Darkest Puzzle of the Cosmos
With one team claiming success, physicists still seek proof of 'dark matter,' which is thought to comprise almost all of the material in the universe.
By USHA LEE McFARLING
LA Times Science Writer
March 20, 2000
PALO ALTO--It's happening again. Some physicists are claiming to have solved
the greatest mystery in the universe.
They say they have found "dark matter"--impossibly elusive particles
that make up nearly all of the universe, yet have never been captured,
created in a lab or even detected.
Previous claims have popped up regularly since the early 1980s. But as
the charmingly named dark-matter candidates--neutrinos, monopoles,
MACHOs, black holes and dwarfs of various colors--fail to stand up to
experimental and theoretical proofs, they fade faster than presidential
"It's like Elvis. There are sightings every so often that are never
confirmed," said Rocky Kolb, who heads the cosmology department at the
Fermi National Accelerator Laboratory in Batavia, Ill.
Without dark matter, the universe does not make sense. Some unseen
material appears to be exerting a powerful gravitational pull that
explains the current structure of the universe and the speeds of the
objects within it.
But because no one can see it, the questions loom: What is this exotic
dark matter? How can we detect it?
A group of Italian and Chinese physicists is claiming to have detected
the curious dark matter in a clever experiment nearly a mile below
ground. Members announced their finding in 1997, and last month
reintroduced it with additional supporting data.
Other physicists doubt, and at times fiercely criticize, the team,
accusing it of making an extraordinary claim without adequate proof.
The controversy, and the many fits and starts of earlier searches,
illustrate just how competitive, difficult and uncharted this
cosmological game of hide and seek really is. Some liken it to trying to
find a black cat in a dark cellar at midnight--with no flashlight.
"It's taking a lot longer than I thought it would," said Vera Rubin,
an astronomer at the Carnegie Institution of Washington who unofficially
set off the search for dark matter in the 1970s with the finding that
something invisible was causing galaxies to rotate at high speeds.
Nevertheless, there is now a palpable excitement among those who
created the theories that predict the nature of dark matter and those who
seek the stuff. Even if the Italian-Chinese team has fallen short, the
theoreticians believe somebody is on the verge of snaring their
quarry--thanks to spectacular leaps in technology that can detect barely
perceptible flickers of energy and motion.
"We've ruled out a lot of suspects and now an arrest is imminent,"
said Mike Turner, an astrophysicist at the University of Chicago. "When
you're working a big case--think JonBenet Ramsey or O.J.--you've got to
check out every lead."
The Nature and Fate of Our Universe
The answer to the dark matter mystery literally wouldn't alter a hair
on your head. But it could create an entirely new type of physics filled
with mind-expanding ideas about things we take for granted, like the
power of gravity. It could also help explain the nature and ultimate fate
of our universe.
"It's about the most important thing we could possibly be looking
for," said David Caldwell, a professor emeritus of physics at UC Santa
Barbara who founded one team now hunting for dark matter. Adds UC Santa
Cruz physicist Joel Primack: "We are, after all, talking about what most
of the universe is made of."
The leading suspects are theoretical particles called WIMPs, or weakly
interacting massive particles.
The particles are thought to be heavy compared with elementary atomic
building blocks like protons. But because WIMPs are believed to race
right through ordinary matter without leaving a trace, they are nearly
impossible to detect.
Physicists around the world are so sure that WIMPs exist that many are
staking their careers on finding them. Twenty international teams are
searching for WIMPs in all manner of strange places: caves, tunnels and
mine shafts, where detectors can be protected from most stray signals
caused by cosmic rays and are therefore able to focus solely on WIMPs.
They're using superheated droplets, magnetic grains, vibrating wires
and huge crystals cooled to the limits of cryogenic science in hopes of
detecting just a fraction of the millions of WIMPs postulated to pass
through an area the size of our thumbnail each second.
While it's possible that the Italian-Chinese team has already found
WIMPs--as it claims--or that a dark-horse team could claim that prize,
most bets are on a team based at Stanford that is perfecting exquisitely
sensitive new WIMP detectors.
Dubbed CDMS for cryogenic dark matter search, the project relies on
ultra-pure superconductor crystals cooled to nearly 500 degrees below
zero. The atomic structure of crystals at such temperatures is so still
that any perturbations caused by particles like WIMPs should be
measurable as slight increases in temperature.
"Like a pool where there's no storm, when you drop a pebble in, you
see the waves," said Blas Cabrera, a physicist who previously led, and
abandoned, the search for a dark-matter candidate called a monopole. He
now co-directs the $25-million CDMS project, funded by the Department of
Energy and the National Science Foundation.
But the team must still determine if the signals they see are caused
by WIMPs or by other particles from cosmic rays. To date, it appears that
the signals recorded by the detectors here have almost certainly come
from ordinary particles like neutrons and not from WIMPs, Cabrera said.
The Italian-Chinese team used an entirely different technique. For the
last three years, its members have looked for signs of a seasonal
difference in the number of WIMPs that would be an expected effect of the
Earth's orbit around the sun. In comparing the total number of particles
detected at different times of the year, the researchers inferred that
any differences would be due to increases or decreases in the number of
They found an increase each June--but only of 1%--compared with the
level in December. Other scientists said they'd be more convinced if a
larger difference had been detected, and if there had been more evidence
that the variation didn't have an earthly cause, like temperature
A conclusive answer may arrive only with the next generation of
experiments. The 10-university collaboration running the CDMS project at
Stanford is now moving its experiment to the former Soudan iron mine in
northeastern Minnesota. Placing the experiment nearly half a mile beneath
the surface and adding more sophisticated detectors should make it
possible to screen out nearly all background signals, including pesky
When those detectors go on line in Minnesota next spring--an event the
scientists dub "First Dark"--team members anticipate being able to
clearly differentiate any WIMPs from background signals for the first
time. WIMP-hunting physicists say these experiments will be the first
time they reach what some call the promised land: the place where they
truly have a chance to catch their prey.
But they also might find nothing at all.
Carrying On Einstein's Quest
That's a risk Bernard Sadoulet is willing to take--and has been
willing to take for the 14 years he's been hunting for WIMPs. A native of
Nice, France, Sadoulet started his physics career by helping run Nobel
Prize-winning particle-smashing experiments that captured fundamental
(but not dark) particles like the charm quark and W and Z bosons.
Sadoulet, who now directs the Center for Particle Astrophysics at
Berkeley, says he's skeptical of the many "grand ideas" that theoretical
physicists cook up to explain the universe. "I'm not ready to spend 10 or
15 years of my life testing an idea some theorist came up with at
breakfast," he said.
But 15 years ago, Sadoulet became captivated by dark matter--and
convinced that the theoretical arguments predicting its existence were
sound. Finding dark matter, he says, would be the "ultimate Copernican
revolution"--proof that not only are we not at the center of our
universe, but we're not even made of the same material as most of it.
The find would also confirm decades of theoretical work that attempt
to explain inconsistencies in physical laws that govern our universe.
Einstein hoped to formulate a unified model that could weave all of the
universe's various forces and materials into one grand theory. He failed,
but many of the physicists following his lead think dark matter may
provide an all-important handhold to such a complete theory.
But scientists are also reluctantly bending to the idea that their
explanations of the universe may not be so elegant after all. "In physics
we always like simple solutions," said Harry Nelson, a physicist at UC
Santa Barbara and a member of the CDMS team. "It almost makes you feel
sick to think dark matter is made up of different types of things, but it
Said Frank Avignone III, a physicist at the University of South
Carolina and a longtime WIMP hunter: "We may not be looking for one
suspect. Maybe we're dealing with a gang."
While WIMPs are the leading contender, there are a few others in the
A search based at the Lawrence Livermore National Laboratory is
looking for another theoretical particle called an axion. The axion
should weigh millions and millions of times less than a single electron.
Although proof of its existence could solve several fundamental problems
in physics--from the dark-matter mystery to the way neutrons are held
together--they have been sidelined because of the current furor over
"There's a bandwagon effect that this theory [that predicts the
existence of WIMPs] must be true because it's so beautiful and explains
everything. People argue for it based on its aesthetics," said Leslie
Rosenberg, a physicist at the Massachusetts Institute of Technology who
co-directs the US Axion Search with Karl Van Bibber, a physicist at
On the other end of the size spectrum from the axion is an extremely
heavy particle proposed by Fermilab's Kolb called a WIMPzilla. But even
Kolb admits that he can't dream of a way to detect the monster.
It's also possible that dark matter could be made of something so
strange that it hasn't yet been theorized, something so elusive that
humans may never detect it.
"There's no guarantee that it will be in a form that we can ever
discover," said Kolb. "That's what wakes me up at night in a cold sweat."
* * *
Searching for Dark Matter
Dark matter makes up more than 90% of the universe but has never been
detected using ordinary methods. A leading candidate for dark matter is
the WIMP ,or weakly interacting massive particle. WIMPs are hundreds of
times heavier than protons, but rarely interact and pass through most
material, including our bodies, without leaving a trace. About 20 groups
are searching for evidence of WIMPs, which exist only in theory.
The illustration below is of an Italian experiment:
Hunting for WIMPs
A WIMP-hunting team based at Stanford will soon move its experiment
nearly a half-mile below the earth's surface to a defunct iron mine in
north-eastern Minnesota. Placing the experiment underground will protect
the detectors, shown at right, from numerous stray particles that come
from cosmic rays but rarely penetrate the ground. Wrapping the detectors
in layers of lead will protect them from natural radioactivity from the
The detectors are hockey puck-sized superconducting crystals of
germanium and silicon. These pure crystals are cooled to about 500
degrees below zero. A particle hitting a detector disturbs the molecular
structure of the crystal and registers as a slight temperature increase.
Because WIMPs easily pass through most matter, they can pass through the
shields and register a signal. To date, the detectors at Stanford have
registered a handful of signals, but an analysis suggests that these were
caused by stray particles that originally came from cosmic rays and
managed to penetrate the 35 feet of rock over the detectors. By improving
the detectors and moving them into the deep mine, scientists hope to
decrease the number of false signals hitting the detectors and be more
certain that the signals they do find are WIMPs.
© Copyright 2000 The Los Angeles Times