  Any one-field inflation is actually realized in a multifield configuration
because the inflaton must have couplings with other fields to reheat the
universe and is coupled to all other fields at least gravitationally. In all
single inflaton models, it is explicitly or implicitly assumed that the heavier
fields are stuck to their potential minima during inflation, which are
time-dependent in general. We present a formalism to calculate curvature
perturbations in such a time-dependent background and show that the proper
expression can be obtained using a single-field analysis with a reduced
potential in which all these heavy fields are situated at their respective,
time-dependent minima. Our results provide a firm ground on the conventional
calculation.
