Non-Gaussianities from Perturbing Recombination

TL;DR

This paper calculates the non-Gaussian features in the CMB caused by perturbations in recombination, highlighting a specific bispectrum signal that could be marginally detectable by Planck.

Contribution

It provides an approximate computation of the bispectrum from recombination perturbations, focusing on electron density fluctuations and their non-Gaussian signatures.

Findings

Bispectrum peaks on squeezed triangles near the first acoustic peak

Effective f_NL ~ -3.5 indicating potential detectability by Planck

Non-Gaussianity mainly due to delayed recombination and altered photon diffusion

Abstract

We approximately compute the bispectrum induced on the CMB temperature by fluctuations in the standard recombination epoch. Of all the second order sources that can induce non-Gaussianity during recombination, we concentrate on those proportional to the perturbation in the free electron density, which is about a factor of 5 larger than the other first order perturbations. This term induces some non-Gaussianity by delaying the time of recombination and by changing the photon diffusion scale. We find that the signal is not scale invariant, peaked on squeezed triangles with the smaller multipole around the scale of the first acoustic peak, and that its size corresponds to an effective f_NL ~ -3.5, which could be marginally detected by Planck if both temperature and polarization are measured.