Cosmologists have presented a theoretical breakthrough that may explain both the nature of invisible dark matter and the large-scale structure of the universe known as the cosmic web.
The research, led by the University of Toronto and published in the Journal of Cosmology and Astroparticle Physics, suggests that the “clumping problem,” which centers on the unexpectedly uniform distribution of matter on a large scale throughout the cosmos, may be a sign that dark matter is composed of hypothetical ultralight particles called axions. The implications of proving the existence of hard-to-detect axions extend beyond the understanding of dark matter and could address fundamental questions about the nature of the universe itself.
“If confirmed by future telescope observations and laboratory experiments, finding axion dark matter would be one of the most important discoveries of this century,” said lead author Keir Rogers, from the Dunlap Institute for Astronomy and Astrophysics at the University of from Toronto.
“At the same time, our results suggest an explanation for why the universe is less lumpy than we thought, an observation that has become increasingly clear over the past decade and currently leaves our theory of the universe uncertain.”
Dark matter, which comprises 85 percent of the mass in the universe, is invisible because it doesn’t interact with light. Scientists study its gravitational effects on visible matter to understand how it is distributed in the universe.
One leading theory proposes that dark matter is made of axions, described in quantum mechanics as “fuzzy” due to their wave behavior. Unlike discrete point particles, axions can have longer wavelengths than entire galaxies. This blurring influences the formation and distribution of dark matter, which could explain why the universe is less lumpy than predicted in an axionless universe.
This lack of clumps has been observed in large surveys of galaxies, challenging the other prevailing theory that dark matter consists only of weakly interacting heavy subatomic particles called WIMPs. Despite experiments such as the Large Hadron Collider, no evidence has been found to support the existence of WIMP.
“In science, it’s when ideas break down that new discoveries are made and old problems are solved,” says Rogers.
For the study, the research team analyzed observations of relic light from the Big Bang, known as the Cosmic Microwave Background (CMB), obtained from the Planck 2018 surveys, the Atacama Cosmology Telescope, and the South Pole Telescope.
The researchers compared this CMB data with galaxy clustering data from the Baryon Oscillation Spectroscopic Survey (BOSS), which maps the positions of approximately one million galaxies in the nearby Universe. By studying the distribution of galaxies, which reflects the behavior of dark matter under gravitational forces, they measured fluctuations in the amount of matter throughout the universe and confirmed its reduced clumps compared to predictions.
The researchers then ran computer simulations to predict the appearance of relics of light and the distribution of galaxies in a universe with long waves of dark matter. These calculations were aligned with the CMB data from the Big Bang and the galaxy clustering data, supporting the notion that fuzzy axions could explain the clumping problem.