A bias to CMB lensing measurements from the bispectrum of large-scale structure

TL;DR

This paper analyzes how the non-Gaussianity of large-scale structure, specifically the bispectrum, introduces a bias in CMB lensing measurements, which is significant for upcoming high-precision experiments.

Contribution

It provides an analytical estimate of the bias in CMB lensing power spectrum caused by the bispectrum of large-scale structure, highlighting the need for bias correction in future analyses.

Findings

Bias is approximately 1% of the lensing signal for upcoming experiments.

The bias is negative and can shift measurements by multiple standard deviations.

Only the largest bias terms were evaluated, indicating the need for further research.

Abstract

The rapidly improving precision of measurements of gravitational lensing of the Cosmic Microwave Background (CMB) also requires a corresponding increase in the precision of theoretical modeling. A commonly made approximation is to model the CMB deflection angle or lensing potential as a Gaussian random field. In this paper, however, we analytically quantify the influence of the non-Gaussianity of large-scale structure lenses, arising from nonlinear structure formation, on CMB lensing measurements. In particular, evaluating the impact of the non-zero bispectrum of large-scale structure on the relevant CMB four-point correlation functions, we find that there is a bias to estimates of the CMB lensing power spectrum. For temperature-based lensing reconstruction with CMB Stage-III and Stage-IV experiments, we find that this lensing power spectrum bias is negative and is of order one percent of the signal. This corresponds to a shift of multiple standard deviations for these upcoming experiments. We caution, however, that our numerical calculation only evaluates two of the largest bias terms and thus only provides an approximate estimate of the full bias. We conclude that further investigation into lensing biases from nonlinear structure formation is required and that these biases should be accounted for in future lensing analyses.