Four hundred years after Galileo Galilei pointed the first telescope at the stars, astronomers devised instruments that could look beyond what optical telescopes reveal. The Chandra X-Ray Observatory, launched on July 23, 1999, aboard the space shuttle Columbia, is a highly sophisticated descendant of the early X-ray instruments. Orbiting 85,000 miles above Earth and operated by the Smithsonian’s Astrophysical Observatory in Cambridge, Massachusetts, the telescope is designed to detect X-ray emissions from extraordinarily hot regions of the universe. Over nearly two decades, Chandra has revealed a universe of violent and extreme environments. With it, scientists have studied intense gravitational and magnetic fields around black holes, supernova shock waves, and titanic collisions between clusters of galaxies.
Text adapted from Chandra’s Cosmos: Dark Matter, Black Holes, and Other Wonders Revealed by NASA’s Premier X-ray Observatory by Wallace H. Tucker, to be published on March 28, 2017, by Smithsonian Books.
Lighting Up Dark Matter
The most graphic evidence for the existence of dark matter comes from Chandra observations that use a technique called gravitational lensing: when dense matter causes the nearby space to warp so light from background galaxies can be seen bending around it. This image of the Bullet Cluster shows hot, X-ray-producing gas as pink and optical light from stars as orange and white. The X-rays show that the Bullet Cluster is composed of two smaller clusters of galaxies colliding at high speeds in one of the most energetic events ever observed. Using gravitational lensing, astronomers have discovered that the heaviest mass concentrations in the clusters, shown in blue, are separate from the mass concentration of the hot gas. Scientists have interpreted the image to explain that as the blue clusters collided, the pink hot gas was slowed, but the dark matter kept on traveling because it interacts weakly, if at all, with itself or the gas except through gravity. The clusters of dark matter passed through one another like ghosts and moved beyond the hot gas.
Although such violent collisions between clusters are rare, Chandra’s observations of a half-dozen other clusters have shown results similar to the Bullet Cluster. Calculating the mass of dark matter compared to that of regular matter in these findings makes it very difficult to avoid the conclusion that most matter in the universe is dark.