Scientists have known for decades that galaxies across the universe are tied together by an elusive cosmic web of dark matter. The structures are massive, stretching over millions of light-years but despite the huge scale involved, imaging the dark matter filaments has proven challenging. Finally, using modern tools and techniques researchers have made the first composite image of a dark matter filament.
“This image moves us beyond predictions to something we can see and measure,” said Mike Hudson, a professor of astronomy at the University of Waterloo in Canada, co-author of the study published in the Monthly Notices of the Royal Astronomical Society.
As its name implies, dark matter is an elusive substance that's invisible to direct observations because it doesn't interact with light in any way. While dark matter is practically invisible, most scientists agree it exists because it exerts a gravitational tug and can warp the light of distant galaxies, a phenomenon called gravitational lensing. Nothing other than dark matter can explain these observations. In fact, it's so abundant that estimates suggest dark matter makes up around 27 percent of the universe.
Hudson and colleagues like Seth Epps, at the time a master's student at the University of Waterloo, measured light that was slightly bent as its path traveled along mass. This technique called weak gravitational lensing is often used by astronomers to image galaxies that appear slightly warped in the presence of cosmic masses, like dark matter. This effect was measured for the first time in 1919 during a solar eclipse, showing that the apparent positions of stars on the sky change due to light deflection by the Sun's gravitational field.
This very powerful statistical framework enabled the researchers to 'not only see that these dark matter filaments in the universe exist', but also 'see the extent to which these filaments connect galaxies together', the researchers wrote.
Weak lensing signals were studied for two massive data sets: a catalog of galaxy cluster pairs and a catalog of background source galaxies whose distance measurements were vetted. The analysis included a map of luminous red galaxies (LRGs) which are some of the most distant and oldest galaxies in the universe. They're also very bright galaxies and astronomers expected the dark matter bridge or filament between them to be more massive than the average.
Overall, some 23,000 galaxy pairs all of which are located 4.5 billion light-years away were combined to create a composite map showing the presence of dark matter in between the galaxies. Essentially, the dark matter bridge we're seeing in the image is the average of all 23,000 pairs.
According to the map, the dark matter bridge is the strongest between pairs less than 40 million light years apart.
“By using this technique, we’re not only able to see that these dark matter filaments in the universe exist, we’re able to see the extent to which these filaments connect galaxies together,” said Epps.
The stacking technique developed by Hudson and Epps will likely be used to measure filaments between other groups of clusters of galaxies. Though still mysterious, such observations might one day lift the fog around dark matter.