Non-Gaussian perturbations from multi-field inflation

We show how primordial bispectrum of density perturbations from inflation may be characterized in terms of manifestly gauge-invariant cosmological perturbations at second order. The primordial metric perturbation, ζ, describing the perturbed expansion of uniform-density hypersurfaces on large scales is related to scalar field perturbations on unperturbed (spatially flat) hypersurfaces at first and second order. The bispectrum of the metric perturbation is thus composed of (i) a local contribution due to the second-order gauge transformation, and (ii) the intrinsic bispectrum of the field perturbations on spatially flat hypersurfaces. We generalize previous results to allow for scale dependence of the scalar field power spectra and correlations that can develop between fields on super-Hubble scales