As astronomers struggle to understand dark matter, they keep pushing up against a contradiction of evidence. While there is a great deal of indirect evidence for dark matter within galaxies and galactic clusters, there is as yet no direct evidence of dark matter particles. Even worse, the standard model of particle physics, which accurately predicts the particles we observe, has no room for any undiscovered particle that could be dark matter.
For this and other reasons, theorists have proposed extensions of the standard model with an even larger range of theoretical particles. It’s an effort to create some grand theory of everything. One popular extension includes a type of particle known as axions.
Axions are pretty controversial in physics. Theorists tend to like them because they would solve some bothersome issues with quantum theory. Some astronomers like them because some axions behave just like cold dark matter. Many experimentalists don’t like them because there is some evidence to disprove them. Measurements of nuclear spin have eliminated many axion models,1 and spectral observations of galaxies rule most of them out as a candidate for dark matter.2
But axions would be so gosh darn useful that as soon as one type of axion is ruled out folks start looking for the ones that haven’t been eliminated. That’s where a new study comes in.3 This one is a bit different because it uses distant galaxies to do particle physics.
If axions exist, then they would be produced by high energy interactions. These are the kind of interactions produced in particle accelerators, but they also occur naturally near black holes. So the team looked at x-ray signals coming from an active black hole in the galaxy NGC 1275.
Because axions would have more mass than known particles within the standard model, they should decay into lighter particles. Very low mass axions would decay directly into photons with specific wavelengths. So the team looked at the spectral pattern of x-rays from NGC 1275, and found no evidence for axions. So more axion models are ruled out, though as the authors point out there are still some axion models that haven’t been disproven.
So once again it looks like axions don’t exist. This goes to show that no matter how elegant your model is, it can still be completely wrong.
Abel, Christopher, et al. “Search for axionlike dark matter through nuclear spin precession in electric and magnetic fields.” Physical Review X 7.4 (2017): 041034. ↩︎
Ajello, M., et al. “Search for spectral irregularities due to photon–axionlike-particle oscillations with the Fermi Large Area Telescope.” Physical Review Letters 116.16 (2016): 161101. ↩︎
Reynolds, Christopher S., et al. “Astrophysical limits on very light axion-like particles from Chandra grating spectroscopy of NGC 1275.” The Astrophysical Journal 890.1 (2020): 59. ↩︎