dark matter

Theoretical Breakthrough Could Finally Solve the Mystery of Invisible Dark Matter

A team of Canadian researchers says that the lack of clumpiness in the structure of the universe could help to shed new light on the composition of dark matter.

A hypothetical form of matter believed to represent as much as 85% of the known universe, dark matter consists of particles that do not produce, reflect, or absorb light, making it undetectable by conventional methods astronomers use to observe electromagnetic radiation.

Although it is essentially invisible, the existence of dark matter can be inferred from the effect it has on objects that can be seen throughout the universe. There are several theories about what forms this hypothetical substance might take, which include “cold dark matter” comprised of weakly interacting particles and “hot” forms resulting from the movement of high-energy particles whose origins go all the way back to the beginning of the universe.

However, according to new research led by Keir Rogers with the University of Toronto’s Dunlap Institute for Astronomy and Astrophysics, dark matter may really consist of a variety of ultra-light particles known as axions, based on the lack of clumpiness in the universe.

The team believes this insight could ultimately reveal the longstanding mystery of invisible dark matter, along with providing new insights into what is known by cosmologists as the cosmic web, by establishing new links between these phenomena.

In the past, astronomers have noted how the distribution of matter throughout the universe is very even—perhaps surprisingly so—even at large scales. This goes against our expectations about how the matter should be distributed and could suggest that dark matter is composed of axions, a hypothetical form of very light, electrically neutral particles.

Rogers says that future experiments which may confirm this could be “one of the most significant discoveries of this century,” adding that his team’s research not only suggests the role axions may play in the mysterious nature of dark matter but also points to “an explanation for why the universe is less clumpy than we thought.”

Rogers adds that the apparent lack of clumpiness is “an observation that has become increasingly clear over the last decade or so and currently leaves our theory of the universe uncertain.”

Because of their wave-like behavior at wavelengths that can be even larger than a galaxy, axions are often characterized as “fuzzy,” a feature that has a direct influence on the formation of dark matter, as well as its distribution. This would not make sense in a universe without axions, Rogers and the team says, and is suggestive of the presence of axions in the dark matter equation.

The new research presents challenges to past cold dark matter theories involving weakly interacting subatomic particles, or WIMPs.

“In science, it’s when ideas break down that new discoveries are made,” Rogers said in a statement, “and age-old problems are solved.”

Analyzing Cosmic Microwave Background (CMB) emanating from the earliest moments of the universe and collected by a series of large-scale surveys conducted in 2018, the CMB data was compared with data on galaxy clustering obtained from the Baryon Oscillation Spectroscopic Survey (BOSS), which provides information on the positions of close to one million galaxies. The galactic distribution is a corollary for the influence of gravitational forces on dark matter throughout the universe.

Using this data to measure variances in the amount and distribution of matter in the universe, Rogers and his team were first able to confirm the lack of clumpiness, then apply computer simulations that allowed them to predict the manifestation of the earliest “relic light” emanating from the Big Bang, as well as the distribution of galaxies within the universe based on where they would be positioned in a universe with long dark matter waves.

Rogers and the team found that the simulations were a good match for the CMB data they analyzed and appear to lend further weight to the notion that axions are indeed a component behind the lack of clumpiness observed throughout the universe. In the future, additional large-scale surveys could help to provide data that will allow researchers like Rogers and his team to conduct even more precise measurements.

“We have the tools now that could enable us to finally understand something experimentally about the century-old mystery of dark matter,” Rogers said in a statement, adding that such insights may occur within the next two decades.

Insights which, Rogers hopes, “could give us hints to answers about even bigger theoretical questions.”

The team’s paper, “Ultra-light axions and the S8 tension: joint constraints from the cosmic microwave background and galaxy clustering,” was written by lead author Keir K. Rogers, and co-authors Mikhail M. Ivanov, Alex Laguë, Oliver H.E. Philcox, Giovanni Cabass, Renée Hložek, David J.E. Marsh, and Kazuyuki Akitsu, and appeared on June 14, 2023, in the Journal of Cosmology and Astroparticle Physics.

Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on Twitter: @MicahHanks