Finding this fingerprint in the universe's afterglow was easier said than done. The local universe hurls an overwhelming amount of radio waves at Earth, drowning out this muted signal. On top of that, the scientists were using a fairly simple instrument -- a single radio detector roughly 6.4 feet long, about the size and shape of a dining table. With this single antenna, looking for a signal in one particular part of the sky would have been impossible.
Instead, they looked at the average radio spectrum across the entire sky and searched for discrepancies. They also placed their detector in a remote region of Australia, in the hopes of being as far away from human-generated radio waves as possible.
Sure enough, the scientists discovered a drop in the radio waves at 78 megahertz -- a wavelength of light that had been dramatically stretched, thanks to the universe's expansion, from the original frequency of 1,420 megahertz. (The higher a wave's frequency, the shorter its wavelength.) This wavelength must be missing, scientists say, because it was absorbed by the hydrogen gas that was primed by the light from those early stars.
"I think it's a little bit like winning the lottery, in a sense," said study co-author Alan Rogers, a radio astronomer at the Massachusetts Institute of Technology. At the same time, he admitted, luck often favors the prepared. "We spent a lot of time improving the calibration of the instrument."
The results show that these first stars were already shining just 180 million years after the Big Bang. As those early stars died, they likely left behind black holes, neutron stars and supernovas, producing X-rays that further heated the hydrogen gas. Thanks to all this heating, the telltale absorption signal disappears around 90 million years later.
"This is a huge potential result that's really a breakthrough in the more-than-a-decade-long effort to detect signals from the very early universe," said Gregg Hallinan, a California Institute of Technology radio astronomer who was not involved in the work. "This measurement is our first step to begin to understand that era where the first stars and galaxies actually formed."
While the signal's location matched theoretical predictions, its shape did not. The dip in the light curve was flat-bottomed, like a U, and also twice as deep as scientists had predicted. That depth appears to imply that the hydrogen was much cooler than it should have been, at that point in time.
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In a separate paper, theorist Rennan Barkana of Tel Aviv University presents a possible explanation: The hydrogen may have interacted with dark matter. If it turns out to be true, this would be groundbreaking, because until now dark matter has only been known to interact with normal matter through its gravitational influence. (That gravitational influence is pretty clear at large scales because there is more than five times as much dark matter as normal matter in the universe, even though it can't be seen or touched.)
But the first step, scientists said, would be for independent experiments to confirm that this signal really is out there.
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