Smoothing of the cosmic background radiation by multiple gravitational scattering

Abstract

We investigated the smoothing of the cosmic background radiation (CBR) through gravitational scatterings. The CBR is gravitationally scattered by galaxies, clusters of galaxies, and superclusters during the travel from the last scattering surface. Although the effect of the gravitational scatterings was thought to be unimportant, we found that the scatterings by superclusters reduce the anisotropy by a large factor. This result is explained by the fact that the distance between two light rays increases exponentially through multiple scatterings. This exponential growth occurs if the distance is smaller than the critical distance determined by the number density of the scattering objects. We found that the gravitational scatterings by superclusters can reduce the temperature anisotropy of the CBR at present time by a large factor, in angular scales up to a few degrees.