Since the 1930s, astronomers have known that there is invisible stuff out there that is gravitationally affecting its surroundings while neither absorbing nor emitting light. For the past many, many years, scientists have been looking for weakly interacting massive particles (WIMPs) that could explain this missing mass. So far nothing has been found, and many, many theories have been ruled out as these massive particles fail to show up in underground vats or high energy particle collisions.
One theory that is slowly starting to gain traction is the possibility that the theoretical particle the axion might be that missing dark matter particle. Axions were first theorized in the 1970s as the particles that are behind a field that causes neutrons to always be electrically neutral and otherwise helps explain why we don’t see particles violating the charge-parity relationship. Axions, if they have a small amount of mass, could also be a constituent of dark matter – a very, very low-mass constituent.
In large enough numbers, even the lowest of mass particles could be dark matter. The issue has been that theories couldn’t find a way to generate enough axions to generate dark matter. Until now.
By assigning a nonzero initial velocity to the axion field, the team discovered a mechanism—termed kinetic misalignment—producing far more axions in the early universe than conventional mechanisms. The motion, generated by breaking of the axion shift symmetry, significantly modifies the conventional computation of the axion dark matter abundance. Additionally, these dynamics allow axion dark matter to react more strongly with ordinary matter, exceeding the prediction of the conventional misalignment mechanism.
Two members of the research team, Keisuke Harigaya and Raymond Co, previously explored the concept of axion dynamics in the study “Axiogenesis,” which explained how the excess of matter over antimatter could be due to a nonzero initial velocity of the QCD [quantum chromodynamics] axion field. This study also provided a framework for generating new insights into the questions surrounding dark matter.
What I really like about this theory is that axions were already needed for reasons that have nothing to do with dark matter, and there have been some observations that hint at seeing the effects of axions, but the quality of the data isn’t enough to say they exist with certainty. I will take a particle that has multiple reasons to exist and low signal / not entirely believable observations over a custom-needed particle with no observations any day. This is starting to remind me of the early days of looking for the Higgs boson where teams were seeing hints of it in data, but the noise was such that they couldn’t say anything with certainty. This is the first time in a long time I’ve felt hope that we might be able to identify dark matter.
We still have a long way to go. We need definitive observations, and the theories need more work, but this feels like the start of something, and we will be following this story and bringing you new results as we have them.
“Axion Kinetic Misalignment Mechanism,” Raymond T. Co, Lawrence J. Hall & Keisuke Harigaya, 2020 June 26, Physical Review Letters