Editing Cosmic inflation (section)

===Horizon problem===
{{Main |Horizon problem}}
The [[horizon problem]] is the problem of determining why the universe appears statistically homogeneous and isotropic in accordance with the [[cosmological principle]].<ref>{{cite journal |title=The isotropy of the universe |doi=10.1088/0264-9381/15/2/008 |first1=Charles W. |last1=Misner |date=1968 |journal=[[Astrophysical Journal]] |volume=151 |issue=2 |pages=431 |last2=Coley |first2=A A |last3=Ellis |first3=G F R |last4=Hancock |first4=M |bibcode=1998CQGra..15..331W |s2cid=250853141 }}</ref><ref name="mtw">{{Cite book |last=Misner |first=Charles |author2=Thorne, Kip S. |author3=Wheeler, John Archibald |name-list-style=amp |title=Gravitation |url=https://archive.org/details/gravitation00whee |url-access=limited |location=San Francisco |publisher=W. H. Freeman |date=1973 |isbn=978-0-7167-0344-0 |pages=[https://archive.org/details/gravitation00whee/page/n521 489]–490, 525–526}}</ref><ref name="weinberg">{{Cite book |first=Steven |last=Weinberg |title=Gravitation and Cosmology |publisher=John Wiley |date=1971 |isbn=978-0-471-92567-5 |pages=[https://archive.org/details/gravitationcosmo00stev_0/page/740 740, 815] |url=https://archive.org/details/gravitationcosmo00stev_0/page/740 }}</ref> For example, molecules in a canister of gas are distributed homogeneously and isotropically because they are in thermal equilibrium: gas throughout the canister has had enough time to interact to dissipate inhomogeneities and anisotropies. The situation is quite different in the big bang model without inflation, because gravitational expansion does not give the early universe enough time to equilibrate. In a big bang with only the [[matter]] and [[radiation]] known in the Standard Model, two widely separated regions of the observable universe cannot have equilibrated because they move apart from each other faster than the [[speed of light]] and thus have never come into [[causal contact]]. In the early Universe, it was not possible to send a light signal between the two regions. Because they have had no interaction, it is difficult to explain why they have the same temperature (are thermally equilibrated). Historically, proposed solutions included the ''Phoenix universe'' of Georges Lemaître,<ref>{{Cite journal |last=Lemaître |first=Georges |title=The expanding universe |journal=Annales de la Société Scientifique de Bruxelles |volume=47A |pages=49 |date=1933}}, English in ''Gen. Rel. Grav.'' '''29''':641–680, 1997.</ref> the related [[oscillatory universe]] of [[Richard Chase Tolman]],<ref>{{Cite book |author=R. C. Tolman |title= Relativity, Thermodynamics, and Cosmology |location=Oxford |publisher=Clarendon Press |date=1934 |isbn=978-0-486-65383-9 |lccn=34032023}} Reissued (1987) New York: Dover {{ISBN|0-486-65383-8}}.</ref> and the [[Mixmaster universe]] of [[Charles Misner]]. Lemaître and Tolman proposed that a universe undergoing a number of cycles of contraction and expansion could come into thermal equilibrium. Their models failed, however, because of the buildup of [[entropy]] over several cycles. Misner made the (ultimately incorrect) conjecture that the Mixmaster mechanism, which made the Universe ''more'' chaotic, could lead to statistical homogeneity and isotropy.<ref name="mtw" /><ref>{{Cite journal |title=Mixmaster universe |doi=10.1088/1751-8113/41/15/155201 |first1=Charles W. |last1=Misner |date=1969 |journal=[[Physical Review Letters]] |volume=22 |issue=15 |pages=1071–74 |last2=Leach |first2=P G L |bibcode=2008JPhA...41o5201A |s2cid=119588491 }}</ref>