New data from the South Pole Telescope indicates that the birth of the first massive galaxies that lit up the early universe was an explosive event, happening faster and ending sooner than suspected. Extremely bright, active galaxies formed and fully illuminated the universe by the time it was 750 million years old, or about 13 billion years ago, according to Oliver Zahn, a postdoctoral fellow at the Berkeley Center for Cosmological Physics (BCCP) at the University of California, Berkeley, who led the data analysis.

The data provide new constraints（约束，限制） on the universe's first era of galaxy formation, called the Epoch of Reionization. Most astronomers think that early stars came to life in massive gas clouds, generating the first galaxies. The energetic light pumped out by these stars is thought to have ionized the hydrogen gas in and around the galaxies, creating "ionization bubbles" millions of light years across that left a lasting, telltale signature in the cosmic background radiation (CMB). This relic light from the early universe is visible today everywhere in the sky and was first mapped by UC Berkeley physicist and Nobel laureate George Smoot, founder of the BCCP.

"We find that the Epoch of Reionization lasted less than 500 million years and began when the universe was at least 250 million years old," Zahn said. "Before this measurement, scientists believed that reionization lasted 750 million years or longer, and had no evidence as to when reionization began."

The first epoch of ionization occurred after the universe was born in the Big Bang. Everything was so hot that all the gas, mostly hydrogen, was ionized. The universe only cooled enough for electrons to latch onto protons to form neutral hydrogen atoms when the universe was about 400,000 years old.

"Studying the Epoch of Reionization is important because it represents one of the few ways by which we can study the first stars and galaxies," said study co-author John Carlstrom of the University of Chicago.

The epoch's short duration also suggests that monster galaxies with more than a billion stars played a key role in the reionization（再电离）, since smaller galaxies would have formed much earlier.

Zahn and UC Berkeley post-doctoral fellow Christian Reichardt, along with colleagues at the University of Chicago, which operates the telescope, will report their findings in the Sept. 1 print edition of The Astrophysical Journal.