Editing Observable universe (section)

== "The universe" versus "the observable universe" ==
Both popular and professional research articles in cosmology often use the term "universe" to mean "observable universe".{{citation needed|date=September 2015}} This can be justified on the grounds that we can never know anything by direct observation about any part of the universe that is [[Causality (physics)|causally disconnected]] from the Earth, although many credible theories require a total universe much larger than the observable universe.{{citation needed|date=September 2015}} No evidence exists to suggest that the boundary of the observable universe constitutes a boundary on the universe as a whole, nor do any of the mainstream cosmological models propose that the universe has any physical boundary in the first place. However, some models propose it could be finite but unbounded,<ref group=note>This does not mean "unbounded" in the mathematical sense; a finite universe would have an upper bound on the distance between two points.  Rather, it means that there is no boundary past which there is nothing. See ''[[Geodesic manifold]]''.</ref> like a higher-dimensional analogue of the 2D surface of a sphere that is finite in area but has no edge.

It is plausible that the [[galaxy|galaxies]] within the observable universe represent only a minuscule fraction of the galaxies in the universe. According to the theory of [[cosmic inflation]] initially introduced by [[Alan Guth]] and [[D. Kazanas]],<ref>{{cite journal|doi=10.1086/183361|title=Dynamics of the universe and spontaneous symmetry breaking|journal=The Astrophysical Journal|volume=241|pages=L59–L63|year=1980|last1=Kazanas|first1=D.|bibcode=1980ApJ...241L..59K|doi-access=free}}</ref> if it is assumed that inflation began about 10<sup>−37</sup> seconds after the Big Bang and that the pre-inflation size of the universe was approximately equal to the speed of light times its age, that would suggest that at present the entire universe's size is at least {{val|1.5|e=34}} light-years — this is at least {{val|3|e=23}} times the radius of the observable universe.<ref>{{cite book |author=Guth |first=Alan H. |url=https://archive.org/details/inflationaryuniv0000guth |title=The inflationary universe: the quest for a new theory of cosmic origins |publisher=Basic Books |year=1997 |isbn=978-0201328400 |pages=[https://archive.org/details/inflationaryuniv0000guth/page/186 186]– |access-date=1 May 2011 |url-access=registration}}</ref>

If the universe is finite but unbounded, it is also possible that the universe is ''smaller'' than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. Bielewicz et al.<ref>{{Cite journal|last1=Bielewicz |first1=P. |last2=Banday |first2=A. J. |last3=Gorski |first3=K. M. |date=2013 |arxiv=1303.4004 |title=Constraints on the Topology of the Universe|journal=Proceedings of the XLVIIth Rencontres de Moriond|editor-first1=E. |editor-last1=Auge|editor-first2=J.|editor-last2=Dumarchez|editor-first3=J.|editor-last3=Tran Thanh Van|volume=2012 |issue=91 |bibcode=2013arXiv1303.4004B }}</ref> claim to establish a lower bound of 27.9 gigaparsecs (91 billion light-years) on the diameter of the last scattering surface. This value is based on matching-circle analysis of the [[WMAP]] 7-year data. This approach has been disputed.<ref>{{cite arXiv |eprint=1007.3466 |last1=Mota |first1=B. |last2=Reboucas |first2=M. J. |last3=Tavakol |first3=R. |title=Observable circles-in-the-sky in flat universes |class=astro-ph.CO |date=1 July 2010}}</ref>