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. We consider different scenarios for neutrino self-interactions distinguished by the fraction of neutrino states allowed to participate in self-interactions and how the relativistic energy density, Neff, is allowed to vary. Specifically, we study cases in which: all neutrino states self-interact and Neff varies; two species free-stream, which we show alleviates tension with laboratory constraints, while the energy in the additional interacting states varies; and a variable fraction of neutrinos self-interact with either the total Neff fixed to the Standard Model value or allowed to vary. In no case do we find compelling evidence for new neutrino interactions or non-standard values of Neff. In several cases we find additional modes with neutrino decoupling occurring at lower redshifts zdec ~ 10³⁻⁴. We do a careful analysis to examine whether new neutrino self-interactions solve or alleviate the so-called H₀ tension and find that, when all Planck 2018 CMB temperature and polarization data is included, none of these examples ease the tension more than allowing a variable Neff comprised of free-streaming particles. Although we focus on neutrino interactions, these constraints are applicable to any light relic particle.