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2022-May-31

Vera Cooper Rubin: Uncovering Dark Matter, a Missing Chunk of the Universe

This article is part of the “Building from Diversity” project.

Written by Anjana Kaushik Talluri, PhD student at the University of Minnesota

 

A whopping 95% of the Universe is hidden! The visible, or baryonic, matter that we are familiar with accounts for a mere 5% of the Universe, while the rest comprises “dark matter” (27%) and “dark energy” (68%).

 

Vera Rubin gave the first strong evidence for the existence of dark matter in the 1970s by measuring the rotational rate of galaxies from their spectra. Credit: Vera C. Rubin Photograph Collection, circa 1942-2012, Department of
Terrestrial Magnetism, Carnegie Institution of Washington, Washington, D.C.

While the fact that dark matter exists is supported with irrefutable evidence today, just a few decades ago, this idea was unthinkable. In 1933, astronomer Fritz Zwicky discovered a glaring discrepancy in the mass of galaxies in the Coma Cluster inferred from the light observed and the total mass calculated from the rotational velocities of the galaxies, which for obscure reasons ended up being much higher. Though Zwicky correctly attributed this difference in mass to invisible matter, there was, unfortunately, insufficient technology available to back up his claims. Decades later, Vera Rubin, a preeminent American astronomer, opened a window to the field of dark matter by presenting concrete observational evidence to the astronomy community, which eventually forced them to take the argument seriously. The story of how the life of Vera Rubin is intriguing and an inspiration for many generations to come.

 

 

 

In 1963, on a clear night at the Kitt Peak Observatory in Arizona, Vera Rubin and her collaborator, Kent Ford, looked at the spectra of young, hot stars in our nearest neighbor, the Andromeda galaxy, to measure their speeds about the center of the galaxy. Having shown in her prior work that galaxies rotate about a central point, Rubin was passionate to learn more about the motion of the stars in a galaxy. Thanks to Ford’s new image tube spectrograph, there was a drastic reduction in the exposure time, and they were able to obtain multiple (4-5) spectra each night. On the first night, as Rubin alternated between developing the images that Ford observed and eating ice cream, she realized that she had stumbled upon a puzzling mystery— the rotational curves she obtained seemed to indicate that the speeds of the stars in the outer parts of the galaxy were quite high, enough to fling them out of the gravitational pull of the galaxy! And yet, these stars remained in stable orbits. In a spiral galaxy like the Andromeda, where most of the light (and therefore, the corresponding mass) is concentrated in the central regions, one would expect from Newtonian physics that objects at larger radii would have lower orbital velocities due to a reduced gravitational force, much like the planets in our solar system. However, the flat rotational curves, indicating a constant orbital speed with distance, hinted at the existence of a form of matter in the outer regions of the galaxies that was invisible. Through painstaking work, Rubin continued to measure the speeds of more than 60 galaxies, all of which also showed flat rotation curves. While it was far from a smooth ride, eventually, Rubin was successful in throwing light on the existence of dark matter.

 

On the left, a galaxy governed only by visible (baryonic) matter. On the right, the actual rotation speed of a galaxy as observed by Vera Rubin, which demonstrated the existence of a non-visible matter (dark matter). Credit Wikimedia Commons (User Ingo Berg/Forbes/E.Siegel)

Rubin’s curiosity and love for the Cosmos was highly palpable right from a young age; 11-year-old Rubin loved looking at the stars from her bedroom window, and she took pride in building a telescope from scratch with the help of her father. After completing her bachelor’s degree as the only astronomy student at the all-women’s Vassar College, Rubin applied for and was denied admission at Princeton University, which was not accepting female students at the time. She instead pursued her master’s at Cornell University, where she focused on studying the large-scale velocity distribution of galaxies and later earned her PhD in Astronomy from Georgetown University.

 

Throughout Rubin’s career, as was typical of the time and, sadly, to a large extent even today, gender discrimination was highly prevalent. In fact, this was apparent in Rubin’s life quite early on. Not only did her high school physics teacher ignore the girl students in the class, but when Rubin informed him of her acceptance into Vassar College, her teacher replied, “You should do OK as long as you stay away from science.” She did not let such incidents deter her, however, and was an active champion of women’s rights and gender equality. At conferences, for example, she would call ahead to make sure that women were included in the mix of keynote speakers. In 1965, a time when women were not granted access to state-of-the-art telescopes such as the Palomar, Rubin not only became the first woman to gain access to Palomar, but she also played a key role in helping women gain access to the bathrooms and living quarters at the observatory, which were otherwise reserved only for men. With abundant passion and the support of her family, Rubin overcame every obstacle and kept pushing the boundaries of the male-dominated academia.

 

Vera Cooper Rubin, in 2009 at the Women in Astronomy and Space Science Conference sponsored by NASA. Credit: NASA

Rubin’s work was the cornerstone of dark matter research. Her findings revealed a large missing chunk of the Universe and opened up a new field for the following generations of astronomers to explore. Her unparalleled contributions to this field should have truly won her a Nobel prize!

 

With the aid of cutting-edge telescopes, we know today that dark matter does not emit or absorb light, which makes it invisible to conventional detectors. It is present in large halos around galaxies and binds luminous matter together in gravitationally bound structures. Dark matter is most likely made of non-baryonic, exotic particles such as Weakly Interacting Massive Particles (WIMPs) which still require detection. Powerful, next-generation observatories with superior sensitivities such as the CTAO will be instrumental in helping us understand the true nature and distribution of dark matter in the Universe.

 

A much respected and beloved mentor, Rubin was heavily involved in ensuring her students received credit for their work. In fact, in her biography, she recalls a time when she refused to get her paper published when she was informed that her students’ names would not be included in it. Always caring for and willing to lend a helping hand to others, Vera Rubin will forever be remembered as a kind person and an inspiring mentor.

 

Rubin always said, “Don’t let anyone keep you down for silly reasons such as who you are, and don’t worry about prizes and fame. The real prize is finding something new out there.”

And she was a living embodiment of her advice.

 

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  1. Rubin, V. C. (n.d.).AA49-Frontmatter Ari 9 August 2011 16:7 – annualreviews.org. Retrieved May 30, 2022, from https://www.annualreviews.org/doi/pdf/10.1146/annurev-astro-081710-102545 
  2. Pinkerton, B., & Hassenfeld, N. (2021, August 17).Astronomers were skeptical about dark matter – until Vera Rubin came along. Vox. Retrieved May 29, 2022, from https://www.vox.com/22576927/vera-rubin-dark-matter-astronomy-biography 
  3. Siegel, E. (2019, July 23).Happy birthday to Vera Rubin: The mother of our dark matter universe. Forbes. Retrieved May 29, 2022, from https://www.forbes.com/sites/startswithabang/2019/07/23/happy-birthday-to-vera-rubin-the-mother-of-our-dark-matter-universe/?sh=2d8542306135 
  4. Neta A. Bahcall. Dark matter universe. Proceedings of the National Academy of Sciences, 112(40):12243–12245, 2015. doi: 10.1073/pnas.1516944112.
  5. Childers, T. (2019, June 11).Vera Rubin: The astronomer who brought dark matter to light. Space.com. Retrieved May 29, 2022, from https://www.space.com/vera-rubin.html 
  6. Vera Rubin on dark matter: A factor of ten: AMNH. American Museum of Natural History. (n.d.). Retrieved May 29, 2022, from https://www.amnh.org/learn-teach/curriculum-collections/cosmic-horizons-book/vera-rubin-dark-matter