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 108M⊙. 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. 1 Published in ApJ, 420, 486, January 10, 1994. Submitted March 18 1993, accepted July 2. Available from h t t p://www.sns.ias.edu/ ̃max/reion.html (faster from the US) and from h t t p://www.mpa-garching.mpg.de/ ̃max/reion.html (faster from Europe).