Limits to Global Rotation and Shear From the COBE DMR 4-Year Sky Maps

Small departures from a homogeneous isotropic spacetime create observable features in the large-scale anisotropy of the cosmic microwave background. We cross-correlate the maps of the cosmic microwave background anisotropy from the Cosmic Background Explorer (COBE) Differential Microwave Radiometers (DMR) 4-year data set with template maps from Bianchi VII_h cosmological models to limit global rotation or shear in the early universe. On the largest scales, spacetime is well described by the Friedmann-Robertson-Walker metric, with departures from isotropy about each spatial point limited to shear sigma/H_0 < 10^{-9} and rotation omega/H_0 < 6 x 10^{-8} for 0.1 < Omega_0 < 1.