Delayed Geometric Response in Cluster Mergers: Predictive Evidence from the Bullet Cluster and Abell 520

Abstract

When galaxy clusters collide, their gas slows—but does the gravitational field stop with it? We explore a possibility: spacetime retains a memory of its pre-collision configuration, continuing outward before readjusting toward the new central concentration of matter. Using gravitational lensing—the distortion of distant galaxy images—we test this picture in the Bullet Cluster, Abell 520 and other merging systems. Measurements outside Abell 520’s collision centre predict the lensing signal inside it. In the Bullet Cluster, the galaxy distribution predicts that signal better than the shocked gas. Across systems at different collision stages, the gas–lensing ordering reverses as expected from the proposed advance-and-return sequence. These results support a gravitational response with memory, without introducing separate dark-matter halo profiles into the predictive models.