AAS Post #6: The cosmological not-so-constant

What's powering the Universal acceleration? This may be the biggest question is modern cosmology, the study of the structure and evolution the Universe. We've known for nearly a century that the Universe is expanding. Distant galaxies appear to be moving away from us, indicating the Universe gets bigger every day. This was a hugely shocking result in the early 20th Century, but bigger shocks were in the works. In the 1990s, two independent teams of astronomers determined that not only is the Universe expanding, but that expansion is accelerating, growing. Not only is the Universe bigger every day, it's getting bigger faster. This was completely unexpected; everyone assumed that the gravity of all the combined matter in the Universe acted as a cosmic brake, slowing the expansion. Some people even theorized the gravity would grind the expansion to a halt, and the Universe would recollapse on itself in the dim ...