Maybe science really is back in vogue. Or maybe "dark matter" is a case of remarkably successful scientific branding—who wouldn't be drawn in by a name like that? Then again, maybe people just want to know what the heck makes up the vast majority of the universe, a question to which science has provided only sketchy answers.
Whatever the reason, a dark matter lecture by physicist Peter Fisher at the American Museum of Natural History in New York drew a sellout crowd April 12 in a theater that seats more than 400, with museum staff turning away disappointed comers and at least one gentleman trying to talk his way in as if he were working to get past the velvet rope at a nightclub.
Fisher, of the Massachusetts Institute of Technology, gave his sizable audience a tidy roundup of what we know about dark matter and what we hope to find out in the coming years. He began by explaining a well-known but nonetheless perpetually jaw-dropping consequence of modern cosmological theory: The matter we can see and touch, all the atoms and molecules in existence, accounts for only about 4 percent of the universe. The rest is dark matter, an invisible substance discernable only by its gravitational effects on large-scale structures such as galaxy clusters, and dark energy, under whose influence the expansion of the universe appears to be accelerating.
Fisher chose to focus on the former issue. "I'm not going to talk too much about dark energy, because no one has the first clue of what dark energy is," Fisher said. That's not all that surprising given that the case for dark energy was not made until the late 1990s. Dark matter, on the other hand, is on firmer footing, thanks to a long scientific history stretching back to the 1930s, when California Institute of Technology astronomer Fritz Zwicky noticed that the galaxies of the Coma cluster moved as if they had far more mass than was observable.
Physicists might even identify a particle culprit for dark matter in the near future. "We could get at it in a number of different ways," Fisher said, adding that "the person who finds it is going to get a quick trip to Stockholm," where the Nobel Prizes are awarded.
For starters, the Large Hadron Collider (LHC) in Europe could actually produce dark matter in its record-breakingly powerful particle collisions. The problem, Fisher said, is that LHC physicists might not be able to recognize the signature of dark matter if they created it. In more targeted experiments, physicists are on the trail of ambient dark matter—Fisher noted that, according to prevailing theories, the theater in which he spoke held about three dark matter particles per liter of air. That may seem like a lot, but among the roughly 1020 other particles in each liter, dark matter remains elusive.
In experiments such as XENON100 and CDMS, physicists have placed cryogenic sensors in underground laboratories, shielded by rock from cosmic rays, in the hopes that passing dark matter particles will induce subtle but telltale recoil or ionization effects when they bump into atoms in the detectors. Those experiments have set some limits on the strength of dark matter's interactions with normal matter, but Fisher thinks the detectors will have to grow 100 times more sensitive before a conclusive dark matter signal is found. Given that their sensitivity has increased by a factor of 10 every six or seven years, in part by increasing the mass of the detector, "I think we'll be at it another decade," Fisher said.
One problem with those searches is that particles known as neutrinos can mimic the kind of weakly interacting massive particle, or WIMP, that is the most likely candidate for dark matter. If the underground detectors grow to a certain size—about 10 times as large as the current generation—without spotting the effects of dark matter, Fisher said, neutrino interactions will generate an irreducible level of background noise in which any putative dark matter signal would be lost. He is part of a group seeking to circumvent that issue by building a detector that can discriminate the direction of an incoming particle, whether neutrino, dark matter or something else entirely. The DMTPC experiment, which will be installed underground in New Mexico, should therefore be able to better discern neutrinos, which stream at us from the sun, from dark matter, which models suggest should pass through Earth in a predictable direction as the planet rotates and the solar system moves through the galaxy.
A composite image of the Bullet Cluster reveals the presence of dark matter through the misalignment of normal matter (pink) and inferred mass (blue). Image credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al.