Status of Cosmological Parameters: Can $Ω=1$?

Abstract

I discuss the Hubble parameter $H_0$, the age of the universe $t_0$, the density $Ω_0$, and the cosmological constant $Λ$. To focus the discussion, I concentrate on the issue of the value of the density $Ω_0$ in currently popular cosmological models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM) and flat low- $Ω$ CDM with a Cosmological Constant ($Λ$CDM). The evidence would favor a small $Ω_0 \approx 0.3$ if (1) the Hubble parameter actually has the high value $h \approx 0.8$ favored by many observers, and the age of the universe $t_0 \geq 13$ Gy; or (2) the baryonic/total mass ratio in clusters is actually $\sim 20$\%, about 3-4 times larger than expected for standard Big Bang Nucleosynthesis in an $Ω=1$ universe, and standard BBN is actually right in predicting that the density of ordinary matter $Ω_b$ lies in the range $0.009 \leq Ω_b h^2 \leq 0.02$. 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 report from LSND indicating a neutrino mass $\geq 2.4$ eV is right, since that would be too much hot dark matter to allow significant structure formation in a low-$Ω$ $Λ$CDM model. Statistics on gravitational lensing of quasars provide a strong upper limit on $Λ$. Reliable data on all of these issues is becoming available so rapidly today that a clear decision between these alternatives may be possible within the next few years.