Post-inflationary thermal histories and the refractive index of relic gravitons

TL;DR

This paper explores how different post-inflationary thermal histories influence the spectrum of relic gravitons, revealing potential detectable signals across various frequency bands and clarifying the refractive index's frame dependence.

Contribution

It introduces a detailed analysis of the impact of thermal histories on graviton spectra and clarifies the refractive index's behavior in different conformal frames.

Findings

Spectral energy distribution can be red, blue, or violet depending on parameters.

Potential signals detectable by terrestrial and space-borne interferometers.

Refractive index description is clarified in conformally related frames.

Abstract

We investigate the impact of the post-inflationary thermal histories on the cosmic graviton spectrum caused by the inflationary variation of their refractive index. Depending on the frequency band, the spectral energy distribution can be mildly red, blue or even violet. Wide portions of the parameter space lead to potentially relevant signals both in the audio range (probed by the advanced generation of terrestrial interferometers) and in the mHz band (where space-borne detectors could be operational within the incoming score year). The description of the refractive index in conformally related frames is clarified.