How did dark matter come about?

Back to overviewPrintable version

Weight problems:
Physicists on the trail of dark matter
Astronomers and cosmologists do not know where 90 percent of the mass in the universe comes from - a fairly substantial amount of ignorance that would make any student rattle through their exams. But the perplexity offers the opportunity to consider exotic solutions to the problem. Some of them were created in collaboration with colleagues from particle physics. For example, heavy particles like the WIMPs could be responsible for the weight problem. The AMANDA experiment at DESY is also on the lookout for these exotic species.

How do you know about matter that cannot be seen?
The universe is big and therefore certainly heavy. But how do you measure your mass? Or better: How do you get the idea that it should be heavier than everything you see? You don't just put stars on a scale, insert a coin and read the result. Rather, indirect methods for long-distance weight measurement are in demand.

And these methods suggest so impressively that galaxies are surrounded by enormous accumulations of dark (non-luminous) matter that only a few are left Scientists doubt it.
One of these methods concerns the rotation of galaxies: galaxies are constantly in motion, revolving around themselves. Otherwise they would collapse due to gravity. However, as a result of the rotation, the stars are pushed outwards like in a carousel, which keeps them on their way. Our solar system also retains its shape in this way.

It is noticeable, however, that the further inside the planets are, the faster they revolve around the sun. The earth is around three times as fast as Saturn. Measurements on galaxies, on the other hand, show that the speed distribution as in the solar system no longer applies to the star clusters. It can be seen here that the stars are all almost identical Speed ​​- no matter how far you are from the center.

Such a speed distribution could be achieved in our planetary system if there is also a lot of matter between the planets. Then Earth and Saturn could be on the move at the same time. The same argument now points to the existence of dark matter: All galaxies are enveloped by a spherical, massive accumulation of dark matter.

What is dark matter made of?
Physicists have identified numerous candidates who could contribute to dark matter: stardust, for example, or brown dwarfs. These are suns that are too light for nuclear fusion to begin. Even stars that have already died cannot be seen. There are also far heavier candidates, such as the black holes, which - the name suggests - are now also quite invisible. These dark matter contenders are a matter for astronomers. But there is another class of "dark" objects that fall into the field of particle physics: WIMPs.

WIMPs
WIMP stands for "weakly interacting massive particle", it has never been observed and it does not appear in the standard model of particle physics.

That’s actually a horse’s foot to begin with. Because with this model it concerns the state of affairs when it comes to the smallest particles and their interactions. It is considered unbeaten so far. But physicists still believe that it cannot be the last word in wisdom. They looked for extensions and found one in something called supersymmetry. According to this, there is a supersymmetric partner for every particle, which is usually much heavier (!).

Some of the particles of the Standard Model. WIMPs are not (yet) among them.

WIMPs are now the supersymmetric partners of the neutrinos. Even the neutrinos are extremely shy particles that almost never interact with other matter. Not least because of this, it took until the summer of 2000 that the last neutrino (tau-nü) could be directly detected for the first time. This would actually make the neutrino a perfect candidate for dark matter. Especially since there are innumerable many of them. But unfortunately neutrinos are far too light and may even weigh nothing - a question that has still not been finally clarified.

The situation is different with the supersymmetric partners of the neutrinos. According to calculations, these particles could be 50 to a few hundred times heavier than the proton. They would weigh as much as heavy atoms, gold for example.

Italian physicists believe they have found the first signs of WIMPs. The AMANDA neutrino telescope from DESY in the Eternal Ice of the South Pole is also looking for the still hypothetical particles. The researchers do not concentrate on the cosmos. Rather, it is also assumed that WIMPs are located deep inside the earth. The final proof has not yet been given. WIMPs shy away from confrontation, as the English word "wimp" means "wimp".