Having studied some properties of spiral and elliptical galaxies in detail, we now take a step back to examine the mix and properties of galaxies as a whole.
In recent year, there have been a number of galaxy redshift surveys,
for example the two degree-field survey (2dF), and the Sloan Digital
Sky survey (SDSS). In a galaxy redshift survey, one first identifies
galaxies from images of the sky, and then takes a spectrum for each of
them. The shift of the spectral lines from the laboratory frame to what
is observed - the redshift - allows one to determine the position
along the line-of-sight. This is because the Universe is exapanding,
and hence there is a relation between how much the line is shifted, and
how distant the galaxy is. Doing this for all galaxies in the survey,
we obtain a three dimensional map of the positions of the
galaxies. Such surveys provide data on several hundred-thousands of
galaxies, and so enable one to investigate in great detail statistical
properties of the galaxy population. So what kind of general trends do
they uncover?
The distribution of galaxies probably traces to some extent the
distribution of dark matter in the Universe. For example, we've seen
that there is evidence for a lot of dark matter in the Universe as a
whole (from the cluster argument), but also in individual galaxies,
groups and clusters. And so, since we find that galaxies are
distributed very inhomogeneous in groups and clusters, presumably
also the dark matter is distributed in an inhomogeneous fashion. How?
And what process determines this in the first place?
We have some good idea of how the dark matter is distributed on a
cosmological scale, from some general considerations, but also
backed-up from observations of the micro-wave background (i.e. the sea
of photons left-over from when the Universe was still hot). Given this
input, we can use numerical simulations to predict the growth of
structure (at least in the dark matter). The nice part of the story is
that this theory is highly predictive, and does predict structures in
the galaxy distribution very similar to what we observe.