How Vera Rubin broke barriers and convinced the astronomy community that dark matter exists
03 Nov 2021
Taken from the November 2021 issue of Physics World where it first appeared under the headline "Illuminating dark matter". Members of the Institute of Physics can enjoy the full issue via the Physics World app.
Emma Chapman reviews Bright Galaxies, Dark Matter, and Beyond: the Life of Astronomer Vera Rubin by Ashley Jean Yeager
In the early 2000s, I sat down at a desk in Durham University library and, behind a tower of ageing books, I began researching my undergraduate dissertation. My topic was the discovery that the matter in the Milky Way galaxy, and indeed the universe, is mostly composed of a mysterious and invisible substance called “dark matter”. I chose the subject out of curiosity. Who had figured dark matter out? How could we have missed it? Well, the answer to the former is Vera Rubin, and the answer to the latter is that, in her words, the universe had been “unkind” and “played a trick” on us all.
But perhaps Rubin was the subject of another trick too. “A flip of a switch, and astronomer Vera Rubin disappeared.” This is the first line of the biography Bright Galaxies, Dark Matter, and Beyond: the Life of Astronomer Vera Rubin by Ashley Jean Yeager, associate news editor at Science News. The line directly refers to Rubin beginning a cold night of observations, but it also hints that the book will not shy away from Rubin’s struggle to gain visibility in the astronomical community. This is Yeager’s first book, and it benefits from her background in journalism and science writing, leaving no stone unturned as it rigorously illuminates Rubin and her discoveries.
As Yeager shows, Rubin – who died in 2016 at the age of 86 – was not a scientist who was content for her career path to be steered by more powerful figures, even her own supervisors. She declared she had no interest in the PhD project she was assigned, which was identifying the Sun’s spectral lines. Instead, Rubin wandered around for something else, searching out the infamous George Gamow and settling on exploring faraway galaxies. The subject of galaxies was one she would stick with long-term, eventually focusing on the motion of stars within them.
The stars in a galaxy orbit its centre of mass, which is usually a supermassive black hole, and Rubin could measure the speed at which those stars were orbiting using the spectrum of light they were emitting. The laws of gravity are clear on how the speeds should change: they should travel more slowly with increasing distance from the galactic centre, as the mass of luminous matter decreases. No-one was interested in proving something so obvious. Why would anyone question it?
Well, Rubin did, and for two reasons. First, she did not accept assumptions readily, and second, because it was a nice, quiet research topic. She liked to work at her own pace because she “would rather drop dead than have another astronomer find I made a mistake”. Rubin measured these stellar speeds with no small contribution from Kent Ford, who built the required technology and observed alongside her for years. What they found came as a shock: the plots of observable speeds versus distance (rotation curves) were flat, implying there was a lot of invisible mass at outer radii, changing the field of astronomy forever.
This is one of the most significant, paradigm-shifting events in astrophysics, and Yeager follows the discovery in a way I have not come across before, keeping me hooked on each page. Most coverage suggests that Vera Rubin discovered dark matter by observing flat rotation curves in 1978 and that was that. A singular event with a singular character. This is an understandable perspective. Women in science have so often had their work co-opted and their contributions diminished, not to mention endure and face blatant sexism on many occasions, and Rubin is no exception; one of the observatories that refused her access to its telescopes barred her with the excuse that the toilet was only for men. So in hindsight, there is a strong temptation to put hers as the only role.
Yet I appreciate how Yeager presents Rubin’s discovery in a more complex context, giving credit to all those who contributed over the years, while never diminishing Rubin’s towering achievement. There had been suggestions of a hidden form of matter before, primarily from Fritz Zwicky in 1933. Zwicky observed the Coma galaxy cluster and noticed that the rotational velocity of the luminous matter was far greater than that implied from the luminous mass. He even referred to this as dark matter.
Zwicky’s observations were riddled with legitimate uncertainties, though, such as his estimate of the distance to the cluster (he had overestimated the fraction of mass quite significantly, but his method was sound). The field was aware of his results, but not convinced. In addition to Zwicky’s optical observations, the radio community had made observations of flat rotation curves similar to Rubin’s – something that she took pains to acknowledge.
What the book also conveys tangibly is the time it took Rubin to convince the astronomy community of her results, and it is here that her fierce persistence shines through. She produced flat curve after flat curve for over a decade, until her colleagues could no longer fight against such an accumulation of evidence, and the scoffing and denial petered out. Did it take so long only because they were being asked to believe in something they could not see? Or was it also because those contentious observations had been taken and presented by a woman?
Her contemporaries must have thought the latter was probable – her Master’s thesis adviser, William Shaw, had previously offered to present results on her behalf. But she turned this offer down, curtly and firmly. Rubin was bold and undeterred by criticism. Interested only in presenting meticulous results, her voice was loud, and she was in fact an established, if controversial, figure in the astronomical community at the time.
I have never admitted to this before, but an undiscovered gender bias ran deep within me when I sat down at that book-laden table in Durham to write my essay. I had heard the name Vera Rubin before, but despite “Vera” typically being a female name, it surprised me to find out that this legend of astronomy was a woman. This is unbelievable to me now, but it shows how much I had internalized the idea that only men led the history of my chosen subject. I’m pretty sure that my later activism in gender equality is a subconscious penance for this unforgivable blunder.
The thread of sexism faced by Rubin pops up in the book where relevant, and there is a more detailed discussion of it towards the end, but it is Rubin’s love of astronomy (an “extra family member”, as she called it) that Yeager prioritizes. I like this. Rubin would want to be remembered for the science, and Yeager honours that wish.READ MORE
There is one voice I would have liked to hear a little more of, and that is of the author herself. In giving so many others their rightful place, the passion that drove Yeager to write the book gets a little lost. It was only within the acknowledgments that I realized her dedication, interviewing Rubin over a decade ago, and even accompanying her on an observing trip. Yeager’s website mentions that Rubin “didn’t just teach me about dark matter, she taught me how to live life”. I would have loved to hear a bit more of this personal insight, but this is my only gripe of substance.
Bright Galaxies, Dark Matter, and Beyond is a mesmerizing read, and a book I will continue to dip into. Serving first and foremost as a biography, it provides more than enough information for the non-expert to understand the scientific aspects, while even someone well-versed in the story will learn something new on every page (I’m in this game and I counted 60 page-markers by the end). Rubin’s research may have revolved around dark matter, but Yeager has flipped the light switch back on and presented her as an astronomer and human being, leading her out of the shadows for good.
2021 MIT Press $24.95hb 256pp
Emma Chapman is an astrophysicist and author of First Light: Switching on Stars at the Dawn of Time
from physicsworld.com 3/11/2021
Δεν υπάρχουν σχόλια:
Δημοσίευση σχολίου