When I was an undergraduate at Princeton University during the early 1970s, the astrophysics department was abuzz with talk about dark matter. Two faculty members, Jeremiah Ostriker and James Peebles (see Physics Today, December 2019, page 14), had recently completed a series of computerized N-body simulations of rotating disk galaxies, and the results were astonishing. The models, whose mass distribution reflected canonical estimates of luminous material seen in photographs, proved to be grossly unstable, which conflicted with the Milky Way’s acknowledged long-term stability. To steady the rotating disks, Ostriker and Peebles superimposed on each a weighty spherical “halo” of matter—a plumped-up version of the seemingly sparsely populated haloes around observed galaxies.
It was at that time that I first heard of Vera Rubin, a researcher at the Carnegie Institution for Science in Washington, DC, whose observations of galactic rotation substantiated the models’ unexpected findings. Taken together, they provided sound evidence for the presence of a great abundance of dark matter in the universe. Even as a callow undergraduate, I sensed scientific history in the making.
Astronomer Vera Rubin pictured during the 1980s with her collection of antique globes.
Astronomer Vera Rubin pictured during the 1980s with her collection of antique globes.
Rubin’s long and illustrious career—and how her name became synonymous with dark matter—is chronicled in Vera Rubin: A Life. The authors, Jacqueline Mitton and Simon Mitton, have each published prolifically on astronomy and its history, and their latest book is a scientific narrative told from the human perspective.
The book alternates between biography and semitechnical exposition. Thus we encounter tutorials on celestial spectroscopy and Kepler’s laws along with descriptions of life stressors faced by dual-career couples; notably, Vera and her husband, mathematical physicist Robert Rubin, faced the “two-body problem” of securing employment in the same locale. The opening chapters span the 1950s and 1960s, and given the time period, it’s no surprise that Robert’s career initially took precedence, at times to the detriment of Vera’s. In one job inquiry, she felt compelled to explain that science was “too large an interest for me to ever stop work, even though my family will always be uppermost in my life.”
When Rubin began her career in the 1950s, dark matter had long been the Bigfoot of astronomy: rarely glimpsed, and then only inferentially. Its observable ramifications were articulated in 1844 by astronomer Friedrich Bessel, who 10 years earlier had detected a periodic undulation in the proper motion of the bright star Sirius and which he had since concluded was gravitationally tugged by an unseen companion. (In 1862 telescope maker Alvan Graham Clark observed the elusive object—not what we now take to be dark matter but, rather, a dim white dwarf star—while field-testing a new objective lens.) As Bessel wrote to naturalist Alexander von Humboldt, “The visibility of countless stars is no argument against the invisibility of countless others.” Nevertheless, observational astronomers of the late 19th and early 20th centuries paid little heed to the presence of matter they could neither see nor photograph through their telescopes.
In 1932 astronomer Jan Oort reported an unexpectedly large velocity dispersion perpendicular to the galactic plane among stars in the solar neighborhood. To explain that phenomenon, he proposed extrapolating the local stellar mass function to stars below the threshold of visibility. Concurrently, astronomer Fritz Zwicky found a hugely discordant velocity dispersion in members of the Coma galaxy cluster; assuming the system to be gravitationally bound, he asserted that dark matter must be present in a much greater amount than luminous matter.
Other studies followed, with like conclusions, but it was not until the 1970s that dark matter emerged from the shadows. The observational impetus for that belated recognition was the galactic rotation curve—namely, the tendency of a galaxy’s rotation speed to remain relatively constant with distance from the galactic center. The curve’s “flatness” was the ostensible manifestation of copious amounts of dark matter, as predicted by galaxy models of that period. Rubin, in collaboration with Kent Ford, became the key figure in extending rotation curves based on optical-wavelength studies to large galactic radii, where their prevalent flatness dovetailed neatly with results from radio-wavelength observations.
Rubin’s life story is one of perseverance in the face of occupational and societal obstacles. As the Mittons’ meticulous account of Rubin’s career arc demonstrates, no single breakthrough vaulted her to prominence; rather, it was a string of self-directed efforts to wring research results out of cataloged data and to acquire suitable observational data where they did not yet exist. She networked tirelessly on her own behalf and secured access to large telescopes traditionally closed to women astronomers.
The authors vividly depict Rubin’s formative early-career huddles with astrophysical luminaries, such as Margaret Burbidge, Geoffrey Burbidge, George Gamow, Walter Baade, and Allan Sandage. They also describe how she later mentored a younger generation of astronomers. Astronomy enthusiasts will enjoy the book’s behind-the-scenes peeks into the profession—several of them cringeworthy.
Rubin’s story illustrates the resistance of the scientific community to altering an established paradigm—that light is the essential gauge of mass in the universe. Chapter by chapter, one senses the gathering force of theoretical and observational evidence for the existence of dark matter, until its impact on an array of astrophysical problems could no longer be dismissed. How does scientific transformation occur? To quote Ernest Hemingway: “Gradually and then suddenly.”
Alan Hirshfeld is a professor of physics at the University of Massachusetts Dartmouth. He is the author of Starlight Detectives: How Astronomers, Inventors, and Eccentrics Discovered the Modern Universe (2014), among other books.
