Scale dependence of local f_NL

Abstract

We consider possible scale-dependence of the non-linearity parameter f_NL in local and quasi-local models of non-Gaussian primordial density perturbations. In the simplest model where the primordial perturbations are a quadratic local function of a single Gaussian field then f_NL is scale-independent by construction. However scale-dependence can arise due to either a local function of more than one Gaussian field, or due to non-linear evolution of modes after horizon-exit during inflation. We show that the scale dependence of f_NL is typically first order in slow-roll. For some models this may be observable with experiments such as Planck provided that f_NL is close to the current observational bounds.

19 pages, 1 figure; v3: acknowledgments of financial support added, no further changes