Top-Down Theories

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 of galaxies.
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 is essential. As we noted in Chapter 24, dark matter (whatever it is) can be classified into two general types. 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 typically assume 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.