The Formation of Galaxies and Large Structures in the Universe

Abstract

The formation of galaxies and large structures in the universe remains an unresolved problem. The starting point for current scenarios based on the idea of ‘gravitational instability’ is, however, simple. In a medium of uniform density a local density excess (overdensity) of matter will attract nearby matter by the effect of its own gravitation and this effect will accelerate. This type of model, which takes up the ideas of Jeans on the evolution of inhomogeneities of a static gravitational medium, predicts a very rapid (exponential) increase of such irregularities. In fact the application of it to a homogeneous, expanding universe of density ρ with some primordial irregularities ρ _pett = ρ + δρ (where δρ/ρ« 1) shows that the increase is not as rapid. There is in effect competition between the growth of the perturbations, characterized by a gravitational collapse time (t _eff ∝ (Gρ_penrt)^−1/2), and the expansion (t _exp ∝ (Gα) ^−1/2), which tends to dilute all local overdensities. The result is that the growth of fluctuations in an expanding universe is slow. For baryons this increase only begins at recombination (z _rec ≈ 1000) and in the linear phase the density contrast only grows by a factor (1 + z _rec) ≈ 10³. Now to obtain an excess δ = δρ/ρ = (ρ — ρ)/ρ 1 corresponding to existing objects (galaxies, clusters, and so on), an initial inhomogeneity δ_i of the order of 10⁻³ must be assumed. However, there is at least one observational constraint on the initial amplitude δ_i. It is given by observation of the 2.7 K cosmic background radiation. This radiation is homogeneous to better than 10⁻⁵ (δT/T ≤ 10⁻⁵), and as density fluctuations lead to temperature fluctuations (δT/T ≈ δρ/ρ), it follows that at the epoch of recombination the matter must have the same degree of inhomogeneity. This is the main difficulty that confronts models of galaxy formation. The appearance of grand unification theories (GUTs) and inflationary models (Chapter 13) partly resolves these problems by predicting in particular the existence of nonbaryonic particles (‘-inos’) of various masses. On the one hand there is the hidden contribution to the density parameter Ω ₀ (predicted equal to 1 by inflationary theories), and on the other there is the source of fluctuations leading to large structures in the real universe. The growth of these fluctuations can begin before recombination; it is thus possible to postulate a δ_i compatible with observations of the cosmic microwave background. However, even in this setting, many difficulties still persist, but we shall nonetheless try to give a review of this rapidly evolving field.