After three decades of rapid experimental and theoretical progress, CMB research has advanced to the era of nanokelvin-scale measurements of the CMB temperature and polarization anisotropy. This level of sensitivity enables an array of exciting science goals, including but not limited to: detection of the B-mode (odd parity) polarization signature of gravitational waves produced during inflation; detecting the signature of relic particle species in the very early Universe; and the determination of the scale of neutrino mass. A detection of a background of primordial gravitational waves (tensor modes) would push our understanding of fundamental physics into new regimes of time and energy. It would reveal the energy scale of inflation and probe energies exceeding that of the LHC by a factor of more than a trillion. A broad class of inflationary models predicts a ratio of tensor-to-scalar power of r ≳ 0.01. Stage-3 experiments such as those being deployed on the SPT and BICEP/Keck telescopes at the South Pole are sensitive enough to detect the faint signal that these tensor modes leave in the polarization of the CMB. CMB-S4 will be a factor of ten more sensitive to r and will test a wider range of inflationary models.
The anisotropy of the CMB is lensed by the gravitational effects of intervening matter. This lensing is a sensitive probe of large-scale structure, enabling a measurement of the neutrino mass scale from the effects of neutrino free streaming on the growth of structure. Setting the neutrino mass scale is crucial for understanding the origin of neutrino mass, which is another probe of very high energy scales. Lensing of the CMB also transforms some of the E-mode polarization into B-mode polarization, interfering with the detection of the inflation-produced B-mode polarization. In the quest to discover inflation-produced gravitational waves through their B-mode signature, lensing gets in the way and must be removed, through a process referred to as “de-lensing.” In essence, what is learned about large-scale structure is used to remove the lensing-produced B-mode signal.
Cosmology also offers a unique means of searching for new physics through the effect on the CMB of relic particles from the early Universe, including sterile neutrinos, light bosons, and other new particle species. Through measurements of the E-mode (even-parity) polarization power spectrum, the current Stage-3 CMB experiments will determine the energy density of the Universe when the CMB decoupled precisely enough to search for new particle species. The number of relativistic species at this early time, parametrized by Neff and equal to 3.046 in the Standard Model, will be determined at the few percent level. CMB-S4 will be even more sensitive and probe almost any extension of the Standard Model of particle physics with a light particle species.