If the gravitino is sufficiently light and stable it will behave as an effective massless neutrino species at the time of nucleosynthesis .
Depending on the temperature at which it decouples from the thermal bath in the early universe , the gravitino mass will be bounded by the primordial ^ { 4 } He abundance .
Assuming a conservative estimate that the number of neutrino families , N _ { \nu } < 3.6 , superlight gravitinos with a mass m _ { 3 / 2 } \lesssim 10 ^ { -6 } eV are ruled out .
This bound is weaker than previous estimates because the Goldstino annihilation cross section was overestimated .