Dynamical heterogeneity in active glasses is inherently different from its equilibrium behavior

Abstract: Activity-driven glassy dynamics, while ubiquitous in collective cell migration, intracellular transport, dynamics in bacterial and ant colonies, etc., also extend the scope and extent of the as-yet mysterious physics of glass transition. Active glasses are hitherto assumed to be qualitatively similar to their equilibrium counterparts at an effective temperature, $T_{eff}$. Here we combine large-scale simulations and an analytical mode-coupling theory (MCT) for such systems and show that, in fact, an active glass is inherently different from an equilibrium glass. Although the relaxation dynamics can be equilibrium-like at a $T_{eff}$, the effects of activity on the dynamical heterogeneity (DH), which has emerged as a cornerstone of glassy dynamics, are quite nontrivial and complex. With no preexisting data, we employ four distinct methods for reliable estimates of the DH length scales. Our work shows active glasses exhibit dramatic growth of DH and systems with similar relaxation times, and $T_{eff}$ can have widely varying DH. To theoretically study DH, we extend active MCT and find excellent agreement between the theory and simulation results. Our results question the supposedly central role of DH in glassy dynamics and can have fundamental significance even in equilibrium.