An important historical question related to Hubble's momentous 1929 discovery concerns its relationship to contemporary work in the general theory of relativity. That the invention of cosmological solutions based on general relativity occurred at precisely the same time that Slipher and Humason were beginning to detect large systematic nebular red shifts was simply a coincidence. The two developments were largely independent. The advances in telescopic instrumentation that made the nebular research possible followed from improvements in technology and the increased financial support for astronomy in America from government and philanthropic foundations. General relativity, by contrast, developed within a central European scientific culture, with a strong emphasis on advanced mathematics and pure theory. In retrospect, it seems that Hubble's relation would have been detected inevitably with improvements in the size, quality, and location of observing facilities; it could well have been discovered earlier or later. It is nonetheless a fact that throughout the decade leading up to the 1929 breakthrough, speculation about the red shifts was often tied in with theorizing in relativistic cosmology. Hubble was aware of de Sitter's writings and explicitly cited the de Sitter effect in the 1929 paper. It was also the case that general relativists such as Eddington were among the first to explore the implications of Hubble's discovery in terms of dynamical world solutions.
Although the observational discoveries of the period were independent of theoretical work in relativistic cosmology, the converse cannot be said to be true. At the time he wrote his 1917 paper de Sitter was aware of Slipher's findings through a report on them published by Eddington in the Monthly Notices of the Royal Astronomical Society. A more detailed description of these findings was presented by Eddington in his 1920 book Space, Time and Gravitation, where he wrote, "The motions in the line-of-sight of a number of nebulae have been determined, chiefly by Professor Slipher. The data are not so ample as we should like; but there is no doubt that large receding motions greatly preponderate" (161) It is significant that Friedmann, in his 1922 paper, cited both de Sitter's paper and the French translation of Eddington's book. The very high red shifts reported in these sources certainly would have raised doubts about Einstein's assumption of a static universe and suggested the possibility of dynamical cosmological solutions of the field equations. It is also known that Slipher's findings were reported in 1923 in a widely read Russian scientific magazine published in Petrograd, Friedmann's home city. The case of Friedmann is interesting because he, more than Lemaitre, is often seen as someone who was uninfluenced by observation and whose geometric solutions represented a prescient achievement of pure theory. It should be noted that one of the key assumptions of his relativistic solution, the dependence of the scale function only on the time, was later found to hold for the universe as a whole. The relativists were not working in complete isolation from observational work, although it is nonetheless the case that the emergence of dynamic theoretical solutions at precisely this time was a highly unusual event of which there are few parallels in the history of science.
In the work during the 1920s on relativistic cosmology, no one, with the possible exception of Carl Wirtz and Howard Robertson, had predicted a linear red shift-distance relation or made an attempt to configure the spectroscopic data to what was then known about distances to nebulae. The very status of the nebulae, much less their distances, was only being clarified during this period. To understand why an expansionist interpretation of the universe was not generally considered before 1930, it is also important to understand the intellectual atmosphere of the 1920s. What most struck scientists of the period about the spectroscopic data was the fact that it might well consist of a verification of Einstein's radical new theory of gravity. It was this theory and its revolutionary implications that excited scientists. The nebular spectral shifts seemed to offer clear and unequivocal evidence for general relativity, much clearer than the fine discriminations involved in interpreting eclipse observations. The focus of scientific attention was on the meaning of the observational data for general relativity and not on the possible fact of universal expansion.
After 1929, when expansion seemed to be the most probable interpretation of Hubble's law, the general relativists were able to turn to the until then neglected dynamical solutions of Friedmann and Lemaitre. It is worth noting that Hubble regarded the concept of an expanding universe as a notion rooted in the general theory of relativity. In retrospect, it seems clear that if one accepts the red shifts as due to real velocities—and this is the most obvious explanation—then it follows that the universe is expanding, a conclusion that requires for its warrant no particular theory of gravity, much less the formidable machinery of general relativity. In 1933 Eddington wrote that the theorists had for the past 15 years been expecting something "sensational" along the lines of Hubble's discovery (and there could be no finding more sensational than Hubble's) and seemed almost to be taking some credit on behalf of the theorists for the discoveries coming from the great American observatories.
There seems little doubt that Hubble was concerned with emphasizing the purely phenomenological character of his result: its independence from contemporary theorizing in mathematical cosmology. To concede that the red shifts were recessional velocities was in Hubble's view to accept an underlying theoretical approach to cosmology and possibly to suppose that an achievement of skilled observation owed something to the "invented universe" of the theorist. As Hubble (1936, vii-viii) emphasized, "the conquest of the Realm of the Nebulae is an achievement of great telescopes."
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