Gravitational waves from inflation on the brane

TL;DR

This paper studies how gravitational waves evolve during inflation in a brane-world model, finding a massless mode responsible for classical perturbations and showing amplitude enhancement at high energies compared to standard cosmology.

Contribution

It demonstrates the existence of a massless graviton mode during brane inflation and quantifies the amplitude enhancement of gravitational waves at high energies.

Findings

Existence of a discrete massless graviton mode during inflation.

Amplitude of gravitational wave fluctuations is enhanced at high energies.

Standard four-dimensional results are recovered at low energies.

Abstract

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 the 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 massive modes. Only the massless mode is `light' compared with the Hubble scale during inflation, leading to the production of classical perturbations on large scales from vacuum fluctuations on small scales. We calculate the amplitude 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.