A computer simulation of a part of the universe with and without axions, showing how the structure of the cosmic web of dark matter is less cumbersome if it contains axions. For scale, the Milky Way galaxy will sit inside one of the small green dots, which are called haloes. Credit: Alexander Spencer London/Alex Lagué.
Researchers suggest in a new study that the lack of clumps in the universe suggests that dark matter is made up of hypothetical ultralight particles called axions. If confirmed, this could have far-reaching implications for our understanding of the universe and could even provide support for string theory.
In a study published on June 14 in Journal of Cosmology and Astroparticle Physics, University of Toronto researchers reveal a theoretical breakthrough that could explain both the nature of invisible dark matter and the large-scale structure of the universe known as the cosmic web. The result establishes a new connection between these two long-standing problems in astronomy, opening up new possibilities for understanding the cosmos.
The research suggests that the “density problem,” which centers on the unexpectedly uniform distribution of matter on large scales throughout the cosmos, may be a sign that dark matter is made up 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 answer 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 the position of a galaxy and Earth is located in the middle of the map. Credit: Sloan Digital Sky Survey
“If confirmed by future telescope observations and laboratory experiments, the finding of axionic dark matter will be one of the most significant discoveries of this century,” said lead author Keir Rogers, a Dunlap Fellow at the Dunlap Institute for Astronomy and Astrophysics in the School of Arts and Science at the University of Toronto. “At the same time, our results suggest an explanation for why the universe is less cumbersome than we thought, an observation that has become increasingly clear over the past decade or so, and currently leaves our theory of the universe uncertain.”
In forming the universe, gravity builds a vast web-like structure of filaments, connecting galaxies and clusters of galaxies together in invisible bridges hundreds of millions of light-years long. This is known as the Space Network. Courtesy: Volker Springel (Max Planck Institute for Astrophysics) et al.
Dark matter, which makes up 85 percent of the mass of the universe, 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.
A leading theory suggests that dark matter is made up of axions, described in quantum mechanics as “fuzzy” because of their wave-like behavior. Unlike discrete point particles, axions can have wavelengths longer than entire galaxies. This ambiguity affects the formation and distribution of dark matter, potentially explaining why the universe is less clumpy than predicted in an axion-free universe.
A computer simulation of a part of the universe with and without axions, showing how the structure of the cosmic web of dark matter is less cumbersome if it contains axions. For scale, the Milky Way galaxy will sit inside one of the small green dots, which are called haloes. Credit: Alexander Spencer London/Alex Lagué
This lack of clumps has been observed in studies of large galaxies, challenging the other prevailing theory that dark matter consists only of heavy, weakly interacting 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 break down, new discoveries are made and old problems are solved,” says Rogers.
For the study, the research team, led by Rogers and including members of Associate Professor Renee Chlozek’s research group at the Dunlap Institute, as well as the University of Pennsylvania Institute for Advanced Study, Columbia University and King’s College London—relict light observations analyzed by Big bang, known as the cosmic microwave background (CMB), obtained from surveys by Planck 2018, 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 the influence of gravitational forces, they measured fluctuations in the amount of matter in the universe and confirmed its reduced density compared to predictions.
The researchers then ran computer simulations to predict the appearance of relic light and the distribution of galaxies in the universe with long-wavelength dark matter. These calculations are aligned with CMB data from the Big Bang and galaxy clustering data, supporting the idea that fuzzy axions can explain the patchiness problem.
Future research will include large-scale surveys to map millions of galaxies and provide precise measurements of the bumps, 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 in which clumps of dark matter are measured by how much they bend light from distant galaxies, like a giant magnifying glass. They also plan to study how galaxies eject gas into space and how this affects the distribution of dark matter to confirm their results.
Understanding the nature of dark matter is one of the most pressing fundamental questions and a key to understanding the origin and future of the universe.
Currently, scientists do not have a single theory that explains both gravity and quantum mechanics—a theory of everything. The most popular theory of everything in the last few decades is string theory, which puts another level below the quantum level where everything is made of string-like excitations of energy. According to Rogers, the discovery of a fuzzy axion particle could be a hint that the string theory of everything is correct.
“We now have the tools that could allow us to finally understand something experimentally about the age-old mystery of dark matter, even in the next decade or so — and that could give us clues to answers to even bigger theoretical questions.” , says Rogers. “The hope is that the puzzling elements of the universe are solvable.”
Reference: “Ultralight axions and the S8 voltage: joint constraints from the cosmic microwave background and galaxy clustering” by Keir K. Rogers, Renée Hložek, Alex Laguë, Mikhail M. Ivanov, Oliver HE Philcox, Giovanni Cabass, Kazuyuki Akitsu and David JE Marsh , June 14, 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.