Ann. Rev. Astron. Astrophys. 1988. 26: 561~530Copyright © 1988 by Annual Reviews Inc. All rights reservedOBSERVATIONAL TESTS OFWORLD MODELSAllan SandageDepartment of Physics and Astronomy, The Johns Hopkins University,Baltimore, Maryland 21218 and Space Telescope Science Institute,3700 San Martin Drive, Baltimore, Maryland 2121811. THE ELEMENTS OF PRACTICAL COSMOLOGYThe standard model of cosmology, based on what has come to be calledthe Friedmann-Lemaitre-Robertson-Walker (FLRW) model (hereinaftersimply the Friedmann model), is now part of scientific culture. The mostpopular current version leads to the hot big bang (HBB) description events near the beginning of the cosmic expansion, which has often beencalled a creation2 moment at the beginning of physical time. In this reviewa prejudice in favor of the HBB [in contrast to cold beginnings discussed,for example, by Layzer (1987) in his remarkable book on the growth order in the Universe] can hardly be suppressed, successful as the modelhas become in providing an understanding of the abundance of He4 andthe 3-K radiation. Nevertheless, if a description of beginnings in this senseis to be confined within the methods of science rather than to be coloredby teleological metaphysics, the model must pass the tests normal toscience rather than to be accepted as revealed truth. The purpose of this~ Presently at Mount Wilson Observatory of the Carnegie Institution of Washington, 813Santa Barbara Street, Pasadena, California 91101.~ Creation is a flammable word that triggers responses often not intended by writers whouse it. Gamow, in reply to a critic who complained about the title of his famous book "TheCreation of the Universe," advised his reader to interpret creation as something similar to alady’s fashion rather than to misinterpret it as a theological statement. If it were the latter,the inquiry would be removed from the possibility of using the scientific method to discover,rather than some other method to reveal. When creation is used in this review, its meaningis in the Gamow sense. Nevertheless, the subject is possibly as close as science can come tothe questions of origins--hence its enormous appeal.5610066-4146/88/0915-0561 $02.00www.annualreviews.org/aronlineAnnual ReviewsAnnu. Rev. Astro. Astrophys. 1988.26:561-630. Downloaded from arjournals.annualreviews.orgby California Institute of Technology on 04/10/09. For personal use only.562 SANDAGEreview is to discuss the direct tests of observation that lead to the viewthat a hot beginning to a current universe of finite age did occur.Reviews of the theoretical aspects of the FLRW standard model fromvarious viewpoints have appeared previously in this series. Novikov &Zeldovich (1967) surveyed the physical aspects of the HBB early Universe.Harrison (1973) summarized and discussed the various chemical eras,starting from a presumed initial singularity of very high temperature tothe time of decoupling of matter and radiation, with the consequent for-mation of atoms some 30,000 yr after the Creation. Steigrnan (1976)reviewed the evidence and the reason(s) for the present matter-antimatterasymmetry. Boesgaard & Steigrnan (1985) discussed the theory and com-pared its predictions with observations of big bang nucleosynthesis. Thiscomparison of the observed abundances of H, D, He3, He4, and Li7 withthe calculations provides one of the two most powerful proofs of the HBBmodel. The other, of course, is the 3-K microwave background (MWB)radiation itself, discussed in these Reviews by Sunyaev & Zeldovich (1980)from the theoretical standpoint, and by Thaddeus (1972) and Weiss (1980)from the observational. The spectrum of the radiation resembles closelythat of a blackbody. This is an important argument supporting a relicorigin for the radiation, although alternate explanations have been pro-posed (Hoyle et al. 1968, Layzer & Hively 1973, Rana 1981, and referencestherein).Other theoretical aspects of the standard model have been developed inthese pages by Gould (1968) and Field (1972) in their reviews of intergalactic medium, by Gott (1977) in his discussion of galaxy formation,and by Ellis (1984) in his survey of alternatives to the HBB standardmodel.Particularly useful among the many workshop and conference pro-ceedings that give entrance to the extensive archive literature are PhysicalCosrnolo~Ty (Balian et al. 1979), Astrophysical CosmolotTy (Bruck et al.1982), Proyress in CosmolotTy (Wolfendale 1981), Cosmoloyy and Fun-damental Physics (Setti & Van Hove 1983), and Inner Space/Outer Space(Kolb et al. 1986).Most of these discussions center on theoretical consequences of the HBBmodel. There have been only a few systematic reviews of results of theseveral direct (mostly geometrical) tests of the model. To be sure, impor-tant expositions of the principles of some of the classical tests are containedin discussions of the general properties of the models, such as the foun-dational reviews by Robertson (1933, 1955), the comprehensive summaryby Zeldovich (1965), the lectures by Gunn (1978), and the textbooksby McVittie (1965), Peebles (1971), Weinberg (1972), Rowan-Robinson(1981), Narlikar (1983), and Zeldovich & Novikov (1983). But in www.annualreviews.org/aronlineAnnual ReviewsAnnu. Rev. Astro. Astrophys. 1988.26:561-630. Downloaded from arjournals.annualreviews.orgby California Institute of Technology on 04/10/09. For personal use only.OBSERVATIONAL TESTS OF WORLD MODELS 563these accounts, details of the practical methods of the subject are kept asa black art, taken to be known, and therefore not set out in detail.The present review is concerned with the observational aspects of thesubject. This is because no textbook now exists on what every studentshould know if practical cosmology--the linchpin of the laboratory partof the subject--is to become their way of life. The emphasis is on thedetails of the calculations (i.e. the equations) that are necessary to makecomparisons between the models and the data. My aim is to assess