From Encyclopedia of Astronomy & Astrophysics

The Sunyaev–Zeldovich (SZ) effect is a spectral effect on the COSMIC MICROWAVE BACKGROUND (CMB) caused by interaction with the electrons in the hot gas inside a CLUSTER OF GALAXIES. In the Rayleigh–Jeans portion of the spectrum it manifests itself as a small (∼0.5µK) dip in the temperature of the CMB in the direction of the cluster. First proposed in 1970, about 20 yr of observational effort were required before it was reliably detected. Many tens of clusters have now detected in the SZ effect, and early predictions of its usefulness for cosmology are now starting to bear fruit. The basic mechanism of the SZ effect (see figure 1) involves scattering of CMB photons by electrons in the distributed intracluster gas that exists in clusters of galaxies and in fact makes up about a quarter of their total mass. A rich cluster may have a total mass>3 × 1014