Authors

Abstract

CMB-S4the next-generation ground-based cosmic microwave background (CMB) experimentisset to significantly advance the sensitivity of CMB measurements and enhance our understanding ofthe origin and evolution of the Universe, from the highest energies at the dawn of time through thegrowth of structure to the present day. Among the science cases pursued with CMB-S4, the quest fordetecting primordial gravitational waves is a central driver of the experimental design. This work details the development of a forecasting framework that includes a power-spectrum-based semi-analyticprojection tool, targeted explicitly towards optimizing constraints on the tensor-to-scalar ratio, r, inthe presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is uniquein its direct use of information from the achieved performance of current Stage 23 CMB experimentsto robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodologyallows for rapid iteration over experimental configurations and offers a flexible way to optimize thedesign of future experiments given a desired scientific goal. To form a closed-loop process, we couplethis semi-analytic tool with map-based validation studies, which allow for the injection of additionalcomplexity and verification of our forecasts with several independent analysis methods. We document3multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of thecurrent reference design of the primordial gravitational-wave component of the Stage-4 experiment,optimized to achieve our science goals of detecting primordial gravitational waves for r > 0.003at greater than 5, or, in the absence of a detection, of reaching an upper limit of r < 0.001 at 95% CL.