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Hubble s Law, Redshift and the Expansion of the Universe. Extremely rare offprint issue of Hubble s landmark paper which "made as great a change in man s conception of the universe as the Copernican revolution 400 years before" (DSB). This paper "is generally regarded as marking the discovery of the expansion of the universe" (Biographical Encyclopedia of Astronomers). It established what would later become known as Hubble s Law: that galaxies recede from us in all directions and more distant ones recede more rapidly in proportion to their distance. "…the repercussions were immense. The galaxies were not randomly dashing through the cosmos, but instead their speeds were mathematically related to their distances, and when scientists see such a relationship they search for a deeper significance. In this case, the significance was nothing less than the realization that at some point in history all the galaxies in the universe had been compacted into the same small region. This was the first observational evidence to hint at what we now call the Big Bang" (Simon Singh, Big Bang). In the early 1920s, most astronomers believed that the universe was static and unchanging on the large scale. Einstein himself had introduced his cosmological constant in 1917 to allow solutions of the equations of general relativity corresponding to a static universe. Two such solutions were found: Einstein s matter-filled universe and Willem de Sitter s empty universe. The latter model attracted much interest because it predicted redshifts for very distant objects, something which had been observed as early as 1912 by Vesto Slipher. However, De Sitter s model was conceived by astronomers to be no less static than Einstein s. In 1922 Alexander Friedmann developed a model of an evolutionary universe, which could be expanding, and this was re-discovered by Georges Lemaître in 1927. But Lemaître went further: he established theoretically the proportional relationship between the rate of expansion and distance. Important as these theoretical developments were, it was only observational data that could establish which of the models, if any, corresponded to the actual universe. By the late 1920s, Edwin Powell Hubble (1889-1953) had established himself as the leading expert on extragalactic nebulae (now called galaxies ). Trained at Yerkes Observatory, in 1919 Hubble had joined the staff of the Carnegie Institution s Mount Wilson Observatory, the leading astrophysical observatory in the world, where he had access to the largest telescope in the world, the 100-inch Hooker reflector. With its aid, he had established in 1924 and 1925 that the spiral nebulae are external galaxies lying far beyond our own Milky Way galaxy, and that the observed universe is therefore much larger than our own galaxy. "By 1929 Hubble had obtained distances for eighteen isolated galaxies and for four members of the Virgo cluster. In that year he used this somewhat restricted body of data to make the most remarkable of all his discoveries and the one that made his name famous far beyond the ranks of professional astronomers. This was what is now known as Hubble s law of proportionality of distance and radial velocity of galaxies. Since 1912, when V. M. Slipher at the Lowell Observatory had measured the radial velocity of a galaxy (M 31) for the time by observing the Doppler displacement of its spectral lines, velocities had been obtained of some forty-six galaxies, forty-one by Slipher himself. Attempts to correlate these velocities with other properties of the galaxies concerned, in particular their apparent diameters, had been made by Carl Wirtz, Lundmark, and others; but no definite, generally acceptable result had been obtained. In 1917 W. de Sitter had constructed, on the basis of Einstein s cosmological equations, an ideal world-model (of vanishingly small average density) which predicted red shifts, indicative of recessional motion, in distant light sources; but no such systema.
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