Other
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........8f&link_type=abstract
Thesis (PH.D.)--UNIVERSITY OF WASHINGTON, 1995.Source: Dissertation Abstracts International, Volume: 57-02, Section: B, page: 11
Other
Press Schechter Theory
Scientific paper
Large-scale structures in the universe observed today arise from density fluctuations generated in the early universe. Theoretically, we need to understand how initial fluctuations are generated and how they evolve to form structures observed today. Observations, on the other hand, provide tests and constrains for theoretical models. In this dissertation, we first discuss a specific theoretical model of structure formation to demonstrate how the predictions of a model depend on the specific assumptions of the model. Then we study whether a statistical method is appropriate to extract desired information from observations, with particular attention to whether the one-point distribution of individual Fourier modes of the density perturbation field can tell us the statistical properties of the density field itself. Rapid developments of the techniques of numerical simulations allow us to study the nonlinear evolution of a system and make the direct comparison between theoretical predictions and observations possible. On the other hand, a semi-analytical model of structure formation can help us to understand the outcome of numerical simulations physically if the model is reasonably correct and further can be used to study the structure formation without expensive numerical simulations. The validity of a semi-analytical model, however, needs to be tested by numerical simulations. In the last three chapters of the dissertation, we develop a numerical method to identify particle groups from a simulation, and study the validities of the Press-Schechter theory and the peak theory of structure formation. We find on the mass scale of particle groups M > 2 times 10^{12 } Modot, where Modot is the solar mass, the group mass function predicted by Press -Schechter theory agrees well with the simulation result and the groups in the simulation are well correlated with linear density peaks. On smaller mass scales, because of statistical fluctuations and the nonnegligible effect of the 'top-down' structure formation, i.e., larger structures form earlier and then they fragment into smaller structures, the theories are less successful than on larger mass scales.
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