Attempting to look beyond the standard model for

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attempting to look beyond the standard model for candidates to explain dark matter.ASTRONOMY: ROEN KELLYParticle feverIn the next few years, WIMPs could be moreor less ruled out as a source of dark matter.
1018Strong-electroweakunification scale?Planck scaleStrings?Quantum gravity?astronomical targets, the team used morethan 6.5 years of data to analyze the back-ground glow of gamma rays seen all overthe sky.The nature of this light, called theextragalactic gamma-ray background(EGB), has been debated since NASA’sSmall Astronomy Satellite 2 first measuredit in the early 1970s. Fermi has shown thatmuch of this light arises from unresolvedgamma-ray sources, particularly galaxiescalled blazars, which are powered bymaterial falling toward gigantic black holesin their centers. Blazars constitute morethan half of the total gamma-ray sourcesseen by Fermi. EGB gamma rays couldarise from distant interactions of darkmatter particles, such as the annihilationor decay of WIMPs, but Ajello and his col-leagues found that blazars and other dis-crete sources can account for nearly all ofthe emission.“Fermi has done great in cutting intothe parameter space” — that is, shrinkingthe theoretical box — “of dark mattermodels,” Ajello says. That’s because itsLarge Area Telescope surveys the wholesky every three hours, deepening its expo-sure with every orbit. WIMPs can producegamma rays through a variety of mecha-nisms, such as converting into pairs ofquarks, gluons, muons, and other particles,which then decay to emit gamma rays andstable particles. This provides scientistswith many avenues to explore in the huntfor dark matter using Fermi. “Direct detec-tions and collider searches test differentaspects of dark matter and are comple-mentary to indirect searches like Fermi’s,”he says. “These three approaches probedifferent regions of dark matter spaces,cutting even more into realistic models.”Dark matter candidates may fall out offashion, but everything we know about theuniverse seems to require the substanceitself.“I’d say that in the next few yearsWIMPs could be more or less ruled out,but it’s quite plausible that dark matter ismade up of whole families of particles,”says Caputo. “Think how diverse the stan-dard model is — and it’s only 5 percent ofthe matter budget of the universe.”17Francis Reddyis the senior science writer forthe Astrophysics Science Division at NASA’sGoddard Space Flight Center in Greenbelt,Maryland.In 1932, Dutch astronomerJan Oort analyzed the verti-cal motions of stars near theplane of our galaxy and con-cluded the density of knownstars was too low, suggestingunseen matter near thegalactic plane was needed toexplain them.The following year, Swissastronomer Fritz Zwickyshowed that the motions ofgalaxies in the Coma Clusterimplied the presence ofmuch more mass than couldbe accounted for by its glow-ing galaxies alone. Zwickysuggested this extra masswas in the form of “dunkle(kalte) materie” — dark coldmatter. In 1936, Americanastronomer Sinclair Smithfound a similar discrepancy

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Term
Fall
Professor
Robbins

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