Scale dependence of local f_NL

Understand

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.