Optical dispersion of composite particles consisting of millicharged constituents

Composite dark matter (DM) comprised of electrically charged constituents can interact with the electromagnetic field via the particle's dipole moment. This interaction results in a dispersive optical index of refraction for the DM medium. We compute this refractive index for atomic DM and more strongly bound systems, modeled via a harmonic oscillator potential. The dispersive nature of the index will result in a time lag between high and low energy photons simultaneously emitted from a distant astrophysical observable. This time lag, due to matter dispersion, could confound potential claims of Lorentz invariance violation (LIV) which can also result in such time lags. We compare the relative size of the two effects and determine that the dispersion due to DM is dwarfed by potential LIV effects for energies below the Planck scale.