New Look at Einstein Rings Sheds Light on Dark Matter Debate

It is believed that more than 80 percent of the universe consists of dark matter. But what exactly is it? It is not easy to figure that out since the matter is completely invisible. A new look at the Einstein rings sheds light on the matter.

We only know that dark matter exists because we can observe its indirect effects on stars and galaxies. The motions of those galaxies can only be consistent with gravity and relativity if there is such a thing as dark matter. But what exactly it is is completely unclear. Although there are some conditions that black matter must meet: it has mass, it is present in large quantities in all galaxies, it cannot be detected and it is diffuse, i.e. it cannot clump together to form a celestial body. Furthermore, it is suspected that it concerns an as yet unknown elementary particle.

A halo of dark matter
To learn more about dark matter researchers watched of the University of Hong Kong to the bending of light under the influence of gravity.
Most of the distant galaxies we can observe appear to be surrounded by a halo of dark matter. But since the mysterious substance doesn’t give off, absorb or reflect light, it’s incredibly difficult to pinpoint.

To move forward, scientists have come up with two hypotheses, which are now leading: dark matter would consist of relatively heavy particles called ‘weakly interacting massive particles’, or WIMPs. The other option is that these are extremely light particles called axions. In theory, WIMPs would behave like discrete particles, while axions behave much more like waves.

Distorted view of reality
It has been difficult to determine which of these two options makes the most sense, but the bending of light around distant galaxies now offers interesting clues. As light travels through the universe and passes a large object, such as a galaxy, it bends because – according to Einstein’s theory of relativity – the object’s gravity distorts space and time around itself.

This means, for example, that when we look at a distant galaxy, we get a distorted image of other galaxies beyond it. And if that happens in a perfect line, the light from the galaxy behind can be seen as a circle around the closer galaxy.

Einstein already predicted it
This distortion of light is also called a gravitational lens or gravitational lensing and the circles of light that can form in this way are called Einstein rings, so named because Einstein predicted their existence in 1936. By studying the deformation of these rings, astronomers can learn more about the properties of the dark matter halo around the closer galaxy.

And that is exactly what the Hong Kong researchers did. They looked at several systems where multiple copies of the same background object were visible around a foreground galaxy, with a special focus on HS 0810+2554.

Still the axions
Using detailed models, they looked at what the images would look like if the dark matter was made up of WIMPs versus what it would look like if the matter was made of axions. The WIMP model did not closely resemble the real images, but the axion model accurately reproduced all the features of the system. The conclusion is therefore that axions are the most logical candidate for dark matter.

Confirmation of previous research
This new study builds on previous research that also pointed to axions as the most likely form of dark matter. Although this study will not end the debate about the nature of dark matter, it does open up new possibilities for testing and experimentation. This allows future observations of gravitational lensing are used to further investigate the wave-like nature of axions and possibly determine their mass.

A better understanding of dark matter is something scientists have been searching for for a long time. It has implications for what we know about particle physics and the early universe. It could also help us better understand how galaxies form and change over time.