Baryon Acoustic Oscillations in the Ly-\alpha\ forest of BOSS quasars
  We report a detection of the baryon acoustic oscillation (BAO) feature in the
three-dimensional correlation function of the transmitted flux fraction in the
\Lya forest of high-redshift quasars. The study uses 48,640 quasars in the
redshift range $2.1\le z \le 3.5$ 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
$20\hMpc<r<200\hMpc$. A peak in the correlation function is seen at a
separation equal to $(1.01\pm0.03)$ 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 $\da$ 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
$H(z=2.3)=(224\pm8){\rm km\,s^{-1}Mpc^{-1}}$, 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 $z\sim0.5$. 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.
