275Chapter 26Clusters, Galaxies, Black Holes
and Stars
The Super Structure of the Lumpy Universe
Having described dark energy and matter, inflation and the multiverse
in the last chapter we can now turn to a description of the visible
universe made up of stars, galaxies, clusters and superclusters. There
remains for us the task of explaining how the cosmos, at the beginning of
the stellar era one million years after the universe itself began, evolved
into the presently observed universe of superclusters, clusters, galaxies
and stars. The universe in its earliest stages was presumably a uniform
sea of matter, which at the beginning of the stellar era had a density of
10 -21 g/cm^3. It is known from fluid dynamics that a density fluctuation
within a uniform fluid will cause a gravitational condensation of matter.
If the density increases in a certain area, then the gravitational forces of
the matter within this zone will be stronger than the gravitational pull of
matter outside the zone and, hence, the matter within the zone will begin
to collapse forming a structure within the uniform fluid. This explains
how clusters and galaxies formed from the uniform sea of matter, which
composed the universe in the early stages of its existence. The formation
of stars within galaxies follows a similar pattern to be described later.
In the last chapter we described the role of dark matter in giving rise
to the lumpy structures of our universe today. In this chapter we will
describe those structures consisting of stars, galaxies, clusters and
superclusters. The stars emerge and live in larger structures consisting of
1011 stars known as galaxies in which they are held in place by the
gravitational pull of the central core of the galaxy. The galaxies
themselves are part of a cluster of galaxies that are held in place by their