An analytic description of the damping of gravitational waves by free   streaming neutrinos
  We provide an analytic solution to the general wavelength integro-differential equation describing the damping of tensor modes of gravitational waves due to free streaming neutrinos in the early universe. Our result is expressed as a series of spherical Bessel functions whose coefficients are functions of the reduced wave number $Q$. 
Ben A. Stefanek and Wayne W. Repko
hep-ph astro-ph.CO