Galaxies in the Universe
Active galactic nuclei
and the
early history
of galaxies
We begin this chapter
by discussing galaxies with an active nucleus, a compact
central region from
which we observe substantial radiation that is not the light of
stars or emission from the gas heated by
them.
Growth of structure: from small
beginnings
The cosmic background radiation
is almost, but not quite, uniform: across the
sky, its
temperature differs by a few parts in 105. These tiny differences tell us
how lumpy
the cosmos was at the time trec
of recombination, when the radiation
cooled
enough for neutral atoms to form. Quantum fluctuations in the
field
responsible for inflation left their imprint as irregularities in the density
of matter and radiation.
Expansion of a homogeneous Universe
Because the cosmic background
radiation is highly uniform, we infer that the
Universe is isotropic – it is the
same in all directions. We believe that on a large
scale the
cosmos is also homogeneous – it would look much the same if we lived
in any other galaxy. Then,
mathematics tells us that the length s of a path linking
any two points at time t must
be given by integrating the expression
Large-scale structure today
As we look out into the sky, it is quite clear
that galaxies are not spread uniformly
through space.
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