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Gravitational Lensing III:The Weak Lensing Effect |
Principle of the Weak Lens Effect
Multiple images, Einstein rings, and arcs are spectacular
gravitational lens events. In addition to these phenomena of the so-called
strong lens effect, there is also the weak gravitational lens effect, which
has been known for around 10 years. This is because light bundles from
very distant galaxies are not only deflected as a whole, but will also
be distorted due to the tidal part of the gravitational field caused by
mass inhomogeneities. This distortion is in general rather weak, and thus
it cannot be found in individual objects since the intrinsic form of the
sources is unknown. However, a large number density of galaxies can be
found in deep optical images of the sky: up to a magnitude of R<25,
there are about 35 galaxies per square arcminute, whose characteristic
redshift is around z=1. Images of neighboring galaxies will be distorted
by a similar tidal field. Thus if the fact that the instrinsic orientation
of galaxies is random is used, then this tidal field can be determined
from the average alignment of the observable images of galaxies. Conclusions
about the distribution of mass can be drawn from the tidal field. Indeed,
by measuring this field of distortion, the distribution of mass in a cluster
of galaxies can be reconstructed, and one thus obtains a picture of (primarily
dark) matter -- it really can be seen! Fig.2 shows an example of such a
reconstruction of mass.
Do Dark Clusters Exist?
With the weak lensing effect it is possible not only to analyse known galaxy clusters, but also to deliberately attempt to find them. A statistically significant alignment of the images of very distant galaxies around a point indicates the presence of a concentration of mass. In this manner, it is possible to find concentrations of mass without resorting to their properties of luminosity. This is of general cosmological interest. While the theory of structure formation makes very detailed predictions about the distribution of matter (e.g. with numerical simulations), predictions about characteristics of luminosity (galaxy development, hot gas in clusters) can be made only by using simplified assumptions and thus have a much greater degree of uncertainty. The possibility of discovering galaxy clusters or their dark matter without resorting to an excess of galaxies on optical images, or through their x-ray luminosity, would permit a direct comparison of the number density of clusters and their masses with cosmological models. Since the number density of galaxy clusters as a function of mass and redshift is a sensitive measure of the cosmological model, this comparison is of exceptional interest. Indeed, a galaxy cluster has already been discovered using this method (Fig. 4).
Cosmic Shear
When atoms were first being formed out of electrons and
nuclei, dark matter was scattered almost homogeneously throughout the universe
- almost, but not completely, as can be clearly seen from fluctuations
of the cosmic microwave background radiation. From these small fluctuations
in density, the large-scale distribution of material in the universe developed
in the course of cosmic evolution. This can now be simulated very precisely
by using super-computers - but can the large-scale structure of dark matter
be observed directly? The weak lensing effect can be used here as a unique
tool: bundles of light from distant sources will be become weak because
of the large-scale distribution of matter, but in principle measurably
distorted (cosmic shear). Using the statistical characteristics of the
ellipticity distribution of galaxy images in a wide-angle image, the statistical
characteristics of the distribution of dark matter can be deduced. For
many years, several research groups in different countries have been looking
for this effect. The difficulty lies in the minuteness of the effect: practically
each inaccuracy in an image taken with a telescope yields a larger signal
than from the cosmic shear itself. Only after learning how to avoid making
such imaging mistakes or after learning how to correct them, four groups
published their discovery almost simultaneously in March 2000. Thus for
the first time, dark matter was directly observed outside bound structures
such as galaxy clusters. This breakthrough will lead to cosmic shear becoming
a main area of study in cosmology within the next few years. With its help,
one will be able to study in detail the development of the distribution
of matter and use models to compare results.
Gravitational Lenses I : Galaxies as Lenses
Gravitational Lenses II : Galaxy
Clusters as Lenses