Particle Physics Division

These latest results were obtained by an international team, led by researchers from the CEA and CNRS, who studied the distribution of hydrogen gas that makes it possible to measure distances in the young Universe. A second team, led by the Lawrence Berkeley National Laboratory, compares the distribution of quasars to the distribution of hydrogen gas. Combined, the two BOSS analyses establish that 10.8 billion years ago the Universe was expanding at a rate equivalent to that measured today. The deceleration of expansion that prevailed at that time was therefore followed by a recent acceleration.

Measuring the rate of expansion of the Universe throughout its history is therefore the key to determining the nature of the dark energy responsible for the increase in this rate of expansion over the last six billion years. By probing the Universe when it was only a quarter of its present age, when dark energy still had a negligible role, BOSS has established a baseline measure.

BOSS determines the rate of expansion of the Universe into the distant universe by measuring the size of baryon acoustic oscillations (BAOs), a signature imprinted in the way matter is distributed, resulting from the sound waves that propagated through the primordial Universe. This imprint is visible in the distribution of galaxies, quasars, and intergalactic hydrogen throughout the cosmos.

When light from a distant quasar passes through hydrogen gas, distributed throughout the Universe, regions of greater density absorb more light. When the spectrum of the quasar is finally observed on Earth by BOSS, it thus contains absorption peaks corresponding to all dense regions through which the quasar light passes. By combining a sufficient number of good quality quasar spectra, quite close to each other, the position of the gas clouds allows to produce a 3D map. BOSS determines the rate of expansion using these maps to measure the size of the BAOs at different cosmic epochs. 

When BOSS started and precision measurements with quasars were proposed, some scientists in the collaboration were afraid it wouldn't work. But the precision measurements obtained here exceed even the most optimistic assumptions.

Le groupe de participation français à SDSS-BOSS rassemble des entités du CEA et du CNRS :

• Irfu (CEA) ;

• APC (CNRS/CEA/Université Paris Diderot/Observatoire de Paris/CNES) ;

• CPPM, Centre de physique des particules de Marseille (CNRS/Aix-Marseille Université) ;

• IAP (CNRS/Université Pierre et Marie Curie) ;

• LAM, Laboratoire d’Astrophysique de Marseille (CNRS/Aix-Marseille Université).