  We discuss the problem of initial states for a system of coupled scalar
fields out of equilibrium in the one-loop approximation. The fields consist of
classical background fields, taken constant in space, and quantum fluctuations.
If the initial state is the adiabatic vacuum, i.e., the ground state of a Fock
space of particle excitations that diagonalize the mass matrix, the
energy-momentum tensor is infinite at t=0, its most singular part behaves as
1/t. When the system is coupled to gravity this presents a problem that we
solve by a Bogoliubov transformation of the naive initial state. As a side
result we also discuss the canonical formalism and the adiabatic particle
number for such a system. Most of the formalism is presented for Minkowksi
space. Embedding the system and its dynamics into a flat FRW universe is
straightforward and we briefly address the essential modifications.
