From observations of the Milky Way galaxy, we’ve learned that in any given cubic meter of space, even the particular cubic meter that snugly fits your seated form as you read this article, there’s a small amount of matter—only about 50 proton masses worth—passing through in any given moment. But unlike the particles that make up your seated form, this matter doesn’t interact. It doesn’t reflect light, it isn’t repelled by solid objects, it passes right through walls. This mysterious substance is known as dark matter.

Since there’s so little of it in each cubic meter, you would never notice its presence. But over the vast distances of space, there’s a lot of cubic meters, and all that dark matter adds up. It’s only when you zoom out and look at the big picture that dark matter’s gravitational influence becomes apparent. It’s the main source of gravity holding every galaxy together; it binds galaxies to one another in clusters; and it warps space around galaxy clusters, creating a lensing effect.

But despite its importance to the large-scale structure of the Universe, we still don’t know what dark matter really is. Currently, the best candidate is WIMPs, or Weakly Interacting Massive Particles (Which makes sense, now that we know it’s not MAssive Compact Halo Objects, or MACHOs). But WIMPs are not the only option—there are quite a few other possibilities being investigated. Some of them are other kinds of massive particles, which would constitute cold dark matter, while others aren’t particles at all.

Axions, theoretical particles that were originally predicted to solve a tricky problem involving the strong nuclear force, happen to have just the right properties to be a good candidate for dark matter. Leslie Rosenberg, a physicist at the University of Washington, Seattle, recently wrote an overview of the experiments being done to investigate the possibility of axions being dark matter for the journal PNAS.

Hot or Cold?

Among the various models of dark matter, there are two overarching categories: Hot (HDM) and Cold Dark Matter (CDM). The hot variety gets its name because its particles would be whipping around at incredibly high speeds, up to significant fractions of the speed of light. But hot dark matter seems to be a dead end as a possibility. If particles were traveling that fast, most of them would be able to escape the gravitational pull of their host galaxy. Instead, dark matter forms into nice, spherical halos around every galaxy—which means that it's probably cold.

The physical difference between HDM and CDM is mass. If dark matter is composed of low-mass particles, then it would be easy for the particles to accelerate, and since the particles interact so little with other particles, it would be very hard to slow them down; hence the relativistic speeds of HDM. CDM, then, would have to be a higher-mass particle, because those aren’t as easy to accelerate. WIMPs would fall into this category.

Axions, meanwhile, occupy a unique sort of middle ground between HDM and CDM. They are low-mass particles, low enough that they might have been HDM, except that they would have been slowed down gravitationally in the very early Universe. In effect, they now behave like CDM, moving slowly and thus potentially forming the dark matter halos we observe, even though they have the mass of HDM. Crucially, axions interact weakly enough with light and other matter that they fulfill the ‘dark’ part of dark matter.

One advantage to axions as dark matter is that there’s only a very specific mass range of axions that would be consistent with the dark matter we observe. If the axions were much lighter or much heavier, they would produce observable differences—sufficiently observable that we would have already seen them. For example, the supernova explosion sn1987a would have lost energy as axions transported it out of the exploding star, which would have resulted in a noticeably different neutrino flash than the one recorded on Earth.

That narrow range of possibilities makes the axion hypothesis very easy to conclusively test. Since it’s such a narrow range, a test that turns up negative could rule out axions as a possibility altogether. (They might still exist, but they would be ruled out as a dark matter candidate). And in science, testability makes a hypothesis very attractive (at least until the test rules out your favorite model).

So how do we find it?

Another advantage of axions is that they can spontaneously decay into things that might be observed. An axion can turn into two photons, and that light could hypothetically be detected. The reverse process, light turning into an axion, is also possible—and it may even play a role in the propagation of light. The light would briefly become an axion, which would then decay back into two photons, with the briefly-existing axion being considered a virtual axion.

Another effect axions could have would be on the Sun—its seismic activity and energy output could be affected by the interactions of axions. And those Solar axions could scatter off a germanium crystal, producing X-rays that could be observed. Additionally, the dark matter axions in the halos around astronomical objects, like other galaxies, could spontaneously decay and produce photons that we might see in telescopes.

Unfortunately, none of these tests are sensitive enough to detect the expected mass range of axions that would be dark matter. To find axions in the right range, there are a few methods that might work—and some of them are being tried in experiments right now.

Astronomical axions

Astronomical objects can provide an opportunity to observe axions. Supernovae should produce them (as noted above), as should other astronomical objects such as the Sun.

In the core of the Sun, light scatters off of the particles it encounters there, bouncing around from particle to particle until its random path allows it to escape the Sun (some 170,000 years after the light was produced). As the light scatters in this process, it can be converted into an axion. That axion might then turn back into two photons while still inside the Sun. Since the axion was produced in the Sun’s hot core, the photons ultimately observed here on Earth would be in the form of X-rays. Alternatively, we could potentially detect the axions themselves, should they escape the Sun.

But it would be difficult to distinguish whether the axions detected this way are dark matter or simply part of a normal physics process. More energetic events, like supernovae, would also fall short of producing unambiguously detectable dark matter axions.

The best experiment using this method right now is the CERN Axion Solar Telescope. Using a dipole magnet from the Large Hadron Collider on a steerable mount, this device could achieve good sensitivity to axions escaping the Sun—but it's just barely more sensitive to dark matter axions than observations of the supernova sn1987a were. So, while this experiment could not rule out axions by itself, it might further constrain the properties of axion dark matter.

A more sensitive version is being conceived, however, which might provide better insight.

Shining light through walls!

Another technique with a chance of detecting dark matter axions is the “Shining light through walls” technique, which is just what it sounds like. (A name we didn’t make up, in case you were wondering). As we’ve seen, light can convert into axions and axions can be converted into light. So if researchers wanted to create axions in the lab, they might start with some light.

By sending some polarized light through a dipole magnet, some of the light can be converted into axions. The axions would then be able to pass right through a wall, as though it weren’t there, and appear on the other side. If they encounter a second dipole magnet, it will convert the axions back into photons, which are then detected. To be fair, this isn’t a measurement of pre-existing axions, so it doesn’t demonstrate that the dark matter we’re observing is composed of axions—only that axions in the right mass range exist. But that by itself would provide a strong argument that dark matter is axions.

The problem with this technique is that the process happens very infrequently—so infrequently that it would be very hard to tell such a light burst from the surrounding noise. As a result, the technique wouldn’t be sensitive enough to detect axions in the dark matter mass range.

But there are some experiments being constructed that have addressed that problem by adding devices called Fabry-Perot optical resonators to both sides of the wall. This has the effect of increasing the number of photons that decay into axions and vice versa, which should make it a vastly stronger signal—strong enough to stand out from the noise. But despite the improvements, these experiments probably still won’t be sensitive enough to detect axion dark matter, though they might be able to find other forms of axions.

Catching axions

Another approach is known as the Radio Frequency (RF) technique. This relies on an axion’s ability to decay into light, and could allow researchers to catch one. Axions that are part of the Milky Way’s dark matter halo should be passing through the Earth at all times, putting them within reach. The only thing that’s needed is the right catcher’s mitt. Like other dark matter candidates, axions pass right through solid matter, so it’s tricky to devise a device to catch one. But unlike other dark matter candidates, axions might interact with a magnetic field. If so, the axion could be stimulated to decay into microwave photons. Those photons could then be detected.

The catcher’s mitt, in this case, is a device called an RF cavity, a metal cylinder which serves as a resonator, keeping the electromagnetic waves it catches inside.

This approach has been taken by the Axion Dark Matter eXperiment (ADMX). That RF cavity device is four meters tall, but the actual cavity itself, the part where the axion’s photons will be caught, is only about half a meter tall, and surrounded by a powerful, wrap-around magnet. The main difficulty with this experiment, as with so many experiments in astronomy, is reducing noise. Axions that are part of the Milky Way’s halo should produce some extremely weak photons, which are very difficult to distinguish from the background noise.

To deal with this issue, the ADMX device has recently been refitted, replacing its transistor amplifiers with Superconducting QUantum Interference Devices (SQUIDs). The SQUIDs are more effective at amplifying the signal of the microwave photons the device catches, helping them to stand out from the noise. The ADMX, enhanced with the SQUIDs, is sensitive enough that it should be able to detect axions from the Milky Way’s dark matter halo with a high degree of certainty. Over the next few years, this experiment could conclusively rule out axions as the identity of dark matter—or it could confirm this hypothesis.

Conclusions

The possibilities raised by these experiments—especially by ADMX—are exciting, as they represent clear progress toward solving the puzzle that is dark matter. And that’s no trivial puzzle, as an understanding of dark matter is important to our understandings of the Universe as a whole.

But in science, things are often more complicated than they seem at first, as the author cautions in the paper. “it may be that the relation between axion mass and couplings is loosened. In such a case, there could well be surprises,” he writes. Nonetheless, he doesn’t downplay the potential significance of ADMX: “sensitivity to dark matter QCD axions has at last been achieved with the RF cavity technique, and we may know soon whether the dark matter is made of axions.”

If dark matter does turn out to be axions, it will be good news in one sense at least: physicists will be able to directly detect and experiment with dark matter, a boon for cosmology. Considering that it’s not yet certain that dark matter interacts at all—and it would be essentially impossible to directly observe if it doesn’t—that would be good news indeed.

PNAS, 2014. DOI: 10.1073/pnas.1308788112 (About DOIs)