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 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.Comment: 4 pages, 2 figure

The Cosmic Neutrino Background anisotropy — linear theory

“…Furthermore, this component is known to be free-streaming, i.e. to have an anisotropic stress component consistent with what is expected from standard model neutrinos (see [7,8,9,10,11,12,13]). Finally the standard model neutrino decoupling history is also confirmed by Big Bang Nucleosynthesis (BBN), the outcome of which depends on both the energy density and flavour composition of the CνB.…”

Neutrino physics from precision cosmology

“…Furthermore, this component is known to be free-streaming, i.e. to have an anisotropic stress component consistent with what is expected from standard model neutrinos (see [120][121][122][123][124][125][126]). Finally the standard model neutrino decoupling history is also confirmed by Big Bang Nucleosynthesis (BBN), the outcome of which depends on both the energy density and flavour composition of the CνB.…”

Cosmology with self-interacting sterile neutrinos and dark matter: A pseudoscalar model

“…The scenario where all neutrinos are strongly interacting is strongly disfavoured by current data (see e.g. [17,[33][34][35][36][37][38][39][40] for discussions of self-interacting neutrinos and cosmic structure formation). However, this is not necessarily true for models in which standard model neutrinos are free streaming, and the interaction is confined to the sterile sector.…”