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