The most commonly known theories are as follows: The Big Rip " ...a cosmological hypothesis about the ultimate fate of the universe, in which the matter of the universe, from stars and galaxies to atoms and subatomic particles, are progressively torn apart by the expansion of the universe at a certain time in the future. Theoretically, the scale factor of the universe becomes infinite at a finite time in the future." The Big Freeze "...a scenario in which the universe becomes too cold to sustain life due to continued expansion and the decay of free energy due to the action of entropy." The Big Crunch "...one possible scenario for the ultimate fate of the universe, in which the metric expansion of space eventually reverses and the universe recollapses, ultimately ending as a black hole singularity." The Big Bounce "...a theorized scientific model related to the creation of the known universe. It derives from the cyclic model or oscillatory universe interpretation of the Big Bang where the first cosmological event was the result of the collapse of a previous universe." To make the picture a little more simplistic, the first two theories (Rip & Freeze) involve the universe expanding at either a constant or an accelerating rate away from Earth. The latter theories (Crunch & Bounce) requires the expansion of the universe to be at a decelerating rate, an accelerating rate toward Earth, or no velocity at all. In the past, I have fancied the idea of the universe operating in the model of the Big Bounce. It's fun to think about, especially theorize about. I mean, if everything did concentrate itself into a black hole... like the cyclic theory proposes, don't you think it would rip the fabric of space time? And what happens then? Maybe... instead of starting a "new" cycle... we would come back and live our lives in exactly the same way... so it would never be a "new" cycle of measurable accord... it would be the same exact cycle, because the black hole kept ripping the fabric of space-time. It's a little hard to run with the cyclic theory, however, because scientists have found that the outer edges of the universe appear to be expanding at an accelerated rate, rather than slowing down. Which theory do you think is most likely?