CMB-S4 has the potential to make fundamental discoveries about the origin of our universe by detecting primordial gravitational waves. These gravitational waves are an as-yet unverified prediction of cosmological inflation, a model that describes a period of nearly exponential expansion and represents a compelling paradigm for the very early universe. These gravitational waves leave a unique imprint on the CMB, the so-called B-mode polarization. For the foreseeable future, precise measurements of CMB B-modes are our only way to detect these primordial gravitational waves.
The figure shows a map of CMB polarization that contains only a B-mode component. The color represents the B-mode fluctuation measured as a temperature, and the lines show the size and direction of the incoming magnetic field that our telescope would measure. CMB-S4 will search for the signature of inflation by making exquisitely sensitive maps of a small, clean patch of the sky to measure the B-mode polarization. To successfully measure this signal, it will be necessary to separate the primordial CMB from foreground emission in the Milky Way and the distorting effects of gravitational lensing due to the cosmic web of galaxies and larger structures in the universe.
Because gravitational waves are fluctuations in spacetime rather than density, such a detection would open a new window into the early cosmos and transform our understanding of fundamental physics. Even an upper limit from CMB-S4 would provide invaluable insights into the first moment of our Universe.