Interacting Dark-Sector Models with Bulk Viscosity Under Dynamical Stability and Observational Constraints

Although the standard model of cosmology (ΛCDM) has been successful in explaining a wide range of observational phenomena, it still faces significant limitations, particularly regarding the nature and properties of the dark sector of the universe. These challenges motivate the exploration of alternative approaches that can deepen our understanding of cosmic evolution. This study proposes an extension of the ΛCDM model by incorporating irreversible processes through a viscous fluid with out of equilibrium pressure. This fluid interacts dynamically with the dark sector components, offering new possibilities for describing the expansion of the universe. To constrain the proposed scenarios, we perform a Bayesian statistical analysis using the Pantheon+ Type Ia supernova compilation, Cosmic Chronometers, and DESI DR2 Baryon Acoustic Oscillation measurements. The observational results show that all three interaction models successfully reproduce the late-time expansion history while preserving a dark matter barotropic index close to the pressureless limit expected in the standard cosmological scenario. In contrast, the dark energy barotropic index consistently favors positive values, suggesting a mild departure from the cosmological constant description. The incorporation of Cosmic Chronometers and, particularly, BAO measurements substantially improves the constraints on the matter sector, leading to a significant reduction in the uncertainty of the present dark matter density parameter, whereas the bulk-viscosity parameter and the asymptotic ratio remain only weakly constrained. Finally, a Bayesian model comparison based on the Bayes factor shows that the interacting dissipative models remain observationally compatible with the reference ΛCDM cosmology. According to the Jeffreys scale, the resulting Bayesian evidence is inconclusive, indicating that current observations neither favor nor disfavor these extensions with respect to the standard cosmological model.