In these models, the universe starts with a Big Bang and expands for some time, with increasing entropy and a thermodynamic arrow of time pointing in the direction of the expansion.
After the universe reaches a low-density state, it recontracts, but entropy now decreases, pointing the thermodynamic arrow of time in the opposite direction, until the universe ends in a low-entropy, high-density Big Crunch.
There are two models of the universe which support the possibility of a reversed direction of time.
The second, a Gold Universe, posits that entropy will increase only until a moment of contraction, then gradually decrease.
The Gold model has been linked to the possibility of retrocausal change, questions concerning the preservation of information in a time-reversed universe (states of decreasing entropy), and causation in general.