• Black holes
• Hair
• Scalar field
• Dark matter
• Dark energy
• Superradiance
• Quasinormal modes

The detection of gravitational waves by the LIGO-Virgo Collaboration made the dream of studying the details of the spacetime around black holes more concrete than it has ever been. Motivated by this opportunity, we will review the concept of hairy black holes, focusing on the case of scalar hair, and explore analytically three different settings of potential observational relevance.

First, in the wave regime of the scalar dark matter scenario, we will systematically explore the field profile around a Kerr black hole, for different values of the scalar mass, emphasizing the effects due to the spin.

Second, we will study black hole superradiance in the presence of an ultralight scalar field. We will complement, with an analytical approach, what was found with numerical methods in the literature. We will show how hairy black holes form as a result of the superradiant evolution, quantify the maximum mass that can be extracted from the black hole and extend the analytical treatment to include the effects of accretion and gravitational wave emission.

Third and last, we will consider hairy black holes in more general scalar-tensor theories of gravity. In an effective description of quasinormal modes, we will address the issue of stability, proving that the odd modes of static black holes are always stable at leading order in derivatives and giving a strong argument for the extension of this result to sufficiently slowly rotating black holes as well.