Editing Flatness problem (section)

{{short description|Cosmological fine-tuning problem}}
[[File:End of universe.jpg|thumb|275px|The local geometry of the universe is determined by whether the relative density Ω is less than, equal to or greater than 1. From top to bottom: a [[spherical]] universe with greater than critical density (Ω>1, k>0); a [[hyperbolic 3-manifold|hyperbolic]], underdense universe (Ω<1, k<0); and a flat universe with exactly the critical density (Ω=1, k=0). The spacetime of the universe is, unlike the diagrams, four-dimensional.]]

The '''flatness problem''' (also known as the '''oldness problem''') is a [[physical cosmology|cosmological]] [[Fine-tuned universe|fine-tuning]] problem within the [[Big Bang]] model of the universe. Such problems arise from the observation that some of the initial conditions of the universe appear to be fine-tuned to very 'special' values, and that small deviations from these values would have extreme effects on the appearance of the universe at the current time.

In the case of the [[Flatness (cosmology)|flatness]] problem, the parameter which appears fine-tuned is the [[density of the universe|density of matter and energy in the universe]]. This value affects the curvature of space-time, with a very specific [[Critical density (cosmology)|critical value]] being required for a flat universe. The current density of the universe is observed to be very close to this critical value. Since any departure of the total density from the critical value would increase rapidly over [[cosmic time]],<ref name="peacock">{{cite book |last= Peacock|first=J. A. |title= Cosmological Physics |url= https://archive.org/details/cosmologicalphys0000peac|url-access= registration|date= 1998|publisher=Cambridge University Press |location=Cambridge |isbn= 978-0-521-42270-3}}</ref> the early universe must have had a density even closer to the critical density, departing from it by one part in 10<sup>62</sup> or less. This leads cosmologists to question how the initial density came to be so closely fine-tuned to this 'special' value.

The problem was first mentioned by [[Robert Dicke]] in 1969.<ref name="Dicke1970">{{cite book|author=Robert H. Dicke|title=Gravitation and the Universe: Jayne Lectures for 1969|date=1970|publisher=American Philosophical Society|isbn=978-0871690784}}</ref>{{rp|62,}}<ref name="Lightman1993">{{cite book|author=Alan P. Lightman|title=Ancient Light: Our Changing View of the Universe|url=https://books.google.com/books?id=nvk9sqbFe3UC|date=1 January 1993|publisher=Harvard University Press|isbn=978-0-674-03363-4}}</ref>{{rp|61}} The most commonly accepted solution among cosmologists is [[cosmic inflation]], the idea that the universe went through a brief period of extremely rapid expansion in the first fraction of a second after the Big Bang; along with the [[monopole problem]] and the [[horizon problem]], the flatness problem is one of the three primary motivations for inflationary theory.<ref name="Ryden">{{cite book|author=Ryden|first=Barbara|title=Introduction to Cosmology|date=2002|publisher=Addison Wesley|isbn=978-0-8053-8912-8|location=San Francisco|pages=|author-link=Barbara Ryden}}</ref>