Increasing effective number of neutrinos by decaying particles

We present models of decaying particles to increase the effective number of neutrinos Nν after big bang nucleosynthesis but before the structure formation begins. We point out that our scenario not only solves the discrepancy between the constraints on Nν from these two epochs, but also provides a possible answer to deeper inconsistency in the estimation of the matter power spectrum amplitude at small scales, represented by σ8, between the WMAP and some small scale matter power measurements such as the Lyman-α forest and weak lensing. We consider (a) saxion decay into two axions; (b) gravitino decay into axino and axion; (c) Dirac right-handed sneutrino decay into gravitino and right-handed neutrino. (orig.)