• PHYSICAL PROBLEM: what is the origin of the large-scale organization of matter in the universe, and of the galaxy formation

 
• OBSERVATIONS: primordial fluctuations of the universe, statistical analysis of the galaxy distribution.

 
• EQUATIONS: Vlasov-Poisson equation for the dark matter and hydrodynamic equation (Navier-Stokes) for the gas; shocks and radiative processes

 
• NUMERICAL EXPERIMENTS: evolution of a slice of universe of size equal to some hundreds of Megaparsecs cube

 
• CONCLUSION: these experiments show why fragmentation models must be eliminated, while hierarchical models (in which small condensations form first, and then merge to form large-structures) are better. They constraint the nature of dark matter in the Universe: it must be non-relativistic (i.e., cold) very early in the Universe history.

 
• REFERENCE: Simulation numériques en cosmologie. Des grandes structures aux galaxies, J.-M. Alimi, J.-P. Chieze, R. Teyssier, A. Serna, E. Audit, Calculateurs Parallèles, réseaux et systèmes répartis,

Volume 11 Numero 3 page 255 (1999)

The first FILM (mpeg, 6 Mb) shows the evolution of  dark matter starting with small fluctuations up to the present epoch in a large slice of the observable universe.

Matter is seen to form along filaments: galaxy clusters are located at the nodes of the filaments.

In the first two cases, the small scales form first, and the large scales (filaments) after. In the third case, filaments form first. This is less akin to the real Universe. One concludes that the conditions of the two first cases are closer to the properties of the real Universe (cold dark matter, or CDM).

The introduction of a cosmological constant allows (as well as by reducing the hot population of dark matter) to trigger the formation of the large structures.

The second FILM (mov, 2 Mo; gif, 8 Mo) shows a snapshot of the distribution of matter at the end of the simulation: the observer turns around the slice of universe.

Remark: (invisible) dark matter is a necessary ingredient to form the very large structure in the Universe. If universe were only made of its visible (observed) constituent, galaxy forming would have been impossible, and  the universe would have remained in an almost homogeneous, uncondensed, dilute, state.

The nature of Dark matter is not presently known. The dark matter considered here triggers the condensation of ordinary, visible matter (baryons) not through collisions but because it modifies the gravitational field.