Illustration depicting three theories of the evolution of the universe. At the top is the classical 'big bang' theory, the universe was created in one moment from which it will constantly expand. If the total mass of the universe is below a critical value, it will continue to expand forever. With a total mass well above the critical value, the universe becomes 'cylcic' (bottom row). Gravity eventually forces the universe to collapse back on itself, leading to a singularity and another 'big bang' type expansion. At the critical mass value, it is thought that gravity and expansion balance, leading to a 'steady state' universe (middle row).