  We propose a new formulation to calculate the Cosmic Microwave Background
(CMB) spectrum in the Randall Sundrum two-branes model based on recent
progresses in solving the bulk geometry using a low energy approximation. The
evolution of the anisotropic stress imprinted on the brane by the 5D Weyl
tensor is calculated. An impact of the dark radiation perturbation on CMB
spectrum is investigated in a simple model assuming an initially
scale-invariant adiabatic perturbations. The dark radiation perturbation
induces isocurvature perturbations, but the resultant spectrum can be quite
different from the prediction of simple mixtures of adiabatic and isocurvature
perturbations due to Weyl anisotropic stress.
