Since mass is clustered in centre, nearer objects should go faster but they gathered dozens of rotation curves for galaxies & all were flat. Their data was undeniable. Dark matter proposed by Oort (‘32) & Zwicky (‘33) but largely ignored. "One day … I made sketches on a piece of paper, and suddenly I understood it all," Rubin said. A halo of unseen dark matter around galactic cores would spread mass throughout the galaxies, & speeds would remain flat with distance.

@cosmos4u Thats a fair question, and I don’t have an answer that everyone would agree with.

But as far as questioning DM as an explanation for rotation curves, that I do not know. Was it in a paper? Or just general commentary, like being MOND-curious in an interview or at a workshop?

@cosmos4u I said there was already evidence before her work, described some of the folks involved, linked to a more compete discussion of the priority, and tried to be careful to reference her work as “contributing” and “helping” with building the case. I just think if you were going to give a prize for DM in the mid-2010s, which they could have done, her name would have been on there.

@pjfasano I am personally sympathetic to what you are saying, though. It’d be great if we could detect it in a lab or a big vat of cleaning fluid, or whatever.

@pjfasano Well, I’d also love to see some other detection that places DM within a minimally extended SM, but I disagree that gravitational interactions aren’t detections. If someone can modify GR in a manner that removes the need for DE and DM, without spoiling its mountain of precision tests, I’m open to that. But I don’t think it’s unreasonable to conclude, based on everything we know, that GR is correct on large scales. And if that’s the case, then multiple lines of evidence imply DE and DM.

And also David DeVorkin's interview with Vera Rubin for the AIP's oral history series. It’s a great read.

https://www.aip.org/history-programs/niels-bohr-library/oral-histories/5920-1

This bit, especially, does a beautiful job of capturing the germ of basic research. DeVorkin asks Rubin what possessed her as a child to stay up late and study the sky.

Rubin: “I don’t have the vaguest idea. I just found it fascinating.”

You should read @chanda ’s Bitch Media
piece on Vera Rubin from a few years ago.

https://www.bitchmedia.org/article/vera-rubin-dark-matter

Imo, evidence for dark matter has always been as solid as the evidence for dark energy, which earned a Nobel Prize in 2011. But the historically sexist Nobel Committee never recognized Rubin with a share of a prize for dark matter. They just waited until she passed away.

If helping discover roughly 27% of the Universe isn’t enough, what is? The lack of a Nobel doesn't diminish Rubin's contributions, but it did a lot to diminish my opinion of the Nobel.

Image: Carnegie Institution of Washington

Bosma himself wrote a detailed account of all the folks who he felt should receive more credit for their work establishing flat rotation curves.

(Thanks to @astromikemerri for pointing this out to me.)

https://ned.ipac.caltech.edu/level5/Sept03/Bosma/Bosma_app.html

Both Kapteyn and Oort suggested that there was too little visible matter to explain the dynamics of stars in our galaxy. Rubin and Ford's work took place throughout the 1970s; Albert Bosma's 1978 thesis provides radio measurements of flat rotation curves:

http://ned.ipac.caltech.edu/level5/March05/Bosma/frames.html

There's a pretty good list of folks who should share the credit for building the case for dark matter.

Long before Zwicky, Kelvin tried to survey the mass content of "dark bodies" in our galaxy. Poincaré introduced the term "matière obscure" when rebutting him.

The case for dark matter is very strong and extends beyond galactic rotation curves. Dynamics of galaxy clusters, gravitational lensing, the existence of especially old galaxies, and cosmological models all support the existence of dark matter.

Dark matter seems to make up about 27% of the universe! This is perhaps surprising, since the matter we are familiar with – the baryons and fermions of the Standard Model of particle physics – is less than about 5% of what's out there.

To explain the data you need more matter, stuff that doesn’t participate in the electromagnetic interactions we use to detect the luminous stuff. (Luminous *means* something interacts via electromagnetism!)

Non-luminous isn't a very catchy name, so we call it "Dark Matter."

The idea that there may be stuff that doesn’t take part in electromagnetism isn’t too out-there. There are already particles in the Standard Model of Particle Physics that experience some interactions but not others.

Now, I am glossing over a lot of details here, but that's the main idea. You look at how fast things are moving as a function of distance from the galactic center. The amount of luminous matter you see doesn't provide enough of a gravitational tug to explain those observations.

Once you add a non-luminous component like this to your description of the galaxy, Newtonian gravity does a pretty good job of explaining our orbital velocity measurements. Here's what we get for that NGC 3198 data.

Suppose the galaxy is surrounded by a large halo of non-luminous matter with density that falls off like 1/r² as you move out from the center. That would give another contribution to M(r) that is proportional to r.

If the non-luminous halo extends out further than the luminous matter, then this contribution to M(r) is still increasing out past what we previously called the "edge" of the galaxy.

As a result, the orbital velocity levels off for r > R_{L}. The rotation curve flattens.

There's a conceptually simple explanation that’s easy to overlook.

When we were working out that first rotation curve, we assumed that the mass M(r) out to distance r came from all the luminous stuff we could see.

What if there's other matter besides that? That would change the rotation curves!

In particular, we assumed that M(r) stopped increasing out past the point where the luminous stuff ends – what we considered the "edge" of the galaxy. What if the galaxy doesn't end there?

So what's going on here? Why are observations so different than Newtonian predictions?

Maybe Newton's Law of Gravitation, central to the analysis in the previous tweets, is the problem. Could gravity work a little differently on these galactic scales?

That's a fine possibility to investigate, but there's now a pretty good body of evidence indicating it's not the explanation. In all likelihood, Newton's description of gravity is perfectly valid here.

If Newton isn't the problem, what is?

There was already evidence for flat rotation curves via radio observations. Rubin et al took advantage of advances in optical instrumentation to provide unambiguous evidence of the disconnect between rotation rates and the amount of luminous matter in galaxies. (More on this in a moment!)

Here's a rotation curve for galaxy NGC 3198. The green line is the approximate edge of the galaxy. As you can see, there's no significant falloff - the curve is flat.

Data: K. G. Begeman, “HI rotation curves of spiral galaxies,” PhD thesis, U Groningen (1987)

Image: NASA