Effect of neutrino rest mass on ionization equilibrium freeze-out
  We discuss how small neutrino rest masses can increase the expansion rate
near the photon decoupling epoch in the early universe, causing an earlier,
higher temperature freeze-out for ionization equilibrium compared to the
massless neutrino case. This yields a larger free-electron fraction. A larger
ratio of the sound horizon to the photon diffusion length follows, implying a
smaller inferred Neff. This neutrino-mass/recombination effect depends strongly
on the neutrino rest masses. Though below current sensitivity, this effect
could be probed by next-generation cosmic microwave background experiments,
giving an observational handle of neutrino mass physics.
