If the cosmological principle held perfectly and the distribution of matter in the Universe were perfectly uniform and isotropic, there would be no structure formation. In order to explain the presence of structure, in particular galaxies, we clearly need some deviations from perfect uniformity. Unfortunately, the standard cosmology does not in itself provide us with an explanation for the origin of these perturbations. We have to go beyond it to search for an answer.
A classical, general relativistic description of cosmology is expected to break down at very early times when the Universe is so dense that quantum effects are expected to be important..., the standard cosmology has a number of conceptual problems when applied to the early Universe, and the solutions to these problems require an extension of the standard cosmology to incorporate quantum processes. One generic consequence of such an extension is the generation of density perturbations by quantum fluctuations at early times. It is believed that these perturbations are responsible for the formation of the structures observed in today’s Universe.
one particularly successful extension of the standard cosmology is the inflationary theory, in which the Universe is assumed to have gone through a phase of rapid, exponential expansion (called inflation) driven by the vacuum energy of one or more quantum fields. In many, but not all, inflationary models, quantum fluctuations in this vacuum energy can produce density perturbations with properties consistent with the observed large-scale structure. Inflation thus offers a promising explanation for the physical origin of the initial perturbations. Unfortunately, our understanding of the very early Universe is still far from complete, and we are currently unable to predict the initial conditions for structure formation entirely from first principles. Consequently, even this part of galaxy formation theory is still partly phenomenological: typically initial conditions are specified by a set of parameters that are constrained by observational data, such as the pattern of fluctuations in the microwave background or the present-day abundance of galaxy clusters.
