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An atlas for the cosmic velocity (V) web is now available

​During the 13.8 billion-year history of the Universe, the action of attractive and repulsive forces has concentrated matter in certain regions, leaving others increasingly empty. For the first time, mapping of the major structures of the Universe, including those which are not observable by conventional methods, has been undertaken by analyzing the movements of thousands of galaxies. This work, which involves the CEA and the Institute of Nuclear Physics of Lyon, was published on 10th August 2017 in The Astrophysical Journal.

Published on 31 August 2017

A collaborative international operation, involving the CEA (the French Atomic Energy Commission) and the Institute of Nuclear Physics of Lyon (University of Claude Bernard Lyon 1/CNRS), has recently reconstructed a 3D structure of the Universe by the analysis of nearly 8,000 galaxies, the precise distances, but also the velocities of which are known. This reconstruction even includes zones which have not been observable to date.


Visualization of the cosmic velocity web. Current lines indicate flows of matter which run along the filaments (in grey) towards zones of compression at the node points (in re). The point of origin (at the intersection of the 3 red, blue and green vectors) corresponds to the position of the Milky Way. © D. Pomarède/ Y. Hoffman/ R.B. Tully/ H. Courtois

By measuring the movements of thousands of galaxies, it is possible to undertake the mapping of matter, whether "ordinary" or – to a predominant extent – "dark" matter. The matter present in the universe is not distributed uniformly: in certain regions, matter is concentrated within galaxies; other regions are virtually empty. The galaxies are drawn by filaments towards sources of gravitational attraction, and move away from, and thus appear to be repelled by empty regions. This distribution produces a network of node points described as the cosmic web, which has historically been defined by the positions of the galaxies only.

The structure of this cosmic web is consistent with the pathways which have been deduced by researchers from the measurement of the velocity of the galaxies. Moreover, this cosmic velocity web discloses the mapping of certain regions which, to date, have been Terra Incognita, such as the "zone of galactic obscuration", where a number of filaments have been discovered. This zone was previously inaccessible for the observation of galaxies and the determination of their positions, as a result of dust from the disk of Milky Way, which obscures the galaxies beyond.

How is the cosmic velocity web reconstructed?
Properties for the compression and expansion of matter flows have been described by the analysis of a mathematical object termed as the "shear tensor". Accordingly, the compression of this flux in three perpendicular directions reveals the presence of a node point in the web, where matter is concentrated, whereas a general expansion in three directions discloses the presence of a void, from which matter is moving away. An intermediate situation of compression in two directions and expansion in the third indicates a filament (see figure and video).

Visualization is a core element of the mapping process of the cosmic web and its complex three-dimensional structure. This intricate architecture has been analyzed and compared using the visualization capabilities of the SDvision software developed by the CEA[1]. The scientific publication includes a video exploring the structure. The article also features a technical innovation: an interactive 3D visualization, allowing the reader to view the cosmic velocity web by immersive exploration, and to compare it with the distribution of galaxies; this technology is based upon the interactive Sketchfab platform for the sharing of visualizations, developed by a French start-up business.

The agreement of this reconstruction of the cosmic web from velocity values with the mapping of the positions of the galaxies is a further confirmation of the standard cosmology model of structure formation: the major structures of the Universe have developed from the growth of miniscule initial fluctuations, specifically under the influence of gravity.

From the movements of the galaxies, researchers can determine how the total mass of the Universe is distributed, comprised of ordinary matter and dark matter, which is five times more abundant. By reconstructing the cosmic velocity web, the community of astrophysicists is provided with a new resource for the understanding and enhancement of the existing cosmic map.

References: The Cosmic V-Web, D. Pomarède, Y. Hoffman, H.M. Courtois, R.B. Tully, The Astrophysical Journal, 845 (2017) p. 55,

A constantly updated atlas of the Universe
In the same way as our navigational maps, the atlas of the Universe is constantly being updated, specifically by the same international team who, in 2014, discovered the frontiers of the continent of galaxies in which we live: Laniakea. In 2015, researchers intially identified a "cosmic superhighway", on which satellite galaxies travel and are focused in the direction of the major galaxies before identifying the reason for the motion of the Milky Way at 630 km/s: the Dipole Repeller. Today, this definition of the cosmic velocity web has been made possible by the extensive and standardized surveying of distances (obtained using various telescopes) and the velocities of galaxies combined in the series of catalogues: CosmicFlows. The present analysis is based upon a study of 8,000 galaxies in the catalogue Cosmicflows-2. Already, a new collection of 18,000 distances and velocities of galaxies is available in the catalogue Cosmicflows-3. An analysis of the latter will permit the identification of the cosmic velocity web with an even greater wealth of detail.

To find out more: Investigate these results using the exploratory video and interactive visualization

[1] In the Institute of Research on the Fundamental Laws of the Universe, the Department of Electronics, Detectors and IT for Physics applications.


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