From Linear Distance to Curved Time: Reframing the Observable Universe Without Expanding Space

The observable universe is commonly defined as the spatial region from which light has had sufficient time to reach an observer since the early universe, often framed through the metaphor of uniformly expanding space. While operationally effective, this description introduces persistent conceptual tensions concerning distance, causality, and cosmic horizons, particularly when present-day separations exceed naive light-travel expectations. In this paper, we present a structure-first reinterpretation of the observable universe in which observability is defined not by spatial extent but by the admissibility of signal intersections along an observer’s worldline under an evolving spacetime metric. Within this framework, cosmological horizons arise as temporal and structural boundaries rather than spatial edges, and large inferred distances emerge from projecting curved temporal paths onto linear coordinate descriptions. This approach preserves all standard cosmological observations and equations while clarifying the geometric origin of apparent paradoxes associated with expanding space. By restoring curvature and temporal structure to their primary role, the observable universe is more naturally understood as a history-bounded subset of spacetime rather than a growing spatial volume.