Up to 85% of the mass in the universe is effectively invisible. Unlike ordinary matter, this dark matter does not interact with light, thus can not be observed directly.
So how do we measure what cannot be seen? The key is to measure the effect of gravity that the dark matter produces.
A small team of astronomers has found a new way to ‘see’ the elusive dark matter haloes. They come up with a technique that is ten times more precise than the previous best method.
Astronomers mainly focused on an effect called weak gravitational lensing. This effect is a feature of Einstein’s general theory of relativity.
Associate Professor Edward Taylor said, “The dark matter will very slightly distort the image of anything behind it. The effect is a bit like reading a newspaper through the base of a wine glass.”
The team has used the ANU 2.3m Telescope in Australia to map how gravitationally lensed galaxies are rotating.
Pol Gurri, the Ph.D. student at the Swinburne University of Technology who led the new research, explains, “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.”
The study has shown that this velocity information enables much more precise measurement of the lensing effect than is possible using shape alone. Through this new way of seeing dark matter, the team hopes to get a clearer view of where the dark matter is and what role it plays in how galaxies form.
Future space missions such as NASA’s Nancy Grace Roman Space Telescope and the European Space Agency’s Euclid Space Telescope are designed, in part, to make these kinds of measurements based on the shapes of hundreds of millions of galaxies.
Taylor said, “We have shown that we can make a real contribution to these global efforts with a relatively small telescope built in the 1980s, just by thinking about the problem differently.”
- Pol Gurri et al. The first shear measurements from weak precision lensing, Monthly Notices of the Royal Astronomical Society (2020). DOI: 10.1093/mnras/staa2893