Size of a hydrogen atom in the expanding universe

I take a simple model of the hydrogen atom in a universe without spatial cu rvature. The Maxwell equations are formulated on the background cosmic spac etime. For a class of cosmic metrics, which includes the de Sitter universe , these equations admit solutions corresponding to an atom whose radius rem ains strictly constant during the expansion. In the Einstein-de Sitter univ erse approximate calculations show that the atom expands, but at a rate whi ch is negligible compared with the general cosmic expansion.