Abstract
Abstract
Models of stepped dark radiation have recently been found to have an important impact on
the anisotropies of the cosmic microwave background, aiding in easing the Hubble tension. In this
work, we study models with a sector of dark radiation with a step in its abundance, which
thermalizes after big bang nucleosynthesis by mixing with the standard model neutrinos. For this,
we extend an earlier work which has focused on the background evolution only until the dark sector
thermalizes by deriving the full background and perturbation equations of the model and
implementing them in an Einstein-Boltzmann solving code. We expound on the behavior of this model,
discussing the wide range of parameters that result in interesting and viable cosmologies that
dynamically generate dark radiation during a range of epochs. We find that for the strongly
self-coupled regime, there is no large cosmological impact for a tight prior on the mass, whereas
larger mass ranges allow a smooth interpolation between a behavior close to the ΛCDM
cosmological standard model and close to an additional component of strongly self-interacting dark
radiation. In the weakly self-coupled regime we find that we can accommodate a parameter space
relevant for the neutrino anomalies as well as one relevant to easing the Hubble tension.