Primordial gravitational waves in an inflationary braneworld

We study primordial gravitational waves from inflation in the Randall-Sundrum braneworld model. The effect of a small change of the Hubble parameter during inflation is investigated using a toy model given by connecting two de Sitter branes. We analyze the power spectrum of the final zero-mode gravitons, which is generated from the vacuum fluctuations of both the initial Kaluza-Klein modes and the zero mode. The amplitude of fluctuations is confirmed to agree with the four-dimensional one at low energies, whereas it is enhanced due to the normalization factor of the zero mode at high energies. We show that the five-dimensional spectrum can be well approximated by applying a simple mapping to the four-dimensional fluctuation amplitude.