\delta N versus covariant perturbative approach to non-Gaussianity outside the horizon in multifield inflation

Abstract

We compute the super-Hubble evolution of non-Gaussianity of primordial curvature perturbations in two-field inflation models by employing two formalisms: delta N and covariant formalisms. Although two formalisms treat the evolution of fluctuations radically different, we show that the formulas of f_{NL} parameter agree quantitatively with each other within 1 % accuracy. We analytically find that the amplitude of f_{NL} decays no faster than a^{-3} as the inflationary trajectory reaches to the adiabatic limit for generic potentials.

Comments: v4: minor changes, version accepted for PRD; v3: 10 pages, 14 figures, added two examples that produce relatively large non-Gaussianity in Appendix to support the results; v2: 8 pages, 10 figures, added references