A Tale of Two Couplings: Bayesian Selection in the Interacting Dark Sector

Abstract

$\Lambda$CDM provides a remarkably successful description of the Universe, yet it suffers from persistent discrepancies, with one of the most prominent being the $\sigma_8$ tension between early-Universe Cosmic Microwave Background (CMB) measurements and late-time structure growth probes. To address this mismatch, we investigate generalized interacting dark sector models governed by a quintessence field coupled to dark matter via conformal and disformal transformations. By generically engineering a quintessence potential that ensures the combined evolution of dark matter and dark energy exactly mimics $\Lambda$CDM, our models satisfy all background constraints while signaling modified gravity effects within the linear perturbation sector. We perform a statistical analysis using late-time structure growth data anchored to Planck 2018 constraints. Furthermore, we address the initial conditions problem inherent to purely disformal models, demonstrating the necessity of a non-zero initial kinetic energy to initiate dynamical evolution. Our results indicate that purely conformal couplings efficiently transfer energy from dark matter to the quintessence field, suppressing late-time structure formation and successfully accommodating CMB data within late-time modified structure growth. This purely conformal limit yields a strong statistical preference over the $\Lambda$CDM baseline according to both the Akaike and Bayesian Information Criteria. Conversely, while non-zero disformal couplings introduce a kinetic friction that dampens the dark sector energy exchange, their expanded parameter space incurs heavy statistical penalties without providing a decisive improvement over the purely conformal scenario.