Sagan said that if there's enough matter for the gravitational forces to stop the expansion of the universe and start the contraction (Big Crunch) until it reaches a critical point at which big bang occurs and it starts expanding again then the universe might be oscillating and undergo an infinite amounts of cycles in which the universe expands and contracts. But such theory in not reconciled with the second law of thermodynamics and caused cosmologists to abandon the oscillating universe model.

If the universe was cyclic and once started to contract in exactly the same way it expanded, it might be that causality will be inverted and effects will precede causes. Which might not be easily understandable to us where the time might seem to be going backwards. Also a question arises if every universe and the laws of nature are randomly determined or are always the same and perhaps the same cycle is repeated infinitely.
I'd like to believe that same guy like me will live in another cycle of the universe but I know that most likely the universe is open and in many many years there will be nothing but particles left in the universe. But we never know.