  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 solar masses. 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 non-standard CDM, incorporating mixed dark matter, vacuum density
or a tilted primordial power spectrum, early reionization plays no significant
role.
