# A tale of two modes: Neutrino free-streaming in the early universe

## TL;DR

This paper investigates the free-streaming behavior of cosmological neutrinos, constraining their interactions and decoupling epoch using CMB, BAO, and local Hubble data, revealing a bimodal posterior with implications for neutrino physics and the Hubble tension.

## Contribution

It introduces a model considering neutrino self-interactions via a four-fermion coupling and analyzes its cosmological implications, identifying a secondary interacting neutrino mode.

## Key findings

- Standard neutrino decoupling mode consistent with ΛCDM
- A secondary interacting neutrino mode with late decoupling
- Adding polarization and H_0 data increases the significance of the interacting mode

## Abstract

We present updated constraints on the free-streaming nature of cosmological neutrinos from cosmic microwave background (CMB) power spectra, baryonic acoustic oscillation data, and local measurements of the Hubble constant. Specifically, we consider a Fermi-like four-fermion interaction between massless neutrinos, characterized by an effective coupling constant $ G_{\rm eff}$, and resulting in a neutrino opacity $\dot{\tau}_\nu\propto G_{\rm eff}^2 T_\nu^5$. Using a conservative prior on the parameter $\log_{10}\left(G_{\rm eff} {\rm MeV}^2\right)$, we find a bimodal posterior distribution. The first of these modes is consistent with the standard $\Lambda$CDM cosmology and corresponds to neutrinos decoupling at redshift $z_{\nu,{\rm dec}} > 1.3\times10^5$. The other mode of the posterior, dubbed the "interacting neutrino mode", corresponds to neutrino decoupling occurring within a narrow redshift window centered around $z_{\nu,{\rm dec}}\sim8300$. This mode is characterized by a high value of the effective neutrino coupling constant, together with a lower value of the scalar spectral index and amplitude of fluctuations, and a higher value of the Hubble parameter. Using both a maximum likelihood analysis and the ratio of the two mode's Bayesian evidence, we find the interacting neutrino mode to be statistically disfavored compared to the standard $\Lambda$CDM cosmology. Interestingly, the addition of CMB polarization and direct Hubble constant measurements significantly raises the statistical significance of this secondary mode, indicating that new physics in the neutrino sector could help explain the difference between local measurements of $H_0$, and those inferred from CMB data. A robust consequence of our results is that neutrinos must be free streaming long before the epoch of matter-radiation equality.