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

We perform a comprehensive study of cosmological constraints on nonstandard neutrino self-interactions using cosmic microwave background 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 nonstandard values of Neff. In several cases, we find additional modes with neutrino decoupling occurring at lower redshifts zdec∼10^(3–4). 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 are included, none of these examples eases 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.