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CORNELL ASTRO 109 - Dark and Darker

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Database:Back7 page(s) will be printed.Record: 1Dark and Darker. By: Tyson, Nell deGrasse. Natural History, Nov2003, Vol.112 Issue 9, p18-22, 4p, 1 color; Abstract Discusses Coma cluster, anensemble of galaxies that serves as an explanation for the existence ofgravity in the cosmos, based on a study by astrophysicist Fritz Zwicky.Estimation of the average speed of the galaxies; Basis for his conclusion thatthe galaxies posses an enormous amount of gravity; Studies on the problemof dark matter; Role of dark matter in the formation of the universe.; (AN11183500)Academic Search PremierSection: UNIVERSEDark and DarkerThere's a lot more gravity in the cosmos than meets the eye.Gravity, that most familiar of nature's forces, is both the best- and least-understoodphenomenon in the cosmos. Not until Sir Isaac Newton turned his attention to the problem inthe late seventeenth century did anybody figure out that gravity's mysterious "action at adistance" is caused by matter. Newton was the first to realize that a simple algebraic equationcould describe the gravitational attraction between any two bodies, and that from that equationyou could "weigh" the Earth and predict the future orbits of the planets. And not until AlbertEinstein pondered gravity in the early twentieth century did anyone figure out that action at adistance is better understood as a warp of space-time, caused by the presence of matter orenergy or both.Neither Newton nor Einstein thought he was describing any thing other man ordinary matter,me kind you can see, touch, feel, and taste. Yet for nearly three-quarters of a centuryastrophysicists have been waiting for someone to explain why 85 percent of all the gravity inthe 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 fromsomething other than matter. In any event, the experts are clueless--and no closer to ananswer 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 thecosmos than the stars, galaxies, and other visible objects could ever account for. Where wasthe "missing mass"?Zwicky had been studying the Coma cluster, a titanic ensemble of galaxies far beyond the localstars that trace the constellation Coma Berenices (a Latin phrase meaning "hair of Berenice," inhonor of an ancient Egyptian queen who willingly cut off her tresses). Isolated and richlypopulated, the Coma cluster lies more than 300 million light-years from Earth. Thousands ofgalaxies 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 speedis astonishingly high--much too high for the gravity field exerted by all of the Coma cluster'svisible matter to be holding the cluster together. By all rights, the galaxies he observed ought tohave been flung off into deep space--yet they clearly seemed bound by gravity to the rest ofthe Coma cluster. Some matter--at least some source of gravity--seemed to be misbehaving.EBSCOhost http://web.ebscohost.com.proxy.library.cornell.edu/ehost/delivery?vid=4...1 of 5 2/15/2009 10:50 PMRupert Deese, Swimmer, 1988Zwicky based his conclusion on an intimate relation between the total amount of matter in agalaxy cluster and the observed speeds of its orbiting member galaxies. Assuming me cluster isnot in some odd state of expansion or collapse, if you know the size of the cluster, and if youcan estimate its mass, you can invoke Newton's equation to calculate what the orbital speed ofits galaxies should be.You can do a similar calculation for the orbital speed of each planet in the solar system. All youneed to know is the planet's mass, the Sun's mass, and the distance between thetwo--well-known quantities by now. Calculate what the orbital speed of the Earth should be, andthen measure the actual speed. The two figures will agree. But suppose you measured Earth'sspeed and it came out ten times greater than Newton's laws said it should be. Knowing thatEarth's velocity of escape from the solar system is only one-sixth that figure, you'd have towonder 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 hecalculated for them. Hence the cluster should have flung itself apart within several hundredmillion years of its birth, leaving barely a trace of its existence. Yet Coma's symmetrical beehiveshape 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 thesame pattern. That meant no one could dismiss the Coma cluster as a renegade, and thesignificance 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 thecorrective treatment of Einstein's general relativity. Perhaps the absent mass was just ordinarymatter that happened to be dark--burned-out stars, for instance, that were no longer emittingvisible light. For a short time, in fact, investigators named the problem "missing light" ratherthan "missing mass." But even when astrophysicists realized that the true problem was surplusgravity, they hurried to invent its presumed source, bestowing upon it the spooky name "darkmatter."Just as astrophysicists were growing accustomed to their ignorance, the problem of dark matterreared its invisible head somewhere else. During the 1970s and 1980s Vera Rubin, anastronomer at the Carnegie Institution of Washington in Washington, D.C., and her colleaguesdiscovered that individual spiral galaxies present a similar anomaly. Beyond the luminous disk ofsuch galaxies, scattered across the largely empty, "rural" areas of the cosmos, are a few gasclouds and isolated regions where bright stars are being born. By observing such star-formingregions, Rubin could trace the gravity field beyond the galaxy's visible edge. If those regionsand gas clouds were subject only to the gravity of the visible matter in the galactic disk, theirorbital speeds out there in Nowheresville should have dropped. But Rubin discovered that theirspeeds stayed high,


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