UCSD PHYS 10 - Illuminating the Dark Universe

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CREDITS: (CLOCKWISE FROM TOP) SDSS COLLABORATION; GSFC/NASA; NASA/WMAP SCIENCE TEAM19 DECEMBER 2003 VOL 302 SCIENCE www.sciencemag.org2038A lonely satellite spinning slowly through thevoid has captured the very essence of the uni-verse. In February, the Wilkinson MicrowaveAnisotropy Probe (WMAP) produced an im-age of the infant cosmos, of all of creationwhen it was less than 400,000 years old. Thebrightly colored picture marks a turning pointin the field of cosmology: Along with a hand-ful of other observations revealed this year, itends a decades-long argument about the na-ture of the universe and confirms that our cos-mos is much, much strangerthan we ever imagined.Five years ago, Science’scover sported the visage ofAlbert Einstein lookingshocked by 1998’s Break-through of the Year: the ac-celerating universe. Twoteams of astronomers hadseen the faint imprint of aghostly force in the deathrattles of dying stars. Theapparent brightness of a cer-tain type of supernova gavecosmologists a way to mea-sure the expansion of theuniverse at different times inits history. The scientistswere surprised to find thatthe universe was expandingever faster, rather than decelerating, as general relativity—and commonsense—had led astrophysi-cists to believe. This was the first sign of themysterious “dark energy,” an unknown forcethat counteracts the effects of gravity andflings galaxies away from each other.Although the supernova data were com-pelling, many cosmologists hesitated to em-brace the bizarre idea of dark energy. Teamsof astronomers across the world rushed to testthe existence of this irresistible force in inde-pendent ways. That quest ended this year. Nolonger are scientists trying to confirm the ex-istence of dark energy; now they are trying tofind out what it’s made of, and what it tells usabout the birth and evolution of the universe.Lingering doubts about the existence ofdark energy and the composition of the uni-verse dissolved when the WMAP satellitetook the most detailed picture ever of thecosmic microwave background (CMB). TheCMB is the most ancient light in the uni-verse, the radiation that streamed from thenewborn universe when it was still a glow-ing ball of plasma.This faint microwaveglow surrounds uslike a distant wall offire. The writingon the wall—tinyfluctuations in thetemperature (andother properties)of the ancient light—reveals what the uni-verse is made of.Long before there were stars and galax-ies, the universe was made of a hot, glowingplasma that roiled under the competing in-fluences of gravity and light. The big banghad set the entire cosmos ringing like a bell,and pressure waves rattled through the plas-ma, compressing and expanding and com-pressing clouds of matter. Hot spots in thebackground radiation are the images ofcompressed, dense plasma in the coolinguniverse, and cold spots are the signature ofrarefied regions of gas.Just as the tone of a bell depends on itsshape and the material it’s made of, so doesthe “sound” of the early universe—the rela-tive abundances and sizes of the hot and coldspots in the microwave background—dependon the composition of the universe and itsshape. WMAP is the instrument that finallyallowed scientists to hear the celestial musicand figure out what sort ofinstrument our cosmos is.The answer was disturb-ing and comforting at thesame time. The WMAP data confirmed the incred-ibly strange picture of theuniverse that other obser-vations had been painting.The universe is only 4%ordinary matter, the stuffof stars and trees and peo-ple. Twenty-three percentis exotic matter: darkmass that astrophysicistsbelieve is made up of anas-yet-undetected particle.And the remainder, 73%,is dark energy.The tone of the cosmicbell also reveals the age ofthe cosmos and the rate atwhich it is expanding, andWMAP has nearly perfect pitch. A year ago,a cosmologist would likely have said thatthe universe is between 12 billion and 15billion years old. Now the estimate is 13.7billion years, plus or minus a few hundredthousand. Similar calculations based onWMAP data have also pinned down the rateof the universe’s expansion—71 kilometersper second per megaparsec, plus or minus afew hundredths—and the universe’s“shape”: slate flat. All the arguments of thelast few decades about the basic propertiesof the universe—its age, its expansion rate,its composition, its density—have been set-tled in one fell swoop.As important as WMAP is, it is not thisyear’s only contribution to cosmologists’ un-derstanding of the history of the universe.The Sloan Digital Sky Survey (SDSS) ismapping out a million galaxies. By analyz-Breakthrough OnlineFor an expanded versionof this section, with refer-ences and links, see www.sciencemag.org/content/vol302/issue5653/#specialBreakthrough#1The WinnerPortraits of the earliest universe and the lacy pattern of galaxies in today’s sky confirm that the universe is made up largely of mysterious dark energy and dark matter. They also give the universe a firm age and a precise speed of expansion.Through a glass, darkly. Microwavedata observed by the WMAP satellite(upper left), supernovae (lower left),and galaxy clusters (above) all reveal auniverse dominated by dark energy.Illuminating the Dark Universeing the distribution of those galaxies, theway they clump and spread out, scientistscan figure out the forces that cause thatclumping and spreading—be they the gravi-tational attraction of dark matter or the anti-gravity push of dark energy. In October, theSDSS team revealed its analysis of the firstquarter-million galaxies it had collected. Itcame to the same conclusion that theWMAP researchers had reached: The uni-verse is dominated by dark energy.This year scientists got their most directview of dark energy in action. In July, physi-cists superimposed the galaxy-clustering data of SDSS on the microwave data ofWMAP and proved—beyond a reasonabledoubt—that dark energy must exist. Theproof relies on a phenomenon known as theintegrated Sachs-Wolfe effect. The remnantmicrowave radiation acted as a backlight,shining through the gravitational dimplescaused by the galaxy clusters that the SDSSspotted. Scientists saw a gentle crushing—apparent as a slight shift toward shorterwavelengths—of the microwaves shiningnear those gravitational pits. In an uncurveduniverse such as our own, this can happenonly if there is some antigravitationalforce—a dark energy—stretching out thefabric of


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