Cosmic timeline 12

Reionization: 150 million to 1 billion years after the big bang and the beginning of the Stelliferous Era
The first quasars form from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.
So the stelliferous era begins in the so called dark ages and is a term that refers to the formation of stars in the universe. The first stars, most likely Population III stars, form and start the process of turning the light elements that were formed in the Big Bang (hydrogen, helium and lithium) into heavier elements. However, as of yet there have been no observed Population III stars, which leaves their formation a mystery.

How did stars and galaxies begin?
This brings us back to what cosmologists call structure formation. This refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 billion years ago. However, looking in the sky today, we see structures on all scales, from stars and planets to galaxies and, on much larger scales still, galaxy clusters, and enormous voids between galaxies. How did all of this come about from the nearly uniform early universe?

Dark matter and structure formation
One of the key realizations made by cosmologists in the 1970s and 1980s was that the majority of the matter content of the universe was composed not of atoms, but rather a mysterious form of matter known as dark matter. Dark matter interacts through the force of gravity, but it is not composed of baryons and it is known with very high accuracy that it does not emit or absorb radiation. It may be composed of particles that interact through the weak interaction, such as neutrinos, but it cannot be composed entirely of the three known kinds of neutrinos (although some have suggested it is a sterile neutrino). Recent evidence suggests that there is about five times as much dark matter as baryonic matter, and thus the dynamics of the universe in this epoch, where the formation of the structure of the universe is concerned, are dominated by dark matter. Dark matter plays a key role in structure formation because it feels only the force of gravity. Dark matter begins to collapse into a complex network of dark matter halos well before ordinary matter, which is impeded by pressure forces. Without dark matter, the epoch of galaxy formation would occur substantially later in the universe than is observed.

The dark matter halo is the hypothetical gravitational core of a galaxy, consisting of dark matter. This image shows a simulated dark matter halo from a cosmological N-body simulation.

An N-body simulation is a simulation of a dynamical system of particles, usually under the influence of physical forces, such as gravity. In cosmology, they are used to study processes of non-linear structure formation such as the process of forming galaxy filaments and galaxy halos from dark matter in physical cosmology. Direct N-body simulations are used to study the dynamical evolution of star clusters.

Newton’s law of gravity
The n-body problem is the problem of predicting the motion of a group of celestial objects that interact with each other gravitationally. Solving this problem has been motivated by the need to understand the motion of the sun, planets and the visible stars. Its first complete mathematical formulation appeared in Isaac Newton’s Principia .

Reionization
In Big Bang cosmology, reionization is the process that reionized the matter in the universe after the “dark ages,” and is the second of two major phase changes of gas in the universe. As the majority of baryonic matter is in the form of hydrogen, reionization usually refers to the reionization of hydrogen gas. The primordial helium in the universe experienced the same phase changes, but at different points in the history of the universe, and is usually referred to as Helium reionization.

The first stars begin to shine
Population III or metal-free stars are a what big bang cosmologists suppose were the first population of stars believed to have been formed in the early universe. They were extremely massive and hot stars with virtually no surface metals, except for a small quantity of metals formed in the Big Bang, such as Lithium-7. They have not yet been observed directly, but indirect evidence for their existence has been found in a gravitationally lensed galaxy in the very distant universe.

Their existence is proposed to account for the fact that heavy elements, which could not have been created in the Big Bang, are observed in quasar emission spectra, as well as the existence of faint blue galaxies. It is believed that these stars triggered a period of reionization.

Current theory is divided on whether the first stars were very massive or not. One theory, which seems to be borne out by computer models of star formation, is that with no heavy elements from the Big Bang, it was easy to form stars with much more total mass than the ones visible today. Typical masses for Population III stars would be expected to be about several hundred solar masses, which is much larger than the current stars.

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