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Baryons observed in Ly α absorbers contribute to the density parameter Ω0 by Ωbar ≥ 0.06 in close agreement with the value of 0.06 from primordial nucleosynthesis (H0=55 km s−1 Mpc−1, Λ = 0 assumed throughout). A number of methods are known to measure Ω0 from density fluctuations; bound structures tend to yield lower values (Ωm ≈ 0.2–0.4), field galaxies over large scales higher, but still undercritical values (Ωm ≈ 0.6 ± 0.2). The best compromise value is Ω0 ≈ 0.5, but the present methods are blind to diffusely distributed, exotic matter which still could make Ω0 = 1. A satisfactory solution of Ω0 (and Λ) will only come from a fundamental cosmological test (e.g. the Hubble diagram of [evolution-corrected] supernovae type Ia) in combination with the CMB fluctuation spectrum.

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