Is our Local Group being pulled or pushed? or both?

Discussion in 'Astronomy, Exobiology, & Cosmology' started by paddoboy, Jan 30, 2017.

  1. paddoboy Valued Senior Member

    Both push and pull drive our galaxy's race through space
    January 30, 2017

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    Although we can't feel it, we're in constant motion: the earth spins on its axis at about 1,600 km/h; it orbits around the sun at about 100,000 km/h; the sun orbits our Milky Way galaxy at about 850,000 km/h; and the Milky Way galaxy and its companion galaxy Andromeda are moving with respect to the expanding universe at roughly 2 million km/h (630 km per second). But what is propelling the Milky Way's race through space?

    Until now, scientists assumed that a dense region of the universe is pulling us toward it, in the same way that gravity made Newton's apple fall to earth. The initial "prime suspect" was called the Great Attractor, a region of a half dozen rich clusters of galaxies 150 million lightyears from the Milky Way. Soon after, attention was drawn to an area of more than two dozen rich clusters, called the Shapley Concentration, which sits 600 million lightyears beyond the Great Attractor.

    Now researchers led by Prof. Yehuda Hoffman at the Hebrew University of Jerusalem report that our galaxy is not only being pulled, but also pushed. In a new study in the forthcoming issue of Nature Astronomy, they describe a previously unknown, very large region in our extragalactic neighborhood. Largely devoid of galaxies, this void exerts a repelling force on our Local Group of galaxies.

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  3. paddoboy Valued Senior Member


    The dipole repeller

    Our Local Group of galaxies is moving with respect to the cosmic microwave background (CMB) with a velocity 1 of VCMB = 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 motion5,6,7 . 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.

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