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(.). Typical parameter values predict that reionization occurs around z=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.