Why a Cosmological World Model Is Not Enough: On the Overextension of the Unity Claim in Modern Cosmology

Modern cosmology is marked by persistent tensions among different measurement methods, ranges of scale, and cosmological epochs. These tensions are often treated as local inconsistencies within a fundamentally unified theoretical framework. This paper proposes a different methodological reading: persistent tensions, provided they are stably reproducible and arise at transitions between modeling regimes, may indicate an overextension of the cosmological claim to unity. This paper is not offered as a physical alternative to modern cosmology, but as a philosophy-of-science case analysis of domain-bound model validity. It argues that cosmological practice already operates with different modeling regimes at the operational level, but that this domain dependence is not always consistently translated into an explicit theory of validity, domain boundaries, transition rules, and the level at which falsification applies. Cosmological tensions are thereby not relativized, but made legible at a different level: as possible indications that not only parameters or auxiliary assumptions, but model-domain assignments themselves may become subject to assessment. What is proposed is therefore not the abandonment of scientific unity, but its reorganization as second-order unity. Unity then no longer resides exclusively in a single model that is supposed to bear all cosmological domains directly, but in explicit rules of domain assignment, controlled transitions, overlapping testing zones, and shared consistency conditions. The level of falsification thereby shifts: not only individual parameters are at stake, but also the question of whether a model is still appropriately assigned to a particular domain. The paper therefore decides no physical hypothesis, but develops a methodological architecture in which model plurality and scientific unity are related to one another in a controlled way.