The Dipole Repeller
  In the standard (LCDM) model of cosmology the universe has emerged out of an
early homogeneous and isotropic phase. Structure formation is associated with
the growth of density irregularities and peculiar velocities. Our Local Group
is moving with respect to the cosmic microwave background (CMB) with a velocity
631+/-20 km s-1 and participates in a bulk flow that extends out to distances
of at least 20,000 km s-1. Since the discovery of the CMB dipole, the implicit
assumption was that excesses in the abundance of galaxies induce the Local
Group motion. Yet, underdense regions push as much as overdensities attract but
they are deficient of 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 is a significant actor in the local flow. Here we report on
kinematic evidence for such an underdensity. We map the large scale 3D velocity
field using a Wiener filter reconstruction from the Cosmicflows-2 dataset of
peculiar velocities, and identify the attractors and repellers that dominate
the local dynamics. We show here that the local flow is dominated by a single
attractor -associated with the Shapley Concentration- and a single previously
unidentified repeller. Multipole expansion of the local flow provides further
support for the existence and role played by the attractor and repeller. The
bulk flow (i.e. dipole moment) is closely (anti)aligned with the repeller at a
distance of 16,000+/-4,500 km s-1. The expansion eigenvector of the shear
tensor (quadrupole moment) is closely aligned with the Shapley Attractor out to
7,000 km s-1. The close alignment of the local bulk flow with the repeller
provides further support for its dominant role in shaping the local flow. This
Dipole Repeller is predicted to be associated with a void in the distribution
of galaxies.
