Speed ​​of dark matter measured for the first time

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Dark matter can only be observed indirectly. A collision between two large galaxy clusters now shows the speed it reaches in the universe.

Pasadena (U.S.A.). Dark matter can currently only be observed indirectly, for example via the movement of stars and galaxies. According to Emily Silich from the California Institute of Technology (Caltech), large galaxy clusters are particularly suitable for this because they consist of up to 90 percent dark matter. When collisions occur within the galaxy clusters, they reveal how the visible matter and the invisible dark matter interact with each other.

“Mergers between such massive galaxy clusters are a particularly sensitive test case for cosmological models.”

Researchers of the Caltech have now measured for the first time the speed at which dark matter reaches when two galaxy clusters collide. According to their publication in The Astrophysical Journal They determined this in the galaxy clusters MACS J0018.5+1626, both of which contain thousands of galaxies and are located several billion light years away from Earth. In contrast to most known galaxy clusters, which move in space perpendicular to our position, the merger in the galaxy cluster MACS J0018.5+1626 takes place in the direction of our position. According to Jack Sayers, the speed of dark matter can therefore be measured well.

“With Bullet Cluster and others, we are essentially sitting in the stands of a car race and can therefore take beautiful snapshots of the racing cars as they overtake each other on the home straight. In our case, however, we are standing right at the edge of the track and see the racing cars racing towards us. We can therefore measure their speed as if with a radar gun.”

Spectral observation data from telescopes

To determine the speed of dark matter, the astronomers used spectral observation data from telescopes. Objects approaching the Earth produce spectral shifts in the blue wavelength range and objects moving away from the Earth produce spectral shifts in the red wavelength range. The researchers also used the Sunyaev-Zel’dovich effect, which occurs when photons of the cosmic background radiation are scattered by the gas of galaxy clusters. The resulting scattering allows conclusions to be drawn about the speed of the gas in the galaxy clusters.

Speed ​​of dark matter

According to the observational data, when the two large galaxy clusters collide, the dark matter and the visible gases do not move at the same speed, but rather decouple from each other. Both galaxy clusters initially move towards each other at around 3000 km/h. Then the dark matter separates from the gases and reaches a much higher speed. Models show that the hot, visible gas in galaxy clusters takes several hundred million years to catch up with the dark matter.

Like Silich, the process is comparable to a collision between two trucks and sand. When they meet, the two trucks stop first and the loose sand flows between them.

“The dark matter is the sand and flies ahead.”

The significantly higher speed of dark matter arises because it has only minimal interaction with its environment. It is only influenced by gravity, while normal matter, such as hot gas masses, is also influenced by electromagnetism. In the observed collisions, electromagnetic forces caused turbulence that reduced the speed of the gases.

“It took us a long time to put all the pieces of the puzzle together. But now we finally know what is going on there. We now hope that this will pave a new way to study dark matter in galaxy clusters.”

The Astrophysical Journal, doi: 10.3847/1538-4357/ad3fb5