Mike Hudson

Mike Hudson
Professor, Faculty of Science

Waterloo scientists have created the first composite image of the cosmic web of filaments that link galaxies.

The filaments are mostly composed of dark matter, a mysterious substance that doesn’t shine, absorb or reflect light and comprises about 25 per cent of the universe. Although it can’t be seen, dark matter has a gravitational pull that affects the path of light, causing the images of distant galaxies to warp.

Mike Hudson, a professor in the Faculty of Science, and Seth Epps, a former Waterloo master’s student, created the image using a technique called weak gravitational lensing.

Image of cosmic web confirms researchers’ predictions

“For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together,” says Hudson, a professor in the Department of Physics and Astronomy. “This image moves us beyond predictions to something we can see and measure.”

Weak gravitational lensing involves taking a picture of that distorted light, measuring the amount of distortion, and from the those measurements making an image or map of the dark matter that is responsible for them.

Hudson and Epps combined signals from more than 23,000 galaxy pairs, located 4.5 billion light years away, which allowed them to estimate not only the location but also the mass of the filaments they imaged.

Cosmic web has been unobservable until now

Their findings confirm predictions that galaxies across the universe aren’t isolated like islands in space, but are linked together through a web of enormous filaments that connect the dark matter haloes that surround each galaxy – a web which has been unobservable until now.

While Hudson and Epps’ main goal was to produce an image of these low-density filaments, the team was also aiming to provide a foundation for future studies. Hudson and Epps hope the method they developed to produce the composite image can be used to examine other weak-lensing data sets, with the potential to analyze large data sets from upcoming surveys: for example, space-based data-collection missions such as the European Space Agency’s Euclid telescope.

Hudson and Epps’s data came from a multi-year sky survey at the Canada-France-Hawaii Telescope, where another major survey has just begun. Both are being developed to further investigate dark matter’s role in the universe.

Feature image: The first image of the cosmic web, which is mostly dark matter, that links galaxies. Credit: Mike Hudson and Seth Epps, a former Waterloo master's student.