Resonant neutrino self-interactions and the $H_0$ tension

In this work, we study the previously unexplored resonant region of neutrino self-interactions. Current disagreement on late and early time observations of the Universe expansion could be solved with new physics acting before the recombination era. Nonstandard neutrino self-interactions are among the most appealing candidates to solve this issue since they could be testable in the near (or midterm) future. We use linear cosmological datasets to test neutrino self-interactions for a sample of fixed scalar mediator masses in the range $10^{-2}$ eV $\leq m_{\varphi}\leq 10^{2}$ eV. The resonant behavior produces observable effects at lower couplings than those reported in the literature for heavy and light mediators. We observe that in the best case scenario, using the Planck + BAO dataset, the tension with local measurements of $H_0$ eases from 4.9$\sigma$ (for $\Lambda$CDM) down to 2.8$\sigma$. Albeit, this is driven mainly by the addition of extra radiation, with $\Delta N_{\rm eff}\sim 0.5$. The joint dataset which includes Planck, BAO, and $H_0$ prefers a nonzero interaction from 2.3$\sigma$ to 3.9$\sigma$ significance in the range $0.5$ eV $\leq m_{\varphi}\leq 10$ eV. Although, this last result is obtained with data that are still in tension. These results add the last piece in the parameter space of neutrino self-interactions at the linear perturbation regime.