This happened more than 13 billion years ago when the Universe was only a few 100 million years old. These sources were contained in the first galaxies to form in the Universe. These galaxies contained hot stars, which produced extreme ultra-violet radiation and black holes, which produced x-rays by accreting large amounts of gas. The influence of the radiation from these objects changed the entire Universe from a cold and neutral to a hot and ionized state. The process is complicated and requires computer simulations to understand. These simulations are often run on supercomputers. Kai Yan's work will help to make better use of these supercomputers and make predictions for what future observations with for example the Square Kilometre Array will show.

Kai Yan is a PhD student who arrived from Hong Kong at the end of 2011. After his PhD he will return to Hong Kong to pursue a career outside of astronomy.