Dark Matter and Dark Energy: Our Strange Universe

Abstract

In 1933 Swiss-American astronomer Fritz Zwicky (1898–1974) measured the velocities of galaxies in the Coma Cluster (see Fig. 13.1). He discovered that the velocity differences between the galaxies are larger than you would expect on the basis of the gravitational attraction that the galaxies exert on each other. This attraction he estimated from the amount of visible matter, in the form of stars in the galaxies. The amount of light that a galaxy emits is determined by how many stars there are in the galaxy, so from the light he could estimate the number of stars and thus the mass of the galaxy. Zwicky measured the average velocity differences between the galaxies in the cluster to be about 700 km/s, while the escape velocity from the cluster, which he calculated from the estimated masses of the galaxies to be only a few hundred km/s. One would therefore expect the galaxies to escape from the cluster within a few billion years. However, galaxy clusters were already formed early in the universe, over 13 billion years ago, and the galaxies of the Coma cluster have not escaped during these 13 billion years. The only way to explain why the galaxies in the cluster have stayed together and have not escaped is: that the escape velocity from the cluster is larger than 700 km/s. This is possible only if there is more matter with gravity in the cluster than we see in the form of the stars of their galaxies. There is, of course, also gas in clusters of galaxies, and this was discovered later with radio telescopes and particularly with X-ray telescopes (as a large part of the gas is very hot and emits X-rays). However, the contribution of this gas is far too small to explain an escape velocity larger than 700 km/s. There is therefore a large amount of matter with gravity in the cluster that is not observable as “normal” matter, consisting of atoms and molecules that make up the stars and the gas in the cluster. The same effect of missing mass is found in all other galaxy clusters and also in the galaxies themselves. In the 1970s it was found by American astronomer Vera Rubin that the velocities with which the stars in the Andromeda galaxy M31 describe their orbits around the galaxy centre hardly decrease with increasing distance from the centre, as would be expected from Kepler’s laws. The rotation curve of this galaxy is flat. The same phenomenon of flat rotation curves was also found in the early 1970s in other spiral galaxies by Groningen radio astronomer Albert Bosma, by measuring the velocities of hydrogen clouds, using the Westerbork Synthesis Radio Telescope (see Fig. 4.15). These findings were very puzzling, as the amount of starlight in these galaxies decreases rapidly going outwards from their centres. If the amount of starlight would be indicative of the amount of mass present in the galactic disk, one would expect the orbital velocities of stars and hydrogen clouds to decrease quite rapidly outwards, just like the velocities of the planets around the sun in the solar system. The findings of Vera Rubin and Albert Bosma that the rotation curves of these galaxies are almost flat can only be understood if the disks of spiral galaxies contain a lot of invisible matter that exerts a force of gravity.

Man cannot live without seeking to describe and explain the universe

Sir Isaiah Berlin, British social and political scientist and philosopher (1909–1997)