Einstein Rings: The clue to understanding DARK MATTER!

Finally!! We know almost accurately what Dark Matter is made up of. As most of you know, Dark Matter… unlike normal matter does not interact with anything except gravity. Since its detection in 1933, scientists are still searching for the ideal dark matter candidate. Now, fast forward to 2023… according to this new study published in the Journal Nature, the most likely possible candidate for Dark Matter is — AXIONS!

(Image credit: Shutterstock)

I know what you are thinking….

What are Axions??

How are they related to Dark Matter??

WIMPS — A lost cause

Well, for these answers let’s start from the very beginning i.e. Weakly Interacting Massive Particles also known as WIMP. They were originally proposed as a candidate for Dark Matter. Basically, WIMP is the term that particle physicists use for all hypothetical particles that interact very weakly with normal matter…possibly only with the help of the weak nuclear force. They include particles like SUSY, UED, and Little Higgs and are predicted to be at least 1,000 times heavier than protons. Back in the 1970s, there was a realization among particle physicists that new particles beyond the Standard Model could eventually produce new types of stable, neutral particles if there was a new type of symmetry that prevented their decay. Due to this, all the WIMPS have a common story of existence that goes back to the ‘Big Bang.’

During the big bang, when the universe was very hot and dense, all the particles including the ones beyond the Standard Model were created in great abundance. But, as the universe cooled down, extra Standard Model particles like WIMPS decayed into more stable Standard Model particles like quarks, leptons, and bosons. Now, because these Standard Model particles were stable, they have persisted to the present day. But this does not mean that one cannot evaluate the mass and cross-section of these particles. And when scientists tried to evaluate their properties, they found out that these types of particles couldn’t have interacted with electromagnetic or strong nuclear force.

The Standard Model of physics describes the known particles and forces that operate at the tiny quantum scale. (Wikimedia Commons: Miss J)

But, they could possibly interact with the weak nuclear force. This means they could interact with W and Z bosons(which carry the weak force) and thus in turn with normal particles that interact with W and Z bosons. Using this information, particle physicists constructed dark matter experiments looking for normal matter particle interactions. And those interactions not corresponding to normal matter would simply be evidence for the existence of WIMPs or Dark Matter Particles. Such experiments, like LUX and Xenon 1T have now been ongoing for a long time, and are yet to see any evidence of WIMPs.

Axions — A New Hope

So, with WIMPs falling from the lead, there entered another particle candidate that claimed to explain the nature of Dark Matter — Axion. An axion is a hypothetical elementary particle, having low mass and energy. It was originally proposed as a solution to the strong CP problem. Simply stating the CP problem asks: Why Conjugation and Pairity symmetry is preserved in quantum chromodynamics?

According to John Ellios, a particle physicist at LHC,

“There’s this CP symmetry which we know, which distinguishes between particles and antiparticles. We know it’s violated in the weak interactions. It was a puzzle why it was not violated in the strong interactions.”

And to solve this problem, an extension to the Standard Model was proposed in which it made sense that the strong interactions didn't violate this symmetry. This model also predicted the existence of the axion. Fast forward to 2020, a team of physicists found the first direct evidence for axions. This has continued to push forward assertions that axion could be the best dark matter candidate.

Einstein Rings: The Dark Matter Lens

And in a study published recently, scientists analyzed Dark Matter with the help of Einstein Rings. Basically, Einstien Rings are formed when light traveling through the universe passes a massive object like a galaxy and is bent due to its gravity. As a result, we can see distorted images of other galaxies behind a distant galaxy. And if things line up perfectly, the light from the background galaxy will make a perfect circle known as Einstein rings. By studying how the rings or other lensed images are distorted, astronomers can learn about the properties of the dark matter surrounding the closer galaxy.

In a recent study, scientists 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.

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

Using detailed modeling, they worked out how the images would be distorted if dark matter were made of axions. And to their surprise, the axion model accurately reproduced all features of the system. The result suggests that axions are a more probable candidate for dark matter, and their ability to explain lensing anomalies and other astrophysical observations has excited scientists.

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