Explaining a few discoveries

The editorial “Discoveries and explanations ” by Charles Day in the March 2017 issue of Physics Today (page 8) discusses the work of my late colleague Vera Rubin. She showed that the rotation curves of stars in the outskirts of spiral galaxies were flat rather than Keplerian, which implied the presence of large amounts of dark matter. The history of that discovery deserves some elaboration, not to diminish Rubin’s influential work but to highlight its precursors.

Rubin’s first paper reaching that conclusion, ¹ with coauthors W. Kent Ford Jr and Norbert Thonnard, was published in 1978. Over the preceding decade, several researchers had already found that the rotation curves of neutral hydrogen gas in spiral galaxies were flat, and they concluded that those galaxies contained at least as much dark mass in their outskirts as the mass in visible stars and gas. ² In 1974 two independent groups, one in the US ³ and one in Estonia, ⁴ used those results, along with fragmentary evidence from various other sources, to argue that galaxies were surrounded by extended halos of dark matter containing up to 30 times the mass in visible stars. Reference is particularly notable because it estimated that relative to the critical cosmological density, the density Ω of dark and luminous matter was ~0.2, remarkably close to the best current estimate ⁵ of Ω = 0.308 ± 0.012.

Many of the earlier papers are cited in Rubin and coauthors’ 1978 paper, which states explicitly that “[Morton] Roberts and his collaborators deserve credit for first calling attention to flat rotation curves.”

Like Saul after his conversion on the road to Damascus, Rubin accepted a revolutionary idea after it was fully formulated, and she became one of its most effective advocates.