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Very large primordial non-Gaussianity from multiple fields: application to massless preheating

In this paper we derive a generic expression which is valid for scales larger than the Hubble radius and contains only local terms, for the second order curvature perturbations for more than one field, provided that the expansion is sourced from the energy density of a single field. As an application, motivated by our previous paper (Enqvist et al, 2005 Phys. Rev. Lett. 94 161301), we apply our formalism to two fields during preheating, where the inflaton oscillations are sourced from a λφ4 potential which is governing the expansion of the Universe. A second field σ, coupled to the inflaton through g2φ2σ2, is excited from the vacuum fluctuations. The excited modes of σ amplify the super-Hubble isocurvature perturbations, which seed the second order curvature perturbations, giving rise to a significantly large non-Gaussianity. Our results show that within three inflaton oscillations for a range of parameters, 1 < g2/λ < 3, the non-Gaussianity parameter becomes , which is already ruled out by the current WMAP observation.