A low-redshift preference for an interacting dark energy model

We explore an interacting dark sector model in trace-free Einstein gravity where dark energy has a constant equation of state, w = −1, and the energy-momentum transfer potential is proportional to the cold dark matter density. Compared to the standard ΛCDM model, this scenario introduces a single additional dimensionless parameter, ϵ, which determines the amplitude of the transfer potential. Using a combination of Planck 2018 Cosmic Microwave Background (CMB), DESI 2024 Baryon Acoustic Oscillation (BAO), and Pantheon+ Type Ia supernovae (SNIa) data, we derive stringent constraints on the interaction, finding ϵ to be of the order of ∼ O(10−4). While CMB and SNIa data alone do not favor the presence of such an interaction, the inclusion of DESI data introduces a mild 1σ preference for an energy-momentum transfer from dark matter to dark energy. This preference is primarily driven by DESI BAO measurements below redshift 1.4, which favor a slightly lower total matter density Ωm compared to CMB constraints. Although the interaction remains weak and does not significantly alleviate the H0 and S8 tensions, our results highlight the potential role of dark sector interactions in late-time cosmology.