Self-interacting neutrinos, the Hubble parameter tension, and the cosmic microwave background

We perform a comprehensive study of cosmological constraints on non-standard neutrino self-interactions using cosmic microwave background (CMB) and baryon acoustic oscillation data. consider different scenarios for distinguished by the fraction states allowed to participate in how relativistic energy density, N$_{\textrm{eff}}$, is vary. Specifically, we cases which: all self-interact N$_{\textrm{eff}}$ varies; two species free-stream, which show alleviates tension with laboratory constraints, while additional interacting variable neutrinos either total fixed Standard Model value or In no case do find compelling evidence new interactions values N$_{\textrm{eff}}$. several modes decoupling occurring at lower redshifts $z_{\textrm{dec}} \sim 10^{3-4}$. careful analysis examine whether solve alleviate so-called $H_0$ that, when Planck 2018 CMB temperature polarization data included, none these examples ease more than allowing comprised free-streaming particles. Although focus interactions, are applicable any light relic particle.