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 radiation. Specifically, there is less energy than expected below 217 GHz 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 detect dense areas of the distant past such as the intracluster medium (ICM) of 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 the furthest clusters because the magnitude of the effect is basically the same at any distance from the observer (but is an effect on intensity so it can be used accurately only on a cluster that is resolved). 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 a smaller 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 black body, 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).
A third effect is the relativistic Sunyaev-Zel'dovich effect (rSZE or rSZ), essentially the particular effects due to the electrons moving at relativistic speeds. It constitutes a correction to the usual approximation of the tSZ to accommodate relativity, in cases where a galaxy cluster's ICM temperature is very high. Accommodating this effect can be helpful when analyzing SZ effects to figure out the structures that caused them.