Two years ago we lost one of my heroes, astronomer Vera Rubin. There are plenty of articles out there that talk about her work accumulating evidence for the existence of dark matter and about challenges she overcame to become an astronomer at all (including the one linked above).
But there’s one thing I’d like to add to all the other tributes, because Vera Rubin was a friend of my grandparents’, and in 2001 she spent a good chunk of one March evening on the phone with a fourteen-year-old she barely knew except by association, kindly and patiently explaining her work so that said fourteen-year-old could write a paper about it for science class. Because that fourteen-year-old was excited about astronomy, and wanted to learn, and she wanted to do what she could to encourage that.
That conversation, and the paper I wrote about it, remains one of the highlights of my academic career, and so I share that paper here, awkward sentences and crappy diagrams and all*, because Vera Rubin thought it was worth her time.
WHY DO ASTRONOMERS BELIEVE IN THE EXISTENCE OF DARK MATTER?
by Emily Gilman
Understanding the reasons behind some scientists’ belief in the existence of dark matter requires knowledge of Sir Isaac Newton’s Universal Law of Gravitation. It states that all matter exerts a gravitational force which acts on all other objects, and that gravitational force is directly related to the mass of the object. Even a single grain of sand has a gravitational pull, however it has so little mass that human beings cannot feel it and are not greatly affected by it.
The Universal Law of Gravitation also explains that another factor affecting the attraction between two objects is the distance between them. As the two objects get farther away from each other, their gravitational pulls affect each other less and less. This is not to say that if a person were to climb a very tall mountain then that person would weigh significantly less, but Neptune and Pluto, which are farther from our sun than Earth is, are affected by the sun’s gravitational pull less than Earth is.
However, the most important aspect of gravity in relation to dark matter is how gravity affects the orbits of objects in space. For example, if a spaceship is orbiting around the earth, there are two major forces on it. One is the earth’s gravity, which is pulling the spaceship downwards (towards the surface of the earth). The other is the ship’s forward movement that it maintains from positioning itself above the earth. As long as the force downward (gravity) and the force forward are equal, the spaceship is constantly falling in a curve matching the earth’s surface. If the ship’s forward motion were to decrease, gravity would pull it inward towards the earth, where it would crash. Conversely, if the ship’s forward motion were to increase, it would break away from Earth’s gravitational pull and go hurtling off into space. The orbits of plants around the sun works the same way, as does the orbit of stars and their systems around the center of a galaxy.
Astrophysicists monitored the movement of stars in other spiral galaxies and realized that the data collected did not match what was predicted. Most people have heard a plane flying overhead, or a car passing by, and noticed that as the vehicle came closer, its pitch seemed to get higher. This is because of the Doppler Effect, and involves sound waves getting shorter as the object approaches them and then longer as the object moves farther away.
Light works in the same way, but instead of different wavelengths being different pitches, different wavelengths are different colors of light. A shift towards the blue spectrum indicates that a star is moving closer to Earth, and a shift towards the red spectrum indicates that a star is moving away from Earth (blue light is a shorter wavelength than red light).
Scientists could watch as an individual star gets bluer and then redder to see where and how it was moving. But as more and more different galaxies were monitored, scientists realized that the stars at the edges of galaxies were moving so fast that they should have gone hurtling off into space. The gravitational forces of all visible matter (or matter that can be detected using special instruments) aren’t enough to hold a galaxy together.
This presents a problem to scientists, and so far there are two main theories. One: the laws of gravity are wrong, or two: there’s a lot more matter in the universe than originally thought, and human beings cannot detect it at the present moment. If the laws of gravity are wrong, then the law of relativity is also wrong, along with many other aspects of science that are taken for granted, and scientists would generally prefer to believe that the laws of gravity are not wrong, and that the galaxies are acting the way they do because of something else. That means that there must be some form of matter in the universe that is invisible not only to the naked eye but to all of the technological wonders the human race has been able to come up with until now.
Scientists have tried various experiments to discover what this “dark matter” is, but so far no one can say what it is. They can only keep hoping that their next experiment will be the one that works, because until then there is a major gap in our understanding of the universe. (V. Rubin, a personal interview, March 2001)
Bartusiak, Marcia. Through a Universe Darkly: a cosmic tale of ancient ethers, dark matter, and the fate of the universe. New York: Harper Collins Publishers, 1993. pp202-214
Snyder, Robert E. Earth Science: The Challenge of Discovery. Lexington, Massachusetts: D.C. Heath and Company, 1991. p100, 103.
Weidner, Richard T. and Michael E. Browne. Physics. Boston: Allyn and Bacon, Inc., 1985. p395
*As a former school librarian I feel the need to point out that while the above citations make me cringe, they’re pretty good by eighth grade standards, and in the interest of accuracy I am resisting the urge to fix them, just as I am resisting the urge to fix errors in capitalization and grammar. I can’t fix the lack of in-text citations, because I have no idea what I took from those sources (probably some of the stuff about gravitational laws and about the Doppler effect?), but I did fix the placement of diagrams, as well as one outright typo.