CONSTRAINTS ON A PRIMORDIAL MAGNETIC-FIELD

We derive an upper limit of B-0 <3.4 x 10(-9)(Omega(0)h(50)(2))(1/2) G on the present strength of any primordial 50 homogeneous magnetic fie ld. The microwave background anisotropy created by cosmological magnet ic fields is calculated in the most general Bat and open anisotropic c osmologies containing expansion-rate and 3-curvature anisotropies. Our limit is derived from a statistical analysis of the 4-year Cosmic Bac kground Explorer (COBE)data for anisotropy patterns characteristic of homogeneous anisotropy averaged over all possible sky orientations wit h respect to the COBE receiver. The limits we obtain on homogeneous ma gnetic fields are stronger than those imposed by nucleosynthesis.