Evolution of the Universe and Critical Density
The concept is this: If there is enough matter in the universe, then its gravitational pull will eventually overtake the expansion of the universe and it will pull all matter back together again. Nothing will survive and the universe may end for ever (or return in a rebound!) What do they call this? The Big Crunch
If there is NOT enough matter, then the universe will continue to expand for ever. Stars will run out of fuel and be spread by enormous distances. The universe will grow expansive and dark and very, very cold. What do they call this? The Big Chill
It has always been thought that the universe had two main forms of energy: The positive expansive energy from the Big Bang, and the negative binding energy of gravity. Cosmologists have found that these two forces are EXTREMELY CLOSE. This appears to be one of the predictions of the Inflation Theory. The universe is at the critical density which has allowed it to expand for so many billions of years, and to continue expanding.
Critical Density
The most important quantity we need to measure to tell us about the fate
of our universe is the average density. The average density of the
universe will tell us whether it is great enough to reverse the expansion.
Cosmologists have determined that if the average density of our current
universe is greater than the critical density of
1 x 10-29 grams per cubic centimeter, then we have
enough mass to reverse the expansion and the universe will end as a
Big Crunch.
What makes this difficult to determine is that we know a LARGE FRACTION of the matter in our universe is hidden in dark matter (more on that in a bit..). Until we determine just how much dark matter there is, we can't determine the final fate of the universe!
However, current observations, even allowing for all the dark matter we can find, suggests we do not have enough mass to reverse the expansion of the universe. Ironically, mass and dark matter might not be only thing capable of effecting the space-curvature of the Universe. We now know about something called Dark Energy. See more here and here. This combining mass and energy, the universe appears to be FLAT.