Dynamics of interacting scalar fields in expanding space-time

The effective equation of motion is derived for a scalar field interacting with other fields in a Friedmann-Robertson-Walker background space-time. The dissipative behavior reflected in this effective evolution equation is studied both in simplified approximations as well as numerically. The relevance of our results to inflation are considered both in terms of the evolution of the inflaton field as well as its fluctuation spectrum. A brief examination also is made of supersymmetric models that yield dissipative effects during inflation.