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).