Editing Particle horizon (section)

{{Short description|Distance measurement used in cosmology}}

{{merge to|Cosmological horizon|discuss=Talk:Cosmological horizon#Merge proposal|date=April 2025}}

The '''particle horizon''' (also called the '''cosmological horizon''', the '''comoving horizon''' (in [[Scott Dodelson]]'s text), or the '''cosmic light horizon''') is the maximum distance from which light from [[Elementary particle|particle]]s could have traveled to the [[observation|observer]] in the [[age of the universe]]. Much like the concept of a [[horizon|terrestrial horizon]], it represents the boundary between the observable and the unobservable regions of the universe,<ref name="books.google.com">{{Cite book |last=Harrison |first=Edward R. |url=https://books.google.com/books?id=kNxeHD2cbLYC&pg=PA447 |title=Cosmology: the science of the universe |publisher=Cambridge University Press |year=2000 |isbn=978-0-521-66148-5 |edition=2nd |location=Cambridge |pages=447–}}</ref> so its distance at the present epoch defines the size of the [[observable universe]].<ref>{{Cite book |last1=Liddle |first1=Andrew R. |url=https://books.google.com/books?id=XmWauPZSovMC&pg=PA24 |title=Cosmological inflation and large-scale structure |last2=Lyth |first2=David H. |date=2000 |publisher=Cambridge University Press |isbn=978-0-521-57598-0 |location=Cambridge |pages=24–}}</ref> Due to the expansion of the universe, it is not simply the [[age of the universe]] times the [[speed of light]] (approximately 13.8 billion light-years), but rather the speed of light times the conformal time. The existence, properties, and significance of a cosmological horizon depend on the particular [[physical cosmology|cosmological model]].