Jul 20 2023

A Galaxy Without Dark Matter

Published by under Astronomy
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Dark matter is one of the greatest current scientific mysteries. It’s a fascinating story playing out in real time, although over years, so you have to be patient. Future generations might be able to binge the dark matter show, but not us. We have to wait for each episode to drop. Another episode did just drop, in the form of an analysis of the massive relic galaxy NGC 1277, but let’s get caught up before we watch this episode.

The term “dark matter” was coined by astronomer Fritz Zwicky in 1933 as one possible explanation for the rotation of the Coma Galaxy Cluster. The galaxies were essentially moving too quickly, implying that there was more gravity (and hence more matter) present in the cluster than was observed. This matter could not be seen, therefore it was dark. The notion was a mere footnote, however, until the 1970s when astronomer Vera Rubin analyzed the rotation curves of many individual galaxies. She found that galaxies were rotating too quickly. The stars should be flying apart because there was insufficient gravity to hold them together (or alternatively they should be rotating more slowly). There must be more gravity that can be seen. The notion of dark matter was therefore solidified, and has been a matter of debate ever since.

Half a century after Rubin confirmed the existence of dark matter, we still don’t know what it is. It must be some kind of particle that does not interact much with other stuff in the universe, does not give off or reflect radiation, but possesses significant mass and therefore gravity. There are candidate particles, such as wimps (weakly interacting massive particles), MACHOs (massive astrophysical compact halo object), axions (particles with a tiny amount of mass but could be very common) or perhaps even several particles currently not accounted for in the standard model of particle physics.

This is one of the exciting things about dark matter – when we figure out what dark matter is, it could break the standard model, pointing the way to a new and deeper understanding of physics. But how certain are we that dark matter exists? To a degree the existence of dark matter is an argument from ignorance – it is a placeholder filling in a gap in our knowledge. We can only infer its existence because we cannot explain with our current models of gravity how stuff is moving in the universe. Perhaps our current models of gravity are wrong?

That is the other major hypothesis to dark matter, often referred to as MOND, or modified Newtonian dynamics. Classical physics, for example, works just fine on the Earth, in our laboratories. Our calculations did not start to break down until we observed objects under intense gravity, such as the orbit of Mercury around the sun. Eventually Einstein figure out that our equations were actually incomplete – general relativity provided more accurate equations. In these equations the new variable approach zero when not dealing with extreme gravity or relative speed, and so they approach the equations of classical physics, which is why the older equations work on Earth.

Perhaps, the argument goes, the same is true of gravity at massive scale – the scale of galaxies and galaxy clusters. We need to modify the equations so that they still work at the scale of our solar system, but also account for the rotation curves of galaxies (the speed of rotation over distance from the galactic center). This solution would dispense with the need to invent new particles. The question is – are out observations of galactic movement and rotation due to stuff, or due to modified physical laws? How can we tell the difference? Well, one way is to see if the stuff is equally everywhere. The thing about physical laws is that they should be universal, and therefore present everywhere at all times in the universe. But stuff can move, especially stuff that creates and reacts to gravity.

There are now several lines of evidence that dark matter exists, that it is stuff out there in the universe, and that there is therefore no universal modified law of gravity. One such line of evidence is the bullet cluster – two galaxies that collided at high speed. We can image the stars and gas clouds, and we can also image the gravity using gravitational lensing effects. What we see is that the gas clouds in each galaxy, which contain most of the mass, collided and because they have a lot of pressure they slowed down considerably. The stars, on the other hand, just moved through each other will little effect. However, the center of gravity moved with the stars, not the more massive gas clouds. This can be explained if the dark matter in each galaxy moved through each other like stars, and did not slow down like gas clouds. Essentially gravity separated from the visible matter – supporting the idea of dark matter.

I should not that the MOND proponents have not given up, and reject the bullet cluster argument. Physicist Sabine Hossenfelder, for example, who is know for her unconventional thinking, argues that MOND can accommodate the bullet cluster also. She writes:

But modifying gravity works by introducing additional fields that are coupled to gravity. There’s no reason that, in a dynamical system, these fields have to be focused at the same place where the normal matter is.

Perhaps, but that sounds like a bit of special pleading to me. Occam finds this argument…disturbing. Also, she argues that the speed of the collision is very unlikely given dark matter (but not given MOND). My response is – so? Astronomers observe lots of extremely unlikely things, because the universe is a really big place. We aren’t seeing galaxy collisions like the bullet cluster all over the place. She also admits that no one has worked out the math that would explain the bullet cluster using MOND. So get back to me when you do.

I’m actually not trying to sound dismissive. It’s good that there are minority opinions in science, keeping those in the majority honest and forcing them to explain themselves, reconsider evidence, and continuously think in unconventional ways. This is all healthy. But we also have to recognize that in modern science a majority opinion is in the majority for a reason. We have a fascination with minority opinions, like to root for the underdog, and celebrate when minority opinions eventually prevail. But statistically, this is rare. It’s easy to ignore all the times that the majority opinion boringly turned out to be correct. It’s a non-story.

This was all a long windup to the latest episode in the dark matter series. What astronomers found in relic galaxy NGC 1277 is that it is “dark matter deficient”. First, a relic galaxy is a galaxy that has not interacted in its long life with other galaxies, and so retains its ancient structure. In this case the relic galaxy is massive, three times the mass of the Milky Way. They did a spectrographic analysis out to 20,000 light year radius and found that the galaxy contains less than 5% dark matter (and the analysis is compatible with zero dark matter). However, such a galaxy should contain 15-60% dark matter depending on distance from the center. So where did the dark matter go?

The authors hypothesize either that the galaxy formed without dark matter, or that the early components of the protogalaxy somehow lost its dark matter, for example by being stripped away by interactions with other massive objects. Again, like the bullet cluster, this is a rare observation, but the universe is a big place.

If this observation is confirmed, this is yet another line of evidence for dark matter and against MOND. Stuff can be missing from a galaxy, but universal laws are universal. I’ll be interested to see how the MOND proponents explain away this one. Meanwhile, this is likely to strengthen the consensus that dark matter exists. It is stuff that can move and be missing. But I suspect the debate will not end until we demonstrate what dark matter actually is. And again, that is fine and healthy for science. As far as I know, there is no policy decision resting on the existence of dark matter, so we can just look at the science without having to put our nickel down.

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