Even though astronomers have talked about it for decades, researchers at the University of Waterloo in Waterloo, Ontario, claim to have seen—and in fact, detected—something for the first time in a composite image. They claim that it is a representation of dark matter, a node in the vast cosmic web that is thought to contain the billions of galaxies that make up our universe. The composite image, which combines a number of individual images, confirms predictions that galaxies throughout the universe are tied together through a cosmic web connected by dark matter that has up until now remained unobservable, according to a statement released on April 12, 2017, by the Royal Astronomical Society, which published the new work in its peer-reviewed Monthly Notices.
In this false-color image, dark matter filaments connect the galaxies. The white sections indicate the locations of luminous galaxies, and the red areas indicate the presence of a dark matter filament that connects the galaxies. RAS/S. Epps & M. Hudson/University of Waterloo, source of the image.
Why do astronomers believe in dark matter? After all, no one has ever claimed to have spotted it directly before this image, much less to have taken a picture of it. Although it is a crucial component of the Lambda Cold Dark Matter model, also known as the standard model of Big Bang cosmology, dark matter nonetheless has a respected place in astronomical theory – a widely accepted model of how our universe works and a model that agrees well with what astronomers believe they see when they look out into deep space.
Some astronomers think we don’t understand dark matter, or think it doesn’t exist at all. For example, in 2016, physicist Erik Verlinde at the University of Amsterdam released the latest installment of his new theory of gravity, in which he said he doesn’t need dark matter to explain the motions of stars in galaxies. Not long afterward, a team led by astronomer Margot Brouwer of Leiden Observatory in The Netherlands tested Verlinde’s theory by examining the lensing effect of gravity around more than 33,000 galaxies. Her team concluded that Verlinde’s theory “agrees well” with the observations. That sort of one-two punch is often seen in astronomy. A theory suggests something, and observations bear it out (or not). Of course, the theories and observations are always flawed and imperfect. What else could they be? Scientists would have to be gods to comprehend the workings of the universe perfectly.
And so what you’re likely to see in the weeks and months and years ahead are other astronomers either agreeing or disagreeing that this image represents what researchers at the University of Waterloo say it represents.
According to the most accepted theories of the universe, dark energy is thought to contribute 73 percent of all the mass and energy in the universe. Another 23 percent is dark matter, which leaves only 4 percent of the universe composed of regular matter, such as stars, planets, and people. Pie chart via NASA.
In the meantime, know that – according to the most popular models of the universe – dark matter comprises about a quarter of the “stuff” of our universe. This mysterious substance doesn’t shine, absorb or reflect light, although its effects are thought to be recognizable via the workings of gravity. According to these theories, dark matter is integral in creating what astronomers call the cosmic web, the basic structure of our universe. This great web, in fact, is thought to consist of a network of filaments of dark matter. Mike Hudson, the University of Waterloo astronomy professor who led this research said of his team’s work: For decades, researchers have been predicting the existence of dark-matter filaments between galaxies that act like a web-like superstructure connecting galaxies together. This image moves us beyond predictions to something we can see and measure.
This often-seen image is an artist’s illustration of a honeycomb-like structure, sometimes called the “cosmic web.” The bright areas are galaxy clusters and groups, with sparsely populated regions devoid of galaxies in between. Since astronomers nowadays believe that galaxies are made in a process whereby dark matter merges and clumps together, and since dark matter is thought to be so much more abundant in our universe than the sort of matter that composes stars and galaxies (and planets like Earth, and people like you and me), dark matter is what is thought to drive the cosmic web’s structure. Image via Volker Springel, Virgo Consortium.
How did the astronomers at the University of Waterloo get their picture of dark matter? A process known as weak gravitational lensing was used by Hudson and co-author Seth Epps, who was a master’s student at the University of Waterloo at the time. This effect causes images of distant galaxies to slightly warp due to the influence of an unseen mass, such as a planet, a black hole, or in this case, dark matter, according to these researchers. The Canada-France-Hawaii Telescope on Mauna Kea in Hawaii is where they claimed to have measured the effect in photos from a multi-year sky survey. They combined lensing images from more than 23,000 galaxy pairs located 4.5 billion light-years away to create this composite image or map, which, they say, shows the presence of dark matter between the two galaxies. In other words, it’s a dark matter bridge, according to these astronomers. Their results suggest the dark matter filament bridge is strongest between systems less than 40 million light-years apart. Epps said:
We are able to see the extent to which these dark matter filaments connect galaxies by employing this technique, which also allows us to confirm the existence of these filaments of dark matter in the universe. Let’s now check whether other astronomers can confirm their findings and if others concur. If they do, yeah, that’s incredible!