Limits on new superweakly interacting particles from primordial nucleosynthesis

The existence of massless (or low mass) weakly (or superweakly) interacting particles is predicted by various models of unified theories. Such particles (neutrinos, gravitinos, gravitons) would contribute to the early expansion rate of the universe and affect the outcome of primordial nucleosynthesis. We extend previous work on the limits to the number of ordinary neutronos to more weakly interacting particles which decouple earlier in the evolution of the universe. We have evaluated the limits on the number of new particles permitted as a function of their interaction strength. Having been forced to consider the universe at earlier times and higher temperatures than was necessary in previous discussions we have dealt, qualitatively, with the transition from free quarks to confined hadrons. We find that the maximum number of superweakly interacting, light (< 1 MeV) particles is between approx. 1 and approx. 20, depending on the strength of their interaction. These constraints may serve as a useful guide or new unification theories. (orig.)