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

TL;DR

This paper explores how non-monochromatic bubble size distributions in cosmological first-order phase transitions affect gravitational wave signals, primordial black hole formation, and observational signatures like microlensing and gamma-ray emissions.

Contribution

It introduces the impact of extended bubble radius distributions on gravitational wave spectra and observational signals, expanding the understanding of FOPT phenomenology.

Findings

GW spectrum peak shifts to lower frequencies with extended distributions

Extended PBH mass distributions produce detectable signals below $10^{-10} M_\\odot$

Gamma-ray signals may show a spectral break between 5-10 MeV

Abstract

In a cosmological first-order phase transition (FOPT), the true and false vacuum bubble radius distributions are not expected to be monochromatic, as is usually assumed. Consequently, Fermi balls (FBs) and primordial black holes (PBHs) produced in a dark FOPT will have extended mass distributions. We show how gravitational wave (GW), microlensing and Hawking evaporation signals for extended bubble radius/mass distributions 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 PBH mass distributions with average masses below $10^{-10} M_\odot$, which is not possible for monochromatic mass distributions. This expands the FOPT parameter space that can be explored with microlensing.