MadSci Network: Physics |
In the simplest case (with no dark matter or dark energy), the fate of the
universe depends on the balance between 'kinetic energy' of the expansion
of the universe and the gravitational 'potential energy' of all galaxies
and stars pulling on each other. The kinetic energy is positive and the
potential energy is negative. If there is more kinetic energy than
potential energy at the time of the Big Bang, the 'total energy' of the
universe is positive and the universe will continue to expand forever. If
there is more potential energy at the time of the Big Bang, it will
eventually halt the expansion of the universe (that is, make the kinetic
energy zero), and then the universe would recollapse onto itself. The
expansion of the universe (the 'kinetic energy') is the expansion of space,
so spacetime has a positive energy at the time of the Big Bang.
One strange feature of 'dark energy' is that it violates the principle of
conservation of energy. If you have normal matter and you double the volume
it occupies, the density is halved (because no matter is created or
destroyed in the process). With dark energy, if you double the volume of
the universe, the density of dark energy remains the same, so the total
amount of dark energy doubles! This violation of conservation of energy is
actually okay with general relativity: energy is conserved locally, but
energy doesn't have to be conserved for the universe as a whole.
You can find other discussions of these issues here
and here.
Try the links in the MadSci Library for more information on Physics.