The Cosmological Constant as a Present Effective Dark-Sector Value: A Compact Phenomenological Parametrization for Cosmic Acceleration That Need Not Be Eternal

This work proposes a compact phenomenological parametrization in which the quantity observationally identified with the cosmological constant is interpreted as the present effective value, Λeff(t0), of a dark component that remains close to the Lambda cold dark matter (ΛCDM) model over the recent cosmological epoch but is not required to remain constant in the remote future. The purpose is not to introduce dynamical dark energy in general, which has already been extensively investigated, but to formulate a minimal subclass in which the present effective value associated with Λ and the asymptotic fate of the expansion are explicitly separated. Within this framework, the current dominance of the dark component does not require an increase in Λeff: it can arise because the matter density dilutes as a−3, whereas Λeff varies more slowly. A delayed-decay function is introduced, normalized to the present effective value and allowing Λeff→0 as a→∞. In the strong version of the parametrization, identified by nf>2, the effective component is asymptotically unable, by itself, to sustain cosmic acceleration indefinitely. The proposal is not presented as a microscopic theory of dark energy, but as a background-level effective dark-sector parametrization that may serve as a phenomenological benchmark for particle-cosmology scenarios and can be constrained through measurements of H(z) and cosmological distances, together with observational constraints on w(a).