Foreground removal requirements for measuring large-scale CMB B modes in light of BICEP2

Abstract

The most convincing confirmation that the B-mode polarization signal detected at degree scales by BICEP2 is due to the cosmic microwave background (CMB) would be the measurement of its large-scale counterpart. We assess the requirements for diffuse component separation accuracy over large portions of the sky in order to measure the large-scale B-mode signal corresponding to a tensor-to-scalar ratio of r = 0.1-0.2. We use the method proposed by Bonaldi & Ricciardi to forecast the performances of different simulated experiments taking into account noise and foreground removal issues. We do not consider instrumental systematics, and we implicitly assume that they are not the dominant source of error. If this is the case, the confirmation of an r = 0.1-0.2 signal is achievable by Planck even for conservative assumptions regarding the accuracy of foreground cleaning. Our forecasts suggest that the combination of this experiment with BICEP2 will lead to an improvement of 25-45 per cent in the constraint on r. A next-generation CMB polarization satellite, represented in this work by the Cosmic Origins Explorer experiment, can reduce dramatically (by almost another order of magnitude) the uncertainty on r. In this case, however, the accuracy of foreground removal becomes critical to fully benefit from the increase in sensitivity.