Astronomers discover a new link between dark matter and the clumps of the universe

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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

In a study published today in 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 on 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.

“If confirmed by future telescope observations and laboratory experiments, finding axion dark matter 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. Sciences at the University of Toronto.

“At the same time, our results point to an explanation as to why the universe is less lumpy than we thought, an observation that has become more pronounced over the past decade or so, and currently leaves our theory of the universe uncertain.”

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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.

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.

One leading theory proposes that dark matter is made of axions, described in quantum mechanics as “fuzzy” because of their wave-like behaviour. 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.

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.

“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 which includes members of Associate Professor Renee Hluczek’s research group at the Dunlap Institute, as well as from the University of Pennsylvania, the Institute for Advanced Study, Columbia University and King’s College London — analyzed observations of leftover light from the Big Bang, otherwise known as the background. Cosmic Microwaves (CMB), obtained from the Planck 2018 surveys and the Atacama Cosmology and South Pole Telescope.

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.

The researchers compared this CMB data with data for galaxy clusters from the Baryon Oscillation Spectroscopic Survey (BOSS), which maps the locations of nearly a million galaxies in the nearby universe. By studying the distribution of galaxies, which reflect dark matter’s behavior under gravitational forces, they measured fluctuations in the amount of matter throughout the universe and confirmed its lower mass compared to predictions.

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The researchers then ran computer simulations to predict the appearance of leftover light and the distribution of galaxies in a universe with long wavelengths of dark matter. These calculations align with CMB data from the Big Bang and galaxy clustering data, supporting the idea that fuzzy axes could explain the clustering problem.

Future research will include large-scale surveys to map millions of galaxies and provide accurate measurements of clustering, 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 dark matter’s agglomeration is measured by how light from distant galaxies is bent, similar to a giant magnifying glass. They also plan to investigate how galaxies expel gas into space and how this affects the distribution of dark matter to further confirm their findings.

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.

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.

At present, scientists do not have a single theory that would explain both gravity and quantum mechanics at the same time – the theory of everything. The most popular theory of everything over the past few decades is string theory, which posits another, lower level of quantum level, where everything consists of excitations of string-like energy. According to Rogers, the discovery of a mysterious axial particle could be a hint that the string theory of everything is correct.

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

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more information:
Ultralight axes and S8 tension: shared constraints from the cosmic microwave background and galaxy clustering, Journal of Cosmology and Astroparticle Physics (2023). DOI: 10.1088/1475-7516. …475-7516/2023/06/023

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