The place in the Universe no one wants to go to

Cosmology is one of the most impressive field of physics. Its aim is to study the Universe as a whole (nothing less) and to understand its structure and its behaviour. Modern cosmologists are equipped with powerful tools and now have a reasonably good idea of what our Universe looks like. As a matter of fact, the place of our planet within its galaxy, the Milky Way, as well as the neighbouring galaxies are very well known. The Milky Way is part of a cluster of galaxies that is called the Local Group and that is host to at least 80 other galaxies of different shapes and sizes. To give a sense of scale, it would take light (the fastest element in the Universe) 80,000 years to go from one end of the Milky way to the other, and 10 million years to cross the local group. But this is nothing for cosmologists, that treat galaxies as the smallest constituents of the universe and are often dealing with much greater distances.

Around the local group, organised motion?

A question Cosmology tries to answer is that of the motion of galaxies in the Universe. Are these objects simply floating in random directions? Are they all grouping under the effect of gravity, forming a giant cluster of galaxy clusters?

In an article published in 2017 in Nature Astronomy, the authors looked at the galaxies around the Local Group. They estimated their velocities and direction to try and spot any large scale movement. What they discovered was a general motion of galaxies, between two regions of the Universe, as shown in the illustration bellow.

Illustration of the attractor/repeller dipole.
The black circles represent galaxies, with the direction of their velocities indicated by the black lines. The galaxy density is indicated by the colours. The Local Group is shown, in the the middle of the dipole.

They noticed that most galaxies move towards the region of the Shapley Supercluster and away from the region of the Great Repeller, in what looks like a very organised manner. They observed that the Milky Way for instance is travelling towards the Shapley cluster at a velocity of 631 km/s!

Because of the resemblance of this phenomenon with an electromagnetic dipole, that is two charges of opposite polarity placed nearby, it was called the Dipole repeller.

Illustration of an electric dipole. The black lines show the direction of the electric field between the charges of opposite polarity.

The nature of the great repeller

With this exciting unexplained observational evidence in mind, many cosmologist tried to find theory able to describe the observed flow of galaxies from repeller to attractor.

One explanation could be that having high mass concentration, the region of the Shapley attractor generates a strong gravitational pull on nearby galaxies, just like the earth attracts us all on its ground. At the same time, the repeller region being almost empty of matter, would generate a far less important pull. The galaxies would thus simply flow due to a difference in matter distribution.

Another theory, mainly defended by the cosmologist J-P Petit, explains that the region of the Great Repeller is filled with a very special type of matter, matter with negative mass. Petit built an entire cosmological model around this idea of negative masses, and explains in an article published in the Russian Journal of Earth Science in 2023 that the structure of the Universe could be shaped by clusters of negative masses that would repel “normal” matter. According to him, the Great Repeller would be an example of such negative mass cluster. This makes the analogy with the electric dipole even more striking as masses of different “sign” would act as charges of different polarities.

At present, no consensus has been reached on the explanation of the peculiar structure of galaxies around the Local Group, and it remains an open topic in research. Finding the reason why this specific spot in the Universe is so unliked by all galaxies might lead cosmologist to rethink the global structure of the Universe.