LiteBIRD Science Goals and Forecasts: A full-sky measurement of gravitational lensing of the CMB

TL;DR

LiteBIRD's high-sensitivity polarization measurements will enable precise full-sky gravitational lensing maps of the CMB, surpassing previous polarization-only results and facilitating new cross-correlation studies.

Contribution

This paper demonstrates that LiteBIRD can produce a high-fidelity full-sky lensing map from polarization data alone, even with foreground contamination considerations.

Findings

Signal-to-noise ratio of ~40 for lensing measurement

Four-fold improvement over Planck's polarization-only lensing

Negligible bias from Galactic foregrounds after component separation

Abstract

We explore the capability of measuring lensing signals in $LiteBIRD$ full-sky polarization maps. With a $30$ arcmin beam width and an impressively low polarization noise of $2.16\,\mu$K-arcmin, $LiteBIRD$ will be able to measure the full-sky polarization of the cosmic microwave background (CMB) very precisely. This unique sensitivity also enables the reconstruction of a nearly full-sky lensing map using only polarization data, even considering its limited capability to capture small-scale CMB anisotropies. In this paper, we investigate the ability to construct a full-sky lensing measurement in the presence of Galactic foregrounds, finding that several possible biases from Galactic foregrounds should be negligible after component separation by harmonic-space internal linear combination. We find that the signal-to-noise ratio of the lensing is approximately $40$ using only polarization data measured over $90\%$ of the sky. This achievement is comparable to $Planck$'s recent lensing measurement with both temperature and polarization and represents a four-fold improvement over $Planck$'s polarization-only lensing measurement. The $LiteBIRD$ lensing map will complement the $Planck$ lensing map and provide several opportunities for cross-correlation science, especially in the northern hemisphere.