CMB-S4: Forecasting Constraints on Primordial Gravitational Waves

Abstract

CMB-S4—the next-generation ground-based cosmic microwave background (CMB) experiment—is set to significantly advance the sensitivity of CMB measurements and enhance our understanding of the origin and evolution of the universe. Among the science cases pursued with CMB-S4, the quest for detecting 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 semianalytic projection tool, targeted explicitly toward optimizing constraints on the tensor-to-scalar ratio, r, in the presence of Galactic foregrounds and gravitational lensing of the CMB. This framework is unique in its direct use of information from the achieved performance of current Stage 2–3 CMB experiments to robustly forecast the science reach of upcoming CMB-polarization endeavors. The methodology allows for rapid iteration over experimental configurations and offers a flexible way to optimize the design of future experiments, given a desired scientific goal. To form a closed-loop process, we couple this semianalytic tool with map-based validation studies, which allow for the injection of additional complexity and verification of our forecasts with several independent analysis methods. We document multiple rounds of forecasts for CMB-S4 using this process and the resulting establishment of the current 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 > 0.003 at greater than 5σ, or in the absence of a detection, of reaching an upper limit of r < 0.001 at 95% CL.

Analyticity and unitarity for cosmological correlators

“…Phenomenological applications. The next few years will bring us a great amount of new cosmological data from both Cosmic Microwave Background [135,136] and Large Scale Structure [137,138] experiments. One of the goals of these experiments is the search…”

Snowmass 2021 CMB-S4 White Paper

“…We have a historic opportunity to open up a window to the primordial Universe [5]. If the predictions of some of the leading models for the origin of the hot big bang are borne out, CMB-S4 will detect the signature of primordial gravitational waves in the polarization pattern of the CMB [6]. This detection would provide the first evidence for the quantization of gravity, reveal new physics at the energy scale of grand unified theories, and yield insight into the symmetries of nature.…”

Snowmass2021 Cosmic Frontier: Cosmic Microwave Background Measurements White Paper

“…CMB-S4 is a "Stage 4" CMB experiment with a rich and diverse set of scientific goals across four major themes [81,82]: primordial gravitational waves and inflation, the dark Universe, mapping matter in the cosmos, and the time-variable millimeter-wave sky. The CMB-S4 project aims to cross science-driven thresholds for inflation science and light relics.…”