It's been nearly a century since scientists first theorized that the Universe was expanding, and that the farther away a galaxy was from us, the faster it appears to recede. This isn't because galaxies are physically moving away from us, but rather because the Universe is full of gravitationally-bound objects, and the fabric of space that those objects reside in is expanding. But this picture, which held sway from the 1920s onward, has been recently revised. It's been only 20 years since we first realized that this expansion was speeding up, and that as time goes on, individual galaxies will...