0_5187 An excitonic insulator (EI) is an unconventional quantum phase of matter in which excitons, bound pairs of electrons and holes, undergo Bose-Einstein condensation. While several materials have emerged as promising EI candidates, distinguishing a possible EI from a normal insulator remains challenging. To address this, we focus on a clear qualitative difference between these two phases: the spontaneous breaking of a unique excitonic U(1) symmetry gives rise to low-lying Goldstone collective modes in the EI gap. We consider how disorder affects these collective modes. We show that different disorder symmetries lead to qualitatively different collective mode scattering rates. Notably, we find that unlike electrons, collective modes are highly robust against disorder that respects excitonic U(1) symmetry, implying a unique experimental fingerprint: the ballistic propagation of low-lying modes over mesoscopic distances, at electronic-scale velocities. We suggest this feature could affect thermal transport at low temperatures, and might be observed in spatially resolved pump-probe spectroscopy as Goldstone-phonon hybridized modes. In novel two-dimensional EI platforms, our treatment of EI disorder screening may be applicable to the polaron impurity problem and the modelling of device inhomogeneity.      [B. Remez and N. R. Cooper, Physical Review B 101, 235129 (2020)]