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Annu. Rev. Astron. Astrophys. 1992. 30:
311-358
Copyright © 1992 by Annual Reviews. All
rights reserved |
5.4 Large Scale Structure and Intergalactic Medium
A great deal of evidence has accumulated in recent years that the
universe is inhomogeneous on scales of up to 50-100 Mpc (e.g.
Geller & Huchra 1989,
Saunders et al. 1991).
Such inhomogeneity
will affect the propagation of light and lead to fluctuations in the
convergence and shear of light bundles
(Section 3.3). This has been
studied in a number of papers which discuss distortions in the images
of distant sources and changes in the apparent luminosities of the
sources (e.g.
Kantowski 1969,
Dyer & Roeder 1973,
Hammer 1985,
Dyer 1986,
Dyer & Oattes 1988,
Schneider & Weiss 1988ab,
Kasai et
al. 1990,
Jaroszynski
et al. 1990,
Babul & Lee 1991,
Bartelmann &
Schneider 1991,
Tomita 1991).
The discovery of the population of faint blue galaxies now provides a
potential tool to study these effects. Specifically, large-scale mass
inhomogeneities will induce correlated elliptical distortions in
images of background sources, which could be measured by mapping large
numbers of sources over areas of the sky ~ 1 square degree in
size
(Kristian 1967,
Blandford et
al. 1991,
Miralda-Escudé 1991c,
Kaiser 1992).
The magnitude of the correlated ellipticity should be
in the range 1-3% (depending on the source redshift
distribution) if the cold dark matter cosmology is correct, with much
larger signals should mass trace light. The same technique will also
reveal any coherent structures such as walls or voids. Although an
early attempt to measure some of these effects only gave upper limits
(Valdes et
al. 1983),
recent improvements in techniques make it worthwhile to repeat the
observations.
In addition to relatively smooth large-scale inhomogeneities, a
fraction of the mass in the universe may be in the form of compact
dark objects on mass scales ranging from stars to galaxy clusters.
These objects will cause distortions in the images of cosmologically
distant sources on angular scales ~ E
(Press & Gunn
1973).
Roughly, the optical depth to lensing is ~ E,
the density parameter corresponding to mass within the projected
Einstein rings of the lenses. For compact objects in the mass range
10-2 - 105 M, a limit of somewhat
less than unity may be
set on E
merely by the fact that the line-to-continuum
ratio is relatively constant among quasars
(Canizares 1982).
Better limits could be obtained from future observations of
cosmologically distant supernovae
(Schneider &
Wagoner 1987,
Linder et
al. 1988,
Rauch 1991).
With masses larger than
105 M, one would expect occasionally to see image
distortions
(Blandford &
Jaroszynski 1981),
image doubling
(Nemiroff &
Bistolas 1990,
Nemiroff 1991),
rings
(Burke 1992),
or phantom images in radio maps
(Kassiola et
al. 1991).
Current observations limit the 0 in the form of these objects to be
0.1. Future
observations, especially using the VLBA, may be used to place significantly
stronger limits.
Lensing by cosmic strings has been studied by some authors. A
straight string will produce a characteristic lensing pattern
consisting of approximately equal-separation image pairs
(Vilenkin 1984,
1986,
Gott 1985)
with similar parity. A promising example of this was discovered
(Cowie & Hu 1987),
but it may be merely a cluster of binary galaxies
(Hewitt et
al. 1990,
Hu 1990).
The image of an extended source, such as a galaxy, can exhibit a
discontinuity if the source happens to lie behind a string
(Paczynski 1986c),
though this signature may not be obvious if strings exhibit small scale kinky
structure as now thought
(Bennett &
Bouchet 1988).
Small string loops are expected to have lensing
properties similar to those of galaxies
(Hogan & Narayan
1984).