  The dynamics of a gravitational wave propagating through a cosmic gauge field
are dramatically different than in vacuum. We show that a gravitational wave
acquires an effective mass, is birefringent, and its normal modes are a linear
combination of gravitational waves and gauge field excitations, leading to the
phenomenon of gravitational wave -- gauge field oscillations. These surprising
results provide insight into gravitational phenomena and may suggest new
approaches to a theory of quantum gravity.
