On the non-Gaussianity from Recombination

TL;DR

The paper addresses how non-linear recombination-era physics induces a non-Gaussian signal in the CMB that contaminates primordial equilateral NG measurements. Using a semi-analytic transfer-function model and a Fisher-matrix framework, the authors quantify the recombination contribution and its cross-talk with equilateral primordial NG for Planck-like experiments. They find the recombination signal is predominantly equilateral and can shift the detectable threshold by roughly $\Delta f^{\rm equil}_{\rm NL} \sim {\cal O}(10)$, moving from $f^{\rm equil}_{\rm NL} \approx 67$ to $\approx 79$. This work underscores the necessity of incorporating second-order recombination effects to accurately interpret NG constraints from CMB data and informs the expected precision of Planck-like analyses.

Abstract

The non-linear effects operating at the recombination epoch generate a non-Gaussian signal in the CMB anisotropies. Such a contribution is relevant because it represents a major part of the second-order radiation transfer function which must be determined in order to have a complete control of both the primordial and non-primordial part of non-Gaussianity in the CMB anisotropies. We provide an estimate of the level of non-Gaussianity in the CMB arising from the recombination epoch which shows up mainly in the equilateral configuration. We find that it causes a contamination to the possible measurement of the equilateral primordial bispectrum shifting the minimum detectable value of the non-Gaussian parameter f^equil_NL by Delta f^equil_NL= O(10) for an experiment like Planck.