A Short History of Big Bang Theory
Timeline & the Future:
Until 1998, astronomers thought that the expansion of the universe ought to be slowing. The initial impulse from the Big Bang must be being retarded by the inexorable pull of universal gravity. The universe may still collapse, even with a non-zero cosmological constant at work, so long as the density of matter in space is high enough. If the density of matter in space is higher than the Friedmann density, (density = 3H2/8piG), the universe must collapse. Science fiction mavens have speculated that there could be a 'big-bounce' or some kind of rebirth of the universe after a big crunch, even infinite big-bounces.
However, the data of 1998 suggests that, because the universe seems to be accelerating in its expansion, in this epoch in any case, the universe will expand forever to become a cold, dark, boring place in the distant future. A simple timeline of the universe up until now:
Dependant upon the geometry of spacetime, we are living in one of four universes:
If the actual density of the universe is greater than critical, the universe will be closed and begin contracting at some point in the future. If the density is actually equal to the Friedmann density, the geometry of the universe will be in principle open, and the energy from the big bang will be just enough to overcome the gravity between massive objects at large. If we're living in an open universe, the cosmological constant will drive apart all matter in the universe that is not already close together.