News Feature: Reionizing the universe.

In the first fractions of a second after the Big Bang, the cosmos expanded exponentially, then became a blistering soup of fundamental particles and energy, and eventually cooled to a point where protons and electrons could combine to form neutral hydrogen. A few hundred million years later, the universe was filled with billowing clouds of hydrogen gas, entering what cosmologists call the cosmic dark ages. Then everything changed. About 500 million years after the Big Bang, intense UV radiation suddenly began to burn into the gas, creating massive hollow holes that grew and merged. The energy onslaught continued for nearly a half-a-billion more years, ripping the neutral hydrogen back into its constituent protons and electrons and transforming the entire universe. This was the start of the era of reionization, a strange and important epoch that nonetheless remains poorly understood. Cosmologists think that during reionization, the first stars and galaxies switched on and enormous black holes consumed anything within their reach. “The Big Bang was the beginning,” says Richard Ellis, an astronomer at the California Institute of Technology in Pasadena, California. “But the moment when the universe was bathed in starlight is the birth of us, because we’re made of star stuff.” Were these first stars and galaxies and supermassive black holes responsible for the UV light that ripped apart hydrogen? Our current best observatories offer only preliminary answers. But soon, powerful new spaceand ground-based telescopes, along with instruments that can probe the distribution of neutral hydrogen gas throughout the cosmos, will allow astronomers to examine the era of reionization in unprecedented detail and fill in the holes in the story of the universe’s evolution.