IMAGE: NASA’S GODDARD SPACE FLIGHT CENTERT
ick ... tick ... boom? In the center of
a galaxy 1.2 billion light-years from
Earth, astronomers say they have
seen signs that two giant black holes,
with a combined mass of hundreds of
millions of Suns, are gearing up for
a cataclysmic merger as soon as 100 days
from now. The event, if it happens, would
be momentous for astronomy, offering a
glimpse of a long-predicted, but never wit-
nessed mechanism for black hole growth.
It might also unleash an explosion of light
across the electromagnetic spectrum, as
well as a surge of gravitational waves and
ghostly particles called neutrinos that could
reveal intimate details of the collision.
As soon as the paper appeared last week
on the preprint server arXiv, other astrono-
mers, eager to confirm the tantalizing sig-
nals, rushed to secure telescope observing
time, says team member Huan Yang of the
Perimeter Institute in Waterloo, Canada.
“We’ve seen people acting pretty fast,” he
says. Emma Kun of Konkoly Observatory in
Budapest, Hungary, began to scour archives
of radio observations for confirmation of
the signal. “If the boom happens, it will con-
firm many things,” she says.
But the prediction may be a mirage. It’s not
clear that the observed galaxy holds a pair
of black holes, let alone a pair that’s about
to merge, says Scott Ransom of the NationalRadio Astronomy Observatory, who finds the
presented evidence “pretty circumstantial.”
Supermassive black holes are thought to
lurk at the heart of most, if not all, galaxies,
but theorists don’t know how they grow so
big. Some sporadically suck in surrounding
material, fiercely heating it and causing the
galaxy to shine brightly as a so-called active
galactic nucleus (AGN). But the trickle of
material may not be enough to account for
the black holes’ bulk. They could gain weight
more quickly through mergers: After galax-
ies collide, their central black holes could
become gravitationally bound and gradually
spiral together.
Such black hole pairs are not easy to
detect. X-ray telescopes have discovered
a handful of AGNs with two bright, sepa-
rated central sources, but the putative black
holes are hundreds of light-years apart and
wouldn’t collide for billions of years. Once
they get closer, it’s almost impossible to sep-
arate their light with a telescope. But some
AGNs dim and brighten every few years—a
sign, astronomers have argued recently, that
they harbor pairs of black holes orbiting
each other that regularly churn and heat the
surrounding material. Some of these peri-
odic oscillations have faded, however, call-
ing into question the binary interpretation.
“AGNs do all sorts of crazy things we don’t
understand,” Ransom says.
In data from a survey telescope in Cali-
fornia called the Zwicky Transient Facil-ity (ZTF), a team led by Ning Jiang of the
University of Science and Technology of
China stumbled on a periodic AGN called
SDSSJ1430+2303. “My first instinct was it
must be related to a pair of supermassive
black holes,” Jiang says.
Then, the researchers found something
more: a trend they interpret as a binary pair
closing in on a merger. The cycles were get-
ting shorter, going from 1 year to 1 month
in the space of 3 years. It is “the first official
report of decaying periods which reduced
over time,” says Youjun Lu, a theoretical
astrophysicist at the National Astronomical
Observatories of China, who was not part
of the team.
The researchers confirmed the month-
long oscillation in x-ray observations from
NASA’s orbiting Neil Gehrels Swift Obser-
vatory. If this decreasing trend continues,
the black holes, which Jiang says come as
close to each other as the Sun is to Pluto,
will merge in the next 100 to 300 days, they
report in the paper, which has not been
peer reviewed.
If the merger comes to pass, observers
could have a field day. “There should be a
huge burst across the electromagnetic spec-
trum, from gamma rays to radio,” Kun says.
Some also expect a flood of neutrinos, which
the IceCube detector at the South Pole—IN DEPTH
By Daniel CleryASTRONOMYImminent merger of giant black holes predicted
Never-before-seen event could spark cosmic fireworks—if it is not a mirage
In this visualization, a pair of giant black holes is
about to merge—an event astronomers long to see.478 4 FEBRUARY 2022 • VOL 375 ISSUE 6580 science.org SCIENCE