Non-Gaussianities from ekpyrotic collapse with multiple fields

Abstract

We compute the non-Gaussianity of the curvature perturbation generated by ekpyroticcollapse with multiple fields. The transition from the multi-field scaling solution to asingle-field dominated regime converts initial isocurvature field perturbations to an almostscale invariant comoving curvature perturbation. In the specific model of two fields,ϕ1 andϕ2, with exponentialpotentials, −Viexp(−ciϕi), we calculate the bispectrum of the resulting curvature perturbation. We find that thenon-Gaussianity is dominated by non-linear evolution on super-Hubble scales and hence isof the local form. The non-linear parameter of the curvature perturbation is given byfNL = 5cj2/12,where cj is the exponent of the potential for the field which becomes subdominant at late times. Sincecj2 must be large, in order to generate an almost scale invariant spectrum, the non-Gaussianityis inevitably large. Thus, the model is strongly constrained by observational bounds on thespectral index and non-Gaussianity.