  A cosmological gauge field with isotropic stress-energy introduces parity
violation into the behavior of gravitational waves. We show that a primordial
spectrum of inflationary gravitational waves develops a preferred handedness,
left- or right-circularly polarized, depending on the abundance and coupling of
the gauge field during the radiation era. A modest abundance of the gauge field
would induce parity-violating correlations of the cosmic microwave background
temperature and polarization patterns that could be detected by current and
future experiments.
