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Relic gravitational waves

The next generation of gravitational wave detectors holds out the prospect of detecting a stochastic gravitational background generated in the very early universe. In this article, 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, possibily at the end of inflation, and networks fo local cosmic strings are within the sensitivity of proposed detectors. However, backgrouds from slow-roll inflation and the electroweak phase transition are too weak.