Status of Cosmological Parameters: Ω₀≈ 0.3 vs. Ω=1

Original · EN

The cosmological parameters that I discuss are the Hubble parameter H₀ ≡ 100 h km s⁻¹ Mpc⁻¹, the age of the universe t₀, the average density Ω₀, and the cosmological constant Λ. To focus the discussion, I concentrate on the the value of Ω₀ in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat (Ω₀ + ΩΛ=1) low-Ω CDM with a Cosmological Constant (ΛCDM). The evidence would favor small Ω₀ ≈ 0.3 if (1) the Hubble parameter actually has the high value h ≈ 0.75 favored by many observers, and t₀ ≥ 13 Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually 15%, about 3 times larger than expected for standard BBN in an Ω=1 universe, Ωb ≈ 0.0125 h⁻², despite the recent measurement by Tytler of D/H=2.4× 10⁻⁵ in two high-redshift Lyman limit systems, implying Ωb≈ 0.024 h⁻². The evidence would favor Ω=1 if (1) the POTENT analysis of galaxy peculiar velocity data is right, in particular regarding outflows from voids or the inability to obtain the present-epoch non- Gaussian density distribution from Gaussian initial fluctuations in a low- Ω universe; or (2) the preliminary LSND report indicating neutrino mass 2.4 eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-Ω₀ ΛCDM model. Statistics on gravitational lensing of quasars provide an upper limit on Λ, and the preliminary results on the deceleration parameter q₀=Ω₀/2-ΩΛ on very large scales from high-redshift Type Ia supernovae suggest that Ω₀ 1 and ΩΛ is small.