stronomers can’t always just
examine a simple image when
they want to solve a mystery.
Most of the time, they have to
meticulously piece together
tiny bits of evidence like
detectives, often by scouring the heav-
ens for clues. One of the biggest cosmic
cold cases that astronomers have been
attempting to solve for years: When
exactly did the first stars form?
On February 28, a team of astrono-
mers announced in the journal Nature
that, after more than a decade of intense
investigation, they had finally cracked
the case of the first stars. Using a simple
radio antenna (the size of a tabletop) in
the Australian desert, the researchers
discovered the faint fingerprints of the
earliest stars in the infant universe, stars
that formed when the cosmos was just
180 million years old.
“This is exciting because it is the first
look into a particularly important period
in the universe, when the first stars and
galaxies were beginning to form,” said
Colin Lonsdale, director of MIT’s Haystack
Observatory, in a press release. “This is the
first time anybody’s had any direct obser-
vational data from the epoch.”When these stars first began shining
within the pitch-black void of the early
universe, they blasted surrounding neutral
hydrogen with ultraviolet radiation. This
impacted the hydrogen atoms within
the gas, as scattered ultraviolet pho-
tons altered the energy of the electrons
in some of the hydrogen, causing it to
absorb energy from the cosmic microwave
background at one particular frequency
— 1.4 gigahertz, equivalent to a wave-
length of 21 centimeters.
Astronomers knew that they
should be able to detect the
absorption or corresponding
emission from this process,
but until now, they have
been unable to do so.
“The problem is, due to
the expanding universe, this absorption
would be observed at some [unknown]
lower frequency,” said Peter Kurczynski,
a program officer with the National
Science Foundation (which supported
the study), in a video. “Finding that
frequency, finding the absorption that
comes when the first stars turn on, would
be like listening to every station on your
car stereo at once, and being able to tell
that your favorite is missing.”To pinpoint the signal, the researchers
used an Earth-based instrument called a
radio spectrometer to scan the majority
of the southern sky. After collecting radio
waves from all possible astronomical
sources, the team combed through the
data, searching for dips — where energy
is absorbed — in the signal’s power as a
function of frequency.
“We see this dip most strongly at
about 78 megahertz,” said Alan Rogers,
a scientist at MIT’s Haystack Observatory
and a co-author on the paper, “and
that frequency corresponds to roughly
180 million years after the Big Bang. In
terms of a direct detection of a signal
from the hydrogen gas itself, this has got
to be the earliest.”
The results of the study reveal that the
pre-star universe was likely a much colder
place than previously thought. In fact, the
researchers found that the hydrogen in
the early universe was less than half the
temperature they expected to find. This
suggests one of two things:
Either astronomers’ theo-
ries are missing something
major about our universe,
or the study has detected
the first evidence of dark
matter siphoning off
energy from normal matter
— a theory initially proposed by Renna
Barkana of Tel Aviv University.
“If Barkana’s idea is confirmed,” said
Judd Bowman, an astronomer at Arizona
State University and lead author of the
study, “then we’ve learned something
new and fundamental about the mysteri-
ous dark matter that makes up 85 percent
of the matter in the universe, providing
the first glimpse of physics beyond the
standard model.” — J.P.FINGERPRINTING THE VERY FIRST STARS
Using a radio antenna the size of a tabletop, astronomers find evidence of the universe’s earliest suns.
14 ASTRONOMY • JULY 2018BEHIND THE VEIL. This artist’s
concept shows one of the universe’s
first stars. The massive blue sun is
embedded within filaments of gas,
while the cosmic microwave
background (CMB) is shown as a red
glow on the outer edges of the
image. Researchers recently
inferred the existence of these
massive and ancient blue stars by
measuring the dimming of the CMB.
N.R. FULLER/NATIONAL SCIENCE FOUNDATIONWhen exactly
did the first
stars form?