Crinkles in the last scattering surface: Non-Gaussianity from inhomogeneous recombination

TL;DR

This paper investigates how inhomogeneous recombination causes non-Gaussian features in the CMB bispectrum, finding that electron density perturbations can produce signals comparable to primordial non-Gaussianity.

Contribution

It provides the first calculation of the bispectrum from electron density perturbations during recombination, highlighting its potential significance.

Findings

Bispectrum peaks for squeezed triangles.

Amplitude of non-Gaussianity comparable to fNL of 0.05 to -1.

Shape differs from primordial non-Gaussianity.

Abstract

The perturbations in the electron number density during recombination contributes to the Cosmic Microwave Background bispectrum through second order terms. Perturbations in the electron density can be a factor of ~5 larger than the baryon density fluctuations on large scales as shown in the calculations by Novosyadlyj. This raises the possibility that the contribution to bispectrum arising from perturbations in the optical depth may be non-negligible. We calculate this bispectrum and find it to peak for squeezed triangles and of peak amplitude of the order of primordial non-Gaussianity of local type with fNL of 0.05 ~ -1 depending on the l-modes being considered. This is because the shape of the bispectrum is different from the primordial one although it peaks for squeezed configurations, similar to the local type primordial non-Gaussianity.