Dark matter seems to interact with itself

Dark matter may not just be sensitive to gravity. It also appears to interact with itself in other ways, according to observations of colliding galaxies from ESO’s Very Large Telescope and the NASA-ESA Hubble Telescope.

The ESO writes this on its website on Wednesday. Using the Very Large Telescope in Chile and images from the Hubble telescope, a team of astronomers examined a simultaneous collision between four galaxies in the cluster Abell 3827. The researchers were able to trace where the mass is in this system and then the distribution of the masses. compare dark matter with the positions of the bright galaxies.

The team was able to determine the location of the dark matter using what is known as a gravitational lensing technique. Coincidentally, the collision happened right in front of a much more distant, unrelated object. The mass of dark matter around the colliding galaxies distorts spacetime, bending light rays from the distant background galaxy in various ways. The image of the system is thus distorted into characteristic arcs.

ESO explains on its site that according to current understanding, all galaxies are surrounded by a shell of dark matter. The binding effect of the gravity exerted by dark matter keeps galaxies from falling apart due to their rotation. That this does not happen can only be explained if 85 percent of all mass in the universe consists of dark matter, ESO writes. The remaining 15 percent of the mass is made up of ordinary matter. The total amount of mass energy, ie all mass plus energy, in the universe consists of 68 percent dark energy, 27 percent dark matter, and 5 percent ordinary matter. So that 32 percent of the mass-energy makes up 100 percent of the mass in the universe. But what that matter consists of is still a mystery.

The shell of one of the four colliding galaxies in Abell 3827 is lagging behind the galaxy it encloses. The gap is currently 5000 light years, or 50 quadrillion kilometres. Such a lag between a galaxy and its associated dark matter probably arises when the dark matter influences itself by forces other than gravity. Dark matter has never before been observed to interact other than through gravity.

However, caution is still warranted when drawing conclusions. It remains to be investigated what other effects could cause this delay. More observations and computer simulations of similar events are also needed.

Other recent results from the same research team on 72 collisions between galaxy clusters showed that dark matter has virtually no interactions with itself. ESO makes it clear that this research does not concern the clusters as a whole, but the movements of individual galaxies. The researchers themselves say the collisions between these galaxies in the new study may have lasted even longer than the collisions observed in the older study. The collision then lasted so long that even a very small force could cause measurable deceleration.

Both studies thus set limits on the behavior of dark matter, namely that dark matter interactions should be stronger than in the previous observations, but weaker than in the latest observations.

Meanwhile, researchers at CERN are also looking for dark matter and its appearance in the second run of the Large Hadron Collider.