Fluctuation-Induced First-Order Transition to Collective Motion

The transition to collective motion is of paradigmatic importance within active matter, with implications in fields as diverse as animal behavior or biology. However, it remains difficult to assess the nature of this transition as it seems to vary depending on the microscopic interactions at play. Starting from mean-field hydrodynamics predicting a continuous emergence of collective motion, we show that fluctuations generically induce a density-dependent shift of the onset of order, which, in turns, changes the nature of the transition into a phase-separation scenario. Our results apply to a range of systems, including 'metric' models where alignment occurs up to a finite distance. They also hold for 'metric-free', or 'topological', models in which particles interact with their k nearest neighbors and for which the onset of order was so far believed to be continuous. Our analytical predictions are confirmed by numerical simulations of fluctuating hydrodynamics and microscopic models.