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Dark and darker: there's a lot more gravity in the cosmos than meets the eye
Natural History, Nov, 2003 by Neil deGrasse Tyson
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.
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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 seemed bound by gravity to the rest of the Coma cluster. Some matter--at least some source of gravity--seemed to be misbehaving.
Zwicky based his conclusion on an intimate relation between the total amount of matter in a galaxy cluster and the observed speeds of its orbiting member galaxies. Assuming the cluster is not in some odd state of expansion or collapse, if you know the size of the cluster, and if you can estimate its mass, you can invoke Newton's equation to calculate what the orbital speed of its galaxies should be.
You can do a similar calculation for the orbital speed of each planet in the solar system. All you need to know is the planet's mass, the Sun's mass, and the distance between the two--well-known quantities by now. Calculate what the orbital speed of the Earth should be, and then measure the actual speed. The two figures will agree. But suppose you measured Earth's speed and it came out ten times greater than Newton's laws said it should be. Knowing that Earth's velocity of escape from the solar system is only one-sixth that figure, you'd have to wonder why Earth (and all the other planets) hadn't flown the coop long ago.
In the Coma cluster, Zwicky found, galaxies were traveling faster than the escape velocity he calculated for them. Hence the cluster should have flung itself apart within several hundred million years of its birth, leaving barely a trace of its existence. Yet Coma's symmetrical beehive shape bespeaks an age perhaps as venerable as that of the universe itself.
In the decades that followed Zwicky's discovery, other galaxy clusters were found to have the same pattern. That meant no one could dismiss the Coma cluster as a renegade, and the significance of the problem became correspondingly magnified. Who--or what--was to blame? Newton? Not likely. His theory had survived two and a half centuries of testing. Einstein? Nope. Even the formidable gravity operating within galaxy clusters is too weak to require the corrective treatment of Einstein's general relativity. Perhaps the absent mass was just ordinary matter that happened to be dark--burned-out stars, for instance, that were no longer emitting visible light. For a short time, in fact, investigators named the problem "missing light" rather than "missing mass." But even when astrophysicists realized that the true problem was surplus gravity, they hurried to invent its presumed source, bestowing upon it the spooky name "dark matter."
Just as astrophysicists were growing accustomed to their ignorance, the problem of dark matter reared its invisible head somewhere else. During the 1970s and 1980s Vera Rubin, an astronomer at the Carnegie Institution of Washington in Washington, D.C., and her colleagues discovered that individual spiral galaxies present a similar anomaly. Beyond the luminous disk of such galaxies, scattered across the largely empty, "rural" areas of the cosmos, are a few gas clouds and isolated regions where bright stars are being born. By observing such star-forming regions, Rubin could trace the gravity field beyond the galaxy's visible edge. If those regions and gas clouds were subject only to the gravity of the visible matter in the galactic disk, their orbital speeds out there in Nowheresville should have dropped. But Rubin discovered that their speeds stayed high, without a trace of dropping off, even in the most remote locations.
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