Cosmological Constraints on Late-time Entropy Production

We investigate cosmological effects concerning the late-time entropy production due to the decay of nonrelativistic massive particles. The thermalization process of neutrinos after the entropy production is properly solved by using the Boltzmann equation. If a large entropy production takes place at late time $t\simeq 1\sec$, it is found that a large fraction of neutrinos cannot be thermalized. This fact loosens the tight constraint on the reheating temperature $T_R$ from big bang nucleosynthesis and $T_R$ could be as low as 0.5 MeV. The influence on large scale structure formation and cosmic microwave background anisotropies is also discussed.