Editing Big Bang (section)

===Flatness problem===
[[File:End of universe.jpg|thumb|upright=1.5|The overall [[Shape of the universe|geometry of the universe]] is determined by whether the [[Friedmann equations#Density parameter|Omega cosmological parameter]] is less than, equal to or greater than 1. Shown from top to bottom are a [[Shape of the universe#Universe with positive curvature|closed universe]] with positive curvature, a [[Shape of the universe#Universe with negative curvature|hyperbolic universe]] with negative curvature and a [[Shape of the universe#Universe with zero curvature|flat universe]] with zero curvature.]]

The flatness problem (also known as the oldness problem) is an observational problem associated with a FLRW.<ref name="kolb_c8"/> The universe may have positive, negative, or zero spatial [[curvature]] depending on its total energy density. Curvature is negative if its density is less than the critical density; positive if greater; and zero at the critical density, in which case space is said to be ''flat''. Observations indicate the universe is consistent with being flat.<ref name=Filippenko2002>{{cite magazine |last1=Filippenko |first1=Alexei V. |author1-link=Alex Filippenko |last2=Pasachoff |first2=Jay M. |author2-link=Jay Pasachoff |date=March–April 2002 |title=A Universe from Nothing |url=http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |url-status=dead |magazine=[[Mercury (magazine)|Mercury]] |volume=31 |issue=2 |page=15 |bibcode=2002Mercu..31b..15F |access-date=10 March 2010 |archive-url=https://web.archive.org/web/20131022135932/http://www.astrosociety.org/pubs/mercury/31_02/nothing.html |archive-date=22 October 2013}}</ref><ref name="Krauss2009">{{cite AV media |url=https://www.youtube.com/watch?v=7ImvlS8PLIo |title='A Universe From Nothing' by Lawrence Krauss, AAI 2009 |date=21 October 2009 |medium=Video |language=en-us |publisher=[[Richard Dawkins Foundation for Reason and Science]] |location=Washington, D.C. |access-date=17 October 2011 |archive-url=https://ghostarchive.org/varchive/youtube/20211123/7ImvlS8PLIo |archive-date=2021-11-23 |url-status=live |people=[[Lawrence M. Krauss]] (Speaker); R. Elisabeth Cornwell (Producer)}}{{cbignore}}</ref>

The problem is that any small departure from the critical density grows with time, and yet the universe today remains very close to flat.<ref group="notes">Strictly, dark energy in the form of a cosmological constant drives the universe towards a flat state; however, our universe remained close to flat for several billion years before the dark energy density became significant.</ref> Given that a natural timescale for departure from flatness might be the [[Planck time]], 10<sup>−43</sup> seconds,<ref name="HTUW"/> the fact that the universe has reached neither a [[heat death of the universe|heat death]] nor a [[Big Crunch]] after billions of years requires an explanation. For instance, even at the relatively late age of a few minutes (the time of nucleosynthesis), the density of the universe must have been within one part in 10<sup>14</sup> of its critical value, or it would not exist as it does today.<ref>{{harvnb|Hawking|Israel|2010|pp=504–517|loc=chpt. 9: "The big bang cosmology — enigmas and nostrums" by [[Robert H. Dicke]] and [[Jim Peebles|Phillip J.E. Peebles]].}}</ref>