The Sunyaev-Zel'dovich effect and cosmology

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The largest clusters of galaxies are made up of thousands of individual galaxies, and are the most massive gravitationally-bound objects in the Universe. The mass that binds the clusters together is principally provided by dark matter, but a significant component, and the dominant component of 'ordinary matter' is in hot gas which is bound to the cluster by the gravitational field. The hot gas is most easily seen through its X-ray emission, which is mapped by satellite observatories such as Chandra and XMM-Newton, but can also be detected by its effect on radio waves passing through the cluster. Collisions between electrons in the cluster gas and photons in the radiation field heat up the photons, and cause the intensity of radio waves seen through the cluster to appear different from radio waves that are seen in other directions. This effect, the Sunyaev-Zel'dovich effect, can be seen by looking at the almost-uniform cosmic microwave background radiation, which is the left-over thermal radiation from an early, hot, phase of the Universe and has almost the same brightness in all directions on the sky, except where the radiation comes through clusters of galaxies. The purpose of this research programme is to measure the Sunyaev-Zel'dovich effects of a large number of clusters of galaxies, and to use the measurements to study the structures of the atmospheres of the clusters. Comparisons of the Sunyaev-Zel'dovich effects and the X-rays from clusters can tell us about the ratio of hot gas to dark matter that they contain. If this ratio changes with redshift, then we learn something about how clusters of galaxies form. A different comparison of Sunyaev-Zel'dovich effect and X-ray data can be used to measure the distances of clusters of galaxies. This is interesting because the way that distance changes with redshift (which measures the amount that the Universe has expanded) can tell us what the Universe as a whole is made of - and can test some ideas about the mysterious 'dark energy' that is thought to be responsible for the current accelerated expansion of the Universe.