We propose an effective cosmological framework in which the observed accelerated expansion is described by a time-dependent cosmological parameter (t) that is not postulated but arises as a boundary-induced, causally retarded geometric quantity associated with a dynamical horizon. The model is formulated as a minimal extension of standard Friedmann cosmology and does not require a fundamental vacuum energy density. The evolution of (t) is governed by a dimensionless function q (a), defined through the coarse-grained evolution of the horizon scale, which establishes a direct mapping between the cosmological expansion history and an effective dark energy equation of state w (a). In this framework, (t) appears as a geometric response to boundary conditions rather than as a material energy component. We show that homogeneous but anisotropic cosmological solutions dynamically approach an isotropic Friedmann–Robertson–Walker regime, with isotropy emerging as a boundary-driven attractor provided (t) remains positive and slowly varying, ensuring consistency with the observed isotropy of the cosmic microwave background without imposing it as an initial condition. The model is confronted with observational data using cosmic chronometers and measurements of structure growth, and a Bayesian MCMC analysis demonstrates that the framework is statistically consistent with current observations of H (z) and f₈ (z) while naturally satisfying Big Bang nucleosynthesis constraints. The resulting parameter space allows for a mildly evolving (t), with the standard CDM model recovered as a stationary limiting case. Our results indicate that cosmic acceleration can be consistently described as an effective horizon-induced phenomenon with clear observational signatures, providing a viable and testable alternative to a strictly constant cosmological constant.
Tutaev et al. (Mon,) studied this question.