Ultra-light axions as dark matter candidates

Dark matter is one of the great puzzles in physics. One of the possible models is a Bose-Einstein condensate of very light particles. I will consider ultra-light axions and their signatures on galactic scales, focusing on two specific phenomena. One is the backreaction of ultra-light axions, and the second is the resonance between ultra-light axions and binary pulsars. I will explain how coherent oscillations of the axion field backreact on the evolution of the condensate wave function of ultra-light axions in the non-relativistic regime appropriate to cosmic structure formation. I will also discuss the impact of axion oscillations on binary pulsars. Resonances between axions and binary pulsars lead to orbital perturbations, which can be used to constrain the range of viable axion masses.