Editing Big Bang (section)

== Features of the models ==
=== Assumptions ===
Big Bang cosmology models depend on three major assumptions: the universality of physical laws, the [[cosmological principle]], and that the matter content can be modeled as a [[perfect fluid]].<ref name="PDG-2024">{{Cite journal |last=Navas, S. |year=2024 |title=Review of Particle Physics |journal=[[Physical Review D]]  |volume=110 |issue=3 |pages=1–708 |doi=10.1103/PhysRevD.110.030001 |collaboration=[[Particle Data Group]]|hdl=20.500.11850/695340 |hdl-access=free }} 22.1 Introduction to the standard big-bang model</ref>  The universality of physical laws is one of the underlying principles of the [[theory of relativity]]. The cosmological principle states that on large scales the [[universe]] is [[Homogeneous space|homogeneous]] and [[isotropy|isotropic]]—appearing the same in all directions regardless of location.<ref name=Francis2018>{{cite book
 | title=Light after Dark I: Structures of the Sky
 | first=Charles | last=Francis
 | publisher=Troubador Publishing Ltd
 | date=2018 | isbn=9781785897122 | page=199
 | url=https://books.google.com/books?id=TVhiDAAAQBAJ&pg=PA199 }}</ref> A perfect fluid has no viscosity; the pressure of a perfect fluid is proportional to its density.<ref name=KolbTurner2018>{{Cite book |last=Kolb |first=Edward |title=The Early Universe |last2=Turner |first2=Michael S. |date=2018 |publisher=Chapman and Hall/CRC |isbn=978-0-201-62674-2 |location=Boulder}}</ref>{{rp|49}}

These ideas were initially taken as postulates, but later efforts were made to test each of them. For example, the first assumption has been tested by observations showing that the largest possible deviation of the [[fine-structure constant]] over much of the age of the universe is of order 10<sup>−5</sup>.<ref>{{cite journal |last1=Ivanchik |first1=Alexandre V. |last2=Potekhin |first2=Alexander Y. |last3=Varshalovich |first3=Dmitry A. |date=March 1999 |title=The fine-structure constant: a new observational limit on its cosmological variation and some theoretical consequences |journal=[[Astronomy & Astrophysics]] |volume=343 |issue=2 |pages=439–445 |arxiv=astro-ph/9810166 |bibcode=1999A&A...343..439I}}</ref> The key physical law behind these models, [[general relativity]] has passed stringent [[tests of general relativity|tests]] on the scale of the [[Solar System]] and [[binary star]]s.<ref>{{cite journal
 | title=Experimental Tests of General Relativity
 | last=Turyshev | first=Slava G.
 | journal=Annual Review of Nuclear and Particle Science
 | volume=58 | issue=1 | pages=207–248 | date=November 2008
 | arxiv=0806.1731 | bibcode=2008ARNPS..58..207T
 | doi=10.1146/annurev.nucl.58.020807.111839
| s2cid=119199160 }}</ref><ref>{{cite journal
 | title=Testing general relativity in cosmology
 | last=Ishak | first=Mustapha
 | journal=Living Reviews in Relativity
 | volume=22 | issue=1 | id=1 | pages=204 | date=December 2019
 | arxiv=1806.10122 | bibcode=2019LRR....22....1I
 | doi=10.1007/s41114-018-0017-4
| pmid=30613193 | pmc=6299071 }}</ref>
The cosmological principle has been confirmed to a level of 10<sup>−5</sup> via observations of the temperature of the CMB. At the scale of the CMB horizon, the universe has been measured to be homogeneous with an upper bound [[on the order of]] 10% inhomogeneity, as of 1995.<ref>{{cite journal |last=Goodman |first=Jeremy |date=15 August 1995 |title=Geocentrism reexamined |url=https://cds.cern.ch/record/283096/files/9506068.pdf |url-status=live |journal=[[Physical Review D]] |volume=52 |issue=4 |pages=1821–1827 |arxiv=astro-ph/9506068 |bibcode=1995PhRvD..52.1821G |doi=10.1103/PhysRevD.52.1821 |pmid=10019408 |s2cid=37979862 |archive-url=https://web.archive.org/web/20190502001358/https://cds.cern.ch/record/283096/files/9506068.pdf |archive-date=2 May 2019 |access-date=2 December 2019}}</ref>

=== Expansion prediction ===
{{main | Expansion of the universe}}
The cosmological principle dramatically simplifies the equations of general relativity, giving the [[Friedmann–Lemaître–Robertson–Walker metric]] to describe the geometry of the universe and, with the assumption of a perfect fluid, the [[Friedmann equations]] giving the time dependence of that geometry. The only parameter at this level of description is the mass-energy density: the [[shape of the universe|geometry of the universe]] and its [[expansion of the universe|expansion]] is a direct consequence of its density.<ref name=Peacock-1998/>{{rp|p=73}} All of the major features of Big Bang cosmology are related to these results.<ref name=KolbTurner2018/>{{rp|49}}

=== Mass-energy density ===
[[File:UniverseComposition.svg|thumb|right|375px|Estimated relative distribution for components of the energy density of the universe. (In February 2015, the European-led research team behind the [[Planck (spacecraft)|Planck cosmology probe]] released new data refining these values to 4.9% ordinary matter, 25.9% dark matter and 69.1% dark energy.)]]
In Big Bang cosmology, the [[mass–energy equivalence|mass–energy]] density controls the shape and evolution of the universe. By combining astronomical observations with known laws of [[thermodynamics]] and [[particle physics]], cosmologists have worked out the components of the density over the lifespan of the universe. In the current universe, luminous [[matter]], the stars, planets, and so on makes up less than 5% of the density. [[Dark matter]] accounts for 27%  and [[dark energy]] the remaining 68%.<ref name="NASA Planck Mission">{{cite web |url=http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |title=Planck Mission Brings Universe into Sharp Focus |website=NASA Mission Pages |date=21 March 2013 |access-date=1 May 2016 |archive-date=12 November 2020 |archive-url=https://web.archive.org/web/20201112001039/http://www.nasa.gov/mission_pages/planck/news/planck20130321.html |url-status=dead }}</ref>
===Horizons===
{{Main|Cosmological horizon}}
An important feature of the Big Bang spacetime is the presence of [[particle horizon]]s. Since the universe has a finite age, and [[light]] travels at a finite speed, there may be events in the past whose light has not yet had time to reach earth. This places a limit or a ''past horizon'' on the most distant objects that can be observed. Conversely, because space is expanding, and more distant objects are receding ever more quickly, light emitted by us today may never "catch up" to very distant objects. This defines a ''future horizon'', which limits the events in the future that we will be able to influence. The presence of either type of horizon depends on the details of the [[Friedmann–Lemaître–Robertson–Walker metric|Friedmann–Lemaître–Robertson–Walker (FLRW) metric]] that describes the expansion of the universe.<ref name="kolb_c3"/>

Our understanding of the universe back to very early times suggests that there is a past horizon, though in practice our view is also limited by the opacity of the universe at early times. So our view cannot extend further backward in time, though the horizon recedes in space. If the expansion of the universe continues to accelerate, there is a future horizon as well.<ref name="kolb_c3">{{harvnb|Kolb|Turner|1988|loc=chpt. 3}}</ref>

===Thermalization===
Some processes in the early universe occurred too slowly, compared to the expansion rate of the universe, to reach approximate [[thermodynamic equilibrium]]. Others were fast enough to reach [[thermalization]]. The parameter usually used to find out whether a process in the very early universe has reached thermal equilibrium is the ratio between the rate of the process (usually rate of collisions between particles) and the [[Hubble parameter]]. The larger the ratio, the more time particles had to thermalize before they were too far away from each other.<ref>{{cite journal | last1=Enqvist | first1=K. | last2=Sirkka | first2=J. | date=September 1993 | title=Chemical equilibrium in QCD gas in the early universe | journal=Physics Letters B | volume=314 | issue=3–4 | pages=298–302 | doi=10.1016/0370-2693(93)91239-J | arxiv=hep-ph/9304273 | bibcode=1993PhLB..314..298E  | s2cid=119406262 }}</ref>