Reionization and Cosmic Microwave Anisotropies

The effects of reionization, occurring after standard recombination in cold dark matter-dominated models, on CMB anisotropies are investigated. Late-time reionization reduces the CMB anisotropies, in particular, on degree scales. It is found that constraints on cold dark matter-dominated models from the highest frequency channel of the 9-point South Pole data are significantly relaxed for models which are consistent with Big Bang nucleosynthesis if reionization is assumed to have occurred by redshift 20.