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

by Richard Panek

Forty years earlier Edwin Hubble had arrived at the evidence for an expanding universe by studying the behavior of galaxies. By tracing that expansion backward, as if running a film of the outward-flying galaxies in reverse, Georges Lemaitre had arrived at the idea of a primeval atom. Peebles hadn’t believed that the universe could be that simple, but now he was becoming one of the leading interpreters of a simple universe: homogeneous and isotropic—one that looked the same no matter where you were in it and no matter which way you looked. After Penzias and Wilson, as well as Peebles and his colleagues at Princeton, had found evidence for what those initial “Primeval Fireball”* conditions might actually have been, Peebles began using that knowledge to refine his understanding of the expansion itself. It was as if he were running the film of the history of the universe again, only instead of rewinding it to the beginning he would be running it forward to today.