Trust your gut and follow where the science leads.
Did you know that the elements in the periodic table make up less than ⅙ of the matter in our universe?? We call that other 5/6s dark matter because it doesn’t emit light the way normal matter does.
And get this: we don’t even know what dark matter is! But if we can’t see it, and we don’t know what it is, how did scientists discover it in the first place? Well there’s one scientist in particular we should thank for helping discover the other 5/6s of the matter in our universe. Her name is Vera Rubin.
"Through her perseverance, Dr. Rubin completely revolutionized physics, astronomy, and cosmology ... She trusted herself, stood firm, and earned her place as one of the most influential astronomers of all time."
Watch the video or continue reading the transcript below!
Vera's Early Life and Education
In high school Vera felt like an outsider in her physics class of mostly boys, and the teacher mostly ignored them since he wasn’t used to having girls in his class. Even when Vera told her teacher she was planning to go to college, he advised her to stay away from science. But Vera ignored his advice and went to Vassar College to study astronomy on a scholarship.
She graduated from Vassar in 1948, and for her master’s degree she attended Cornell where her husband, Bob, was finishing his PhD in chemistry.
Vera continued working hard at Cornell. Unfortunately, the feedback on her master’s thesis was not very positive… A lot of very famous astronomers were not convinced by her observational data, but she stayed firm. Now, astronomers say that her results were impressive considering the kind of data that was available back then.
In 1951, Vera started working on her PhD in astronomy at Georgetown University by taking night classes two nights a week. She was pregnant with her second child by then! Her parents babysat her son, and her husband waited in their car and ate his dinner while Vera was in class.
Dr. Rubin, a Research Scientist
She finished her PhD three years later, in 1954 and stayed at Georgetown for a few years to do research and give lectures. By the 1960s Dr. Vera Rubin was an established astronomer, and was winning valuable observation time using telescopes in California, Arizona, and Texas. By then Vera and Bob had four children.
Around this time Dr. Rubin met another scientist named Dr. Kent Ford who had built a very nice version of a spectrograph. They could use the spectrograph device to observe the Doppler Effect and therefore measure the speed of the stars in a galaxy. Kent and Vera spent many nights observing...
As the stacks of photographic plates grew taller and taller, Vera began to realize something very strange about the stars they were observing-- they were all moving much faster than expected. When she plotted the star speeds against their distance from the center of the galaxy, Vera found the galaxies had flat rotation curves.
Okay, so wait. What’s the big deal with a flat rotation curve? Let’s take a little detour to talk about forces…
A Short Math Break
Gravity (FG) is an attractive force generated by objects with mass (m).
Everything in the universe that has mass (m) pulls on all other massive things with a force (FG) that is proportional to one over the square of the distance between them
This means that as the distance from the gravitational source (R) increases, the force (FG) decreases according to the square of that distance (R).
As the distance R (on the x-axis) increases, the force (on the y-axis) decreases. Try sliding the blue dot to see how the force responds as the distance changes.
Think for a second about twirling a yo-yo in a circle using its string:
In this case the string is exerting a force (Fc) that continuously pulls the yo-yo (m) toward the center of the circle... The string’s force is what keeps the yo-yo traveling in a circle-- if the string were to break, the yo-yo (m) would fly off because of its velocity (v).
The velocity of an object in circular motion is precisely determined by the centripetal force pulling it towards the center (Fc):
In the case of a galaxy, a star is the "yo-yo" (m) and gravity is the "string" (FG) holding the star in its orbit.
The force of gravity is (FG) is equal to the mass of the star, times the mass of the galaxy, times the gravitational constant, divided by the star’s distance from the center of the galaxy squared.
When set equal to the centripetal force equation, we can rearrange the variables:
And we see that the star’s velocity is equal to the square root of the mass of the Galaxy (M), times the gravitational constant (G), divided by the star’s distance from the center (R).
Flat Rotation Curves
Okay, so remember that Vera was measuring the velocities of stars versus their distance from the center of the galaxy. Based on the calculation we just did, the plot of star velocities should look something like this:
But instead, over and over again Vera found that the curves were flat, like this:
This meant that the stars were moving WAY faster than seemed possible! Let’s look at our equation again to see what could be going wrong.
Well, if we trust our understanding of gravity, then this equation must be true. Big G is a well-measured constant, so it’s unlikely anything can be going wrong there. At this point astronomers were really good at measuring cosmological distances, so there’s no chance that R was measured wrong… The only other option is that something is wrong with big M, the mass of the galaxy.
"Dr. Rubin had made an incredible discovery ... her analysis was some of the first really undeniable evidence of something wonky going on with gravity in galaxies and the universe, and reignited the concept of dark matter."
It turns out that in the 1930s, an astronomer named Dr. Fritz Zwicky had proposed something called “dark matter”, but the idea didn’t really take off. Fritz had noticed something funky was going on with matter in the universe-- there was more gravity than there were stars that could be generating the gravity! Fritz said the missing matter must be out there somewhere, and he called it “dark matter” since it didn’t emit light. While collaborating with Dr. Ford, Dr. Rubin had made an incredible discovery… her analysis was some of the first really undeniable evidence of something wonky going on with gravity in galaxies and the universe, and reignited the concept of dark matter.
Controversy sure seemed to follow Vera and her research, and by this point Vera had a lot of practice pushing through negative feedback and trusting herself and her science. It took a really long time for the scientific community to be convinced of the existence of dark matter. But there was no denying the stacks and stacks of flat rotation curves Drs. Rubin and Ford had observed.
"But Vera knew she loved the stars. She trusted her calculations because she studied hard and worked carefully."
Almost 50 years later, we have a lot of evidence that it must exist, and many many physicists have set their minds to figuring out what dark matter could be.
Pursuing the Unexpected
There were many times throughout her life that people doubted Vera Rubin. They doubted she could be a good scientist, they doubted she could breakthrough and make an impact as an astronomer…
They doubted that a wife and mother could participate in scientific discussions and collaborate with men. They doubted that the analysis of a young scientist could be correct and indicative of stranger-than-fiction truths.
But Vera knew she loved the stars. She trusted her calculations because she studied hard and worked carefully. She surrounded herself with people who asked great questions and taught her new ways to think about things. She didn’t let other scientists take advantage of her youth or gender, and she fought for her and her students’ contributions to be recognized properly!
Through her perseverance, Dr. Rubin completely revolutionized physics, astronomy, and cosmology. She dedicatedly pursued answers to questions she found fascinating and boldly shared her findings, especially when they were unexpected or controversial. She trusted herself, stood her ground, and earned her place as one of the most influential astronomers of all time.
Most images are courtesy of the Carnegie Institution for Science, DTM Archives.
Written by Madelyn Leembruggen
Edited by Ella King, Rachel Glanton
Original artwork and portrait by Sar Carol
Cartoons by Madelyn Leembruggen
Accompanying activities by Rachel Glanton
Primary sources and additional reading:
Vera Rubin's Oral History Session I and Session II from the American Institute of Physics
"Vera Rubin and Dark Matter" from the American Natural History Museum
"Vera Rubin, giant of astronomy" from Symmetry Magazine
Learn more about Dr. Vera Rubin's science with these activities!
Play (20-30 minutes): Make your own "spiral galaxy"
Remember (5-15 minutes): Crossword
Investigate (20-40 minutes): Drs. Rubin and Ford were able to measure the speed of stars by examining the Doppler effect within the stars' spectra. Explore how (relative) star speed can shift the stellar spectrum.
Expand (1 hour): Explore the effects of normal and dark matter on orbital velocity and total enclosed mass in our own Milky Way Galaxy, using this advanced Dark Matter Simulator.
Challenge (2-3 hours): Build your own spectrometer to explore how light can be split into all the colors of the rainbow and give us clues about the motion and makeup of stars.