ATOM & COSMOS
Test casts doubt on dark matter claim
COSINE-100 experiment fails to confirm controversial detection
ATOM & COSMOS
Spacetime ripples reveal
additional black hole mergers
BY EMILY CONOVER
For years, some physicists have rowed
against the tide, claiming they’ve found
the universe’s elusive dark matter, despite
mounting evidence to the contrary. A new
experiment makes that upstream paddling even more of a challenge.
Observations of the cosmos indicate
that an invisible, unknown type of particle pervades the universe. The extra
mass this dark matter provides is necessary to explain the motions of stars within
galaxies and how matter clumps together
BY EMILY CONOVER
Astronomers have now tallied up more
gravitational wave sightings than they
can count on two hands.
Scientists with the LIGO and Virgo
gravitational wave observatories report
four new sets of these ripples in space-
time. Those additions bring the total
count to 11, the researchers say in a study
posted online December 3 at arXiv.org,
Scientists with the DAMA experiment
at Italy’s Gran Sasso National Laboratory
near L’Aquila, however, say they have
strong evidence that dark matter is
interacting in their detector. Now, an
experiment called COSINE-100 has
searched for the particles using the same
type of detector as DAMA and found no
signs of dark matter, scientists report in
the Dec. 6 Nature.
“This is one more nail in the coffin,”
says astrophysicist Dan Hooper of
Fermilab in Batavia, Ill., who was not
involved with the new research. Earlier
experiments using different types of
detectors have likewise tried and failed
to reproduce DAMA’s results.
Both DAMA and COSINE-100 search
for dark matter particles slamming into
atomic nuclei in sodium iodide crystals.
If a collision occurs, it should produce a
tiny flash of light in a crystal. But mundane interactions can produce similar
flashes, such as those caused by minute
amounts of radioactive elements.
The DAMA team monitored crystals
for years to tease out the purported dark
marking major progress since the first
gravitational wave detection in 2015.
The patterns of the waves indicated
that 10 of the 11 sets of waves were stirred
up in black hole collisions. The remaining
detection, reported in 2017, came from
the smashup of two stellar corpses called
neutron stars (SN: 11/11/17, p. 6).
The data are revealing how often such
waves jiggle the cosmos and the properties of the cosmic figures that unleash
the ripples. For example, black holes may
have merged more frequently earlier in
the universe’s history, the scientists say
in a second study posted December 3 at
arXiv.org. The team also concluded that
few mergers involve black holes bigger
than about 50 times the sun’s mass, says
The COSINE-100 detector (shown) in
South Korea found no signs of dark matter
particles interacting in sodium iodide crystals.
matter signature, reporting that the rate
of collisions in the detector rises and
falls with a specific annual pattern. That
pattern, the argument goes, is the result
of Earth’s motion through a stream of
dark matter as the planet orbits the sun.
Previous experiments that tested
for the yearly variation have found
nothing (SN: 2/4/17, p. 15). But those
experiments used a different detector
material. By using sodium iodide crystals, “we’re going to take out any possible
loophole as to why DAMA sees something,” says Yale University physicist
Reina Maruyama, a spokesperson for
the COSINE-100 collaboration.
COSINE-100 scientists did not look for
a yearly pattern. Instead, they compared
the rate of hits in their detector, at the
Yangyang Underground Laboratory in
South Korea, with the number expected
from known sources, such as radioactivity. The team found no sign of extra blips
that could be from dark matter.
DAMA physicists are sticking to their
claims. Rita Bernabei of the University
of Rome Tor Vergata says, “COSINE-100
has no impact on the long-standing
results obtained with the DAMA setups.”
Katherine Freese, a physicist at the
University of Michigan in Ann Arbor, says
what’s really needed is for COSINE-100
to search for annual variation. Maruyama
says that search is in the works. s
LIGO member Daniel Holz, an astrophysicist at the University of Chicago.
The combined mass of two of the
colliding behemoths was the largest
merger yet spotted, with one black hole
weighing about 50 times the sun’s mass
and the other 34 times the sun’s mass.
Those ripples came from farther away
than previous detections: about 9 billion
light-years from Earth. “It stands out in
every possible way,” says Johns Hopkins
University physicist Emanuele Berti.
“It’s super interesting.”
LIGO’s two detectors, located in the
United States, and Virgo, in Italy, are
shuttered for upgrades until the spring.
Improvements could triple the number of
gravitational wave sightings, Holz says. s