The next cosmology seminars take place on Friday 13th of December, seminar room E349 at 9:45am. Our guest speakers are Jibril Ben Achour from Munich U. (Germany) and Hugo Roussille from ENS Lyon (France).
At 9:45am, room E349, Jibril Ben Achour, will be talking about
Classification of gravitational memory effects in a vacuum gravitational plane wave
Gravitational memory effects are permanent and cumulative shifts of relative observables (distance, velocity, angular momentum …) between two test particles induced by the passage of a gravitational wave. They stand as one of the last prediction of general relativity (GR) yet to be confirmed through the gravitational wave astronomy. In turn, they are intimately related to the notion of degeneracy of the vacuum in gauge theory and important efforts have been devoted to characterize these effects through the relation to the symmetries of spacetime. In this talk, I will consider the simplest exact non-linear radiative solution of GR, namely a vacuum gravitational plane wave, and show step by step how to classify the different memory effects occurring in this case. I will show new relation between memories and hidden symmetries, and resolve conflicting claim in the literature. The goal being to illustrate a concrete method based on symmetries and applicable to more complex radiative spacetime, in particular asymptotically flat radiative spacetimes.
At 11am, room E349, Hugo Roussille will talk about
Algebraically special quadratic Schwarzschild perturbations
The equations describing linear perturbations around a Schwarzschild black hole admit analytical solutions that describe waves of specific wavelengths propagating outside the black hole. While perturbations around a Schwarzschild black hole are generally of Petrov type I, these analytical solutions describe spacetimes of Petrov type II, and are thus dubbed ‘algebraically special modes’. The existence of these modes is linked to the isospectrality theorem for Schwarzschild. In this work, I go beyond the linear approximation and construct algebraically special perturbations around a Schwarzschild black hole at the quadratic order, making use of a family of exact twisting vacuum radiative solutions of General Relativity. These quadratic perturbations can still be expressed analytically, similarly to their linear sources. I study their properties and show in particular how static quadratic perturbations deform the Schwarzschild black hole.