Nernst had a point. The notion of a beginning certainly raised the question of an end. Could an expanding universe keep on expanding forever, or would it stop? Would it recollapse? These were precisely the alternate scenarios defined by Einstein’s theory, and he and his contemporaries may well have been despondent at the emergence of hypotheses that seemed patently untestable. Foundations of science indeed.

It turns out the scientists need not have despaired. Their successors in the latter part of the century have filled in many details of what is now known as the Big Bang, to within the tiniest imaginable fraction of a second from time zero. Moreover, researchers are closing in on a definitive answer to the ultimate question, which has until now been most perplexing: Is there sufficient mass in the universe for gravity to halt Hubble’s expansion and reverse it?

At Mount William Observatory, UofC track star Edwin Hubble, AB'10, PhD'17, measured the "red-shift" of starlight from dist-ant galaxies to show an expand-ing universe--pav-ing the way for Big Bang theory.

With a series of clever experiments set for launch on satellites and even on balloons, astrophysicists around the world—including many from the University of Chicago, which has team members on most of these missions—have taken dead aim at an elusive and crucial number: omega, the fate of the universe. Appropriately named for the last letter of the Greek alphabet, this most-unknown unknown is a sort of dimensionless density—the ratio of the density of the universe to the theoretical “critical density” needed to arrest the Hubble expansion. If omega is just shy of unity, or 1, the universe expands forever; if just the other side, the universe collapses. Destiny hangs on a digit.

“There is probably no cosmological observation that is more significant than a determination of omega,” says Edward W. (“Rocky”) Kolb, professor of astronomy and astrophysics. “It’s the question for our time."

As the omega hunters head into the endgame, Kolb’s colleague Michael S. Turner, the Bruce and Diana Rauner distinguished service professor and chair of astronomy and astrophysics, describes the possible outcomes. If omega is greater than 1 (what Turner calls a “long-shot bet”), what happens is a Big Crunch: “The universe stops expanding, starts contracting, and it gets hotter than hell,” he says. The cosmos again becomes infinitely hot and dense as it was at the instant of the Big Bang.

If omega is less than 1, Turner warns of a Big Chill: “That’s the case that’s sort of depressing, because the universe expands forever. It gets colder. Eventually all the stars burn out. The next thing that happens—most of us believe that matter is unstable—all the material in the universe decays, so it becomes a pretty lonesome place. You’re left with black holes, photons, electrons. In a real sense, the universe kind of meets its end.”

How will the cosmologists find omega? In principal, Kolb says, it’s a simple thing to do: Just measure the total density of the universe, determine the critical density theoretically, and see whether their ratio is larger or smaller than 1. The problem, he continues, is that it’s hard to find the average mass density of the universe. “So, you want to weigh a galaxy? Where do you put the scale? How many galaxies are there?” Kolb shrugs. But two things have allowed the cosmologists to pursue omega and finally to corner it: First, they found that they actually could weigh the universe, or at least certain parts of it, and second, they realized that they didn’t even have to.