Black Holes and Gravitational Waves from Slow First-Order Phase Transitions.

Slow first-order phase transitions generate large inhomogeneities that can lead to the formation of primordial black holes. We show that the gravitational wave spectrum then consists of a primary component sourced by bubble collisions and a secondary one induced by large perturbations. The latter gives the dominant peak if β/H_{0}<12, impacting, in particular, the interpretation of the recent pulsar timing array data.