  The puzzle of the thermodynamic arrow of time reduces to the question of how
the universe could have had lower entropy in the past. I show that no special
entropy lowering mechanism (or fluctuation) is necessary. As a consequence of
expansion, at a particular epoch in the history of the universe a state that
was near maximum entropy under the dominant short range forces becomes
extremely unlikely, due to a switchover to newly dominant long range forces.
This happened at about the time of decoupling, prior to which I make no
statement about arrows. The role of cosmology in thermodynamics was first
suggested by T. Gold.
