For decades, astronomers have grappled with their inability to decipher the universe’s darkest secret: the identity of most of its matter.
It’s not the same stuff as the ordinary atomic variety of matter common on Earth. Atoms or their parts — such as protons and neutrons — make up less than 17 percent of the mass in the cosmos. All the rest is “dark” — invisible to eyes and telescopes, its presence deduced by its gravitational tug on stars and galaxies (SN: 8/28/10, p. 22). This mystery matter apparently consists of tiny particles of some exotic species, but efforts to trap them (in underground detectors) or make them (in particle accelerators) have produced frustrating results: Some experiments find hints of such particles; others find nothing.
Yet despite the frustration, physicists offer a message of hope. With a deluge of new data already in hand, and more precise probes in the works, learning the identity of dark matter may just be a matter of time.
“My feeling,” says theoretical physicist Katherine Freese, “is that we’re on the edge of discovering it.”
By discovering it, she means directly detecting the existence of dark matter particles, determining their species and mass and then, ideally, figuring out how they fit into physicists’ broader theories of matter and energy. And as it turns out, the latest twists in the dark matter plot suggest that those particles might not fit where most experts had expected.
Two problems, one particle
Dark matter does have to fit in somehow, though. While its existence has been established only indirectly, few experts now doubt its presence throughout the cosmos. It was first inferred 80 years ago, when Caltech astronomer Fritz Zwicky noticed that clusters of galaxies behave oddly — the speeds of galaxies moving within a cluster cannot be explained by the gravity of the visible mass.
Later, astronomers found that the outer edges of most galaxies rotate much more rapidly than they ought to, if galaxies contain only visible matter. Extra unseen matter was needed to explain those observations, unless the law of gravity had somehow been repealed for long distances (a prospect seriously entertained by some researchers but dismissed by most experts). Further compelling evidence for dark matter came in 2006, when observations of colliding galaxies showed that ordinary matter did not track with the gravity of the collision debris.
As recently as a couple of decades ago, many astronomers suspected that dark matter might be ordinary matter clumped into lightless spheres — massive compact objects whimsically dubbed MACHOs. Such balls of darkness might be “failed stars” known as brown dwarfs, or possibly black holes. But various studies showed that MACHOs couldn’t provide enough mass to explain the galactic spin rates. More likely, each galaxy is embedded in a cloud of massive subatomic particles, invisible because they don’t interact with light.
For that matter, they don’t interact with much of anything — hence their designation as weakly interacting massive particles, or WIMPs. If they exist, billions of these particles pass through you every second, but only about one per minute makes an impact on even a single atom in your body.
Enthusiasm for WIMPs as dark matter candidates was bolstered by theorists’ attempts to unite matter, gravity and other forces in a single mathematical package. Those physicists proposed that each fundamental particle of matter, and each basic force-carrying particle, had a “superparticle” cousin. Such particles would be more massive than their ordinary partners, perhaps just the right mass to explain the dark matter in space.
Here was one of science’s favorite coincidences: a possible solution for one mystery also offering an explanation for another great mystery. With two independent excuses for believing in WIMPs, scientists and funding agencies deemed it worthwhile to conduct experiments to search for them.
One such search strategy was proposed in the 1980s by Freese and collaborators Andrzej Drukier and David Spergel. If galaxies lived in a massive cloud (known as a halo) of WIMPs, each star within a galaxy (and each planet) would constantly smash into those WIMPs as the galaxy rotated. The sun, for instance, travels along a spiral arm of the Milky Way toward the constellation Cygnus, plowing through the swarm of WIMPs in its path the way a car windshield splatters mosquitoes.
Despite the reluctance of WIMPs to interact with ordinary matter, some few would, thus making it possible to record their presence with sensitive detectors. But windshields splatter more than mosquitoes. All sorts of gnats and moths and other bugs get flattened as well, their identities undecipherable from the smudges on the glass. In the same way, other subatomic particles, such as those produced by natural radioactivity, could fool the WIMP detectors. Freese and colleagues realized, though, that those other collisions occur at random. Collisions with WIMPs would be at a peak in June, as the Earth moved in the same direction as the sun toward Cygnus, but at a minimum in December, when the Earth was revolving around the sun in the opposite direction. (Fewer mosquitoes hit your windshield when you’re backing up than when you’re driving forward.)
Sure enough, when an experiment named DAMA, buried deep under an Italian mountain, looked for signs of such collisions, it found more in the summer than winter. But that finding was met with skepticism when other experiments failed to confirm it. More recently, an experiment called CoGeNT, in a Minnesota mine, did find hints of a summer-winter difference (SN: 6/4/11, p. 10). But another detector (CDMS II), operating in the same Minnesota mine, has found no sign of WIMPs. And yet another experiment in the Italian lab (XENON100) also sees nothing. Analysis of 13 months of data “has yielded no evidence for dark matter interactions,” the XENON100 team recently reported in Physical Review Letters.
Despite the apparent inconsistencies among current experiments, Freese, of the University of Michigan, believes evidence for WIMPs will emerge in the next few years.
“There are anomalous results, and they obviously can’t all be right. But I’m predicting that one of the anomalies is right and we’ll know pretty soon,” she said at a recent symposium in Raleigh, N.C., sponsored by the Council for the Advancement of Science Writing.
At first glance, the experiments seem hard to reconcile: Those finding nothing seem to suggest that the searches reporting signs of WIMPs must be wrong. But upon further review, the evidence against WIMPs is not indisputable. For one thing, Freese points out, each experiment uses a different material to detect the WIMPs, so comparisons can be tricky. And other theoretical issues come into play when comparing one experiment with another, Freese says, such as whether the likelihood of a WIMP collision depends on the spin of the atomic nucleus it collides with.
Besides all that, current experiments are pushing the limit of their sensitivity. That’s turning out to be an especially crucial problem — made worse by the realization that WIMPs might not possess the amount of mass that the detectors were designed to look for.
In 1998 the first supposed WIMP sighting, by the DAMA experiment, suggested a WIMP weighing in at about 60 proton masses, based on collisions recorded with iodine atoms in the detector’s sodium iodide crystals. When other experiments found no sign of WIMPs, many physicists dismissed the DAMA results. But further data collected by the more advanced DAMA/LIBRA experiment continue to show the June-December difference — and with a new twist: Analyzing collisions with sodium rather than iodine suggests a much lighter WIMP, perhaps of only five to 10 proton masses.
That’s not what scientists had expected. Although once considered a possibility, such low masses had supposedly been ruled out by particle accelerator searches.
“The idea that this region is back is really a surprise,” Freese says. “The experiments were really designed for the higher-mass region, not this low-mass region.”
With the realization that WIMPs might have relatively low masses, it may be possible to reconcile the incompatibilities among current WIMP-hunting detectors. “It has become clear that the breadth of possibilities for light WIMPs is much larger than previously appreciated,” theorist Dan Hooper of Fermilab and colleagues write in a recent paper. But low-mass WIMPs live on the fringe of what current experiments were designed to find. Establishing the existence of dark matter WIMPs for sure will require more sensitive detectors, or novel detector strategies.
One experiment expected to provide more sensitivity than previous searches will use a combination of liquid and gaseous xenon in a detector called LUX. It is scheduled to begin collecting data in 2013 at a depth of 1.5 kilometers in the Sanford Underground Research Facility (formerly the Homestake Mine) in South Dakota. Another promising approach is now being tested at the South Pole by a collaboration known as DM-Ice. It would use sodium iodide crystals, the same material used in the Italian DAMA/LIBRA experiment. Placing the detector more than 2 kilometers deep in Antarctic ice would shield it from most of the impostor particles. And because it is in the Southern Hemisphere, the peak WIMP rate would be expected in winter rather than summer, a check on whether it is the season, rather than the Earth’s direction of travel, that accounts for the Italian results.
Still, just counting more hits than expected isn’t proof that the colliding particles arrive from the direction of Cygnus. A WIMP striking an atomic nucleus sends it recoiling in the opposite direction of the WIMP’s arrival path. But current detectors measure WIMP impact by recording signals such as flashes of light or a slight increase in temperature, obtaining no information about direction.
With a little help from biology, though, a future detector might be able to track the path of the recoiling nucleus. Freese and collaborators, including Harvard geneticist George Church, recently proposed a detector using single strands of the genetic molecule DNA hanging from a gold target. If the strands of DNA are strategically positioned, they will be sliced by a gold nucleus as it flies away after a WIMP impact (SN: 12/1/12, p. 9). If the order of the DNA building blocks (abbreviated A, T, C and G) is known, the pieces can be put back together to determine the path of the recoiling nucleus, kind of like reassembling the pages of a book that had been cut at a specific angle by scissors.
SUSY steps aside
If these new experiments establish low-mass WIMPs as the dark matter, scientists will no doubt celebrate the confirmation that WIMPs exist. But they will probably want to invite Alanis Morissette to the ceremony to sing “isn’t it ironic.” For if WIMPs turn out to have a low mass, one of the main motivations for seeking them so enthusiastically in the first place will have been a mirage.
All along, most experts assumed that WIMPs would be the superpartner of an ordinary particle, as described by a theoretical framework known as supersymmetry (affectionately called SUSY for short). When accelerator experiments failed to find low-mass superpartners, dark matter hunters jumped to the conclusion that WIMPs must have higher masses. But WIMPs do not have to be superpartners. Any particle that interacts weakly with ordinary matter would do.
Designing experiments to search for superpartner WIMPs may have led physicists astray, and may explain why the current results are so confusing. New detectors, designed without the constraint of faith in SUSY, may have better luck pursuing more speculative possibilities.
“You remove one restriction from your theory,” Freese says, “and a whole new world opens up.”
It wouldn’t be the first time that scientists have let their theories mislead the interpretation of observations. Ancient astronomers didn’t realize that the rotation of stars around the sky at night was observational evidence of the Earth’s spin — the reigning theory held that the Earth was motionless. Albert Michelson thought his experiment to detect the ether had failed — the prevailing theory that light required an ethereal medium blinded him to his discovery that the ether didn’t exist.
Theory is nevertheless important, of course, in guiding the design of experiments and in interpreting their results. But, as Darwin’s friend Thomas Henry Huxley once pointed out, sometimes a beautiful theory is slain by an ugly experimental fact. It’s the law of the scientific jungle. So someday soon, scientists may have to sacrifice the beauty of SUSY to the cause of identifying the dark matter. Whatever the bulk of cosmic mass is made of, sooner or later earthbound scientists are going to figure it out.