Dark matter is required to explain the orbits of stars in galaxies, galaxies in clusters and the detailed structure of the cosmic microwave background. As far as we know, dark matter is made of subatomic particles that do not shine, and do not absorb, reflect or scatter light, or indeed any form of electromagnetic radiation, in any way. So is it possible to “see” dark matter? Not directly, but because dark matter has mass, it interacts with the rest of the Universe through gravity. There is a way to “see” dark matter using the phenomenon called gravitational lensing in which the path of a light ray is bent by gravity. Indeed, Einstein predicted the strength of this effect according to the General theory of Relativity, and this was later confirmed by Eddington after the solar eclipse in 1919.
Prof. Mike Hudson and his research team in the Physics & Astronomy Department at the University of Waterloo have used the gravitational lensing technique to generate maps of dark matter in the cosmos. The images of distant background galaxies are distorted as the rays of light they emit pass by a massive object on the way from the far reaches of the Universe to our telescopes on Earth. Of course, we do not know what an individual galaxy looked like before the distorting effect of gravitational lensing, but we do know that there is no special direction in the Universe: in the absence of lensing, galaxy orientations should be random. By measuring the average orientation of many galaxies, he and his group turn the problem around and use the phenomenon of gravitational lensing to map the dark matter.
For many years computer simulations of the formation of large-scale structures in the Universe have predicted that dark matter clumps into halos and that these halos are connected to each other by filaments making the fabric of the so-called cosmic web. The filaments had never before been mapped through observations. In 2017, Hudson and his graduate student Seth Epps analysed data from a multi-year survey conducted at the Canada-France-Hawaii Telescope. Through careful modelling of the shapes of distant galaxies, they were able to use gravitational lensing to map out the structure of cosmic web filaments for the first time. While the gravitational lensing effect was too weak to image individual filaments, by combining the lensing effect from 20,000 pairs of galaxy haloes, they found that, the galaxy halos were connected by filamentary structures. The filaments studied by Hudson’s group are truly enormous in length, spanning 40 million light-years, and weigh over 1013 times the mass of the Sun.
Dark matter filaments (shown in red) bridge the space between galaxy halos (shown in white) on this false colour composite map.
In January 2020, Hudson, graduate student Tianyi Yang and Prof. Niayesh Afshordi have approached cosmic web filaments from a new angle, answering the question: are the filaments completely dark? They carefully studied the light in galaxies in the same regions of space as the filaments and found that, while the filaments are mostly dark matter, they also contain a sprinkling of galaxies: stars in those galaxies make up less than 1% of the mass of the filaments.
Hudson is looking forward to extending and improving these maps using new data from the much larger Ultraviolet Near Infrared Optical Northern Survey (UNIONS), in which he is leading an international team of scientists specializing in weak lensing. After UNIONS, will come the much-anticipated launch of the billion-dollar Euclid satellite mission scheduled for 2022. Euclid will study the whole sky from the pristine vantage point of outer space where there is no blurring of the atmosphere – a major headache for ground-based telescopes. The future is bright for mapping dark matter in the cosmos -- and the Physics and Astronomy Department at the University of Waterloo is poised to shine new light on this long-standing puzzle.
