Astrophysicists Jamal Islam and Freeman Dyson first theorized the Big Freeze in the 1970s and divided the universe into five eras from the birth of the universe, the primordial era, to the dark era, where everything has decayed. This theory deals with the continual expansion of the universe, which will lead it to gradually cooling until it approaches a temperature of absolute zero As time passes, stars slowly start to fade away while the distance between galaxies continues to grow due to the expansion. After the stars fade away due to lack of fuel, only black holes will remain until they disappear as well due to Hawking radiation. This continues until no further mechanical work is possible, which leads to the final death of the universe.
Another theory, the Big Rip, first published in 2003, theorizes that as the universe expands until all matter is torn apart. This largely stems from the effects of “dark energy” which pulls objects apart. There is evidence dark energy may get stronger as the universe expands. If dark energy gets to the point where it is strong enough to overcome gravity and nuclear forces, it will end up pulling these objects apart. This will lead to clusters of galaxies disbanding, and even all life on Earth being torn apart as the dark energy will overcome their forces of attraction, hence the name, the Big Rip.
The Big Crunch is different from the first two theories in that it deals with the eventual reversal of the universe where it will recollapse back into a black hole singularity. This deals the fact that the expansion speed of the universe may be slow enough that gravity will eventually stop the expansion and cause the universe to contract and implode upon itself. However, this will only occur if the density of the universe is sufficiently large such that the strength of gravity overcomes the expansion.
All of these theories are potentially valid, and will occur in the distant future, billions of years away. It all depends on how dense the universe is and the physical makeup of dark energy, both of which are still uncertain. There are many mysteries still out there dealing with the physics of the universe, and we will have more than enough time to address them, but maybe not solve them.
See:
http://www.universetoday.com/107316/what-is-the-big-rip/
http://hubblesite.org/hubble_discoveries/dark_energy/de-fate_of_the_universe.php
http://www.physicsoftheuniverse.com/topics_bigbang_bigcrunch.html
- Charles Wang
