probably played a role in astronomers overlooking the evidence for dark matter in these radial velocity curves. It is interesting that Vera Rubin was a collaborator with the Burbidges in five of the papers in the last two years of their work on radial velocity curves of galaxies. About the time of this collaboration, Rubin was first author on a paper that did not involve the Burbidges which examined motions of stars within the Milky Way (Rubin, et al. 1962). They found that “for R > 8.5 kpc, the stellar curve is flat, and does not decrease as is expected for Keplerian orbits.” This may have been the first clear indication that a similar problem exists in the Milky Way Galaxy. About this time, Rubin met instrument maker Kent Ford, which a few years later resulted in a decade-long collaboration extending radial velocity curves of spiral galaxies. Ford combined image tubes, the latest technology used in astronomy in the late 1960s, to boost the sensitivity of cameras recording spectra. When combined with the larger telescopes coming into use at that time (especially the two 4-meter telescopes at the recently opened national optical observatories) allowed extending good observations over the supposed Keplerian part of galaxy radial velocity curves for the first time. Being the closest and hence brightest spiral galaxy, M 31 was the first target (Rubin and Ford 1970), in which they confirmed Babcock’s earlier work. It is worth noting that more than a decade earlier, a study of 21-cm radiation of neutral hydrogen in M 31 showed the same thing (van de Hulst, et al. 1957; Schmidt 1957). Additionally, Roberts and Whitehurst (1975) extended the rotation curve of M31 beyond what Rubin and Ford had. Roberts and Whitehurst found that the mass-tolight ratio in the outermost regions of M31 had to be at least 200. Rubin and Ford spent the 1970s investigating the radial velocity curves of many spiral galaxies, culminating in 1980 (Rubin, et al. 1980). While Rubin was pursuing this work optically in the 1970s, radio astronomers were using 21-cm radiation to produce radial velocity curves of galaxies that agreed with the visible light results of Rubin and Ford (Rogstad and Shostak 1972). By the 1980s, this groundbreaking work began to convince most astronomers of the reality of dark matter. It was not until 1984 that Bond, et al. (1984) resurrected Zwicky’s original term dark matter. In the 1980s, cosmologists began to discuss dark matter within big bang models, though that discussion did not begin in earnest until the 1990s. One would think that inclusion of dark matter in cosmological models would have been motivated by the desire to have realistic models. After all, if gravity is the dominant force in cosmology, and if gravity is caused by matter, then cosmological models that omit 90% of the mass of the universe cannot be very good. However, this does not seem to have been the case. One reason why cosmologists began to include dark matter was to explain galaxy formation. The density of the universe in big bang models at the time could not account for galaxy formation. It was hoped that dark matter could help solve this problem. Another reason dark matter was considered in Figure 1. A measured rotation curve of the galaxy M33 superimposed upon an image of the galaxy. The origin of the rotation curve is at the galaxy’s center. The horizontal axis is distance from the galactic center, with radial velocity on the vertical axis. The dashed line is the rotation curve expected from the light distribution, assuming that light and mass are directly related. Beyond the turnover around 9,000 light years from the galactic center, the expected curve approaches Keplerian. Contrast this with the observed orbital velocity consisting of yellow and blue data points fitted to the solid line. FAULKNER Dark matter and dark energy 2023 ICC 3
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