  We measure the cosmological matter density by observing the positions of
baryon acoustic oscillations in the clustering of galaxies in the Sloan Digital
Sky Survey (SDSS). We jointly analyse the main galaxies and LRGs in the SDSS
DR5 sample, using over half a million galaxies in total. The oscillations are
detected with 99.74% confidence (3.0sigma assuming Gaussianity) compared to a
smooth power spectrum. When combined with the observed scale of the peaks
within the CMB, we find a best-fit value of Omega_m=0.256+0.029-0.024 (68%
confidence interval), for a flat Lambda cosmology when marginalising over the
Hubble parameter and the baryon density. This value of the matter density is
derived from the locations of the baryon oscillations in the galaxy power
spectrum and in the CMB, and does not include any information from the overall
shape of the power spectra. This is an extremely clean cosmological measurement
as the physics of the baryon acoustic oscillation production is well
understood, and the positions of the oscillations are expected to be
independent of systematics such as galaxy bias.
