Non-Gaussianity in the Cosmic Microwave Background Anisotropies at Recombination in the Squeezed limit

TL;DR

The paper addresses how second-order CMB anisotropies generated at recombination in the squeezed limit contaminate the local-type primordial non-Gaussianity. It introduces a coordinate-rescaling approach that links long-wavelength modulations to local changes of coordinates, enabling an analytic derivation of the recombination-era bispectrum and its impact on $f_{\rm NL}^{\rm loc}$. The main result is that the induced contamination is $f_{\rm NL}^{\rm con} = \mathcal{O}(1)$ (numerically about $0.9$), with an additional small (~0.3) contribution from other nonlinear effects, yielding a total below the sensitivity of current experiments. This work provides a clear physical picture for squeezed-limit NG generation and strengthens confidence that recombination-era nonlinearities do not masquerade as detectable primordial local NG in present data.

Abstract

We estimate analytically the second-order cosmic microwave background temperature anisotropies at the recombination epoch in the squeezed limit and we deduce the contamination to the primordial local non-Gaussianity. We find that the level of contamination corresponds to f_NL^{con}=O(1) which is below the sensitivity of present experiments and smaller than the value O(5) recently claimed in the literature.