The Oscillatory Universe is the hypothesis, due to Richard Tolman, that the universe undergoes an infinite series of oscillations, each beginning with a big bang and ending with a big crunch. After the big bang, the universe expands for a while before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.

It was once popular amongst cosmologists who thought some force would prevent the formation of a gravitational singularity and connect the big bang to an earlier big crunch: the mathematical singularities seen in calculations were the result of mathematical over-idealizations and would be resolved by a more careful treatment. However, in the 1960s Stephen Hawking, Roger Penrose and George Ellis showed that singularities were a universal feature of cosmologies with a big bang and that no feature of general relativity could prevent them. Theoretically, the oscillating universe could not be reconciled with the second law of thermodynamics: entropy would build up from oscillation to oscillation and cause heat death. Other measurements suggested the universe is not closed. These arguments caused cosmologists to abandon the oscillating universe model.

The theory has been revived in brane cosmology as the cyclic model, which evades most of arguments levelled against the oscillatory universe in the sixties. Despite some success, the theory is still controversial, largely because there is no satisfactory string theoretic description of the bounce in this model.