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.
