Friedmann made two simple assumptions about the universe: that when viewed at large enough scales, it appears the same both in every direction and from every location. From these assumptions (called the cosmological principle) and Einstein’s equations, he developed the first model of a universe in motion. The Friedmann universe begins with a Big Bang and continues expanding for untold billions of years—that’s the stage we’re in now. But after a long enough period of time, the mutual gravitational attraction of all the matter slows the expansion to a stop. The universe then starts to fall in on itself, replaying the expansion in reverse. Eventually all the matter collapses back into a singularity, in what physicist John Wheeler likes to call the “Big Crunch.”

Dark Matter

Carlos Frenk: Where Is the Missing Matter?
 

 Although Friedmann found only this one solution, called a closed universe because the size of the universe is finite, two similar solutions exist. In an open universe, there’s not enough matter to bring the expansion to a halt. Galaxies continue to separate from one another, although more slowly as time passes. Eventually all the stars go out, and the universe becomes cold and dark. Intermediate between the open and closed universes is the flat universe. In this case, the universe expands forever, but the speed at which the galaxies separate eventually approaches zero. What kind of universe do we live in? Observations of the universe’s density should eventually tell us, but they are not yet accurate enough to distinguish among the three possibilities.