Vera Rubin and Dark Matter

In the late 1970s, astronomer Vera Rubin (1928-2016) was perplexed when she analysed the results of her observations of the Andromeda Galaxy at the laboratories of the Carnegie Institution in Washington. The great spiral of our large galactic neighbour had a strange rotation: the stars on the edges moved as fast as the ones at the centre, which violated Newton’s laws of motion (which also regulate how the planets revolve around the Sun). That result also contradicted classical mechanics, unless there was some kind of material that could not be seen. Rubin did not know it yet, but she had found the first evidence of the existence of dark matter.

It is a postulated form of matter that does not interact with common matter, does not emit light, and whose presence can only be inferred from the gravitational effects it causes on visible matter such as stars, galaxies and agglomerations of galaxies. Almost four decades after Rubin’s observation, scientists now know that this enigmatic matter makes up 84% of the Universe, although they still do not know what it is made of. Its invisible particles pass through the entire cosmos and it affects the way stars move within galaxies, how galaxies attract each other, and how the entire Universe came together in the first place.

Dark matter was proposed as a concept by astronomers like Jan Oort in 1932 and Fritz Zwicky in 1933, who also noticed discrepancies in the amount of mass that astronomers could see and the physical quantity that should actually exist, but few paid attention to their work, and their research was considered little more than a cosmological oddity. It was Rubin who realized that if a halo of dark matter adorned each galaxy, matter would spread throughout the galaxy instead of concentrating in the centre and the force of gravity and orbital velocity would be similar in all parts.

Vera Rubin fue la única mujer graduada en astronomía en Vassar College en 1948. Crédito: Vassar College Library

Vera Rubin was the only woman to graduate in astronomy at Vassar College in 1948. Credit: Vassar College Library

A few years after the discovery of dark matter by Rubin, physicists such as Jeremiah Ostriker and James Peebles provided the theoretical framework to support her work and the mysterious substance settled into its celebrated place in science. In 2013, the Planck satellite measured the dark matter content of the Universe by observing microwave background radiation, the radiation left over from the Big Bang that fills the entire Universe. The result showed that dark matter was grouped first and later the common matter was added, forming the agglomerates of galaxies. “This is fundamental in our current concept of astrophysics,” Emily Levesque, an astronomer at the University of Washington, told OpenMind.

In 2016, the Dark Energy Survey, led by Fermilab (Fermi National Accelerator Laboratory) of the United States, published a map with 26 million galaxies that presents the heterogeneous distribution of dark matter in a band billions of light years wide. The objective of the analysis is to seek explanations for the expansion of the Universe. “Either it will expand continuously or there will be enough matter to slow it down. When we look into space, we see a lot of matter, but the Universe acts as if there were more than what is experimentally observable, something with a greater gravitational influence than might be expected,” explains Bruno Fernando Ferreira, astronomer at McKenzie University in Sao Paulo.

A difficult start and an end without recognition

Vera Rubin loved science as a child, but as she progressed in her studies she realized that her great passion, astrophysics, was a field dominated by men. She was the only woman to graduate in astronomy at Vassar College in 1948 and was unable to study a doctorate in astronomy at Princeton simply because the institution did not accept women at that time. She did not give up, however, and eventually got a Ph.D. at Georgetown University. She became, albeit unintentionally, a fighter for the presence of women in science.

Rubin encontró la primera evidencia de existencia de la materia oscura. Crédito: Vassar College Library

Rubin found the first evidence of the existence of dark matter. Credit: Vassar College Library

Gender discrimination was not limited to the early years of her career. Despite decades of work and the relevance of her research and discoveries, Rubin died on December 25, 2016, at the age of 88, without the Nobel Prize that her colleagues believed she deserved. No woman has received the Nobel Prize in Physics since 1963, when Maria Goeppert Mayer shared it with Eugene Wigner and J. Hans Jensen for their work on atomic structure and theory, and the only woman before Mayer was Marie Curie in 1903.

In her final years, some of her colleagues, such as the astrophysicist Chanda Prescod-Weinstein, campaigned for no man to accept the award before Rubin received it. As Emily Levesque puts it: “The existence of dark matter has utterly revolutionized our concept of the Universe and our entire field; the ongoing effort to understand the role of dark matter has basically spawned entire subfields within astrophysics and particle physics at this point. Alfred Nobel’s will describes the physics prize as recognizing ‘the most important discovery’ within the field of physics. If dark matter doesn’t fit that description, I don’t know what does.”

Rubin herself never spoke about her merits to receive the award from the Swedish Academy. She simply continued her scientific work until shortly before she passed away. Her passion was to understand the Universe and, in doing so, she changed everyone’s understanding of it. In the world of science, that achievement, on its own, is more important than any medallion.

Joana Oliveira