So where do we go after years of empty searches for dark matter? We haven’t learned nothing. After decades of searches, we’re narrowing down the range of what dark matter can and cannot be.
We strongly suspect that we’re getting our theory of gravity right at galactic scales. We also strongly suspect that the dark matter is not some abundance of known particles like neutrino. We also strongly suspect it’s not something like black holes (although there are some possibilities there that I will tackle in a future episode).
We strongly believe based on the weight of the accumulated evidence that dark matter is a particle. But that same evidence is leading us to the conclusion that our prime candidate, a weakly interacting massive particle, is not it. It’s not completely ruled out, but as the years go by without a solid detection it makes us seriously doubt our assumptions.
Which is a good thing! This is how science operates. Most ideas in science are wrong, because we’re constantly learning new things. And we only get to learn new things by trying out as many ideas as possible and seeing what sticks. WIMPs aren’t seeming to stick, so it’s time to think of something else.
Which brings me to axions. The gist is that maybe dark matter isn’t some heavy particle, comparable in mass to a quark or a weak boson, or even an electron…or even a neutrino. Maybe dark matter is really, really strange – nature has no obligation to make sense, after all. Maybe dark matter is orders of magnitude lighter than the lightest known particles.
There’s precedent here, at least some vague theoretical motivation for a particle like this. Back in the funky 70’s, around the same time the Vera Rubin was observing rotation rates in galaxies, some theorists noticed that there was a strange issue with quantum chromodynamics, our model of the strong nuclear force.
All experiments show that the strong force obeys some special symmetries in nature, and yet the theory itself doesn’t explicitly state that. To make the fix you have to introduce some unnatural hand-tuning, which is never a fun thing to do from a particle physics viewpoint.
One way to solve this is to introduce a new kind of particle, which was named the axion after a brand of laundry detergent (get it, it cleans up messes). Now the axion is interesting because it’s a) a new particle, b) could be made in abundance in the early universe, and c) doesn’t interact with other particles all that often.
Hey, that’s dark matter! Now in the 80’s and 90’s WIMPs became more popular as a dark matter candidate, but now that they are looking a little sketchy, we’ve started rummaging around in the attic looking for any discarded or overlooked ideas to see if something interesting pops up.
And so now there’s a lot of interest in axions. I’m going to be honest, they’re also not the greatest idea in the world, but they’re interesting enough to be worth pursuing. Axions themselves are just one example of a broader class of ultra-light dark matter, which include other candidates with delightful names like the ultralight scalar field and the dark photon.
And they would be wild. One of the most fun things about axions and their relatives is that they are so incredibly light that their inherent wave nature comes out.
All particles aren’t really particles, they’re mixtures of both waves and particles, something we know about from quantum mechanics. But the more massive a particle is, the more its wave nature is suppressed. But axions are light enough that we shouldn’t think of them as trillions of microscopic bullets buzzing around the galaxies.
Instead, we should think of them as an ocean. A sea of dark matter, with the galaxies as lit-up buoys on the surface. This ocean of dark matter even has waves, pockets of higher dark matter density and regions of low density.
One way that we could access this dark ocean is through the possibility of axions converting directly into photons in regions of extremely strong magnetic fields. This conversion is incredibly rare, otherwise we wouldn’t noticed by now, but it gives us a potential window into the axion world.
Searches for axions are just getting started, and they’ve already produced some limits on what the axion could be. There is no confirmed detection yet, but we’ve only begun to pivot in this direction. We’re even tuning existing dark matter searches to see if they sniff out an axion here and there.
We don’t have any slam-dunk, case-closed explanations for the dark matter mystery, and some of our best ideas haven’t panned out. But the story of dark matter is just getting started. The universe is obviously trying to tell us something. The observational evidence is strong and clear that something is going on in the wider cosmos. Dark matter might be axions, or it might be something more exotic, or it might be something we haven’t even begun to think of yet. Facing this challenge will require patience, persistence, and most importantly, creativity.