In the hunt for dark matter, are axions our best bet?

A composite image of the Bullet Cluster, a much-studied pair of galaxy clusters that have collided head on. One has passed through the other, like a bullet traveling through an apple, and is thought to show clear signs of dark matter (blue) separated from hot gases (pink). (Image credit: X-ray: NASA/ CXC/ CfA/ M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/ U.Arizona/ D.Clowe, Lensing map: ESO/WFI)

Scientists investigating the true identity of dark matter are finding new evidence to support one leading candidate: axions.

Since its existence was first inferred in 1933, dark matter has remained an elusive "white whale" for scientists around the globe. While it is thought to comprise about 85% of all matter in the universe, what the invisible matter actually is remains a mystery.

But some researchers think that dark matter could actually be a strange particle called the axion.

Related: The 11 biggest questions about dark matter

What is an axion?

The axion is a hypothetical elementary particle that has both low mass and low energy. Itwas first proposed as a hypothetical particle in 1977 as a solution to what is called the "strong CP problem" in particle physics. The strong CP problem is a long-standing unsolved question that asks why CP symmetry (conjugation symmetry (C) and parity symmetry (P)) is preserved in quantum chromodynamics (the theory of the strong interaction between elementary particles quarks and gluons).

Within CP-symmetry, the laws of physics should be the same if a particle is swapped with its antiparticle and it is inverted or mirrored. Inquantum chromodynamics, a violation of CP-symmetry could happen in strong interactions. However, no violation has ever been observed. 

"There's this CP symmetry which we know, which distinguishes between particles and antiparticles. We know it's violated in the weak interactions. It was a puzzle why it was not violated in the strong interactions," John Ellis, a particle physicist at CERN (home of the Large Hadron Collider, or LHC), who has studied axions since they were first proposed, told 

In 1977, an extension of the standard model was proposed in which it would make sense that the strong interactions didn't violate this symmetry, Ellis said "And this theory predicted the existence of the axion," he added.

In 2020, a team of physicists found the first direct evidence for axions, launching the particle's legitimacy forward along with the scientific community's interest in the particle. This has continued to push forward assertions that axion could be the best dark matter candidate. 

"Dark matter is most of the matter in the universe, and we have no idea what it is. One of the most outstanding questions in all of science is, 'What is dark matter?'" Benjamin Safdi, an assistant professor of physics at the University of California, Berkeley and lead author of a new study investigating axions, said in a statement

"We suspect it is a new particle we don't know about, and the axion could be that particle. It could be created in abundance in the Big Bang and be floating out there explaining observations that have been made in astrophysics," Safdi added. 

Dark matter "hairs." Some researchers think that dark matter is distributed throughout the universe in this "net-like" pattern. (Image credit: NASA/JPL-Caltech)

Letting go of WIMPs

In two new reviews published to the journal Science Advances Feb. 23, researchers describe how the axion came to be a leading dark matter candidate and how physicists could continue to study the particle and perhaps explain the mystery of dark matter. 

Until now, one particular dark matter candidate has been leading the pack: WIMPs, which stands for weakly interacting massive particles. WIMPs are an umbrella term that describe hypothetical particles that interact only very weakly with matter through the weak  nuclear force. WIMPs are predicted to be 1-1,000 times heavier than protons. 

However, despite their status as a leading candidate, "over the past few years, WIMPs have not shown up at the LHC, they haven't shown up in direct searches for dark matter," Ellis said.

So, with WIMPs losing their luster as a candidate, "we thought [this] would be a good moment to somehow capture this shift in focus," Ellis said about this investigation into axions. 

Ellis did add: "I still hold a fire for WIMPs." But "I think it's certainly reasonable to hedge one's bets a little bit and think about other candidates. So, I must confess that I continue to play the field," Ellis said. 

Catching the axion

So, with WIMPs falling from the lead, the researchers explored what steps could be taken to confirm the existence of the axion and explore further whether or not it could be dark matter. According to the reviews, the team suggests that they could finally "catch" the axion and confirm its existence by predicting its mass. 

In the two new reviews, researchers suggest a number of different approaches that physicists could take in both predicting the axiom's mass and investigating it as a dark matter candidate. These approaches include using haloscopes, instruments which "scope microwave photon signals from the axions in our galactic halo," according to the paper. (A galactic halo is a large, spherical region of space around a galaxy that extends beyond just visible matter.)

Scientists expect that axions would convert into an electromagnetic wave in a microwave cavity during an experiment like this, although it would be very rare. And thus, they would be able to detect that wave. 

But the researchers suggest a number of other methods that physicists currently use and could potentially use to hunt for axions. These include using terrestrial telescopes, using the CERN Axion Search Telescope (CST) to detect axions produced in the sun's core, or even spotting axions in the magnetosphere of neutron stars where they're thought to convert into photons and leave behind distinct spectral features. These are among many different suggestions that the two reviews make. 

Searching for mass

While the researchers in the two reviews explore the cutting edge technology that could allow scientists to detect axions, one of the more popular axion-hunting techniques to-date has been to try and detect electromagnetic waves in a microwave cavity. 

But, in the second new study, published Feb. 25 in the journal Nature Communications, researchers used one of the world's largest supercomputers at Berkeley Lab's National Research Scientific Computing Center (NERSC) to simulate when axions would have been created almost immediately after the Big Bang. 

In their simulation, the team was able to factor in the total mass of dark matter in the universe and the total number of axions produced. This allowed them to estimate what the axion mass could possibly be.

They found that the axion's mass would be more than twice as big as theorists have expected: 40-180 microelectron volts (roughly equivalent to one 10-billionth the mass of an electron, according to a statement.) 

"Our work provides the most precise estimate to date of the axion mass and points to a specific range of masses that is not currently being explored in the laboratory," Safdi said. 

The team also found that axions could produce an electromagnetic wave at a higher frequency than expected, a frequency that is typically outside of the range of the experiments designed to detect axion EM waves. 

"With these axion experiments, they don't know what station they're supposed to be tuning to, so they have to scan over many different possibilities," Safdi said about experiments searching for the waves left behind by axions.

With their simulation, the researchers showed that axions in the early universe might have acted like "riders bucked from a bronco," according to the statement.

"You can think of these strings as composed of axions hugging the vortices while these strings whip around forming loops, connecting, undergoing a lot of violent dynamical processes during the expansion of our universe, and the axions hugging the sides of these strings are trying to hold on for the ride," Safdi said. "But when something too violent happens, they just get thrown off and whip away from these strings. And those axions which get thrown off of the strings end up becoming the dark matter much later on."

The team has not solved the dark matter problem or the axion quandary, but as more researchers continue to push forward with this type of experimentation it brings science as a whole closer to better understanding what exactly these hypothetical particles might be and, ultimately, what is dark matter. 

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Chelsea Gohd
Senior Writer

Chelsea “Foxanne” Gohd joined in 2018 and is now a Senior Writer, writing about everything from climate change to planetary science and human spaceflight in both articles and on-camera in videos. With a degree in Public Health and biological sciences, Chelsea has written and worked for institutions including the American Museum of Natural History, Scientific American, Discover Magazine Blog, Astronomy Magazine and Live Science. When not writing, editing or filming something space-y, Chelsea "Foxanne" Gohd is writing music and performing as Foxanne, even launching a song to space in 2021 with Inspiration4. You can follow her on Twitter @chelsea_gohd and @foxannemusic.