Astrophysicists from HKU uncover the nature of dark matter by studying crinkles in spacetime


26th April 2023 – (Hong Kong) Astrophysicists from The University of Hong Kong (HKU), Hong Kong University of Science and Technology (HKUST), and the Center for Astrophysics | Harvard & Smithsonian (CFA) have provided the most direct evidence yet that dark matter comprises particles so light that they travel through space like waves. This contradicts the long-held belief that dark matter is made up of ultramassive particles, first proposed in the 1970s, known as Weakly Interacting Massive Particles (WIMPs). With 85% of the universe’s mass made up of dark matter, identifying the particle that makes up dark matter is essential in modern physics. The team’s work, published in Nature Astronomy, resolves an outstanding problem in astrophysics first raised two decades ago: why do models that adopt ultramassive dark matter particles fail to correctly predict the observed positions and the brightness of multiple images of the same galaxy created by gravitational lensing?

Dark matter does not emit, absorb or reflect light, making it difficult to observe using traditional astronomical techniques. The most powerful tool scientists have for studying dark matter is through gravitational lensing, a phenomenon predicted by Albert Einstein in his theory of General Relativity. Mass causes spacetime to curve, creating the appearance that light bends around massive objects such as stars, galaxies, or groups of galaxies. By observing this bending of light, scientists can infer the presence and distribution of dark matter – and, as demonstrated in this study, the nature of dark matter itself.

The team led by Alfred Amruth, a PhD student in the team of Dr Jeremy Lim of the Department of Physics at HKU, computed how gravitationally-lensed images generated by galaxies incorporating ultralight dark matter particles differ from those incorporating ultramassive dark matter particles. The general level of disagreement found between the observed and predicted positions and brightness of multiply-lensed images generated by models incorporating ultramassive dark matter can be resolved by adopting models incorporating ultralight dark matter particles. Models incorporating ultralight dark matter particles can reproduce the observed positions and brightness of multiply-lensed galaxy images, revealing the crinkly rather than smooth nature of spacetime around galaxies.

The work also resolves other problems in both laboratory experiments and astronomical observations. Laboratory experiments have been unsuccessful at finding WIMPs, the long-favoured candidate for dark matter. With the planned DARWIN experiment, WIMPs will have no place to hide if not found. If dark matter comprises ultramassive particles, then according to cosmological simulations, there should be hundreds of satellite galaxies surrounding the Milky Way. However, despite intensive searches, only around fifty have been discovered so far. If dark matter comprises ultralight particles instead, then the theory of Quantum Mechanics predicts that galaxies below a certain mass cannot form owing to the wave interference of these particles, explaining why we observe a lack of small satellite galaxies around the Milky Way.

The team’s pioneering work used the supercomputing facilities at HKU, without which this work would not have been possible. The research paves the way for future tests of wave-like dark matter in situations involving gravitational lensing. The James Webb Space Telescope should discover many more gravitationally-lensed systems, allowing for even more exacting tests of the nature of dark matter.

Figure 1: Illustration of gravitational lensing by a galaxy. Light from a more distant and reddish galaxy is bent by a more nearby and bluish galaxy, which acts like a natural cosmic telescope to magnify the more distant galaxy. In this instance, multiple images of the reddish galaxy are created, forming a reddish ring-like feature referred to as an Einstein ring around the bluish galaxy.
(Image credit: ALMA, L Calcada, Y. Hezaveh et al.)
Figure 2: Examples of gravitational-lensed images observed with the Hubble Space Telescope. Left: 2M130-1714, in which the four bright and bluish points comprise the quadruply-lensed images of the bright nucleus of a background galaxy, such that the main body of the background galaxy is lensed and distorted into an Einstein ring. The Einstein ring encircles two yellowish galaxies comprising the foreground lensing galaxies. Image credit: NASA/ESA/Hubble/T.Treu/Judy Schmidt. Right: The Einstein Cross, comprising four bright points corresponding to the quadruply-lensed images of the bright nucleus of a background galaxy. The fifth point near the middle of the cross corresponds to the foreground lensing galaxy. 
(Image credit: NASA/ESA/STSci)
Figure 3: Visualisation of smooth versus crinkly spacetime produced by different forms of Dark Matter around galaxies. Left: Dark Matter comprising ultramassive particles create a smooth curvature in spacetime, such that light from a distant lensed galaxy takes smooth paths around the foreground lensing galaxy. Right: Dark Matter comprising ultralight particles creates crinkly fluctuations in spacetime, such that light from a distant lensed galaxy takes chaotic paths around the foreground lensing galaxy. The multiple images of the background galaxy thus created are predicted to have different positions and brightness for the different forms of Dark Matter around the lensing galaxy, allowing astrophysicists to probe the nature of Dark Matter. Source: HKU