Measuring Omega

Original · EN

We were asked to debate the value of the cosmological mass-density parameter Omega. Is Omegaₘ=1 in accordance with the simplest model? Is Omegaₘ much smaller as indicated by some observations? There is conflicting evidence. We lay out the various methods for measuring Omegaₘ, mention new developments and current estimates, and focus on prospects versus the associated difficulties. We try to shed light on the uncertainties that are responsible for the span of estimates for Omegaₘ. We divide the methods into the following four classes: a. Global measures of the properties of space-time that constrain combinations of Omegaₘ and the other cosmological parameters (Lambda, Ho, to). b. Virialized Systems. Nonlinear dynamics within galaxies and clusters on comoving scales 1-10 Mpc/h. c. Large-scale structure. Mildly-nonlinear gravitational dynamics of fluctuations on scales 10-100 Mpc/h, in particular cosmic flows. d. Growth rate of fluctuations from the last scattering of the CMB or from high redshift galaxies to the present. Methods (b) typically yield low values of Omegaₘ=0.2-0.3. Methods (a) and (c) typically indicate higher values of Omegaₘ=0.4-1.