Accordingly, they calculated that the rate of ionization would be very
slow during this early time, because of this low star-formation rate.
"We were really surprised that the rate of ionization seems so low,
which would constitute a contradiction with the claim of NASA's WMAP
satellite. It concluded that reionization started no later than 600
million years after the Big Bang," Ouchi said. "
http://www.space.com/scienceastronomy/091106-earliest-galaxies-universe.html