Solving the Hubble tension without spoiling Big Bang Nucleosynthesis

Abstract

The Hubble parameter inferred from cosmic microwave background observations is consistently lower than that from local measurements, which could hint towards new physics. Solutions to the Hubble tension typically require a sizable amount of extra radiation $\Delta N^{}_{\rm eff}$ during recombination. However, the amount of $\Delta N^{}_{\rm eff}$ in the early Universe is unavoidably constrained by Big Bang Nucleosynthesis (BBN), which causes problems for such solutions. We present a possibility to evade this problem by introducing neutrino self-interactions via a simple Majoron-like coupling. The scalar is slightly heavier than $1~{\rm MeV}$ and allowed to be fully thermalized throughout the BBN era. The rise of neutrino temperature due to the entropy transfer via $\phi \to \nu\overline{\nu}$ reactions compensates the effect of a large $\Delta N^{}_{\rm eff}$ on BBN. Values of $\Delta N^{}_{\rm eff}$ as large as $0.7$ are in this case compatible with BBN. We perform a fit to the parameter space of the model.