Is the gravitational force repulsive for dark matter

Possible explanation for strange dark matter behavior found

Hamburg - For decades it has been one of the central puzzles in astrophysics: the so-called dark matter, of which there is supposed to be significantly more in the universe than conventional, visible matter, which only interacts with it via gravity. Their gravitational force is said to be of central importance for the formation of stars and galaxies - and ultimately also for our existence. So far, however, she has withdrawn from all attempts at proof.

But not only is the essence of dark matter a mystery - it also seems to have some properties that raise further questions. One of them concerns their uneven distribution - and an explanation for this could now have been found, as scientists from the German Electron Synchrotron (DESY) and the Austrian Academy of Sciences report.

The problem

Astronomers have found that dark matter doesn't seem to clump together as much as previous computer simulations suggest. If the only force through which dark matter interacts is gravity, then the density of dark matter in the center of galaxies should become very high. However, especially in the centers of small, weak galaxies, so-called dwarf spheroids, dark matter does not seem to be as dense as expected.

This behavior could be explained if dark matter is able to interact with itself. Whatever the nature of the dark matter units, they would collide with one another like billiard balls, so that they can be distributed more evenly after a collision. One problem with this idea, however, is that dark matter seems to condense in larger systems such as galaxy clusters, but not in small systems. The question is: Why does dark matter behave differently in dwarf galaxies and galaxy clusters?

Feedback factor

Feedback could be the answer to the question, reports an international team of scientists in the journal "Physical Review Letters". The Chinese physicist Xiaoyong Chu from the Austrian Academy of Sciences explains: "If particles of dark matter only scatter at a low, but very special speed, this can often occur in dwarf spheroids, where the particles move slowly, but rarely in galaxy clusters, where they are moving fast. They have to hit a resonance for a collision to occur. "

However, the team was not convinced from the start that such a simple theory would correctly describe previous measurement data. "At first we were a little skeptical as to whether this idea would explain the data from observations. But when we tried it, it worked out fantastic," says the Colombian researcher Camilo Garcia Cely from DESY.

For their description to work, the resonance must be very close to twice the mass of a dark matter particle. The researchers believe that it is no coincidence that dark matter hits just the right resonance energy for the collisions. "There are many other systems in nature that have similar coincidences: during the production of carbon in stars, for example, alpha particles encounter a resonance from beryllium, which in turn meets a resonance from carbon and thus produces the building blocks that make up all life of the earth, "says Garcia Cely.

"Separated at Birth"

"This behavior of dark matter can also be an indication that our world has more dimensions than we see. When a particle moves in such extra dimensions, it has a certain kinetic energy. We who do not see the additional dimensions , observe the energy as mass, thanks to Einstein's formula E = mc2 - energy and mass are equivalent. Perhaps a particle moves twice as fast in the additional dimension, so that its mass is exactly twice as large as the mass of dark matter, "says Chu.

In the next step, the research team wants to support the theory with observational data. A more detailed observation of various galaxies could show that the scattering of dark matter actually depends on its speed. Hitoshi Murayama of the University of California Berkeley describes the drive for the search for the mysterious physical quantity as follows: "Dark matter is in a way our mother who gave birth to us all. But we never got to know her; somehow we became Separated at birth. Who is she? That is the question we are investigating. " (red, February 27, 2019)