The cosmological parameters:: Can Ω=1?

The cosmological parameters that I will discuss are the traditional ones: the Hubble parameter H-0 = 100h km s(-1) Mpc(-1), the age of the universe t(0), the average density Omega(0) = <(rho)over bar>/rho(c) in units of critical density rho(c), and the cosmological constant Lambda (or Omega(Lambda) = Lambda/(3H(0)(2))). To focus the discussion, I will concentrate on the issue of the value of Omega(0) in currently popular cosmological models in which most of the dark matter is cold, especially Cold + Hot Dark Matter (CHDM), and flat (Omega(0) + Omega(Lambda) = 1) low-Omega CDM with a Cosmological Constant (Lambda CDM). The evidence would favor a small Omega(0) approximate to 0.3 if (1) the Hubble parameter actually has the high value h approximate to 0.7 favored by many observers, and the age of the universe t(0) greater than or equal to 13 Gy; or (2) the baryonic/total mass ratio in dusters of galaxies is actually similar to 15%, about 3 times larger than expected for standard Big Bang Nucleosynthesis in an Omega = 1 universe, and standard BBN is actually right in predicting that the density of ordinary matter is Omega(b) approximate to 0.0125h(-2), based mainly on He-4 and 7Li data, despite the recent measurement by Tytler of D/H = 2.4 x 10(-5) in two high-redshift Lyman limit systems, implying Omega(b) approximate to 0.024h(-2). The evidence would favor Omega = 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-Omega universe; or (2) the new measurement of the the deceleration parameter q(0) = Omega(0)/2 - Omega(Lambda) on very large scales using high-redshift Type Ia supernovae is confirmed - the preliminary results suggest that Omega(0) approximate to 1 and Omega(Lambda) is small. Statistics on gravitational lensing of quasars also provide a strong upper limit on Lambda. The era of structure formation is another important discriminant between these alternatives, low Omega(0) favoring earlier structure formation, and Omega = 1 favoring later formation with many clusters and larger-scale structures still forming today. Reliable data on all of these issues is becoming available so rapidly today that there is reason to hope that a clear decision between these alternatives will be possible within the next few years.