For decades, astronomers have sought to discover the ultimate fate of the universe. Will the cosmos continue forever in its expansion, which began billions of years ago with the big bang? Or will gravity someday reverse the process, producing a "big crunch?" Two groups of astronomers have recently announced a discovery that seems to resolve the issue but that also shakes the science of cosmology to its very foundation: the expansion of the universe appears to be accelerating. The Runaway Universe is the story of these astronomers who have stood the world of cosmology on its ear—:and of their competitive race to discover the future of the universe.