As an observational cosmologist, I study how structure forms in the Universe. As part of this, I have pioneered techniques to find the earliest galaxies. These are key objects in astronomy, they are seen when the Universe is young and so unlike galaxies seen today, there is little time for many complicated processes to affect them.
I also study the early stages of the formation of galaxy clusters, collections of tens or hundreds of galxaies bound together by their mutual gravity. The clusters discovered to-date show little sign of strong evolution in their properties with cosmic time. This is a puzzle, these systems are meant to form when the Universe is only half its present age, why do we see no sign evolution in their properties at around this time? I search for clusters observed ever earlier in the history of the Universe in order to find the epoch when they form.