Dark Matter Isn’t Warm And Fuzzy

In Cosmology by Brian Koberlein

Dark matter is one of the big mysteries of cosmology. Theoretically it explains cosmic phenomena such as the scale at which galaxies cluster, and observationally we see its effect through things like gravitational lensing, but it hasn’t been observed directly. This means we have a limited understanding its exact nature. As a result, there have been lots of theoretical ideas about what dark matter could be. But we now know that whatever dark matter is, it isn’t warm and fuzzy. 

There are two broad aspects about dark matter that no one disagrees about (assuming it exists). The first is that it must be dark, meaning that it doesn’t interact much with light. If it did interact with light, we would see its effects through the absorption or scattering of light from stars and distant galaxies. The second is that it must have mass, since the models require that it interacts with regular matter gravitationally. Beyond that, almost anything goes.

The most models assume that dark matter is cold. In this case, cold vs warm refers to the speed at which dark matter particles typically move. In cold dark matter models, the particles are relatively heavy, with a mass similar to that of protons or more. Because of their high mass, these dark matter particles would move relatively slowly, at much the same speed as the gas and dust in our galaxy. Neutrinos, on the other hand, are warm dark matter. Neutrinos don’t interact strongly with light, and they do have mass, so they meet the basic requirement of dark matter. But neutrino mass is minuscule, and they typically move at speeds approaching the speed of light. Thus, neutrinos are an example of warm or hot dark matter. One of the things we observe about galaxies is that they have far more mass than their visible matter would suggest, so they must contain a lot of dark matter. This means dark matter clumps together just as gas and dust clumps to form galaxies. Warm dark matter such as neutrinos move much too quickly to clump together in this way, so it would seem that dark matter must be cold. While cold dark matter is a central part of the standard “concordance model” of cosmology, it isn’t without problems. One of the biggest problems is that cold dark matter predicts that large spiral galaxies like our Milky Way should have hundreds of small satellite galaxies surrounding it. We’ve only found about a dozen satellite galaxies. Even the distribution of stars within these dwarf galaxies doesn’t fit the dark matter model very well.

While warm dark matter like neutrinos doesn’t fit the data well, there are other warm models that might. They solve some of the issues with warm dark matter by suggesting dark matter is also “fuzzy.” This refers to its quantum nature. All matter has a quantum aspect to it. For example, an electron doesn’t orbit the nucleus of an atom like a planet around the Sun. Instead, the electron is in a “fuzzy” quantum state within the atom. Normally the fuzzy nature of quantum particles only acts at short distances, on the scale of a few atoms, but under the right conditions this kind of fuzzy quantum behavior can occur over large distances. In the fuzzy dark matter model, the dark matter particles can interact quantum mechanically over great distances, thus allowing them to behave in ways similar to cold dark matter.

Several computer simulations of the universe agree with the cold dark matter model on large scales, but a new study specifically looked at how the warm fuzzy model compares. To do this the team used observations from more than 100 quasars. Quasars are distant objects powered by the supermassive black holes in the centers of galaxies. They give off tremendous amounts of light and energy, and so we can see them across billions of light years. As the light from these quasars travels across the cosmos to reach us, it is distorted by diffuse filaments of hydrogen gas between galaxies, known as the intergalactic medium. The distribution of hydrogen in the intergalactic medium allows us to study how clusters of galaxies formed. The team compared this data to both cold and warm-fuzzy dark matter models. They found the warm-fuzzy model didn’t agree with observation. That doesn’t mean that warm-fuzzy dark matter doesn’t exist, but if it does exist it must be so diffuse and have such an extraordinarily tiny mass that it couldn’t have caused the clustering of galaxies we observe.

Cold dark matter still has its own problems, and the nature of dark matter still holds many mysteries. But we now know that for the most part dark matter isn’t warm and fuzzy.

Paper: Vid Iršič, et al. First Constraints on Fuzzy Dark Matter from Lyman-α Forest Data and Hydrodynamical Simulations. Phys. Rev. Lett. 119, 031302 (2017)