  We present a general and model-independent method to obtain an effective
Markovian quantum kinetic equation for the expectation value of a slowly
evolving scalar field in an adiabatically evolving background from first
principles of nonequilibrium quantum field theory. The method requires almost
no assumptions about the field's interactions and the composition of the
background, except that 1) the coupling constants shall be small enough for
perturbation theory to be applicable, 2) there is a clear separation between
microphysical time scales and the rate at which bulk properties change, and 3)
higher time derivatives of the field remain small. The resulting Markovian
equation of motion is expressed in terms of an effective potential and friction
coefficients. Motivated by cosmological applications we focus on spatially
homogeneous and isotropic systems, but the approach could also be applied to
spatial gradients.
