Delensing CMB B-modes using galaxy surveys: the effect of galaxy bias and matter clustering non-linearities

Abstract

The B-mode of polarization of the CMB is a uniquely powerful probe of gravitational waves produced in the very early Universe. But searches for primordial B-mode anisotropies must contend with gravitational lensing, which induces late-time B-modes not associated with gravitational waves. These lensing B-modes can be removed -- i.e., delensed -- using observations of the E-modes and a proxy of the matter fluctuations along the line of sight that caused the deflections. The number density and redshift reach of galaxy surveys such as the upcoming Rubin observatory offer attractive prospects for using them to delens B-mode data from CMB experiments such as the Simons Observatory, LiteBIRD or CMB-S4. However, stochasticity and non-linear galaxy bias may in principle decorrelate the galaxy field from the matter distribution responsible for the lensing effect, thus hindering efforts to delens B-modes. In addition, non-linear gravitational evolution and bias introduce non-Gaussianities in the large-scale structure which further complicate the modelling. We quantify these effects by populating an N-body simulation with a magnitude-limited, photometric sample of galaxies similar to Rubin's gold selection, and using them to delens CMB maps lensed by the same matter distribution. We find that pipelines that treat the galaxy overdensity as a Gaussian field will incur negligible bias on the inferred tensor-to-scalar ratio, r. Moreover, we show that even in a highly conservative scenario where only the linear bias of the galaxies can be determined, the bias on r arising from this simplification is well within statistical uncertainties for a cosmic-variance limited scenario where Rubin-like galaxies are used for delensing.