The hunt for dark matter continues — now axions are leading candidates

A new study contributes to unraveling the riddle of dark matter.
Tejasri Gururaj
An Einstein rings as seen by the Hubble telescope
An Einstein rings as seen by the Hubble telescope

ESA/Hubble & NASA/Wikimedia Commons 

Dark matter is one of the greatest mysteries in astronomy. It makes up 85 percent of the universe and is next to impossible to detect as it doesn't interact with electromagnetic radiation. The only way it interacts with other matter is through gravitational force. 

Two leading candidates for dark matter are weakly interacting massive particles (WIMP) and axions. Axions are ultralight bosons, which scientists predict would behave like waves. 

A new study by Alfred Amruth from the University of Hong Kong observed the gravitational lensing of a distant quasar. Their study was published in Nature Astronomy.

When a ring of light passes near large structures like galaxies, the light gets distorted by gravity. This distorted light causes objects in the background to get magnified and appear as a ring around the object being observed, in a process called gravitational lensing. The rings are called Einstein rings, named after Albert Einstein

This is also the same method that allows telescopes like the SuperBIT to map and observe dark matter. 

So, what did they find?

The team wanted to understand why lensed objects like galaxies and quasars fluctuate in brightness. For this, they chose to study the quasar named HS 0180+2554. 

They found that the axionic dark matter model showed no anomalies in the observed and predicted brightness of the quasar, and the model was perfectly able to reproduce the quasar system. Whereas the dark matter model based on WIMPs showed anomalies and could not reproduce all aspects of the quasar system.

Their study provided valuable evidence into why the axionic dark matter model should be more accepted. Currently, dark matter has not been accounted for in the Standard Model of particle physics, which aims to explain the fundamental forces and particles of the universe. Unfortunately, neither is gravity.

The inability of the Standard Model to explain gravity is one of the greatest mysteries of our time. For many years, scientists have tried to work on a theory of everything which would unify the Standard Model with Einstein's general relativity. But this would only account for 15 percent of the visible universe. 

It seems like the rest of the universe is governed by dark matter. This study may not change our understanding of physics, but it certainly encourages us forward on our quest to understand dark matter. We still have a long way to go. 

Study abstract:

Unveiling the true nature of dark matter, which manifests itself only through gravity, is one of the principal quests in physics. Leading candidates for dark matter are weakly interacting massive particles or ultralight bosons (axions), at opposite extremes in mass scales, that have been postulated by competing theories to solve deficiencies in the Standard Model of particle physics. Whereas dark matter weakly interacting massive particles behave like discrete particles (ϱDM), quantum interference between dark matter axions is manifested as waves (ψDM). Here, we show that gravitational lensing leaves signatures in multiply lensed images of background galaxies that reveal whether the foreground lensing galaxy inhabits a ϱDM or ψDM halo. Whereas ϱDM lens models leave well documented anomalies between the predicted and observed brightnesses and positions of multiply lensed images, ψDM lens models correctly predict the level of anomalies remaining with ϱDM lens models. More challengingly, when subjected to a battery of tests for reproducing the quadruply lensed triplet images in the system HS 0810+2554, ψDM is able to reproduce all aspects of this system whereas ϱDM often fails. The ability of ψDM to resolve lensing anomalies even in demanding cases such as HS 0810+2554, together with its success in reproducing other astrophysical observations, tilt the balance toward new physics invoking axions.

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