Distorted Galaxy Hints at the Nature of Dark Matter

An international team of astrophysicists has provided the most direct evidence yet that dark matter does not consist of ultramassive particles. The work, based on supercomputer simulations, suggests that instead dark matter is made up of axions — hypothetical particles so light that they travel through space like waves. If correct, it would not only reveal what 85% of matter in the universe is made of but could also lead to new physics beyond the Standard Model. The study is published in Nature Astronomy.

Dark matter is tricky to study because it doesn't emit, absorb, or reflect light. So instead, a team led by graduate student Alfred Amruth (University of Hong Kong) looked for the material’s influence on gravitational lensing.

In this effect, a galaxy and its massive dark matter halo curves the fabric of spacetime around it. Light from a more distant source follows this curvature and bends around the galaxy as if passing through a lens.

When the foreground lens and the distant light source are closely aligned, astronomers see multiple images of the same background object. The positions and brightnesses of these images depend on the distribution of dark matter in the lens, providing a powerful probe of the mysterious substance.

For the past two decades, astrophysicists have struggled to correctly reproduce the positions and brightness of these multiple images, because they’ve assumed that dark matter is made of weakly interacting massive particles (WIMPs). If WIMPS are dark matter, then galaxies’ density should decrease smoothly as you move out from the center. Except that's not what astronomers actually infer from the lensed images.  

Amruth's team turned to an alternative dark matter candidate instead: axions. These ultralight particles were first proposed in the 1970s to solve a problem in particle physics regarding the strong force. Quantum theory says axions travel through space as waves rather than particles, leading to random density fluctuations as the waves interfere with each other. These random fluctuations would make the distribution of dark matter around a galaxy bumpy. (It’s worth noting that neither WIMPs nor axions have been detected directly.)

After assuming that dark matter is made of axions, Amruth and colleagues were able to recreate the observed positions and brightnesses of the quadruply lensed system HS 0810+2554: a foreground elliptical galaxy that splits the light from a background galaxy into four images.

‘We have reached a point where the existing paradigm of dark matter needs to be reconsidered,” says Amruth. “Waving goodbye to ultramassive particles, which have long been heralded as the favored candidate for dark matter, may not come easily, but the evidence accumulates in favor of dark matter having wave-like properties as possessed by ultralight particles.”

However, Edward Hardy (University of Liverpool, UK), who was not involved in the research, urges caution. “This paper is only a first step and extensive further analysis is needed before the nature of dark matter is settled,” he says. According to team member George Smoot, the James Webb Space Telescope should discover many more gravitationally-lensed systems, offering a more stringent test of the idea.

If that future work cements the conclusions of Amruth's research, Hardy says it would be a spectacular discovery. “There would be major implications,” he says, “not just for astrophysics but also for high-energy fundamental particle physics more generally.”

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