Posted: Fri, Oct 14, 2011, 6:16 AM ET (1016 GMT)
Astronomers looking at the oldest galaxies in the universe have been able to establish a date for when a "cosmic fog" of hydrogen dissipated. Astronomers used the Very Large Telescope at the European Southern Observatory to determine the concentrations of neutral hydrogen in the vicinity of galaxies less than a billion years after the Big Bang. They found the amount of neutral hydrogen dropped significantly between 780 and 980 million years after the Big Bang, and conclude that the "reionization" of the universe, when ultraviolet light broke down clouds of neutral hydrogen in the universe, took place during that period. That ultraviolet light most likely came the first generation of very large stars in those early galaxies. The observations mark the first time astronomers have been able to determine the timing for reionization, a key event in the early history of the universe.
