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

by Richard Panek

In 1917, in considering the implications of general relativity, Einstein saw that the universe was inherently unstable. Just as Newton had invoked God to keep his version of the universe from collapsing, so Einstein added a symbol to his equations—arbitrarily, the Greek letter lambda, A. Whatever lambda was, it was counteracting gravity, because, in Einstein’s idea of a stable universe, something had to be. It was the reason that a universe full of matter attracting other matter through gravity wasn’t collapsing. After Hubble’s discovery of evidence for the expansion, the universe didn’t need lambda, and Einstein abandoned it. Unlike Newton’s God, however, you couldn’t altogether ignore it. Lambda was, after all, in the equation. What you could do instead was set lambda to zero. That’s what generations of observers and theorists had done. Sometimes they left the assumption implicit, simply failing to mention lambda. Often they stated the assumption explicitly: “Assume Λ = 0.” For most observers and theorists, lambda was there and it wasn’t there. It occupied a parallel existence, like a ghost in the attic.