Charged-Particle Decay and Suppression of Primordial Power on Small Scales

We study the suppression of the small-scale power spectrum due to the decay of charged matter to dark matter prior to recombination. Prior to decay, the charged particles couple to the photon-baryon fluid and participate in its acoustic oscillations. However, after these charged particles decay to neutral dark matter, the photon-baryon fluid is coupled only gravitationally to the newly created dark matter. This generically leads to suppression of power on length scales that enter the horizon prior to decay. For decay times of $\sim 3.5\mathrm{yr}$ this leads to suppression of power on subgalactic scales, bringing the observed number of galactic substructures in line with observation. Decay times of a few years are possible if the dark matter is purely gravitationally interacting, such as the gravitino in supersymmetric models or a massive Kaluza-Klein graviton in models with universal extra dimensions.