Researchers reveal a theoretical breakthrough that may explain both the nature of invisible dark matter and the cosmic web, the large-scale structure of the universe. The result forges a new link between these two long-standing astronomical problems, opening up new avenues for understanding the universe.
According to the findings, the ‘clumpiness problem,’ which concerns the unexpectedly even distribution of matter on large scales throughout the universe, could be a sign that dark matter is made up of hypothetical, ultra-light particles known as axions. The implications of proving the existence of difficult-to-detect axions go beyond understanding dark matter and may raise fundamental questions about the nature of the universe itself.
Researchers at the University of Toronto reveal a theoretical breakthrough in a study published in the Journal of Cosmology and Astroparticle Physics that may explain both the nature of invisible dark matter and the large-scale structure of the universe known as the cosmic web. The result forges a new link between these two long-standing astronomical problems, opening up new avenues for understanding the universe.
Finding axion dark matter would be one of the most significant discoveries of this century if confirmed by future telescope observations and lab experiments.
Keir Rogers
The research suggests that the “clumpiness problem,” which centres on the unexpectedly even distribution of matter on large scales throughout the cosmos, may be a sign that dark matter is composed of hypothetical, ultra-light particles called axions. The implications of proving the existence of hard-to-detect axions extend beyond understanding dark matter and could address fundamental questions about the nature of the universe itself.
“Finding axion dark matter would be one of the most significant discoveries of this century if confirmed by future telescope observations and lab experiments,” says lead author Keir Rogers, Dunlap Fellow at the Dunlap Institute for Astronomy & Astrophysics in the Faculty of Arts & Science at the University of Toronto. “At the same time, our results suggest an explanation for why the universe is less clumpy than we thought, an observation that has become increasingly clear over the last decade or so, and which currently leaves our theory of the universe uncertain.”
Dark matter, comprising 85 percent of the universe’s mass, is invisible because it does not interact with light. Scientists study its gravitational effects on visible matter to understand how it is distributed in the universe.
According to one leading theory, dark matter is made of axions, which are described as “fuzzy” in quantum mechanics due to their wave-like behavior. Axions, as opposed to discrete point-like particles, can have wavelengths larger than entire galaxies. This fuzziness influences the formation and distribution of dark matter, which could explain why the universe is less clumpy than predicted in an axion-free universe.
This lack of clumpiness has been observed in large galaxy surveys, calling into question the other dominant theory that dark matter is only made up of heavy, weakly interacting subatomic particles known as WIMPs. Despite experiments such as the Large Hadron Collider, no evidence for the existence of WIMPs has been discovered.
“In science, it’s when ideas break down that new discoveries are made and age-old problems are solved,” says Rogers.
For the study, the research team — led by Rogers and including members of associate professor Renée Hložek’s research group at the Dunlap Institute, as well as from the University of Pennsylvania, Institute for Advanced Study, Columbia University and King’s College London — analyzed observations of relic light from the Big Bang, known as the Cosmic Microwave Background (CMB), obtained from the Planck 2018, Atacama Cosmology Telescope and South Pole Telescope surveys.
The CMB data were compared to galaxy clustering data from the Baryon Oscillation Spectroscopic Survey (BOSS), which maps the positions of about a million galaxies in the nearby universe. They measured fluctuations in the amount of matter throughout the universe and confirmed its reduced clumpiness compared to predictions by studying the distribution of galaxies, which mirrors the behavior of dark matter under gravitational forces.
Following that, the researchers used computer simulations to predict the appearance of relic light and the distribution of galaxies in a universe with long dark matter waves. These calculations agreed with CMB data from the Big Bang and galaxy clustering data, lending credence to the idea that fuzzy axes could account for the clumpiness issue.
Future research will include large-scale surveys to map millions of galaxies and provide precise measurements of clumpiness, including observations with the Rubin Observatory over the next decade. The researchers hope to compare their theory to direct observations of dark matter via gravitational lensing, a phenomenon in which the clumpiness of dark matter is measured by how much it bends light from distant galaxies, similar to a giant magnifying glass. They also intend to investigate how galaxies expel gas into space and how this affects the distribution of dark matter to confirm their findings.
Understanding the nature of dark matter is one of the most pressing fundamental questions and is critical to understanding the universe’s origin and future. Scientists currently lack a single theory that explains gravity and quantum mechanics simultaneously – a theory of everything. String theory, which proposes another level below the quantum level where everything is made of string-like excitations of energy, has become the most popular theory of everything in recent decades. Detecting a fuzzy axion particle, according to Rogers, could be an indication that the string theory of everything is correct.
