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Extragalactic void pushes Milky (a)Way

​An international team involving the CEA, Université Claude Bernard Lyon 1 and the CNRS has discovered why our galaxy moves at a speed of 630 km/s: the reason is the Dipole Repeller. This research, to be published on January 30, 2017, and selected by Nature Astronomy, the Nature group’s brand new journal devoted to the sciences of the universe, answers an enigma that has puzzled astrophysicists for more than 40 years.

Published on 30 January 2017
In our extragalactic neighborhood lies a vast empty region that might exert a repulsive force on our galaxy, the Milky Way. This “repeller” contributes to the gravitational forces that drive us at a speed of nearly 2.3 million km/h through the cosmic web, the large-scale structure on which matter is organized and comprising filaments that interconnect galaxies and separate voids.

Attraction and repulsion combine to make us move

The movement of our galaxy (and its companion, the Andromeda galaxy) was already familiar to astrophysicists, who had been searching for its cause for 40 years. Researchers first sought an explanation in the possible existence of an excess of galaxies located in the general direction of our motion. The initial suspect was called the Great Attractor, a region containing half a dozen galaxy-rich clusters and located 150 million light years away. Attention then shifted to a larger entity, still in the same line of sight but directly behind the Great Attractor, known as the Shapley Supercluster, located 600 million light years away from us. Over the years, however, the debate became stuck on the relative importance of these two attractors, neither of which offers a sufficient explanation for our movement, particularly as we are not moving exactly in the direction of Shapley, as we should be doing. The hypothesis of a sub-dense region or extragalactic “void” was then put forward to shed light on this phenomenon.

It is, however, extremely difficult to confirm the observation of such a void. Which is why the researchers decided that instead of focusing on just the visible part of the mass - in other words the galaxies - they should map out in 3D the movement of all matter, namely visible or baryonic matter and invisible matter, known as dark matter, although it is in fact transparent. 

CEA1 research engineer, Daniel Pomarède, explains: “It is microwave radiation emitted more than 13 billion years ago that enables us to detect the movement of the Milky Way. This light reaches us from every direction, but because we are moving, it is observed with a blue shift in the direction we are heading, and with a redshift in the opposite direction. And we can analyze this dipole effect to deduce our speed of 630 km/s.” Hélène Courteous, an astrophysicist at Université Lyon 12, adds: “By analyzing the velocity fields of thousands of galaxies in our local universe, we identified the movement of “rivers of matter”, such as those flowing through Lamiae,” our super cluster of galaxies discovered by the same team in 2014. “These rivers are a direct consequence of the distribution of total mass, which moves away from empty regions and towards those with higher densities.” 
 1 (Institut de Recherche sur les Lois Fondamentales de l’Univers -Irfu[CEA, Saclay]).
 2 (Université Claude Bernard Lyon 1/ Institut de Physique Nucléaire de Lyon CNRS).

The team discovered the presence in our galaxy of repulsive and attractive forces of comparable strength coming from distant entities. It deduced that the major influences behind our movement are the Shapley Attractor, and a vast, previously unidentified, empty region (i.e., devoid of visible and invisible matter) that they have called the Dipole Repeller. This discovery, published on January 30, 2017 in Nature Astronomy, could indeed explain the existence of the dipole (see box below) observed in the cosmic wave background, one of the vital components of the standard model of cosmology.

This map showing flows of matter (the directional arrows) and equipotential gravitational surfaces (regions of space that “feel” the same gravitational attraction, shown in green and yellow) materializes the Dipole Repeller region by illustrating its influence, together with the nodes and filaments of the cosmic web (red and gray surfaces). It thus portrays the large-scale structure of our local universe. The yellow arrow is located in the Milky Way, our galaxy, and points to the cosmic wave background dipole. The map covers a region of our universe roughly 2.5 billion light years wide. © Y. Hoffman, D. Pomarède, R.B. Tully, H. Courteous

Astrophysicists have finally learned what path our galaxy is following, as it is pulled by the distant Shapley Supercluster, and repelled by the Dipole Repeller, a region made up of “nothing”, not even invisible matter, and about which almost nothing is known. That is why astrophysicists are now preparing ultrasensitive optical, near-infrared and radio readings to identify the rare galaxies that may be found in and around such a void to increase our knowledge.

VideoExplication in video of the article on The Dipole repeller

Further reading:

Video of the results of the article and downloadable high-resolution illustrations (© Y. Hoffman, D. Pomarède, R.B. Tully, H. Courtois)

What is a dipole?

The cosmic wave background radiation of the Big Bang is distributed almost identically in all directions. By studying deviations with respect to its mean level, a regular modulation emerges that seems to indicate that the cosmological black body is slightly warmer near one celestial hemisphere than the other. This is known as dipolar or dipole anisotropy. For astrophysicists, this dipole is generally easy to observe and they use it to check or calibrate their instruments. (See box on the cosmic wave background radiation).

Cosmic wave background radiation - an essential component of the standard model of cosmology

​The radiation emitted when the universe was 380,000 years old is measured here extremely accurately by Europe’s Plank satellite to produce a map of the cosmic wave background radiation. It shows traces of the initial density fluctuations that gave birth to existing structures (super clusters, galaxy clusters, galaxies, stars, planets and so on). Analyzing these fluctuations plays a vital role in the standard model of cosmology.​To produce a map of the cosmic wave background radiation, we must first subtract the dipole observed in this radiation, shown here as measured by the American satellite COBE. This dipole is due to the fact that our galaxy is moving at a speed of 630 km/s, creating a blueshift in the direction of travel and a redshift in the opposite direction. The discovery of the Dipole Repeller at last helps us understand the reason for this movement.

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