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Scientists know that just over a quarter of everything in the universe is in the form of dark matter, something we cannot see directly. We know that dark matter has to be there because of the way galaxies move, which shows they have much more mass than we can observe. So we call dark matter of unknown mass remaining.
But how do you study something you don’t see? The next generation of dark matter instruments will use new techniques and highly accurate hardware to measure the motions of distant galaxies. But for now, a small group of astronomers at Swinburne University of Technology have come up with a way to “see” dark matter using current telescopes.
The method works by looking for the gravitational effects of dark matter, rather than the supposed particles themselves. “It’s like looking at a flag to try and know how much wind is there,” lead author Pol Gurri explained in a statement. “You can’t see the wind, but the movement of the flag tells you how hard the wind blows.”
The research uses a technique called weak gravitational lensing, in which they observe distant galaxies and wait for another galaxy to pass between it and us. When this happens, the intermediate galaxy bends the light waves of the distant galaxy due to its gravity. “Dark matter slightly distorts the image of anything behind it,” explained associate professor Edward Taylor, who was also involved in the research. “The effect is a bit like reading a newspaper through the base of a wine glass.”
This technique has been used in the past to investigate dark matter. But it usually requires very precise telescopes, which measure the shape of distant galaxies. The team’s innovation was instead to observe how galaxies rotate.
“Because we know how stars and gas are supposed to move inside galaxies, we know pretty much what that galaxy should look like,” Gurri said. “By measuring how distorted real images of the galaxy are, we can figure out how much dark matter it would take to explain what we see.”
This means that even older telescopes, such as the ANU 2.3m telescope in Australia, can be used to “see” dark matter, more accurately than if they were not watching the rotation.
“With our new way of seeing dark matter,” said Gurri, “we hope to have a clearer picture of where dark matter is and what role it plays in how galaxies are formed.”
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