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 $z_i$. The latest cosmological data imply $z_i\lesssim 1500$, the exact constraint depending somewhat on the assumed prior on $z_i$. This bound can be translated into a bound on the coupling strength between neutrinos and majoronlike particles.

Figure 1

Two-dimensional marginal 68% and 95% contours for $z_i$ and $N_{\nu }$. Top: Linear prior for $z_i$. Bottom: Logarithmic prior, i.e., linear in $\log (1+z_i)$.

Figure 2

Two-dimensional marginal 68% and 95% contours for $z_i$ and ${\Omega }_{\mathrm{M}}{h}^2$, using the linear prior for $z_i$ and using $N_{\nu }$ as a fit parameter. Top: Full data set. Bottom: WMAP5 data only.