A Conceptual Note on Response-Defined Mass from Black Hole Ringdown to Cosmological Inference

Mass plays a central role across gravitational physics, from black hole dynamics to cosmological parameter inference. In most applications, mass is implicitly treated as an intrinsic andobserver-independent property. In this conceptual note, we revisit this assumption and proposethat, in a broad class of observational contexts, mass is more appropriately interpreted as aresponse-defined coefficient, emerging from how a system reacts to external or environmentalprobes. Focusing first on black hole ringdown as a well-controlled local observable, we thenextend the same interpretational framework to cosmological inference, where mass and energyparameters are extracted indirectly from large-scale observations. We emphasize that no newdynamics or modified equations are introduced. Rather, this work aims to clarify the conceptual status of mass across scales and to highlight potential implications for the interpretation ofobservational uncertainties and apparent parameter inconsistencies.