Editing Flatness problem (section)

===Post inflation===
Although inflationary theory is regarded as having had much success, and the evidence for it is compelling, it is not universally accepted: cosmologists recognize that there are still gaps in the theory and are open to the possibility that future observations will disprove it.<ref>{{cite book |last=Albrecht |first=Andreas |title=Proceedings of the NATO Advanced Study Institute on Structure Formation in the Universe, Cambridge 1999 <!--|journal=Nato Asic Proc. 565: Structure Formation in the Universe -->|volume=565 |page=17 |date=August 2000 |arxiv=astro-ph/0007247 |bibcode=2001ASIC..565...17A |isbn=978-1-4020-0155-0}}</ref><ref>{{cite journal|last=Guth |first=Alan |title=Was Cosmic Inflation the 'Bang' of the Big Bang? |date=1997 |journal=The Beamline |volume=27 |url=http://nedwww.ipac.caltech.edu/level5/Guth/Guth_contents.html |access-date=2008-09-07}}</ref> In particular, in the absence of any firm evidence for what the field driving inflation should be, many different versions of the theory have been proposed.<ref name="Bird et al.">{{cite journal|author1=Bird, Simeon |author2-link=Hiranya Peiris |author2=Peiris, Hiranya V. |author3=Easther, Richard |title=Fine-tuning criteria for inflation and the search for primordial gravitational waves |date=July 2008 |journal=Physical Review D|doi=10.1103/PhysRevD.78.083518|volume=78|issue=8|pages=083518|bibcode = 2008PhRvD..78h3518B |arxiv = 0807.3745 |s2cid=118432957 }}</ref> Many of these contain parameters or initial conditions which themselves require fine-tuning<ref name="Bird et al."/> in much the way that the early density does without inflation.

For these reasons work is still being done on alternative solutions to the flatness problem. These have included non-standard interpretations of the effect of dark energy<ref>{{cite journal|last=Chernin |first=Arthur D. |title=Cosmic vacuum and the 'flatness problem' in the concordant model |date=January 2003 |journal=New Astronomy |volume=8|issue=1 |pages=79–83 |bibcode=2003NewA....8...79C|doi=10.1016/S1384-1076(02)00180-X|arxiv = astro-ph/0211489 |s2cid=15885200 }}</ref> and gravity,<ref>{{cite journal|last=Nikolic |first=Hrvoje |title=Some Remarks on a Nongeometrical Interpretation of Gravity and the Flatness Problem |date=August 1999 |journal=General Relativity and Gravitation |volume=31|issue=8 |page=1211 |bibcode=1999GReGr..31.1211N|doi=10.1023/A:1026760304901|arxiv = gr-qc/9901057 |s2cid=1113031 }}</ref> particle production in an oscillating universe,<ref>{{cite journal|last=Anderson |first=P. R. |author2=R. Schokman |author3=M. Zaramensky |title=A Solution to the Flatness Problem via Particle Production in an Oscillating Universe |date=May 1997 |journal=Bulletin of the American Astronomical Society |volume=29 |page=828 |bibcode=1997AAS...190.3806A}}</ref> and use of a [[Bayesian statistics|Bayesian statistical]] approach to argue that the problem is non-existent. The latter argument, suggested for example by Evrard and Coles, maintains that the idea that Ω being close to 1 is 'unlikely' is based on assumptions about the likely distribution of the parameter which are not necessarily justified.<ref>{{cite journal|last=Evrard |first=G |author2=P. Coles |title=Getting the measure of the flatness problem |date=October 1995 |journal=Classical and Quantum Gravity |volume=12|issue=10 |pages=L93–L97 |bibcode=1995CQGra..12L..93E|doi=10.1088/0264-9381/12/10/001|arxiv = astro-ph/9507020 |s2cid=14096945 }}.</ref> Despite this ongoing work, inflation remains by far the dominant explanation for the flatness problem.<ref name="peacock"/><ref name=Ryden />  The question arises, however, whether it is still the dominant explanation because it is the best explanation, or because the community is unaware of progress on this problem.<ref>{{cite journal|last=Holman |first=Marc |title=How Problematic is the Near-Euclidean Spatial Geometry of the Large-Scale Universe?  |date=November 2018 |journal=Foundations of Physics |volume=48|issue=11 |pages=1617–1647 |bibcode=2018FoPh...48.1617H|doi=10.1007/s10701-018-0218-4|arxiv=1803.05148 |s2cid=119066780 }}</ref>  In particular, in addition to the idea that Ω is not a suitable parameter in this context, other arguments against the flatness problem have been presented: if the universe collapses in the future, then the flatness problem "exists", but only for a relatively short time, so a typical observer would not expect to measure Ω appreciably different from 1;<ref>{{cite journal|last=Helbig |first=Phillip |title=Is there a flatness problem in classical cosmology? |date=March 2012 |journal=Monthly Notices of the Royal Astronomical Society |volume=421|issue=1 |pages=561–569 |bibcode=2012MNRAS.421..561H|doi=10.1111/j.1365-2966.2011.20334.x|doi-access=free |arxiv=1112.1666 |s2cid=85526633 }}</ref> in the case of a universe which expands forever with a positive cosmological constant, fine-tuning is needed not to achieve a (nearly) flat universe, but also to avoid it.<ref>{{cite journal|last=Lake |first=Kayll |title=The Flatness Problem and Λ  |date=May 2005 |journal=Physical Review Letters |volume=94|issue=20 |page=201102 |bibcode=2005PhRvL..94t1102L|doi=10.1103/PhysRevLett.94.201102|pmid=16090234 |arxiv=astro-ph/0404319 |s2cid=40500958 }}</ref>