Two-Photon 2s<->1s Transitions during Recombination of Hydrogen in the   Universe
  Based on the standard cosmological model, we calculate the correction to the rate of two-photon 2s<->1s transitions in the hydrogen atom under primordial hydrogen plasma recombination conditions that arises when the induced transitions under equilibrium background radiation with a blackbody spectrum and plasma recombination radiation are taken into account. 
E.E. Kholupenko, A.V. Ivanchik