Cosmological Tensions and the Interacting Dark Sector: Observational Motivation and Theoretical Constraints

Abstract

The ΛCDM model has been remarkably successful in describing the large-scale evolution of the Universe, yet persistent discrepancies in key cosmological parameters increasingly challenge its completeness. In particular, the growing tension between early- and late-Universe measurements of the Hubble constant (H₀), along with inconsistencies in the amplitude of matter clustering quantified by S₈, suggests that the standard assumption of non-interacting dark components may require revision. This review examines the Interacting Dark Sector (IDS) hypothesis, in which dark matter and dark energy are allowed to exchange energy and momentum while preserving total energy–momentum conservation. We survey phenomenological coupling models, including density-dependent interactions and running vacuum scenarios, and discuss their impact on the expansion history, structure formation, and cosmological observables. By synthesizing recent theoretical developments with constraints from cosmic microwave background measurements, large-scale structure surveys, and distance-ladder observations, we assess the extent to which IDS models can simultaneously alleviate the H₀ and S₈ tensions. We further examine theoretical challenges associated with stability, thermodynamic consistency, and the lack of a microphysical origin for the coupling. We conclude by outlining observational prospects for testing dark sector interactions with forthcoming surveys and discuss whether the interacting paradigm represents a viable extension of ΛCDM in the era of precision cosmology.