The Scientific Council of the A.F.Ioffe Physical-Technical Institute of Russian Academy of Sciences, one of the largest scientific institutes in Russia, nominated Rashid Sunyaev for a series of papers, in which he and Ya. B. Zel'dovich predict the existence of two important milestones in the evolution of the Universe: (1) the last scattering surface of photons and (2) the blackbody photosphere of the universe. In addition, the papers also describe the existence of inevitable distortions of the CMB spectrum from a pure Planck spectrum (for references see below).
According to the Big Bang model, the universe started in an infinitely dense and hot state, rapidly expanding in its first few moments. As it stared to cool, a hot plasma formed and eventually the electrons combined with protons forming hydrogen (the so-called recombination) after about 380 000 years (or at a redshift of 1090). This leads to the presence of the last scattering surface, i.e. after the recombination photons can move more or less freely through space and one can observe them as the cosmic microwave background (CMB). While very homogeneous on the whole, there are tiny angular fluctuations and acoustic peaks in the power spectrum of the CMB, which have been observed by the cosmological satellites WMAP (2001-2010) and PLANCK (2009-2013). These observations demonstrated that Zel'dovich and Sunyaev (1970a) evaluated the location and effective width of the last scattering surface with an accuracy of a few percent.