How large are curvature perturbations from slow first-order phase transitions? A gauge-invariant analysis

TL;DR

This paper presents a gauge-invariant analysis of super-horizon curvature perturbations generated by slow first-order phase transitions in the early universe, showing they are unlikely to produce primordial black holes and discussing observational constraints.

Contribution

It introduces a gauge-invariant multi-fluid formalism to accurately compute super-horizon perturbations from slow FOPTs, resolving previous gauge ambiguities.

Findings

Super-horizon perturbations from slow FOPTs are unlikely to form primordial black holes.

Provides a fitting formula for curvature perturbations from FOPTs.

Discusses observational limits from primordial curvature perturbations and gravitational waves.

Abstract

When strongly supercooled cosmological first-order phase transitions (FOPTs) are sufficiently slow, super-horizon inhomogeneities can be generated. We compute these super-horizon curvature perturbations by employing a gauge-invariant, multi-fluid formalism. By resolving the gauge ambiguities inherent in conventional separate-universe simulations, we demonstrate that Primordial Black Holes are unlikely to be produced by these super-horizon inhomogeneities. We also derive a fitting formula for the resulting curvature perturbations and discuss potential observational constraints on FOPTs imposed by limits on primordial curvature perturbations and associated scalar-induced gravitational waves.