Stepped Partially Acoustic Dark Matter (SPartAcous): A new step in interacting dark sector cosmologies for the Hubble tension and Large Scale Structure

We propose Stepped Partially Acoustic Dark Matter (SPartAcous), a new class of interacting dark sector models with a qualitatively new cosmological behavior that can simultaneously address the two most important tensions in cosmological data, the H₀ and S₈ tensions. The defining element of this class of models is an interacting dark radiation that includes a component with a light mass, which becomes non-relativistic close to the time of matter-radiation equality, and which interacts with a fraction of dark matter. This generalizes the scenario known as Partially Acoustic Dark Matter (PAcDM) by adding a mass to one of its massless dark radiation components. As the light component annihilates, the remaining dark radiation heats up, leading to a step-like increase in N_eff, which has been shown to significantly reduce the H₀ tension. In addition, the interaction with dark matter reduces the matter power spectrum at small scales and thus alleviates the S₈ tension, while its eventual decoupling leaves the power spectrum at larger scales identical to ΛCDM.