The spiral galaxy NGC 6384. Image: ESA/Hubble & NASA
Astronomers have discovered a potential challenge to the established physics of the universe by studying the evolution of a mysterious type of matter at the center of hundreds of distant galaxies. The results from a new study in the journal Astronomy & Astrophysics suggest that dark matter, an unidentified yet abundant substance in the universe, might be directly interacting with the regular “baryonic” matter that makes up stars, planets, and our bodies. This finding directly conflicts with a well-corroborated theory called the Lambda cold dark matter (ΛCDM) model, or the standard model of cosmology, which predicts that dark matter particles do not interact with regular particles, except as a gravitational influence.
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Scientists led by Gauri Sharma, who conducted this research while at Italy’s Scuola Internazionale Superiore di Studi Avanzati (SISSA), concluded that the observations available to scientists “can help us open a new portal to the nature of dark matter,” according to the new study. It could also potentially test the limits of the standard model, though it will take more observations and research to be sure if the new findings pose a real threat to this established view of the universe.“In order to totally rule out the ΛCDM model, which is one of the biggest and simplest models for defining the universe, we have to really work on this a lot more,” said Sharma, who is now a South African Radio Astronomy Observatory (SARAO) fellow at the University of the Western Cape. Sharma and her colleagues serendipitously discovered this interesting twist in our understanding of physics while they were combing through observations of spiral galaxies, which are galaxies with distinctive coiled arms, like the Milky Way. Studies of spiral galaxies have shown that are surrounded by huge “halos” of dark matter that emanate from their centers and maintain a constant density up to a certain radius.Scientists know that dark matter exists because it exerts a huge gravitational pull on regular matter all around the universe. However, this elusive matter doesn’t emit light and doesn’t appear to undergo any other interactions with regular matter, which leads to a lack of observable clues to test out various theories about the particles within it. As a result, the search to identify the true nature of dark matter particles is “one of the major efforts of astro-particle physics,” though it “has been unsuccessful so far,” as Sharma and her colleagues note in the study.
Whereas past research has mapped out dark matter halos in nearby spiral galaxies, Sharma and her colleagues observed the halos of hundreds of distant galaxies, some as far as seven billion light-years away. Because looking into deep space is also a way of looking back in time, the researchers saw many of these objects as they appeared seven billion years ago. This approach revealed a surprising finding: The distant halos appear to be more compact than those closer to the Milky Way, suggesting that these structures may be expanding very slowly over time. This was very odd behavior, which the scientists believed may be caused by dark matter interacting with feedback from regular matter in a way that alters the density of these halos. Such observations are only possible by staring deep into space, because not enough time has elapsed to measure this expansion in other nearby galaxies.“We did not expect it when we saw this data, because at that time, we just wanted to solve the problem of whether these galaxies have dark matter halos or not,” said Sharma. “These were unexpected results that have come out. They are telling us a lot about the nature of the dark matter particle—how it could be, how it could not be, and which kind of particle could be the dark matter particle.”For instance, the new observations might be explained by a scenario in which dark matter particles are interacting with regular baryonic matter in ways that transcend the gravitational influences that are already known. The researchers suggest that dark matter particles spill out of central galaxies due to the pull of stars or other galactic forces, which drives this slow advance of the halo. It would be hard to square this situation with the standard model of cosmology, which predicts that dark matter particles do not interact with regular matter in this way.
However, as Sharma noted, it would take a lot more than this study to dethrone the established framework of the universe as we currently know it. That’s why she will be following up on this study with even more precise observations of these distant galactic haloes, so that they can be compared to simulations of galactic evolution under various models. “The main challenges are finding the right density profile model for dark matter, and then comparing it with the observations,” Sharma said. “This is the main thing we must do. The problem is that the inner regions of galaxies are still unresolved in our observations.” “The [James Webb Space Telescope], which is a new telescope, might help us more in resolving the inner region of galaxies,” she concluded. “Once we are able to resolve it, we can actually disentangle the dark matter profile and then try comparing it with the simulations.”Regardless of the results of these future studies, the new research confirms that observations of deep space and time are always full of surprises.