The Cosmological Constant Λ, the Age of the Universe and Dark Matter: Clues from the Lyα-Forest

Evidence for a positive cosmological constant $\Lambda$, derived from the Ly$\alpha$-forest in high-resolution spectra of quasars, leads to a closed, low-density, $\Lambda$-dominated universe. The analysis is based on the assumption of a universal shell structure expanding predominantly with the Hubble flow. Supporting evidence comes from two pairs of very wide absorption lines in the spectra of two quasars separated by 8 arcmin on the sky. These results contradict the higher values of the density parameter $\Omega_{M,0}$ derived, for example, from clusters of galaxies by the pure gravitational instability theory. Implications thereof and for the amount of non-baryonic dark matter in the universe are discussed with some $\Lambda$-dominated models, in particular with $\Omega_{M,0} = 0.2$ and with pure baryonic models. The evidence points to low-density, closed models with spherical metric, expanding forever.