The ‘Stage-4’ ground-based cosmic microwave background (CMB) experiment, CMB-S4, consisting of dedicated telescopes equipped with highly sensitive superconducting cameras operating at the South Pole, the high Chilean Atacama plateau, 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. This will test many of the simplest models of inflation, including those based on symmetry principles, that occur at high energy and large inflaton field range. The r requirements have been translated into measurement requirements consistent with projecting out foregrounds and other contamination.
The total project cost for the CDT design concept is $412 Million in 2017 dollars, including 45% contingency.
State-of-the-art CMB observations require the highest and driest sites on Earth. The two best sites developed for CMB are South Pole in Antarctica and Cerro Toco in the Chilean Andes. Both of these sites have been in use for several decades and have hosted many CMB
At these sites, the atmosphere is almost transparent to microwave radiation in a series of windows bracketed by oxygen and water emission lines. These windows are 40 GHz and below, around 90 GHz, around 150 GHz, and from 200 to 300 GHz.
The CMB-S4 strawperson instrument can be developed based on existing technology that is being deployed in Stage-3 experiments.
Measurements from ground and space have been complementary in the angular scales and the range of frequencies measured. Ground-based experiments have focused on degree to arcminute angular scales, whereas space-based measurements have focused on scales as large as the dipole to five-arcminute angular scales. The 10-meter class telescopes required to resolve arcminute scales are more practical in cost on the ground, whereas the clear view from space with no atmospheric interference and access to the full sky is optimal for measuring the largest angular scales. Recently, WMAP and Planck in space and ACT and SPT on the ground together have made the state-ofthe-art temperature anisotropy measurements from dipole to arcminute angular scales.
There is also a complementarity in the frequency coverage of ground-based and space-based observations. Ground-based measurements are sensitive in atmospheric windows up to ∼300 GHz, which simulations suggest is adequate to reach the science goals of CMB-S4. Space-based measurements can give additional information at frequency bands across the entire spectrum that are not accessible from the ground, and at frequencies over 300 GHz, which are important for understanding the properties of Galactic dust.
The CMB-S4 deep search for inflationary physics and rich science goals for Neff, neutrino masses, and dark energy are self-contained in that they do not require auxiliary data from a future CMB satellite. But CMB-S4’s science would be enhanced if satellite data are available. JAXA is supporting a phase-A study for the LiteBIRD mission, and NASA has recently commissioned a study for a ‘Probe’ class mission. The combination of data from CMB-S4 and a satellite would increase confidence in the inflation measurements through the complementarity of CMB-S4’s measurements at degree scales and the satellite’s measurements at degree and larger angular scales. High-resolution CMB-S4 observations can be used to delens coarser resolution space data, improving the depth of the space-based inflation search. Also highly complementary, the space mission’s measurement of the optical depth τ could greatly increase the precision of the measurement of the sum of neutrino masses from CMB-S4 lensing analysis.