  Phase transitions in the early universe can readily create an observable
stochastic gravitational wave background. We show that such a background
necessarily contains anisotropies analogous to those of the cosmic microwave
background (CMB) of photons, and that these too may be within reach of proposed
gravitational wave detectors. Correlations within the gravitational wave
anisotropies and their cross-correlations with the CMB can provide new insights
into the mechanism underlying primordial fluctuations, such as multi-field
inflation, as well as reveal the existence of non-standard ``hidden sectors" of
particle physics in earlier eras.
