The dark side

摘要

The story of dark matter began in 1933. Fritz Zwicky, one of the greatest astronomers of the twentieth century, announced that the Coma cluster of galaxies ought to contain more matter than is inferred from optical observations. He had found that many of the thousands of galaxies in the cluster move at speeds faster than the escape velocity expected from the amount of visible matter. Zwicky realized that the cluster had to contain more, unseen mass to keep these galaxies bound together. In the early 1970s, astronomers discovered that the speed of stars and clouds of hydrogen atoms rotating in a galactic disk is nearly constant all the way out to the edge of the galaxy (Fig. 1, overleaf). By Newton's law of gravitation, this implied that the amount of matter at increasing radius is not falling away―and yet starlight fades towards the edge of the galaxy. This suggested that the disk of a galaxy might be surrounded by a massive halo of dark, unseen matter. Further studies of our own Galaxy, the Milky Way, have indicated that its dark halo may extend to more than ten times the radius that is optically observed. The existence of a dark halo is also suggested by the stability of the Milky Way. When viewed edge-on, the Milky Way (and other similar spiral-arm galaxies) appears as a thin disk (shown above). This disk would be unstable against its own gravity, unless it was embedded in a more massive, sphere-like, dark halo of matter.