The search for dark matter and dark energy

Tuesday 17th May, 2016

Our speaker was Professor Lance Miller from the Physics Department at Oxford University who came to talk to us about "The search for dark energy and dark matter". He said that he has been at Oxford University for 20 years and considers himself an observer rather than a theoretician as he studies observational evidence.

He began by explaining that dark matter was first formally proposed by the Swiss astrophysicist Professor Franz Zwicky in 1933 whilst he was working at the California Institute of Technology. Prof Zwicky was studying the movement of galaxies in a cluster of galaxies called the Coma Cluster and found that they were moving so fast that the cluster should not be able to stay together as it did — the galaxies should be flung off into the surrounding space. As the gravitational force that kept all the galaxies together is related to the amount of mass the cluster has as a whole he reasoned that there must be some missing mass present in the cluster that he could not detect. He called this invisible material "dunkle Materie" or dark matter.

Professor Miller then went on to say that the presence of dark matter only can be inferred by its gravitational effects on the normal matter we can actually observe. The mysterious material known as dark matter does not interact with normal matter and neither does it emit or absorb light. However, by studying how galaxies move within a cluster or the motions of stars within a galaxy, scientists can calculate the amount of dark matter needed to keep everything made of normal matter in stable orbits.

Another way of studying how much dark matter is present in a cluster of galaxies is by measuring how light from a distance source from behind the galaxy cluster is distorted as it moves past the cluster. The mass of the galaxy cluster actually distorts the space around it just like a heavy ball on a rubber sheet, so the paths of any light rays travelling near the galaxy cluster are also bent. This effect, known as weak gravitational lensing is what Professor Miller specialises in to study dark matter. He told us that the current consensus for the ratio of dark matter to normal matter is roughly five to one with ordinary matter making up 4.9% of the Universe's mass and dark matter adding another 26.8%. He added that this leaves another 68.3% still unaccounted for.

In 1929 the astronomer Edwin Hubble used his observations of distant galaxies to confirm that the Universe was expanding in all directions. Then in 1998 a team of scientists studying distant exploding stars found that the acceleration was not constant but was actually speeding up. The name of this new property of the Universe i.e. "dark energy" was given by the cosmologist Michael Turner from the University of Chicago in the same year as its discovery.

Professor Miller admitted that scientists do not know or understand what this mysterious force is but is hoping that two projects in particular will help. The first is the Large Synoptic Survey Telescope which is currently being constructed in Chile and the other is a European Space Agency spacecraft named Euclid that will probably launch in 2021.