In the article at https://phys.org/news/2017-01-galaxy-space.html it says that the Milky Way is being pushed away by a great void. How does gravity repel or is this to do with dark energy?
An abstract from the original paper:The Dipole Repeller
Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity 1 of V CMB = 631 ± 20 km s−1 and participates in a bulk flow that extends out to distances of ~20,000 km s−1 or more 2,3,4 . There has been an implicit assumption that overabundances of galaxies induce the Local Group motion 5,6,7 . Yet underdense regions push as much as overdensities attract 8 , but they are deficient in light and consequently difficult to chart. It was suggested a decade ago that an underdensity in the northern hemisphere roughly 15,000 km s−1 away contributes significantly to the observed flow 9 . We show here that repulsion from an underdensity is important and that the dominant influences causing the observed flow are a single attractor — associated with the Shapley concentration — and a single previously unidentified repeller, which contribute roughly equally to the CMB dipole. The bulk flow is closely anti-aligned with the repeller out to 16,000 ± 4,500 km s−1. This ‘dipole repeller’ is predicted to be associated with a void in the distribution of galaxies.
The movement of a galaxy, such as the Milky Way, is based on the relative abundance and velocity of the galaxies surrounding them. The term attractor is used to describe regions of relatively dense concentrations of galaxies and the term repeller refers to voids, which because of tbeir relatively low concentration of mass, are effectively acting to push galaxies away from them, (not literally pushing and not involving dark energy), just a mismatch in gravitational effects.
The point that may really hit home here, it certainly did to me, is the vast scale of the observable universe, which is better captured in some ways by these mapping techniques than by photography.
It is shown here by means of the flow streamlines (in black–blue, left panel) and of the anti-flow (in yellow–red, right panel). Anti-flow is defined here by the negative (namely, the reverse) of the velocity field. The same streamlines are seeded on a regular grid and are coloured according to the magnitude of the velocity.
Image Source and Extract : Nature Dipole Repellers