National Geographic : 2015 Jan
118 national geographic • january 2015 Dark matter Distant galaxy Distorted image of single distant galaxy Light from distant galaxy matter. The Large Hadron Collider, scheduled to resume operations in 2015 after a shutdown for maintenance and upgrades, may attain high enough energy levels to produce a few dark matter particles. But the odds are difficult to es- timate, because the masses of the sought-after particles are not well understood. WIMP hunting is not for wimps. Weird as the dark matter riddle may be, it looks almost pedestrian in comparison with the mysterious phenomenon of dark energy, which physicist Steven Weinberg calls the “central problem for physics” and astrophysicist Michael Turner nominates as the “most profound mys- tery in all of science.” Turner coined the term “dark energy” after two teams of astronomers announced in 1998 that the rate at which the universe was expand- ing appeared to be accelerating. The astronomers reached this conclusion by studying a particular and normal matter here on Earth. They’re buried deep underground, to minimize intrusions by high-velocity normal matter particles flying in from space. Some consist of a supercooled set of crystals or a tank of liquid xenon or argon surrounded by detectors and onionskin layers of shielding materials, ranging from polyethylene to copper to lead. (Recently mined lead tends to be mildly radioactive, so two experiments—one in Soudan, Minnesota, and the other in L’Aquila, Italy—use the melted-down ballast of ancient Roman shipwrecks. Mined thousands of years ago, the old lead emits less radioactivity.) America’s Large Underground Xenon detec- tor, the most sensitive of its kind, is situated in Lead, South Dakota, right off Main Street and 4,850 feet down by elevator. It started operat- ing in 2013 but came up empty-handed; it’s currently resuming the search at a higher sen- sitivity. Other searches produced ghostly clues, but none has found definitive evidence of dark It’s Dark, but It Bends Light The gravity of a massive object can bend light the way a lens does, as Einstein first realized. Dark matter, though invisible, can reveal its presence when its gravitational force greatly distorts the image of a distant galaxy.