Abstract
This lecture describes the acute problem of structure formation in an expanding universe. Gravitational collapse is very slow, yet initial density fluctuations are extremely small. The density contrast develops as the expansion factor, in the comoving frame of reference. In the absence of exotic dark matter, collapse can only begin when the temperature of the universe drops below 3000K, and the plasma recombines into hydrogen atoms, so that the baryons are decoupled from the photons, whose pressure prevents collapse. Then density contrasts will never approach 1, and the universe should not be structured into galaxies and clusters today. Dark matter is really the key to galaxy formation, and must be made up of particles that decouple very early, in the epoch of equivalence between matter and radiation, as these particles do not interact with photons. Primordial fluctuations would be due to inflation, which is also a necessary theory for solving the universe's flatness problem, and the horizon problem: how is it that cosmological radiation at 3K is homogeneous and isotropic to within 10-5 throughout the universe observable today, when it contains a large number of horizons at the epoch of emission, i.e. non-causally connected regions?
