Evading no-hair theorems: Hairy black holes in a Minkowski box

Abstract

We find hairy black holes of Einstein-Maxwell theory with a complex scalar field that is confined inside a box in a Minkowski background. These regular hairy black holes are asymptotically flat and thus the presence of the box or mirror allows to evade well-known no-hair theorems. We also find the Israel surface stress tensor that the confining box must have to obey the energy conditions. In the zero horizon radius limit, these hairy black holes reduce to a regular asymptotically flat hairy soliton. We find our solutions using perturbation theory. At leading order, a hairy black hole can be seen as a Reissner-Nordstrom black hole placed on top of a hairy soliton with the same chemical potential (so that the system is in thermodynamic equilibrium). The hairy black holes merge with the Reissner-Nordstrom black hole family at the onset of the superradiant instability. When they co-exist, for a given energy and electric charge, hairy black holes have higher entropy than caged Reissner-Nordstrom black holes. Therefore, our hairy black holes are the natural candidates for the endpoint of charged superradiance in the Reissner-Nordstrom black hole mirror system.