Annu. Rev. Astron. Astrophys. 1992. 30: 311-358
Copyright © 1992 by . All rights reserved


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1. INTRODUCTION

1.1 History

The discovery by Walsh et al. (1979) of the first bona fide gravitational lens, the doubly-imaged quasar, Q0957+561, happened at an opportune time, following several prescient theoretical papers, and just preceding the completion of radio and optical quasar surveys that have since yielded over a dozen examples of this phenomenon. Interest in gravitational lenses stretches back over more than seventy years (Eddington 1919, Lodge 1919). Zwicky (1937a, b) appears to have been the first to realize that gravitational lensing ought to have a major impact on cosmology, specifically by ``weighing'' nebulae and providing crude telescopes to magnify lensed sources. The discovery of quasi-stellar ``point'' sources added two more possible uses of lenses, for distance measurement (Klimov 1963, Liebes 1964, Refsdal 1964b) and as probes of the stellar composition of lenses (Chang & Refsdal 1979), both of which may be just coming to fruition. These four topics constitute the primary theme of this review.

The observational challenge of gravitational lensing is great and, in fact, many of the difficulties had already been recognized prior to 1979. (a) Multiple-imaging due to lensing is a comparatively rare phenomenon, affecting only a fraction of a per cent of distant sources (Press & Gunn 1973). (b) Large magnification can disguise the nature of the source (Barnothy 1965, 1966). (c) The lens mass distribution is uncertain and this is reflected in the lensing properties (Bourassa & Kantowski 1976). (d) Regions of different size in the source can be magnified to differing degrees (Sanitt 1971). (e) Perturbations due to inhomogeneities along the line of sight can introduce additional distortions (Zeldovich 1964, Gunn 1967a, b). Despite these many problems, the past decade has seen gravitational lenses produce pleasing corroboration of many basic ideas in extragalactic astronomy and geometrical optics. We expect that the next decade will see lenses playing a more central role in cosmological research.

1.2 This Review

The field of gravitational lensing has expanded so much in recent years that it is not practical to encompass it entirely in a single Annual Review article. We therefore concentrate only on those aspects of the subject that promise to impact cosmology, dealing successively with cosmography (Section 4), the inferences that can be drawn about the perturbing masses (Section 5), deductions that can be made concerning the source structure (Section 6), and future prospects for enlarging the sample of known lenses (Section 7). For completeness, however, we begin with abbreviated discussions of two basic topics: the observational status of individual gravitational lenses (Section 2), and the theoretical concepts used to understand the optics of lensing (Section 3). We give only limited bibliography for these two sections and refer the reader to more detailed reviews by Blandford & Kochanek (1987a), Canizares (1987), Refsdal & Kayser (1988), Fort (1990), Surdej (1990), Narayan & Wallington (1992a), the recent conference proceedings edited by Moran et al. (1989), Mellier et al. (1990), and Kayser & Schramm (1992) and, in particular, the excellent monograph of Schneider et al. (1992) (cf also Bliokh & Minakov 1989).

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