and Cosmology

8.4 Cosmic Shear
ity one can separately investigate SNe Ia that occur in
early-type galaxies, in which only little dust exists, and
compare these to events in spiral galaxies. In this test,
no systematic differences are found, neither in events at
high redshift nor in nearby SNe Ia.
One possibility that has been discussed is the existence
of “gray dust”: dust that causes an absorption
independent of wavelength. In such a case extinction
would not reveal itself by reddening. However, this
hypothesis lacks any theoretical explanation for the
physical nature of the dust particles. In addition, the
observation of SNe Ia at z 1 shows that the evolution
of their magnitude at maximum is compatible
with a Λ-universe. In contrast, in a scenario involving
“gray dust”, a monotonic decrease of the brightness
with redshift would be expected, relative to an empty
universe.
Although it cannot be completely ruled out that the
results from SN Ia investigations are affected by system-
atic effects that mimic a cosmological effect, all tests
that have been performed for such systematics have been
negative. For this reason, the results are a very strong
indication of a universe with finite vacuum energy density.
The confirmation of this conclusion by the CMB
anisotropies (see Fig. 8.6) is indeed impressive.
8.4 Cosmic Shear
On traversing the inhomogeneous matter distribution in
the Universe, light beams are deflected and distorted,
where the distortion is caused by the tidal gravitational
field of the inhomogeneously distributed matter.
As was already discussed in the context of the reconstruction
of the matter distribution in galaxy clusters
(see Sect. 6.5.2), by measuring the shapes of images of
distant galaxies this tidal field can be mapped. From
probing the tidal field, conclusions can be drawn about
329
Fig. 8.18. As light beams propagate through the Universe they
are affected by the inhomogeneous matter distribution; they
are deflected, and the shape and size of their cross-section
changes. This effect is displayed schematically here – light
beams from sources at the far side of the cube are propagating
through the large-scale distribution of matter in the
Universe, and we observe the distorted images of the sources.
In particular, the image of a circular source is elliptical to
a first approximation. Since the distribution of matter is highly
structured on large scales, the image distortion caused by light
deflection is coherent: the distortion of two neighboring light
beams is very similar, so that the observed ellipticities of
neighboring galaxies are correlated. From a statistical analysis
of the shapes of galaxy images, conclusions about the statistical
properties of the matter distribution in the Universe can
be drawn. Hence, the ellipticities of images of distant sources
are closely related to the (projected) matter distribution, as
displayed schematically in the right panel