Vera C. Rubin: Pioneering American astronomer (1928-2016).

Vera Cooper Rubin, an icon of astronomy whose work revolutionized our understanding of the universe by confirming the existence of dark matter, passed away on December 25, 2016 at the age of 88. Vera had a lifelong love of the cosmos. “How could you possibly live on this Earth and not want to study the universe?” shewondered. Vera was a devoted champion of women in science and a beloved role model to generations of astronomers. The astronomical community has lost a pioneering scientist; her colleagues have lost an inspiring leader and a friend. Vera’s pivotal work in the 1970s established that the orbital speeds of stars in the outer parts of galaxies remain constant to large distances (known as “flat rotation curves”), rather than decline as expected from Newton’s law, and as observed in the outer parts of our Solar system. This excess speed requires excess mass beyond what we see in stars and gas. Vera’s observations provided crucial and convincing evidence for the existence of this excess “dark matter,” a suggestion first made by astronomer Fritz Zwicky in 1933 based, similarly, on large velocities observed in clusters of galaxies. Vera’s flat rotation curves implied that galaxies are embedded in large dark matter halos; we now know that these halos contain most of the mass in the universe. Vera’s observations were carried out in close collaboration with her colleague Kent Ford, who built the sophisticated spectrograph that enabled these precise measurements. Their first results, in 1970 (1), were for our nearby Andromeda galaxy. At that time, rotation curves were also being observed with radio telescopes, tracing the rotation of the neutral hydrogen gas disks that surround spiral galaxies; these observations, carried out by Morton Roberts (2, 3) and others using the 21-centimeter line of neutral hydrogen, found similar results. Vera and Ford’s observations of the stellar rotations, using optical telescopes, were less prone to interpretation of gas dynamics. Both sets of observations ultimately revealed the striking constant rotation speeds of galaxies, thus the need for dark matter. Rubin and Ford’s observations of a large sample of galaxies in 1978 (4), all showing the same remarkable flat rotation to large distances, were crucial in confirming the existence of dark matter and the large dark halos around galaxies. We now know that most of the mass in the universe, 85%, is dark matter; it profoundly affects the evolution of the universe and the formation of structure. The dark matter is believed to be a new, exotic, yet undetected nonbaryonic particle, not the familiar atomic elements that make up stars, gas, planets, and people. We don’t yet know what these particles are. Many experiments have been searching for these particles; detecting and understanding their nature is one of the most important open quests in science. When asked what she thought of a theoretical suggestion that dark matter may not exist but instead a change in Newton’s gravity law was needed, Vera was open minded: “I don’t know if we have dark matter or need a change in gravity or need VeraC.Rubin. ImagecourtesyofMarkGodfrey (photographer).