Artificial satellites: mysteries and tests of general relativity

It was a satellite that played a key role in the discovery of one of the "deepest problems of modern physics": the incredible acceleration in the expansion of the universe. The Hubble Space Telescope (NASA) has indeed revolutionized cosmology and our understanding of the universe. This accelerated expansion is explained by the existence of the so-called "Dark Energy" which together with the "Dark Matter" in galaxies and galaxy clusters is estimated to constitute about 95% of the mass-energy of our universe. The riddle on the nature of Dark Energy is profound! Theories alternative to Einstein's gravitational theory, General Relativity (GR), have also been proposed in an attempt to explain the accelerated expansion of the universe and the rotational speed of galaxies (explained by Dark Matter). A renewed interest has therefore arisen for more and more accurate experimental tests of GR on which today's cosmology is based.