According to physicists, the majority of the universe’s matter is composed of an invisible material that humans can only detect indirectly by its effects on the visible galaxies and stars.
We are not insane! The universe as we currently understand it would not make sense without this “dark matter.”
But the nature of dark matter is a longstanding puzzle. The gravitational bending of light is used in a new study by Alfred Amruth at the University of Hong Kong and colleagues that was published in Nature Astronomy to advance our understanding.
Invisible but omnipresent
We believe dark matter exists because we can observe the consequences of its gravitational pull on how galaxies behave. Particularly, dark matter appears to account for around 85% of the universe’s mass, and the majority of the distant galaxies that we can observe seem to be encircled by a halo of the unidentified material.
Dark matter is so-called because it doesn’t emit light or absorb or reflect it, making it very difficult to find.
So what is this stuff? Beyond that, we are unsure, but it must be some type of undiscovered fundamental particle. Physicists have been arguing the nature of dark matter for decades, despite the fact that all laboratory studies to date have failed to discover dark matter particles.
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. Theoretically, axions would act much more like waves due to quantum interference than WIMPs, which would behave like discrete particles.
It has been challenging to tell these two possibilities apart, but recently light bent around far-off galaxies has provided a hint.
Gravitational lensing and Einstein rings
According to Albert Einstein’s theory of general relativity, the gravity of a huge object bends space and time around it, thus as light moving through the universe passes a massive object like a galaxy, its course is bent.
As a result, occasionally we can see distorted images of other galaxies behind a distant galaxy. And if everything falls into place, a circle of light from the background galaxy will surround the galaxy that is closer.
This distortion of light is called “gravitational lensing,” and the circles it can create are called “Einstein rings.”
Astronomers can discover information about the characteristics of the dark matter halo encircling the nearby galaxy by examining how the distortions in the rings or other lensed images are produced.
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.
With the aid of intricate modeling, they determined how the images would differ if dark matter were composed of WIMPs as opposed to axions. The axion model faithfully replicated all aspects of the system while the WIMP model did not closely like the real thing.
The finding points to axions as a more likely candidate for dark matter, and scientists are giddy about its potential to explain lensing abnormalities and other astronomical data.
Particles and galaxies
The latest findings support earlier studies that suggested axions were the most likely type of dark matter. For instance, one study investigated how axion dark matter affected the cosmic microwave background, while another investigated how dark matter behaved in dwarf galaxies.
The scientific dispute over the nature of dark matter is still ongoing, but this finding does open up new areas for investigation and experimentation. Future gravitational lensing observations, for instance, could be used to examine the wave-like properties of axions and possibly determine their mass.
Knowing more about dark matter will affect our knowledge of particle physics and the early cosmos. It might also aid in our understanding of the evolution and formation of galaxies.
