The 4% Universe: Dark Matter, Dark Energy, and the Race to Discover the Rest of Reality

by Richard Panek

Ostriker had been working on rotating celestial objects since he was a graduate student at Cambridge; he had written his thesis on rotating stars. Scientists had known since the nineteenth century that if you rotated an initially spherical liquid drop it would become oblate, increasingly so, and eventually compress into a bar shape. Ostriker had treated stars as liquid drops—as compressible objects—and found that they, too, would become oblate over time. Recently, he told Peebles, he had looked at a rendering of the Milky Way—a flat disk like the other spiral galaxies that astronomers had been collecting by the thousands. He could see at a glance that it should have become bar-shaped or broken up into two galaxies after one rotation. Yet by now the Milky Way was old enough to have completed a dozen rotations. “There’s something wrong here,” Ostriker said. Peebles agreed. He began work on creating an N-body simulation for the Milky Way. He laid the points into a spiral shape and set it rotating. Sure enough, it wobbled catastrophically during its first 200-million-year rotation. He and Ostriker would need something else to stabilize it: a surrounding mass of something to hold it together gravitationally. Something you couldn’t see with a telescope—at least not yet—but something that had to be there. Something that Peebles and Ostriker would now add to the computer program. For the first simulation of the rotation of the galaxy with this missing component, the amount of mass...