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.

Furthermore, Sunyaev and Zel'dovich found an analytical solution that describes how the CMB is blurred and deviates from a pure Planck black-body spectrum due to the combined action of bremsstrahlung (or double Compton) at low frequencies and comptonization of photons on Maxwellian electrons, which leads to a redistribution of photons in frequency. The numerical calculations of recent years have proved faithful to the classical result: the energy release in the early universe (at redshifts greater than 2 million) does not leave traces on the observed spectrum of the CMB. However, the noticeable energy release of any nature at lower redshifts leaves typical spectral distortions. Several satellite projects, capable of searching for this spectral distortion of the CMB, were submitted for consideration to NASA and ESA. Furthermore, recent papers of Sunyaev and R. Khatri demonstrated that the analytical solution of Zel'dovich and Sunyaev (1970b) describes the location of the blackbody photosphere of the universe with an accuracy of 10%.

Papers:
1. Sunyaev, R.A., Zel'dovich, Ya.B., 1970a, Small scale fluctuations of relic radiation, Astrophysics and Space Science, Vol.7, pp. 3-19 (554 citations)
2. RA Sunyaev, Zel'dovich, Ya.B., 1970b, The interaction of matter and radiation in the hot model of the universe, II, Astrophysics and Space Science, Vol. 7, pp.20-30 (189 citations)
3. RA Sunyaev, Zel'dovich, Ya.B., 1970c, The spectrum of the primordial radiation, its distortions and their significance, Comments on Astrophysics and Space Physics , Vol. 2, pp. 66-73 (152 citations)
4. RA Sunyaev, Zel'dovich, Ya.B., 1980, Microwave background radiation as a probe of the contemporary structure and history of the universe, Annual Review of Astronomy and Astrophysics, Volume 18, pp. 537-560 (510 citations)

R. Sunyaev, Khatri, R., 2013, Unavoidable CMB Spectral Features and Blackbody Photosphere of our Universe, International Journal of Modern Physics D, Vol. 22, Issue 7, id. 1330014
Khatri, R., R. Sunyaev, 2012, Creation of the CMB spectrum: precise analytic solutions for the blackbody photosphere, Journal of Cosmology and Astroparticle Physics, Issue 06, id. 038