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Horizon problem
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{{Short description|Cosmological fine-tuning problem}} {{about|the astronomical "horizon problem"|the problem relating to artificial intelligence|Horizon effect}} [[File:Horizon problem.svg|thumb|300px|When we look at the [[cosmic microwave background radiation|CMB]] it comes from [[Observable universe#Size|46 billion]] comoving [[light-year]]s away. However, when the light was emitted the universe was much younger (300,000 years old). In that time light would have only reached as far as the smaller circles. The two points indicated on the diagram would not have been able to contact each other because their spheres of causality do not overlap.]] The '''horizon problem''' (also known as the '''homogeneity problem''') is a [[physical cosmology|cosmological]] [[Fine-tuning (physics)|fine-tuning]] problem within the [[Big Bang]] model of the [[universe]]. It arises due to the difficulty in explaining the observed homogeneity of [[Causality (physics)|causally]] disconnected regions of space in the absence of a mechanism that sets the same initial conditions everywhere. It was first pointed out by [[Wolfgang Rindler]] in 1956.<ref name="CarriganTrower1983">{{cite book|last1=Carrigan|title=Magnetic Monopoles |first1=Richard A.|last2=Trower|first2=W. Peter|year=1983|doi=10.1007/978-1-4615-7370-8|isbn=978-1-4615-7372-2 }}</ref> The most commonly accepted solution is [[cosmic inflation]]. Different solutions propose a [[Cyclic model|cyclic universe]] or a [[variable speed of light]].
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