The Impact of Inhomogeneous Reionization on Cosmic Microwave Background Anisotropy

Hydrogen atoms in the Universe that initially formed at redshift z = 1100 were reionized at 5 < z < 60. It is likely that this transition proceeded through a mixed phase of partial ionization. We develop an analytic approach to calculating the effect on the cosmic microwave background (CMB) of Thomson scattering off this inhomogeneous distribution of free electrons. We study two models of the reionization process, one of which associates ionized patches with overdense regions of the smoothed density field. We find the generated anisotropy to have a power spectrum which peaks at angular scales corresponding to the extent of the ionized regions, and has a width that reflects the correlations between them. There is considerable uncertainty in the amplitude, which depends on how efficiently collapsed objects can reionize the surrounding medium. We calculate the effect of neglecting inhomogeneous reionization in the determination of cosmological parameters from high resolution CMB maps and find that it may be a significant source of systematic error.