The late-time accelerated expansion of the Universe is an empirically established phenomenonwhose onset occurred at a cosmic lookback time of approximately seven billion years. This timing isoften interpreted as a coincidence or as the outcome of model-dependent dynamics associated withdark energy or modified gravity.In this work, we demonstrate that the onset epoch of late-time acceleration is neither contingentnor microphysically determined. Instead, it is a structural consequence of generally covariant cosmological dynamics once two empirically established facts are imposed: the existence of a prolongedmatter-dominated era and the observed present-day energy budget of the Universe.We derive a model-independent acceleration condition and establish a no-go theorem: in anyspatially homogeneous and isotropic Universe governed by covariant field equations with a conservedenergy–momentum tensor, accelerated expansion cannot begin arbitrarily early or arbitrarily latewithout violating either the existence of the matter-dominated epoch or the observed expansionhistory. Under minimal assumptions, the transition to acceleration is forced to occur at redshift oforder unity.We further show that all observationally admissible explanations of late-time acceleration reduceto distinct realizations of a single structural condition. The observed timing follows inevitably fromthe scaling hierarchy of cosmological components rather than from detailed microphysics.This article constitutes the first part of a three-paper series. The companion works address,respectively, the admissible magnitude of the effective acceleration-driving contribution and thestructural origin of its tracking of the cosmological horizon Structural Explanation of Late-Time Cosmic Acceleration A Three-Paper Series in Relativistic Cosmology This research program presents a model-independent, structurally closed explanation of late-time cosmic acceleration within the framework of relativistic cosmology. The series consists of three tightly connected articles, each addressing a logically distinct aspect of the phenomenon. Rather than proposing new dynamical fields, modified gravity models, or speculative microphysics, the series identifies those features of cosmic acceleration that are already fixed by general covariance, cosmological history, and observational accessibility. Article I Late-Time Cosmic Acceleration as a Structural Necessity of Relativistic Cosmology The first article establishes that the timing of late-time cosmic acceleration is not contingent or model-dependent. Under minimal assumptions—spatial homogeneity and isotropy, general covariance, conservation of the energy–momentum tensor, and the existence of a prolonged matter-dominated era—the transition from decelerated to accelerated expansion is forced to occur at redshift of order unity. This result closes the logical space of explanations for when acceleration begins. Article II On the Admissible Magnitude of the Effective Acceleration-Driving Contribution in Relativistic Cosmology The second article addresses the long-standing question of the magnitude of the acceleration-driving contribution. It demonstrates that values substantially larger or smaller than the observed one are either empirically excluded, dynamically irrelevant, or observationally indistinguishable. The observed magnitude is shown to lie within a narrow admissible window defined by structural constraints rather than microphysical tuning, reframing the cosmological constant problem as a question of microscopic origin rather than macroscopic admissibility. Article III Why the Vacuum Scale Tracks the Cosmological Horizon: Structural Constraints from Covariance and Observability The third article explains why the effective vacuum scale relevant for cosmic acceleration tracks the cosmological horizon. It is shown that any gravitationally active vacuum contribution compatible with cosmic history must be infrared-dominated and covariant, leaving the Hubble scale as the unique admissible geometric scale. This result closes the remaining conceptual gap between vacuum physics and cosmological acceleration. Structural Closure Together, the three articles provide a complete, internally consistent, and model-independent explanation of late-time cosmic acceleration at the level of relativistic cosmology: Article I: inevitability of timing Article II: admissible magnitude Article III: horizon-scale origin The series establishes which aspects of cosmic acceleration are structurally fixed and which questions remain genuinely open at the level of microscopic physics.
Serge Kolesnyak (Tue,) studied this question.