Last scattering surface and black body photosphere of the Universe are two very important milestones in the history of our Universe. Hydrogen recombination at redshifts z ~ 1100 - 1300 leads to rapid decrease of the optical depth of the Universe due to Thomson scattering. At a result the acoustic peaks in the observed CMB angular distribution are formed in the vicinity of the last scattering surface. The question remains how and at which redshifts the observed practically ideal black body spectrum of CMB was formed. Why we dream that proposed space missions like PIXIE will be able to detect traces of any significant energy release in our Universe at redshifts smaller than 2 million? Why spectral distortions can not originate behind black body photosphere or at redshifts higher that 2 million ?