The Sunyaev-Zel'dovich effect (SZ effect, SZ, SZE) is an effect of inverse Compton scattering on the cosmic microwave background (CMB). Free electrons in a plasma cloud can impart energy to photons according to the electron's velocity, due to the cloud temperature. It produces a recognizable change in the spectral energy distribution (SED), resulting in a spectrum that diverges from a normal black body. Specifically, there is less energy than expected below 217GHz and more than expected above that frequency. One way to quantify the effect is to determine the associated Compton Y-parameter. The mechanism was worked out in 1969-1980 by Rashid Sunyaev and Yakov Zel'dovich and observed in 1983.
The effect can be used to observe dense areas of the early universe such as galaxy clusters, which produce a pattern of a peak in the magnitude of the effect in the direction of the cluster. It is very useful for detecting distant clusters because the magnitude of the effect is basically the same at any distance from the observer. Other cosmological information can be inferred such as estimates of the Hubble constant. The value of such observations is one motivation for continuing efforts to improve CMB observations.
A further effect can happen when the dense area (e.g., cloud) is in motion relative to the Hubble flow, which affects the average relative velocity of the electrons. This is known specifically as the kinematic Sunyaev-Zel'dovich effect (kSZE or kSZ), which produces less change to the SED but may be distinguished by the fact that its maximum effect is at a different frequency. Analysis can reveal information about the peculiar velocity of such a cloud. It leaves the CMB spectrum as that of a blackbody, but showing a different temperature:
ΔTSZE ————— = -τe(ν/c) TCMB
The non-kinematic type is sometimes referred to as the thermal Sunyaev-Zel'dovich effect (tSZE or tSZ).