Massive neutrino self-interactions with a light mediator in cosmology

TL;DR

This paper investigates how massive neutrino self-interactions mediated by a light scalar affect cosmological observations, providing constraints on interaction strength and implications for the Hubble tension.

Contribution

It offers new constraints on neutrino self-interaction strength using diverse cosmological data and explores their impact on the Hubble tension and neutrino parameters.

Findings

Upper bounds on the effective coupling constant g_eff from cosmological data.

Self-interactions reduce the H_0 tension from 4.3σ to 3.2σ.

Models with self-interactions are favored when including local H_0 measurements.

Abstract

Nonstandard self-interactions can alter the evolution of cosmological neutrinos, mainly by damping free streaming, which should leave traces in cosmological observables. Although overall effects are opposite to those produced by neutrino mass and a larger $N_{\rm eff}$, they cannot be totally canceled by these last. We harness cosmological data that includes Cosmic Microwave Background from Plank 2018, BAO measurements, local $H_0$, Ly-$\alpha$ and SNIa, to constrain massive neutrino self-interactions with a very light scalar mediator. We find that the effective coupling constant, at the 95\% C.L., should be $g_{\rm eff}< 1.94 \times 10^{-7}$ for only Planck 2018 data and $1.97\times10^{-7}$ when Planck + BAO are considered. This bound relaxes to $2.27\times 10^{-7}$ ($2.3\times 10^{-7}$) for $H_0$ ($H_0$+SNe+Ly-$\alpha$) data. Using the Planck + BAO dataset, the $H_0$ tension lowers from 4.3$\sigma$ (for $\Lambda$CDM) to 3.2$\sigma$. The Akaike Information Criterion penalizes the self-interacting model due to its larger parameter space for Plank or Planck + BAO data, but favors the interacting model when we use local $H_0$ measurements. A somewhat larger value for $H_0$ is preferred when we include the whole data pool, which comes accompanied with a larger value of $N_{\rm eff}$ and a more constricted bound on $\Sigma m_\nu$.