Relic Gravitational Waves and Cosmology

TL;DR

This paper reviews the theoretical foundations and current observational efforts to detect relic gravitational waves, discussing their expected amplitudes across different frequencies and clarifying misconceptions about inflationary gravitational waves.

Contribution

It provides a comprehensive overview of relic gravitational waves, including their quantum generation, detection prospects, and clarifies misconceptions about inflationary predictions.

Findings

Relic gravitational waves have different amplitudes across frequency ranges.

Current detection efforts include direct and indirect methods via CMB measurements.

Inflationary gravitational waves are predicted to be negligibly small, contrary to some claims.

Abstract

This is an expanded version of my talk given at the international conference "Zeldovich-90". I start with a brief recollection of interactions with Zeldovich in the context of the study of relic gravitational waves. I then summarise the principles and early results on the quantum-mechanical generation of cosmological perturbations. The expected amplitudes of relic gravitational waves are different in different frequency windows, and therefore the techniques and prospects of their detection are different. One section of the paper describes the present state of efforts in direct detection of relic gravitational waves. Another section is devoted to indirect detection via the anisotropy and polarisation measurements of the cosmic microwave background radiation (CMB). It is emphasized throughout the paper that the conclusions on the existence and expected amount of relic gravitational waves are based on a solid theoretical foundation and the best available cosmological observations. I also explain in great detail what went wrong with the so-called `inflationary gravitational waves', whose amount is predicted by inflationary theorists to be negligibly small, thus depriving them of any observational significance.