A Polarizable Vacuum Approach to Cosmic Dynamics: Horizon-controlled infrared vacuum response within general relativity

The purpose of this article is threefold. First, we formulate a consistent relativistic description of infrared vacuum polarisation in terms of an effective stress--energy tensor. Second, we show that, in the late-time adiabatic regime, this framework leads to a minimal and testable deformation of the standard $\Lambda$CDM background, while preserving early-Universe physics. Third, we explore the extension of the same mechanism to the primordial Universe, where the rapid evolution of the horizon can naturally give rise to a phase of accelerated expansion without introducing ad hoc degrees of freedom. Throughout this work, the emphasis is placed on conceptual coherence and effective consistency rather than on detailed data fitting. No specific microphysical model of the vacuum is assumed, no modification of the Einstein equations is introduced, and all results are derived within the standard framework of general relativity. The infrared vacuum is treated as an emergent gravitational medium whose response depends on scale and causal structure, offering a unified perspective on galactic dynamics, cosmic acceleration, and early-Universe expansion.