Our galaxy may have two arms, or perhaps four. It was thought to be four until 2008, when it was reduced to two. Then, in 2015, it was expanded again to four arms, but recent research suggests it’s only two again. About 70% of galaxies have arms, easily counted from the outside, as in the picture below. Apparently it’s hard to get a good view from the inside.

Logically speaking, we should not expect a galaxy to have arms at all. For a galaxy to have arms, it must rotate as a unit. Otherwise, even if the galaxy had arms when it formed, it would lose them by the time the outer rim rotated even once. As it happens we know the speed of rotation and age of galaxies; they’ve all rotated 10 to 50 times since they formed.

For stable rotation, the rotational acceleration must match the force of gravity and this should decrease with distances from the massive center. Thus, we’d expect that the stars should circle much faster the closer they are to the center of the galaxy. We see that Mercury circles the sun much faster than we do, and that we circle much faster than the outer planets. If stars circled the galactic core this way, any arm structure would be long gone. We see that the galactic arms are stable, and to explain it, we’ve proposed the existence of lots of unseen, dark matter. This matter has to have some peculiar properties, behaving as a light gas that doesn’t spin with the rest of the galaxy, or absorb light, or reflect. Some years ago, I came to believe that there was only one gas distribution that fit, and challenged folks to figure out the distribution.

The mass of the particles that made up this gas has to be very light, about 10^-7 eV, about 2 x 10¹² lighter than an electron, and very slippery. Some researchers had posited large, dark rocks, but I preferred to imagine a particle called the axion, and I expected it would be found soon. The particle mass had to be about this or it would shrink down to the center of he galaxy or start to spin, or fill the universe. Ina ny of these cases, galaxies would not be stable. The problem is, we’ve been looking for years, and not only have we not seen any particle like this. What’s more, continued work on the structure of matter suggests that no such particle should exist. At this point, galactic stability is a bigger mystery than it was 40 years ago.;

So how to explain galactic stability if there is no axion. One thought, from Mordechai Milgrom, is that gravity does not work as we thought. This is an annoying explanation: it involves a complex revision of General Relativity, a beautiful theory that seems to be generally valid. Another, more recent explanation is that the dark matter is regular matter that somehow became an entangled, super fluid despite the low density and relatively warm temperatures of interstellar space. This has been proposed by Justin Khoury, here. Either theory would explain the slipperiness, and the fact that the gas does not interact with light, but the details don’t quite work. For one, I’d still think that the entangled particle mass would have to be quite light; maybe a neutrino would fit (entangled neutrinos?). Super fluids don’t usually exist at space temperatures and pressures, and long distances (light years) should preclude entanglements, and neutrinos don’t seem to interact at all.

Sabine Hossenfelder suggests a combination of modified gravity and superfluidity. Some version of this might fit observations better, but doubles the amount of new physics required. Sabine does a good science video blog, BTW, with humor and less math. She doesn’t believe in Free will or religion, or entropy. By her, the Big Bang was caused by a mystery particle called an inflateon that creates mass and energy from nothing. She claims that the worst thing you can do in terms of resource depletion is have children, and seems to believe religious education is child abuse. Some of her views I agree with, with many, I do not. I think entropy is fundamental, and think people are good. Also, I see no advantage in saying “In the beginning an inflateon created the heavens and the earth”, but there you go. It’s not like I know what dark matter is any better than she does.

There are some 200 billion galaxies, generally with 100 billion stars. Our galaxy is about 150,000 light years across, 1.5 x 10¹⁸ km. It appears to behave, more or less, as a solid disk having rotated about 15 full turns since its formation, 10 billion years ago. The speed at the edge is thus about π x 1.5 x 10¹⁸ km/ 3 x 10¹⁶ s = 160km/s. That’s not relativistic, but is 16 times the speed of our fastest rockets. The vast majority of the mass of our galaxy would have to be dark matter, with relatively little between galaxies. Go figure.

Robert Buxbaum, May 24, 2023. I’m a chemical engineer, PhD, but studied some physics and philosophy.