Cosmic variance in inflation with two light scalars

TL;DR

This paper investigates how a light scalar field coupled to the inflaton affects the primordial bispectrum's squeezed limit, revealing that mode-coupling causes cosmic variance in fluctuation statistics but not in the inferred mass of the hidden field.

Contribution

It demonstrates that higher order correlation functions induce cosmic variance in the fluctuation statistics without altering the local mass measurement of the hidden scalar.

Findings

Mode-coupling causes statistical variation in sub-volumes.

The local bispectrum's squeezed limit remains unaffected.

Mass estimation of the hidden field is robust against cosmic variance.

Abstract

We examine the squeezed limit of the bispectrum when a light scalar with arbitrary non-derivative self-interactions is coupled to the inflaton. We find that when the hidden sector scalar is sufficiently light ($m\lesssim0.1\,H$), the coupling between long and short wavelength modes from the series of higher order correlation functions (from arbitrary order contact diagrams) causes the statistics of the fluctuations to vary in sub-volumes. This means that observations of primordial non-Gaussianity cannot be used to uniquely reconstruct the potential of the hidden field. However, the local bispectrum induced by mode-coupling from these diagrams always has the same squeezed limit, so the field's locally determined mass is not affected by this cosmic variance.