Scientists have known that the Shapley Supercluster has been pulling on the Milky Way Galaxy and its nearest neighbor, Andromeda. This supercluster is a massive collection of galaxies about 750 million light-years from Earth and is huge enough to have an accelerating effect on our galaxy from that distance. However, its gravitational pull was never enough to account for the Milky Way’s speed, about two million kilometers per hour.
The answer may lay in a cosmic “dead zone” in the constellation of Lacerta that appears to be relatively empty of galaxies. This dead zone may be pushing on our galaxy from the opposite side of the Milky Way from the Shapley Supercluster with the same repulsive force that it used to repel any galaxies that may have inhabited it at one time.
Cosmologist Yehuda Hoffman of Hebrew University in Jerusalem worked with scientists in France and Hawaii to study the movement of more than 8,000 galaxies. They built a 3D map of the region these galaxies reside in and found that they seem to be generally moving away from the relatively empty region and toward the Shapely Supercluster. The seeming combination of pulling by Shapely’s gravitational force and pushing by the repulsive force in the relatively empty zone led to the dead zone being dubbed a “dipole repeller.”
“We show that the Shapley attractor is really pulling, but then almost 180 degrees in the other direction is a region devoid of galaxies, and this region is repelling us,” said Hoffman.
Could the effect of the dipole repeller be overstated? Some astronomers, such as Michael Rowan-Robinson of Imperial College London, seem to think so. In 2000, Rowan-Robinson led a team that studied surveys conducted by the Infrared Astronomical Satellite to demonstrate the ways that superclusters and empty zones like the dipole repeller affect the movement of galaxies. He stated that the effect of the Shapely Supercluster may be overstated.
In the meantime, Hoffman expressed his hope that scientists would be able to use more sensitive instruments to study the dead zone in more detail. Future studies could include a survey that could reveal whether the empty zone is truly devoid of galaxies.