  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.
