Saul Perlmutter, Brian Schmidt and Adam Riess for the discovery of the accelerating expansion of the universe. The expansion of the universe has been known for many years now. However Perlmutter, Schmidt and Riess, along with their supporting cast took those measurements to the next level.
Perlmutter and Schmidt investigated the light from Type Ia Supernovae explosions by setting up two rival teams to search for these energetic explosions. Riess, working for Schmidt, took the data and converted the amount of light we received from the explosion into what the Type Ia's intrinsic luminosity. With that in hand he could calculate the distance to these objects and compare that against their redshift. This allowed for an extremely precise measurement of the expansion of the universe and its evolution in time.
What they found was amazing. Instead of a fixed expansion like was expected from a simple Hubble Law, they found that the expansion changed overtime. It changed in such a way that as one got closer to the present time the universe was expanding faster and faster. Thus the universe's expansion was accelerating.
This discovery set off a flurry of activity in the cosmological community to explain this expansion as well as figure out what will happen if it continues. If the expansion continues to accelerate, and given the fact that the universe appears to have a flat geometry, it seems that the eventual fate of the universe is that we will expand forever to the point where we will no longer see any other stars in the sky. Naturally this will take hundreds of billions of years. This also assumes that the acceleration continues and does not reverse itself in the future. Regardless of the future fate of the now incorrectly named Hubble constant, we know for sure that the expansion is variable and currently accelerating.
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