The oscillating universe theory is a modification of the famous big bang theory.
According to this theory, there are successive cycles of expansion and contraction that occurs in the universe and due to the contraction, the universe gets confined into a very small volume which has a large density.
When the universe gets confined in such a way then, it is said that a 'bounce' will occur and, it would thus oscillate between the big bang and the big crunch.
Thus, this theory states that for every big bang there is a big crunch.
