© NASA, CXC, M. Weiss.
When cosmic microwave photons pass through a galaxy cluster, about 1 percent of them scatter off of hot electrons in the gas filling the space between galaxies in the cluster. This is called the "Sunyaev-Zel'dovich effect." When the photons scatter, they gain energy from the electrons. Thus, galaxy clusters appear in a map of the CMB as cold spots at the original photon frequency, and hot spots at a higher frequency. Microwave telescopes such as the South Pole Telescope can detect galaxy clusters using the Sunyaev-Zel'dovich effect, while x-ray telescopes such as Chandra can map the hot gas that scattered the CMB photons. These measurements together allow astronomers to measure the growth of structure in the universe. (Unit: 11)