Our galaxy would be special and would represent a kind of anomaly in the Universe, according to an international scientific team using an advanced computer model. Our Milky Way is too big for its “cosmic wall”, which would be rare.
But where do we live? The question is complex. Since the Big Bang, our Universe has been expanding. It is vast and you have to imagine the material that constitutes it distributed a little like a gigantic spider’s web not as regular as if it were woven by an arthropod coming from planet Earth. There are solids and voids, bubbles, lumps and filaments, cosmic walls and sheets. All arranged according to laws that cosmology and astrophysics try to break through (read box).
“Matter is distributed in clusters of galaxies and in superclusters… or in clusters of clusters. Our Milky Way, for example, is found in a concentration of superclusters called Laniakea”, explains Stéphane Paltani, professor at the Astronomical Observatory of the University of Geneva, reached by telephone by RTSinfo.
Heaven is a kind of continent; this gigantic system, whose name in Hawaiian means “vast celestial horizon” or “immeasurable paradise”, was discovered in 2014 and would contain alone more than 100,000 galaxies. To get an idea of the size of the structure, you should know that Laniakea reaches 500 million light-years in span; the Milky Way measures “only” 100,000.
>> A Nature video in English explaining what Laniakea looks like:
Rounded and flattened structures
“Galaxies are somehow linked to their clusters: everything keeps a round shape. The clusters arrange themselves in larger structures, the superclusters, which, sometimes, are organized in a more flattened way”, notes the astrophysicist .
And science has a visual vocabulary to further define these cosmic gatherings: “A ‘wall’ is a flattened cluster of clusters. A small cluster is called a ‘cluster’ and a ‘leaf’ is a cluster of ‘clusters'” .
With all these definitions, it is now possible to appreciate the news of the day: the international team which was interested in the place of our galaxy in the Universe realized – thanks to a cutting-edge computer simulation, the project Illustrious TNG –, that the Milky Way is far too large compared to its “cosmic wall”. Consider its direct environment consisting of a flattened arrangement of galaxies.
Around, there are also particularly deserted regions – aptly named “empty” – on either side of this wall. And the novelty is that this has not yet been observed in other galaxies: “If we observed a similar phenomenon once more elsewhere, this would call into question our understanding of the formation of cosmic structures”, according to Stéphane Paltani .
>> The Universe within a billion light years:
Galaxy superclusters (in blue) and cosmic voids (in red) over a distance of one billion light-years from the Milky Way. Laniakea is in yellow. [Richard Powell – Wikimedia/CC BY-SA 2.5]
Furthermore, these voids appear to be crushing the galaxies together in what looks like a pancake shape – a thick pancake popular in the United States for breakfast – to achieve a flat arrangement: “The phenomenon is due to the absence of mass in the voids, which means that nothing prevents the collapse in their direction”, specifies Stéphane Paltani.
To better understand this study published at the end of December in the Monthly Notices of the Royal Astronomical Society, one more notion: our cosmic home, the Milky Way, is near the center of a flat “wall” of galaxies called the “local sheet”; its diameter is 30 million light-years. The term “leaf” here translates to a much smaller size than “wall”.
Although “sheet” environments are common cosmic structures, very few of them harbor a galaxy as massive as the Milky Way, at least in extensive computer simulations: “In reality, we don’t know” , notes Stéphane Paltani.
>> Watch the video “The Milky Way and its Local Leaf”:
The animation above shows our Milky Way as a spiral galaxy in the center. The gray spheres correspond to nearby galaxies scaled by their luminosity. For clarity, only the brightest galaxies are shown. The blue circles indicate the distance from the Milky Way in 3 million light-year intervals.
A precise organization
The environment of the “cosmic wall” – in this case, the “local sheet” – influences how the Milky Way and surrounding galaxies rotate around their axis. And this is done in a more organized way than if we were in any place in the Universe, without a wall (read box).
In general, galaxies tend to be significantly smaller than this “wall”. The Milky Way, on the other hand, is surprisingly massive compared to his.
The Illustris TNG simulation allowed scientists to recreate a volume of the Universe representing a distance of regarding one billion light-years, containing millions of galaxies; only a small fraction of them – one millionth – were as special as the Milky Way.
That is to say, they had the characteristics of being embedded in a “cosmic wall” such as the “local sheet” and of being as massive as the galaxy that hosts us. And Stéphane Paltani concludes: “It’s a simulation, which means that it will have to be confirmed by observations. There are also possibilities of interpretation: are there any imperfections in the computer simulation? “Are we really in some weird place in the Universe? Is there some physical mechanism that we haven’t figured out yet?”
For now, one thing is certain, it is our address in the Universe: Earth, Solar System, Milky Way, Local Group, Local Leaf, Virgo Supercluster, Laniakea. A very chic precision on a business card.
Stephanie Jaquet