One of the more morbid questions that astronomers have debated over the last few decades has been the possibility of the end of the universe. With the widespread acceptance of a model of the universe that is in some way finite, there are questions about how the state of that universe could change over time. There are largely two options given the state of astrophysics, depending on the relative sizes of the forces of dark energy and gravity.
The first that was originally popularized was the “Big Crunch”, a sort of reverse big bang. This was rather romantic, as it provided a nice sort of symmetry, and also inspired thoughts of the crunch resulting in a new big bang, thus creating the possibility of an infinitely recreated universe. In this model, the forces of gravity eventually overwhelm the expansion of the universe, causing it to shrink in on itself, accelerating inward until all mass ends up in a singular point, the crunch.
The second is the “big rip”, or the “big freeze”. In this model, the forces causing the expansion of the universe are never reversed by gravity, and the finite amount of energy in the universe causes all matter eventually to be torn away from itself. This was called the “big freeze” because it means that all matter will eventually be isolated, unable to interact with all other matter, meaning the universe would eventually enter a static state, with each particle eternally separated from every other.