[Physicists believe most of the matter in the universe is made up
of an invisible substance that we only know about by its indirect
effects on the stars and galaxies we can see. But the nature of dark
matter is a longstanding puzzle.]
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NEW LOOK AT ‘EINSTEIN RINGS’ JUST GOT US CLOSER TO SOLVING THE
DARK MATTER DEBATE  
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Rossana Ruggeri
May 20, 2023
The Conversation
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_ Physicists believe most of the matter in the universe is made up of
an invisible substance that we only know about by its indirect effects
on the stars and galaxies we can see. But the nature of dark matter is
a longstanding puzzle. _

, ESA / Hubble & NASA

 

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

We’re not crazy! Without this “dark matter”, the universe as we
see it would make no sense.

But the nature of dark matter is a longstanding puzzle. However, a new
study [[link removed]] by Alfred
Amruth at the University of Hong Kong and colleagues, published in
Nature Astronomy, uses the gravitational bending of light to bring us
a step closer to understanding.

Invisible but omnipresent

The reason we think dark matter exists is that we can see the effects
of its gravity in the behaviour of galaxies. Specifically, dark matter
seems to make up about 85% of the universe’s mass, and most of the
distant galaxies we can see appear to be surrounded by a halo of the
mystery substance.

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

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

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

It has been difficult to distinguish between these two possibilities
– but now light bent around distant galaxies has offered a clue.

Gravitational lensing and Einstein rings

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

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 be smeared out
into a circle around the closer galaxy.

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

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

Axions vs WIMPs

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

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Multiple images of a background image created by gravitational lensing
can be seen in the system HS 0810+2554. Hubble Space Telescope / NASA
/ ESA
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Using detailed modelling, they worked out how the images would be
distorted if dark matter were made of WIMPs vs how they would if dark
matter were made of axions. The WIMP model didn’t look much like the
real thing, but the axion model accurately reproduced all features of
the system.

The result suggests axions are a more probable candidate for dark
matter, and their ability to explain lensing anomalies and other
astrophysical observations has scientists buzzing with excitement.

Particles and galaxies

The new research builds on previous studies that have also pointed
towards axions as the more likely form of dark matter. For example,
one study [[link removed]] looked at the effects
of axion dark matter on the cosmic microwave background, while another
[[link removed]] examined the behaviour of dark
matter in dwarf galaxies.

Although this research won’t yet end the scientific debate over the
nature of dark matter, it does open new avenues for testing and
experiment. For example, future gravitational lensing observations
could be used to probe the wave-like nature of axions 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 could also
help us to understand better how galaxies form and change over time.
[The Conversation]

Rossana Ruggeri
[[link removed]],
Research Fellow in Cosmology, _The University of Queensland
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This article is republished from The Conversation
[[link removed]] under a Creative Commons license. Read
the original article
[[link removed]].

* Science
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* physics
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* astronomy
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* Dark Matter
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* light
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* axions
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* gravity
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