CMB anisotropies in the reionized universe

Abstract

Observations of Cosmic Microwave Background (CMB) anisotropies smaller than the 10 degree scale axe more and more important after the discovery of cosmic microwave background (CMB) anisotropies by COBE satellite (Smoot et al. 1992). If we assume the standard thermal history, one degree scale is just correspond to so-called doppler peak on CMB power spectrum because it is the horizon scale of the last scattering surface, i.e, the horizon scale at the standaxd recombination. Supposing possible reionization after the epoch of standard hydrogen recombination, however, we find the place of last scattering becomes much closer to present and the horizon scale increases. Hence, this peak is very sensitive to the thermal history of the universe. Readers will find the motivation, the physical mechanism and the constraint from compton y-parameter of reionization after the standard recombination in a paper by 3. Silk of this proceeding. Here, we would like to point out the fact that reionization reduces CMB anisotropies. Recently, several authors suggest cold dark matter dominated models (CDM models) are no more preferable (Gorski et al. 1993) or are given very severe constraints (Dodelson and Jubas 1993) from the resent observations by Gaier et al. (1992) at South Pole(hereafter as SP91). The beam of this experiment had a sinusoidal throw on the sky with amplitude 1.°5. Though the doppler peak of the CDM model without reionization is slightly smaller than this scale, this experiment is still sensitive to this peak. If reionization occurs, however, such a high peak is smeared out and CDM models may be saved. There is another model motivated by reionization. From the numerical simulations by several authors (Suginohaxa and Suto 1992 and Cen, Ostriker and Peebles 1993), the primeval isocurvature baxyonic models (hereafter as PIB models) are known as one of the most attractive candidate of the large scale structure formation. Because PIB model has very steep spectrum, first objects created in the very early stage and the universe kept ionizing till present. The most preferable spectrum index n from numerical simulations is about -0.5. Then when we