
- Big Freeze
- Big Crunch
- Big Rip
- Evaporation
Big Freeze: Big Freeze is a condition when the universe may keep on expanding resulting in an absolute zero temperature. Here the entropy will continuously increase, leading to even distribution of everything in the universe. As the entire heat dies, this theory is also called as "Heat death". Big freeze takes place if the measured density of the universe is less than the critical density. But you never know when it's going to attain this state just from the study of today's universe. Theoretical work by Eric Chaisson and David Layzer concluded that the space time is expanding and thus it gives rise to an increasing entropy-gap saying the universe may end up with a Heat death.

Big Rip: is a state where the universe and every thing present in the universe are going to get ripped into small elementary particles due to the Phantom dark energy. According to the present expansion studies of the nearby supernovae, its been known that the universe is expanding at an accelerating rate and the dark energy is filling the space resulting in this expansion. So as the increase in the quantity of the dark energy speeds up the expansion, and will continue to expand forever.
The Fourth one, I call it the Evaporation, as stated at the very beginning. Whats bothering me is why it can't end up evaporating the whole matter into the medium around it and leaving just the vacuum in place of the present universe. May be you call it a wrong one just because the matter exists somehow in the evaporated state but again I have something to state at this point. Why are the other three theories been called possible when according to those theories also, the matter exists, even though in singularity and spread-up.
Thus there are evidences to say that each and every theory is a valid one and none is yet been proved. Lets wait till someone's gonna find it out or wait till the end of the universe ;)
Pranu :)