National Geographic : 2015 Jan
first glimpse 115 <1% Dark energy <1% <1% Dark energy 68% 13% Radiation <1% Matter 4% Dark matter <1% 86% 13% 12% 75% Dark matter 27% Radiation 99%* Matter <1% <1% <1% 1% 15% 84% Dark energy Composition of the universe Dark matter recent experimental results obtained with CERN’s Large Hadron Collider, near Geneva, Switzer- land, ruled out some versions of supersymmetry. The mood among the theorists, says one of them, is “fairly somber.” Rather than speculate about the precise identity of dark matter, most scientists on the hunt just say they’re looking for WIMPs, or “weakly interacting massive particles.” Evidence of just how weakly dark matter interacts not just with normal matter but also with itself has turned up three billion light-years from Earth, in the Bullet cluster—which is actu- ally two galaxy clusters in the act of colliding. Astronomers mapping the Bullet with the aid of NASA’s Chandra X-ray Observatory found massive clumps of hot gas at its center, which they attributed to collisions of clouds of normal matter. But when the astronomers charted the Bullet’s gravitational field, they discovered two more huge concentrations of mass, one for each of the original clusters, farther from the center of the collision. They concluded that although the normal matter frames of the two clusters were colliding and merging as spectacularly as two munitions trains, their heavier cargoes of dark matter were sailing through the carnage uninvolved and unscathed. Dark matter’s aloofness makes it challenging for experimenters to catch—even if, as some scientists estimate, dark matter particles are so commonplace that billions of them pass through every human being every second. The dark mat- ter detectors currently operating are so techno- logically sophisticated as to resemble Fabergé eggs constructed for the delectation of future archaeologists. One of them, the two-billion-dollar Alpha Magnetic Spectrometer, is perched on the Inter- national Space Station and hunts for evidence of dark matter particles colliding near the center of our galaxy. Most of the detectors, however, look for interactions between particles of dark matter The expansion slows 1 billion years after the big bang Stars clump into galaxies, galaxies into clusters along a scaffolding of dark matter. The mass of all matter, most of it dark, is so great that its gravity slows cosmic expansion. Dark energy rises 4-8 billion years after the big bang After slowing for billions of years, the expansion accelerates again. Why? A mysterious repulsive force, dubbed dark energy, has begun to counteract the pull of dark matter. Ever outward Today The universe hurtles outward toward an uncertain future. JASON TREAT, NGM STAFF. SOURCE: TOM ABEL *Percentages do not add up to 100 due to rounding.