Published on 25 July 2019
schéma abysse cosmique © D. Pomarede. CEA

An international team including Daniel Pomarède from Université Paris-Saclay (CEA) has just published new revelations about the immense cosmic void, called “The local void”, at the borders of which our galaxy, the Milky Way, is located. He is answering our questions.

Can you clarify this new discovery on our galaxy?

The large-scale Universe is composed of clusters of galaxies connected by filaments, separated by large voids. They form a structure called the Cosmic Web, a three-dimensional network that includes not only ordinary matter but also the mysterious dark matter whose nature still escapes our understanding. In a new study published in The Astrophysical Journal, our team mapped the void located in the immediate vicinity of our galaxy and which influences its cosmic fate. It is by using the observations of the movements of galaxies located in our neighborhood that we have been able to deduce the distribution of the mass responsible for these movements and to reconstruct three-dimensional maps of our local universe.

What consequences for our understanding of the universe?

We analyzed the movements of the 18,000 galaxies belonging to the Cosmicflows-3 catalog, by building a cosmographic map that highlights the limits of the matter and void zones within our super galaxy of galaxies. In 2014, we discovered the supercluster to which our galaxy belongs, a structure composed of more than a hundred thousand galaxies, to which we gave the name of Laniakea, which means "Huge Heavenly Horizon" in Hawaiian.

For 30 years, astronomers have been trying to understand the origin of the movement of the Milky Way, which along with its neighbor, the Andromeda galaxy and their satellite galaxies, rush into the Cosmos at a speed of 630 km / s, nearly 2.3 million km / h! This new study shows that nearly half of this displacement is generated by the attraction of the cluster of galaxies in the constellation Virgo combined with the evacuation of the local void matter, a void that is becoming larger and less dense over time.

The galaxies do not move only with the general expansion of the Universe, they also react to the gravitational pull of their neighbors and to very massive regions.

How did this collaboration with the University of Hawaii come about?

In Ouagadougou! Brent Tully, astronomer at the Institute of Astronomy at the University of Hawaii, and I met at a conference on the origin of galaxies. When he saw our data visualization work for the Coast project, he found what he dreamed of doing dynamic mapping of the Universe. And our small collaboration of four people began, with Hélène Courtois, astrophysicist at the University of Lyon and Yehuda Hoffman, gravitational theorist at the Hebrew University of Jerusalem.

What was the role of Université Paris-Saclay in this work?

There are many teams of astrophysicists at Université Paris-Saclay but we have entered into collaboration with our know-how in the exploitation of data from high performance calculations. We first developed tools to exploit the many simulation results that we then used for real data. The transdisciplinarity between astrophysicists, particle physicists, computer scientists, researchers and engineers found in the Institute of Research into the fundamental laws of the Universe, associated with the development of high performance calculations, was a perfect breeding ground to optimize these tools.

Can our readers also see this local void?

Empty representations can be viewed in a video and alternatively with an interactive model (below). With the interactive model, a viewer can pan, zoom, rotate and activate or pause the temporal evolution of the movement of galaxies along their paths. These trajectories are shown in a frame of reference that suppresses the global expansion of the Universe. What we are seeing is the deviations from cosmic expansion caused by interactions with local sources of gravity.

See the article in The Astrophysical Journal


See the local void


Interactive model


Picture : © D. Pomarede. CEA