MSGID: 1:317/3 6448a8e5   
   PID: hpt/lnx 1.9.0-cur 2019-01-08   
   TID: hpt/lnx 1.9.0-cur 2019-01-08   
    Astrophysicists reveal the nature of dark matter through the study of   
   crinkles in spacetime    
      
     Date:   
         April 25, 2023   
     Source:   
         The University of Hong Kong   
     Summary:   
         Astrophysicists have provided the most direct evidence yet that   
         Dark Matter does not constitute ultramassive particles as is   
         commonly thought but instead comprises particles so light that   
         they travel through space like waves. Their work 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?   
      
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   FULL STORY   
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   Most of the matter in the universe, amounting to a staggering 85%   
   by mass, cannot be observed and consists of particles not accounted   
   for by the Standard Model of Particle Physics (see remark 1). These   
   particles are known as Dark Matter, and their existence can be inferred   
   from their gravitational effects on light from distant galaxies. Finding   
   the particle that makes up Dark Matter is an urgent problem in modern   
   physics, as it dominates the mass and, therefore, the gravity of galaxies   
   -- solving this mystery can lead to new physics beyond the Standard Model.   
      
   While some theoretical models propose the existence of ultramassive   
   particles as a possible candidate for Dark Matter, others suggest   
   ultralight particles. A team of astrophysicists led by Alfred AMRUTH, a   
   PhD student in the team of Dr Jeremy LIM of the Department of Physics at   
   The University of Hong Kong (HKU), collaborating with Professor George   
   SMOOT, a Nobel Laureate in Physics from the Hong Kong University of   
   Science and Technology (HKUST) and Dr Razieh EMAMI, a Research Associate   
   at the Center for Astrophysics | Harvard & Smithsonian (CFA), has provided   
   the most direct evidence yet that Dark Matter does not constitute   
   ultramassive particles as is commonly thought but instead comprises   
   particles so light that they travel through space like waves. Their work   
   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? The research   
   findings were recently published in Nature Astronomy.   
      
   Dark Matter does not emit, absorb or reflect light, which makes it   
   difficult to observe using traditional astronomical techniques. Today,   
   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 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.   
      
   When the foreground lensing object and the background lensed object --   
   both constituting individual galaxies in the illustration -- are closely   
   aligned, multiple images of the same background object can be seen in the   
   sky. The positions and brightness of the multiply-lensed images depend   
   on the distribution of Dark Matter in the foreground lensing object,   
   thus providing an especially powerful probe of Dark Matter.   
      
   Another assumption of the nature of Dark Matter In the 1970s, after the   
   existence of Dark Matter was firmly established, hypothetical particles   
   referred to as Weakly Interacting Massive Particles (WIMPs) were proposed   
   as candidates for Dark Matter. These WIMPs were thought to be ultramassive   
   -- more than at least ten times as massive as a proton - - and interact   
   with other matter only through the weak nuclear force. These particles   
   emerge from Supersymmetry theories, developed to fill deficiencies in the   
   Standard Model, and have since been widely advocated as the most likely   
   candidate for Dark Matter. However, for the past two decades, adopting   
   ultramassive particles for Dark Matter, astrophysicists have struggled   
   to correctly reproduce the positions and brightness of multiply-lensed   
   images. In these studies, the density of Dark Matter is assumed to   
   decrease smoothly outwards from the centres of galaxies in accordance   
   with theoretical simulations employing ultramassive particles.   
      
   Beginning also in the 1970s, but in dramatic contrast to WIMPs, versions   
   of theories that seek to rectify deficiencies in the Standard Model,   
   or those (e.g., String Theory) that seek to unify the four fundamental   
   forces of nature (the three in the Standard Model, along with gravity),   
   advocate the existence of ultralight particles. Referred to as axions,   
   these hypothetical particles are predicted to be far less massive than   
   even the lightest particles in the Standard Model and constitute an   
   alternative candidate for Dark Matter.   
      
   According to the theory of Quantum Mechanics, ultralight particles   
   travel through space as waves, interfering with each other in such large   
   numbers as to create random fluctuations in density. These random density   
   fluctuations in Dark Matter give rise to crinkles in spacetime. As might   
   be expected, the different patterns of spacetime around galaxies depending   
   on whether Dark Matter constitutes ultramassive or ultralight particles   
   -- smooth versus crinkly -- ought to give rise to different positions   
   and brightness for multiply-lensed images of background galaxies.   
      
   In work led by Alfred AMRUTH, a PhD student in Dr Jeremy LIM's   
   team at HKU, astrophysicists have for the first time computed how   
   gravitationally-lensed images generated by galaxies incorporating   
   ultralight Dark Matter particles differ from those incorporating   
   ultramassive Dark Matter particles.   
      
   Their research has shown that the general level of disagreement found   
   between the observed and predicted positions as well as the brightness of   
   multiply- lensed images generated by models incorporating ultramassive   
   Dark Matter can be resolved by adopting models incorporating ultralight   
   Dark Matter particles.   
      
   Moreover, they demonstrate that models incorporating ultralight Dark   
   Matter particles can reproduce the observed positions and brightness of   
   multiply- lensed galaxy images, an important achievement that reveals   
   the crinkly rather than smooth nature of spacetime around galaxies.   
      
   'The possibility that Dark Matter does not comprise ultramassive   
   particles, as has long been advocated by the scientific community,   
   alleviates other problems in both laboratory experiments and astronomical   
   observations,' explains Dr Lim.   
      
   'Laboratory experiments have been singularly unsuccessful at finding   
   WIMPs, the long-favoured candidate for Dark Matter. Such experiments are   
   in their final stretch, culminating in the planned DARWIN experiment,   
   leaving WIMPs with no place to hide if not found (see remark 2).'   
   Professor Tom BROADHURST, an Ikerbasque Professor at the University   
   of the Basque Country, a Visiting Professor at HKU, and a co-author   
   of the paper adds, '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. On the other   
   hand, if Dark Matter comprises ultralight particles instead, then the   
   theory of Quantum Mechanics predicts that galaxies below a certain mass   
   simply cannot form owing to the wave interference of these particles,   
   explaining why we observe a lack of small satellite galaxies around the   
   Milky Way.'  'Incorporating ultralight rather than ultramassive particles   
   for Dark Matter resolve several longstanding problems simultaneously 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 favoured candidate for Dark Matter, may not   
   come easily, but the evidence accumulates in favour of Dark Matter   
   having wave-like properties as possessed by ultralight particles.'   
   The pioneering work used the supercomputing facilities at HKU, without   
   which this work would not have been possible.   
      
   The co-author Professor George SMOOT added, 'Understanding the nature   
   of particles that constitute Dark Matter is the first step towards   
   New Physics.   
      
   This work 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 us to   
   make even more exacting tests of the nature of Dark Matter.'  Remarks:   
   1. The Standard Model of Particle Physics is the theory describing three   
   of the four known fundamental forces (electromagnetic, weak and strong   
   interactions -- excluding gravity) in the universe and classifying all   
   known elementary particles. Although the Standard Model has met with huge   
   successes, it leaves some phenomena unexplained -- e.g., the existence   
   of particles that interact with known particles in the Standard Model   
   only through gravity -- and falls short of being a complete theory of   
   fundamental interactions.   
      
       * RELATED_TOPICS   
             o Space_&_Time   
                   # Dark_Matter # Astrophysics # Astronomy # Sun #   
                   Solar_Flare # Black_Holes # Galaxies # Northern_Lights   
       * RELATED_TERMS   
             o Dark_matter o Ultimate_fate_of_the_universe o Dark_energy   
             o Subatomic_particle o Galaxy o Interstellar_medium o   
             Spitzer_space_telescope o Big_Bang   
      
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   Story Source: Materials provided by The_University_of_Hong_Kong. Note:   
   Content may be edited for style and length.   
      
      
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   Related Multimedia:   
       * Figures_showing_gravitational_lensing_and_space-time_models   
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   Journal Reference:   
      1. Alfred Amruth, Tom Broadhurst, Jeremy Lim, Masamune Oguri, George F.   
      
         Smoot, Jose M. Diego, Enoch Leung, Razieh Emami, Juno Li, Tzihong   
         Chiueh, Hsi-Yu Schive, Michael C. H. Yeung, Sung Kei Li. Einstein   
         rings modulated by wavelike dark matter from anomalies in   
         gravitationally lensed images.   
      
         Nature Astronomy, 2023; DOI: 10.1038/s41550-023-01943-9   
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   Link to news story:   
   https://www.sciencedaily.com/releases/2023/04/230425111243.htm   
      
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