There was a time after Edwin Hubble discovered the universe was expanding that I think a lot of people thought the universe was doomed to end. Gone was the idea that the universe was static and eternal. So it was either the Big Freeze, or the Big Crunch. Either the great heat death of the universe, or gravity would eventually halt the expansion and everything would come crashing back down into a singularity again.
But then they found out that the expansion was actually accelerating, which lead to the concept of dark matter and revived Einstein’s idea of the cosmological constant, and I don’t think it’s anywhere as clear as to what the ultimate fate of the universe is.
True, the Big Freeze is still a viable possibility. All the matter and energy will get spread thin across the voluminous void of space, and it will become more and more difficult to exploit matter-energy gradients to perform useful work, and eventually the whole universe will reach a temperature close to absolute zero. Of course, because of quantum mechanical effects, the thermodynamic gradient will never be entirely zero, but nevertheless, the universe will be a cold, lightless place, and as close to static and eternal as Heisenberg’s uncertainty principle will allow it. Certainly, life as we know it will be impossible.
But it’s possible that that isn’t the entirety of the story. The horizon problem showed that the Big Bang theory by itself couldn’t explain the state of the observable universe, so Alan Guth came up with the inflation theory to explain why the universe looks essentially homogenous in all directions. The problem with inflation is we have no idea why it stopped. So there’s a possibility that we just live in a metastable pocket universe that could collapse at any moment, transforming either into true vacuum, or yet another metastable false vacuum with entirely different forces and constants. This would obviously end life as we know it, but the universe would continue on.
And there’s the possibility that inflation hasn’t really stopped, except for our tiny patch of observable universe, and that multitudes and multitudes of bubble universe are collapsing out of the an eternal inflaton field, inflation without end, Eternal Inflation.
Of course, then we get to string theory and the concept of branes, and the possibility that the Big Bang is just an illusion caused by the fact that we can only perceive three spatial dimensions and one temporal dimension, and that what we perceive as the Big Bang (or, more directly observe as cosmic microwave background radiation) is really just two 3-branes crashing into each other, and then bouncing away. And, like the idea of the Big Crunch, the mutual gravitational attraction of the two 3-branes will eventually halt the rebound, and force the two to come crashing back together again, initiating yet another Big Bang, annihilating this entropy-laden universe to create a new one, a process which is summarized by the ekpyrotic universe model.
Some of these concepts are actually testable (although we may not have the current technology to do so, and we probably don’t have the political will to fund the research to create the necessary technology.) While all these theories explain why the observable universe looks the way it does, they also make specific predictions. It’s likely that we’ll have an even better idea of whether or not the universe is a one-time thing, or whether it’s just one out of many (either spatially or temporally.) There’s something to be said about not knowing how it all ends.