What is special about dark matter

"A new candidate for dark matter"

As early as the 1930s, astronomers realized that numerous phenomena in space could not be explained with the matter known to us. Since then, researchers have been looking for an invisible form of matter that is mainly noticeable through gravity. But what this dark matter could consist of is still unclear. Now Hermann Nicolai from the Max Planck Institute for Gravitational Physics and Krzysztof Meissner from the University of Warsaw proposed a new candidate for a dark matter particle. In an interview with Welt der Physik, Hermann Nicolai explains what special properties this so-called Gravitino has and how it can possibly be proven.

World of physics: why do physicists assume that there is dark matter?

Hermann Nicolai: If you look at the movement of galaxies in galaxy clusters, for example, you can see that the galaxies are much faster than expected. Because if there were only ordinary matter - also called baryonic matter - the galaxies should actually fly apart. From this, researchers conclude that there must be a lot more matter in the universe that we have not yet been able to observe - dark matter. This assumption has been confirmed again and again by various astronomical observations.

What do we know about dark matter so far?

It is usually assumed that dark matter is a particle that interacts with ordinary matter mainly via gravity. It is still uncertain whether the dark matter particles interact with the baryonic matter in any other way. Scientists have been looking for theoretical models that describe both ordinary matter and dark matter particles for many years. And meanwhile some hypothetical particles have already been proposed for it.

Can you give a few examples?

A possible candidate is the so-called axion, which, in addition to gravitation, also interacts via electromagnetic force. However, not much is known about this hypothetical particle, not even exactly what mass an axion has or how strongly it interacts. Heavier candidates like the “Weakly Interacting Massive Particle” - translated: weakly interacting massive particle - or WIMP for short are also in the running. WIMPs also interact with ordinary matter via the weak force. Like neutrinos, they are therefore very difficult to detect. So trillions of these particles could fly through the human body every second without being noticed.

Do you try to prove these particles anyway?

Krzysztof Meissner and Hermann Nicolai

For example, researchers are trying to detect WIMPs with the XENON1T detector in the Gran Sasso underground laboratory - but so far in vain. Physicists at the CERN research center also want to use accelerator experiments to generate and measure dark matter particles. So far, no suitable particle has been found there either. In addition, attempts are being made to use telescopes to observe traces of processes in space in which dark matter was involved. So far this search has also been unsuccessful. It looks back on a thirty-year history of experiments. But the researchers agree that the dark matter theory can only be validated if a dark matter particle can be observed.

How do you intend to solve the mystery of dark matter?

We propose a completely new candidate for a dark matter particle. Originally, we wanted to develop a unified theory to explain the structure and build of the Standard Model of particle physics. In doing so, we used an approach that requires another besides the baryonic particles - the so-called gravitino. And with their special properties, these Gravitinos are compatible with observations on dark matter. Our new candidate is extremely difficult and - in contrast to the candidates discussed so far - would even interact with ordinary matter via the strong and electromagnetic force.

And why has no Gravitino been observed so far?

We know from simulations how much dark matter there must be in our universe. From this, a density of dark matter particles can be predicted. For comparison: if dark matter consisted of protons, for example, you would need about one proton per cubic centimeter to provide the required mass of dark matter. However, since the Gravitino is much heavier than a proton, there would only be about one particle per ten thousand cubic kilometers. Thus, Gravitinos would appear much less often than the easier candidates and that makes it more difficult to observe them.

How could a Gravitino possibly be proven anyway?

One possibility would be to search for gravitinos with detectors deep underground on our earth. The gravitinos postulated by us do interact with matter, but due to their high mass - unlike most particles - penetrate the earth undisturbed. But other particles, such as muons, also penetrate underground. But due to their special properties, the Gravitinos can be distinguished from the other particles. However, Gravitinos are so rare that you would probably have to wait a long time for a signal.

What would be an alternative to underground measurements?

Our earth has been flying through the solar system for about 4.5 billion years. During this time, some Gravitinos may have flown through the earth and may have left traces. So one could go on a kind of fossil search in which our earth serves as a natural detector. Perhaps Gravitinos have left a trace in otherwise very stable and long-lasting crystal structures such as the British crown diamond. This is very speculative, but it might be worth taking a look.