Hairy black holes and bosonic stars: linear and non-linear solutions and their properties

The Kerr black hole paradigm reigns supreme in explaining the nature of the most

compact astrophysical objects in the Universe. However, lingering theoretical questions and

the limitations of current observations motivate the search for alternatives, whose properties

defy the current paradigm. Among the many possibilities, only a few appear to be sound and

of potential astrophysical relevance. This is the case of black holes with synchronized hair

and bosonic stars, two families of exotic compact objects that spring from the existence of as

yet hypothetical bosonic fields.

This doctoral thesis is a compilation of research papers on the physics of these

compact objects. The chapters are, for the most part, reprints of research papers published

in peer-reviewed journals. Chapter 2 studies in detail the ground state of the Proca hairy

black holes, namely their linear strand, known as stationary clouds Chapter 3 discusses the

rationale behind the existence of stationary clouds, which requires a feedback mechanism.

Chapter 4 presents an upper limit on the “hairiness” of black holes with synchronized hair

formed through the growth and saturation of the dominant superradiant mode of Kerr black

holes. Chapter 5 considers these hairy black holes from a thermodynamic point of view.

Despite being entropically favored, they are found to be locally unstable in the canonical

ensemble even when branched off from locally stable Kerr black holes and thus correspond

to a new phase. Finally, Chapter 6 delves into linear radial perturbations of spherically

symmetric bosonic stars in the frequency domain, confirming and extending previous results

in the literature. It includes the computation of their fundamental normal modes as well as a

discussion about criteria for linear stability.