Probing dark radiation with inflationary gravitational waves

TL;DR

This paper explores how dark radiation influences inflationary gravitational wave spectra, revealing potential signatures like spectrum suppression and dips that can inform us about dark radiation's origin and production epoch.

Contribution

It demonstrates that dark radiation leaves distinctive imprints on gravitational wave spectra, offering a new observational probe for dark radiation properties and production history.

Findings

High frequency gravitational wave modes are suppressed by dark radiation.

A dip in the spectrum indicates the horizon entry during dark radiation production.

Gravitational wave observations can reveal dark radiation's origin and production timing.

Abstract

Recent cosmological observations indicate the existence of extra light species, i.e., dark radiation. In this paper we show that signatures of the dark radiation are imprinted in the spectrum of inflationary gravitational waves. If the dark radiation is produced by the decay of a massive particle, high frequency mode of the gravitational waves are suppressed. In addition, due to the effect of the anisotropic stress caused by the dark radiation, a dip in the gravitational wave spectrum may show up at the frequency which enters the horizon at the time of the dark radiation production. Once the gravitational wave spectrum is experimentally studied in detail, we can infer the information on how and when the dark radiation was produced in the Universe.