O-V-S-Z and friends: Non-Gaussianity from inhomogeneous reionization

TL;DR

This paper calculates the CMB bispectrum caused by inhomogeneous reionization, accounting for all relevant terms and matter-density correlations, and assesses its detectability and impact on primordial non-Gaussianity estimates.

Contribution

It provides a comprehensive calculation of the reionization-induced CMB bispectrum, including all first-order products and third-order matter correlations, which was not fully addressed before.

Findings

The bispectrum is of local type as expected.

Detection signal-to-noise ratios are low for typical reionization models.

Reionization can bias primordial non-Gaussianity estimates by approximately -0.1 to -0.2 in f_NL.

Abstract

We calculate the cosmic microwave background (CMB) bispectrum due to inhomogeneous reionization. We calculate all the terms that can contribute to the bispectrum that are products of first order terms on all scales in conformal Newtonian gauge. We also correctly account for the de-correlation between the matter density and initial conditions using perturbation theory up to third order. We find that the bispectrum is of local type as expected. For a reasonable model of reionization, in which the Universe is completely ionized by redshift z_{ri} ~ 8 with optical depth to the last scattering surface \tau_0=0.087 the signal to noise for detection of the CMB temperature bispectrum is S/N ~ 0.1 and confusion in the estimation of primordial non-Gaussianity is f_{NL} ~ -0.1. For an extreme model with z_{ri} ~ 12.5, \tau_0=0.14 we get S/N ~ 0.5 and f_{NL} ~ -0.2.