10 Cosmic Structures
After the decoupling of matter and radiation described in Chapter 6, we followed the
fate of the free-streaming CMB in Chapter 8. Here we shall turn to the fate of matter
and cold nonradiating dust. After recombination, when atoms formed, density per-
turbations in baryonic matter could start to grow and form structures, but growth in
weakly interacting nonbaryonic (dark) matter could have started earlier at the time
of radiation and matter equality. The time and size scales are important constraints
to galaxy-formation models, as are the observations of curious patterns of filaments,
sheets and voids on very large scales.
In Section 10.1 we describe the theory of density fluctuations in a viscous fluid,
which approximately describes the hot gravitating plasma. This very much parallels
the treatment of the fluctuations in radiation that cause anisotropies in the CMB.
In Section 10.2 we learn how pressure and gravitation conspire so that the hot
matter can begin to cluster, ultimately to form the perhaps 10^9 galaxies, clusters and
other large-scale structures.
10.1 Density Fluctuations
Until now we have described the dynamics of the Universe by assuming homogeneity
and adiabaticity. The homogeneity cannot have grown out of primeval chaos, because
a chaotic universe can grow homogeneous only if the initial conditions are incredibly
well fine-tuned. Vice versa, a homogeneous universe will grow more chaotic, because
the standard model is gravitationally unstable.
But the Universe appears homogeneous only on the largest scales (a debatable
issue!), since on smaller scale we observe matter to be distributed in galaxies, groups
of galaxies, supergalaxies and strings of supergalaxies with great voids in between. At
the time of matter and radiation equality, some lumpiness in the energy density must
have been the ‘seeds’ orprogenitorsof these cosmic structures, and one would expect
to see traces of that lumpiness also in the CMB temperature anisotropies originating
Introduction to Cosmology, Fourth Edition. Matts Roos
© 2015 John Wiley & Sons, Ltd. Published 2015 by John Wiley & Sons, Ltd.