Gravitinos as the cold dark matter in an Ω = 1 universe

In attempts to simultaneously have galaxy formation with adiabatic fluctuations, dark galactic halos and a critical Ω = 1, recent models have been proposed with a heavy (cold) species which decays and a lighter (hot or warm) one which is stable. Surprisingly enough, these models are very constraining. Independent of the particle physics model, the decay width of the heavy particle must be O(10^-40) GeV. Such a scale is only natural in theories involving gravity where one expects Γ ∼ M_H ³/M_P ², where M_H is the mass of the heavy particle and M_P is the Planck mass. In this paper we suggest that the heavy particle might be the gravitino which is present in all supergravity theories. This model would then require that the gravitino decay products not include photons, indicating that the lightest supersymmetric particle (LSP) must be something other than the photino. An acceptable candidate for the LSP might be the axino, the supersymmetric partner of the axion.