Using radio antennas the size of coffee tables, a small team of astronomers has glimpsed the cosmic dawn, the moment billions of years ago when the universe's first stars began to shine. The observation also serves up surprising evidence that particles of dark matter—the unseen stuff that makes up most of the universe's matter—may be much lighter than physicists thought.
If it holds up, the result could sharpen cosmologists' picture of the early universe and shake up the search for dark matter. "It's going to generate a huge amount of interest," says Kevork Abazajian, a theoretical cosmologist at the University of California (UC), Irvine. But others worry that the subtle radio signal reported by the team could be an artifact. "I don't think that right now, at least in my mind, it's a clear discovery," says Aaron Parsons, an experimental cosmologist at UC Berkeley.
The data come from the Experiment to Detect the Global Epoch of Reionization Signature (EDGES), a $2 million array of three radio antennas in the outback of Western Australia. The five EDGES researchers searched for signs that the hydrogen atoms that pervaded the newborn universe had absorbed microwaves lingering from the big bang.
The absorption marks the moment just after the first stars began to shine. Before that moment, the atoms' internal states were in equilibrium with the microwaves, emitting as much radiation as they absorbed. But light from the first stars jostled the atoms' innards, disrupting the equilibrium and enabling the atoms to absorb more of the microwaves than they emit.
The expansion of the universe stretches the absorption signal from its original 21-centimeter wavelength to longer radio wavelengths. However, radio noise from our galaxy is 30,000 times more intense. To subtract it, EDGES researchers relied on the noise's smooth, precisely predictable spectrum. This week in Nature, they report detecting the tiny absorption signal—the cumulative shadows, they conclude, of hydrogen clouds that existed between 180 million and 250 million years after the big bang.
It's the first thing scientists have seen in the time between the cosmic microwave background, 380,000 years after the big bang, and the oldest known galaxy, which shone 400 million years later, says EDGES leader Judd Bowman. "This is really the only possible probe that we have of the time before the stars," says Bowman, who is an experimental astrophysicist at Arizona State University in Tempe. Ultimately, scientists hope to use the absorption signal or the fainter emission of 21-centimeter radiation from gas clouds at slightly later times to map the 3D distribution of hydrogen during these so-called cosmic dark ages, tracing its evolution into embryonic galaxies.
The absorption is more than twice as strong as predicted, which suggests that the hydrogen was significantly colder than previously thought. The gas must have lost heat to something even colder, and the only colder thing around was dark matter, which was coalescing into the clumps that would seed the formation of galaxies, reasons Rennan Barkana, an astrophysicist at Tel Aviv University in Israel. In a second paper in Nature, Barkana argues that to cool the hydrogen, the dark matter particles must have been less than five times as massive as a hydrogen atom. Otherwise the atoms would have bounced off them without losing energy and getting colder, just as a Ping-Pong ball will bounce off a bowling ball without slowing down.
Many dark matter searches have targeted hypothetical weakly interacting massive particles, which are generally expected to weigh hundreds of times as much as a hydrogen atom. As those searches have come up empty, some physicists have begun searching for lighter dark matter particles. The new result may encourage them, Abazajian says.
However, it's too early to rule out a more mundane explanation for the unexpectedly strong absorption, cautions Katherine Freese, an astrophysicist at the University of Michigan in Ann Arbor. "Is [this scenario] the only way to explain this? Of course not."
A more pressing question is whether the signal is an experimental artifact, Parsons says. The measurements rely on calibrations that could produce false signals if they are off by just a few hundredths of a percent, he says. Bowman says he and his colleagues "have gone as far as we can go to ensure that there isn't an error, but, of course, we're eager for others to confirm the result."
Confirmation could come from other experiments that are probing the dark ages. Parsons leads one, called the Hydrogen Epoch of Reionization Array in South Africa, which is trying not just to detect the faint signals, but to map them across the sky. They may soon show whether cosmic dawn has really broken.