Editing Comoving and proper distances (section)

{{Short description|Measurement of distance}}
{{Redirect|Physical distance|the general concept|Distance (physics)}}
{{Cosmology}}

In [[Big Bang|standard cosmology]], '''comoving distance''' and '''proper distance''' (or physical distance) are two closely related [[distance measures (cosmology)|distance measures]] used by cosmologists to define distances between objects. ''Comoving distance'' factors out the [[expansion of the universe]], giving a distance that does not change in time except due to local factors, such as the motion of a galaxy within a cluster.<ref name=Huterer-2023>{{cite book |last1=Huterer |first1=Dragan |title=A Course in Cosmology |date=2023 |publisher=Cambridge University Press |isbn=978-1-316-51359-0}}</ref> ''Proper distance'' roughly corresponds to where a distant object would be at a specific moment of [[cosmological time]], which can change over time due to the expansion of the universe. Comoving distance and proper distance are defined to be equal at the present time. At other times, the Universe's expansion results in the proper distance changing, while the comoving distance remains constant.