It’s often said that we haven’t yet detected dark matter particles. That isn’t quite true. We haven’t detected the particles that comprise cold dark matter, but we have detected neutrinos. Neutrinos have mass and don’t interact strongly with light, so they are a form of dark matter. While they don’t solve the mystery of dark matter, they do play a role in the shape and evolution of our universe.
From the evidence we have of dark matter, such as the clustering of galaxies and gravitational lensing, we know that most dark matter must be cold. That means it is likely made of heavy particles. A range of possibilities has been proposed, from exotic particles called axions to tiny primordial black holes. So far, no such solution has been found. But while most dark matter must be cold, other dark matter that is warm or hot could also play a role.
Neutrinos are a form of hot dark matter. The temperature of a material is determined by the speed of its particles. Since neutrinos move at nearly the speed of light, they are a form of hot matter. For a long time it was thought that neutrinos were massless, and thus wouldn’t be a part of dark matter. Then in the 1990s, they were found to have a tiny amount of mass. Their mass is so small that we don’t know what it is. We only know that neutrinos have mass because the state of a neutrino can change over time through a process known as oscillation. This wouldn’t be possible if they were massless and moved at the speed of light.
So neutrinos are a part of dark matter, but what role do they play? That is the question recently explored in the Astrophysical Journal. The team ran computer simulations on how neutrinos interact on a cosmic scale. Since they don’t know the mass of neutrinos, they created a simulation where they could vary the mass to study different outcomes. They found that while neutrinos do tend to clump with galaxies, they actually work to hinder the amount of clustering by cold dark matter. The amount of hindrance depends on the mass of the neutrinos.
Earlier studies have shown how neutrino mass could affect cosmic evolution, but this study shows how neutrinos can affect cold dark matter. Further research could even allow astronomers to use galactic clustering to pin down the mass of neutrinos, thus using the most massive objects in the universe to measure particles with the tiniest mass. It’s a hot idea that would be pretty cool.
Reference: Yoshikawa, Kohji, et al. “Cosmological Vlasov–Poisson Simulations of Structure Formation with Relic Neutrinos: Nonlinear Clustering and the Neutrino MassKo.” The Astrophysical Journal 904.2 (2020): 159.