Are cosmological neutrinos free-streaming?

TL;DR

Cosmological data constrain neutrinos to have recoupled before redshift 1500, limiting their interaction strength with hypothetical particles and supporting their free-streaming behavior during recombination.

Contribution

This study provides the first direct cosmological constraint on neutrino recoupling redshift and coupling strength to majoron-like particles.

Findings

Neutrinos recoupled before redshift 1500.

Coupling strength between neutrinos and majoron-like particles is tightly constrained.

Neutrinos are consistent with free-streaming during recombination.

Abstract

Precision data from cosmology suggest neutrinos stream freely and hence interact very weakly around the epoch of recombination. We study this issue in a simple framework where neutrinos recouple instantaneously and stop streaming freely at a redshift z_i. The latest cosmological data imply z_i < 1500, the exact constraint depending somewhat on the assumed prior on z_i. This bound translates into a limit on the coupling strength between neutrinos and majoron-like particles phi, implying tau > 1 x 10^10 s (m_2/50 meV)^3 for the decay nu_2 -> nu_1+phi.