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

Abstract

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 $O({\epsilon }^2)$, where $\epsilon$ is the expansion parameter associated with a spatial derivative, which includes both the scalar and tensor modes. In particular, the $O({\epsilon }^2)$ correction terms to the nonlinear curvature perturbation, which become important in models with a non-slowroll stage during inflation, are explicitly obtained.

13 pages, no figures, typos corrected

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