In some theories, dark energy is not the cosmological "constant" of Einstein's original formulation, but varies over time and space. If dark energy grows, it will cause the universe to unravel in about 20 billion years in a crescendo called the "Big Rip". First galaxies, then stars, and finally atoms will be torn asunder by dark energy. Nothing can survive; it's an outcome of crushing finality. Absent the big rip, cosmic acceleration will steadily remove galaxies from view. After 100 billion years, most galaxies will recede faster than the speed of light, leaving frozen final images on the edge of our horizon as if at the boundary of a black hole. The Milky Way and Andromeda galaxies will merge and our view of the universe will end at the edge of this super-galaxy. On even longer time scales, familiar gravitational structures become unglued. In about 10^15 years, planets detach from their dead stars and drift through interstellar space. In about 10^19 years stars detach from galaxies and float off into intergalactic space. In most theories that unify fundamental particles in terms of a single super-force, the proton is not stable and will decay in something like 10^35 years. This vast time scale is to the age of the universe what the age of the universe is to a millisecond. The decay of protons heralds a final drawn-out phase of disintegration in the universe, as everything falls apart. After protons decay, there are no stable atoms, presenting a challenge for life.