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90 Percent of the Universe

If gelperin’s research is a fruitful mixture of the basic and applied, Tony Tyson’s seems, at first blush, to be purely fundamental. Tyson is one of the world’s pre-eminent astrophysicists. When his name comes up, Cherry Murray deadpans: “He’s discovered 90 percent of the universe-what can you say?”

Her statement is only somewhat glib, since what the Bell Labs researcher has done is find a way to image cosmic dark matter, the invisible “missing mass” thought to make up some 90 percent of the universe’s total mass. Tyson has made a start on filling in the details. But, he figures, “at the rate we’re currently going it will take me another 50 years.”

The idea that invisible dark matter exists has been around since the 1930s. But the theory attracted only a fringe following until the late 1970s, when modern techniques proved that the visible universe doesn’t contain nearly enough mass to explain the movements of galactic gas and dust-a sure indication something else is out there exerting a strong gravitational effect. Early theories tapped neutrinos for the missing mass, but these particles have since been ruled out as major players. Tyson’s bet is for a combination of unfamiliar objects and events, including weakly interacting massive particles, or WIMPs, magnetic entities called axions, cosmic strings and breakdowns in the uniformity of the space-time continuum.

The 29-year veteran of Bell research has been hunting cosmic dark matter since 1977. “I’m a prospector,” Tyson says. “I should have a donkey, a hat, a canteen and a pickax.” His work makes use of what are called gravitational lenses to map this invisible dark matter. Any mass exerts a gravitational pull that bends or deflects the light from something behind it with respect to an observer. It’s a very imperfect lens-like looking through a Coke bottle. So, if something lies between the Earth and some distant galaxy, for example, astronomers equipped with the right camera sensitivity and processing software will detect multiple images of that galaxy. The distribution of those images makes it possible to figure out how much mass is out there affecting the light.

Dark matter often congregates around visible objects like galaxies. In one of Tyson’s experiments, the Hubble Space Telescope was trained on a cluster of several hundred galaxies some 2 billion light-years from Earth in the constellation Pisces that seemed a good bet for a gravitational lens. Sure enough, Tyson picked up at least eight images or partial images of another galaxy “behind” the cluster, a systematic distortion that revealed the presence of a good deal of dark matter. Aided by the fact that individual galaxies inside the cluster served as smaller lenses, revealing fine details of their masses, Tyson and collaborators Greg Kochanski and Ian Dell’Antonio created a map showing the distribution of cosmic dark matter at unprecedented resolution. Their map was published this May in Astrophysical Journal Letters, with more data to come from Hubble and the special Big Throughput Camera built by Tyson and University of Michigan astronomer Gary Bernstein. Installed on a telescope in northern Chile, it offers 200 times Hubble’s field of view.

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