Cosmological thermal decoupling and primordial molecules

Primordial chemistry began, at the recombination epoch, when the adiabatic expansion caused the temperature of the radiation to fall below 4000K. The chemistry of the early Universe involves the elements hydrogen, its isotope deuterium, helium with its isotopic forms and lithium. In this contribution I will discuss the influence of the primordial molecules on the cosmological decoupling. In the framework of the gravitational instability theory, each protostructure started as a tiny local overdensity. As long as these inhomogeneities are small, their evolution can be studied by the classical linear perturbation theory. Once the deviations become large, the linear theory is no more valid. we present the role played by these molecules on the transition between the linear regime and the non-linear regime, and show that the molecules can lead to a thermal change at the turn-around point between these two regimes.