2023-06-30 14:41:25
To be honest, no one knows what dark matter is. All that is certain is that it is written with a capital D and that it is there. Exactly ninety years ago, this was noticed by an astronomer observing galaxy clusters. In a sense, these are the largest objects in the universe: clusters of hundreds to thousands of galaxy-like star clouds dancing around each other in a common gravitational field.
Ulf von Rauchhaupt
Editor in the “Science” section of the Frankfurter Allgemeine Sunday newspaper.
The problem: you dance too fast. At their speed, the gravitational field that whirls them around must be stronger than that generated by the galaxies themselves with their stars and gas clouds – at least when gravity works the way it is thought to work today. If this is true, then among the galaxies in a galaxy cluster there must be a clump of matter that has a gravitational field but does not emit, reflect, or even swallow light. It is completely dark and transparent. That’s why the researchers called it dark matter.
Dark energy is the bigger problem
In the meantime, the researchers have other findings that can only be satisfactorily explained by such dark matter. Accordingly, there is around five times more dark matter in the universe than normal material, i.e. such that is composed of atoms. Because there is so much, dark matter plays a major role in the processes that have produced the cosmos we see today. The European Space Agency ESA is therefore sending a new space telescope into space on July 1st. “Euclid” is intended to determine more precisely than has previously been possible how dark matter is distributed over a large area in space. At the same time, the physicists are also hoping for new clues as to what dark matter is all about.
Our column “How do I explain it to my child?”: Image: FAZ
One could imagine that dark matter simply consists of objects that do not glow or are illuminated, otherwise one could see them, but which are nevertheless accumulations of normal atoms, perhaps interstellar rocks. But physicists can now rule out this idea, at least for the largest amount of dark matter. It must be something completely different.
One possibility is that there are myriad black holes. These objects are too heavy for their size to allow even light to escape from their gravitational field. This is theoretically to be expected, and it has already been observed in many places. Very heavy stars collapse into black holes when they run out of fuel. But in the quantities that would be necessary to explain dark matter with it, they could only occur if they were remnants of the Big Bang.
Most researchers consider it more likely that a previously unknown type of elementary particle forms dark matter. They already have a name: “axions” is what the researchers call the particles they have only assumed so far, which, if they exist, must be very light. An axion would be around a hundred billion times lighter than an electron. However, very heavy particles could also be considered, which nevertheless have hardly any effect on light or atoms. A theory that has long been very popular with elementary particle physicists had predicted exactly such particles, but despite decades of effort, no such “wimps”, as they are also called, have been detected.
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Dark matter is thus confronting physicists with one of their greatest mysteries today. Actually, only that of the “dark energy” is bigger, again with a capital D, about which “Euclid” is also supposed to find out more. It also fills the observable cosmos, but completely evenly. Dark energy is, so to speak, a property of empty space, but with the effect of inflating this space and thus constantly multiplying itself. It therefore makes itself felt by further accelerating the expansion of space that has been ongoing since the Big Bang. Ever since astronomers discovered this some 25 years ago, dark matter has been by far the smaller problem.
Even more answers to curious children’s questions
An illustrated selection of articles from our column “How do I explain it to my child?” has been published by Reclam.
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