and Cosmology

8. Cosmology III: The Cosmological Parameters
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because it consists predominantly of dark matter. If it is
assumed that the distribution of galaxies traces the underlying
distribution of dark matter fairly, the properties
of the LSS of matter could be studied by observing the
galaxy distribution in the Universe. Quite a few good
reasons exist for this assumption not to be completely
implausible. For instance, we observe a high galaxy
density in clusters of galaxies, and with the methods
discussed in Chap. 6, we are able to verify that clusters
indeed represent strong mass concentrations. Qualitatively,
this assumption therefore seems to be justified.
We will later modify it slightly.
In any case, the distribution of galaxies on the sphere
appears inhomogeneous and features large-scale structure.
Since galaxies have evolved from the general
cosmic density field, they should contain information
about the latter. It is consequently of great interest
to examine and quantify the properties of the galaxy
distribution.
In principle, two possible ways exist to accomplish
this study of the galaxy distribution. With photometric
sky surveys, the two-dimensional distribution of galaxies
on the sphere can be mapped. To also determine the
third spatial coordinate, it is necessary to measure the
redshift of the galaxies using spectroscopy, deriving the
distance from the Hubble law (1.6). It is obvious that
we can learn considerably more about the statistical
properties of the galaxy distribution from their threedimensional
distribution; hence, redshift surveys are of
particular interest.
The graphical representation of the spatial galaxy positions
is accomplished with so-called wedge diagrams.
They represent a sector of a circle, with the Milky Way
at its center. The radial coordinate is proportional to
z (or cz – by this, the distance is measured in km/s),
and the polar angle of the diagram represents an angular
coordinate in the sky (e.g., right ascension), where
an interval in the second angular coordinate is selected
in which the galaxies are located. An example for such
a wedge diagram is shown in Fig. 8.1.
8.1.2 Redshift Surveys
Performing redshift surveys is a very time-consuming
task compared to making photometric sky maps,
because recording a spectrum requires much more ob-
Fig. 8.1. The CfA redshift survey, in equatorial coordinates.
Along its radial axis, this wedge diagram shows the escape
velocity cz up to 12 000 km/s, and the polar angle specifies
the right ascension of a galaxy. The Great Wall extends from
9 h to 15 h . The overdensity at 1 h and cz = 4000 km/s isthe
Pisces–Perseus supercluster
serving time than the mere determination of the apparent
magnitude of a source. Hence, the history of redshift
surveys, like that of many other fields in astronomy,
is driven by the development of telescopes and instruments.
The introduction of CCDs in astronomy in the
early 1980s provided a substantial increase in sensitivity
and accuracy of optical detectors, and enabled
us to carry out redshift surveys of galaxies in the
nearby Universe containing several thousand galaxies
(see Fig. 8.1). Using a single slit in the spectrograph
implied that in each observation the spectra of only
one or very few galaxies could be recorded simultaneously.
The situation changed with the introduction of
spectrographs with high multiplexity which were designed
specifically to perform redshift surveys. With
them, the spectra of many objects (up to a thousand)
in the field-of-view of the instrument can be observed
simultaneously.
The Strategy of Redshift Surveys. Suchasurveyisbasically
defined by two criteria. The first is its geometry: