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Abstract

The goal of this thesis are numerical studies of the era of reionization, which took place at about a 150 million years to a billion years after the Big Bang. Since reionization is a process driven by radiation, a major fraction of this work is dedicated to numerical methods for radiative transfer. In particular, we develop the Sweep method, which allows us to study reionization within large cosmological simulations. We begin by in- troducing the basics of the Sweep algorithm and its implementation in the simulation code Arepo. We discuss the motivation behind it, how it integrates with the rest of Arepo and perform a number of tests to assess its performance and physical accuracy. We find that the Sweep method does not only produce physically accurate results, but does so in a very efficient manner, even when applied to large simulations on a large number of processors. We then proceed by introducing the standalone radiative trans- fer postprocessing code Subsweep in which we add a variety of improvements to the original Sweep method, in particular the addition of sub-timesteps. We perform a number of additional tests to verify that Subsweep correctly solves a number of physi- cal problems and show that sub-timesteps can drastically improve performance of the Sweep algorithm when applied to problems with heterogeneous environments without sacrificing accuracy. Finally, we apply Subsweep to the cosmological simulation suite TNG in order to recreate the era of reionization in the TNG universe. We find that Subsweep allows us to study the spatial structure of reionization in detail and that we can reproduce the observational constraints on the history of reionization reasonably well.