The Impact of Cross-Covariances Between the CMB and Reconstructed Lensing Power

TL;DR

This paper quantifies how cross-covariances between the lensed CMB and reconstructed lensing power affect cosmological parameter constraints, revealing that neglecting these correlations can lead to significant biases in some models.

Contribution

It provides a detailed analysis of the impact of CMB-lensing cross-covariances on parameter estimation across various experimental setups, highlighting their importance.

Findings

Neglecting cross-covariances shifts constraints by up to 3% in ΛCDM models.

Cross-covariances can impact some extended models by more than 10%.

The effect varies with experimental noise levels and model complexity.

Abstract

Weak gravitational lensing of the Cosmic Microwave Background (CMB) changes CMB statistics in a nontrivial way, allowing for reconstruction of the lensing potential and the use of these reconstructed maps in determining cosmological parameters that affect the formation of intervening large-scale structures. Although in principle there are correlations between the primary CMB and the reconstructed lensing potential due to the lensing procedure itself, in practice CMB analyses treat these as negligible when combining these band powers in likelihoods. In this paper we quantify explicitly the impact on parameter constraints due to these cross-covariances between the lensed CMB and reconstructed lensing power, and we compare to the effect of including all lensing-induced non-Gaussian covariances, which have previously shown to impact parameter constraints on the order of 10%. We perform our analysis for a range of experimental setups, scanning over instrumental noise levels of 0.5 to 10.0 $\mu$K-arcmin in temperature assuming fully polarized detectors, and using a fixed beam size of 1.4 arcmin. When the correlations between the lensed CMB and lensing power are neglected, we find that forecasted constraints shift by at most 3% of the error bar for a 6-parameter $\Lambda$CDM model, and for the noise levels considered in this paper. For some of the $\Lambda$CDM extensions considered here, however, these correlations have a nontrivial impact, in some cases more than 10% of the error bar, even for current experimental noise levels.