Abstract
We report a detection of the baryon acoustic oscillation (BAO) feature in the
three-dimensional correlation function of the transmitted flux fraction in the
Łya forest of high-redshift quasars. The study uses 48,640 quasars in the
redshift range $2.1z 3.5$ 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 $Łambda$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, by themselves, require the existence of dark energy.
Combined with constraints derived from Cosmic Microwave Background (CMB)
observations, this result implies $H(z=2.3)=(224\pm8)km s^-1Mpc^-1$,
indicating that the time derivative of the cosmological scale parameter
$a=H(z=2.3)/(1+z)$ 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.
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