318 Highlights in modern observational cosmology
(a)(b)Figure 11.1. (a) Lookback time as a function of the redshift for three reference FRW
models (Einstein–de Sitter, open, flat). Atz=20 the lookback time is approximately 99%
of the age of the universe in all models. (b) Derivative of the comoving volume element,
per unit solid angle, as a function of redshift for the same models.