Astronomers Develop New Way To “See” Dark Matter More Accurately

An exaggerated impression of a foreground galaxy warping the shape of distant galaxies due to dark matter. Swinburne Astronomy Productions - James Josephides

Dark matter is the hypothetical substance that cocoons galaxies and outweighs regular matter in the universe five to one. We can’t directly observe it because it doesn’t interact with light but we can measure its gravitational effects, which is how we “see” it, especially by looking at the motion of galaxies and clusters of galaxies.

Researchers have reported a new approach that improves how we estimate, or "see", the presence of dark matter. As reported in the Monthly Notices of the Royal Astronomical Society, this method uses gravitational lensing. The gravity of massive objects warps space-time in such a way that any distant light source behind it appears to the observer as though through an optical lens.

The effect used in this specific study is known as weak gravitational lensing. In this case, the shape and size of distant galaxies are only slightly altered.

“The dark matter will very slightly distort the image of anything behind it,” explained Associate Professor Edward Taylor in a statement. “The effect is a bit like reading a newspaper through the base of a wine glass.”

Detecting these changes requires high-precision observations but leads to very detailed maps of dark matter.

“It’s like looking at a flag to try to know how much wind there is. You cannot see the wind, but the flag’s motion tells you how strongly the wind is blowing,” lead author Pol Gurri, a graduate researcher from the Swinburne University of Technology, added.

The team went a step further than standard weak gravitational lensing, however. The new approach used the ANU 2.3m Telescope in Australia to measure the rotational speed of gravitationally lensed galaxies. This provides an even more precise view of the distortion, in turn allowing the team to work out a more refined estimate of how much dark matter must be present in the foreground to create the lensing effect seen.

“Because we know how stars and gas are supposed to move inside galaxies, we know roughly what that galaxy should look like. By measuring how distorted the real galaxy images are, then we can figure out how much dark matter it would take to explain what we see. said Gurri. "With our new way of seeing the dark matter, we hope to get a clearer picture of where the dark matter is, and what role it plays in how galaxies form.”

Future missions such as NASA's Nancy Grace Roman Space Telescope and ESA's Euclid Space Telescope are expected to carry out these observations for hundreds of millions of galaxies.

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