In this review I will concentrate on the values of the cosmological parameters. The other key questions in cosmology today concern the nature of the dark matter and dark energy, the origin and nature of the primordial inhomogeneities, and the formation and evolution of galaxies. I have been telling my theoretical cosmology students for several years that these latter topics are their main subjects for research, since determining the values of the cosmological parameters is now mainly in the hands of the observers.
In discussing cosmological parameters, it will be useful to distinguish between two sets of assumptions: (a) general relativity plus the assumption that the universe is homogeneous and isotropic on large scales (Friedmann-Robertson-Walker framework), or (b) the FRW framework plus the CDM family of models. In addition to the FRW framework, the CDM models assume that the present matter density _{m} plus the cosmological constant (or its equivalent in ``dark energy'') in units of critical density _{} = / (3 H_{0}^{2}) sum to unity (_{m} + _{} = 1) to produce the flat universe predicted by simple cosmic inflation models. These CDM models assume that the primordial fluctuations were adiabatic (all components fluctuate together) and Gaussian, and had a Zel'dovich spectrum (P_{p}(k) = A k^{n}, with n 1), and that the dark matter is mostly of the cold variety.
Although the results from the Long-Duration BOOMERANG [30, 75] and the MAXIMA-1 [54, 5] CMB observations and analyses [59] were were not yet available at the Dark Matter 2000 conference, I have made use of them in preparing this review. The table below summarizes the current observational information about the cosmological parameters, with estimated 1 errors. The quantities in brackets have been deduced using at least some of the CDM assumptions. The rest of this paper discusses these issues in more detail. But it should already be apparent that there is impressive agreement between the values of the parameters determined by various methods.
H_{0} | = 100 h km s^{-1} Mpc^{-1} , h = 0.65 ± 0.08 |
t_{0} | = 9-16 Gyr (from globular clusters) |
= [9-17 Gyr from expansion age, CDM models] | |
_{b} | = (0.045 ± 0.0057) h_{65}^{-2} (from D/H) |
> [0.04 h_{65}^{-2} from Ly forest opacity] | |
_{m} | = 0.4 ± 0.2 (from cluster baryons) |
= [0.34 ± 0.1 from Ly forest P(k)] | |
= [0.4 ± 0.2 from cluster evolution] | |
> 0.3 (2.4, from cosmic flows) | |
= [0.5 ± 0.1 from flows plus SN Ia] | |
3/4 _{} - 1/4 ± 1/8 from SN Ia | |
_{m} + _{} | = 1.11 ± 0.07 (from CMB peak location) |
_{} | = 0.71 ± 0.14 (from previous two lines) |
< & 0.73 (2) from radio QSO lensing | |
_{} | 0.001 (from Superkamiokande) |
[0.1] | |