Relic Cosmological Vector Fields and Inflationary Gravitational Waves

We show that relic vector fields can significantly impact a spectrum of primordial gravitational waves in the post-inflationary era. We consider a triplet of U(1) fields in a homogeneous, isotropic configuration. The interaction between the gravitational waves and the vector fields, from the end of reheating to the present day, yields novel spectral features. The amplitude, tilt, shape, and net chirality of the gravitational wave spectrum are shown to depend on the abundance of the electric- and magnetic-like vector fields. Our results show that even a modest abundance can have strong implications for efforts to detect the imprint of gravitational waves on the cosmic microwave background polarization. We find that a vector field comprising less than two percent of the energy density during the radiation dominated era can have a greater than order unity effect on the predicted inflationary gravitational wave spectrum.