A new link has been discovered between dark matter and the clumps of the universe

In a new study, researchers suggest that the universe’s lack of mass indicates that dark matter is composed of hypothetical ultralight particles called axions. If confirmed, this could have broad implications for our understanding of the universe and could even provide support for string theory.

In a study published June 14 in the Journal of Cosmology and Astroparticle PhysicsResearchers at the University of Toronto have revealed a theoretical breakthrough that may explain both the invisible nature of dark matter and the large-scale structure of the universe known as the cosmic web. The result establishes a new link between these two long-standing problems in astronomy, opening up new possibilities for understanding the universe.

The research suggests that the “clumping problem,” which centers around an unexpectedly even distribution of matter on large scales throughout the universe, 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 axes extend far beyond understanding dark matter and could address fundamental questions about the nature of the universe itself.

A map of galaxies in the local universe as seen by the Sloan Digital Sky Survey, which researchers used to test the axion theory. Each point is a galactic location and the Earth is in the middle of the map. Credit: Sloan Digital Sky Survey

“If confirmed by future telescope observations and lab experiments, finding dark matter from axions will be one of the most important discoveries of this century,” says lead author Keir Rogers, a Dunlap fellow in the Dunlap Institute for Astronomy and Astrophysics in the College of Arts and Astrophysics of Science at the University of Toronto. . “At the same time, our results point to an explanation for why the universe is less lumpy than we thought, an observation that has become increasingly apparent over the past decade or so, and currently leaves our theory of the universe uncertain.”

In shaping the universe, gravity builds a cobweb-like structure of filaments that bind galaxies and galaxy clusters together along invisible bridges hundreds of millions of light-years long. This is known as the cosmic web. Credit: Volker Sprengel (Max Planck Institute for Astrophysics) et al.

Dark matter, which makes up 85% of the mass of the universe, is invisible because it does not interact with light. Scientists study the effects of gravity 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” because of their wave-like behavior. Unlike discrete point-like particles, axions can have longer wavelengths than entire galaxies. This fuzziness affects the composition and distribution of dark matter, which may explain why the universe is less lumpy than would be expected in a universe without axes.

A computer simulation of a part of the universe with or without axes showing how the structure of the cosmic web of dark matter is less lumpy if it has axes. As for size, the Milky Way will sit inside one of the tiny green dots called halos. Credit: Alexander Spencer London/Alex Lago

This lack of clumping has been observed in surveys of large galaxies, challenging the other prevailing theory that dark matter consists solely 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 WIMPs.

Keir Rogers, lead author of the study and a Dunlap fellow at the Dunlap Institute for Astronomy and Astrophysics. Credit: Courtesy Keir Rogers

“In science, when ideas are unraveled, new discoveries are made and old problems are solved,” says Rogers.

For the study, the research team — led by Rogers and including members of the research group of Associate Professor Rene Hluczek at the Dunlap Institute, as well as from the University of Pennsylvania, Institute for Advanced Study,[{” attribute=””>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 researchers compared these CMB data with galaxy clustering data from the Baryon Oscillation Spectroscopic Survey (BOSS), which maps the positions of approximately a million galaxies in the nearby universe. By studying the distribution of galaxies, which mirrors the behavior of dark matter under gravitational forces, they measured fluctuations in the amount of matter throughout the universe and confirmed its reduced clumpiness compared to predictions.

The researchers then conducted computer simulations to predict the appearance of relic light and the distribution of galaxies in a universe with long dark matter waves. These calculations aligned with CMB data from the Big Bang and galaxy clustering data, supporting the notion that fuzzy axions could account for the clumpiness problem.

Future research will involve large-scale surveys to map millions of galaxies and provide precise measurements of clumpiness, including observations over the next decade with the Rubin Observatory. The researchers hope to compare their theory to direct observations of dark matter through gravitational lensing, an effect where dark matter clumpiness is measured by how much it bends the light from distant galaxies, akin to a giant magnifying glass. They also plan to investigate how galaxies expel gas into space and how this affects the dark matter distribution to further confirm their results.

Understanding the nature of dark matter is one of the most pressing fundamental questions and key to understanding the origin and future of the universe.

Presently, scientists do not have a single theory that simultaneously explains gravity and quantum mechanics — a theory of everything. The most popular theory of everything over the last few decades is string theory, which posits another level below the quantum level, where everything is made of string-like excitations of energy. According to Rogers, detecting a fuzzy axion particle could be a hint that the string theory of everything is correct.

“We have the tools now that could enable us to finally understand something experimentally about the century-old mystery of dark matter, even in the next decade or so—and that could give us hints to answers about even bigger theoretical questions,” says Rogers. “The hope is that the puzzling elements of the universe are solvable.”

Reference: “Ultra-light axions and the S8 tension: joint constraints from the cosmic microwave background and galaxy clustering” by Keir K. Rogers, Renée Hložek, Alex Laguë, Mikhail M. Ivanov, Oliver H.E. Philcox, Giovanni Cabass, Kazuyuki Akitsu and David J.E. Marsh, 14 June 2023, Journal of Cosmology and Astroparticle Physics.
DOI: 10.1088/1475-7516/2023/06/023

National Aeronautics and Space Administration, Natural Sciences and Engineering Research Council of Canada, David Dunlap family and University of Toronto, Connaught Fund.