Dark matter is a topic that most people, even non-scientists have heard of, yet how many actually know anything about it? The name itself seems more fitted to science fiction than anything factual. Dark matter is ‘sexy’ science, one of the topics brought out in a bid to convince high school students that physics is exciting. Yet how much evidence is there to support even its existence?
Firstly what is dark matter? Its given its name because it does not reflect or emit electromagnetic radiation. This means that unlike ordinary visible matter it cannot be directly detected by any sort of optical instrument. Its presence can only be inferred from its effects on visible matter. The first person to hypothesise the existence of dark matter was Swiss astrophysicist Fritz Zwicky in 1933. He used the Virial theorem which describes the relationship between the average kinetic energy of a system and the average potential energy of the same system. Zwicky applied this to the Coma cluster of galaxies. The kinetic energy of the cluster could be found from measurements of the Doppler shift. Using the Virial theorem this gave a value for the gravitational potential energy of the cluster which then provided an expected value for the total mass of the cluster. The mass of the cluster could also be inferred from observations of its luminosity. Surprisingly it was found that the mass calculated using the virial theorem was about 400 times larger than that calculated using the luminosity of the cluster.These observations led Zwicky to be the first person to suggest the presence of some unseen or dark matter. This would contribute to the mass of the cluster but not to the luminosity.
Further evidence for the existence of dark matter was discovered in the 1970’s by Vera Rubin an astronomer at the Carnegie Institute in Washington. She used new spectrograph technology to carry out precise measurements of the rotation speeds of spiral galaxies. These measurements produced results which again suggested that the visible matter present in galaxies is only a small proportion of the total matter.
Gravitational lensing provides yet more evidence that there is ‘missing mass’ in the universe. This occurs when light from a distant object is bent by the presence of a massive object between the distant object and an observer. This leads to the distant object appearing distorted. The distortion of these galaxies can be used to determine the location and amount of mass in the cluster. Again the measurements suggest a much larger amount of mass than can be seen.
Dark matter is also part of the Big Bang model of cosmology. This explains structure formation of the universe as occurring in several stages. Initially the universe is believed to have been almost totally uniform. The few temperature fluctuations that existed would have been of the order ten to the minus five. These tiny fluctuations later led to the formation of large scale structures in the universe. The fluctuations have been seen by COBE the cosmic background explorer satellite which studies the cosmic microwave background relic electromagnetic radiation from about 100,000 years after the Big Bang. These fluctuations can be seen amplified in the picture. Big bang nucleosynthesis of lighter elements then occurred followed by linear structure formation.During the linear growth of structure gravitational collapse occurred to form the structures seen today. Ordinary matter experiences both gravity and radiation pressure but dark matter experiences only gravity. This meant that dark matter collapsed to form dark matter halos long before any other matter collapsed. The gravitational pull of the dark matter halos then attracted ordinary matter providing the basis for galaxy formation.
These results may seem to suggest that the existence of dark matter is certain. However Dark matter is a theory which has been developed to explain physical observations. It can be argued that any theory could be made to fit measurements with enough caveats added. An example of this is the theory that the solar system rotated around the Earth. Originally it was hypothesized that the planets traveled in circular orbits around the Earth. However once observations had proved this to be incorrect, the shapes of the orbits were gradually hypothesized to be more and more complicated in order to fit the measurements. The underlying theory was mistaken, yet with enough work it could be made to agree with observations. Is Dark matter another case of scientific wishful thinking?
There are alternative theories which could explain the ‘missing mass’ problem. These assume that our understanding of the gravitational effects of mass are flawed. Theories such as Modified Newtonian Dynamics (MOND) and Tensor Vector Scalar gravity (TeVeS) remove the need for dark matter. All of these theories accurately match observations, so how can scientists conclude which is correct?
The way in which one scientific theory gains support over others, is in its ability to predict as yet unobserved phenomena. If these can be subsequently observed then the theory gains significant amounts of credibility. In the case of Dark matter scientists are trying to detect directly or indirectly some of the particles that it is hypothesized that it is made of. These include neutrinos and Weakly interacting massive particles (WIMPS). Indirect detection includes looking for products of reactions of these particles. Direct detection looks for energy depositions from rare elastic scattering events. Although some evidence for these have been found this is still very much a current research topic. Over the next few years scientists will hopefully be able to find further evidence to support or dismiss the theory of dark matter.