This month's provocative results on the acceleration of the universe raise an interesting issue: what can we say about our universe's ultimate fate? In the old days (like, when I was in grad school) we were told a story that was simple, compelling, and wrong. It went like this: matter acts to slow down the expansion of the universe, and also to give it spatial curvature. If there is enough matter, space will be positively curved (like a sphere) and will eventually collapse into a Big Crunch. If there is little matter, space will be negatively curved (like a saddle) and expand forever. And if the matter content is just right, space will be flat and will just barely expand forever, slowing down all the while.
This story is wrong in a couple of important ways. First and foremost, the assumption that the only important component of the universe is ...
