Editing Big Bang (section)

=== 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>