Collective modes of axionic dark matter

The axion is a dark matter candidate which displays novel properties because of its small mass. It has self-interactions, which lead to collective modes. We develop a theory of these interacting modes inside a dense body of axions. It takes a various forms, depending on the background we linearize about. The common key feature of these modes is that their dispersion relation is dictated by the coupling strength. Based on work with Alexander Balatsky and Henrik Roising.