Future cosmic microwave background delensing with galaxy surveys
  The primordial B-modes component of the cosmic microwave background (CMB)
polarization is a promising experimental dataset to probe the inflationary
paradigm. B-modes are indeed a direct consequence of the presence of
gravitational waves in the early universe. However, several secondary effects
in the low redshift universe will produce \textit{non-primordial} B-modes. In
particular, the gravitational interactions of CMB photons with large-scale
structures will distort the primordial E-modes, adding a lensing B-mode
component to the primordial signal. Removing the lensing component
("delensing") will then be necessary to constrain the amplitude of the
primordial gravitational waves. Here we examine the role of current and future
large-scale structure surveys in a multi-tracers approach to CMB delensing. We
find that, in general, galaxy surveys should be split into tomographic bins as
this can increase the reduction of lensing B-modes by $\sim 25\%$ in power in
the most futuristic case. Ongoing or recently completed CMB experiments
(CMB-S2) will particularly benefit from large-scale structure tracers that,
once properly combined, will have a better performance than a CMB internal
reconstruction. With the decrease of instrumental noise, the lensing B-modes
power removed using CMB internal reconstruction alone will rapidly increase.
Nevertheless, optical galaxy surveys will still play an important role even for
CMB S4. In particular, an LSST-like survey can a achieve a delensing
performance comparable to a 3G CMB experiment but with entirely different
systematics. This redundancy will be essential to demonstrate the robustness
against systematics of an eventual detection of primordial B-modes.
