Delensing CMB Polarization with External Datasets
  One of the primary scientific targets of current and future CMB polarization
experiments is the search for 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 be
removed through use of so-called delensing algorithms. We forecast prospects
for delensing using lensing maps which are obtained externally to CMB
polarization: either from large-scale structure observations, or from
high-resolution maps of CMB temperature. We conclude that the forecasts in
either case are not encouraging, and that significantly delensing large-scale
CMB polarization requires 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 computationally fast and
agrees well with Monte Carlos.
