Astronomers believe that they may have, just maybe, observed dark matter interacting with a force other than gravity for the first time. If true, it would mean that dark matter isn’t as completely dark as previously thought.
Dark matter is thought to make up about 85 percent of the mass of the universe. Because it does not absorb or reflect light however, it has only been observable through its gravitational effect on objects.
Now, an international team of scientists led by Dr Richard Massey, Royal Society Research Fellow with the Institute for Computational Cosmology at Durham College, UK believe they have found evidence of dark matter being influenced by other forces.
Last month, Massey’s group published observations stating that dark matter interacted very little, even with other dark matter. The team had studied images showing 72 collisions between galaxy clusters, each containing as many as 1,000 galaxies, and found no observable reaction even when dark matter collided with other clusters of dark matter.
However, using the Hubble Space Telescope to study the simultaneous collision of four galaxies at the center of a galactic cluster, the researchers have observed an apparent dark matter reaction.
In research published in the journal Monthly Notices of the Royal Astronomical Society, the authors say that one clump of dark matter appeared to lag behind the galaxy it surrounds.
According to the researchers, the clump was offset from it’s galaxy by 5,000 light years. Such a reaction is only predicted if dark matter has interacted with a force other than gravity.
In computer simulations such a lag was produced by friction from the collision. In the simulation the dark matter slowed, and was eventually left behind. Scientists believe that galaxies exist inside clumps of dark matter, with the dust and gas the form the galaxies being pulled together by the gravitational influence of the dark matter.
The scientists were able to see the dark matter clump because of gravitational lensing, a distortion to the light of the galaxy which is 1.3 billion light years from Earth.
According to the researchers, their findings could rule out the standard theory of Cold Dark Matter, which holds that it only interact with gravity.
We used to think that dark matter sits around, minding its own business. But if it slowed down during this collision, this could be the first dynamical evidence that dark matter notices the world around it. Dark matter may not be completely ‘dark’ after all,” said Massey.
The researchers admit that further investigation will be required into other possible causes of the lag before the offsetting of the dark matter can be confirmed.
“Our observation suggests that dark matter might be able to interact with more forces than just gravity. The parallel Universe going on around us has just got interesting. The dark sector could contain rich physics and potentially complex behaviour,” said Professor Liliya Williams of the University of Minnesota.
The researchers also suggested that the collisions between the four galaxies could have allowed frictional force to build up more than collisions between individual galaxy. If the findings hold, it could set a standard for the amount of energy required to cause a reaction in dark matter.
We are finally homing in on dark matter from above and below – squeezing our knowledge from two directions. Dark matter, we’re coming for you,” said Massey.
