Dark photons and visible photons can mix — quantum mechanics allows transitions between them when their masses are small and their kinetic terms overlap. In free space, this mixing is a constant: a fixed fraction of visible photons oscillate into dark photons and back, independent of position.
Kaloper (arXiv:2602.20248) shows that domain walls in a dark sector change this. When a visible photon crosses a dark domain wall — a boundary between regions of different dark field values — it generically converts into a dark photon. Not probabilistically. Not through rare quantum tunneling. Through the classical field equations: the domain wall's gradient couples the two photon species at the boundary, and the photon that enters visible exits dark.
The conversion is frequency-dependent, with a UV cutoff. High-frequency photons convert more efficiently than low-frequency ones. This means domain walls act as frequency-selective screens — transparent to long wavelengths, opaque to short ones. A photon crossing multiple domain walls gets progressively reddened, not by expansion or scattering but by selective conversion of its high-frequency components into an invisible sector.
The cosmological consequence is specific: this breaks Etherington's reciprocity theorem, which relates luminosity distance to angular diameter distance. If photons are lost to a dark sector during propagation, supernovae look dimmer than expected without looking smaller. This is exactly the pattern attributed to accelerating expansion and dark energy. Not all of the apparent dimming needs to be geometric. Some could be conversion.
The general point: when two species can mix, a domain wall in one species' sector acts as a converter for the other. The wall doesn't need to couple directly to visible photons. It couples to dark photons, and the mixing does the rest. The visible sector feels the dark sector's topology through the mixing channel. Boundaries in an invisible field create effects in visible light.