Page 13 - Science

In the Australian outback,
small radio antennas
were used to detect a
13.6-billion-year-old signal.

COSMOLOGY

Cosmic dawn signal holds clue to dark matter

Microwaves from the big bang probe primordial gas clouds as the first stars turn on

By Adrian Cho librium and enabling the atoms to absorb in Nature, Barkana argues that to cool the Downloaded from
more of the microwaves than they emit. hydrogen, the dark matter particles must
U tables, a small team stars began to the absorption signal from its original as a hydrogen atom. Otherwise the atoms
expansion
The
less than
the universe stretches
have been
of
five times
sing radio antennas the size of coffee
as
massive
of astronomers
the
to longer radio
21-centimeter wavelength
has glimpsed the cosmic dawn,
would have bounced off them without los-
moment billions of years ago
wavelengths. However, radio noise
when
from
ing energy and getting colder, just as a Ping-
the universe’s first
bowling ball
Pong ball will bounce off a
our galaxy is 30,000 times more
intense.
observation also serves
shine. The
up surprising evidence that particles of To subtract it, EDGES researchers relied without slowing down. http://science.sciencemag.org/
on the noise’s smooth, precisely predictable
Many dark matter searches have targeted
dark matter—the unseen stuff that makes spectrum. This week in Nature, they report hypothetical weakly interacting massive
up most of the universe’s matter—may be detecting the tiny absorption signal—the cu- particles, which are generally expected to
much lighter than physicists thought. mulative shadows, they conclude, of hydro- weigh hundreds of times as much as a hy-
If it holds up, the result could sharpen gen clouds that existed between 180 million drogen atom. As those searches have come
cosmologists’ picture of the early universe and 250 million years after the big bang. up empty, some physicists have begun
and shake up the search for dark matter. It’s the first thing scientists have seen in searching for lighter dark matter particles
“It’s going to generate a huge amount of the time between the cosmic microwave (Science, 24 March 2017, p. 1251). The new on March 1, 2018
interest,” says Kevork Abazajian, a theoreti- background, 380,000 years after the big result may encourage them, Abazajian says.
cal cosmologist at the University of Califor- bang, and the oldest known galaxy, which However, it’s too early to rule out a more
nia (UC), Irvine. But others worry that the shone 400 million years later, says EDGES mundane explanation for the unexpect-
subtle radio signal reported by the team leader Judd Bowman. “This is really the edly strong absorption, cautions Katherine
could be an artifact. “I don’t think that right only possible probe that we have of the Freese, an astrophysicist at the University of
now, at least in my mind, it’s a clear discov- time before the stars,” says Bowman, who Michigan in Ann Arbor. “Is [this scenario]
PHOTO: COMMONWEALTH SCIENTIFIC AND INDUSTRIAL RESEARCH ORGANISATION
ery,” says Aaron Parsons, an experimental is an experimental astrophysicist at Arizona the only way to explain this? Of course not.”
cosmologist at UC Berkeley. State University in Tempe. Ultimately, sci- A more pressing question is whether the
The data come from the Experiment to entists hope to use the absorption signal or signal is an experimental artifact, Parsons
Detect the Global Epoch of Reionization the fainter emission of 21-centimeter radia- says. The measurements rely on calibrations
Signature (EDGES), a $2 million array of tion from gas clouds at slightly later times that could produce false signals if they are
three radio antennas in the outback of West- to map the 3D distribution of hydrogen dur- off by just a few hundredths of a percent,
ern Australia. The five EDGES researchers ing these so-called cosmic dark ages, tracing he says. Bowman says he and his colleagues
searched for signs that the hydrogen atoms its evolution into embryonic galaxies. “have gone as far as we can go to ensure
that pervaded the newborn universe had The absorption is more than twice as that there isn’t an error, but, of course, we’re
absorbed microwaves lingering from the strong as predicted, which suggests that eager for others to confirm the result.”
big bang. the hydrogen was significantly colder than Confirmation could come from other ex-
The absorption marks the moment just previously thought. The gas must have lost periments that are probing the dark ages.
after the first stars began to shine. Before heat to something even colder, and the only Parsons leads one, called the Hydrogen Ep-
that moment, the atoms’ internal states colder thing around was dark matter, which och of Reionization Array in South Africa,
were in equilibrium with the microwaves, was coalescing into the clumps that would which is trying not just to detect the faint
emitting as much radiation as they ab- seed the formation of galaxies, reasons signals, but to map them across the sky.
sorbed. But light from the first stars jostled Rennan Barkana, an astrophysicist at Tel They may soon show whether cosmic dawn
the atoms’ innards, disrupting the equi- Aviv University in Israel. In a second paper has really broken. j
SCIENCE sciencemag.org 2 MARCH 2018 • VOL 359 ISSUE 6379 969
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