DID THE UNIVERSE RECOMBINE ? NEW SPECTRAL CONSTRAINTS ON REHEATING yMax

One still cannot conclusively assert that the universe underwent a neutral phase, despite the new COBE FIRAS limit y < 2:5 10 ?5 on Compton y-distortions of the cosmic microwave background. Although scenarios where the very early (z 1; 000) ionization is thermal, caused by IGM temperatures exceeding 10 4 K, are clearly ruled out, there is a signiicant 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 b ?1=4 0 < 0:1 for fully ionized models. This means that BDM models with 0 0:15 and reionization at z 1; 000 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.