The Sunyaev-Zel'dovich Effect (SZE)
The Sunyaev-Zel'dovich effect describes the interaction of hot electrons in the dense intracluster medium of rich galaxy clusters with CMB photons. Since the plasma temperature in rich cluster galaxies (6X107 K) is much higher than that of the CMB (~3 K), approximately 1% of CMB photons traversing a dense cluster of galaxies are inverse Compton scattered from low to high frequency leaving a decrement in the amount of power seen on the Rayleigh-Jeans side of the blackbody spectrum with a compensating increment on the Wien side. This spectral distortion is independent of redshift, providing a unique way to probe the largest cosmological scales.
The fact that the SZ effect is redshift independent can be exploited in order to produce a distance-ladder independent measurement of the Hubble constant. The CMB distortion for a given cluster depends on the density of hot intracluster electrons integrated along the line of sight. The X-ray emission depends on the density squared integrated along the line of sight. Therefore, assuming equilibrium conditions and a spherical cluster, the comparison of SZ data and X-ray observations provides a direct measurement of the cluster size and therefore the distance. This distance estimate is used to constrain the Hubble constant without making assumptions about cluster luminosity evolution.
Since clusters of galaxies collapse from large volumes of order 1000 Mpc 3, the ratio of baryons to dark matter should be a reasonable approximation to the mix in the universe as a whole. Therefore, observations of the SZ effect allow for the determination of the gas mass fraction in clusters. Making the approximation that the cluster gas mass fraction reflects the gas mass fraction of the universe as a whole, and taking constraints on the baryon density imposed by nucleosynthesis, measurements of the SZE provide an estimate of the matter density in the universe.
In addition, since the SZ effect is independent of redshift, it is possible
to produce a large survey of high redshift clusters selected only by their
masses (a very difficult thing to do using X-ray observations). The number
of expected such clusters as a function of redshift depends on the cosmological
model. Observations of SZ clusters at high redshift therefore can be used
to distinguish between cosmological models.
For more about the SZE, check out Birkinshaw's review (astro-ph/9808050) or the more recent review by Carlstrom, Holder, and Reese (astro-ph/0208192)