Most of the content of the Universe, about 70%, is dominated by an energetic component that is neither matter nor radiation: dark energy. This mysterious component, first observed in 1998 with supernovae, revolutionized our vision of the evolution of the Universe and is one of the major discoveries of the end of the 20th century.
A characteristic scale of about 500 million light-years, acting as a "standard meter", is present in the distribution of matter on a cosmic scale. This scale is the result of the baryonic acoustic oscillations (BAO) that propagated through the primordial universe and became fixed at the time of recombination, when the universe was a few hundred thousand years old. The study of the distribution of galaxies for different ages in the Universe allows us to measure this "standard meter". The dark energy that today constitutes the main component governing the large-scale evolution of the Universe is thus probed. This method has already given promising results for large optical galaxy surveys (SDSS).
Irfu is participating in the second and third generations of SDSS optical surveys, with the BOSS spectroscopic survey, in the framework of the international collaboration SDSS-III and eBOSS in the framework of SDSS-IV. One aspect of the BOSS and eBOSS programs is the analysis of the Lyman-alpha spectrum of distant quasars, which allows the study of the distribution of matter in the universe through the absorption of hydrogen along the line of sight. The BOSS program observed its first quasars in September 2009 and is expected to continue until 2014, when the spectrum of more than 100,000 quasars will be completed.
The BAO group is also interested in another method, which consists of detecting galaxies by radio, using the 21 cm (~1.4 GHz) line emitted by neutral hydrogen. The shift of this line due to the expansion of the Universe gives access to the third coordinate, along the line of sight. Recent developments in electronics in the frequency range around GHz, used by mobile telephony, make this project technologically feasible. A project recently under study, the CRT (Cylindrical Radio Telescope), for example, could probe a volume of more than five billion light-years in radius. Other programs of this type are currently in the R&D phase. The 21 cm line is thus a promising probe of the 3D distribution of matter in the cosmos. By exploiting the universal length scale BAO, it is hoped to use this probe to study the acceleration of cosmic expansion in the relatively recent history of the Universe.