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

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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 higherdimensional Kaluza–Klein (KK) graviton. It is shown that in much of the viableparameter space such dark matter emerges naturally warm/hot or mixed. Inparticular, decay produced gravitinos (KK gravitons) may only be considered cold forthe mass of the decaying particle in the several TeV range, unless the decayingparticle and the dark matter particle are almost degenerate. Such dark mattercandidates are thus subject to a host of cosmological constraints on warm andmixed dark matter, such as limits from a proper reionization of the Universe, theLyman-α forest, and the abundance of clusters of galaxies. It is shown that constraints from an earlyreionization epoch, such as are indicated by recent observations, may potentially limit suchwarm/hot components to contributing only a very small fraction to the dark matter.