Vera Rubin, a world-reknowned physicist credited as a co-discoverer of dark matter, spoke on the matter Friday in a lecture entitled “A brief history of dark matter.”
Rubin opened the lecture by saying that her title was a misnomer. “We don’t know what dark matter actually is,” she explained, “so we can’t tell its history.” She went on to rename the lecture “A brief history of how we learned about dark matter.”
Her history began in 1784, with the mention in a scientific paper of a dark object so massive that it pulls in the light around it–a black hole. The next mention of “dark objects” came in a 1885 paper that theorized that dark stars could exist. However, it wasn’t until 1931 that cosmologist Fritz Zwicky argued that dark matter was required in order to hold certain clusters together. Zwicky’s theory attracted some attention at that time, but was not largely accepted, Rubin noted.
Hubble came to a similar conclusion in 1936 when he calculated the mass/luminosity ratio of different galaxies. His calculation led to a large discrepancy between the observed mass of the galaxies and the theoretical mass. Oort discovered the same discrepancy in other galaxies four year later. Still, the idea of dark matter remained on the fringe.
According to Rubin, the real proof for dark matter came in the 1960s, when astronomers began studying the rotation curves of stars in galaxies. These are plots of the velocity of stars relative to their position in the galaxy. According to Kepler’s laws, the curves should peak close to the center of the galaxy where the majority of the mass is accumulated and drop off as you reach the outer limits of the galaxy.
Rubin then mentioned her experience studying the rotation curve of our own galaxy in 1962. Surprisingly, her data showed that the rotation curve was flat, not downward as expected. Thoughout the 1970s, Rubin went on to study many other galaxies and found the same downward curve in all of them. By this time the astronomy community had finally taken noticed and realized that the masses of galaxies have been underestimated by up to 10 times.
The presence of dark matter was solidified by the publications of two papers in the late 1970s: one by Rubin and her colleagues Ford and Thonnard in 1978, and another by Faber and Gallagher in 1979.
With the history part completed, Rubin then explained the modern view of dark matter. It is believed that baryonic visible matter–essentially everything we can see–makes up only 3% of the matter in the universe. Baryonic dark matter makes up 14% and non-baryonic dark matter is the remaining 83%. This consists of particles we haven’t discovered. Furthermore, she went on to explain that all this matter only makes of 30% of the universe, while the other 70% consists of the unknown dark energy.
Rubin acknowledged that very little is known about dark matter, but she added that continued studies with rotation curves, as well as the more modern technique of gravitational lensing is consistently yielding new information.