Primordial Gravitational Waves from Phase Transitions during Reheating

TL;DR

This paper investigates how first-order phase transitions during cosmic reheating can generate primordial gravitational waves with spectra that differ significantly from standard cosmology, potentially detectable by future observatories.

Contribution

It introduces a minimal particle physics model to analyze phase transition parameters during reheating and computes the resulting gravitational wave spectra, highlighting their dependence on cosmological evolution.

Findings

GW amplitude and peak frequency can vary by orders of magnitude due to reheating effects

Certain cosmological scenarios allow delayed and prolonged phase transitions

Predicted GW signals may be detectable by next-generation observatories

Abstract

We study primordial gravitational waves (GWs) generated from first-order phase transitions (PTs) during cosmic reheating. Using a minimal particle physics model, and a general parametrization of the inflaton energy density and the evolution of the Standard Model temperature, we explore the conditions under which PTs occur and determine the corresponding PT parameters (the PT temperature, duration and strength), which depend on the evolution of the background during reheating. We find that, in certain cosmological scenarios, PTs can be delayed and prolonged compared to the standard post-inflationary evolution. Incorporating these PT parameters, we compute the resulting GW spectrum generated from the various processes occurring during a first-order PT: bubble collisions, sound waves, and magneto-hydrodynamic turbulence. We find that, in comparison to the standard cosmological history, the GW amplitude and peak frequency can be modified by several orders of magnitude due to the additional enhancement or suppression arising from the cosmological evolution during reheating. In particular, the GW spectra could be within the reach of next-generation GW and CMB observatories.