Solving the Hubble tension without spoiling Big Bang Nucleosynthesis
  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.
