The

**flatness problem** is an issue of

cosmology that the theory of

cosmological inflation answers. It is also closely related to the

horizon problem.
Our

universe began with a

big bang and has been expanding ever since, it has neither recollapsed on itself in a

big crunch, of flown off to be (nearly)

infinitely large. The speed of

expansion is

moderated by the total amount of

mass present in the universe, and therefore there is a

critical density of matter in the universe, above which the universe recollapses, below which the universe expands forever. This relationship; the ratio of the average density of the universe, to the critical density is often termed as

*Omega*. Omega values of less than one give an ever expanding universe, greater than one, a collapsing universe and when

*exactly* one a universe that neither grows or collapses. Cosmologists refer to a universe with an omega value of one, as being

*flat*.
If you model the

big bang 'traditionally' you find that during the initial expansion, the average density of the universe and the critical density fluctuated hugely. It would have only taken a very small difference (only about 10

^{-15}) in the early stages to give rise to a universe we couldn't live in today. The problem of what could so finely tune our universe to have a omega value that is near to one is the

*flatness problem*.
With an

inflationary model of the big bang however the 10

^{30} (at least!) expansion factor quickly dampened any irregularities in space, making omega near one, no matter what it's initial value.
NB. Current

observations can only find enough matter to give an omega value of 0.1, making it increasingly likely the universe will carry on expanding until the

final heat death of the universe.