Baryon Acoustic Oscillations in the Ly-\alpha\ forest of BOSS quasars

We report a detection of the baryon acoustic oscillation (BAO) featurein the three-dimensional correlation function of the transmitted fluxfraction in the \Lya forest of high-redshift quasars. The study uses48,640 quasars in the redshift range $2.1\le z \le 3.5$ from the BaryonOscillation Spectroscopic Survey (BOSS) of the third generation of theSloan Digital Sky Survey (SDSS-III). At a mean redshift $z=2.3$, wemeasure the monopole and quadrupole components of the correlationfunction for separations in the range $20\hMpc200\hMpc$. A peakin the correlation function is seen at a separation equal to$(1.01\pm0.03)$ times the distance expected for the BAO peak within aconcordance $\Lambda$CDM cosmology. This first detection of the BAO peakat high redshift, when the universe was strongly matter dominated,results in constraints on the angular diameter distance $\da$ and theexpansion rate $H$ at $z=2.3$ that, by themselves, require the existenceof dark energy. Combined with constraints derived from Cosmic MicrowaveBackground (CMB) observations, this result implies$H(z=2.3)=(224\pm8){\rm km s^{-1}Mpc^{-1}}$, indicating that the timederivative of the cosmological scale parameter $\dot{a}=H(z=2.3)/(1+z)$is significantly greater than that measured with BAO at $z\sim0.5$. Thisdemonstrates that the expansion was decelerating in the range$0.72.3$, as expected from the matter domination during thisepoch. Combined with measurements of $H_0$, one sees the pattern ofdeceleration followed by acceleration characteristic of a dark-energydominated universe.