The ultimate fate of the universe is a topic in physical cosmology. Many possible fates are preicted by rival scientific theories, including futures of both finite and infinite duration. Once the notion that the universe started with a Big Bang became accepted by a consensus of scientists, the ultimate fate of the universe became a valid cosmological question, one depending upon the physical properties of the mass/energy in the universe, its average density, and the rate of expansion. By extension, the fate of the universe is also a significant theme in science fiction.