12
2 Aggregation
Fig. 2.1 An image of a spiral galaxy taken by the Hubble Space Telescope
as weak inhomogeneities in the cosmic background radiation, were sufficient to
seed the sharp density contrasts that shaped the new world dominated by matter.
Whatever we say about the ill effects of extreme wealth and poverty in the social
context, the Universe would be boring, dull, and unsuitable for life if it had not
developed sharp density and temperature contrasts between stars, planets, and the
interstellar void.
Density inhomogeneities develop in a hierarchical way, from galaxy clusters to
galaxies, to stellar systems, to stars and planets. The order in which this hierarchy was created is not evident: it depends on the original spectrum of temperature
and density inhomogeneities and on the speed with which perturbations with different wavelengths were growing. One possibility is a “top-down” scenario with
widespread denser areas giving rise to gas clouds condensing into galaxy clusters,
which break up into individual galaxies and further into still denser clouds where
stars are born. The opposite “bottom-up” scenario would be a faster growth on short
scales, with small denser clumps assembling due to gravitational attraction to larger
entities. The young Universe likely became structured in a top-down way, as indicated by a correspondence between the extent of inhomogeneities in the background
radiation and the size of galaxy clusters.
In the beginning, even denser regions would still have been “without form”, and
the first morphing would have been invisible, as matter, falling inward and cooling,
left behind a wider halo of dark matter, which would not have cooled due to the lack
of interactions sucking up its energy. Free fall would make the collapse complete if
motion was strictly centripetal – but velocities of gas molecules and cloudlets are
haphazard; just as a satellite does not fall onto Earth when it has a sufficient orbital
velocity (depending on its elevation) and planets do not fall onto the Sun, gas clouds
start rotating around the central core, and this gives rise to flattened galactic disks.
Any uniform distribution of matter also remains unstable on shorter scales. Density
waves propagate through the rotating disk, and, since rotation is slower in outlying
regions, they bend into spiral arms spreading out of the central bulge (Fig. 2.1).
2 Aggregation
Fig. 2.1 An image of a spiral galaxy taken by the Hubble Space Telescope
as weak inhomogeneities in the cosmic background radiation, were sufficient to
seed the sharp density contrasts that shaped the new world dominated by matter.
Whatever we say about the ill effects of extreme wealth and poverty in the social
context, the Universe would be boring, dull, and unsuitable for life if it had not
developed sharp density and temperature contrasts between stars, planets, and the
interstellar void.
Density inhomogeneities develop in a hierarchical way, from galaxy clusters to
galaxies, to stellar systems, to stars and planets. The order in which this hierarchy was created is not evident: it depends on the original spectrum of temperature
and density inhomogeneities and on the speed with which perturbations with different wavelengths were growing. One possibility is a “top-down” scenario with
widespread denser areas giving rise to gas clouds condensing into galaxy clusters,
which break up into individual galaxies and further into still denser clouds where
stars are born. The opposite “bottom-up” scenario would be a faster growth on short
scales, with small denser clumps assembling due to gravitational attraction to larger
entities. The young Universe likely became structured in a top-down way, as indicated by a correspondence between the extent of inhomogeneities in the background
radiation and the size of galaxy clusters.
In the beginning, even denser regions would still have been “without form”, and
the first morphing would have been invisible, as matter, falling inward and cooling,
left behind a wider halo of dark matter, which would not have cooled due to the lack
of interactions sucking up its energy. Free fall would make the collapse complete if
motion was strictly centripetal – but velocities of gas molecules and cloudlets are
haphazard; just as a satellite does not fall onto Earth when it has a sufficient orbital
velocity (depending on its elevation) and planets do not fall onto the Sun, gas clouds
start rotating around the central core, and this gives rise to flattened galactic disks.
Any uniform distribution of matter also remains unstable on shorter scales. Density
waves propagate through the rotating disk, and, since rotation is slower in outlying
regions, they bend into spiral arms spreading out of the central bulge (Fig. 2.1).