Detecting relics of a thermal gravitational wave background in the early Universe

TL;DR

This paper explores how a thermal gravitational wave background from the early Universe could leave detectable imprints on the CMB power spectra, offering insights into pre-inflationary conditions.

Contribution

It demonstrates the potential detectability of a thermal graviton background in CMB data for certain inflationary scenarios, linking early Universe physics to observable signals.

Findings

Thermal graviton background can be detected if inflation lasts fewer than 65 e-folds.

Detection prospects improve with sub-exponential inflation models.

Planck, PolarBear, and EPIC experiments could observe these signals.

Abstract

A thermal gravitational wave background can be produced in the early Universe if a radiation dominated epoch precedes the usual inflationary stage. This background provides a unique way to study the initial state of the Universe. We discuss the imprint of this thermal spectra of gravitons on the cosmic microwave background (CMB) power spectra, and its possible detection by CMB observations. Assuming the inflationary stage is a pure de Sitter expansion we find that, if the number of e-folds of inflation is smaller than 65, the signal of this thermal spectrum can be detected by the observations of Planck and PolarBear experiments, or the planned EPIC experiments. This bound can be even looser if inflation-like stage is the sub-exponential.