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COPYRIGHT 2003 Natural History Magazine, Inc.
Gravity, that most familiar of natures forces, is both the best- and least-understood phenomenon in the cosmos. Not until Sir Isaac Newton turned his attention to the problem in the late seventeenth century did anybody figure out that gravity's mysterious "action at a distance" is caused by matter. Newton was the first to realize that a simple algebraic equation could describe the gravitational attraction between any two bodies, and that from that equation you could "weigh" the Earth and predict the future orbits of the planets. And not until Albert Einstein pondered gravity in the early twentieth century did anyone figure out that action at a distance is better understood as a warp of space-time, caused by the presence of matter or energy or both.
Neither Newton nor Einstein thought he was describing any thing other than ordinary matter, the kind you can see, touch, feel, and taste. Yet for nearly three-quarters of a century astrophysicists have been waiting for someone to explain why 85 percent of all the gravity in the universe originates in a substance that no one has ever seen, touched, felt, or tasted. There's no guarantee that it even is a substance: maybe "excess" gravity emanates from something other than matter. In any event, the experts are clueless--and no closer to an answer today than they were in the 1930s. That's when the colorfully contentious Swiss-American astrophysicist Fritz Zwicky discovered the first sign that there is far more gravity in the cosmos than the stars, galaxies, and other visible objects could ever account for. Where was the "missing mass"?
Zwicky had been studying the Coma cluster, a titanic ensemble of galaxies far beyond the local stars that trace the constellation Coma Berenices (a Latin phrase meaning "hair of Berenice," in honor of an ancient Egyptian queen who willingly cut off her tresses). Isolated and richly populated, the Coma cluster lies more than 300 million light-years from Earth. Thousands of galaxies revolve about its center, moving in every possible orbit like bees circling a beehive.
By measuring the motion of a few dozen galaxies, Zwicky discovered that their average speed is astonishingly high--much too high for the gravity field exerted by all of the Coma cluster's visible matter to be holding the cluster together. By all rights, the galaxies he observed ought to have been flung off into deep space--yet they clearly...
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