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 ( 3.0 \sigma 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 .