# Program

General Schedule

The book of abstracts can be found here

INVITED TALKS

# The Einstein Constraint Equations

Rodrigo Ávalos

The Einstein Constraint Equations The Einstein constraint equations (ECE) have their roots in the evolution problem of initial data in general relativity (GR) providing necessary and sufficient conditions for well-posedness. As such, their analysis has become a central area of research within mathematical GR. In particular, their conformal formulation naturally relates them to generalised scalar curvature deformation problems and Yamabe-type equations. This conformal formulation has provided classifications for the set of constant mean curvature (CMC) solutions in different settings, although many problems still remain open, especially in the case of far from CMC solutions. Within this mini-course, the objective is to provide an introduction to this topic, reviewing the origins of the ECE, introducing their conformal formulation, and then focusing on CMC classifications where current open questions will be highlighted.

# Global Hiperbolicity Revisited

Annegret Burtscher

Global hyperbolicity is a ubiquitous notion in general relativity. It is crucial for both the PDE theoretic perspective as well as the Lorentzian geometric aspect. In this talk, we review the results that characterize globally hyperbolic spacetimes or assume this notion and add a new characterization via metric completeness. This is joint work with L. García-Heveling

# Coordinates are messy in General (Relativity)

Carla Cederbaum

We consider asymptotically flat Lorentzian manifolds which are foliated by asymptotically Euclidean time-slices with suitably decaying lapse and shift. Each of these time-slices then comes with ten “charges” (energy, linear and angular momentum, and center of mass); the first four of these are well-known to be well-defined under now standard “optimal decay assumptions” on the metric and second fundamental form of the time-slice. For the last six charges, one typically assumes extra decay assumptions called the Regge—Teitelboim conditions, taking the form of asymptotic parity conditions in “Regge—Teitelboim coordinates”. We will first give an introduction to the charges and their (expected) properties and transformation behavior under the asymptotic Poincaré group of the Lorentzian manifold. Next, we will discuss the relevance of the decay conditions for the transformation behavior; this relies on joint works with Christopher Nerz, with Anna Sakovich, and with Saradha Senthil Velu. We will then give examples of asymptotically Euclidean time-slices which do not possess any Regge--Teitelboim coordinates and explain the consequences of this result for the charges; this is joint work with Melanie Graf and Jan Metzger.

# Spacetime (in-)extendibility and questions of uniqueness

Melanie Graf

Questions of (low-regularity) spacetime (in-)extendibility have a long history within Lorentzian Geometry and are closely related to several important physical problems. If a spacetime (M,g) turns out to be extendible natural follow up questions are how much uniqueness may be expected from purely geometric considerations (i.e., without assuming that the extension continues to obey a specific physical model or similar) and if and how at least the boundary of M in the extension could be constructed from M itself. Understanding the structure of the boundary of M in any potential extension also played a key role in several general results on low regularity inextendibility. In my talk I want to try and give an overview of some of the newer (and older) inextendibility results (including joint work with Eric Ling) before considering the question of uniqueness of extensions and discussing local results by Jan Sbierski as well as recent joint work with Marco van den Beld Serrano on trying to obtain a more global statement and comparing our work to some general boundary constructions for spacetimes.

# Finsler structures applied to the study of spacetimes with symmetries

Miguel Ángel Javaloyes

We will show that when a spacetime admits a Killing field (a one-parameter family of local isometries), it is possible to define a Finsler structure in a hypersurface transversal to this Killing field that retains all the information about the causality of the spacetime. If this Killing field is not timelike, then the Finsler structure is not defined in all the directions. In particular, this can be used to study Killing horizons and black holes as well as Cauchy developments, and it allows one to describe all the steps in the Causal ladder. We will also describe how the Finsler invariants are related to the causal invariants of the spacetime.

# What is a spacetime?

Olaf Müller

Usually, a spacetime is defined to be a (connected) time-oriented Lorentzian manifold. However, from the category of (say, globally hyperbolic) spacetimes there are injective functors into other categories, e.g. into the one of sets with Lorntzian distance functions or into the one of ordered measure spaces. There is even an injective map of the category of Cauchy slabs into the category of Riemannian manifolds, preserving curvature and other geometric bounds. There is also a spectral description of spacetimes in analogy to Connes’ spectral triples. According to what structure is taken to be the fundamental one, there are different notions of maximal extensions and maximal Cauchy developments. The talk will explore the different notions and their relations to each other as well as relations to questions of singularities in mathematical relativity.

Rigidity of mass-preserving 1-Lipschitz maps from integral current spaces into Euclidean space.

Raquel Perales

We will prove that given an n-dimensional integral current space and a 1-Lipschitz map, from this space onto the n-dimensional Euclidean ball, that preserves the mass of the current and is injective on the boundary, then the map has to be an isometry. We deduce as a consequence the stability of the positive mass theorem for graphical manifolds as originally formulated by Huang--Lee--Sormani. (Joint work with G. Del Nin).

Kundt spacetimes

Abdelghani Zeghib

To any semi-Riemannian metric one associates the set of its scalar curvature invariants. One may ask if this system of functions fully determines the metric. In particular, (locally)homogeneous semi-Riemannian spaces have all their scalar curvature invariants constant. One then asks if the converse is true, that is, constancy of these invariants implies local homogeneity? This turns out to be true in the Riemannian case, but not in the higher signature cases. In the Lorentz case, constancy seems related to being of Kundt type, that is having (locally) a geodesic null vector field with all the scalar invariants of its covariant derivative vanishing.