Astronomers studying the collision of four galaxy clusters have come across an anomaly. Dark matter appears to be interacting with itself through the medium of some previously unidentified force, which isn’t gravity and which doesn’t interact with normal matter.
Dark matter has never been seen, because it doesn’t absorb, emit or reflect light and, therefore, it can’t be definitively identified.
However, astronomers have calculated the “weight” of many galaxies, and, in all cases, galaxy clusters have been found to have huge amounts more mass or matter than can be accounted for.
Furthermore, all matter has gravity, but it has been calculated that the gravitational power of the visible matter in galaxies is insufficient to hold galaxies them together.
This missing weight and extra gravitational force is accounted for by dark matter, which, together with dark energy, is thought to make up more than 80 percent of the universe.
All galaxies and galaxy clusters are thought to be surrounded by dark matter, which holds them together through its powerful gravitational effect. It is this effect, which can be identified, even if dark matter can’t.
This effect can be seen, because the gravity from dark matter can bend light like normal matter. This is called gravitational lensing and causes light to curve, so we see arcs of light or double images. Since dark matter has the same effect, we can deduce that it is present.
In this case, astronomers were observing collisions between different galaxy clusters and measuring the amount of dark matter involved. Phys org says, that from analyzing the gravitational lensing, the researchers were able to “see” a particular dark matter clump because of the distorting effect its mass had on the light from background galaxies.
In doing so, they discovered that this dark matter clump was “lagging behind” one of the galaxies, while it was heading for collision with the others. The dark matter had become separated from its component stars and planets by about 5,000 light-years.
Since, this couldn’t be accounted for by the relationship between the gravitational force of the ordinary matter in the galaxy and the dark matter surrounding it — some other force had to be interacting with the clump.
The scientists believe that the dark matter from another galaxy is pulling it away or slowing it down. This would mean that dark matter is interacting with itself and some unknown force other than gravity must be the conduit for this interaction. This has never been observed before, and, if it is true, current theories of dark matter will have to be completely revised.
Scientific American explains that, until now dark matter has been thought of as a single particle. But, if it is interacting with itself, there must be different dark matter particles and a “dark force”, which is responsible for their interaction.
This would mirror our understanding of normal matter. For example, protons interact through the electromagnetic forces carried by particles called photons, which are particles of light. When they come into proximity, two protons will be repelled from one another by emitting a photon that is absorbed by the other.
But, since dark matter doesn’t interact with the electromagnetic force, there has to be some new dark force carried by dark-photons or dark boson particles, which acts in a similar way on dark matter particles as normal photons do on protons.
Consequently, the dark matter from other galaxies would be repulsing the dark matter from the other through the conduit of some unknown, new dark force. This would contradict the standard theory of Cold Dark Matter, where dark matter interacts only with gravity.
“This result, if confirmed, could upend our understanding of dark matter,” said Don Lincoln of the Fermi National Accelerator Laboratory.
Quoted in ExtremeTech, Liliya Williams of the University of Minnesota, a member of the team on the project said;
“We know that dark matter exists because of the way that it interacts gravitationally, helping to shape the Universe, but we still know embarrassingly little about what dark matter actually is. Our observation suggests that dark matter might interact with forces other than gravity, meaning we could rule out some key theories about what dark matter might be.”
The European Southern Observatory website also quotes the lead author of the study, Richard Massey at Durham University, UK, who explained:
“We used to think that dark matter just sits around, minding its own business, except for its gravitational pull. But if dark matter were being slowed down during this collision, it could be the first evidence for rich physics in the dark sector — the hidden Universe all around us.”
The researchers used the MUSE (Multi Unit Spectroscopic Explorer) instrument on the Very Large Telescope in Chile, along with the Hubble Space Telescope to examine the Abell 3827 cluster, and their findings were published in Monthly Notices of the Royal Astronomical Society.
More investigations into other galaxies are underway to see if the theory can be confirmed and it may lead us to finally answering the puzzle of what dark matter really is.
Dr Massey said that;
“We are finally homing in dark matter from above and below – squeezing our knowledge from two directions.
“Dark matter, we’re coming for you.”
