  Cosmological issues are examined when gravitino is the lightest superparticle
(LSP) and R-parity is broken. Decays of the next lightest superparticles occur
rapidly via R-parity violating interaction, and thus they do not upset the
big-bang nucleosynthesis, unlike the R-parity conserving case. The gravitino
LSP becomes unstable, but its lifetime is typically much longer than the age of
the Universe. It turns out that observations of diffuse photon background
coming from radiative decays of the gravitino do not severely constrain the
gravitino abundance, and thus the gravitino weighing less than around 1 GeV can
be dark matter of the Universe when bilinear R-parity violation generates a
neutrino mass which accounts for the atmospheric neutrino anomaly.

High Energy Physics - Phenomenology