Two of the most pressing questions in cosmology are the nature of dark matter, and how the Universe began. When we say we do not know what dark matter is, what we mean is that we cannot characterise it as a particle – the mass, spin, self interaction and couplings to standard model matter are unknown. When we say we do not know how the Universe began, we mean that accepted paradigms like inflation still leave many questions unanswered at the earliest moments after the Big Bang. This project has pushed forward our knowledge in these areas, highlighting the model dependence of inflationary models and clarifying the behaviour of dark matter environments around black holes.
A key highlight this year has been the extension of our studies of light dark matter candidates to the binary case, where we have found density enhancements between the black holes that impact on the merger part of the gravitational wave signal. Such effects are small and need to be distinguished from other parameters of the binary, but have the potential to shed light on the particle nature of dark matter.