The study of the distribution of matter and, more generally, of the large structures of the Universe, is an important field in cosmology, because it makes it possible to better constrain the theoretical models describing the evolution and dynamics of our universe, but also to understand how it is globally arranged. This pattern of structural organization is called the "cosmic web".
In 1956, the Franco-American astrophysicist Gérard de Vaucouleurs, published an article in which he highlighted the presence of a structural organization in the distribution of several galaxies listed in the Shapley-Ames Galactic Catalog. He gives this organization the name "supergalaxy". In 1981, Russian astrophysicists V. Arnold, S. Shandarin and I. Zeldovich (University of Moscow) demonstrated that this distribution is the product of the gravitational collapse of primordial large-scale density fluctuations.
With the improvement in the performance and sensitivity of instruments, observations make it possible to refine more and more the structure of the observable universe, to the point of highlighting a network organization , the superclusters of galaxies being connected by galactic filaments and separated by voids . These many works, of which the American astrophysicist John Huchra was one of the pioneers, combined with complex simulations, will contribute to the appearance of the term "cosmic web to describe the filamentous distribution of matter in the Universe.
The structure of the cosmic web is today explained by the Standard Model of cosmology, the Λ-CDM model. In this model, after inflation, primordial quantum fluctuations turn into large-scale density fluctuations which, under the effect of gravity, have shaped this homogeneous distribution of matter in the observable Universe, while accompanying its expansion. The data collected by the Planck mission made it possible to detect traces of these density fluctuations in the cosmic microwave background.
The cosmic web breaks down into several constituent elements. The superclusters of galaxies such as Laniakea, Berenice's Hair or the Perseus-Poissons supercluster constitute the "points" of the network. These points contain "nodes which are local concentrations of mass like the Great Attractor, the Southern Wall, the Knot of Hercules or the Hare's Knot.
These nodes are interconnected by galactic filaments , that is, threadlike structures composed of galaxies or clusters of galaxies, such as the Big Dipper filament or the Perseus-Pegasus filament. Between the galactic filaments are cosmic voids , i.e. areas with an extremely low density of matter, such as the Local Void or the Bouvier Void.
Despite its relative precision, the representation of the cosmic web suffers from certain observational problems such as obscuration (the disc of the Milky Way hides certain areas of the Universe from us), the quantity of galaxies which decreases proportionally to the distance, or even areas simply inaccessible to instruments.
To circumvent these problems, astrophysicists are also establishing a cosmic web of speeds (Cosmic V-Web ) in the following way:thanks to the velocities of the galaxies listed in the Cosmicflows-2 catalog, the scientists reconstruct in 3D the different galactic densities, then build a shear tensor allowing to describe the way in which the galactic properties vary spatially, and finally use this tensor to reconstruct the missing parts of the cosmic web.
The cosmic web also takes into account the distribution of dark matter in the observable universe. According to theoretical models, it acts as a "galactic cement" and stabilizes the hierarchical structural organization of the Universe on large scales. To do this, cosmologists are forced to perform powerful simulations integrating current dark matter data, in order to obtain a consistent 3D distribution of baryonic matter and dark matter.
