In 1915, Einstein introduced his theory of gravitation: General Relativity, explaining the nature of the force of gravity by linking the geometry of the Universe and its mass distribution. The “expansion” of the Universe measured by Hubble in 1929 follows naturally the equations of Friedman-Lemaître that predict the topology and evolution of the Universe from General Relativity. Since then astronomers have used many different techniques to probe with increasing precision the expansion parameters of the Universe to learn the growth of the Universe from the past to the future.
Just before our new millennium, observations of 50 type Ia Supernovae (ranged between 1 and 6 billion light years) have completely revolutionized our understanding of the evolution of the universe. The Universe is not only expanding, its expansion is accelerating! The engine of this expansion have the form of a mysterious component nicknamed : Dark Energy and that represent more than 70% of the mass-energy balance of the Universe.
This somewhat unexpected discovery has shaken the fundamental physics and astrophysics communities and many observational projects aim to confirm the accelerating expansion and measure it accurately throughout the history of the Universe . The underlying idea is that precise measurements may constrain the equation of state of Dark Energy and thus reveal its true nature.
Ten years ago, observation of the spatial distribution of galaxies, allowed to develop a new technique for measuring the expansion of the universe: the measurement of baryonic acoustic oscillations. At the beginning of the universe when the light and matter were in constant interaction (up to 380,000 years after the Big Bang), acoustic waves were propagating through the primordial plasma. Recombination of atomic particles froze these waves. Since galaxies formed preferentially in the over-density of matter (at the top of the density waves), by measuring accurately the 3D distribution of galaxies one can measure precisely the acoustic wavelength at a given time thus probing the expansion velocity and its acceleration.
The key is to measure the 3D position of a multitude of galaxies … about 30 millions, a small number compared to the estimated number of 100 billion galaxies in the whole visible Universe. Easy? To date, we do not know the exact 3D position of more than ~2 million galaxies. The last million was measured during the last 3 years … at this rate the goal of 30 millions will be obtained at the end of this century … so we need a technological revolution to achieve our goal faster, we need robots!