Decades after it is clear that the visible universe is built on a framework of dark matter, we still don’t know what dark matter actually is. On large scales, a variety of evidence points to so-called WIMPs: weakly interacting massive particles. But there are a variety of details that are hard to explain with WIMPs, and decades of searching for the particles turned up nothing, leaving people open to the idea that something other than a WIMP is made of dark matter.
One of the 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 consistent with dark matter, which have kept a low level of interest in them. Now, a new paper argues that there are features in gravitational lensing (largely a product of dark matter) that may be better explained by axion-like properties.
Particle or wave?
So, what is an axion? At its simplest level, it is a very light particle with no spin and acts as a force carrier. They were originally proposed to ensure that quantum chromodynamics, which describes the behavior of the strong force that binds protons and neutrons together, does not break the conservation of charge parity. Enough work has been done to ensure that axes are compatible with other theoretical frameworks, and some research has been done to try and figure them out. But axions have mostly weakened as one of a number of potential solutions to a problem we haven’t yet figured out how to solve.
However, they have attracted some interest as potential dark matter solutions. But dark matter’s behavior is best explained by a heavy particle – specifically a weakly interacting massive particle. The axions were expected to be on the lighter side and could be as light as near-massless neutrinos. Searches on axions tend to exclude many heavy masses as well, which makes the problem even more obvious.
But the axions may reappear, or at least remain stationary while the WIMPs faceplant. There have been a number of detectors built to try and identify indicators of weak interactions for WIMPs, and they came up empty. If WIMPs are Standard Model particles, we can infer their existence based on the mass lost in particle colliders. No evidence of this has been shown. This has led people to reconsider whether WIMPs are the best solution to dark matter.
On cosmic scales, WIMPs continue to fit the data very well. But once you get down to the levels of individual galaxies, there are some anomalies that don’t work out well unless the halo of dark matter surrounding a galaxy has a complex structure. Similar things sound true when you try to map the dark matter of individual galaxies based on its ability to create a gravitational lens that warps space so that it magnifies and distorts background objects.
The new work attempts to relate these potential anomalies to the difference between the properties of WIMPS and axions. As its name suggests, WIMPs must behave like discrete particles, interacting almost entirely through gravity. By contrast, axions must interact with each other through quantum interference, which creates wave-like patterns in their frequency throughout the galaxy. So, while the frequency of WIMPs should decrease gently with distance from the galactic core, the axions should form a standing wave (technically, a soliton) that boosts their frequency near the galactic core. Beyond this, complex interference patterns should create regions where axes are essentially absent and other regions where they are present at twice the average intensity.
Difficult to locate
With some possible exceptions, dark matter makes up the majority of a galaxy’s mass. Given this, these interference patterns must cause the gravitational pull from different regions of the galaxy to be uneven. If the differences between regions are large enough, this will likely manifest as slight deviations in the expected behavior of gravitational lensing. Therefore, objects behind the galaxy must still appear as lenticular images; It may not be formed the way we expect it to or exactly in the location we expect it to be.
Modeling indicates that these aberrations are small enough that not even the Hubble Space Telescope has been able to capture them. But it may be possible to detect them at radio wavelengths by merging data from widely separated radio telescopes into what is essentially one giant telescope. (This approach enabled 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. Gravitational lensing created by the foreground galaxy creates four images of the active galaxy, each with a bright galactic core and two large jets of material extending from it. It is possible to compare the location and distortion of these four images to 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: a gradual fall in dark matter levels as you move away from the galactic core. Lens predictions based on this distribution do a poor job of matching real-world data of where images appear to lens lenses.
The challenge is to perform the same analysis based on the interference patterns of chaotic axions: run the model twice with different initial conditions, and you get a different interference pattern. So the odds of actually getting those in the real world galaxy doing the lenses are pretty slim. Instead, the research team ran 75 different models with initial conditions chosen at random. By accident, I created some of these distortions similar to those seen in real-world data, usually affecting only one of the four images with a lens. Therefore, the researchers concluded that the distortions in the lenticular images are consistent with a dark matter halo formed by quantum interference of axions.
So, are they really axions?
Analyzing a single galaxy isn’t going to be a critical jab at anything, and there are plenty of reasons to be extra careful here. For example, researchers have made some assumptions about the distribution of ordinary and visible matter in the galaxy, which also has a gravitational influence. It is believed that elliptical galaxies are the result of mergers of smaller galaxies, which may affect the distribution of dark matter in subtle ways that are difficult to detect by tracing the distribution of normal matter.
Finally, this kind of overlapping pattern only works for unusually light axes – on the order of 10-22 electron volt. By contrast, the mass of the electron itself is about 500,000 electron volts. This would make axions much lighter than even neutrinos.
The authors of the new paper themselves are mostly cautious about the evidence here, concluding their paper with the sentence: “Determine whether [WIMP- or axion-based dark matter] Better reproducing the astrophysical observations would tilt the balance towards one of the two similar classes of theories of the new physics. But their caution slips in the last sentence of the summary, where they write, “Ability.” [axion-based dark matter] The resolution of lens anomalies even in challenging cases such as HS 0810+2554, along with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axes. “
We will see, no doubt soon, whether physicists share these sentiments beyond the authors and peer reviewers of this paper.
Natural Astronomy, 2023. DOI: 10.1038 / s41550-023-01943-9 (about DOIs).
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