Adapted from P. Coles, 1999, The Routledge Critical Dictionary of the New Cosmology, Routledge Inc., New York. Reprinted with the author's permission. To order this book click here:

When photons from the cosmic microwave background radiation travel through a hot plasma (with a temperature of, say, around 108 K) they collide with energetic electrons and get scattered up to X-ray energies. If we look at the cosmic microwave background radiation through such a plasma cloud, we therefore see fewer microwave photons than we would if the cloud were not there. Paradoxically, this means that the plasma cloud looks like a cool patch on the microwave sky. This photon deficiency is the essence of the Sunyaev-Zel'dovich effect, named after Rashid Alievich Sunyaev (1943 - ) and Yakov Zel'dovich.

Quantitatively, the relative temperature dip DeltaT/T depends on the temperature and number density of the scattering electrons (Te and ne) according to the formula

DeltaT/T = - 2 integ(nekTe sigma / mec2) dl

where the integral is taken along the line of sight through the cloud; me is the mass of the electron, and sigma is the Thomson scattering cross-section. This effect has been detected in observations of rich clusters of galaxies: the size of the temperature decrement DeltaT/T is around 10-4. Future fine-scale experiments designed to map the fluctuations in the cosmic microwave background radiation with an angular resolution of a few arc minutes are expected to detect large numbers of Sunyaev-Zel'dovich contributions from individual clusters.

A particularly interesting aspect of this method is that it is possible, at least in principle, to use it to obtain measurements of the distance to a cluster of galaxies in a manner that is independent of the cluster's redshift. To do this we need X-ray measurements of the cluster (see X-ray astronomy) which give information about ne and Te. Comparing these with the measured DeltaT/T yields an estimate of the total path length (L = integ dl) traversed by the photons on their way through the cluster. Assuming the cluster to be spherical, or by using a sample of clusters with random orientations, we can use L to estimate the physical size of the cluster. Knowing its apparent angular size on the sky then leads to an estimate of its distance; knowing its redshift then leads to a value of the Hubble constant H0.

Attempts to apply this idea in practice have not been overwhelmingly successful, rather low values being obtained for H0. On the other hand, it is a potentially important method because it does not rely on the complicated overlapping series of calibrations from which the extragalactic distance scale is usually constructed.


Jones, M. et al., `An image of the Sunyaev-Zel'dovich Effect', Nature, 1993, 365, 320.