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from the something-unobservable-made-of-something-unobserved,-it's-axionmatic dept.
In an article Friday on Universe Today, Paul M. Sutter, an astrophysicist at Ohio State, discusses one tantalizing possibility for explaining dark matter, which is that it may be comprised of particles called axions.
Axions are an exotic hypothetical particle invented to explain a conundrum in high energy physics having to to with [sic] charge-parity symmetry and the strong nuclear force. Like dark matter, we have not actually observed axions.
The conundrum is that by all rights the strong nuclear force should violate [CP-symmetry]. There are terms in the mathematics that very obviously break CP-symmetry, and yet we don't see any signs of symmetry breaking with the strong nuclear force in any of our experiments. So something must be going on to restore this symmetry when it ought to be broken.
The answer – or at least one potential answer – is a new kind of particle called the axion. The axion restores a certain kind of balance in the force (yes I'm aware of the Star wars reference here) so that the CP-symmetry is preserved and everyone can go about their daily lives. Of course experiments to date haven't directly revealed the existence of the axion, and there's a range of possible masses and properties that the axion could have.
Based on the relationship of galactic core objects to galaxy sizes, a team of astronomers was able to place upper bounds on axion particle mass, which will help guide future experiments.
It turns out that some of the range of possible axion properties allow that hypothetical particle to be a candidate for the dark matter.
The Dark Axions
If we let the axion be the dark matter it can generally explain all the usual dark matter observations. It can explain the rotation curves inside of galaxies. It can explain the motions of galaxies within galaxy clusters. It can be manufactured in sufficient abundance in the early Universe to fit observations of the cosmic microwave background. And so on.
Axions acting as dark matter also present a potential alternative for black holes in the center of galaxies
axions in the cores of galaxies can bundle together tightly enough to form a single massive ball that would at first blush look a lot like a supermassive black hole. It would be small, it wouldn't interact with light, and it would be incredibly massive.
He also notes that the recent imaging of Sagittarius A* does not rule out axion cores.
(Score: 5, Interesting) by maxwell demon on Monday May 20 2019, @06:16AM
Guess what? The Higgs boson also hadn't been seen for quite some time. And it (or rather the corresponding Higgs field) was invented to "explain away" a discrepancy between theory (which says that Z and W particles belong to gauge fields, and that gauge field particles have to be massless) and observation (which says that W and Z particles have a quite substantial mass). You probably would have claimed that we should just accept the defeat of gauge theory, instead of inventing a new field with properties like no other known field to save it.
Also the neutrino was initially unobserved, and it was used to "explain away" the observed violations of energy conservation in beta decays, violating the theoretical predictions that energy is conserved. Indeed, it is a quite relevant example here, as the properties of the neutrino basically match those of dark matter, with the exception that neutrinos have too little mass to explain dark matter. You probably would have claimed that we should just have to accept defeat of energy conservation, instead of inventing such a ghostly particle that at the time wasn't even expected to be ever practically observable.
So all in all, I'd say we've been quite successful with "inventing" unobserved particles to "explain away" failures of the theories to correctly predict observed behaviour.
Of course it could be that dark matter doesn't exist, and the explanation requires a modification of gravity. But as of now, dark matter is the best explanation we have (in particular, modified gravitation theories have a hard time to explain why the apparent distribution of dark matter isn't completely determined by the distribution of observable matter).
The Tao of math: The numbers you can count are not the real numbers.