Quark-hadron phase transition in the early Universe: Isothermal baryon-number fluctuations and primordial nucleosynthesis

We study the quark-hadron transition in the early Universe and compute the amplitude of isothermal baryon-number fluctuations that emerge from this transition along with their effects on primordial nucleosynthesis. We find that such fluctuations are a natural consequence of a first-order phase transition occurring in the strongly interacting system. Fluctuation-generating mechanisms are discussed. We estimate the nucleation rate and derive the mean separation between fluctuations. It is shown that the amplitude of the fluctuations depends sensitively on the phase coexistence temperature $T_c$ and on the baryon transmission probability ${\Sigma }_h$ at the phase boundary. For realistic values of ${\Sigma }_h$ and $T_c$ the fluctuations are large and have a significant effect on primordial nucleosynthesis yields. With these fluctuations the limit on the baryonic contribution to Ω depends primarily on what is taken to be the primordial ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ abundance: Ω=1 in baryons would require large ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ and ${}_{}{}^2{}_{}{}^{}\mathrm{H}$ destruction factors during the evolution of the Galaxy.