When CMB photons pass through giant clusters of galaxies filled with hot gas of electrons, they gain energy by scattering off the hot electrons. This effect, known as Sunyaev-Zeldovich, is very well known and it is one of the most important tools for cosmological observations. In particular, it helps to detect the existence of galaxy clusters, and therefore enable all the incredible science we can do with observations of clusters’ properties.
The SZ signal will be extremely important to investigate still unknown galactic physics and evolution. For example, star formation in galaxies is known to be inefficient: less than 10% of the available gas is ever turned into stars. This means that there must be very powerful processes that prevent gas from efficiently cooling and forming stars (feedback processes). The SZ signal is able to tell us something about the properties (pressure and density) of the gas in clusters and therefore to provide information about the feedback processes that delay star formation.
Simons Observatory will inform models of galaxy evolution for the first time with very high accuracy. In fact, these important aspects of galaxy formation are currently very feebly constrained.