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

We perform a comprehensive study of cosmological constraints on nonstandard neutrino selfinteractions 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, N-eff, is allowed to vary. Specifically, we study cases in which all neutrino states self-interact and N-eff 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 N-eff 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 N-eff. In several cases, we find additional modes with neutrino decoupling occurring at lower redshifts z(dec) similar to 10(3-4). We do a careful analysis to examine whether new neutrino self-interactions solve or alleviate the so-called H-0 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 N-eff comprised of free-streaming particles. Although we focus on neutrino interactions, these constraints are applicable to any light relic particle.

Automated summary

The study comprehensively explores cosmological constraints on nonstandard neutrino self-interactions, revealing no compelling evidence for new neutrino interactions or nonstandard values of N-eff. In some cases, additional modes with neutrino decoupling occurring at lower redshifts were found. The effectiveness of solving the H-0 tension is not as strong as allowing a variable N-eff comprised of free-streaming particles.