Graviton cosmology in universal extra dimensions

In models of universal extra dimensions, gravity and all standard model fields propagate in the extra dimensions. Previous studies of such models have concentrated on the Kaluza-Klein (KK) partners of standard model particles. Here we determine the properties of the KK gravitons and explore their cosmological implications. We find the lifetimes of decays to KK gravitons, of relevance for the viability of KK gravitons as dark matter. We then discuss the primordial production of KK gravitons after reheating. The existence of a tower of KK graviton states makes such production extremely efficient: for reheat temperature T-RH and d extra dimensions, the energy density stored in gravitons scales as T-RH(2+3d/2). Overclosure and big bang nucleosynthesis therefore stringently constrain T-RH in all universal extra dimension scenarios. At the same time, there is a window of reheat temperatures low enough to avoid these constraints and high enough to generate the desired thermal relic density for KK weakly interacting massive particle (WIMP) and superWIMP dark matter.