Black hole apparent horizons become comoving with the universe

Abstract

It has been shown that static black hole event horizons cannot exist in time-dependent backgrounds without becoming naked singularities. In light of this, we are left with the question ``how does a black hole horizon evolve in an expanding universe?'' Here we offer a solution to this puzzle by showing in full generality that regular black hole apparent horizons tend to become comoving, thus confirming the necessity of a cosmological coupling and raising intriguing possibilities for the early growth of black holes.