One still can not conclusively assert that the universe underwent a neutral phase , despite the new COBE FIRAS limit y < 2.5 \times 10 ^ { -5 } on Compton y -distortions of the cosmic microwave background .
Although scenarios where the very early ( z \sim 1000 ) ionization is thermal ( caused by IGM temperatures exceeding 10 ^ { 4 } K ) 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 ^ { 4 / 3 } \Omega _ { igm } \Omega _ { 0 } ^ { -0.28 } < 0.06 for fully ionized models .
This means that BDM models with \Omega _ { 0 } \approx 0.15 and reionization at z \approx 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 .