Relic gravitational waves

The next generation of gravitational-wave detectors holds out the prospect of detecting a stochastic gravitational-wave background generated in the very early Universe. In this paper, we review the various cosmological processes which can lead to such a background, including quantum fluctuations during inflation, bubble collisions in a first-order phase transition and the decay of a network of cosmic strings. We conclude that signals from strongly first-order phase transitions, possibly at the end of inflation, and networks of local cosmic strings are within the sensitivity of the proposed detectors. However, backgrounds from standard stow-roll inflation and the electroweak phase transition are too weak.