#### 2. December 2021

**Video**

## STEFAN CZIMEK

(ICERM, *Brown University*, USA)

**Title:** The characteristic gluing problem of general relativity

**Abstract:** In this talk we introduce and solve the characteristic gluing problem for the Einstein vacuum equations. We prove that obstructions to characteristic gluing come from an infinite-dimensional space of conservation laws along null hypersurfaces for the linearized equations at Minkowski. We show that this obstruction space splits into an infinite-dimensional space of gauge-dependent charges and a 10-dimensional space of gauge-invariant charges. We identify the 10 gauge-invariant charges to be related to the energy, linear momentum, angular momentum and center-of-mass of the spacetime. Based on this identification, we explain how to characteristically glue a given spacetime to a suitably chosen Kerr black hole spacetime. As corollary we get an alternative proof of the Corvino-Schoen spacelike gluing to Kerr. Moreover, we apply our characteristic gluing method to localise characteristic initial data along null hypersurfaces. In particular, this yields a new proof of the Carlotto-Schoen spacelike localization where our method yields no loss of decay, thus resolving an open problem in this direction. We also outline further applications. This is joint work with S. Aretakis (Toronto) and I. Rodnianski (Princeton).

#### 4. November 2021

**Video**

## MELANIE GRAF

(University of Tübingen, Germany)

**Title:** Singularity theorems in low regularity

**Abstract:** The singularity theorems of R. Penrose and S. Hawking from the 1960s show that a spacetime satisfying certain physically reasonable curvature and causality conditions cannot be causal geodesically complete. Despite their great success these classical theorems still have some drawbacks, one of them being that they require smoothness of the metric while in many physical models the metric is less regular. In my talk I will present work on singularity theorems based on distributional energy conditions for metrics that are merely continuously differentiable – a regularity where one still has existence but not uniqueness for solutions of the geodesic equation. We will see that an approximation-based approach to the low-regularity issue is closely linked to establishing singularity theorems under weakened energy conditions and while the improvements necessary are still entirely straightforward in the present case, attempting to lower the regularity further would require some new methods.

#### 7. October 2021

**Video**

## MIHALIS DAFERMOS

(Princeton University, USA) and (University of Cambridge, UK)

**Title:** The black hole stability problem in general relativity

**Abstract:** I will review the current status of the black hole stability problem in general relativity and discuss joint work Holzegel, Rodnianski and Taylor.

#### 1. July 2021

**Video**

## GEORGIOS MOSCHIDIS

**Title:**The instability of Anti-de Sitter spacetime for the Einstein-scalar field system

**Abstract:**The AdS instability conjecture provides an example of weak turbulence appearing in the dynamics of the Einstein equations in the presence of a negative cosmological constant. The conjecture claims the existence of arbitrarily small perturbations to the initial data of Anti-de Sitter spacetime which, under evolution by the vacuum Einstein equations with reflecting boundary conditions at conformal infinity, lead to the formation of black holes after sufficiently long time. In this talk, I will present a rigorous proof of the AdS instability conjecture in the setting of the spherically symmetric Einstein-scalar field system. The construction of the unstable initial data will require carefully designing a family of initial configurations of localized matter beams and estimating the exchange of energy taking place between interacting beams over long periods of time, as well as estimating the decoherence rate of those beams. I will also discuss possible paths for extending these ideas to the vacuum case.

#### 3. June 2021

**Video**

## PIOTR CHRUŚCIEL

(University of Vienna, Austria)

**Title:**The hyperbolic positive energy theorem

**Abstract:**I will review the notion of mass of asymptotically locally

hyperbolic manifolds, and sketch the proof of positivity of the energy for manifolds with spherical conformal infinity.

Talk based on joint work with Erwann Delay in arXiv: 1901.05263 [math.DG]

#### 6. May 2021

**Video**

## LAN-HSUAN HUANG

(University of Connecticut, USA)

**Title:**Existence of static vacuum extensions

**Abstract:**The study of static vacuum Riemannian metrics arises naturally in general relativity and differential geometry. A static vacuum metric produces a static spacetime by a warped product, and it is related to scalar curvature deformation and gluing. The well-known Uniqueness Theorem of Static Black Holes says that an asymptotically flat, static vacuum metric with black hole boundary must belong to the Schwarzschild family. In contrast to the rigidity phenomenon, R. Bartnik conjectured that there are asymptotically flat, static vacuum metric realizing certain arbitrarily specified boundary data. I will discuss recent progress toward this conjecture. It is based on joint work with Zhongshan An.

#### 1. April 2021

**Video**

## MARTÍN REIRIS

(University of the Republic, Uruguay)

**Title:**New results on compact Cauchy horizons of smooth vacuum spacetimes

**Abstract:**Cauchy horizons are rather unique and peculiar objects that have been studied for decades. In this talk I will first review old and new breakthroughs on the subject by Isenberg and Moncrief, and by Petersen and Rácz, and then discuss joint recent work together with I. Bustamante where it is proved that non-degenerate Compact Cauchy horizons on smooth vacuum spacetimes (shortly CHs) have indeed constant non-zero temperature. It then follows by the work of Petersen and Petersen-Rácz that CHs are Killing horizons, and by the work of Beig-Chruściel-Schoen that such objects are non-generic on the initial data, (in agreement with the Cosmic Censorship conjecture). A null-orbital and topological classification of CHs will also be commented.

#### 4. March 2021

**Video**

## IGOR RODNIANSKI

**Title:**On naked singularities in Einstein equations

**Abstract:**I will describe recent and ongoing work with Y. Shlapentokh-Rothman on a construction of solutions to the Einstein vacuum equations corresponding to a naked singularity forming from a regular past. The talk will focus on a new geometric phenomenon, corresponding to twisting of null geodesics, new type of self-similarity, dynamical approach to the problem, and on the comparisons with the naked singularity solutions constructed by Christodoulou for the spherically symmetric Einstein-scalar field model.

#### 4. February 2021

**Video**

## HARVEY REALL

**Title:**Well-posed formulation of Lovelock and Horndeski theories

**Abstract:**Lovelock theories are the most general diffeomorphism invariant theories of gravity in higher dimensions with second order equations of motion. Horndeski theories are the most general diffeomorphism invariant theories of gravity coupled to a scalar field in four dimensions with second order equations of motion. In this talk I will discuss well-posedness of the initial value problem for these theories. Previous work has shown that (generalised) harmonic gauge does not give a well-posed initial value problem. I will describe recent work with Aron Kovacs in which we introdued a modification of harmonic gauge that does give a well-posed initial value problem provided that the theory remains “weakly coupled”. Our modified harmonic gauge may also have applications in conventional GR.

#### 3. December 2020

**Video**

## LYDIA BIERI

(University of Michigan, Ann Arbor, USA)

**Title:**New Structures in Gravitational Radiation

**Abstract:**Gravitational waves are transporting information from faraway regions of the Universe. A new era began with the first detection of gravitational waves by Advanced LIGO in September 2015, and since then several events have been recorded by the LIGO/VIRGO collaboration. New challenges await us to unravel the interesting interplay between physics, astrophysics and mathematics. Most studies so far have been devoted to sources like binary black hole mergers or neutron star mergers, or generally to sources that are stationary outside of a compact set. We describe these systems by asymptotically-flat manifolds solving the Einstein equations. These sources have in common that far away their gravitational field decays fast enough towards Minkowski spacetime. In particular, far away from the source, the decay behavior can be described by a term that is homogeneous of degree -1 and lower order terms. I will present new results on gravitational radiation for sources that are not stationary outside of a compact set, but whose gravitational fields decay more slowly towards infinity. A panorama of new gravitational effects opens up when delving deeper into these more general spacetimes. In particular, whereas the former sources produce memory effects that are of purely electric parity (permanent displacement only), the latter in addition generate memory of magnetic type, thus allowing for rotation in the system. These new effects emerge naturally from the Einstein equations.

#### 5. November 2020

**Video**

## GREG GALLOWAY

**Title:**Initial data rigidity results

**Abstract:**We present several rigidity results for initial data sets motivated by the positive mass theorem. An important step in our proofs is to establish conditions that ensure that a marginally outer trapped surface is “weakly outermost”. A rigidity result for Riemannian manifolds with a lower bound on their scalar curvature is included as a special case. Relevant background on marginally outer trapped surfaces will be discussed. This talk is based on joint work with Michael Eichmair and Abraão Mendes.

#### 1. October 2020

**Video**

## HANS RINGSTRÖM

**Title:**On highly anisotropic big bang singularities.

**Abstract:**In cosmology, the universe is typically modelled by spatially homogeneous and isotropic solutions to Einstein’s equations. However, for large classes of matter models, such solutions are unstable in the direction of the singularity. For this reason, it is of interest to study the anisotropic setting.