The Fate of the Universe

The past history of the Universe is one of an early, energetic time. As the
Universe expanded and cooled, phenomenon became less violent and more
stable.
This ruling law of Nature during the evolution of the Universe has been
entropy, the fact that objects go from order to disorder. There are local regions
of high order, such as our planet, but only at the cost of greater disorder
somewhere nearby.
If the Universe is open or flat (as our current measurements and theories
suggest) then the march of entropy will continue and the fate of our Universe is
confined to the principle of heat death, the flow of energy from high regions to
low regions.

With this principle in mind, we predict the future of the Universe will pass
through four stages as it continues to expand.
The Stellar Era is the time we currently live in, where most of the energy of the
Universe comes from thermonuclear fusion in the cores of stars. The lifetime of
the era is set by the time it takes for the smallest, lowest mass stars to use up
their hydrogen fuel.
The lower mass a star is, the cooler its core and the slower it burns its hydrogen
fuel (also the dimmer the star is). The slower it burns its fuel, the longer it lives
(where `live' is defined as still shining). The longest lifetime of stars less than
1/10 a solar mass (the mass of our Sun) is 1014 years.

New stars are produced from gas clouds in galaxies. However, 1014 years is
more than a sufficiently long enough time for all the gas to be used up in the
Universe. Once the gas clouds are gone, all the matter in the Universe is within
stars.
Degenerate Era :
Once all the matter has been converted into stars, and the hydrogen fuel in the
center of those stars has been exhausted, the Universe enters its second era, the
Degenerate Era. The use of the word degenerate here is not a comment on the
moral values of the Universe, rather degenerate is a physical word to describe
the state of matter that has cooled to densities where all the electron shell orbits
are filled and in their lowest states.
During this phase all stars are in the form of white or brown dwarfs, or neutron
stars and black holes from previous explosions. White and brown dwarfs are
degenerate in their matter, slowly cooling and turning into black dwarfs.

During this era, galaxies dissolve as stars go through two-body relaxation.
Two-body relaxation is when two stars pass close to one another, one is kicked
to high velocity and leaves the galaxy, the other is slowed down and mergers
with the Galactic black hole in the center of the galaxy's core. In the end, the
Universe becomes filled with free stars and giant black holes, leftover from the
galaxy cores.

The Universe would evolve towards a vast soup of black dwarf stars except for
process known as proton decay. The proton is one of the most stable
elementary particles, yet even the proton decays into a positron and a meson on
the order of once per 1032 years. Thus, the very protons that make up black
dwarf stars and planets will decay and the stars and planets will dissolve into
free leptons. This all takes about 1037 years.
Black Hole Era :
Once all the protons in the Universe have decayed into leptons, the only
organized units are black holes. From Hawking radiation, we know that even
black holes are unstable and evaporate into electrons and positrons.