Neutrinos have played a huge role in the evolution of the universe

It is often said that we have yet to detect dark matter particles. This is not entirely true. We haven’t detected the particles that make up cold dark matter, but we have detected neutrinos. Neutrinos have mass and do not strongly interact with light, so they are a form of dark matter. While they do not 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, like the galaxy cluster and the gravitational lens, we know that most dark matter must be cold. This means that it is probably made up of heavy particles. A range of possibilities have been offered, from exotic particles called axions to tiny primordial black holes. So far, no such solution has been found. But while most dark materials should be cool, other dark materials that are hot 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 almost the speed of light, they are a form of hot matter. For a long time, it was thought that neutrinos were massless and therefore would not be part of dark matter. Then in the 1990s, it was discovered that they had a small 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 called oscillation. This would not be possible if they were massless and moved at the speed of light.

Neutrinos are therefore part of dark matter, but what role do they play? This is the question recently explored in the Astrophysics Journal. The team performed computer simulations on the interaction of neutrinos 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 results. They found that while neutrinos tend to clump together with galaxies, they actually work to hinder the amount of clumping by cold dark matter. The amount of annoyance depends on the mass of the neutrinos.

Previous studies have shown how neutrino mass can affect cosmic evolution, but this study shows how neutrinos can affect cold dark matter. Further research could even allow astronomers to use galactic aggregation to determine the mass of neutrinos, thus using the most massive objects in the universe to measure particles of the smallest mass. It’s a great idea that would be pretty cool.

Reference: Yoshikawa, Kohji et al. “Vlasov – Poisson cosmological simulations of the formation of structures with relic neutrinos: nonlinear clustering and the MassKo neutrino.” The astrophysical journal 904.2 (2020): 159.