A phenomenon in which the apparent temperature of the cosmic microwave background is reduced when the radiation passes through hot, ionized gas in a cluster of galaxies. Paradoxically, the effect is caused by an increase of the mean photon energy due to inverse Compton scattering, which appears as a reduction in brightness temperature of no more than 0.001 K at centimetre and millimetre wavelengths. At submillimetre wavelengths the brightness temperature increases. This effect has been observed in many clusters of galaxies. In combination with X-ray observations of clusters, it allows an independent determination of the Hubble constant. The effect is named after the Russian astrophysicists Rashid Alievich Sunyaev (1943– ) and Yakov Borisovich Zel'dovich (1914–87), who first drew attention to it.