**Helvi Witek (Cambridge) — Wednesday 12th March — Maths Room 103 — 4.30pm**

The next Relativity and Cosmology seminar of term takes place on Wednesday 12th March, and will be given by Helvi Witek (Cambridge) speaking on:

**Black holes as observatories for beyond-standard model physics**

Black holes are key players in a wide range of fundamental physics with a rich phenomenology. Here, I will focus on the superradiant effect of Kerr BHs which can yield to instabilities when surrounded by a “mirror”. This setup arises naturally in asymptotically anti-de Sitter spacetimes or in the presence of massive fields surrounding the BH, which can model condensates of ultra-light bosonic fields such as axion-like particles or dark matter candidates. Thus, exploring the superradiant instability of Kerr BHs opens up the exciting possibility to better understand fundamental fields by observing astrophysical BHs. In this talk, I will present recent results concerning massive fields in BH backgrounds as well as first numerical simulations of the fully non-linear case. We have explored massive scalar fields surrounding Kerr BHs and we have found interesting signatures in the scalar and gravitational wave channel. The BH’s response hints a superradiant effects at the non-linear level.

The seminar will be in the usual place, Maths Room 103 on the Mile End Campus, at the usual time of 4.30pm.

**Eugene Lim (KCL) — Wednesday 5th march — Maths Room 103 — 4.30pm**

The next Relativity and Cosmology seminar of term takes place on Wednesday 5th March, and will be given by Eugene Lim (KCL) speaking on ultrarelaticistic solitons and his numerical relativity programme:

**Ultrarelativistic Soliton Collisions, Numerical Experiments and all that**

In contrast to their reputation of being “non-perturbative, strongly coupled and hard to calculate”, the ultra-relativistic limit of soliton/defect collisions are actually weakly coupled and hence analytically tractable. I will present an analytic derivation of the phase and velocity shifts in general 1+1 soliton collisions, and check that these expressions are accurate with numerical simulations.

This project is part of a general program that I am working on to understand non-perturbative physics with gravity backreaction, which involves the development of a generic adaptive mesh general relativity code GRCHOMBO. I will discuss the general goals of the program, and the current state of GRCHOMBO.

The seminar will be in the usual place, Maths Room 103 on the Mile End Campus, at the usual time of 4.30pm.

Ellie Nalson (QML) — Wednesday 26th February — Maths Room 103 — 4.30pm

The next Relativity and Cosmology seminar of term takes place on Wednesday 26th February, and will be given by our own Ellie Nalson (QML) speaking on:

Magnetic fields are present on all scales in the Universe and recently have even been found within voids in the large scale structure. At present there is no single “natural” mechanism to generate the initial seed fields needed to produce these inter-galactic magnetic fields. We start by reviewing the main possible generation mechanisms. Then by using fully relativistic cosmological perturbation theory we show analytically how magnetic fields can be generated. We present the amplitude and scale dependence one would expect from such a generation mechanism.

The seminar will be in the usual place, Maths Room 103 on the Mile End Campus, at the usual time of 4.30pm.

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