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

by Richard Panek

Ever since Hubble’s discovery of evidence for an expanding universe, astronomers had known how to measure how much the expansion was slowing down, at least in principle. Hubble had used Henrietta Swan Leavitt’s period-luminosity relation for Cepheid variable stars to determine distances to nearby galaxies. And he had used the redshifts for those galaxies as equivalent to their velocities as they moved away from us. When he graphed those distances against those velocities, he concluded they were directly proportional to each other: the greater the distance, the greater the velocity. The farther the galaxy, the faster it was receding. This one-to-one relationship showed itself as a straight line on a 45-degree angle. If the universe were expanding at a constant rate, that straight line would continue as far as the telescope could see. But the universe is full of matter, and matter is tugging gravitationally on other matter, so the expansion can’t be uniform. At some far reach of the Hubble relation, the galaxies would have to deviate from the straight line. The graph of their points would begin to gently curve toward brighter luminosity. How much they deviated from the straight line would tell you how much brighter they were at that particular redshift than they would be if the universe were expanding at a constant rate. And how much brighter they were—how much nearer they were—would tell you how much the expansion was slowing.