Structure formation

Structure formation is the study of how the universe created its complex shapes. This includes everything from single stars to massive galaxy clusters. It also includes the giant, empty spaces called voids that separate these structures. This process relies on gravity and the movement of gases. Scientists use models of gravitational instability to explain how small ripples in mass density grew into these shapes.

The process began approximately 13.8 billion years ago. At that time, the universe was in a hot, dense, and nearly uniform state. A mechanism called cosmic inflation is thought to have established the initial conditions. Inflation helped create a universe that is homogeneous, isotropic, and flat. It also amplified tiny quantum fluctuations into ripples of overdensity and underdensity. These ripples served as the essential seeds for all future structures.

As the universe expanded, it underwent several distinct stages. In the very early universe, radiation was the dominant force. During this time, density fluctuations larger than the cosmic horizon grew proportional to the scale factor. However, smaller structures remained frozen because radiation impeded their growth. About 50,000 years after the Big Bang, the universe reached matter-radiation equality. At this crossover point, the density of radiation dropped faster than the density of matter. This allowed dark matter ripples to grow freely and create seeds for baryonic matter to follow.

Another critical stage is known as recombination. This occurred less than 400,000 years after the beginning of the universe. Before this, intense heat kept hydrogen and helium fully ionized. This meant electrons were not bound to nuclei, and photons could not travel far without scattering. As the universe cooled to about 3000 K, protons captured electrons to form neutral hydrogen atoms. Once these neutral atoms formed, photons were free to travel through space. We can still detect these ancient photons today as the Cosmic Microwave Background radiation (CMB).

Dark matter is the primary driver of how these structures grow. Unlike baryonic matter, dark matter does not interact with light or radiation pressure. It only feels the force of gravity. This allows dark matter to collapse into a complex network of dark matter halos very early on. These halos then act as gravitational wells that attract regular baryonic matter, such as hydrogen. Without the influence of dark matter, the epoch of galaxy formation would have happened much later than what we observe.

As matter continues to clump, the physics moves from linear to nonlinear stages. In the linear stage, small deviations from a uniform density are easy to calculate. But as regions become much denser than the average, the physics becomes highly complicated. Dark matter begins to form caustics where particle trajectories cross. To understand these complex movements, scientists use N-body simulations. These computer models simulate millions or billions of particles to recreate the cosmic web. One famous example is the Millennium simulation. These simulations show that the universe is composed of large filaments and halos surrounding massive voids.

The final stage involves the evolution of gas within these dark matter halos. Baryonic matter condenses in the centers of these halos to form galaxies, stars, and quasars. This is a difficult process to model because it involves many different types of physics. It includes gravity, magnetohydrodynamics, atomic physics, and even nuclear reactions. While dark matter forms large structures, it cannot form small, dense objects because it cannot dissipate angular momentum. Only baryonic matter can do this through radiative cooling. This cooling allows gas to become dense enough to ignite stars.

Today, researchers use several methods to study these ancient processes. Space-based missions like COBE, WMAP, and Planck have measured tiny temperature variations in the CMB. These variations are only a few parts in 100,000, but they trace the original density seeds. Scientists also use large redshift surveys, such as the Sloan Digital Sky Survey, to map the distribution of galaxies. These surveys show that galaxies are more numerous in dense regions and scarce in voids. This confirms that the large-scale distribution of galaxies closely mirrors the underlying distribution of dark matter.