Stochastic backgrounds of relic gravitons: a theoretical appraisal

TL;DR

This paper reviews the theoretical foundations and observational prospects for detecting stochastic backgrounds of relic gravitons, highlighting the complementarity of CMB polarization measurements and interferometers across different frequency ranges.

Contribution

It provides a comprehensive theoretical overview of relic graviton backgrounds and discusses how current and future experiments can constrain their properties.

Findings

CMB experiments constrain low-frequency relic graviton spectrum

Interferometers are sensitive to high-frequency spectrum

Spectral energy density depends on post-inflationary expansion rate

Abstract

Stochastic backgrounds or relic gravitons, if ever detected, will constitute a prima facie evidence of physical processes taking place during the earliest stages of the evolution of the plasma. The essentials of the stochastic backgrounds of relic gravitons are hereby introduced and reviewed. The pivotal observables customarily employed to infer the properties of the relic gravitons are discussed both in the framework of the $\Lambda$CDM paradigm as well as in neighboring contexts. The complementarity between experiments measuring the polarization of the Cosmic Microwave Background (such as, for instance, WMAP, Capmap, Quad, Cbi, just to mention a few) and wide band interferometers (e.g. Virgo, Ligo, Geo, Tama) is emphasized. While the analysis of the microwave sky strongly constrains the low-frequency tail of the relic graviton spectrum, wide-band detectors are sensitive to much higher frequencies where the spectral energy density depends chiefly upon the (poorly known) rate of post-inflationary expansion.