The Sunyaev-Zel'dovich, or S-Z, effect creates a distinctive pattern of temperature variation in the cosmic microwave background as CMB photons pass through galaxy clusters. Between the galaxies in a cluster, there is a gas containing energetic electrons that scatter about 1 percent of the CMB photons. The scattered photons gain energy from the electrons, leaving a cold spot in the CMB when observed at the original photon energy. The more energetic scattered photons appear as a hot spot when the CMB is observed with a receiver tuned to a slightly higher radio frequency. The S-Z effect allows astronomers to identify galaxy clusters across the entire sky in a manner that is independent of redshift. See: cosmic microwave background, galaxy cluster.