Neutrinos played a huge role in the evolution of the universe

It is often said that we have not yet detected dark matter particles. This is not entirely true. We did not detect the particles that make up cold dark matter, but we did detect neutrinos. Neutrinos have mass and do not interact strongly with light, so they are a form of dark matter. While they don’t solve the mystery of dark matter, they play a role in the shape and evolution of our universe.

From the evidence we have of dark matter, such as the cluster of galaxies and gravitational lenses, we know that most of the dark matter must be cold. This means that it is probably composed of heavy particles. A number of possibilities have 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 needs to be cold, other hot or hot dark matter could play a role as well.

Neutrinos are a form of hot dark matter. The temperature of a material is determined by the speed of its particles. Since neutrinos move almost at the speed of light, they are a form of hot matter. For a long time, neutrinos were thought to be massless and therefore not part of dark matter. Then, in the 1990s, it turned out 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 known as oscillation. This would not be possible if they were massless and moved at the speed of light.

So neutrinos are part of dark matter, but what role do they play? This is the question recently explored in Astrophysical Journal. The team ran computer simulations of how neutrinos interact on a cosmic scale. Because 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 cluster with galaxies, they actually work to obstruct the amount of clustering by cold dark matter. The amount of the obstacle depends on the mass of the neutrinos.

Previous studies have shown how neutrino mass can influence cosmic evolution, but this study shows how neutrinos can affect cold dark matter. Further research could even allow astronomers to use galactic clustering to define the mass of neutrinos, thus using the most massive objects in the universe to measure particles with the smallest 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 Grouping and the MassKo Neutrino.” The Astrophysical Journal 904.2 (2020): 159.