B. The Problem of Large-Scale Structure
In contrast to the horizon problem, the fact that the Big Bang predicts no inhomogeneity is a problem as well. How are galactic structures to form in a perfectly homogeneous universe? The fact that galaxies have been shown to cluster locally with great voids on the order of 100 Mpc, is proof of the inhomogeneity of the universe. Moreover, the 10-5 anisotropies (temperature differences) on angular scales of 10 degrees as measured by the COBE satellite, form a blueprint of the seeds of formation at the time of decoupling. However, there is no mechanism within the Big Bang theory to account for these `seeds', or perturbations, that result in the large-scale structure. Not only does the Big Bang predict homogeneous structure, but it also had to `explode' in just the right way to avoid collapse. This is often called the fine-tuning problem. Cosmologists would like to have a theory that does not require specific parameters to be put in the theory ad hoc. The density, the expansion rate, and the like, prove to be other unfavorable aspects of the hot Big Bang.