IN 1977, physicist took a walk that would change the course of particle physics forever. “On that walk, I had the germs of two really good ideas,” he recalls. The first was how a theoretical particle, later dubbed the Higgs boson, might interact with other particles. This would be how the Higgs was found decades later. The second idea, however, has taken a little longer to catch on.
Wilczek had imagined a way that very light – essentially massless – particles could be made. He talked to his colleague, the late Steven Weinberg, who had been thinking along the same lines. Together, they predicted a class of particles we now call axions.
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Weinberg was optimistic, convincing Wilczek, now at the Massachusetts Institute of Technology, that axions would be easy to find. But nearly half a century later, we are still looking. In the intervening years, interest in axions – largely fuelled because they could be the hard-to-find dark matter that makes up 85 per cent of the matter in the universe – dwindled in favour of other explanations.
Today, amid our failure to track down dark matter and a slew of theoretical and experimental breakthroughs, axions are resurgent. “They’re very much back in fashion,” says Wilczek.
And now, there is far more than the mysterious nature of dark matter up for grabs, because axions offer a solution to a whole host of cosmological mysteries, including the elusive dark energy thought to drive the universe’s expansion. They are, in short, the particles that could solve the universe. Which is why efforts to search for them are ramping up like never before.
A few years before his fateful walk, Wilczek was at the supermarket with his mother when he spotted a detergent called Axion. He liked the name. “When this opportunity arose for a new particle that cleaned up a problem, the stars were aligned,” he says. “Weinberg had been calling it a ‘Higglet’, but he agreed that ‘axion’ was a better name.”
What are axions?
Axions, if they exist, would abide by the strange rules of quantum mechanics, meaning they could act as both waves and particles. As particles, they would have an exceptionally low mass, ranging from hundreds to billions of times smaller than the lightest particles we know of today. Because they would be so light, their wavelength could be as long as the width of a galaxy.
Not that axions were originally meant to be a solution to cosmological mysteries. Instead, the first issue they resolved was something called the strong charge-parity problem. This is a mystery in particle physics related to the strong force, one of the four fundamental forces. Unlike the other three, which affect matter and antimatter in different ways, the strong force affects subatomic particles called quarks to the same extent as their antimatter counterpart, antiquarks. But if there were some kind of new field pervading the universe, composed of light particles like axions, it could explain this unusual symmetry.
Then, in 1983, various physicists, , began to realise that this new kind of field could also solve another major problem. “It just came for free,” says . This happened because, in the early universe, there was lots of energy. “As the universe cools, this [axion] field will start oscillating,” says Marsh. In other words, it would give off energy in the form of pulsating light and heat. “The energy carried in those oscillations turns out to behave just like dark matter,” he says. So axions became a candidate for dark matter.
Axion dark matter
Axions are good at hiding. This is mostly because they could come in a dramatically wide range of possible masses, so knowing where to look for them is tricky. As energy and mass are interchangeable, physicists measure the masses of particles using a unit known as an electronvolt, with the mass of a proton being a billion electronvolts. “The heaviest the axion could possibly be is about one-hundredth of an electronvolt,” says Marsh. “But it can go all the way down to one-trillionth of an electronvolt.” Because of this huge range of masses, nobody knew where to start. There was very subdued enthusiasm for axions, says Wilczek.
At the same time, an alternative explanation for dark matter had been gaining favour. Weakly interacting massive particles (WIMPs) were estimated to be much larger than axions, around the mass of the proton, which meant experiments to find them were easier to do. On the other hand, “the experiments to search for axions were really difficult”, says Marsh.
“For many years, WIMPs were dominant and axions got less attention,” says . In the decades since WIMPs were proposed, however, there have been many searches for them, all of which have come up short. And over the past few years, research has suggested that, when it comes to dark matter, axions could make more sense than WIMPs. “They have essentially become the preferred dark matter candidate,” says .
One such hint comes from the way light bends around galaxies due to their gravity. Most galaxies are thought to be surrounded by halos of dark matter. We see this in the way they rotate, which appears to be uniform. From this, astronomers deduce that they must be pulled equally, in all directions, by some unseen force.
This is where axions come in. Their wavelength, the distance over which they oscillate, could be up to 3000 light years, says . But like ripples in a pond, these oscillations would interact with each other, causing interference. In a dark matter halo, that might form noticeable regions of higher-density and lower-density axions. “It’s a very lumpy halo,” says Hui. Earlier this year, a team of astronomers led by , in the way light bent around a galaxy on its way to Earth.
Dark matter clumping
This is an exciting development, because if axions are the answer to dark matter, they could solve a separate, but related mystery. Namely, that dark matter appears to clump together less than we would expect. Observations of the cosmic microwave background (CMB), the light left over from the inflation of the early universe given off some 380,000 years after the big bang, don’t match up with the distribution of clusters of galaxies we see today.
“We have a model of the universe to predict how it should look right after the big bang and today, but those two observations don’t agree with each other,” says in the UK. Heymans was the first to spot this problem, known as the sigma-8 tension, and has worked on it for years. If dark matter were even partly , meaning those at the lower end of the potential mass range, she says, these axions might suppress the clumping.
Evidence for this could come soon, if we are lucky. Such axions might show up in experiments seeking to find gravitational waves, ripples in space-time caused by massive objects spinning or colliding. In June, a handful of experiments measuring the arrival times of light pulses from rapidly spinning stars, known as pulsar timing arrays, revealed the first evidence for a gravitational wave background pervading the universe. These waves turn out to be a great way to measure the distribution of dark matter in the cosmos, too.
“Oscillating axions would leave imprints in these gravitational wave signatures,” says Rogers. So far, the datasets aren’t sensitive enough to detect such imprints. “They didn’t find anything in the first run,” he says. But the results should continue to improve as more pulses are measured, narrowing down a potential signal of axion dark matter.
If detected, it is possible that axions make up all dark matter, solving the mystery in one swoop. But it could be that they comprise only a fraction of it, the rest being WIMPs or other candidates – things like primordial black holes left over from the big bang. “There’s no reason dark matter should be one thing,” says Marsh. “The standard model of the universe is very complicated, with protons, neutrons, quarks, electrons, all of them. Why should dark matter be simple?”
Axions themselves probably wouldn’t come in just one guise, either. It is likely that if one type of axion exists, then there is a range of them, from the ultralight to the relatively heavy, like a family of waves and particles, with each applicable to different problems. “It would be very natural to have axions of different masses,” says . “But we have to be wary of using that to say we’ve got an axion for every problem when we haven’t had any direct experimental evidence for them.”
Not yet, anyway. But an observation by the European Space Agency’s has provided some of the best evidence so far. And this time, axions might help solve a completely different mystery in cosmology by offering a potential explanation for dark energy, the unknown force causing the universe to fly apart.
In 2020, and at the Max Planck Institute for Astrophysics in Germany were looking at measurements of the CMB from the Planck satellite when they found something intriguing. They looked at a property called polarisation – a measure of light’s orientation when it travels as a wave – and . It seemed as if something had interfered with the light between it being released and reaching the satellite.
What’s even more intriguing, says Marsh, is that this strange effect . In the 1980s, researchers had been looking into the axion field and found that, as particles of light, or photons, moved through it, the field would cause them to interact with themselves. The polarisation of light detected by Planck is “very naturally explained by axions between the CMB and us detecting it”, says Marsh.
Dark energy
It just so happens that the kind of axion being proposed would behave like dark energy, but only for a certain period of time. This makes it part of a class of dark energy known as early dark energy, so-called because it was only around in the early universe. There are lots of theoretical kinds of early dark energy, but this one is particularly strange: the axions would have acted like dark energy after the light of the CMB was released, interfering with it on its way to our telescopes. But then, at later times, they would have started to act like dark matter. “They behave a bit like dark energy and a bit like dark matter,” says Marsh.
Axions offer potential explanations for a range of other mysteries, too (see “All the answers”). But the Planck polarisation result is the most exciting, says Marsh. A Japanese space observatory called , planned for launch in 2028, is specifically designed to measure the polarisation of the CMB. This will confirm whether the strange rotation is there or not. If the signal does hold up, these strange axions that behave like dark energy and then dark matter might exist. “We can try and search for them, and if we’re lucky, they may be just below our current upper limits,” says Marsh.
Detecting axions
In the meantime, back on Earth, experiments to find axions are picking up pace. Arguably the best effort to date has been the Axion Dark Matter Experiment (ADMX) at the University of Washington in Seattle, which uses magnets to try to catch axions decaying into photons, which theory says they should do when influenced by a magnetic field. But this is a slow-burn approach because it requires a lot of measurements, says Gray Rybka, who leads the experiment. It is “like an AM radio”, he says – but instead of a radio station, they are tuning for axions.
So far, ADMX, which has run since 1996, has explored only a small fraction, around 5 per cent, of the potential mass range of the axion. But the experiment continues to run and expand its search range, and a detection could come at any moment. “They could discover the axion tomorrow,” says Marsh.
Other, smaller experiments are popping up too, taking advantage of the very trait that made axions so unappealing in the first place: their wide range of possible masses. At the University of Sheffield, for example, a new effort called Quantum Sensing for the Hidden Sector is set to begin a hunt for axions in 2024. The experiment can run colder than other axion searches, at close to absolute zero, says Daw, who leads the project. “That’ll put us essentially in the quantum regime,” he says. “We should be more sensitive than other experiments.”
“There are now ideas to cover the entire mass range we think the axion could have,” says Marsh. “It is very, very exciting.” On the dark matter front, there is some competition between WIMPs and axions, says Laura Baudis at the University of Zurich in Switzerland. Searches for WIMPs continue, but the number of locations where they could be hiding gets smaller and smaller every year. For axions, many of the possibilities have yet to be explored. “There’s a race between axion and WIMP experiments, but they’re also complementary because they are not looking for the same particle,” says Baudis.
If axions do turn up, they could clean up a whole number of messes with our universe. In that regard, they would truly live up to their detergent-inspired name. Wilczek, for one, is convinced that we will find them, eventually. “Theoretically, there’s no other way to solve the problems it addresses,” he says. “This particle more or less has to exist.”
All the answers
Theoretical particles called axions could provide answers to two more mysteries in cosmology, on top of dark matter and dark energy (see main story). The first is the Hubble tension. This is the question of why, when we predict the acceleration of the expansion of the universe based on analysis of light left over from the big bang, known as the cosmic microwave background (CMB), we get a different rate to what we see when looking at the speeds of stars moving away from us locally. "Axions may alter the history of the universe," says Francesca Chadha-Day at Durham University, UK. If certain kinds of axions existed in the early universe, they could change what we predict based on the CMB, removing the tension.
The second mystery is why there is anything at all. When the universe was born, matter and antimatter should have been made in equal amounts, annihilating to leave nothing. That didn't happen, of course. And it turns out that a rotating axion field could explain the imbalance.
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