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Tribute to Astrophysicist Vera Rubin, the “Mother” of Dark Matter

Tribute to Astrophysicist Vera Rubin, the “Mother” of Dark Matter

Astrophysicist Professor Vera Rubin, National Medal of Science awardee who confirmed the existence of dark matter, died on 25 December 2016.

Dark matter is “the invisible material that makes up more than 90% of the mass of the universe.” Rubin’s pioneering work progressed from 1965 to the late 1970s. Her webpage describes the beginning of this discovery:

“By the late 1970s, after Rubin and her colleagues had observed dozens of spirals, it was clear that something other than the visible mass was responsible for the stars’ motions. Analysis showed that each spiral galaxy is embedded in a spheroidal distribution of dark matter — a “halo.” The matter is not luminous, it extends beyond the optical galaxy, and it contains 5 to 10 times as much mass as the luminous galaxy. The stars’ response to the gravitational attraction of the matter produces the high velocities. As a result of Rubin’s groundbreaking work, it has become apparent that more than 90% of the universe is composed of dark matter.”

Rubin’s research remained prolific until the early 2000s, as she continued to study various models for the composition of the dark halos. Among her most recent publications was an examination of the rotation curves of spiral galaxies.

Until her retirement, Rubin worked at the Carnegie Institution for Science Department of Terrestrial Magnetism in Washington, D.C. She was awarded the National Medal of Science in 1993. She was also a member of the National Academy of Sciences and in 1996, she received the Royal Astronomical Society’s Gold Medal, the first woman to do so 168 years after Caroline Hershel (1828).

Neta Bahcall of Princeton University describes Rubin’s scientific significance: “A pioneering astronomer, the ‘mother’ of flat rotation curves and dark-matter, a champion of women in science, a mentor and role model to generations of astronomers.”

Carnegie Science describes Rubin’s scientific impact extends far beyond her pioneering research: “She was an ardent feminist, advocating for women observers at the Palomar Observatory, women at the Cosmos Club, Princeton, and she even advised the Pope to have more women on his committee.”

See Yonatan Zunger’s tribute to Professor Rubin in the linked post.

Learn more

Read some background on Rubin from Carnegie Science:

See Rubin’s biography and publications: #stemwomen #astrophysics #astronomy

Originally shared by Yonatan Zunger

And in the continuing march of the Angel of Death, I am sad to report that Vera Rubin died today at the age of 88. Rubin was most famous as the discoverer of dark matter: the invisible and still-mysterious substance which makes up 85% of the mass of the universe.

Dark matter had been hypothesized back in the 1930’s, but it wasn’t until the 1970’s that it was finally observed. Rubin was studying distant galaxies when she noticed that the rotation speed of their outer edges didn’t jibe with the speed they should have based on the amount of visible matter.

You can tell how fast something is moving relative to you using the Doppler effect: the same thing that makes a siren sound higher-pitched as it moves towards you and lower-pitched as it moves away. It works because sound looks like a sine wave of rising and dropping pressure, and pitch corresponds to the time between successive peaks. When the source is moving towards you, the first peak emitted by the siren is already moving towards you at the speed of sound, but the second peak will get there sooner than expected, because it had the benefit of moving towards you at the siren’s speed for one more period and then being sent off at the speed of sound. This means that if you know the original pitch of the siren, you can even figure out how fast it’s moving based on the pitch you hear.

The same trick works with light, only now instead of pitch, it’s color that depends on the time between peaks; things appear bluer when they approach, and redder when they recede. Since starlight contains a lot of easily measured standard lights in it – colors like those that Hydrogen and Helium emit when heated, and which have a very distinct pattern when viewed through a prism – we can measure the speed of distant stars and galaxies. And by comparing the speed of the left and right edges of a galaxy, you can tell how fast it’s spinning.

But we’ve known how to calculate the orbits of stars since Kepler, and from the amount of light a galaxy emits, we can make a pretty good guess at how heavy it is. From that, you would conclude that the stars at the outside of a galaxy should be moving more slowly than the ones at its center, in a nicely predictable way.

But that’s not what Rubin saw! Instead, she discovered that the stars at the outside were moving at the same speed as the ones at the center – something only possible if there was some extra, invisible mass pulling them.

What Rubin discovered was that there is an invisible halo of “dark matter” surrounding each galaxy, nearly ten times as massive as the galaxy itself. It’s “dark” in the plainly literal sense: unlike stars, it’s not actively on fire and glowing.

In the decades since, dark matter has become a core area of study in astrophysics. Using the same techniques and ever-more-sophisticated telescopes, including dedicated satellite observatories, we’ve mapped the presence and motion of dark matter in greater detail, and discovered that it’s far more mysterious than we first suspected. For example, we know it’s not made up of ordinary atomic or molecular stuff, because its dynamics is all wrong; neither is it made up of massive neutrinos or any other kind of matter we understand.

(There’s also dark energy, an even more widespread and invisible field, discovered a few decades later. Unlike dark matter, which attracts things by gravity, dark energy seems to provide a universe-spanning, diffuse, but very distinctly measurable repulsive force. It’s even less understood than dark matter; most scientists suspect that if we understood these things well, we’d know a lot more about the nature of the universe)

Rubin therefore sits in the pantheon of the great astronomers of the 20th century. Alas, her death means she will not get the Nobel Prize that many have been arguing she deserves for a very long time: the prize cannot (by the terms of its founding grant) be awarded posthumously. But she remains one of the most important researchers in the field, and her work will continue to have a profound impact on our understanding of Nature for generations to come.


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