Structure Formation - Overview

Overview

Under present models, the structure of the visible universe was formed in the following stages:

• The very early universe In this stage, some mechanism, such as cosmic inflation is responsible for establishing the initial conditions of the universe: homogeneity, isotropy and flatness. Cosmic inflation also would have amplified minute quantum fluctuations pre-inflation into slight overdensities post-inflation. These acted as seeds around which the dark matter could begin to gravitationally congregate, even as the normal, baryonic matter was still in thermal equilibrium - way too hot to allow gravity to get any purchase on it.
• The primordial plasma The universe is dominated by radiation for most of this stage, and due to the intense heat and radiation, the helium nuclei fused in the first few minutes, along with the remaining hydrogen nuclei (essentially protons), cannot capture and hold onto an electron before the radiation blasts it away. The Universe is very hot, dense, but expanding rapidly, and therefore cooling. In this hot, dense situation, the radiation (photons) cannot travel far before interacting with one of these charged particles. Finally at about 400,000 years after the 'bang', it's cool enough for the nuclei to capture their electrons, forming neutral-charge atoms. As the charged particles pair up, the photons no longer interact with them, they are free to propagate, and we currently detect those photons as the Cosmic Microwave Background Radiation (CMB), because they fill the Universe. After remarkable space-based missions, we have detected very slight variations in the density or temperature in the CMB, otherwise it is nearly the same in every direction. These variations were essentially early "seeds" upon which subsequent structure formed.

From there, the theory is one of hierarchical structure formation: the smaller gravitationally bound structures form first – large early stars collapse to form black holes, which may join other black holes to form quasars, active galactic nuclei, and galaxies, followed by groups, clusters and superclusters of galaxies.