Lithium hydride in the early Universe and in protogalactic clouds

We examine the processes of the formation and destruction of LiH molecules in the primordial gas at temperatures T ≤ 5000 K in the framework of standard Friedmann cosmological models, and we compute the optical depth of the Universe due to Thomson scattering of cosmic background photons on LiH. With the help of a simple model of the evolution of a spherical density perturbation, we follow the linear growth and the subsequent collapse of protogalactic clouds of various masses, evaluating consistently their physical characteristics and chemical composition. We determine the expected level of anisotropy of the cosmic background radiation arising from the presence of LiH molecules in high-redshift protogalactic clouds. We conclude that the LiH spectral features generated in the primordial gas are hardly detectable with current techniques and instrumentation.