The Revolutions of our Vision of the Universe
during the 20th Century

Prof. Alain Omont

The 20th Century was marked by a complete revolution of our vision of the Universe. I can identify six major breakthroughs:
- The discovery of the existence of galaxies in the first quarter of the century, which later developed in a deep understanding of their world, their properties and their history up to the boundaries of the Universe.
- The realisation that the whole Universe is expanding, with the unavoidable conclusion that it originated in a singular, extremely compact and energetic state: the Big Bang. Detailed developments of the Big Bang model , from recent observations, have strongly confirmed it, but have revealed the existence of mysterious “dark matter” and “dark energy”.
- The comprehensive understanding of the detailed organization of stars, including the Sun, their life, birth and death, and the way they produce their enormous energy, and synthesize most of the chemical elements which are found through the Universe, including our terrestrial environment and our bodies.
- The revelation of a world of fantastically energetic objects and cosmic events, including supernova explosions, hyper-compact neutron stars, mysteriously singular black-holes and hyper-luminous quasars.
- The long dreamed direct exploration of the planets and satellites of our Solar System, from the Moon to Titan, Mars and Jupiter.
- The long waited discovery of planets around other stars than the Sun; with the continuing questioning about the possibility of life outside of the Earth.

The two last points will be treated by T. Encrenaz and A. Vidal-Madjar. J. Stenflo and K. Shibata will partially address the ones about stellar physics and energetic processes, respectively. I will specially highlight the others.