Black holes with dark-photon hair

The dark photon, a massive vector particle, is a dark-matter candidate which has gained wide popularity within the cosmology community. In its ultralight regime, such a vector field has been shown to exhibit a superradiant instability around rotating black holes, which would allow it to settle into nontrivial "hairy" configurations in astrophysical situations. In this work, we consider hairy vector-field solutions resulting not from superradiance, but rather from the accretion of particles from a cloud of non-relativistic vector dark matter. Such a mechanism is compelling, as it provides hairy solutions for black holes of any mass and spin. We present our recent perturbative results, explicitly constructing solutions for a Schwarzschild black hole, then discuss our work extending these to Kerr spacetime and studying their dynamical growth using numerical relativity. This complements similar work done for massive scalar fields. These bosonic condensates, if sufficiently common, could affect the dynamics of typical binary mergers, making this work important for understanding environmental effects in gravitational-wave observations.