From Vacuum Tension to Dynamical Dark Energy - A Porous-Vacuum Framework Consistent with the Late-Time Evolution Favoured by Current Cosmological Data

What if dynamical dark energy were not a new cosmic component, but the observable signature of a vacuum still relaxing towards equilibrium? This technical note develops that possibility within a porous-vacuum framework. A macroscopic residual vacuum tension, S, is treated as a coarse-grained property of a structurally non-saturated vacuum. When S is allowed to evolve, covariant conservation leads directly to an effective dynamical-dark-energy behaviour: increasing vacuum tension corresponds to w < -1, a stationary point of S corresponds to w = -1, and decreasing tension corresponds to w > -1. A crossing from w < -1 to w > -1 therefore identifies a local maximum of the macroscopic vacuum tension, without requiring the introduction of a fundamental phantom field. The resulting late-time branch is consistent with the w0 > -1, wa < 0 region currently favoured, with dataset-dependent significance, by DESI DR2 combinations with CMB and Type Ia supernova data. In this framework, that phenomenological sector maps onto a vacuum tension that has increased, passed through a recent maximum, and is decreasing at the present epoch. The mature porous-vacuum state requires S to approach a finite residual value S∞ > 0, while w tends back to -1 and the Hubble rate approaches a finite asymptotic value Hmin > 0. If the present evolution of w is confirmed, it therefore cannot continue indefinitely in the same direction: it must eventually reach a future turning point and return towards -1. The framework is explicitly testable and falsifiable: it identifies specific observational and theoretical outcomes that would contradict its late-time interpretation. It is presented as an effective framework, not as a completed microscopic theory. Its central relation follows from covariant conservation; its observational compatibility is explicitly distinguished from confirmation; and the remaining open problems — the dynamical law governing S(t), perturbative stability, and a full likelihood-level cosmological fit — are stated as the next decisive tests of the model.