SunyaevZeldovich

The Sunyaev–Zeldovich effect, commonly abbreviated as SZ effect, is a distortion of the cosmic microwave background (CMB) radiation caused by inverse Compton scattering of CMB photons off high-energy electrons in the intracluster medium of galaxy clusters. It was predicted independently by Rashid Sunyaev and Yakov Zeldovich in 1969–1970.

There are two main components: the thermal SZ effect, produced by hot electrons at temperatures of about 10^7–10^8 kelvin transferring energy to CMB photons, and the kinetic SZ effect, arising from the bulk motion of a cluster along the line of sight with respect to the CMB rest frame. Relativistic corrections become relevant for very hot gas, slightly altering the spectral features.

Spectral and observational signatures differ between the components. The thermal effect creates a frequency-dependent distortion: a decrement in CMB intensity at lower frequencies and an increment at higher frequencies, with a null near 217 GHz. The kinetic effect induces a roughly frequency-independent temperature shift in the non-relativistic limit, proportional to the cluster’s line-of-sight velocity and optical depth.

Cosmological significance and uses are broad. The SZ effect is nearly redshift independent in surface brightness, enabling detection of distant clusters. When combined with X-ray data, it yields estimates of gas mass, total mass, and pressure profiles. SZ-selected cluster catalogs inform measurements of cosmological parameters such as the matter density and the amplitude of fluctuations, and aid in deriving the Hubble constant when used with other observables.

Notable experiments that have leveraged the SZ effect include Planck, the Atacama Cosmology Telescope (ACT), and the South Pole Telescope (SPT), which have produced extensive cluster samples and advanced studies of large-scale structure.