Detailed calculation of primordial black hole formation during first-order cosmological phase transitions

Primordial black holes could potentially form during a first-order cosmological phase transition due to a buildup of particles which are predominantly reflected from the advancing bubble walls. After discussing the general mechanism, we examine the criteria that need to be satisfied for a black hole to form. We then set out the Boltzmann equation that describes the evolution of the relevant phase space distribution function, carefully describing our treatment of the Liouville operator and the collision term. Assuming a spherical false-vacuum pocket of sufficient size and a constant wall velocity, we find that black holes can form in a range of different scenarios.