Possible models of the expanding Universe

The expansion of the Universe is a key feature of the Big Bang cosmology and is a generic property of the universe we inhabit. Since Edwin Hubble"s observation in 1929, which led to the discovery of the expansion of the Universe, astronomers try to understand the behavior of it. Several models were created up today which shall explain the past and the future expansion:

A decelerating universe reaches its current size in the least amount of time. The universe could eventually contract and collapse into a "big crunch" or expand indefinetly. A coasting universe is older than a decelerating universe because it takes more time to reach its present size and expands forever. An accelerating universe on the other hand is older still. The rate of expansion actually increases because of a repulsive force that pushes galaxies apart.