On the Euler–Poisson Equations with Variable Background States and Nonlocal Velocity Alignment

Abstract. We study the 1D pressureless Euler–Poisson equations with variable background states and nonlocal velocity alignment. Our main focus is the phenomenon of critical thresholds, where subcritical initial data lead to global regularity, while supercritical data result in finite-time singularity formation. The critical threshold behavior of the Euler–Poisson-alignment (EPA) system has previously been investigated under two specific setups: (1) when the background state is constant, phase-plane analysis was used in [Bhatnagar, Liu, and Tan, J. Differential Equations, 375 (2023), pp. 82–119] to establish critical thresholds; and (2) when the nonlocal alignment is replaced by linear damping, comparison principles based on Lyapunov functions were employed in [Choi, Kim, Koo, and Tadmore, Ann. Inst. H. Poincaré C, 43 (2025), pp. 203–237]. In this work, we present a comprehensive critical threshold analysis of the general EPA system, incorporating both nonlocal effects. Our framework unifies the techniques developed in the aforementioned studies and recovers their results under the respective limiting assumptions. A key feature of our approach is the oscillatory nature of the solution, which motivates a decomposition of the phase plane into four distinct regions. In each region, we implement tailored comparison principles to construct the critical thresholds piece by piece.