Mitigating Cosmic Microwave Background Shadow Degradation of Tensor-to-scalar Ratio Measurements through Map-based Studies

It has been pointed out that the spurious cosmic microwave background (CMB) B -mode polarization signals caused by the absorption of the CMB monopole component due to the Galactic interstellar matter, called the CMB shadow, degrade the accuracy of detecting the CMB B -mode polarization signals imprinted by primordial gravitational waves. We have made a realistic estimation using simulated sky maps of how the CMB shadow affects forthcoming high-precision CMB B -mode experiments for the first time. The delta-map method, an internal template method taking into account the first-order spatial variation of foregrounds’ spectral parameters, is applied as a foreground-removal method. We show that if the CMB shadow effects are not taken into account in the foreground-removal process, future observations would lead to the false detection of the CMB B -mode polarization signals originating from primordial gravitational waves. We also show that the effect of the CMB shadow can be mitigated by our revised delta-map method to target the CMB B -mode polarization signals at the level of tensor-to-scalar ratio r = 0.001.