MPIDS Seminar: Birth, Death, and Flight: the hydrodynamics of Malthusian flocks

I'll present the hydrodynamic theory of "Malthusian Flocks": moving aggregates of self-propelled entities (e.g., organisms, cytoskeletal actin, microtubules in mitotic spindles) that reproduce and die. Long-ranged order (i.e., the existence of a non-zero average velocity ⟨v⃗ (r⃗ ,t)⟩≠0⃗ ⟨v→(r→,t)⟩≠0→) is possible in these systems, even in spatial dimension d=2d=2. Their spatiotemporal scaling structure can be determined exactly in d=2d=2; furthermore, they lack both the longitudinal sound waves and the giant number fluctuations found in immortal flocks. Number fluctuations are very {\it persistent}, and propagate along the direction of flock motion, but at a different speed. I'll also present recent results for the three dimensional version of this problem, which required the first full blown dynamical renormalization treatment of a flocking system in its ordered phase.