INTERNATIONAL SCHOOL OF COSMOLOGY AND GRAVITATION «PETER G. BERGMANN»

The School of Cosmology and Gravitation consists of the study of the universe in its immense entirety. One conception of cosmology sees the universe of galaxies as a kind of diffuse fluid in which the galaxies themselves play the role of fundamental particles. Modern cosmology is based on Einstein's famous work on general relativity. Early cosmological models were based on this new theory of gravitation. Einstein was guided in his work by Mach's principle, according to which the inertial properties of matter are determined by the behavior of matter on a cosmic scale. With its development, cosmology, starting from cosmological models based on the general theory of relativity, continued to grow into a branch of physics that is both broader and more specialized. After Hubble and Humanson's discovery of the redshift of spectral lines in distant galaxies and its subsequent interpretation in terms of the general expansion of the universe, models of the universe involving a singular initial state prevailed. These models ensure that it is completely meaningless to investigate the behavior of the universe before this point. An alternative hypothesis has also been proposed, which envisages a stationary universe in which the constant creation of matter is supposed to compensate for its dispersion during the expansion of the universe. It is obvious that in this case we cannot speak of the age of the universe. Instead, we can speak of the average age of a part of the universe. The hypothesis of the universe as a steady state seems to have been contradicted by recent discoveries and observations, in particular by the cosmological interpretation of the redshift of quasars and background radiation. Accepting the fact that the large redshifts of quasars are cosmological leads to the conclusion that quasars are the most distant objects ever observed. This in turn would lead to the conclusion that the universe was different in the past from its current state. The discovery of background radiation in 1965 (its existence had been predicted by Gamow as early as 1948) seems to imply the existence of a very hot initial state, similar to that hypothesized in big bang theories, of which background radiation is a remnant. This is where speculation about the early stages of the universe begins, particularly when, as Gamow pointed out in 1948, radiation predominated over matter. The study of blackbody radiation at high temperatures (between 400 and 1000 MeV), where strong interactions play a key role between pions, nucleons, and antinucleons, seems to be of primary importance. At these high temperatures, blackbody radiation consists of photons, electrons, positrons, neutrons, muons, pions, nucleons, antinucleons, and possibly kaons and hyperons. When discussing thermal equilibrium at this stage of the universe, strong interactions must be taken into account. It is easy to understand the value and importance of studying elementary particles in a course on cosmology and cosmological models. It is also quite easy to understand the importance of studying those phases of the universe's development in which the presence of a high density of neutrinos was important; it seems that, due to the viscosity of neutrinos, even a highly anisotropic universe soon becomes isotropic. Another very interesting topic concerns the origin of galaxies and the related problem of the intergalactic medium. However, this problem requires knowledge of plasma physics. This aspect of cosmology and the related problem of angular momentum deserve great attention.  Attention should also be paid to the discovery of gravitational waves which, due to their cosmological implications, allow for some interesting speculation about what happens in the nuclei of galaxies. Some galactic nuclei, in addition to being strong emitters of infrared radiation, show extremely violent events occurring; all these phenomena suggest that super-dense matter, collapsed matter, or black holes must exist in galactic nuclei. The cosmological problems raised by recent discoveries are numerous. Many important branches of physical science are brought together in cosmology, such as elementary particle physics, thermonuclear reactions, super-dense matter, and many fundamental concepts such as the geometry of space-time, Mach's principle, the direction of time, and others.