Are cosmological neutrinos free-streaming?
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 zi. The latest cosmological data imply zi < 1500, the exact constraint depending somewhat on the assumed prior on zi. This bound translates into a limit on the coupling strength between neutrinos and majoron-like particles phi, implying tau > 1 x 1010 s (m2/50 meV)3 for the decay nu2 -> nu1+phi.
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