Quick– What’s the most renowned physics equation? If you said e=mc2, congrats, that’s exactly what I was hoping you’d say to make this intro work. If you thought of F=ma or P1 + ρgy1 + ½ρv12 =P2 + ρgy2 + ½ρv22 , good job, you obviously know your physics, but pretend you thought of e=mc2. It has a lot of other implications that I’m not going to even try to explain, but one of the ideas that we can gather from it is that energy (E) and matter (in this equation we use mass, which is designated by M), can be thought of as the same thing. Everything in our universe is composed of come combination of the two. Energy is surprisingly complicated, but it should be thought of as synonymous with matter for now.
But what is “matter”? It’s everything you can see, smell, touch, etc. Your cat? Fluffy, sometimes angry matter. My English Breakfast tea with far too much cream and sugar? Sweet, life giving matter. So, all the “stuff” in the universe is made of of cats and tea, right? Not so fast. But isn’t that what I just said? Yes, but that would be too simple. If I learned anything from my astrophysics classes, it’s that nothing can ever be that simple.
It turns out that less than 5% of the universe is the stuff puppies and Joss Whedon is actually made up of. We call this “visible matter” (or light matter, or baryonic matter if you’re feeling fancy), to differentiate it from the other 95% of the universe. The rest is aptly called dark matter and dark energy, with dark matter being about 23% of the universe. That leaves dark energy to be about 72% of the universe, but because we know even less about that than dark matter, I’m going to pretend that it doesn’t exist for the rest of this article. Because that’s how I live my life.
So what is this dark matter? No one really knows. We don’t even know for sure that it exists. We call it “dark” because it doesn’t interact in anyway with our beloved light matter, so we literally can’t see it. The weird thing is that the only fundamental force it does interact with is gravity, which seems pretty random. This is, in fact, the only reason we know it exists.
Enter Vera Rubin, aka one of my biggest inspirations and the reason I’m telling you about dark matter. My iPod is named after her. (All my electronics are named after female astronomers: my phone is Mary Sherman Morgan, my Kindle is Sally Ride, and my iPad is Hypatia*. I’m a dork, I know.)
Vera Rubin was born in 1928, which means she was 88 when she died in December. (That’s why I’m writing this article about her.) She was the only person to graduate with a degree in astronomy in her 1948 class at Vassar College. She wanted to get her Ph.D. from Princeton but they wouldn’t let her because, you know, sexism. So she said “boy bye,” and got her degree from Georgetown instead. The 1950s were a rough time for most working women, and astronomy was certainly no exception.
There’s a long list of ways she got discriminated against because of her sex, but she persevered. There’s a story that I really like about when she was working at an observatory with no female restrooms. She cut out a little triangle of black paper and taped it to the little plastic person on the door that for some reason we all recognize as the sign for “male restroom”. I can’t help but be charmed by that.
In the 1960s she teamed up with Kent Ford to study the rotation of galaxies. While looking at Andromeda, she was surprised to find the stars on the very outside of the galaxy were rotating almost as quickly as the stars much closer to the center. This is not what you would think should happen. Closer to the center there is more matter, so more gravity, which causes the inner stars (and dust and other space gunk) to zip by. Similarly, stars farther away should orbit much slower.
There are two reasons this might happen: either our theory of gravity is wrong, or there is more mass in these galaxies than we can see. More observations have been conducted since (she studied more than 60 galaxies), and most astronomers agree that the most likely explanation is that there is in fact more mass, which they’ve named dark matter. That is basically all we know about it, though. We know it reacts with gravity, but somewhat confusingly, it doesn’t seem to clump into dark planets. Why? It as well be cosmic fairly, for all we know. The only way the galaxies would be moving like they are is if the dark matter is in a ring around the middle of the center.
Interestingly, Rubin herself held no strong views either way. To her, we didn’t have enough evidence to satisfy either possibility. I find it fascinating that she could disagree with the New York Times when they called her “[the woman] who transformed modern physics and astronomy with her observations showing that galaxies and stars are immersed in the gravitational grip of vast clouds of dark matter”. That takes a level of humility I can only aspire to.
She was not the first person to recognize this anomaly, but she was the one to bring it to the attention of the scientific community. It was first noticed in the early 1930s by Fritz Zwicky, who was looking at clusters of galaxies. Alas, because astronomers were at the beginning stages of understanding galaxies at all, no one paid his research much attention.
According to many people, this is the kind of field-changing discovery that should warrant winning a Nobel. But because awards are never given posthumously, now she never will. Why didn’t she? There is a lot of controversy about this. It’s not because we don’t know enough about dark matter– the 2011 Nobel in physics was about dark energy, which we know even less about. Some say it’s because Fritz Zwicky found it first, but science is all about building on prior knowledge. Maybe it has to do with the fact that only two out of 204 physics Nobel laureates have been women. We’ll probably never know, but we can’t deny that she was a major contributor to our current understanding of our universe.