Birth, Death and Flight: A Theory of Malthusian Flocks

I study “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 $<\vec{v}(\vec{r},t)>\mathrm{\ne }\vec{0}$) is possible in these systems, even in spatial dimension $d=2$. Their spatio-temporal scaling structure can be determined exactly in all spatial dimensions; 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.