Regime-Dependent Spacetime: A Conceptual Framework for Environment-Dependent Propagation Effects in Cosmology

Modern cosmology assumes that spacetime on large scales is sufficiently homogeneous and isotropic such that the propagation of light over cosmological distances can be treated as occurring through a uniform metric. This assumption underpins distance inference, redshift interpretation, and the mapping between observed signals and cosmic history. This work proposes a conceptual framework—Regime-Dependent Spacetime (RDS)—in which spacetime may exhibit distinct effective propagation regimes depending on environmental factors such as matter density, gravitational structure, or large-scale void dominance. Under this hypothesis, local spacetime physics remains consistent with established theory, while ultra-low-density regions may exhibit subtly different effective metrics for signal propagation without invoking violations of local causality or known physical constants. The framework does not propose faster-than-light transmission or modification of local physical laws, but instead explores whether cumulative propagation effects across different spacetime regimes could bias cosmological distance inference, potentially influencing interpretations of late-time acceleration or large-scale structure observations. This paper is explicitly exploratory and non-mathematical, intended to clarify conceptual assumptions, identify falsifiable implications, and outline observational domains where regime-dependent effects—if present—would be constrained or ruled out. The goal is not to replace existing cosmological models, but to examine whether an implicit uniform-metric assumption warrants further scrutiny in the era of precision cosmology. _________ Version note: Version 2 adds a short section clarifying potential falsification paths and observational relevance. No substantive claims or conclusions have been altered.