Superamas de Laniakea superposés aux orbites et aux surfaces de densité de masse. Crédit : Université d’Hawaïʻi
Tout dans notre univers bouge, mais les échelles de temps nécessaires pour voir le mouvement sont souvent largement supérieures à la durée de vie humaine. Dans une nouvelle étude majeure, une équipe d’astronomes de l’Institut d’astronomie de l’Université d’Hawaïʻi (IfA), de l’Université du Maryland et de l’Université de Paris-Saclay a retracé le mouvement de 10 000 galaxies et amas de galaxies, les congrégations dominantes de matière, dans un rayon de 350 millions d’années-lumière. Leurs mouvements ont été suivis sur une période de 11,5 milliards d’années, depuis les origines des galaxies, lorsque l’univers n’avait que 1,5 milliard d’années, jusqu’à aujourd’hui, à un âge de plus de 13 milliards d’années.
L’étude a été acceptée pour publication dans The Astrophysical Journal.
Calcul des trajectoires des galaxies
Grâce à une technique mathématique appelée méthode d’action numérique, l’équipe a calculé ces trajectoires en se basant sur la luminosité et les positions actuelles des galaxies, ainsi que sur leur mouvement actuel qui les éloigne de nous. Les astronomes ont pris en compte la physique du Big Bang theory, including the idea that galaxies initially start out expanding from each other almost precisely at what is called the Hubble expansion rate. Throughout time, gravity alters galaxy motions, so they are not just moving apart as the universe expands, but are drawn together into filaments, walls and clusters, while also emptying out other regions that are now voids. Over the eons of time, galaxies typically deviate from pure Hubble rate expansion by millions of light-years over a billion years. In regions of high density, the galaxy orbits can become quite complicated and involve collisions and mergers.
Slice of local universe showing orbits that galaxies have followed in white and contours of regions of high density in shades of yellow-orange. Milky Way (near center) Great Attractor core of Laniakea Supercluster (left) Perseus-Pisces (right). Credit: University of Hawaiʻi
“We are bringing into focus the detailed formation history of large-scale mass structures in the universe by reverse engineering the gravitational interactions that created them,” said Ed Shaya, Associate Research Scientist at the University of Maryland.
The Great Attractor
There are several particularly interesting vast regions of high matter and galaxy density the astronomers explore. One, which has been called “the Great Attractor,” is the core of the Laniakea Supercluster, an immense supercluster of galaxies containing our own Milky Way. Galaxies can be seen flowing toward a location within a nest of four rich clusters.
Milky Way Galaxy. Credit: Thomas Ciszewski
A second fascinating region is in the adjacent Perseus-Pisces filament of galaxies, which stretches for nearly a billion light-years and is one of the largest known structures in the universe. The vicinity of the Virgo Cluster, the closest large cluster, is also seen, and can be studied in detail because it is nearby.
“For more than 30 years, astronomers have considered a ‘Great Attractor’ to be the primary source of gravity that makes the whole region near us move with a high peculiar velocity relative to uniform cosmic expansion, but the nature of that source has been obscure,” said R. Brent Tully, an astronomer at IfA who co-authored the study. “Our orbit reconstructions have provided the first good look at this previously enigmatic region.”
Across the entire expanse, the orbits can be projected into the future as well. The accelerating expansion of the universe dominates the overall picture, causing most galaxies to move apart. However, some coalescence and merging will continue in localized regions.
A video of the paths of galaxies in this vast region, starting from the epoch of early galaxy formation and continuing until the universe is almost twice as old can be viewed here. On the large scales depicted in this simulation, only a few major mergers, all in very dense regions, are seen to occur in the next 10 billion years.
The technical article is accompanied by four interactive models. and four videos:
Reference: “Galaxy flows within 8,000 km/s from Numerical Action methods” by Edward Shaya, R. Brent Tully, Daniel Pomarède and Alan Peel, Accepted, The Astrophysical Journal.
DOI: 10.3847/1538-4357/ac4f66
arXiv:2201.12315
The research team is composed of Shaya (University of Maryland), Tully (University of Hawaiʻi), Daniel Pomarede (University of Paris-Saclay) and Alan Peel (University of Maryland).