Damping of primordial gravitational waves from generalized sources

It has been shown that a cosmological background with an anisotropic stress tensor, appropriate for a free-streaming thermal neutrino background, can damp primordial gravitational waves after they enter the horizon, and can thus affect the cosmic microwave background B-mode polarization signature due to such tensor modes. Here we generalize this result and examine the sensitivity of this effect to nonzero neutrino masses, extra neutrino species, and also a possible relativistic background of axions from axion strings. In particular, additional neutrinos with cosmologically interesting neutrino masses at the O(1) eV level will noticeably reduce damping compared to massless neutrinos for gravitational wave modes with kτ₀≈100-200, where τ₀≈2/ H₀ and H₀ is the present Hubble parameter, while an axion background would produce a phase-dependent damping distinct from that produced by neutrinos.