Phenomenology of bubble size distributions in a first-order phase transition

Abstract: Fermi balls (FBs) and primordial black holes (PBHs) can be copiously produced during a dark first-order phase transition (FOPT). The radius distribution of false vacuum bubbles causes FBs and PBHs to have extended mass distributions. We show how gravitational wave (GW), microlensing and Hawking evaporation signals deviate from the case of monochromatic distributions. The peak of the GW spectrum is shifted to lower frequencies and the spectrum is broadened at frequencies below the peak frequency. Thus, the radius distribution of true vacuum bubbles introduces another uncertainty in the evaluation of the GW spectrum from a FOPT. The extragalactic gamma-ray signal at AMEGO-X/e-ASTROGAM from PBH evaporation may evince a break in the power-law spectrum between 5~MeV and 10~MeV for an extended PBH mass distribution. Optical microlensing surveys may observe PBHs lighter than $10^{-10}M_\odot$, which is not possible for monochromatic mass distributions. This expands the FOPT parameter space that can be explored with microlensing.