Delensing CMB polarization with external datasets

Abstract

One of the primary scientific targets of current and future CMB polarization experiments is the searchfor a stochastic background of gravity waves in the early universe. As instrumental sensitivity improves,the limiting factor will eventually be B-mode power generated by gravitational lensing, which can beremoved through use of so-called ``delensing'' algorithms. We forecast prospects for delensingusing lensing maps which are obtained externally to CMB polarization: either from large-scale structureobservations, or from high-resolution maps of CMB temperature. We conclude that the forecasts in eithercase are not encouraging, and that significantly delensing large-scale CMB polarizationrequires high-resolution polarization maps with sufficient sensitivity to measure the lensing B-mode.We also present a simple formalism for including delensing in CMB forecasts which is computationallyfast and agrees well with Monte Carlos.