Editing Observable universe (section)

== Overview ==
{{Physical cosmology|comp/struct}}
[[File:Home in Relation to Everything-Observable Universe.png|thumb|upright=1.6|Observable Universe as a function of time and distance, in context of the [[expanding Universe]]]]
The universe's size is unknown, and it may be infinite in extent.<ref>{{cite book |first=Andrew |last=Liddle |date=2015 |publisher=John Wiley |title=An Introduction to Modern Cosmology |isbn=978-1118502143 |url=https://books.google.com/books?id=4lPWBgAAQBAJ&dq=infinite+universe+observable&pg=PA33}}</ref> Some parts of the universe are too far away for the light emitted since the [[Big Bang]] to have had enough time to reach Earth or space-based instruments, and therefore lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so one might expect that additional regions will become observable. Regions distant from observers (such as us) are expanding away faster than the speed of light, at rates estimated by [[Hubble's law]].<ref group="note"> [[Special relativity]] prevents nearby objects in the same local region from moving faster than the speed of light with respect to each other, but there is no such constraint for distant objects when the space between them is expanding; see [[Comoving and proper distances#Uses of the proper distance|uses of the proper distance]] for a discussion.</ref> The [[Accelerating expansion of the universe|expansion rate appears to be accelerating]], which [[dark energy]] was proposed to explain.

Assuming dark energy remains constant (an unchanging [[cosmological constant]]) so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter the observable universe at any time in the future because light emitted by objects outside that limit could never reach the Earth. Note that, because the [[Hubble's law#Interpretation|Hubble parameter]] is decreasing with time, there can be cases where a galaxy that is receding from Earth only slightly faster than light emits a signal that eventually reaches Earth.<ref name=ly93 /><ref>[http://curious.astro.cornell.edu/question.php?number=575 Is the universe expanding faster than the speed of light?] (see the last two paragraphs).</ref> This future visibility limit is calculated at a [[comoving distance]] of 19 billion parsecs (62 billion light-years), assuming the universe will keep expanding forever, which implies the number of galaxies that can ever be theoretically observed in the infinite future is only larger than the number currently observable by a factor of 2.36 (ignoring redshift effects).<ref name="mapofuniverse2" group=note>The comoving distance of the future visibility limit is calculated on p. 8 of Gott et al.'s [http://www.astro.princeton.edu/universe/ms.pdf A Map of the Universe] to be 4.50 times the [[Hubble radius]], given as 4.220 billion parsecs (13.76 billion light-years), whereas the current comoving radius of the observable universe is calculated on p. 7 to be 3.38 times the Hubble radius. The number of galaxies in a sphere of a given comoving radius is proportional to the cube of the radius, so as shown on p. 8 the ratio between the number of galaxies observable in the future visibility limit to the number of galaxies observable today would be (4.50/3.38)<sup>3</sup> = 2.36.</ref>

In principle, more galaxies will become observable in the future; in practice, an increasing number of galaxies will become extremely [[redshift]]ed due to ongoing expansion, so much so that they will seem to disappear from view and become invisible.<ref>{{cite journal |last1=Krauss |first1=Lawrence M. |last2=Scherrer |first2=Robert J. |date=2007 |title=The Return of a Static Universe and the End of Cosmology |journal=General Relativity and Gravitation |volume=39 |issue=10 |pages=1545–1550 |arxiv=0704.0221 |bibcode=2007GReGr..39.1545K |doi=10.1007/s10714-007-0472-9 |s2cid=123442313}}</ref><ref>[https://www.npr.org/templates/story/story.php?storyId=102715275 Using Tiny Particles To Answer Giant Questions]. Science Friday, 3 Apr 2009. According to the [https://www.npr.org/templates/transcript/transcript.php?storyId=102715275 transcript], [[Brian Greene]] makes the comment "And actually, in the far future, everything we now see, except for our local galaxy and a region of galaxies will have disappeared. The entire universe will disappear before our very eyes, and it's one of my arguments for actually funding cosmology. We've got to do it while we have a chance."</ref><ref>See also [[Faster than light#Universal expansion]] and [[Future of an expanding universe#Galaxies outside the Local Supercluster are no longer detectable]].</ref> A galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its history, say, a signal sent from the galaxy only 500 million years after the Big Bang. Because of the universe's expansion, there may be some later age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future, so, for example, we might never see what the galaxy looked like 10 billion years after the Big Bang,<ref name=Loeb2002/> even though it remains at the same comoving distance less than that of the observable universe. 

This can be used to define a type of cosmic [[event horizon]] whose distance from the Earth changes over time. For example, the current distance to this horizon is about 16 billion light-years, meaning that a signal from an event happening at present can eventually reach the Earth if the event is less than 16 billion light-years away, but the signal will never reach the Earth if the event is further away.<ref name=ly93 />

The space before this cosmic event horizon can be called "reachable universe", that is all galaxies closer than that could be reached if we left for them today, at the speed of light; all galaxies beyond that are unreachable.<ref>{{Cite web |last=Siegel |first=Ethan |title=How Much Of The Unobservable Universe Will We Someday Be Able To See? |url=https://www.forbes.com/sites/startswithabang/2019/03/05/how-much-of-the-unobservable-universe-will-we-someday-be-able-to-see/ |access-date=2023-04-04 |website=Forbes |language=en}}</ref><ref>{{Cite web |last=Siegel |first=Ethan |date=2021-10-25 |title=94% of the universe's galaxies are permanently beyond our reach |url=https://medium.com/starts-with-a-bang/94-of-the-universes-galaxies-are-permanently-beyond-our-reach-293c29e771be |access-date=2023-04-04 |website=Starts With A Bang! |language=en}}</ref> Simple observation will show the future visibility limit (62 billion light-years) is exactly equal to the reachable limit (16 billion light-years) added to the current visibility limit (46 billion light-years).<ref>Ord, Toby. (2021). The Edges of Our Universe. [https://www.researchgate.net/publication/350647191_The_Edges_of_Our_Universe]</ref><ref name="mapofuniverse" />

[[File:Home in Relation to Everything-Reachable Universe.png|thumb|upright=1.6|The reachable Universe as a function of time and distance, in context of the expanding Universe.]]