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Birth of the universe

The Big Bang: 13.7 billion to 13.4 billion years ago:

The instant in which the universe is thought to have begun rapidly expanding from an extremely high energy density is known as the Big Bang.

The Big Bang describes the history of the universe according to the prevailing scientific theory of how the universe came into being, using the cosmological time parameter of comoving coordinates.

The best available measurements as of 2011 suggest that the initial conditions occurred about 13.7 billion years ago. The evolution of the universe is divided into three phases.

First phase: The very early universe was so hot that particles had energies higher than those currently accessible in particle accelerators on Earth.

Second phase: The formation of the first protons, electrons and neutrons then nuclei and finally atoms.

Third phase: The formation of neutral hydrogen and the cosmic microwave background. Matter then continued to aggregate into the first stars and ultimately galaxies, quasars, clusters of galaxies and superclusters.

The evolution of the universe has proceeded in accordance with the tenets of high-energy physics.

Cosmology is the branch of astronomy that deals with the origin, structure, and space-time relationships of the universe.


Recombination: 13.4 billion years ago:

Recombination refers to the epoch at which charged electrons and protons in the universe first formed electrically neutral hydrogen atoms. After the Big Bang, the universe was a hot, dense plasma of photons, electrons, and protons. The interaction of photons with the plasma made the universe effectively opaque to radiation. As the universe expanded, it also cooled. Eventually, the universe cooled to the point that the formation of neutral hydrogen was energetically favored, and the fraction of free electrons and protons as compared to neutral hydrogen decreased to about 1 part in 10,000.

Shortly after, photons decoupled from matter in the universe, which leads to recombination sometimes being called photon decoupling, although recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter, and constitute what we observe today as cosmic microwave background radiation. Recombination occurred when the universe was roughly 380,000 years old, or at a redshift of z = 1,100.

This time is also called the Dark Ages because there are no light sources yet other than the gradually darkening cosmic background radiation.


Reionization: 13.5 billion to 12.5 billion years ago:
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.

The universe was opaque before recombination because photons scatter off free electrons (and, to a significantly lesser extent, free protons), but it became transparent as more and more electrons and protons combined to form hydrogen atoms. While electrons in neutral hydrogen (or other atoms or molecules) can absorb photons of some wavelengths by going to an excited state, a universe full of neutral hydrogen will be relatively opaque only at those wavelengths, and transparent over most of the spectrum.

The first stars and quasars form and light up from gravitational collapse. The intense radiation they emit reionizes the surrounding universe. From this point on, most of the universe is composed of plasma.

Plasma is a state of matter similar to gas in which a certain portion of the particles are ionized. Lightning is an example of plasma present at Earth’s surface.

In this period, matter has been diluted by the expansion of the universe, and scattering interactions are much less frequent than before recombination. Thus a universe full of low density ionized hydrogen is transparent, as is the case today.

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