Forecasting the Impact of Atmosphere and Foregrounds on Future Rayleigh Scattering Measurements

In recent decades, measurements of the CMB angular power spectra have yielded ever-narrowing constraints on cosmological parameters. As measurements of the CMB signal approach the cosmic variance limit, a new source of information will become necessary to further improve constraints. The Rayleigh scattering of CMB photons off of the neutral hydrogen produced during recombination is a promising option. Rayleigh scattering effectively creates an additional scattering surface after recombination that encodes new cosmological information, including the expansion and ionization history of the universe. Thus a first detection of Rayleigh scattering is a tantalizing target for next-generation CMB experiments. We have developed a Rayleigh scattering forecasting pipeline that includes instrumental effects (e.g., atmospheric noise) and astrophysical foregrounds (e.g., Galactic cirrus, CIB, thermal Sunyaev-Zel'dovich effect). We forecast the Rayleigh scattering detection significance for several upcoming ground-based experiments, including SPT4, Simons Observatory, and CMB-S4, and examine the limitations from different astrophysical foregrounds and potential mitigation techniques. We further discuss how to optimize the detection significance in future experiments.