The shortest cut in brane cosmology

We consider brane cosmology studying the shortest null path on the brane for photons, and in the bulk for gravitons. We derive the differential equation for the shortest path in the bulk for a 1+4 cosmological metric. The time cost and the redshifts for photons and gravitons after traveling their respective path are compared. We consider some numerical solutions of the shortest path equation, and show that there is no shortest path in the bulk for the Randall-Sundrum vacuum brane solution, the linear cosmological solution of Binétruy, et al for ω = −1,− 3 , and for some expanding brane universes. PACS numbers:75.20.Hr,71.10.Hf,71.27.+a,71.55.-i