Weak Lensing of the Primary CMB Bispectrum

TL;DR

Weak gravitational lensing modifies the CMB bispectrum by smoothing and reducing its amplitude, significantly impacting the estimation of primordial non-Gaussianity parameters like f_NL in current and future experiments.

Contribution

This paper quantifies the lensing effect on the CMB bispectrum and highlights its importance for accurate non-Gaussianity measurements, which was not fully accounted for previously.

Findings

Lensing decreases the bispectrum amplitude by smoothing between multipoles 100 and 2000.

Neglecting lensing biases the f_NL estimate low by about 6% for WMAP.

For Planck, lensing increases the minimum detectable f_NL from 5 to 7.

Abstract

The cosmic microwave background (CMB) bispectrum is a well-known probe of the non-Gaussianity of primordial perturbations. Just as the intervening large-scale structure modifies the CMB angular power spectrum through weak gravitational lensing, the CMB primary bispectrum generated at the last scattering surface is also modified by lensing. We discuss the lensing modification to the CMB bispectrum and show that lensing leads to an overall decrease in the amplitude of the primary bispectrum at multipoles of interest between 100 and 2000 through additional smoothing introduced by lensing. Since weak lensing is not accounted for in current estimators of the primordial non-Gaussianity parameter, the existing measurements of $f_{\rm NL}$ of the local model with WMAP out to $l_{\rm max} \sim 750$ is biased low by about 6%. For a high resolution experiment such as Planck, the lensing modification to the bispectrum must be properly included when attempting to estimate the primordial non-Gaussianity or the bias will be at the level of 30%. For Planck, weak lensing increases the minimum detectable value for the non-Gaussianity parameter of the local type $f_{\rm NL}$ to 7 from the previous estimate of about 5 without lensing. The minimum detectable value of $f_{\rm NL}$ for a cosmic variance limited experiment is also increased from less than 3 to $\sim$ 5.