• Question: Could dark matter be made up of axioms? Or is dark matter more likely to be composed of WIMPs?

    Asked by Anon to Sam, Helena, Anne, Adam on 22 Jun 2020.
    • Photo: Anne Green

      Anne Green answered on 22 Jun 2020:

      Short answer: we really don’t know what dark matter is. Axions and WIMPs are both good candidates, and axions have started getting more attention over the last few years.

      Long answer:
      WIMPs and axions have some things in common: they both come from particle physics models which were proposed to solve other problems. And they both could be produced in the early Universe, just after the Big Bang, in the right amount to be the dark matter. However their properties, and therefore how we try to detect them, are very different. WIMPs are relatively heavy and interact only weakly with each other and normal matter. Axions are extremely light and extremely weakly interacting.
      Up until a few years ago WIMPs were a lot more popular than axions (in the sense that more researchers were working on them). However that’s changing. One specific model that predicts WIMPs is supersymmetry-it was proposed to solve various problems with the standard model of particle physics. We were hoping that the LHC would detect supersymmetric particles, but it hasn’t (so far). Which means that most of the supersymmetric particles (if they exist) have to be heavier than expected. We’ve also been looking for WIMPs directly, by looking for the very rare interactions between WIMPs and normal matter. So far these direct detection experiments haven’t seen any (convincing) sign of WIMPs. This doesn’t yet rule WIMPs as dark matter out, but it does mean they have to be a bit more weakly interacting than we originally thought. Because axions are extremely weakly they’re harder to detect. Axion detection experiments have only just reached the sensitivity to detect axion dark matter.