Aspects of black holes in alternative theories of gravity

Black holes are among the simplest objects in the Universe, yet they are also the perfect probes of the strong-field regime of gravity, where alternative theories of gravity are expected to yield new predictions for the physics of compact astrophysical objects. In the era of gravitational wave astronomy, some of the predictions can finally be put to a test. We start by investigating the Horndeski class of scalar-tensor theories. It incorporates many alternative theories of gravity that feature a scalar degree of freedom in addition to the metric, either explicitly or through a specific representation. Such scalars have been employed to explain the puzzles of cosmology and they often arise in the effective field theories of more fundamental physics. With the additional field comes the possibility of black-hole solutions that differ from those of general relativity. We investigate the dynamics of a theory that admits only solutions of this kind and find evidence suggesting that such black holes form through gravitational collapse. Moreover, we study the causal structure of any solution with a non-trivial scalar, as the equations permit superluminal propagation for scalar perturbations. We find that the notion of a horizon that bounds all field excitations persists. This is not the case in theories with a preferred frame where black holes have multiple nested horizons. We address the claim that superluminal modes, the hallmark of Lorentz-violating theories, allow for processes that violate the generalized second law of black hole thermodynamics. We derive the necessary conditions for such a violation and find those unlikely to arise from a theory where gravity is attractive. Finally, we sketch a path to advance the research programme featured in this thesis in view of the recent progress in gravitational wave astronomy.