Cosmic timeline 10

Matter domination
Matter domination began about 70,000 years after the Big Bang.

In the photon epoch which began 10 seconds after the Big Bang photons are still interacting frequently with charged protons, electrons and (eventually) nuclei, and continue to do so for the next 300,000 years. Then between 3 minutes and 20 minutes after the Big Bang the temperature of the universe falls to the point where atomic nuclei can begin to form. Protons (hydrogen ions) and neutrons begin to combine into atomic nuclei in the process of nuclear fusion. Nucleosynthesis only lasts for about seventeen minutes, after which time the temperature and density of the universe has fallen to the point where nuclear fusion cannot continue. At this time, there is about three times more hydrogen than helium-4 (by mass) and only trace quantities of other nuclei.

Matter domination and dark matter
Matter domination began about 70,000 years after the Big Bang. At this time, the densities of non-relativistic matter (atomic nuclei) and relativistic radiation (photons) are equal. At this stage, cold dark matter dominates, paving the way for gravitational collapse making dense regions denser and rarefied regions more rarefied. However, present theories as to the nature of dark matter are inconclusive, but the thinking is that it needs to exist in order for the Big Bang theory to work.

Strong gravitational lensing as observed by the Hubble Space Telescope in Abell 1689 indicates the presence of dark matter
.

What is dark matter?
Most of the mass of the observable universe probably! In astronomy and cosmology, dark matter is a hypothetical form of matter that is undetectable, but whose presence can be worked out from looking at gravitational effects on visible matter. According to present observations of parts of the universe larger than galaxies, as well as Big Bang cosmology, dark matter and dark energy could account for the vast majority of the mass in the observable universe.

Dark matter plays a central role in state-of-the-art modeling of structure formation and galaxy evolution, and has measurable effects on the differences that exist in all directions of our universe (called anisotropies) observed in the cosmic microwave background. All these lines of evidence suggest that galaxies, clusters of galaxies, and the universe as a whole contain far more matter than that which interacts with electromagnetic radiation: the remainder is frequently called the “dark matter component,” even though there is a small amount of baryonic dark matter. The largest part of dark matter, which does not interact with electromagnetic radiation, is not only “dark” but also, by definition, utterly transparent.

The vast majority of the dark matter in the universe is believed to be nonbaryonic, which means that it contains no atoms and that it does not interact with ordinary matter via electromagnetic forces. The nonbaryonic dark matter includes neutrinos, and possibly hypothetical entities such as axions, or supersymmetric particles.

The first known observation of a neutrino, on November 13, 1970. A neutrino hit a proton in a hydrogen bubble chamber. The collision occurred at the point where three tracks emanate on the right of the photograph.

Neutrinos
Neutrinos, meaning “small neutral one”, are elementary particles that often travel close to the speed of light, are electrically neutral, and are able to pass through ordinary matter almost undisturbed and are thus extremely difficult to detect. Neutrinos have a minuscule, but nonzero mass.

Experiments at neutrino detectors like SNO and Super-Kamiokande (pictured here) have established that neutrinos oscillate among various flavors, each with a different tiny mass.

The Kamioka Observatory, Institute for Cosmic Ray Research is a neutrino physics laboratory located underground in the Mozumi Mine of the Kamioka Mining and Smelting Co. near the Kamioka section of the city of Hida in Gifu Prefecture, Japan.

A set of groundbreaking neutrino experiments have taken place at the observatory over the past two decades. All of the experiments have been very large and have contributed substantially to the advancement of particle physics, in particular to the study of neutrino astronomy and neutrino oscillation.

Neutrino astronomy
Neutrino astronomy is still very much in its infancy: so far, the only confirmed extraterrestrial neutrino sources are the Sun and supernova SN1987A.

Recombination
This occurred about 377,000 years after the big bang. Hydrogen and helium atoms begin to form and the density of the universe falls. Hydrogen and helium are at the beginning ionized, i. e., no electrons are bounded to the nuclei, which are therefore electrically charged (+1 and +2 respectively). As the universe cools down, the electrons get captured by the ions, making them neutral. This process is relatively fast (actually faster for the helium than for the hydrogen) and is known as recombination. At the end of recombination, most of the atoms in the universe are neutral, therefore the photons can now travel freely.


The universe has become transparent.
The photons emitted right after the recombination can now travel undisturbed and are those that we see in the cosmic microwave background (CMB) radiation. Therefore the CMB is a picture of the universe at the end of this epoch.

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