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. The gravitational wave signal associated with a particular primordial black hole population is stronger than in typical scenarios because of a negative non-Gaussianity of the perturbations, and it has a distinguishable shape with two peaks.