Big Bang

The **Big Bang** theory is a scientific theory that models our universe based on the assumptions that the universe's laws of physics apply everywhere and that the universe is homogenous and isotropic which describes the universe in its earliest known stages after its initial state about 13.799 billion years ago. During the universe's beginning, it was extremely hot, curved, and dense but as the universe's space expanded, the universe cooled down. More generally, "Big Bang" is often used to refer to the formation and rapid expansion of a universe from its initial state or the initial state itself.

The Big Bang of our universe is commonly described as the entire universe exploding from a single, infinitesimally small point called a "singularity" from which all matter and energy originated. While it is true that the observable universe was essentially infinitesimal in size during the universe's beginning, it's possible that the entire universe (likely an infinitely large flat universe) was not and the universe's space was always infinitely large during the Big Bang (just much, much denser. This also holds for if our universe was a hyperbolic realmic open universe). For finite hypersphere-shaped closed universes, the entire universe in its initial state is a point from which the universe expanded from. Though the Big Bang is commonly interpreted as an "explosion", this is not actually the case and rather than exploding, the universe just expands extremely rapidly during its Big Bang state.

## Timeline of our universe's beginning

### Superunified epoch (Planck epoch)

The **Planck epoch** of the universe occurred from the Big Bang to about one Planck time (5.391247 × 10-44) after the Big Bang. During the Planck epoch, the universe was a particle desert and the background radiation temperature is thought to have been over the Planck temperature (1.416784 × 1032 K). All four fundamental forces were unified during this epoch. Theories such as general relativity and quantum mechanics cannot be used to predict the properties of the universe during this epoch and as such, not very much overall is known about this epoch. Effects of quantum gravity are assumed to have been dominant during the Planck epoch.

### Electronuclear epoch (Grand unification epoch)

The **Grand unification epoch** of the universe was the epoch that directly followed the Planck epoch. Gravity split off as its own force from the single force the Planck epoch possessed, leaving the three forces of the Standard Model unified as the electronuclear force. Characteristics of particles such as mass, charge, flavour, and colour charge were meaningless during this epoch. The epoch ended around 10-36 s (1 tenamicrosecond) after the Big Bang.

### Electroweak epoch

The **electroweak epoch** was an epoch that followed the Grand unification epoch. It is characterized by the strong force splitting off from the electronuclear force. Large numbers of exotic particles such as Higgs bosons, W bosons, and Z bosons were produced throughout the universe from energetic particle interactions. The epoch ended around 10-12 s (1 picosecond) after the Big Bang.

#### Inflationary epoch

During the **inflationary epoch**, the universe underwent an extremely rapid exponential cosmic expansion that scaled the observable universe's radius by a factor of at least but possibly much larger than a hundred septillion (1026) and its volume by at least a quinvigintillion (1078). Expansion by such a factor is equivalent to scaling a one nanometer long object to an object about 10.6 light-years long. The universe was supercooled during this rapid expansion from 1027 K to 1022 K. The inflationary epoch started alongside the electroweak epoch around 10-36 s after the Big Bang but ended much earlier at around 10-32 s after the Big Bang.

### Quark epoch

The **quark epoch** was the epoch that directly followed the electroweak epoch. It is characterized by the weak force splitting off from the electroweak force, leaving the universe with the four fundamental forces as they are in the present. During the quark epoch, the universe was entirely a hot, dense quark-gluon plasma containing quarks, leptons, and their antiparticles. Baryogenesis, the process that allowed for the universe to have a higher concentration of matter than anti-matter, likely occurred during this era. The epoch ended around 10-5 s (ten microseconds) after the Big Bang.

### Hadron epoch

The **hadron epoch** was the epoch that directly followed the quark epoch. During this era, the quark-gluon plasma cools down enough for hadrons and anti-hadrons such as protons and neutrons to form. As the universe's temperature cooled, hadrons and anti-hadrons could no longer be produced and the particles eventually annihilated each other. As there was in general more matter than anti-matter in any given finite volume of the universe, virtually all of the anti-hadrons were destroyed and hadrons were left over. This epoch ended about one second after the Big Bang.

### Lepton epoch

The **lepton epoch** was the epoch that directly followed the hadron epoch. During this era, the mass of the universe was predominantly leptons and anti-leptons such as electrons and positrons leftover from hadrons and anti-hadrons annihilating each other. This epoch ended about ten seconds after the Big Bang when leptons and their antiparticles began to annihilate each other.

### Photon epoch

The **photon epoch** was the epoch that directly followed the lepton epoch. During this epoch, most of the universe's mass-energy was in the form of photons. This epoch ended about 370,000 years after the Big Bang and was succeeded by the cosmic "Dark Age".

#### Big Bang nucleosynthesis

Between 10 seconds and 17 minutes from the Big Bang, atomic nuclei were able to form very quickly from the protons, neutrons, and electrons leftover from the hadron and lepton epochs. The most commonly formed nuclei were of protium and helium-4. Trace amounts of stable nuclei deuterium, helium-3, and lithium-7 also formed during this time period. Tritium, beryllium-7, and beryllium-8 may have also been formed but were unstable and could not be synthesized again until the formation of stars. By mass, the resulting matter within the observable universe was about 75% hydrogen nuclei and about 25% helium nuclei.

#### Matter-dominated era

47,000 years into the photon epoch, the universe would enter the "matter-dominated era" it is now presently in. The universe's mass energy became about 31.7% matter (of which 84.5% was dark matter and 15.5% baryonic matter), about the value it is in the present.

100,000 years after the Big Bang, the first molecule (helium hydride) would form.

## References

* https://en.wikipedia.org/wiki/Big\_Bang
* https://en.wikipedia.org/wiki/Chronology\_of\_the\_universe