I promised earlier that I would talk about the Big Rip, but never got around to it, so here you go.
Since Edwin Hubble discovered in 1929 that the universe is expanding, cosmologists have suggested two possible ultimate fates for our currently-expanding universe: it could cease expanding and recollapse, eventually reaching zero size in a Big Crunch, or it could expand forever, but gradually more slowly (the Big Fizzle?). In a universe dominated by ordinary matter and radiation, these were really the only choices. Now we know that most of energy in the universe is some persistent dark energy that doesn’t diminish as the universe expands, and can continue to feed the expansion rate; therefore another possibility becomes likely, that the expansion will continue at a constant rate forever. Confusingly, a universe with constant expansion rate is said to be “accelerating,” because any individual galaxy appears to be accelerating away from us. That’s because Hubble’s Law says the recession velocity is the expansion rate times the distance (v = Hd), so a constant expansion rate H implies an increasing velocity v.
But there’s a lot we don’t know about dark energy, so it’s prudent to keep an open mind. The simplest dark energy model is an absolutely constant “vacuum energy,” but we can consider dynamical models in which the energy density is slowly decaying. Robert Caldwell and others have even suggested that the dark energy density (the amount of energy per cubic centimeter) might be increasing with time, a possibility he dubs phantom energy.
This opens the possibility of a Big Rip, in which the expansion rate increases without bound until it reaches infinity at some finite time in the future. This scenario was explored by Caldwell, Kamionkowski, and Weinberg. The consequences are dramatic: first galaxies, then stars and planets, then atoms and nuclei are ripped apart by the expansion of the underlying spacetime. The recent supernova results that were in the news indicate that the dark energy density is changing very slowly, if at all, so the Big Rip would have to be some time in the distant future (if, once again, at all).
It’s important to realize that the quoted numbers depend on an enormous extrapolation, one we have little reason to trust. Phantom energy density increases as the universe expands; but the above analyses made the simplifying assumption that the increase proceeded at a constant rate. In other words, imagine hopping in your car and accelerating to the speed limit, but then estimating your arrival time by guessing that your acceleration (rather than your speed) remains constant. You’d be able to get across the country awfully quickly if that were true; but it’s not a safe assumption.
The truth is, we can’t predict the future of the universe with any reliability at all until we understand much more about the underlying physics of the dark energy. If it is increasing, it might only do so temporarily, before leveling off to a constant value (see my paper with Hoffman and Trodden). If it’s currently a constant, it may decay completely away in the future. So, the choice between Crunch, Fizzle, and Rip is one we have no way to decide between right now. (But “Crunch, Fizzle, and Rip” is a great name for a band, or maybe a law firm.)