A new study of Einstein rings suggests that dark matter behaves like a wave, not a particle

Physicists believe that most of the matter in the universe is made up of invisible matter that we only know about through its indirect effects on stars and galaxies.

We are not crazy! Without this “dark matter”, the universe as we see it would have no meaning.

But the nature of dark matter is a long-standing puzzle. However, a new study by Alfred Amruth of the University of Hong Kong and colleagues, nature astronomy, Uses gravitational bending of light to bring us closer to understanding.

Invisible but omnipresent

The reason we think dark matter exists is because we can see its gravitational effects in the behavior of galaxies. In particular, dark matter makes up about 85 percent of the mass of the universe, and most of the distant galaxies we can see appear to be surrounded by halos of mysterious matter.

But it’s called dark matter because it doesn’t emit light, or absorb it, or reflect it, which makes it incredibly difficult to detect.

So what is this stuff? We think it must be some kind of unknown elementary particle, but beyond that we’re not sure. All attempts to detect dark matter particles in laboratory experiments have so far failed, and physicists have debated its nature for decades.

Scientists have proposed two leading hypothetical candidates for dark matter: relatively heavy particles called weakly interacting massive particles (or WIMPs), and extremely lightweight particles called axons.

In theory, WIMPs behave like discrete particles, while axons behave more like waves due to quantum interference.

Distinguishing between these two possibilities is difficult – but now the light curling around distant galaxies has provided a clue.

Gravitational lensing and Einstein rings

When light traveling through the universe passes through a massive object like a galaxy, its path is bent because—according to Albert Einstein’s theory of general relativity—the massive object’s gravity distorts space and time around it.

As a result, sometimes when we look at a distant galaxy we see distorted images of other galaxies behind it. And if things line up perfectly, the light from the background galaxy will emerge in a circle around the nearby galaxy.

This distortion of light is called “gravitational lensing” and the circles it can create are called “Einstein rings”.

By studying how rings or other lensed images are distorted, astronomers can learn about the properties of the dark matter halo surrounding nearby galaxies.

Axions vs WIMPs

And that’s what Amrit and his team did in their new study. They looked at several systems where multiple copies of the same background object appeared around the foreground lensing galaxy, focusing in particular on one called HS 0810+2554.

Using detailed modelling, they worked out how the images would be distorted if the dark matter was composed of WIMPs versus how it would be if the dark matter was composed of axons. The WIMP model did not look like the real thing, but the Axion model accurately reproduced all the features of the system.

The results suggest that axes are a more likely candidate for dark matter, and scientists are buzzing with excitement over their potential to explain lensing anomalies and other astrophysical observations.

Particles and galaxies

The new research builds on previous studies that have also pointed to axons as a more likely form of dark matter.

For example, one study looked at the effects of axial dark matter on the cosmic microwave background, while another investigated the behavior of dark matter in dwarf galaxies.

Although this research will not yet end the scientific debate on the nature of dark matter, it opens new avenues for testing and experimentation. For example, future gravitational lensing observations could be used to probe the wave-like nature of the axes and potentially measure their mass.

A better understanding of dark matter will have implications for what we know about particle physics and the early universe. It can also help understand how galaxies form and change over time.

Rosanna Ruggeri, Research Fellow in Cosmology, University of Queensland

This article is republished from The Conversation under a Creative Commons license. Read the original article.