Question: Is the universe heading towards a Big Freeze, a Big Rip, a Big Crunch or a Big Bounce? Or is it part of an infinitely recurring cyclic model?

James Holloway answered on 18 Mar 2013:

This depends on what the density of matter in our universe is.

If it is above the critical density, then there is enough stuff for gravity to win out and its a big bang -> big crunch / big bounce scenario.
If it is equal to the critical density, then there isn’t stuff for gravity to win out, it only slows the expansion of the universe (to 0 at infinite time) and its a big bang -> big freeze scenario.
If it is below the critical density, then there is enough stuff for gravity to win out and the expansion of the universe will speed up. This is a big bang -> big rip scenario.

We are still trying to get an accurate measurement of what the matter density of the universe is but it seems to be very close to the critical density – we just do not have the accuracy to say whether it is definitely above, below or equal to that value!

It would have to be the first scenario to be the cyclic model. Personally somehow this one makes the most sense to me, it means the universe has no true beginning or end and thus it wasn’t create, it has always been.