A new look at “Einstein’s rings” around distant galaxies has brought us closer to resolving the dark matter debate

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Multiple images of the background image created by the gravitational lensing of the system HS 0810+2554 can be seen. Credit: Hubble Space Telescope/NASA/ESA

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 that we can see.

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 an ancient mystery. but, New study Written by Alfred Amroth of the University of Hong Kong and colleagues, published in natural astronomyuses the gravitational bending of light to bring us one step closer to understanding.

Invisible but omnipresent

The reason we think dark matter exists is because we can see its gravitational effects on the behavior of galaxies. Specifically, dark matter appears to make up about 85% of the mass of the universe, and most of the 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, which makes it very difficult to detect.

So what are these things? We think it must be some kind of unknown fundamental particle, but we’re not sure yet. All attempts to detect dark matter particles in laboratory experiments have so far failed, and physicists have been debating their nature for decades.

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Scientists have proposed two main hypothetical candidates for dark matter: relatively heavy characters called weakly interacting massive particles (or WIMPs), and extremely lightweight particles called axions. In theory, WIMPs behave like discrete particles, while axions behave a lot like waves due to quantum interference.

It was difficult to distinguish between these two possibilities – but a slight detour around distant galaxies provided a clue.

Gravitational lensing and Einstein rings

When light passes through the universe by a massive object such as a galaxy, its path is curved 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 can see distorted images of other galaxies behind it. And if things line up perfectly, the light from the background galaxy will circle around the nearest galaxy.

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

By studying how rings or other lenticular images distort, astronomers can learn about the properties of the halo of dark matter surrounding the nearest galaxy.

Axions vs. WIMPs

And that’s exactly what Amroth and his team did in their new study. They looked at several systems where multiple copies of the same object were visible in the background around the foreground lensing galaxy, with a particular focus on a system called HS 0810+2554.

Using detailed modeling, they worked out how the images would distort if the dark matter were made of WIMPs versus how it would be if the dark matter were made of axions. The WIMP model didn’t look much like the real thing, but the axion model accurately reproduced all of the system’s features.

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The finding indicates that axions are a more likely candidate for dark matter, and their ability to explain lensing anomalies and other astrophysical observations is irritating scientists.

particles and galaxies

The new research builds on previous studies that also indicated axions are the most likely form of dark matter. For example, one study looked at the effects of axion dark matter on the cosmic microwave background, while last Examining the behavior of dark matter in dwarf galaxies.

Although this research will not end the scientific debate about the nature of dark matter, it does open up new avenues for testing and experimentation. For example, future gravitational lensing observations could be used to probe the wave-like nature of axons and possibly 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 us better understand how galaxies form and change over time.

more information:
Alfred Amroth et al., Einstein’s loops modulated by wave-like dark matter from aberrations in gravitational lensing images, natural astronomy (2023). DOI: 10.1038/s41550-023-01943-9

Journal information:
natural astronomy

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