HDR : Generic and universal properties of active systems

Active systems break detailed balance at the scale of the single constituent. Examples range from living systems such as animals, bacteria or algae to systems locally out of equilibrium due to non-equilibrium chemical reactions, self-propelled colloids, or at completely different spatial and temporal scales, ecological or social systems. They contrast with types of nonequilibrium system more often studied in the past, where driving is imposed at the boundary while the bulk dynamics respects detailed balance in its absence. As such, active systems display, at macroscopic level, a series of collective phenomena that are impossible in equilibrium systems. Being very complex at microscopic level, one of the key questions in understanding them is to describe which universal and generic properties can emerge, respectively meant as quantitative and qualitative properties that are independent of system-details. This thesis addresses this question in a series of different active systems, among which: active systems undergoing phase separation, displaying polar or nematic order, and in dilute suspensions of microswimmers.