Invisible, foreign and reclusive, the dark matter that courses
quietly through the universe may finally be within grasp. With the
construction of a new detector for dark matter, UCLA scientists and
their international collaborators are participating in the
beginnings of a potential revolution in physics. The detection of
this mysterious material may hold the key to teaching scientists
about the fundamental laws of the universe, said David Cline, UCLA
professor of physics and astrophysics and a main initiator of the
design and construction of the detector. Collaborators at
Rutherford Laboratory near Oxford University in England are putting
the final touches on the detector, which is due to start collecting
data this September in an underground laboratory in Yorkshire,
England. At the size of a small office refrigerator (about 2 meters
tall), the ZEPLIN II detector is the largest dark matter detector
in the world, Cline said.
Shining the light on dark matter “We call it dark because
it’s not supposed to absorb or emit any electromagnetic waves
(visible and invisible light), because if (it) did, you would see
it,” said Hanguo Wang, an associate research scientist in the
UCLA Department of Physics and Astronomy and collaborator with
Cline on ZEPLIN II. Making up nearly 80 percent of our universe,
dark matter (and its counterpart dark energy) are the dominant
materials of the universe, Cline said. Its massive abundance has
made it an important factor in the creation of galaxies, said Mark
Morris, professor of physics and astronomy at UCLA. “We
wouldn’t be here without dark matter,” Morris said.
Morris explains that their abundance would have caused the dark
matter particles to clump together early in the development of the
universe, forming the seeds that allowed baryons ““ the matter
composed of protons and neutrons which forms everything from stars
and black holes to people and tuna fish ““ to clump together
to form galaxies and other cosmic structures much faster than they
would have been able to on their own. “They’ll do it
““ they’ll start gravitating toward each other ever so
slowly. But we’d still be waiting for structures to form.
We’d still be in the cosmic dark ages,” Morris said.
Scientists have inferred the existence of dark matter for 70 years,
when scientists noticed that stars were orbiting much faster around
the centers of their respective galaxies than they should have,
Wang said, adding that at those speeds, the stars should have been
flung off into space like discuses from a discus thrower.
Scientists realized, Wang said, that there must be some sort of
invisible mass holding the galaxies together. “That’s
the only way that we so far have of inferring that there’s
dark matter,” Morris said. “It doesn’t absorb
light; it doesn’t emit light.” Light and dark matter
are ships passing in the night; they absolutely don’t know of
each other, or see each other, or care about each other.”
According to Morris, scientists still do not know what dark matter
is, though they have inferred several of its properties indirectly.
Dark matter is anything that does not emit light. It could
technically, Morris said, be made from objects such as black holes
or clouds of intergalactic gas, which are composed of protons and
neutrons, but which do not emit light. But calculations of the
amount of baryons in the universe since the Big Bang tell
scientists that there is not enough of this matter floating around
to account for these observations. “We don’t know what
it is. It sort of leaves us philosophically in a hole. We’re
a minor constituent of the universe ““ it’s kind of nice
to know what’s going on around us,” Morris said.
Catching the elusive WIMP The particles that scientists have put
forth as candidates for dark matter are composed of a completely
foreign material. Named “WIMPs””“ Weakly
Interacting Massive Particles ““ these particles rarely
interact with the stuff that humans and stars are made of, Wang
said. As their name implies, Wang explained, WIMPs interact
“weakly” with other particles, allowing them to pass
through Earth millions of times without interacting with it. For a
detector like ZEPLIN II, “that means maybe in 1000 days we
get one event,” Wang said. To find this elusive matter,
Cline, Wang and their colleagues from the United Kingdom, Russia
and Portugal are using a detector filled with liquid xenon, which
Cline said is ideally suited to dark matter detection. The
xenon’s physical properties will allow scientists to
distinguish between a reaction with a dark matter particle and a
reaction with a known particle. When a dark matter particle
collides with an atom of xenon, it will leave a
“recoil” signature, Wang said, as when one billiard
ball imparts energy to another. Another telltale sign of dark
matter, which flows through the solar system in a single direction
like a cosmic jet stream of particles, would be a signal that comes
from one direction during half the year and the opposite direction
during the other half. This regular annual variation should
distinguish the dark matter from background noise, Cline said,
allowing scientists to admit the new particle as a possible
candidate for dark matter. “These are the key signatures that
we’ve discovered dark matter, and not some random background
or some mistake in our detector,” Cline said. They are also
placing the detector 4,000 feet under the ground, in the Boulby
salt mine in Yorkshire, England, in order to get rid of distracting
background noise from cosmic rays that hit Earth from space.
“This room has dark matter in it. … It’s coursing
through you and me now,” Cline said, referring to his office.
“But you couldn’t detect dark matter in this room
because there’s too much background (noise). So you have to
go deep underground ““ thousands of meters underground,”
Cline added.
A renaissance in physics? Cline said that the search for dark
matter has become a worldwide effort within the past 15 years, with
detectors on every continent. “If a dark matter particle
could be detected, it would be the biggest revolution in particle
physics since the electron, proton and neutron were
discovered,” Morris said. “I think we’ll find it
in our lifetime,” he added. “We know it’s there;
how can we not find it eventually?” Cline said UCLA and other
dark matter groups around the world could be very close to
discovering dark matter. “It’s the most important
question we need to answer today in physics,” Wang said.
Cline added that the discovery of dark matter would be even more
profound since it would tell scientists about the most dominant
matter in the universe. “It would be a wonderful development
in our field because it would be a renaissance in our field. It
would be like going back a hundred years almost, to the time when
people went up on mountains and did experiments.” Cline said.
“So of all the fields of science right now that I know of,
this is one of the most exciting fields because it may be on the
cusp of a discovery.”