Public Lectures for the Session 2002-2003 held in Room E7 of the Renold Building, UMIST.
Delivered to the Manchester Astronomical Society

20 March 2003

''Dark Matter''

Dr. Neil Spooner
(Sheffield University)

In our third lecture on modern cosmology the subject of missing mass, 'dark matter', is not only topical but is essential to our understanding of the universe.

Dr. Spooner went to great lengths in explaining how the mass of the visible universe was measured. Galaxies are the fundamental building blocks in the universe, yet without exception the rotation rates at various radial distances from their centres did not obey Newtonian and Keplarian laws of orbital motion. The implication was that galaxies are surrounded by much more mass than is visible in the form of stars and nebulae.

Within the past few weeks, Dr Spooner said that our understanding of the early years of the universe had been confirmed by data from the MAP satellite that showed that at resolutions of 1/10,000 degree, the background radiation at the time of photon decoupling, 300,000 years after the big bang, was not uniform and corresponded closely with the known distribution of baryonic (visible or implied) matter in the cosmos. From this confirmatory evidence, theory predicted that only ~5% of the matter in the universe is baryonic.

At present it is thought that the total mass in visible stars is ~0.5%, 3-4% is found in galaxies and galactic clusters (measured by the motion of galaxies within clusters and via gravitational lensing) and about 4% can be accounted for via the motion of galaxy superclusters. This leaves 90-95% of the mass of the universe as invisible, unidentified matter. Although consistent with theoretical models, the missing mass was largely inexplicable.

Sheffield University is a world leader in dark matter research. Dr Spooner spends his research time 1200m below the surface of the earth in the Boulby Mine in North Yorkshire, the deepest mine in Europe. Shielded against cosmic rays by over a kilometre of rock and surrounding salt deposits, world-leading instruments are detecting Weakly Interactive Massive Particles (WIMPS) that may constitute most of the missing mass of the universe. Like neutrinos, WIMPS pass through ordinary matter like ghosts but can, very occasionally, interact with atomic nuclei in special liquid Xenon detectors, shielded by hundreds of meters of rock and lead shielding made from old lead, to minimise radioactive contamination, from the roof of Salisbury cathedral. These detectors are a specialility of Dr. Spooner's team.Research is ongoing. New detectors are being constructed to refine observations of these elusive particles and results are encouraging. The missing mass will eventually be identified and described.

What perplexed Einstein into including his cosmological constant into his general theory of relativity nearly ninety years ago is now being thoroughly investigated. Although the universe is now thought to be flat, recent observations that the expansion is increasing are considered to be due to energy from non-baryonic dark matter. 'Dark energy' is the next fundamental property of the universe still to be understood and UK astronomers are at the forefront of this research.

Synopsis by Kevin J. Kilburn (Secretary)

Maintained by Michael Oates
Page modified 28 October, 2006