The first observation of the earliest stars in the Universe suggests they were forming about 180 million years after the Big Bang. The radio signal used to make this observation, though indirect, backs up some theoretical models about the evolution of the early Universe.
In the beginning, the Universe was made mainly of gas — mostly hydrogen — and a heavy, mysterious material known as dark matter. Over time, pockets of hydrogen gas collapsed to form the first stars, and there was light. But no one knew when exactly these cosmic lights first turned on, until a team of astronomers picked up a faint radio signal that traveled 13.6 billion years to reach Earth.
The radio signal, described today in the journal Nature, tells us that early stars were already forming 180 million years after the Big Bang. That’s because ultraviolet light from these stars irradiated the hydrogen gas surrounding them, causing a telltale dip in the spectrum of radiowaves detected here on Earth. The signal gives scientists an indirect look into the mysterious period of time when the Universe was still in its infancy.
The reason scientists don’t know for certain when the stars first started shining is because traditional telescopes can’t see that far back in time. And while theoreticians predicted that hydrogen gas illuminated by UV light might produce a distinct radio signal, no one had been able to detect it.
That’s what makes this new study “groundbreaking,” says Lincoln Greenhill, a radio astronomer at the Smithsonian Astrophysical Observatory who wrote an editorial about the study, but was not involved in the research. “It fills in a gap in what I’d call the cosmological record.” Still, he cautions that because this is such a potentially huge find, it will be even more important to replicate it using different equipment and analyses. “We really have to work extra hard to make sure it’s right,” he says.
Since it’s difficult to see so far back in time, a team of astronomers turned to radio waves to listen in on the early Universe, using an antenna deep in the Australian desert. The idea was hydrogen gas floating through the early Universe absorbed ultraviolet light from the first generation of stars. That transformed the hydrogen gas, making it soak up background radiation left over from the Big Bang — and the transformation caused a telltale dip in the spectrum of radio waves that reached Earth 13.6 billion years later.
The radio signal was tiny, though, and our planet is noisy — our whole galaxy is. So to separate the signal from all that background noise, a team of astronomers trained their antenna on the sky for hundreds of hours to learn what signals came from nearby, and which signals came from far away.
Two years ago, the team picked up the signal they were expecting to find. “Since then we have conducted all kinds of tests to convince ourselves,” says Raul Monsalve, an experimental cosmologist at the University of Colorado Boulder and an author of the study. The timing of the radio signal makes sense based on theoretical models. “It’s the first stars that create the trigger that enable us to see this weird spectral signature that’s reported,” Greenhill agrees.
But there was something unexpected about the results: The size of the signal, while tiny, was beefier than expected. One possible explanation is that the hydrogen gas may have been colder than models predicted. That finding produced a second paper published today in Nature, in which Rennan Barkana, an astrophysicist at Tel Aviv University, proposes that hydrogen gas interacting with dark matter at the beginning of the Universe could explain the unexpected temperature. That means that this new radio signal could help scientists probe new properties of dark matter in the early Universe, and gives scientists a new clue where to search for it . “So this goes from being a really important finding — if verified,” Greenhill says, “to perhaps revolutionary.”
But first, the measurement needs to be confirmed. “I hope I don’t go down in history as the curmudgeon that rained all over this,” Greenhill says. But he’d like to see another team of investigators use their own instruments to replicate the finding. “And if they both see the same thing, then, ‘Voila!’” he says. Monsalve agrees. “Now, it feels exciting of course — but it feels like the beginning of a process,” he says. “We are eager to hear from other experiments.”