There are two very interesting theories on the end of the
universe:
1:
Heat Death. If the
Hubble Constant (the rate at which the universe is expanding) is greater than the
universal gravitation constant (the accelleration force that acts on the universe to slow expansion and draw itself back into it's center), then the universe will expan forever. This theory is also called an
open universe theory, because it means that the universe is free to exist forever. As the universe expands, eventually all the
hydrogen,
helium, and other light
elements will be used up as fuel for the last remaining stars. When all nuclear fusion of stars and other celestial objects stops, the universe will exist at only a few billionths of a degree Kelvin. Then (per
Stephen Hawking's theories), the
gluon bonds in protons and nutrons will break down, leaving the universe a sea of
quarks. Afterward, the quarks would break down into tiny ammounts of energy, and the universe would exist for eternity filled with zero matter close to, if not at,
Absolute Zero.
2:
The Giant Crunch. If the aforementioned Hubble Constant is less than the universal gravitation constant, the universe will eventually stop expanding, and begin to
contract. The universe would compress into one point in three-dimensional space.
Einstein stated that
matter and
energy cannot be created or destroyed, so when the universe converges on itself, it will have exactally the same properties as it did when the universe began (the same ammount of matter and energy, though not in the same proportions because matter and energy can be changed from one form to the other.) Then, if all goes right, the universe will start over. This theory is called a
closed universe theory because the universe can only expand to a certain size before compressing.
Either way, I won't be around to see it. If it's a choice between living in a universe at 0 Kelvin, living in a universe that occupies one 3-d point, or dying, I'd chose dying any
day.