If the acceleration of the universe continues in many billion years galaxies beyond the Milky Way and its companions will become invisible because their apparent velocity as seen from Earth would exceed the speed of light. In some models dark energy would rip apart all galaxies and solar systems, and eventually become so strong that it could tear apart atoms and even nuclei. The universe would end in what has been called the “Big Rip.” In other models gravity would again take over, causing the universe to contract and end in the “Big Crunch.” More measurements of acceleration are needed to decide between these two ultimate ends.