Cosmological parameters

Abstract

The determination of the fundamental cosmological parameters has achieved dramatic progress over the past few years. Using Cepheids and secondary distance indicators, the expansion rate of the Universe is constrained to H 0 = 60–75 km s −1 Mpc −1. More stringent constraints can be expected from independent determinations based on the Sunyaev-Ze'dovich effect in clusters of galaxies. The total matter-energy density, measured from the first acoustic peak in the angular power spectrum of cosmic microwave background fluctuations (CMB) assuming initial adiabatic Gaussian density fluctuations, results Ω tot = 1.01 −0.03 +0.03 (68% confidence levels are quoted hereafter). The classical determination of cosmic baryonic density from big bang nucleosynthesis and abundances of light elements has been fully confirmed with comparable accuracy by measurements of peaks in the CMB power spectrum. The combined results give Ω b h 2 = 0.022 −0.0017 +0.0017 . The index of the power spectrum of primordial fluctuations is found n s = 1.00 −0.03 +0.03. Estimates of the average matter density in the universe from clusters of galaxies, gravitational lensing, cosmic flows and power spectra of galaxies, clusters and Lyman α forest give as likely values Ω m ∼ 0.3−0.4 , but with large uncertainties. Similar results are found from CMB angular power spectrum measurements and large scale structure data. Observations of distant Type Ia supernovae suggest an accelerated expansion of the universe, setting new tight constraints on Ω m and Ω Λ which combined with the total matter-density energy constraints from CMB, lead to Ω m ∼ 0.3 and Ω Λ ∼ 0.7 with uncertainties of ∼20%. The expansion age of the universe inferred from the previous set of cosmological parameters, t o = 13 ± 1.5 Gyr, is fully consistent with current determinations from globular clusters, radioactive nuclei and CMB anisotropies.