Chiral Imprint of a Cosmic Gauge Field on Primordial Gravitational Waves
  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. 
Jannis Bielefeld and Robert R. Caldwell
astro-ph.CO gr-qc