What are the Big Freeze, the Big Crunch, and the Big Rip/Crackup?
The Big Freeze is one scenario that could describe the end of the Universe. In this scenario, everything in the Universe keeps getting farther and farther apart because of the Universe's expansion. When things get farther apart, they will become colder, because their atoms will no longer interact and cause fission and fusion, which create explosions, and thus energy and heat. So, in this scenario, when a star runs out of fuel, it dies, and that death does not support the beginning of another star, because there are no other particles nearby. So, one by one, the stars die, and they do not produce any heat, and the universe becomes very cold. This is the expected fate of our Universe given what we know about cosmology today.
The Big Crunch is what would happen if the Universe were more dense than the "critical density", which we now know it isn't. If the Universe were denser than this, eventually the gravitational force of the matter in the Universe would cause it to stop expanding (overcoming the force of dark energy), and then the Universe would begin to collapse. As it collapses, matter would be pushed closer and closer together, eventually collapsing back into one singularity. At this point it is possible under certain conditions that the collapse could "bounce" and the expansion could begin again. There is no enough matter in the Universe for the collapse to occur, there just isn't enough pulling it back together.
The Big Rip is an end scenario that most scientists do not believe will happen. The current theory of the cosmology states that the expansion is accelerating, but that it is accelerating at a constant rate. In the Big Rip scenario, the Universe's acceleration is increasing as time goes on. Eventually, it will be accelerating so fast that the fundamental forces (like the force that holds atoms together) will not be able to have an effect, and everything will rip apart. While this isn't theoretically disallowed, there is no evidence at present for this model.
This page was last updated by Mike Jones Feb 10th 2016