  We study the impact of modifying the vector sector of gravity on the CMB
polarization. We employ the Einstein-aether theory as a concrete example. The
Einstein-aether theory admits dynamical vector perturbations generated during
inflation, leaving imprints on the CMB polarization. We derive the perturbation
equations of the aether vector field in covariant formalism and compute the CMB
B-mode polarization using the modified CAMB code. It is found that the
amplitude of the B-mode signal from the aether field can surpass the one from
the inflationary gravitational waves. The shape of the spectrum is clearly
understood in an analytic way using the tight coupling approximation.
