An invisible mass seems to be pulling galaxies towards it at millions of kilometres an hour. It’s Force is so great that it is influencing galaxies all across the Universe. What could this invisible mass be? A greater part of our universe or a whole another universe?
A known reason for the motion of galaxies is the expansion of the universe. Movement of galaxies caused by this expansion is known as the “Hubble Flow”. In addition to the Hubble Flow, galaxies have their own peculiar (random) motion in other directions. But, this peculiar motion is insignificant on a large scale.
When you look at the universe, you see that there is no one preferred direction for random motion of galaxies – galaxies are not moving in one specific direction. But, when astronomers factor out the Hubble flow, we observe that all galaxies are moving towards one part of the universe at speeds of upto 600-1000 kilometers/second (3 million kilometers an hour). A great gravitational pull is being exerted on galaxies across the universe. The motion of galaxies due to this mysterious force is called the Dark flow.
What is this invisible matter that is making its gravitational presence felt throughout the universe? Scientists have a few speculations.
1. The Great Attractor
Scientists know of a gravitational anomaly in the region where dark flow is pulling galaxies. They speculated that the point pulling galaxies towards it was the centre of the Laniakea supercluster. The supercluster is a collection of many galaxy clusters. it is home to the Milky Way and about a 100,000 other galaxies.
Later scientists concluded that the Laniakea’s gravitational force could not influence galaxies on the scales observed and therefore, this was not the answer.
2. A Greater Part of Our Universe
Another possible explanation for dark flow Is the existence of a part of the universe beyond the cosmic horizon where there is much more mass-density than our visible part. This greater mass’s gravitational influence is pulling the galaxies in our region towards it. Scientists say that in the early universe, this dense region was much more close to our today’s observable part. But expansion caused that region to move away from us. Even though it is beyond our cosmic horizon, its gravitational influence is so great that it is still influencing our universe today.
A more exciting explanation this mystery involves the existence of other universes. Scientists say that inflation gave rise to not one but many universes. A second universe close to our universe may theoretically influence our universe. The matter in this other universe is pulling matter in our universe towards it.