Dark matter has a profound effect on our universe, shaping galaxies and even leaving its fingerprints on the energy left over from the Big Bang. Despite its relevance, dark matter is also extremely hard to detect — rather than observe it directly, astronomers instead look for clues based on its gravitational interaction with normal matter (the protons, electrons, and neutrons that make up everything we see and touch). Recent observations made with NASA’s Chandra X-ray Observatory have hinted that dark matter may be “fuzzier” than previously thought.
The study, which was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society, focuses on X-ray observations of 13 galaxy clusters. The authors use observations of the hot gas that permeates galaxy clusters to estimate the amount and distribution of dark matter within the clusters and test its properties against current leading models, looking for the model that best fits the data.
The current standard cosmological model includes “cold dark matter” as a major component. In this case, “cold” simply means that dark matter travels slowly when compared to the speed of light. However, cold dark matter models indicate that dark matter — and normal matter, which is drawn to the dark matter via gravity — should clump together in the centers of galaxies. But no such increase in matter, normal or dark, is seen. Additionally, cold dark matter models predict that the Milky Way should have many more small satellite galaxies than we currently see. Even accounting for the fact that some satellites may be challenging to find, the cold dark matter models still over-predict our satellites by a considerable amount.
However, cold dark matter is only one of several dark matter theories. By contrast, “fuzzy dark matter” is a model in which dark matter has a mass about 10 thousand trillion trillion times smaller than an electron. In quantum mechanics, all particles have both a mass and a corresponding wavelength. Such a tiny mass would actually cause the wavelength of dark matter to stretch 3,000 light-years between peaks. (The longest wavelength of light, which is radio, stretches just a few miles between peaks.)
With a wavelength this long, dark matter would not clump in the centers of galaxies, which could explain the reason this is not observed. But while simple fuzzy dark matter models fit observations of small galaxies, larger galaxies may require a slightly more complex explanation. And galaxy clusters are larger test beds still, which is why researchers turned Chandra to several massive galaxy clusters for observations.
The results show that while a simple fuzzy dark matter model still didn’t explain the cluster observations well, a more complex and “fuzzier” model did. In this model, dark matter occupying several quantum states at once (think an atom with many electrons, some of which are at higher energy levels) creates overlapping wavelengths that further spread out the effect, which changes the distribution of dark matter expected throughout the galaxy cluster as a whole.
The predictions from this model match the observations of the 13 galaxy clusters much more closely, indicating that fuzzier dark matter may be the best model to incorporate into our cosmological models. However, further study and more precise measurements are needed to better test this theory and ensure it truly reflect what we see throughout the cosmos.