ABSTRACT
The speculations about the history of the Universe have provided crucial and well motivated 'hints' as to the appropriate initial data, hints that have led to a renaissance in the study of structure formation. On small scales the Universe today is very lumpy. Of course on very large scales, say much larger than 100 Mpc, the Universe is smooth, as evidenced by the isotropy of the cosmic microwave background radiation, the isotropy of the x-ray background, and number counts of radio sources. Overdense regions will behave like small closed-Universe models and will recollapse before pressure forces can respond to prevent black hole formation. The growth of linear perturbations in both a radiation-dominated and curvature-dominated Universe is inhibited. Adiabatic fluctuations either are present initially as fluctuations in the curvature or can be produced at early times provided that some mechanism allows sub-horizonsized scales to become super-horizon-sized.
