The goal of the observational cosmologist is to utilize astronomical objects to derive cosmological parameters. The transformation from the observables to the key parameters usually involves many assumptions about the nature of the objects, as well as about the nature of the dark matter. Below we outline the physical processes involved in each probe, and the main recent results. The first two subsections concern probes of the homogeneous Universe, while the remainder consider constraints from perturbations.
3.1. Direct measures of the Hubble constant
In 1929 Edwin Hubble discovered the law of expansion of the Universe by measuring distances to nearby galaxies. The slope of the relation between the distance and recession velocity is defined to be the Hubble constant H0. Astronomers argued for decades on the systematic uncertainties in various methods and derived values over the wide range, 40 km s-1 Mpc-1 H0 100 km s-1 Mpc-1.
One of the most reliable results on the Hubble constant comes from the Hubble Space Telescope Key Project . The group has used the empirical period-luminosity relations for Cepheid variable stars to obtain distances to 31 galaxies, and calibrated a number of secondary distance indicators (Type Ia Supernovae, Tully-Fisher, surface brightness fluctuations and Type II Supernovae) measured over distances of 400 to 600 Mpc. They estimated H0 = 72 ± 3 (statistical) ± 7 (systematic) km s-1 Mpc-1. 3 The major sources of uncertainty in this result are due to the metallicity of the Cepheids and the distance to the fiducial nearby galaxy (called the Large Magellanic Cloud) to which all Cepheid distances are measured relative to. Nevertheless, it is remarkable that this result is in such good agreement with the result derived from the WMAP CMB and large-scale structure measurements (see Table 2).
3 Unless stated otherwise, all quoted uncertainties in this article are one-sigma/68% confidence. It is common for cosmological parameters to have significantly non-Gaussian error distributions. Back.