Shedding Light on the Dark

The existence of dark matter (DM), the mysterious substance that makes up 80 per cent of the universe’s mass, was first reliably inferred in the 1930s, although not fully recognised until the 1970s. What particle makes up DM, however, remains an enigma. For decades, physicists have hypothesised that weakly interacting massive particles (WIMPs) are the strongest candidate, but laboratory experiments have failed to find evidence to back the theory.

The new study adds to the increasing evidence against WIMPs that has been gathered from astronomical observations. “Our findings make way for a new paradigm where ultralight DM particles are strong contenders for dark matter, deserving the full weight of scrutiny as has been paid to WIMPs,” said Mr Alfred Amruth, lead author of the study.

Dr Jeremy Lim, Associate Professor in the Department of Physics and Mr Amruth’s supervisor on the research, said: “At present, the Standard Model does not allow for any particle having the properties of dark matter. There are many theoretical extensions to the Standard Model, predicting particles over a wide range of masses – from ultralight particles (such as the ones we infer from our study) to ultra-massive particles. The question is which of these theoretical extensions are correct: if we can identify the correct theoretical extension, then we would know the correct path towards new physics.”

The findings have attracted a great deal of interest, an example being the study’s selection as the cover story of Nature Astronomy earlier this year.

“That was something we were very happy about,” said Mr Amruth, who has just completed his PhD defense and is remaining at HKU on a Dissertation Year Fellowship. He first came to HKU from Sri Lanka in 2013 as an undergraduate, and for those 10 years, Dr Lim has been his research supervisor. The two of them collaborated on this work with Professor Thomas Broadhurst, Ikerbasque Research Professor at Spain’s University of the Basque Country; Professor George Smoot, a Nobel Laureate in Physics from the Hong Kong University of Science and Technology; and Dr Razieh Emami, Research Associate at the Center for Astrophysics, Harvard & Smithsonian.

“Our demonstration that ultralight particles are a strong candidate for DM will now help mobilise further research (theoretical and observational) into ultralight DM to assess whether it truly is a correct description,” said Mr Amruth. “The possible existence of such ultralight particles was first proposed by String Theory, and this particle is capable of resolving the astrophysical problems faced by massive DM. Much research has already been spurred by our work showing ultralight DM’s ability to resolve the long-standing lensing anomalies in astronomy.”

Gravitational lensing

The evidence the team use in particular, arises from the phenomenon of gravitational lensing (courtesy of Einstein’s general theory of relativity), where anything with mass can bend the path of light. “Over the past two decades, when we looked at astronomical observations of quasars (the very bright nuclei of galaxies constituting visible evidence for vigorous accretion on to their central supermassive black holes) which are lensed by a foreground galaxy, it was typically difficult to reproduce the observed positions and brightnesses (also known as lensing anomalies) of the lensed galaxies if one used a massive particle DM model,” explained Mr Amruth.

“However, when one uses ultralight DM, we can resolve these lensing anomalies and reproduce the observations of the lensed galaxies. Our motivation was the long-standing problem of lensing anomalies, as well as the fact that no one has attempted to calculate the lensing properties of ultralight DM before. We take these together, in the spirit of the scientific method, and make theoretical predictions which can be compared with the observations of lensed galaxies.”

Their theory has already inspired more research papers which further investigate the possible mass range of ultralight DM particles from the perspective of particle physics experiments. “There has also been research in astronomy looking at observations of lensed galaxy clusters to place further constraints on the mass of the ultralight DM particle,” said Mr Amruth. “In addition, we are currently working on a follow-up paper which finds that ultralight DM can resolve lensing anomalies in the first type-1a lensed supernova [a type of supernova that occurs due to an accreting white dwarf approaching a mass of 1.4 times that of the Sun].”

Asked about his initial interest in astrophysics, Mr Amruth said: “I’ve always been very passionate about the universe and how it works – I focussed on research about DM since it composes the majority of mass in our universe and we still don’t know what it is! This is of fundamental importance since DM, much like fire and electricity, has the potential to be the next discovery which propels humanity into a spacefaring civilisation! Ultimately, what this is all about, is to identify the path to new physics, which will be an incredible revolution for modern science.”