Investigation of the electromagnetically induced transparency in era of   cosmological hydrogen recombination
  Investigation of the cosmic microwave background formation processes is one of the most actual problem at present time. In this paper we analyze the response of the hydrogen atom to the external photon fields. Field characteristics are defined via conditions corresponding to the recombination era of universe. Approximation of three-level atom is used to describe the "atom - fields" interaction. It is found that the phenomena of the electromagnetically induced transparancy takes place in this case. Consideration of EIT phenomena makes it necessary to update astrophysical description of the processes of the cosmic microwave background formation and, in particular, Sobolev escape probability. Additional terms to the optical depth in definition of Sobolev escape probability on the level about 1% are found. 
D. Solovyev, V. K. Dubrovich and G. Plunien
physics.atom-ph astro-ph.CO