Testing the Kerr hypothesis: the examples of synchronisation and scalarisation

by Carlos A.R. Herdeiro

The Kerr hypothesis is that astrophysical black hole candidates are very special objects, with only two degrees of freedom and well described by the Kerr metric. Theoretically, this hypothesis is based on the uniqueness theorems for electro-vacuum. But in the presence of other types of matter or modified gravity are there any viable alternatives? In this talk I will illustrate some examples of black holes with “hair” that could co-exist with Kerr black holes, but emerge dynamically (and be preferred) at particular scales, either in General Relativity with ultralight bosonic matter or in modified gravity with higher curvature corrections, commenting on their theoretical and phenomenological differences (e.g. shadows) and on their phenomenological viability.