We report a detection of the baryon acoustic oscillation ( BAO ) feature in the three-dimensional correlation function of the transmitted flux fraction in the Ly \alpha forest of high-redshift quasars .
The study uses 48,640 quasars in the redshift range 2.1 \leq z \leq 3.5 from the Baryon Oscillation Spectroscopic Survey ( BOSS ) of the third generation of the Sloan Digital Sky Survey ( SDSS-III ) .
At a mean redshift z = 2.3 , we measure the monopole and quadrupole components of the correlation function for separations in the range 20 { h ^ { -1 } Mpc } < r < 200 { h ^ { -1 } Mpc } .
A peak in the correlation function is seen at a separation equal to ( 1.01 \pm 0.03 ) times the distance expected for the BAO peak within a concordance \Lambda CDM cosmology .
This first detection of the BAO peak at high redshift , when the universe was strongly matter dominated , results in constraints on the angular diameter distance D _ { A } and the expansion rate H at z = 2.3 that , combined with priors on H _ { 0 } and the baryon density , require the existence of dark energy .
Combined with constraints derived from Cosmic Microwave Background ( CMB ) observations , this result implies H ( z = 2.3 ) = ( 224 \pm 8 ) { km s ^ { -1 } Mpc ^ { -1 } } , indicating that the time derivative of the cosmological scale parameter \dot { a } = H ( z = 2.3 ) / ( 1 + z ) is significantly greater than that measured with BAO at z \sim 0.5 .
This demonstrates that the expansion was decelerating in the range 0.7 < z < 2.3 , as expected from the matter domination during this epoch .
Combined with measurements of H _ { 0 } , one sees the pattern of deceleration followed by acceleration characteristic of a dark-energy dominated universe .