Clearly, there's no easy answer when it comes to predicting the fate of the universe. But let's imagine for a moment that the density of the universe is above the critical value required to stop expansion. This would lead to the big crunch, which in many ways would be like hitting the rewind button on a VCR. As gravity within the universe pulled everything back, galaxy clusters would draw closer together. Then individual galaxies would begin to merge until, after billions of years, one mega-galaxy would form.
Inside this gigantic cauldron, stars would meld together, causing all of space to become hotter than the sun. Eventually, stars would explode and black holes would emerge, slowly at first and then more rapidly. As the end drew near, the black holes would suck up everything around them. Even they would coalesce at some point to form a monstrous black hole that would pull the universe closed like a drawstring bag. At the end, nothing would remain but a super-hot, super-dense singularity -- the seed of another universe. Many astronomers think the seed would germinate in a "big bounce," starting the whole process over again.
That's not the only theory. A few cosmologists, led by Paul J. Steinhardt of Princeton University and Neil Turok of Cambridge University, have recently argued that the big chill and the big crunch are not mutually exclusive. Their model works like this: The universe began with the big bang, which was followed by a period of slow expansion and gradual accumulation of dark energy. This is where we are today. What happens next is highly speculative, but Steinhardt and Turok believe that the dark energy will continue to accumulate and, as it does, will stimulate cosmic acceleration. The universe won't ever stop expanding, but will spread out over trillions of years, stretching all matter and energy to such an extreme that our one universe will be separated into multiple universes. Inside these universes, the mysterious dark energy will materialize into normal matter and radiation. This will trigger another big bang -- perhaps several of them -- and another cycle of expansion.
If you're disconcerted by all this talk of crunching and expanding, you can take comfort in knowing that the fate of the universe won't be determined for billions, maybe even trillions, of years. That gives you plenty of time to focus on things that are a bit more certain, such as your own life cycle of birth, growth and death.