Evidence could someday be detected by radio telescopes, study suggests

By Nadia Drake

Web edition: June 20, 2012
Print edition: August 11, 2012; Vol.182 #3 (p. 9)

A 3-D simulation of the early universe suggests that the first stars left a cosmic signature large enough to be read by radio telescopes.

“It’s a new way to probe the universe when it was very young,” says Zoltan Haiman, a cosmologist at Columbia University, who was not involved in the new work. “We have very few ways to do that.”

Studying early star formation is challenging because the first galaxies were so small — and, because of the universe’s expansion, are now so distant — that even the most sensitive eyes in the sky can’t see them.

But the new simulation, described online June 20 in Nature, suggests that a stellar signature exists in the form of fluctuating radio waves, oscillations produced when young stars and nascent galaxies warm and excite surrounding hydrogen gas. The stars and galaxies in the period simulated, when the universe was 180 million years old, are distributed in a distinct, detectable pattern.

“There’s a prominent weblike structure, meaning that you have clumps of galaxies and nearly empty voids,” says study coauthor Eli Visbal, a graduate student at Harvard University. The clumps and voids should be readily discernible, he notes. “Turns out, it’s much easier than was previously thought to observe these first stars using radio.”

Visbal and his colleagues simulated a cube of space measuring 1.3 billion light-years across. They filled it with hydrogen gas and dark matter, the invisible counterpart to normal matter, and accounted for the recent observation that the two kinds of matter travel at different speeds. These different rates, when combined with varying densities of each substance, affect star formation by stunting growth in some places and promoting it in others. “The dark matter collapses into clumps,” Visbal says. “And the gas, due to the force of gravity, falls into these clumps and forms stars and galaxies.”

But not where the gas is moving too quickly relative to the dark matter clumps, which then have to tug harder to get the gas to come inside. A paucity of gas produces a star-forming void, while dense gas congeals to form clusters of stars and galaxies. Those clusters then heat up and excite the surrounding sea of neutral hydrogen atoms, which emit radiation detectable by radio telescopes.

The telescopes, though, have to be scanning the sky at the right frequency — lower than the band that’s typically used by the most powerful radio detectors.

But future telescopes — maybe even the enormous Square Kilometre Array, under development on two continents  — could do the job. Another option, says UCLA astrophysicist Steven Furlanetto, would be a proposed project called the Dark Ages Radio Explorer, a lunar satellite that could use the moon as a shield against interference by technologies like television and radio.

In the simulation, the researchers focused on a critical epoch in the early universe, Furlanetto notes. “That’s the first moment in which complexity appears,” he says. “Once those first stars form and you get radiation and nuclear fusion and explosions, it gets very, very complex almost instantaneously.”