|The Expanding Universe|
When we look at the galaxies in the Universe around us, we find a fascinating thing: nearly every galaxy appears to be moving away from us. Also, the further away a galaxy is, the faster it is moving away from us. This fact was first determined in 1929 by Ed Hubble (for whom the Hubble Space Telescope is named), and it marks the starting point for modern cosmology.
That galaxies appear to be moving away from us at faster and faster speeds the further out we look is a sign that our universe is expanding. It's not that there was some explosion in space that sent galaxies flying away from us; instead, space itself is expanding, carrying all galaxies away from one another. An observer in any galaxy would see every other galaxy rushing away from them; it's not that we are at the center of the expansion (and in fact, there is no center of the expansion).
If space is expanding, let's think about what happened in the past. If we "run the clock backwards" space gets smaller and smaller until you have gone back so far in time that space occupies an infinitesimally small point -- this is the time of the Big Bang, the beginning of the universe. When did this happen? If we knew the expansion history of the Universe, we could solve for the universe's age. There are three dominant factors which control the expansion:
If we can determine these three parameters, we can solve for the age of the Universe, as well as determine its ultimate outcome. Will it expand forever? Will it ultimately recollapse?