Recent seminars

Europe/Lisbon
Online

Rita Teixeira da Costa

Rita Teixeira da Costa, Princeton University
The Teukolsky equation on Kerr in the full subextremal range

The Teukolsky equation is one of the fundamental equations governing linear gravitational perturbations of the Kerr black hole family as solutions to the vacuum Einstein equations. I will discuss joint work with Yakov Shlapentokh-Rothman (Toronto), where we show that solutions arising from suitably regular initial data decay inverse polynomially in time. Our proof holds for the entire subextremal range of Kerr black hole parameters ($|\alpha|\lt M$).

Europe/Lisbon
Online

Mariem Magdy

Mariem Magdy, Instituto Superior Técnico
Conformal methods and asymptotic charges on vacuum spacetimes

One of the common approaches in the studies of the asymptotic structure of spacetimes involves the use of conformal transformations. These transformations allow us to study the behaviour of the gravitational field ‘at infinity’ using local differential geometry by mapping points at infinite distances in one manifold to finite distances in another. In recent years, the subject of asymptotic symmetries/charges has acquired renewed interest, in part due to its relation with soft theorems, the gravitational memory effect and the information loss paradox. During my talk, I will focus on the role of Friedrich’s formulation of spatial infinity in evaluating asymptotic charges near spatial infinity and the implications of our analysis in the recent articles: arXiv:2311.07294 and arXiv:2112.03890


Seminar room (2.8.3), Physics Building

Martin Kološ

Martin Kološ, Silesian University in Opava
Vibrating string loop around black hole

A thin circular structure, a string loop, vibrating in the central plane of a Schwarzschild black hole will be investigated. String loop stability and frequencies of its vibrational modes will be provided for different string equations of state. String are objects with tension and hence they can help us to understand the problem of relativistic elasticity; they can be also considered as simplified models for thin magnetic flux tubes from plasma physics. We used the vibrating string loop model to fit the quasi-periodic oscillation observed in X-ray signals coming from some compact sources.

This seminar is joint with CENTRA, and will take place on the Physics Department (seminar room, 2nd floor).

Europe/Lisbon
Online

Spyros Alexakis

Spyros Alexakis, University of Toronto
Squeezing a fixed amount of gravitational energy to arbitrarily small scales, in U(1) symmetry

We discuss a construction of polarized $U(1)$ space-times of a “fixed size”, where the initial data contains a given amount $\eta\gt 0$ of incoming gravitational energy which is allowed to be supported at arbitrarily small scales, without affecting the time of existence. More specialized examples can be constructed where any chosen fraction of this initial incoming energy can stay focused along a null geodesic, in the $U(1)$ picture. We discuss the relationship of this result with one direction of Thorne’s hoop conjecture. The work relies on a special structure of the Einstein equations in this symmetry class, a new re-scaling of the equations and large-time control, along with a new local Klainerman-Sobolev estimate. Time permitting, some comparisons of the result and the methods with large-data constructions for the Einstein equations over the last decade will also be presented.

Europe/Lisbon
Online

Rick Schoen

Rick Schoen, University of California, Irvine
How minimal hypersurface and MOTS singularities affect relativity theorems

Some theorems in relativity work directly only in low dimensions because of the possibility of singularities in area minimizing hypersurfaces and MOTS. These include both Riemannian and spacetime positive mass theorems as well as the Riemannian Penrose inequality. In some cases the singularities are relatively easy to work around and in others they present significant problems to overcome. This will be a general lecture which will attempt to give perspective and more clarity to this issue.