Fate of the Universe - Astronomy

Which scenario will be worse for the ultimate fate of the universe?

Well, that's a wee bit opinionated. It depends on your point of view - that is, how you take a liking to the following two scenarios. Everyone has their own opinion; from the details I give below, I'll let you answer that yourself.

To infinity - and . . . well, more infinity: The perpetually expanding universe

There are actually three types of universes that could fit this model: The accelerating expansion one, the constant expansion one, and the decelerating expansion one. At this point in time, we appear to be in a universe undergoing accelerated expansion, thanks to dark energy. However, all three scenarios give us a similar end result: A dead universe.

In the far future, as long as the universe does not curl back into a Big Crunch, slowly all processes as we know them will begin to end. Star formation will eventually cease, and black holes will slowly start to gobble up a lot of matter. In the even farther future, black holes are evaporate to decay via Hawking radiation, leaving the universe a collection of various subatomic particles. (If protons decay, then perhaps there will no longer be any atoms). The temperature of the universe will drop until everything is about thermodynamically equivalent. Not a great place to be (well, there won't be any life, so nobody will be around to see it, but still. . .).

Beyond this. . . well, there are a few different ideas regarding what could happen. The Big Freeze idea states that the universe will eventually just cool off to this low-temperature state. The Big Rip idea says that matter itself could be torn apart bye extreme expansion of space. Cosmological false vacuum theories suggest that quantum tunneling effects could make some interesting expansion possible. However, I emphasize that we do not know whether any of these long-term theories are correct. They are too far in the future, and there is too much uncertainty among the relevant factors.

In the short term, however, it appears our universe will expand forever.

Back to the beginning: The Big Crunch universe and all its friends

The Big Crunch scenario says that if the density of the universe is greater than the critical density, the universe will fall back onto itself into a singularity - just like the Big Bang. This leads to other theories, such as the epykrotic universe, or a Big Bounce, which postulate that our current universe is just undergoing one of infinitely many stages of expansion and collapse.

This isn't exactly a great outcome, either, but again, I'll leave you to judge.