|
| Multiple images of a background image created by gravitational lensing can be seen in the system HS 0810+2554. Credit: Hubble Space Telescope / NASA / ESA |
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
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 behavior 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 traveling 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.
Using detailed modeling, 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 looked
at the effects of axion dark matter on the cosmic microwave background,
while another examined the behavior 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.
Reference:
Alfred Amruth et al, Einstein rings modulated by wavelike dark matter from
anomalies in gravitationally lensed images, Nature Astronomy (2023). DOI:
10.1038/s41550-023-01943-9