On the inevitability of reionization: Implications for cosmic microwave background fluctuations

Early photoionization of the intergalactic medium is discussed in a nearly model-independent way, in order to investigate whether early structures corresponding to rare Gaussian peaks in a CDM model can photoionize the intergalactic medium sufficiently early to appreciably smooth out the microwave background fluctuations. We conclude that this is indeed possible for a broad range of CDM normalizations and is almost inevitable for unbiased CDM, provided that the bulk of these early structures are quite small, no more massive than about 10^8^ M_sun_. Typical parameter values predict that reionization occurs around z = 50, thereby suppressing fluctuations on degree scales while leaving the larger angular scales probed by COBE reasonably unaffected. However, for nonstandard CDM, incorporating mixed dark matter, vacuum density, or a tilted primordial power spectrum, early reionization plays no significant role.