Non-gravitational dark matter

A recent study of 72 clusters of interacting galaxies provides a test to examine the non-gravitational forces acting on dark matter. The results contradict at least one of the major theories and place limits on the nature of the particles, excluding those similar to the proton. The results are published in Science.
Thanks to collaboration between the École Polytechnique Fédérale de Lausanne (EPFL) and the University of Edinburgh, a group of researchers studied 72 clusters of galaxies interacting to understand how dark matter behaves when they collide in the course of billions of year old. The findings, published in Science, contradict at least one of the major theories of this enigmatic component of the Universe.


Credit: NASA, ESA, the Hubble SM4 ERO Team and ST-ECF

Dark matter is one of the greatest mysteries of modern physics. It makes up most of the matter in the Universe and it is invisible and does not match anything else that is part of our experience. Different theories compete to find an explanation, but so far none have been successful. Although dark matter takes into account the 90% of all matter in the Universe, and more than a quarter of the content matter-energy, we still know very little about its origin and nature. One of the ideas more followed in the scientific community is that dark matter consists of a new subatomic particle that we have not discovered yet. Most extravagant and exotic theories suggest, however, that it is a kind of “quantum defect”, appeared since the birth of the Universe, or it can be a kind of extra-dimensional mass or even a modified form of gravity.
What we know for sure is that dark matter interacts with cosmic structures through the gravitational interaction, shaping them and giving them a certain form. We know, for example, that dark matter “fold” when the light passes through, distorting the images of distant objects according to an effect predicted by general relativity that is known as gravitational lensing. Furthermore, the dark matter can influence the motion of the galaxies of a cluster, consisting of 90% dark matter, and this makes them ideal laboratories to study in detail, especially when they begin to collide forcing the respective distributions of dark matter to interact.


Credit: NASA/Chandra X-ray Observatory

The research team, led by David Harvey EPFL’s Laboratory of Astrophysics, examined 72 clusters of galaxies interacting to find new clues about the nature of dark matter. These “galactic clashes” are very slow because they take place in the course of billions of years and begin when their gigantic masses begin to interact because of the mutual gravitational attraction. When that happens, the dark matter in each cluster interacts with that of the other, a fact which offers a unique opportunity to study these particular astrophysical phenomena.

Data were obtained as a result of a series of observations made with the Chandra Space Observatory and the Hubble Space Telescope. They include, inter alia, the so-called Bullet Cluster, an example of interaction between two clusters of galaxies where the gas has been modeled in the form of a “bullet”, hence the name, the best evidence on the existence of dark matter. The research objective was to measure the change of the speed of dark matter when two clusters collide. The experiments on Earth, such as those that are made at the Large Hadron Collider (LHC), suggest that when two subatomic particles interact they change their speed value. Therefore, taking into account what happens to dark matter after the interaction between the two clusters, astronomers may be able to derive some conclusions about its physical properties.
To verify the theory that the dark matter is constituted by particles, the authors have proposed two possible scenarios: in the first, it is assumed that the dark matter particles interact so frequently but suffer less speed variation; in the second, it is assumed that they interact rarely undergoing a greater speed variation.


Credit: D. Harvey et al. 2015/Science

In the first case, the dark matter should slow down immediately after the collision, since the frequent interactions between the particles that cause an additional “resistance”. In the second scenario, the dark matter could be widespread during the interaction and thus be dispersed in space. However, and to our surprise, the scientists found that the dark matter in a cluster passes through undisturbed, and without interaction, the dark matter of another cluster. This implies that the particles do not interact with themselves otherwise the process would result in a slowdown of the motion associated with dark matter. In other words, the dark matter could interact in a “non-gravitational” manner with ordinary matter, that is visible, a process which does not happen when it interacts with itself instead.
This study seems to contradict the thought that the dark matter consists of particles similar to the protons, or perhaps some other similar type. “We believe that the probability of interaction of two dark matter particles is less than that relating to the case of two protons. This implies that dark matter is made up of ‘dark protons’ as if this were true we would expect to see them ‘bounce’ to each other, “says Harvey. Finally, according to the authors, further research on galactic collisions should allow in the future to reveal new clues, in general, the distribution and the presence of dark matter in the Universe.

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