Status of Cosmological Parameters: $Ω_0\approx 0.3$ vs. $Ω=1$

Abstract

The cosmological parameters that I discuss are the Hubble parameter $H_0 \equiv 100 h$ km s$^{-1}$ Mpc$^{-1}$, the age of the universe $t_0$, the average density $Ω_0$, and the cosmological constant $Λ$. To focus the discussion, I concentrate on the the value of $Ω_0$ in currently popular models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat ($Ω_0 + Ω_Λ=1$) low-$Ω$ CDM with a Cosmological Constant ($Λ$CDM). The evidence would favor small $Ω_0 \approx 0.3$ if (1) the Hubble parameter actually has the high value $h \approx 0.75$ favored by many observers, and $t_0 \geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters of galaxies is actually $\sim 15$\%, about 3 times larger than expected for standard BBN in an $Ω=1$ universe, $Ω_b \approx 0.0125 h^{-2}$, despite the recent measurement by Tytler of $D/H=2.4\times 10^{-5}$ in two high-redshift Lyman limit systems, implying $Ω_b\approx 0.024 h^{-2}$. 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 $\gsim 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-$Ω_0$ $Λ$CDM model. Statistics on gravitational lensing of quasars provide an upper limit on $Λ$, and the preliminary results on the deceleration parameter $q_0=Ω_0/2-Ω_Λ$ on very large scales from high-redshift Type Ia supernovae suggest that $Ω_0 \sim 1$ and $Ω_Λ$ is small.