The primordial "f_NL" non-Gaussianity, and perturbations beyond the present horizon

TL;DR

This paper investigates how primordial non-Gaussianity, specifically the f_NL parameter, affects CMB anisotropy predictions for different primordial power spectra, constraining models with low wavenumber transitions.

Contribution

It analyzes the impact of primordial non-Gaussianity on CMB anisotropy for various power spectrum models, deriving constraints on the transition scale and spectral index.

Findings

Negative running spectral index models are favored.

Transition scale must satisfy log(k_c/k_0)> -184.

Large f_NL can cause unphysically high CMB anisotropy.

Abstract

We show a primordial non-linear "f_NL" term may produce unphysically large CMB anisotropy for a red-tilted primordial power spectrum (n<1), because of coupling to primordial fluctuation on the largest scale. We consider a primordial power spectrum models of a running spectral index, and a transition at very low wavenumbers. We find that only negative running spectral index models are allowed, provided that there is no transition at a low wavenumbers (i.e. k<<1). For models of a constant spectral index, we find log(k_c/k_0)> -184, at 1 sigma level, on the transition scale of sharp cut-off models, using recent CMB and SDSS data.