Simulations of Structure Formation in the Universe: Hot vs Cold Dark Matter

Sammanfattning: In the last century, new observational techniques and discoveries such as the Cosmic Microwave Background Radiation have brought a new dimension of knowledge about the Universe. Therefore new theories and models have been proposed to explain the observed Universe. Computer simulations are a very important tool because they lay a bridge between theory, often over-simplified, and observations, which reveal the complexity of our Universe.

In this thesis, it is given a review of observations including the most important discoveries and results that help to describe the Universe and have been used to develop the models considered nowadays. The cosmological theory behind the large-scale structure formation is explained, from the basis of the Friedman model to the formation of structures through the linear, quasi-linear and non-linear regime, including the Zeldovich approximation and the spherical collapse model. Furthermore, the different types of codes used for cosmological simulations are introduced, focusing on the N-body codes and presenting the code used in this thesis, developed by Klypin & Holtzman (1997). The tools used to analyse the results: density plots, power spectrum and mass variance are described as well.

Three main sets of simulations have been performed: a basic simulation (RUN0) with standard cosmological parameters, simulations of ΛCDM and simulations of Hot+Cold Dark Matter (HCDM). All the simulations use 32³ particles, while dierent cosmological parameters have been changed e.g. σ₈, Ω_m, Ω_λ and n. Thus, it is observed that higher values of Ω_m and low values of Ω_λ lead to more clustering and hence more developed structures. Moreover, the effect of σ₈ appears to be critical, since it determines the amplitude of the density fluctuations at the initial redshift of the simulation. When studying the presence of hot dark matter, the main difference comes from the cut-off in the power spectrum due to the hot dark matter free-streaming, resulting in less developed structures. Similarly to the previous case, the effects of the cosmological parameters are explained forthis model.

Finally, some additional simulations regarding dark halos populations and density profiles are included in the Appendix.