The arrow of time arises from the universe being far from equilibrium in a state of low entropy . The Second Law of Thermodynamics requires that the entropy of the universe does not decrease . A homogeneous universe with a perfect fluid expands adiabatically with constant total entropy : this means that the total entropy remains constant, but it could be converted into entropy variation due to irreversibility (entropy production, also called entropy generation), which is always a growing quantity. This is a usual case in an adiabatic system in Engineering thermodynamics, as for example a real mass flow in a pipe or in a turbine: in this science this kind of irreversibility is usually named internal irreversibility .