Neutrino heat conduction and inhomogeneities in the early Universe

Constraints on parameters of inhomogeneous nucleosynthesis, namely, the overdensity and size of baryon lumps, are found by calculating the blackbody neutrino heat conduction into the lumps, which tends to inflate them away. The scale size for efficient heat conduction is determined by the mean free path $\lambda$ of the neutrino, and so we compute $\lambda$ in our case of a high-temperature plasma with low chemical potential, and find a general result that many-body effects are unimportant, simplifying the calculation. We find that in region of interest for nucleosynthesis, neutrino inflation is important for overdensities > ${10}^4$.