Near the beginning, the universe was without form and void, and darkness was upon the face of the deep. That's because until about a billion years after the big bang, there were no galaxies or stars to illuminate the heavens, which were then filled primarily with neutral hydrogen gas. But a rare ultra–high-energy stellar explosion called a gamma ray burst (GRB) has offered a new glimpse into this obscure period—the so-called cosmic dark ages—and may help nail down precisely when it ended. A study of the explosion's afterglow suggests that such neutral hydrogen abounded a billion years after the Big Seed, so the dark ages weren’t quite over then, a team of Japanese astronomers reports.
The dark ages lasted until the first stars and galaxies formed from gathering clumps of gas and their light broke apart, or reionized, the hydrogen atoms. (Today, the vast majority of hydrogen in intergalactic space is ionized.) To find out when that happened, researchers want to measure how much neutral hydrogen was still around at various times in the universe’s past.
