Friday, December 30, 2016

Raising a Glass to Vera Rubin & Dark Matter

“Fame is fleeting. My numbers mean more to me than my name. If astronomers are still using my data years from now, that’s my greatest compliment,” astronomer Vera Rubin told Discover in 1990. In honor of her passing on Christmas day, this post will focus on her data.

Astronomer Vera Rubin at her "measuring engine" used to examine photographic plates, 1974.
Image Credit: Courtesy Carnegie Institution of Washington

It’s hard to imagine a physics blog without dark matter. Dark matter is one of the greatest mysteries of the universe, a shadowy force pulling countless researchers, students, and NOVA-watchers deep into the worlds of astronomy and physics. As with all scientific advancements, the history of dark matter can be traced back generations as one scientist’s work builds on another. However, there is no question that Vera Rubin was at its center.

Rubin began work at the Department of Terrestrial Magnetism of the Carnegie Institution for Science in Washington, DC in the 1960s. Along with colleague Kent Ford, she took an up-close and personal look at the Andromeda galaxy (known as M31). Using a tool Ford had recently developed, the two were able to observe stars previously too faint to study and map how fast the stars rotated around the center of the galaxy.

Imagine a galaxy as a two-dimensional disc of stars and gas rotating around its center. As you move farther out from the center, the objects in the galaxy should rotate around the center more slowly than the ones closer in. That’s what Newton’s law of gravity predicts and what we observe in similar systems like planets orbiting a star—if Mercury moved as slowly as Neptune, it'd crash into the sun, rather than falling around it and staying in orbit. By observing the rotation curve of a galaxy (the orbital speed of objects versus their distance from the center), you can learn about the mass of the galaxy. That was the goal of this project.

In a 2003 article for Physics Today, Rubin wrote, “Standing at the telescope in a black dome during a many-hour exposure, guiding by a nearby star, I found the greenish glow of the M31 nucleus exhilarating and a little spooky. Often I wondered if an astronomer in M31 was observing us. Always I wished we could exchange views.”

The breakthrough came with the data. The stars rotating within M31 did not follow the curve predicted by the accepted theory of gravity. Those around the outside of the galaxy moved much faster than expected, leading to a rotation curve that flattened out as you moved away from the center instead of decreasing. Measurement of galaxy after galaxy led this same result.

When asked in a 1989 interview by Alan Lightman whether anyone doubted this data, she responded, “No, no one did. It just piled up too fast. Soon there were 20, then 40, then 60 rotation curves, and they were all flat. My recollections are that no one doubted the data. . . The interpretation was more complicated.”

Today, nearly all scientists have come to the consensus that these flat rotation curves are explained by dark matter. In other words, galaxies do obey the law of gravity, but they contain a huge amount of mass that we can’t see. The composition and properties of dark matter are still up for debate, and we have yet to detect a particle of dark matter, but it’s all part of the adventure. Rubin was driven by curiosity, so it seems fitting that her data is driving one of the biggest unanswered questions in science today.

Although numbers may have been more important to Rubin than her name, her character and fierce support for women in science are equally worth remembering. Google her name and you’ll see stories liberally using adjectives like brilliant, badass, graceful, icon, pioneer, game-changing, ground-breaking, kind. She unapologetically believed that women could do science as well as men, and that scientific literacy was essential in politics. The American Institute of Physics has a nice collection of photos, interviews, and articles remembering Vera Rubin that reflect her contributions to both the data and culture of science.

It’s been a year of loss for science. As 2016 comes to a close, many people are mourning the loss of role models and mentors. During this period of reflection, let’s take a moment to raise a glass to the members of the community that, like Vera Rubin, spent their careers pushing the limits of our knowledge and equipping the next generation to go farther. Cheers, and may the mysteries of the universe continue to drive our curiosity in honor of Dr. Rubin.

Kendra Redmond

Interview of Vera Rubin by Alan Lightman on 1989 April 3, Niels Bohr Library & Archives, American Institute of Physics, College Park, MD USA, www.aip.org/history-programs/niels-bohr-library/oral-histories/33963

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