Decades after it became clear that the visible Universe is built on a framework of dark matter, we still don’t know what dark matter really is. On a large scale, various evidence points to what are called WIMPs: weakly interacting massive particles. But there are a variety of details that are difficult to explain using WIMPs, and decades of particle research have turned up nothing, leaving people open to the idea that something other than a WIMP includes dark matter.
One of many candidates is something called an axion, a force-carrying particle that has been proposed to solve a problem in a field unrelated to physics. They are much lighter than WIMPs but have other properties that are consistent with dark matter, which has kept interest in them low. 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 the simplest level, it is an extremely light, spinless particle that acts as a force vector. They were originally proposed to ensure that quantum chromodynamics, which describes the behavior of the strong force that holds protons and neutrons together, does not break the conservation of charge parity. Enough work has been done to ensure that axions were compatible with other theoretical frameworks, and some research has been done to try to detect them. But axions have mostly languished as one of many potential solutions to a problem we haven’t figured out how to fix.
They have, however, attracted attention as potential solutions to dark matter. But the behavior of dark matter was best explained by a heavy particle, especially a weakly interacting massive particle. Axions were expected to be lighter and could potentially be as light as the nearly massless neutrino. Searches for axions also tended to exclude many of the heavier masses, making the problem even more pronounced.
But the axions can make a comeback, or at least stay stable while the WIMPs cope. There have been a number of detectors built to try to pick up indications of weak WIMP interactions, and they have been left empty. If WIMPs are Standard Model particles, we could have inferred their presence based on the missing mass in particle colliders. No proof of this was provided. This has caused people to re-examine whether WIMPs are the best solution to dark matter.
At the cosmological scale, WIMPs continue to adapt extremely well to the data. But once you get to the levels of individual galaxies, there are certain quirks that don’t work as well unless the dark matter halo surrounding a galaxy has a complicated structure. Similar things seem to be true when trying to map the dark matter of individual galaxies based on its ability to create a gravitational lens that distorts space in order to magnify and distort background objects.
The new work attempts to link these potential quirks to a difference between the properties of WIMPS and axions. As their name suggests, WIMPs are expected to behave like discrete particles, interacting almost entirely by gravity. In contrast, axions should interact with each other through quantum interference, creating wave patterns in their frequency throughout an entire galaxy. So while the frequency of WIMPs should decrease gently with distance from a galaxy’s core, axions should form a standing wave (technically, a soliton) that increases their frequency near the galactic core. Further, complex interference patterns should create areas where there are virtually no axions and other areas where they are present at twice the average density.
Hard to spot
With few exceptions, dark matter makes up the majority of a galaxy’s mass. Given this, these interference patterns should make the gravitational pull of different areas of the galaxy unequal. If the differences between the regions are large enough, this could potentially appear as minor deviations in the expected behavior of the gravitational lens. Thus, objects behind a galaxy should always appear as lensed images; they just might not be shaped the way we expected or exactly where we intended.
Modeling indicates that these deviations are small enough that even the Hubble Space Telescope cannot 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 a single 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 massive elliptical galaxy that lies between us and an active black hole at the heart of another galaxy. The gravitational lens created by the galaxy in the foreground creates four images of the active galaxy, each with a bright galactic core and two large jets of matter extending from it. It is possible to compare the location and distortion of these four images to what one 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, because there’s only one pattern we expect: dark matter levels gradually dropping as you move away from the galactic core. Lens predictions based on this distribution do a poor job of matching real-world data on where lens images appear.
The challenge is to do the same analysis based on the axion interference patterns, which are chaotic: run the model twice with different initial conditions and you will get a different interference pattern. So the chances of getting whoever is actually present in the real-world galaxy making the lens are pretty slim. Instead, the research team ran 75 different models with the initial conditions chosen at random. Luckily, some of these created distortions similar to those seen in the real-world data, usually only affecting one of the four lens images. Thus, the researchers conclude that the distortions in the lensed images are consistent with a dark matter halo structured by the quantum interference of the axions.
So, are they really axions?
Scanning a single galaxy will never be a decisive slam dunk for anything, and there are several reasons to be extremely careful here. On the one hand, the researchers hypothesized 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 influence the distribution of dark matter in ways that are subtle and hard to detect by tracing the distribution of normal matter.
Finally, this type of interference model only works with extraordinarily light axions – of the order of 10-22 electronVolts. By contrast, the electron itself has a mass of about 500,000 electronVolts. 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 phrase: “Determining whether [WIMP- or axion-based dark matter] better reproduces astrophysical observations will tip the balance toward one of two corresponding classes of theories for 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 demanding cases such as HS 0810+2554, along with its success in reproducing other astrophysical observations, tips the balance towards new physics invoking axions.”
We will see, no doubt shortly, whether this sentiment is shared by the physicists behind the authors and peer reviewers of this paper.
Nature Astronomy, 2023. DOI: 10.1038/s41550-023-01943-9 (About DOIs).
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