Flocking round and round
I will discuss the new features that arise when polar flocking takes place on the round 2-sphere rather than in the plane. The topology of the 2-sphere requires that flocking states exhibit defects even in the steady state. In the presence of spontaneous flow the system also supports long-wavelength propagating sound modes gapped by the (constant) spatial curvature. I will discuss the steady state profile of such an active polar flock and show that curvature and active flow together result in symmetry-protected topological modes localized to a great circle. These modes are the analogue of edge states in electronic quantum Hall systems and provide unidirectional channels for information transport in the flock, robust against disorder and backscattering.