Generation of curvature perturbations with extra anisotropic stress

Abstract

We study the evolution of curvature perturbations and the cosmic microwave background (CMB) power spectrum in the presence ofan hypothesized extra anisotropic stress whichmight arise, for example, from the dark radiation term in brane-world cosmology.We evolve the scalar modes of such perturbations before and after neutrino decoupling and analyze their effects on the CMB spectrum. A novel result of this work is that the cancellation of the neutrino and extra anisotropic stress could lead to a spectrum of residual curvature perturbations which is similar to the observed CMB power spectrum.This implies a possible additional consideration in the determination of cosmological parameters from the CMB analysis.