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

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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 apprecia- bly 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 unaf- fected. However, for non-standard CDM, incorporating mixed dark matter, vacuum density or a tilted primordial power spectrum, early reionization plays no significant role.

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