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

Abstract

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.

32 pages, incl 10 figures. Stone-age postscript replaced by LATeX. Latest version at http://www.sns.ias.edu/~max/reion.html (faster from the US), from http://www.mpa-garching.mpg.de/~max/reion.html (faster from Europe) or from max@ias.edu