Reheating signature in the gravitational wave spectrum from self-ordering scalar fields

TL;DR

This paper studies how the reheating phase of the universe influences the gravitational wave spectrum generated by scalar fields after a phase transition, revealing potential signatures to distinguish different early universe scenarios.

Contribution

It provides the first detailed analysis of reheating effects on gravitational wave spectra from scalar field self-ordering using large-scale lattice simulations.

Findings

Reheating alters the spectral slope of gravitational waves.

The spectrum shape encodes information about the reheating history.

Potential to distinguish inflation from phase transitions via gravitational wave observations.

Abstract

We investigate the imprint of reheating on the gravitational wave spectrum produced by self-ordering of multi-component scalar fields after a global phase transition. The equation of state of the Universe during reheating, which usually has different behaviour from that of a radiation-dominated Universe, affects the evolution of gravitational waves through the Hubble expansion term in the equations of motion. This gives rise to a different power-law behavior of frequency in the gravitational wave spectrum. The reheating history is therefore imprinted in the shape of the spectrum. We perform $512^3$ lattice simulations to investigate how the ordering scalar field reacts to the change of the Hubble expansion and how the reheating effect arises in the spectrum. We also compare the result with inflation-produced gravitational waves, which has a similar spectral shape, and discuss whether it is possible to distinguish the origin between inflation and global phase transition by detecting the shape with future direct detection gravitational wave experiments such as DECIGO.