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Rashid A. Sunyaev (Space Research Institute of the Russian Academy of Sciences, Moscow; Max-Planck Institute for Astrophysics, Garching; Institute for Advanced Study, Princeton)
Date & Time
17 January 2019, 17:00 to 18:00
Ramanujan Lecture Hall, ICTS Bangalore

Lecture 1: Thursday, January 17 at 17:00 hrs

Title: X-Ray and microwave cosmology: synergy and competition. 
What do we expect from the next generation X-ray and microwave telescopes?

Abstract: Our Universe is filled with cosmic microwave background radiation (CMB) which is isotropic and has the black body spectrum with temperature 2.7 Kelvin. No spectral deviations from a black body have been detected in the CMB monopole until now. However the physics of interaction of the CMB photons with hot electrons predicted the presence of  "shadows" in the angular distribution of CMB in the directions where clouds of very hot electrons Te~106-108 K exist in our Universe. Today we know that such objects exist and they are clusters of galaxies containing thousands of galaxies each, a lot of dark matter, and  hot gas filling the huge potential well. The "shadows" with very peculiar frequency spectrum arise due to the Thomson scattering interaction of the CMB photons with the hot electrons. Today this method has permitted us to discover several thousands of clusters of galaxies at relatively high redshifts 0.25 < z < 2. Behind practically every new discovered rich cluster of galaxies we see the extremely distant galaxies with their shapes distorted and brightness increased due to gravitational lensing by the huge gravitational potential of the invisible "dark matter" present in a cluster. The amplitude of the CMB brightness shadow corresponds only to a few tens to hundreds of microKelvin.

There is another way to observe the same hot gas. Russia plans to launch in March-April of 2019 the SRG spacecraft with German eRosita X-Ray telescope which would have grazing incidence optics. This telescope is expected to discover more than 100,000 clusters of galaxies (i.e. all rich clusters of galaxies in the observable Universe) during the 4 years of surveying the full sky. At the same time, ground-based millimeter wavelenghts telescopes on the South Pole of the Earth and in the Atacama desert at 5 km altitude, equipped with thousands of cryogenic bolometers in their focal planes, promise to detect majority of these clusters due to their "shadows" in the CMB.

These two data sets will be very complimentary and there will be a lot of synergy. At the same time there is competition: who will be the first to discover the most interesting clusters of galaxies? Ensemble of 105 clusters, their distribution in space, mass and redshift will provide a unique sample of data for testing cosmological models.

Interaction of CMB photons with free electrons opens a unique way to measure the peculiar velocity of a cluster of galaxies relative to the unique system of coordinates in which the CMB is isotropic. Observers dream to measure peculiar velocities and even bulk and turbulent velocities inside the clusters of galaxies at any distance from us because both effects (thermal and kinematic) do not depend on the redshift of the object.

Lecture 2: Friday, January 18 at 16:30 hrs

Title: Physics of the radiation spectra formation due to Thomson scattering of low frequency photons on hot Maxwellian electrons.
Why the spectra formed in the infinite expanding Universe differ strongly from the spectra observed from accretion disks around black holes?

Abstract: Thomson scattering of a low frequency photon on the electron at rest does not change its frequency according to the classical physics. However in the thermal plasma electrons move, having thermal velocities distributed according to the isotropic Maxwellian distribution. A narrow laser line will be broadened due to the Doppler effect even after a single scattering. The presence of the first order in kTe/mc^2 corrections gives a slight asymmetry to the broadened line profile and the high energy wing of the line becomes stronger compared to the low energy wing. Multiple scatterings in the optically thick plasma lead to the formation of specific kinds of the broadband continuum spectra. In 1956 A.S. Kompaneets was permitted to publish a kinetic equation, describing the interaction of radiation field with the hot Maxwellian electrons. This equation was derived in the Zeldovich group while working on the Soviet hydrogen bomb project, but was found useless for it.  However, this process turned out to be one of the most important processes for the formation of the radiation spectra in the astrophysical objects of interest for the high energy astrophysics and for the Universe as a whole.  I plan to demonstrate typical power law continuum spectra arising in the astrophysical environments: in the accretion disks around black holes of stellar mass and supermassiv eones and in the plasma accreting onto neutron stars with "weak" H < 10^8 Gauss magnetic fields. I will demonstrate several broad band spectra of the brightest X-Ray celestial sources like Cyg X-1 and X-Ray transients observed by Mir-Kvant, GRANAT, RXTE and INTEGRAL orbital observatories. Special attention will be given to observational consequences of the same physical process in the early Universe and in the hot plasma of the clusters of galaxies. The CMB spectral distortions have been already observed by the Planck spacecraft, South Pole Telescope and Atacama Cosmology Telescope in the directions of many hundreds of clusters of galaxies. The problem of down-Comptonization of gamma ray photons emitted due to radioactive decay of Ni-56 and Co-56 and experiencing multiple recoil effects during Compton scatterings on the electrons in the expanding envelopes of both SN Ia and core collapse types supernovae will also be mentioned. These processes lead to the specific power law hard X-Ray spectrum which has already been observed from the SN 1987A, using X-Ray telescopes aboard the Mir-Kvant space station.

Lecture 3: Wednesday, January 23 at 17:00 hrs

Title:Two important milestones in the history of the Universe:
the last scattering surface and the black body photosphere of the Universe.

Abstract: 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.

This lecture series is part of the program "Cosmology - The Next Decade".