Gravitational waves from inflation on the brane

We discuss the evolution of gravitational waves in a brane-world cosmology embedded in five-dimensional anti-de Sitter spacetime. We show that during slow-roll inflation, modelled as a period of quasi-de Sitter expansion on t he brane, there is a discrete normalizable massless graviton mode. There is a mass gap due to the expansion, above which there is a continuum of massi ve modes. Only the massless mode is 'light' compared with the Hubble scale during inflation, leading to the production of classical perturbations on l arge scales from vacuum fluctuations on small scales. We calculate the ampl itude of these fluctuations at horizon-crossing and show that the standard four-dimensional result is recovered at low energies, but the amplitude of the perturbations is enhanced at high energies. (C) 2000 Elsevier Science B .V. All rights reserved.