Astrophysicists unravel the nature of dark matter through the study of wrinkling in spacetime

April 25, 2023

(Nanowerk News) Most of the matter in the universe, at 85% by mass, is unobservable and consists of particles that are not accounted for by the Standard Model of Particle Physics (see description). These particles are known as Dark Matter, and their existence can be inferred from the effect their gravity has on light from distant galaxies. Finding the particles that make up Dark Matter is a pressing problem in modern physics, because they dominate the mass and, therefore, the gravity of galaxies – solving this mystery could lead to new physics beyond the Standard Model.

While some theoretical models propose the existence of ultramassive particles as possible candidates for Dark Matter, others suggest ultralight particles. The team of astrophysicists led by Alfred AMRUTH, a PhD student in the team of Dr Jeremy LIM from the Department of Physics at The University of Hong Kong (HKU), collaborated with Professor George SMOOT, a Nobel Laureate in Physics from the Hong Kong University of Science and Technology (HKUST) and Dr Razieh EMAMI, Research Fellow at the Center for Astrophysics | Harvard & Smithsonian (CFA), has provided the most direct evidence yet that Dark Matter is not ultramassive particles as is commonly thought, but instead consists of particles so light that they travel through space like waves.

Their work resolves a remarkable problem in astrophysics first raised two decades ago: why do models adopting ultramassive Dark Matter particles fail to correctly predict the observed positions and brightness of multiple images of the same galaxy created by gravitational lensing?

Research findings recently published in Natural Astronomy (“Einstein ring modulated by dark matter wave-like from an anomaly in gravitational lensing image”).

Dark Matter does not emit, absorb, or reflect light, making it difficult to observe using traditional astronomical techniques. Currently, 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. In this theory, mass causes spacetime to bend, creating the appearance that light bends around massive objects such as stars, galaxies, or clusters of galaxies. By observing this bending of light, scientists can infer the existence and distribution of Dark Matter – and, as this study shows, the nature of Dark Matter itself. Figure 1: Illustration of gravitational lensing by a galaxy. The light from the reddish, more distant galaxies is bent by the bluish closer ones, which acts like a natural cosmic telescope to magnify more distant galaxies. In this case, many images of reddish galaxies are made, forming reddish ring-like features known as Einstein rings around bluish galaxies. (Image: ALMA, L Calcada, Y. Hezaveh et al.)

As illustrated in Figure 1, when the foreground lensing object and the background lensing object – both individual galaxies in the illustration – are closely aligned, multiple images of the same background object can be seen in the sky. The position and brightness of a multi-lens image depend on the distribution of Dark Matter in the foreground lensing objects, making for a very robust Dark Matter probe.

Another assumption about the nature of Dark Matter

In the 1970s, after the existence of Dark Matter was firmly established, hypothetical particles called Weakly Interacting Massive Particles (WIMPs) were proposed as Dark Matter candidates. These WIMPs are considered ultramassive – at least ten times larger than protons – and interact with other matter only via the weak nuclear force. These particles emerged from the theory of supersymmetry, were developed to fill gaps in the Standard Model, and have since been widely advocated as the most likely candidate for Dark Matter. However, over the past two decades, adopting ultramassive particles for Dark Matter, astrophysicists have struggled to correctly reproduce the position and brightness of multi-lens images as shown in Figure 2. In this study, the density of Dark Matter is assumed to smoothly descend outward from the galactic center. according to theoretical simulations using ultramassive particles. Example of a gravitationally lensed image observed with the Hubble Space Telescope Figure 2: Example of a gravitationally lensed image observed with the Hubble Space Telescope. Left: 2M130-1714, where four bright, bluish dots comprise a quadruple lensed image of the bright core of the background galaxy, such that the main body of the background galaxy is lensed and distorted into Einstein rings. Einstein’s rings surround two yellowish galaxies which consist of the foreground lensing galaxy. (Image: NASA/ESA/Hubble/T.Treu/Judy Schmidt). Right: Einstein’s cross, consisting of four bright spots that correspond to a quadruple lensed image of the bright core of the background galaxy. The fifth point near the center of the cross corresponds to the foreground lensing galaxy. (Image: NASA/ESA/STSci)

Beginning also in the 1970s, but in stark contrast to WIMP, versions of the theory which seek to correct deficiencies in the Standard Model, or those (e.g., String Theory) which seek to unify the four fundamental forces of nature (three in the Standard Model, along with gravity). , advocating the existence of ultralight particles. Called axions, these hypothetical particles are thought to be much smaller than the lightest particles in the Standard Model and are alternative candidates for Dark Matter.

According to the theory of Quantum Mechanics, ultralight particles travel through space as waves, interfering with each other in large numbers thereby creating random density fluctuations. These random density fluctuations in Dark Matter give rise to wrinkles in spacetime, as illustrated in Figure 3 below for galaxies around Dark Matter. As expected, the different spacetime patterns around galaxies depending on whether the Dark Matter is ultramassive or ultralight particles – smooth versus wrinkled – should appear to differ in position and brightness for the dual-lens image of the background galaxy, as illustrated in the same image. number. Smooth versus wrinkled visualization of spacetime produced by various forms of Dark Matter around the galaxy Figure 3: Visualization of smooth versus wrinkled spacetime produced by various forms of Dark Matter around the galaxy. Left: Dark Matter composed of ultramassive particles creates a subtle curvature in spacetime, such that light from distant lensed galaxies takes a smooth path around the foreground lensing galaxy. Right: Dark Matter composed of ultralight particles creates crimped fluctuations in spacetime, such that light from distant lensed galaxies takes chaotic paths around the foreground lensing galaxy. The different images of the background galaxy predictably have different positions and brightness for different forms of Dark Matter around the lens galaxy, allowing astrophysicists to investigate the nature of Dark Matter.

In work led by Alfred AMRUTH, a PhD student in Dr Jeremy LIM’s team at HKU, astrophysicists have for the first time calculated how gravitationally lensed images produced by galaxies that incorporate ultralight Dark Matter particles differ from those that incorporate ultramassive Dark Matter particles.

Their research has shown that the general degree of disagreement found between observed and predicted positions and brightness of multi-lens images produced by models incorporating ultramassive Dark Matter can be resolved by adopting models incorporating ultralight Dark Matter particles. In addition, they show that a model incorporating ultralight Dark Matter particles can reproduce the observed position and brightness of a dual-lens image of a galaxy, an important achievement that reveals the wrinkled rather than smooth nature of spacetime around galaxies.

‘The possibility that Dark Matter is not composed of ultramassive particles, as the scientific community has long advocated, overcomes another problem in both laboratory experiments and astronomical observations,’ explained Dr Lim. ‘Laboratory experiments have been very unsuccessful at finding WIMPs, long-favored candidates for Dark Matter. Such experiments are in their final stages, culminating in the planned DARWIN experiment, leaving WIMP with nowhere to hide if not found (see comment 2).’

Professor Tom Broadhurst, an Ikerbasque Professor at the University of the Basque Country, a Visiting Professor at HKU, and co-author of the paper added, ‘If Dark Matter is composed of ultramassive particles, then according to cosmological simulations, there should be hundreds of satellite galaxies circling the Milky Way. However, despite intensive searches, only about fifty have been found so far. On the other hand, if Dark Matter is composed of ultralight particles, then the theory of Quantum Mechanics predicts that galaxies below a certain mass cannot form due to the interference of these particle waves, explaining why we observe a lack of small satellite galaxies in the vicinity. Milky Way.’

‘Incorporating ultralight rather than ultramassive particles for Dark Matter simultaneously solves several old problems in both particle physics and astrophysics,’ said Amruth Alfred, ‘We have reached a point where the existing paradigm of Dark Matter needs to be reconsidered. Waving goodbye to ultramassive particles, which have long been heralded as the favorite candidate for Dark Matter, may not be easy, but evidence is accumulating to support Dark Matter having wavelike properties similar to those of ultralight particles.’ The pilot work uses the supercomputing facilities at HKU, without which this work would not be possible.

Co-author Professor George SMOOT added, ‘Understanding the nature of the particles that make up Dark Matter is the first step towards New Physics. This work paves the way for future testing of Wavelike Dark Matter in situations involving gravitational lensing. The James Webb Space Telescope will discover many more gravitational lensing systems, allowing us to carry out more rigorous tests of the nature of Dark Matter.’


The Standard Model of Particle Physics is a theory that explains three of the four known fundamental forces (electromagnetic interactions, weak and strong interactions — excluding gravity) in the universe and classifies all known elementary particles. Although the Standard Model has achieved great success, it leaves some phenomena unexplained – for example, the existence of particles that interact with particles known in the Standard Model only through gravity – and does not become a full-fledged fundamental theory of interaction.

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