Light Primordial Black Holes from a First-Order Phase Transition near the End of Inflation: Baryogenesis and Gravitational Wave Signatures

Abstract

We investigate the formation and cosmological consequences of light primordial black holes (PBHs) from vacuum bubbles nucleated during a spectator sector first-order phase transition in inflation. We construct a phenomenological model in which the spectator potential evolves with the rolling inflaton, allowing vacuum bubbles to nucleate and grow during inflation. The vacuum ordering subsequently reverses near the end of inflation, leaving the bubble interiors in a metastable false vacuum state. We examine their stability against reverse vacuum tunneling and impose benchmark conditions to suppress this process before and after reheating. Sufficiently large bubbles can subsequently undergo supercritical evolution and form PBHs. In this framework, the PBH abundance is determined by the vacuum tunneling history, while the characteristic mass depends on the nucleation epoch and bubble growth during inflation. For sufficiently light PBHs that evaporate before big bang nucleosynthesis, Hawking radiation can create locally superheated regions in which electroweak symmetry is restored, potentially generating a baryon asymmetry in the presence of additional CP-violating interactions. We also investigate the direct gravitational wave (GW) signals from Hawking radiation and the induced GWs from PBH density fluctuations, the latter arising if an early PBH dominated epoch occurs. For representative benchmark parameters, the resulting baryon asymmetry can reach the ten-percent level of the observed value, while the induced GW signal can reach a peak amplitude of $Ω_{\rm GW,0}h^2\sim10^{-7}$ at frequencies of order $10$-$10^2\,{\rm Hz}$.