He is called a dark matter detective, hired and dispatched by the European Space Agency (ESA). The Euclid space telescope will scan no less than a third of the cosmic sky over a period of six years. His assignment? Spying on a mysterious type of matter that is as ubiquitous in the universe as it is invisible. Yet we know that so-called dark matter is more common in some places than elsewhere. For example, in the Perseus cluster, a group of a thousand galaxies held together by their mutual gravity. Excuse me, due to the attraction of the enormous amount of dark matter hiding there.
“Dark matter forms the basic structure of the material universe,” says Henk Hoekstra, professor of observational cosmology at Leiden University and closely involved in the Euclid mission. ‘Without dark matter, galaxies would not have formed, and we would not exist.’ However, dark matter may be fundamental, it is invisible and elusive and therefore one of the great mysteries in the universe. It is estimated to make up 85 percent of all matter, leaving only 15 percent for the ordinary matter that makes up all stars, planets and other celestial bodies. Yes, also black holes. Compared to that mysterious matter, even they are ordinary.
Yet dark matter is not completely elusive – logical, otherwise we would not know about its existence. Just like normal matter, it is sensitive to gravity, with which masses attract each other. Already in the first half of the last century, cosmologists discovered that galaxies must contain much more matter than can be observed. After all, most galaxies are spinning around. For example, our solar system is zooming around the center of the Milky Way at more than 800,000 km/h. In addition, the gravity of the visible matter in galaxies is not enough to compensate for the centrifugal forces and hold everything together. So there must be something like dark matter, which with its extra attraction prevents galaxies from flying apart.
However, not much more is known about dark matter. For example, it is not known what type of particles it consists of. That frustrates cosmologists and physicists, one of the reasons why the ESA launched Euclid last summer. The space telescope has been in place for several weeks, one and a half million kilometers away (near the James Webb telescope). On Tuesday, the ESA released a first selection of images of Euclid, showing that the dark matter detective is ready to start its assignment.
One of Euclid’s assets is that he looks at the cosmic sky with both a very broad and sharp view. Take the photo of the Perseus cluster, the ‘system of galaxies’ that, at 240 million light years, is not that far away from us. It took just five hours to create this statue. The cluster’s thousand galaxies can be recognized by their sharply defined, oval shape, often with a white or yellow halo of light surrounding them. But there are many more galaxies in the photo, more than a hundred thousand. From our perspective they lie behind the Perseus cluster, some many billions of light years. They can still be seen in the photo as very weak, red points of light.
Looking back in time
Clusters like Perseus help Euclid spy dark matter. Such structures can only arise in places where dark matter has accumulated. This involves a lot of dark matter, so much that the light from the underlying galaxies that passes through it is bent (because the gravity of the dark matter curves space) and the image we see of the galaxies becomes distorted. Cosmologists call this effect a gravitational lens. It allows them to study dark matter in clusters and determine how it is distributed over them, for example. By combining observations from many clusters, cosmologists can then get a picture of the distribution of dark matter across the entire universe. Conveniently, they can also look back in time: the further the galaxies are behind the gravitational lens of the cluster, the longer ago they radiated their light.
What can we expect from Euclid when his assignment ends at the end of this decade? Completely clearing up the mystery of dark matter may be a bit much to ask. By then, cosmologists want to know how dark matter is distributed across the universe. “We think that dark matter forms a web of diffuse threads that come together in nodes,” says Hoekstra. ‘There is more dark matter in those places and clusters such as Perseus could arise. With Euclid we want to map that web of dark matter.’