Anisotropic stresses in inhomogeneous universes - art. no. 043502.

Anisotropic stress contributions to the gravitational field can arise from magnetic fields, collisionless relativistic particles, hydrodynamic shear v iscosity, gravitational waves, skew axion fields in low-energy string cosmo logies, or topological defects. We investigate the effects of such stresses on cosmological evolution, and in particular on the dissipation of shear a nisotropy. We generalize some previous results that were given for homogene ous anisotropic universes, by including small inhomogeneity in the universe . This generalization is facilitated by a covariant approach. We find that anisotropic stress dominates the evolution of sheer, slowing its decay. The effect is strongest in radiation-dominated universes, when there is slow l ogarithmic decay of shear, [S056-2821(99)07602-X].