Gravitino, axino, and Kaluza–Klein graviton warm and mixed dark matter and reionization

Stable particle dark matter may well originate during the decay of long-lived relic particles, as recently extensively examined in the cases of the axino, gravitino, and higher dimensional Kaluza–Klein (KK) graviton. It is shown that in much of the viable parameter space such dark matter emerges naturally warm/hot or mixed. In particular, decay produced gravitinos (KK gravitons) may only be considered cold for the mass of the decaying particle in the several TeV range, unless the decaying particle and the dark matter particle are almost degenerate. Such dark matter candidates are thus subject to a host of cosmological constraints on warm and mixed dark matter, such as limits from a proper reionization of the Universe, the Lyman-α forest, and the abundance of clusters of galaxies. It is shown that constraints from an early reionization epoch, such as are indicated by recent observations, may potentially limit such warm/hot components to contributing only a very small fraction to the dark matter.