the important consequences of considering interactions between the quanta of geometry for static or evolving GFT condensates and I will then discuss anisotropic GFT configurations.
Slides: pdf

Harold Steinacker
Title: Covariant Quantum Spaces, the IKKT Matrix Model and Gravity
Abstract:
I will discuss geometrical properties and physical perspectives of covariant quantum spaces, focusing on the fuzzy 4-sphere. These spaces realize noncommutative 4D geometries with local and global symmetries. The fluctuations on such a background within the IKKT matrix model lead to a higher spin theory, due to the underlying twisted bundle structure. Some aspects of the gravitational sector are elaborated, and the linearized Einstein equations modified by an IR cutoff are obtained under some assumptions.
Slides: pdf

Silke Weinfurtner
Title: Observation of superradiance in a vortex flow
Abstract:
Wave scattering phenomena are ubiquitous to almost all Sciences, from Biology to Physics. When an incident wave scatters off of an obstacle, it is partially reflected and partially transmitted. Since the scatterer absorbs part of the incident energy, the reflected wave carries less energy than the incident one. However, if the obstacle is rotating, this process can be reversed and waves can be amplified, extracting energy from the scatterer. Even though this phenomenon, known as superradiance, has been thoroughly analysed in several theoretical scenarios (from eletromagnetic radiation scattering on a rotating cylinder to gravitational waves incident upon a rotating black hole), it has never been observed. Here we describe in detail the first laboratory detection of superradiance. We observed that plane waves propagating on the surface of water are amplified after being scattered by a draining vortex. The maximum amplification measured in the experiment was 20%, obtained for 3.70 Hz waves, in a 6.25 cm deep fluid. Our results are consistent with superradiant scattering caused by rapid rotation. In particular, a draining fluid can transfer part of its rotational energy to incident low-frequency waves. Our experimental findings will shed new light on Black Hole Physics, since shallow water waves scattering on a draining fluid constitute an analogue of a black hole. We believe, especially in view of the recent observations of gravitational waves, that our results will motivate further research (both theoretical and experimental) on the observation of superradiance of gravitational waves.

Peter West
Title: E theory
Abstract:
I will briefly review Kac-Mooody algebras and non-linear realisations. I will show that the non-linear realisation constructed from $E_{11}$ and its vector representations leads to a set of $E_{11}$ invariant equations of motion that contain and infinite number of fields which live on an infintie dimensional spacetime. However, if one keeps only the low level fields and coordinates one finds a set of equations, which are essentially unique, and are precidely those of eleven dimensional supergravity. I will explain that the non-linear realisation is a unified theory in that it also contains all the other maximal supergravities in the different dimensions and also all the gauged supergravities. This encourages us to believe that the non-linear realisation is the complete low energy effective action for strings and branes.
Slides: pdf

Christof Wetterich
Title: Scale symmetry - a link from quantum gravity to cosmology
Abstract:
Scale symmetry is a central ingredient in the functional renormalisation group approach to quantum gravity. It arises from fixed points of running couplings - no intrinsic mass or length scales are present anymore. Such a world can be realised in the infinite past and future of our Universe. We explore dilaton quantum gravity as a quantum field theory for the metric and a scalar field, with ultraviolet and infrared fixed points. The ultraviolet fixed point makes gravity asymptotically safe and non-perturbatively renormalizable. The approximate scale invariance close to the fixed point is reflected in cosmology in the almost scale invariant spectrum of the observed primordial cosmic fluctuations. The approach to the infrared fixed point in the asymptotic future is characterized by the dynamics of an almost massless cosmon - the pseudo Goldstone boson of spontaneously broken scale symmetry. It is responsible for dynamical dark energy. Both inflation and the present dynamical dark energy arise from the same scalar field. The history of the Universe describes a crossover from a „past fixed point“ where all particles are massless, to a „future fixed point“ where exact scale invariance is spontaneously broken, generating the particle masses. The cosmological solution can be extrapolated to the infinite past in physical time - the Universe has no beginning and no physical singularity. A simple model is compatible with all present cosmological observations. It could be tested by the observation of huge lumps in the cosmic neutrino background , the detection of early dark energy , or rather large primordial graviton fluctuations generated during inflation.
Slides: pdf

John Wheater
Title: Solvable matter on 2D causal triangulations
Abstract:
Progress towards exact solutions for matter on 2D causal triangulations has been slow. I will describe solutions for restricted dimer models and discuss what is currently known about other matter systems.
Slides: pdf

Edward Wilson-Ewing
Title: Emergent Friedmann dynamics from quantum gravity condensates
Abstract:
I will explain how cosmological dynamics emerge from the hydrodynamics of isotropic group field theory condensate states in the Gross-Pitaevskii approximation. The correct Friedmann equations are recovered in the classical limit for some choices of the parameters in the action for the group field theory, and quantum gravity corrections arise in the high-curvature regime causing a bounce which generically resolves the big-bang and big-crunch singularities.
Slides: pdf