Published online by Cambridge University Press: 19 July 2016
The history of modern cosmology has been divided into two periods. The primary discovery phase extended from 1920 to ∼1950 when most of the classical tests were developed. The subsequent consolidation period from 1950 to ∼1980 saw a deeper understanding of the tests and the introduction of concepts of stellar populations that led to estimates of evolutionary trends with look-back time. The only test not affected by evolutionary effects is the comparison of the Hubble time, Ho−1, with independent estimates of the age of the Universe. The N(m), m(z), θ(z), θ(m), and HoTo (qo, Λ) tests are reviewed as they have been discussed in the archive literature. The problem of observational bias is emphasized, as is the difficult recent search for secular evolutionary effects. Current problems which have prospects for solutions in the next decades are set out in the final section.