Evrim Yazgin
Cosmos science journalist
Gravitationally lensed “Einstein rings” show the tell-tale signs of wave-like dark matter according to research from the University of Hong Kong (UHK).
Dark matter is the invisible substance that physicists tell us we can’t see but makes up five-sixths of the stuff in our universe. It is one of the Holy Grails of modern physics to directly measure dark matter, which we only know about indirectly through its gravitational effects throughout the universe.
Actually, our observations of the universe wouldn’t make sense without dark matter. But what this enigmatic stuff really is has puzzled physicists for decades.
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The main contenders are weakly-interacting massive particles (WIMPs) or axions – extremely lightweight bosons. In theory, the axions should appear to act more like waves, while WIMPs should appear to act more like discrete particles.
Short of directly finding dark matter in an experiment (like SABRE, one half of which is in Victoria in Australia and the other half in Italy), it is very difficult to distinguish between the two possibilities.
Now, light bent around a galaxy nearly nine billion light-years away gives us a new angle.
A team led by UHK PhD student in astrophysics Alfred Amruth focused on the quasar at the centre of galaxy HS 0810+2554 to test the axion vs WIMP theory.
HS 0810+2554 was discovered in 2002 by the Hubble Space Telescope and is a great example of gravitational lensing. This is the effect caused by the curvature of spacetime described in Einstein’s General Theory of Relativity.
A massive object (like a galaxy) bends the path of light coming from sources behind it. The effect results in a ring of warped objects known as an “Einstein ring”.
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Simulating what the Einstein ring around HS 0810+2554 would look like for both a WIMP and an axion based dark matter model, Amruth was able to determine that the WIMP model didn’t match up while the axion picture reproduced all the features of the system.
While not definitive, the study does suggest an axion model for dark matter is more probable.
Amruth’s team also note that an axion-based model of dark matter is also better at predicting many other observed astronomical effects.
The study is published in Nature Astronomy.