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Cosmic Microwave Background

A significant fraction of the Astrophysics Group is involved in work on the Cosmic Microwave Background, both experimental and theoretical. We are currently operating the Arcminute Microkelvin Imager, an arcminute-scale survey telescope, and developing analysis techniques for the Planck satellite.

Over the last few years the detection of fluctuations in the CMB on degree scales, and the measurement of their power spectrum, has revolutionized observational cosmology. The Cosmic Anisotropy Telescope (CAT) was the first CMB telescope to detect structure in the microwave background radiation on angular scales smaller than the main peak in the power spectrum. The CAT was also the first interferometer to detect structure in the CMB, a technique now being used in several groups around the world. Our next generation instrument in this area, the Very Small Array (VSA), was operational until 2008 on its site in Tenerife. The VSA made images of the anisotropies on a range of angular scales between ~15 arcmin and 2 degrees. From this, it has been possible to deduce the density and age of the universe to a relative accuracy of a few percent. The Group is now involved in the Quijote project, which is an experiment designed to measure the polarisation signal from primordial CMB anisotropies and is currently being comissioned in Tenerife. This instrument has the capability to measure the signature in the CMB of primordial gravitational waves, and thereby determine the energy scale of inflation.

On smaller angular scales, the main features visible in the CMB are due to clusters of galaxies, whose hot atmospheres scatter the CMB radiation via the Sunyaev-Zel’dovich effect. The Ryle Telescope was the first instrument able to make images of galaxy clusters via the SZ effect. This information enables us to measure the Hubble constant and to search for proto-clusters forming at very high redshift. Gravitational-lensing inversion techniques are also being developed to compare the total mass in clusters to the gas mass measured via the SZ effect. Our current project in this field is the Arcminute Microkelvin Imager (AMI), which can detect high-redshift clusters and proto-clusters over large areas of sky, as well as search for fine structures in the primary CMB anisotropies due to, for example, re-ionization or cosmic strings. AMI began observing in mid 2005 and, having now completed several surveys which are now published, is available to guest observers – details of the application procedure will be published shortly.

Future plans include involvement in observations of the 2.7-K background from space,  made by the European Planck Surveyor satellite launched in 2008. The Cambridge Planck Analysis Centre is a designated centre for scientific analysis for this satellite, and the Astrophysics Group has made a major contribution to the development of  the data analysis techniques for this mission.