The first evidence of a large amount of Dark Matter came from observations made by Fritz Zwicky in the 1930s. He studied the Coma cluster of galaxies, 321 million light-years distant, and observed that the outer members of the cluster were moving at far higher speeds than were expected. Suppose a cluster of galaxies was created which was not in motion. Gravity would cause it to collapse down into a single giant body. If, on the other hand, the galaxies were initially given very high speeds relative one to another, their kinetic energy would enable them to disperse into the universe and the cluster would disperse, just as a rocket traveling at a sufficiently high speed could escape the gravitational field of the Earth. The fact that we observe a cluster of galaxies many billions of years after it was created implies that there must be an equilibrium balancing the gravitational pull of the cluster’s total mass and the average kinetic energy of its members. This concept is enshrined in what is called the virial theorem so that, if the speeds of the cluster members can be found, it is possible to estimate the total mass of the cluster. Zwicky carried out these calculations and showed that the Coma cluster must contain significantly more mass than could be accounted for by its visible content.
NASA’s Chandra X-ray Observatory has revealed that the elliptical galaxy NGC 4555 is embedded in a cloud of gas having a diameter of about 400 000 light-years and a temperature of 10 million°C (Figure 9.11). At this temperature, the molecules of gas would be traveling at very high speeds and the mass of the stars within the galaxy would be far too low to prevent its escape. For the gas to remain in the vicinity of the galaxy the total mass of the system must be about 10 times the combined mass of the stars in the galaxy and about 300 times that of the gas cloud.
Earlier, the formation of multiple images of a distant object by a foreground galaxy has been discussed. On a much larger scale, the mass of a cluster of galaxies can distort the images of more distant objects. The image of the Abell 2218 cluster is a wonderful example showing images of more distant galaxies that have been distorted into arcs (Figure 9.12). The amount of distortion will be a function of the total mass of the intervening cluster, so this gives a way of estimating the total mass of galaxy clusters, confirming the existence of dark matter. Using this technique, astronomers have even shown how the distribution of dark matter has become more ‘clumpy’ over the last 6 billion years.
How much non-baryonic dark matter is there?
There are several ways of estimating the amount of dark matter. One of the most direct is based on the detailed analysis of the fluctuations in the CMB. The percentage of dark matter has an observable effect, and the best fit for current observations corresponds to dark matter making up ∼23% of the total mass/energy content of the universe. Other observations support this result. This then leaves two further questions: what is dark matter and what provides the remaining 73% of the total mass/energy content?
What is dark matter?
The honest answer is that we do not really know. The standard model of particle physics does not predict its existence and so extensions to the standard theory (which have yet to be proven) have to be used to predict what it might be and suggest how it might be detected. Dark matter can be split into two possible components: hot dark matter would be made up of very light particles moving close to the speed of light (hence hot) whilst cold dark matter would comprise relatively massive particles moving more slowly. Simulations that try to model the evolution of structure in the universe – the distribution of the clusters and superclusters of galaxies – require that most of the dark matter is ‘cold’ but astronomers do believe that there is a small component of hot dark matter in the form of neutrinos. There are vast numbers of neutrinos in the universe but they were long thought to have no mass. However, recent observations attempting to solve the solar neutrino problem discussed earlier, show that neutrinos can oscillate between three types (electron, tau, and muon). This implies that they must have some mass but current estimates put this at less than one-millionth of the mass of the electron. As a result, they would only make a small contribution to the total amount of dark matter – agreeing with the simulations. Further confirmation of the fact that hot dark matter is not dominant is that, if it were, the small scale fluctuations that we see in the WMAP data would have been ‘smoothed’ out and the observed CMB structure would have shown far less detail.