|
Annu. Rev. Astron. Astrophys. 1992. 30:
311-358
Copyright © 1992 by Annual Reviews. All
rights reserved |
1. INTRODUCTION
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.
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).