In the heavyweight championship of the cosmos--to decide who gets to be 90 percent of everything--WIMPs have MACHOs on the ropes.
Astronomers can see to the edge of creation and probe the hearts of galaxies, but they can’t account for 90 percent of the mass in the universe. They discovered this somewhat embarrassing state of affairs while observing the rotation of spiral galaxies like our own. If the mass of a galaxy is concentrated where its light is, in the bright core, then just as the sun’s gravity pulls distant Pluto around much more slowly than it pulls Mercury, stars in the outer reaches of a galaxy should orbit much more slowly than stars in the center. But they don’t. The inescapable conclusion is that a galaxy’s mass is not concentrated in its center: there must be some form of matter surrounding the galaxy, in a spheroidal halo, that doesn’t show up in telescopes.
The nature of this dark matter has puzzled astronomers for decades. But now they may be on the verge of identifying what it is--by virtue of knowing what it is not. New evidence indicates there is a paucity of MACHOs in the universe. That suggests the dark matter is mostly WIMPs.
The pithy acronyms are shorthand for the two leading candidates for dark matter. MACHOs are massive compact halo objects--planet-size clumps of ordinary matter that are not quite massive enough to ignite nuclear fusion in their cores and shine as stars. In contrast, WIMPs, or weakly interacting massive particles, are not ordinary at all. They are hypothetical subatomic particles that zip around the universe in great abundance. But they interact so infrequently with the normal matter in particle detectors that they have so far eluded detection altogether.
In 1986, though, Princeton theorist Bohdan Paczy´nski proposed an ingenious method for detecting MACHOs. The method was based on a discovery of Einstein’s--that concentrations of mass bend light. Whole galaxies have been seen to act as gravitational lenses, focusing the light of other galaxies that lie behind them. But Paczy´nski calculated that a much smaller mass could focus light from a single background star and make it look brighter on Earth, just as a flashlight lens makes the light from a small bulb appear brighter. If a MACHO in the Milky Way’s halo were to pass directly between Earth and a star in another galaxy, this gravitational microlensing effect would cause the starlight to brighten gradually and fade over the course of a month or so. In principle, Paczy´nski argued, this fluctuation should be detectable.
At the time I proposed the idea, I thought it was more or less science fiction, recalls Paczy´nski. Two major technological developments made it possible. First, electronic light detectors called CCDs (there’s one in every camcorder) became not only sensitive enough to detect individual stars in other galaxies but large enough to scan hundreds of thousands of stars at a time. Second, computers became fast enough (and cheap enough) to sift through all those star images for the few microlensing events that Paczy´nski predicted should occur every year--to distinguish, for instance, the perfectly symmetrical brightening and fading caused by the passage of a MACHO from the more erratic fluctuations of a star blinking on its own.
By 1992 two groups of astronomers, one American and one European, were searching for MACHOs. They pointed their telescopes at the Large Magellanic Cloud (LMC for short), a neighboring galaxy containing roughly 15 billion stars. In October 1993 both groups announced they had detected gravitational microlensing events. The announcements generated a flurry of excitement: Had the dark matter been found at last?
Apparently not. Since last year, the Americans have detected only three more microlenses--for a grand total of four--while the Europeans have seen none at all since the initial two. If MACHOs make up a significant fraction of the dark matter in our galaxy’s halo, both groups should have detected many more of them by now. Astronomers on the American team caution that their results are still preliminary. But the European group is pulling no punches. If the halo was completely composed of objects of the mass of Jupiter, or ten times smaller or larger, says Alain Milsztajn of the Saclay Research Center in France, then we should already have observed 15 events. The indications now are that there are not enough MACHOs to fill up the halo.
Astronomers not directly involved with either project tend to agree. We’re waiting for them to come out with a conclusive statement like, ‘We know the halo is not made of MACHOs,’ says University of Washington astronomer Craig Hogan. Any day now I expect them to have the data to say that.
Bolstering this view is the fact that both the Americans and a group of Polish astronomers led by Paczy´nski have detected a large number of microlensing events while looking toward the bulge of stars at the center of the galaxy, rather than out through the halo. The American group has detected two dozen, the Polish group 11. This is no help in the search for dark matter--it has to be in the halo if it is to account for the rapid rotation of the outer reaches of the galaxy. But the observation of abundant microlenses toward the galactic bulge does prove that the lack of MACHOs in the halo can’t be blamed on Paczy´nski’s detection technique. I think there’s no doubt in anybody’s mind that microlensing has been seen, says University of California at Berkeley astronomer Douglas Scott. The fact that there are many more toward the bulge than toward the LMC says to most people that the dark matter in the halo is not MACHOs.
Indeed, it is possible that no MACHOs have been seen at all. Some researchers believe the few microlenses observed in the direction of the LMC were not MACHOs in our galaxy but ordinary stars in the LMC itself that were passing in front of more distant LMC stars. The same probably goes for the microlenses seen toward the galactic bulge. Paczy´nski thinks there are so many in that direction because the bulge is shaped like a bar that points toward us--something astronomers have suspected for 30 years. Most of the events are caused by stars on the near side of the bar lensing stars on the far side, says Paczy´nski.
MACHOs aren’t dead yet, though. If they are smaller than Earth, the current searches might have missed them. But if so, a survey begun in September by Columbia University astronomer Arlin Crotts should find them. Crotts is pointing his telescope at the Andromeda galaxy rather than at the LMC. Since Andromeda is 2.2 million light-years away, 13 times farther than the LMC, its stars appear smaller; thus a less massive object between them and us--either in Andromeda’s halo or in the Milky Way’s--can act as a lens. In theory, Crotts should be able to detect lensing masses as small as Earth’s moon. He should have plenty of chances: Andromeda contains some 300 billion stars, 20 times more than the LMC.
If Crotts’s search and the two LMC searches come up MACHO-less, then WIMPs may be declared the winners by default. It’s not a resolved issue, but it’s certainly looking to be in that direction, and I think all the people working on this sort of admit that, some more strongly than others, says Scott. I hope we’ll know the answer in 1995, adds Crotts.
Whatever the answer, gravitational microlensing is providing an invaluable new tool for astronomers. A technique for finding planet-size MACHOs, for instance, is also a way of looking for planets around other stars. Thus in the end, Paczy´nski’s technique may help answer two of the greatest questions in astronomy--the nature of the dark matter and the existence of Earth-like planets outside the solar system. That would be pretty good for a scheme that once seemed preposterous. Says Paczy´nski: It’s always a theoretician’s dream to see one’s ideas being implemented.