How accurately can we measure the hydrogen 2S->1S transition rate from   the cosmological data?
  Recent progress in observational cosmology, and especially the forthcoming PLANCK mission data, open new directions in so-called precision cosmology. In this paper we illustrate this statement considering the accuracy of cosmological determination of the two-quanta decay rate of 2s hydrogen atom state. We show that the PLANCK data will allow us to measure this decay rate significantly better than in the laboratory experiments. 
Viatcheslav Mukhanov, Jaiseung Kim, Pavel Naselsky, Tiziana Trombetti,   Carlo Burigana
astro-ph.CO physics.atom-ph