Beyond the Hubble scale: Super cosmic variance and nongaussianity as a portal to the superhorizon

Abstract

If cosmological perturbations in our Hubble sized volume are nongaussian, then they will be coupled to any larger perturbation modes outside our Hubble volume. This has important consequences for modeling inflation: the scalar power spectrum and spectral index measured in our Hubble volume would depend on an adjacent background of super Hubble perturbations. In other words, a detection of nongaussianity implies a possible portal to the superhorizon. By the same token, ruling out nongaussianity would rule out the possibility that the power spectrum's size and running are accidents of super cosmic variance. In this note, we provide a compact derivation of super cosmic variance, survey recent results, and show how experimental limits on nongaussianity help to rule it out.