Gradient expansion approach to nonlinear superhorizon perturbations. II - A single scalar field -

Abstract

We formulate nonlinear perturbations of a scalar field dominated universe on superhorizon scales. We consider the case of a single scalar field. We employ the gradient expansion approach. We adopt the uniform Hubble slicing and derive the general solution valid to O(ε2), where e is the expansion parameter associated with a spatial derivative, which includes both the scalar and tensor modes. In particular, the O(ε2) correction terms to the nonlinear curvature perturbation, which are important in models with a non-slowroll stage during inflation, are explicitly obtained.