1997 - 2000 activity report
Computational cosmology
The development of new computing facilities has triggered the development of large numerical simulations. The results of cosmological observations should be compared to numerical simulations to get meaningful physical constraints in term of cosmological model. In 1996, Jean Pierre Chieze joined the SAp. This has boosted new activities in this domain.
In the plasma models used to study clusters, it is generally assumed that there is equilibrium between ions and electrons. Jean Pierre Chieze and Romain Teyssier have conducted simulations of plasma in clusters by separately taking into account the various components: ions, electrons and neutrals. They were able to show that there are situations without equilibrium between ions and electrons. These effects are especially prevalent on the edges of the cluster and in impact regions resulting from fusion between clusters (Teyssier et al., 1998, Chieze et al., 1998).
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Figure 9: Example of a numerical simulation performed using the Adaptive Mesh Refinement code RAMSES developed by R. Teyssier and co-workers. This simulation starts in a cubic box of 100 Mpc h-1 length, with initially 2563 equal cells and 16.7x106 particles. The cells can be divided according to their particle number on 7 scale levels. At the end of this simulation, the minimum cell size has a resolution of 3 kpc h-1, which permits to study cluster of galaxies and galaxy formation within the dark matter large-scale structures |
Digital simulation can be used to tackle the problem of the formation of large structures in the universe. SAp has made a major contribution to developments in this field. Its essential contribution lies in the development of "fluid" codes. These codes allow the study of the gravitational collapse of spherical structures made up of gas and black matter, by using hydrodynamic models. The equations that determine the fluid dynamics of black matter were obtained on a purely thermodynamic basis (Chièze, Teyssier and Alimi, 1997) or by introducing a natural closing of the series of moments of the Vlasov equation (Teyssier, Chièze and Alimi, 1997). The collapse of structures in an Einstein-de Sitter universe could thus be studied at very high resolution of 0.1 pc in central regions. This new method was successfully tested on the auto-similar solutions of Fillmore & Goldreich (1984), and on new solutions found in the context of that work. In these models, the gas does not strictly follow the black matter but, on the contrary, the gas mass fraction is an increasing function of the radius. This result is interpreted in the context of a spherical model, noting that the number of degrees of freedom of the particles of black matter is reduced to one unit, and not three, for ordinary matter.
The main limitation of these simulations remains the necessarily finite size of the number of meshes, which can be included in the calculation. With a fixed mesh, this results in a limitation on the final space resolution of the simulation and this, in practice, prevents us from examining at smaller scale than that for superclusters or for clusters, at best. Romain Teyssier developed a new digital simulation code with an adaptive mesh operating according to the principle of successive dichotomies: this is the RAMSES code. This can be used to follow the formation of structures and their fragmentation in the context of a non-collisional gas, 2563 particles and 190 106 points in the grid (Teysier, 2001). This code can be used to follow the fragmentation of structures down to the scale of galaxies (fig. 9). A modification of this code has been obtained with the addition of a gaseous component in addition to particles of black matter. This has made it possible to study the evolution of hot gas in clusters and the importance of the SZ effect on measurements of cosmological diffused background (Refrégier and Teyssier, 2001).
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| DSM/DAPNIA/Service d'Astrophysique | mise à jour : 15/10/2001 |
| Computational cosmology | © CEA 2001 - Tous droits réservés |
