Vera Rubin: Shining "light" on Dark Matter
M.V.N. Murthy, Chennai
Vera Rubin (born 23rd July, 1928), one of the leading American Astronomers, passed away on 25th December, 2016. She was a giant among astronomers in the last sixty years. Her observations over two decades provided the crucial evidence for the existence of a major matter content of the universe, namely Dark Matter. In her honour, the Large Synoptic Survey Telescope (LSST) under construction in Chile, South America, was renamed Vera Rubin Observatory on 20 December, 2019.
Early days Vera Rubin was interested in science, especially astronomy, from her childhood. When she started her college education there were very few women in Physics and Astronomy. She was inspired by Maria Mitchell, a 19th century astronomer who is credited with the discovery of a comet in 1840s. She was the first female professor to work at Vassar College in New York. After hearing about Mitchell, Very Rubin decided to go to Vassar College since it was a school where she could study astronomy, although her high school science teacher told her to stay away from science. Much later, in an interview, she recalled her experiences ``It takes an enormous self-esteem to listen to things like that and not be demolished."

What is in the Universe?
For thousands of years, we believed that what we see in the sky is what is there--the moons, planets, stars and galaxies and all that. This picture changed when Rubin and her collaborator Kent Ford discovered that there is much more than that meets the eye in the sky. In fact it is five times more! The seemingly vast empty spaces between the shining stars are filled with matter that is invisible to the traditional methods of detection.
This invisible matter, called the ``Dark Matter", is seen indirectly through the gravitational effect it has on ordinary matter. While we know what this ordinary matter is made up of atoms, molecules, etc, we do not know what this dark matter is except that it obeys the laws of gravitation as every thing else. It is called "dark" because it gives no light from which it can be seen or detected.
Much before Vera Rubin, in 1932, astronomers Jan Oort and Fritz Zwicky independently noticed that there are some puzzles in the measurement of the speed of the stars in our galaxy as they travel in their orbits. This is called orbital velocity. To understand this puzzle, let us look at the solar system: The orbital speed of Earth (as it orbits the Sun) is about 30 km/s whereas Mercury goes around at a speed of 47 km/s and Jupitor goes around at a speed of 13 km/s. One can look at all other planets and it is clear that as the distance from the Sun increases, the orbital speed gradually reduces. In fact, if we make the simple assumption that all orbits are circular, then the orbital speed is also independent of the mass of the planet and only depends on its distance from the cenre. [See the Box for a detailed explanation].
Similar to the solar system, in a spiral galaxy, the stars go around the central bulge where most of its mass is located. Here again, as you go away from the galactic centre, you expect the velocities to decrease. However, instead of rotational speeds decreasing as you move to the edge of the galaxy, they appear to become constant, after the initial rise. This happens even though the visible matter density is close to zero at the edge. See the figure (courtesy Mario De Leo, https://en.wikipedia.org/wiki).
This lead Zwicky to conclude that there is much more matter in the galaxy than is visible to the naked eye (or optical devices). He named this missing matter as ``Dark Matter" but whose precise confirmation had to wait for Vera Rubin's work some three decades later. [See the box for a simple explanation.]
Vera Rubin joined the Carnegie Institution in Washington after obtaining Ph.D from Georgetown University. Here she met her collaborator Kent Ford with whom she did all her work on the rotational speed curves of galaxies for the next two decades. Balancing the family duties with two of her children, she did quite a bit of her work from home. She was one of the first women scientists who was allowed to do her observations in the Mt. Palomar Observatory in California. In fact when she went there, there was not even a toilet for women. 
In the late sixties and early seventies she worked with an accurate spectrograph devised by Ford which could measure the rotational speeds with greater accuracy than ever before. She and her colleagues systematically measured the velocity curve of stars in spiral galaxies; our own Milky Way galaxy is also a spiral galaxy.
Vera Rubin discovered that most stars in spiral galaxies which lay outside the bulge, orbited roughly at the same speed instead of decreasing as the distance increased from the centre. This implied that the mass densities in these galaxies were more or less uniform. The figure shows a typical rotation speed curve from a spiral galaxy. The dashed line shows what is expected from the visible matter distribution. As the visible matter is concentrated in the centre, the velocities are expected to decrease as you go far away from this centre. The observations show that the speeds are increasing or at worst flattening and do not show any decreasing tendency. Why is this so?
This means that there is actually a lot of mass even far away from the centre, although it may not be visible to us. This additional mass causes the observed behaviour of the velocities. If we work out the mass distribution required to explain the observed rotational curve, it becomes clear that most galaxies contain about six times the mass that can be accounted for by the visible stars. Since there is no sign of the curve going down, it is clear that the dark matter distribution must extend beyond the galactic shape as visible from the central bulge containing ordinary matter and is almost uniformly distributed.
There is now independent evidence for dark matter through what is called gravitational lensing. Just as light bends when going through an optical medium like a lens, it also bends around a massive gravitating object. This one of the central results of Einstein's theory of gravitation. A distant bright star behind a massive object may become visible because its light can bend around the massive object. This is known as gravitational lensing. If dark matter exerts gravity as shown by the rotational curves, they should also cause gravitational lensing. This is hard to observe in our own galaxy but was observed in other astronomical objects through powerful space based telescopes.
The presence of dark matter was therefore firmly established by the work of Vera Rubin and confirmed beyond doubt. It is clear from the observations that the dark matter behaves like ordinary matter as far as its gravitational properties are concerned. But that is all we know until now. What is it made of --- dark particles/nuclei/atoms...? It remains one of the great mysteries of science, at least for the present.
The legacy of Vera Rubin goes beyond her contributions to astronomy. Over time she became an inspiration to women scientists and paved the way for them to become astronomers. Many present day women astronomers recognise her as the ``guiding light" in their own careers in astronomy. On her death, she was described as a national treasure by the president of the Carnegie Institution where she did her most important work.

BOX Women Physicists and Nobel Prize in Physics
There is a long history of gender discrimination in Nobel prizes awarded, as in most such prestigious awards. Vera Rubin did receive many awards but not the Nobel prize, though she was nominated several times in recent years. Many famous women scientists such as Lise Meitner (helped discover nuclear fission), Cecilia Paine (astrophysicist), Chien-Shiung Wu (contributed to deeper understanding of fundamental interactions), and Jocelyn Bell (discovered the first pulsar) were all overlooked for this prize.
In all only about fifty women have received Nobel prize in various categories (nearly a third for Peace). Of these only three women, Marie Curie (1903), Maria Mayer (1963) and Donna Strickland (2018), have received the prize for physics. 
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