Cosmic Dawn and the Epoch of Reionization
About 100 million years after the Big Bang, gravitational collapse from the initial density fluctuations in the Universe produced the first structures (proto-galaxies) capable of forming stars. The radiation produced in these proto-galaxies partly escaped and started to ionize the immediate surroundings of each proto-galaxy. As more and more proto-galaxies form, these regions start to overlap and grow. This process is known as the Reionization of the Universe and lasted until 1000 million years after the Big Bang, when all of the material between the galaxies had become ionized. This is the state it has remained in until the present day. This process is not well understood and currently at the forefront of cosmological research as a number of observational experiments such as the European LOFAR radio telescope and the future global Square Kilometre Array (SKA) aim at detecting signposts from the Reionization process. In this project we perform large scale simulations of the reionization process, by calculating the transfer of ionizing photons through the evolving three-dimensional density field of a representative volume of the Universe. These are obtained from (previously run) large cosmological simulations. The radiation sources (proto-galaxies) are also found from these cosmological simulations. In a typical volume, towards the end of Reionization, tens of millions of sources are present. Numerical simulations are the only way to properly deal with the complex structure of the intergalactic density field, and the spatial distribution of sources, which is not random but clustered. The result of the simulations is a history of the distribution of ionized material as well as the temperature of the gas from when the first sources formed to end of Reionization. Such histories are subsequently used to characterize the expected observational signals, which helps in the design and future analysis of the observational experiments. In this project we will specifically investigate how X-ray radiation produced in very first (proto-)galaxies heated the then still mostly neutral medium, a period known as the Cosmic Dawn. The heating process is important for the both the SKA and LOFAR measurements as a cold medium will give an absorption signal whereas a heated medium will give an emission signal. We also develop a framework for the interpretation of the LOFAR results which relies on the combination of fast approximate simulations and slow detailed simulations.