Cosmological probes of Dark Radiation from Neutrino Mixing

TL;DR

This paper investigates models of dark radiation with a step in abundance caused by neutrino mixing, analyzing their effects on cosmic microwave background anisotropies and potential to address the Hubble tension.

Contribution

It extends previous work by deriving full background and perturbation equations for these models and explores their cosmological implications across various parameter ranges.

Findings

Strongly self-coupled regime shows minimal cosmological impact with tight neutrino mass priors.

Larger neutrino masses allow models to interpolate between ΛCDM and self-interacting dark radiation.

Weakly self-coupled models can address neutrino anomalies and the Hubble tension.

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 $\Lambda$CDM 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.