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

We formulate nonlinear perturbations of a scalar field dominated universe on super-horizon scales. We consider the case of a single scalar field. We take the gradient expansion approach. We adopt the uniform Hubble slicing and derive the general solution valid to $O({ϵ}^2)$, where $ϵ$ is the expansion parameter associated with a spatial derivative, which includes both the scalar and tensor modes. In particular, the $O({ϵ}^2)$ correction terms to the nonlinear curvature perturbation, which become important in models with a non-slowroll stage during inflation, are explicitly obtained.