Decades after it became clear that the visible Universe is built on a dark matter framework, we still don’t know what dark matter really is. On a large scale, a variety of evidence points to what are called WIMPs: massive, weakly interacting particles. But there are a variety of details that are difficult to explain using WIMPs, and decades of particle research have yielded nothing, leaving people open to the idea that anything other than a WIMP comprises dark matter.
One of many candidates is something called an axion, a force-carrying particle that has been proposed to solve a problem in an unrelated area of physics. They are much lighter than WIMPs but have other properties that are consistent with dark matter, which has maintained low-level interest in them. Now, a new paper argues that there are features in a gravitational lens (largely the product of dark matter) that are best explained by axion-like properties.
Particle or wave?
So what is an axion? At its simplest level, it is an extremely light, spinless particle that serves as a force carrier. They were originally proposed to ensure that quantum chromodynamics, which describes the behavior of the strong force holding protons and neutrons together, does not break the conservation of charge parity. Enough work has been done to make sure that axions are compatible with other theoretical frameworks, and some research has been done to try to detect them. But axions have mostly languished as one of several potential solutions to a problem we haven’t figured out how to solve.
However, they have attracted some attention as potential solutions to dark matter. But dark matter’s behavior was best explained by a heavy particle, especially a massive particle that interacts weakly. The axions were expected to be on the lighter side and could potentially be as light as the nearly massless neutrino. The searches done for the axions tended to exclude many of the heavier masses as well, making the problem even more pronounced.
But axions could make a comeback, or at least hold their own as WIMPs riot. A number of detectors have been built to try to capture indications of WIMPs’ weak interactions and have come up empty. If WIMPs are Standard Model particles, we could have inferred their presence based on the missing mass in the particle colliders. No proof of this was forthcoming. This has caused people to re-examine whether WIMPs are the best solution to dark matter.
On a cosmological scale, WIMPs continue to fit the data very well. But once you get down to individual galaxy levels, there are some quirks that don’t work as well unless the dark matter halo surrounding a galaxy has a complicated structure. Similar things appear to be true when trying to map the dark matter of individual galaxies based on its ability to create a gravitational lens that warps space in a way that magnifies and distorts background objects.
The new work attempts to relate these potential quirks to a difference between WIMPS properties and axions. As the name suggests, WIMPs are expected to behave like discrete particles, interacting almost entirely through gravity. Instead, the axions should interact with each other through quantum interference, creating wave patterns in their frequency throughout the galaxy. So while the frequency of WIMPs should gradually decrease with distance from a galaxy’s nucleus, the axions should form a standing wave (technically, a soliton) that increases their frequency near the galactic nucleus. Further out, complex interference patterns should create areas where there are essentially no axions and other areas where they are present at twice the average density.
Hard to locate
With a few possible exceptions, dark matter makes up most of a galaxy’s mass. That said, these interference patterns should make the gravitational pull from different areas of the galaxy uneven. If the differences between the regions are large enough, this could potentially manifest itself as minor deviations in the expected behavior of gravitational lensing. Thus, objects behind a galaxy should still appear as slow images; they may simply not be shaped the way we would expect or exactly the position we would have expected.
Modeling indicates that these deflections are small enough that not even the Hubble Space Telescope could detect them. But it might be possible to detect them at radio wavelengths by combining data from widely separated radio telescopes into what is essentially one giant telescope. (This approach allowed the Event Horizon Telescope to create an image of a black hole.)
And, in at least one case, we have that data. HS 0810+2554 is a huge elliptical galaxy that lies between us and an active black hole in the core of another galaxy. Gravitational lensing created by the foreground galaxy creates four images of the active galaxy, each with a bright galactic nucleus and two large jets of material extending from it. It is possible to compare the position and distortion of these four images with what we would expect based on the presence of a typical dark matter halo in the foreground galaxy.
It’s a relatively simple thing to do with WIMPs, since there’s only one pattern we’d expect: the gradual lowering of dark matter levels as one moves away from the galactic core. Lensing predictions based on that distribution do a poor job of matching real-world data of where lensed images occur.
The challenge is to perform the same analysis based on axion interference models, which are chaotic: run the model twice with different initial conditions and you will get a different interference model. So, the odds of getting the one that’s actually present in the real-world galaxy that’s making the goal are pretty slim. Instead, the research team ran 75 different models with the initial conditions chosen at random. By chance, some of these created distortions similar to those seen in real-world data, usually affecting only one of the four images obtained with the lens. Hence, the researchers conclude that the distortions in the slow images are consistent with a dark matter halo structured by axion quantum interference.
So, are they really axions?
Analyzing a single galaxy will never be a slam-dunk for anything, and there are several reasons to be extra cautious here. First, the researchers made some assumptions about the distribution of normal visible matter in a galaxy, which also exerts a gravitational effect. And elliptical galaxies are thought to be the result of merging smaller galaxies, which could affect the distribution of dark matter in subtle ways that are difficult to detect by tracking the distribution of normal matter.
Finally, this kind of interference pattern only works with extraordinarily light axions on the order of 10-22 electronVolt. In contrast, the electron itself has a mass of about 500,000 electron volts. This would potentially make axions much lighter than even neutrinos.
And the authors of the new paper themselves are mostly cautious about the evidence here, concluding their paper with the sentence: “Determining whether [WIMP- or axion-based dark matter] better reproduces astrophysical observations will tip the balance toward one of the two corresponding classes of theories for the new physics.” But their caution slips into the last sentence of the abstract, where they write: “The ability to [axion-based dark matter] to resolve lens anomalies even in challenging cases like HS 0810+2554, together with its success in reproducing other astrophysical observations, tips the balance towards a new physics that invokes axions.”
We will see, undoubtedly shortly, if this sentiment is shared by the physicists behind the authors and reviewers of this article.
Nature Astronomy, 2023. DOI: 10.1038/s41550-023-01943-9 (DOI information).