My favorite thing to write for this blog has always been ‘History’s Hidden Heroes’, in which I showcased the lives and accomplishments of scientists of color, LGTB scientists, and female scientists. This feature fell by the wayside when I started writing Kickass Women for Smart Bitches Trashy Books. Guest writer Max Fagin is bringing the Heroes back with his contribution about scientist Vera Rubin. bonus – unlike me, Max is actually a scientist, so his explanations of Rubin’s accomplishments are far more sophisticated than my own, which run along the lines of, “IDK, she discovered stuff, it was cool.” Enjoy!
It’s no secret that STEM has a major problem with obtaining gender parity, and astronomy is not immune to that problem. Astronomy doesn’t have it as bad as, say, computer science (with 18% women at the PhD level) but it isn’t doing as well as biology (58% women, again at the PhD level). Astronomy sits somewhere in the middle of the STEM pack with 35% of new PhDs going to women in 2012.
But as arguably the oldest science, astronomy has also been a field of many firsts for women in STEM. America’s first woman to be hired as a college professor was Maria Mitchell, who was hired by Vassar College in 1843 as a professor of astronomy. At around the same time, the first woman to receive a salary for her work as a professional scientist was the German astronomer Caroline Herschel.
The subject of today’s History’s Hidden Heroes is one of those women: Vera Rubin. A woman who overcame the biases of her day to discover an even bigger bias in the universe itself…
Up until the late 1970’s, if you were to ask an astronomer what the universe was made of, a complete list would be composed of:
1) Stars (living and dead)
2) Interstellar gas and dust
3) Black holes
5) Whatever detritus happens to be on that planet’s surface
(Note, that last category would include us). These are the things we typically think of as “normal” matter, where “normal” means stuff composed of protons, neutrons and electrons, sometimes in the form of atoms and chemicals, or ionized plasma.
However, ask an astronomer today what the universe is made of, and you will probably hear a list containing something that isn’t on that 1970’s list: Dark Matter. Vera Rubin was the first scientist to uncover observational evidence that Dark Matter was a real thing, and to reveal that “normal” matter was far from normal, but comprised only 20% of the matter in our universe. All past detection methods had been heavily biased towards detecting this normal matter, but that didn’t mean Dark Matter wasn’t real, and didn’t mean it hadn’t played a profound part in the formation and evolution of our universe.
Vera Rubin earned her BA in astronomy from Vassar college in 1957 (the college that had hired Maria Mitchell, the first woman to hold a professorship in the United States). Vassar had been founded as an all girls school in 1861 (and would remain so until 1969) but even before going co-ed, it had a reputation for producing smart and driven graduates, many of whom has already made significant contributions to science and engineering, including Admiral Grace Hopper, one of the inventors of COBOL, an early programing language. (Vassar also had a reputation in the fictional realm as well, counting among its many fictional alumni the smartest James Bond girl, NASA scientist Dr. Holly Goodhead, who proudly represented Vassar in the stupidest James Bond movie, Moonraker.)
Rubin graduated from Vassar as the only astronomy major in her class, and went on to complete her Masters in Astronomy at Cornell (after being rejected from Princeton on the grounds that their astronomy department did not admit women). Rubin then completed a PhD at Georgetown University under the famous cosmologist George Gamow.
At this era in astronomy, before the invention of space based telescopes, cosmology was focused on studying galaxies in our corner of the universe to see what their structure and distribution could tell us about the universe at large. This was the field that Dr. Rubin made her greatest discovery in during the 1970’s, while conducting observations of galactic rotation curves.
Under the influence of gravity, objects behave in extremely predictable ways. Since the days of Kepler, it had been understood that the further away an object (like a planet) was from the body it was orbiting (like a star) the slower it would be traveling in its orbit. The embodiment of this was captured in what became known as Kepler’s 3rd law, that the square of a planet’s period (the time it takes to complete one orbit) was proportional to the cube of the object’s semimajor axis (its distance from the star).
Although the geometry was more complicated on galactic scales, these rules applied to stars orbiting around the center of their galaxies as well. The farther away a star was from the center of the galaxy, the slower it should be orbiting.
But when Rubin measured the speed of stars in the nearby Andromeda galaxy, this was not what she saw. Outside the galactic core, the stars did not continue to slow down as one looked further away from the center of the galaxy. Instead, the velocity of the stars plateaued into a flat line, all the way to the edge of the galaxy where the stars stopped and intergalactic space began.
What could cause the stars to behave like that? By playing with the distribution of mass, it was quickly noticed that this “flat rotation curve” could be explained if there was some “missing mass” distributed in a spherical halo around the galaxy. The idea that astronomers might have missed some of the galaxies mass was not so far fetched. After all, galaxies contain more than just luminous stars. They also contained giant clouds of interstellar gas and dust, which can only be seen in the visible band by the starlight it reflects, or blocks out. But astronomers knew where gas and dust tended to be in a galaxy: In the galactic plane. Much like stars, it never tended to wander very far from the flat disk of the galaxy. And besides, if the missing mass was just gas and dust, an enormous amount of starlight would be obscured. Orders of magnitude more than what was actually observed in nearby galaxies.
NGC 891: A galaxy seen edge on, where dust is clearly visible from the starlight it obscures.
Astronomers began to consider more exotic possibilities. What if galaxies were surrounded by swarms of super compact dead stars? If the missing mass was composed of very small very dense objects orbiting the galaxy in a spherical cloud, their small size wouldn’t necessarily block the light from the stars (unlike the diffuse distributed clouds of gas and dust).
But in order to account for the amount of mass that was missing, these objects (and others like them, eventually referred to as MACHOS, for MAssively Compact Halo ObjectS), would have to be so numerous that they would still occasionally transit (pass in front of) a background star, causing the star’s light to briefly fluctuate in a very characteristic way. Search after search for these transiting MACHOs over the past few decades has come up empty.
Perhaps the missing mass could be explained by some very massive subatomic particle? If it was, this particle would have to be very weakly interacting, or else we would have seen it in our detectors by now. Unfortunately, these particles (now called WIMPs, for Weakly Interacting Massive Particles) have also eluded detection by every effort mounted so far to find them.
The seeming futility of the search even drove some scientists to suggest that we don’t understand gravity as well as we thought we did. Perhaps, at galactic scales, gravity required some additional correction factor that would explain the rotation of galaxies. These possibilities (called MONDs, for MOdifications to Newtonian Dynamics) originally showed a great deal of promise for explaining galactic rotation curves, but observations in the early 00’s of galactic clusters, and of elemental abundances in the early universe effectively ruled it out as an option as well.
The bullet cluster. Two clusters of colliding galaxies, the observations of which provided some of the first evidence that modifications to gravity could not explain the behavior attributed of Dark Matter.
Our existing model of gravity has withstood the test, leaving WIMPs as the best candidate for Dark Matter (though entirely by default). In the decades since Dr. Rubin’s discovery, we have simply ruled out anything else it really could be. However we slice it, ~80% of the mass in our universe is composed of “something” that doesn’t emit or obscure light (i.e. is invisible) doesn’t decay or radiate in anyway we can yet detect, and betrays its presence solely by exerting a pull of gravity on the normal matter around it (Though the term “normal” matter could now be said to be a misnomer. If anything, the matter that makes up the stars, dust, gas, planets, rock and squishy stuff that composes us is the unusual kind of matter. Physicists now prefer the term “Baryonic matter” to describe this type of everyday matter.)
The Nobel prize in physics is the most male dominated of all the original Nobel prizes. It has been awarded to a woman only twice since it was established in 1901: Most recently in 1963 when it was shared by three physicists, including Maria Goeppert Mayer for work on deriving a successful theoretical model of the nuclear shell. Before that, the only other woman to win the prize was Marie and Pierre Curie for their work on radiation.
Dr. Rubin is perhaps the best candidate to break this 50 year dry spell, and many would say that Dr. Rubin is long overdue for her prize, the original discovery being made almost 45 years ago. Such a long wait is not unprecedented (the 2013 prize for the discovery of the Higgs Bozon was the culmination of a prediction made 50 years before the awarding of the prize). However, in 2012, the Nobel Prize in physics was awarded for the discovery of the (similarly named but entirely unrelated) Dark ENERGY. While no one in the astrophysics community doubted that the discovery of Dark Energy (which outnumbers both Dark Matter and baryonic matter in our universe by 4 to 1) was an incredibly significant discovery, many people were surprised that the prize was awarded so early. The discovery of Dark Energy was only 13 years only in 2013. Some might say Dr. Rubin and the discovery of Dark Matter is overdue for its medal.
In the meantime, Dr. Rubin has retired from astronomy, but remains an active proponent of women in STEM. I had the privilege of hearing her talk about her work when she returned to Vassar while I was a student there in 2007. It was the first time I really accepted that Dark Matter was a real thing (in my defence, early 2007 was before some of the clinching observations were made of galactic clusters and cosmology that made MOND no longer a tenable theory.) Every year, when nomination season rolls around, I hope that Dr. Rubin will receive the call from the Nobel Committee, and I still think she will. Astronomy, by the nature of the subject, only attracts those who can learn to be patient, but waiting that long for the recognition a discovery like that must be a maddening prospect, even for a mind as tuned to astronomy as Vera Rubin’s.