Weird dark matter waves seem to warp the light from distant galaxies

Evidence is growing for an ultralight dark matter particle called the axion. A study of light warped by galaxies has shown that it is better explained by axion dark matter than weakly interacting massive particles (WIMPs), which have long been the leading candidate for dark matter.

Researchers are fairly sure that dark matter exists because of its gravitational effects, but so far all efforts to detect any dark matter particle directly have failed. Amruth Alfred at the University of Hong Kong and his colleagues took another indirect look at dark matter by examining an effect called gravitational lensing. This is where light from a distant object is warped and magnified by the gravitational field of a relatively nearby galaxy, creating several images of the background object around the nearby galaxy in what is called an Einstein ring.

Galaxies are expected to be surrounded by haloes of dark matter, so the properties of that dark matter should affect how the light is stretched. Axions are many orders of magnitude less massive than WIMPs, so they are expected to behave differently – while WIMPs behave like standard particles, axions are so light that quantum effects should make them behave more like waves.

So if the foreground galaxy in gravitational lensing is surrounded by axions, we would expect that to affect how the images of the background galaxies appear once they have been lensed. “If you have a pool with waves in it, and you put a stone in it, you are able to see the oscillations in the wave when you look at the stone,” says Razieh Emami at the Harvard-Smithsonian Center for Astrophysics in Massachusetts, who was part of the research team. “In these observations, those wave structures would be directly translated to the position of the lensed images and their brightness.”

We know that there are anomalies between the patterns we have seen in gravitational lensing and WIMP models, but the researchers found that when the WIMP models were switched out for axion ones, those anomalies went away. They also tested the models on a real lensing system and found that the axion model fit much better.

“Since dark matter only interacts via gravity (and maybe the weak force in some models), this is one of the purer tests that can be done to investigate the nature of dark matter,” says Alfred. “Wave-like dark matter… holds up to the scrutiny that we put it under.”

This is good news for axions, which have been overshadowed by WIMPs as dark matter candidates for decades. “The observations from gravitational lensing do tilt the scale from heavier particles towards lighter ones,” says Emami. “So far, there are no other explanations for this phenomenon.”

“I don’t think this counts as proof that ultralight axions exist, but it’s a compelling result,” says Chanda Prescod-Weinstein at the University of New Hampshire, who wasn’t involved in the work. “It provides further evidence that axions as a class of dark matter candidates are compelling.”

Immense effort has gone into detecting WIMPs with no luck, so this work is part of a renaissance for axions and other dark matter candidates that haven’t been as thoroughly explored. “Axions represent one of the simplest extensions to the standard model of particle physics, and a large portion of their plausible search space has not been explored yet,” says Jae Sub Hong, also at the Harvard-Smithsonian Center for Astrophysics and not involved in the work. “In other words, there are low-hanging fruits waiting to be picked, relative to what’s been done for other candidates like WIMPs.”

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