Account is taken of the dissipation of anisotropy due to neutrino viscosity and nearly collisionless radiation in a Bianchi type-I cosmology during primordial nucleosynthesis. For experimentally allowed cross sections, and for moderate anisotropy, the final ${}_{}{}^4{}_{}{}^{}\mathrm{He}$ mass fraction is significantly less than in the standard model, while the D fraction increases much less than would be expected in a low-helium isotropic cosmology.

• Received 8 October 1981

DOI:https://doi.org/10.1103/PhysRevLett.48.1565