In top-down theories of
structure formation, large objects the size of
superclusters of galaxies form first and these then fragment to form
smaller structures like galaxies. The adjacent figure illustrates the basic idea.
Theories of this kind predict that supercluster size blobs collapse into
pancake-like structures and that small density condensations within this pancake
collapse to form galaxies and clusters
Hot Dark Matter
Top-down structure formation is favored by hot dark matter.
To form structure quickly in the early Universe we have seen that the gravitational effect of dark matter
As we noted in Chapter 24, dark matter (whatever it is) can be classified into two general
Hot dark matter
consists of particles that have very low mass or are massless and that move at
velocities comparable to that of light (relativistic velocities).
Cold dark matter consists of massive particles or objects
that move at
velocities small compared with that of light (nonrelativistic velocities).
In the early Universe, hot dark matter
streams through space
at relativistic velocities. Calculations indicate that this does not
permit structure to form initially on scales smaller than superclusters because the streaming hot dark
matter would destroy all
structure on smaller scales such as that of galaxies.
Therefore, top-down theories
that structure formation in the early Universe was dominated by hot dark matter.
Top-Down Theories and Observations
Top-down theories can account for much observed
structure, but they have two serious difficulties when confronted with the data on structure formation.
First, such theories have difficulty in forming galaxies quickly, which is required by the
high redshift observations showing the early onset of galaxy formation.
Second, these theories predict that significant galaxy formation should be
going on today, which is in contradiction to observations. Finally, cosmological
parameters determined from experiments like WMAP on the anisotropy of the cosmic
microwave background suggest that hot dark matter, which is essential to top-down
theories, probably played little role in initial structure formation.