Editing Physical cosmology (section)

==History of the universe==
{{see also|Timeline of the Big Bang}}
The history of the universe is a central issue in cosmology. The history of the universe is divided into different periods called epochs, according to the dominant forces and processes in each period. The standard cosmological model is known as the [[Lambda-CDM model]].

===Equations of motion===
{{main|Friedmann–Lemaître–Robertson–Walker metric}}

Within the [[standard cosmological model]], the [[equations of motion]] governing the universe as a whole are derived from general relativity with a small, positive cosmological constant.<ref>
{{cite journal |author=Ojeda |first1=P. |last2=Rosu |first2=H. |date=June 2006 |title=Supersymmetry of FRW barotropic cosmologies |journal=Int. J. Theor. Phys. |volume=45 |issue=6 |pages=1191–1196 |arxiv=gr-qc/0510004 |bibcode=2006IJTP...45.1152R |doi=10.1007/s10773-006-9123-2 |s2cid=119496918}}
</ref> The solution is an expanding universe; due to this expansion, the radiation and matter in the universe cool and become diluted. At first, the expansion is slowed down by [[gravitation]] attracting the radiation and matter in the universe. However, as these become diluted, the cosmological constant becomes more dominant and the expansion of the universe starts to accelerate rather than decelerate. In our universe this happened billions of years ago.<ref name=Springel2006>{{cite journal | title=The large-scale structure of the Universe | last1=Springel | first1=Volker | last2=Frenk | first2=Carlos S. | last3=White | first3=Simon D.M. | journal=Nature | volume=440 | issue=7088 | pages=1137–1144 | year=2006 | doi=10.1038/nature04805 | pmid=16641985 | bibcode=2006Natur.440.1137S | arxiv=astro-ph/0604561 | citeseerx=10.1.1.255.8877 | s2cid=8900982 }}</ref>

===Particle physics in cosmology===
{{main|Particle physics in cosmology}}

During the earliest moments of the universe, the average energy density was very high, making knowledge of [[particle physics]] critical to understanding this environment. Hence, [[scattering]] processes and [[particle decay|decay]] of unstable [[elementary particles]] are important for cosmological models of this period.

As a rule of thumb, a scattering or a decay process is cosmologically important in a certain epoch if the time scale describing that process is smaller than, or comparable to, the time scale of the expansion of the universe.{{clarify |date=April 2018 |reason=Isn't the opposite true – departures from equilibrium occur only when the process takes longer than the time scale? }} The time scale that describes the expansion of the universe is <math>1/H</math> with <math>H</math> being the [[Hubble parameter]], which varies with time. The expansion timescale <math>1/H</math> is roughly equal to the age of the universe at each point in time.

===Timeline of the Big Bang===
{{main|Timeline of the Big Bang}}
Observations suggest that the universe began around 13.8 billion years ago.<ref>{{cite web
|title = Cosmic Detectives
|url=http://www.esa.int/Our_Activities/Space_Science/Cosmic_detectives
|publisher = The European Space Agency (ESA)
|date = 2 April 2013
|access-date = 2013-04-25}}
</ref> Since then, the evolution of the universe has passed through three phases. The very early universe, which is still poorly understood, was the split second in which the universe was so hot that [[subatomic particle|particles]] had energies higher than those currently accessible in [[particle accelerator]]s on Earth. Therefore, while the basic features of this epoch have been worked out in the Big Bang theory, the details are largely based on educated guesses.

Following this, in the early universe, the evolution of the universe proceeded according to known [[high energy physics]]. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the [[cosmic microwave background]] was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first [[star]]s and [[quasar]]s, and ultimately galaxies, [[clusters of galaxies]] and [[supercluster]]s formed. The future of the universe is not yet firmly known, but according to the [[ΛCDM]] model it will continue expanding forever.