Programme

Norbert Bodendorfer (University of Warzav, Warzav): Strings meet Loops via AdS/CFT

It has been notoriously difficult to compare loop quantum gravity and string theory due to their very different underlying principles and basic observables. Whereas (perturbative) string theory has much to say about scattering amplitudes and the low energy limit of the theory, loop quantum gravity so far mainly gave insights about a possible quantum geometry at the Planck scale. New tools from the AdS/CFT correspondence, such as holographic entanglement entropy, however allow to investigate non-perturbative string theory, including geometric observables such as co-dimension 2 areas, which are in turn among the most basic operators in loop quantum gravity. We argue that more detailed investigations along this route could potentially be very fruitful and might offer not only a change to better understand quantum gravity, but also dual quantum field theories.

Sylvain Carrozza (University of Bordeaux 1, Bordeaux): Group Field Theories: motivations and renormalization

I will start with a review of the main features of the Group Field Theory (GFT) approach to quantum gravity, which can be best understood as an extension of standard Loop Quantum Gravity (LQG). The GFT formalism provides a natural prescription for summing over Spin Foam amplitudes and hence completing the definition of the dynamics of LQG. GFTs can also be interpreted as QFTs creating and annihilating spin-network states over a “no-space” vacuum, they have therefore the potential to greatly facilitate the study of the many-body sector of LQG, and hence of its continuum limit. I will finally review recent results on GFT renormalization, which is necessary to the overall consistency of the formalism and is expected to play a central role in the exploration of its various physical phases.

Astrid Eichhorn (Imperial College, London): The asymptotic safety paradigm for quantum gravity

I will introduce the main idea of the asymptotic safety paradigm. It generalizes the success-story of asymptotic freedom, underlying the construction of parts of the Standard Model of particle physics, to a quantum gravitational setting. I will then review the evidence that we have for the consistency of this scenario in the case of pure quantum gravity, and under the coupling to matter fields. Finally, I will explain how quantum-gravity effects on matter in asymptotically safe quantum gravity could provide a way to test the consistency of the model with observations.

Lisa Glaser (University of Nottingham, Nottingham): A partially ordered introduction to Causal Set theory

In causal set theory, space-time is reduced to a partially ordered set. The only remaining structure are causal curves, and discrete events. On first glance this seems to be very little structure to encode an entire universe, but on closer examination it reveals a rich structure. Recent work in the theory has given rise to proposals of Lorentz invariant non-local phenomenology.

Valentina Giangreco Puletti (University of Iceland, Reykjavík): Holographic Rényi entropy and Lovelock gravity

I will review basic concepts of gauge/gravity duality (AdS/CFT), introduce the definition of Rényi entropy in quantum field theories and the so-called Casini-Huerta-Myers prescription employed to holographically compute Rényi entropy. The second part of the talk is devoted to the analysis of the holographic Rényi entropy in Lovelock gravity theories and its unusual features.

Hal Haggard (Bard college, New York): Dynamical polyhedra and the atoms of space in quantum gravity

A quite revolution in the study of polyhedra is brewing along the road to a quantum theory of gravity. Despite having been studied for centuries polyhedra are rarely taken to be dynamical. Doing so ties together Einstein’s dynamical geometry and discrete models of quantum space in a compelling fashion. I will give an overview of these ideas and to the many open questions in this area, such as: Given a dynamical description of a polyhedron, how do you determine the adjacency of its faces? or How can we describe the geometry of discrete grains of space in a spacetime with a cosmological constant?

Sabine Hossenfelder (Frankfurt Institute for Advanced Studies, Frankfurt): Quantum Gravity Phenomenology

Why are theoreticians so convinced that gravity must be quantized and how do you work in a field where you neither have a theory nor experiment? These are the questions I will take on in my talk. I will explain the relevance of phenomenological models and their guiding role in theory development.

Jeff Hnybida (Radboud University, Nijmegen): Spin foams and their combinatorial solution

I will introduce spin foam models and show how they can be expressed in terms of statistical loop models.  An example of a statistical loop model is the 2d Ising model, where the loops are the domain walls.  I will then discuss the relation between the exact combinatorial solution of these models and spin geometry.

Miklos Långvik (University of Helsinki, Helsinki): Twistors, LQG and the conformal symmetry of flat Minkowski spacetime

I will give an introduction to twistors and focus on their application to the spinnetwork states in LQG. A realization leading to twisted geometry, a generalization of Regge geometry. I will then introduce a null octahedron, using twistors, that seems to play a central role for the geometric description of SU(2,2), the isometry group of flat Minkowski spacetime. Finally, the possible applications of this construction will be highlighted.

Jarmo Mäkelä (Vaasa University of Applied Sciences, Vaasa): The Problem of the Cosmological Constant in Loop Quantum Gravity

In this talk we consider the thermodynamics and the statistical physics of the de Sitter spacetime. We show that with a specific counting of states loop quantum gravity predicts for the cosmological constant a value, which is small, but still non-zero and positive. In this sense, both the presence and the smallness of the cosmological constant, together with the observed accelerating expansion of the Universe, may be viewed as quantum effects of gravitation.

Ilkka Mäkinen (University of Warsaw, Warsaw): Scalar field as a physical time variable in loop quantum gravity

A possible approach to the issue of dynamics in quantum gravity is to use a matter field as a relational time variable, with respect to which the evolution of the quantum state of the gravitational field is described. The aim of my talk is to show how a concrete definition of the dynamics can be achieved in loop quantum gravity, when the role of the time variable is played by various kinds of a scalar field. After starting with a short introduction to the question of dynamics in general relativity and quantum gravity, I will describe the Hamiltonians which govern the dynamics in models of loop quantum gravity coupled to a scalar field, and show numerical results on the time evolution of some simple states of quantum geometry.

Alejandro Perez (Centre de Physique Théorique, Marseille): Black hole entropy, 2d CFTs, and quantum geometry

I will show that there is a natural relationship between quantum geometry degrees of freedom on an (isolated) horizon and 2-dimensional CFTs. This relationship might play a central role in the understanding of BH entropy in LQG.

Andreas Pithis (King’s College, London): Towards nonlinear effective interactions in GFT condensate cosmology

The Group Field Theory (GFT) proposal to quantum gravity is closely related to and draws from Loop Quantum Gravity, simplicial quantum gravity, matrix models as well as condensed matter theory. Its Group Field Cosmology (GFC) spin off aims at providing a framework for quantum cosmology. Its main conceptual idea and conjecture is that continuum spacetime is a thermodynamical phase of an underlying GFT system, that is obtained through a phase transition (“geometrogenesis”) in the quantum gravity analogue of the thermodynamic limit used in condensed matter systems. In this context, we discuss the main aspects of the GFC condensate picture exemplified through a particular free model and report progress on the analysis of several effectively interacting GFC models.

Matti Raasakka (Tampere): Banishing the Culprit: A spacetime-free framework for quantum physics

Spacetime is a serious troublemaker in modern physics, well-known and dreaded for resisting quantization for over half a century by now. As an alternative to the attempts to contain the brawler, we suggest expelling him. To that end, we develop a background-geometry-free algebraic approach to quantum theory. The basic idea is that spacetime need not be a fundamental ingredient in our description of Nature, but instead an effective organization of its dynamics. Importantly, we realize in our framework a mechanism for the quantum state to influence the causal relations and evolution of the observables, which is a prerequisite for gravitational phenomena. We also consider some ideas and methods for the (re)construction of effective spacetime geometry from the more fundamental quantum ingredients.

Julian Rennert (University of Waterloo, Waterloo): Timelike simplicity constraints and 4d Lorentzian Spinfoam models

I will give a brief summary of 4d Lorentzian spinfoam models with an emphasis on the description of timelike faces and their quantization. Based on the twistorial parametrization of loop gravity I will then present a new `generalized’ spinfoam model and discuss its potential implications.

Simone Speziale (Centre de Physique Théorique, Marseille): From loop quantum gravity to exploding black holes

I will briefly review the present structure of spin foam transition amplitudes for loop quantum gravity, mentioning also its description in terms of twistors. I will then present some recent results on the evaluation of the amplitudes using SL(2,C) Clebsch-Gordan coefficients and an application to computing the tunnelling amplitude for a black hole to “explode” into a white hole.

Marika Taylor (University of Southampton, Southampton): Holography and quantum gravity

The holographic principle states that any quantum theory of gravity should be described as a theory without gravity in one less spatial dimension. We will discuss why quantum gravity in negatively curved anti-de Sitter spacetimes is believed to be equivalent to a scale invariant quantum field theory in one less dimension, before discussing recent ideas on holography for other spacetimes, including in particular flat spacetime.

Wolfgang Wieland (Perimeter Institute, Waterloo): Spinors, twistors and the quantisation of geometry

Edward Wilson-Ewing (Albert Einstein Institute, Potsdam): Bouncing cosmologies from condensates of quantum geometry

We study the effective cosmological dynamics, emerging as the hydrodynamics of simple condensate states, of a group field theory model for quantum gravity coupled to a massless scalar field and reduced to its isotropic sector. The quantum equations of motion for these group field theory condensate states are given in relational terms with respect to the scalar field, from which effective dynamics for spatially flat, homogeneous and isotropic space-times can be extracted. The result is a generalization of the Friedmann equations, including quantum gravity modifications, in a specific regime of the theory. The classical Friedmann equations of general relativity are recovered in a suitable semi-classical limit for some range of parameters of the microscopic dynamics. An important result is that the quantum geometries associated with these GFT condensate states are non-singular: a bounce generically occurs in the Planck regime. For some choices of condensate states, these modified Friedmann equations are very similar to those of loop quantum cosmology.

Antonia Zipfel (University of Warzaw, Warzaw): How to derive predictions from LQG?

By now, the foundations of loop quantum gravity (LQG) are well understood and some magnificent predictions, e.g. the replacement of the big bang by a big bounce, have been made. Due to the complexity of the theory, most of these results have, however, been derived from simplified toy models, whose relation to the full theory is not sufficiently understood. I will discuss some ideas how to overcome this situation and make LQG more predictive and thus falsifiable. Hereby, I will focus especially on the following two aspects: a) How can we build explicit physical states in canonical LQG? and b) How can we identify the physically interesting subsectors?

In this talk I hope to give a brief overview based on my recent work for the non-experts, but at the same time stimulate a discussion over the problems.