9.3. Gravitinos

The gravitino provides another intriguing candidate for dark matter from the zoo of particle physics. Predicted by supersymmetric theories as the fermion partner to the graviton, the gravitino may have mass and decouple much earlier (at > 100 MeV) than ordinary left-handed neutrinos (which decouple at ~ 1 MeV). The cosmological bound on the gravitino mass is about 1 keV, since the number density of gravitinos is suppressed relative to that of neutrinos by an amount equal to the increase (by a factor ~ 10) in the number of particle species among which the entropy is shared (Pagel and Primack, 1982). Whereas there is no significant period of radiation-dominated growth for massive neutrinos, since they become non-relativistic only when the-universe is close to being matter-dominated, the situation may be quite different for gravitinos. In fact, gravitino fluctuations on scales between that of the maximum Jeans length (corresponding to, a mass- scale of ~ 10¹¹ M for 1 keV gravitinos) and the horizon scale when equal mass densities of matter and radiation occur (~ 10¹⁵ M) are suppressed in growth (Bond, Szalay and Turner, 1982; Blumenthal et al., 1982; Peebles, 1982b).

The net effect is that primordial gravitino fluctuations imprint scales down to galactic mass in the initial fluctuation spectrum from which galaxies originated. Subsequent evolution proceeds with baryonic dissipation and fragmentation to subgalactic scales, with gravitino infall to form dark halos. The wide range of mass-scales tends to result in the formation of galactic potential wells before larger scale structures develop, leading to a scenario that resembles the hierarchical development of large-scale structure. A similar scheme for galaxy formation might result with other weakly interacting particles such as the photino, the supersymmetric partner of the photon (Sciama, 1982), GUT monopoles or axions (Ipser and Sikivie, 1983).