Cosmological Probes of Dark Radiation from Neutrino Mixing

Models of stepped dark radiation have recently been found to have animportant impact on the anisotropies of the cosmic microwave background, aidingin easing the Hubble tension. In this work, we study models with a sector ofdark radiation with a step in its abundance, which thermalizes after big bangnucleosynthesis by mixing with the standard model neutrinos. For this, weextend an earlier work which has focused on the background evolution only untilthe dark sector thermalizes by deriving the full background and perturbationequations of the model and implementing them in an Einstein-Boltzmann solvingcode. We expound on the behavior of this model, discussing the wide range ofparameters that result in interesting and viable cosmologies that dynamicallygenerate dark radiation during a range of epochs. We find that for the stronglyself-coupled regime, there is no large cosmological impact for a tight prior onthe mass, whereas larger mass ranges allow a smooth interpolation between abehavior close to the ΛCDM cosmological standard model and close to anadditional component of strongly self-interacting dark radiation. In the weaklyself-coupled regime we find that we can accommodate a parameter space relevantfor the neutrino anomalies as well as one relevant to easing the Hubbletension.