Primordial Gravitational Waves: a Probe of the Very Early Universe

We discuss the potential cosmological role of gravitational wave astronomy as a probe of the very early universe. The next generation of detectors—now in production—may be able to observe a stochastic background of gravitational waves produced by violent processes during the earliest moments after the creation of the universe. Viable theoretical scenarios within detector sensititivity include strongly first-order phase transitions, possibly at the end of inflation, and networks of cosmic strings. At this stage, other primordial backgrounds from slow-roll inflation, global topological defects and the standard electroweak phase transition appear to be out of range. The discovery of any of these possible cosmological sources will have enormous implications for our understanding of the very early universe and for fundamental physics at the highest energies.