Cloaking Cosmic Light

Light crossing dark domain walls that source a top form coupled to gauge Chern--Simons terms mixing visible and dark $U(1)$ gauge fields generically converts into dark photons. The effect is entirely localized on the wall and requires no additional ingredients. The conversion rate is a sharp function of the photon frequency in the wall rest frame, vanishing above the ultraviolet cutoff of the top form sector. Partial cloaking may also induce a rotation of the polarization of transmitted light of order $\Delta\vartheta \sim 10^{-3}$ radians, modify the cosmic microwave background power spectrum, and violate Etherington's reciprocity relation at low frequencies. These effects can impact cosmological determinations of the Hubble rate.