The Dark Side of the Universe

Abstract

The invisible came into astronomy in the nineteenth century when Friedrich Bessel concluded, from tiny motions of Sirius in the sky, that it is circled by a dark body. The companion of Sirius , a white dwarf star, was discovered later, in 1862, by the talented American telescope maker Alvan Clark when he tested a new half-meter size objective lens. Bessel did not live to see the discovery, but he was convinced that the universe has its dark secrets: “There is no reason to assume that brightness is an essential property of celestial bodies. Countless visible stars do not exclude numberless invisible ones.” As we have discussed earlier, the faint high-density white dwarf companion of Sirius is not totally devoid of light, but modern astronomy speaks about genuinely dark substances whose physical nature is still unknown.