Abstract
One still cannot conclusively assert that the universe underwent a neutral phase, despite the new COBE FIRAS limit y<0.000025 on Compton y-distortions of the cosmic microwave background. Although scenarios where the very early (z=1000) ionization is thermal, caused by IGM temperatures exceeding 10000K, are clearly ruled out, there is a significant loophole for cosmologies with typical CDM parameters if the dominant ionization mechanism is photoionization. If the ionizing radiation has a typical quasar spectrum, then the y-constraint implies roughly h^{3/2}\Omega_b Omega_0^{-1/4}<0.1 for fully ionized models. This means that BDM models with Omega_0 around 0.15 and reionization at about z=1000 are strongly constrained even in this very conservative case, and can survive the y test only if most of the baryons form BDM around the reionization epoch.
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