Large non-Gaussianity in multiple-field inflation

Abstract

We investigate non-Gaussianity in general multiple-field inflation using the formalism we developed in earlier papers. We use a perturbative expansion of the non-linear equations to calculate the three-point correlator of the curvature perturbation analytically. We derive a general expression that involves only a time integral over background and linear perturbation quantities. We work out this expression explicitly for the two-field slow-roll case, and find that non-Gaussianity can be orders of magnitude larger than in the single-field case. In particular, the bispectrum divided by the square of the power spectrum can easily be of O(1-10), depending on the model. Our result also shows the explicit momentum dependence of the bispectrum. This conclusion of large non-Gaussianity is confirmed in a semi-analytic slow-roll investigation of a simple quadratic two-field model.

21 pages, 9 figures. v4: Minor textual changes to match published version. In addition, and superseding the published version, a small error in X and X-bar has been corrected; no significant changes to the final results. Note that an extended (no slow roll) numerical treatment superseding section V.D is available in astro-ph/0511041