On second-order superhorizon perturbations in multifield inflationary models

We present a method for the study of second-order superhorizon perturbations in multifield inflationary models with nontrivial kinetic terms. We utilize a change of coordinates in field space to separate isocurvature and adiabatic perturbations generalizing previous results. We also construct second-order gauge-invariant variables related to them. It is found that with an arbitrary metric in field space the isocurvature perturbation sources the gravitational potential on long wavelengths even for 'straight' trajectories. The potential decouples from the isocurvature perturbations if the background fields' trajectory is a geodesic in field space. Taking nonlinear effects into account shows that, in general, the two types of perturbations couple to each other. This is an outline of a possible procedure to study nonlinear and non-Gaussian effects during multifield inflation.