Galaxies in the first billion years of the Universe's expansion

The Universe started its expansion 13.8 billion years ago. A hundred million years later, the gas component of the Universe's matter ‘matured’ to the point of forming stars. This epoch should be inspected to search for the beginning of the emergence of galaxy populations in the Universe—large gravitationally bound star systems. Modern astronomical observation tools—ground-based interferometers and space-borne telescopes—allow the properties of galaxies to be studied directly at the earliest stages of their evolution, i.e., in the first billion years of the Universe's expansion, including their shapes, sizes, masses, star formation rates, and nuclear activity. This brief review presents the latest results of such studies.