Non-Gaussian Mode Coupling and the Statistical Cosmological Principle

Abstract

Local-type primordial non-Gaussianity couples statistics of the curvature perturbation \zeta on vastly different physical scales. Because of this coupling, statistics (i.e. the polyspectra) of \zeta in our Hubble volume may not be representative of those in the larger universe -- that is, they may be biased. The bias depends on the local background value of \zeta, which includes contributions from all modes with wavelength k ~< H_0 and is therefore enhanced if the entire post-inflationary patch is large compared with our Hubble volume. We study the bias to locally-measured statistics for general local-type non-Gaussianity. We consider three examples in detail: (i) the usual fNL, gNL model, (ii) a strongly non-Gaussian model with \zeta ~ \zeta_G^p, and (iii) two-field non-Gaussian initial conditions. In each scenario one may generate statistics in a Hubble-size patch that are weakly Gaussian and consistent with observations despite the fact that the statistics in the larger, post-inflationary patch look very different.