Astrophysics Group

Cavendish Laboratory


Mullard Radio Astronomy Observatory

Much of the effort of the Observatory is directed towards imaging the cosmic microwave background radiation. This radiation is a very faint relic of the stage when the Universe was a small hot fireball with an age of only some 300,000 years. In the time since it was emitted the expansion of the Universe has caused the radiation to cool to a temperature of only 2.7 degrees above absolute zero, and it is now detectable only at radio wavelengths. By mapping differences in radiation intensity from point to point across the sky, one can find out about the growth of material structures in the Universe as the fireball phase was ending. This tells us about the formation of the galaxies and galaxy clusters that populate the Universe now, and allows us to infer the physical conditions in the Universe when it was only a fraction of a second old.

Cambridge Anisotropy Telescope The difficulty with microwave background imaging, however, is that the point-to-point intensity variations are only about one part in one hundred thousand, requiring extremely sensitive purpose-built telescopes. The first MRAO telescope designed specifically to map the microwave background was the Cosmic Anisotropy Telescope – CAT – shown on the right. This had 3 receiving horn aerials working at 2 cm wavelength and mapped the fluctuations on a scale of about half a degree. The success of this design was exploited for a more ambitious collaborative project to build the Very Small Array – VSA – a 14-horn telescope now sited on Mount Teide in Tenerife.


The upgraded Ryle Telescope was also used for microwave background mapping. It was sensitive to scales from 1 second of arc to several minutes. It produced the image below of a faint dip (shown black) in the intensity of the background (shown in red) as the radiation travels towards us through a galaxy cluster. The size of the dip and the cluster’s recession speed have enabled us to estimate the age of the Universe as nearly 20 billion years. The Ryle Telescope has now been reconfigured and incorporated into the most recent addition to the telescopes at MRAO, the Arcminute Microkelvin Imager – AMI – which is designed to make surveys of the microwave background looking for other such dips in intensity due to galaxy clusters. Because this so-called SZ effect is independent of redshift, AMI has the potential to find some of the earliest galaxy clusters formed in the Universe.

Sunyaev-Zel'dovich Effect

For more information about our work, follow the links on the left.