  How do topological defects affect the degree of order in active matter? To
answer this question we investigate an agent-based model of self-propelled
particles, which accounts for polar alignment and short-ranged repulsive
interactions. For strong alignment forces we find collectively moving
polycrystalline states with fluctuating networks of grain boundaries. In the
regime where repulsive forces dominate, the fluctuations generated by the
active system give rise to quasi-long-range transitional order, but---unlike
thermal system---without creating topological defects.

Soft Condensed Matter