A low-redshift preference for an interacting dark energy model

Abstract 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 1 σ 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 Ω m compared to CMB constraints. Although the interaction remains weak and does not significantly alleviate the H 0 and S 8 tensions, our results highlight the potential role of dark sector interactions in late-time cosmology.