Editing Horizon problem (section)

==Inflationary model==
{{further|Inflation (cosmology)}}
[[File:Spacetime Diagram with Inflation.png|thumb|400x400px|This spacetime diagram shows how inflation changes the light cones for two light particles spaced some distance apart at the time of last scattering (ls) to allow them to intersect. In this scenario, they are in causal contact and can exchange information with one another. The horizontal axis is comoving distance, the vertical axis is conformal time, and the units have the speed of light as 1. For [http://courses.las.illinois.edu/spring2020/astr507/ reference].]]
The theory of cosmic inflation has attempted to address the problem by positing a 10{{sup|−32}}-second period of exponential expansion in the first second of the history of the universe due to a scalar field interaction.<ref>[http://ned.ipac.caltech.edu/level5/Watson/Watson_contents.html An Exposition on Inflationary Cosmology, Gary Scott Watson, Dept. of Physics, Brown University]</ref> According to the inflationary model, the universe increased in size by a factor of more than 10{{sup|22}}, from a small and causally connected region in near equilibrium.<ref name="RemmenCarroll2014">{{cite journal|last1=Remmen|first1=Grant N.|last2=Carroll|first2=Sean M.|title=How many e-folds should we expect from high-scale inflation?|journal=Physical Review D|volume=90|issue=6|pages=063517|year=2014|issn=1550-7998|doi=10.1103/PhysRevD.90.063517|arxiv = 1405.5538 |bibcode = 2014PhRvD..90f3517R |s2cid=37669055}}</ref> Inflation then expanded the universe rapidly, isolating nearby regions of spacetime by growing them beyond the limits of causal contact, effectively "locking in" the uniformity at large distances. Essentially, the inflationary model suggests that the universe was entirely in causal contact in the very early universe. Inflation then expands this universe by approximately 60 e-foldings (the scale factor increases by factor <math>e^{60}</math>). We observe the CMB after inflation has occurred at a very large scale. It maintained thermal equilibrium to this large size because of the rapid expansion from inflation.

One consequence of cosmic inflation is that the [[Anisotropy|anisotropies]] in the Big Bang due to [[quantum fluctuation]]s are reduced but not eliminated. Differences in the temperature of the cosmic background are smoothed by cosmic inflation, but they still exist. The theory predicts a spectrum for the anisotropies in the microwave background which is mostly consistent with observations from [[WMAP]] and [[Cosmic Background Explorer|COBE]].<ref>[http://www.sciam.com/article.cfm?articleID=0009F18C-C8F1-12DB-882283414B7F0000 Starkman, Glenn D. and Dominic J. Schwarz; Scientific American (subscription required)]</ref>

However, gravity alone may be sufficient to explain this homogeneity.<ref>{{cite web |url=https://medienportal.univie.ac.at/presse/aktuelle-pressemeldungen/detailansicht/artikel/gravity-causes-homogeneity-of-the-universe/ |title=Gravity causes homogeneity of the universe |date=22 September 2020 |author=Fajman, David }}</ref>