Cosmic 'enlightenment' dawned slowly

The end of the universe's "dark age" was long and drawn out, according to the first direct measurement of the period when the first stars and galaxies heated up intergalactic gas.

Right after the big bang, the universe was a roiling soup of subatomic particles. These cooled and coalesced into neutral atoms within 400,000 years, beginning the cosmic dark age. This only ended when ultraviolet light from the first stars and giant black holes had once again ionised the fog of neutral atoms filling the universe. How long this process of "re-ionisation" took isn't clear.

To find out, Judd Bowman of Arizona State University in Tempe and Alan Rogers of the Massachusetts Institute of Technology deployed a small radio antenna called EDGES in Western Australia.

The telescope detects radio waves that have been emitted by neutral hydrogen atoms. These have a wavelength of 21 centimetres when they are emitted, but this gets stretched as they travel across space due to the universe's expansion.
Slow and steady

Based on the amount of stretch, the team knew that EDGES measured light released when the universe was a few hundred million to a billion years old. It did not find a sudden decrease in the brightness of the light emitted by neutral hydrogen atoms at any point in that period, suggesting that re-ionisation did not occur suddenly.

"I'm excited," says Avi Loeb of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "It's the first time we have a constraint on the duration of re-ionisation."

"It's also nice to see two people getting a result before the big teams that have much more money," Loeb adds. A range of large radio telescope arrays are under construction, such as the LOFAR telescope in the Netherlands and Germany. These projects, which consist of hundreds or thousands of antennas, will attempt to take high-resolution maps of the hydrogen.

EDGES, which measures the total amount of radiation from hydrogen atoms over a large swathe of the sky, cannot image the gas in detail. But the small telescope may be better at looking even farther into the past than the larger arrays, allowing it to look at hydrogen atoms heated by the very first stars, Bowman says.