Squeezed bispectrum from multi-field inflation with curved field space metric

Abstract

We investigate influences of the curved field-space metric of multi-field inflationary models on the squeezed bispectrum. The reduced bispectrum in squeezed limit is computed using the δN formalism. The calculation is performed under the slow-roll approximation and assumption that field derivative of the field-space metric is sufficiently small such that the contributions from Riemann tensor of the field-space can be approximately ignored. Based on these approximations, We compute the analytic expressions for the reduced bispectrum in squeezed limit, and find that, for such a nearly flat field-space metric, the field dependence of the metric can significantly alter both amplitude and shape of the reduced bispectrum. The reduced bispectrum from this nearly flat field-space metric can lead to spectral index of the halo bias which amplitude is 2 -- 4 times larger than that from the flat field-space model. This modification of the spectral index of the halo bias due to the curved field-space metric could leave observable imprints in future galaxy surveys.