Physicists believe that most of the matter in the universe is made up of dark matter, a mysterious substance we only know about because of its indirect effects on observable stars and galaxies. But a new study brings us one step closer to understanding it, writes The Conversation.

Without this dark matter, the universe as we know it would not make sense, but the nature of dark matter has puzzled physicists for decades. But a new study published today by researchers at the University of Hong Kong in the journal Nature Astronomy has used the gravitational distortion of light to bring us closer to understanding it.

The reason why scientists believe that dark matter is real is because we can see the effect of its gravity on the behavior of galaxies. In particular, dark matter appears to make up about 85% of the mass of the universe, and the most distant galaxies we can see appear to be surrounded by a halo of mysterious matter.

But it is called dark matter because it does not emit, absorb, or reflect light, making it incredibly difficult to detect.

So what is this substance?

Two hypotheses of dark matter

Scientists believe that it must be some kind of fundamental particle that has not yet been discovered, but otherwise opinions are divided. All attempts to detect dark matter in laboratory experiments have so far failed, and physicists have debated its nature for decades.

There are currently two favored hypotheses for the origin of dark matter: relatively heavy particles called WIMPs from “weakly interacting massive particles” and extremely light particles called axions.

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

Astrophysicists have trouble distinguishing between the two, but researchers in a new study got a clue by analyzing the bent light around very old (and therefore distant) galaxies.

“Gravitational lensing” and “Einstein rings”

When light traveling through the universe passes a massive object such as a galaxy, its path is distorted because – according to Albert Einstein’s general theory of relativity – the massive object’s gravity distorts the space-time around it.

As a result, when we look through powerful telescopes at a distant galaxy, we see a distorted image of other galaxies behind it. And if everything is perfect, the light of the background galaxy will form a circle around the nearest one.

This distortion is known as “gravitational lensing” (or gravitational mirage), and the circles it can create are called “Einstein rings.”

By studying how these types of “rings” or other objects are distorted, astronomers can learn about the properties of the dark matter halo around the nearest galaxy.

Artist’s impression of a gravitational lens around a black hole (PHOTO: Handout / AFP / Profimedia)

This is exactly what scientists from the University of Hong Kong did in their new study. They looked at several systems where multiple foreground images of the same background object were visible, focusing in particular on one called HS 0810+2554.

Using sophisticated simulations, they were able to figure out what the images would look like if they were distorted by dark matter made up of WIMP particles, and what they would look like if they were made up of axions.

The WIMP model bore little resemblance to real images, but the one with axions accurately reproduced all the features of the system under study.

Axions that explain dark matter

These results suggest that axons are a more likely candidate for dark matter, and their ability to explain gravitational anomalies and other observations made by astrophysicists has excited the scientific community, especially since the new study is consistent with previous research that pointed to axons as more a likely candidate for dark matter.

For example, a study published in August 2017 in Monthly Notices of the Royal Astronomical Society looked at the “behaviour” of dark matter in dwarf galaxies, while another study published in February 2018 in the same journal looked at the effects of dark matter on axions. cosmic background radiation.

Although the new study by scientists at the University of Hong Kong will not end the debate about the nature of dark matter, it opens up new avenues for testing and experimentation.

Two possibilities are for future gravitational lensing observations to be used to study the wave nature of axions, or even to attempt to measure their mass.

A better understanding of dark matter will have major implications for everything that particle physics means and for what we understand about the universe.