13.6 billion years later, astronomers have found clues to our earliest stars

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The first observation of the first stars in the Universe suggests that they were forming around 180 million years after the Big Bang. The radio signal used to make this observation, although indirect, supports some theoretical models about the evolution of the early Universe.
At first, the Universe was composed mainly of gas, mainly hydrogen, and a heavy and mysterious material known as dark matter. Over time, the bags of hydrogen gas collapsed to form the first stars, and there was light. But no one knew exactly when these cosmic lights came on, until a team of astronomers detected a weak radio signal that traveled 13.6 billion years to reach Earth.
The radio signal, described today in the journal Nature, tells us that the first stars were already forming 180 million years after the Big Bang. This is because the ultraviolet light of these stars radiated the hydrogen gas that surrounded them, causing a deep fall in the spectrum of radio waves detected here on Earth. The signal gives scientists an indirect look at the mysterious period of time when the Universe was still in its infancy.

A timeline of the universe, updated with the results of today's study. Credit: N.R.Fuller, National Science Foundation

The reason why scientists do not know for sure when the stars began to shine is that traditional telescopes can not see so far back in time. And although theorists predicted that hydrogen gas illuminated by ultraviolet light could produce a different radio signal, no one had been able to detect it.
That's what makes this new study "innovative," says Lincoln Greenhill, a radio astronomer at the Smithsonian Astrophysical Observatory who wrote an editorial on the study, but did not participate in the research. "It fills a void in what I would call the cosmological record." Still, he cautions that because this is such a potentially large finding, it will be even more important to replicate it using different equipment and analysis. "We really have to work harder to make sure he's okay," he says.
Since it is difficult to see in the past, a team of astronomers turned to radio waves to listen to the Universe early, using an antenna in the Australian desert. The idea was that hydrogen gas floating through the early Universe absorbed the ultraviolet light of the first generation of stars. That transformed the hydrogen gas, absorbing it by the background radiation that remained of the Big Bang, and the transformation caused a deep fall in the spectrum of radio waves that reached the Earth 13.6 billion years later.
The radio signal was small, however, and our planet is noisy, our entire galaxy is. Then, to separate the signal from all that background noise, a team of astronomers trained their antenna in the sky for hundreds of hours to learn what signals came from up close, and what signals came from far away.

EDGES terrestrial radio spectrometer, Murchison Radioastronomy Observatory of CSIRO in Western AustraliaCredit: CSIRO Australia

Two years ago, the team picked up the signal they expected to find. "Since then we have carried out all kinds of tests to convince ourselves," says Raul Monsalve, an experimental cosmologist at the University of Colorado at Boulder and author of the study. The timing of the radio signal makes sense according to the theoretical models. "They are the first stars that create the trigger that allow us to see this rare spectral signature that is reported," agrees Greenhill.
But there was something unexpected about the results: the size of the signal, although small, was more robust than expected. One possible explanation is that hydrogen gas may have been cooler than the predicted models. That finding produced a second article published today in Nature, in which Rennan Barkana, an astrophysicist at Tel Aviv University, proposes that hydrogen that interacts with dark matter at the beginning of the Universe could explain the unexpected temperature. That means that this new radio signal could help scientists test new properties of dark matter in the early Universe, and gives scientists a new clue as to where to look for it. "So this happens to be a really important finding, if it is verified," says Greenhill, "maybe even revolutionary."
But first, the measurement must be confirmed. "I hope I do not go down in history like the curmudgeon that rained on this," says Greenhill. But he would like to see another team of researchers use their own instruments to replicate the finding. "And if they both see the same thing, then, & # 39; Voila! & # 39;" he says. Monsalve agrees. "Now, it feels exciting, of course, but it feels like the beginning of a process," he says. "We are anxious to hear about other experiments."


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