Gravitino dark matter without R-parity

Cosmological issues are examined when the 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 the diffuse photon background coming from radiative decays of the gravitino do not severely constrain the gravitino abundance, and thus a gravitino weighing less than around 1 GeV can be the dark matter of the Universe when bilinear R-parity violation generates a neutrino mass which accounts for the atmospheric neutrino anomaly.