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 <= z <= 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 +/- 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 observations, this result implies H(z = 2.3) = (224 +/- 8) km s(-1) Mpc(-1), indicating that the time derivative of the cosmological scale parameter (a) over dot = H(z = 2.3)/(1 + z) is significantly greater than that measured with BAO at z similar to 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.
