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The Velocity of Our Galaxy: the End of a 40-Year Mystery

​In a study published in Nature Astronomy, the new magazine by the Nature Group devoted to universe sciences, an international team involving IRFU finally revealed why our galaxy is moving at a velocity of 630 km/s.

Published on 31 January 2017

​For the past four decades, scientists have observed that our galaxy, the Milky Way, and its neighbor, Andromeda, are moving at a velocity of 630 km/s with respect to the expanding universe. It was first thought that this was due to the overabundance of galaxies observed in the direction of our motion. The first suspect was a region of half a dozen galaxy-rich clusters located at a distance of 150 million light-years, which was given the name "Great Attractor". After the Great Attractor, scientists shifted their focus to the Shapley concentration, an area containing more than two dozen massive clusters located 600 million light-years away. Over the years, there was a debate about the relative importance of these two attractors and whether they sufficed to explain our galaxy's movement—a debate that eventually reached an impasse.

Now, instead of observing the distribution of galaxies in detail, the researchers are studying 3D maps of galaxy flows. These flows are the gravitational responses of galaxies to the distribution of surrounding matter. Galaxies are moving away from empty regions toward the most massive regions.

The astrophysicists discovered that we are subject to repulsive and attractive forces that add up. These forces are due to the Shapley Attractor and the "dipole repeller", attributed to a vast area with very low density and unknown until now. The term "dipole" refers to the Doppler effect induced by the motion of our galaxy with respect to the cosmic microwave background (CMB), and which makes the spectral mapping of the CMB appear as dipolar.      

The 3D reconstruction of the galaxy flow field makes it possible to visualize the driving forces of our cosmic movement. A video explains precisely how the combined effects of the Shapley Attractor and the dipole repeller cause the Milky Way and Andromeda to move with a velocity of 630 km/s.

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