Einstein’s rings suggest that dark matter behaves like a wave

We have been looking the wrong way, dark matter might not be a particle and rather behave like a wave.

Dark matter is the biggest enigma in the cosmos, it seems to be covering absolutely everything, but it is impossible for us to see it. We only know about it because of its extreme density that causes space-time to bend in ways that baryonic matter (all that we can see) cannot explain. Physicists have been putting a lot of effort into discovering dark matter but nothing has worked so far, although there are still more ways to go and new research suggests that dark matter may be behaving more like a wave than a particle.

Invisible but ubiquitous

As far as we believe, dark matter makes up 85% (some say 80%) of the entire cosmos. Our Universe is made up mainly of material that we do not know and have never seen, but that we know is there. Otherwise the Universe itself would not make sense, the puzzle of what we know about the behavior of galaxies, stars and planets simply would not have an explanation.

The only problem is that this type of slippery matter lives up to its name and is incapable of emitting any kind of energy that reflects light. That’s why our telescopes can’t detect it no matter how hard they search. Astrophysicists have been chasing mysterious dark matter for decades and believe this madness may be made up of some hitherto unknown fundamental particle, though they aren’t entirely sure. Various particles have been proposed as hypothetical candidates, some of them have been ruled out such as the sterile neutrino; but two particles still remain on the map.

On one side are the weakly interacting massive particles (WIMP) and on the other corner we have the extremely light particles called axions. The former would behave more like discrete particles, axions on the other hand would behave much more like waves due to quantum interference.

Einstein’s rings and their relationship with dark matter

When light traveling through the cosmos passes a massive object like a cluster of galaxies, its path is deflected because the object’s gravity distorts the spacetime around it. This phenomenon called gravitational lensing was described by Albert Einstein in his Theory of General Relativity over a century ago, where light forms the so-called Einstein rings.

Researchers at the University of Hong Kong took a close look at gravitational bending, where multiple copies of the same distant object were seen due to gravitational lensing. They then used a detailed model to calculate how such images would be distorted if the dark matter were made up of WIMPs or axions.

The simulation of the WIMP model was not very close to Einstein’s ring, while the model made by axions perfectly reproduced the characteristics of the gravitational bending system. This is why the researchers believe that dark matter probably behaves more like a wave and this is the reason why we have not been able to find it, beyond the fact that it is invisible.

References: Amruth, A., Broadhurst, T., Lim, J. et al. Einstein rings modulated by wavelike dark matter from anomalies in gravitationally lensed images. Nature Astronomy (2023). DOI