Possible cosmological scenario with an unstable 17-keV neutrino

Abstract

Simpson has recently reported evidence for a heavy neutrino of ∼17.1 keV mass. Cosmological bounds on stable neutrino species imply that this neutrino (ν_H) must be unstable. The most likely decay mode, ν_H ν → _Lf where ν_L is a light neutrino and f is a scalar boson, leads to the cosmological scenario presented here, which is quite different from the conventional one. In this scenario, the Universe becomes matter, dominated at a redshift of z∼10⁷ and becomes radiation-dominated (by the decay product ν_L of ν_H) at z∼310. The kinematic constraints on the lifetime of ν_H do not lead to any contradictions. On the other hand, the growth of baryonic perturbations is severely limited in this model because virtually no growth can take place in the radiation-dominated region z<310 and decay of ν_H is likely to disrupt and smooth-out past growth by a large factor. It is doubtful whether there is a simple way to avoid this difficulty.