Three directly measurable quantities are available in observational cosmology:
- apparent flux fv, integrated flux or apparent magnitude m (broadband, narrowband, colours)
- angle - position and extent
- redshift z
- combinations, such as surface brightness B (flux/solid angle)
Polarization measurements are employed only implicitly through the astrophysical properties of the objects used in cosmological studies. The fifth measureable quantity of astronomy, time - here the age of the universe - is obtained indirectly (Sect. 3.1.3).