The Sunyaev-Zel'dovich effect is an apparent temperature dip in the cosmic microwave background (CMB) radiation, in certain directions, caused by inverse Compton scattering of CMB photons by electrons in the hot, thin plasma in clusters of galaxies. Measurement of the effect, which amounts to less than 0.001 K at centimeter and millimeter wavelengths but is greater at submillimeter wavelengths, is an important way of determining absolute distances and hence the Hubble constant. It is named after the Uzbekistani astrophysicist Rashid Sunyaev (1943–) and Yakov Zel'dovich.