Cold Dark Matter - is is cold is it dark and is it matter?

Tuesday 16th February, 2021

Our speaker (on Zoom) was Dr Julian Onions from Nottingham University and he talked to us about "Cold Dark Matter: is it cold, is it dark and is it matter?". He introduced himself saying that he specialises in computer models to simulate how dark matter interacts with ordinary matter and light to make the Universe appear as it does.

He began by saying that there are many particles that have been proposed only in theory and then experimental results have actually confirmed their existence. For example, in 1930 Wolfgang Pauli suggested the existence of a particle called a neutrino. At the time there was a mystery concerning a type of radioactive decay known as "beta decay", where a nucleus becomes stable after emitting an electron. Physicists could not understand why the ejected electron had a range of energies when quantum theory predicted it should always have the same amount of energy.

Pauli's answer to the problem was to suggest that another particle was emitted with the electron, and that would account for the energy deficit. He initially named this particle a "neutron" but later the Italian physicist Enrico Fermi would rename it the neutrino - as the name neutron fitted better with the 1932 discovery of a chargeless counterpart of the proton. Although Pauli is reported to have said, "I have done a terrible thing, I have postulated a particle that cannot be detected", it was detected in emissions from a nuclear reactor in 1956 at the Savannah River Plant in South Carolina, USA.

The term "dark matter" was introduced by the Swiss astronomer Fritz Zwicky at California's Institute of Technology in 1933 after he studied the Coma Galaxy Cluster. This cluster is made up of many galaxies that are held together by their gravitational attraction as they circle each other. The problem was that, if you just measured the amount of mass due to objects that emitted light, such as stars, there was not enough there to keep the galaxies in their orbits. They were travelling so fast that they should leave the cluster and fly off further into the Universe. This speed anomaly was also confirmed on smaller scales in the 1970s when Vera Rubin of the Carnegie Institution found that the stars at the edge of a single galaxy, M33, were moving too fast to remain in orbit.

Dr Onions continued by saying that scientists currently think that the ratio of dark matter to ordinary matter is about five to one, and it only interacts with ordinary matter through the force of gravity. There are many different theories as to what dark matter actually is, and the most likely candidate is a particle known as a WIMP or Weakly Interacting Massive Particle that would have up to a thousand times the mass of a proton.

He concluded his talk by mentioning a couple of current experiments set up to try and detect these elusive particles. The LUX-ZEPLIN detector is located nearly a mile underground in a former gold mine in South Dakota, USA. It consists of a 5-foot cylindrical tank filled with ten tons of liquid xenon. If a dark matter particle interacts with the xenon it should create a flash of light and an electric charge.

The other experiment, known as SuperCDMS, is again sited deep underground (6,880 feet down) in a nickel mine in Ontario, Canada. It consists of four towers that each contain six detectors made from silicon germanium crystals. Eventually a further 24 towers will be added to increase the likelihood of a detection. In this design if a WIMP hits one of the detectors it will disturb electrons in the crystal lattice and this signal will be recorded.