Non-local bispectra from super cosmic variance

TL;DR

This paper demonstrates how super cosmic variance can produce non-Gaussian bispectra mimicking various templates, complicating the distinction between single and multi-field inflation scenarios due to cosmic variance effects.

Contribution

It provides examples of non-Gaussian statistics that can produce bispectra similar to local and non-local templates in biased sub-volumes, highlighting challenges in interpreting inflation models.

Findings

Super cosmic variance can induce bispectra matching various templates.

Biasing effects can influence power spectrum, bispectrum, or both.

Distinguishing single-field from multi-field inflation is more difficult than previously thought.

Abstract

We present examples of non-Gaussian statistics that can induce bispectra matching local and non-local (including equilateral) templates in biased sub-volumes. We find cases where the biasing from coupling to long wavelength modes affects only the power spectrum, only the bispectrum or both. Our results suggest that ruling out multi-field scenarios is quite difficult: some measurements of the scalar correlations, including the shape of the bispectrum, can be consistent with single-clock predictions even when cosmic variance from super-horizon modes is at work. Furthermore, if observations of the density perturbations rule out single-clock inflation, we will face a serious cosmic variance obstacle in drawing any further conclusions about the particle physics origin of the scalar fluctuations.