....and possibly northern hemisphere sites, will provide a dramatic leap forward in our understanding of the fundamental nature of space and time and the evolution of the Universe.

CMB-S4 will be designed to cross critical thresholds in testing inflation, determining the number and masses of the neutrinos, constraining possible new light relic particles, providing precise constraints on the nature of dark energy, and testing general relativity on large scales.

The first goal and requirement for CMB-S4 is to measure the imprint of primordial gravitational waves on the CMB polarization anisotropy, quantified by the tensor-to-scalar ratio r. Specifically, CMB-S4 will be designed to provide a detection of r ≥ 0.003. In the absence of a signal, CMB-S4 will be designed to constrain r less than 0.001 at the 95% confidence level, nearly two orders of magnitude more stringent than current constraints.