Decaying Particles and the Reionization History of the Universe

We investigate the possibility that the Universe is significantly reionized by the decay products of heavy particles. The ionization produced by decaying particles implies a high optical depth even if the maximum level of ionization ever produced is low (10(-2)). As a consequence, a high ionization fraction (x approximately equal to 0.5) at high redshifts (z approximately equal to 20) fails to fit the cosmic microwave background (CMB) spectra at l> or =30. Recent CMB data limit the primordial abundance of the decaying particles, favoring long decay times. Other significant sources of reionization are still needed at z approximately equal to 13. The decay process heats up the medium, bringing the expected y distortion to unobservable levels.