Editing Cosmic inflation (section)

===Flatness problem===
{{Main|Flatness problem}}
The [[flatness problem]] is sometimes called one of the [[Robert H. Dicke|Dicke]] coincidences (along with the [[cosmological constant problem]]).<ref>
{{cite book
 |last=Dicke |first=Robert H.
 |title=Gravitation and the Universe
 |location=Philadelphia, PA
 |publisher=American Philosophical Society
 |date=1970
}}
</ref><ref>
{{cite conference
 |last1=Dicke   |first1=Robert H.
 |last2=Peebles |first2=P.J.E.
 |year=1979
 |title=The big bang cosmology – enigmas and nostrums
 |editor=Hawking, S.W.
 |editor2=Israel, W.
 |book-title=General Relativity: An Einstein centenary survey
 |publisher=Cambridge University Press
}}
</ref> It became known in the 1960s that the density of matter in the Universe was comparable to the [[Critical density (cosmology)|critical density]] necessary for a flat universe (that is, a universe whose large-scale [[geometry]] is the usual [[Euclidean geometry]], rather than a [[non-Euclidean geometry|non-Euclidean]] [[hyperbolic geometry|hyperbolic]] or [[spherical geometry]]).<ref name="Lightman1993">
{{cite book
 |first=Alan P. |last=Lightman
 |title=Ancient Light: Our Changing View of the Universe
 |date=1 January 1993
 |publisher=Harvard University Press
 |isbn=978-0-674-03363-4
}}
</ref>{{rp|style=ama|p= 61}}

Therefore, regardless of the [[shape of the universe]], the contribution of spatial curvature to the expansion of the Universe could not be much greater than the contribution of matter. But as the Universe expands, the curvature [[redshift]]s away more slowly than matter and radiation. Extrapolated into the past, this presents a [[Fine-tuning (physics)|fine-tuning]] problem because the contribution of curvature to the Universe must be exponentially small (sixteen orders of magnitude less than the density of radiation at [[Big Bang nucleosynthesis]], for example). Observations of the cosmic microwave background have demonstrated that the Universe is flat to within a few percent.<ref>
{{cite web |title=WMAP – Content of the Universe |url=http://map.gsfc.nasa.gov/universe/uni_matter.html |website=nasa.gov}}
</ref>