Entropy Production during Reheating at Late Times and Neutrino Decoupling

Small Recent theories have proposed a variety of massive particles, like the moduli, whose abundance or decay endangers standard cosmological results. To dilute them, thermal inflation has been proposed, with its own massive scalar flaton field which goes on decaying in the era of Mev-scale temperatures. In this paper, the effect of late-time entropy production on neutrino decoupling during such reheating is investigated by including a term, arising from the rate of entropy production due to scalar decay, in the Boltzmann equation for the neutrino number density. The effect on the decoupling temperature of massless neutrinos is studied. It is found that a lower bound to the scalar decay rate constant can be set at 10^{-22} Gev.