Two Important Milestones in the History of the Universe: The Last Scattering Surface, the Black Body Photosphere of the Universe and Distortions of the CMB Spectrum

Our Universe is filled with Cosmic Microwave Background (CMB) radiation having an almost perfect black body spectrum with a temperature of To=2.7K. The number density of photons in our Universe exceeds the number density of electrons by a factor of more than a billion. In the expanding Universe the temperature at early times was higher than today: Tr = To (1+z), where z is the redshift.

Hydrogen recombination at redshifts z ~ 1100 - 1300 leads to a rapid decrease in the Thompson scattering optical depth of the Universe. When this optical depth becomes close to or lower than unity, the mean free path of the photons starts to exceed the horizon (~ ct) and they can reach us directly carrying information about the inhomogeneities in distribution of the density of matter, the gravitational potential and the velocities of electrons at that time. The WMAP and PLANCK spacecrafts have measured, with enormous accuracy, the traces of these inhomogeneities in the angular distribution of CMB, originating during the epoch of hydrogen recombination due to existence of the last scattering surface. The width of this surface is defined by the rate of two photon decay of 2s level of hydrogen atom and the escape of the Ly-alpha photons in the distant low frequency wing of this line due to the expansion of the Universe. The epoch of hydrogen recombination in the Universe defines the properties of the observed angular anisotropy and E-mode polarization of the CMB and leads to tiny distortions of the CMB spectrum from the black body spectrum.

There is an obvious question: "How and at which redshifts the observed, practically ideal, black body spectrum of Cosmic Microwave Background Radiation was created?"

I plan to describe the nature of the black body photosphere of the Universe and demonstrate why the production of low frequency photons due to the double Compton effect and their redistribution over a broad energy range due to Comptonization allows creation of an ideal black body spectrum at redshifts higher than z~ 2 $10^6$. This explains why we will never observe, in the spectrum of CMB, the traces of the giant energy release connected with the annihilation of positrons and electrons at redshifts z ~ $10^9$.

Nevertheless, spectral features in the CMB energy spectrum contain a wealth of information about the physical processes in the early Universe at redshifts below 2 x $10^6$. The CMB spectral distortions are complementary to all other probes of cosmology. In fact, most of the information contained in the CMB spectrum is inaccessible by any other means. Among the unavoidable reasons for the existence of the spectral distortions are: emission of the hydrogen line photons during cosmological recombination, energy release due to Silk damping of small scale sound waves and heating of primordial gas during the epoch of reionization. In addition scientists are looking for the CMB spectral distortions arising, for example, due to decay of unknown elementary particles with lifetime significantly shorter than the lifetime of our Universe or due to the evaporation of small mass primordial black holes.