Rayleigh scattering and microwave background fluctuations

During the recombination epoch, cosmic background photons coupled not only to free electrons through Thompson scattering, but also to neutral hydrogen through Rayleigh scattering. This latter is a similar to2% effect for phot ons near the peak of the photon energy distribution at z = 800 and a simila r to0.2% effect at z = 1100. Including Rayleigh scattering in the calculati on reduces Silk damping at fixed redshift, alters the position of the surfa ce of last scattering, and alters the propagation of acoustic waves. We est imate the amplitude of these effects. For the Microwave Anisotropy Probe (M AP), Rayleigh scattering increases the anisotropy spectrum by 0.1% at the m ost. For the highest frequencies of the Planck Surveyor, the effects of Ray leigh scattering are much more dramatic (decreasing the anisotropy spectrum by 3% at v similar to 550 GHz and multipole number l similar to 1000). The relative difference between the spectra of low and high frequencies is imp osed by an oscillation with a function of multipole l, and the oscillation amplitude is up to 0.5% between 100 and 550 GHz. Rayleigh scattering also s lows the decoupling between radiation and matter, but the effect is undetec tably small.