Bottom-Up Theories

In bottom-up theories it is assumed that masses the size of star clusters collapse first, and these then assemble through collisions and mergers into galaxies, clusters of galaxies, and superclusters. The adjacent figure illustrates schematically. The initial collapses are assumed to begin very early in the history of the Universe.
Cold Dark Matter
Bottom-up theories of structure formation are favored by cold dark matter. To form small structures the dark matter must have low velocity, since hot dark matter streams at relativistic velocities and destroys small-scale structure. Therefore, bottom-up theories of structure formation generally assume that the initial formation structure in the early Universe was dominated by the influence of cold dark matter.

Symmetry and Age

One rather general observation in astronomy is that structures tend to become more symmetric with age. This is because, on average, interactions tend to erase irregularities (there are exceptions, but we are speaking of the typical behavior). This is one indication that structure is still growing on the largest scales. Many clusters and superclusters are very irregular in shape, which suggests that they are younger than the clusters from which they are being assembled.

Bottom-Up Theories and the Observations
The implication in bottom-up theories that mergers are essential to the formation of larger structures has strong resonance with our discussion of evidence for galaxy collisions in the early Universe and their influence on the evolution of galaxies (Chapter 24). In particular, we noted that giant elliptical galaxies have probably grown large by absorbing their neighbors, with the resulting star formation and gas ejection leaving the elliptical without star-forming material. More generally, bottom-up theories make two key predictions that seem to be supported by observations: galaxy formation should have been more common earlier in the history of the Universe rather than later, and structure on the largest scales (clusters and superclusters of galaxies) should still be assembling itself today (see the box). This supercomputer simulation illustrates initial structure formation under the influence of gravity from cold dark matter, while this simulation illustrates mergers in a cluster of galaxies, also aided by cold dark matter.
Cold versus Hot Dark Matter
The present observational data appear to support a bottom-up assembly of structure more so than a top-down picture. This in turn implies a more important role for cold dark matter than for hot dark matter in structure formation. As we have noted, cosmic microwave background data also indicate that hot dark matter played little role in the formation of initial structure. We shall see that the most successful computer simulations of initial structural growth typically use either cold dark matter or cold dark matter with an admixture of some hot dark matter to seed the initial growth.