Hubble's Cosmology

2nd Crisis in Cosmology Conference, CCC-2 ASP Conference Series, Vol. 413, c 2009 Frank Potter, ed. Hubble’s Cosmology: From a Finite Expanding Universe to a Static Endless Universe A. K. T. Assis,1 M. C. D. Neves,1,2 and D. S. L. Soares3 1. Institute of Physics “Gleb Wataghin,” University of Campinas UNICAMP, 13083-970 Campinas, SP, Brazil email: assis@ifi.unicamp.br 2. Departamento de F´ısica, Funda¸cão Universidade Estadual de Maringá — FUEM, 87020-900 Maringá, PR, Brazil email: [email protected] 3. Departamento de F´ısica, ICEx, Universidade Federal de Minas Gerais, C. P. 702, 30123-970 Belo Horizonte, MG, Brazil email: dsoares@fisica.ufmg.br Abstract. We analyze the views of Edwin Hubble (1889–1953) as regards the large scale structure of the universe. In 1929 he initially accepted a finite expanding universe in order to explain the redshifts of distant galaxies. Later on he turned to an infinite stationary universe and a new principle of nature in order to explain the same phenomena. Initially, he was impressed by the agreement of his redshift-distance relation with one of the predictions of de Sitter’s cosmological model, namely, the so-called “de Sitter effect,” the phenomenon of the scattering of material particles, leading to an expanding universe. A number of observational evidences, though, made him highly skeptical with such a scenario. They were better accounted for by an infinite static universe. The evidences he found were: (i) the huge values he was getting for the “recession” velocities of the nebulae (1,800 km s−1 in 1929 up to 42,000 km s−1 in 1942, leading to v/c = 1/7), with the redshifts interpreted as velocity-shifts. All other known real velocities of large astronomical bodies are much smaller than these. (ii) The “number effect” test, which is the running of nebulae luminosity with redshift. Hubble found that a static universe is, within the observational uncertainties, slightly favored. The test is equivalent to the modern “Tolman effect,” for galaxy surface brightnesses, whose results are still a matter of dispute. (iii) The smallness of the size and the age of the curved expanding universe, implied by the expansion rate that he had determined, and, (iv) the fact that a uniform distribution of galaxies on large scales is more easily obtained from galaxy counts, when a static and flat model is considered. In an expanding and closed universe, Hubble found that homogeneity was only obtained at the cost of a large curvature. We show, by quoting his works, that Hubble remained cautiously against the big bang until the end of his life, contrary to the statements of many modern authors. In order to account for redshifts, in a non-expanding universe, Hubble called for a new principle of nature, like the “tired-light” mechanism proposed by Fritz Zwicky in 1929. On the other hand, he was aware of the theoretical difficulties of such a radical assumption. Hubble’s approach to cosmology strongly suggests that he would not agree with the present status of the modern cosmological paradigm, since he was, above all, driven by observations and by the consequences derived from them. 255 256 Assis, Neves, and Soares 1. Introduction The concept of an expanding universe, as we are familiar with nowadays, was invented independently by the Russian scientist Alexander Friedmann and by the Belgian cosmologist Georges Lemaˆıtre, with their solutions of Einstein’s theory of general relativity applied to the cosmic fluid. Their pioneering papers on the subject were published in 1922 and 1924 (AF), as well as 1927 and 1931 (GL). The redshift-apparent magnitude (or distance) relation discovered by Edwin Hubble in 1929 (Hubble 1929), fitted nicely the new theoretical picture. The so-called Hubble’s law was precisely the one predicted by both Friedmann’s and Lemaˆıtre’s models. It was immediately raised to the status of an “observational” discovery of the expanding universe. This, of course, is not the case. The idea of an expansion is, first of all, a theoretical idea — a strange “effect” in the early empty de Sitter model. Hubble’s observations are consistent with the idea, but are not necessarily a proof of it. Hubble himself was aware of this and sought during all his life for the correct answer to the question posed by his discovery: What does cause redshifts? The two possibilities considered by him were the expanding relativistic models and the tired-light paradigm. The latter, incidentally, is not characterized by a particular physical theory, which has not yet been found. The idea was originally suggested by one of Hubble’s greatest friends, Fritz Zwicky (1929). He and Richard Tolman are gratefully acknowledged in the preface of Hubble’s The Realm of the Nebulae with these impressive words, [p. ix] (Hubble 1958): In the field of cosmology, the writer has had the privilege of consulting Richard Tolman and Fritz Zwicky of the California Institute of Technology. Daily contact with these men has engendered a common atmosphere in which ideas develop that cannot always be assigned to particular sources. The individual, in a sense, speaks for the group. By the way, this is the aim of any successful collaboration. In his Principia, Book III, Isaac Newton presented the “Rules of Reasoning in Philosophy.” His third rule reads as follows, [p. 398] (Newton 1934): The qualities of bodies, which admit neither intensification nor remission of degrees, and which are found to belong to all bodies within the reach of our experiments, are to be esteemed the universal qualities of all bodies whatsoever. In his com- ments on this rule he stated that “We are certainly not to relinquish the evidence of experiments for the sake of dreams and vain fictions of our own devising.” As we show below, this requirement was definitely fulfilled by Hubble’s approach to cosmology. 2. Hubble and His Changing Views about Cosmology Edwin Powell Hubble (1889–1953) established in 1924 that many nebulae are stellar systems outside the Milky Way, when he discovered Cepheid variables in the Andromeda Nebula using the 100-inch telescope on Mount Wilson. In 1929 he established the famous distance-velocity relation which is also called nowadays the law of redshift or Hubble’s law, (Hubble 1929). The title of his paper reads: “A relation between distance and radial velocity among extra- galactic nebulae.” In Table I Hubble presented a symbol v and called it the “measured velocities in km./sec.” As a matter of fact he and his collaborator Hubble’s Cosmology: From a Finite Expanding Universe 257 Milton L. Humason (1891–1972) never measured velocities directly. What they measured were the redshifts of these extra-galactic nebulae. But in this crucial paper Hubble considered these redshifts as representing real radial velocities of these nebulae. The main conclusion of the paper appeared on page 139: “The data in the table indicate a linear correlation between distances and velocities, whether the latter are used directly or corrected for solar motion, according to the older solutions.” The latter paragraph of the paper presented Hubble’s interpretation of his findings, the meaning he gave them in 1929, namely: The outstanding feature, however, is the possibility the velocity- distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space. In the de Sitter cosmology, displacements of the spectra arise from two sources, an apparent slowing down of atomic vibrations and a general tendency of material particles to scatter. The latter involves an acceleration and hence introduces the element of time. The relative importance of these two effects should determine the form of the relation between distances and observed velocities; and in this connection it may be emphasized that the linear relation found in the present discussion is a first approximation representing a restricted range in distance. Willem de Sitter (1872–1934) was a Dutch mathematician, physicist and astronomer. In 1916–17 he had found a solution to Einstein’s field equations of general relativity describing the expansion of the universe. Hubble had met him in 1928 in Leiden, where de Sitter was both professor of astronomy at the Uni- versity of Leiden and director of the Leiden Observatory, [p. 198] (Christianson 1966). This last paragraph of Hubble’s paper shows that in 1929 he thought of the expansion of the universe as a real possibility. However, it must be pointed out that, as early as 1935, Hubble was much more cautious when referring to velocities of recession. In a paper with R. Tol- man (Hubble & Tolman 1935), already in the introductory section, the authors made a plain statement of their worries about the proper nomenclature: Until further evidence is available, both the present writers wish to express an open mind with respect to the ultimately most satisfac- tory explanation of the nebular red-shift and, in the presentation of purely observational findings, to continue to use the phrase “apparent” velocity of recession. They both incline to the opinion, however, that if the red-shift is not due to recessional motion, its explanation will probably involve some quite new physical principles. Hubble was invited to deliver eight Silliman Lectures at Yale University in 1935. These lectures formed the basis of his book The Realm of the Nebulae, published in 1936. In this book he was more careful in stating what was really measured and what was interpreted. On pages 2 and 3 he mentioned that the observer accumulates data of apparent luminosities of nebulae and red-shifts of their spectra.

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