The bias of galaxies and the density of the universe from the 2dF galaxy redshift survey

By studying the bispectrum of the galaxy distribution in the 2dF galaxy redhsift survey (2dFGRS), we have shown that 2dFGRS galaxies are unbiased tracers of the mass distribution. This allows us the break the degeneracy intrinsic to power spectrum studies, between the matter density parameter Omegaₘ and the bias parameter b, and to obtain an accurate measurement of Omegaₘ: Omega₌, ₙ_₄₅₅=0. 27 0. 06, a measurement obtained from the 2dFGRS alone, independently from other data sets. This result has to be interpreted as Omegaₘ at the effective redshift of the survey z₄₅₅=0. 17. Extrapolated at z=0 we obtain Omegaₘ=0. 2 0. 06 for the LCDM model. This constraint on the matter density parameter when combined with cosmic microwave background constraints on the flatness of the Universe, show firm evidence that the Universe is dominated by a vacuum energy component.