Are cosmological neutrinos free-streaming?

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 zᵢ. The latest cosmological data imply zᵢ1500, the exact constraint depending somewhat on the assumed prior on zᵢ. This bound can be translated into a bound on the coupling strength between neutrinos and majoronlike particles.