Seeing the dark matter in the new light

A small team of astronomers has found a new way to ‘see’ the lure of intricate dark objects around galaxies, which, with new technology, is 10 times more accurate than the previous best method. The work is published Monthly instructions of the Royal Astronomical Society.

Scientists currently estimate that about 85% of the mass in the universe is effectively invisible. This “dark matter” cannot be observed directly, as it does not interact with light like the ordinary matter that makes up the stars, planets and life on Earth.

So how do we measure what we can’t see? The key is to measure the effect of gravity that a dark object produces.

Paul Guri, Ph.D. The University of Swinburne student who led the new research explains: “It’s like looking at a flag trying to figure out how much wind there is. You can’t see the wind, but the speed of the flag tells you how much the wind is progressing.”

New research focuses on the effect of a weak gravitational lens, characterized by Einstein’s general theory of relativity. “The dark matter will slightly distort the image of anything behind it,” says Edward Taylor, an associate professor of research. “The effect is a bit like reading a newspaper through a wine glass base.”

Weak gravity lensing is one of the most successful ways to map the dark matter content of the universe. Now, the Swinburne team has used the ANU 2.3M telescope in Australia to map how gravitationally orbiting galaxies rotate. “Because we know how stars and gases come to orbit inside galaxies, we know roughly what a galaxy should be,” says Gurari. “After measuring how distorted real galaxy images are, we can figure out how much dark matter it will take to explain what we want.”

New research shows how this velocity information enables a more accurate measure of the lensing effect than is possible with the help of shapes alone. “With our new way of looking at dark matter, we hope to get a clearer picture of where the dark matter is, and what role it plays in how galaxies form,” says Gururi.

Future space missions, such as NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s Euclid Space Telescope, have made such measurements based on the shapes of millions of galaxies. Taylor adds, “We have shown that we can make a real contribution to these global efforts with relatively small telescopes built in the 1980s, just by thinking differently about the problem.”