The Dipole Repeller

Abstract : 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 . There has been an implicit assumption that overabundances of galaxies induce the Local Group motion. Yet underdense regions push as much as overdensities attract , 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 . 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.
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Y. Hoffman, D. Pomarède, R. Brent Tully, H. Courtois. The Dipole Repeller. Nature Astronomy, Nature Publishing Group, 2017, 1 (2), pp.36. ⟨10.1038/s41550-016-0036⟩. ⟨in2p3-01468457⟩

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