2015 Oskar Klein Memorial Lecture - "Unavoidable distortions in the spectrum of CMB and the Blackbody Photosphere of our Universe"

Spectral features in the CMB spectrum contain a wealth of information about physical processes in the early Universe at redshifts z < 2 10^6, i.e. when Universe was older than 2 months. The Cosmic Microwave Background Radiation (CMB) spectral distortions are complementary to other probes of cosmology. In fact, most of the information contained in the CMB spectrum is inaccessible by any other means. This talk outlines the main physics behind the spectral features in the CMB produced throughout the history of the Universe. I will concentrate on the distortions which are inevitable and must be present at the level observable by the next generation of CMB experiments. The spectral distortions considered here include spectral features from cosmological recombination of hydrogen and helium, resonant scattering of CMB by metals during reionization which allows us to measure their abundances, y-type spectral distortions produced during and after reionization and μ – type distortions created at redshifts z > 10^5 due to any significant energy release (for example: due to decay or annihilation of the dark matter particles or due to viscous decay of the primordial sound waves). Special attention will be given to existence of the Blackbody Photosphere of our Universe at redshift z ~ 2 10^6, behind which Comptonization, double Compton and Bremsstrahlung are able to wash out any spectral distortions arising due to arbitrary strong energy release. CMB spectral distortions detected on the sky by Planck spacecraft, South Pole Telescope and Atacama Cosmology Telescope permitted us to discover more than thousand unknown before clusters of galaxies (most massive gravitationally bound objects in the Universe, containing thousands of galaxies, hot (kTe > 1 KeV) intracluster gas, gravitational lenses and huge amount of dark matter). These clusters of galaxies are serving today as probes for modern cosmology tracing the growth of the Large Scale Structure with time and containing strong gravitational lenses.