June 1993 On the Inevitability of Reionization :

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 suuciently early to appreciably smooth out the microwave background uctuations. 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. Typical parameter values predict that reionization occurs around z = 50, thereby suppressing uctuations on degree scales while leaving the larger angular scales probed by COBE reasonably unaaected. However, for non-standard CDM, incorporating mixed dark matter, vacuum density or a tilted primordial power spectrum, early reionization plays no signiicant role.