ASTRONOMY COLLOQUIUM (Greenstein Lecture)
Our Universe is filled with the isotropic Cosmic Microwave Background (CMB) Radiation, which has a blackbody spectrum with a temperature of 2.7 (1+z) K, where z is the redshift. No deviations of the monopole CMB spectrum from the blackbody at the level of 10^-5 have been detected. However, in 1970 theorists predicted the shadows in CMB brightness in the directions to the massive clusters of galaxies with the hot intergalactic gas. Such shadows permitted ground based South Pole and Atacama Cosmology Telescopes and Planck spacecraft to discover more than nine thousand clusters of galaxies during the last decade. Today, we observe strong competition in the search for the clusters of galaxies between ground based SPT and ACT and SRG/eRosita X-Ray Space Telescope on the halo orbit around L2. The great success of Planck spacecraft increased the interests of cosmologists to the weak spectral deviations of CMB spectrum from black body.
The photon density of CMB is 411 cm^-3 (1+z)^3. The average density of baryons in our Universe is billions of times lower than the density of photons Nb = 2 10^-7 (1+z)^3 cm^-3 , i.e., the plasma in our Universe at the early stages of its expansion was radiation-dominated.
The interaction between matter and radiation at redshifts greater than z=3000 was determined by Thomson scattering of numerous photons which, taking into account the Doppler and recoil effects, set the temperature of electrons and baryons close to the radiation temperature.
Simultaneously photons were changing their energies. Any release of energy in the early Universe might leave a tiny characteristic deviation in the spectrum of CMB, which could inform us about the hidden processes in early Universe.
During this lecture, I will talk about three important milestones in the life of our universe;
1. The blackbody photosphere of the Universe. At redshifts z > 2 10^6 (when the age of the Universe was close to 18 years) Thomson scattering together with the double Compton effect had time to create a blackbody spectrum of radiation at any noticeable release of energy in the Universe due to any physical processes, including for example decay or annihilation of unknown particles, evaporation of primordial black holes, etc. (there are hundreds of possibilities).
Such energy release at later stages of expansion of the Universe (at smaller z) should inevitably leave characteristic traces on the spectrum of microwave background radiation, leading to its difference from the black body radiation spectrum.
2. The surface of the last photon scattering. The second most important stage in the life of the observed Universe was the recombination of hydrogen at redshift z ~ 1100. Before this period (when the age of the Universe was 380,000 years) hydrogen in the Universe was strongly ionized and opaque to photons because the probability of Thomson scattering within the horizon ct was large and photons continuously changed the direction of their propagation. Adiabatic reduction of the radiation and plasma temperature to 3000K led to recombination of hydrogen atoms but, because of the enormous optical depth of the Universe for the Lyman-alpha line, Ly-alpha photons maintained high population of 2p and 2s levels in hydrogen atom and delayed recombination of primary plasma in comparison with the equilibrium one at full thermodynamic equilibrium. But with time, the forbidden two-quantum decay of the 2s level in the hydrogen atom came into play, which allowed the hydrogen plasma to recombine, but at appreciably lower temperatures than at full thermodynamic equilibrium. The density of free electrons by redshift 1100 was greatly reduced and hydrogen and helium atoms became transparent for CMB photons. Photons could propagate freely from the surface of the last scattering to the present-day observer. The Planck cosmological spacecraft investigated the properties of this surface and confirmed the validity of the predictions of the theory created in 1968 and an estimate of the redshift at which the Universe became transparent to relic radiation.
3. Period of reionization of the Universe (z ~ 6 - 12). During this period, according to the observations of the PLANCK satellite, orbital observatories James Webb, Hubble, Chandra, XMM, SRG/eRosita, and ground-based radio telescopes, astronomers begin to see the appearance of the first galaxies and supermassive black holes in the Universe. Their ultraviolet and X-ray radiation reionizes the gas in the Universe, the synthesis of heavy elements facilitates observations of newly formed objects and molecular gas, and a rapid evolution of the physical properties of the Universe around us begins.