Illustration of three theories of the origin and nature of the universe. At top is the Big Bang and expanding universe. Matter formed after the Big Bang (far left), the initial expansion of the universe from an infinitely compact state 13.8 billion years ago. On the largest scale, galaxies are observed to be moving away from each other. This is due to the expansion of the universe. At centre is the oscillating, or cyclic, universe. Following the Big Bang there is enough mass to reverse the expansion towards a Big Crunch. This is then followed by a rebound and a new cycle of existence. Between each cycle is a singularity. At bottom is the steady-state theory, where the Universe has no beginning or end. It is constantly expanding and maintains a constant density through the formation of new matter.