Dipole repeller

The universe is filled with massive movements of galaxies. Our own Local Group of galaxies moves through space at a specific speed. This motion is part of a much larger pattern called a bulk flow. Astronomers have studied this flow across distances of over 500 million light-years. In 2017, scientists identified a major factor in this movement. They named it the Dipole Repeller.

The Dipole Repeller is a massive region of space with very little matter. It is considered a large supervoid, which is a vast empty zone. This region is not an active force that pushes objects. Instead, it acts as a center of effective repulsion. This means that galaxies appear to move away from it. This phenomenon is part of the large-scale flow of galaxies near the Milky Way.

To understand how this works, we must look at the relationship between density and gravity. Gravity is an attractive force that pulls matter toward dense areas. The Dipole Repeller is an underdense region, meaning it has very little matter. Because it lacks matter, it has very little gravitational pull. On the other side of this void lies the Shapley Supercluster. This is an overdensity of galaxies known as the Shapley Attractor.

The mechanism of the Dipole Repeller is based on this imbalance. Matter in the surrounding area is pulled toward the Shapley Attractor. At the same time, there is very little gravity coming from the direction of the void. This lack of counteracting pull causes galaxies to move away from the empty zone. This movement makes the void seem as if it is pushing matter back. The apparent repulsion is actually the result of being pulled toward a more dense area.

Researchers first detected this phenomenon in January 2017. A team of scientists published their findings in the journal Nature Astronomy. These researchers included Yehuda Hoffman, Daniel Pomarède, R. Brent Tully, and Hélène Courtois. They used distance and velocity measurements to study the motion of galaxies. They discovered that no single concentration of matter could explain all observed velocities. This led them to conclude that an additional force was influencing the direction of stars and galaxies.

Specific measurements help define the scale of this cosmic structure. The Dipole Repeller is located at a distance of about 250 million light-years. It sits directly opposed to the Shapley Attractor. This single center of attraction and the single repeller are significant. They appear to be the most important contributors to the cosmic microwave background dipole. This dipole refers to a specific pattern in the radiation left over from the early universe.

Scientists have found that these voids are part of a larger system. In September 2017, the same research team identified a second void. They called this the Cold Spot Repeller. These voids are considered main components of the cosmic V-Web. This V-Web describes the large-scale structure of the universe. The voids act as repulsive forces through the inverse of gravitational attraction. They help shape how matter is distributed across the cosmos.

There has been some discussion regarding the term "repulsive force." Some astrophysicists and journalists have noted that gravity is strictly an attractive force. They argue that the void does not actually push. Instead, the apparent repulsion is due to the lack of a force to balance the pull from the Shapley Supercluster. Even with this distinction, the Dipole Repeller remains a vital concept. It explains the complex dance of galaxies as they flow through the vastness of space.