Annu. Rev. Astron. Astrophys. 1999. 37:
127-189
Copyright © 1999 by . All rights reserved |

The idea that mass condensations and the geometry of the Universe
can alter light bundles and distort the images of distant galaxies
was emphasized by
Kristian & Sachs
(1966), and
later by
Gunn (1967),
Blandford & Jaroszynski
(1981),
who first gave a quantitative estimate of the amplitude of this effect.
Kristian (1967)
looked at this effect on photographic plates of six clusters of galaxies
using the Palomar Telescope, but found nothing significant.
Valdes et al (1983)
were the first to attempt to measure a coherent alignment of distant
galaxies generated by large-scale structures. They used about 40,000
randomly selected field galaxies with *J* magnitudes
between 22.5 and 23.5, but, like Kristian, they did not find any conclusive
signal. These negative measurements were not definitely interpreted as
important cosmological constraints on the curvature and the mass
distribution in our Universe, but instead as a result of technical
limitations related to the poor image quality of the photographic
data. Indeed, the recent
weak lensing analysis produced by a supercluster candidate done
by Kaiser et al (1998)
seems to show that large-scale
structures produce gravitational shear which is already
detectable. Numerical simulations by
Schneider & Weiss
(1988) using
point-mass models, or by
Babul & Lee (1991)
using a
smooth mass distribution, showed that both the ellipticity distribution
and the apparent luminosity function of distant galaxies could be
modified, in particular if the fraction of small-scale structures such
as clusters of galaxies is important
(Webster 1985).
Therefore, two
different effects produced by the cosmological distribution of structures
in the universe are expected: a change of the galaxy number count
correlated with the mass distribution, namely a magnification bias; and
a change of the ellipticity distribution, namely a shear pattern,
correlated with the mass distribution as well. Because the expectation
values strongly depend on the fraction of non-linear
systems and the redshift distribution of the galaxies, it is clear that
the analysis of weak lensing effects by large-scale structures is an
interesting test of cosmological scenarios.