Impact of polarisation on the intrinsic CMB bispectrum

TL;DR

This paper calculates the intrinsic bispectrum of the CMB including polarization, showing it enhances detection prospects and biases primordial non-Gaussianity measurements, with implications for future CMB experiments.

Contribution

It provides the first detailed computation of the intrinsic CMB bispectrum with polarization using second-order Boltzmann code SONG, highlighting its impact on non-Gaussianity analysis.

Findings

Adding polarization increases the signal-to-noise ratio fourfold.

The intrinsic bispectrum can bias local non-Gaussianity measurements by f_NL=0.66.

Lensing reduces the bispectrum signal and affects detectability.

Abstract

We compute the bispectrum induced in the cosmic microwave background (CMB) temperature and polarisation by the evolution of the primordial density perturbations using the second-order Boltzmann code SONG. We show that adding polarisation increases the signal-to-noise ratio by a factor four with respect to temperature alone and we estimate the observability of this intrinsic bispectrum and the bias it induces on measurements of primordial non-Gaussianity. When including all physical effects except the late-time non-linear evolution, we find for the intrinsic bispectrum a signal-to-noise of $S/N=3.8,\,2.9,\,1.6$ and $0.5$ for, respectively, an ideal experiment with an angular resolution of $\ell_\text{max}=3000$, the proposed CMB surveys PRISM and COrE, and Planck's polarised data; the bulk of this signal comes from the $E$-polarisation and from squeezed configurations. We discuss how CMB lensing is expected to reduce these estimates as it suppresses the bispectrum for squeezed configurations and contributes to the noise in the estimator. We find that the presence of the intrinsic bispectrum will bias a measurement of primordial non-Gaussianity of local type by $f_{NL}^\text{bias}=0.66$ for an ideal experiment with $\ell_\text{max}=3000$. Finally, we verify the robustness of our results by reproducing the analytical approximation for the squeezed-limit bispectrum in the general polarised case.